Alvin, there are several ways prevent corrosion.  One is passivation (a passive
layer), however a passive layer is different from other corrosion free layers.
A passive layer remains thermodynamically capable of diffusion, only the
diffusion rates are very slow, combine this with the simple fact that some part
of your surface will be polarized and accelerated degradation and diffusion can
take place in the most unwanted spots.  In the ideal case you want to prevent
any form of anodic reaction. The only method to assure this is to make it
thermodynamically unfavorable to annodically react.
Rob
therefore the only 100% certian to prevent any form of anodic reaction is to
cathoddically protect the die.  In simple terms, the potential on the die must
be about 800 mv difference to a metallic package component connected to the
packaged die and conductive electrolite.  In this example the die package system
is designed to force all anodic reactions on the package and zero reactions on
the die.  We would have to list all materials, list all half cell reactions,
calculate and compare the various free energies and monitor the conditions.

> From: Alvin Barlian 
> Date: 2004/04/22 Thu PM 08:39:46 GMT
> To: [email protected]
> Subject: [mems-talk] silicon nitride passivation for saline solution/water
>
> Hi all,
> I'd like to operate my device underwater (and most likely saline solution),
thus I need to deposit a passivation layer so that it would protect metal
(aluminum) and the silicon substrate itself. Currently, I have 2500 Angstrom of
nitride in mind (after reading some literatures, I know that 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
it? Thank you so much and I greatly appreciate your help and info.
>
> Alvin
>
>
>
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