Origins and Signatures of Tail Bit Failures in Ultrathin MgO Based STT-MRAM

In this study, the time-dependent dielectric breakdown (TDDB) phenomenon has been investigated in a macro array of nominally identical ultrathin MgO based magnetic tunnel junction (MTJ) devices by bipolar pulsed voltage endurance test. The first of such comprehensive study shows that the MTJ breakdown distribution can be differentiated into stress-induced and process-induced (tail bit) failures. Three unique signatures are observed from the tail bit (TB) failed devices which are attributed to pinhole formation due to interface roughness (TB-1), sidewall redeposition (TB-2) and unintended back hopping (TB-3). We propose the bimodal clustering model to best describe breakdown statistics accounting for the unique TB patterns and localized degradation events. Approaches to resolve these TB failure modes to achieve endurance failure rate <; 1ppm after 1M cycles in 40Mb embedded-MRAM product are also presented.