Ion-beam sputtering of (Ta2O5)x− (SiO2)1−x composite thin films

Abstract

Composite (Ta2O5)x−(SiO2)1−x thin films were deposited by ion-beam sputtering using a single ion-beam gun. The composition of the composite films was adjusted by simple movement of the superposed tantalum and silicon targets. Optical properties of the films — refractive index n, extinction coefficient k and optical energy bandgap Eg — were determined as a function of composition of the films. Optical properties of the individual oxide films were also determined (for λ = 550 nm) for Ta2O5 (n = 2.12, k ⩽2 × 10−4, Eg = 4.3 eV) and for SiO2 (n = 1.485, k < 2 × 10−4, Eg = 9 eV).Composite films of (Ta2O5)x−(SiO2)1−x with uniform thickness exhibited a continuous change in their optical properties as a function of composition ratio x/(1− x). The refractive index of the composite film was found to follow an exactly linear relationship as a function of composition ratio of the film. Spectrophotometry and ellipsometry were used to measure refractive index (and other optical properties), while Rutherford back scattering spectroscopy was used to determine the composition of the films. Argon incorporation was detected in the films, originating from the Ar neutrals reflected from the sputtering targets. Its value (0.1–2.5%) changed with the composition of the composite films, being higher for tantalum-oxide-rich films. The optical bandgap position (4.3–9 eV) changed non-linearly with the linear change in the composition ratio of the film. X-Ray spectroscopy peaks of composite films (Si 2p for SiO2 and Ta 4d, Ta 4f for Ta2O5) revealed that all oxygen was bonded, confirming that films were fully mixed. All films were amorphous as detected by X-ray diffraction. Films with linear and V-shape composition variations with thickness were produced and are discussed. A single-layer anti-reflection coating was produced with n = 1.88 for silicon substrate, achieving zero reflection at λ = 1060 nm.Stress in the films was measured. All films were under compressive stress, in the range (3.2–5.5) × 108Pa, depending upon composition of the films.