MECH 466
Microelectromechanical Systems
University of Victoria
Dept. of Mechanical Engineering
Lecture 17:
Advanced Microfabrication Technologies
© N. Dechev, University of Victoria
1
Overview
EFAB Surface Micromachining Process
LIGA Process
Laser Micromachining
Micro EDM (Electro Discharge Machining)
Flip-Chip Assembly
Self-Assembly
Robotic Assembly
© N. Dechev, University of Victoria
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Advanced Microfabrication of MEMS
There are inherent limitations with Surface Micromachining, and
Bulk Micromachining for creating different types of
microstructures.
A number of different advanced microfabrication technologies
have been developed in recent years to create micro-parts that
cannot be made with traditional approaches.
It is important to note that many of these processes can only
produce ‘micro-scale parts’, and cannot produce microsystems.
© N. Dechev, University of Victoria
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EFab Process
The EFab was developed by MicroFabrica Inc.
It is a multi-layer surface micromachining process, where each
and every layer is ‘planarized’ before the next layer is added.
This allows the process to create 20+ layers of surface
micromachined materials to build up very tall, 3D MEMS
microstructures.
More information about the EFAB process is available at: http://
www.microfabrica.com/
© N. Dechev, University of Victoria
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EFab Process
The process works as follows:
A three-step process is used to generate each layer. This is
repeated as many times as there are layers to build the desired
complex devices. These steps are:
1. Patterned layer deposition
2. Blanket layer deposition
3. Planarization
© N. Dechev, University of Victoria
Images of EFab Process [From Microfabrica]
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EFab Process
© N. Dechev, University of Victoria
Images of EFab Process
[From Microfabrica]
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Examples of Devices Produced by EFab Process
Fluid-driven Impeller with Reduction Gearing
[Microfabrica]
Articulated micro-hand (flexed) [Microfabrica]
© N. Dechev, University of Victoria
Rotary Varactor (24 layers) [Microfabrica]
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LIGA Process
The LIGA (acronym from German words for lithography,
electroplating, and molding) process is based on the creation of
plastic micro-molds, which are electroplated with metal to create
micro-parts.
The process uses PMMA (polymethylmethacrylate) (otherwise known
as acrylic) as the plastic for the mold.
The mold is created by lithographically patterning the PMMA. The
unique aspect of LIGA is that the pattern is exposed with high
intensity X-Ray radiation, usually from a Synchrotron.
X-Ray exposure causes very ‘Deep’ exposure of the PMMA , which
allows for very deep molds with straight sidewalls to be created.
the molds can be 100 um to 1000 um deep.
The aspect ratio (depth to width) for the molds can be as high as 20:1
The electroplated material is usually nickel metal.
© N. Dechev, University of Victoria
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LIGA Process
Note: LIGA is essentially a 2D process, in that it produces very tall
micro-parts, that have a 2D cross-section
Picture of LIGA Process Steps [Sandia National Laboratories]
© N. Dechev, University of Victoria
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Examples of LIGA Process
Comb-drive
Electro-Static Motor
E-S Linear Motor
Micro Heat Exchangers [Sandia National Labs]
Images of Various LIGA Micro-Parts
LIGA Stator for Stepper Motor [Sandia National Labs]
© N. Dechev, University of Victoria
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Laser Micromachining
Laser micromachining makes use of laser light to selectively vaporize
areas of a material to create a desired micro-part.
The main advantage of lasers is the ability to micromachine a very
wide variety of materials, such as most metals, ceramics, and
plastics.
There are two main types of laser micromachining:
(a) ‘Focal point’ or ‘direct write’ systems
(b) ‘Mask Projection’ laser etching systems
Direct Write Laser [Image from Resonetics]
Mask Projection Laser [Image from Resonetics]
© N. Dechev, University of Victoria
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Direct Write Laser Micromachining
The ‘direct write laser’ system is the traditional method of focusing a
beam of laser light to a point and moving the focal point along the
x, y and z directions. In this manner, a path is ‘burned into’ the
material, by selectively vaporizing material to create the desired
shape.
This method is good for cutting out parts, making holes, and other
features, as shown in the images below.
The disadvantage is that the cut-path tends to be non-smooth, since
the pulsing beam of light produces paths as a sequence of
overlapping circles.
Exaggerated image of cutting a
straight line with a series of
overlapping holes
© N. Dechev, University of Victoria
Unfocused (left) and focused (right)
hole drilling [TeoSys Engineering]
30 um hole in Teflon
[Image from TeoSys Engineering]
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Mask Projection Laser Micromachining
The ‘mask projection laser’ system uses a ‘mask’ in the laser path to
selectively block out certain areas. The mask is placed in a wide
(low flux) laser beam, and the ‘masked light’ them passes through a
lens to focus the image (high flux) onto the target.
