I am spinning SU-8 2025 onto 4" silicon wafers and I have two possibly
1. Unless I pour SU-8 to cover essentially the entire wafer, there is
a visible thickness transition between the region it was poured onto
and the region it was dispersed onto by spinning.
2. Soft baking the wafers on a hotplate creates millimeter wide
craters in the SU-8 layer. They don't extend all the way to the wafer
but there is an obvious, large thickness variation. A few craters
sometimes form during the 65C bake but many form during the 95C bake.
Prior to the bake the SU-8 looks nice, no bubbles.
I'm using Microchem's recommended process parameters. I've tried
nanostripping the wafers beforehand, ashing them, letting the SU-8 sit
overnight before baking, nothing seems to help. Any help or ideas
would be most appreciated!
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