Three-dimensional atom probe investigation of boron distribution in CoFeB/MgO/CoFeB magnetic tunnel junctions

Significant lateral compositional variations have been revealed in a three-dimensional atom probe (3DAP) study of the MgO barriers of magnetic tunnel junctions. High resolution electron microscopy of the CoFeB∕MgO∕CoFeB∕Ru∕CoFe∕PtMn multilayer indicates that the MgO barrier is smooth, uniform, and with good crystallinity. Nevertheless, the 3DAP data show lateral variations in the MgO composition over length scales on the order of tens of nanometers. Boron is not uniformly distributed within the CoFeB layers, but has tended to segregate to the interfaces. Annealing has no significant effect on the distribution of B, indicating that the segregation occurring during growth is relatively stable.