Try Phosphoric acid @ 155C used for etching of LPCVD, i.e. silicon rich SiN film
in Semiconductor/MEMS industry. For process details, open the Stanford Nano
Fabrication (SNF) Process link as follows;

  http://www-snf.stanford.edu/Equipment/wbgeneral/NitrideEtchWBGen.html


Ciro Chiappini  wrote:

I have to etch a 2000A thick silicon rich nitride film under a
photoresist (AZ5209) mask. I need to get a nitride mask since the next
step is electrochemical etching in EToH:HF 3:7. I am currently RIE
etching it, but the process is very time consuming.

I tried wet etch both HF:H2O 1:10 AND HF:HCL 1:1 but the resist gets
peeled off in few minutes.

Do you have any alternative recipe to suggest?