nitride using KMER resist

Dear Colleagues:

I am forwarding one latest reply from Mr. Gary Turner about silicon
nitride about the use of H3PO4 reflux to pattern nitride.

"We/I use H3PO4 regularly. The refluxing is not a problem once you
have the equipment set up. You need to initially adjust the boiling
point by adding/evaporating water, but then once it is at the right
point, when you are done, you can cool and seal up the acid, and it
is ready to use the next time. We aim for 165 degC. Etch rate is
about 50A/min, and absolutely selective to oxide.

As a masking material, we use a technique from edinburgh university,
which is to convert the surface of LPCVD nitride to oxide in a very
wet steam oxidation. We pattern the oxide with BOE, and then strip
the resist. The H3PO4 is so selective to oxide, that the invisible
layer of oxide- maybe only a few nanometres - is enough to act as an
etch mask. The oxidation is a lot simpler than an evaporation or
sputter coating.

It's quite spectacular to see the colour of the masked pattern
developing during the etch, from a totally featureless wafer...

--- Gary Turner

Industrial Research Ltd,      Work: +64-4-569-0444 x 4220
P.O.Box 31310,                 Fax: +64-4-569-0754
Lower Hutt,  (NZST : +1200)   Home: +64-4-475-8844
New Zealand.              Internet: g.turner@irl.cri.nz