MEMS Cell Adhesion Device Andrea Ho Mark Locascio Owen Loh Lapo Mori December 1, 2006

PVDF (Piezoelectric) SiO 2 Si Parylene PDMS PLA Top Electrode Bottom Electrodes Via 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 of traces 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 pattern by e-beam lithography 5.Etch through SiO2 by RIE 7.Sputter with Ni 8.Spincoat with e-beam resist and pattern by 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

Parametric Study

Response

Inverse analysis

FEM analysis Model geometryMesh

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

Additional results Resonance frequencyTip displacement

Frequency Response Lumped element model Long, thin Ni wires in and out of pillar Electrode of pillar modeled as parallel resistor & capacitor R wire R PVDF C PVDF

Frequency Response Circuit element values calculated from material properties

Frequency Response Combine impedances Take output across Z P ZwZw ZwZw Z PR Z PC ZwZw ZwZw ZPZP Z EQ

Frequency Response Bode plot shows ω C >> any frequency we will be sensing

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 Noise Consider noisy resistor to be a noiseless resistor an a voltage source R PVDF C PVDF V OUT V NOISE R PVDF C PVDF

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

Actuation Piezoelectrics allow for both actuation and sensing Electromechanical coupling factor k k PVDF ≈ 0.1 to 0.3 Easy to run in reverse to stimulate cell

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

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

Sensitivity Analysis

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/ = 6.54 nN

Sensitivity Analysis Worst case scenario:  At F=100nN, output voltage varies over a total range of mV = 57.8 mV  ΔF = 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