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.