The production of thin or thick films of metals or ceramics by chemical vapour deposition has often been achieved by the use of halide gas precursors. In certain cases, this choice was made purely for reasons of simplicity: gas cylinder available, gas species already used in another field, etc. Experience has subsequently shown, however, that this choice can give rise to significant changes in the nature and proportions of deposited phases. These are highly dependent upon:

• – the value of the oxidiser:reducer ratio in the gas phase,

• – the degree of metal oxidation in the halide considered,

• – possible competition between two reducing agents designed to reduce the halide.

The formalism for the accurate modeling of chemical vapor deposition (CVD) processes has matured based on the well established principles of transport phenomena and chemical kinetics in the gas phase and on surfaces. The utility and limitations of such models are discussed in practical applications for high temperature structural materials. Attention is drawn to the complexities and uncertainties in chemical kinetics. Traditional approaches based on only equilibrium thermochemistry and/or transport phenomena are defended as useful tools, within their validity, for engineering purposes. The role of modeling is discussed within the context of establishing the link between CVD process parameters and material microstructures/ properties. It is argued that CVD modeling is an essential part of designing CVD equipment and controlling/optimizing CVD processes for the production and/or coating of high performance structural materials.

Equilibrium calculations are reported for conditions typical of silicon carbide (SiC) deposition from mixtures of silane and hydrocarbons. Included are 34 molecules containing both silicon and carbon, allowing an assessment to be made of the importance of organosilicon species (and organosilicon radicals in particular) to the deposition process. The results are used to suggest strategies for improved operation of SiC CVD processes.

A kinetic model is presented for the deposition of silicon carbide through decomposition of methyltrichlorosilane (MTS). The developed model includes gas phase (homogeneous) reactions that lead to formation of deposition precursors and surface (heterogeneous) reactions that lead or can lead to deposition of silicon carbide, silicon, and carbon. The kinetic model is incorporated in a transport and reaction model for a tubular hot-wall reactor, and the overall reactor model is used to obtain some preliminary results on the effects of pressure and distance in the reactor on the rate of deposition and the composition of the deposit. The results show that the model can reproduce most of the experimental observations of the literature.

The deposition rate from mixtures of methyltrichlorosilane (MTS), hydrogen and methane was measured thermogravimetrically using a hot wall vertical reactor and planar SiC substrates. Below 850 °C and at sufficiently high gas velocities, the rate of the phase boundary reaction could be determined. In the absence of CH4 and at H2 :MTS=55, Si was deposited together with SiC. Addition of CH4 lowered the Si content, pure SiC being deposited at CH4 :MTS above 10. The deposition rate j in the range 750 to 850 °C follows the equation with E(Si) = 160 and E(SiC) = 300 kJ/mol. Reaction mechanisms are presented to account for the observed reaction orders with respect to MTS. Between 900 and 970 °C, the reaction rate decreased with temperature indicating a change in the deposition mechanism.

The CVD of Si3N4 from SiF4 and NH3 gaseous precursors was studied using a hotwall reactor in the temperature range of 1340 to 1490°C. The effects of temperature, time, flow rate, and SiF4/NH3 molar ratio on deposition rate and axial and radial deposition profiles were identified. The decomposition characteristics of pure NH3 and SiF4 were studied utilizing mass spectroscopy and compared to thermodynamic predictions.

A combined experimental/theoretical study is presented of the onset conditions for gas phase reaction and particle nucleation in hot substrate/cold gas CVD of transition metal oxides. Homogeneous reaction onset conditions are predicted using a simple high activation energy reacting gas film theory. Experimental tests of the basic theory are underway using an axisymmetric impinging jet CVD reactor. No “vapor phase ignition” has yet been observed in the TiCl4/O2 system under accessible operating conditions (below substrate temperature Tw=1700 K) and further experiments are planned using more reactive feed materials. The goal of this research is to provide CVD reactor design and operation guidelines for achieving acceptable deposit microstructures at the maximum deposition rate while simultaneously avoiding homogeneous reaction/nucleation and diffusional limitations.

Particle-enhanced chemical vapor deposition (PECVD) is capable of producing ceramic films at high deposition rates. A mathematical model of the particle-vapor codeposition process has been developed and has been applied to PECVD processes to predict deposition rate enhancements and deposit properties.

