Role of Elastic Scattering in Low-Energy Neutron-Induced SEUs in a 40-nm Bulk SRAM

The contribution of mymargin elastic scattering to upsets caused by 1–10-MeV terrestrial neutrons in a 40-nm bulk SRAM and the influence of cascade recoils are investigated using Geant4 simulations. It is shown that elastic scattering, rather than (
$n$
,
$p$
) or (
$n$
,
$\alpha$
), is a dominant mechanism, contributing ~66% of the total upsets induced by 1–10-MeV neutrons. The dominance is attributed to the major role it plays in upset cross sections below ~6 MeV, and the abundance of lower energy neutrons due to
$E^{- 1}$
dependence characteristic of the terrestrial neutron differential flux below 10 MeV. After considering ionization energy deposition of cascade recoils in Geant4 simulations, remarkable rises in upset cross sections of 27%, 18%, and 9% are observed for reactor, 2.5- and 1–10-MeV neutrons, which are the consequence of cascade recoils induced by primary Si ions. In-depth TRIM calculations on the ionization energy deposition of incident Si ions and cascade recoils have been performed, revealing that the underlying mechanism is the reduction in the threshold energy of Si ions from ~50 to ~30 keV if cascade recoils are taken into account.