Study of the evolution of refractive inhomogeneity in HfO₂ thin films

To study the evolution of refractive inhomogeneity in hafnia thin films, single layers with different thickness were prepared on BK7 substrates by Electron Beam(EB) evaporation process. The spectral measurements showed that the inhomogeneity of HfO2 thin films was significantly affected by the layer thickness. X-ray diffraction(XRD) measurements showed that the crystalline microstructure of HfO2 thin films had direct influence on their inhomogeneity and the microstructure was determined by thin film growth mechanism. Due to amophous surface of BK7, thinner films are hard to crystallize and the refractive index of the HfO2 thin films are prone to the positive inhomogeneity. If the deposition temperature is high enough, films reaching a certain thickness begin to crystallize, which causes more and bigger voids in the film and results in negative inhomogeneity. When film grows thick engough, the microstructure tends to being stable, and the layer inhomogeneity is independent of thickness and fixed to a certain value.