Ditto what Shay said. In the first etch-rates paper I listed a variety of reasons for etch-rate *variation*. One is a galvanic effect when two metal are on contact with each other and also with the etchant. The less electronegative metal (aluminum in this case) gives up electrons more easily and will probably etch much faster when in contact with copper than aluminum sitting on an insulator. --Kirt Williams -----Original Message----- From: mems-talk-bounces+kirt_williams=sbcglobal.net@memsnet.org [mailto:mems-talk-bounces+kirt_williams=sbcglobal.net@memsnet.org] On Behalf Of Shay Sent: Wednesday, June 15, 2011 6:47 AM To: 'General MEMS discussion' Subject: Re: [mems-talk] aluminum etchant vs. copper vs. aluminum Hi, You should be careful with etch rates of structures. Two materials that are in contact behave very different in etching environment because of the different electro chemical potential. I suggest that you run a test on a real small area. Also, try using copper etchant. The FeCl stuff. It might work better. Shay