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. © 2003 American Institute of Physics.