Hi Vishwa,
I'll run through a list of tips/suggestions you could try and hopefully
something will be useful:
1) try not heating the PDMS to cure, try just letting it cure for 24-48
hours at room temperature. When you heat cure PDMS it contracts, and the
hotter you cure it, the more it contracts.
2) sometimes people use vapor deposited silane coatings to help avoid
PDMS-mold adhesion issues. If you look through literature where a Si
mold was used, rather than SU8, you will probably find mention of some
long-named silane chemical which I cannot recall of the top of my head.
Or you could try HMDS.
3) try the cold water soaking process suggested earlier to remove the
PDMS from the mold. I've also found that soaking/spraying with IPA or
methanol helps while peeling the PDMS because PDMS loves alcohol and it
lubricates everything to make separation easier. Plus solvents cause
PDMS to swell. Alcohol is a mild solvent so you can maybe get the PDMS
to pop off that way. There are super aggressive solvents that will make
the PDMS swell considerably (like toluene) but I'm not sure what that
would do to the SU8 (although cured SU8 should be fairly indestructible).

you said in your first email that you plasma treat the first layer
before spinning the second, what are you settings for that treatment?

-Joe

Vishwa wrote:
> Hello,
> As a follow up to the earlier posting, I would like to add a couple
> more expts and observations that I performed today. I desperately need
> some help as I am not getting adhesion
>
> Recipe followed
> 1) Bare Silicon wafer - Rinse with Ethanol, followed by DI water,
> Dried with N2 and then plasma treated with oxygen (1 Torr, 22w, 1
> min). When I don't plasma treat the wafer, then the SU 8 moves towards
> the center of the wafer during soft bake
>
> 2) First Layer spin coat - ramp to 500 at 100rmp/s and hold there for
> 10 sec, then ramp to 1500 rpm at 300rmp/s and hold for 30 sec. I use
> SU 8 -2025 and this rmp is used to fetch 60um thick coating
>
> 3) SEB -
> 4) Exposure - I use a film which is attached to a glass by means of
> scotch tape. I expose it according to the application notes provided
> by microchem
> 5) PEB
> 6)Second layer spin coat - at 3000 rpm to yield 30 um
> 7)SEB
> 8) Exposure
> 9)PEB
> 10) develop
>
> The SEB and PEB are done according to individual layers.
>
> The mold looks ok. When I pour PDMS( mixed in 10:1 and degassed for 1
> hr) and heat on hot plate at 100 degrees for 12 mins, and then peel,
> the second layer comes off.
>
> So, I tried to create on the second layer on the wafer and the result
> is the same. The second layer is an array of rectangular structures
> whose dimensions are 50um x 2.5mm x 30um.
>
> When I was going thru the archives, I did see that heating the wafer
> before spinning SU-8 helps. Should I do that in spite of plasma
> treating the wafer?
>
> I need some help as have no idea how to address this issue.