You can use custom made SU-8 resist with modified PAG content.
Use SU-8 with lower PAG content for the first layer and standard SU-8 for
the second layer.
After exposing the first layer, spin and bake the second, expose, do the
postbake and develop both layers together.
search for papers from Zhong-Geng Ling and Kun Lian on this topic or contact
me for the exact paper references.

Regards

J.K.

-----Original Message-----
From: Chen-Han Lee
Sent: Donnerstag, 11. August 2005 14:39
To: Mems-Talk
Subject: [mems-talk] multi-layer SU-8


Hi mems-talk members,

I need some advice.
I am trying to fabricate a two-layer SU-8 structures.
the bottom layer has 15um holes and the top layer has 5um holes. the problem
is that becuase SU-8 is a negative resist, if the structure is done from
normal two layer process, the bottom will have 5um holes as the top layer.
when exposing the top layer, UV light will penetrate through the top and
exposing the bottom one too.

could anyone help me with these obstacles?