You will always get some undercut due to the finite etch rate of the
<111> plane.

The undercut is given by H = D*R111/[R100*Sin(54.74)]

Where D is the <100> etch depth and R111 and R100 are the etch rates of
the <111> and <100> planes.  The angle between the <100> and <111>
planes is 54.74 degrees.

You may be able to minimize the undercut by adjusting the temperature
and composition of the etch bath (improve the <100> to <111>
selectivity).

Roger Shile

-----Original Message-----


I always get some undercut when etching grooves on (100) Si wafer using
KOH+Isopropyl alcohol. I use PECVD nitrate mask. Is that a regular
result? Is there any analytic equation for this undercut? Can it be
estimated?
Comments and/or references are appreciated.
Thanks.
===================================

Isa Kiyat