Junction area scaling and statistical description of dc breakdown of ferromagnetic tunnel junctions

Magnetoresistive random access memory (MRAM) with ferromagnetic tunnel junctions (MTJs) has been recognized as a successful candidate for next generation solid-state memory. There are, however, still significant problems to realize MRAM. In particular, reliability of an ultrathin dielectric layer composing tunnel junctions is of great importance for MRAM developments. In this article, we performed systematical studies of the dielectric breakdown of MTJs. The samples have a double tunnel barrier structure with IrMn/CoFe/AlOx/CoFe/AlOx/CoFe/IrMn. Its width is ranging from 0.4 to 30 μm. Different from previous reports, the breakdown voltage gradually decrease with decreasing junction area. The breakdown voltage only shows weak dependence on junction area in the sub-μm region. In addition, the breakdown voltage is inversely proportional to the junction perimeter. Results of time-dependent failure measurements demonstrate that, within the limit of this experiment, the failure function of the MTJs obeys a Weibull distribution. The shape parameter of the Weibull distribution is less than unity, indicating early device to failure. We consider that process-induced defects at the sidewall of the junctions may serve a certain role in their early failure and lead the failure of the junction area scaling of the breakdown voltage.