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The formation of the insulating A1O[sub 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 A1O[sub x] insulating layer in this study. In the natural oxidation, the A1 layer was exposed to pure oxygen gas at 20 Torr for up to 50 min to produce the A10[sub x] tunnel barrier. It was revealed by high resolution transmission electron microscopy that the oxidation occurred preferentially through the grain boundary of A1 grains. Also, the A10[sub x] grains expanded isotropically when fully oxidized, thereby making the surface of the A1O[sub 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 A10[sub x] layer formed uniformly on the A1 layer. It had sharp interfaces with the underlying metallic A1 and the rate of oxide layer growth decreased as the oxidation proceeded. [ABSTRACT FROM AUTHOR]

In a magnetic tunnel junction, the formation of an insulator is sensitive and critical to the stable performance and reproducibility of the junction. The oxidation path and the microstructural change with time of the insulator in natural oxidation have been studied by the high-resolution transmission electron microscopy. It has been observed that the oxidation path is primarily through the grain boundary at an early stage of oxidation and then through the grains at a later stage. The morphology of the oxide layer was rugged and modulated. There also occurred an isotropic volume expansion with increased oxidation. It was observed that the ferromagnetic Co layer below an insulator was partially oxidized because of the preferred grain boundary oxidation. When this multilayer was annealed, the locally oxidized Co layer was reduced and the metallic layer formed as a continuous film type, thereby improving the interface. [ABSTRACT FROM AUTHOR]