Reactive deposition of low loss Al2O3 optical waveguides by modified dc planar magnetron sputtering

Abstract

A modified planar magnetron system has been used to dc reactively sputter Al2O3 films with optical waveguide losses of 1 dB/cm and refractive indices of 1.65±0.005 at 632.8 nm. The target and substrate are separated by two 176×45 mm graphite grid plates with either 2 or 4 mm diam apertures in a hexagonal array. When the Al target was sputtered in argon and the oxygen flow rate increased, the value f2c at which an Al2O3 layer formed on the target depended upon the configuration of the apertures and the rf power coupled to the substrate holder; for a sputtering power of 1.25 kW, f2c increased from 11 to 13 ml/min when the aperture diameter was reduced from 4 to 2 mm or when the rf power increased from 0 to 50 W, due to increased gettering of oxygen outside the target region. The low optical loss films were obtained by controlling the ‖oxygen flow rate between 40% and 70% of f2c; at low flow rates, the films were absorbing due to oxygen deficiency whereas scattering loss increased near f2c due to particle ejection caused by arcing of the partially oxidized target. Although the deposition rate of Al was reduced by the grids, the Al2O3 deposition rate obtained was approximately three times the value obtained by rf reactive sputtering without grids to obtain the same optical loss. The flux through the grid structures was calculated and agreed with the measured deposition rates; for the 4 mm apertures, the calculated flux was 8.3% of the value without apertures and the experimental value was 10%. The modified magnetron system provides a higher deposition rate method for depositing low optical loss Al2O3 films than is obtained by rf reactive sputtering.