Hello everyone,

We are doing the following process to obtain 40 um tall structures using
SU-8 2035-

1. Piranha wash
2. Dry 4" Si Wafer at 150C for 30 min
3. Coat SU-8 10s at 500 rpm, 30s at 3000 rpm
4. Immediately transfer to hot plate at 65C for 3 min and 95C for 5 min
5. Expose at 17 mW/cm2 for 11s using an Abet exposure source through a
PL-360 filter placed directly on top of the mask such that the mask is
sandwiched between the filter and the coated wafer
6. PEB for 1 min at 65C and 5 min at 95C
7. Develop for 10 min in SU-8 developer and wash with IPA

We get good resolution of the features down to 10 um.  Our problem is that
we are getting white spots and streaks on the SU-8 structures making the
surfaces look distinctly rough.  Note that our development time is 10 min
(longer than the recommended 6 min) so we don't think under development is
our problem.  Our hot plates seem to be level and uniformly heated too.
Does anyone have any recommendations about how we can eliminate these
spots?  Are our soft bake times too low or could the filter being placed
in direct contact with the wafer be causing a problem?

Thanks in advance.

Best,
Dhananjay