One method I know consists of growing a thin (~ 50 nm) SiO2 layer on top
of the Si3N4 through wet oxidation (Si3N4 is very resistant to
oxidation). Then mask and etch the SiO2 with BOE, then strip the PR.
Then use hot Phosphoric acid to etch the nitride. The SiO2 will mask
efficiently the nitride against phosphoric etch. It has to be thick
enough not to have many pinholes and resist the phosphoric etch. The
oxide on top of the nitride is necessary because PR adhesion to nitride
is not good, and also the PR would be by the hot phosphoric acid.

Regards,
Pierre x286