Assembling 3D MEMS Structures by Folding, Aligning and
Latching 2D Patterned Films
by
Nader S. Shaar
B.S., American University of Beirut
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
MASSACHUSETTS INSiTRE
OF TECHNOLOGY
at the
MAY 082014
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
LIBRARIES
February 2014
@
Massachusetts Institute of Technology 2014
/
Signature of A uthor.....
............................................
Department of Mechanical Engineering
77
Certified bK.
September 26, 2013
...
. ..... ........ ............ ...............................
George Barbastathis
Professor of Mechanical Engineering
Research Head
/I
Certified by. ..
....................
Carol Livermore
Associate Professor, Northeastern University
Thesis Supervisor
A ccepted by ..............
................
......................................
David Hardt
Chairman, Department Committee on Graduate Students
Assembling 3D MEMS Structures by Folding, Aligning and Latching 2D
Patterned Films
by
Nader S. Shaar
Submitted to the Department of Mechanical Engineering
on September 26, 2013, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Abstract
The techniques used in the fabrication of micro-electro-mechanical systems (MEMS) were
adopted from the integrated circuits (IC) industry and are mostly limited to patterning thin
films on a flat substrate. As a consequence, micro-machined devices mostly comprise sets of
flat two-dimensional (2D) membranes with etched patterns and undercuts that enable them to
serve their intended functions. However, many mechanical, optical and biological applications,
such as corner-cube retro reflectors, micro-scale magnetometers, 3D microfluidic systems and
3D photonic crystals, require three-dimensional (3D) geometries for their functionality. In addition, 3D circuits have also emerged as a way of improving connectivity and reducing power
dissipation in electronic chips. However, the creation of fully 3D structures via conventional
MEMS fabrication techniques typically requires processes that have low throughput, limited
control over the final geometry, and higher costs. A promising alternative to 3D microfabrication that addresses these challenges while requiring minimal investment in a new infrastructure
is to use the existing technologies to pattern in 2D, and then assemble the patterned segments
into 3D structures. Demonstrated methods to achieve that objective have been limited in scope,
requiring manual assembly or with limited applicability to specific architectures. This thesis
presents a coherent modular system for folding, aligning and latching 2D-patterned precursors
into prescribed 3D structures.
The system presented here comprises flexure hinges to enable relative motion among the
2D precursors, a cascaded alignment system to provide progressively better alignment among
precursors as they approach their final positions, and systems of reversible latches to retain the
assembly in its final configuration while, optionally, permitting disassembly and reassembly of
the structure. In particular, two types of systems are considered. First, the design, fabrication
and testing of polymer structures with metal hinges, cascaded alignment features and integrated latching mechanisms are presented for perpendicular assembly of structures. Second, an
alternative latching technique using controlled melting of photoresist polymer adhesive pads is
analyzed and tested for the parallel assembly of structures. The structures discussed in this thesis consist of SU-8 polymer segments patterned on silicon wafers and linked with an underlying
thin gold pattern that defines the hinges. The elasto-plastic bending of the hinges is analyzed
and simulated to predict the trajectory and angular position of the membranes during folding.
The design of cascaded alignment features, consisting of triangular protrusions and corresponding rhombic holes, is discussed. A kinematic model of the alignment mechanism is presented to
2
demonstrate the effectiveness of the cascading aspect of the design to achieve a large range of
angular correction and high alignment accuracy at the same time. The design of micro snap-fit
latches that work in conjunction with the alignment system is also presented, and quasi-static
simulations of the elastic bending of latches is used to evaluate their strength. Experimental
measurements were conducted to characterize the behavior of the gold hinges during bending,
demonstrating good agreement with models. The integrated folding-alignment-latching system
was demonstrated by assembling corner-cube structures. The alignment process was found to
be accurate to within 1 from measurements of the final assembled position of the corner cube
structure. The system was also shown to support fabricating reconfigurable devices by demonstrating the ability to unlatch and re-latch segments. The latching and unlatching forces were
measured to be 9.7 pN and 12.3 pN respectively.
Research Head: George Barbastathis
Title: Professor of Mechanical Engineering
Thesis Supervisor: Carol Livermore
Title: Associate Professor, Northeastern University
3
Contents
1
Introduction
1.1
Overview of MEMS Development .......
1.2
3D Microfabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3
2
3
16
...........................
17
18
1.2.1
Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.2.2
Direct 3D writing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.2.3
Building 3D structures from 2D components
. . . . . . . . . . . . . . . . 21
......................................
Thesis outline ..........
23
26
Design
2.1
A ctuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2
Hinge Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3
Numerical Simulations of Rectangular Hinges . . . . . . . . . . . . . . . . . . . . 38
2.4
Constricted Hinges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.5
Edge-to-Face Alignment and Latching . . . . . . . . . . . . . . . . . . . . . . . . 46
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.5.1
Alignment Features
2.5.2
Micro Snap-Fit Latches
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
53
Fabrication
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
3.1
Process overview
3.2
Metal layer patterning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3
SU-8 patterning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.4
Patterning photoresist pads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4
3.5
4
5
6
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Dry release
Experimental Setup and Measurement Tools
66
4.1
Experimental Setup ............
66
4.2
Magnetic Field Characterization
68
4.3
Angular Measurement . . . . . .
70
4.3.1
GUI Interface and Usage
73
4.3.2
GUI Evaluation . . . . . .
75
77
Metal Hinge Folding
5.1
Fabricated Devices
. . . . . . .
77
5.2
Folding Measurements . . . . .
78
5.3
Discussion . . . . . . . . . . . .
82
Cascaded Mechanical Alignment
84
. . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.1.1
Coupling measurement .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.1.2
Final angle measurement
. . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.1
6.2
Measurement Protocols
Experimental Results . . . . . . . . . . .
88
92
7 Micro Snap-Fit Latches
7.1
Fabrication Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
7.2
Latching and Unlatching Measurements
100
8 Face-to-Face Latching
8.1
. . . . . . . . . . . . . . . . . . . . . . . 96
Concept and Design Considerations . . .
101
8.1.1
Addition of micro-heaters
. .
101
8.1.2
Structural layer modifications
102
8.1.3
Patterning the adhesion pads
102
8.2
Thermal simulations . . . . . . . . . . .
103
8.3
Experimental results . . . . . . . . . . .
104
5
9
109
Conclusion
9.1
Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
9.2
Error analysis and reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
9.3
Roadmap to production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
9.3.1
Face-to-face interaction
. . . . . . . . . . . . . . . . . . . . . . . . . . . .113
9.3.2
Integration of face-to-face and edge-to-face systems . . . . . . . . . . . . . 114
9.3.3
Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
A Lithography mask layouts
122
B Fabrication Processes
125
C Software Code
127
C.1 MATLAB GUI code for angular measurements . . . . . . . . . . . . . . . . . . . 127
C.2 Arrow-head latch strength simulations . . . . . . . . . . . . . . . . . . . . . . . . 146
C.3 Thermal simulation of heat pads
. . . . . . . . . . . . . . . . . . . . . . . . . . . 151
6
List of Figures
2-1
Schematic showing how various interactions of 2D patterned membranes can be
reduced to a combination of edge-to-face and face-to-face interactions.
2-2
. . . . . . 27
Lorentz force actuation of a released segment. With the current running into the
page, a vertical or horizontal force can be generated by applying a horizontal or
vertical magnetic field, respectively . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2-3
Force and bending moment acting on the gold hinge . . . . . . . . . . . . . . . . 30
2-4
Shear force and bending moment distribution along the hinge length . . . . . . . 32
2-5
Strain distribution in a beam under bending . . . . . . . . . . . . . . . . . . . . . 32
2-6
Stress vs. strain diagram of gold
2-7
Stress distribution in a beam section in the elastic regime . . . . . . . . . . . . . 33
2-8
Stress distribution across a hinge section in plastic and elastopoastic deformation 35
2-9
Diagram of a segment during folding . . . . . . . . . . . . . . . . . . . . . . . . . 37
. . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2-10 Tip angle deflection of a hinge as a function of the applied vertical force. . . . . . 39
2-11 Bending moment distribution for vertical loads . . . . . . . . . . . . . . . . . . . 40
2-12 Profile of bent hinge for vertical loads. . . . . . . . . . . . . . . . . . . . . . . . . 40
2-13 Force (at 450) vs fold angle of the membrane. . . . . . . . . . . . . . . . . . . . . 41
2-14 Bending moment distribution for loads at 450 . . . . . . . . . . . . . . . . . . . . .
42
2-15 Profile of the bent hinge for loads at 450 . . . . . . . . . . . . . . . . . . . . . . . 42
2-16 Force vs. deflection angle for horizontal loading . . . . . . . . . . . . . . . . . . . 43
2-17 Bending moment distribution for horizontal loads . . . . . . . . . . . . . . . . . . 43
2-18 Bent hinge profile for different horizontal loads . . . . . . . . . . . . . . . . . . . 44
2-19 Force vs deflection angle for loads in multiple directions . . . . . . . . . . . . . . 45
7
2-20 Top views of two fabricated devices showing (a) the straight rectangular gold
hinges and (b) the constricted hinges. The dark grey areas are the SU-8 structural
segments of the devices and the light grey is the underlying silicon substrate . . . 46
2-21 Schematic diagram of a corner cube structure with three alignment feature pairs
(a) in its flat as-fabricated configuration and (b) during the assembly process
. . 47
2-22 Front and side views of the cascaded alignment system at the onset of alignment.
The lower alignment feature pair is engaged while the upper pair is not in contact
yet ...........
48
...........................................
2-23 Schematic of corner-cube in its final assembled position with a close up view of
an alignment feature. Section views of the feature pair show the final relative
position of the rhombus and the traingle . . . . . . . . . . . . . . . . . . . . . . . 49
2-24 Plot of the distance from the hinge to the alignment feature vs. range of correction (left axis) and the sensitivity of the alignment feature to variations in the
film thickness (right axis)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2-25 Schematic diagram of a latching feature showing the cantilever and arrowhead
tip in their free-standing and loaded configurations . . . . . . . . . . . . . . . . . 51
2-26 Simulated insertion and extraction forces for a micro snap-fit latch as they vary
with its position relative to the slit in the mating segment . . . . . . . . . . . . . 52
3-1
Schematic of the final fabrication process: gold lift-off, SU-8 spinning and photolithography and XeF 2 isotropic dry release etch.
3-2
. . . . . . . . . . . . . . . . . 55
Optical micrographs of a sample device (a) before and (b) after the XeF 2 dry
isotropic etch; the last step of the fabrication process. The outline of the resleased
segments is highlighted with white lines in (b) for clarity, since the structure is
transparent..........
3-3
........................................
Schematic of the gold wet etching process.
56
Gold is evaporated on a Silicon
susbtrate, photoresist is patterned on top of the gold layer, the gold layer is
etched in sulfuric acid, and, finally, the photoresist is stripped by in an asher
with
3-4
oxygen plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Optical images of 81Lm gold features patterned (a) by wet etching with a photoresist hard mask and (b) with a liftoff process . . . . . . . . . . . . . . . . . . . 57
8
3-5
Optical images of the SU-8 layer during processing. The pattern edges start to
appear at the first stage of crosslinking, after the post-exposure bake . . . . . . . 59
3-6
Delamination of the SU-8 layer due to prolonged development. The developer
seeps under the polymer layer detaching it from the underlying layers
3-7
Schematic of the patterning steps of the photoresist pads, between patterning
the SU-8 layer and the dry release etch in XeF 2 plasma
3-8
. . . . . . 60
. . . . . . . . . . . . . . 61
Thickness profile of a photoresist pad after spinning one layer of AZ4620 over
the SU-8 layer (lower curve) and after spinning a second layer (upper curve) . . . 62
3-9
Sample images during the XeF 2 release etching step.
Devices at 5 different
locations of the wafer were observed after each of 3 rounds of etching. Each
round consisted of 90 cycles, each 60 sec long
4-1
. . . . . . . . . . . . . . . . . . . . 64
Images showing the test setup with (a) the device mounted on the ceramic chip
holder, (b) the magnetic stack attached to the chip with lead wires connected to
the pins of the chip holder, and (c) the circuit board placed under the microscope
for m easurement
4-2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Lateral measurements of the magnetic field taken on and off the center axis of
the magnet, along the x and y directions.
. . . . . . . . . . . . . . . . . . . . . . 69
4-3
Normalized magnetic field measurements along the lateral axes (x and y).
4-4
Decay of the magnetic field along the vertical, z-axis, away from the magnet.
The zero reference position was chosen to be the surface of the chip holder.
4-5
. . .
70
. . . 71
Extrapolated lateral profile of the normalized magnetic field values at different
distances from the surface of the chip holder. The profile becomes flatter as the
magnetic flux drops, with increased distance.
4-6
. . . . . . . . . . . . . . . . . . . . 72
Front-end of the MATLAB GUI used for measuring the angular position of the
membranes from optical imagines.
The different regions of the interface are
highlighted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4-7
Plot of GUI measurements of the test images samples. The actual values of the
angles of the drawn lines are superimposed showing accurate overlap of the data
4-8
75
Means and standard deviations of the errors in the GUI measurement of the
control im age set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
9
5-1
Optical micrographs of the two types of patterned hinges: (a) straight uniform
width and (b) constricted width in the middle section. SU-8 appears as dark
grey, hinges appear as a light gold, and the surrounding exposed silicon surface
appears as light grey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5-2
Optical image of the patterned gold layer, before adding the SU-8 structural
layer on top, showing the hinges as well as the wires used to actuate the device
segm ents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5-3
Sample set of images taken using the microscope camera during the folding of a
segment. The bright line in the image is the edge of the membrane seen from
the side. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5-4
Current and voltage measurements for a device segment during folding and release. The data is fitted with a line to predict the resistance in the circuit. . . . . 81
5-5
Tangential component of the Lorentz actuation force vs. deflection angle of the
folded segment for a sample device . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5-6
Tangential actuation force vs. deflection angle measurements for several devices . 82
6-1
SEM micrograph of a cornercube structure in its initial unfolded configration
showing the three aligmnent feature pairs distributed along the edges of the
segments to be folded
6-2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Plot of the tangentail actuation force vs. deflection angle of a membrane showing
the elasto-plastic deformation from the initial 380 to the platically deformed state
at 700 followed by repeatable elastic cycling of the membrane in a range up to
1050 ...............
6-3
...........................................
...
87
Optical snapshots of a device during alignment. The aligning segments is seen
coming fully into focus as it is folded from the back into the imaging plane. The
target segment being aligned is seen from the side, as a translucent blur, with
its far edge being in focus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6-4
Close-up SEM image of an alignment feature pair with the triangular protrusion
fully inserted into the rhombus hole and aligned to its central axis. . . . . . . . . 89
6-5
Optical images of a fully assembled cornercube structure from different angles
.
90
6-6
Histogram of the angular measurements of the final corner cube assemblies. .
.
91
10
7-1
Mask layout of the first generation latching features showing the dimensions of
an arrowhead latch and its corresponding slit . . . . . . . . . . . . . . . . . . . . 93
7-2
Overlay of the CAD mask pattern onto SEM images of the fabricated latches
and etch holes. The only significant mismatch is the rounding at the corners.
7-3
. . 94
Mask layout of the second generation latching features showing (a) the dimensions of the arrowhead profile and (b) an overlay of the cross-section of the
corresponding slit in its latched state . . . . . . . . . . . . . . . . . . . . . . . . . 95
7-4
SEM images of the two latch designs . . . . . . . . . . . . . . . . . . . . . . . . . 95
7-5
SEM image of a fully latched corner-cube structure with 3 alignment pairs and
3 latches..........
7-6
.........................................
96
Schematic of the measurement setup with two independant currents used to
actuate the two segments in a vertical external magnetic field. . . . . . . . . . . . 97
7-7
Raw angle and force measurements of a corner cube segment during latching:
(a) A chronological plot of the angular position (left axis) and the actuation
force (right axis). (b) A plot of the force vs. angular position for two cycles of
latching/unlatching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7-8
Actuation force vs. angular position during a latching/unlatching cycle with the
force values offset to account for the spring-back force in the hinge. . . . . . . . . 99
8-1
Schematic of (a) the overall layout of a device with micro-heaters and (b) a zoomin onto the corner of the device segment showing one of the micro-heaters and
the folding actuation wire passing around it . . . . . . . . . . . . . . . . . . . . . 101
8-2
Profile measurements of the photoresist pads after the first spin showing (a) a
relatively uniform thickness in a device with a smaller pad close to the center
axis of the wafer, and (b) a sloped profile of the resist in a device with a larger
hole that is off-axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
8-3
Simulated temperature profiles plotted vs. the radial position along the photoresist pad. Temperature profiles are shown for a series of times after the current
flow begins (from 0 to 14msec) to capture the profile as it progresses from a
starting room temperature profile towards its equilibrium profile. . . . . . . . . . 104
8-4
Step response of the microheater-pad system with an input current of 23mA . . . 105
11
8-5
SEM images of a microscale capacitor prior to folding (top) and with one electrode folded and latched on top of the other (bottom)
8-6
. . . . . . . . . . . . . . . 105
Optical and SEM images of a photoresist polymer pad in its patterned state (left)
and after melting (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
8-7
A histogram showing the currents required in experiments for melting the AZ
P4620 polymer pads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
9-1
Representative diagram of a 24-channel LabView-controlled circuit for automating the assembly of the 3D structures. The LabView panel is connected to a data
acquisition card that sends serial signals to digital to analog converters, which
control the current in a particular channel via a MOSFET transistor circuit. . . . 115
A-i Grid of 14x14 dies patterned on a single 6 inch wafer. Two cross-hair alignment
patterns were placed in row #7 of each layer and used to align the mask of a
particular layer to a previously patterned film on the wafer. . . . . . . . . . . . . 123
A-2 Mask layout of the structural layer of a corner-cube device showing the two
sidewall segments, marked with 'M' and 'T' patterns.
The segments have 3
alignment feature pairs and 3 micro snap-fit latches pattered at the mating edge.
The pattern also shows the array of etch holes in the SU-8 layer. . . . . . . . . . 123
A-3 Mask layout of super-capacitor devices showing the six contact pads on the periphery and the two rectangular electrodes in the middle. The close-up view
shows the resistor wire pattern used to heat the photoresist pads. . . . . . . . . . 124
12
List of Tables
2.1
Material properties for gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.1
Product specifications of the B662-N52 block magnets (courtesy of http://kjmagnetics.com) 68
13
Acknowledgements
This thesis summarizes my research work during my Ph.D. studies at MIT. My experience in
the program would not have been as rewarding and enjoyable without the contributions of a
great number of people and organizations that have gotten integrated into my daily life at MIT
and in the Boston area.
I owe my advisors and thesis committee a great debt of gratitude for their advice, guidance
and patience over the last few years. I thank Professor Barbastathis for giving the opportunity
to join his research group, and for the numerous mind-stimulating discussions he initiated
at group meetings and research discussions. My co-advisor, Professor Livermore, played an
instrumental role in jump-starting my research project, keeping it on track, and ensuring that
it was completed successfully. I am utterly grateful for her contributions to my education, and I
give her a lot of credit for guiding me to the point of finishing my degree. I also thank Professor
Slocum for the great perspective that he brought to our meetings and the constructive and
supportive feedback he has provided as a member of my thesis committee. I have learned so
much about mechanical design from him, and his mode of thinking has been inspirational.
My colleagues in the 3D Optical Systems Group and the Livermore Group have been a great
source of knowledge and support. Dr. Tony Nichol and Dr. Hyun Jin In helped me find my way
in the microfabrication world and around the fabrication lab. Their feedback was invaluable
in growing my knowledge in the domain. My good friend and labmate, Dr. Nick Loomis, was
amazingly generous in sharing his knowledge with me, ranging from complex Fourier optics to
photography and cycling. I am grateful for all that he has taught me both inside and outside
the lab.
Besides the academics, I have been lucky to get involved with various student groups at MIT.
14
Through my involvement with the Graduate Association of Mechanical Engineers, the Lebanese
Club at MIT, and the Graduate Student Council I have forged friendships, built relationships,
and developed skills that shaped who I am. I feel lucky to have had the opportunity to meet so
many brilliant and wonderful people, through those organizations, who made my stay at MIT
enjoyable. I am also grateful to my teammates in the MIT Cycling Club and the Boston Team
Handball Club for their support and encouragement. Their presence around me was fun and
uplifting during my down times.
My family and friends provided support in their unique way. While some did not quite
know what I was working on or why it took a long time, their support has been unconditional.
