I remove Al from GaAs with Shipley Microposit MF-319 developer, which is
the developer for S1813 resist - I think there should be a pretty wide
range of developers which are chemically similar though (it's TMAH + H2O
+ surfactant).  It works much better than HCl, in my experience.

I haven't tried it with Al on top of PMMA, but would be fairly surprised
if it caused a problem.

Andy

David Casale wrote:
> 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