How to cure diluted PDMS with solvent


I am doing some work with adding on a thin layer of PDMS that is diluted with a solvent (DCM, THF, 1,4-Dioxane) on a silicon wafer by spin-coating or drop coating. My problem is that I have noticed that I have bubbles on the backside of the layer and I am having problem with curing it without getting bubbles. It is either cavities of the solvent getting evaporated during curing or bubbles as I might have to high temperature?

I usually cure at 100 deg. but I have also tried curing at boiling temperature of the solvent and then later at 100 deg.

I usually have the ratio of 1:10 or 1:15 of the agent:elastomer. I add 1:6 ratio of the PDMS:solvent. DCM has boiling point of 40 deg. and THF has a boiling point of around 66 deg.

I am not sure what to do? I need a thin (few micrometer) nice homogeneous layer (without cavities or bubbles) that is molded into my structures of the wafer. My structures are in the size of micrometers in size.

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If you're using a platinum-catalyzed addition cure, those might be hydrogen gas bubbles from reaction of the silicon hydride component with itself. This could be caused by attempting to cure it on an incompatible surface. Lots of materials can inhibit the catalyst. If it's a raw silicon wafer, you should be okay, but if there's a coating like photoresist on it, that might contain a cure inhibitor. Dow Corning has an app note on cure inhibition which lists likely culprits. Their site is difficult to use, so you'll have to hunt it up yourself.

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Mark Thorson

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