485lecture5.pdf
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Electrostatically Driven &Sensed MEMS Devices
How theory from last lecture is
applied in practice
ECE/ME 485
Lecture 5
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Outline B i it r nfi r ti n
Parallel plates
Applications examples
Actuators
ow to counteract t e snap own e ect
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Basic Principles Sensin
capacitance between moving and fixed plates change as
distance and position is changed
media is replaced
Actuation electrostatic force (attraction) between moving and fixed plates as
a voltage is applied between them
Two major configurations
parallel plate capacitor (out of plane)
interdi itated fin ers - IDT in lane
dA
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Parallel plate configuration
Interdigitated finger configuration
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Examples Parallel Plate Capacitor
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Parallel Plate Capacitor
dAV
E
d
Frin e electric field
V
QC
AQE /
(ignored in first order
analysis)
dd
A
Q
QC
Equations without considering fringe electric field.
A note on fringe electric field: The fringe field is frequently
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Lecture 5
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gnore n rs -or er ana ys s. s none e ess mpor an .Its effect can be captured accurately in finite elementsimulation tools.
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Fabrication Methods Surface micromachining
Wafer bonding
3D assembly
Flip and
Movable
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Will discuss more details on fabrication later
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Forces of Capacitor Actuators
Stored energy W 12
CV2 1
2
Q2
C
Force is derivative of energywith respect to pertinent F
W1 C
V2
Plug in the expression for Acapacto
d
for force
W AV2 1
1 A 2 1 CV2
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d 2d d
2 d2
2 d
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Relative Merits of Capacitor Actuators
Pros
Nearly universal
Cons
Force and distanceinversel scaled - to
no need for specialmaterials.
obtain larger force, thedistance must be small.
.driven by voltage, not
current.
,vulnerable to particles
as the spacing is small - High speed. Use
charging anddischarging, therefore
.
Vulnerable to stickingphenomenon due to
realizing full mechanicalresponse speed.
mo ecuar orces. Occasionally, sacrificial
release. Efficient and
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clean removal of
sacrificial materials.
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Capacitive Accelerometer
Proof mass area 1x0.6 mm2,Device Operation
Micromachined
Fabrication Method
.
Net capacitance 150fF External IC signal
J .C. Cole, A new sense
accelerometer subsystems,Transducers91, pp. 93-06,1991
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Sealed Cavity Pressure Sensor
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Differential Capacitance Flow Sensor
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Capacitive Tactile Sensors ens v y . p gram o
normal force, 0.32 pF/gramto shear force.
pa a reso u on . mm
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Deformable Mirrors for Adaptive Optics
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Deformable Mirrors for Adaptive Optics
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Deformable Mirrors for Adaptive Optics
2 m surface normal stroke
for a 300 m square mirror, the displacement is 1.5
T. Bifano, R. Mali, Boston University(http://www.bu.edu/mfg/faculty/homepages/bifano.html)
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Adaptive Optics
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Optical Micro Switches
Texas InstrumentDLP
capacitor support Two stable positions
-respect to rest)
All aluminum structure o process s eps
above 300-350 oC toavoid damage to
circuits
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Digital Light Mirror Pixels
center-to-center spacing
Ga s are 1.0 m nominal Mirror transit time is
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Digital Micromirror Device (DMD)
Mirror
-10 degMirror
+10 deg
Hinge
Yoke
Substrate
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Lecture 519
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3 Pixel Image
on Screen
Light Source Projection
Lens
Light
Absorber
3 DMD
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Lecture 520
Micromirrors(Actual Top
View)
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3 Pixel Image
on Screen
Light Source Projection
Lens
Light
Absorber
3 DMD
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Lecture 521
Micromirrors(Actual Top
View)
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3 Pixel Image
on Screen
Light Source Projection
Lens
Light
Absorber
3 DMD
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Lecture 522
Micromirrors(Actual Top
View)
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DMD Pixel Exploded ViewDMD Pixel Exploded View
Landing Tip Torsion HingeMirror Layer
Mirror AddressElectrode
Yoke
Yoke and
Yoke Address
ElectrodeVia 2 Contact
to CMOS
Hinge Layer
Bias/Reset Bus Landing Site
Metal-3
Layer
Memory Cell
(CMOS SRAM)
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Lecture 523
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Sensors and Actuators based
MOVING SIDE
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Interdigitated Comb Capacitance
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Electric Field Profile
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b i i
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Transverse Comb Drive Devices
rec on o nger movemen s or ogona o e rec on o ngers. Pros: Frequently used for sensing for the sensitivity and ease of
fabrication
.
