Hi Darren, Could you remind how thick is the first layer and second layer? Also, when you say 'the resist flow away from the edges into the centre', did you mean the resist at the edge shows poor wettability and completely flows off the first layer surface? Or, did you mean the resist at the edge just flows back a bit and the edge is thinner than the centre? If it is the latter case, I think that's normal situation for thick SU8 due to the high surface tension. But, it would be strange to see the SU8 to 'ball up'. I would suspect the first layer was baked too long. Yingtao -----Original Message----- From: mems-talk-bounces+y.tian=bham.ac.uk@memsnet.org [mailto:mems-talk- bounces+y.tian=bham.ac.uk@memsnet.org] On Behalf Of Darren Alvares Sent: 03 July 2012 03:07 To: mems-talk@memsnet.org Subject: [mems-talk] Su-8 2075 Process Hi All, SU-8 is a pretty well studied and commonly used resist and it seems in most instances I can achieve the types of results I want. However, recently I have encountered a few problems with multilayer structures. The main issue I think stems from the uniformity of the thickness. However, I could be wrong on this. I am after making a freestanding su-8 mould for use in pdms moulding. The su-8 is applied to a silicon wafer. I would like to have a thin first layer followed by my patterned top layer. The first layer seems to work ok. I use a high spin speed of 4000RPM and a long softbake (gradual ramp up and down). The exposure is done in steps to avoid any over heating and a shortened prebake is done after. The problem occurs when I spin on the second layer. During the softbake (which is designed to be around 2hrs) the resist flows away from the edges into the center. This is not an issue I think that stems from the hotplate incline as that would cause the the resist to flow to one edge. The only explainations I had were * The edge effect of the first layer means it is thicker on the outside then on the inside which means that the second layer when heated flows inward. I tried increasing the spin speed of layer 1 but this still happened. * The surface tension of the SU-8 uncured is very high causing it to ball up. I had a search online for multilayer papers and found quite a few but none mentioned this issue. Anyone experience this or heard of it in the literature? Would be great to get your thoughts. Regards, Darren Alvares _______________________________________________ Hosted by the MEMS and Nanotechnology Exchange, the country's leading provider of MEMS and Nanotechnology design and fabrication services. Visit us at http://www.mems-exchange.org Want to advertise to this community? See http://www.memsnet.org To unsubscribe: http://mail.mems-exchange.org/mailman/listinfo/mems-talk _______________________________________________ Hosted by the MEMS and Nanotechnology Exchange, the country's leading provider of MEMS and Nanotechnology design and fabrication services. Visit us at http://www.mems-exchange.org Want to advertise to this community? See http://www.memsnet.org To unsubscribe: http://mail.mems-exchange.org/mailman/listinfo/mems-talk