Fabrication Process PDMS Electrode Array

1
Fabrication Process PDMS Electrode Array

• ME342 MEMS Laboratory
• Jennifer Blundo
• Gretchen Chua
• Yong-Lae Park
• Ali Rastegar

2
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)

hESC-CMs organized in embryoid body
Contractility Electrophysiology Mechanical force
Undifferentiated hESCs-Fluc-eGFP (DAPI nuclear
stain)
bioMEMS device
3
Current Microscale Devices
Thin-film gold strain gauges (200nm) encapsulated
in PDMS (50µm). Wen et al, 2005.
Thin-film stretchable (015) gold electrodes
(25nm) on PDMS. Lacour 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.
4
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) 100µV1mV
4. Area of mechanical deformation A lt 1cm2
5. Size of electrodes diameter 20µm
6. Inter-electrode spacing spacing 250µm
7. Area of cell culture A gt 1cm2
8. Thickness of substrate t lt 1mm
5
BioMEMS Device Design
B. Strained state
A. Unstrained 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
6
Fabrication Baseplate

• Step 1 Clean Pyrex 7740 4 glass wafer (300µm
  thick), dehydrate 5min _at_ 200C
• Equipment Acetone/Methanol/IPA/DI rinse
  Location MERL

Glass
7
Fabrication SU-8 Processing

• Step 2 Spin 1st layer SU-8-100 (100µm thick),
  prebake 10min _at_ 65C, softbake 30min _at_ 95C,
  expose, postbake 1min _at_ 65C, 10 min _at_ 95C
• Equipment Spin coater, hot plate, UV
  Location MERL

Channels to apply vacuum pressure to PDMS membrane
Glass
Glass Exposed SU-8 Unexposed SU-8
8
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
9
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
10
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
11
Fabrication SU-8 Processing

• Step 6 Develop SU-8, IPA/DI rinse
• Equipment Location MERL

Glass Exposed SU-8
12
Fabrication SU-8 Processing

• Step 7 Pipette tetrafluoropolymer (PS200 or
  T2494) to prevent PDMS membrane stiction
• Equipment Location MERL

Glass Exposed SU-8 Tetrafluoropolymer
13
Fabrication Baseplate Assembly

• Step 8 Laser cut quartz 4 wafer (300µm thick)
  and bond quartz over SU-8
• Equipment Laser cutter Location MERL

20µm clearance between loading post and PDMS
membrane
Glass/Quartz Exposed SU-8
14
Process Option 1Top to Bottom
PDMS Electrode Array

• Photoresist

15
Fabrication PDMS Membrane

• Step 1 Clean 4 silicon wafers
• Equipment wbdiff Location SNF

Silicon
16
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

¼ Kapton tape at edge, removed after bake to
prevent lift-off of PDMS during processing
Silicon PAA
17
Sacrificial LayersPDMS Micromachining

• Advantages of water-soluble films
• Deposited by spin-coating
• The solvent removed at a low temperature
  (95150C)
• 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
  metalsmaterials that might be damaged during
  typical surface micromachining processes

18
Sacrificial LayersPAA Dextran
19
BioMEMS Fabrication

• Step 3 Spin thick photo resist 10µm
• Equipment SVGcoat Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
20
BioMEMS Fabrication

• Step 4 Expose, develop, postbake
• Equipment KarlSuss, SVGdev Location SNF

200µm interelectrode distance
Silicon PAA PDMS
Shadow Mask Ti Au
20µm diameter electrodes
21
BioMEMS Fabrication

• Step 5 Gold deposition (2µm thick)
• Equipment Metallica Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
22
BioMEMS Fabrication

• Step 6 Resist strip
• Equipment Wbgeneral2 Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
23
BioMEMS Fabrication

• Step 7 Spin photo-patternable silicone (PPS)
  WL5153 30sec _at_ 2500rpm (6µm thick), prebake 110C
• Equipment Headway Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
24
BioMEMS Fabrication