0 CltNC
)( 0
0
fsr Clt
NC
xx
0
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Longitudinal Comb Drive Actuators
Total capacitance isproportional to the overlaplength and depth of the
,
proportional to the distance.d
Frequently used inactuators for its relatively
long achievable drivings ance.
Cons
force output is a function of
Ctot N[2
0tx
0
d Cf ]
.
thicker the fingers, thelarger force it will be.
Relatively large footprint.
N = number of fingers (4 in above diagram)
t= comb finger depth
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Fx 0
x
1
2
CTOTV2
NOTE CORRECTION!
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Devices Based on Transverse Comb Drive
Analog Devices ADXL accelerometer A movable mass supported by cantilever beams
move in response to acceleration in one specificdirection.
Relevant to device performance sidewall vertical profile
of -axis movement com ensation
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temperature sensitivity
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Sandia electrostatically driven gears- translating linear motion into continuous rotary motion
Longitudinal comb drive banks
ec an ca
springs
Optical shutter
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http://www.mdl.sandia.gov/micromachine/images11.html
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Where linear motion turns into rotary motion
driving Disengaging
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Sandia Gears Use five layer polysilicon toincrease the thickness t in
actuators.
Mechanical springs
Positionlimiter
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More Sophisticated Micro Gears
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Actuators that Use Fringe Electric Field -
Three phase electrostatic actuator. Arrows indicate electric field and electrostatic force.
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.
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Three Phase Motor Operation Principle
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Starting Position -> Apply voltage to
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Motor tooth aligned to A -> Apply
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M t t th li d t B A l
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Motor tooth aligned to B -> Apply
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M t t th li d t A A l
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Motor tooth aligned to A -> Apply
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Some variations
Large out of plane rotation
Low voltage
near movemen
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Actuators thatUse Fringe Field Micro Mirrors
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Actuators that Use Fringe Field - Micro Mirrors
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44R. Conant, A flat high freq scanning micromirror, IEEE Sen &Act
Workshop, Hilton Head Island, 2000.
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Curled Hin e Comb Drives
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Electrostatic Driven Leverage Actuator
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u e a ., a ona s ng ua nvers y, rans ucers ,
Implications of Pull in Effect
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Implications of Pull-in Effect
Vsnap 8kgo
3
27Agsnap
2
3g0
For electrostatic actuator, it is impossible to controlthe displacement through the full gap. Only 1/3 of
.
Electrostatic micro mirrors
reduced range of reliable position tuning
Electrostatic tunable capacitor
reduced range of tuning and reduced tuning range
unng s ance ess an , unng capac ance ess an
50%.
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C t ti P ll I Eff t
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Counteracting Pull-In Effect
everage en ng or u ap os onng
E. Hung, S. Senturia, Leveraged bending for full gap positioning with
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, ,Island, p. 83, 2000.
Counteracting Pull-in Effect: Variable Gap Capacitor
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Counteracting Pull in Effect: Variable Gap Capacitor
x s ng una e apac or
Suspension
spring
Counter
capacitor plate
Tuning range: 88%(with parasitic capacitance)
0
apac tor
plate
ctuat on
electrode
ctuat on
electrode NEW DESIGN
Sus ension
Variable Gap Variable Capacitor
spring
capacitor
plated0