I have utmost respect for their understanding, openness and trust. Among them, Zeina, my
wife, Wissam, my brother and my dad deserve the most credit for having to deal with my mood
swings and grumpiness, particularly during the writing phase of the project. Hiba and Nazih
never stopped to believe in my ability to achieve any goals I set for myself, even when they
seemed out of reach. Their perseverance was contagious and helped me stay focused when I
found myself derailed.
Lastly, there are two people without whom I would not be where I am today - writing the
acknowledgements section of a Ph.D. thesis at MIT. Those are my late mom, Hoda Hamdan,
and my advisor, Carol Livermore. Unsurprisingly, besides being mothers, they shared quite a
few traits from my perspective. They knew me almost better than I knew myself. They were
mentors when I needed guidance, and friends when I needed to talk. They shaped who I am
and how I think. They had to deal with my flaws and still smiled even when I was giving them
a hard time. My mom made sure that I secured my path to start a Ph.D. at MIT, and Carol
made sure I left MIT with a Ph.D. I dedicate this thesis to them.
*Thnding for this research work was provided by the Institute for Soldier Nanotechnologies
and the National Science Foundation.
*All device fabrication was done at MIT's Microsystems Technology Laboratories (MTL)
15
Chapter 1
Introduction
In a world where almost every item in our day-to-day life is three-dimensional, it is hard to
imagine what it would be like to have tools and objects that can only be a stack of slabs of material that are cut out using a waterjet cutter and assembled in stacks. The functionality would
be constrained that usage of such tools would be limited. That had been the case, to a large
extent, with Microelectromechanical Systems (MEMS) for a couple of decades after they came
about - tools known to the consumer as: airbag sensors, smartphone tilt sensors, smartphone
digital compasses and inkjet cartridge nozzles, to mention a few. For the technically-oriented,
MEMS are miniature devices ranging from sub-micrometer and up to a few millimeters, in total
size, with functional mechanical elements controlled by electronic circuitry at the micrometer
(micron) and nanometer scales.
The MEMS fabrication technologies were an extension of the integrated circuits (ICs) manufacturing technologies. The IC fabrication was two-dimensional (2D) in nature in part because
the conventional circuits market required thin flat layouts of conducting wires, sometimes separated by insulating layers - just like macro-scale printed circuit boards (PCBs). Another reason
for the confinement of the fabrication methods to 2D was the adoption of photolithography as
the standard method for patterning semiconductor material. Photolithography is a process by
which a pattern is transferred from a mask to a photo-sensitive film on a substrate by exposing
the film to UV light through the mask then developing it in a chemical. The shallow depth of
focus, at high resolutions, implies that a well-defined pattern has to be in the focal plane of the
exposure system; hence limiting the patterning to a single plane at a time.
16
However, the fast growth in the MEMS markets over the last several years and the inherent
three-dimensional (3D) nature of MEMS devices has resulted in the development of a greater
interest in 3D fabrication [1] [2]. Ironically, even the electronic circuits' development, which
had limited MEMS to 2D, has also taken a "3D turn" with the emergence of Thru-Silicon-Via
(TSV) technology to enable 3D circuit architectures [3] [4] [5]. Beyond the circuits, the use of the
third dimension in building micro-devices provides opportunities for applications that were not
possible before, particularly in fields that are inherently 3D, such as biology (BioMEMS) [6] [7],
optics [8] [9] [10], microfluidics [11] [12] [13], energy [14] [15] [16] and sensing [17] [18] [19].
Several approaches have been demonstrated in fabricating 3D micro and nano devices. An
approach that has the biggest potential of making it into large scale production is one that
builds on the existing well-developed 2D fabrication technologies to pattern precursors that are
then assembled into 3D structures. The work presented in this thesis is an example of that
approach. This chapter sets the stage for presenting the developed technique. The following
section puts the MEMS manufacturing into perspective by shedding light on the development
of the fabrication technologies over the last half-century. After that a brief survey of other
3D fabrication approaches is presented, paving the way for the discussion of the 3D assembly
method proposed in this thesis.
1.1
Overview of MEMS Development
Looking at the historical progression of MEMS fabrication without regard to that of the IC
industry, from which it was bred, would be incomplete, if not unjust. The invention of the first
transistor at Bell Labs in 1947 triggered the birth of the microelectronics industry [20]. About
a decade later, in 1958, the first integrated circuit was created using germanium (Ge) devices at
Texas Instruments [21], and soon after that a silicon-based (Si) IC was announced by Fairchild
Semiconductor. The latter announcement marked the start of a slow, but full, transition of
the IC fabrication technologies to silicon, a transition that spanned about a decade. From that
point onward, microfabrication technologies revolving around the patterning of Si-based devices
were developed, many of which have remained in use since.
The first MEMS-like devices that emerged were micro-scale sensors, well before the term
17
"MEMS" was established. In 1954, the first pressure sensor was manufactured based on piezoresistive effects in Si. The semiconductor's resistivity proved to be more sensitive to strain than
metals, which were used for strain sensing prior to that [22]. By 1958, Si strain gauges were
commercially available, leading up to commercial pressure sensor manufacturing that grew into
a huge industry - one of the classic examples of a MEMS success story.
The term micromachining emerged to describe the act of carving out silicon patterns using
previously developed isotropic and anisotropic etching (1960-1967). Two types of micromachining, bulk micromachining and surface micromachining, were distinguished from one another
with the former involving etching into the substrate, while the latter was limited to etching of
films grown and deposited on the substrate, often making use of 'sacrificial material' layers, the
purpose of which is to provide structural support for the device layers during the fabrication
process which are removed at the end of the processing.
Despite all the above-mentioned build up in technologies and the utilization of the IC
microfabrication tools developed in the 1950's and 1960's to create mechanical sensors as early
as the 1950's, it was not until the late 1970's that MEMS, as we know them spread. In 1982, the
famous paper by Kurt Petersen, titled "Silicon as a mechanical material," presented a vision of
integrating mechanical components patterned into silicon along with the circuits and processors
being built on the semiconductor material [23].
The term MEMS - Microelectromechanical
Systems - was later established in 1987 in the context of microdynamics workshops. It was used
to describe surface micromachined mechanical components that were connected with hinges and
moved to perform particular tasks. The term evolved over the last two decades to include multibillion dollar industries ranging from microfluidic devices to inertial sensors that often are a
hybrid of bulk and surface micromachining among many other emerging technologies developed
over the last few years.
1.2
3D Microfabrication Techniques
A natural consequence of the development of MEMS from the microelectronics fabrication
technologies was that the production tools were good at producing very well defined thin films.
The films are typically created with a sequence of two steps starting with an additive process,
18
where a layer of material is added on top of the substrate, followed by a subtractive process
in which parts of the layer are removed and the remaining parts form the patterned layer. In
most cases, an optics-based step is performed between the two steps to define the pattern to be
'subtracted'. In some cases, defining the pattern optically in an intermediate film is done prior
to the additive step. Based on the pattern, material is selectively added to certain areas of the
substrate and not others.
Additive processes include evaporation, sputtering, film growth, electroplating, nano-imprinting
and centrifugal spinning of viscous material on a wafer. Subtractive steps include all types of
etches, categorized in different ways such as isotropic vs. anisotropic, dry vs. wet and chemical
vs. physical. The most common methods used in the optical patterning step are projection
lithography, electron beam writing and interference lithography.
Moving to the third dimension has taken several approaches that can be classified into
three main categories. One category involves layering 2D patterns in stacks along the third
dimension, another is directly writing into a medium to form a 3D object, in a way analogous
to lithography in 3D, and a third category consists of assembling thin film segments in 3D
orientations to form the desired structure.
1.2.1
Layering
Since the state-of-the-art fabrication techniques are two-dimensional, creating 3D structures
simply by stacking 2D patterns on top of one another seems the most natural step towards
3D fabrication. While the techniques following the layering approach implement the classical
fabrication methods to define the individual layers, they vary significantly when it comes to
how they assemble the layers. The most straight-forward of the techniques is to simply pattern
layers on top of one another, with the additive step of a layer following the subtractive step of
the previous layer.
Qi et al. demonstrated the fabrication of a 3D photonic crystal using such a technique
[24].
Their fabrication method consisted of iteratively patterning 2D silicon films, creating
a stacked structure with a 3D pattern of cavities in the silicon. Patel et al. presented an
improvement to that fabrication technique demonstrating a significant improvement in yield
(66%) and reduction in processing time from months to days [25].
19
The alternative method
still relied on layering; however, instead of patterning the layers on top of one another, on the
same substrate, layers were patterned separately, inspected for defects and stacked if they were
defect-free. The technique was recently demonstrated for large-area stacking by Lu et al. [9]
Aoki et al. have successfully demonstrated a similar technique using micro positioners to pick
up segments of a patterned layer and stack them on top of one another [26]; a method utilized
by Tandaechanurat et al. to create high-Q 3D photonic crystals [27].
A third technique, within the layering category, is transfer-patterning of imprinted films,
where each layer is patterned on a base substrate and transferred to the destination substrate
sequentially creating a stack of the patterned layers, as demonstrated by Han et al. [28].
The biggest advantage of these approaches is the high resolution and reliability of patterning
the individual layers, particularly for MEMS scales, because the technology involved in the
patterning is tailored to smaller scale industries such.as electronics and photonics. However,
the weaknesses lie in the effect of the multiple layers on compromising that quality. In the
first technique, one has to choose between removing the sacrificial material at every step, which
results in lower layers creating a non-flat topology of the surface for subsequent layer patterning
steps, and keeping the sacrificial material, which would require access to all the cavities in the
final 3D structure - a limitation on the architecture of the device, as was the choice of Qi et
al. For the latter two techniques, the limitations lie in the ability to align the layers with highenough accuracy and at a high enough pace to match the fast throughput of the fabrication of
the individual layers.
1.2.2
Direct 3D writing
Another approach to creating 3D patterns is to simply write the patterns in 3D. This is possible
by using nonlinear optical sensitivity of photo-patternable material to expose a 3D medium.
One technique is to use a 3D optical pattern to expose the resist [29] [30] [31]. Divliansky et al.
demonstrated using a single mask with one light source that generates a 3D diffraction profile to
expose the resist [29]. Lai et al. followed a different approach by splitting the exposure source
beam and using interference lithography with multiple exposures to create the 3D pattern [31].
Another way of writing in 3D is to raster scan the volume, writing one voxel at a time.
Focusing a laser beam onto one spot in a block of photoresist and controlling the intensity
20
allows polymerizing the resist within that voxel only. The photoresist is then scanned in 3D
to create the desired polymerized 3D structure. Kawata et al. demonstrated the ability to
produce arbitrary shapes in 3D, comparing the results of scanning the full volume to the faster
- but lower quality - approach of patterning the shell then curing the inner parts by flood
exposure [32]. Deubel et al. have also demonstrated fabricating a 3D photonic crystal by direct
laser write [33]. Similar approaches use the focused laser beam to initiate additive or subtractive
processes that are localized to the voxel at the focal point [8] [34].
These techniques compare to the common projection lithography as 3D bench-top printing
compares to injection molding, in classic macroscale manufacturing, in terms of cost structure.
The former has lower capital costs but is not a good model for mass production, since the
cost of manufacturing each product is relatively high. In contrast, projection lithography involves setups that are multi-million dollars in cost, so, in the extreme case where one device is
manufactured using the set up, the cost of that device would be extremely large; however, the
variable cost is low that when millions of devices are produced the cost per device can go down
to a few cents.
So, while direct 3D writing is great for making a few custom micro prototypes, it is not
suited for mass production. Another limitation of the technique is that patterning enclosed
cavities is not possible because the photoresist inside the cavity needs to be developed.
1.2.3
Building 3D structures from 2D components
A third approach to creating 3D microstructures captures the benefits of the layering approach
without compromising on the misalignment and potential speed of production throughput. It
uses 2D patterning to define high quality thin film with high resolution, using conventional
fabrication, and utilizes techniques to create 3D structures from those building blocks. The
assembly method presented in this thesis falls into this category. Within this approach the
methods used can be classified into two main subcategories. The first is based on the robotic
assembly of the 2D components, while the second relies on self-assembly of the patterned
segments.
The robotic assembly technique involves the use of microfabricated micro-scale grippers
that are mounted on micropositioners and used to pick up fabricated components and assemble
21
them on a substrate. Contact points are implemented in the components to allow the micro
grippers to hold on to them. Breakable tethers are used to hold the components in place until
they are picked up. On the assembly side, patterned slits in the substrate are used to place the
components while mechanical stoppers in the patterns allow the pieces to support one another
[35] [36]. Attempts to automate the robotic assembly in a manner similar to how electronic
components are assembled on printed circuit boards have been made [37] [36]; however, while
the automation reduces the fabrication cost, it falls short of matching the production rate of
the components, given the batch fabrication capabilities of foundries.
The Nanostructured Origami T M project is a classic example of the self-assembly technique
[38].
Nanopatterned films are shown to fold about creases upon their release [39]. Films of
different types have been demonstrated including ceramics (SiN) [39], polymers (SU-8) [40] and
silicon [41]. Several methods for folding are also demonstrated. Stressed bilayers of chromium
on silicon nitride films are shown to fold the membranes to angles specified by the dimensions
of the chromium pattern. Nanomagnets patterned in plane as well as magnetic tips of carbon
nanotubes are shown to provide enough magnetic torque in a rotating external magnetic field
to rotate the membranes [42]. Stresses in SiN films are also shown to be induced by helium (He)
ion implantation and fold the hinges as well [43]. Other 3D self assembly methods have also
been demonstrated. Surface tension of melted solder patterns and thermal polymer bimorphs
have been demonstrated as effective folding mechanisms. Nanomagnet arrays have also been
demonstrated to provide in-plane alignment and latching between membranes with arrays of
matching magnetizations [44].
Of all the approaches used to fabricate in 3D, the one that is most promising is the origamilike self-assembly approach. The method capitalizes on the fabrication capabilities of the current
state-of-the-art technologies in the field, eliminating the need to investment in new technologies.
However, the techniques demonstrated in the literature only focus on one aspect of the assembly
process. A black-box solution that takes a 2D pattern and produces a 3D structure has still not
been shown. Moreover, while the out-of-plane folding techniques have been extensively studied
and impressive results have been shown, the alignment of the folded membranes and methods
of reliably fixing them in their final configurations are not as well developed.
22
1.3
Thesis outline
This thesis presents an integrated system for assembling 3D micro devices from 2D patterned
precursors. The design, modeling, fabrication and testing of the folding, alignment and latching
components of the system are discussed as follows. In Chapter 2, the theoretical modeling and
mechanical design of the hinges, alignment features and micro snap-fit latches is discussed.
Section 2.1 describes the Lorentz force actuation method used in the assembly process and
defines the loading effects of those forces on the hinges. Subject to those loads, the mechanics of
the hinge bending are modeled in Section 2.2, addressing both the elastic and plastic regimes of
the hinge deformations to predict their profiles. Numerical simulations, based on the developed
models, are then presented in Section 2.3, and a modified design of hinges with constrictions
is adopted allowing for better controlled folds with localized bending of the film. The design
and modeling of alignment and latching features are also presented. The kinematic constraints
defining the behavior of the alignment system is discussed in Section 2.5.1, highlighting the
importance of cascading the features in achieving a combination of high accuracy and large
correction range. Section 2.5.2 addresses the mechanics of the micro snap-fit latches used to
hold the folded segments together and presents a simple elastic model of the small deformations
they undergo in the latching/unlatching processes.
Chapter 3 focuses on the clean-room fabrication process used to manufacture the 2D membranes before they are folded. A general overview of the fabrication process is first presented
(Section 3.1), then the detailed processing steps and parameters for each layer are discussed.
Section 3.2 compares the two investigated methods of patterning the metal layer, wet etching
and lift-off, showing the superiority of the latter in producing cleaner patterns with better
controlled feature dimensions. An optimized process for patterning the structural layer, SU-8
2015, is outlined in Section 3.3, including recommendations for the exposure, development and
baking of the film to minimize residual stress in the film and maximize adhesion to the underlying metal layer. The protocol for defining adhesion pads, used in some devices, is covered in
Section 3.4. That step is particularly interesting, because the photoresist used to make the pads
is patterned into existing trenches that are twice as deep as its spun thickness, which requires a
double-spin of the resist. Lastly, the isotropic XeF 2 dry release etch is analyzed in Section 3.5.
Etch rates are compared at different positions on the wafer and across different etch hole sizes.
23
Chapter 4 describes the experimental setup as well as the measurement tools and procedures
used in the testing of the fabricated devices. Section 4.1 compares two test assemblies and
evaluates the pros and cons of each, and explaining the choice of using a stack of magnets
under the device's chip holder as the magnetic source for the Lorentz actuation. Then, Section
4.2 presents the characterization of the magnetic field distribution in the device cavity, for the
chosen setup. Section 4.3 provides an overview of the image processing MATLAB tool used
to measure the angular position of the folded membranes. The tool uses the user's input to
trace the edge of the membrane and calculate its angle. The tool's measurement accuracy was
evaluated using CAD-generated images and found to be within 0.150 (~2 mrad).
Chapters 5, 6 and 7 present the experimental results obtained from testing the different
aspects of the systems. For the hinge folding, a characterization of the fabricated patterns of
the hinges and actuation wire loops is first introduced, describing the achieved variations in film
thicknesses and pattern dimensions (Section 5.1). The details of the measurement procedures
and the collected results are then discussed in Sections 5.2 and 5.3. A correlation between the
applied force and the fold angle for the hinges is established in order to allow open loop control
of the folding to within the alignment correction range. The alignment system testing is then
presented in Chapter 6. For the fabricated devices, it is shown that the system's cascaded
features allowed for angular corrections from misalignments of up to
+/
- 110 to within 0.5'
nominal accuracy. Potential source of error in the final alignment and suggestions to improve
on that are presented in Section 7.2. The test results of the latching mechanisms that hold
the aligned segments together, in their final configuration, are discussed in Chapter 7. The
resistive force of the latches as well as their retention force are characterized and the ability
to produce re-configurable devices is demonstrated, by repeatedly latching and unlatching the
same devices.
Chapter 8 is a stand-alone chapter that addresses the particular case of assembling segments
into stack-like structures, in which the faces of the mating come into contact. The presented
solution is based on locally heated adhesion pads that melt and fuse together. First order
thermal modeling is used to predict the currents required to melt the pads (Section 8.2) and
experimental support for the model is presented, based on melting currents for fabricated pads
(Section 8.3. The demonstrated strength of adhesion is found to exceed 6.8 pN.
24
The thesis concludes with a summary of contributions and proposed advancements to take
this fabrication technique to the level of large volume production
25
Chapter 2
Design
The objective of assembling 3D micro devices from 2D patterns can be divided into three
fundamental functional requirements: folding, alignment and latching. This chapter focuses on
the design considerations taken in achieving those three functions in fabricated microstructures.
However, before diving into the design, we need to define some of the terminology used and
lay out some basic assumptions about the devices at hand. The devices are assumed to consist
of flat thin membranes of finite uniform thickness, by virtue of the photolithographic thin film
patterning used. Upon the completion of the fabrication process, the devices consist of a 2D
layout of patterned pieces of film. Those pieces are referred to as segments and are connected
to one another with hinges. The device segments are folded about the hinges and consequently
assembled into a 3D structure. To make the design process manageable, we make the following
two observations:
1. The interaction between device segments can be itemized as interactions between pairs of
segments at a time. Three segments engaging together can be thought of as two segments
each engaging with a common third segment.
2. As two segments engage in some sort of interaction, be it mutual folding, relative alignment
or latching to one another, they may do that in two modes. The first is an edge-to-face
interaction, with the edge of one segment interacting with the face of the other, as is
the case with assembling a T-like geometry. The second is face-to-face interaction, as
is the case with building a stack of segments. Figures 2-1a and 2-1b illustrate the two
26
(b)
(a)
(C)
(d)
Figure 2-1: Schematic showing how various interactions of 2D patterned membranes can be
reduced to a combination of edge-to-face and face-to-face interactions.
27
modes. An edge-to-edge interaction can be reduced to an edge-to-face or a face-to-face
interaction by adding an extension to one of the membranes or adding an intermediate
assembly-supporting membrane (Figure 2-1c-d).
The focus of the work reported in this thesis is on the edge-to-face interaction mode. Assemblies with face-to-face interactions were demonstrated and are reported, as well, but to a
smaller extent. In this chapter we start with a quick overview of the actuation method used
in the assembly process and a discussion of the design of the hinges about which the segments
fold, both of which are at the heart of the concept of making 3D objects from the 2D patterns.
Then, we discuss the alignment and latching features designed for the edge-to-face interaction
of segments. Chapter 8 reports on the design of the photoresist adhesive pads and the use of
micro-heaters to selectively activate the pads for the face-to-face interaction.
