Introduction to Microeletromechanical Systems (MEMS)tjs/4700/lec19/memslecture7.pdf ·...

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Texas Christian University Department of Engineering Ed Kolesar

Introduction toMicroeletromechanical Systems

(MEMS)Lecture 7 Topics

• Thermal TransducersThermal SensorsThermal Actuators

• Thermoresistive Effect• Shape Memory Alloy (SMA)• Thermal Actuator Devices and

Applications


MEMS Overview

Micromachining: lithography, deposition, etching, …

Processes & Foundries

Devices & Structures

Methodology

History & Market

Introduction &

Background

22


Thermal Transducers

• Thermal sensor: heat causes measurable (electric) effect

Change in resistivityThermocouple …

• Thermal actuator: heat causes motionThermal expansion…

Advantages: small size, high force/volume, … Disadvantages: high power, …

thermalsensor

Vout

thermalactuator

Vin


1kg+1K

Thermal TransducersHeat capacity, c : ability to hold thermal energy; energy needed to

change unit mass by unit temperature. Usually measured in J/kg·K. Analogous to electrical capacitor.

Thermal conductivity, κ : ability to transfer heat, usually measured in W/m·K. Analogous to electrical conductivity.

Coefficient of thermal expansion (CTE), α : relative change in length per unit temperature change, usually measured in 1/K.

∆l = l0 α ∆T

Note: thermal strain εT= α ∆Tthermal stress σT = E εT= E α ∆T

1W +1K

1m

+1K

l0 ∆l

1J

33


Thermal Transducers

c (J/kg·K) κ (W/cm·K) α (10-6/K)

Si 712 1.49 2.6Polysilicon 920 (?) 0.34 2.33SiO2 745 0.0138 0.35Al 903 2.37 25.0Au 129 3.18 14.2GaAs 325 0.56 5.4Polyimide (Dupont PI2611D) 3.00Polyimide (Amoco Ultradel 1414) 191

c (J/kg·K) κ (W/cm·K) α (10-6/K)

Si 712 1.49 2.6Polysilicon 920 (?) 0.34 2.33SiO2 745 0.0138 0.35Al 903 2.37 25.0Au 129 3.18 14.2GaAs 325 0.56 5.4Polyimide (Dupont PI2611D) 3.00Polyimide (Amoco Ultradel 1414) 191


Thermal Transducers

Note: Different coefficients of thermal expansion cause stress in bonded layers and thin films.

[After Madou 1997]

T/K

W

200 400 600 800

α/10-6K8

6

4

2 Si

SiO2

Pyrex

Ni Co Fe Alloy

44


Thermal Transducers

Heat flux, q : thermal energy flowing across unit area per unit time (“heat current”)

m) (inflow heat of direction in distance K)(in etemperatur

)m (inarea sectional-crossW/mK) (inty conductivi thermal

J) (in ferredheat trans ofquantity

in measured :common more

W in measured :law sFourier'

2

2

===

==

∇−=

−==

xTA

QmWTq

dxdTA

dtdQq

κ

κ

κ

r


Thermal Transducers

Temporal diffusion of thermal energy:

Thermal diffusivity: ratio of thermal conductivity to “normalized” thermal capacity

/s)m(in y diffusivit thermal

K)(in etemperaturK)J/kg(in capacity heat )kg/m(in density mass

K)(in W/mty conductivi thermal

:Equation Diffusion

2

3

2

=

=⋅=

=

⋅=

∇=

c

Tc

Tcdt

dT

ρκ

ρ

κρκ

55


Thermal Pressure Sensor

Thermal conductivity changes with temperature

Want to measure pressure in volume with dimensions < λ:gas molecules collide mostly with heater/sensorlinear relationship between conductivity κ and pressure P

[Kovacs 1998, p. 596]

K)(J/kg gas ofheat specific(m/s) molecules gas ofity mean veloc

)(1/m eunit volumper molecules ofnumber (m)path freemean

(kg) massmolecular average

3

31

⋅===

==

=

V

V

cvn

mcvnm

λ

λκ


Resistivity TCR

C (graphite) 1390 -500

Al 2.83 3600

Au 2.4 8300

(µΩ·cm) (10-6/K)

Resistivity TCR

C (graphite) 1390 -500

Al 2.83 3600

Au 2.4 8300

(µΩ·cm) (10-6/K)

