The impact of delta-rays on single-event upsets in highly scaled SOI SRAMs
King, Michael Patrick
Orbiting spacecraft experience harsh radiation environments that may affect microelectronics in undesirable ways. Ionizing radiation interacts with microelectronics in a variety of ways. One such effect is known as single event effects (SEEs), which is the response of semiconductor device to a single ionizing particle event. Ensuring the integrity of scientific data in harsh radiation environments is critical for the success of missions that require recording and storing large volumes of data. A single event upset (SEU) is an erroneous change in the state of the memory cell. The radiation effects community has developed many experimental and simulation based techniques to determine the sensitivity of memory cells to SEUs. The role of ion track structure has been a concern in the SEEs community for more than twenty years. Linear energy transfer (LET), the rate of energy lost by the incident ion per unit path length within a material, has been used to relate the space environment to the ground test environment and has been the traditional metric for much of SEE analysis. In more recent years, additional physical mechanisms, for example SEUs resulting from proton direct ionization and nuclear reactions, have been required to describe the conventional cross section versus LET curves obtained by ground based experiments. These trends suggest that technology nodes are becoming sensitive to direct ionization and effects from secondary particles produced by the incident particle’s track structure. Silicon-on-Insulator (SOI) technology has long been advantageous in SEE mitigation due to its small active device area and isolating buried oxide layer (BOX). However, these benefits are not without cost, as the threshold LET, the LET at which saturation occurs in the upset cross section, for SOI technologies is typically lower than equivalent bulk technologies. With memory cells becoming increasingly sensitive to effects from ionizing particle events, there is concern about the contribution from secondary particles related to track structure of the incident heavy ion. Energetic secondary electrons, δ-rays, are frequently generated in ionizing radiation events. These δ-rays undergo scattering events resulting in localized charge generation comparable to the critical charge of modern SOI technology nodes. In this work, we use Monte-Carlo radiation transport simulations to evaluate the impact of δ-rays on highly scaled silicon-on-insulator (SOI) technologies. A 22 nm SOI SRAM is used to estimate the geometry and critical charge. Results suggest that long-range δ-rays can deposit sufficient energy to cause single event upsets (SEUs) in SRAM cells separated by many micrometers. We discuss the implications of δ-ray induced SEU on hardening techniques and technology computer aided design device simulations.