• Step 8 Expose, postbake _at_ 150C, develop,
  hardbake 150C
• Equipment Metallica Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
25
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
PPS
Requires characterization
26
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
27
Fabrication Electrode Array

• Step 11 Evaporate Ti adhesion layer (10nm
  thick), Au layer (100nm thick), Ti adhesion
  layer (10nm thick)
• Equipment Innotec Location SNF

Maintain 200µm interelectrode distance
30µm width horseshoe tracks for electrode
connections
Silicon PAA PDMS
Shadow Mask Ti Au
30µm diameter to allow 20µm diameter electrodes
28
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
29
Fabrication Electrode Array

• Step 13 Spin 201 Sylgard 184
  poly(dimethylsiloxane) (PDMS) 90sec _at_ 1200 rpm
  (50µm thick), bake (60C, 1 hr)
• Equipment Location MERL

Silicon PAA PPS
Ti Au PDMS
30
Fabrication Electrode Array

• Step 14 Dissolve sacrificial layer PAA in water
• Equipment wbgeneral Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
PPS
31
Fabrication Electrode Array

• Step 15 Air dry device and transfer with handle
  wafer (glass)
• Equipment N2 gun Location SNF

PPS PDMS
Ti Au
Glass
32
Fabrication Assembly

• Step 16 O2 plasma PDMS and quartz surfaces
• Equipment Drytek

PDMS PPS
Ti Au
Glass/Quartz SU-8
33
Fabrication Assembly

• Step 2 Bond PDMS membrane to glass

Ti Au SU-8
Glass/Quartz PDMS PPS
34
Process Option 2Top to Bottom
PDMS Electrode Array

• Skip PhotoresistPattern PPS right on PAA,
  expose, deposit metal

35
Fabrication Electrode Array

• Step 4 Spin photo-patternable silicone (PPS)
  WL5153 30sec _at_ 2500rpm (6µm thick), prebake
  110C, expose, postbake _at_ 150C, develop,
  hardbake 150C
• Equipment Hot plate, Spin coater, Karl Suss,
  BlueM oven, wbgeneral

Silicon PAA PDMS
Shadow Mask Ti Au
PPS
Proximity exposure Need to characterize in
BlueM Oven
36
Fabrication Electrode Array

• Step 5 O2 plasma (5 min) to etch and round PPS
• Equipment Gasonics

Silicon PAA PDMS
Shadow Mask Ti Au
PPS
Requires characterization
37
Process Option 3Bottom to Top
PDMS Electrode Array

• Pattern PDMS right on PAA, deposit metal, spin
  PPS, expose, O2 plasma etch down OR HCl dip if
  use Ti layer

38
Fabrication PDMS Membrane

• Step 1 Clean 4 silicon wafers
• Equipment wbdiff Location SNF

Silicon
39
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

¼ Kapton tape at edge, removed after bake to
prevent lift-off of PDMS during processing
Silicon PAA
40
Fabrication PDMS Membrane

• Step 3 Spin 201 Sylgard 184 poly(dimethylsiloxa
  ne) (PDMS) (50µm thick), bake (60C, 1 hr), O2
  plasma (1 min)
• Equipment Location MERL

2mm gap at edge of wafer to prevent lift-off of
PDMS during processing
Silicon PAA PDMS
41
Fabrication Electrode Array

• Step 4 Align beryllium copper shadow mask and
  temporarily bond.
• Equipment EV aligner Location SNF

200µm interelectrode distance
30µm width tracks for electrode connections
Silicon PAA PDMS
Shadow Mask Ti Au
30µm diameter to allow 20µm diameter electrodes
42
Fabrication Electrode Array

• Step 5 Evaporate Ti adhesion layer (10nm thick)
  and Au layer (100nm thick)
• Equipment Innotec Location SNF

30µm width tracks for electrode connections
Silicon PAA PDMS
Shadow Mask Ti Au
30µm diameter to allow 20µm diameter electrodes
May want second layer of Ti to promote adhesion
to PPS on top layer! Use an HCl dip to dissolve
this
43
Fabrication Electrode Array

