You could try a long, low temperature (e.g. 45C) PEB to avoid the wrinkles
(I'm assuming the wrinkling occurs in the unexposed resist which generally
indicates insufficient softbake - if it's in the exposed resist try
increasing the exposure dose).

I guess some of the solvent from the top layer is migrating into the
previously baked bottom layer which could be why you have trouble.
Maybe vacuum baking is an option as suggested.

On Tue, Dec 2, 2008 at 02:19, Shao Guocheng  wrote:

> Hi, All:
>
>    So what kinda softbaking process u guys using for ultra-thick SU-8
> layer. I'm working with 1mm thick SU-8. My process is:
>
> Spincoating first layer(around 500um), softbake at 65C for about 2 hours
> and slowly ramp to 110c, hold for 6 hours and slowly ramp to room temp. for
> the first layer I can bake out aroun 28% of the solvent which SU-8 50 should
> have 31%. But the remaining 3% seems not a big problem and I can finish my
> first layer exposure pretty well.
>
> for the second layer, I use almost the same process, but hold at 110c for
> 8hr, but I can only bake out around 24% of solvent. Because of that, during
> my PEB process, severe wrinkle happen around 50-60C, although it will smooth
> out after a while, some of my fine structures r destorted by these wrinkles.
> I tried to lower the baking temperature to 90C and bake for 15hrs, but
> still, only 24%-25% of solvent can be bake out.
>
> Any one has a cure for this prolem? thx a lot
>
> Guocheng Shao