I am trying to create stamps for microcontact printing of protein substrates onto glass, and one of the features includes channels (or in the case of the features on the PDMS, pillars) 10um wide, 50um apart and 850 um wide. I am having difficulty getting these channels to peel up properly; I spin coat SU-8 3025 at ~25 micron thickness, but when I measure the channel depth with a profilometer the channels are only ~1um deep. The master looks fine whenever I check it before casting the PDMS, so I think it must be an issue with peeling. I've tried both spin coating and vapor depositing the wafer with HMDS before I cast the PDMS, and this hasn't helped. I've also tried decreasing my polymer:curing agent ratio with the PDMS to make it less sticky, and I've played around with curing times, even forgoing putting it in the incubator and just letting it cure for 24 hours. I have a few more things in mind to try: decreasing the aspect ratio by using a polymer that will coat thinner, using a positive photoresist, and using methanol to help peel the PDMS, but I am interested to know if there is anything else I could try or anything I am overlooking to help me get these features.
hi,
It may sound little orthodox and non scientific but you may try some regular oils. I used hair oil. Put (dip, spin, manually coat) a very thin layer of the oil on the master. Then coat the pdms on the master by whichever method you want and then peel of the structures after the curing. The oils can be easily washed from the master and the pdms surfaces using ethanol.
The procedure was so simple and elegant that it provided beautiful results to me.
Regards
Try using something like perfluorooctyltrichlorosilane instead of the hexamethyldisilizane. I usually use the HMDS as an adhesion promoter. What type of profilometer are you using? If you are using an old one (we have an old AlphaStep 200) it may have a stylus that will not fit down in your 10um width channel. What does your master look like after you peel of the PDMS? Try running your profilometer over the master before and after casting. This will let you know if you are really damaging the master.
I have used trimethylchlorosilane which works (it's still tricky). You can look up aspect ratio & protocol on doi:10.1016/B978-0-12-407863-5.00004-6. The polymer:curing agent ratio also determines the stickiness of PDMS (also baking time). These depend on what kind of PDMS you are using (RTV vs. Sylgard).....Another good resource for same is http://www.princeton.edu/prism/microfluidics/training/boot-camp/2015__Materials/bootcamp1_1_for_printing.pdf
I assume that you have an access to the cleanroom. You can try to put down a thin fluorocarbon layer on your masters by using RIE (reactive ion etching) system. A couple of minutes in CHF3 plasma leaves a hydrophobic coating on the masters, which will aid peeling. I use this to passivate the surface of 3D printed masters, and it works well for bigger features (~150um), so it might be worth a shot.
Thank you for all of these helpful answers. I was able to just decrease the aspect ratio and increase my PDMS ratio to 5:1 and everything peeled off nicely. I will keep these suggestions in mind if I run into any more problems.
Hi, I have a silicon master with 100um deep posts (100um diameter) and I've been trying to peel off my PDMS from it. I haven't tried any of your suggested techniques yet - I poured on PDMS already and it cured, however some of the wells have PDMS stuck in them. Is there a way I can dissolve out the PDMS from the wells. If I can do this, I can try your treatments to keep my PDMS from sticking int he future. Thanks!
Hi Manisha
Unfortunately dissolving PDMS is not usually feasible. The closest I've come to getting that to work was putting an etched silicon wafer in pirhana (sulfuric acid + hydrogen peroxide) at 80 C. Even so, it did not get all the PDMS out.
I suggest you make a new master and apply a fluoropolymer coating. Or use a commercial provider for your stamps so you can focus on your direct research.
http://www.gelest.com/wp-content/uploads/Goods-PDF-brochures-microfluidics.pdf
Gelest sells a chemical for this purpose, but your mileage may vary.
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