TITLE: Photolithography for microfluidics
AUTHORS: Nadanai Laohakunakorn

[DESCRIPTION]
Photolithography for microfluidic mold fabrication, successfully tested at the Scottish Microelectronics Centre. Produces rounded flow layer using positive resist SPR 220-7 with ~12um features, and rectangular control layer using negative resist SU-8-3035 with ~35um features). 

Adapted from dx.doi.org/10.17504/protocols.io.46wgzfe / Laohakunakorn et al. 2021 (https://doi.org/10.1007/978-1-0716-1032-9_9). 

[STEPS]
SECTION: Flow Layer
1. HMDS priming
SECTION: Control layer
9. O2 plasma treatment
SECTION: Flow Layer
2. Prebake
SECTION: Flow Layer
3. Spin coat with SPR 220-7 (Megaposit)   

SECTION: Flow Layer
4. Softbake
SECTION: Flow Layer
5. Exposure
SECTION: Flow Layer
6. Post-exposure bake
SECTION: Flow Layer
7. Develop with MF26A
SECTION: Flow Layer
8. Reflow
SECTION: Flow Layer
1.1. Prime a clean Si wafer in a sealed box with HMDS for 10 min  

SECTION: Flow Layer
2.1. Transfer to hotplate and carry out pre-bake at 120 °C for 12 min.

SECTION: Flow Layer
3.1. Transfer wafer to spin coater and run following programme using SPR 220-7 resist:
1. 400 rpm, 30 s 
2. 1000 rpm, 100 s 
at 200 rpm/s. This coats the wafer with ~12 um.

SECTION: Flow Layer
4.1. Transfer to hotplate and carry out soft-bake at 105 °C for 6 min . Remove promptly when done.

SECTION: Flow Layer
5.1. Expose with the following settings (for a Karl Suss MA8 mask aligner):

Multiple exposure steps: [ 80 s exposure + 10 s  wait] x 2 cycles, for a total of 160s (=638mJ/cm^2)

WEC=cont
Expose type = prox
40um alignment gap
5um expose gap
WEC offset OFF
N2 purge NO

The Karl Suss machine has a flux of 3.99 mW/cm^2 as measured from last maintenance. We would like an exposure of 660 mJ/cm^2. 
SECTION: Flow Layer
5.2. Wait: leave the wafer in a sealed wafer box for 120 min.

SECTION: Flow Layer
6.1. Transfer to a hotplate and hold at 110 °C  for 5 min 

SECTION: Flow Layer
6.2. Wait: leave the wafer in a sealed wafer box for 45 min 

SECTION: Flow Layer
7.1. Develop right side up for up to 5 min, rinse with DI water, and dry gently with compressed N2.

SECTION: Flow Layer
8.1. Place the wafer on a hotplate at room temperature, then ramp up to 170 °C (at a ramp rate of ~0.5*C/s). Hold at 170 °C for 40 min , then turn off hotplate and allow to ramp down to room temperature (takes a few hours).

    

SECTION: Control layer
10. Spin coat with SU-8-3035 (Kayaku)   

SECTION: Control layer
11. Softbake 
SECTION: Control layer
12. Exposure
SECTION: Control layer
13. Post-exposure bake
SECTION: Control layer
14. Wait for a minimum of 10 min .

SECTION: Control layer
15. Develop
SECTION: Control layer
16. Hardbake
SECTION: Control layer
9.1. Treat with O2 plasma for 10 minutes in barrel asher. (At SMC, Electrotech 508 with typical parameters: forward power 350W, flow 32%, pressure 0.8 torr)
SECTION: Control layer
10.1. Spin coat with SU8-3035 using the following programme with ramp rate of 100rpm/s for all steps:
1. 500 rpm, 10 s 
2. 3000 rpm, 65 s = 25s ramp + 40s hold
3. 4000 rpm, 1 s 
3. 3000 rpm, 5 s 
3. 0 rpm, 30 s 
Make sure the spin coater has a plastic liner installed. If necessary clean back and edge of wafer with wipe soaked in PGMEA.

SECTION: Control layer
11.1. Place on hotplate for 750 s at 95 °C 
Let wafer come to room temp (few mins) before exposure. If there are wrinkles, place on hotplate until wrinkles disappear. 


SECTION: Control layer
12.1. Expose with the following settings (for a Karl Suss MA8 mask aligner):

Multiple exposure steps: [ 18 s exposure + 10 s  wait] x 5 cycles, for a total of 90s (=359mJ/cm^2)

WEC=cont
Expose type = soft
30um alignment gap
5um expose gap
WEC offset OFF
N2 purge NO
SECTION: Control layer
13.1. a. Place on hotplate at 65 °C for 1 min . 
b. Move to second hotplate and hold at 95 °C for 4 min . 
SECTION: Control layer
15.1. Develop upside down in PGMEA for 2 min , and exchange with new PGMEA for a further 2 min . Check for underdevelopment each time on the microscope (white film). If necessary continue development for up to 6 min.
  
  

SECTION: Control layer
15.2. Rinse with IPA and dry gently with compressed air.
SECTION: Control layer
16.1. Ramp to 135 °C , and hold at 135 °C for 120 min . 

SECTION: Silanization of wafers
17. Silanization must be carried out before first use, to prevent PDMS sticking to wafers.
Leave 500 µL of chlorotrimethylsilane (Sigma 386529) in an upturned Falcon cap, inside a sealed box with the wafers until the solvent has completely evaporated.