I am trying to coat SU-8 on a photodiode (400x400 um) via spray coating. The coating isn't uniform and the SU-8 doesn't seem to cover all the surface; it looks like it is affected by the surface tension. I tried to activate the surface using oxygen plasma but the edges still are not covered perfectly. Does anyone have any experience with spray coating SU-8?
Dear Hanie Kavand , I´m afraid that your substrate´s size is quite small. Your photodiode is just 400x400 um, and therefore the surface tension of SU-8 will cause a non uniform thin film. Usually when you are trying to obtain a thin film with a liquid photoresist, using spincoating, spraying, casting, etc over a large wafer or substrate, some problems could happen at the border, but the idea is to get a very uniform thin film, so that you can apply a lithographic process to obtain a collection of structures which will be cut into dices.
Therefore starting with a tiny wafer or substrate dice is not the best approach normally.
Surface tension is always a concern, but with tiny substrates it is even more problematic, because you have the sample edges very close.
May be you could embed your photodiode in some material with similar surface properties, so that when you apply SU-8 by spraying the edges do not cause these problems, which would allow you to obtain a uniform thin film while it would make easier to handle the sample. Then you can carry on further treatments or just removing the photodiode from the embedding material.
SU-8 is a solid, its different formulations are based on its different conentrations (some could varie the solvent too) what if afecting the formulation viscosity what together with the spraying nodle could have an effect on the final result. If you just need a very thin SU-8 film, a diluted SU-8 formulation and an homegenous spray formed by the tiniest droplets and applied for a short time could be a good strategy. If you need a thick layer of SU-8, then you could go for repeated applications of a thin film or just casting, cure and polishing till the thickness you need.
Hope this helps. Good luck with your research. Best!
Thanks Manuel Gómez
Right, but we buy them off the shelve and they come diced. Embedding and casting sounds nice but we don't have much space to play with and also considering the dimensions, it would be very hard to control it.
I am diluting my SU-8 using cyclopentanone (1:10) but the results vary every time. I have even sprayed while having the sample on a hot plate to help with evaporation.
I have tried HMDS this time, don't know if it actually is good for SU-8 but the results weren't as awful as before :)
Well, I still have to go for smaller dimensions and tougher coating...
Dear Hanie Kavand , has you tried to embed your chip into some wax or polymer, so that you could spray the SU-8 over a larger surface avoiding the edge problems?
There are also a large variety of other resists apart of SU-8 that could do the job, but probably most of them will cause similar problems due to the small size of your substrate. You have also solid resists ( a kind of tape) you can cut as you want or probably better, to attach your photodiode to the tape-like resist and then cut the unwanted tape, so that it only remains over the chip.
Hope this helps. Keep it up, research is also trial and error, but you learn a lot along the process. Regards!
Well, these chips need to be electrically connected, so it would be very tricky to coat them in a polymer first and then mount and connect. The thing is I am not using SU-8 just as a photoresist for patterning; it serves as a matrix for an optical filter. I am totally open to new suggestions for a material that can host some dyes, is optically transparent, and can be polymerized :)
Actually the tape resist sound VERY interesting, never heard about them! Can you please tell me more?
We tried to laser cut Kapton tapes to mount them on top of the substrate. It wasn't successful as there was a problem with detaching and placing it on our sample. I am taking about 400 um and 80 um alignments with hands :))
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