Hi
At the moment Im trying to create gratings with SU8 3050 in the 50-100um range with a positively sloped sidewall. To achieve this I have been performing backside exposure of SU8 through a 1mm glass substrate. Ideally for my work, the sidewall angle should be 70 degrees or greater, however I seem to be fabricating gratings with around a 45 degree angle.
The gratings are typically around 100um width with spacing of 300um. I was just wondering if anyone has any experience with exposing SU8 from the back or would have any helpful advice to make the sidewall more vertical.
Im following a similar fabrication process as for silicon with a 95o soft-bake for 30 mins and the recommended post bake time, the only major difference is the exposure dose. When using a similiar exposure to silicon, I was getting an overexposed profile like the one in the attached image, Ive divided the dose by 3 which seems good results other than the angle.
I was intending to use thinner glass substrates (around 100um) to see if the problem will improve. Otherwise, any suggestions would be greatly appreciated!
Thin glass does help to solve the issue that you have. However, i would suggest to use fused silica glass which have very high transmittance in UV region. This will pass through more UV light than normal glass on the back side of the substrates.
Are you sure that the wall profile looks like what you show in the figure? This could be a measurement artifact. If this is a profilometry measurement and the structures are relatively tall, then perhaps what you are imaging is your profilometer tip shape rather than the wall profile, or more correctly - their convolution.
Yeah pretty sure the profile looked like the plot as lower exposure doses lead to the square wave pattern I was anticipating. Done a test run today with 100um thick glass and got a sidewall angle of 70 degrees which is a vast improvement!
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