Hongwei,

Aluminum can not be electroplated from aqueous solutions directly due to its
tendency to form oxides that are thermodynamically quite stable. Under such
circumstances the plating industry  has developed a technique that enables
indirect electroplating by depositing Zn as an intermediate step. In a
"zincate" process", for example, in an alkaline environment (Sodium
Hydroxide and Zn salt mixture) aluminum oxide is chemically dissolved while
Zn is deposited on exposed Al sites. This Zn deposit acts as an active
substrate to be electroplated with Cu. I do not know if this can be applied
in your case. Some other methods are even more complex and aggressive to be
considered for MEMS or microelectronics applications. Perhaps the simplest
method for you would be the plasma or other type of "dry" deposition of Cu
seed layer on an oxide free Al.

Igor Kadija
www.fibrotools.com