MEMS Cell Adhesion Device

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MEMS Cell Adhesion Device

• Andrea Ho
• Mark Locascio
• Owen Loh
• Lapo Mori
• December 1, 2006

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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
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Fabrication - Alignment Features

1. Si wafer

1. Deposit silicon nitride by LPCVD

1. Spincoat with resist

1. Pattern alignment features in resist

1. Etch silicon nitride using RIE

1. Strip resist in oxygen plasma

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Fabrication - Pillar Mold

1. Spincoat with resist

1. Pattern resist by e-beam lithography

1. Etch Si using DRIE

1. Strip resist

1. (Silanize wafer to improve PLA release)

1. Pour PLA

1. Deposit common top electrode by e-beam
   evaporation

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Fabrication - Piezoelectric Elements

1. Spincoat with PVDF (piezoelectric)

1. Spincoat with resist

1. Pattern using e-beam lithography

1. Etch PVDF using RIE

1. Strip resist

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Fabrication - Electrodes

1. Spincoat with PDMS

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

1. Deposit SiO2 dielectric layer by PECVD

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

1. Etch through SiO2 by RIE

1. Strip resist in acetone

1. Sputter with Ni

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

1. Etch exposed Ni

1. Strip resist

1. Deposit parylene by CVD

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Fabrication - Wafer Bonding

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

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

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PDMS Membrane

1. Begin with Si wafer

1. Spincoat with photoresist

1. Spincoat with diluted PDMS

1. (Treat in oxygen plasma)

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Mold Release

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

1. Peel away support wafer

1. (Treat in oxygen plasma)

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Parametric Study

• Dependence of output voltage on
• pillar geometry
• Diameter
• Height
• Electrode geometry
• material properties

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Parametric Study
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Parametric Study
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Parametric Study
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Response
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Inverse analysis
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FEM analysis
Model geometry
Mesh
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FEM results
It is reasonable to assume constant sz over the
piezoelectric material.
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Additional results
Resonance frequency
Tip displacement
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Frequency Response

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

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Frequency Response

• Circuit element values calculated from material
  properties

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Frequency Response

• Combine impedances
• Take output across ZP

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Frequency Response

• Bode plot shows ?C gtgt any frequency we will be
  sensing

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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 ltlt output
  voltages

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Thermal Noise

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

RPVDF
VNOISE
VOUT
RPVDF
CPVDF
CPVDF
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Thermal Noise

• Calculate noise bandwidth
• Calculate thermal noise
• This is acceptable, since our outputs will be
  hundreds of mV

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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

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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

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Sensitivity Analysis

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

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Sensitivity Analysis
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Sensitivity Analysis
Resolution where system noise is the limiting
factor
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Sensitivity Analysis
Resolution affected by fabrication processes

• Effect of variation in pillar diameter on output
  voltage

Diameter varies by 10nm ? Output voltage varies
mV
?V (30mV/µm)(0.06 µm) 1.8 mV
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Sensitivity Analysis

• Effect of PVDF layer uniformity (4 )
• At F 100nN, ?VmV 450?xµm
• This results in an output voltage range of 36 mV
• ?F 36 mV/5.5061 6.54 nN

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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 F100nN, output voltage varies over a total
  range of 20 36 1.8 mV 57.8 mV
• ?F 10.50 nN (10 error)

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Questions