Magnetic tunnel junctions consisting of Ta(50 Å)/NiFe(50 Å)/IrMn(150 Å)/CoFe(50 Å)/Al(13 Å) –O/CoFe(40 Å)/NiFe(400 Å)/Ta(50 Å) with a 100×100 μm2 junction area were prepared. The AlOx tunnel barrier was produced by oxidizing the 13 Å thick Al metal using inductively coupled plasma (ICP) for 30–360 s and the ensuing junction properties were characterized as a function of oxidation time. It was found that a junction oxidized for 80 sec exhibited the highest magnetoresistance ratio, 30.3%, at room temperature. It was also shown that the junctions with an ICP oxidized tunnel barrier maintained the tunneling magnetoresistance ratio over 15% even when the insulator layer was oxidized for a prolonged period, well beyond the optimal oxidation time. The large processing window for the insulator oxidation was attributed to the dense amorphous AlOx structure formed by the ICP oxidation.
Magnetic tunnel junctions (MTJ) with the tunnel barrier oxidized in two steps with plasma were fabricated to obtain a structurally uniform AlO x insulator. Plasma oxidation of 10 Å-thick Al layer for 5~20 sec formed the initial oxide barrier on top of which a second oxide layer was deposited by oxidizing 13 Å-thick Al for 120 sec. The doubly oxidized junctions exhibited a magnetoresistance (MR) ratio of 27~31 % depending on the oxidation period of the initial oxide layer while only 24 % was obtained for the junction deposited in one-step oxidation with a similar stack configuration. The junction resistance of the MTJ increased monotonically with oxidation time while minimal deterioration of the MR ratio was observed when oxidation time increased from 5 sec to 20 sec. Transmission electron microscopy of the junctions also confirmed that the AlO x thickness was thinner for the doubly oxidized junctions compared to the singly oxidized MTJ. X-ray photoelectron spectroscopy of the double junction also strongly suggested that the initial oxide layer prevents over-oxidation of the bottom electrode. Our results suggest that the AlO x oxidized in two steps produces improved junction performance as well as a wider processing window.
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