A two—dimensional finite element model was developed to study the step coverage of submicron trenches with arbitrary shape under chemical vapor deposition processes. Parameters that characterize the step coverage were found to be the surface Damkohler number, ratio of diffusion coefficients inside and outside of the trench, and aspect ratio of the trench geometry. Efforts were concentrated on studying the step coverage of SiO2 film deposited from SiH4/O2 precursors within rectangular shape trenches. The model predictions were found to be in good agreement with reported experimental results.

The present study outlines the importance of carbon active sites in controlling the kinetics of pyrolytic carbon deposition on a graphitized pitch fiber at 1025°C. Blockage of active sites with chemisorbed hydrogen retards deposition rates substantially. Removal of the chemisorbed hydrogen from the occupied sites raises deposition rates to the normal values noted on a fresh “clean” surface. The effect of surface activation prior to deposition is discussed. Activating the surface generates additional active sites and enhances the rates. However, not all the newly developed sites contribute to the kinetics. After the deposition of the first carbon layer, a fraction of those newly developed sites is instantaneously blocked and does not further contribute to subsequent deposition. The remaining fraction, along with the original sites available before activation, keeps replicating during the remaining course of deposition. That is, the disappearance of one active site after carbon deposition is associated with the generation of a new site. This trend continues up to the deposition of 60 carbon layers.

The reactivity of several fluorine-containing molecules on a polycrystalline silicon nitride (Si3N4) surface is studied under ultrahigh vacuum (UHV) conditions using temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). The chemistry of fluorine on Si3N4 is of interest in understanding the high temperature chemical vapor deposition (CVD) of Si3N4, which uses SiF4 as a starting material. XeF2 is reacted with a Si3N4 surface to prepare and characterize various surface SiFx (1 ≤ × ≤ 3) species. These are identified by the chemical shift induced by the fluorine atoms in the Si (LMM) Auger peak and by changes in the TPD. Of these species, SiF2 is stable to the highest temperature. SiF2 is also formed by the reaction of SiF4 with a Si3N4. Because SiF2 is so stable, its decomposition is proposed as a rate-determining step in the CVD deposition of Si3N4 from SiF4. Gaseous HF, which is a product of the CVD process, does not dissociate on Si3N4 and is therefore unlikely to cause the etch-like marks on the Si3N4 coating that are observed under certain conditions.

A laser scattering apparatus was developed for the determination of surface roughness and other surface statistical parameters of chemically vapor-deposited coatings. Visual examination of HeNe laser scattering patterns reflected from polished sapphire and CVD titanium nitride surfaces showed a sensitivity to roughness differences of tens of nanometers. The scattering apparatus was integrated with a cold-wall CVD reactor. The root mean square roughness of silicon carbide deposits on silicon in the early stages of growth was determined from the intensity of the specularly reflected beam. Changes in roughness and the spatial arrangement of depositing crystallites were monitored in situ by angular resolution of the scattered light spectra. Both ex situ and in situ results were in good agreement with profilometric examinations of the rough surfaces.

Light-scattering was used to monitor the chemical vapor deposition of silicon carbide from methyltrichlorosilane. The nucleation and growth of the SiC features caused changes in the surface topography that altered the angular scattering spectrum that was generated with a He-Ne laser. These scattering spectra were then analyzed to obtain information about the nucleation and growth processes that are occurring.

Both electron microscopy and light scattering have played an important role in elucidating the processes of inception, growth, and oxidation of carbonaceous particles in flames. The techniques developed have application to the various pyrogenic materials including the metallic oxides, carbides, etc. Thermophoretic sampling has been developed to afford efficient extraction of particle samples from hot reaction zones. The sampling procedure preserves the particle morphology for subsequent analysis by transmission electron microscopy (TEM). Studies using this technique have shown aggregate structures with fractal dimensions of 1.6 to 1.8, a result that is consistent with the computer simulations of the cluster-cluster aggregation process. Diverse morphologies, including microparticles found in the particle inception zone, reveal the evolution of these aggregates. The optical cross sections for polydisperse aggregates which are used to interpret the laser scattering/extinction tests (LSE) are described. Population averaged properties - volume fraction, volume mean diameter, monomer and aggregate number concentrations, mean-square radius of gyration - are derived. The interactive use TEM and LSE data leads to a global description of the aggregate dynamic processes which are found to be regionally partitioned within the laminar hydrocarbon diffusion flame.

