Electrical properties of thin rf sputtered aluminum oxide films

Thin films of aluminum oxide (Al2O3) were fabricated by rf magnetron sputtering. Different sputter conditions, e.g., composition of the sputter gas (Ar:O2), sputter gas pressure, deposition rate, and preparation of the Al2O3 sputter target before deposition were investigated with the aim to achieve good insulating films with high electrical breakdown fields. The Al2O3 films had a thickness of 160 nm and were deposited on ITO covered glass. By evaporation of Au electrodes on top of the Al2O3 films thin film capacitors were fabricated. Current voltage (I–V) measurements were performed under high vacuum and temperatures between 4 and 300 K. Significant scattering of the I–V curves and “burn-in” effects are observed. We find that an admixture of 1% of O2 in the sputter gas improves the electrical properties, but higher breakdown fields and smaller leakage currents are obtained for sputtering in pure Ar, using a sputter target conditioned in an Ar:O2 mixture. Impedance spectra, revealed a dielectric constant of ∼7 for all Al2O3 films. Atomic force microscopy experiments reveal that the surfaces are rather rough with grain sizes in the order of 0.3–0.5 μm.