Ned,

I would choose tungsten over the chromium. The galvanic couple is still
going to exist, but the peroxide is not going to damage the Al, whereas the
ceric nitrate acid mixture could pit the Al especially if it is an alloy
like Al-1%Cu.

Glenn Silveira had a better solution if you have the capability to deposit a
non conducting silicon dioxide or glass. In this case there will be no
galvanic cell. Since you mentioned only Cr and W I assumed you only had
sputtering capability.

Regards,

Bill
----- Original Message -----
From: "Ned Flanders" 
To: "General MEMS discussion" 
Sent: Tuesday, January 03, 2012 6:30 AM
Subject: [mems-talk] Chromium as a mask for wet etching of aluminum


> Hey all,
>
>
> this may sound a bit weird, but I have my reasons: I need to wet-etch
> (phosphoric acid as enchant) an aluminum wafer. I must use an
> inorganic mask for the purpose, but my favorite would be chromium:
> deposit a thin layer of chromium on my aluminum wafer, pattern it with
> ceric ammonium nitrate and then etch the aluminum with phosphoric
> acid. I need to etch some 100 um of aluminum.
>
> The big problem I have is, I don't know whether there will be some
> electrochemical effects that would prevent either of the etching
> processes. Let me know before I go and waste several hours in a futile
> attempt.
>
> By the way, happy and fruitful New Year - good health and successful
> experiments going forward!
>
> m