The only alternative I can offer you would be to etch approximately 90% of
the aluminum film in rie system and finish it off with a wet etch. This will
allow you much better control of your critical dimensions as the rie etch
will be anisotropic in nature. There will be a small amount of undercut by
using wet etchant but as you have found out chlorine and or bcl3 will etch
GaAs and aluminum at around the same rate. Good luck.  Bob Henderson