Hi Geng
Your problem is difficult to resolve: any Al-etchant will etch Ti in
principle. Ti is very reactive to acids and bases. Gold, plasma enhanced
oxide, nitride are resistant to many etchants and can 'survive' easily in
an etching process.
I have once observed that acetone etches Al. This requires that acetone is
warmed up and therefore this can be dangerous, both for the health and
because of fire/explosions risks.
One problem encountered when etching Al is Al-oxide that is naturally
formed on the surface of Al. This layer can be difficult to get rid off and
its thickness can vary causing different etching times for different
Al-structures. Besides etching  Al-structure should be done in one run to
avoid the formation of Al-oxide between etching processes.
I hope I have been to any help.
Mounir Ennabli.