Chapter 9:
Micro Structure Technology and
Micromachined Devices
Picture shows the
interior chip
assembly of the
SA30 Crash
Sensor, a
microsystem from
SensoNor, Norway
The course material was developed in INSIGTH II, a project
sponsored by the Leonardo da Vinci program of the European Union
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 1
Definitions
• MICRO STRUCTURE TECHNOLOGY can be defined as a
group of three-dimensional micromachining techniques enabling
feature dimensions with accuracy in the micrometer range.
• MICROMACHINED DEVICES can be defined as devices made
by Micro Structure Technology.
– These micromachining techniques are mainly based upon batch organised
microelectronic process technology, either directly adapted techniques like
photolithographics, or modified techniques such as anisotropic etching
techniques.
– Some micromachining techniques are specially developed for this field,
e.g., anodic bonding of micromachined devices.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 2
Example: SP80 Pressure Sensor
• Developed at SINTEF (earlier Center for Industrial Research),
Norway and manufactured by Capto as (earlier SensoNor AS,
earlier ame), Borre, Norway.
• This sensor visualises the main features and limitations of
micromechanical sensors, and points out pressure sensing as a
main application for these kinds of sensors.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 3
SP80 Principal Design
• A piezoresistive integrated pressure sensor with the pressuresensitive diaphragm micromachined in a silicon chip by
anisotropic etching.
• Ion implanted piezoresistors in a full Wheatstone bridge
configuration as the electronic sensing element.
• Temperature measuring resistor and a heating resistor are
implanted on the same chip, to compensate or thermostat the
chip to minimise thermal drifts.
• By varying the area and the thickness of the diaphragm,
pressure ranges from 0.5 Bar full scale pressure up to 60 Bar
full scale pressure can be achieved
• Packaged in a transistor header
• Main application areas are within general instrumentation,
metrology and aerospace application.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 4
The SP80 Silicon Chip Set - Drawing
• Consists of diaphragm chip sealed to a support chip which is mounted on
top of a glass tubing acting as a mounting stand as well as a pressure port.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 5
The SP80 Silicon Chip Set - Picture
• Consists of diaphragm chip sealed to a support chip which is mounted on
top of a glass tubing acting as a mounting stand as well as a pressure port.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 6
Dimensions and Processing
• The size is 4*4 mm, thickness approximately 0.3 mm, the diaphragm area is
typical 2*2 mm and the diaphragm thickness is typical 30 micrometers.
• The diaphragm is manufactured by stripping off the surface oxide of the
silicon wafer by means photolithographic technique in the areas we want
the diaphragm cavity.
• Then the wafer is etched in an anisotropic etching solution with the
remaining oxide as masking film.
– This etching solution attacks the single crystal silicon with different
speed in the different crystal directions.
– The etch is extremely slow in the <1-1-1> direction: The etch is therefore
stopped towards the (1-1-1) planes.
• The chip material is (1-0-0) silicon
– Therefore, the etch cavity is surrounded by four (1-1-1) planes which
have an angle of inclination of 54.7 degrees relative to the (1-0-0) surface
plane, rendering a cavity with four sloped walls.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 7
SP80 Package
• Cross-sectioned view of the SP80 Pressure Sensor packaged in
a transistor header.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 8
SP80 Package, continued
• Cross-sectioned view of the SP80 Pressure Sensor packaged in a transistor
header with a top chip containing a vacuum reference chamber.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 9
SP80 Schematic
• The SP80 schematic consists of 4 ion implanted piezoresistors
in a full Wheatstone bridge configuration as the electronic
sensing element. In addition, a temperature measuring resistor
and a heating resistor are implanted on the same chip, to
compensate or thermostat the chip to minimise thermal drifts.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 10
Picture of SP80 in Transistor Package
• Comment: The Norwegian coin is approximately the size of Ø10 mm
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 11
Main Features of SP80
•
•
•
•
Low non-linearity ( < +- 0.1% )
Negligible hysteresis ( < +- 0.005% of full scale output )
Low long term drift ( typical less than 0.1% per year )
Active thermal compensation by utilising the on-the-chip
heating resistor.
• Small size.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 12
Drawbacks of SP80
• Reference pressure medium must be non-conducting and noncorrosive to be compatible with the on-chip sensing elements
and electronics.
