Erkin,

The problem here is that, especially with spin coating, the topography of the
posts will cause eddies to occur in the resist film leading to non-planarity.

There are a couple of approaches to look into:

1) Try aiming for a thicker layer.  However, this can be difficult, since with
spin coating, the resist film will tend to need to be much thicker than the
topography (probably >5 µm).

2) Spray coating: if you have this capability available to you, you would be
able to place down a 2-3µm layer of material that should maintain a good degree
of planarization during the soft bake.  We've had some pretty good results with
planarization of some pretty extreme topography.

I know I've listed this link before, but to keep it easy, take a look at the
following link to Micro Magazine:

http://www.micromagazine.com/archive/04/03/brubaker.html

Specifically, you'll want to look at figure 8b.

Best Regards,
Chad Brubaker
-----Original Message-----
From: erkin seker
Subject: [mems-talk] Surface Planarization

I have 1-um-thick, 100um-by-100um square SU8 posts, and I want to fill
around them with a sacrificial layer preferably photoresist.  When I spin
coat the photoresist (approximately 1-um-thick), and develop away the resist
over the posts, raised lips appear at the edges of the posts.  I tried
oxygen plasma to planarize the surface to some degree while depositing
multiple photoresist layers to reduce the lip formation, but it doesn't work
too well.