ELEC9703: MST&D W3&4
© 2025 A/Prof. A Michael
1
SURFACE MICROMACHINING
•Surface Micromachining ---- W4
•Bulk Silicon Micromachining --- W3
[Please refer to UNSW Moodle to download the relevant
references on silicon surface micromachining]
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2
What is surface micromachining?
a technique for fabricating 3 dimensional micromechanical structures
from multi-layers of deposited/grown thin film materials that can be
patterned.
basic requirements:
Substrate
sacrificial layer that can be removed by etching
structural layer from which the micromechanical structure is to be
patterned
what is the processing sequence?
Issues:
choice of material for structural/sacrificial layers
silicon based or non-silicon based materials
mechanical properties of the thin films
influence of thin film processing on the mechanical properties
characterisation of mechanical properties
Applications?
SURFACE MICROMACHINING
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Basic processing sequences
substrate
substrate
substrate
Sacrificial layerAnchor cut
Structural layer patterned
Sacrificial layer removedReleased
Cantilever Released bridge
•Deposit isolation layer on substrate
•Deposit sacrificial layer
•Pattern anchor cut
•Deposit structural layer
•Pattern structural layer
•Etch sacrificial layer
•Cantilever & bridge released
• Rinsing and drying procedures
Isolation layer
SURFACE MICROMACHINING
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4
Bulk micromachining versus Surface micromachining
Smaller structures
Better dimensional control
SURFACE MICROMACHINING
[4]
SCS stress free
• Isotropic etching of sacrificial layer
•But anisotropic etching of the structural
layer
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Choice of materials
Sacrificial layer composition depends on choice of structural layer
Structural layer must not be affected by etchant for sacrificial layer ( or at
least have an etch rate for the structural layer several orders lower that
the sacrificial layer)
Most common choice for silicon surface micromachining:
Structural material: polysilicon
Sacrificial layer: PSG/ SiO2
SURFACE MICROMACHINING
[4]
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6
Removal of Sacrificial Layer
Usually done by wet isotropic etch (dry etching techniques also used)
High selectivity
Model for sacrificial layer etching process
Example - HF removal of PSG/SiO2
(1) Mass transfer of reactant by diffusion from the
bulk to the external etch opening
(2) Diffusion of the reactant from the etch
opening through the etch channels to vicinity of
the internal catalytic surface
(3) Adsorption of the reactant onto the catalyst
(4) Reaction on the surface of the catalyst
(5) Desorption of the products from the surface
SURFACE MICROMACHINING
[6]
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SURFACE MICROMACHINING
Removal of Sacrificial Layer (cont.)
(6) Diffusion of the products from the interior of the etch channels
(7) Subsequent mass transfer of the products from the etch opening to
the bulk fluid
Overall reaction: SiO2 + 6HF → H2SiF6 + 2H2O
•Two elementary steps involved:
• Protons break-up the siloxane bonds (Si-O-Si) to from silanol
species (Si-OH) at the surface.
• Attachment of F ions on the Si in the silanol. Leading to
SiF4 formation.
• Dissolve in aqueous solution as H2SiF6
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SURFACE MICROMACHINING
Removal of Sacrificial Layer (cont.) polySi
oxideC(x,t)Co
bulk
conc.
x
(t)
position of etch front
0
J
dt
td
HF

−=
)(


2
2
6
1
SiO
SiOm
=
0
2
2
21
=
+=−=
C
CkCkCDJ HF

+

=

 


