I don't think it is feasible, the etch rate of nitride in BHF is low, even if it
is PECVD nitride.

>From: "Aleksandar Tomovic" 
>Reply-To: General MEMS discussion 
>To: mems-talk@memsnet.org
>Subject: Re:[mems-talk] wet etch of SiN revisited
>Date: Mon, 28 Dec 2009 12:27:40 +0100
>
>Dear All,
>
>i need to do a wet etch of SiN (don't have equipment for dry etch),i read that
>it is  possible to do a wet etch with BHF, while using photoresist as etch
>mask, but i also read somewhere that the photoresist will last only for a
>short period of time (20 min). My SiN layer is 180nm thick. If anyone could
>give me more data on this subject i would appreciate it.
>
>Thanks in advance!
>
>A.Tomovic