Thin film reactions and interface formation in advanced high-kappa-metal-gate stacks containing rare-earth oxides have been studied. In particular, interfacial reactions of Dy2O3 and HfSiO4 with TiN- or TaCN-based metals were studied. It is shown that Dy diffusion is considerably stronger than Hf diffusion. It is further demonstrated that for TaCN-based materials, the material density and the microstructure are of much larger influence than the chemical composition.