Microstructural observation on effect of oxidation method of AlOx in magnetic tunnel junction by high resolution transmission electron microscopy

Abstract

The formation of the insulating AlO x tunnel barrier is a critical and sensitive process in magnetic tunnel junctions. Both the natural oxidation and the plasma oxidation methods were employed to fabricate the AlO x insulating layer in this study. In the natural oxidation, the Al layer was exposed to pure oxygen gas at 20 Torr for up to 50 min to produce the AlO x tunnel barrier. It was revealed by high resolution transmission electron microscopy that the oxidation occurred preferentially through the grain boundary of Al grains. Also, the AlO x grains expanded isotropically when fully oxidized, thereby making the surface of the AlO x layer modulated. In plasma oxidation, the oxygen plasma was used at 20 mTorr of pure oxygen gas for up to 30 s and the flat AlO x layer formed uniformly on the Al layer. It had sharp interfaces with the underlying metallic Al and the rate of oxide layer growth decreased as the oxidation proceeded.

Thermally Stable Magnetic Tunnel Junctions for High Density MRAM

“…As is known well, oxidation occurs via grain boundaries at the initial stage and later proceeds to grain interiors [5,6]. There is a tendency for metallic Al to remain in the interior of the grain of more than 1 nm-thick Al film because of the kinetics of Al oxidation [5][6][7]. We found that the remaining Al caused a serious change in the junction resistance during the high temperature processes.…”

“…The tunnel barrier of AlO x is usually formed by oxidation of Al. As is known well, oxidation occurs via grain boundaries at the initial stage and later proceeds to grain interiors [5,6]. There is a tendency for metallic Al to remain in the interior of the grain of more than 1 nm-thick Al film because of the kinetics of Al oxidation [5][6][7].…”

Tunnel spectra for Al/Al-oxide/Ni80Fe20 junctions under the superconducting transition temperature of Al films

“…Oxidation with weak power (weak plasma, radical, natural oxidation) proceeds preferentially through the grain boundary of Al film, compared with inside of the grain [4,5]. As a result, the oxidized layer is inhomogeneous.…”

“…We could obtain the junction with low ZBC even using natural oxidation [6]. On the other hand, oxidation with strong power proceeds homogeneously because oxidation process is not different between the boundary and inside of the grain [5]. It is considered that for strong oxidation, the oxidized layer is homogeneous and Al/Al-oxide interface can be kept sharp with increasing oxidation time.…”