Wet etch of an isotropic substrate is isotropic itself. All the none
crystalline and fine grain polycrystalline materials can be considered
isotropic.

So when predicting the shape of a feature after wet etching, you merely
need to draw a radius at the edge of the resist opening of your feature
with a length set by the etch time X etch rate.

For example, if you had an opening of 2 microns in resist, and you had a
film 1.0 thick, and you did a 50% overetch, your etch length would be
1.5 microns and your feature would be 5.0 at the top, and 3.0 at the
bottom. (5 = 1.5 + 1.5 + 2.0) (3 = .5 + .5 + 2.0 roughly).

For anisotropic etches you have different etch rates depending on the
direction. For cystalline substances it would be perpendicular to the
different crystal planes. You would have to modify accordingly. Taking
the above example, assuming the etch rate latterly was half of the
vertical etch rate, then you would have a feature with the top 3.5
microns, and 2.5 at the bottom.

I would try to do some rough calculations by hand. Engineering existed
prior to simulation software.

Ed Sebesta

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Evelyn B
Sent: Sunday, March 22, 2009 10:49 AM
To: General MEMS discussion; Evelyn Benabe
Subject: [mems-talk] Aspect Ratio


Hi all,

In reviewing my process flow using Coventorware I have noticed some
aspect ratios that seem quite low.  They vary from 0.25 and can get as
low as 0.01.  Can anyone provide a good rule of thumb for an aspect
ratio using wet processes?