Anna,
I've noticed that you mentioned using the oven and observing a layer of 'skin'
on the SU-8.  Try not to use the oven as oven baking tends to harden the outer
layer of SU-8 leaving the bulk of the SU-8 unbaked.  That could explain the
'skin' you saw.  Try using hotplate baking throughout.

Also, when building multiple layers SU-8, try to ramp up to 65'C from room
temperature for the first temperature bake, then ramp up to 95'C on the same
hotplate, then cool down again to 60-65'C for a short period before ramping down
to room temperature.  This process takes a much longer time but it introduces
lesser thermal shock stress than if you were to introduce the SU-8 to 65'C and
then to 95'C suddenly.

If your holes are to be through-holes from the 2010 layer down to the 2100
layer, then you can lay the layers on top of each other.  Otherwise, if your
holes are to stop on the 2100 layer itself, then you will need a UV blocking
layer such as Chromium thermally evaporated on the first layer of SU-8.  SU-8
absorbs i-line UV light quite well and will be crosslinked if there is no UV
blocking layer from the areas which are not intended to be crosslinked.

Hope this helps.

Jeffrey

________________________________

From: [email protected] on behalf of Dubnisheva, Anna
Sent: Wed 4/25/2007 1:57 AM
To: [email protected]
Subject: [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.