fabrication process pdms electrode array

Fabrication Process PDMS Electrode

Array

ME342 MEMS Laboratory

Jennifer Blundo

Gretchen Chua

Yong-Lae Park

Ali Rastegar

Project Goal

• Design a bioMEMs substrate to apply and

measure electromechanical forces in the

differentiation of human embryonic stem cell-

derived (hESC)-cardiac myocytes (CM)

Undifferentiated hESCs-Fluc-eGFP

(DAPI nuclear stain)

hESC-CMs organized in embryoid body

bioMEMS device

Contractility

Electrophysiology

Mechanical force

Current Microscale Devices

Thin-film stretchable (0—15%) gold electrodes (25nm) on PDMS. Lacour et al, 2005.

Thin-film gold strain gauges (200nm) encapsulated in PDMS (50μm). Wen et al, 2005.

64 Electrode array for extracellular recording, Multi Channel Systems

Pressure actuated PDMS membrane (120μm) with S-shaped SiO2 traces. Lee et al, 2004.

BioMEMS: Engineering Specs

Device Requirement Target Value

1. Apply mechanical strain Up to 10%

2. Apply electric field ~O(1) V/cm

3. Measure electric potential (ECG) 100μV—1mV

4. Area of mechanical deformation A < 1cm2

5. Size of electrodes diameter = 20μm

6. Inter-electrode spacing spacing = 250μm

7. Area of cell culture A > 1cm2

8. Thickness of substrate t < 1mm

BioMEMS: Device Design

A. Unstrained state B. Strained state

Glass/Quartz: Optically transparent baseplate PDMS: A biocompatible elastomeric polymer

PPS: A biocompatible elastomeric polymer

Ti: Adhesion layer for electrodes

Gold: Biocompatible thin film electrodes

SU-8: Transparent polymer

Fabrication: Baseplate

Step 1: Clean Pyrex 7740 4” glass wafer (300μm thick), dehydrate

5min @ 200°C

Equipment: Acetone/Methanol/IPA/DI rinse Location: MERL

Glass

Fabrication: SU-8 Processing

Glass

Channels to apply vacuum pressure to PDMS membraneGlass

Exposed SU-8

Unexposed SU-8

Step 2: Spin 1st layer SU-8-100 (100μm thick), prebake 10min @

65°C, softbake 30min @ 95°C, expose, postbake 1min @ 65°C,

10 min @ 95°C

Equipment: Spin coater, hot plate, UV Location: MERL


Step 3: Spin 2nd layer SU-8 (100μm thick), prebake, expose,

postbake


Loading post to support PDMS membrane

Glass

Exposed SU-8

Unexposed SU-8


Step 4: Spin 3rd layer SU-8 (100μm thick), prebake, expose,

postbake


Glass

Exposed SU-8

Unexposed SU-8


Step 5: Spin 4th layer SU-8 (80μm thick), prebake, expose,

postbake


Glass

Exposed SU-8

Unexposed SU-8


Step 6: Develop SU-8, IPA/DI rinse

Equipment: Location: MERL

Glass

Exposed SU-8


Step 7: Pipette tetrafluoropolymer (PS200 or T2494) to prevent

PDMS membrane stiction


Glass

Exposed SU-8

Tetrafluoropolymer

Fabrication: Baseplate Assembly

Step 8: Laser cut quartz 4” wafer (300μm thick) and bond quartz

over SU-8

Equipment: Laser cutter Location: MERL

Glass/Quartz

Exposed SU-8

20μm clearance between loading post and PDMS membrane

Process Option 1—Top to Bottom

Photoresist

PDMS Electrode Array

Fabrication: PDMS Membrane

Step 1: Clean 4” silicon wafers

Equipment: wbdiff Location: SNF

Silicon


Step 2: Spin sacrificial layer 5% (w/v) poly(acrylic acid) (PAA)

(3000 rpm, 15 s) and bake (150C, 2 min)

Equipment: Spin coater, Hot plate Location: MERL

Silicon

PAA

¼” Kapton tape at edge, removed after bake to prevent lift-off of PDMS during processing

• Advantages of water-soluble films – Deposited by spin-coating– The solvent removed at a low temperature (95–150C)– The resulting layer can be dissolved in water– No corrosive reagents or organic solvents– Faster release of features by lift-off

• Compatible with a number of fragile materials, such as organic polymers, metal oxides and metals—materials that might be damaged during typical surface micromachining processes

Sacrificial Layers—PDMS Micromachining

Sacrificial Layers—PAA & Dextran

BioMEMS: Fabrication

Step 3: Spin thick photo resist ~ 10μm

Equipment: SVGcoat Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


Step 4: Expose, develop, postbake

Equipment: KarlSuss, SVGdev Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au

20μm diameter electrodes

200μm interelectrode distance


Step 5: Gold deposition (2μm thick)