2.1
Actuation
The patterned devices were actuated by running a current through a wire in the suspended
sections and applying an external magnetic field, resulting in a Lorentz force on each wire
segment given by
F =iL x B
(2.1)
where L is a vector whose magnitude is equal to the length of the wire segment shown in Figure
2-2, and whose direction is the same as that of the current flowing through it. B is the applied
external magnetic field.
By virtue of the cross product relationship, applying a horizontal
magnetic field while running a current through the wire loop generates a vertical force that
folds the segment out of plane (Figure 2-2b). Similarly, a vertical magnetic field results in a
horizontal actuation force. (Figure 2-2c).
This actuation method has several advantages over other methods found in the literature,
such as stressed by-layers [39], nanomagnets [44] and thermal bimorphs [45]. For a set external
magnetic field, this method allows to selectively apply forces to specific segments, in specific
directions, by running a current throw those segments' actuation wire-loops. It also allows for
a highly accurate control of the force, independently of the device structure, simply by varying
the magnitude of the current, which can be used to de-couple the applied force and the position
28
(a)
d v is F
BI
F(
B
(side views)
(c)
(b)
Figure 2-2: Lorentz force actuation of a released segment. With the current running into
the page, a vertical or horizontal force can be generated by applying a horizontal or vertical
magnetic field, respectively
29
F
E IN
Ls
Mt
Figure 2-3: Lorentz actuation force translates to a force and a bending moment at the hinge
tip
of the folded segment. The price to pay for all those features is mostly in real estate. Supplying
independent wire loops for each segment of a complex device may be challenging, particularly
when a segment is connected to the substrate through multiple other segments. In the latter
case, the wire that actuates the segment needs to be navigated through the segments that
connect it to the substrate where the contact pads typically are.
Having adopted this method of actuation, our analysis of the folding mechanism assumes a
force acting at the tip of the folded segment, distributed along the edge parallel to the hinge.
2.2
Hinge Design
For the purpose of modeling the system, given the micro-scale of the actuated segments, the
variation of the magnetic field along their tip is assumed to be negligible. Hence, the distributed
actuation force is considered to be uniform along the wire segment. To avoid any twisting
of the segments during the folding process, the hinges are distributed symmetrically along
the segments' edges. That results in a symmetrical distribution of the load on the multiple
gold hinges connecting the two structural segments. Subsequently, the analysis of the folding
mechanism can be reduced to a two-dimensional analysis as depicted in Figure 2-3.
A generic force applied to the actuated segment at an angle
OF from
the vertical, translates
to a transverse force Ft, an axial force F and a moment Mt at the tip of the hinge where, for
30
a segment of length L, in the starting flat configuration,
Ft
=
Fcos(OF)
Ft'= F sin(OF)
Mt = FL
8 cos(OF).
The axial force results in minimal axial elongation and does not affect the profile of the bent
hinge; however, the transverse force and moment generate a distribution of internal shear force
and internal bending moment along the hinge length, which govern its bending proffle.
The gravitational force is ignored in the analysis, since the weight of a 1mm x 1mm x 10pm
SU-8 membrane is on the order of 0. 1 pN, while the magnetic forces during the assembly are
on the order of 1 0pN - about 2 orders of magnitude larger.
At a distance x from the base of the hinge, the internal shear force V and bending moment
M are, hence, given by
V(x) = Ft
M(x) = Mt +
J
V(x)dx = Mt + (Lh - x) Ft
(2.2)
(2.3)
where Lh is the length of the hinge. Figure 2-4 illustrates the internal shear force and bending
moment distribution along the hinge. The moment diagram suggests that, under the assumptions that the hinge is made of a homogeneous material and has a uniform cross-sectional area,
the hinge section subject to the largest bending moment is at its base.
Analysis of beam bending is very common in structural mechanics, and the failure mechanism of ductile metals is a well understood process too. The internal moment in the gold hinge
induces a linear axial strain distribution across the beam thickness that varies from tensile
(positive) in the lower section of the beam to compressive (negative) in its upper section. By
virtue of symmetry, the unstrained neutral axis for a hinge with a rectangular cross section is
at its center axis (Figure 2-5).
Taking a look at the stress versus strain diagram of gold (Figure 2-6) [RR], we distinguish
between two regimes: elastic and plastic. The transition point between the two regimes is at
the yield stress
ay,
which, for ductile material, is determined by drawing a line parallel to the
31
Lh
v
-Mt
Ft
.
C1~
I.
10 X
Lh
M
M + Lt
Mt
Lh
Figure 2-4: Internal shear force and bending moment distributions for the hinge subject to a
tip force and moment
f
C
lt"
Figure 2-5: Linear strain distribution profile in a beam with rectangular section subject to a
bending moment
32
Elastic Plastic
Figure 2-6: Stress strain diagram of gold (typical of a ductile material). The yield stress is
determined by the offset method; drawing a line parrallel to the linear section with a 0.002
strain-axis intercept.
Figure 2-7: Linear stress distribution across the thickness of a beam (a) before yielding and (b)
at the onset of plastic deformation
linear section of the plot through the 0.2% strain point.
In the elastic range, the stress is directly proportional to the strain (a = EE), so the stress
distribution in the beam section is also linear (Figure 2-7). At equilibrium, the magnitude of
the maximum stresses, at the top/bottom, is
Umax = MC
(2.4)
where c is distance from the neutral axis to the top/bottom of the beam, and I is the area
moment of inertia of the section (for a rectangular beam of thickness 2c and width b, I =
1b (2c) 3 ). The elastic stress decays linearly from a magnitude of -max at the top and bottom
33
surfaces to 0 at the neutral axis, so, at a distance y from the neutral axis, the axial stress is
given by
My
(2.5)
o(y)
and the radius of curvature of the beam is
2Ebc3
3M
3M
El
M
(2.6)
where E is the Young's modulus of elasticity of the material.
We define the yield internal moment My as the moment required to initiate yielding in a
beam section. Rearranging the terms of Eq. 2.4, My can be calculated as
My
I
2 bc2ay.
c
(2.7)
3
Any section of the beam subject to a bending moment larger than My will start to deform
plastically. Assuming material continuity, the strain profile is independent of whether the stress
in the material is in the elastic regime or the plastic regime. It continues to be linear; however,
the stress distribution is linear only in the elastic range. The profile of the stress versus strain
curve of the material, including the non-linear plastic part, needs to be taken into account.
Figure 2-8a shows the stress distribution for a generic ductile material in plastic deformation.
A common simplification is to assume that the metal is perfectly elastic in the first phase and
fails in a perfectly-plastic manner, as shown in the stress vs. strain curve of Figure 2-8b. The
stress profile in the beam section is, hence, trapezoidal, where the stress in the top and bottom
sections saturates at ay in the plastic regime. As we increase the moment beyond My, the
thickness of the elastic core decreases while that of the plastic sections increases. Equating the
moment of the stresses to the applied moment at the section gives
M()
=
2
=
c2c.y
b
y
y dy + 2
JkyY}Y+Y~~Y
1 - y
).
y (y) dy
(2.8)
We have already established that the strain in the beam section remains linear during plastic
34
(b)
(a)
...
...
In
. ....... .
....
s-i
N;IJ
Figure 2-8: Stress distribution along the thickness of a hinge made up of (a) a generic ductile
metal and (b) an idealized elastoplastic metal
35
deformation. The strain gradient can be calculated from the slope of the stress in the elastic
core section
(y)E =
=
'-(y)core
(2.9)
_
yY
the radius of curvature is thus
P
y
Eyy
(2.10)
Combining Eq.s 2.8 and 2.10 relates the radius of curvature of the beam to the bending moment
at a particular section
M(X) = c2.
( -
(cTyp)
2
)
(2.11)
rearranging the terms of Eq. 2.11
J
[ 1 -
=
.bc2
(2.12)
Eq.s 2.3, 2.6 and 2.12 determine the profile of the gold hinges for any tip loading condition.
The deflection angle of the tip of the hinge 0, which is equal to the angle of the folded segment,
can be calculated by integrating the angles of small beam sections along the length of the hinge
dOx
1
_
dx
(2.13)
P(X) cos
where
1/2
1/3E
-
PL\ =
0 < X < xy
(2.14)
M
-E-
xy < x
< Ls
and xy is the transition point between the elastically and plastically deformed sections of the
hinge.
In theory, the applied force can be in any direction as dictated by the cross product of the
unit vectors of the external magnetic field and the current in the wire loop. To simplify the
experimental procedures, we assume that the applied field is kept at a fixed angle, hence the
actuation force is at a fixed angle OF (measured from the vertical). Based on that, the moment
it exerts on the hinge, Mt, and the transverse component of the force, F, vary as the hinge
deforms significantly, as depicted in Figure 2-9. At a folded angular position 0, the moment
36
..... . .........
MtL
Figure 2-9: Diagram of a folded segment subject to a tip force F at an angle OF from the
vertical, at a deflection angle of 0.
and transverse force at the tip of the hinge are given by
Mt = FL, cos(0 - OF)
(2.15)
Ft = F cos(O - OF)
(2.16)
The bending moment distribution is then
M(,o) = F cos(9 -
OF) (Ls - Lh - x)
(2.17)
By setting
M = My = 2bc2oy,
3
(2.18)
the point of transition between elastic and plastic deformation, xy, is
2bc~gy
- ca
3F cos(O - OF)
xy = L,, + Lh
(2.19)
Eq. 2.17 shows that the moment distribution along the hinge is a function of the hinge's
tip deflection angle itself. However, the moment distribution determines the curvature of the
beam, which, itself, defines the tip angle. The dependence is also nonlinear, so, to find the
37
220MPa
55GPa
0.44
27GPa
UY
E
v
G
Table 2.1: Material properties for gold
deflection angle 0 due to a force F, one needs to simultaneously solve Eq.s 2.13, 2.14 and 2.17
numerically.
2.3
Numerical Simulations of Rectangular Hinges
MATLAB was used to solve the equations numerically. The MATLAB scripts are included
in Appendix C for reference, and Table 2.1 lists the material properties of gold used in the
simulations .
Figure 2-10 is a plot of the applied force vs. tip deflection angle of a 600nm thick hinge
that is 300pm wide and 100pm long. The force is applied vertically at the end of the folded
membrane, whose length is assumed to be
1mm, and its magnitude and direction are maintained
constant throughout the fold. As the hinge angle increases, the moment arm and the transverse
component of the force both decrease. They are maximum in the starting flat configuration,
which explains the rapid rise in the angle for small deflections. The tip angle then saturates at
around 750 with a force of 14.5pN. The sharp drop in the curve at that point occurs because
the bending moment at the base of the hinge is high enough to plastically deform the whole
beam section. The radius of the elastic core, in the elasto-plastic model goes to zero, and the
trapezoidal stress profile shown in Figure 2-8 becomes a rectangle. The radius of curvature at
that section, given by equation 2.14, becomes imaginary, and the simulation terminates. The
slope of the graph is a measure of the effective compliance of the hinge; the hinge starts out as
being very compliant, and it becomes stiffer with increasing 0.
This saturation effect is evident in the moment distribution and hinge profile plots as well.
In Figure 2-11, the shift in the moment curves for a constant increment of the applied force
decreases with the increasing value of that force. For a force of 14.40IN, part of the hinge,
that is closer to the base (~30pm in length), starts to deform perfectly plastically. Shortly after
38
Hinge tip angle - OF 0
80
70-
o,60 -
(U
40
-
030--
2010
0
0
10
5
15
Applied force -MN
The direction of
Figure 2-10: Variation of the tip angle of a bent hinge with the applied force.
process.
bending
the force is kept vertical throughout the
deform the entire
that, the moment at the base of the hinge grows high enough to plastically
tends to bend
section. The profile plots in Figure 2-12 confirm those observations. The hinge
shows that for
less for a constant force increment at higher angles. The hinge profile plot also
predicted by
small deflections, where linearization is a valid approximation, the hinge profile
the numerical simulation matches the analytical solution.
of folding
As expected, a force with finite amplitude in the vertical direction is not capable
of the force
a segment to a 90 deg angle. Figure 2-13 shows the hinge-tip angle as a function
of up to 1000 can
amplitude when the force is maintained at a 450 angle from the vertical. A fold
the components
be achieved with this form of loading. In this particular loading configuration,
force Ft and the
of the load force contributing to the bending of the hinge, namely the transverse
to bending
bending moment Mt, increase in amplitude between 0' and 450, but the resistance
due to the increased radius of curvature also increases.
So the net effect is a flattening of
to being
the curve between those two limits. After 45', the curvature of the plot transitions
to decrease
downwards again, because the components of the load contributing to bending start
decreases.
again, and the material resistance continues to increase; so the effective compliance
hinge profile for a
Figures 2-14 and 2-15 show the moment distribution along the hinge and the
39
Moment distribution along the beam-F=O
4500
E.
............
... . . ........................... ................ y ..
4000,
Z
=- 35003000-
F =
N
0 2500
E
0) 2000F
-
=
f.lpN
C 1500
.0
ii 1000-
500~
0
0 AN
F
20
40
60
Distance along the beam -
80
100
pm
Figure 2-11: Bending moment distribution along the hinge for different values of vertical loads
applied at the tip of the folded segment.
Hinge profile - 0F=0
60
F = 11.7piN
F =
N
50-
E
=L 40-
30-
Analytica:
F = 3.4N
:E
-
20
Anaiytic:
10
0' ---0
F
20
40
80
60
Horizontal distance -
F = 0.4pN
-
100
prm
Figure 2-12: The profile of the bent hinge for different values of a vertical load at the end of
the folded segment.
40
Hinge tip angle - 0 F= 4 5
100-
a>
800-
CU
60-
40-
20-
0
1
0
2
3
4
Applied force -
5
6
7
8
pN
from the
Figure 2-13: Magnitude of the applied force vs deflection angle for a load force at 450
vertical.
range of applied forces. For an amplitude greater than 51LN, the moments throughout the beam
are above the yield moment threshold, so the whole beam is in the partially-plastic deformation
regime.
the
A horizontal load results in a singularity in the starting horizontal configuration where
load translates to a pure axial force on the hinge. To avoid the singularity of applying a perfectly
horizontal force
(OF = 900),
at the starting point when the segment is horizontal, a horizontal
= 89 deg. That results in an interesting force-angle relationship,
deflection
shown in Figure 2-16. Because of the nonlinear dependence of Ft and Mt on the
radius of
angle (Eq.s 2.16 and 2.15) as well as the nonlinear dependence of the tip angle on the
switches
curvature of the beam (Eq. 2.13), the change in the effective compliance of the hinge
the second
from increasing in the first portion of the fold (0' to about 25') to decreasing in
is bigger
portion. This is due to the fact that in the first part, the rate of increase of Ft and Mt
force is simulated with
OF
the second
than that of the internal resistive moment of the hinge, while the opposite is true in
moment in
part of the fold. Figure 2-17 shows that for loads up to about 2.7piN the bending
of the
the hinge is well below the yield moment. It also shows an abrupt jump in the values
point
moments between 2.7puN and 3.6ptN, which corresponds to the sharp rise at the inflection
41
0 F=4 5
Moment distribution along the beam 5000
E4
2
F
5N
4 000 .........................................................................
L 3500
C
r3 000
0
E 2500
-
2000
-
500-
E
C
C 000 F0.AN
500
0
20
40
80
60
Distance along the beam -
100
Figure 2-14: Bending moment distribution along the hinge length for different magnitudes of a
load force acting at the tip of the folded membrane at an angle of 450 with the vertical.
Hinge profile - 0F=45
7n.
F = 6.! N
F
60-
5. IN
3.tN
E
L 50-
C 40 -
0
30Analytical:
F = 2.4tN
20
10
;:5;:...--Analytical:
..
g'..F
0
20
40
F=0
'F0.4tN
60
Horizontal distance-
80
100
pm
Figure 2-15: The bent profile of the hinge for different magnitudes of a load acting at the tip
of the folded segment, at an angle of 450 with the vertical.
42
Hinge tip angle -OF =89
140
120* 100-
-)
0)
80-
o
60*4020
0
2
1
0
6
5
4
3
Applied force -
7
pN
Figure 2-16: Applied force vs fold angle for a horizontal loading configuration
Moment distribution along the beam -
0F 89
6000
E
5000
40
44N
F
F =M3.UN
E
0
E 3000
I)
2000.0
E10000
F
'
0
20
2AN
80
60
40
Distance along the beam -
100
gm
of a load acting
Figure 2-17: Bending moment distribution along the hinge for different values
in the horizontal direction.
43
Hinge profile - 0 F=8 9
80
70-
E 60
*
50.40S3020-......
10
......
0
0
20
40
60
Horizontal distance -
80
100
pm
Figure 2-18: Profile of the bent hinge for different values of the load acting in the horizontal
direction.
in the force-angle curve of Figure 2-16.
That jump is also evident in the plots of the hinge
profile. Figure 2-18 shows a large change in the bending profile of the hinge between 2.7p'iN
and 3.6pN, as opposed to the change between 3.6pN and 4.5p-N.
Comparing the force-angle plots for the three loading directions discussed above shows that
the plots overlap in a way that makes each configuration optimal in a particular range of fold
angles. Figure 2-19 shows that, for the simulated device, to achieve a 90' fold with minimum
external force, the direction of the force has to be switched from vertical to 450 to horizontal,
with the transition points being at 220 and 67' respectively.
As shown in Equation 2.17 and is evident in Figures 2-11, 2-14 and 2-17, the bending
moment along the hinge has a constant component equal to the moment at its tip and a linear
component from the transverse shear force. Since the length of the folded segment is typically
significantly larger than the length of the hinge the constant part of the bending moment ends
up dominating.
For a simple rectangular hinge with a uniform cross section, this results in
a relatively constant stress distribution and hence radius of curvature along the length of the
hinge; rendering the profile of the bent hinge into a circular shape (Figures 2-12, 2-15 and
2-18). However, this also implies that if a particular section of the hinge has a defect from the
44
Hinge tip angle
140
F
120
0>
100
.
o80
0 =45deg
0
60 -F
a4020
0
0
1
2
3
4
5
6
Applied force - pN
directions
Figure 2-19: Force vs deflection angle for loads in the vertical, horizontal and 45 deg
would
fabrication process, that particular section would be a weak point and the hinge bending
bending proffle of
get concentrated at that point. That creates a level of uncertainty about the
of the folded
the hinge and, hence, about the final position of its tip, which dictates the position
where a
membrane. To reduce that uncertainty, an alternative hinge design was investigated
section.
section of the hinge is weakened, by design, to localize the bending at a specific
2.4
Constricted Hinges
can be modiTo induce localized bending in a hinge at a particular section, several parameters
of a section
fied. Equation 2.6, in Section 2.2, shows the dependence of the radius of curvature
as the geomof the beam on the bending moment, the material's modulus of elasticity, as well
and, to avoid
etry of the section. The moment is dictated by the device's overall architecture,
and have
complex fabrication processes, the hinge is assumed to be made of the same material
of introducing a
the same thickness throughout its length. Hence, the only straightforward way
at a particular
weak section in the hinge is to add a lateral constriction in the width of the hinge
point (Figure 2-20).
45
1
(b)
(a)
Figure 2-20: Top views of two fabricated devices showing (a) the straight rectangular gold
hinges and (b) the constricted hinges. The dark grey areas are the SU-8 structural segments of
the devices and the light grey is the underlying silicon substrate
2.5
Edge-to-Face Alignment and Latching
The Lorentz force actuation method allows for controlled folding of the device segments; however, variations in material properties and the unpredictable of mechanical defects in the hinges
make it impossible to rely on it for accurately positioning segments relative to one another in
an open loop control method. The correlation between the applied current and the angle of
the fold for devices of identical designs will vary across dies from different parts of a wafer and
across different wafers in a processing batch. To ensure that the fabricated devices always assemble in their prescribed configuration after folding, an alignment system that is less sensitive
to fabrication errors was designed to create an enregy minimum for the segments during the
actuation phase of the assembly process. A set of latching features were also designed to work
in conjunction with the alignment system to maintain the state of minimum energy for the
system when the actuation forces are removed.
