We've had both of these problems. The first you may be able to reduce, but
probably not completely eliminate, by letting the SU8 rest before the soft
bake. We have left 3" wafers rest in 100mm petri dishes on a level
surface, covered, for 24 hours.
The second is probably due to small bubbles, as Andrew said. I just had
this experience. Sometimes small bubbles can get trapped while pouring.
Larger craters can be from contaminants in the air. We partially cover our
wafers during the soft bake.
Eric Johnston (email@example.com)
Hammer Lab, Department of Bioengineering
University of Pennsylvania
On Fri, Nov 9, 2012 at 5:43 PM, Tom Yuzvinsky wrote:
> Hi all,
> I am spinning SU-8 2025 onto 4" silicon wafers and I have two possibly
> related problems:
> 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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