[mems-talk] Oxygen precipitates & KOH etching

[Shivalik Bakshi]

Hi folks:

I am told that if a silicon wafer has a high oxygen content, the oxygen can 
form a precipitate during a high-temperature process (1000C - 1100C). And if 
that wafer is etched in KOH subsequently, the etchant tends to attack the 
(111) sidewalls resulting in larger cavity sizes than expected and rough 
(111) sidewalls. I have observed this on 2 of my wafers recently.

I spoke with a couple of wafers vendors regarding oxygen content of wafers 
and was told that typical silicon wafers for MEMS applications have a oxygen 
content in the 10 - 30 ppma range.

Can anyone recommend a spec for Oxygen content so that it does not form a 
precipitate and cause subsequent KOH etching issues?

Thanks a lot,

Shivalik
Cambridge MA