Nitridation of SiO 2 thin films at low ammonia pressures: Composition and electrical properties

Abstract

Thin thermal SiO 2 films on crystalline silicon substrates were nitrided at low ammonia pressures (10 −6 mbar ⩽ P NH 3 ⩽ 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 SiO 2 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 SiO 2Si 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).