Plasma oxidation of thin aluminum layers for magnetic spin-tunnel junctions

Abstract

This article presents results of a study initiated to characterize the plasma-oxidation process of very thin Al films, a technology commonly used to produce good barrier layers for magnetic spin-tunnel junctions. The behavior of oxygen in the oxidizing Al layer is determined using both quantitative (Rutherford backscattering spectrometry, transmission electron microscopy) and qualitative (x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry) analytical techniques. We have applied in situ XPS and experimented with O218 to unravel details of the oxidation mechanism. In addition, the influence of the oxygen pressure on the oxidation rate was established, both with and without a plasma being present. From optical emission spectra it is concluded that this pressure has a minor effect on the relative abundance of excited species in the oxygen plasma. When combined, these data constitute the basis of a model that distinguishes several steps in the plasma oxidation of Al. At the start, oxygen penetrates rapidly throughout the total Al layer, followed by a period of increasing oxygen concentration but constant oxide thickness. Finally, the Co underlayer becomes involved in the oxidation process, which marks the deterioration of the spin-tunnel junction. Evidence is obtained that for the thicker initial Al layers the Co electrode layer starts to oxidize before completion of the Al oxidation. This explains why for 0.8-nm-thick Al films the highest tunnel-magnetoresistance effect is obtained for stoichiometric Al2O3, whereas for 1.5 nm Al this occurs while the oxide is still substoichiometric.