The information on the subtle local structural modifications around the Hf atom in HfO2 high-k dielectric stacks has been demonstrated to be crucial in determining the resulting electronic properties of the complementary metal oxide (CMOS) devices. In this work, using extended X-ray-absorption fine structure (EXAFS) spectroscopy, the local structural characterization of Hf-based advanced gate stacks thin films were investigated. The thin film stacks used in this project are Al2O3 capped HfO2 thin films deposited on silicon substrates. EXAFS simulations and fits were applied to the data in order to extract crucial structural modifications on these films upon postdeposition annealing (PDA). The local crystal symmetry and coordination around the Hf atom were investigated under various annealing conditions. Specifically, in Hf-based dielectric thin films on silicon substrates capped with Al2O3 layers, the local crystal symmetry, and coordination around the Hf atom were investigated under various annealing conditions. The questions addressed are the effects of various annealing mechanisms to the diffusion mechanisms in the cap layers and modifications on the emerging local structures around the Hf atom. The diffusion of Al into HfO2 films were monitored through the EXAFS simulations. The non-linear least-squares fitting to the EXAFS data revealed that the PDA in NH3 (or N2) ambient after HfO2 deposition had prevented Al diffusion to the HfO2 layers.