Optical EngineeringIon-beam and dual-ion-beam sputter deposition of tantalum oxide films
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Ion-beam sputter deposition (IBS) and dual-ion-beam sputter deposition (DIBS) of tantalum oxide films was investigated at room temperature and compared with similar films prepared by e-gun deposition. The optical properties, i.e., refractive index and extinction coefficient, of IBS films were determined in the 250- to 1100-nm range by transmission spectrophotometry and at λ = 632.8 nm by ellipsometry. They were found to be mainly sensitive to the partial pressure of oxygen used as a reactive gas in the deposition process. The maximum value of the refractive index of IBS deposited tantalum oxide films was n = 2.15 at = 550 nm and the extinction coefficient of order k = 2 x 10-4. Films deposited by e-gun deposition had refractive index n = 2.06 at λ = 550 nm. Films deposited using DIBS, i.e., deposition assisted by low energy Ar and O2 ions (Ea = 0 to 300 eV) and low current density (Ji = 0 to 40 μA/cm2), showed no improvement in the optical properties of the films. Preferential sputtering occurred at Ea(Ar) = 300 eV and Ji = 20 μA/cm2 and slightly oxygen deficient films were formed. Different bonding states in the tantalum-oxide films were determined by x-ray spectroscopy, whereas composition of the film and contaminants were determined by Rutherford backscattering spectroscopy (RBS). Tantalum oxide films formed by IBS contained relatively high Ar content (≈ 2.5%) originating from the reflected argon neutrals from the sputtering target whereas assisted deposition slightly increased the Ar content. Stress in the lBS-deposited films was measured by the bending technique. IBS-deposited films showed compressive stress with a typical value of s = 3.2 x 109 dyn/cm2. Films deposited by concurrent ion bombardment showed an increase in the stress as a function of applied current density. The maximum was s ≈ 5.6 x 109 dyn/cm2 for Ea = 300 eV and Ji = 35 μA/cm2. All deposited films were amorphous as measured by the x-ray diffraction (XRD) method.