THE RESPONSE OF N-CHANNEL EPROMS TO RADIATION AND ANNEALING

The performance of n-channel EPROMs, type 2716 and 2732, under differe nt types of radiation environment and high temperature annealing was s tudied. Variations in the number of programmed logic ''0'' states and unprogrammed logic ''1'' states of memory MOS transistors of EPROMs we re measured as functions of doses of CO-60 gamma-rays, fluences of 1 a nd 6 MeV electrons and EPROM temperature up to 500 degrees C. In the E PROMs, when they were exposed to radiation, the programmed ''0'' state s gradually underwent a transition to ''1'' states, and finally the st ored data was erased. With increased radiation doses, though all the m emory MOS transistors appeared to be in ''1'' states, the EPROM was no t programmable. After this stage when the exposure to radiation was co ntinued, at a dose of 116 krad (Si) of Co-60 gamma-rays or at a fluenc e of, 2.2 X 10(13) e/cm(2) of 1 MeV electrons or 8 x 10(12) e/cm(2) of 6 MeV electrons, all the ''1'' states transitioned to ''0'' states. T he transition from the ''1'' to the ''0'' states was also seen in the biased EPROMs. This rebound phenomenon for the ''0'' states was observ ed for the first time in the EPROMs irradiated either at room temperat ure or at liquid nitrogen temperature. Since in the case of cooled EPR OMs the transition from ''1'' state to ''0'' state was observed at rel atively low electron fluence, it is shown that negatively charged inte rface states do not influence the observed rebound phenomenon. Detaile d analysis of the results and observations lead to conclusion that the positive oxide-trap charge, in the memory MOS transistors is responsi ble for the observed unprogrammable ''1'' states and that in the MOS t ransistors of the control circuitry for the observed radiation induced ''0'' states. An irradiated EPROM, when annealed at 450 degrees C for 30 min, ail its radiation-induced ''0'' states recovered to ''1'' sta tes and the EPROM was found functioning normally. The activation energ y of 1.2 eV is found to be effective for removal of the induced damage in an EPROM.