> I'd like to operate my device underwater (and most likely
> saline solution), thus I need to deposit a passivation
> layer...nitride
> is probably the best passivation scheme in this case). Does
> anybody know any reference/paper/info on the thickness of
> nitride needed in order for the nitride layer to be a good
> passivation layer for the metal and the silicon underneath

   I've found these papers to be useful:

Schmitt et al, 1999, "Passivation and corrosion of microelectrode
arrays," Electrochimica Acta 44 pp3865-3883 (especially recommended).
Buss et al, 1999, "Modifications and characterization of a silicon-based
microelectrode array," Electrochimica Acta 44 pp3899-3910.

   To avoid problems with step coverage at the edges of your metal
features, it's a good idea for your nitride coating to be at least 150%
as thick as the metal.  You'll probably get the best film from PECVD at
>300degC.  To check if your step coverage is adequate, you could immerse
the device in something that etches Al (I think HCl, HNO3, or H2SO4
would work) and see if the metal is attacked.
   PECVD silicon carbide is also reported to be a useful dielectric
coating.  If you can deposit nitride, you may also be able to deposit
carbide.  See:

Flannery et al, 1998, "PECVD silicon carbide as a chemically resistant
material for micromachined transducers," Sensors Actuators A70 pp48-55.
Stoffel et al, 1996, "LPCVD against PECVD for micromechanical
applications," J Micromech Microeng 6 pp1-13.
Cogan, U.S. Pat. No. 5,755,759.
Eriksen et al, 1996, "Protective coatings in harsh environments," J
Micromech Microeng 6 pp55-57.

Good luck,
John Maloney
Senior MEMS Engineer
MicroCHIPS, Inc.
http://www.mchips.com