46
Target
Segment
Alignment
Feature Pairs
n
n
-rn--~~~~~-
Aligning
Segment
n
Target
Segment
Aligning
Segment
Figure 2-21: Schematic diagram of a corner cube structure with three alignment feature pairs
(a) in its flat as-fabricated configuration and (b) during the assembly process
2.5.1
Alignment Features
The alignment features for the edge-to-face interaction mode consist of pairs of rhombic holes
in one segment - the aligning segment - and corresponding triangular protrusions on the other
- the target segment. Both the holes and the protrusions are patterned in the plane of the
structural layer in a single lithographic step. That makes their relative position to the overall
structures and to one another is accurate to the same order as the lithographic mask, which
can be down to the nanometer range for high quality masks. Figure 2-21 is a schematic of
a structure showing the alignment features. The left image is a top view of the patterned
structure in its 2D configuration prior to folding. On the right side is a snapshot of the device
during the assembly process, right as the lower-most alignment feature pair is about to engage.
As the segments are folded, the features engage sequentially, starting with the pair closest
to the hinge and ending with the one farthest from it. Figure 2-22 shows front and side views
of the same corner-cube right as the segments are about to start aligning. At the captured
instant, the lower alignment features, which are closer to the hinge, have just started engaged.
47
~JL
Front
Right
Figure 2-22: Front and side views of the cascaded alignment system at the onset of alignment.
The lower alignment feature pair is engaged while the upper pair is not in contact yet
The triangular protrusion has just started to get pushed into its corresponding hole. Notice
that at this point, the second pair looks out of aligmnent in the front view. However, as the
aligning segment is actuated further, the lower alignment features bring the two segments closer
to alignment. By the time the surface of the aligning segment reaches the tip of the second
triangular protrusion, the target segment would be aligned enough to allow the second pair to
engage. This behavior applies to subsequent alignment features; thus the alignment progresses
as a cascade or in a "zipper"-like manner.
When engaged, as the aligning segment is folded, the edge of the rhombus hole of the
engaged pair applies force onto the triangle it is in contact with, hence transferring a portion
of the actuation force to the target segment - driving it into alignment. The progression of
the relative alignment of the segments proceeds until the triangle is centered with the hole and
can no longer penetrate it any further. Figure 2-23 depicts the corner-cube in its assembled
configuration. The section views of the alignment feature pair demonstrate the final position
of the triangle relative to the hole. In the 'section-front' image, the rectangular cross-section of
the protrusion is centered along the vertical diagonal of the rhombus. The 'section-left' image
48
Left
ection-Front
Front
Section-Left
Figure 2-23: Schematic of corner-cube in its final assembled position with a close up view of
an alignment feature. Section views of the feature pair show the final relative position of the
rhombus and the traingle
shows how the triangular profile of the protrusion sets the limit to how far it can go into the
hole.
The design of mechanical systems often involves a trade-off between range and accuracy. The
alignment feature pairs are an example of such a trade-off. For a given size of a protrusion-hole
pair, the closer the pair is to the folding axis of the segments, the larger is the angular range of
misalignment it can correct. However, by virtue of the same geometric relationship, for a given
error in the relative position of the alignment feature pair, the variation in the final angular
position of the folded segments is also inversely proportional to the feature pair's distance from
the folding axis. Figure 2-24 shows the trade-off between the range and error-sensitivity of an
alignment feature pair placed at various distances from the hinge. The advantage of cascading
the alignment features is that the features closer to the hinge covers a wide angle while the
features further from the hinge fine tune the final alignment with higher accuracy. Jointly, the
cascaded alignment features provide both a large range of alignment and a low sensitivity to
errors.
49
' C"
0.3
x Sensitivity
- Range
16,
0.25
0.2
*12-
0
10-
.1
6
4
200
30
Soo
4Wo
X
-0.1
600
700
2*
Distance from the hinge to the alingment feature (ptm)
Figure 2-24: Plot of the distance from the hinge to the alignment feature vs. range of correction
(left axis) and the sensitivity of the alignment feature to variations in the film thickness (right
axis)
2.5.2
Micro Snap-Fit Latches
Once aligned,
The
slits.
latching features took care of holding the segments in their final folded positions.
latches consisted of pairs of cantilevers with pointy arrowhead tips that squeezed through
The
latching-unlatching performance of the system was predicted from quantitative,
analytically-based models and used to design the geometry of the latches.
The choice of cantilevers with arrowhead tips allowed for a de-coupled design that achieved
several functional requirements of the latching system. The ratio of insertion force to extraction
force was set by choosing the profle of the arrowhead slants on the tip side and the cantilever
side. The final minimum-energy state of the
latches was chosen such that the cantilevers are
not in their fully-relaxed state, to eliminate backlash. The overall strength of the
latch was
controlled, independently, by the cantilever design.
For small elastic deflections of the cantilevers, the
lateral and angular deflections of the
beam's tip, at the base of the arrow head, are given by
j = Fb3+ -b2(2.20)
3EI
50
2EI
L
FFv
Fa
Fh
Figure 2-25: Schematic diagram of a latching feature showing the cantilever and arrowhead tip
in their free-standing and loaded configurations
a=
FbL 2
2EI
+
MbL
EL
(2.21)
EI
where E is the Young's modulus of the material, I is the moment of inertia, L is the length of
the cantilever (Figure 2-25).
When a snap-fit latch is subject to a contact force, F, at a position (xy) relative to the tip
of the cantilever, the beam is subject to a tip force and bending moment given by
Fb = F
(2.22)
Mb = Fox - Fhy
(2.23)
where the subscripts v and h indicate the vertical and horizontal components of the contact
force. The deflection of the contact point is
d = 6 + xtan(a).
(2.24)
The deflection of the latch is assumed to happen in a quasi equilibrium manner, since the
response of the cantilever bending is much faster than the latching/unlatching speeds. A force
balance at the contact point, between the arrowhead and the slit edge, correlates the horizontal
51
Insertion
40 ...... ........
Extraction
......---
-0-20
Di*U~
from Uip - ym
Figure 2-26: Simulated insertion and extraction forces for a micro snap-fit
with its position relative to the slit in the mating segment
latch as they vary
insertion/extraction force to the vertical bending force as
Fa
sin(0) + it cos(0)
Pb
cos (0) - y sin (0)
where tan(O) is the slope of the arrowhead profile at the contact point and p is the coefficient
of friction between the two surfaces.
Simulating the
latching process requires solving the inverse problem of predicting what
insertion force is required to deflect the cantilever such that the contact point on the arrowhead
face coincides with the edge of the corresponding slit in the mating membrane. Figure shows
a plot of the insertion and extraction forces for a
micro snap-fit latch simulated in MATLAB.
The plot shows a peak insertion force of 32piN and a peak extraction force of 83pN.
52
Chapter 3
Fabrication
The devices discussed in this theses were fabricated on a silicon substrate using a polymer
structural layer on top of a patterned metal layer that defined the electrical connections as well
as the hinges about which the segments were folded. In some devices an additional photoresist
polymer layer was patterned on top of the structural layer to latch parts together, as described
in chapter 8. This chapter presents an overview of the fabrication processes used then goes
into more detail in discussing the critical steps and criteria for increasing yield and throughput.
The last section describes the additional photoresist patterning step. Process parameters are
tabulated in Appendix B.
3.1
Process overview
The fabrication process involved three main steps; first, electrical wires and mechanical hinges
were patterned in a gold metal layer on top of a silicon <100> substrate. Then, the structural
layer was patterned with SU-8 polymer, and finally the devices were released using a dry
isotropic etch. The end result was a two-layered 2D pattern defining the fixed and foldable
structural parts of the devices. Embedded in the structural segments were metal hinges and
wires used were to run currents for magnetic actuation as well as melting of polymer gluing
pads for latching purposes.
In some devices, the metal layer also included other electrical
features such as gold electrodes for a super capacitor, as a potential application of the 3D
MEMS technology. Some of the gold features were also used to test the latching strength of
53
the gluing-pads; as described in Chapter 8.
Figure 3-1 shows the steps of the fabrication process. Negative photoresist was spun on the
silicon wafer after coating it with an adhesive hexamethyldisilazane (HMDS) layer. The resist
was then patterned with the gold pattern (refer to Appendix A for mask layouts). A
0.6pim
gold layer was evaporated on top of the patterned surface and the resist was then stripped by
soaking the wafers in acetone. In the areas that were not covered with the photoresist patterns
the gold was evaporated onto the silicon and stuck to it, while the gold that was deposited on
the photoresist got washed away in the acetone bath. A minimum line spacing of 4pm was
demonstrated with 8pum line widths.
After the gold metal layer was patterned, an SU-8 layer was spun on top of it.
SU-8
is a photo-sensitive polymer, so it was patterned simply by lithography. The 15pm SU-8
layer was pre-baked at 95'C, exposed with UV-light with a proximity mask, developed in
Methoxypropanol Acetate (PM-Acetate), and then post-baked at 140'C. The final fabrication
step was releasing the patterned SU-8 structures. An isotropic dry etch of silicon with Xenon
DiFloride (XeF 2 ) gas etchant was used. XeF 2 provided a relatively high etch rate of silicon as
well as a high selectivity to gold, SU-8, and photoresist, which are the only materials that were
used in the devices. Typical device segments where about 800Im in size, so etch holes were
used to speed up the release step. The size and placement of the etch holes is discussed in more
detail in section 3.5.
Figure 3-2a shows one of the devices before the release step. The dark grey areas are SU-8
structures. The underlying gold pattern defining the hinges and electrical wire loops is visible
as yellow features in the image. The lighter grey areas (top right) are exposed silicon regions.
Figure 3-2b shows a similar device after release. The SU-8 segments with the 'M' and 'T'
patterns are hanging over a trench created by the XeF 2 isotropic etch and are held up by the
gold hinges that connect them to the base SU-8 pattern. The edges of the structure are outlined
in the image as a visual aid, since the SU-8 films are transparent.
3.2
Metal layer patterning
54
Step: Pattern the Gold metal layer by lift-off $
Step2: Spin-on and pattem the SU-8 structural layer
Step3: Release the structure with an Isotropic etch step
XOF2
Figure 3-1: Schematic of the final fabrication process: gold lift-off, SU-8 spinning and photolithography and XeF 2 isotropic dry release etch.
55
Figure 3-2: Optical micrographs of a sample device (a) before and (b) after the XeF 2 dry
isotropic etch; the last step of the fabrication process. The outline of the resleased segments is
highlighted with white lines in (b) for clarity, since the structure is transparent.
The first attempt to pattern the gold metal layer was done using wet etching; Figure 3-3 shows
the processing steps involved. A 1pm thick gold layer was deposited on a (100) silicon wafer
using electron beam evaporation. A 30nm chromium layer was evaporated prior to the gold to
promote adhesion to silicon. Photoresist was spun on top and patterned using photolithography.
The patterned photoresist was then used as a hard mask to etch the underlying gold by placing
the wafers in a gold etchant. At an etch rate of about 0.2pLm/ min, the wet etching step took
about 5 min only; however, it had major disadvantages. The first was the wet etch's inherent
multi-directionality that resulted in an undercut of the gold layer beneath the photoresist mask
and jagged pattern edges. The second was the non-uniform etch rate across the wafer and thus
the need to over-etch in order to make sure that all the unwanted gold was stripped. That
led to a larger undercut of the film. Figure(a) shows a wet-etched gold pattern that is 1im
thick. The dark line outlining the pattern is the photoresist mask. The columnar structure of
the evaporated gold favors etching in the lateral direction, and the measured ratio of lateral to
vertical etch rates was about 2; that is why the etching of the 1pm film, along with some degree
of over etch, resulted in about 2.5pm of undercut, as seen in the figure. With a gold layer of
0.6 - 1.Opm, patterning 4ptm features proved to be very difficult. Agitation of the wafers in
the etchant reduced the depth of the undercut but did not eliminate the problem. To avoid
all these issues, a lift-off process was adopted as an alternative for patterning the gold layer.
Figure 3-4 compares 8pim wide features patterned using a wet etch and the lift-off process. A
56
Si
PR
Figure 3-3: Schematic of the gold wet etching process. Gold is evaporated on a Silicon susbtrate,
photoresist is patterned on top of the gold layer, the gold layer is etched in sulfuric acid, and,
finally, the photoresist is stripped by in an asher with oxygen plasma
I,,
Figure 3-4: Optical images of 8pm gold features patterned (a) by wet etching with a photoresist
hard mask and (b) with a liftoff process
57
photoresist layer of 2.2pm thickness was found to be optimum for lifting off the
0.6pm gold
layer. Thicker resist made it difficult to pattern small features due to the absorption of UV
waves and diffraction of light from the edges of the mask pattern. Thinner photoresist did not
provide a high enough step to ensure discontinuity of the gold film between the sections that
were on the silicon surface and the sections that were on top of the resist.
3.3
SU-8 patterning
The main criteria for the structural layer were rigidity, thickness uniformity and pattern edge
quality. The choice of SU-8 as a structural layer was based on the easy of patterning by simple
photolithography, since SU-8 is a negative photoresist, and the ability to pattern high aspect
ratio structures, due to the relatively low absorption and scattering of UV electromagnetic
waves in SU-8. With a target thickness of 15pim, SU-8 2015, produced by MicroChem Corp,
was chosen.
While transferring the mask pattern to SU-8 was done by mere exposure, the high viscosity
of the polymer and the degree of cross-linking during exposure and post-baking made the
spinning and baking steps quite challenging. To avoid forming bubbles in the polymer layer
and to maximize the adhesion of SU-8 to silicon, the wafers were first dry baked at 150'C for
2 min to get rid of any moisture in the underlying layers. They were then spun at a speed
of 100rpm for about 10 sec, as the SU-8 was poured slowly onto the wafers, starting from the
center and moving outwards towards the edge. The spinning speed was gradually ramped up
to 500rpm over a time period of 5 sec, to fully spread the polymer on the wafers. Over the next
10 sec, the spinning speed was ramped up again from 500rpm to 3000rpm and was held at the
final speed for 30 sec. That resulted in a nominal thickness of 15pim for the SU-8 2015 layer.
After spinning, the wafers were baked on a hot plate at 65'C for 1 min followed by 95'C
for 2 min. They were then exposed with UV light using a chrome mask for two consecutive
intervals of 5 sec each separated by a
5 sec break. The two-interval exposure was adopted to
avoid stress gradients in the SU-8 film that may be created by temperature gradients induced
by the exposure. Low stress gradients in the film enhances adhesion and reduces the risk of
delamination of the SU-8 film. After exposure, the wafers were baked again on a hot plate.
58
(a)
(c)
(b)
(d)
Figure 3-5: Optical images of the SU-8 layer during processing. The pattern edges start to
appear at the first stage of crosslinking, after the post-exposure bake
The bake temperature was increased over 3 steps: 1 min @45"C - 1 min @650C - 2 min @95C.
The wafers were then put back on a hot plate at 450C for 1 min before being cooled down to
room temperature.
Following the post-exposure bake, the wafers were developed in PM-Acetate. Mild agitation
of the wafer during development was found to reduce the development time from 5 min down
to 2 min, which drastically reduced the seeping of the developer under the SU-8 layer, hence
the delamination of the layer at the pattern edges. Finally, the wafers were hard-baked on a hot
plate with step-wise temperature ramping: 1 min
85"C -
1 min @1100C -+ 1 min @140C --
1 min @85C -+ 1 min @45C. Figure 3-5 shows optical images of the SU-8 films during the
patterning process.
Avoiding large steps in temperatures during baking and cooling proved to be critical in
producing good quality SU-8 films. The two step exposure as well as the reduced development
time with agitation of the wafer in development consistently reduced the premature detachment
of the SU-8 film from the gold layer and the silicon substrate. Failing to adhere to the protocol
presented above results in a film of poor quality as a result of residual stress build-up. Figure
3-6 shows an example of the SU-8 layer delamination, eventually leading to the detachment of
the SU-8 segments from the hinges, as a result of stress gradients in the film and the seeping
of the developer under the SU-8 layer.
59
Figure 3-6: Delamination of the SU-8 layer due to prolonged development. The developer seeps
under the polymer layer detaching it from the underlying layers
3.4
Patterning photoresist pads
In some devices, such as the ones described in Chapter 8, photoresist pads were patterned
inside cavities in the SU-8 layer, on top of underlying gold patterns. With the SU-8 layer
fully crosslinked, the photoresist layer was patterned using photolithography without having
any effect on the SU-8. Figure 3-7 shows the patterning steps of the photoresist pads before
the final dry release etch. Two layers of thick photoresist (AZ P4620) were used to fill up the
cavities in the SU-8, which were 15pm deep. The first layer was hardened by a soft baked for
20 min at 90'C before spinning on the second layer. The proffle of a pad is shown in Figure 3-8.
The plots in the figure show the thickness of the patterned photoresist after a single spin and
a soft-bake (green) and after a second spin and another soft-bake. The scales on the axes are
not identical, to make the plot clearer. The trench is actually wide and shallow, with an aspect
ratio 1:14 for the first spin and 1:28 for the second. The slopes of the profile near the edges are
due to the accumulation of the resist in the trench corners, right before the sharp edges. The
irregularities in the surface profile are due to the shrinkage of the resist during the baking as
the solvent evaporates.
60
Photo-
resist
SU-8
I:
XeF2
Figure 3-7: Schematic of the patterning steps of the photoresist pads, between patterning the
SU-8 layer and the dry release etch in XeF 2 plasma
61
Photoresist Pad Thickness Profile
-TWo
LayerThidess (um)
-One
Layer Thickness (um)
30
25
E
-..........-.....-..
..-..-..............
.-..-......
.....
^-
. ..-.
20
FA
$A
10
.
5
0
0
24
48
72
96
167 191
120
143
Horizontal Distance - urn
215
239
263
287
Figure 3-8: Thickness profile of a photoresist pad after spinning one layer of AZ4620 over the
SU-8 layer (lower curve) and after spinning a second layer (upper curve)
62
3.5
Dry release
The last step of the fabrication process was a dry release of the structures in a XeF 2 plasma.
Williams et al. have reported a large range of etch rates for silicon <100> in XeF 2 . That is
because the rate depends to a large extent on the object being etched. The larger the etch area
the slower the etch rate. That is mainly due to the decrease in the rate of the chemical reaction
between the etchant and the substrate as the ratio of products to reactants near the surface
of the wafer increases. Etch holes in the SU-8 layer were used to increase the rate of the etch,
hence speed up the overall fabrication process and lower its cost. The minimum size of the etch
holes was set based on the mean free path of the XeF 2 molecules in the plasma. The mean free
path, 1, of a gas molecule at a pressure P and temperature T, is given by
kBT
v'-7rd2p'
where d is the diameter of the gas molecule and kB is the Boltzman constant. For XeF 2 , whose
molecular diameter is ~480pm, at room temperature and a pressure of ~2500mTorr, the mean
free path of the gas particles is ~12pm. Hence, etch holes were designed to have diameters of
15pm and 30pm, with 50pLm and 60pm respectively. Figure 3-9 shows the progress of the etch
depth at five locations on a wafer.Based on the collected data, the following observations where
noted:
1. The etch rate was not uniform across the wafer. That is most likely due to the nonuniformity of the gaseous plasma distribution in the etch chamber. The etch rate at the
center of the chuck holding the wafer was consistently higher than at the perimeters.
2. The etch rate of convex surfaces was significantly higher than the concave ones. That was
expected and applies to any isotropic etch, since the surface to volume ratio of the etched
material is higher.
3. Doubling the etch hole diameter did not impact the etch rate by allowing more etchant to
diffuse into the hole; however, it did speed up the release process by reducing the distance
between the edges of adjacent holes. The closer distance between holes sped up the release
step in two ways. First, it simply reduced the distance needed to connect the growing
63
Device Position 90 min - XeF2
on the wafer
(90 x 60sec cydes)
180 min - XeF2 270 min - XeF2
(I80 x 6Osec cydes) (270 x 60sec Cydes)
-
-
_I.
~ -
L
Figure 3-9: Sample images during the XeF 2 release etching step. Devices at 5 different locations
of the wafer were observed after each of 3 rounds of etching. Each round consisted of 90 cycles,
each 60 sec long
64
circular undercuts, and that created islands of sharp corners that etched faster by virtue
of their geometry.
65
Chapter 4
Experimental Setup and
Measurement Tools
Chapter 2 presented the design and modeling of various features of the assembly system. An
experimental setup was constructed to characterize how well those features performed their
functional requirements. Software tools and hardware setups were developed to aid with the
measurement processes as well. The main parameters that needed to be measured were the
force used for actuating the segments of the devices and the angular position of the segments.
Chapters 5-8 describe the specific measurements made to characterize each of the design
features. This chapter focuses on the general experiment setup, the electronic circuitry used in
the measurements and the software tools developed to process the acquired data.
