Hi Arnaud

Unfortunately, your GaAs is likely to be etched significantly by anything
you can use to etch the Al layer.  Al will passivate effectively if you have
any fluorine present, so that should be avoided.  Your best hope is to try
wet etching with something like orthophosphoric acid.  This should remove
the Al at a reasonable rate.  As far as I know, it does not attack GaAs.

Regards, Martin

Martin Walker BSc(Tech) MSc
Tactical Marketing Engineer
Oxford Instruments Plasma Technology
North End, Yatton,
Bristol BS49 4AP  UK
T. +44 (0) 1934 837031
F. +44 (0) 1934 837001
E. 
W. 

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