485lecture5.pdf

7/27/2019 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

ECE/ME 485

Lecture 5

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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

ECE/ME 485

Lecture 5

3

Parallel plate configuration

Interdigitated finger configuration


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Examples Parallel Plate Capacitor

ECE/ME 485

Lecture 5

4


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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

ECE/ME 485

Lecture 5

5

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

ECE/ME 485

Lecture 5

6

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

ECE/ME 485

Lecture 5

7

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

ECE/ME 485

Lecture 5

8

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

ECE/ME 485

Lecture 5

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Sealed Cavity Pressure Sensor

ECE/ME 485

Lecture 5

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Differential Capacitance Flow Sensor

ECE/ME 485

Lecture 5

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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

ECE/ME 485

Lecture 5

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Deformable Mirrors for Adaptive Optics

ECE/ME 485

Lecture 5

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ECE/ME 485

Lecture 5

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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)

ECE/ME 485

Lecture 5

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Adaptive Optics

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Lecture 5

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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

ECE/ME 485

Lecture 5

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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

ECE/ME 485

Lecture 519


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3 Pixel Image

on Screen

Light Source Projection

Lens

Light

Absorber

3 DMD

ECE/ME 485

Lecture 520

Micromirrors(Actual Top

View)


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3 Pixel Image

on Screen


Lens

Light

Absorber

3 DMD

ECE/ME 485

Lecture 521


View)


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3 Pixel Image

on Screen


Lens

Light

Absorber

3 DMD

ECE/ME 485

Lecture 522


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)

ECE/ME 485

Lecture 523


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Sensors and Actuators based

MOVING SIDE

ECE/ME 485

Lecture 524


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ECE/ME 485

Lecture 525


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ECE/ME 485Lecture 5

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Interdigitated Comb Capacitance

ECE/ME 485Lecture 5

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Electric Field Profile

ECE/ME 485Lecture 5

28

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

ECE/ME 485Lecture 5

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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

ECE/ME 485Lecture 5

30

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

ECE/ME 485Lecture 5

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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

ECE/ME 485Lecture 5

32

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

ECE/ME 485Lecture 5

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Actuators that Use Fringe Electric Field -

Three phase electrostatic actuator. Arrows indicate electric field and electrostatic force.

ECE/ME 485Lecture 5

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.


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Three Phase Motor Operation Principle

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Starting Position -> Apply voltage to

ECE/ME 485Lecture 5

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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

ECE/ME 485Lecture 5

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M t t th li d t A A l


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Motor tooth aligned to A -> Apply

ECE/ME 485Lecture 5

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Some variations

Large out of plane rotation

Low voltage

near movemen

ECE/ME 485Lecture 5

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Actuators thatUse Fringe Field Micro Mirrors


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Actuators that Use Fringe Field - Micro Mirrors

ECE/ME 485Lecture 5

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

ECE/ME 485Lecture 5

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Electrostatic Driven Leverage Actuator

ECE/ME 485Lecture 5

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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%.

ECE/ME 485

Lecture 5

47

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

ECE/ME 485

Lecture 5

48

, ,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

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