Vishwa,

Based on the fact that you are trying to create a PDMS mold from this
structure, I would gather that the exposed portions of the second layer
of SU-8 are all located on top of portions that had been exposed in the
first layer (creating a pyramid type structure).  If this wasn't true,
you would create over-hangs that would tear the PDMS.

The biggest problem is the inert nature of SU-8 after it has been fully
crosslinked.  What you may want to do is coat and expose the first layer
as you are doing now, and then, without performing the PEB or
developing, coat your second layer over it.  The photoactive components
in the SU-8 are not prone to migration (explaining the high aspect ratio
possibilities), and the soft bake for the second layer will cause the
first layer to crosslink.  You would then proceed with the patterning of
the second layer normally.

If some issue with re-dissolution of the first layer occurs, you could
play with the concept of partial crosslinking (reduced exposure and/or
reduced PEB).

Best Regards,
Chad Brubaker


-----Original Message-----
From: Vishwanath Somashekar
Subject: [mems-talk] Reg: problems with SU-8 bonding to the silicon wafer

Hi,
I have been trying to put two layers of SU-8 on to a silicon wafer.

Cleaning of wafer includes rinsing in ethanol and then with DI water
followed by heating oven at 120 for about an hour.

I then plasma treated(o2) the wafer to improve the adherence of SU-8
to the wafer.

I then exposed it and finally developed it after baking it.

Now to lay the second layer of SU-8, I plasma treated the developed
wafer and then spun coated this wafer with SU-8 and then followed it
by Soft bake followed by exposure to UV and then baking it and finally
developing it.

When I cast PDMS to it, a part of the second layer came with the PDMS
after baking. Could anyone of you let me how to over come this
problem.