Team:SUSTech Shenzhen/Notebook/Fabrication

Fabrication

Detailed Protocol

Microfluidic Chips Production

Work done by Yu Zhenming (sophomore) with the help of Chen Rifei (junior). Assistant professor Jaewon Park, docter Wang Yao and docter Zhang Rui have offered their advices.

After reading existing papers about the processing technology typically used in microfluidic chips production and endless trial experiment, we have formed our own processing protocol based on our equipment.

The microfluidic chips used in our project were all produced using this protocol.

Process Overview

Fig. 1 Process Overview

SU-8 Photoresist can cross-link to a network of polymer when exposed to short wavelength light. If the cross-linked network has formed, it cannot be dissolved by the SU-8 developer. So if we place a photo mask to shade the light shine on the photoresist, we can make a copy of the pattern on the photo mask.

Both PDMS and glass contains silicon element. When they were treated by oxygen plasma, unstable hydroxyl group will form on the treated surface. When they get close enough, two hydroxyl group will dehydrate and bond together covalently.

Detailed Protocol

1. Wash the glass mask holder by socking it in dilute ammonia water for over 1 hour, and refresh it with deionized water.
2. Wash the glass mask holder by socking it in dilute hydrochloric acid for over 1 hour, and refresh it with deionized water.
3. Wash the glass mask holder by socking it in positive photoresist remover for over 1 hour, and refresh it with deionized water.
4. Dry the glass mask holder in the drying oven.
5. Clean the mask film by wiping the two sides carefully on one direction with a clean paper socked in acetone.
6. Clean the mask for the second time film by wiping the two sides carefully on one direction with a clean paper socked in alcohol or isopropyl alcohol.
   

   Fig. 2 Cleaning Photomask
7. Fix the mask film on to the mask holder with tape.
   

   Fig. 3 Fixing Photomask

   The tape can only touch the edges of the mask film in order to prevent contamination.

   

   Fig. 4 Fixed Photomask

8. Pour some SU-8 2100 photoresist in a small bottle, keep it still in the refrigerator for at least a week(two weeks recommend) to remove the bobbles inside the photoresist.
9. Take the SU-8 photoresist out from the refrigerator, wait until the bottle hit room temperature, and carefully pour it on to a clean silicon wafer. The bottle should be held as close to the wafer as possible to prevent bobble from generating.
   

   Fig. 5 Pouring Photoresist
10. Spin the wafer with photoresist on the spin coater with the following procedure.
    1. Spin at 500 rpm for 10 seconds with acceleration time of 5 seconds.
    2. Spin at 2000 rpm for 30 seconds with acceleration time of 5 seconds.
    3. Spin at 500 rpm for 5 seconds with acceleration time of 5 seconds.
    4. Stop with acceleration time of 5 seconds.
       

       Fig. 6 Prepared to be Spin Coated
       

       Fig. 7 Spin Coating
11. Bake the wafer on 65℃ hotplate for 5 minutes.
    

    Fig. 8 Soft Bake at 65℃
12. Bake the wafer on 95℃ hotplate for 30 minutes.
    

    Fig. 9 Soft Bake at 95℃
13. Keep the wafer still in room temperature for more than 1 hour and wait until the photoresist is dry enough.
14. Clean the backside of the wafer with a clean paper socked in SU-8 developer to remove excessive photoresist.
    

    Fig. 10 Cleaning the Backside
15. Fix the wafer to the mask aligner.
    

    Fig. 11 Fixing the Wafer
16. Place the mask holder on the wafer carefully.
    

    Fig. 12 Placing the Photo Mask
17. Expose the wafer with the mask aligner with the exposure energy of 220 mJ/cm².
18. Bake the wafer on 65℃ hot plate for 5 minutes.
    

    Fig. 13 Post Exposure Bake at 65℃

    A visible pattern identical to the photomask will be seen in the film within 5 seconds after being placed on the hotplate.

    

    Fig. 14 Pattern Seen During Post Exposure Bake

19. Bake the wafer on 95℃ hot plate for 12 minutes.
20. Keep the wafer still in room temperature for 15 minutes and wait until the photoresist is dry enough.
21. Pour fresh SU-8 developer into the container. Place the container on the shaking table and set the speed to 400rpm. Start the shaking table and wait for the speed to be stabilized.
    

