Hello Sudarshan,

Unfortunately, you cannot etch high aspect ratio holes in 110 silicon
using wet chemical anisotropic etching (I assume this is what you are
trying).  The slanted faces are buried 111 etch-stop crystal planes.
However, you can make long narrow trenches through the wafer with 111
faces, as the buried planes will only show up at each end.  If you think
about the geometry of the 111 planes in the Si diamond lattice, this
effect is fairly clear.

It sounds like your requirements are better suited to an RIE (reactive ion
etch) process.  An electrochemical etching process may also work, but I
think this would require considerable effort to optimize (If interested,
you can check the "porous silicon" literature.  You may be able to form a
porous silicon column in the dimension of your holes.  Then take your pick
of etchants to remove the porous material - KOH, TMAH, EDP, etc.).  If you
have access to an RIE system, that's the preferred option, though.

Good luck!!

---Chris

Christopher C. Striemer, Ph.D.
Research Associate
Department of Electrical and Computer Engineering
University of Rochester
Rochester, NY 14627

-----Original Message-----
From: hegdesudarshan@gmail.com
To: mems-talk@memsnet.org
Sent: Mon, 16 Apr 2007 10:26 AM
Subject: [mems-talk] Etching Vertical Holes in Si 110 wafer


The objective is to etch through vertical holes of 100 micron dimension in
Si wafer of 300 micron thickness.
I am using Si 110 wafer.
I have tried out a parallelogram with one of the angles equal to 70.6 and
the other as 109.4.
But each time, I am getting slanted faces which results in stopping of
etching.
Please help me regarding regarding this if at all vertical holes are
possible.