Bilal,

I think Transene's CRE-473 or something similar (usually just called "Chrome
Etch") will work for you. Most cleanrooms keep this in stock for Cr etching..
It's an orange solution, usually slightly viscous.

If I recall correctly, it will take off the Cu pretty quickly. I know it
won't touch the Ag, and I'm pretty confident it won't touch the Ni at any
appreciable rate either.

I'm not fully confident of this, so try it with an patterned wafer first.
And, of course, if it does work, please post the results back on the list.

- Kevin

> From: Sardar Bilal 
> Reply-To: General MEMS discussion 
> Date: Wed, 28 May 2008 21:47:16 +0300
> To: 
> Subject: [mems-talk] Selective etching between Cu and Ni / Ag
>
> Hello folks !
>
> I have a question related to metal etching. I want to etch my Cu seed layer
> (100 nm by sputtering). Its a bit tricky since I also have about 100 nm of
> Ni and 100 nm of Ag on my wafer which I want to keep. I have tried 10%
> nitric acid at room temperature, but it etches Cu and Ni at almost the same
> rate, though silver is not etched significantly. I have also tested 10% HCl
> and i observed the loss of adhesion of some Ni structures (strange!) and Cu
> was not etched significantly. When the sample was taken out of HCl, it
> oxidized and Cu color changed to purple shades. I would appreciate if
> someone can recommend some wet etchants that can selectively etch Cu and
> have low etch rate for Ni and Ag.