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.

Oxidation process of AlOx-based magnetic tunnel junctions studied by photoconductance

“…The junction resistance shows ͑roughly͒ an exponential increase with oxidation time, which is related to the growth of the barrier layer. 2 The TMR ratio, however, shows a maximum around an oxidation time of 105-120 s. Such a maximum in the TMR has also been observed by others, 2,6,7 and has been attributed to the full oxidation of the Al layer, without oxidizing the underlying magnetic electrode.…”

“…Several techniques have previously been used to investigate the oxidation process. [2][3][4] However, most of these techniques use special samples or incomplete junctions, and could therefore not be directly related to other junction properties. Previously we have shown that Al can be used as an efficient source for photoexcited hot electrons.…”

The formation of amorphous alloy oxides as barriers used in magnetic tunnel junctions

“…Indeed, Bae et al reported the initial oxidizing paths of the crystalline precursors were the grain boundaries and then through the grains, leading to the formation of nonuniform interfaces and other defects 26,27 as well as the partial oxidation of the ferromagnetic layer below the tunnel barrier layer before completion of the Al oxidation. 28 However, since an amorphous phase was found in Al-Zr alloys via different experimental techniques, 22,[29][30][31][32][33] we choose to use this binary as a model system for both theoretical and experimental investigations in the present study. We will first present a thermodynamic formulation to predict alloy compositions of Al-Zr with tendencies to form amorphous thin films when these films are prepared using rapid quenching techniques such as sputter deposition.…”

Real-time monitoring of plasma oxidation dynamics of subnanometer Al2O3 barriers for magnetic tunnel junctions

“…After stoichiometric Al 2 O 3 is formed, oxygen accumulates at the CoFe/ Al interface and when there is an excess of oxygen, the CoFe starts to oxidize. 7 The features of the oxidation rate in Fig. 1͑b͒ are analyzed in terms of this stepwise oxidation model.…”