Asymmetric dielectric breakdown behavior in MgO based magnetic tunnel junctions

Abstract

The time-dependent dielectric breakdown phenomenon (TDDB) has been investigated in a series of nominally identical MgO based magnetic tunnel junctions (MTJs) by pulsed voltage endurance test. Results from the pulsed endurance test reveal that the breakdown voltage is dependent on the polarity of the applied voltage. MTJs with “UP” current stress (Iup) (flowing from reference layer (RL) to free layer (FL)) show higher endurance than that of MTJs with “DOWN” current stress (Idown). We also found that bipolar stressing could result in reduced cumulative stress time before failure as compared to unipolar stressing. This could be explained by increased charge trapping/detrapping effects during bipolar stressing. The asymmetric breakdown behavior for different polarity was further supported by the different field acceleration slopes observed in the mean time to failure (MTTF) – voltage bias trend line. Symmetric pulse scheme breakdown measurements were also carried out at different temperatures ranging from 25–85°C. The time-dependent clustering model is applied here to best describe the breakdown statistics in view of the non-uniformity in percolation breakdown due to thickness variations (interface roughness) and other process-induced damages to the ultra-thin MgO barrier layer.