[mems-talk] Hairlines on crosslinked SU-8 2002 (~8um)

[Yue Mun Pun, Jeffrey]

Hi,
I have deposited 4 layers of SU-8 2002 on conductors of Gold-Titanium.  Each layer was spin coated at 3000rpm to obtain a nominal thickness of 2um.  So I should get a film of about 8um.  After exposure at 140mJ/cm2 and post-exposre bakinbg at the recommended temperature of 95'C for 4mins, I observed that there were always hairlines on the crosslinked SU-8.  Are these micro-cracks?  How do I get rid of them?
 
Is it safe to spin coat another layer of SU-8 2150 on this layer to build a multiple layer SU-8 film and develop without the whole film cracking or delaminating?
 
Jeffrey