Study of aging effects on optical properties and residual stress of HfO2 thin film

Hafnium dioxide (HfO2) thin films are generally used as high refractive index layers in optical coating devices. Aging effects on optical and mechanical properties of the films are very crucial for long-standing performance and stability of the devices, therefore knowledge of the aging effects is very important. In the present work, HfO2 thin film has been prepared using electron beam evaporation. The optical properties and residual stress of the film have been investigated in six months interval over a period of one-year in ambient condition to probe its aging behaviour. The measured refractive index increases with aging and its change is Delta n = 0.011 at wavelength of 550 nm. The dispersion parameters such as epsilon(L), N/m*, epsilon(s) and E-d increase with aging, while E-0 decreases. The measured film residual stress is tensile in nature. The residual stress decreases with aging and the change in stress in the film is around 78% in a year. A simple model has been utilized to establish and explain a correlation between the measured stress and the film porosity which varies with aging time. The results suggest that the change in the residual stress is significant in contrast to a very small change in the refractive index with aging.