An understanding of the chemical processes occurring in the gas phase during metalorganic chemical vapor deposition is needed to design novel precursors and for the subsequent control of the composition and the microstructure of the solid product. Tunable diode laser absorption spectroscopy provides a means to precisely monitor specific bond rupture in the precursor during pyrolysis. Methylsilazane [CH3SiHNH]n,, a precursor to silicon-based ceramic thin films, was used to investigate the potential of this technique. Below the decomposition temperature, the intensity of the absorption line at 871.6±0.1 cm−1 corresponding to one of the harmonics from Si-CH3, increased linearly with the vapor pressure of methylsilazane up to 800 Pa and then decreased exponentially. The typical linewidths of the absorption line was approximately 0.006 cm−;1, orders of magnitude narrower than would be observable using conventional infrared techniques. The absorption line was detectable over a pressure range from less than 1 Pa to 10 kPa.

Silicon nitride (Si3N4) has been demonstrated to be an effective high temperature anti-oxidant when deposited in its α-crystalline form. The Materials Technology Laboratory at UTRC has developed a pilot-scale chemical vapor deposition (CVD) reactor capable of depositing α-Si3N4 from ammonia (NH3) and silicon tetrafluoride (SiF4) at 1.8 torr and 1440 C. Coherent anti-Stokes Raman spectroscopy (CARS) has been applied to measure H2 produced in this reactor. Axial concentration measurements have been performed both in the presence and absence of SiF4. Previous CARS measurements demonstrated the importance of surface (Si3N4) catalyzed decomposition of NH3:

as a competing reaction to:

in the CVD reactor under deposition conditions. The observed hydrogen concentration profiles confirm these measurements and allow quantitative comparison between the competing reactions. NH3 decomposition is suppressed 20% by the addition of SiF 4 in a 6:1 (NH3:SiF4) molar ratio. No decomposition is observed in the absence of Si3N4.

OH vibrational Thermally Assisted Fluorescence (THAF) temperature measurements have been demonstrated in both premixed and diffusion flames. The accuracy of the measurements is generally better than 100 K over a wide range of flame conditions for which the collisional quenching rate varies considerably. Application of this technique for temperature measurement in Chemical Vapor Deposition (CVD) flows, for which the quenching rate is relatively constant, should exhibit greater accuracy. THAF measurements in these flows are limited by signal to noise considerations, and should be possible down to pressures of 103-104 Pa or less.

The first optical emission spectra of Ti, TiN and TiSi2 plasma during growth of thin films by pulsed laser evaporation has been reported. A KrF excimer laser (248nm) with pulsewidth of 45ns operating on 5Hz repitition rate was used for deposition of refractory metal thin films using varying laser fluence of 4J/cm2 to 15J/cm2. Most of the radiative species seen in plasma belongs to atomic neutrals and ionic species such as Ti I, Ti II, Si I, Si II and N 11. Emission spectra was mostly dominated by neutral and ionic emission from Titanium (Ti I and Ti II).

The coadsorption and reaction of diborane with ammonia and with hydrazine on Ru(0001) have been studied using X-ray photoelectron spectroscopy (XPS) and thermal desorption mass spectroscopy (TDS). Diborane is found to decompose to atomic boron and hydrogen upon adsorption at T>200K. Multilayers of diborane and ammonia, deposited at 90K on Ru(0001), react when annealed to 600K. The XPS results indicate that boron-nitrogen adlayers can be formed by this reaction. These boron-nitrogen films are boron-rich and.decompose at temperatures higher than 1100K. Our TDS studies reveal that hydrazine decomposes extensively to NH3, N2, N and H on Ru(0001). Due to its higher reactivity, boron-nitrogen films of B/N stoichiometric ratio near unity are obtained when hydrazine is used rather than ammonia. In our studies, these films were formed by either simultaneously dosing B2H6 and N2H4 at 450K or by coadsorption of the reactants at 90K and subsequent annealing to 450K. These studies have shown that diborane and hydrazine can be successfully used as molecular precursors in the low temperature deposition of boron nitride thin-films.

A novel near-room temperature CVD process has been developed using H-atoms reaction with WF6 to produce tungsten and tungsten oxide films. The chemical, physical and electrical properties of these films were studied. Good adhesion and low resistivity of W films were measured. Conformal W03 films were obtained on columnar tungsten using a small amount of molecular oxygen in the gas stream. A reaction mechanism was evaluated on the basis of experimental results. The advantages of the method include deposition of adherent films in a plasma-free environment, near-room temperature, with a low level of impurity