Hi David,

If you are seeing "distortion" of convex edges into many-faceted
round corners with undercutting of the masking layers there, and if
this is the same at all convex corners, then this has nothing to do
with your masking material and is a normal effect of the preferential
etch of different crystal planes.
You can improve things by re-designing your mask to include corner
compensation patterns, but you will not get a 90 deg corner out of
a 90 deg mask.
If you meant concave edges, then the above does not apply.
KOH does not attack Si Nitride and the underlying Oxide is etched
only slowly.

Greetings,
        Frank Berauer
        Senior R&D Engineer
        Hewlett-Packard Singapore


-----Original Message-----
From: Yanjun(David) Tang [mailto:yta001@coes.latech.edu]
Sent: Monday, June 03, 2002 2:22 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] (100) Si deep etching


Hi, MEMS folks:

     This is David Tang, a graduate student in Louisiana Tech University.
I'm
doing (100) Silicon deep etching (500mm). The Si wafer is coated with 0.6mm
SiO2 layer and 0.2mm Silicon Nitrate layer is coated on SiO2 layer(Silicon
Quest company). The etchant is KOH, temp 800C, stirring 220RPM and Silicon
Nitrate layer is used for masking. During the etching, I found Silicon
Nitrate
stripped at some edge area, leading to the severe distortion of expected
pattern. Was it because 0.2mm Silicon Nitrate isn't sufficient to mask
against
KOH etchant? Could you help me to improve process to obtain best pattern?
Did
someone ever deposit Au/Cr layer on Silicon Nitrate layer to help mask
against
KOH etchant? Thank you for any help.




Best regards

Yanjun(David) Tang

Graduate student
Institute for Micromanufacturing
Louisiana Tech University

1401 Tech Farm Rd. Apt. #238, Ruston, LA 71270, USA
Email: yta001@coes.latech.edu
          tang_yanjun@yahoo.com
Homepage: http://www.latech.edu/~yta001/index.htm
Tel: 1-318-255-5133
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