TITLE: Device Fabrication Using Soft Lithography Technique
AUTHORS: Jann Gamboa

[DESCRIPTION]
Standard Operating Procedure
Institute of Biomedical Engineering
University of Toronto

Name of the procedures: Device Fabrication using Soft Lithography Technique
Location: MB308A
PI: Dr. Freeman Lan
Instructor: Jann Gamboa

[STEPS]
SECTION: Equipment Preparation
2. Hot Plates and Oven
SECTION: Equipment Preparation
2.1. The first step consists of turning one hot plate to 95 °C and another hot plate to ~200 °C

SECTION: Potential Hazards
1.   is a reproductive toxin and goes through gloves!

2. Wear cut resistant gloves when using the razor!

3. Always put the lid on when using the 
 
 
Note: The wafer could come off and cut you, or  could spray everywhere.

4.   burns the eyes! Put on safety goggles when using  

SECTION: Equipment Preparation
2.2. Turn on the 
 
 by turning the timer to "stay on" and the temperature to 60 °C.

SECTION: Equipment Preparation
3. Equipment Checklist:

1. Tweezers (2)
2. Pyrex dish for the  
3.   waste beaker
4.   waste beaker
5.   solid Waste Container
6. Aluminum foil
7. Double sided tape
8. Glass Slabs (for exposure)
9. Timers

SECTION: Equipment Preparation
4. 
 

SECTION: Equipment Preparation
4.1. Ensure that the spin coater cover and base are covered with aluminum foil.
SECTION: Equipment Preparation
4.2. Ensure that the spin coater is programmed to the desired 2-step rpm (1000 rpm for 10 s and the appropriate rpm for 30 s; refer to Figure 1)


 

SECTION: Equipment Preparation
4.3. Place a piece of double sided tape on the spin coater. 
The tape should cover the diameter of the spin coater stage and be centered.
SECTION: Equipment Preparation
4.4. Note: There may be times when the tape is not sticking to the spin coater stage. To mitigate this issue, clean the stage with   to wash off chemicals that are preventing it from sticking:

Unscrew the stage portion of the spin coater
Place it on a separate clean beaker.
Pour   onto it, just enough to submerge the surface of the stage. You can obtain the   either from the "Used PGMEA bottle" so long as the number of times that it has been recycled is <10. Otherwise, obtain it from the fresh PGMEA squeeze bottle.
Agitate the beaker gently for 5 min
After  5 min, pick up the stage using tweezers and wash it off for the last time with fresh   over the same beaker.
Wash the stage with   over the   waste beaker.
Blow dry the spin coater stage using the Dyson technique.
Proceed to place the double sided tape on the spin coater.
Discard the   to the "Fully depleted PGMEA waste bottle".
SECTION: Equipment Preparation
5. 
 

SECTION: Equipment Preparation
5.1. Locate or prepare a beaker for the waste  .

SECTION: Equipment Preparation
5.2. Pick up the silicon wafer using the tweezer located in the fume hood.
SECTION: Equipment Preparation
5.3. Rinse the wafer with  .
Make sure that the   is ending up in the waste beaker for alcohol.
SECTION: Equipment Preparation
5.4. Subsequently, use compressed air to blow the alcohol off of the wafer (using the Dyson technique; ask Jann or Freeman if you are unsure)
SECTION: Spin-coating and Soft Baking
6. Centre the silicon wafer on the spin-coater stage.
SECTION: Spin-coating and Soft Baking
6.1. Note: It is imperative to centre the silicon wafer onto the spin coater stage as it can lead to uneven coating. If this occurs, proceed to wash off the   on the wafer with    for 5 min using the instructions above over a Pyrex dish, not the beaker.

SECTION: Spin-coating and Soft Baking
7. Obtain the   NOT from the main SU-8 bottle but from the pre-made aliquots.

SECTION: Spin-coating and Soft Baking
8. Place about 1 mL or 2 mL (the size of a quarter coin) of the   on the centre of the wafer, taking care not to create air bubbles. Make sure also that you do not get any   on the neck of the bottle because it would then be hard to close the bottle.
SECTION: Spin-coating and Soft Baking
8.1. Note 1: DO NOT let the   bottle be open for a longer period of time because the compound quickly evaporates and changes the compound in the bottle.
SECTION: Spin-coating and Soft Baking
8.2. Note 2: Note that   solid wastes should only be placed in the yellow container that is marked "SU-8 Solid Waste"

SECTION: Spin-coating and Soft Baking
9. Spin the wafer according to the appropriate rpm (1000 for 10 s and refer to Figure 1 for the appropriate rpm for 30 s)

SECTION: Spin-coating and Soft Baking
10. After spinning, pick up the wafer on its edge and place it on the 95 °C hot plate for 10 min

SECTION: Spin-coating and Soft Baking
10.1. Note 1: Make sure that the double sided tape from the spin coater is not stuck on the bottom of the wafer before placing it on the hot plate.
SECTION: Spin-coating and Soft Baking
10.2. Note 2: Adjust the soft bake times according to the thickness of the channel

 
 
SECTION: Photolithographic Exposure
11. Place the wafer and the print out of the mask in between two pieces of glass such that the order is: 

Glass | wafer | mask | Glass <––– UV Light
SECTION: Photolithographic Exposure
11.1. Note 1: Make sure that the mask is facing up – that is, the description (writings) on the mask is facing up. You can tell that the mask is facing up if you can read the description.
SECTION: Photolithographic Exposure
12. Important: Wear the UV glasses.
SECTION: Photolithographic Exposure
13. Ensure that the 
 
 is on full battery, that is, 4/4 bars.
SECTION: Photolithographic Exposure
14. Place the UV light apparatus over on top of the glasses. Make sure that the box is covering the entirety of the glasses. Make sure that you line up the UV light on the centre of the wafer and the mask.
SECTION: Photolithographic Exposure
15. Inform everyone in the lab before turning on
SECTION: Photolithographic Exposure
16. Expose for 30 s
SECTION: Post-Exposure Baking (PEB)
17. Heat the wafer again by placing it on the 95 °C hot plate for 3 min to 5 min

