Silicide reaction process

The rate at which silicides form can yield insights into the reaction process and is important for practical applications. The temperature dependence of the reaction rate, R, is given by ... 

The deposition processes discussed so far typically operate such that all the material required for the growing film comes from the overlying gas or liquid phase. Other deposition reactions involve reaction (and therefore consumption) of the underlying substrate itself. Examples of such deposition processes include thermal oxidation, nitridation, or silicidation of silicon, which can be accomplished by exposing a silicon wafer at high temperature to oxygen, ammonia, or titanium tetrachloride, respectively, to form silicon dioxide, silicon nitride, or titanium disilicide. Solid-phase diffusion and reaction processes are involved in each case. 

Suppose we are attempting to nucleate a solid-phase reaction between a metal and a silicon substrate to produce a silicide intermetallic compound and cany the reaction to completion. Further suppose that we have a dopant in the solid which we do not wish to have diffusing as the silicide reaction takes place. Suppose the two processes both have rates defined by Equation 4.26. Given the kinetic parameters for the reactions as follows ... 

The maximum temperature at which the silicide reaction can take place is 850°C and the minimum temperature is 450°C. If the product of rate and time for the desired process must be 2x10 cm, calculate ... 

Figure 10.2 Shows the basic reaction process for forming a silicide by solid-phase reaction. A metal is deposited directly on a silicon layer and the couple is heated to permit the reaction. The first event is nucleation of one of the silicide phases. This grows thicker and slower until a new more stable phase has the chance to nucleate.

If the rules for volatiles and thermodynamics of the haUdes are followed, the reaction can be used for aluminizing, silicidizing, chromizing, and similar processing. 

Similar results are obtained for dre deposition of the carbides of these metals using methane as a source of carbon, atrd silicon tetrahalides for the preparation of silicides. These reactions are more complex than dre preparation of the diborides because of the number of carbides atrd silicides that the tratrsition metals form, some of which have wide ranges of non-stoichiometry. The control of the ratio of the partial pressures of dre ingoing gases is therefore important as a process variable. 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

An older process to form silicides is siliconizing which is a relatively simple CVD process used to provide oxidation and chemical resistance to refractory metals. The siliconizing reaction uses the substrate itself, (such as Mo or Ti), as the metal source. Silicon diffuses readily in... 

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 many cases, metal silicides may very well be the catalysts. For example, FeSi2 is being considered to be the catalyst in Fe-assisted nanowire synthesis. This is similar to the silicon mono-oxide case, although it is much easier to understand the mechanisms in the FeSi2 case. It is also possible that during the catalytic processes that silicon diffuses relatively freely through the metal catalyst and consequently, the observed silicides at the end of reaction may be different from those during the catalytic reaction. No direct evidence is available to show whether metal or metal silicide nanoparticles are the tme catalyst. 

Another preparative method involves treating magnesium silicide with ammonium bromide in liquid ammonia in a current of hydrogen. The process forms 70 to 80% yield of mono- and disilanes. The reaction is shown below ... 

T. Shibata, A. Wakita, T. W. Sigmon and J. F. Gibbons, Metal-Silicon Reactions and Silicide Y. I. Nissim and J. F. Gibbons, CW Beam Processing of Gallium Arsenide... 

The obtained results on the reaction mechanism can be summarized as follows The metal silicides form cluster structures which represent electron buffer systems. They can be oxidized or reduced easily by surface reactions. The adsorption of SiCl4 molecules at the cluster surface is immediately followed by an electron transfer from the cluster to the silicon atom of SiCl4, the cluster is oxidized. As a result of such a process a silylene species is formed at the surface of the catalyst. Chloride ions act as counter ions to the positive cluster, supporting the redox step (Eq. 4). 

The catalyst has to be created before the hydrodehalogenation reaction starts because disperse metals are inactive. An induction period is always observed when the H2/SiCl4 mixture is passed over the metal powders. This is caused by the gradual formation process of the silicides which are stable under the specific reaction conditions. In the Ni/Si system at 600 °C this is the Ni2Si phase. Only after this pheise has been formed at least on the particle surface (identified by X-ray diffraction), the silylene group concentration appears to be high enough for the oxidative addition of HCl to take place. Then the fast... 

Inorganic non-oxide materials, such as III-V and II-VI group semiconductors, carbides, nitrides, borides, phosphides and silicides, are traditionally prepared by solid state reactions or gas-phase reaction at high temperatures. Some non-oxides have been prepared via liquid-phase precipitation or pyrolysis of organometallic precursors. However, amorphous phases are sometimes formed by these methods. Post-treatment at a high temperature is needed for crystallization. The products obtained by these processes are commonly beyond the manometer scale. Exploration of low temperature technique for preparing non-oxide nanomaterials with controlled shapes and sizes is very important in materials science. 

Non-oxide ceramic nanomaterials, such as carbides, nitrides, borides, phosphides and silicides, have received considerable attention due to their potential applications in electronics, optics, catalysis, and magnetic storage. In contrast with the traditional processes, such as solid state reactions, CVD, MOCVD and PVD, which involve using high temperatures, toxic organometallic precursors, or complicated reactions and posttreatments, solvothermal method is a low temperature route to these materials with controlled shapes and sizes. 

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