Three comments:

1. I presume you increase you exposure dose for the second layer? If you
don't you will underexpose the second layer due to the lower reflection
from the substrate. Try increasing the exposure dose by 50% and see what
happens.

2. For your second layer your thermal gradient is considerably lower
than for the first layer, due to the thermal isolation of the first
layer. I presume you use a hotplate for the soft-bake. Try increasing
the bake time, but don't increase temperature beyond 115 C as the
SU-8 will autocrosslink at higher temperatures.

3. For your PEB you can try baking at 35 C overnight before doing you
normal PEB. The trick is to cause the SU-8 to crosslink sufficiently
below the Tg of un-crosslinked SU-8 (50 C). If you exceed the Tg
before it has crosslinked sufficiently, it will flow and then crosslink
in the distorted shape.

Regards

Jacques Jonsmann

________________________________

Fra: [email protected] p=E5 vegne af Shao Guocheng
Sendt: ti 02-12-2008 03:19
Til: General MEMS discussion
Emne: Re: [mems-talk] very thick layer of SU-8



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