The aluminum barrier in exchange biased (Mn-Ir/Co-Fe/AlOx/Py) magnetic tunnel junctions was formed by oxidation with an electron cyclotron resonance plasma source. This technique allows, in contrast to commonly used floating substrate, an independent control of the energy of the ions bombarding the Al film by adjusting a direct current bias voltage at the sample. Here,we show that low energy ions and oxidation times between 50 and 200 s lead to optimum barrier properties and a maximum tunnel magneto resistance (TMR) ratio of 46% at room temperature and 73% at 10 K. Shorter oxidation time gives underoxidized samples with strongly reduced TMR. Ions with energy above 30 eV turn out to strongly overoxidize the sample resulting in large junction resistance and reduced TMR values. (C) 2003 American Vacuum Society.