[mems-talk] Multi layer SU-8 process

[Dubnisheva, Anna]

Hello to everyone,
 
I would like to ask for advice/help in processing of multi layer SU-8. 
 
I use three layer process: 100 um SU-8 2100 + 10 um SU-8 2010 + 100 um SU-8 2100; where I spin, bake and expose each layer separately and then develop all of them together. This process used to work just fine, but then our lab got a new aligner MA/BA-6 and I also have noticed that chemically SU-8 differ from bottle to bottle - every time I open a new bottle I have to "readjust" my process all over again. I can make one layer of each SU-8 on bare Si wafer and pattern it just fine - no problem, but when I tried to put two layers together and pattern my top 10 um thick layer of SU-8 2010 (on top of 100 um thick patterned layer of SU-8 2100) with 10 um holes, the holes are either underexposed or all closed (overexposed?)  - exposure time difference in 0.5 sec gives such different results! 
The whole process I do looks like that:
1 layer SU-8 2100 - spin at 2600 rpm for 45 sec; bake 5 min @65C hotplate + 55 min @ 95C in the oven; 18 sec exposure @15mW/cm-2; PEB 30  <mailto:min at 95C> min @ 95C in the oven; then spin SU-8 2010 at 2500 rpm for 45 sec; bake 1min @65C hotplate + 6 min @95C oven; either 5 or 5.5 or 6 sec  <mailto:exposure at 15> exposure @15 mW/cm-2; PEB 8 min @95C in the oven. When I develop these two layers for 15 min the holes are either closed or underexposed or there is a '"skin" overfloating the openings (I do SEM analysis after each run) and I can't get nicely defined holes... I do not use UV filter and I was thinking to get one and try it. Also wanted to mention that bake in the oven proved to be optimal for me - I bake wafers lying on the solid metal shelf inside the oven and it seems to work fine. 
I can't get to my third layer of SU-8 until I make two layers work and so far I have no success.
Any help or suggestion would be greatly appreciated.
 
Thank you,
Anna