• Safe overload is limited to 3 times rated pressure as no
mechanical overload stop is implemented.
• The devices have no normalised output signal. Each device has
to be individually calibrated when system installed.
• Temperature range is limited (-55 - +125 °C) and
uncompensated thermal sensitivity drift is relative high
( = -0.2%/°C).
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 13
•
Applications for Micromachined Sensors
and
Microsystems:
The biomedical market
–
•
The space, defence and avionics markets
–
–
–
•
–
–
Acceleration microsystems for air bag systems
Tire pressure microsystems
The data and peripheral market
–
•
High pressure measurement in oil wells
Sea wave sensor
The automotive market
–
•
Automotive sensors used in tractors, harvesters etc.
The off-shore oil exploitation market
–
•
Accelerometers for rocket navigation
Micro gravity sensor
Gyroscopes for navigation
The agriculture electronics market
–
•
Blood pressure sensors
Disk drive write and read heads
The consumer market
–
–
Photo diodes in cameras
Level measurement in white goods appliances.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 14
Top10 Success Factors
• 1. Batch organised processing technology
• 2. Microelectronics manufacturing infrastructure
• 3. Research results from solid state technology and other
related fields of microelectronics
• 4. Micromachining
• 5. Wafer and chip bonding
• 6. Mechanical material characteristics
• 7. Sensor effects
• 8. Actuator functions
• 9. Integrated electronics
• 10. Combination of features
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 15
Bottom10 Limiting Factors
1.
2.
3.
4.
5.
6.
7.
Slow market acceptance
Low production volumes
Immature industrial infrastructure
Poor reliability
Complex designs and processes
Immature processing technology
Immature packaging and interconnection
technologies
8. Limited research resources
9. Limited human resources
10. High costs
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 16
Milestones in the Planar Silicon Processing
Technology (and some other related breakthroughs):
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
1890: Punched cards invented
1939: Vacuum tubes and mechanical computing
1948: The invention of the transistor
1959: The invention of the planar silicon processing
1959: The invention of the integrated circuit
1964: Mainframe computing
1971: The invention of the microprocessor
1981: introduction of personal computers
1985: 1 Megabit random-access-memory chips available
1991: 64 Megabit random-access-memory chips available
1994: Internet in widespread use
1994: 256 Megabit random-access-memory chips available
1995: Microprocessors with more than 3 million transistors available
2000: Microprocessors with more than 100 million transistors available
2005: 1 Gigabit random-access-memory chips available
2006: Digital consumerisation (Video on mobile phones etc)
2007: The Intel Itanium microprocessor with 1.2 billions transistors.
2008: 4 Gigabit random-access-memory chips available
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 17
Manufacturers of Micromechanical
Devices
• The industry structure is highly diversified both in size,
technological basis and organisation type.
– Traditional sensor manufacturers have seen micromechanical sensors as
a natural expansion of their technological basis, and have taken up
research and production of these sensors as a part of their activity.
– Semiconductor companies have entered this market as an expansion of
their integrated circuit activity, since they already have most of the
needed equipment and the appropriate marketing channels.
– System companies or original equipment manufacturers which see
micromechanical devices as a way to boost their systems.
– "Start ups", companies having micromechanical devices as their main
business idea.
