Hi

I tried longer softbakes and it seemed to help in one run (only a
20micron layer) but not my most recent run (with a 100micron layer on
top of a 20micron layer).

A diffraction / air gap effect could be the cause. I am also wondering
about my mask contrast. We use a photoplotter to print the masks onto
acetate film which we then stick using tape to a 5inch glass wafer to
load into the mask aligner (I take care to ensure it is flat and no
tape contacts the wafer which could cause a gap).

Examining the structures tells me there is underexposed SU-8 close to
my features but in regions which should be masked. My structures are
not very sharply defined and there is often a film of partially
exposed resist.

I think I have two things to try:

1. Eliminate any possible air gap between mask and wafer
2. Transfer my mask to a chrome mask

If anyone has tips on how to eliminate any air gap in the aligner that
would be great (NB I don't think I have a problem with edge bead as
this seems to disappear during a 1 hour relaxation step). Has anyone
had success in solving this problem using a chrome mask?

Cheers

Gareth

On Mon, Jul 28, 2008 at 20:32, Peng Li  wrote:
> Hey Gareth,
>
> Air gap between the mask and the resist could also cause such T profile in
> SU-8 due to diffraction. If you have large edge beads or non-uniform
> surface, air gap may exist when you exposing your wafer.
>
> Peng
>