Regarding the craters (for searching the problem also try dimples and
divots!), I believe there are a number of possible causes.
Surface contamination is one but perhaps unlikely in your case as you have
taken steps to solve this. (In the past, I had solved the problem by
changing the conc. H2SO4 I was using for wafer prep. but in another
cleanroom we had the same problem which we couldn't get rid of.)
Micro bubbles are another cause as Andrew pointed out. To help the bubbles
come out you can also try placing the closed bottle in an oven at 60C for a
few hours. Aliquoting into smaller vials would be a good idea in this case
too.
Particulate contamination is also another possibility. Often dried resist
from the neck of the bottle can fall back in. Filtering may be an option
but I am not sure how exactly.
In my experience we have had far more trouble with the 2000 series than
with old GBL series. In one cleanroom we pretty much gave up as we still
had these craters after ordering a fresh bottle from Microchem. I have a
feeling it could have been airflows and particulate contamination in the
air around the spinner / hotplate but never really got to the bottom of it.


On 11 November 2012 11:47, Andrew Sarangan  wrote:

> 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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