)(
)(
2
21 CkCk
t
•molecular weight
•density
•Fick’s 1st Law + empirical rate law
•Fick’s 2nd Law, neglecting
convective component
and instantaneous rate change
of concentration.
•Solve for (t) and t()
[WP Eaton, et.al, Transducer ’97, p249]
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Calculated underetch length of SiO2 as a function of time for HF
solutions of different concentrations and for structures of varying
dimensions ( based on a combined first and second order reaction model
etc.)
Diffusion limitations observed at about 200µm etch lengths.
Etch rate of PSG is higher and increases with phosphorus content.
SURFACE MICROMACHINING
[6]
Removal of Sacrificial Layer (cont.)
HF conc.
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Problem of Stiction
Problem of thin film structure coming into permanent contact with the substrate after
removal of the sacrificial layer rendering the surface micromachined structure unusable
Attempts to physically detach the structure from the substrate will be destructive.
Mechanisms of stiction:
• forces that pull the structure down to the surface is probably due to surface
tension between the liquid in which the wafer is rinsed.
liquid
substrate
Released structure Opposing forces
Surface Tension Forces
• as liquid evaporates, structure and substrate are bounded by meniscus - cause of the
attractive force between structure and substrate
• structure collapses and comes in permanent contact with substrate :
held together by either van der Waals forces, hydrogen bridging etc.
• Hydrophobic surfaces -- van der Waals forces responsible for stiction
• Hydrophilic surfaces -- hydrogen bridging responsible for stiction
SURFACE MICROMACHINING
•Capillary action in wet release
•Utotal=Ubending+Ustretching+Usurfacetension
Van der Waals forces: attractive and repulsive electrostatic dipole
interaction between molecules. Hydrogen bridging: attraction
between a H atom of one molecule and a pair of unshared electrons
of another molecule.
H
H O: H O:
H
Two situations: in-use or during release
During release
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How to minimise stiction after wet release?
1. Critical Point Drying
◼ under suitable conditions, the liquid and
vapour phases cease to exist as distinct states
in the supercritical region
◼ CO2, supercritical region exist above 31.1°C
and at 1073 psi
 Structure is on LTO oxide etched in HF
 DI (deionised) water rinse without
drying
 Water exchanged by methanol, then
transfer to pressure vessel and methanol
exchanged for CO2 at 25ºC and 1200psi.
 Heat up to 35°C and CO2 vented -
structures released
 L → SF → G
SOLID
GAS
Supercritical
region
Temperature
P
re
ss
u
re
SURFACE MICROMACHINING
Which methods are useful for in-use stiction free conditions?
❑ Freeze drying - L → S → G
❑ Direct
2025 A/Prof A Michael
evaporation L → G : serious stiction problem
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SURFACE MICROMACHINING
• Hydrophilic:
A surface that invites wetting by water
Get stiction
Occurs when the contact angle θwater < 90o
•Hydrophobic:
A surface that repels wetting by water
Avoids stiction
Occurs when the contact angle θ water > 90o
Prof Clark Nguyen
http://www-
inst.eecs.berkeley.edu/~ee245/fa11/modules/LecM5.SurfaceMicromachining.
ee245.f11-1.pdf
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SURFACE MICROMACHINING
How to minimise stiction after wet release? (cont.)
2. Other phases-change release methods: ‘freeze-drying’ - eg.
▪ replace water by tertiary butyl alcohol and frozen
▪ sublimated at low vacuum
▪ others use multi-solvent drying process - methanol, ethyl ether and Flourinet
(low surface tension perflourinated hydrocarbon liquid)
▪ Rinse-freeze-sublimation procedure:
(i) Rinse in DI water after HF(hydroflouric acid) etch to remove
etchant (maintain wet)
(ii) Add IPA(iso-propyl alcohol) to keep maintain wafer surface
hydrophobic
(iiii)Place wafer in beaker of IPA
(iv) Final rinse in cyclohexane
(v) Place on Peltier element already cooled to -10°C , passing N2
helps in the sublimation process as cyclohexane vapor is
removed.
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How to minimise stiction after wet release? (Cont.)
3. Reduction of contact area - use stand-off bumps or increase surface
roughness
4. Geometry modification : ‘anti-stiction structure
SURFACE MICROMACHINING
Sacrifical oxide
•cantilever structure •bumps
substrate
short long cantilever
modifier long cantilever
‘anti-stiction
structure
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5. Surface modifiers: for release and in-use stiction