• There are of course companies that does not fit into any of these types and
some are someplace in between these types.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 18
Manufacturers
• USA:
– Honeywell, Microswitch, SenSym, IC Sensors, Motorola, Delco, Foxboro/ICT,
Endevco, Kulite, Lucas/NovaSensor, Michigan Microsensors
• Japan:
– Hitachi, Toshiba, NEC, Yokagawa Hokushin, Toyota Motor Company
• Europe:
–
–
–
–
–
–
–
Germany: Infineon, Bosch,
The Netherlands: Philips, Microtel, Xensor Integration
UK: Druck
Switzerland: Keller, Kistler
Finland: Vaisala
Sweden: Radi Medical Systems
Norway: SensoNor
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 19
Research Centers
• USA
– Stanford University, Case Western Reserve University, University of Michigan,
University of California at Berkeley, University of Wisconsin, MIT
• Japan:
– Tohoku University, Kyoto University, Fudan University,
• Europe:
–
–
–
–
–
–
–
–
The Netherlands: Delft University, Twente University
Belgium: IMEC, Catholic Un of Leuven
Switzerland: University of Neuchâtel, CSEM
Germany: Fraunhofer Institute, IFT Munich, Fraunhofer Institute, IMT
Itzehoe, Techn. Un of Berlin
Denmark: Techn. Un of Denmark
Finland: VTT
Sweden: Uppsala University, KTH/Acreo
Norway: SINTEF
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 20
Batch Processes Adapted from
Microelectronics/IC Technology with no
or Minor Modifications
•
•
•
•
•
•
•
•
•
Photolithography
Spin coating
Etching techniques
Diffusion of dopants
Implantation
Epitaxy
Chemical vapour deposition (CVD)
Thin film technology
Thick film technology
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 21
Batch Processes Modified from
Microelectronics/IC Technology
Processes
•
•
•
•
Double-sided photolithography
Wafer fusion bonding
LIGA and LIGA-like techniques
Laser micromachining
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 22
Batch Processes Adapted or Modified
from Other Technologies than
Microelectronics/IC Technology
• Micro stereo lithography
• Micro electro discharge machining
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 23
Batch Processes Mainly Developed for
Micromachined Devices
•
•
•
•
Bulk micromachining
Surface micromachining
Anodic wafer bonding
Fusion bonding (Direct bonding)
These technologies will be commented on the
following slides
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 24
Bulk Micromachining in Silicon
• Bulk Micromachining in Silicon is here
defined as three-dimensional
micromachining in single crystal silicon by
means of photolithographic etching
techniques.
• It is also called Bulk Micromechanics in
Silicon or Silicon Micromachining
• To understand this technology, some basic
insight in single crystal silicon is needed
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 25
Crystal Structure of Single Crystal
Silicon
• It is a face-centered
cubic structure
(diamond structure)
with two atoms
associated with each
lattice point of the unit
cube. One atom is
located in position
with xyz coordinates
(0, 0, 0), the other in
position (a/4, a/4, a/4),
a being the basic unit
cell length.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 26
Miller Indices for a Plane in a Crystal
2
1
z
x
1
2
y
1
2
3
The orientation of of different crystal planes in the basic unit cell can be
described by the Miller indices (hkl) between parentheses with each plane
defined by a vector description (hx + ky + lz) of the direction
perpendicular to that plane. This is related to a coordinate system oriented
in parallel with the side edges of the basic cell, with the Miller indices
reduced to the smallest possible integers with the same ratio.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 27
Important Crystal Planes in the
Silicon Crystal
• (100), (110) and (111) are the three most
important crystal planes of the silicon
crystal structure.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 28
Silicon as a Mechanical Material
Characteristics
Single Crystal
Silicon
Stainless Spring
Steel
Units
Ultimate Strength
ca. 7,0*109
ca. 2,0*109
N /m 2
Yield Strength
ca. 7,0*109
ca. 1,0*109
N /m 2
Young's Modulus
of Elasticity
ca. 1,7*1011
ca. 1,9*1011
N /m 2
Shear Modulus
ca. 6,2*1010
ca. 8,2*1010
N /m 2
Knoop Hardness
ca. 8,5*109
ca. 6,6*109
N /m 2
D ensity
2,3*103
7,9*103
Kg/m 3
1,6*102
0,33*102
W/m°C
3*10-6
12*10-6
m/m°C
Excellent
Fair
Excellent
Fair
Thermal Conductivity
Thermal Expansion
Chemical Resistance
Corrosion Resistance
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 29
Silicon as an Electronic Material
Property
Value
Unit
Atomic N umber
14
-
Atomic Weight
28,1
-
Atoms per Unit Volume
22
5 x 10
#/cm
Energy Gap ( 300 °K )
1,1
Intrinsic Resistivity
5
2,3 x 10
3
eV
ohm cm
Relative D ielectric Constant
11,7
-
Electron Mobility
1350
2
cm /Vsec
Hole Mobility
480
2
cm /Vsec
Breakdow n Voltage
30
V/µm
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 30
Principles of Micromachining in Silicon
• Micromechanics in silicon is here defined as
three-dimensional micromachining in single
crystal silicon by means of photolithographic
etching techniques.
–This definition covers most techniques used to
make micromechanical sensors, although in some
cases additive structures such as polysilicon and
silicon dioxide also have been micromachined by
selective etching techniques, and in some cases
mechanical drilling or other machining methods
are used.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 31
Wet Chemical Etching of Silicon
using Alkaline Etchants
• The fundamental reactions are electrochemical in nature.
