Effect of annealing temperature on structure and stress properties of Ta<sub>2</sub>O<sub>5</sub>/SiO<sub>2</sub> multilayer reflective coatings

In the optical system of spaceborne laser altimeter, dielectric mirror is an indispensable optical film element. Its surface shape quality directly affects the resolution and accuracy of distance measurement of the detection system. It is pressing and necessary to carry out research on the surface shape control technology of dielectric mirror to eliminate or reduce the effect of film stress on surface shape. The Ta₂O₅/SiO₂ multilayer reflective coatings are deposited on quartz substrates by using the ion beam assisted electron beam evaporation (IBE), and then annealed in air in a temperature range from 200 to 600 ℃. The effect of annealing temperature on the structure, optical and stress properties of Ta₂O₅/SiO₂ multilayer reflective coatings are systemically investigated by using x-ray diffraction, atomic force microscope, spectrophotometer and laser interferometer. The results show that all the Ta₂O₅/SiO₂ multilayer reflective coatings, after being annealed, are amorphous in structure. The annealing temperature has a great influence on the surface roughness of reflective coating. With the increase of annealing temperature, the surface roughness of reflective coating first decreases and then gradually increases, but is still smaller than that of as-deposited sample. After being annealed, the reflectance spectrum of reflective coating shifts slightly toward the long-wave direction, and the reflectivity increases a little. When being annealed at 500－600 ℃, the compressive stress of reflective coating could be transformed into tensile stress, and the surface is changed from convex to concave shape. It can be concluded that annealing at an appropriate temperature can effectively release residual stress of Ta₂O₅/SiO₂ multilayer reflective coating and eliminate the deformation of substrate caused by film stress, and thus improving the surface shape quality of dielectric mirror., After being annealed, the reflective coating still possesses the stable structure and spectral properties, so that dielectric mirror can meet the application requirements of spaceborne laser altimeter. In this paper, the experimental results are of great significance for applying the annealing technology to the surface shape control technology of dielectric mirrors.