Reliability Degradation in NMOS Devices Subjected to High Energy Gamma-Ray Irradiation

Abstract

Introduction The effects of radiation on memory devices is of great concern to manufacturers and users in high-tech industry. With the increase in space and nuclear applications of computer circuitry, such devices are likely to be exposed, circumstantially or accidentally, to high doses of radiation and may therefore exhibit failures or be electrically degraded. Radiation has been known to cause charge leakage leading to soft errors due to alpha-particle emissions in DRAMs. Since EPROMs utilize ultra-violet light for erasure, some effect on the volatility due to radiation was suspected. The test described in this article involved samples of 4116 DRAMs, 2114 SRAMs and 2716 EPROMs. Gamma rays were initially discovered by Becquerel and Curie around 1896. They are photons which were first defined as grains of light and later as quanta of electromagnetic radiation. In the electromagnetic spectrum, gamma-rays occupy the highest energy region. The energy of rays is measured in eV. Xrays are in the 100eV-100keV range, whereas gamma-rays lie in the 100keV-100MeV region. Gamma rays can be generated by means of radioactive isotopes, such as colbalt, or in this case by an electron accelerator. When the electrons are accelerated they absorb energy. Only high energy particles are able to penetrate nuclei and create reactions. These high energy electrons are channelled into an electron beam. When the electrons hit a target, they decelerate abruptly thus releasing all of their stored energy. An atom penetrated by such an electron is left in a more exited state. Gamma-rays are emitted when the nucleus reverts from its exited state to its ground state. Analysing the effects of radiation on semiconductor devices is of a complex nature. Since the experimental results are a function of various factors such as dose-rates, total doses, energy of rays, temperature, chip uniformity, etc. no concrete theoretical model for radiation damage has been arrived at to date.