I've seen the first problem happen if the SU8 and the wafer are not at the
same temperature. When the SU-8 first contacts the wafer surface,
temperature gradients will be created which will cause solvent gradients. A
small amount of SU-8 may solidify on the wafer surface. Subsequent spinning
will coat over this area so the transition will not go away no matter how
much you spin.

The second problem is most likely related to tiny bubbles in the film
invisible to the eye. During softbake these bubbles will expand and become
visible. The bubbles won't shrink when you cool down because the film will
be a lot denser after the solvent evaporates. You could try transferring
the SU-8 into small vials and let it sit for days or even weeks to allow
the bubbles to disappear before using it.


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!
>
> Thanks,
> Tom
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