Hello Folks,

I have a problem with SiO2/Si etching.

After Bosch etching from the back side of 400 um thick SOI wafer, 120 um x
120 um square membranes (SiO2 100 nm/Si device layer 3 um thick) were made.
Buried oxide layer was HF-vapor etched.
On the front side, I have a 10 um long, 250 nm wide Pt/Cr metal line and
want to suspend this metal beam by etching SiO2/Si.
Only 10 um long 5 um wide rectangle around the metal beam must be etched,
not the whole membrane area.

After electron beam lithography to define a etching window, the sample was
immersed in 1:6 BOE for 90 s (90 nm/min etch rate) and 25% TMAH at 70C for
12-13 min.
The problem is after TMAH etching, the whole 120 um square membrane was
damaged or etched maybe because once a window opens up on the bottom of Si
device layer, TMAH etches Si from the back side as well.

Any idea to control Si etching?
I desperately need to have SiO2/Si membrane with 10 um/5um rectangle
thru-etched window.

Thanks

TK