Effect of oxygen flow on the optical properties of hafnium oxide thin films by dual-ion beam sputtering deposition

Hafnium oxide (HfO${}_2$) is an important high-refractive-index material used in the optical thin films. Dual beam ion sputtering is one of the most important methods for preparing HfO${}_2$ films. This study systematically investigated the optical, structural, and compositional properties of HfO${}_2$ films by dual ion beam sputtering deposition. The influence and mechanism of the oxygen flow of the assist ion source and vacuum chamber to prepare hafnium oxide film with higher refractive index, lower extinction coefficient, and lower surface roughness were researched. Based on microstructure measurements obtained by atomic force microscopy, scanning electron microscopy and X-ray diffraction, it was found that the film with 15 sccm oxygen flow rate in assisted ion source and zero in vacuum chamber had the smallest root mean square roughness and highest degree of crystallization. Results of optical studies showed that the film exhibited the highest refractive index and lowest extinction coefficient, whereas the oxygen flow rate of the assist ion source and vacuum chamber were 15 sccm and zero, respectively. The film was polycrystalline and comprised monoclinic, tetragonal, and orthorhombic phases. We believe this study was important for choosing proper oxygen flow parameters for the fabrication of hafnium oxide film with different applications.