Did I understand that SiC was deposited on the SiO2?

One potential cause is that the deposition of SiC reduced the SiO2 layer at the
surface, making it unetchable by flourides. I don't know what the etch rate of
SiO would be, but I suspect it would be slow.

With HF or BOE you should get some etch of the SiO2 even with 1 um contacts.
Especially with BOE, it is always sold as "super wet" or some such thing full of
surfactants.

Also, how would you be absolutely sure that you completely etched off the SiC?

Just some thoughts.

Ed