I want to etch away the Si base substrate of SOI wafer from back side and
remain the top thin Si layer and buried oxide layer.  I try the wet etching
in both ways (using TMAH or KOH solution). The SiO2 or Si3N4 deposited by
PECVD were used as protection mask layers (both side: top and back). However
SiO2 or Si3N4 can not survive very well in the hot sulution for long term
(several hours). Some local areas of top Si layer of SOI was etched
away. Could you guys kindly give me some suggestions? Is this due to the
quality of SiO2 or Si3N4 layer? The quality of SiO2 or Si3N4 by PECVD may be
not very good , so the SiO2 or Si3N4 can not survive.
Some dry etching suggestion is aslo welcome.

Thank you!

Weiquan Yang
EE department
University of Texas at Arlington