Crystal phase transition of HfO2 films evaporated by plasma-ion-assisted deposition

HfO(2) is a well-known high-refractive-index material for optical interference coatings from the infrared down to the ultraviolet (UV). Dense, homogeneous, and stoichiometric HfO(2) film is critical for low-loss UV optics where the spectral region is close to its fundamental absorption edge. We prepare HfO(2) films by plasma-ion-assisted deposition with various amounts of plasma-ion momentum transfer. The correlation between the HfO(2) film structure and the optical properties are evaluated by a variable-angle spectroscopic ellipsometry, indicating inhomogeneous to homogeneous to inhomogeneous film structural evolution with increasing momentum transfer during deposition. The HfO(2) film inhomogeneity, surface roughness, and crystal phase are confirmed by scanning electron microscopy cross-sectional, atomic force microscopy, and x-ray diffraction measurements. The results suggest that the HfO(2) film structural evolution corresponds to crystal phase transition from weak monocline amorphous to strong monocline depending on the amount of plasma-ion momentum transfer during film deposition.