Hi,
I am trying to wet etch SOI wafer from back side and stop the etching at
the SiO2 layer. To do that, I first PECVD a 5000 A silicon nitride layer
on both side. Then I opened window on backside and used KOH to etch it.
What suprised me was that after one hour etching, the silicon layer under
the top silicon nitride protecting layer was attacked. And I can see a lot
of small crack formed in the silicon nitride layer, while the silicon
nitride layer on back side looked much better (But also have some crack).
There are two possible reason I am suspecting. One is that before silicon
nitride PECVD, I did not clean the wafer specially (take it out directly
from the shipping box and put it into the reactor). The other is, just my
guess, the difference thermal expand coefficient between top silicon layer
and the sio2 layer under it might form some small crack on the silicon
during the high temperature of PECVD process.
I don't know which one is more possible. And if it is the first one, does
any of you know how to get rid of the si3n4 layer which has been grown
without attacking the silicon layer underneath.

Thanks a lot!!!

Peng Yao
DOEs lab
Electrical Engineering Dept.
Univeristy of delaware
Newark D.E 19716