The response of n-channel EPROMs to radiation and annealing

Abstract

The performance of n-channel EPROMs, type 2716 and 2732, under different types of radiation environment and high temperature annealing was studied. Variations in the number of programmed logic “0” states and unprogrammed logic “1” states of memory MOS transistors of EPROMs were measured as functions of doses of CO-60 gamma-rays, fluences of 1 and 6 MeV electrons and EPROM temperature up to 500°C. In the EPROMs, when they were exposed to radiation, the programmed “0” states gradually underwent a transition to “1” states, and finally the stored data was erased. With increased radiation doses, though all the memory MOS transistors appeared to be in “1” states, the EPROM was not programmable. After this stage when the exposure to radiation was continued, at a dose of 116 krad (Si) of Co-60 gamma-rays or at a fluence of, 2.2 × 10 13 e/cm 2 of 1 MeV electrons or 8 × 10 12 e/cm 2 of 6 MeV electrons, all the “1” states transitioned to “0” states. The transition from the “1” to the “0” states was also seen in the biased EPROMs. This rebound phenomenon for the “0” states was observed for the first time in the EPROMs irradiated either at room temperature or at liquid nitrogen temperature. Since in the case of cooled EPROMs the transition from “1” state to “0” state was observed at relatively low electron fluence, it is shown that negatively charged interface states do not influence the observed rebound phenomenon. Detailed analysis of the results and observations lead to conclusion that the positive oxide-trap charge, in the memory MOS transistors is responsible for the observed unprogrammable “1” states and that in the MOS transistors of the control circuitry for the observed radiation induced “0” states. An irradiated EPROM, when annealed at 450°C for 30 min, all its radiation-induced “0” states recovered to “1” states and the EPROM was found functioning normally. The activation energy of 1.2 eV is found to be effective for removal of the induced damage in an EPROM.