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, Thin-film gold strain gauges (200nm) encapsulated in PDMS (50μm). Wen et al, Electrode array for extracellular recording, Multi Channel Systems Pressure actuated PDMS membrane (120μm) with S-shaped SiO 2 traces. Lee et al, 2004.

BioMEMS: Engineering Specs Device RequirementTarget Value 1. Apply mechanical strainUp to 10% 2. Apply electric field~O(1) V/cm 3. Measure electric potential (ECG)100μV—1mV 4. Area of mechanical deformationA < 1cm 2 5. Size of electrodesdiameter = 20μm 6. Inter-electrode spacingspacing = 250μm 7. Area of cell cultureA > 1cm 2 8. Thickness of substratet < 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 ” glass wafer (300μm thick), dehydrate 200°C Equipment: Acetone/Methanol/IPA/DI rinse Location: MERL Glass

Fabrication: SU-8 Processing Glass Channels to apply vacuum pressure to PDMS membrane Glass Exposed SU-8 Unexposed SU-8 Step 2: Spin 1 st layer SU (100μm thick), prebake 65°C, softbake 95°C, expose, postbake 65°C, 10 95°C Equipment: Spin coater, hot plate, UV Location: MERL

Fabrication: SU-8 Processing Step 3: Spin 2 nd 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 3 rd 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 4 th 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 cutterLocation: 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: wbdiffLocation: 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 plateLocation: 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: SVGcoatLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au

BioMEMS: Fabrication Step 4: Expose, develop, postbake Equipment: KarlSuss, SVGdevLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au 20μm diameter electrodes 200μm interelectrode distance

BioMEMS: Fabrication Step 5: Gold deposition (2μm thick) Equipment: MetallicaLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au

BioMEMS: Fabrication Step 6: Resist strip Equipment: Wbgeneral2Location: SNF Silicon PAA PDMS Shadow Mask Ti Au

BioMEMS: Fabrication Step 7: Spin photo-patternable silicone (PPS) WL rpm (6μm thick), prebake 110°C Equipment: HeadwayLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au

BioMEMS: Fabrication Step 8: Expose *, 150°C **, develop, hardbake 150°C Equipment: MetallicaLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au

Fabrication: Electrode Array Step 9: O 2 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 alignerLocation: 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: InnotecLocation: 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, O 2 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) 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: wbgeneralLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au PPS

Fabrication: Electrode Array Step 15: Air dry device and transfer with handle wafer (glass) Equipment: N 2 gunLocation: SNF PPS PDMS Ti Au Glass

Fabrication: Assembly Step 16: O 2 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) WL rpm (6μm thick), prebake 110°C, expose *, 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: O 2 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: wbdiffLocation: 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 plateLocation: 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), O 2 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 alignerLocation: 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: InnotecLocation: 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, O 2 plasma Equipment: DrytekLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au

Fabrication: Electrode Array Step 7: Spin photo-patternable silicone (PPS) WL rpm (6μm thick), prebake 110°C, expose *, 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: O 2 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: wbgeneralLocation: SNF Silicon PAA PDMS Shadow Mask Ti Au PPS

Fabrication: Electrode Array Step 10: Air dry device and transfer with handle wafer (glass) Equipment: N 2 gun Silicon PAA PDMS Shadow Mask Ti Au PPS

Fabrication: Assembly Step 1: O 2 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). O 2 plasma (1 min) Equipment: SpinnerLocation: 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), O 2 plasma (1 min) Equipment: Spinner, ovenLocation: 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. 4 th IEEE Symposium on Bioinformatics and Bioengineering. 2004

1 st Generation: The Sepertine Strain Contour PlotStress Contour Plot

2 nd 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

2 nd Generation: The Horseshoe Strain Contour PlotStress 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?

3 rd Generation: Horseshoe Hairpin Strain Contour PlotStress 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.

4 th Generation: Angled Horseshoe Hairpin Strain Contour PlotStress 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.) O 2 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 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 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 CompanyType PhotosciencesBeryllium Copper FotofabStainless Steel Thin Metal PartsCopper

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