Chemical vapour deposition of coatings

Chemical Vapour Deposition (CVD) of films and coatings involve the chemical reactions of gaseous reactants on or near the vicinity of a heated substrate surface. This atomistic deposition method can provide highly pure materials with structural control at atomic or nanometer scale level. Moreover, it can produce single layer, multilayer, composite, nanostructured, and functionally graded coating materials with well controlled dimension and unique structure at low processing temperatures. Furthermore, the unique feature of CVD over other deposition techniques such as the non-line-of-sight-deposition capability has allowed the coating of complex shape engineering components and the fabrication of nano-devices, carbon-carbon (C-C) composites, ceramic matrix composite (CMCs), free standing shape components. The versatility of CVD had led to rapid growth and it has become one of the main processing methods for the deposition of thin films and coatings for a wide range of applications, including semiconductors (e.g. Si, Ge, Si_1-xGe_x, III-V, II-VI) for microelectronics, optoelectronics, energy conversion devices; dielectrics (e.g. SiO₂, AlN, Si₃N₄) for microelectronics; refractory ceramic materials (e.g. SiC, TiN, TiB₂, Al₂O₃, BN, MoSi₂, ZrO₂) used for hard coatings, protection against corrosion, oxidation or as diffusion barriers; metallic films (e.g. W, Mo, Al, Au, Cu, Pt) for microelectronics and for protective coatings; fibre production (e.g. B and SiC monofilament fibres) and fibre coating. This contribution aims to provide a brief overview of CVD of films and coatings. The fundamental aspects of CVD including process principle, deposition mechanism, reaction chemistry, thermodynamics, kinetics and transport phenomena will be presented. In addition, the practical aspects of CVD such as the CVD system and apparatus used, CVD process parameters, process control techniques, range of films synthesized, characterisation and co-relationships of structures and properties will be presented. The advantages and limitations of CVD will be discussed, and its applications will be briefly reviewed. The article will also review the development of CVD technologies based on different heating methods, and the type of precursor used which has led to different variants of CVD methods including thermally activated CVD, plasma enhanced CVD, photo-assisted CVD, atomic layer epitaxy process, metalorganic assisted CVD. There are also variants such as fluidised-bed CVD developed for coating powders; electrochemical vapour deposition for depositing dense films onto porous substrates; chemical vapour infiltration for the fabrication of C-C composites and CMCs through the deposition and densification of ceramic layers onto porous fibre preforms. The emerging cost-effective CVD-based techniques such as electrostatic-aerosol assisted CVD and flame assisted CVD will be highlighted. The scientific and technological significance of these different variants of CVD will be discussed and compared with other vapour processing techniques such as Physical Vapour Deposition.     

Chemical vapour deposition of silanes

Chemical surface modification of oxide-covered surfaces by silanization is a well-known technique in such fields as gas and liquid chromatography, electro-chemistry and immobilization of biomolecules. In the commonly used silanization technique the silane is reacted with the surfaces in a liquid phase. If strict anhydrous conditions do not prevail, however, this technique often results in polymerization, irreproducibility and instability of the silane films. We report here on a gas phase silanization of silicon surfaces at elevated temperatures. The method comprises a washing and surface activation step followed by silanization at about 0.5–1 Nm^?2 and 80–190 °C depending on the type of silane. The silanized surfaces were characterized by ellipsometry, contact angle measurements and scanning electron microscopy, which revealed smooth, stable and reproducible silane films of monolayer character. A comparison of surfaces that were silanized in the gas phase with those that were silanized in the liquid phase was also made.     

Chemical vapour deposition of chromium

The chemical vapour deposition of chromium and the partial transformation of chromium into chromium carbide on carbon-containing substrates such as tool steels was investigated.The process using CrCl₂ as the reactive gas is described. Of the different process parameters studied, the substrate composition was found to have an important influence on the nature and properties of the chromium layer. In the early stages of deposition the reaction is controlled by the exchange of Fe from the substrate for Cr. As the H₂ reduction process becomes increasingly important, a nearly linear relationship exists between the growth rate and the deposition time. The composition, structure, thickness, hardness, roughness, corrosion resistance and tribological properties of the coatings are described. Some applications of chromium coatings are also given.     

