Neutron-Induced Multiple-Cell Upsets in 20-nm Bulk SRAM: Angular Sensitivity and Impact of Multiwell Potential Perturbation

Neutron-induced multiple-cell upsets (MCUs) are studied in a 20-nm bulk SRAM by irradiation experiments using atmospheric-like neutrons at several angles of incidence. The evaluation of soft error rates reveals that the multiple-bit upset rate significantly increases at grazing incidence, indicating the possibility that the efficiency of error-correction codes is reduced depending on the device orientation in the terrestrial environment. The analyses of the characteristics of the MCU events confirm that the forward emission of secondary ions, the geometry of SRAM cells, and the parasitic bipolar effects are the key factors in determining the angular sensitivity of MCUs. Our analysis unveils the unique data pattern dependence in the occurrence probability of gap structures in fail bit patterns. Based on the consideration of potential perturbations in multiple p-wells, multi-well coupled potential unbalancing is found to be the plausible mechanism underlying this observation. The detailed analyses of fail bit maps with internal node voltages demonstrate that observed gap structures are consistent with the configuration induced by this mechanism.