[mems-talk] Dimples in SU-8 film

[Yue Mun Pun, Jeffrey]

Hi,
I am trying to coat 2-3 layers of SU-8 2050.  My process is aas follows:
 
1.  Dehydration bake SiO2 wfr at 120'C for 2mins 
2.  Spin coat SU-8 2050 at 500rpm and ramp up to 3000rpm for 30sec.
3.  Soft bake the first layer at 65'C for 3mins (ramped from 50'C to 65'C and hold for 3mins)
4.  Return wafer to spin coat another layer at the same conditions stated in 2.
5.  Soft bake wafer at 65'C for 5mins and ramp to 95'C for 20mins, since the layers now have a nominal thickness of 100 microns.
 
I noticed that while baking at 95'C the SU-8 film developed dimples.  These may or may not be an issue for my process, but they are cosmetically ugly.  Can anyone tell me how do I solve this dimple issue?
 
Jeffrey