4.1
Experimental Setup
Given the size of the micro devices, measurements were conducted under a conventional microscope or at a probe station, when a large working distance was needed. Dies from the fabricated
wafers were glued and wire-bonded to ceramic chip holders. The chip holders were mounted on
the test circuit board, which was placed on the microscope stage. Since the actuation method
used is based on Lorentz force, external permanent magnets were used to supply the needed
magnetic field. The electric current was controlled using the test circuit and the images from
the microscope camera were processed to read the position data of the actuated devices. Figure
66
Figure 4-1: Images showing the test setup with (a) the device mounted on the ceramic chip
holder, (b) the magnetic stack attached to the chip with lead wires connected to the pins of the
chip holder, and (c) the circuit board placed under the microscope for measurement
4-1 shows images of the setup.
Two kinds of test assemblies were attempted.
The first consisted of mounting a small
magnet inside the chip holder cavity, fixing the device die on top of the magnet and wirebonding the device to the chip holder's contact pads, as shown in Figure 4-la. The second,
shown in Figure 4-1b, had a stack of 7 permanent magnets pressed against the bottom of the
chip holder, transmitting the magnetic field through the ceramic before getting to the device.
The first setup provided the highest proximity of the device to the magnetic field source,
providing a large magnetic field, since the field decays with distance from the magnet. It also
made swapping devices on the testing board much easier, since each device was packaged with
its own magnetic source and only needed to be "plugged in" to be folded. However, having a
magnet attached to each chip holder meant that each device would be tested using a different
magnet, which would introduce a variable quantity across measurements and would require a
measurement of the magnetic field for each device being tested. Having a magnet attached to
67
Product Name
Dimensions
Tolerances
Material
Plating/Coating
Magnetization Direction
Weight
Surface Field
Max Operating Temp
Residual Flux Density
Product specifications
Table 4.1:
http://kjmagnetics.com)
of
B662-N52
3/8" x 3/8" x 1/8" thick
±0.004 in x ±0.004 in x ±0.004 in
NdFeB, Grade N52
Ni-Cu-Ni (Nickel)
Thru Thickness
0.0762 oz (2.16 g)
3798 Gauss
176 OF (80 -C)
14,800 Gauss
the
B662-N52
block
magnets
(courtesy
of
the device also meant that they could no longer be imaged in a scanning electron microscope
(SEM) after testing.
In contrast, the second setup, which was used for the final measurements, used the same
stack of magnets across the different measurements and allowed the imaging of devices in
an SEM. Since the chip holder material is non-magnetic, that only meant a reduction in the
magnetic flux due to the gap between the device and the stack without alteration of the magnetic
flux lines. Figure 4-1b shows an image of the assembled components. With that setup, the same
magnets were used to test all the devices.
Block Neodymuim (NdFeB) magnets, from "K&J Magnetics, Inc.," were used in both setups.
The strongest available magnets that could fit in the die cavity of the chip holder were chosen.
The specifications of the magnets, as provided by the manufacturer are listed in Table 4.1.
4.2
Magnetic Field Characterization
Before conducting the tests, the magnetic field in the die cavity of the chip holder was characterized. A magnetometer mounted on a micro-positioner was used to scan the space where the
die is placed along the lateral and vertical directions.
Figure 4-2 shows the variation of the magnetic field along the lateral - x and y - directions.
The top curve was taken along the center axis of the square magnets, where the magnetic field
is maximum. The center axis was located by scanning the probe along the orthogonal direction
and locating the peak magnetic field value. As expected, the magnetic field values drop away
68
a Along x (Off-center)
30000
350
I.
I
a
2000
A Along y (Off-center)
-
...........---
....
2s00-
ID
* Along y (Center axis)
---
0...0..
.
.........- ..... ......
'a
U
S
we
a
2
20
3000
s..........
4000
40
-WO
-
2000
0
-2000
Lateral Position of Probe (pm)
4000
S00
8000
Figure 4-2: Lateral measurements of the magnetic field taken on and off the center axis of the
magnet, along the x and y directions.
from the center axis. However the over profile of the curve does not change much.
The same data normalized by the peak values of the magnetic field is shown in Figure 43. Normalizing each lateral scan by its peak value resulted in the same profile, whether the
measurement was along x or y, and whether it was along the center axis of the magnet or
off-center.
The magnetic field variation along the vertical z-axis was also measured. Figure 4-4 is a
plot of the magnetic field variation along the z-axis, with the z=O plane being the top surface
of the chip holder. Negative z values correspond to the die cavity. A curve fit of the data points
showed a cubic dependence of the field on the distance, which is consistent with the theoretical
models [RR].
Given the consistency in the normalized lateral profile of the field variation, a 2D map of the
magnetic field was extrapolated from measurements along the x-axis and y-axis. The 2D map
was then scaled by the vertical decay along the z-axis to generate a 3D mapping of the field
in the die cavity area. Figure 4-5 shows the normalized extrapolated data at different values
69
a Along x (Off-center)
+ Along y (Center axis)
A Along y (Off-center)
1.2-
E
..
... ......
-....
4
:
E
0.4
0.2
-
............
-
--
0
-8000
46000
-4000
0
-2000
2000
4000
6000
8000
Lateral Position of Probe (pm)
Figure 4-3: Normalized magnetic field measurements along the lateral axes (x and y).
of z. The data was normalized by the maximum field value, which is the peak of the curve at
z =-0.5 mm. The 2D distribution of the magnetic field becomes flatter with increasing z. The
profile approaches the zero-plane asymptotically as z goes to
oo.
The generated 3D map of the field was used to calculate the applied Lorentz force with
higher accuracy, as opposed to the assumption that the field was uniform.
For the tested
devices, the typical travel distance of a segment during folding was close to 1 mm
laterally and
vertically. Based on the plots of Figures 4-3 and 4-4, not accounting for the variation of the
field would have introduced errors of up to 25% in the force calculations.
4.3
Angular Measurement
During the assembly process, optical images of the side of the segment being folded were taken
using the microscope camera. The images were collected and processed afterwards to calculate
the angular position of the segment at each
captured frame. A MATLAB Graphical User
Interface (GUI) and supporting scripts were developed to perform those angular measurements.
70
Vertical Decay in the Magnetic Field
y =-12.789x + 124.87x 2 - 830.SSx+ 3220.3
3000
3 500
R2
0
2000
......
......
- ......
-.
2500-
100-
-0.5
0
1
0.5
Distance (mm)
1.5
2
Figure 4-4: Decay of the magnetic field along the vertical, z-axis, away from the magnet. The
zero reference position was chosen to be the surface of the chip holder.
71
U05
3 ktW "M
4mm
w-
044
Diatonac - mmn
D~Wn
- MM
U
141
E
100
'IU.c
rM
m
~
Dsac.
Figure 4-5: Extrapolated lateral profile of the normalized magnetic field values at different
distances from the surface of the chip holder. The profile becomes flatter as the magnetic flux
drops, with increased distance.
72
4.3.1
GUI Interface and Usage
A snapshot of the front end of the GUI is shown in Figure 4-6. The user interface consists of 6
main sections that are listed below, along with their functionality:
1. File/Folder Selection: Allows the user to select a single image or a folder containing images
with a '.jpg' extensions to be processed. Once selected, the path of the file or folder was
shown for verification
2. Start Push Button: A button to start the processing
3. Image Display Panel: A frame that displays the current image and allows the user to
interact with it, when appropriate.
4. Image Scrollbar: A scrollbar used to navigate between images, when a folder of many
images was being processed
5. Image Information Table: A tabular display of the image information including the name
of the image file, its measured angle and a label indicating whether that image had been
selected as the zero-angle 'reference' position. In addition to the displayed information,
the table also included a check box, per image, to select the measurement for plotting,
and a button to set the current image as the reference.
6. System Display Panel: A panel displaying user prompts, errors and messages about the
progress of the processing.
To perform a set of measurements, the folder containing the images was selected and the
'Start' button was pressed. The GUI then looped through all the images in the folder, displaying
one image at a time in the Image Display Panel. The user was prompted to trace the displayed
edge by clicking with the mouse at multiple locations along the edge, before moving to the next
image. Once all the images were traced, the user was prompted to scroll to the image with the
reference angle and click the 'Set Reference' button. Finally, data was extracted to a CSV file
in order to be matched with the corresponding force measurements.
The inner-workings of the GUI are quite simple. The angle of an edge is calculated from a
linear curve fit through the points that the user clicks during the processing. Clicking the 'Set
73
Figure 4-6: Front-end of the MATLAB GUI used for measuring the angular position of the
membranes from optical imagines. The different regions of the interface are highlighted.
74
Evaluation of MATLAB GUI Measurement Data
-- Measured Angle (deg)
120
-
Actual Angle (AutoCAD)
*
-
--
10.
.......
.-
-
-
90 41---
-
--------
-
.....
-......
1 00
--
112.741
-.....
-
so
--
--
10
50
-~~
40.~0.0
~
a]04k
30.00:
-31
00.00.
200
10 4
..
--
20.
~
0~~..
0
2
.044
............
6
4
8
10
Image Index
Figure 4-7: Plot of GUI measurements of the test images samples. The actual values of the
angles of the drawn lines are superimposed showing accurate overlap of the data
Reference' button, stores the value of the current angle in memory. That value is then used to
offset all the other angles to get the angular position of the segments relative to the reference
orientation. The code for the GUI is attached in Appendix C.
4.3.2
GUI Evaluation
Nine (9) images with lines drawn at known angles were used to evaluate how well the measurement tool worked. Figure 4-6 shows one of the images loaded in the GUI's image display panel.
The images were created in AutoCAD with a length-to-thickness ratio of 70: 1 - the same ratio
as the actual device edges. The images were then fed to the GUI, which was configured to
prompt for 4 clicks per image, from the user. Figure 4-7 shows a plot of the measured angles
along with the real values of the angles. The results showed that the GUI performed pretty
well.
To check the consistency of the tool's performance, six more measurements of the same
set of images were performed.
For each set of measured angles, the difference between the
75
Means and Standard Deviations of Errors for Multiple
Measurements of the 'Control' Image Set
a Std Dev
Absolute Mean
0.30 r
-
-
0.25
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.30
- -
0.25
1'
0
1!
LUU
0 .1
.0
0.15
..
...............
0.0
0.0
05
i
0
.- ... .- . .....
...
- - ~~..- .............
-
0.00
0
1
2
3
-.
4
Data Set
- ..-- ......
-....
5
6
- - 0.00
----.....
7
8
Figure 4-8: Means and standard deviations of the errors in the GUI measurement of the control
image set.
measured and actual values of the angles was calculated. The means and standard deviations
of those errors were then compared across the measurements. Figure 4-8 shows a plot of those
values. The average error in the angle read-out of the GUI seems to be within 0.1 ' in 6 out of
7 measurements, and the standard deviations of the errors hovers around 0.15*.
76
Chapter 5
Metal Hinge Folding
Chapter 2 presented an analysis of the forces acting on the metal hinges and defined a set of
equations governing the folding of the device segments. A relationship was established between
the applied current and the force applied on the segment in a set external magnetic field.
Chapter 2 also described how those forces translate into shear and axial stresses in the hinges
resulting in their bent profile. This chapter presents the experimental results associated with
the folding phase of the assembly process.
5.1
Fabricated Devices
Following the protocol described in Chapter 3, several devices were fabricated on a 6-inch <100>
silicon wafer, with gold as the metal layer and SU-8 as the structural layer. Devices with straight
rectangular hinges as well as constricted hinges were patterned and released. Figure 5-1 shows
the two types of fabricated hinges on two different devices. The dark grey patterns are the
SU-8 structures on top of the gold hinges. The effective length of the hinges was 50 [Lm, with
75 am of overlap with the SU-8 layer to establish good adhesion. The constricted hinges had
reduced widths of 20 Im at their mid-length section. The length of the constricted part was
10 pm. The corners of the pattern were rounded to minimize stress concentration.
The gold layer was patterned using a lift-off process with a target nominal thickness of
600 nm. The thickness of the gold was measured after the evaporation process at different
points of the wafers to enable the subsequent analysis by accounting for the variation in the
77
(b)
(a)
Figure 5-1: Optical micrographs of the two types of patterned hinges: (a) straight uniform
width and (b) constricted width in the middle section. SU-8 appears as dark grey, hinges
appear as a light gold, and the surrounding exposed silicon surface appears as light grey.
film thickness, which is dependent on the position of the wafer in the evaporator and offset in the
evaporator's thickness-control crystal. Variations between 10% and 25% of the film thickness
were recorded. The lateral dimensions, on the other hand, were accurate to within 5%, with
the measured width of the hinges showing variations under 1 pm.
The wire loops used to fold the devices were patterned along with the hinges in the same
lift-off step.
The patterns included 500 pm square contact pads and wires that are 50 pm
wide leading to the segments, used to actuate folding (Figure 5-2). The hinges formed by the
actuation wires were not constricted to avoid reaching the breakdown current density of gold
when current passes through those small cross-sectional areas. The nominal resistance of each
loop was measured to be on the order of 3 Q.
5.2
Folding Measurements
Each of the fabricated devices was mounted onto a 48-pin chip holder and the contact pads
on the die were wire-bonded to the holder leads' pads, using 100 im diameter gold wires. The
chip holder was then clamped onto the circuit testing board, under a microscope, and oriented
side-ways so that the side edge of the segment being folded remained in the focal plane of the
78
Figure 5-2: Optical image of the patterned gold layer, before adding the SU-8 structural layer
on top, showing the hinges as well as the wires used to actuate the device segments.
microscope as the segment was actuated. The voltage range of the power supply used was
0 V - 6 V, with a read-out resolution of 10 mV. For a target range of actuation currents of up
to 50 mA, a 100 Q resistor was used in series with the inherent 3 Q resistance of the wire loop.
An ammeter was connected in series with the device to measure the current running through
it.
The voltage was gradually incremented to drive more current through the loop, hence increasing the folding force. At each increment in the voltage, a snapshot of the segment was
taken using the microscope camera, and both the supply voltage and ammeter current readings
were recorded. An external light source was used to illuminate the folded segment such that
the light scattered from the edge of the membrane was highly contrasted with the background.
Figure 5-3 shows a typical set of images taken as the device segment was folded. At the beginning of each measurement, a reference point for the angular position was established by taking
a snapshot of the edge of the base SU-8 layer on the silicon substrate. Each image set was then
post-processed using the MATLAB tool described in section 4.3 to calculate the angle of the
membrane at each point.
A plot of the current vs. voltage measurements for a sample device (serial: WF05D59Q1M)
79
IWA4
E--r-'-.
26"
toom
3$k%4
~
J61IN Of
Figure 5-3: Sample set of images taken using the microscope camera during the folding of a
segment. The bright line in the image is the edge of the membrane seen from the side.
is shown in Figure 5-4. The resistance was assumed to be constant, and its value was calculated
based on a linear curve fit of the data. Based on the fitted resistance value, corrected values
of the current readings were then calculated by dividing the voltage by the resistance. This
technique gave a theoretical resolution of the current readings of 0.1 mA - the voltage reading
resolution (10 mV) divided by the ~100 Q of the resistance. That was a significant improvement
over the standalone ammeter, which had a read-out resolution of 1 mA.
The actuation force was calculated from the current readings and the magnetic field values
of the particular experimental setup. The spatial variation of the magnetic field characterized in
section 4.2 was used to calculate the Lorentz force at each measurement position. The tangential
component of the force, which is primarily responsible for the folding, was then calculated by
projecting the force along an axis normal to the membrane's measured edge orientation.
The tangential force data points, combined with the angular displacement measurements
from the image processing tool, formed the characteristic folding plot of the hinge. Figure 5-5
shows the characteristic curve of the same device as above (WF05D59Q1M).
80
Current-Voltage Measurements (WF05D59Q1M)
3
y = 0.1051x - 0.0035
2.5
R=0.999
2.R5
1.5
be
0.5
30
25
20
15
10
5
-0.5'
Current (mA)
Figure 5-4: Current and voltage measurements for a device segment during folding and release.
The data is fitted with a line to predict the resistance in the circuit.
Tangential Force vs Deflection Angle (WF05D59Q1M)
-
140
-
-
-
-
-
-
- ---
120
0
0-5
10
150
2.00
2.50
3.0
3.50
Tangential Force(piN)
Figure 5-5: Tangential component of the Lorentz actuation force vs. deflection angle of the
folded segment for a sample device
81
-WFOSDQiF -4-WFSDdQ1 -0-WFSOBAQ
-WF5DSdQ2
-4-WF05099Q3M
-WFSDBAQ3
-WFO5099Q1M
140
120
0
Figure
5.3
0s
1
1.5
2
2.5
3
3.5
4
4.5
s
5-6: Tangential actuation force vs. deflection angle measurements for several devices
Discussion
Several nominally identical devices were tested using the protocol outlined above, and the forceangle data were collected for each. Figure 5-6 shows plots of the tangential component of the
actuation force plotted against the measured displacement angle for each of the devices. As
predicted, the plots start out as linear, during the elastic stage of the bend, then curve as the
plastic deformation starts. During the plastic deformation phase, the hinges is significantly more
compliant and, hence, smaller increments in the applied force induce large angular deflections.
Upon releasing the segment, the elastic component of the deformation is restored and the hinge
unfolds. Since the 'unfolding' is elastic the return path of the force-angle curve is close to linear.
The curves do not fully overlap in the linear section, which suggests that the hinges have
different stiffnesses across devices, despite the fact that they all have the same hinge pattern.
More notably, the hinges start to deform plastically at different
loads and angular displacements
as evident in the spread of the points where the plots deviate from being
82
linear.
The variation in the hinge stiffnesses can be attributed to the variation in the gold film
thickness during the evaporation process. Other factors, such as nano-scale cracks and defects
in the film structure, create stress concentration points during the folding process resulting in
weaker hinges. The variation in the onset of plastic deformation, on the other hand, is caused
by the different initial states of the folded segments, which is a result of the influx of air into
the etching chamber of the XeF 2 etcher at the end of the release step. Overall, though, the
devices exhibit similar behavior that is highly repeatable within the elastic regime, and the final
angular positioning is accurate to within +/-100, which sets the requirement for the range of
the alignment system.
83
Chapter 6
Cascaded Mechanical Alignment
The alignment features play two complimentary roles in the assembly process. The first is
providing a coupling between the movement of the aligning membrane and the target membrane
by physical contact. The second is determining the final relative position of the membranes
based on the respective pattern of the alignment features on each. Hence, characterizing the
performance of the alignment system involved the assessment of its performance in each of the
two roles.
The procedures followed for conducting each of the two measurements were significantly
different. This chapter describes the measurement procedures for each, then presents the results.
6.1
Measurement Protocols
Corner-cube structures with three identical alignment feature pairs were fabricated for testing
purposes. The alignment features were distributed along the edges of the aligning and target
segments. Figure 6-1 shows a top view SEM image of a sample device marking the alignment
feature set. Overlaid on the figure are also illustrations of what was designated the 'front' and
'side' viewing directions. The front is the side facing the aligning segment, when folded to 900.
The side view is the one facing the target segment, with the triangular protrusions.
84
Figure 6-1: SEM micrograph of a cornercube structure in its initial unfolded configration showing the three alignment feature pairs distributed along the edges of the segments to be folded
85
6.1.1
Coupling measurement
Characterizing the coupling of the alignment feature pairs of the devices during the assembly
process required measuring the angular position of two orthogonal segments simultaneously.
However, the experimental setup was limited to measuring the position of one folding segment
at a time. So, separate consecutive measurements were made of the two segments during folding,
and the data were correlated using the voltage/current measurements. The following steps were
taken to conduct the measurements:
1. Each of the two segments was folded, independently, past the vertical position - up to
~120 0
2. The segments were then allowed to unfold and settle in their plastically deformed position.
3. Folding measurements of the aligning segment, with the 'M' cut-out, were conducted by
imaging the device from the its side to establish a reference point of the uncoupled folding.
4. The target segment, with the 'T' cut-out, was folded up to a position that is close to
900 and held at that position. The accuracy of this fold did not matter much, since the
alignment features were going to reposition the membrane.
5. The aligning segment was folded until contact was established between the alignment
features. The angle of the target segment was subsequently measure as it was driven into
its final 900 position by the aligning segment
The measurement procedure relies on the demonstrated repeatability of the hinge folding
behavior within an elastic region around its final plastically deformed position, as illustrated
in Figure 6-2. The plot shows the correlation between the tangential force applied at the tip of
the segment and its angular position. After the initial elasto-plastic deformation, the segment
settles in its plastically deformed state at an angle of 70 . Cycling the segment between that
final position and 1050 is shown to be repeatable, because it falls within the elastic phase
of the elasto-plastic deformation. The cycling, hence, does not introduce any further plastic
deformation, and the behavior of the segment in that range is fairly predictable.
86
Tangential Force vs. Angular Postision
140
Elastic Range
220
Final Position
V so ........ . .Desired
V
-...
