The formation of the insulating $AlO_x$ tunnel barrier is a critical and sensitive process in magnetic tunnel junctions (MTJ). 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 (HRTEM) that the oxidation occurred preferentially through the grain boundary of Al grains. Also, the $AlO_x$ grains expanded upward 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 AlOx 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.
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 upward 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 The annealing treatment was performed in rapid thermal annealing (RTA) furnace.
The influence of the natural oxidation and the plasma oxidation on density and stoichiometry of the $AlO_x$ insulating layer was studied. The density was analyzed by X-ray reflectivity and it showed little difference with oxidation methods. From the analysis by synchrotron and X-ray photoelectron spectroscopy, (XPS) and transmission electron microscope (TEM) energy dispersive X-ray spectroscopy (EDS) it was revealed that oxidation occurred more rapidly in the plasma oxidation than in the natural oxidation. The ratio of Al and O of the oxide layer in the natural oxidation was almost the same regardless of oxygen pressure levels. In the plasma oxidation, the oxidation proceeded more rapidly at the reasonable oxygen pressure level and resulted in a higher ratio of O/Al than that achieved at high oxygen pressure. The result that the ratio of O/AL in the plasma oxidation is higher than in the natural oxidation is implicitly consistent with a previous finding that the MR ratio of MTJ fabricated by plasma oxidation is generally higher than that by natural oxidation.
