Hi,
        Actually, you may want to try using Phosphosilicate Glass (PSG)
as a possible alternative SACRIFICIAL LAYER, which has a high selectivity
to SILICON NITRIDE, when used in conjunction with 25-49 Wt% HF.

The PSG etchrate was a strong function of HF dilution. There was not much
discernable differnce in PSG etchrate at room temperature and at 40degC.

I came across a paper few years ago by David Monk in the IEEE conference on
solid state sensors and actuators, 1991/2(?).

Phil


> ----------
> From:         X. Yuan[SMTP:xyuann@yahoo.com]
> Reply To:     mems-talk@memsnet.org
> Sent:         02 October 2002 01:31
> To:   mems-talk@memsnet.org
> Subject:      [mems-talk] SiO2:Si3N4 etch selectivity in HF
>
> Hi,
>
> I released my polySi device (LTO as the sacrificial
> layer) in concentrated HF. However, due to
> the long release time, the field nitride (LPCVD
> 850C) got completely etched away. Does anyone
> know a good recipe which can etch oxide fast
> while has a very good selectivity over nitride?
>
> Thanks.
>
> Xyuan
>
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