--
-
'--40
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
Tangential Force (pN)
Figure 6-2: Plot of the tangentail actuation force vs. deflection angle of a membrane showing the
elasto-plastic deformation from the initial 380 to the platically deformed state at 700 followed
by repeatable elastic cycling of the membrane in a range up to 1050
87
6.1.2
Final angle measurement
To evaluate the performance of the alignment system in accurately positing the structure in its
desired final configuration, images of the front, side and top of the fully assembled cornercube
structures were taken and processed using the MATLAB image processing tool. Since the number of images per device to be processed was fairly small, for this setup, multiple measurements
of the same image sets were conducted using the image processing tool. The measurements
were then averaged to minimize random human errors generated by the clicking aspect of the
, as was demonstrated in the calibration measurements of the MATLAB GUI in section 4.3.
6.2
Experimental Results
Figure 6-3 shows front-side sample snapshots of a device as the aligning segment is folded from
the back into the focal plane of the microscope. The target segment is mostly out of focus
with the far edge being in the focal plane. The alignment features of the two membranes
first engage at frame #3. Up to that point, the target segment is in a static position. Further
actuation of the aligning segment drives the target segment into its final vertical position (frame
#6).The alignment features physically limit the movement of both segments beyond the final
designated position, even if the actuation current is increased further. For the devices at hand,
the final configuration had the triangular protrusions aligned with the center axis of the rhombus
holes, placing the target segment at a perpendicular orientation to the substrate. The physical
limitation imposed by the profile of the protrusions on the depth of penetration into the holes
positioned the aligning segment perpendicular to the substrate (Figure 6-4).
While the alignment system brings the actuation progression to a halt in the prescribed 3D
configuration, the elastic spring-back in the hinges would unfold the assembly upon removal of
the driving Lorentz forces. Therefore, to measure the accuracy of the final assembly, latched
structures were imaged from the front, side and top. Figure 6-5 shows optical images of a device
taken from the three orientations.
Angular measurements based on the captured images are presented in Figure 6-6.
The
mean value of the measured angles was 90.4 , with the target angle, by design, being 900.
Initial offset of the target segment of up to
+/
88
- 11 ' were successfully brought to alignment
I
4
Figure 6-3: Optical snapshots of a device during alignment. The aligning segments is seen
coming fully into focus as it is folded from the back into the imaging plane. The target segment
being aligned is seen from the side, as a translucent blur, with its far edge being in focus.
Figure 6-4: Close-up SEM image of an alignment feature pair with the triangular protrusion
fully inserted into the rhombus hole and aligned to its central axis.
89
Figure 6-5: Optical images of a fully assembled cornercube structure from different angles
using the cascaded alignment features; a result that is consistent with the theoretical kinematic
constraints of the structure.
Variation in the final angles are attributed to the torsional compliance of the hinges, which,
despite being designed to be torsionally-stiff, had finite stiffness. Since the segments in the
tested devices were held only from one side - at the inner corner of the cube - in the final
configuration, the spring-back force from the hinge is evenly distributed along the edge, while
the retaining force created a torsional load on the hinge. This phenomenon would not be a
problem in more complex structures with multiple segments latched to one another, such as a
closed box.
Another source of error was the compliance of the latches. Upon removing the actuation
forces, the assembly relaxes into a minimum energy configuration, where the total energy is
the sum of elastic energy stored in the hinges and the latches.
Solving this issue involves
a compromise with the re-configurability aspect and, hence, is a decision that is application
specific. Designing the latches to have a steeper slope on the back end of the arrowhead would
reduce this error, but it would make the unlatching of the assembly more difficult.
90
Angular Measurement of Final Assembly
5
4,
.a
....
...
0
88
88.5
89
89.5
90
90.5
91
91.5
92
Angle (degrees)
Figure 6-6: Histogram of the angular measurements of the final corner cube assemblies.
91
Chapter 7
Micro Snap-Fit Latches
To demonstrate the ability to latch the folded assemblies, devices were fabricated with micro
snap-fit latches patterned into the SU-8 structural layer. The two generations of latches described in section 2.5.2 were tested; however, due to the minimum feature size limit, the simple
cantilevers proved to be too stiff to latch using the magnetic force. Attempts to latch those
devices required too high of a current in the actuation wire loop, which heated up the SU-8
segment and created a thermal bimorph that deformed the device.
Though the first generation of latches was not characterized mechanically, the fine features
of its arrowhead profiles served as a good platform for optimizing the SU-8 fabrication processes,
laying the groundwork for the optimal fabrication of the second generation latches. As a result,
the final processing protocol consistently produced SU-8 films with minimal residual stress
gradients or shrinkage in the SU-8 pattern. This chapter presents the fabrication results of
the micro snap-fit latches as well as the measurements of the latching strength of the second
generation, flexure-based, latching components.
7.1
Fabrication Results
Like the alignment features described in Chapter 6, the micro snap-fit latches were patterned
into the SU-8 structural layer using the same lithography step. The male arrow-head feature
was patterned on the target segment alongside the triangular alignment protrusions, and corresponding curved slits were patterned in the aligning segment along with the rhombic alignment
92
~0
0
0
00
00
10um
>
l1um
ii
E
=I
/
40um
UI 00-/0
7z
-----
/
0 o 00o 0
0
00
40
v+~4ziz
0
0
(V)I1
00
0
0 0
Figure 7-1: Mask layout of the first generation latching features showing the dimensions of an
arrowhead latch and its corresponding slit
holes. Variations of devices were fabricated with different combinations of the number and
position of latches on each device. Devices with a single latch were used to isolate the latch
component for the purpose of characterizing it, while three latches per device were patterned
on other devices to create a more stable final structure in order to evaluate how accurately
positioned the final assembly was.
The first generation of latches consisted of a pair of cantilevers that were 85 /Lm long and
about
10pjm thick, with an arrowhead profile at their tips. The width of the cantilevers was
15pjm, as dictated by the thickness of the SU-8 film.
A gap of 10 jim separated the two
in the
cantilevers to allow them to deflect inwards, as they penetrated their corresponding slits
and
aligning segment. Figure 7-1 shows the mask layout of the arrowhead latch on one segment
profile was
its corresponding slit on the other segment. The maximum width of the arrowhead
40 pim, and the corresponding slit width was 33 pim.
Figure 7-2 compares SEM images of a fabricated device with its mask layout. The distance
and was
between etch holes was measured under an optical microscope with micropositioners
was also
used to scale the mask pattern to match the actual scale on the image. The scaling
93
(a)
(b)
Figure 7-2: Overlay of the CAD mask pattern onto SEM images of the fabricated latches and
etch holes. The only significant mismatch is the rounding at the corners.
verified using the scale bar from the SEM magnification settings. After fitting the etch holes'
pattern to the image, the profile of the latches was inspected to check how well the mask
pattern matched the fabricated feature. Except for the rounding of the corners, an accurate
match between the two profile was observed.
The mask layout of a second generation latching feature is show in Figure 7-3. The twobeam flexure consisted of 7 1m thick beams that were 30 pm and 112 pm in length, with the
last 40 [Lm of the long section forming the arrowhead tip. The gap between the arrowhead
pair was 8 pim and the tip-to-tip distance was 40 jim. The corresponding slit height was set at
33 jim. Figure 7-4 compares SEM images of fabricated latches of the two different designs along
with adjacent alignment features. A higher quality chromium mask was used for the second
generation latches, rendering better defined corners of the latch tip. The use of the flexures as
the support structure for the arrowheads instead of the simple cantilevers provided a significant
improvement in the compliance of the latches without taking up much real estate on the device.
An SEM image of a fully-assembled corner cube structure is shown in Figure 7-5. The
structure in the figure is imaged from the outer side of the corner. The alignment features can
94
o
112um
------I 30um
0 i70
o
0 0
I
-~
o
0
000
0
0iiW,
0 0
0
0
0
01
ooOI
E
El
0O
E
CY) I
0 0
(a)
(b)
Figure 7-3: Mask layout of the second generation latching features showing (a) the dimensions
of the arrowhead profile and (b) an overlay of the cross-section of the corresponding slit in its
latched state
Figure 7-4: SEM images of the two latch designs
95
Figure 7-5: SEM image of a fully latched corner-cube structure with 3 alignment pairs and 3
latches
be seen in their final position and the micro snap-fits are fully latched into their corresponding
slits. While the elastic component of the hinges' bending would want to unfold the two pieces,
the profile of the arrowhead tip shifts the minimum energy configuration to the assembled state.
7.2
Latching and Unlatching Measurements
To measure the latching/unlatching strength of the micro snap-fits, devices with one latch were
tested. The target segment, with the arrowhead feature, was held as close as possible to its
final prescribed angle to eliminate any interaction forces between the aligning segment and the
target segment through the alignment features. In that configuration, the only forces acting on
the aligning segment were the actuation force, the hinge's resistive force and the contact force
at the latch; the latter being present only while the arrowhead feature was in contact with its
corresponding slit.
The devices were mounted under the microscope such that the edge of the aligning segment
lay in the imaging plane. A magnetic field orthogonal to the substrate surface was applied
using a stack of magnets placed under the chip holder, and the two segments of the device
96
F4e
TFge
FAligni
Target
External
Figure 7-6: Schematic of the measurement setup with two independant currents used to actuate
the two segments in a vertical external magnetic field.
were connected to separate power sources (Figure 7-6). Since the test devices were corner-cube
structures with orthogonal sides, placing the target segment in its final 90-degree position was
done by folding it up until the face was entirely in focus.
Before taking the measurements, both segments of the devices were folded past their plastic
limit and were allowed to spring back into equilibrium. Doing that placed the final 90-degree
position of the segments within the elastic range of their deformed state. The target segment
was then folded up until its face was fully in focus, and the aligning segment was actuated
and brought into contact with the target segment.
Meanwhile, snapshots were taken of the
segments' progression and were processed afterwards to measure the angle. The voltage and
current were also recorded to calculate the applied force in the manner described in section 5.2.
A raw data set of the actuation force and angular position of the aligning segment is shown
in Figure 7-7. It is worth noting that the data points in Figure 7-7b are connected with lines
merely for illustrating the latching/unlatching cycle. The data covers two cycles that can be
97
-Mesured
Ange
(dg)
+
Actuation Force (uN)vs Membrane Angle (d*gW
Force~i4)
10.000
0
8.00
..........
2)~
........
40
30
20
40
10
30
-2.000
0
4
6
S
10
UZ
4
14
0
4.000
4.000
000
2.0
4.000
.000
000
Force(uN)
Oa Pots
(b)
(a)
Figure 7-7: Raw angle and force measurements of a corner cube segment during latching: (a)
A chronological plot of the angular position (left axis) and the actuation force (right axis). (b)
A plot of the force vs. angular position for two cycles of latching/unlatching
broken down into the following 5 stages:
1. Folding: The segment is actuated and the angular position grows linearly with the increasing force. This stage ends at the point when the edge of the slit on the aligning
segments comes in contact with the arrowhead tip on the target segment
2. Latching: Between the point of initial contact of the latching features, the aligning segment is actuated further to push the arrowhead into the slit. Due to the resistive force
from the latch, the slope of the Force-Angle curve is decreased significantly from Stage 1
3. Latched Release: The external actuation force is brought down to zero.
The angular
position of the segment remains at 900, because the device is latched.
4. Unlatching: An external unlatching force is applied until the segments get separated.
While the external force is applied, the aligning segment is held at an angle that is past
its equilibrium position prior to folding.
5. Unlatched Release: The external unlatching force is removed, and the actuated segment
resets in its equilibrium position.
98
IO.JOG
Actuatdon Force (u N) vs Membrane Angle (deg)
.......
......... 10
00.....
.................. ................... .............
r-~-
Unlatchlng
t/
Force
-4.00
4.00
Force
-4.00
-2.00
0.00
2.00
4.00
6.00
8.00
Force (uN)
Figure 7-8: Actuation force vs. angular position during a latching/unlatching cycle with the
force values offset to account for the spring-back force in the hinge.
Since the spring-back force of the hinge is constantly trying to unfold the segment, to
calculate the force from the latch only, the data of Figure 7-7 were offset by the value of the
force at the end of Stage 1. Figure 7-8 shows the offset data set. The latching force is measured
at the point right before the segments falls into its final latched position (~4.5 AN) and the
unlatching force is at the point where the segment springs back out of the latched configuration
(~5.7 pN). Since the ratio of the moment arms of the point of application of the actuation force
and that of the latch is 2.15, the actual latching and unlatching forces of the micro snap-fit are
9.7 pN and 12.3 pN respectively.
99
Chapter 8
Face-to-Face Latching
In Chapter 2, we introduced the design challenges involved in assembling 3D micro-structures
and, based on the nature of their fabrication process, we reduced the problem to addressing
two types of interactions: edge-to-face and face-to-face. The alignment and latching elements
discussed thus far are ideal for the edge-to-face interaction of micro-patterned membranes, because the features themselves are in-plane patterns. Chapter 3 highlights how such features
were added to the devices, simply by modifying the mask of the structural layer of the devices;
with no additional fabrication steps. In contrast, using a similar method for aligning and latching membranes that are interacting in a face-to-face mode would require patterning protrusions
that are orthogonal to the wafer surface, which can be quite challenging and would involve
multiple additional fabrication steps, as is the case with the MEMS velcro [46].
Besides the additional cost and the reduction in production yield imposed by the added
fabrication steps, most of those extra steps would exposes the devices to chemicals and, hence,
limit the range of material that could be used for the device structure, due to chemical incompatibility with etchants and etch selectivity. Therefore, for the face-to-face interactions we
chose a different approach.
The solution presented here keeps the fabrication process simple by simply adding heated
adhesion pads to the faces of the segments. Furthermore, it allows for control sequencing of
latching through selective activation of the pads by means of local micro heaters that are built
into the device structures.
100
(a)
(b)
Figure 8-1: Schematic of (a) the overall layout of a device with micro-heaters and (b) a zoom-in
onto the corner of the device segment showing one of the micro-heaters and the folding actuation
wire passing around it
8.1
Concept and Design Considerations
For the SU-8 devices being tested, the addition of the adhesive pads involved adding one
noninvasive fabrication step of patterning thick photoresist (AZ P4620), right before the final
release, and introducing adjustments to the patterning masks of each of the gold metal layer
and SU-8 structural layer to accommodate the added pads.
8.1.1
Addition of micro-heaters
To melt the adhesive pads, micro-heaters were patterned as part of the devices' gold metal layer.
The heaters consisted of winding thin gold wires concentrated into a small area at the tips of the
device segments that needed to be latched. The layout of the metal wiring of a typical device,
with the micro-heaters and the actuation wire-loop, is shown in Figure 8-1. Separate wires and
101
contact pads were used to actuate the heaters than the ones used to fold the segments.
The wires leading to the micro-heaters were designed to be wide where space was not an
issue in order to localize the resistive heating at the pads. The total resistance between the
contact pads for a typical device, such as the one shown in Figure 8-1, was 22Q, with 1/3 of
the resistance concentrated in each of the two micro-heaters (~7i
each). This minimized the
heating of the device frame when the micro-heaters were activated.
8.1.2
Structural layer modifications
Since the photoresist pads were spun-on on top of the SU-8 layer, access sink-in holes were
added in the SU-8 layer, so that the photoresist is in direct contact with the gold wires when
patterned. Adding the holes simply meant adding dark field areas to the SU-8 mask right on
top of the underlying resistor pattern in the gold layer. The profile of the holes, and hence the
resistors, was chosen to be elliptical, with no corners, to avoid stress concentration effects in
the film (Figure 8-1b).
8.1.3
Patterning the adhesion pads
The two factors involved in choosing photoresist as the material for the adhesion pads were its
ease of fabrication and low melting temperature. The nominal thickness of the SU-8 structural
layer is 15pim, so two spins of thick photo resist were used, as was described in section 3.4.
The chrome mask pattern used for the photoresist layer was a replica of that used to create
the holes in the SU-8 layer. To facilitate the alignment of the layers, a positive photoresist was
used so that the mask is a light field mask with small opaque spots. After photolithography
and development, the areas shielded by the mask retained the photoresist to form the pads.
The first spin of the resist was critical for providing good coverage inside the SU-8 trenches,
given the height of the hole edges. The centrifugal force of the spin also results in an inclined
profile of the resist in the holes that is dependant on the device's position on the wafer. Figure
8-2 shows the profile measurements of the photoresist after the first spin. In Figure 8-2a, device
WB07DV39D is positioned on the center axis of the wafer such that the centrifugal force is
along them minor axis of the elliptical shape. Device WB07DV33D, on the other hand, is off
axis and the centrifugal force has a component along the major axis. Furthermore, the size
102
Prof WAUaM oRUM PadM07OV390
PionieofAZ462
roft ofAZ*S1
PiofleofAZU2O
bo~c~u~st ati W5P~VO3
Phot Res
4I4MA
IMM4lf~1
)
144
13Is
(a)
(b)
Figure 8-2: Profile measurements of the photoresist pads after the first spin showing (a) a
relatively uniform thickness in a device with a smaller pad close to the center axis of the wafer,
and (b) a sloped profile of the resist in a device with a larger hole that is off-axis
of the hole in the latter is 1.5 times longer than the former, which reduces the ability of the
surface tension forces around the perimeter of the ellipse to hold the resist and prevent it from
getting pushed against the outer side of the hole.
8.2
Thermal simulations
The thermal behavior of a radially symmetric a photoresist pad was simulated numerically in
MATLAB. The ratio of pad thicknesses to their lateral dimensions is on the order of 1:10, so
the temperature profile along the thickness was approximated as uniform, since its variation is
much smaller than that along the radial direction. Since the photoresist is spun on as a liquid
and fills up the trenches in the SU-8, the model assumed no gap between the outer edges of
the photoresist and the inner edges of the SU-8 trenches that it filled i.e. no insulating air gap
between the two materials. The model also assumes a uniform flux of heat coming from the
heaters, since the spacing between the winding wires of the heaters is small (4pm) compared
to the dimensions of the pad.
103
ISO.
120
~100.
1wreusing time
(towards stc-dy state
40:
20"
10
20
30
40
10
60
70
80
90
Radial Distance (um)
Figure 8-3: Simulated temperature profiles plotted vs. the radial position along the photoresist
pad. Temperature profiles are shown for a series of times after the current flow begins (from
0 to 14msec) to capture the profile as it progresses from a starting room temperature profile
towards its equilibrium profile.
The melting temperature of soft-baked AZ P4620 was found to be ~170'C in a bench
level experiment. The model predicted that a current of 23mA was required to bring up the
temperature of the photoresist to that melting point at the center of the pad. Figure 8-3 shows
the simulated temperature distribution along the radial direction of the pad for an input current
of 23mA. The lower-most plot is the starting room-temperature profile and subsequent plots
show the rise in the temperate over a time period of a few milliseconds, settling into an inverted
parabolic profile in its steady state. Figure 8-4 is a plot of the mean temperature of the pad
for the system's dynamic response to a step input in the current. The system reaches a steady
state within 4msec.
8.3
Experimental results
Micro-capacitors were fabricated using gold electrodes on two SU-8 frames. Two pairs of oval
photoresist gluing pads (100pm x 200pm) were then patterned at the tips of the frames. One
frame was released by an isotropic XeF 2 etch and manually folded about the gold hinges on top
of the other electrode, using micro-positioners and probe tips under a probe station. Figure 8-5
104
160
140-
I
23mA
e 120-
0
20 -
4 8 ( 8
lime (msOc)
10
12
14
Figure 8-4: Step response of the microheater-pad system with an input current of 23mA
Figure 8-5: SEM images of a microscale capacitor prior to folding (top) and with one electrode
folded and latched on top of the other (bottom)
105
shows SEM images of a device before and after folding. The spring-back in the gold hinges was
used to check if the device latched successfully. Without the latches, upon folding the electrode
to 180', the spring-back of the elasto-plastic gold hinges unfolds the membrane back to an angle
in the 140 - 150' range, due to the elastic component of the hinge deformation.
To latch the electrodes together, the current was increased in increments of
1mA while
the top frame was held down on top of the substrate with the probe tip. At each increment,
the probe tip was lifted off the top electrode, and the electrode was visually monitored for
spring-back. The device was considered latched when the top electrode stayed flat on top of
the substrate even after the probe tip was moved away. In Figure 8-5, the bottom image is an
SEM micrograph of a device with the fused adhesion pads holding the membranes together.
The back side of the segment shows the wires leading up to the micro-heaters.
