Mikas,

For etching Al I have in my lists 10% potassium ferrocyanide (cautions
to avoid any acids due to the generation of poisonous hydrogen cyanide),
19:1:1:2 of phosphoric acid (85%): acetic acid: nitric acid (70%):
water. The second one may still etch GaAs because of phosphoric acid, so
I would try the K3Fe(CN)6.

Best of luck,

David Casale
R&D Engineer
www.maxlevy.com
Max Levy Autograph
2710 Commerce Way
Philadelphia, PA 19154 USA
tel: 215-842-3675