Characterization of HfO2 Optical Coatings Deposited by MF Magnetron Sputtering

The aim of this work is to determine the influence of medium frequency magnetron sputtering powers on the various properties of hafnium dioxide (HfO2) thin films. Microstructure observations show that an increase in the sputtering power has a significant influence on HfO2 films' microstructure. As-deposited hafnia thin films exhibit nanocrystalline structure with a monoclinic phase, however the rise of the sputtering power results in an increase of crystallite sizes. Atomic force microscopy investigations show that the surface of the deposited films is smooth, crack-free, and composed of visible grains. The surface roughness and the value of the water contact angle increase with the increase of the sputtering power. Measurements of the optical properties show that HfO2 coatings are transparent in the visible wavelength range. A higher sputtering power causes a decrease of an average transmittance level and a simultaneous increase of the real part of the refractive index. Nanoindentation measurements reveal that the thin film hardness and Young's elastic modulus increase with an increase in the sputtering power. Moreover, the results of plasticity index H/E and plastic resistance parameter H-3/E-2 are discussed. Based on the obtained results, a correlation between the sputtering power and the structural, surface, and optical properties, as well as the hardness and Young's elastic modulus, were determined.