Preparation Of HfO₂/SiO₂ And Nb₂O₅/SiO₂ Laser High-reflective Films On SiC Mirror

In recent years, with the rapidly development of laser technology applied in controlled nuclear fusion, simulated explosions and laser weapons, the design and implementation of high-power laser systems brings forward rigorous demands for the high reflectance and laser-induced damage threshold of reflector components. In this work, the preparation and processing of the mirror substrate, damage characteristics of single laser film, the design, preparation and damage properties of multilayer high reflective films were systematically studied. Our results have great significance and practical value for improving the reflectance and laser damage threshold of laser high-reflective films.Silicon carbide（Si C）, with excellent physical and chemical properties, has become an ideal material for high performance mirrors in special environment.However, there was seldom report about its use as the laser mirror substrate to deposit high reflective laser films. According to previous research results of our laboratory, the surface-modified Si C by compressive/tensile stress laminated Si coatings with low residual stress was polished as the mirror substrate. For the sputtered defects in the Si film surface, pre-melting process was optimized to obtain the high-quality surface.Hafnium oxide（Hf O₂） has been one of the most promising anti-laser film materials and widely used in high power laser systems due to high refractive index,low absorption at the wavelength from 351 nm to 1064 nm, good thermal stability,chemical stability and mechanical properties. There are few reports on the study of the intrinsic relation among the deposition mechanism, microstructure, and laser ablation characteristics of Hf O₂ thin films. Therefore, this work focused on the preparation of high-LIDT Hf O₂ films deposited by reactive plasma assisted electron beam evaporation from metal hafnium Hf, in which nodule defects could be avoided.However, strong absorption caused by the mismatch of stoichiometric ratio（O/Hf）and the cracking damage induced by high residual stress decrease the LIDT of Hf O₂ films. The impact of the growth conditions, including but not limited to the deposition rate, bias voltage and discharge current of APS ion-source, substrate temperature, and oxygen flow-rate, on the residual stress, stoichiometric ratio and laser induced damage threshold,was systemically investigated in orthogonal experimental method. Large deposition rate（>1.5nm/s） makes poor uniformity of membrane component. High APS bias voltage（160V） benefits the O/Hf ratio but leads to remarkable crystallization orientation to m（-1 1 1） which brings about high residual stress（-1700MPa）, and introduces impurities such as Ar which results in strong absorption. Temperature combines complex effects on films in terms of the promotion of chemical reaction, the transform of crystalline structures, therevaporization of films and the improvement of uniformity. The laser induced damage mechanism of Hf O₂ films were then discussed. The laser damage properties has a dependency on the microstructure of films. The damage morphology of Hf O₂ films appears in the form of agglomerations of craters with a few hundreds of nanometers. The craters can be attributed to the strong absorption and accumulation of laser energy at the grain-boundaries, which results in the evaporation of grains.Lower bias voltage, and increasing the deposition temperature benefit the crystallization orientation of Hf O₂ films from m（-1 1 1） to m（0 0 2） and uniform microstructure with less lattice defect and lower compressive stress, which brings about higher LIDT. Thermal annealing treatment and low-power ion beam bombardment can promote the regeneration of films to increase the LIDT. We believe that further regulation of APS source and the oxygen filling is expected to obtain amorphous Hf O₂ film, which is very favorable for the realization of high LIDT.Finally two kinds of high-reflective films were deposited on the Si C mirror. It has been proved that the modification of Si C surface improves both the reflectance and LIDT of laser films. The residual stress of Si O₂ can be calculated by stress model and theoretical formula, which accorded with the testing values. we believe that increasing the oxygenation and reducing the deposition temperature are expected to reduce the compressive stress of Si O₂ layer, thus regulating the residual stress of the mutiplayer. The good-quality surface can be obtained by the use of block Si O₂ ring as the evaporation source instead of Si O₂ particle. The damage morphology of the two films both appear in the form of diffused melting zones with a crater in the center. Hf O₂/Si O₂ films get higher laser damage threshold of14.97J/cm2 than Nb₂O₅/Si O₂ films with LIDT of 4.56 J/cm2, but needs more numbers of layers to realize the high reflectance than the later. Therefore we put forward that maybe the Hf O₂/Si O₂/Nb₂O₅/Si O₂ films with lower layers has important significance on the achievement of laser high reflective mirrors.