Au and Ag have similar crystal structures so they will interdiffuse
fairly easily.  If Ag does stick strongly to silicon (I wouldn't
expect it to, but I don't have experience with this) at least
initially, then after fabrication Ag will diffuse into the Au.  Once
all the Ag is gone from the interface, the Au will most likely peel
off, or there is so much Ag in the Au, that the Au isn't usable any
more.  For this application, you wouldn't want to use two metals whose
phase diagram is a simple lens.

Cr will diffuse into Au to some extent, though the solubility of Cr in
Au is much less than Ag.  Cr's failure mechanism is to diffuse to the
Au surface and form brown oxide grains at a fairly low temperatures.

For products, people not only use adhesion layers, but
diffusion-barrier stacks, i.e., Cr-Pt-Au, Ti-TiN-Au, Ti-Ni-Au, etc. to
prevent this type of problem even more.

Gabriel


---------- Forwarded message ----------
From: [email protected]
To: General MEMS discussion 
Date: Mon, 13 Mar 2006 13:30:19 -0500
Subject: [mems-talk] can Ag be a adhesive layer between Au and Si?
Hello All,

people usually use Ti or Cr as an adhesive layer between Au and Si,

as far as I know, Ag can stick on silicon too, why people do not use
silver as an adhesive layer?

best

Yu Chen