MEMS Cell Adhesion Device

Andrea HoMark LocascioOwen LohLapo Mori

December 1, 2006

PVDF(Piezoelectric)

SiO2

Si

Parylene

PDMS

PLA

Top Electrode

Bottom ElectrodesVia

Ni Traces (Layer 2)

Ni Traces (Layer 1)

Summary of Fabrication

Based on passive PDMS pillar arrays

Add 3-axis force sensitivity on each pillar

Thin membrane over pillars

Alignment is critical Pillars, piezoelectric

elements, electrodes Use single set of

alignment marks for all layers

PDMS Membrane

[Roure, et al. PNAS 2005]

1. Si wafer

2. Deposit silicon nitride by LPCVD

3. Spincoat with resist

4. Pattern alignment features in resist

5. Etch silicon nitride using RIE

6. Strip resist in oxygen plasma

Fabrication - Alignment Features

1. Spincoat with resist

2. Pattern resist by e-beam lithography

3. Etch Si using DRIE

4. Strip resist

6. Pour PLA

7. Deposit common top electrode by e-beam evaporation

5. (Silanize wafer to improve PLA release)

Fabrication - Pillar Mold

1. Spincoat with PVDF (piezoelectric)

2. Spincoat with resist

3. Pattern using e-beam lithography

4. Etch PVDF using RIE

5. Strip resist

Fabrication - Piezoelectric Elements

1. Spincoat with PDMS

2. Pattern bottom electrodes and first set oftraces by e-beam lithography and liftoff

3. Deposit SiO2 dielectric layer by PECVD

11.Deposit parylene by CVD

4. Spincoat with e-beam resist and patternby e-beam lithography

5. Etch through SiO2 by RIE

7. Sputter with Ni

8. Spincoat with e-beam resist and patternby e-beam lithography

9. Etch exposed Ni

6. Strip resist in acetone

10.Strip resist

Fabrication - Electrodes

Electrodes

PVDF

1. Flip over and bond parylene layer to Si wafer with low heat and pressure

2. Peel off top Si wafer and SU-8 mold

Fabrication - Wafer Bonding

1. Begin with Si wafer

2. Spincoat with photoresist

3. Spincoat with diluted PDMS

4. (Treat in oxygen plasma)

PDMS Membrane

1. Flip over PDMS-coated wafer and bond to pillars

2. Peel away support wafer

3. (Treat in oxygen plasma)

Mold Release

Parametric Study

Dependence of output voltage on pillar geometry

Diameter Height Electrode geometry

material properties

2 2

334

material geometry of electrode and pillar

3 6 4 sin / 264 23 x

hD D s s h L

dV F

e D s D

Parametric Study

Parametric Study

Parametric Study

Response

2 μmD

1 μmh

20 μmL

80

m 5.0 s

Inverse analysis

1 3

2 2 21 3 1 1 2 2 2 3 3

1 3

1 3

2 2 21 1 2 2 2 3 3

2

2 2 2

2

arccos2 2 2 2

zz

V VF

V V V V V V V V VF

V V

V V

V V V V V V V

33

2 2

334

2

3 6 4 2 sin / 232

3

z

d h

e s D s

D D s s h L hd

e D s D

FEM analysisModel geometry Mesh

FEM results

It is reasonable to assume constant z over the piezoelectric material.

