> 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?

Three suggestions:
(1) Use PSG (LTO SiO2 with 5-10% P2O5) instead of undoped LTO.
It etches several times faster in HF solutions.
(2) Use a weaker HF or BHF (say 5:1 BHF) solution. Based on my data,
the selectivity of etching LTO over nitride improves in more dilute solutions.
Of course, the release time will be even longer.
(3) Use a little thicker LPCVD nitride.

        --Kirt Williams Agilent Technologies