Selective etch of SiO2 over SiCr

Hi,
Does anyone know of a good selective etch (either dry or wet) that etches SiO2 (or SiNx) but stops on SiCr (or other thin film resistor
materials such as WSi2)? Will BOE work?
thanks Erji
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BOE would work but would most likely attack the SiCr. Why not put an "etch stop" layer between these films ---something that BOE would not etch and then etch the etch stop away with another etch that does not attack Si Cr??
Kitty
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