Photolithography for microfluidics
Feb 01, 2023
Open access
Protocol CitationNadanai Laohakunakorn 2023. Photolithography for microfluidics. protocols.io https://dx.doi.org/10.17504/protocols.io.5jyl89n5dv2w/v1
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License,  which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: Working
We use this protocol and it's working
Created: Dec 15, 2021
Last Modified: Feb 01, 2023
PROTOCOL integer ID: 55952
Funders Acknowledgement:
Carnegie Trust Research Incentive Grant
Grant ID: RIG009858

Public workspacePhotolithography for microfluidics

  • 1University of Edinburgh
Abstract
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).

MATERIALS
  • Masks: from Compugraphics. Master-Si, NFR5009, grade 2160A2.

1X Master, Soda Lime, 5"x5"x0.09"
Minimum critical feature: 2.0um
Defect density: 1 per in^2 (0.2 per cm^2)
Registration: SPC
CD tolerance: 0.25um
Defect size: 2um
Data design: rectilinear
Write area: 1" smaller than mask size

Compugraphics product code: 2160A2

Flow layer (SPR220, positive resist)
Digitised data = Dark
Data parity chrome up = Wrong
Title parity chrome up = Wrong
CD size = SPC
Data top cell = TOP
Data format = CIF
Data window = Compugraphics default

Control layer (SU8, negative resist)
Digitised data = Clear
Data parity chrome up = Wrong
Title parity chrome up = Wrong
CD size = SPC
Data top cell = TOP
Data format = CIF
Data window = Compugraphics default

Double-check these settings very carefully!

  • Wafer properties: from Inseto.

Diameter: 100mm
Material: Si
Type: N
Orientation: 100
Dopant: P-doped
Grade: Test?
Lower resistivity: 1 ohm.cm
Upper resistivity: 10 ohm.cm
Thickness: 525 um
Polish: SSP

Flow Layer
1
HMDS priming
10m
1.1
Prime a clean Si wafer in a sealed box with HMDS for Duration00:10:00

2
Prebake
12m
2.1
Transfer to hotplate and carry out pre-bake at Temperature120 °C for Duration00:12:00 .

3
Spin coat with SPR 220-7 (Megaposit) Download SPR_220_DATA_SHEET_RH.pdfSPR_220_DATA_SHEET_RH.pdf

2m 10s
3.1
Transfer wafer to spin coater and run following programme using SPR 220-7 resist:
1. Centrifigation400 rpm, 00:00:30
2. Centrifigation1000 rpm, 00:01:40
at 200 rpm/s. This coats the wafer with ~12 um.

4
Softbake
6m
4.1
Transfer to hotplate and carry out soft-bake at Temperature105 °C for Duration00:06:00 . Remove promptly when done.

5
Exposure
1m 30s
5.1
Expose with the following settings (for a Karl Suss MA8 mask aligner):

Multiple exposure steps: [ Duration00:01:20 exposure + Duration00:00:10 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.
2h
5.2
Wait: leave the wafer in a sealed wafer box for Duration02:00:00 .

6
Post-exposure bake
5m
6.1
Transfer to a hotplate and hold at Temperature110 °C for Duration00:05:00

45m
6.2
Wait: leave the wafer in a sealed wafer box for Duration00:45:00

7
Develop with MF26A
5m
7.1
Develop right side up for up to Duration00:05:00 , rinse with DI water, and dry gently with compressed N2.

8
Reflow
40m
8.1
Place the wafer on a hotplate at room temperature, then ramp up to Temperature170 °C (at a ramp rate of ~0.5*C/s). Hold at Temperature170 °C for Duration00:40:00 , then turn off hotplate and allow to ramp down to room temperature (takes a few hours).

Before reflow
Before reflow
After reflow
After reflow

Control layer
1h 22m 40s
9
O2 plasma treatment
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)
10
Spin coat with SU-8-3035 (Kayaku) Download KAM-SU-8-3000-Datasheet-7.10-final.pdfKAM-SU-8-3000-Datasheet-7.10-final.pdf

1m 51s
10.1
Spin coat with SU8-3035 using the following programme with ramp rate of 100rpm/s for all steps:
1. Centrifigation500 rpm, 00:00:10
2. Centrifigation3000 rpm, 00:01:05 = 25s ramp + 40s hold
3. Centrifigation4000 rpm, 00:00:01
3. Centrifigation3000 rpm, 00:00:05
3. Centrifigation0 rpm, 00:00:30
Make sure the spin coater has a plastic liner installed. If necessary clean back and edge of wafer with wipe soaked in PGMEA.

11
Softbake
12m 30s
11.1
Place on hotplate for Duration00:12:30 at Temperature95 °C
Let wafer come to room temp (few mins) before exposure. If there are wrinkles, place on hotplate until wrinkles disappear.


12
Exposure
28s
12.1
Expose with the following settings (for a Karl Suss MA8 mask aligner):

Multiple exposure steps: [ Duration00:00:18 exposure + Duration00:00:10 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
13
Post-exposure bake
5m
13.1
a. Place on hotplate at Temperature65 °C for Duration00:01:00 .
b. Move to second hotplate and hold at Temperature95 °C for Duration00:04:00 .
10m
14
Wait for a minimum of Duration00:10:00 .

15
Develop
4m
15.1
Develop upside down in PGMEA for Duration00:02:00 , and exchange with new PGMEA for a further Duration00:02:00 . Check for underdevelopment each time on the microscope (white film). If necessary continue development for up to 6 min.
Underdevelopment after 2+1 min
Underdevelopment after 2+1 min
Correct development after 2+2 min
Correct development after 2+2 min

15.2
Rinse with IPA and dry gently with compressed air.
16
Hardbake
2h
16.1
Ramp to Temperature135 °C , and hold at Temperature135 °C for Duration02:00:00 .

Silanization of wafers
17
Silanization must be carried out before first use, to prevent PDMS sticking to wafers.
Leave Amount500 µL of chlorotrimethylsilane (Sigma 386529) in an upturned Falcon cap, inside a sealed box with the wafers until the solvent has completely evaporated.