Since the smaller structures seem to come off preferentially I would
suspect your exposure dose is insufficient. Examine a section of the
PDMS structure to see what kind of profile you have - a negative slope
could indicate insufficient dose.
Also, it is not recommended to do the PEB in an oven. Use a hotplate
with a two stage bake and ensure you ramp slowly both up and down
(usually it's OK to let it heat up and cool down with the hotplate. Also
forget about the hard bake as it really isn't necessary for PDMS
moulding and can actually contribute to poor adhesion rather than
prevent it.


Abhishek Jain wrote:
> Hi
> I am coating SU8 2025 on Si wafer with a target depth of 30um.
> I prebake in a hot plate using the ramp up procedure, i.e. 65C for 2
> min and 95C for 4 min.
> My exposure time is around 100sec and I post-bake the wafer after
> exposure in an 80C oven for 90 min. Because I want to do PDMS replica
> molding with SU8 masters, I also hard bake the SU8 wafers for 1 hour
> in 100C after development for better adhesion of resist.
> When I lift-off the PDMS, I see that my thinnest features, ~10um size
> features peel off along with the PDMS. Thicker features stay intact.
> Please advice if you may have any clue to this problem.
> r