Solid phase crystallisation of HfO₂ thin films

In this paper, we report on the solid phase crystallisation of carbon-free HfO₂ thin films deposited by plasma ion assisted deposition (PIAD). After deposition, the HfO₂ films were annealed in N₂ ambient for 3 h at 350, 550 and 750 °C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) were used for the physical characterisation of as-deposited and annealed HfO₂. XRD has revealed that the as-deposited HfO₂ film is in an amorphous-like state with only traces of crystalline phase and that the annealed films are in a highly crystalline state. These results are in good agreement with the SE results showing an increase of refractive index by increasing the annealing temperature. XRR results show a significant density gradient over the as-deposited film thickness, which is characteristic of the PIAD method. The AFM measurements show that the HfO₂ layers have a smooth surface even after annealing at 750 °C. The present study demonstrates that the solid phase crystallisation of HfO₂ PIAD thin films starts at a temperature as low as 550 °C.