This method will burn an image ‘simultaneously’ across an area, to a
small depth. Repeated exposure will slowly burn downward
through the material.
Step 1: Original Substrate
Step 2: Expose Area with Mask 1
Step 4: Expose Area with Mask 2
Step 5: Result of 2nd Exposure
© N. Dechev, University of Victoria
Step 3: Result of 1st Exposure
Step n: Result of nth Exposure
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Mask Projection Laser Micromachining
This method can be used to create parts with ‘smooth’ edges.
The disadvantage of this method is that ‘masks’ are needed for each
type of part that must be made.
SEM of micromachined Polyimide, 10x10 um
[Resonetics]
© N. Dechev, University of Victoria
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Micro Electro Discharge Machining (Micro EDM)
An electric-discharge process using electrical ‘arcing’ to melt and
destructively remove material.
A high voltage potential is created between an electrode and a
workpiece submerged in a dielectric fluid.
Depending on the process used, the electrode is ‘pushed into’ the
material and the desired shape is ‘burned into’ it.
30-micron shafts and 50-micron holes
Microelectrodischarge Machining Schematic
produced by micro-EDM.
[Center for Non Traditional Manufacturing Research, University of Nebraska]
[www.mikrotools.com]
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Micro Electro Discharge Machining (Micro EDM)
The electrode can have a specific shape pre-machined into it, to create
a ‘reverse copy of itself’ when it is ‘pushed into a material’.
Alternatively, the electrode can be ‘spun’ in a chuck, to create a
circular point source.
The work-piece is mounted on a worktable, that can be positioned in
x,y and z, to create the desired pattern/shape.
Advantages: Can be used to machine hard, high strength and
temperature resistant materials such as hardened steel, ultra-hard
polycrystalline diamond and ceramics.
Disadvantages: Resolution is lowest of all micromachining methods at
around 50 um to 5 um. Electrode wear requires continuous
monitoring to maintain precise dimensions.
Good EDM web site from University of Nebraska: http://
www.unl.edu/nmrc/equipment.htm
© N. Dechev, University of Victoria
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Microassembly
A tool that can be used in the fabrication of MEMS devices
Microassembly is a ‘post-processing’ process, meaning that it occurs
after surface, bulk, or other micromachining processes are
completed.
Microassembly is simply the assembly of a collection of parts, but
done at the micro-level with micro-scale parts.
© N. Dechev, University of Victoria
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What is Microassembly?
Microassembly can be described as a process of moving micro-parts from
their original fabrication location, manipulating them by translation and/or
rotation through space, and fixing them at a ‘final’ assembly location.
Interface
Feature
Micro-Part Body
Slot
Base
Structure
Micro-Parts Before Assembly [N. Dechev]
© N. Dechev, University of Victoria
Snap-Lock
Joint
Micro-Parts After “Snap-Lock” Assembly [N. Dechev]
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Why Use Microassembly?
Microassembly for MEMS is necessary when:
(a) Micro-parts for the assembled device originate from two or
more different sources. (Due to fabrication incompatibilities)
(b) Tall or non-planar microstructures are required.
(c) Complex 3D microstructures, perhaps consisting of subassemblies of micro-parts are required.
© N. Dechev, University of Victoria
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Microassembly Approaches
There are a number of different microassembly approaches,
which can be split into two main strategies:
(a) Parallel (Batch) Microassembly
- Simultaneous microassembly occurs at multiple
sites, such as flip chip assembly.
(b) Self-Assembly (Batch)
- An applied force ‘drives’ the assembly to take place,
such as heat, magnetism, or centrifugal force.
(c) Serial (Sequential) Microassembly
- Step by Step microassembly, at a single site.
Performed by manual micro-manipulators, or teleoperated robotic systems.
