Mechanical and thermoelastic characteristics of optical thin films deposited by dual ion beam sputtering

Abstract

Mechanical and thermoelastic properties of optical films are very important to ensure the performance of optical interference filters and optical coating systems. We systematically study the growth and the mechanical and thermoelastic characteristics of niobium oxide (Nb(2)O(5)), tantalum oxide (Ta(2)O(5)), and silicon dioxide (SiO(2)) thin films prepared by dual ion beam sputtering. First, we investigate the stress (sigma), hardness (H), reduced Young's modulus (E(r)), and scratch resistance. Second, we focus on the methodology and assessment of the coefficient of thermal expansion (CTE) and Poisson's ratio (nu) using the two-substrate method. For the high refractive index films, namely, Nb(2)O(5) (n at 550 nm=2.30) and Ta(2)O(5) (n at 550 nm=2.13), we obtained H approximately 6 GPa, E(r) approximately 125 GPa, CTE=4.9x10(-6) degrees C(-1), nu=0.22, and H approximately 7 GPa, E(r) approximately 133 GPa, CTE=4.4x10(-6) degrees C(-1), and nu=0.27, respectively. In comparison, for SiO(2) (n at 550 nm=1.48), these values are H approximately 9.5 GPa, E(r) approximately 87 GPa, CTE=2.1x10(-6) degrees C(-1), and nu=0.11. Correlations between the growth conditions (secondary beam ion energy and ion current), the microstructure, and the film properties are discussed.