The etching of La2O3, Al2O3, HfSiON, SiO2 and photoresist films in various concentrations of HCl and H2SO4 solutions was studied. The wet etch selectivity of La2O3 to other thin films was very high. The etch rate of La2O3 is strongly related to the H+ concentration in the both HCl and H2SO4 solutions. Further, the etching mechanism of La2O3 in HCl was investigated. The study's two major new findings are: i) etching of La2O3 thin films occurs in H2O containing LaCl3 without addition of HCl, which may result from the production of HCl in solution, and ii) addition of La(OH)(3) in HCl suppresses the etch rate of La2O3. Based on these experimental results, an etching reaction of La2O3 in HCl is proposed. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.085111jes] All rights reserved.