Path One

Interdigitated arrays were fabricated on a 2” x 3” glass slide according the process flow shown here

Pros: Does not require extra tools for insulator deposition, uses negative photoresist as insulator

Cons: Does not hold up well in organic solutions, not compatible with oxidizing conditions

SU8 with a sacrificial underlayer of OmniCoat was lithographically patterned

NOTE: SPR220 or otherpositive resist can work wel in place of SU8 and omnicoat. However mask was inverted so this process uses negative resist
2x3in glass slide is substrate
Degrease
Spin omnicoat - 1mL - recipe 3
200C bake, 1min
Spin SU8-2002 - 3mL. Make sure its spread over the whole glass slide
- Spin Recipe #3
Prebake 110C for 2min
Expose - 70mJ/cm^2
Post exposure bake 2-3min until latent image visible
Develop - SU8 Developer - 1.5min
- IPA quench, then rinse with IPA, then dry. (no water)
- Water lifts off the omnicoat

Plasma clean, sputter, lift-off

1min 100W, O2&Ar (2 O2, 1 Ar) plasma clean in RIE March
Sputter coat 5nm (1min) Ti then 100nm (3min) Pt
Lift-off in 917MIF Developer for 5min without sonication, then sonicate for 30-60seconds to remove remaining metal
DI water rinse sample
Move sample to fresh beaker with water 30-60seconds sonicate
DI water rinse sample

Insulation Spin, Pattern

An insulating layer of KMPR photoresist was patterned, with an OmniCoat base layer to improve poor KMPR-glass adhesion. Substrate was briefly

etched in O2 plasma to remove residual OmniCoat, and then hard baked at 300 °C to ensure adhesion of the KMPR layer.

Degrease
Spin omnicoat - 1mL - recipe 3
200C bake, 1min
Spin KMPR1010- 3-4mL. Make sure its spread over the whole glass slide
- Spin Recipe #3
Prebake 110C for 5min (check for wrinkles. Bake more if wrinkles found)
Expose - 300-400mJ/cm^2 (normally 200mJ/cm^2 is enough. but a few months ago needed to double to get consistent adhesion)
Post exposure bake 2-3min until latent image visible
Develop - SU-8 Dev - 2min (no 917 because it lifts off the omnicoat)
- IPA quench, then rinse with IPA, then dry. (no water)
- Water lifts off the omnicoat
250C 15-20min

Spin PR and Dice

Spin SPR220- 3-4mL. Make sure its spread over the whole glass slide
- Spin Recipe #3
- 1min 110C bake
Then Dice

Path Two

Interdigitated arrays were fabricated on a 2” x 3” glass slide according the process flow outlined in Figure

2A. A spin coated layer of SU8 photoresist with a sacrificial underlayer of OmniCoat was lithographically

patterned and then plasma-cleaned briefly to ensure adhesion of sputtered metal to the exposed substrate.

Substrates were sputtered with a 5 nm Ti adhesion layer followed by 100 nm of Pt, and liftoff was carried

out by ultrasonication in a TMAH developer solution. Substrates were plasma cleaned thoroughly, and then

a plasma enhanced chemical vapor deposition (PECVD) process was used to deposit a 1 μm thick layer of

SiO2 to act as the insulating layer. SU8 photoresist was patterned, WITH NO OMNICOAT (SU8 will

have good adhesion to the PECVD oxide). Substrate was etched for 30 minutes in a CF4 plasma to expose

the electrodes. Device was then plasma cleaned in O2 to remove any remaining negative resist. A protective

layer of SPR220 was spun on prior to dicing.

Same as above, however KMPR deposition is replaced by PECVD of Oxide

degrease
5min 100W oxygen plasma (2 O2, 1 Ar flow) March RIE
HMNTL oxford PECVD - basic SiO2 deposition for 15min (~ 1um)
(might add anealing step here 600-700C)
Degrease
Spin omnicoat - 1mL - recipe 3
200C bake, 1min
Spin SU8-2002 - 3mL. Make sure its spread over the whole glass slide
- Spin Recipe #3
Prebake 110C for 2min
Expose - 70mJ/cm^2
Post exposure bake 2-3min until latent image visible
Develop - SU8 Developer - 1.5min
- IPA quench, then rinse with IPA, then dry. (no water)
- Water lifts off the omnicoat
1min 100W oxygen plasma (2 O2, 1 Ar flow) March RIE
RIE Axic ~30min CF4 isotropic etch to remove oxide where necessary
Spin SPR220- 3-4mL. Make sure its spread over the whole glass slide
- Spin Recipe #3
- 1min 110C bake
Then Dice

Pros: Gives a more robust device that can handle intensive oxidizing conditions and organic solutions.

Cons: Requires tooling that can do CVD if insulators and also requires extra steps, has lower throughput,

and is more difficult to get thick films.

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