Etch behavior of ALD Al₂O₃ on HfSiO and HfSiON stacks in acidic and basic etchants

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 Al ₂O₃, 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 H ₃PO₄, 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/SiO ₂/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^-.