Equipment: Metallica Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


Step 6: Resist strip

Equipment: Wbgeneral2 Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


Step 7: Spin photo-patternable silicone (PPS) WL5153 30sec @

2500rpm (6μm thick), prebake 110°C

Equipment: Headway Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


Step 8: Expose*, postbake @ 150°C**, develop, hardbake 150°C

Equipment: Metallica Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au

Fabrication: Electrode Array

Step 9: O2 plasma (several min)* to etch and round PPS as well as

promote adhesion of metal deposition

Equipment: Gasonics

Silicon

PAA

PDMS

Shadow Mask

Ti

Au *Requires characterization

PPS


Step 10: Align beryllium copper shadow mask and temporarily

bond.

Equipment: EV aligner Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


Step 11: Evaporate Ti adhesion layer (10nm thick), Au layer

(100nm thick), Ti adhesion layer (10nm thick)

Equipment: Innotec Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au

30μm diameter to allow 20μm diameter electrodes

30μm width horseshoe tracks for electrode connections

Maintain 200μm interelectrode distance


Step 12: Remove shadow mask, O2 plasma to clean and promote

adhesion

Equipment: Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


Step 13: Spin 20:1 Sylgard 184 poly(dimethylsiloxane) (PDMS)

90sec @ 1200 rpm (50μm thick), bake (60°C, 1 hr)


Silicon

PAA

PPS

Ti

Au

PDMS


Step 14: Dissolve sacrificial layer PAA in water

Equipment: wbgeneral Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au

PPS


Step 15: Air dry device and transfer with handle wafer (glass)

Equipment: N2 gun Location: SNF

PPS

PDMS

Ti

Au

Glass

Fabrication: Assembly

Step 16: O2 plasma PDMS and quartz surfaces

Equipment: Drytek

PDMS

PPS

Ti

Au

Glass/Quartz

SU-8


Step 2: Bond PDMS membrane to glass

Glass/Quartz PDMS

PPS

Ti

Au

SU-8

Process Option 2—Top to Bottom

Skip Photoresist—Pattern PPS right

on PAA, expose, deposit metal




2500rpm (6μm thick), prebake 110°C, expose*, postbake @

150°C**, develop, hardbake 150°C

Equipment: Hot plate, Spin coater, Karl Suss*, BlueM oven**,

wbgeneral

Silicon

PAA

PDMS

Shadow Mask

Ti

Au *Proximity exposure

**Need to characterize in BlueM Oven

PPS


Step 5: O2 plasma (5 min)* to etch and round PPS

Equipment: Gasonics

Silicon

PAA

PDMS

Shadow Mask

Ti

Au *Requires characterization

PPS

Process Option 3—Bottom to Top

Pattern PDMS right on PAA, deposit

metal, spin PPS, expose, O2 plasma

etch down OR HCl dip if use Ti layer



Step 1: Clean 4” silicon wafers

Equipment: wbdiff Location: SNF

Silicon


Step 2: Spin sacrificial layer 5% (w/v) poly(acrylic acid) (PAA)

(3000 rpm, 15 s) and bake (150C, 2 min)

Equipment: Spin coater, Hot plate Location: MERL

Silicon

PAA

¼” Kapton tape at edge, removed after bake to prevent lift-off of PDMS during processing



(50μm thick), bake (60C, 1 hr), O2 plasma (1 min)


Silicon

PAA

PDMS

2mm gap at edge of wafer to prevent lift-off of PDMS during processing


Step 4: Align beryllium copper shadow mask and temporarily

bond.

Equipment: EV aligner Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


30μm width tracks for electrode connections

200μm interelectrode distance


Step 5: Evaporate Ti adhesion layer (10nm thick) and Au layer

(100nm thick)*

Equipment: Innotec Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au


30μm width tracks for electrode connections

*May want second layer of Ti to promote adhesion to PPS on top layer! Use an HCl dip to dissolve this


Step 6: Remove shadow mask, O2 plasma

Equipment: Drytek Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au



2500rpm (6μm thick), prebake 110°C, expose*, postbake @

150°C**, develop, hardbake 150°C

Equipment: Hot plate, Spin coater, Karl Suss*, BlueM oven**,

wbgeneral

Silicon

PAA

PDMS

Shadow Mask

Ti

Au *Proximity exposure

**Need to characterize in BlueM Oven

PPS


Step 8: O2 plasma (5 min)* to etch and round PPS as well as

promote adhesio

Equipment: Gasonics

Silicon

PAA

PDMS

Ti

Au

PPS *Requires characterization

PPS


Step 9: Dissolve sacrificial layer PAA in water

Equipment: wbgeneral Location: SNF

Silicon

PAA

PDMS

Shadow Mask

Ti

Au

PPS


Step 10: Air dry device and transfer with handle wafer (glass)