© N. Dechev, University of Victoria
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Parallel Microassembly Examples
Wafer to Wafer or Flip Chip Bonding using Indium Solder [A. Singh, et al]
Parallel “Self-Assembly”
Polyimide in Silicon V-Joints
[T. Ebefors, et al]
© N. Dechev, University of Victoria
Parallel “Self-Assembly”
Plastic Deformation Magnetic Assembly
[J. Zou, C. Lui, et al]
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Serial “Sequential” Microassembly Examples
Fold-Out Microassembly
[E. E. Hui, et al]
Ortho-Tweezers Assembly
[E. Shimada, et al]
Serial Microassembly
[G. Yang, B. Nelson, et al]
© N. Dechev, University of Victoria
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Microassembly Work by Others
R.S. Fearing, R.S. Muller, et al., have demonstrated ‘hinged’ microassembly
were micro-parts are folded ‘out-of-plane’. (1992)
R.T. Howe, A.P. Pisano, et al., have demonstrated flip chip microassembly,
whereby two chips are aligned face to face, and pressed together. (1997)
V.M. Bright, K.F. Harsh, et al., have demonstrated solder-reflow selfassembly. (1999)
W. J. Li, K. W. C. Lai, et al., have demonstrated inertial self-assembly. (2002)
R.S. Fearing, E. Shimada, et al., have developed a robotic workstation making
use of ‘orthogonal micro-tweezers’ for ‘serial’ microassembly. (1998)
B.J. Nelson, G. Yang, et al., have demonstrated a robotic workstation making
use of a ‘micro-gripper’ for ‘serial’ microassembly. (2000)
G.D. Skidmore, M. Ellis, et al., are developing a microassembly system using
a ‘micro-gripper’ for ‘serial’ microassembly. (2003)
© N. Dechev, University of Victoria
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Microassembly vs Micro-manipulation
Micro-manipulation is the act of translating and/or rotating micro-objects for
various purposes, such as inspection, alteration, or assembly.
The end effectors (e.g. probes, or grasping tools such as tweezers or grippers)
used for micro-manipulation are usually designed to handle a variety of microobjects of a certain type or class.
For example, the end effector used for the manipulation of biological cells
would have to accommodate a range of sizes, and must accommodate for the
various initial positions and orientations of the cells.
Therefore, the end effector design must be versatile, to handle micro-objects
of varying properties, shapes, initial or final positions and orientations
In Other Words: The micro-objects are the ‘variables’ and cannot be altered,
while the end-effector must be designed to accommodate them.
© N. Dechev, University of Victoria
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End Effectors for Micromanipulation or Microassembly
Micro-EDM Tweezers
[B.J. Nelson, et al]
[C. G. Keller, et al]
S. Micromachined Microgripper
© N. Dechev, University of Victoria
Silicon Crystal Micro-Tweezers
[C. G. Keller]
S. Micromachined Microgripper
[N. Dechev, et al]
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Microassembly vs Micro-manipulation
Microassembly ‘uses micro-manipulation’ during the assembly of microcomponents.
The goal of microassembly is to build/construct/erect two or more microcomponents into a permanent microstructure.
Since the geometry of the completed microstructure is known in advance, it
follows that the constituent micro-components of a microassembly have a
known shape, size, material property, final position and behavior.
In Other Words: We can ‘control’ the design of the micro-components and
therefore we can design the end-effector very specifically.
© N. Dechev, University of Victoria
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Soldering a Microgripper to the Contact Head
The microgripper is specifically designed to be removed from the chip
substrate prior to use. Microgrippers are arranged in rows, and are attached to
the substrate via tethers.
Microgrippers Arranged on Chip Substrate
[N. Dechev]
SEM Image of Chip [N. Dechev]
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Microgripper UV-Bonded to Probe Pin
1 mm
SEM Images of Microgripper Bonded to Probe Tip of a Micromanipulator [N. Dechev]
© N. Dechev, University of Victoria
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Grasping Micro-Parts
The MEMS chip under the microgripper is translated along the x, y and z axes to
align the gripper tips with the interface feature of the micro-part.
After a successful grasp, the microgripper must remove the micro-part from the
substrate, by breaking its tethers.
© N. Dechev, University of Victoria
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Movie of Grasping Micro-Parts
Micro-Parts
Target Joint
Sites
© N. Dechev, University of Victoria
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Grasping Micro-Parts
Micro-parts used in this microassembly system are designed in advance with a
‘specific geometrical feature’ built-in, such that they can accommodate the
microgripper.
The microgripper is specifically designed to interface to the geometrical feature.
Geometric Feature (Simplified)
Microgripper (Simplified)
© N. Dechev, University of Victoria
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Passive, Compliant Microgripper Design
An important question arises. How can you:
(a) grasp micro-parts, and (b) release micro-parts,
with a microgripper that cannot be powered to open or close?
Tether
Micro-Part
Interface Feature (Simplified)
Joint Target
© N. Dechev, University of Victoria
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Video of Grasping Operation
100 um
Grasping Operation (Tele-operated, Actual Speed, ! 20 sec. )
© N. Dechev, University of Victoria
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Micro-Parts
Each micro-part incorporates three modular design features:
(a) An ‘interface feature’ specifically designed to be grasped by the microgripper.
(b) A ‘tether feature’, that secures the micro-part to the substrate and breaks away after
the microgripper has grasped the micro-part.
(c) A ‘snap-lock feature’, which is used to create joints between a micro-part and the
chip substrate, or other micro-parts.
