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Fabrication Process PDMS Electrode Array. ME342 MEMS Laboratory Jennifer Blundo Gretchen Chua Yong-Lae Park Ali Rastegar. Project Goal. - PowerPoint PPT Presentation
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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
Fabrication: SU-8 Processing
Step 3: Spin 2nd layer SU-8 (100μm thick), prebake, expose,
postbake
Equipment: Spin coater, hot plate, UV Location: MERL
Loading post to support PDMS membrane
Glass
Exposed SU-8
Unexposed SU-8
Fabrication: SU-8 Processing
Step 4: Spin 3rd layer SU-8 (100μm thick), prebake, expose,
postbake
Equipment: Spin coater, hot plate, UV Location: MERL
Glass
Exposed SU-8
Unexposed SU-8
Fabrication: SU-8 Processing
Step 5: Spin 4th layer SU-8 (80μm thick), prebake, expose,
postbake
Equipment: Spin coater, hot plate, UV Location: MERL
Glass
Exposed SU-8
Unexposed SU-8
Fabrication: SU-8 Processing
Step 6: Develop SU-8, IPA/DI rinse
Equipment: Location: MERL
Glass
Exposed SU-8
Fabrication: SU-8 Processing
Step 7: Pipette tetrafluoropolymer (PS200 or T2494) to prevent
PDMS membrane stiction
Equipment: Location: MERL
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
Fabrication: PDMS Membrane
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
BioMEMS: Fabrication
Step 4: Expose, develop, postbake
Equipment: KarlSuss, SVGdev Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
20μm diameter electrodes
200μm interelectrode distance
BioMEMS: Fabrication
Step 5: Gold deposition (2μm thick)
Equipment: Metallica Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
BioMEMS: Fabrication
Step 6: Resist strip
Equipment: Wbgeneral2 Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
BioMEMS: Fabrication
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
BioMEMS: Fabrication
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
Fabrication: Electrode Array
Step 10: Align beryllium copper shadow mask and temporarily
bond.
Equipment: EV aligner Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
Fabrication: Electrode Array
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
Fabrication: Electrode Array
Step 12: Remove shadow mask, O2 plasma to clean and promote
adhesion
Equipment: Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
Fabrication: Electrode Array
Step 13: Spin 20:1 Sylgard 184 poly(dimethylsiloxane) (PDMS)
90sec @ 1200 rpm (50μm thick), bake (60°C, 1 hr)
Equipment: Location: MERL
Silicon
PAA
PPS
Ti
Au
PDMS
Fabrication: Electrode Array
Step 14: Dissolve sacrificial layer PAA in water
Equipment: wbgeneral Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
PPS
Fabrication: Electrode Array
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
Fabrication: Assembly
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
PDMS Electrode Array
Fabrication: Electrode Array
Step 4: Spin photo-patternable silicone (PPS) WL5153 30sec @
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
Fabrication: Electrode Array
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
PDMS Electrode Array
Fabrication: PDMS Membrane
Step 1: Clean 4” silicon wafers
Equipment: wbdiff Location: SNF
Silicon
Fabrication: PDMS Membrane
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
Fabrication: PDMS Membrane
Step 3: Spin 20:1 Sylgard 184 poly(dimethylsiloxane) (PDMS)
(50μm thick), bake (60C, 1 hr), O2 plasma (1 min)
Equipment: Location: MERL
Silicon
PAA
PDMS
2mm gap at edge of wafer to prevent lift-off of PDMS during processing
Fabrication: Electrode Array
Step 4: Align beryllium copper shadow mask and temporarily
bond.
Equipment: EV aligner Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
30μm diameter to allow 20μm diameter electrodes
30μm width tracks for electrode connections
200μm interelectrode distance
Fabrication: Electrode Array
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 diameter to allow 20μm diameter electrodes
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
Fabrication: Electrode Array
Step 6: Remove shadow mask, O2 plasma
Equipment: Drytek Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
Fabrication: Electrode Array
Step 7: Spin photo-patternable silicone (PPS) WL5153 30sec @
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
Fabrication: Electrode Array
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
Fabrication: Electrode Array
Step 9: Dissolve sacrificial layer PAA in water
Equipment: wbgeneral Location: SNF
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
PPS
Fabrication: Electrode Array
Step 10: Air dry device and transfer with handle wafer (glass)
Equipment: N2 gun
Silicon
PAA
PDMS
Shadow Mask
Ti
Au
PPS
Fabrication: Assembly
Step 1: O2 plasma PDMS and quartz surfaces
Equipment: Drytek
Silicon
PDMS
PPS
Ti
Au
Glass/Quartz
SU-8
Fabrication: Assembly
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
PDMS Electrode Array
Fabrication: PDMS Membrane
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
20μm clearance between loading post and PDMS membrane
Step 2: Spin 20:1 Sylgard 184 poly(dimethylsiloxane) (PDMS)
(50μm thick), bake (60C, 1 hr), O2 plasma (1 min)
Equipment: Spinner, oven Location: MERL
Glass/Quartz
Exposed SU-8
20μm clearance between loading post and PDMS membrane
Fabrication: PDMS Membrane
Final Device: Deposit metal, spin PPS, expose, O2 etch
Glass/Quartz
Exposed SU-8
Fabrication: PDMS Membrane
Final Device: Dissolve PAA
Glass/Quartz
Exposed SU-8
Fabrication: PDMS Membrane
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
Strain Contour Plot Stress Contour Plot
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
Strain Contour Plot Stress Contour Plot
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
Strain Contour Plot Stress Contour Plot
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