J.C. Bureau

Nitridation of SiO2 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 2 Si 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).

Thermal behaviour of WO3 and WO3/TiO2 materials

The knowledge of the interdependence between the structural, thermal and electric properties of oxide semiconductors contributes to widen their practical application. In this work the thermal behaviour of WO 3 and WO 3 /TiO 2 powders and thick films - studied by DSC...- is compared with the results of X-ray diffraction (XRD) investigation in the temperature range 25-500°C. The X-ray analyses show a reversible phase transition (from γ-WO 3 into β-WO 3 ) both in WO 3 and in WO 3 /TiO 2 . However, while in the WO 3 the transition takes place between 300 and 400°C, in the WO 3 /TiO 2 system it is not complete even at 400°C, during the measurement time. The phase transition in the WO 3 was identified between 320 and 360°C also by DSC, but in the bicomponent material no sharp transition could be detected. This may mean that titanium can thermally stabilize the WO 3 , influencing also its electric properties.

Effects of addition of oxygen and water vapor in high thermal nitridation processes of SiO2 thin films

Abstract Thin thermal SiO 2 films deposited on crystalline silicon substrate have been nitrided at high temperature (1050°C) and low ammonia pressures (10 −1 mbar) for times varying from 2 to 7 h. The effects of additional oxygen or water vapor to the nitridation reaction gases have been investigated. Nitrogen depth profiles in the nitrided films have been determined by Auger electron spectroscopy (AES) in conjunction with argon ion sputtering. Elemental composition as well as chemical bondings have been studied by means of AES and infrared absorption spectrometry. No nitrogen pile-up was found at the silicon/insulator interface. Compared to the pure NH 3 reaction gas, addition of water vapor led to an increase of the dielectric quality and a best resistance to energetic (3 keV) electron bombardment. The electrical properties (flat-band shift density of interface states, conduction and breakdown) of the nitrided films have been compared with those of the initial SiO 2 films. Barriers height values have been assigned to the metal/insulator interface. SiO 2 films reacted at low (NH 3 + H 2 O) pressures exhibit lower currents and interface state densities than the films reacted at low (NH 3 + O 2 ) pressures. Infrared absorption analyses have been performed on as-grown films: OH and NH species have been detected in SiO 2 films reacted at low (NH 3 + O 2 ) pressures, but the same species have not been observed in SiO 2 films reacted at low (NH 3 + H 2 O) pressures. It is concluded that the presence of oxygen in nitridation gases favors the increase of interfacial disorder; on the other hand, the presence of water decreases it.

High and low thermal nitridation of SiO2 thin films

Thin thermal SiO2 films on crystalline silicon substrate were nitrided at low ammonia pressures (10-6⩽PNH3⩽10-1 mbar) for times varying from 1 to 10 h, by means of two techniques. (i) Surface nitridation has been achieved by thermal activation at high temperature (HT), in the range 800–1100°C. (ii) A new process at low temperature (LT), was employed at T ≈ 30°C, under electron-beam irradiation; the nitridation-reaction rate depends on the electron energy (reaching a maximum within the energy range 1 to 2 keV), and on the electron flux. Conduction and electron trapping on the nitrided oxide films depends on the chemical compositions and on the amount of nitrogen incorporated into the bulk of the films and/or at the SiO2-Si interface.

Low-energy (3–100 eV) electron-bombardment-induced nitridation of thin SiO2 films: physicochemical and electrical analyses

Abstract Low-energy (3–100 eV) electrons are shown to be efficient promoters of the superficial nitridation of thin SiO2 films grown on boron-doped Si(100) using low pure NH3 pressures (⩽2 × 10−4 mbar) at ambient temperature. The initial concentration (≈1015 at. cm−3) of ionized acceptors in silicon near the SiO2/Si interface is first lowered by the nitridation process and then restored by hydrogen annealing at 450°C. The ultrahigh vacuum experimental set-up permits us to uncouple two important effects from a thermally-assisted (700–950°C) r.f. NH3 plasma nitridation process: the role played by low-energy (3–5 eV) electrons and the temperature. It is found that the existence of charge particles near the sample surface must be taken into account in the plasma reaction process and that the main effect of thermal activation is to enhance the reaction and the diffusion of nitrogen species through the SiO2 layer up to the SiO2/Si interface.

Nitridation of thin SiO 2 films induced by low energy (3–100 eV) electron bombardment

Abstract Low energy (3–100 eV) electrons are shown to be efficient promoters of the superficial nitridation of thin SiO 2 films grown on boron-doped Si(100) using low pure NH 3 pressures ( -1 mbar) at ambient temperature. The initial active boron concentration in silicon (≌10 15 cm -3 ) is first lowered near the SiO 2 /Si interface by the nitridation process and then restored by a hydrogen annealing at 450°C. The ultra-high vacuum experimental set-up permits us to uncouple two important effects from a thermally-assisted RF NH 3 plasma nitridation process : the role played by low energy (3–5 eV) electrons and the temperature. It is found that the presence of electrons near the sample surface must be taken into account in the plasma reaction process and that the main effect of thermal activation is to enhance the reaction and the diffusion of nitrogen species through the SiO 2 /Si interface.

Physicochemical characterization by means of IR absorption spectroscopy of Si3N4 thin films obtained by chemical vapour deposition assisted by in situ electrical disharge

An original process has been developed enabling the fabrication by chemical vapour deposition of insulating (Si 3 N 4 , SiO 2 ,...) thin dims, by means of in situ activation of the reactions at low temperature (T<400 o C) and under low pressure (P=1-2 Torr). The activation was performed by a d.c. electrical discharge. The substrate was not used as an electrode, and was placed parallel to the electric field. During the deposition process, several activation mechanisms can interfere, for exemple UV activation, photo-sensibilization, and direct activation by impacts (as is observed during thermal activation), and various thin films can be obtained

FT-IR, SIMS and electrical characterization of Si3N4 thin films obtained from CVD, assisted by in situ electrical discharge

Abstract An original process has been developed enabling the fabrication of CVD insulating (Si 3 N 4 ) thin films by means of an in situ activation of the reactions at T C and under P = 1 to 2 torr. Mono-Si substrates were nitrided using a mixture of argon containing SiH 4 and NH 3 . O 2 has also been added to the reaction gases. The activation was performed by a DC electrical discharge. The substrate was not used as an electrode, and was placed parallel to the discharge current. This configuration minimized the contamination of the films during their formation. The layers have been analyzed using FT-IR and SIMS spectroscopy. MIS structures have been realized using these nitrided layers. The flat-band shift ΔV FB (about −1 V) and the interface state density N it has been measured. Good electrical characteristics are obtained with T ⩽ 225 ° C .