CVD processes

Rhenium hexafluoride is used for the deposition of rhenium metal films for electronic, semiconductor, laser parts (6—8), and in chemical vapor deposition (CVD) processes which involve the reduction of ReF by hydrogen at elevated (550—750°C) temperatures and reduced (<101.3 kPa (1 atm)) pressures (9,10). 

Rhenium hexafluoride is a cosdy (ca 3000/kg) material and is often used as a small percentage composite with tungsten or molybdenum. The addition of rhenium to tungsten metal improves the ductility and high temperature properties of metal films or parts (11). Tungsten—rhenium alloys produced by CVD processes exhibit higher superconducting transition temperatures than those alloys produced by arc-melt processes (12). 

Copper is an attractive metallisation element because of its high conductivity. It has been added to Al in low concentrations (AlSi(l%)—Cu(0.5%)) to improve conductive priorities. Selective, low temperature copper CVD processing, using copper(I) P-diketonate compounds, has been carried out (23). 

G. Lucovsky, D. E. Ibbotson, and D. W. Hess, eds.. Characterisation of Plasma-PInhanced CVD Processes Materials Kesearch Society Symposium Proceedings, Vol. 165, Materials Research Society, Pittsburgh, Pa., 1990. 

Another growing apphcation that overlaps the electrically functional area is the use of transparent conductive coatings or tin oxide, indium—tin oxide, and similar materials in photovoltaic solar ceUs and various optic electronic apphcations (see Photovoltaic cells). These coatings are deposited by PVD techniques as weU as by spray pyrolysis, which is a CVD process. 

Refractory compound coatings of carbides, nitrides, and oxides on cemented carbide cutting tools, mainly by the CVD process, are estimated at 300 X 10 annually worldwide. 

The CVD process is accomplished using either a hot-wall or a cold-wall reactor (Fig. 13). In the former, the whole chamber is heated and thus a large volume of processing gases is heated as well as the substrate. In the latter, the substrate or substrate fixture is heated, often by inductive heating. This heats the gas locally. 

CVD processing can be accompanied by volatile hot-reaction by-products such as HCl or HF, which, along with unused precursor gases, must be removed from the exhaust gas stream. This is done by scmbbing the chemicals from the gas using water to dissolve soluble products or by burning the precursor gases to form oxides. 

Film stress arises owing to the manner of growth and the coefficient of expansion mismatch between the substrate and film material (4). In many CVD processes, high temperatures are used. This restricts the substrate-coating material combinations to ones where the coefficient of thermal expansions can be matched. High temperatures often lead to significant reaction between the deposited material and the substrate, which can also introduce stresses. 

CVD processing can be used to provide selective deposition on certain areas of a surface. Selective tungsten CVD is used to fill vias or holes selectively through siUcon oxide layers in siUcon-device technology. In this case, the siUcon from the substrate catalyzes the reduction of tungsten hexafluoride, whereas the siUcon oxide does not. Selective CVD deposition can also be accompHshed using lasers or focused electron beams for local heating. 

Safety is often a primary concern in CVD processing because of the ha2ardous nature of some of the gases and vapors that are used and the hot reaction products generated. 

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. 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

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. 

In spray pyrolysis, very fine droplets are sprayed onto a heated substrate. The limitations of this process are the same as for spin-on coating. The same is often the case for preparing solid electrolytes by chemical vapor deposition (CVD) processes, which in addition are more expensive, and the precursors are often very toxic. 

The author is fortunate to have the opportunity, as a consultant, to review and study CVD processes, equipment, materials and applications for a wide cross-section of the industry, in the fields of optics, optoelectronics, metallurgy and others. He is in a position to retain an overall viewpoint difficult to obtain otherwise. 

The book is divided into three major parts. The first covers a theoretical examination of the CVD process, a description of the major chemical reactions and a review of the CVD systems and equipment used in research and production, including the advanced subprocesses such as plasma, laser, and photon CVD. 

Until recently, most CVD operations were relatively simple and could be readily optimized experimentally by changing the reach on chemistry, the activation method, or the deposition variables until a satisfactory deposit was achieved. It is still possible to do just that and in some cases it is the most efficient way to proceed. However, many of the CVD processes are becoming increasingly complicated with much more exacting requirements, which would make the empirical approach too cumbersome. 

Such an analysis requires a clear understanding of the CVD process and a review of several fundamental considerations in the disciplines of thermodynamics, kinetics, and chemistry is in order. It is not the intent here to dwell in detail on these considerations but rather provide an overview which shouldbe generally adequate. More detailed investigations of the theoretical aspects of CVD are given in Refs. 1-3. 

It should be first realized that any CVD process is subject to complicated fluid dynamics. The fluid, in this case a combination of gases, is forced through pipes, valves, and various chambers and, at the same time, is the object of large variations in temperature and to a lesser degree of pressure before it comes in contact with the substrate where the deposition reaction takes place. The reaction is heterogeneous which means that it involves a change of state, in this case from gaseous to solid. 

The various CVD processes comprise what is generally known as thermal CVD, which is the original process, laser and photo CVD, and more importantly plasma CVD, which has many advantages and has seen a rapid development in the last few years. The difference between these processes is the method of applying the energy required for the CVD reaction to take place. 

Coatings are by far the largest area of application of CVD at the present but by no means the only CVD process. Other areas of CVD, such as production of powders, fibers, monoliths, and composites, are growing rapidly. 

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