Since the pads are between the two SU-8 layers in the final latched configuration, it was
difficult to confirm that the latching was due to the fusing of the melted photoresist. To verify
that, separate experiments were conducted on unassembled devices with the photoresist pads
exposed. The pads were melted by running currents through the underlying micro heaters.
The devices were observed under a microscope during the process, and the current supply was
discontinued when air bubbles formed in the photoresist pad. The devices were later inspected
in an SEM. Figure 8-6 shows optical and SEM images of a patterned photoresist pad before
and after heating. The rounded surface profile of the pad after heating is characteristic of
surface tension, indicating that the photoresist had melted and re-flowed before solidifying.
The soft-baked photoresist is almost fully transparent before melting. As the resist is heated,
the remaining solvent starts to evaporate and form bubbles in the film rendering the film more
opaque and increasing the volume of the pad. The opacity of the pad was used as a visual aid
indicating the melting state of the pad. As predicted by the simulations, the temperature is
highest in the middle, so the opacity of the pad grows from the center outwards, as the current
is increased in the heater. The pads were considered 'melted' when most of the underlying wire
pattern was no longer visible. Figure 8-7 shows a histogram of the measured melting current
for the photoresist pads in the unassembled melting tests.
A lower bound on the strength of the latches was measured by applying Lorentz force on a
wire loop on the assembled electrode. The latches were found to sustain a force that exceeded
106
Figure 8-6: Optical and SEM images of a photoresist polymer pad in its patterned state (left)
and after melting (right)
543
0
22 23 24
25
28 27 28
29 30
31
32
33 34
35
Melting current (mA)
Figure 8-7: A histogram showing the currents required in experiments for melting the AZ P4620
polymer pads
107
6.8pN, the maximum Lorentz force that the wire loop could sustain before the heat from the
actuation current damaged the device.
108
Chapter 9
Conclusion
Extending the microfabrication technologies into the third dimension is a growing trend in the
MEMS field that promises to provide new functionality the 2D world could not meet and improved performance for applications that existed in the conventional 2D architecture. While
some new 3D microfabrication technologies are being developed from the ground up, the majority of the advancements have built on the existing state-of-the-art manufacturing techniques
and introduced innovative ways to transform the 2D patterns into 3D structures. The work
that was presented in this thesis falls into the latter category. What follows is a summary of
this work's contribution to the field and a suggested path of moving forward.
9.1
Contributions
A complete 3D assembly solution was demonstrated with the capability of folding 2D-patterned
thin films, positioning them at prescribed arbitrary angles in 3D and latch the assembled pieces
in their final configuration. This work contributes to the collective effort in several aspects:
9 Integrated yet modular approach
The three fundamental components of the developed assembly system - folding, alignment
and latching - build on one another; without the folding, alignment would not be possible,
and the latches would only work if the arrowhead tips and their corresponding slits are
aligned. However, the components were designed with modularity in mind. That was
done by providing functional integration while maintaining structural independence. In
109
other words, the aligmuent features would work equally as well, regardless of how the
segments were folded or what the hinges were, as long as the folding mechanism provided that functionality. Similarly, the micro snap-fit latches would latch the segments
regardless of how they are brought into alignment, provided that they are aligned. It
follows that the designs of the individual components could be incorporated into other
assembly techniques, if properly integrated. Presenting the three aspects of the assembly
as working coherently is equally as valuable, though.
" Optimized SU-8 fabrication
Since its first development, SU-8 has grown to be a popular material in MEMS technologies. That was largely due to two main characteristics of the material: (1) it is a
photoresist, which makes patterning it as simple as a lithography step (spin-expose-bake)
and (2) the optical properties of the material in the UV range result in minimal scattering
and, hence, allow for smooth vertical side walls of feature with fairly high aspect ratios.
However, those great properties come at a cost. SU-8 patterning is a very sensitive process
and stress build-up due to the cross-linking is hard to avoid. In the process of fabricating
the devices used to demonstrate the assembly system, an optimized fabrication process
for SU-8 2015 was developed. The processing parameters listed in Section 3.3 consistently
produced high quality films with no noticeable residual stress.
" Elasto-plastic design, modeling and testing of gold film hinges
The gold hinges characterized in Chapter 2 and tested in Chapter 5 provide a compromise
between the classic MEMS pin hinges that have high compliance but are susceptible to
wear and stiction, on one hand, and the elastic brittle aspect of silicon-based flexural
hinges.
The plastic aspect of the deformation increases the compliance of the hinge,
while the elastic part allows for repeated predictable folding within a range around the
plastically-deformed equilibrium state. The constricted design modification also adds
value to the hinge functionality by localizing the bending to a predictable section of the
hinge, thus reducing uncertainty in the trajectory of the folded segments.
" Cascaded alignment system design
Compromising between range and accuracy is a common phenomenon in many engineering
110
domains. A classic example of that would be the two focusing knobs on a microscope
stage. The outer knob typically has a large range of motion and places the stage at a
position reasonable close to the focal plane, while the inner knob is used for fine tuning
the position. The designed cascaded alignment system essentially provides that same
functionality, but in a more continuous fashion. That, combined with the simplicity of
the design, makes for a powerful tool in the 3D assembly process.
" Flexibility and robustness of the latch design
While the demonstrated functionality of the micro snap-fit latches is of great importance,
particularly when it comes to reversibility and eliminating backlash, the most powerful aspect of the micro snap-fit latches lies in the flexibility of the feature's design. In particular,
the decoupling between the ratio of insertion to extraction force from the overall strength
of the latch enables controlling the values of latching and unlatching independently of
one another; within the limitation of the minimum feature size and the geometrical limits
on the arrowhead tips. Two latches can be equally as easy to latch with one harder to
unlatch than the other. Using that aspect, one wire loop could be used to fold and latch
multiple segments sequentially, with the ones hardest to unlatch being assembled first,
then using forces smaller than the unlatching force of the assembled pieces to fold and
latch subsequent segments.
* Controlled face-to-face latching
While solder bumps had long been developed and demonstrated for the flip-chip technology, and melted polymer and metal alloy patterns had been used to align dies and attach
wafers, the use of adhesive pads at the component level within a 3D MEMS assembly was
an important contribution that filled a void in the scope that the micro snap-fits latches
could not cover. The presentation of the photoresist pads contributes to the completeness
of the work in this thesis.
While relying on classic fabrication methods, the combined capabilities of the various aspects
of the proposed MEMS assembly system present a viable solution for production of 3D MEMS,
with minimal investment in new equipment or development of new technologies. Furthermore,
the modularity and simplicity of the design makes this technique a good resource for future
111
development of 3D MEMS fabrication, in general.
Error analysis and reduction
9.2
The angular errors observed in the alignment system can be attributed to three main sources:
the compliance of the latches, the offset in positioning of the alignment features, and the
torsional compliance of the hinges. The alignment error is sensitive to each of the three sources
to various degrees and different countermeasures can be taken to reduce the error from each
source.
The finite stiffness of the latches implies that upon releasing the actuation current, the
spring-back force in the hinges pulls the latches partially out of their slits. The correlation
between how much that pull-out distance is and the error in the angle is a function of how
far the latch is from the hinge. A latch at a distance of 800 pm from the hinges that is pulled
out by 3 pim results in an offset of 0.2
in the angular position of the membrane. That error
can be reduced by making the latches stiffer, but that increases the force needed to latch the
devices. Alternatively, making the slope on the back side of the arrowhead steeper increases
the effective stiffness of the latches during the unlatching phase; however, that makes it harder
for the devices to be unlatched and reconfigured. The decoupled design discussed in Chapter 2
allows the latches to be tailored for particular applications.
The second source of error stems from simple geometric constraints. Offsets in the dimensions of the alignment feature pairs from their nominal design values translate into error in
the angular position of the aligned segments. The sensitivity of the error to those offsets is
a function of the position of the alignment feature, as was discussed in Section 2.5.1. In a
system of cascaded alignment features, having identical feature pairs may increase the angular
error unnecessarily, particularly if all features are offset by the same amount. That is because
the feature closest to the hinge dominates by generating a larger angular error. It is preferred
that the sides of the triangular protrusions closest to the hinge be flattened at the depth where
the subsequent feature pair engages. With that, the advantage of covering a wider angle of
correction using features close the hinge is not lost, while neutralizing those features when it
comes to final angular positioning error.
112
The third potential source of error is the torsional compliance of the hinges. If a segment
is latched on one end only in the final assembly, the retaining latching force exerts a torsional
moment about the hinges. Although the hinges have high torsional stiffness, because of their
large effective width, which spans almost the entire edge, the structure is finite and hence
has some compliance. The use of alternative hinge structures, such as surface micromachined
pinned hinges, could be a solution in some applications.
9.3
Roadmap to production
More often than not, creating real impact in people's lives requires going beyond the scientific
innovation and the fabrication and testing in a university lab, and MEMS technologies are not
an exception to that rule. One of the main requirements for a successful MEMS product is the
ability to produce it in very large volumes - as a matter of fact, that applies to any microfabricated product. The rationale is that both the capital and variable costs of microfabrication
are significantly high, and need to be spread over a large number of produced items to make
the ratio of the product price to functionality value as low as possible.
A few hurdles still exist for the assembly system presented here, before it can be put into
mass production, even to a limited level of throughput. The next steps on the path towards
mass production mainly include further investigation of face-to-face alignment and latching,
integrating the face-to-face and edge-to-face systems and fully-automating the assembly process.
9.3.1
Face-to-face interaction
As mentioned earlier, the investigation of the face-to-face latching using adhesive photoresist
pads was mainly to cover the shortcoming of the edge-to-face alignment and latching system.
The purpose was to demonstrate the ability to predict the current needed to activate the
pads, as well as the main function of latching and evaluating how strong of a latch could be
achieved. A better understanding of the mechanism would include investigating whether the
lateral component of the surface tension is sufficient to align the two segments or if the device
would require separate alignment features. Moreover, a better evaluation of the strength of the
latch would be necessary for more complex assemblies where subsequent segments are actuated
113
after some segments are latched. When those two aspects of the process are better understood,
a comparison with other demonstrated techniques, such as nanomagnet arrays, would show the
better choice for that interaction mode.
9.3.2
Integration of face-to-face and edge-to-face systems
The segments in both of the demonstrated edge-to-face and face-to-face assemblies were actuated using Lorentz forces. The fabrication process for both was also identical, with the exception
of the additional photoresist patterning step for the adhesion pads. Hence, integrating both
methods into the same device does not introduce any fabrication challenges like material incompatibility considerations or processing step synchronization. Creating a structure with both
face-to-face and edge-to-face would require the following:
e Adjusting the gold layer mask to include micro heaters and their corresponding connecting
wires and contact pads
e Adjusting the SU-8 layer mask to include alignment features, the micro snap-fit latches
and the photoresist pad trenches
e Adding the photoresist patterning step for the pads before the release process.
9.3.3
Automation
Automation is an absolute requirement for mass production. Production fabrication facilities
are already automated, so the only missing part is automating the assembly. In the case of a
permanent magnet, the only thing that needs to be automated is the control of the currents
through the actuation wire loops to fold the device and through the micro heaters to activate the pads. However, a permanent magnet constrains the magnetic field to one direction,
so a complete automation would better utilize a magnetic field generated by electromagnets
where the magnitude of the field can be controlled. Adding three coils along three orthogonal
axes would also allow the rotation of the field simply by varying its x, y and z components
independently.
A first step towards automation has been taken, as part of a supervised undergraduate
research project by Rane Nolan. A 24 channel circuit was designed and built for the purpose of
114
Current Output
to Device
12CSraBs
Analog Voltage
24xZVN3306
MOSFET Transistor
3x MAX520
8-channel DAC
National Instruments
Digital VO
Figure 9-1: Representative diagram of a 24-channel LabView-controlled circuit for automating
the assembly of the 3D structures. The LabView panel is connected to a data acquisition card
that sends serial signals to digital to analog converters, which control the current in a particular
channel via a MOSFET transistor circuit.
actuating up to 24 different segments at one time. The circuit consisted of MOSFET current
drain transistors at every channel, controlled by 8-channel digital to analog converters (DACs).
A LabView virtual instrument was used to control the DACs, hence the current in each channel.
Figure 9-1 is a schematic diagram illustrating the control signal flow from the UI panel, through
the data acquisition card (DAQ), to the circuit components. the DACs used the I2 C serial
protocol to receive the commands from LabView.
115
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Appendix A
Lithography mask layouts
122
A Ark,
07.
16
a a:a:R RiA Art,
1A
A. X, A
PL
jrj.
I, f'4rin ff I ON Or
k
91
1 _06
IN A7 It 04PIN iR 1* 0 -A rV 16 OIN
06
IL *A
. ....
.......
...........
JPF
.....
.....
...
.....
..
..........
Mi !,M:4Idan.
I...........
.
............
I.."
,..........
rtrirtrdn.rH
..
........
..
...
.....
A A PI JR 01
Figure A-1: Grid of 14x14 dies patterned on a single 6 inch wafer. Two cross-hair alignment
patterns were placed in row #7 of each layer and used to align the mask of a particular layer
to a previously patterned film on the wafer.
Figure A-2: Mask layout of the structural layer of a corner-cube device showing the two sidewall
segments, marked with 'M' and 'T' patterns. The segments have 3 alignment feature pairs and
3 micro snap-fit latches pattered at the mating edge. The pattern also shows the array of etch
holes in the SU-8 layer.
123
I
U
Figure A-3: Mask layout of super-capacitor devices showing the six contact pads on the periphery and the two rectangular electrodes in the middle. The close-up view shows the resistor
wire pattern used to heat the photoresist pads.
124
Appendix B
Fabrication Processes
The fabrication of all devices reported in this thesis was conducted in the Microsystems Technology Laboratories (MLT) at MIT. The main fabrication process including its parameters and
designated equipment is included here.
Step Description
Equipment
Specs
Facility
Start with 6" (100) Silicon Wafer
Patterning Metal Layer
TRL
1
HMDS
2
Spin Photo Resist
2 um AZ5214
Coater
TRL
3
Soft Bake
30 min
PR Oven
TRL
4
Expose
EV1
TRL
5
Develop
Photo-Wet-L
TRL
6
Hard Bake
30 min
PR Oven
TRL
7
Metal Deposition
Cr 300A, AU 600nm
E-beam Au
TRL
8
Lift-off
Acetone + Ultrasound bath
Photo-Wet-AU
TRL
125
Step Description
Specs
Equipment
Facility
Patterning SU-8 Layer
9
Spin SU-8
15 um
SU-8 Spinner
TRL
10
Soft Bake
1 min
Hot Plate
TRL
11
Expose
EV1
12
Post Exposure Bake
13
Develop
14
Rinse
15
Hard Bake
TRL
1 min 30 sec w/ aggitation
Hot Plate
TRL
Photo-Wet-AU
TRL
TRL
Hot Plate
TRL
Patterning Photoresist for Adhessive pads
16
Spin Photo Resist
10um AZ P4620
Coater
TRL
17
Soft Bake
30 min, 95C
SU-8 Oven
TRL
18
Spin Photo Resist
10um AZ P4620
Coater
TRL
19
Soft Bake
30 min, 95C
SU-8 Oven
TRL
20
Expose
EVI
TRL
21
Develop
Photo-Wet-AU
TRL
22
Rinse
TRL
Release Isotropic Etch
23
XeF2 etcher
XeF2 etching
126
TRL
Appendix C
Software Code
The software code included in this appendix is a representative sample of key tools and simulations used in this document. A more comprehensive collection of code files can be downloaded
from http://www.mems3d.org/, after the publishing of this document.
C.1
MATLAB GUI code for angular measurements
function varargout = digital-protractor(varargin)
% DIGITALPROTRACTOR M-file for digital.protractor.fig
%
DIGITALPROTRACTOR, by itself, creates a new DIGITALPROTRACTOR or raises the existi:
%
singleton*.
%
H = DIGITALPROTRACTOR returns the handle to a new DIGITALPROTRACTOR or the handle
%
the existing singleton*.
%
DIGITALPROTRACTOR('CALLBACK',hObject,eventData,handles,...) calls the local
%
function named CALLBACK in DIGITALPROTRACTOR.M with the given input arguments.
%
DIGITALPROTRACTOR('Property','Value',...) creates a new DIGITALPROTRACTOR or raise;
%
existing singleton*.
%
are
Starting from the left, property value pairs
127
applied to the GUI before digital-protractorOpeningFcn gets called.
An
unrecognized property namce or invalid value makes property application
stop.
All inputs are passed to digital-protractorOpeningFcn via varargin.
*See GUI Options on GUIDE's Tools menu.
Choose "GUI allows only one
instance to run (singleton)".
% See also: GUIDE, GUIDATA, GUIHANDLES
% Edit the above text to modify the response to help digital-protractor
% Last Modified by GUIDE v2.5 29-Jun-2012 01:29:20
% Begin initialization code - DO NOT EDIT
guiSingleton =
1;
guiState = struct('guiName',
'guiSingleton',
mfilename,
guiSingleton,
'guiOpeningFcn', Odigital-protractorOpeningFcn,
'guiOutputFcn',
@digital-protractorOutputFcn,
'guiLayoutFcn',
[]
'guiCallback',
[]);
if nargin && ischar(varargin{1})
guiState.guiCallback = str2func(varargin{1});
end
if nargout
[varargout{1:nargout}] = gui-mainfcn(guiState, varargin{:});
else
gui-mainfcn(guiState,
varargin{:});
end
128
...
...
% End initialization code - DO NOT EDIT
% ---
Executes just before digital-protractor is made visible.
function digital-protractorOpeningFcn(hObject,
eventdata,
handles,
varargin)
% This function has no output args, see OutputFcn.
% hObject
handle to figure
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% varargin
command line arguments to digital-protractor
(see VARARGIN)
% Clear the main console screen
cdc;
% Set the number of clicks per line for measurement
handles.numberOfClicks = 5;
% Add library paths
addpath('libraries/nssjlib');
% Initialize the text displays
set(handles.edit-statusLog,'String',{'GUI Initialized...'});
set(handles.textpath,'String','');
% Initialize the ImageSelection scrollbar and hide it
set(handles.slider-imageSelector,'Min',1);
set(handles.slider-imageSelector,'Max',2);
set(handles.slider-imageSelector,'Value',1);
set(handles.slider-imageSelector,'Visible','off');
129
% Clear all axes of content
axes(handles.axes-original);
hold off;
cla;
set(handles.axes-original,'Visible','off');
axes(handles.axes-plotData);
hold off;
cla;
set(handles.axes-plotData,'Visible','off');
% Choose default command line output for digital-protractor
handles.output = hObject;
% Update handles structure
guidata(hObject, handles);
% UIWAIT makes digital-protractor wait for user response (see UIRESUME)
% uiwait(handles.figurel);
% ---
Outputs from this function are returned to the command line.
function varargout = digital-protractorOutputFcn(hObject,
eventdata,
% varargout
cell array for returning output args (see VARARGOUT);
% hObject
handle to figure
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Get default command line output from handles structure
varargout{1} = handles.output;
130
handles)
function editstatusLog-Callback (hObj ect,
eventdata,
handles)
% hObject
handle to editstatusLog (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Hints: get(hObject,'String') returns contents of editstatusLog as text
str2double(get(hObject,'String')) returns contents of editstatusLog as a double
%
% ---
Executes during object creation, after setting all properties.
function editstatusLogCreateFcn(hObject,
eventdata, handles)
% hObject
handle to editstatusLog (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
empty - handles not created until after all CreateFcns called
% Hint: edit controls usually have a white background on Windows.
%
See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor')
%
set(hObject,'BackgroundColor','white');
end
% ---
Executes on button press in buttonselectFile.
function buttonselectFileCallback(hbject, eventdata, handles)
% hObject
handle to buttonselectFile (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
131
% Open a browing window to select a file
[nameSelectedFile pathSelectedFile] = uigetfile('*.*');
set(handles.text-path,'String',[pathSelectedFile nameSelectedFilel);
handles.sourceDirectory = pathSelectedFile;
handles.sourceFile = nameSelectedFile;
% Add 'File Loaded' to the statusLog text box
appendText(handles.edit-statusLog, 'File loaded...');
% Update handles structure to store the handles.soucePath
guidata(hObject, handles);
% ---
Executes on button press in buttonselectFolder.
function buttonselectFolderCallback(hObject, eventdata, handles)
% hObject
handle to buttonselectFolder (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
pathSelectedDirectory = uigetdir(pwd);
set(handles.text-path,'String',pathSelectedDirectory);
handles.sourceDirectory = [pathSelectedDirectory '/'];
handles.sourceFile =
'
% Add 'Directory Loaded' to the status log text box
appendText(handles.editstatusLog, 'Directory loaded...');
% Update handles structure to store the handles.soucePath
guidata(hObject, handles);
132
. ---
Executes on slider movement.
function sliderimageSelectorCallback(hObject, eventdata, handles)
% hObject
handle to slider-imageSelector (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Hints:
get(hObject,'Value') returns position of slider
%
get(hObject,'Min') and get(hObject,'Max') to determine range of slider
% Get the current slider position
sliderPosition = int8(get(hObject,'Value'));
% Load the data structure to work with
dataStruct = handles. dataStruct;
% Display the image in the axes-original
axes(handles.axesoriginal);
image(dataStruct(sliderPosition).rgb);
axis image;
tabledata = cell(1,5);
tabledata(1) = {num2str(sliderPosition)};
tabledata(2) = {dataStruct(sliderPosition).filename};
if(dataStruct(sliderPosition).isRef)
tabledata(3) = {'Yes'};
else
tabledata(3) = {'No'};
end
dataStruct(sliderPosition).angle;
tabledata(4) = {num2str(dataStruct(sliderPosition).angle +...