SECTION: Post-Exposure Baking (PEB)
17.1. Note 1: At this point you should be able to see the outlines of the channel on the wafer. No visible latent image during or after PEB means that there was insufficient exposure, temperature or both.
SECTION: Channel Development
18. For this step, you would need a compound called  . Locate first the bottle that says "Used PGMEA". If the number of times that the product has been reused is less than 10, proceed to use the "Used PGMEA". Otherwise, use fresh   from the squeeze bottle or the actual bottle.
SECTION: Channel Development
18.1. WARNING 1:   goes through gloves – make sure that you double glove and/or change your gloves if you feel it necessary

SECTION: Channel Development
19. Pour   onto the Pyrex dish, just enough to submerge the wafer, thereby developing it.

SECTION: Channel Development
18.2. WARNING 2:   is a reproductive toxin – it is not good for pregnant women

SECTION: Channel Development
20. Cover the Pyrex dish using aluminum foil.
SECTION: Channel Development
19.1. If the   has been used more than 10 times, dispose of it in the PGMEA waste container and obtain it from the original bottle.

SECTION: Channel Development
21. Manually agitate (gentle) the Pyrex dish every once in a while.
SECTION: Channel Development
22. The development time depends on the thickness (height) of the channels. See the table below for guidance.

 
 

SECTION: Channel Development
23. After the appropriate development time has passed, take out the wafer using tweezers.
SECTION: Channel Development
24. Rinse the wafer with fresh   (over the Pyrex dish)

SECTION: Channel Development
25. Rinse the wafer with   over the IPA waste beaker

SECTION: Channel Development
26. If white precipitates arise, wash with fresh   over the PGMEA waster beaker to wash off   – NOT over to the Pyrex dish containing  , then back into Pyrex dish to develop longer.

SECTION: Channel Development
26.1. Repeat steps 24-25
SECTION: Channel Development
27. If   doesn't generate white precipitates, then blow dry off using the Dyson technique

SECTION: Channel Development
28. Recycle the   by pouring it back to the used PGMEA bottle (use the PGMEA funnel). Indicate that the   has been used once more by writing a line on the bottle.

SECTION: Channel Development
28.1. Note: If the number of lines has reached 10, discard the   in the PGMEA waste container.

SECTION: Hard Baking
29. Place the wafer on the 200 °C hot plate for 2 min

SECTION: Hard Baking
30. At this point you should be able to view the channels on the device under the microscope. Place the device on a clean petri dish after baking and view the channel from there using the microscopes.
SECTION: Hard Baking
31. Label the petri dish with the name of the dimensions of the channels, ID of the device, the date, your initials.
SECTION: Molding a Device
32. Prepare the   by mixing the   and   in 1:11 ratio
SECTION: Molding a Device
33. Mix vigorously using the Dremel tool or manually using the mixing sticks until you get an opaque colour and a lot of bubbles.
SECTION: Molding a Device
32.1. For 30 g, you would need 2.5 g of the   and fill the rest with the    (desired final amount x 1/12 = amount of elastomer needed)

SECTION: Molding a Device
34. Degas or desiccate until the   is clear is free from air bubbles using the 
 

SECTION: Molding a Device
35. While waiting, place the wafer on a petri dish. Cover it so that it is free from dust.
SECTION: Molding a Device
36. Once the de-gassing is done, pour the   on top of the wafer.
SECTION: Molding a Device
37. Make sure that the temperature of the oven is around 60 °C. Place it in the oven for several hours or even 2 min
SECTION: Molding a Device
37.1. Important: since the temperature of the oven fluctuates drastically, place the cover of the petri dish at the bottom by placing the device on top of it so that it does not melt.
SECTION: Harvesting the Device
38. After several hours, take the device out of the oven.
SECTION: Harvesting the Device
39. IMPORTANT: Wear the cut resistant gloves.
SECTION: Harvesting the Device
40. Cut the device mold using a scalpel. Make sure that you are not cutting into the channels and don't push too hard on the wafer or it will splinter
SECTION: Harvesting the Device
41. Peel the device mold using tweezers or your hands
SECTION: Harvesting the Device
41.1. Note 1: It is very easy to break the silicon wafer so when you are cutting or peeling off the device, be very careful not to break the wafer.
SECTION: Harvesting the Device
42. Punch the inlet and outlets of the device using the manual puncher.
SECTION: Harvesting the Device
43. Tape the device so that it would be free from dust.
SECTION: Plasma Bonding the Device
44. Speak to Jann or Freeman about this or refer to the SOP for the Harrick Plasma Cleaner PDG-32G from the Matsuura Lab
SECTION: Aquapel Treatment
45. IMPORTANT: Wear safety glasses!
SECTION: Aquapel Treatment
46. To make the device mold hydrophobic – cut a piece off of the aquapel syringe tubing.
SECTION: Aquapel Treatment
47. Stick the tubing into the channel, push just enough to get the liquid into the droplet making a nozzle.
SECTION: Aquapel Treatment
48. Push air through the channel to clear it out.
SECTION: Aquapel Treatment
49. Put in the oven for 5 min to dry.

SECTION: Aquapel Treatment
50. Seal the aquapel syringe tubing by melting the tubing end for a FEW seconds on the hot plate or lamp and sealing it with your gloved fingers.