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.

Thank you,
Anna