INSITU MONITORING OF RADIATION-DAMAGE TO THERMAL SILICON DIOXIDE FILMS EXPOSED TO DOWNSTREAM OXYGEN PLASMAS

Field effect transistors with exposed SiO2 gate insulators were used as in situ monitors of the effects of downstream oxygen plasma exposure on the Si/SiO2 System. The source to drain current of the field effect transistor was monitored before, during, and after plasma processing. At a fixed source to drain potential difference, devices displayed a drain-current decrease during plasma exposure; this indicates an increase in the fixed negative charge in the oxide and/or a decreased transconductance due to an increase in the density of interface traps. A portion of the drain-current decrease was recovered after exposure to room air or in situ exposure to water vapor suggesting that the drain-current decrease was caused by electron-trapping at water-related traps in the oxide. The remainder of the drain-current decrease was recovered after annealing at 400-degrees-C in forming gas and was caused most likely by electron-trapping at neutral electron traps. Substrate currents were generated during plasma exposure indicating that electron injection from the substrate was the probable source of electrons trapped in the oxide. Vacuum UV photons with energies greater than the band gap of SiO2 (approximately 9 eV) and electron injection into the SiO2 due to surface-charging most likely were responsible for formation of neutral electron traps.