I've looked at a lot of etch rates of silicon dioxides and silicon nitrides
deposited with various methods.
Solutions that do not contain HF (or create their own HF upon mixing) do not
etch SiO2 or do so very slowly, so you'll have to use an HF-based etch.

Now, HF solutions do etch silicon nitride, but the etch rate is slow in most
cases (PECVD nitrides etch faster), so the good news is that you can get a
high selectivity of oxide over nitride.

Look online for two papers entitled "Etch Rates For Micromachining
Processing" in JMEMS for recipes, etch rates, and discussion.

    --Kirt Williams

----- Original Message -----
From: "Andrea Mazzolari" 
To: "'General MEMS discussion'" 
Sent: Wednesday, November 29, 2006 9:04 AM
Subject: [mems-talk] Silicon oxide etch

> Hi all,
> I need to etch silicon oxide using a silicon nitride mask. Any suggestion
> about a wet etchant which will etch only silicon oxide preserving silicon
> nitride?
>