Thermoresistive EffectResistivity changes with temperature

yresistivit oft coefficien etemperaturdifference etemperatur

tureat temperay resistivit tureat temperay resistivit

)1(

0

00

0

==−=∆

==

∆+=

R

T

RT

TTTTRTR

TRR

α

α

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Shape Memory Alloy (SMA)

Materials that exhibit a temperature-dependent phase transition (“martensite” - “austensite” crystal structure)Mostly Ni/Ti alloy, but also Au/Cu, In/Ti

Advantages:

• considerable temperature dependent expansion/contraction

• relatively linear control

• very high stress (> 200 MPa)

• arbitrary shapes

• simple actuation

• life time: millions of cycles (?)

Advantages:

• considerable temperature dependent expansion/contraction

• relatively linear control

• very high stress (> 200 MPa)

• arbitrary shapes

• simple actuation

• life time: millions of cycles (?)

Disadvantages:

• special alloy material

• high temperature annealing

• low efficiency (≈3%)

• thermal: long time constants (?)

Disadvantages:

• special alloy material

• high temperature annealing

• low efficiency (≈3%)

• thermal: long time constants (?)


02

21 µB

221

bE⋅ε

2PZT2

1bE⋅ε

Energy densities for different actuation principles

Actuation Maximum Energy Density Equation

Magnetic 0.95

SMA 10.4 -

Electrostatic 0.4

Piezoelectric 52[106J/m3]

Shape Memory Alloy (SMA)

[After Madou 1997]

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“Chevron” Actuator

• Current flows through expansion members (legs)

• Legs expand and push shuttle towards right

Anchor to Substrate

Anchor to Substrate

Shuttle

Legs

[Cragun and Howell, ASME IMECE, 1999]


Thermal Actuator

• “Pseudo bimorph”• Current runs through

loop of released structure (usually polysilicon, “cold arm” with gold layer)

• Regions with higher current density (“hot arm”) are heated up more and expand more (“pseudo bimorph”)

Substrate

Hot Arm

Cold Arm

[Allen, Howard, Ruff and Kolesar, TCU, 1997]

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Thermal Bimorph

Two layers of materials with different coefficients of thermal expansion

Approximation: t1

t2

R

iit

ttR

i

i

layer ofstrain thermallayer of thickness

12

21

==

−+

=

ε

εε


Polyimide Cilia Array

Thermobimorph and electrostatic actuator:length: 430 µm, height (out-of-plane) ≈ 120 µmrange of motion ≈ 10 µm in plane, 40 µm out of planelifting capacity ≈ 76 µN (250 µ N/mm2)

Thermobimorph and electrostatic actuator:length: 430 µm, height (out-of-plane) ≈ 120 µmrange of motion ≈ 10 µm in plane, 40 µm out of planelifting capacity ≈ 76 µN (250 µ N/mm2) [From Suh et al., 1996]

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Polyimide Bimorph MEMS

• Surface Micromachining of two polyimides with very different CTE

• Al connectors, bonding pads, and sacrificial layer for release of structures

• TiW heater loop• Nitride masking material• Spin-on deposition of

polyimide layers• Lower temperature → higher

curvature

[Figure: Suh et al., 1996]


Polyimide Cilia Array

• 4 actuators per “motion pixel”

• 8 x 8 motion pixels per chip

• Up to 4 chips per array:1024 cilia total

Right: MEMS cilia array moving an ADXL50 accelerometer chip.

[From Suh et al., 1996]

• 4 actuators per “motion pixel”

• 8 x 8 motion pixels per chip

• Up to 4 chips per array:1024 cilia total

Right: MEMS cilia array moving an ADXL50 accelerometer chip.

