NITRIDATION OF SIO2 THIN-FILMS AT LOW AMMONIA PRESSURES - COMPOSITION AND ELECTRICAL-PROPERTIES

Thin thermal SiO2 films on crystalline silicon substrates were nitrided at low ammonia pressures (10-6 mbar ≤ PNH ≤ 10-1 mbar) for times varying from 1 to 15h held at short intervals (5-30 min) by means of two techniques: (i) surface nitridation has been achieved by thermal activation at high temperature (HT), in the range 800-1200 °C; (ii) a new process at low temperature (LT) was carried out at T ≈ 30 °C, under electron beam irradiation. The nitridation reaction rate, the nitrogen distribution in the film and the chemical composition were found to be a function of pressure, temperature and time for the HT process, and a function of electron flux and energy, pressure and time for the LT process. The electrical properties of nitrided films were compared with those of thin SiO2 films. Conduction, electron trapping, fixed charge, interface trapped charge densities and the breakdown behaviour of nitrided oxide films depend on the amount of nitrogen incorporated into the bulk of the films and/or at the SiO2Si interface. We have shown that metal-insulator-semiconductor structures with nitrided oxide as the gate insulator exhibit two attractive electrical properties: a sufficiently low density of interfacial fast states can be achieved by optimization of nitridation parameters and a high density of shallow and/or deep traps exists; however, there is a balance between the trapping of injected electrons and detrapping by tunnel emission (saturation of the flat band voltage shift under bias stress). © 1990.