Plasma etching is the most common method of Si3N4 in the semiconductor
industry.  Depending on the amount of undercutting you can tolerate and the
underlying film,  CF4 with 4% O2 is the least costly.
We can locate new or used equipment.
What are your requirements?
Photoresist used and critical dimensions?
Size and type substrate?  How many to processed per run?

Jack Shelton,  gwht@aol.com
SWS
Phoenix, AZ