Additional resultsResonance frequency Tip displacement

3 3 6

4

20.077 μm

3 3x x

x

F L F L

E I E D

22 2

0

2 2

20

33* ( )

4 560

( ) 3*

128

L

L

Dm d y dy L D

d d y Dk EI dy E

dy L

2

2

1 * 77017.41 Hz

2 * 88

k EDf

m L

Frequency Response

Lumped element model Long, thin Ni wires in and out of pillar Electrode of pillar modeled as parallel

resistor & capacitorRwire

Rwire

RPVDFCPVDF

Frequency Response

Circuit element values calculated from material properties

41.2m106.523

m1020m101.63215

69

AR Ni

wire

18

2

18

182

0

1091.1μm 5236.0

)μm 1)(μm10(

F103.34μm 1

μm 5236.0**7.4

PVDF

PVDF

R

C

Frequency Response

Combine impedances Take output across ZP

Zw

Zw

ZPR ZPC

Zw

Zw

ZP ZEQ

Frequency Response

Bode plot shows ωC >> any frequency we will be sensing

sRCRRR

R

Z

ZsH

PPWWP

P

EQ

P

22)(

Thermal Noise

The electrodes and PVDF form an RC system

As in Senturia, this arrangement will create thermal noise in the system

Need to ensure RMS thermal noise << output voltages

fTRkV BRMS 4

Thermal Noise

Consider noisy resistor to be a noiseless resistor an a voltage source

RPVDF

CPVDFVOUTVNOISERPVDFCPVDF

RCssH

1

1)(

Thermal Noise

Calculate noise bandwidth

Calculate thermal noise

This is acceptable, since our outputs will be hundreds of mV

mV 1364.04 fTRkV BRMS

Hz0038.04

1

)2(1

10 2

RCdf

RCff

Actuation

Piezoelectrics allow for both actuation and sensing

Electromechanical coupling factor k

kPVDF ≈ 0.1 to 0.3 Easy to run in reverse to stimulate cell

energy mechanicalinput

energy electrical storedor

energy electricalinput

energy mechanical stored 22 kk

Actuation

Applied voltages will have to be roughly 10x the voltage out for a corresponding deflection

This puts it at a reasonable value for actuation voltage

Actuation would have to be calibrated experimentally

2

22

energy electricalinput

energy mechanical stored

V

dk

Sensitivity Analysis Change in voltage output for a given change in

force: Slope of linear parametric plots

y = 24.85x

0

200

400

600

800

1000

1200

0 10 20 30 40 50

Height of the pillar, L (um)

Ou

tpu

t v

olt

ag

e r

an

ge

(m

V)

y = 469.71x

0

200

400

600

800

1000

1200

1400

1600

1800

0 0.5 1 1.5 2 2.5 3 3.5 4

Distance between electrodes, h (um)

Ou

tpu

t vo

ltag

e ra

ng

e (m

V)

Sensitivity Analysis

0

100

200

300

400

500

600

700

0 0.2 0.4 0.6 0.8 1 1.2

Radial width of the electrode, s (um)

Ou

tpu

t vo

ltag

e ra

ng

e (m

V)

Sensitivity Analysis

y = 5.5061x

0

100

200

300

400

500

600

0 20 40 60 80 100 120

Applied Force (nN)

Vo

ltag

e O

utp

ut

(mV

)

5061.5

][][

mVVnNF

nNmV

F 0248.05061.5

1364.0

Resolution where system noise is the limiting factor

Sensitivity Analysis

Effect of variation in pillar diameter on output voltage

ΔV = (30mV/μm)(0.06 μm) = 1.8 mV

Diameter varies by ~10nm → Output voltage varies ~mV

Resolution affected by fabrication processes

Sensitivity Analysis

Effect of PVDF layer uniformity (4% )

At F = 100nN, ΔV[mV] = 450Δx[μm]

This results in an output voltage range of 36 mV

ΔF = 36 mV/5.5061 = 6.54 nN

y = 5.5061x

0

100

200

300

400

500

600

0 20 40 60 80 100 120

Applied Force (nN)

Vo

ltag

e O

utp

ut

(mV

)

Sensitivity Analysis

Worst case scenario: At F=100nN, output voltage varies over a total

range of 20 + 36 + 1.8 mV = 57.8 mV ΔF = 10.50 nN (~10% error)

Effect of variation in pillar height

DRIE allows pillar height to vary ~μm

At F = 100nN, output voltage can range over 20 mV

Questions