Ultrathin dielectric capping layers are a prominent route for threshold voltage control in advanced Si devices. In this work the position of an Al 2O 3 layer inside a HfO 2-based stack is systematically varied and investigated following a low and a high temperature anneal. Electrical results are compared with a sub-nanometer resolution materials characterization, showing a diffusion of Al to the bottom HfO 2 interface. A correlation is found between the presence of Al at the bottom interface and a flatband voltage increase. Based on these findings, we propose to use the position of the Al 2O 3 for fine-tuning the threshold voltage.