[mems-talk] Multi layer SU-8 process

[Gareth Jenkins]

Hi

A filter may be the answer. What did you use for exposure before?
If you have deep UV, you will be preferentially cross-linking the top of 
the layer. I have not heard of this causing a skin (normally T-topping 
or negative side walls) but it could be possible that there are some 
strange propagation effects between the layers or that differences in 
stress during PEB due to unevenly cross-linked layers could cause problems.
I would agree with Jeffrey that hotplates are better than ovens but 
since it was fine but then you had problems after you got the new 
aligner it seems more likely optical effects are to blame!
Gareth

Dubnisheva, Anna wrote:
> 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.