Chemical vapour deposition of tantalum diboride

Tantalum diboride (TaB₂) was deposited on a quartz substrate from a gas mixture of TaCl₅, BCl₃, H₂, and Ar at a temperature between 900 and 1300° C. When the atomic ratio (B/Ta) in source gas was held above 1.0 at 1000° C, TaB₂ with a composition of between TaB_1.90 and TaB_1.95 was obtained in a single phase. The deposits grew to grain crystals with an increase in temperature and with an increase in the atomic ratio (B/Ta) in the source gas. The mass transfer of TaCl₅ was supposed to be the rate-determining step. The Vickers microhardness values for the coating deposited at 1100° C from a source gas with atomic ratio (B/Ta) above 1.0 were 3500 to 4100 kg mm^–2. Dispersing Ni or Pd on the substrate as an impurity, woolly crystals of up to 100m in length were grown in 30 min at 1050° C, and the growth mechanism was thought to be that of tip-VLS.     

Chemical vapour deposition of chromium onto nickel

The chemical vapour deposition (CVD) of chromium onto a pure nickel substrate was investigated in the presence of hydrogen at three temperatures (950, 1000 and 1050°C) using a pack cementation method.Of the thermochemical reactions generally involved in CVD, such as thermal decomposition, reduction, displacement and disproportionation, the only significant reaction which occurs for this system is a reduction of CrCl₂ by hydrogen according to

$\text{CrCl}{}_2\text{(g)+H}{}_2\text{(g)→Cr(s)+2HCl(g)}$

Two diffusion layers appear during the process. (1) In the early stages of deposition a nickel-rich layer with a face-centred cubic structure develops. (2) Depending on the time and the temperature of the coating, an outer chromium-rich layer with a body-centred cubic structure develops progressively on the sample surface and grows from or together with the inner layer.The deposition rate is controlled by diffusion of the chromium and the nickel atoms through the coating layers, mainly through the inner layer.     

Chemical vapour deposition of molybdenum on graphite

Transmission electron microscopy was used to investigate the Au-Sn thin film system. Evaporated films with total thickness ranging from about 300 to about 1000 Å and with composition ranging from 5 to 90at.% Sn were studied with respect to intermetallic compound phases, texture and microscopic structure. It was found that the unannealed films contained one or more of the phases AuSn, AuSn₂ and AuSn₄, depending on the overall composition. The texture and structure of the films were dependent on the order of evaporation as well as the composition. In many cases a considerable grain growth took place during annealing.     

Chemical vapour deposition of epitaxial ZnO films

Heteroepitaxial growth of ZnO on sapphire has been studied, using the (H₂HClZnO) vapour growth technique. A systematic study of the influence of the substrate orientation and temperature of growth on the quality of the films has been carried out. The surface morphology of the grown layer was strongly dependent on substrate orientation and growth anisotropy.     

Chemical vapour deposition densification of plasma-sprayed oxide coatings

Chemical vapour deposition (CVD) was used to produce an additional protective layer on the surface of plasma-sprayed alumina coatings. This layer was deposited from an AlCl₃-CO₂-H₂ gas mixture at a temperature of 1000°C. The thickness of the layer varied from 10 to 20 μm and the grain size varied from about 3 to 30 μm. The CVD-produced alumina had the crystallographic structure of stable α-Al₂O₃. The protective quality of the composite coatings was verified with electrochemical measurements. These indicated that completely tight coatings could be obtained.     

Chemical vapour deposition of nitrogen-doped titanium dioxide thin films

Nitrogen-doped titanium dioxide is often considered as a promising nanomaterial for photocatalytic applications. Here we report the first results of a study of APCVD of N-doped TiO2 thin films prepared with the use of ammonia as a source of nitrogen and titanium tetraisopropoxide (TTIP) as a source of Ti and O atoms. The obtained films were analyzed with X-ray diffraction, infrared spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, UV-Vis spectroscopy, and ellipsometry. It was found that the film growth rate in the TTIP-NH3-Ar reaction system varied insignificantly with substrate temperature in the range of 450,..., 750 degrees C and did not exceed 4.4 nm/min. Yellow and orange layers with nitrogen content of about 7.6% were formed at the deposition temperature higher than 600 degrees C. The results of the structure analysis of the deposited films showed that addition of ammonia led to stabilization of the amorphous phase in the films. The effect of ammonia on optical and photocatalytic properties was also considered.     