Equipment: N2 gun

Silicon

PAA

PDMS

Shadow Mask

Ti

Au

PPS


Step 1: O2 plasma PDMS and quartz surfaces

Equipment: Drytek

Silicon

PDMS

PPS

Ti

Au

Glass/Quartz

SU-8


Step 2: Bond PDMS membrane to glass

Glass/Quartz PDMS

PPS

Ti

Au

SU-8

Process Option 4—Entire Device

Pattern PDMS right on top of

baseplate with PAA sacrifical layer,

follow process option 3



Step 1: Fill baseplate with sacrificial layer—5% (w/v)

poly(acrylic acid) (PAA). Spin, squeegy off, bake (150C, 2

min). O2 plasma (1 min)

Equipment: Spinner Location: MERL

Glass/Quartz

Exposed SU-8



(50μm thick), bake (60C, 1 hr), O2 plasma (1 min)

Equipment: Spinner, oven Location: MERL

Glass/Quartz

Exposed SU-8



Final Device: Deposit metal, spin PPS, expose, O2 etch

Glass/Quartz

Exposed SU-8


Final Device: Dissolve PAA

Glass/Quartz

Exposed SU-8


The Meander Evolution…

Challenge: To maintain electrical

connections under strain

Material Properties & Geometry

• Material Properties– PDMS: E = 500kPa, υ = 0.5

– Gold: E = 78GPa, υ = 0.44

• Geometry– PDMS: t = 100μm

– Gold: t =100nm, w = 30μm,

L = 240μm, pitch (p) = 120μm

• Loading Condition– Plane Strain

– Biaxial Loading—10% Strain

G. Yang, et al. Design of Microfabricated Strain Gauge Array to Monitor Bone Deformation In Vitro and In Vivo. Proc. 4th IEEE Symposium on Bioinformatics and Bioengineering. 2004

1st Generation: The Sepertine

Strain Contour Plot Stress Contour Plot

2nd Generation: The Horseshoe

• Geometry– PDMS: t = 100μm

– Gold: t =100nm, w = 30μm,

R = 60μm , H = 45°

D. Brosteaux, et al.. Elastic Interconnects for Stretchable Electronic Circuits using MID (Moulded Interconnect Device) Technology. Mater. Res. Soc. Symp. Proc. Vol. 926. 2006

2nd Generation: The Horseshoe


Results: Stresses are distributed in a wider region, instead of being concentrated in a small zone at the crests and troughs.

Strains are lower at the boundaries, decreasing potential of delamination

The Meander Evolution…

Challenge: What if an electrode breaks?

How do we know if a connection is compromised?

3rd Generation: Horseshoe Hairpin


Results: Stresses are distributed in across the area of the electrode, however, stresses are higher in the immediate turn.

Strains are lower at the electrode.

4th Generation: Angled Horseshoe Hairpin


Updates

• Training done: – Wbgeneral – Innotec – Amtetcher– Laser Ablator

• Training still needed: – Litho Solvent Bench

(if PPS is allowed)– EV Aligner

Updates

• Fabrication: – Spinning of PDMS on Si Wafer

• 10:1, 50 ums, 1000 RPM, 90 secs (G. Yang, et al.) • O2 Plasma in Gasonics for 25 secs (program A, no lamp)• Problems – PDMS is a challenge to peel off • Possible Solution – PAA sacrificial layer

– Spinning PR on PDMS (Backup method)• SPR 3612 1.6 um, baked in the 90ºC for 30 mins• Problems – cracking of PR• Possible solution – ramping of temperature instead of baking

(suggested by Vikram). Similar to SU-8 stacking

Updates

• Fabrication: – Exposure of PR on PDMS

• Karlsuss 2 (down during the weekend ruined 3 wafers)

• Tried exposure times of 1.6-2 seconds. Contact pads were overexposed, but tracks were not defined

– Cr/Au deposition in Innotec• 100 A Cr/ 1000 A Au

• Purpose – check adhesion

• Still need to strip the PR to lift the unwanted metal

Updates

• Fabrication: – Spinning of PAA

– Spinning of SU-8

Updates

• Masks: – SU-8 Masks are already here

– Shadow Masks Vendors

Company Type

Photosciences Beryllium Copper

Fotofab Stainless Steel

Thin Metal Parts Copper

Updates

• Transparency Mask Design:

Updates

• Shadow Mask Design:

Action Items

• Get training on more machines

• Check actual thickness of PDMS on Dektak

• Send in shadow masks once finalized

• Characterize photo-patternable silicone (MERL)– Still waiting for SPECMAT, looks like yes from MT but

might need an official yes from Ed Meyers and Mahnaz, too

• Laser cut quartz wafers

• Trial of Ti and Au adhesion on PDMS

fabrication process pdms electrode array

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