Snap-Lock
Tether
Interface Feature
SEM Images of Micro-Parts used in Microcoils [N. Dechev]
© N. Dechev, University of Victoria
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Micro-Part Tether Design
The tethers serve four important functions:
Secure the micro-parts during the oxide release process (fluid forces).
Secure the micro-parts during the transport process (vibratory forces).
Must adequately restrain the micro-parts during the grasping operation.
Must reliably and predictably break-away after the micro-part is grasped.
© N. Dechev, University of Victoria
35
Micro-Part Tether Design
Anchor Pad
Tether
Break Notch
(2 Mm x 3 Mm)
Flexible Notch
(2 Mm x 6 Mm)
150 Mm
High Stress
Low Stress
11 Mm
© N. Dechev, University of Victoria
Applied Force
x-direction
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Manipulation of Micro-Parts
© N. Dechev, University of Victoria
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Joining Micro-parts
To create a joint, the ‘barbed plug’ of the tips is brought into focus. The micro-part is
aligned in the x and y axes, with the slot located on the base structure, and the distal
arm is commanded down in the z-axis to make the joint.
Next, the distal arm is commanded back up in the z-axis, which causes the
microgripper to release the micro-part, which remains perpendicularly joined to the
base structure.
© N. Dechev, University of Victoria
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Joining Micro-parts
The video microscope system used for this work has a resolution of 0.7 um, a field of
view of 320 x 240 um and a depth of focus of 1.5 um.
Due the low depth of focus, it is difficult to see objects that are perpendicular to the
substrate, such as the microgripper and micro-part shown below.
Snap-Lock Tips
(in Focus)
Snap-Lock Tips
(in Focus)
Microgripper
Loop Element
LE with snap-lock tips in focus.
All other entities are out of focus.
Tips are 80 um above joint target.
LE with snap-lock tips in focus.
Tips are 30 um above joint target.
© N. Dechev, University of Victoria
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Joining Micro-parts
SEM Image of 6-Loop Micro-coil
(200 um x 140 um x 150 um TxWxL)
[N. Dechev]
© N. Dechev, University of Victoria
SEM Close-Up of Snap-Lock Joints [N. Dechev]
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Snap-Lock Micro-Joint
68 Mm
28 Mm
Offset
2 Mm
4 Mm
6 Mm
2 Mm
4 Mm
Plug Tip
Base Structure
Insertion
Force
2 Mm
Fe
Slot
Substrate
© N. Dechev, University of Victoria
Substrate
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Snap-Lock Micro-Joint
© N. Dechev, University of Victoria
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Key-Lock Joint Design
Micro-Component
Body
Key
Base Structure
Substrate
(a) Before Insertion
(b) After Insertion
Joint is formed when the Key is inserted into the Wide Slot, and then translated
into the Narrow Slot. The joint relies on stiction to remain in place.
© N. Dechev, University of Victoria
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Video of Key-Lock Joining
100 um
MANUAL, Key-Lock Joining Operation
(Real Time, ! 40 sec. )
© N. Dechev, University of Victoria
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Key-Lock Joint Design
MicroComponent
Square Slot
Key-Lock
Joint
Gold Layer
Round Slot
Narrow Slot
Wide Slot
SEM image of T-Slot and Hole Slot KeyLock Joints [N. Dechev]
SEM image of Key-Lock Joint [N.
Dechev]
© N. Dechev, University of Victoria
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Key-Lock Micro-Coil Application
SEM Image of 4-Loop Key-Lock Micro-coil
[N. Dechev]
© N. Dechev, University of Victoria
Close-Up of Snap-Lock Joints [N. Dechev]
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Inter-Lock Joint Design
Upper
MicroComponent
Slit
Slit
Key
Lower
MicroComp.
Base
Structure
(a) Before Insertion
(b) After Insertion
Slot
© N. Dechev, University of Victoria
47
Inter-Lock Joint Design
Slits
InterLock
Joint
T-Slot
Base Structure
Key-Lock
Joint
Lower
Micro-Component
Inter-Lock
Joint
Empty Base Structures
© N. Dechev, University of Victoria
Lower Micro-Parts Joined
Upper Micro-Part Joined
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Fully 5-DOF Assembly
SEM Image of Joints of Lower-Micro-Parts
[N. Dechev]
SEM Image of Joints of Upper Micro-Parts
[N. Dechev]
© N. Dechev, University of Victoria
49
Fully 6-DOF Assembly
SEM Image of Joints of Lower-Micro-Parts
[N. Dechev]
© N. Dechev, University of Victoria
SEM Image of Joints of Upper Micro-Parts
[N. Dechev]
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