I am in search of a wet chemical etchant, preferably commercially
available,  that will etch doped oxides but will have a low silicon nitride
etch rate. Selectivity on the order of 500 or higher desired, but  ~200
selectivity may be acceptable

   - Nitride of interest is PECVD and has larger than 2.5KA/min etch rate in
   49% HF.
   - Oxides of interest are mainly doped (PSG, BPSG) but etchant should also
   have decent etch rate of oxides deposited  by PECVD, TEOS, HDP, SAPCVD etc.
   - Dry etch is almost impossible in our case (very deep undercuts
   desired),
   - Prefer an etchant that will not attack metals (Al, Ti, W etc) but not a
   must


LPCVD or Si rich nitride is not an option for our devices, but if you know
an oxide etchant  with low etch rate of LPCVD/Si rich nitride (e.g
<10A/min), that may be something to check out.

A quick search yielded numerous patents from especially memory and capacitor
manufacturers such as Micron, Samsung, as well as others TI, Sandia,
Motorola etc.

I am hoping there may be a commercially available etchant that I can buy off
the shelf, but any hints are welcome.

Best regards,

Erhan