Influence of thermal annealing on mechanical and thermoelastic characteristics of SiO₂ films produced by DIBS

Mechanical and thermoelastic charecteristics of optical films are important to ensure the performance of optical coating systems. SiO2 films were prepared by dual ion beam sputtering (DIBS) on Si 110 and Schott Q1. The mechanical and thermoelastic properties of films as-deposited and annealed were systematically investigated. The results show that reduced Young's modulus (Er) is elevated to 72 GPa while the film was annealed at 750℃, and hardness (H) increases to over 10 GPa. The as-deposited films show compressive stress and the stress could be dramatically released while annealed over 450℃, incicating that heat treatment could improve the internal stress of SiO2 film. Poisson's ratio (vf) of annealed SiO2 films is around 0.18, and Young's modulus (Ef) of as-deposited and annealed films is larger than that of fused silica, and elevated over 50 GPa while annealed at 750℃. Thermal expansion coefficient (f) decreases from 6.7810-7℃-1 to the minimum value 5.2210-7℃-1 while annealed at 550℃.