For the integration of advance gate stacks, selective wet etching of an Al2O3 capping layer on top of high-k dielectrics was studied. From the fundamental etch study on the single-layer Al2O3, HfSiO and HfSiON thin films were prepared by atomic layer deposition (ALD). Using the etch rate information of each single layer, several optimized etch conditions in acidic and basic etchants including H3PO4, NH4OH, and TMAH resulted in wet etch selectivities of Al2O3 to high-k materials higher than 50:1. As a result, the Al2O3 capping layer on Si/SiO2/high-k multi-layer gate stack could be completely removed without thinning of the underlying high-k thin films. Finally, the etch mechanisms of Al2O3 in acidic and basic etchants were studied and the etch rates of Al2O3 were determined as functions of [H+] and [OH-]. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3554729] All rights reserved.