An experimental study of single-event effects induced in commercial SRAMs by neutrons and protons from thermal energies to 500 MeV

Irradiations have been performed, with neutrons and protons over a wide particle energy range, on six different commercial-grade 4-Mbit SRAM parts from the manufacturers Hitachi, Toshiba, Mitsubishi, and Samsung. At energies above 20 MeV, the single-event upset (SEU) cross sections are similar for protons and neutrons. While the proton cross sections fall off rapidly below this energy due to package shielding and Coulomb barrier effects, the neutron cross sections at 14.5 MeV are close to the high energy plateau values. Four of the devices show significant thermal neutron SEU cross sections and in two cases these exceed the high energy plateau values. The most modern of the parts tested is susceptible to latchup from both protons and neutrons but the response is shifted to higher energies compared with SEU.

I. Introduction

Neutrons are a very significant component of the natural particle radiation environment in the atmosphere at passenger aircraft cruise altitudes. They are produced by the nuclear interactions between atmospheric nuclei of oxygen and nitrogen and cosmic rays and solar particles that have sufficient momentum to penetrate the Earth's magnetic field. The atmospheric neutron energy spectrum extends to many GeV and has a differential flux spectrum (with respect to energy) which is approximately inversely proportional to particle energy. The neutron flux peaks at about 9 particles cm s at heights of around 18 km at high latitudes. Increases by up to factors of 300 are possible for several hours during occasional very large solar particle events [1], [2]. Aircraft structures and loads, in particular fuel and passengers, can provide a significant thermal neutron component which is absent in the external atmospheric spectrum. These are neutrons that have scattered sufficiently to reach thermal equilibrium with their surroundings and for room temperature this implies an average kinetic energy of 0.025 eV. Thermal neutron fluxes of around 1 cm s have been predicted and observed in a Boeing-747 structure [1]. Sea level fluxes of thermal neutrons of 4 cm hr have recently been reported [3]. Secondary neutron fluxes are also of significance in heavy spacecraft structures and fast and thermal neutron fluxes of order 2 cm s and 0.3 cm s respectively have been observed on the Space Shuttle [4].