Energetic electron-induced single event upsets in static random access memory

Abstract

Energy deposition in ionizing radiation events can cause errors in static random access memories (SRAMs). The resulting errors can negatively impact nominal systems operation. These errors, termed single-event upsets (SEUs), are a concern for SRAM data integrity in space and terrestrial applications. This work presents evidence of single electron-induced SEU in 28 and 45 nm CMOS SRAMs.

Exposing test chips to a source of X-rays generates energetic photoelectrons. The experimental SEU cross-sections depend exponentially on applied bias, consistent with previous results obtained with muons and low-energy protons. Data analysis indicates the observed errors have unique memory address locations and occur randomly during X-ray exposure. The SRAM test chips were demonstrated to be functionally stable during parametric bench testing before and after irradiation under all applied bias conditions. Therefore, the SRAMs remained stable while exposed to X-rays. The errors are not due to "weak bits" or photocurrents generated during X-ray irradiation. Instead, results and analysis suggest the observed errors are the result of single energetic electron scattering events within SRAM cells.

Error rate predictions of electron-induced SEU are low under nominal bias conditions in the geosynchronous environment around Earth. Similarly, electron-induced SEUs in the Jovian environment are predicted to be rare events occurring slightly more frequently than in the near-Earth environment. Simulation results indicate proton-induced SEUs are likely to remain a larger contributor to error rates in the Jovian environment in modern sub-65 nm SRAMs operating at nominal supply voltage conditions. Microelectronic systems operating in power-saving or quiescent modes would experience a significant increase in the rate at which electron-induced SEUs contribute to anomalous behavior in onboard electronics systems while within the Jovian electron belts.

Electron-induced SEUs have only been observed under reduced bias conditions for SRAMs fabricated in 28 and 45 nm technology nodes. This suggests the overall contribution of energetic electrons to error rates is small in current-generation technology. Electronics designed to operate with ultra-low power likely will exhibit higher relative sensitivity to energetic electron-induced SEUs. This represents an additional design concern for the space and terrestrial environment in order to avoid high error rates from lightly ionizing particles.