The Chemical Vapor Deposition Process

CVD is nowawell-established process that has reached major production status in areas such as semiconductors and cutting tools. It is a vapor-phase process which reiies on the chemical reaction of a vapor near or on a heated surface to form a solid deposit and gaseous by-products. The process is very suitable to the deposition of carbon, as reviewed below.l l 

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Naimpally, A., A Study of the Annealing Processes for Doped Glasses Deposited by the Chemical Vapor Deposition Process, Y4MPE J., pp. 24-5 (Sept/Oct 1988)... 

The chemical vapor deposition processes obtained from the laser induced multiphoton decomposition of neat organogermanes or sensitized by SFg have also been characterized for several systems. For example, the decomposition of Ge(OMe)4 leads to the formation of organoxogermanium polymers, while EtOGeMcs leads to materials rich in Ge and containing small amounts of oxygen and carbon. In the latter case, two primary processes have been proposed to be responsible for the chain reactions leading to the final products (equations 40a and 40b)... 

The constituents of the precursors for most of the ceramics exist in the form of heavy molecular gases or volatile liquids such as halides, hydrides, organo-meta11ics, hydrocarbons and ammonia complexes, which can be dissociated into highly reactive fragments by providing photon, electron or thermal excitation. These fragments can easily react with the second component of the desired ceramic and form a solid product. Most of the chemical vapor deposition processes work on this basic principle. They are known as Thermally Assisted CVD, Plasma Assisted CVD and Photo CVD for thermal, electron and photon excitation respectively. 

Viswanathan R, Thompson D L and Raff L M 1984 Theoretical investigations of elementary processes in the chemical vapor deposition of silicon from silane. Unimolecular decomposition of SiH J. Chem. Phys. 80 4230 0... 

In chemical vapor deposition processing, the principal source of residual stress is from a coefficient of expansion mismatch. One of the principal criteria for CVD processing is the matching of the coefficient of expansions of the film and substrate, which limits the possible film—substrate combinations that can be used. 

Tertiary stibines have been widely employed as ligands in a variety of transition metal complexes (99), and they appear to have numerous uses in synthetic organic chemistry (66), eg, for the olefination of carbonyl compounds (100). They have also been used for the formation of semiconductors by the metal—organic chemical vapor deposition process (101), as catalysts or cocatalysts for a number of polymerization reactions (102), as ingredients of light-sensitive substances (103), and for many other industrial purposes. 

Many attempts have been made to synthesi2e cubic BN at low pressures by some sort of chemical vapor deposition process in analogy with the low pressure deposition of diamond from methane in the presence of H atoms (see Diamond, synthetic). However, the amounts of cubic BN produced in this fashion in 1991 were miniscule, and were at best thin layers only a few do2en atoms thick (12). 

Of the many forms of carbon and graphite produced commercially, only pyrolytic graphite (8,9) is produced from the gas phase via the pyrolysis of hydrocarbons. The process for making pyrolytic graphite is referred to as the chemical vapor deposition (CVD) process. Deposition occurs on some suitable substrate, usually graphite, that is heated at high temperatures, usually in excess of 1000°C, in the presence of a hydrocarbon, eg, methane, propane, acetjiene, or benzene. 

Diamonds also occur in meteorites, probably as a result of high pressures produced dynamically by impact (10,11). The shock or explosive mode of synthesis is a viable process for fine diamond powders of both the cubic and hexagonal (lonsdaleite) polymorphs (12) naturally or otherwise. Some diamonds in space appear to have formed by processes more closely related to the low pressure chemical vapor deposition processes described later (see... 

Several patents dealing with the use of volatile metal amidinate complexes in MOCVD or ALD processes have appeared in the literature.The use of volatile amidinato complexes of Al, Ga, and In in the chemical vapor deposition of the respective nitrides has been reported. For example, [PhC(NPh)2]2GaMe was prepared in 68% yield from GaMes and N,N -diphenylbenzamidine in toluene. Various samples of this and related complexes could be heated to 600 °C in N2 to give GaN. A series of homoleptic metal amidinates of the general type [MIRCfNROilnl (R = Me, Bu R = Pr, BuO has been prepared for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. The types of products are summarized in Scheme 226. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. 

Chemical vapor deposition processes are complex. Chemical thermodynamics, mass transfer, reaction kinetics and crystal growth all play important roles. Equilibrium thermodynamic analysis is the first step in understanding any CVD process. Thermodynamic calculations are useful in predicting limiting deposition rates and condensed phases in the systems which can deposit under the limiting equilibrium state. These calculations are made for CVD of titanium - - and tantalum diborides, but in dynamic CVD systems equilibrium is rarely achieved and kinetic factors often govern the deposition rate behavior. 

Metal and polysilicon films are formed by a chemical-vapor deposition process using organometallic gases that react at the surface of the IC structure. Various metal silicide films may also be deposited in this manner by reaction with the surface of the silicon wafer to form metal silicides. Glass and pol3uner films are deposited or spin cast or both, as are photoresist films (those of a photosensitive material). This process is accomplished by applying a liquid polymer onto a rapidly rotating wafer. The exact method used varies from manufacturer to manufacturer and usually remains proprietary. 

In Section 3.4.2, we introdnced the concept of chemical vapor infiltration, CVI, in which a chemical vapor deposition process is carried out in a porous preform to create a reinforced matrix material. In that section we also described the relative competition between the kinetic and transport processes in this processing technique. In this section we elaborate npon some of the common materials used in CVI processing, and we briefly describe two related processing techniques sol infiltration and polymer infiltration. 

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