You cannot do serious SU-8 lithography without a UV-filter. Get one and =
most of your problems will go away!
The underexpose/overexpose/surface skin variations are typical when =
exposing with shortwave UV.

Regards

Jacques Jonsmann

________________________________

Fra: mems-talk-bounces@memsnet.org p=E5 vegne af Dubnisheva, Anna
Sendt: ti 24-04-2007 19:57
Til: mems-talk@memsnet.org
Emne: [mems-talk] Multi layer SU-8 process

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  =
 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   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.