S. Rigo

An 18O study of the thermal oxidation of silicon in oxygen

The mechanism of thermal oxidation of silicon in dry oxygen was studied using 18O as the tracer. SiO2 layers first grown in natural oxygen (1300–3000 A) were further grown in highly 18O‐enriched oxygen for 8.5 h at 930 °C. 18O profiling was carried out using the 629‐keV narrow resonance in the nuclear reaction 18O(p,α)  15N. The resulting SiO2 films consist of two 18O‐rich layers, 7% near the SiO2 surface and 93% near the Si‐SiO2 interface, while the bulk 18O concentration is very low. The results suggest that the oxide grows mainly through long‐range migration of oxygen, favoring models based on the transport of molecular oxygen.

The thermal oxidation of silicon the special case of the growth of very thin films

Abstract The thermal oxidation of silicon is generally modelled by Deal and Groves theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO2 interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.

Oxidation of silicon

Abstract We describe a unified model for silicon oxidation in dry and wet conditions. It goes beyond the current models of kinetics and of ellipsometric and spectroscopic data by explicitly addressing the issues raised by isotope experiments. The model concentrates especially on the problems indicated in recent discussions, notably the very early stages of oxidation, the reactions at the Si/oxide interface, and the origin and nature of stress. Three features emerge as central: first, the several roles of stress and stress relaxation; second, the specific properties of the oxide adjacent to the silicon and the concept of the ‘reactive’ layer different in structure and composition, and third, the processes which encourage the insertion of interstitial oxygen into the silicon/oxygen network of the oxide. This ‘reactive layer’ or ‘altered layer’ is central to our analysis of dry oxidation through its role in the conversion of interstitial oxygen molecules into network oxygens. This conversion can be stimulate...

Structural evolution of very thin silicon oxide films during thermal growth in dry oxygen

We have studied the structural characteristics of very thin silicon oxide films thermally grown (at 930 °C, 10 Torr) in 18O enriched dry oxygen for times ranging from 0.5 to 22.5 h (corresponding to equivalent thicknesses ranging from 3.2 to 20.8 nm). Chemical etching combined with nuclear microanalysis, x‐ray photoelectron spectroscopy (XPS), and reflection high‐energy electron diffraction (RHEED) has been applied to the samples. A layer that is very slow to dissolve (of equivalent thickness ∼1.1 nm) was observed for oxide films grown for oxidation times of less than 1 h (equivalent thicknesses under 4.6 nm). As indicated by XPS this layer seems to be related to a structure different from what is observed for thicker films; its existence is also correlated to a structural order as seen by RHEED.

Study of atomic transport mechanisms during thermal nitridation of silicon in ammonia using 15N and D labelled gas

Thermal silicon nitridation mechanisms in ammonia gas at 950°C and under 2 × 10−5 Torr were investigated by using 15N as a tracer. Nitride layers first grown in 14NH3 were further grown in 15NH3. 15N profiling was obtained by combining nuclear microanalysis with step-by-step chemical dissolution of the nitride layer. Thus one can show that the nitride film consists of a 14N layer located near the Si/nitride interface and a 15N layer located near the external surface of the film. It was also shown that there is no isotope exchange between the nitrogen of the nitride network and of the NH3 gas. These results indicate that the growth does not occur by interstitial transport through the nitride film of the NH3 molecules nor by any other nitrogenated species resulting from a decomposition of the ammonia gas. A possible incorporation of hydrogenated species was also examined using ND3 nitridation. As the relative concentration of deuterium in such a film was less than 10−4, ammonia most probably dissociates at the outer surface and hydrogenated species do not react appreciably with the network. From these experiments, it can be concluded that the growth of the silicon nitride layer occurs by either: - transport of silicon atoms through the layer which then react with the nitridant gas; - or a step-by-step migration of a nitrogen network defect (vacancy or interstitialcy) in only one direction. In such a case, the defect should be electrically charged and the hoping direction should be imposed by a strong electrical field arising from the charging of nitrogenated species adsorbed at the external nitride surface by electrons coming from the silicon substrate because of the very small thickness of the nitride film (< 40 A) (tunnel or thermionic effects) .

