How thick is the SU-8 you are using, and what are the sizes of your features?
I have encountered this problem in SU-8 due to under-exposing the
resist. One way to check this is to verify the dimension of the top
part of the exposed structure. If it is the same as the mask size,
then you have an under-exposed lower portion. I have also found the
dose specified on the spec sheet to be on the low side. If your
features are large, then it doesn't hurt to over-expose.


On Mon, Jul 28, 2008 at 5:30 AM, Gareth Jenkins
 wrote:
> Hi
>
> I'm currently trying to resolve a number of problems with our SU-8
> processes in a new cleanroom. One thing I have noticed in particular
> is extreme mushroom shaped structures and extreme negative sidewalls
> when using a Suss MA/BA6 mask aligner. I don't believe this is
> overexposure of the top layer due to deep UV (I believe it should be
> filtered but need to check that) since the structures are so much
> wider at the top and the effect is very inconsistent across the wafer.
> I now suspect the resist is being physically squashed during contact
> mode (it occurs in both soft and hard contact modes).
> I intend to try the following:
>
> 1. Increase my softbake to make the layer more rigid.
> 2. Try a proximity exposure.
>
> Has anyone encountered similar problems and is there any other tricks
> I could try?
> I worry about using proximity exposure since with thick layers this
> could make things worse. I have always been concerned about using a
> rigid mask with thick layers of SU-8 as good conformal contact may be
> difficult. This may not be a problem with single layers but I need to
> do multiple layers and previous layers which have been post-baked can
> cause the wafer to bow slightly.
>
> I would love to hear from anyone with experience of doing thick layers
> with a contact mask aligner.
> Best regards
>
> Gareth