Effect of substrate bias and oxygen partial pressure on properties of RF magnetron sputtered HfO2 thin films

Abstract

Among all dielectric materials, hafnium oxide qualifies to be one of the most significant candidates as high index optical coating material due to its excellent chemical and thermal stability with SiO2, the low refractive index material, apart from its excellent laser radiation resistance. In this article, microstructural, physical, and optical properties of two sets of hafnium oxide thin films deposited by radio frequency (RF) magnetron reactive sputtering under a mixed ambient of argon and oxygen have been investigated: one set at various oxygen partial pressure and with substrate biasing by 50 W pulse direct current (DC) and another set at similar gaseous ambient as above but without any substrate bias. Structure of all the HfO2 thin films have been found to be monoclinic through grazing incidence x-ray diffraction measurements. Mass density of the samples has been estimated by grazing incidence x-ray reflectivity measurements and compared with the atom density of the samples estimated through Rutherford back scattering study. The evolutions of optical properties of the films with respect to the variation of oxygen partial pressure and substrate biasing have been investigated by transmission spectrophotometry and spectroscopic ellipsometry. It has been observed that RF sputtering with pulse DC substrate bias and with 15–30% oxygen partial pressure in the ambient helps in achieving better quality HfO2 films with low void fraction and high refractive index.