[mems-talk] Re: su8 cracking

[oygm]

Hi, 
 
In my case, after 400mJ/cm2 exposure, only few small cracks appeared on su8 surface. These cracks disappeared after hard bake. I don¡¯t think su8 reflowed during hard bake. But the hard bake did anneal su8 and removed the small cracks. 
 
About the bubbles, no matter how carefully I operated, there is still one or two tiny bubbles produced. In my case, this did not matter much, since I only used su8 as a mask. 
 
There is another thing I do not understand. When I exposed 800mJ/cm2, the su8 can be developed more than 5mins without any cracks. Of course my structure looks ugly with this exposure dose. On the other hand if I use 400mJ/cm2 exposure, I will get nice features after 2mins developing. But if I keep developing to 3mins or longer, big and deep cracks show up again. Is this phenomenon reasonable?  The developing time should be controlled very precisely?
 
Sonic