Methods for Proton Direct Ionization SEU Characterization and Orbital Error-Rate Estimation

In space environments, protons can cause single-event effects either by direct ionization or by nuclear reactions, whose secondary products deposit charges in sensitive regions. Given the low linear energy transfer value of the proton, the direct ionization contribution was usually considered negligible in technologies larger than 65 nm. However, as a result of the technology scaling and the voltage reduction, the minimum amount of charge required to invert the state of a memory cell has significantly decreased, and the direct ionization mechanism can even dominate the low Earth orbit error rates in some specific conditions. Proton direct ionization (PDI) occurs only at very low energies, near the ionization Bragg peak. Therefore, its experimental characterization requires specific beam characteristics and the corresponding methods to evaluate the orbital upset rates. This work proposes two methodologies to characterize the sensitivity of SRAM devices to PDI and details the procedures for orbital error-rate calculation. Both approaches were applied to a set of four SRAM devices, ranging from 65- down to 16-nm FinFET and the resulting upset rates were cross-validated to evaluate their accuracy. The respective advantages and limitations of each methodology are discussed, taking into account the specific constraints related to the test facilities and the device under test (DUT).