Chemical vapour deposition (CVD) of borophosphosilicate glass films

Chemical vapour deposition (CVD) has become the standard method for the fabrication of microelectronic devices for use in the semiconductor industry. In this investigation, it has been used to grow films of silicon dioxide (SiO₂) and borophosphosilicate glass (BPSG) at both atmospheric and low pressures under various conditions. The growth behaviour of SiO₂ and BPSG films has been investigated as a function of the O₂/SiH₄ ratio. Both processes give a similar trend, with the growth rates of BPSG being somewhat higher than SiO₂. The variation in the growth rate with O₂/SiH₄ ratio has been explained in terms of relative transport and kinetic reaction rates. The effects of temperature on the deposition rate have also been studied and the activation energy calculated showed two distinct regions corresponding to mass transport control and kinetic control regimes. Both BPSG and SiO₂ have been annealed under various furnacing conditions. It has been shown that the addition of boron and phosphorous results in much lower reflow temperatures and times. This has a significant bearing on the performance characteristics of devices. Initial results from rapid thermal annealing (RTA) work are also presented, and RTA is shown to be a viable annealing process.     

Chemical vapour deposition of tungsten and TiN on SiC fibres

Handling of uncoated SiC/W (Sigma) fibres resulted in a drop in their strengths. Coating with TiB₂ or boron at about 1000 °C by chemical vapour deposition resulted in a further reduction in strength. Coating with tungsten or TiN by lower temperature chemical vapour deposition processes did not degrade the fibre and the coated fibres also retained their strengths after heating in hydrogen at 1040 °C.     

Chemical vapour deposition of titanium nitride on plasma nitrided steel

Ferritic and austenitic nitriding by the plasma nitriding technique were investigated for the modification of steel substrates prior to the chemical vapour deposition of titanium nitride at 1273 K. It was confirmed that prenitriding enhances the growth of the titanium nitride layer and it was found that a TiN coating can be formed using substrate derived nitrogen only. Control of porosity, arising during austenitic nitriding, was investigated and it was found that in practice this phenomenon could not be avoided.     

Chemical vapour deposition of tungsten films for metallization of integrated circuits

New materials and processes are required for the metallization of very large- scale integrated circuits. Tungsten offers several advantages as a contact barrier, low resistance gate material and metal for interconnections. Conventional, plasma- enhanced and laser-induced chemical vapour deposition processes used to produce tungsten films are described. The basic reactions involved are either pyrolysis of the carbonyl (W(CO)₆) or reduction of halides (WCl₆ and WF₆). The mechanism of tungsten deposition via the H₂ and silicon reduction of WF₆ is discussed. The physical properties of tungsten films (resistivity, W-Si contact resistance) and the charateristics of Schottky barrier diodes and W/SiO₂/Si structures are reviewed. The chemical properties of tungsten films, including W-Si reactivity (thermal stability of W-Si contacts) and suitable etching solutions are presented. Several applications of tungsten films for metallization of integrated circuits are examined.     

Chemical vapour deposition of molybdenum carbides: aspects of phase stability

Thin films of different molybdenum carbides (δ-MoC_1−x, γ′-MoC_1−x and Mo₂C) have been deposited from a gas mixture of MoCl₅/H₂/C₂H₄ at 800°C by CVD. The H₂ content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H₂ contents lead to the formation of nanocrystalline δ-MoC_1−x films while coarse-grained γ′-MoC_1−x is formed with an H₂-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that γ′-MoC_1−x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the γ′-MoC_1−x phase is stabilized by oxygen but that the difference in energy between e.g. δ-MoC_0.75 and oxygen-free γ′-MoC_0.75 is small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable δ-MoC_1−x and γ′-MoC_1−x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.