• Holes are injected from the etching solution into the silicon and
Si-atoms are ionized to Si+.
• Hydroxyl (OH-) from the etching solution reacts with Si+ to
hydrated silicon.
• Hydrated silicon reacts with a complexing agent in the etching
solution to form a soluble reaction product.
• The soluble reaction product is dissolved into the etching
solution and carried away from the etching site on the silicon
surface into the solution.
• All in all, silicon is etched and the reactant products are diluted
into the etching solution.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 32
Isotropic Etching of Silicon
• Typical wet isotropic silicon etches are either organic or inorganic
acids such as acetic acid (CH3COOH) or hydrogenfluorid (HF) or
mixtures together with water. Often a complexing agent is needed
transforming the oxidized product into soluble species.
• By using selective etching techniques in combination with etching
time some sort of dimensional control of the etched structure can
be obtained. By using spray etching, agitation or light enhanced
etching preferred etching directions can be obtained.
• Generally, dimensional accuracy below approximately 30 µmeters
are very hard to achieve, making wet isotropic etching a less
favourable and less used method for micromechanics in silicon
compared to anisotropic etching.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 33
Isotropic Etching of Silicon
• This table shows some popular isotropic etches
Etchant
(D iluent)
Typical
Compositions
Temperature
[ °C ]
Etch Rate
[ µm/ min ]
HF
HN O 3
CH 3COOH + w ater
10 ml
30 ml
80 ml
22
0,7 - 3,0
HF
HN O 3
CH 3COOH + w ater
25 ml
50 ml
25 ml
22
40
HF
HN O3
CH 3COOH + w ater
9 ml
75 ml
30 ml
22
7,0
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 34
A Typical Isotropic Etch Cavity
Isotropic etch cavity in a silicon chip with a square masking film
opening. The result is an underetched etch pit with rounded
structures.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 35
Anisotropic Etching of Silicon
• An anisotropic etching solution or orientation -dependent etching solution
will attack the various crystal directions in single crystal silicon with different
speed. Orientation effects during this type of preferential etch have been
attributed to crystallographic properties. One explanation is that the atomic
bonds in some planes are more exposed than in some others. A suitable
designed etching agent will thus attack and strip away certain plane
orientations more quickly than others.
• Typical wet anisotropic silicon etches are organic or inorganic alkaline
solutions used at elevated temperatures, such as a mixture of ethylene
diamine, pyrocatechol and water (EDP-etch) or potassium hydroxide and
water (KOH-etch), or tetra-methyl-ammonium-hydroxide (TMAH)..
Hydrazine-water mixture are also popular anisotropic silicon etchants. In the
following table some examples of anisotropic etchants are given, including
appropriate masking films.
• These typical anisotropic etching solutions are all characterized by an
extremely slow etching speed in the <111> directions of single crystal silicon,
as shown in the example given in the following figure.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 36
Anisotropic Etching of Silicon
Etchant
(Diluent)
Ethylenediamine
Pyrocatechol
Water
Ethylenediamine
Pyrocatechol
Water
KOH
Water &
Isopropyl
KOH
Water &
Isopropyl
TMAH
Water &
Isopropyl
Typical
Temp.
Composition [ °C ]
750 ml
120 gram
100 ml
750 ml
120 gram
240 ml
44 gram
115
115
85
Etch
Rate
µm/min
0,75
1.25
1.4
Anisotropic
Etch Ratio
(100)/(111)
35:1
35:1
1.0
780ml
1.
SiO2 (2Å/min)
Si3N4 (1Å/min)
Metals : Au, Cr, Cu, Ta
400:1
400:1
SiO2 (14Å/min)
Si3N4 (1Å/min)
100:1
SiO2 (??Å/min)or
Si3N4 (0,5 – 2,5 Å/min)
100ml
220 gram
90
SiO2 (2Å/min)
Si3N4 (1Å/min)
Metals : Au, Cr, Cu, Ta
SiO2(14Å/min)
Si3N4 (1Å/min)
100ml
50 gram
50
Masking
Film
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 37
Anisotropic Lateral Etch Rate
• Lateral etch rate as a
function of crystal direction
on (110) silicon wafers for
an EDP-etch.