    Fig. 15 Pouring Developer
22. Put the wafer in the container and develop for 4 minutes.
    

    Fig. 16 Developing
23. Spray and wash the developed image with fresh developer for approximately 10 seconds.
    

    Fig. 17 Spraying Developer
24. Spray and wash the developed image with isopropyl alcohol for approximately 10 seconds.
    

    Fig. 18 Spraying Isopropyl Alcohol
25. Air dry the wafer with nitrogen gun.
26. Bake the wafer on 150℃ hot plate for 20 minutes.
27. Check the developed image.
    

    Fig. 19 Checking Developed Image
    

    Fig. 20 Curing
28. Wait for the wafer to cool down.
29. Check the final image of the photoresist. The final image usually has a lot of defects at the edges of the channels. And due to the low transmittance of SU-8 photoresist to the low-wavelength light, the photoresist at the top side of the channel will absorb higher energy than the photoresist at the bottom, causing a trapezoidal cross section profile.
    

    Fig. 21 Topside View of Microfluidic Channels (5X)
    

    Fig. 22 Topside View of Microfluidic Channels (20X)
    

    Fig. 23 Optical Transmittance of SU-8 Photoresist
    

    Fig. 24 Cross Section Profile of Microfluidic Channels under SEM
    

    Fig. 25 Cross Section Profile of Microfluidic Channels under SEM
    

    Fig. 26 Cross Sectin Profile of Microfluidic Channels under SEM
30. Mix about 30g PDMS base with about 3g curing agent (at the ratio of 10:1).
31. Wrap the wafer with tinfoil and pour the PDMS mix on to the wafer.
32. Put the wafer in the vacuum dryer to remove the bobbles inside.
    

    Fig. 27 Removing Bobbles
33. Bake the wafer in the 80℃ drying oven for 3 hours.
    

    Fig. 28 Baking PDMS
34. Take the wafer out and rip off the tinfoil.
    

    Fig. 29 Baked PDMS
35. Carefully remove PDMS from the wafer and cut it into pieces on a piece of clean paper.
    

    Fig. 30 Cutting PDMS
    

    Fig. 31 PDMS Slices
36. Spray and wash the PDMS with alcohol.
    

    Fig. 32 Spraying Alcohol
37. Spray and wash the PDMS with deionized water.
    

    Fig. 33 Spraying Deionized Water
38. Air dry the wafer with nitrogen gun.
    

    Fig. 34 Air Drying
39. Soak glass slides in 100g/L NaOH solution for 14~24 hours.
    

    Fig. 35 Soaking Slides
40. Take the glass slides out, spray and wash them with deionized water.
    

    Fig. 36 Spraying Deionized Water
41. Spray and wash the glass slides with alcohol.
    

    Fig. 37 Spraying Alcohol
42. Spray and wash the glass slides with deionized water.
    

    Fig. 38 Spraying Deionized Water
43. Air dry the wafer with nitrogen gun.
    

    Fig. 39 Air Drying
44. Place the clean slides on a piece of tinfoil.
    

    Fig. 40 Clean Slides
45. Dry the glass slides in the drying oven
    

    Fig. 41 Drying Glass Slides
46. Clean the glass slide in the ultra-violet ozone cleaner for 15 minutes.
    

    Fig. 42 Cleaning in Ultra-Violet Ozone Cleaner
47. Take the slides out and wrap them with another piece of tinfoil.
    

    Fig. 43 Wrapped Slides
48. Punch the holes for the tube on the PDMS.
    

    Fig. 44 Punching Holes
    

    Fig. 45 Punching Holes
49. Open the plasma cleaner and place the PDMS and the slide in the meddle. A piece of tinfoil should be placed under the PDMS.
    

    Fig. 46 Glass Slide and PDMS in The Plasma Cleaner
50. Treat them with oxygen plasma for 1.5 minutes at 300W and the oxygen flow rate of 0.6mL/min.
    

    Fig. 47 Plasma Cleaning
51. Stop the plasma cleaner and open the cover as soon as possible. Press the treated sides together and apply pressure within 5 seconds.
    

    Fig. 48 Bonding

Made by from the iGEM team SUSTech_Shenzhen.

Licensed under CC BY 4.0.

• ©2016 SUSTech_Shenzhen