A few years ago, I plated Cu to Al foil (ref: W. K. Chan, A. Yi-Yan and
R. Bhat, Electronics Letts, vol 28, 1730 (1992)), and found that a thin
(100 nm) layer of evaporated Ni on the Al was needed to get even plating
and good adhesion.

We did not care about the conductivity between the Cu and the Al foil,
so we did not bother measuring it.  What you can try to get rid of the
aluminum oxide layer (I assume it is just the native oxide) is evaporate
about 100 nm of Ti followed by 100 nm of Ni and heat the sample to about
300-400 C for a few minutes.  I don't have the appropriate book here,
but I think that the oxygen would rather form titanium oxide than
aluminum oxide.  During the anneal, the oxygen will diffuse from the
thin aluminum oxide leaving metallic aluminum into the thick titanium
layer forming titanium oxide dispersed in a titanium matrix.  An
alternative which is easy but not very elegant is to not do anything
about it during processing but to burn through the oxide after you're
done with a voltage pulse; the oxide is thin enough that a couple of
volts will break it down.

BTW, if you are using a copper sulfate bath, photoresist is a perfectly
adequate plating mask.

Good luck,

Winston Chan

Alan Wilson wrote:
>
> I want to Cu plate small pillar/buttons, about 50um X 50um, onto Al on a
> simulated CMOS chip.
> The idea is to test plate onto an Al coated SiO2/Si wafer, with an oxide
> mask to define the plating area.
> The problem is/will be getting good adhesion and electrical conductivity
> between the Cu and Al. The Al surface will need to be clean/deoxidized to
> achieve this. Commercial processes for doing this look rather agressive and
> include some unfortunate chemicals.
> Has anybody done this? and how did they prepare the Al surface without
> compromising any CMOS (which is protected by an oxide/Al/oxide film)
> or
> does anyone have any ideas?
>
> Thanks
>
> Alan W
>
> Dr Alan Wilson
> Defence Science and Technology Organization, Australia
>                on attachment to the
>        Center for Integrated Systems, Room 206X,
>    Stanford University, Stanford CA 94305-4075, USA.
>
> E'mail: alan@sensors.stanford.edu  (alan.wilson@dsto.defence.gov.au)
>
> Tel (+1 415) 725 1682  Fax (+1 415) 725 9020
>
>

--
Winston Chan                            | phone: (319) 353-2398
Dept. of Electrical Engineering         | fax: (319) 353-1115
University of Iowa                      | email: wkchan@icaen.uiowa.edu
Iowa City, IA 52242