Hi All,

SU-8 is a pretty well studied and commonly used resist and it seems in most
instances I can achieve the types of results I want. However, recently I have
encountered a few problems with multilayer structures. The main issue I think
stems from the uniformity of the thickness. However, I could be wrong on this.

I am after making a freestanding su-8 mould for use in pdms moulding. The su-8
is applied to a silicon wafer.  I would like to have a thin first layer followed
by my patterned top layer. The first layer seems to work ok. I use a high spin
speed of 4000RPM and a long softbake (gradual ramp up and down). The exposure is
done in steps to avoid any over heating and a shortened prebake is done after.
The problem occurs when I spin on the second layer. During the softbake (which
is designed to be around 2hrs) the resist flows away from the edges into the
center. This is not an issue I think that stems from the hotplate incline as
that would cause the the resist to flow to one edge.

The only explainations I had were

  *   The edge effect of the first layer means it is thicker on the outside then
on the inside which means that the second layer when heated flows inward. I
tried increasing the spin speed of layer 1 but this still happened.
  *   The surface tension of the SU-8 uncured is very high causing it to ball
up.

I had a search online for multilayer papers and found quite a few but none
mentioned this issue. Anyone experience this or heard of it in the literature?
Would be great to get your thoughts.


Regards,

Darren Alvares
_______________________________________________
Hosted by the MEMS and Nanotechnology Exchange, the country's leading
provider of MEMS and Nanotechnology design and fabrication services.
Visit us at http://www.mems-exchange.org

Want to advertise to this community?  See http://www.memsnet.org

To unsubscribe:
http://mail.mems-exchange.org/mailman/listinfo/mems-talk