• The composition of the
etchant was 1l ethylenediamine, 133 ml water, 160
gram pyrocatechol and 6
gram pyrazine.
• The dashed (111) directions
are all equivalent with the
(111) direction in single
crystal silicon.
80 micrometer/hour is around 1.3 micrometer/min
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 38
Anisotropic Etch Cavity in (100) Silicon
Anisotropic etch cavity in (100) silicon with a square masking film opening
oriented in parallel with the <110> direction. Due to the four-fold symmetry of
the slow-etching (111) planes, sideways etching is stopped giving a cavity with
four sloped sidewalls. The photography shows such an etched cavity.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 39
Understanding Anisotropic Underetching
• Anisotropic underetching of mask openings
nonparallel with <110> direction, and
anisotropic underetching of convex corners.
• (a) is a typical pyramidal pit, bounded by the
(111) planes, etched into silicon with an
anisotropic etch through a square hole in an
oxide mask.
• (b) is a type of pit which is expected from
anisotropic etch with a slow convex undercut
rate.
• (c) is the same mask pattern resulting in an
substantial degree of undercutting using an
etchant with a fast undercut rate such as
EDP.
• In (d), further etching of (c) produces a
cantilever beam suspended over the pit.
• (e) is an illustration of the general rule for
anisotropic etch undercutting assuming a
"sufficiently long" etching time. The reader
who understands (e) has understood the main
principles.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 40
Selective Etching of Silicon
• There are four different techniques in use:
– Calculate the needed etching time on the basis of the etching speed of the
used etch. This is an easy, but inaccurate method, as etching speed varies
with the chemical condition of the etch and upon geometrical factors limiting
the agitation of the etch. Typical accuracy: ±20 micrometer.
– Inspect etch cavity depth in appropriate time intervals until needed depth is
reached. More time consuming than the above method, but improved
accuracy. Uneven etching depth from cavity to cavity due to chemical and
geometrical factor is still a problem limiting accuracy, which is typical ±10
micrometer.
– Chemical selective techniques stopping the etch when an impurity doped
chemical resistive layer is reached. Accuracy is typical ±3 micrometer.
– Electrochemical selective techniques stopping the etch towards a biased p-n
junction. This enhances passivation very effectively, giving a typical
accuracy of ±1 micrometer.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 41
Chemical Selective Etching of Silicon
Chemical selective etching with EPD-etch as a function of
boron doping concentration. The boron stop layer can be
made by diffusion deposition or implantation on the opposite
side of the wafer compared to the etch cavity, which are both
well known processing techniques.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 42
Epitaxial Layer Atop Boron Doped Stop Layer
n-type epitaxial layer
p+ type boron stop layer
n-type substrate
• The shortcoming of not being able to integrate electronics in
the boron stop layer can be avoided by depositing an epitaxial
layer atop the stop layer, with doping appropriate as
substrate material for integrated devices.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 43
A Typical Etching Dewar for
Wet Chemical Etching of Silicon
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 44
Electrochemical Selective Etching of
Silicon
• Low-doped material can be
passivated, both p-type and n-type.
This gives more processing
flexibility, and low-doped silicon can
be used as substrate material for
integrated components such as
piezoresistors.
• High accuracy of thickness of
unetched layer can be achieved,
typical ±1micrometer, by using wellcontrolled implantation and
diffusion techniques for making the
p-n- junction.
• This method makes KOH a useful
selective etch, avoiding the health
dangers of EDP-etch.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 45
Surface Micromachining
• Surface micromachining can be defined as a set of
methods to make three-dimensional surface
structures, with deposition of thin films as
additive technique and selective etching of the
deposited thin films as subtractive techniques.
• In practice, single crystal silicon wafer is the
dominant substrate material, and chemical vapor
deposited (CVD) polysilicon is mostly used as the
material making up the three-dimensional surface
structures.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 46
Surface Micromachining, continued
• A main advantage, compared to bulk micromachining, is that it
does not need double sided processing (back side processing) of
the wafers.
• The main additive deposition techniques are evaporation,
sputtering, chemical vapor deposition (CVD), and variants of
these.
• The main subtractive methods are selective wet etching and
dry plasma etching.
• Photolitography is used for pattern definition.
