Electron irradiation effects on thin MOS capacitors

The charge trapping properties of 8 nm thick oxide layers of metal-oxide-se miconductor capacitors have been studied before and after a MeV electron ir radiation as a function of the radiation dose. The positive charge induced by ionizing radiation increases with the radiation dose without saturation up to 20 Mrad(Si). It does not disappear at room temperature even 10 days a fter the irradiation, but it annihilates by recombination if electrons are injected into the oxide by Fowler-Nordheim tunneling. After the positive ch arge recombination, we have observed, by capacitance-voltage and current-vo ltage measurements, increased electron trapping in the irradiated oxides, d ue to the neutral traps generated by radiation. The negative trapped charge increases linearly with the radiation dose, and its centroid is close to t he center of the oxide but shifted to the gate/SiO2 interface. Finally, the oxide degradation of irradiated and unirradiated devices by constant curre nt stress has been measured: differences between irradiated and unirradiate d devices disappear for fluences larger than 10(19) electrons/cm(2). (C) 19 99 Elsevier Science B.V. All rights reserved.