Stability of amorphous alumina layers deposited on platinum films by r.f. magnetron sputtering in water solutions. A microscopic approach based on scanning electrochemical microscopy

A series of bilayers, formed by amorphous Al2O3 (thickness in the range 60-1000 nm) and Pt (thickness 300 nm) films, deposited by r.f. magnetron sputtering on a silicon substrate, was investigated by scanning electrochemical microscopy (SECM) to establish the stability of Al2O3 in contact with aqueous solutions, containing various electrolytes at different pH. Under mild acidic conditions (pH 6.5-4.6), SECM responses indicated that the Al2O3 layers were stable. To achieve more acidic (pH < 3) and basic (pH > 9) conditions, avoiding the use of harmful strong acids or bases, local H+ and OH- challenges were electrogenerated at the SECM tip through water electrolysis at constant currents. pH 2.3 and 11.6 were attained on the Al2O3 surface, as predicted by finite element simulation. At low pH, a slow dissolution kinetic was involved. At high pH, the formation of the soluble AlO2- species triggered a faster alumina dissolution, which led to the formation of etch pits. The geometric features of the etch pits, evaluated by SECM as a function of the electrolysis time, afforded to a mass dissolution rate of 6.7 (+/- 0.6) x 10(-)(1)(2) molesmin(-)(1) and to a vertical dissolution rate of 308 (+/- 15) nmh(-)(1).