Investigation of the Composition of Sputtered Silicone Nitride Films by Nuclear Microanalysis

The composition and uniformity of silicon nitride thin films deposited by reactive sputtering in a mixture of argon and nitrogen eventually contaminated by oxygen traces were studied. Both direct observation of nuclear reactions on nitrogen and oxygen and backscattering of 4He+ ions were used to analyze the deposits. The films appear uniform as a function of depth. They are practically oxygen free but contain large amounts of argon. Their stoichiometry changes with the partial pressure PN2 of nitrogen. The ratios N/Si and Ar/Si vary in the opposite direction; for smaller values of PN2 there is an excess of silicon, whereas for larger values an excess of nitrogen is observed with respect to the stoichiometric composition Si3N4.

Study of atomic transport mechanism of oxygen during thermal nitridation of silicon dioxide

Abstract Silicon dioxide films were nitrided at atmospheric pressure in ammonia gas at 1100°C. The atomic transport mechanism of oxygen during the nitridation was studied by using isotopic oxides (18O labeled oxides). Physico-chemical characterization of the films was carried-out by nuclear reaction analysis (NRA), secondary ion mass spectrometry (SIMS) and ellipsometry. The nitrogen incorporation in the oxide film is accompanied by a decrease of the oxygen amount. It is shown that this decrease of the oxygen amount is due to an autodiffusion phenomenon. A good fit between the calculated and experimental curves is obtained for a self-diffusion coefficient D ∗ of 3 × 10-15 cm2 s-1.

Thermal oxidation of silicon. The special case of the growth of very thin films

Abstract The thermal oxidation of silicon is generally modelled by Deal and Groves theory based on the assumption that the oxygen molecules dissolve in silicon in interstitial positions and migrate to the Si-SiO2 interface where they react with the silicon substrate. Experimental results for oxidation in dry oxygen agree with this theory only for thick oxide films. The growth of very thin oxide films exhibits particular features which are discussed in this paper. For these films, the growth mechanism is different from that of thick films; this difference is possibly associated with the transport of oxygen atoms through the silica network. The effect of hydrogenated impurities is also discussed.

Thermal nitridation of silicon dioxide at atmospheric pressure: Physico-chemical and electrical characterization

Abstract Thermal nitridation of silicon dioxide films was performed at atmospheric pressure in a furnace under NH 3 and at a temperature of 1100°C. Physico-chemical characterizations of the grown films were carried out by nuclear methods (NRA and ERD), electron spectroscopies (AES and ESCA) and ellipsometry. NRA measurements give quantitative results about nitrogen and oxygen concentrations and on the same samples AES and ESCA give the distribution of these elements throughout the films. The variation of the stoichiometry with the depth is determined. It is shown that the resulting nitroxide film is inhomogeneous with a nitrogen-rich surface layer and an interface pile-up of nitrogen. Nitridation is studied versus nitridation time and oxide thickness. The incorporation of nitrogen at the surface is higher when the initial oxide is thinner. As regards the bulk, the incorporation kinetics of nitrogen depends on the initial oxide thickness. Electrical characterizations of MIS structures realized with these nitroxide films show their good quality: flat-band voltage shifts are low; the difference in nature of interface charges is shown; conduction in the film is enhanced by nitridation as well as break-down electrical field.

Effect of pressure on thermally induced diffusivity and reactivity of water in thin amorphous silica films

Abstract We have studied the effect of water pressure on the exchange phenomena between 18O-labelled water and thin thermal silica films. We have shown that the Doremus model applies; that is, water molecules diffuse in interstitial positions while exchanging their oxygens with those of the silica network. H2O diffusivity is not affected by pressure variations, while reactivity seems to be. We discuss the latter point.