[From Suh et al., 1996]

1010


Fully Addressable Cilia Array

• Polyimide on top of CMOS

• 8 x 8 motion pixels

• Serial / parallel PC interface

• Interactive control software

• Polyimide on top of CMOS

• 8 x 8 motion pixels

• Serial / parallel PC interface

• Interactive control software

[From J. Suh et al., 1999]


Related Work• Micro Actuator Arrays

Pister, Fearing and Howe (UC Berkeley) 1990 micro-air bearing

Ataka, Omodaka, Konishi and Fujita (U. Tokyo) 1993 and 1995 various µ-actuators

Böhringer, Donald, Mihailovich and MacDonald (Cornell) 1994 and 1996 electrostatic

Liu and Will (USC ISI) 1995 magnetic

Suh, Glander, Darling, Storment and Kovacs (Stanford) 1996 thermobimorph

Yonezawa et al., 1997 magnetic

Hirata et al. (Neuchâtel) 1998 micro-pneumatics

Ebefors et al. (RIT Stockholm) 1999polyimide thermal actuator

• Macroscopic (Robotic) Actuator ArraysLuntz, Messner and Choset (CMU) 1997

macro-actuator arrayBerlin et al. (PARC) 1997

air jets for paper transport Frei, Wiesendanger, Büchi and Ruf (ETH Zurich) 1998

electromotors

1111


Kinematics of Actuation


2-Phase and 4-Phase Gaits

4 cilia per pixel: n,e,w,sN = north cilium is upn = north cilium is down

4 cilia per pixel: n,e,w,sN = north cilium is upn = north cilium is down

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Diagonal (Virtual) Gaits

Motion in diagonal direction:couple two neighboring cilia of each pixel to implement “virtual cilia”(by analogy to virtual robot legs [Raibert’93])

“virtual” 4-phase gait: news neWS NEWS NEws

Motion in arbitrary direction:alternate principal and diagonal gaits(by analogy to Bresenham line scan algorithm)


Orienting with Cilia Array

squeeze force

Automatic positioning and orienting with a“squeeze field”

Force at location (x,y): f(x,y) = - (sign(x),0)

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Rotating with Cilia Array

squeeze line

squeeze force

Automatic Centering and rotating with a skewed “squeeze field”

Force at location (x,y): f(x,y) = sign(x) (-1,ε)


MEMS Cilia Array in Action

MEMS cilia moving a 3 x 3 silicon chip• Translation• Rotation • Centering and aligning

[Suh et al., S&A-A, 1997; JMEMS 12, 1999][Böhringer et al., IJRR 2, 1999]

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Cilia Summary• MEMS cilia actuator array with integrated control

circuitry

• Electrostatic and thermal bimorph sensing and actuation

• High lifting capacity (≈ 70 µN / per cilium)

• Large motion range (up to ≈ 25 µm horizontal and ≈ 125 µm vertical)

• Implementation of open-loop manipulation strategies (transport, rotate, position, orient, ...)

• Interactive PC control interface

• Abstraction: force vector field


Omni-Directional WalkingMicrorobot

• Cilia chips turned upside down• Matthew H. Mohebbi and Mason L. Terry

(UW MEMS Lab)

• Two cilia chip designStabilityEnables rotation

• Wiring9 Wires - Chip 1 (N,S,E,W), Chip 2 (N,S,E,W) and ground0.05 A per channel25 µm insulated Cu 99.99%

[Mohebbi et al., ASME IMECE 2001]

1515


Video Motion

• Side view of MEMS robot moving in NW direction


• Displacement of micro robot:

• Velocity of micro robot:

• Constants dependent on the properties of the cilia and operating conditions

• Constants found using Monte Carlo search method to minimizing the least squares fit

Data Modeling

ctbceatf −+=)('

)1()( ctebattf −−−=

0,, ≥cbabcafvo +== )0(' atfv =∞→=∞ )('

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Experimental Results• Maximum velocity of 635µm/s (Northwest at 110 Hz)• Carrying capacity of 1.448 g

[Mohebbi et al., ASME IMECE 2001]


• Larger displacement is seen with the northwest motion• Near linear correspondence between control frequency

and velocity below 60 Hz

Experimental Results

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MEMS Robot Summary

• Thermal bimorph cilia arrays work for micro robot locomotion

• Maximum velocity of 635µm/s and carrying capacity of 1.4 g observed

• Each cilium has undergone close to 1 million actuations with no failures

• Accurate method for tracking robot investigated• Linear plus negative exponential model closely

describes movement of the robot• Further work includes:

Temperature – velocity correspondenceWalking surface effectsActuation voltage effects

Introduction to Microeletromechanical Systems (MEMS)tjs/4700/lec19/memslecture7.pdf ·...

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Transcript of Introduction to Microeletromechanical Systems (MEMS)tjs/4700/lec19/memslecture7.pdf ·...