Dear KC

SiO2 exhibits excellent masking property in TMAH solution. I use thermally grown
oxide layers of about 0.9~1um thickness in 25 wt% TMAH at 80 C for more 10 hours
in order to etch {100}Si.

I think, in your case, oxide layer is very thin (500 A as you mentioned) and
porous. I suggest you to deposit thick oxide layer and do the etching in 5-10
wt% TMAH if etched surface roughness is not the issue, as the low concentration
provides high etch rate but rough etched surface morphology. If you have the
thermal annealing facility, anneal you oxide deposited wafers at 1000 C for
about one hour.

Cr-Au metal layer can use as mask in both KOH and TMAH. I think in your case
adhesion of deposited layer is not good. So before using the Cr-Au as mask,
check the adhesion of metal layer.  It can simply be done by scotch tape test
method.

Best regards
Prem