• Step 6 Remove shadow mask, O2 plasma
• Equipment Drytek Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
44
Fabrication Electrode Array

• Step 7 Spin photo-patternable silicone (PPS)
  WL5153 30sec _at_ 2500rpm (6µm thick), prebake
  110C, expose, postbake _at_ 150C, develop,
  hardbake 150C
• Equipment Hot plate, Spin coater, Karl Suss,
  BlueM oven, wbgeneral

Silicon PAA PDMS
Shadow Mask Ti Au
PPS
Proximity exposure Need to characterize in
BlueM Oven
45
Fabrication Electrode Array

• Step 8 O2 plasma (5 min) to etch and round PPS
  as well as promote adhesio
• Equipment Gasonics

PPS
Silicon PAA PDMS
Ti Au PPS
Requires characterization
46
Fabrication Electrode Array

• Step 9 Dissolve sacrificial layer PAA in water
• Equipment wbgeneral Location SNF

Silicon PAA PDMS
Shadow Mask Ti Au
PPS
47
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
48
Fabrication Assembly

• Step 1 O2 plasma PDMS and quartz surfaces
• Equipment Drytek

Silicon PDMS PPS
Ti Au
Glass/Quartz SU-8
49
Fabrication Assembly

• Step 2 Bond PDMS membrane to glass

Ti Au SU-8
Glass/Quartz PDMS PPS
50
Process Option 4Entire Device
PDMS Electrode Array

• Pattern PDMS right on top of baseplate with PAA
  sacrifical layer, follow process option 3

51
Fabrication PDMS Membrane

• Step 1 Fill baseplate with sacrificial layer5
  (w/v) poly(acrylic acid) (PAA). Spin, squeegy
  off, bake (150C, 2 min). O2 plasma (1 min)
• Equipment Spinner Location MERL

20µm clearance between loading post and PDMS
membrane
Glass/Quartz Exposed SU-8
52
Fabrication PDMS Membrane

• Step 2 Spin 201 Sylgard 184 poly(dimethylsiloxa
  ne) (PDMS) (50µm thick), bake (60C, 1 hr), O2
  plasma (1 min)
• Equipment Spinner, oven Location MERL

20µm clearance between loading post and PDMS
membrane
Glass/Quartz Exposed SU-8
53
Fabrication PDMS Membrane

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

Glass/Quartz Exposed SU-8
54
Fabrication PDMS Membrane

• Final Device Dissolve PAA

Glass/Quartz Exposed SU-8
55
The Meander Evolution

• Challenge To maintain electrical connections
  under strain

56
Material Properties Geometry

• Material Properties
• PDMS E 500kPa, ? 0.5
• Gold E 78GPa, ? 0.44
• Geometry
• PDMS t 100µm
• Gold t 100nm, w 30µm,
• L 240µm, pitch (p) 120µm
• Loading Condition
• Plane Strain
• Biaxial Loading10 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
57
1st Generation The Sepertine
Strain Contour Plot
Stress Contour Plot
58
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
59
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
60
The Meander Evolution

• Challenge What if an electrode breaks?
• How do we know if a connection is compromised?

61
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.
62
4th Generation Angled Horseshoe Hairpin
Strain Contour Plot
Stress Contour Plot
63
Updates

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

• Training still needed
• Litho Solvent Bench (if PPS is allowed)
• EV Aligner

64
Updates

• Fabrication
• Spinning of PDMS on Si Wafer
• 101, 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

65
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

66
Updates

• Fabrication
• Spinning of PAA
• Spinning of SU-8

67
Updates

• Masks
• SU-8 Masks are already here
• Shadow Masks Vendors

Company Type
Photosciences Beryllium Copper
Fotofab Stainless Steel
Thin Metal Parts Copper
68
Updates

• Transparency Mask Design

69
Updates

• Shadow Mask Design

70
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