Reducing optical loss of dual-ion beam sputtered HfO2 films via optimization of coating and annealing parameters

HfO2 films are widely used for optical coatings due to the high refractive index and low absorption, especially in the ultraviolet (UV) band. In this work, HfO2 film samples were prepared with the optimized assistant source power and deposition temperature by dual-ion beam sputtering (DIBS), followed by annealing treatments in vacuum and atmosphere, respectively. For samples with different annealing temperatures from 200 to 450 degrees C, the microstructure, morphology, film stress and optical properties from 200 to 1000 nm were systematically investigated. A monoclinic phase, a refractive index inhomogeneity along the film thickness and an absorption of shoulder-shape in the 250-300 nm band were found in the as-deposited samples. For samples annealed in vacuum, 400 degrees C annealing leaded to more oxygen defects, which in turn caused aggravated UV absorption. For samples annealed in atmosphere, the shoulder-shaped absorption weakened obviously above 300 degrees C annealing, which was suspected due to the reduction of oxygen defects during the crystallization process with sufficient oxygen. Scattering loss was investigated and found negligible for as-deposited and annealed samples. Additionally, film stress varied from compressive state to tensile state with increasing annealing temperature, and the zero-stress temperature is between 300-350 degrees C, which is due to the obvious crystallization behavior. Production methods and physical mechanisms for low absorption and scattering loss DIBS deposited HfO2 films were proposed and discussed in detail.