Then your nitride is of poor quality - perhaps PECVD? I've measured the ethcrate
of Si-rich LPCVD sini in 5% BHF to 0.4 nm/min. (giving etch-rate ratio between
oxide and nitride of ~200!)

regards,
Peter Andreas Rasmussen
Postdoctoral Research Fellow
MIC - Institute for Nano and Microtechnology
Technical University of Denmark

Phone: (+45) 4525 5732 Mail: pra@mic.dtu.dk
http://www.mic.dtu.dk/