[mems-talk] Su8-2100 soft bake

[Gareth Jenkins]

Barbara Cortese wrote:
> Dear all
>
> Recently we met a problem using SU-8 2100. When spinning (at 1000rpm),after softbake treatment, waves appear on the layer. The hotplate I use is levelled, and I am using as a substrate silicon. The soft bake lasts 15 minutes at 65°and 60 min at 95°. I tried longer times of soft bake but the problem is always the same, and the layer is always soft.
> Can you help me to solve this problem? What do you think the problem is caused by?
>
>   

Do the ripples occur during the PEB? Do they occur only in exposed 
regions or in both exposed and unexposed. If they occur only in exposed 
regions, then you need to increase you exposure dose. If they occur in 
both regions then it is likely you softbake is insufficient.
I have recently been trying exactly the same process (SU-8 2100 at 
1000rpm) and found I needed about twice the recommended baking time (2 
hours at 95C). This is related to the fact that I am getting thicker 
than expected layers (see my previous thread).
There is a way to determine the optimum baking time which is to take 
your substrate off during the 95C bake and allow it to cool to room 
temperature. Then put it back on the hotplate and if you see ripples 
occur then leave it on for longer (the glass transition temperature of 
uncrosslinked SU-8 is around 60C. Above this temperature ripples should 
planarise so you should see ripples appear and then disappear). Repeat 
this process untill you do not observe ripples.
Also, have you measured the thickness of your layers? I would be 
interested to know if you have a similar problem as myself with thicker 
than expected layers.

Gareth