133
dataStruct(sliderPosition).angleOffset)};
% Update the displayed values in the table to reflect the current image
% data (including the checkbox status
set(handles.tablecurrentImage,'Data',tabledata);
set(handles.checkbox-plot,'Value',dataStruct(sliderPosition).plot);
% Save the data structure back into handles
handles.dataStruct = dataStruct;
% Update handles structure
guidata(h~bject, handles);
% Display the position of the slider
% appendText(handles.editstatusLog, ['Image:
% ---
'
num2str(sliderPosition)]);
Executes during object creation, after setting all properties.
function sliderimageSelectorCreateFcn(hObject, eventdata, handles)
% hObject
handle to slider-imageSelector (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
empty - handles not created until after all CreateFcns called
% Hint: slider controls usually have a light gray background.
if isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
set(hObject,'BackgroundColor',[.9 .9 .9]);
end
% ---
Executes on button press in buttonstart.
function button_startCallback(hObject, eventdata, handles)
134
% hObject
handle to buttonstart (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Get the current time
c = clock;
timestamp-string = [sprintf('%02d',round(c(4)))
sprintf('X02d',round(c(5))) ':'
':'...
sprintf('%02d',round(c(6)))];
% Display the time stamp in the status box
appendText(handles.editstatusLog,
['----------------------------'));
appendText(handles.edit-statusLog,
['Processing Started at
timestamp-string;
''1);
drawnow
% Clear all axes and table of content
axes(handles.axesoriginal);
hold off;
cla;
set(handles.axes-original,'Visible','off');
axes (handles. axes-plotData);
hold off;
cla;
set(handles.axes-plotData,'Visible','off');
set(handles.table-currentImage,'Data',cell(1,5));
sourcePath = [handles.sourceDirectory handles.sourceFile];
% Check to see if the selection was a file or directory
handles.pathIsDirectory = exist(sourcePath, 'dir');
% If the path is NOT a directory, check for a valid file
135
'
...
if(~handles.pathIsDirectory)
handles.pathIsFile = exist(sourcePath,'file');
end
% Processing of a directory
if(handles.pathIsDirectory)
searchterm
listfiles
='*.jpg;
=
dir([handles.sourceDirectory searchterm]);
numberoffiles = length(list-files);
elseif(handles.pathIsFile)
numberof_files = 1;
end
% Create an empty dataStructure to store the image data in
dataStruct = struct('filename',{},'rgb',{},'isRef',{},...
'angleOffset',O,'pts',zeros(4,2),'plot',O);
for(i-files = 1:numberoffiles)
%
Construct the file path of the image being handled
if(handles.pathIsDirectory)
imageFileName = list-files(i-files).name;
imageFilePath = [handles.sourceDirectory imageFileName];
elseif(handles.pathIsFile)
imageFileName = handles.sourceFile;
imageFilePath = sourcePath;
end
7X
Read the image
RGB = imread(imageFilePath);
136
% Display image with true aspect ratio in the axes-original
axes(handles.axes-original);
image(RGB);
axis image
%
Display an update on which image is being processed
appendText(handles.edit-statusLog, ['Processing Image
num2str(i-files)
%
'
...
' of ' num2str(numberoffiles)]);
Prompt the user to click on the edge of the segment
appendText(handles.edit-statusLog, ['Click on the edge..']);
drawnow
% Use ginput to select n points along the edge
p = myginput(handles.numberOfClicks,'crosshair');
% Fit selected points into a linear regression
linfit = polyfit(p(:,1),p(:,2),1);
%
Store the image information in the data Structure
dataStruct(i-files).filename = imageFileName;
dataStruct(i-files).rgb = RGB;
dataStruct(i-files).isRef = 0;
dataStruct(i-files).angleOffset = 0;
dataStruct(i-files).angle = roundn(radtodeg(atan(lin-fit(1))),-2);
dataStruct(i-files).pts
dataStruct(i-files).plot
=p;
=
1;
end
%
Enable the scroll bar if the number of files is more than one
137
set(handles.sliderimageSelector,'Value',1);
if(number-offiles > 1)
sliderMin = 1;
sliderMax = numberoffiles;
sliderStep = [1, 1] /
(sliderMax - sliderMin);
set(handles.slider-imageSelector,'Min',sliderMin);
set(handles.slider-imageSelector,'Max',sliderMax);
set(handles.slider-imageSelector,'Value',1);
set(handles.slider-imageSelector,'SliderStep',sliderStep);
set(handles.slider-imageSelector,'Visible','on');
end
% Update status log
appendText(handles.edit-statusLog, ['Processing Ended at
'
...
timestamp.string]);
% Store the imagesData in handles for global access
handles.dataStruct = dataStruct;
% Call the slider callback function to update the display in the table
sliderimageSelectorCallback(handles.sliderimageSelector,eventdata, handles);
set(handles.axes-original,'Visible','on');
% Update handles structure
guidata(hObject,
handles);
138
% ---
Executes on button press in buttonsetReference.
function buttonsetReferenceCallback(hObject, eventdata, handles)
% h~bject
handle to buttonsetReference (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
imgIndex = get(handles.slider-imageSelector,'Value');
% Load the data structure to work with
dataStruct = handles . dataStruct;
% Read the angle to offset from the current image data
angleOffset = -dataStruct(imgIndex).angle;
% Set the angleOffset of all the structure to -ve the current angle
for i=1:numel(dataStruct)
dataStruct(i).angleOffset = angleOffset;
dataStruct(i).isRef = 0;
end
% Set the 'isRef' flag to high and plot flag to low for the reference img
dataStruct(imgIndex).isRef
dataStruct(imgIndex).plot
=
=
1;
0;
% Store the imagesData in handles for global access
handles.dataStruct = dataStruct;
% Call the slider callback function to update the display in the table
sliderimageSelectorCallback(handles.slider-imageSelector,eventdata, handles);
% Update handles structure
guidata(h0bject, handles);
139
% ---
Executes on button press in button-exportCSV.
function button-exportCSV-Callback(hObject, eventdata, handles)
% hfbject
handle to button-exportCSV (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Load the structure data
dataStruct = handles.dataStruct;
% Get the current time to use to create the filename
timestamp-string = datestr(now, 'yyyymmddHHMMSS');
% Open a file for writing in the directory of the images
datafilename = [timestamp-string '.csv'];
fileId = fopen([handles.sourceDirectory datafilename] ,'w');
fprintf(fileId,'Image File Name,Current(mA),Angle(deg)\n');
% Loop through the data structure and extract plotting data
x = [I; y = [];
for i=1:numel(dataStruct)
if(dataStruct(i).plot)
yi = dataStruct(i).angle + dataStruct(i).angleOffset;
if(yi < 0)
yi = 180 + yi;
end
y = [y; yi];
f_name = dataStruct(i).filename;
xi = str2double(f-name(str2double(get(handles.editfromChar,'String')):str2double(g
140
x
=
[x; xi];
fprintf(fileId,'%s,%4.2f,%4.2f\n',f-name,xi,yi);
end
end
% Close the file
fclose(fileId);
% ---
Executes on button press in checkbox-plot.
function checkbox-plotCallback(hObject, eventdata, handles)
% hObject
handle to checkbox-plot (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Hint: get(hObject,'Value') returns toggle state of checkbox-plot
% Get index of current image
imgIndex = get(handles.slider-imageSelector,'Value');
% Load the data structure to work with
dataStruct = handles.dataStruct;
% Set the plot flag of the particular image to the state of the checkbox
dataStruct(imgIndex).plot = get(hObject,'Value');
% Store the imagesData in handles for global access
handles. dataStruct = dataStruct;
% Call the slider callback function to update the display in the table
sliderimageSelectorCallback(handles.sliderimageSelector,eventdata, handles);
141
% Update handles structure
guidata(hObject, handles);
% ---
Executes on button press in button-plotData.
function button-plotDataCallback(hfbject,
eventdata,
handles)
% hObject
handle to button-plotData (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Load the structure data
dataStruct = handles.dataStruct;
% Loop through the data structure and extract plotting data
x = []; y = [];
for i=1:numel(dataStruct)
if(dataStruct(i).plot)
yi = dataStruct(i).angle + dataStruct(i).angleOffset;
if(yi < 0)
yi = 180 + yi;
end
y = [y; yi];
f_name = dataStruct(i).filename;
xi
x
=
=
str2double(f-name(str2double(get(handles.edit_fromChar,'String')):str2double(g
[x; xi];
end
end
% Plot the data
142
axes (handles. axesplotData);
hold off;
p = plot(x,y);
set(handles.axesplotData, 'Visible' ,'on');
% hold for 2 seconds and then hide
pause(2);
set (handles.axesplotData, 'Visible','off');
set(p,'Visible','off');
% Save the plotted data to a jpeg
% We use jpEg extension to avoid using that image when measuring angles
fh = figure(1);
plot(x,y,'+--');
jpegPlotFile = [handles. sourceDirectory
'plotData.jpeg']
print(sprintf('-f%d',fh),'-djpeg','-r72',jpegPlotFile);
close(fh);
function edit_fromCharCallback (hObj ect,
eventdata,
handles)
% hObject
handle to editfromChar (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Hints: get(h~bject,'String') returns contents of editfromChar as text
%
str2double(get(hObject,'String')) returns contents of editfromChar as a double
% Round any decimal input
set(hObject,'String',num2str(round(str2double(get(hObject,'String')))));
% Update the edit-toChar to be one character more
143
set(handles.edittoChar,'String',num2str(round(str2double(get(hObject,'String')))+1));
% Update handles structure
guidata(hObject, handles);
% ---
Executes during object creation, after setting all properties.
function editfromCharCreateFcn(hObject, eventdata, handles)
% hObject
handle to edit-fromChar (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
empty - handles not created until after all CreateFcns called
% Hint: edit controls usually have a white background on Windows.
%
See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor')
set(hObject,'BackgroundColor','white');
end
function edit_toCharCallback (hObj ect, eventdata, handles)
% hfbject
handle to edit-toChar (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
structure with handles and user data (see GUIDATA)
% Hints: get(hObject,'String') returns contents of edittoChar as text
%
str2double(get(hObject,'String')) returns contents of edittoChar as a double
% Round any decimal input
set(hObject,'String',num2str(round(str2double(get(hbject,'String')))));
144
Executes during object creation, after setting all properties.
% ---
function edit toCharCreateFcn(hObject,
eventdata, handles)
% hObject
handle to edit-toChar (see GCBO)
% eventdata
reserved - to be defined in a future version of MATLAB
% handles
empty - handles not created until after all CreateFcns called
% Hint: edit controls usually have a white background on Windows.
See ISPC and COMPUTER.
%
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor')
set(hObject,'BackgroundColor','white');
end
% % --
%
---
Helper functions for the callbacks
Add a string to the a text box
function appendText(textBoxHandle,
textToAppend,
newLineFlag)
% textBoxName
gui name of the textbox to append to
% textToAppend
text string to append to the textbox
% Check if the newLineFlag is set or default to TRUE
% Fill in unset optional values.
switch nargin
case 2
newLineFlag = 1;
end
% Add textToAppend to the textBoxName text box
if(newLineFlag == 1)
145
set (textBoxHandle, 'String' , [textToAppend; get (textBoxHandle, 'String')]);
else
set (textBoxHandle, 'String' , [get (textBoxHandle, 'String')
end
C.2
Arrow-head latch strength simulations
close all;
cdc;
11 = 70;
bi = 4.5;
al = 36;
ti = 7;
H1 = 16;
hbl = 11.5;
hal = 8;
yOl = 12;
scale
=
1;
kolor
% Flag to scale thickness or not
scalethickness = 0;
1
11*scale
b
b1*scale
a
al*scale
t
tl*scale
H
H1*scale
146
'
'
textToAppend]);
hb = hbl*scale
ha = hal*scale
L = 1+a+b
peakindex = 0;
Ca = ([0 0 1; a^2/4 a/2 1; a^2 a 1])\[0;ha;H];
Cb = ([a^2
a 1; (a+b/2)^2 (a+b/2)
1; (a+b)^2 (a+b)
% returns a vector [C(1) C(2) C(3)]
1])\[H;hb;t];
% where y = C(3)*x^2 +
% set the resolution of the plot (# of divisions of the arrow length)
res = 1000;
x = 0:(L)/res:L;
n = length(x);
y = zeros(n,1);
for count = 1:n
if (x(count)<=a)
y(count) = Ca(1)*x(count)^2 + Ca(2)*x(count)
+ Ca(3);
peak-index = count;
else if (x(count)<=a+b)
y(count) = Cb(1)*x(count)^2 + Cb(2)*x(count) + Cb(3);
else
y(count) = t;
end
end
147
end
x-peak = x(peak-index)
y-peak = y(peak-index)
figure(1);
subplot(2,2,1);
plot(x,y);
% axis equal;
title('Arrow profile','Fontsize',16);
xlabel('Distance - \mum' ,'Fontsize',14);
ylabel('Height - \mum','Fontsize',14);
y-prime = zeros(n-1,1);
for count = 1:n-1
y-prime(count) = (y(count+1)-y(count))/(x(count+1)-x(count));
end
figure(1);
subplot(2,2,2);
plot(x(1:n-1),y-prime);
title('Arrow slope','Fontsize',16);
xlabel('Distance - \mum','Fontsize',14);
ylabel('Slope','Fontsize',14);
%
Calculating the vertical bending force based on the hole edge heigh yO
E = 4;
X
Modulus of SU8
w = 14.5;
%
Film thickness
if(scale-thickness)
148
w
=
w*scale;
end
I
t^3*w/12;
=
yO
fy
yOl*scale;
=
zeros(n,1);
for count
1:n
=
if (yO
-
y(count) > 0)
fy(count) = 0;
else
fy(count) = 6*le3*(E*I*(y(count)-yO))/(L-x(count))^2/(2*L+x(count));
end
end
figure(1);
subplot(2,2,3);
plot(x,fy);
title('Vertical Bending Force - \muN','Fontsize',16);
xlabel('Distance - \mum','Fontsize',14);
ylabel('Force - \muN','Fontsize',14);
% Calculating the horizontal insertion force needed for bending
ratio-approx=zeros(n-1,1);
ratioreal
for count
=
1:n-1
=
if (yO
zeros(n-1,1);
-
y(count) > 0)
%
ratio-approx(count) = y-prime(count);
%
ratioreal(count) = y-prime(count);
ratio-approx(count) = 0;
ratioreal(count) = 0;
149
X
units: uN
else
ratio-approx(count) = y-prime(count);
def-slp = fy(count)*le-3/E/I*(L-x(count))*(L-(L-x(count))/2);
ratio-real(count) = y-prime(count) + def-slp;
% Slope due to th,
% Total slope
end
end
fin-approx = ratioapprox.*fy(1:n-1);
finreal = ratioreal.*fy(1:n-1);
figure(1);
subplot(2,2,4);
plot(x(1:n-1),fin.approx,'b');
hold on;
plot(x(1:n-1),fin-real,'r');
xlabel('Distance - \mum','Fontsize',14);
ylabel('Force - \muN','Fontsize',14);
title('Insertion and Extraction Forces - \muN', 'Fontsize',16);
figure(1);
% title(['L=' num2str(L) '\mum, H=' num2str(H)
'\mum, t=' num2str(t) '\mum, a=' num2str(a)
figure (3)
title(['L='
num2str(L)
'\mum, H=' num2str(H)
figure(2);
% plot(x(1:n-1),fin-approx,':b');
hold on;
p = plot(x(1:n-1),fin-real,'r');
plot(x(1:n-1),finapprox,'b');
150
'\mum, t=' num2str(t)
'\mum, a=' num2str(a) '\i
xLabel('Distance - \mui'
,'Fontsize',14);
ylabel('Force - \muN' , 'Fontsize',14);
title('Insertion and Extraction Forces - \muN',
'Fontsize',16);
grid on;
C.3
Thermal simulation of heat pads
%==
Nader S.
%==
nshaarmit . edu
%==
Thermal Modeling of Local melting of Material
Shaar
April 05,
2005
==
(617) 458-0649
==
==
cdc;
clear;
sigma-s = 0.2;
sigma-f = 0.2;
density-s = 1200;
density-f = 1200;
Cm-s = 0.18;
% 0.18 W/m.K Ref: M.-T. Hung and Y.
S. Ju: Process dependence of the therma
Cm-f = 0.18;
t_s = 15e-6;
t_f = 12e-12;
Diam-f = 100e-6;
Diam_0 = 10*Diam-f;
% Diameter at which T is considered to be room temperature
151
n-r = 20;
n_theta = 90;
A = zeros(10*n-r,10*n-r);
B = zeros(10*n-r,10*n-r);
Q = 2e-4;
for countl = 1:10*nr-1
L_i = 1/2*Diam-f/n-r;
w_i = 2*pi*(countl*L-i)/ntheta;
Rt-f = L-i/sigma-f/w-i/t-f;
Rts = L-i/sigma-s/w-i/t-s;
Cts = density-s*Cm-s*(w-i*Li*t-s);
Ctf = density-f*Cm-f*(w-i*Li*t-f);
if(countl<=n_r)
C_eq = Ct-s+Ct-f;
R_eq = 1/(1/Rt-f+1/Rt-s);
I_i = Q*w-i*L-i/(1/4*pi*Diam.f^2);
else
C_eq = Ct-s + Ct-f;
R_eq = 1/(1/Rt-f+1/Rt-s);
I_i = 0;
end
152
B(countl,countl) = I-i/Ceq;
if(count1==1)
A(countl,countl) = -1/R-eq/C-eq;
A(countl,count1+1)= 1/R-eq/C-eq;
else
A(countl,countl-1) = 1/R-eq/C-eq;
A(countl,countl) = -2/R-eq/C-eq;
A(counti,countl+1)= 1/R-eq/C-eq;
end
end
C = eye(10*n-r);
D = zeros(10*n-r);
sys = ss(A,B,C,D);
timesteps =
0:5e-5:le-3;
[Y,t] = step(sys,timesteps);
Nt
Lin
length(t);
=
=
length(Y(1,1,:));
Lout= length(Y(1,:,1));
% sum up the effects of all inputs
Ys=zeros(Nt,Lout);
7. Ys=Y(:,:,1);
for i=1:length(Y(1,1,:))
Ys=Ys+Y(:,:,i);
153
end
%Plot of temperature distribution at different times
x = L-i/2:Li:Diam_0/2;
% size(x)
% size(Ys(1,:))
for i = 1:3:Nt
plot(x*1e6,Ys(i,:)+300-273);
hold on;
end
xlabel('Distance - \mum');
ylabel('Temperature - C');
axis tight;
axislimit = axis;
axis([0 Diamf*1e6*1.2/2 axis-limit(3) axisjlimit(4)1);
%Average Temperature over time
for i=1:Nt
sum=0;
for j=2:Lout/2-1
sum=sum+Ys(i,j)+300-273;
end
MeanYs(i)=1/(Lout/2-2)*sum;
end
figure(2)
plot(t,MeanYs);
xlabel('Time - sec');
ylabel('Mean Temperature - C');
154
sr= [I
;
for coun
=
1:length(x)
for coun2 =
1:10:length(x)
d = (sqrt(x(coun1)^2+x(coun2)^2))*1e6;
search=l;
while((1e6*x(search)-d<5)&&search<length(x))
search=search+1;
end
sr(counl,coun2) = Ys(117,search);
end
end
dim=[];
sur=[];
stepl=l;
for coul = 1:10:length(x)
dim(stepl)=x(coul)*1e6;
step2=1;
for cou2 = 1:10:length(x)
sur(stepl,step2)=sr(coul,cou2);
step2=step2+1;
end
step1=stepl+1;
end
155