Dear colleagues,
I will have to pour liquid PDMS on a substrate having 300 micron thick pin-structures on it. The difficulty is, that the pins later have to break through the hardened PDMS, so the PDMS gets wholes.
Do you have some basic tips to get a uniform PDMS layer or do you have experience in coating on patterned substrates?
Hi, I used to spin coat pdms from n-Hexane Solutions (under fume Hood; Hexane is extremely toxic); I used different ratios, depending on desired tickness. I did NOT have structures on surface though.
Hexane is not considered toxic in the normal understanding of the word. It's volatile, inflammable, and a reasonable anesthetic plus will exclude O2 if in high quantity. See: https://en.wikipedia.org/wiki/Hexane
"NIOSH has set a recommended exposure limit (REL) for hexane isomers (not n-hexane) of 100 ppm (350 mg/m3) over an 8-hour workday" (also from wiki above)
But, yes, work under a fume hood to remove the vapors...
Adding Hexane, as suggested above, should reduce the viscosity of the PDMS enough to spin coat a layer thin enough to expose the pins. You will want to experiment with the ratio to find the best condition.
For example test spin coating pure PDMS and PDMS:Hexane onto plain glass (microscope slides would work) at 1:20 , 1:10 , 1:5 ratios. Make sure they are well mixed before coating. Then, after curing, measure the thickness with a profilometer or AFM (if available) and pick the appropriate thickness. I would aim for 150um thickness to make sure the pins are exposed.
When I spin-coat PDMS, I use 3000 rpm @ 1500 rpm/s acceleration as my spin coat recipe and get ~500um thickness with pure PDMS. keep in mind increasing the spin rpm can also decrease thickness, so you may not even need much hexane mixed in. Also, I highly recommend treating your substrates, especially the glass pins, with either UV Ozone (10min) or O2 plasma (1min) cleaning if you have it available. The treatment will increase the surface wetting (very important for a patterned substrate) and give a more uniform film.
Best of luck, and I also recommend to work under a fume hood!
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