Comparison of Single and Two-Photon Absorption for Laser Characterization of Single-Event Upsets in SOI SRAMs

The laser pulse energy thresholds for single-event upset measured by single photon and two photon absorption are measured and compared for Sandia SRAMs and DPSRAMs, and IBM 45-nm SRAMs for devices with and without the back substrate removed. These results are also compared to heavy-ion results taken on the same devices. Sandia SRAM data taken on different test dates resulted in considerably different TPA laser pulse energy thresholds even though the TPA system was calibrated using standard techniques each test date. These differences are believed to be due to changes in laser spot size. This shows that it is imperative to develop a calibration procedure that monitors all relevant laser parameters if TPA is to be used as a routine quantitative tool. Removing the back substrate makes a very large difference in TPA laser pulse energy threshold. This large difference is likely due to either displacement currents generated in the back substrate by TPA and/or nonlinear optical effects which can reduce the laser pulse irradiance in the active region. Nevertheless, the mechanism does not appear to affect the qualitative nature of TPA measurements. Both SPA and TPA laser measurements were used to estimate the heavy-ion threshold LETs of the Sandia DPSRAMs and 45-nm IBM SRAMs. Both SPA and TPA overestimated the heavy-ion threshold LET of the IBM 45-nm SRAMs (likely due to the large laser spot size compared to the size of the SRAM cell), but reasonably estimated the threshold LETs of the Sandia DPSRAMs. For the first time, TPA laser pulse energy (squared) is directly compared to SPA laser pulse energy at threshold. There is reasonable quantitative agreement between the charge required to induce upsets by TPA and SPA with the back substrate removed.