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MEMS Cell Adhesion Device. Andrea Ho Mark Locascio Owen Loh Lapo Mori December 1, 2006. PDMS Membrane. PLA. Top Electrode. PVDF (Piezoelectric). Bottom Electrodes. PDMS. Via. SiO 2. Ni Traces (Layer 2). Parylene. Ni Traces (Layer 1). Si. Summary of Fabrication.

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MEMS Cell Adhesion Device

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Mems cell adhesion device

MEMS Cell Adhesion Device

Andrea Ho

Mark Locascio

Owen Loh

Lapo Mori

December 1, 2006

Summary of fabrication

PDMS Membrane


Top Electrode


Bottom Electrodes




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

[Roure, et al. PNAS 2005]

Fabrication alignment features

Fabrication - Alignment Features

  • Si wafer

  • Deposit silicon nitride by LPCVD

  • Spincoat with resist

  • Pattern alignment features in resist

  • Etch silicon nitride using RIE

  • Strip resist in oxygen plasma

Fabrication pillar mold

Fabrication - Pillar Mold

  • Spincoat with resist

  • Pattern resist by e-beam lithography

  • Etch Si using DRIE

  • Strip resist

  • (Silanize wafer to improve PLA release)

  • Pour PLA

  • Deposit common top electrode by e-beam evaporation

Fabrication piezoelectric elements

Fabrication - Piezoelectric Elements

  • Spincoat with PVDF (piezoelectric)

  • Spincoat with resist

  • Pattern using e-beam lithography

  • Etch PVDF using RIE

  • Strip resist

Fabrication electrodes



Fabrication - Electrodes

  • Spincoat with PDMS

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

  • Deposit SiO2 dielectric layer by PECVD

  • Spincoat with e-beam resist and patternby e-beam lithography

  • Etch through SiO2 by RIE

  • Strip resist in acetone

  • Sputter with Ni

  • Spincoat with e-beam resist and patternby e-beam lithography

  • Etch exposed Ni

  • Strip resist

  • Deposit parylene by CVD

Fabrication wafer bonding

Fabrication - Wafer Bonding

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

  • Peel off top Si wafer and SU-8 mold

Pdms membrane

PDMS Membrane

  • Begin with Si wafer

  • Spincoat with photoresist

  • Spincoat with diluted PDMS

  • (Treat in oxygen plasma)

Mold release

Mold Release

  • Flip over PDMS-coated wafer and bond to pillars

  • Peel away support wafer

  • (Treat in oxygen plasma)

Parametric study

Parametric Study

  • Dependence of output voltage on

    • pillar geometry

      • Diameter

      • Height

      • Electrode geometry

    • material properties

Parametric study1

Parametric Study

Parametric study2

Parametric Study

Parametric study3

Parametric Study



Inverse analysis

Inverse analysis

Fem analysis

FEM analysis

Model geometry


Fem results

FEM results

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

Additional results

Additional results

Resonance frequency

Tip displacement

Frequency response





Frequency Response

  • Lumped element model

  • Long, thin Ni wires in and out of pillar

  • Electrode of pillar modeled as parallel resistor & capacitor

Frequency response1

Frequency Response

  • Circuit element values calculated from material properties

Frequency response2









Frequency Response

  • Combine impedances

  • Take output across ZP

Frequency response3

Frequency Response

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

Thermal noise

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

Thermal noise1

Thermal Noise

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







Thermal noise2

Thermal Noise

  • Calculate noise bandwidth

  • Calculate thermal noise

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



  • 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



  • 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

Sensitivity analysis

Sensitivity Analysis

  • Change in voltage output for a given change in force: Slope of linear parametric plots

Sensitivity analysis1

Sensitivity Analysis

Sensitivity analysis2

Sensitivity Analysis

Resolution where system noise is the limiting factor

Sensitivity analysis3

Sensitivity Analysis

Resolution affected by fabrication processes

  • Effect of variation in pillar diameter on output voltage

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

ΔV = (30mV/μm)(0.06 μm) = 1.8 mV

Sensitivity analysis4

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

Sensitivity analysis5

Sensitivity Analysis

  • Effect of variation in pillar height

  • DRIE allows pillar height to vary ~μm

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

  • 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)



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