SURFACE MICROMACHINING
How to minimise stiction after wet release? (Cont.)
• good for stiction free release & in –use stiction free
•Chemical modification of surface - reduce adhesive energy of in-use
stiction by 4 orders of magnitude compare to SiO2 coated surfaces
•Use self-assembled monolayer (SAM) from the precursor molecule of
DDS, OTS (Octadecyltrichlorosilane), FDTS (Perflourodecyltrichlorosilane)
•Good temperature independence
•Deposits onto hydrophilic polysilicon
DS: dicholorosilane TS: trichlorosilane
DDS: R2SiCl2 -- dialkyl-dichlorosilane
MTS: RSiCl3 - monoalkyl-dichlorosilane
DDMS: (CH3)2SiCl2
OTS: C18H37SiCl3 - octadecyltrichlorosilane
MTS
FDTS: C10H4F17SiCl3 - 1H,1H,2H,-2H-perfluorodecyltrichlorosilane
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SURFACE MICROMACHINING
How to minimise stiction after wet release? (Cont.)
5. Surface modifiers: for release and in-use stiction (cont.)
Process flow for polySi release
sacrificial etch
in 49% HF
D I water
rinse
surface
oxidation
H2O2 dip
Dip1: 2m
Dip2: 8min
IPA rinse
Dip: 1 min
Iso-octane
Dip: 1min
DDS coating
Dip: 15 min.
Iso-octane –
Remove pre-cursor
Dip: 2 min.
Dry
1 min.
[ref: BK Kim, et.al.,Proc. MEMS’99, p189]]
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• Can reduce stiction by tailoring
surfaces so that they induce a water
contact angle > 90°
• Self-Assembled Monolayers (SAM’s):
• Monolayers of “stringy” molecules
covalently
bonded to the surface that then raise
the contact angle
• Beneficial characteristics:
Conformal, ultrathin
Low surface energy
Covalent bonding makes them wear
resistant
Thermally stable (to a point)
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Polysilicon as a structural layer/member
Key issues:
▪ Deposition methods and conditions (temp., pressure, power)
▪ Post-deposition treatment (annealing)
▪ Film stress in relation to deposition conditions and post-deposition
treatment
▪ Test structures & characterisation techniques (mechanical)
Elwenp153 risticp106
Examples of film stress:
SURFACE MICROMACHINING
[6]
[4]
ELEC9703: MST&D W3&4
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• Under tensile stress, a film
wants to shrink with respect
to its substrate.
Caused, e.g., by differences
in film vs. substrate thermal
expansion coefficients
If suspended above a
substrate and anchored to it
at two points, the film will
be “stretched” by the
substrate.
•Under compressive stress, a
film wants to expand with respect
to its substrate.
If suspended above a
substrate and anchored to it
at two points, the film will
buckle over the substrate
Tensile and compressive stress
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Polysilicon as a structural layer/member
APCVD (Atmospheric Pressure CVD)
PECVD (Plasma Enhanced CVD)
LPCVD (Low Pressure CVD)
- most commonly used
- pyrolysis of SiH4
Typical conditions of deposition:
• 530°C - 850°C ( 950°C -1000° for ‘epi-poly’)
• Total pressure 10-3 –10-2 Torr
• Process parameters: temperature, SiH4 partial pressure
• Amorphous, pseudo amorphous (partially polycrystalline),
polycrystalline
• Transition temp.: 575°C -600°C
Below 575°C - amorphous
Above 600°C - polycrystalline
• Temp. spread due to incubation time for
nucleation / crystallisation rate versus total deposition
time
SURFACE MICROMACHINING
Deposition Methods and conditions
[4]
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Importance of transition regime to
micromachined structures: affects film
morphology, has strong bearing on ultimate
average residual stress and stress gradient
through thickness of the polySi film.
eg. Constrained structures like bridges and
diaphragms - tensile stress films are desirable
to avoid buckling but excessive tensile stress can
fracture the beam.
As-deposited polySi films do not usually give the
desired stress requirements – need to conduct
post-deposition thermal treatment on as
deposited amorphous films/polycrystalline film
Grain size a strong function of deposition
condition (temp) and annealing temp. - large
grain size obtained from annealing of films
deposited at low temp. and high deposition rates.
SURFACE MICROMACHINING
Polysilicon as a structural layer/member
Deposition Methods and conditions (cont.)
[4]
Deposit above 600°C, grains have columnar
structure.
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risticp107
• As deposited film stress:
➢ average residual stress peaks at around
630°C and rapidly drops off at higher deposition
temperature
➢ Stress gradient over the film thickness results in
