Hello,

I have three questions about selectivity of wet etchants between
silicon and silicon oxide/nitride mask.

First, some background: A friend of mine wants to etch through a
0.5-mm-thick Si(001) wafer using KOH or TMAH. Both sides of the wafer
have 2–micron-thick, SiO2. He wants to pattern the back of the wafer,
and etch holes all the way through to expose the SiO2 on the front from
behind. Without losing all the SiO2 on the front. I think he wants a
few hundred nanometers of the SiO2 left, but the more, the better.

My first question is: Where can I find information on the selectivity
of KOH and TMAH for different concentration and for different
temperature, and about adding isopropanol to the etchant?

I looked on the archives, and I see that with 30%, 80-deg KOH, I get
SiO2:Si selectivity of 100:1, and for 8% TMAH about 1000:1. I also saw
a recommendation for using 14% KOH to get the best selectivity.
Finally, I saw a reference to the Handbook of Microlithography,
Micromachining, and Microfabrication, vol 2, chapter by DL Kendall and
RA Shoultz. Do you all recommend this reference for useful information?
I will look for it.

My second question is: are there pros and cons between using silicon
oxide or silicon nitride as the etch mask?

My third question is: Is there any common tricks or pitfalls when
etching through a wafer using wet etching? I already know about the
(111) limiting planes. I don't think the shape of the holes is a
problem, but if (111) planes slow down etching too much, then does it
work to use (110) wafer?

Sorry for putting too many questions in one email! My feeling is that
5-8% TMAH is best, but if anyone has advice or experience on this kind
of etching, I would be overwhelmingly grateful.

PeAcE
Simon Garcia