Mechanism of the chemical vapour deposition of Si3N4 films from SiH2Cl2 and NH3 under diffusion-controlled conditions

At medium carrier gas flow rates the rate of chemical vapour deposition of Si₃N₄ films is limited by diffusion in the gaseous phase. The mechanism of Si₃N₄ deposition from, for example, the SiH₂Cl₂-NH₃ system in the diffusion regime is explained on the basis of the stagnant gas layer model. The applicability of the general stagnant gas layer model is based on the following simplifying assumptions: the limitation of the deposition rate by the diffusion of the single silicon-containing species, i.e. SiH₂Cl₂, and the insignificant contribution of the homogeneous reaction in the gaseous phase. The experimental results are in reasonable agreement with the theoretical predictions of the model.     

Calculation of deposition conditions for Si3N4 from a SiL4-NH3 gas phase (L=H,Cl, Br,CH)

Deposition conditions yielding silicon nitride are calculated for a set of initial gaseous systems, by complex thermodynamic equilibria computations. The influence of temperature, total pressure and reactant gas ratios on the composition and yield of the condensed phase are shown for SiH₄/NH₃, SiCl₄/NH₃, SiBr₄/NH₃, Si(CH₃)₄/NH₃ and SiH₄/ CH₄/NH₃ systems. The interest of such calculations is to give an efficient approach for experimental studies of vapour-deposition systems.     

Preparation of amorphous Si3N4-C plate by chemical vapour deposition

Chemical vapour deposition of a Si-N-C system has been studied by using SiCl₄, NH₃, H₂ and C₃H₈ as source gases at deposition temperatures (T _dep) of 1100 to 1600° C, and total gas pressures (P _tot) of 30 to 100 torr. To control the amount of carbon in these deposits the propane gas flow rate [FR(C₃H₈)] was varied from 0 to 200 cm³ min^–1. Homogeneous plate-like amorphous deposits were successfully prepared atT _dep=1100 to 1300° C,P _tot=30 to 70 torr andFR(C₃H₈)=25 to 100 cm³ min^–1. The deposits were composed of amorphous silicon nitride and carbon and the carbon content increased up to 10 wt% with increasingFR(C₃H₈). The surfaces of the deposits had a pebble-like structure.     

Hybrid plasma chemical vapour deposition of aluminium nitride from Al and NH3/N2 mixtures

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal plasma chemical vapour deposition for SiC powders from SiCH3Cl3-H2

Ultrafine -SiC powders were synthesized by introducing trichloromethylsilane and hydrogen into the high temperature RF thermal plasma argon gas. Powders were characterized by XRD, TEM, TGA, FT-IR and wet chemical analysis. Two different positions of reactant gas injection, i.e., upstream and downstream of the plasma flame, were compared in terms of the powder characteristics. The optimum concentration of hydrogen was found out to be about 3 to 4 mol % for the upstream injection. Amorphous SiC with free carbon was formed when the hydrogen concentration was lower than optimum and -SiC with free silicon was formed when it was higher than the optimum. For the downstream injection, free silicon formation was not significant and free carbon formation was suppressed when the hydrogen concentration was higher than 7 mol %. Chemical reaction pathways were suggested which could explain these observations.     

High temperature evaporation characteristics of amorphous Si3N4-C composite prepared by chemical vapour deposition

Evaporation characteristics of amorphous Si₃N₄ and amorphous Si₃N₄-C composite (6 wt % C) prepared by the chemical vapour deposition (CVD) were investigated in the temperature range of 1400 to 1650°C in a vacuum of about 10^?6 torr. The weight loss due to the evaporation was linear with time for all the samples tested. Evaporation rate of the amorphous CVD-(Si₃N₄-C) composite was 50 to 70% of that for the amorphous CVD-Si₃ N4. The activation energy for evaporation, calculated from the temperature dependence of the evaporation rates, was about 160 kcal mol^?1 for both samples. The carbon dispersed in the amorphous CVD-(Si₃N₄-C) composite reacted at the time of heat-treatment with the amorphous Si₃N₄ matrix and formedβ-SiC particles. Theβ-SiC particles were found to be about 100 nm in diameter and connected each other to form a three-dimensional network structure.     

On the chemical vapour deposition of Ti3SiC2 from TiCl4-SiCl4-CH4-H2 gas mixtures

In the ternary system Ti-Si-C, the ternary compound Ti₃SiC₂ seems to exhibit promising thermal and mechanical properties. Its synthesis as a thin film from the vapour phase is very difficult owing to the complexity of the system. A contribution to the knowledge of the CVD of Ti₃SiC₂ from a TiCl₄-SiCl₄-CH₄-H₂ gas mixture is proposed on the basis of a thermodynamic approach. This approach is based on a reliable estimation of Ti₃SiC₂ thermodynamic data in good accordance with recent experimental results on its thermal stability. A first equilibrium calculation for the deposition on an inert substrate shows the influence of the experimental parameters on the composition of both the deposit and the gas phase. As a result, the deposition of Ti₃SiC₂ can be favoured by an excess of TiCl₄ ( 45%), a rather low pressure (10–20 kPa), high temperature ( 1273 K) and low H₂ dilution ratio. On the basis of equilibrium calculations for various reactive substrates, complex mechanisms of Ti₃SiC₂ deposition are pointed out, with intermediate steps of substrate consumption, e.g. the formation of TiC from a carbon substrate or TiSi₂ from a silicon substrate.     