• The use of sacrificial layers is important. With this method,
etching of the sacrificial layers underneath non-etched thin film
structures can be done. In this way several three-dimensional
surface structures can be made, such as cavities, supported
microbeams, microstrings, diaphragms, lateral mobile
microelements etc.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 47
Micrograph of a Surface
Micromachined Structure
• Lateral mobile polysilicon microwheels on a silicon substrate fabricated by
surface micromachining. Each wheels is free to rotate around its axis at the
center of the stud element, which is fixed against the substrate and thus
keeps the wheel in place. The wheels have gear teeth to show a possible gear
function.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 48
Process Sequence
• The process sequence for fabricating laterally mobile elements,
such as the microwheel shown in Photo XIII.2, is schematically
depicted in Figure XIII.12.
– First (a), an oxide film is grown on the silicon wafer.
– Then (b), a polycrystalline film is deposited by chemical vapor
deposition (CVD), and openings are defined and etched out using
standard photolithography (c).
– A second oxide layer is deposited by CVD (d), an opening in the oxide is
etched using a second lithographic mask, and a second polysilicon film is
deposited and patterned with a third mask (e).
– Finally (f), the sacrificial oxide layers are removed by selective etching in
hydrofluoric acid (HF), leaving the first polysilicon film free to move
laterally, and the second polysilicon film as a supporting element fixed to
the substrate.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 49
Process Sequence Diagram
• Figure XIII.12 Process sequence
for the fabrication of laterally
mobile structures using surface
micromachining and sacrificial
layer technique.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 50
Examples of Sensor Elements Using
Surface Micromachining
• Sensor elements can be made by surface micromachining by either using
thin films with sensing effects, such zinc oxide ZnO with piezoelectric field,
or using mechanical sensing properties such as variable air gap elements
and/or vibrating structures.
• An example of such a sensor, the Berkeley Polysilicon Microbridge
Integrated Vapor Sensor. This sensor has a surface micromachined
polysilicon microbridge. This sensor uses the vibrating structure sensing
principle, with vibration activation and vibrating sensing by means of the
capacitance between the bridge and the substrate. (Coulomb force
activation and capacitance change sensing)
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 51
Anodic Wafer Bonding
• Can be defined as a method of electrostatically bonding two dissimilar
materials together to form a strong, hermetic seal that involves little
alteration in the shape, size, and dimensions of the members making up the
joint.
• It is a high yield wafer-to-wafer sealing method that makes it possible to
obtain hermetic seals. The technique was first developed for silicon-to-glass
anodic wafer bonding, and has later been further developed to silicon-tosilicon anodic wafer bonding and silicon-to-thin film anodic wafer bonding.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 52
Anodic Wafer Bonding –Schematic
View
-50 - 200V
T - 400 °C
-100 - 1200V
T - 400 °C
Silicon
Pyre x Borosilic a te Gla ss
Sputtere d Pyre x
Borosilic ate Gla ss
0V
Silicon
0V
Silicon
• Schematic view of silicon-to-silicon anodic bonding and siliconto-glass anodic bonding.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 53
Example: Digital Micromirror Device
(DMD) from Texas Instruments
• The device is using very advanced surface micromachining of
thin Al alloys on Si substrates containing CMOS drive
electronics
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 54
Picture of the packaged DMDs
• The DMDs are pixel devices
• Here are the VGA (640x480), the SVGA (800x600) and the
XGA (1024x768) devices shown
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 55
Principle of Operation for the DMD
• The hinge system of each pixel structure enables electronic
control mirror position.
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 56
Picture of Digital Micromirror Device
• The device is packaged in an elastomer connect package with a
glass window. Here shown mounted on a PCB with back end
drive electronics
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 57
The Davis DPX 16 Projector using the
TI Digital Micromirror Device
• XGA resolution (1024 x 768 pixels)
• 2.3 kg weight
• 1000 Lumens brightness
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 58
The Zeiss Optical Engine for the DP
X16 Projector
• Advanced optics
• Small size and low weight
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 59
The Zeiss Optical Engine for the DP
X16 Projector: Modelling
• Mechanical modelling using ProEngineer Design Tools
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 60
Example: The SA30 Crash Sensor from
SensoNor
• This is a good example of the features of microsystems
– please refer to the separate slide presentation
Electronic Pack….. Chapter 9 Micro Structure Technology and Micromachined Devices
Slide 61