The development of magnetic random access memory requires very reliable magnetic tunnel junctions (MTJs). However these devices are prone to dielectric breakdown. To investigate the reliability of the MTJs, we performed constant voltage stress tests. This study shows the area scaling and the voltage dependence of the time-to-breakdown tBD of exchange biased, naturally oxidized MTJs. The fraction of broken devices F is plotted on a Weibull scale. The Weibull fit of our data shows clearly that tBD scales with the area, meaning that the breakdown sites are randomly distributed. Moreover, the Weibull distributions have a constant slope β (=0.35) and show no tails. This suggests that only one breakdown mode is observed. It is not clear yet whether an intrinsic or extrinsic failure mechanism is causing the breakdown, but considering the small Weibull slope, extrinsic breakdown caused by randomly distributed process-induced defects in the oxide is most plausible. Second, the voltage dependence of tBD is studied. To fit the data, a linear dependence of log(tBD) on stress voltage is assumed. It is however not yet proven whether this model is applicable for ultrathin Al2O3 barriers. © 2002 American Institute of Physics.