On the chemical vapour deposition of Ti3SiC2 from TiCl4-SiCl4-CH4-H2 gas mixtures

An experimental study of the deposition of Ti₃SiC₂-based ceramics from TiCI₄-SiCI₄-CH₄-H₂ gaseous precursors is carried out under conditions chosen on the basis of a previous thermodynamic approach, i.e. a temperature of 1100°C, a total pressure of 1 7 kPa, various initial compositions and substrates of silicon or carbon. Ti₃SiC₂ is deposited within a narrow composition range and never as a pure phase. A two-step deposition process is observed, in agreement with the thermodynamic calculations. For a silicon substrate, TiSi₂ is formed as an intermediate phase from consumption of Si by TiCI₄ and then is carburized by CH₄ into Ti₃SiC₂. For a carbon substrate, the first step is the formation of TiC_x either from consumption of carbon by TiCI₄ or by reaction between TiCI₄ and CH₄ and then TiC_x reacts with the gaseous mixture to give rise to Ti₃SiC₂. In most cases, Ti₃SiC₂ is obtained as small hexagonal plates oriented perpendicular to the substrate surface. These nano- or micro-crystals are usually co-deposited with TiC_x and to a lesser extent SiC, and their size is increased by increasing the dilution of the gaseous mixture in hydrogen.     

Effect of gas phase on SiC and Si3N4 formations from SiO2

During the synthesis of SiC, Si₃N₄ and sialon whiskers by carbothermal reduction of SiO₂, a localized formation of amorphous phases or Si₂N₂O powders was observed beneath these whiskers. Because these whiskers were formed by the vapour/solid mechanism, the controlling gas phase was of primary importance to obtain whiskers of tailored morphology and chemistry. To elucidate the effect of the gas phase composition on the reaction mechanisms of SiC and Si₃N₄, the oxygen partial pressure was measured during the synthesis with a ZrO₂ solid electrolyte. The carbothermal reduction of SiO₂, as well as evolution of gases, were accelerated by a formation of a molten fluorosilicate with an auxiliary halide bath. The oxygen partial pressure steadily increased with increasing temperature and reached a maximum level of 10^–1110^–12 atm in the early stage of reaction at 1623 K, then decreased again towards the end of reaction in both cases. Effects of the gas phase on the SiC and Si₃N₄ formations were not the same: p _CO and and their ratio were important factors in the SiC formation, while the higher formed an oxynitride phase in the Si₃N₄ formation.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Chemical vapour deposition of PbTiO3 films onto TiO2-Si

Ferroelectric lead titanate thin films were deposited on the TiO₂ coated Si(100) substrate by chemical vapour deposition under atmospheric pressure. We have used Pb powder and tetraethyle-orthotitanate [Ti(C₂H₅O)₄] as the source material of Pb and Ti vapour. We investigated the variations of the phase of deposited films with the Ti(C₂H₅O)₄ input fractions. When the mole fraction was 0.021, PbTiO₃ single phase was obtained. For the other Ti(C₂H₅O)₄ input fractions, we could find PbO solid solutions phase in addition to the PbTiO₃ phase. The results of AES analysis revealed that the TiO₂ layer on the Si substrate acted as diffusion barriers between Si substrate and PbTiO₃ films.     

Synthesis and characterization of a new p-phenylenediammonium dihydrogenmonophosphate [p-NH3C6H4NH3][H2PO4]2

Chemical preparation, crystal structure and NMR spectroscopy of a new organic cation p-phenylenediammonium monophosphate [p-NH₃C₆H₄NH₃][H₂PO₄]₂ are presented. This new compound crystallizes in the orthorhombic system, with the space group Pnma and the following parameters: a = 7.970 (2) Å; b = 22.770 (7) Å; c = 7.000 (7) Å, V = 1270.3 (11) Å³, Z = 4 and D_x = 1.590 g cm⁻³. The crystal structure has been determined and refined to R = 0.043 and R_(w) = 0.057 using 2623 independent reflections. The structural arrangement can be described as inorganic layers of (H₈P₄O₁₆)⁴⁻ units, parallel to (a, c) planes. The organic groups (p-H₃NC₆H₄NH₃)²⁺are anchored between the phosphoric layers to form a three-dimensional infinite network. This compound is also investigated by IR and solid-state ¹H, ¹³C and ³¹P MAS NMR spectroscopies. The ab initio method is used in the calculation of chemical shifts.     

Hot corrosion of chemical vapour deposited SiC and Si3N4 in molten Na2SO4 salt

The corrosion behaviour of SiC and Si3N4 prepared by chemical vapour deposition (CVD) technique was investigated in molten Na2SO4 salt under argon and argon–oxygen mixture gases. The CVD-SiC was more attacked than the CVD-Si3N4 in molten Na2SO4 salt. This was in good agreement with the results of the thermodynamic equilibrium calculations. The corrosion of both materials in argon–oxygen mixture was less severe compared to that in argon. This was attributed to the formation of amorphous SiO2 acting as a protective film against the corrosion. The apparent activation energies of the CVD-SiC and CVD-Si3N4 in molten Na2SO4 under argon gas were 167 kJ mol-1 and 595 kJ mol-1, respectively. © 1998 Chapman & Hall