>
> 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?
>

David Monk did some nice work on release etching while working on his
dissertation in the early 90s. Much of his work was published in Journal of
the Electrochemical Society (references to follow at end of this message).
He tried adding HCl to HF with good results. If you use concentrated HF (49
wt%) and concentrated HCl (37 wt%), a 1:1 solution by volume works well.

I should mention a few things about safety here. HF and HCl are both strong
and dangerous acids, so you should be careful with them. One thing I was
never sure about was which acid to pour first. The mnemonic for mixing
chemicals is AAA: Always add acid. But I was never sure whether to pour HF
first. Also, with both of these acids, good exhaust ventilation is
important. It's easy to get a whiff of HF or HCl in your lungs.

Finally, if it's possible you may want to think about design changes for the
next version of your device. For slowing etching oxides like LTO, the etch
rate is fairly linear with undercut distance (i.e. no diffusion
limitations). So the time to release is governed by the longest path HF must
travel to remove all of the oxide. The classic way is to punch release holes
in your device.

Good luck.

D.J. Monk et al, J ECS, Vol 140, No 8, pp. 2339-2346 (1993).
D.J. Monk et al, J ECS, Vol 141, No 1, pp 264-269 (1994).
D.J. Monk et al, J ECS, Vol 141, No 1, pp 270-274 (1994).


Bill Eaton, Ph.D.
Materials & Analysis Manager
NP Photonics
5706 Corsa Avenue, Suite 100
Westlake Village, CA  91362
www.parvenutech.com