a bending moment -- deflection of cantilever
beam - type of structures
➢ Stress gradient can also be reduced at higher
deposition temperatures.
➢Average residual stress and stress gradient appear to be a function of the
dominant orientation in fully polycrystalline films, with the <110> orientation
the highest in magnitude and randomly oriented films, the lowest.
➢ At 700°C (0.1Torr –1 Torr) film texture is mainly <100> and stress is
considerably reduced.
SURFACE MICROMACHINING
Polysilicon as a structural layer/member
[4]
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• Post-deposition annealing
➢Annealing of as-deposited polycrystalline films below 1100°C
show little change in microstructure but above 1100°C re-
crystallisation and grain growth occurs.
➢Annealing (650°C-950C) of as-deposited amorphous/psuedo-
amorphous (initially compressive) to become tensile -- could be
due to contraction of volume due to crystallisation of the
amorphous Si layer on the top surface.
➢Annealing at high temp. induces re-crystallisation which allows
intrinsic stresses in the Si film to relax.
SURFACE MICROMACHINING
Polysilicon as a structural layer/member
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SURFACE MICROMACHINING
Polysilicon as a structural layer/member
• Post-deposition annealing
• Annealing above
1000°C - almost
complete relaxation
of internal stress.
[4]
[4]
• comparing doped and undoped film
annealing behaviour
• curvature of doped and undoped polySi
cantilever as a function of annealing temp.
• residual stress gradient tends to be
significantly more sensitive to phos. predeposition
than ave. residual stress.
merge
Indicates increase in
compressive stress at the polySi
surface.
Deposited at 580°C
• compressive as deposited
•Low temp. anneal will
reduce strain over a long
period.
• mod. Temp anneal will
cause film to become
tensile
[6]
• high temp.
reduces stress
quickly to zero.
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• Fracture Strain
➢ How does the mechanical properties of polySi compare with SCS?
➢ Fracture strain of polySi - unannealed : 1.72 +/- 0.09%
- annealed at 1000°C (1Hr) polySi: 0.93 +/- 0.04%
➢Young’s modulus: nearly similar to SCS and not affect by annealing
➢ Fracture stress: unannealed: 3.2 x 109 N/m2
annealed: 1.8 x 109 N/m2
SCS : 7 x 109 N/m2
➢ Conclusion: unannealed polySi is stronger, BUT not as strong as SCS
SURFACE MICROMACHINING
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Test structures and characterisation
•Surface micromachined structures tend to have large residual stress fields
•Very sensitive to deposition conditions and post-deposition treatment
•Residual stress affect load response behaviour, frequency response etc.
•Need test structures:
In-situ characterisation
• Distribute across wafer
• Fabricate in parallel with
devices.
• Process monitor
External load characterisation
Two kinds of stress deform released micromachined structures:
(1) Ave. compressive/tensile axial stress - important for bridges/diaphragms
(2) Vertical stress gradients (across the film thickness)
Specially designed test structures to monitor the forms of stress present
SURFACE MICROMACHINING
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Test structures and characterisation (cont.)
1. Cantilever length changes: l
ll
0
0
−
=
• measurement technique is problematic
• change in length extremely difficult to
measure over a long length –not practical
2. Bridges:
• fixed at both ends, under sufficient compressive axial stress, the structure will buckle
• Euler buckling strain for a thin beam under an axial load (Critical)
KL
t
cr 2
22
 =
• where t is film thickness, L is beam length and K is a constant (values ranging from
3-12, depending on the shape and type of beam support)
• need to fabricate a array of bridges of varying length and observe the point
at which the critical strain is exceeded. Observe using optical intereference
microscope.
SURFACE MICROMACHINING
[Guckel: J Applied Phys., vol.57,
No.5,1985, p1671]
[4] [4]
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Test structures and characterisation (cont.)
3. Ring structure:
• for characterising tensile stress
• relaxation of of the internal stress in the film results in application of tensile stress at the
two opposing anchor points
• ring contracts radially at the point where the ring joins the internal crossbar, applying a
compressive stress to the crossbar that is related to the average residual strain it film
critical = G film
• critical value of residual strain need to buckle crossbar:














=
GR
t
cr
cr
film
1
12
2
22

• G is specific to geometry, Rcr is the critical radius of ring
• Need array of rings to determine upper and lower bound of residual tensile strain.
G: Geometry dependant
ratio
SURFACE MICROMACHINING
[6] [4]
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Test structures and characterisation (cont.)
4. Wafer curvature:
• simple, requires no patterning or etching
• Measure radius of curvature of wafer covered on one side by thin film
• Use profilometer
•Stoney’s formula:
( ) Rt
Et
f
s
61
2
0  −
=
Young’s Mod.
Substrate thickness
Film thickness
Radius of curvature of substrate
Poisson’s ratio
SURFACE MICROMACHINING
• Poisson’s ratio, ν: defined as
the ratio of transverse strain to
axial strain under condition of
uniform and uniaxial
longitudinal stress in the elastic
region.
F F
e’
e
L
d
 =
2e
L
’ =
d
2e’
= 
’>0 <0
>0
original
final
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Test structures and characterisation (cont.)
5. Cantilever curvature:
• residual strain gradient introduces eccentricity to axial
loading – due to variation of stress along the direction of film
growth.
• in plane stress varies across the thickness of the film, causing
a effective bending moment ( )ywdyyM
t
t
x=
−
2/
2/

• Bending moment causes film to curl when released
eg. Released cantilever structure
• Assuming residual stress gradient does not change
along the length of the cantilever, the vertical deflection
(x) at any point along the length
Mx
EI
K
x
x
2
)1()( 2 −
+=
risticp132K = Constant determined by boundary condition
E/(1-v2) = biaxial modulus of the film (compensating Young’s modulus
for stiffening of the beam due to stretching
I = moment of inertia of cantilever about the z-axis
M = internal bonding moment
Slope gives M(1-v2)/EI
- process sensitive
SURFACE MICROMACHINING
-t/2
t/2
x
{see ref. [10]}
dAz
2
[4]
[4]
y
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Effects of stress gradient
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SURFACE MICROMACHINING
Test structures and characterisation (cont.)
6. Archemedian spiral structure: residual stress gradient measurement
• spiral structure tends to expand or contract when released
• end point rotation, endpoint height and lateral
contraction - related to residual strain gradient (tedious
to measure)
• only single data point, but occupies a
large area for good sensitivity
Anchor
outside
Anchor
inside
[4]
•+ve stress gradient (increasingly tensile
towards film surface) – open bowl shape
for center anchored spiral
•-ve stress gradient (increasingly
compressive towards film surface
– dome shape for edge anchored spiral
[6]
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Test structures and characterisation (cont.) [10]
7. Response characteristics to external loading on structure:
risticp134
risticp135
(i) Deflection under load
•Mechanically deflect beam while
continuously monitoring the applied load
Young’s modulus
Yield strength
Poisson’s ratio
(x) = 4 P x3(1 –2)
E w t3 Film thickness
Film width
SURFACE MICROMACHINING
in the elastic region
•load
controlled
sub-
micrometer
[4]
[4]
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Test structures and characterisation (cont.)
(ii) Resonant frequency measurements
risticp136
ristic137
risticp138








−=
Q
ff
rcr
4
1
1
2
2/1














=
LA
IE
f
r
 42
52.32
Dynamic properties used to evaluate mechanical
properties
Av
voltage
→ freq.
sweep
Amplitude
of
vibration
measured
optically –
ampl. vs
freq
Resonant freq.
vibrational amplitude goes through max. at fcr
critical freq. - underdamped cantilever
reflects the energy loss due to damping
→ Q extracted from the width of resonant
freq.
•Function of geometry and mechanical property – Plot ƒr vs 1/L
2
[Ref: K.Petersen;
IEEE ED – 25 ,
1978, p1241;
JAP, vol.50,1979,
P6761]
SURFACE MICROMACHINING
[4]
[4]
fr = mechanical resonant frequency
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Test structures and characterisation (cont.)
risticp139 risticp140






+=−
a
t
Bfd
a
t
APP 4)(0201 
(iii) Membrane deflection under pressure
risticp139
measure height of diaphragm
bulge/ deflection of membrane
Applied pressure
Differential
pressure
Residual
stress
2a = width of diaphragm Function of Poisson’s ratio
A, B, constants
determined by
geometry
Centre displacement
E d3
1 – v2
SURFACE MICROMACHINING
thickness
Biaxial modulus
[4][4]
• diaphragm is bowed out due to differential pressure
• optical measurement of diaphragm centre displacement d
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Applications of surface micromachining: examples
Electrostatic comb drive
SURFACE MICROMACHINING
[4]
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Applications of surface micromachining: examples (cont.)
Electrostatic comb drive
elwenp161a
elwenp161b
elwenp163a
SURFACE MICROMACHINING
[6]
[6]
[6]
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elwenp163b
elwenp165a
SURFACE MICROMACHINING
Applications of surface micromachining: examples(cont.)
Link mechanism of micromanipulator [6]
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Microgripper
elwenp165b
elwenp166 elwenp167
SURFACE MICROMACHINING
Applications of surface micromachining: examples
[6]
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Variable Capacitance motor
elwenp168a
elwenp168b
elwenp169
SURFACE MICROMACHINING
Applications of surface micromachining: examples
[6]
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© 2025 A/Prof. A Michael
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Surface micromachined accelerometer: vertical movement
SURFACE MICROMACHINING
Applications of surface micromachining: examples
[4]
ELEC9703: MST&D W3&4
© 2025 A/Prof. A Michael
42
Surface micromachined accelerometer ADXL: lateral movement
W1:sentp5
W1:sentp514a
W1:sentp514b
W1:sentp514c
SURFACE MICROMACHINING
Applications of surface micromachining: examples
[1]

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