Mechanical silicides

More than half of the elements in the Periodic Table react with silicon to form one or more silicides. The refractory metal and noble metal silicides ate used in the electronics industry. Silicon and ferrosilicon alloys have a wide range of applications in the iron and steel industries where they are used as inoculants to give significantly improved mechanical properties. Ferrosilicon alloys are also used as deoxidizers and as an economical source of silicon for steel and iron. 

The expected oxidation mechanisms of carbides and silicides can be analysed from a thermodynamic viewpoint by a comparison of the relative stabilities... 

The deposition temperature is above 1200°C and the deposit usually consists of an outer layer of MoSi2 and an intermediate layer of MoSi.PlP l Such reactions are difficult to control and often result in mechanical stresses and voids at the interface, which may cause adhesion failure. The direct deposition of the silicide is often preferred. This is accomplished by reacting a gaseous silicon compound with a gaseous metal compound, as shown in the following sections. 

MOSFETT s, and silicon oxide is deposited. The source/drain positions where electrical contact is to be made to the MOSFETs are defined, using the oxide-removal mask and an etch process. For shallow trench isolation, anisotropic silicon etch, thermal oxidation, oxide fill and chemical mechanical leveling are the processes employed. For shallow source/drains formation, ion implantation techniques are still be used. For raised source/drains (as shown in the above diagram) cobalt silicide is being used instead of Ti/TLN silicides. Cobalt metal is deposited and reacted by a rapid thermal treatment to form the silicide. Capacitors were made in 1997 from various oxides and nitrides. The use of tantalmn pentoxide in 1999 has proven superior. Platinum is used as the plate material. 

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. 

Proposed Growth Models. Summarizing the results given above, the growth mechanisms are proposed as follows. Because of the location of nanoparticles at the tips of SiNW, we conclude that growth must occur in the gas phase. More precisely, we think that Co silicide SAN play a role here. They help convert Si and H2 into SiH4 (gas). Unreacted Co nanoparticles left on the surface, possibly due to the... 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

A fuzing application for Ca silicide is discussed in a current Brit patent (Ref 18) which claims mechanically insensitive and thermally stable timing fuses with burning rates of 100—500 sec/m (with a variation of 52%) can be manufd with a pyrot type powder core consisting of 20—67.3% K nitrate, with the remainder Ca silicide. The silicide is also used as a fuel in priming compns. Meyer (Ref 1) suggested that it be used to the extent of 15% with MF (25%),... 

The formation of silicides in reaction couples, for example, of the MesAl-SiC type, where Me is a transition metal, is more complicated. In this case, in addition to the Me2Si layer, the MeAl layer (or some other aluminide layer) also grows, i.e. the Me3Al-MeAl-(Me2Si+C)-SiC system is formed. The mechanism of its occurrence is probably as follows. The Me3Al phase is decomposed at the Me3Al-MeAl interface by the reaction... 

If the initial solid substance is a chemical compound (an intermetallic, a silicide, etc), then its oxidation can proceed via two different mechanisms, depending on the experimental conditions. Two oxides are formed in the severe oxidation (combustion) usually resulting in the disintegration of the compact solid phase. In the partial (soft) oxidation the chemical compound undergoes a partial decomposition giving another chemical compound of the same class and an oxide. 

J.E.E. Baglin, H.A. Atwater, D. Gupta, F.M. d Heurle. Radioactive Ni tracer study of the nickel silicide growth mechanism // Thin Solid Films - 1982 - V.93 - P.255-264. 

The generation of all silicide phases in dependence on temperature can be thermodynamically explained by means of equilibrium calculations. Taking into accoimt our experimental results for the H2/SiCl4/Ni system, a picture of the mechanism of hydrodehalogenation is developed which combines thermodynamic and kinetic results. 

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). 

Based on the above ideas on the reaction mechanism, it is expected that the silylene species at higher temperatures display an increased electron acceptor ability compared to the silicide cluster. Evidence for that is provided by investigations of catalyst samples using ferro-magnetic resonance (FMR). 

In accordance with our thermodynamic calculations we have observed the generation of the NiSi phase at temperatures above 700 °C. The incorporated silicon atoms originate from the gas phase. After the generation period the trichlorosilane conversion increases to the level determined by the equilibrium calculations. Taking into account the proposed mechanism of hydrodehalogenation, this leads to the same consequences as in the case of Ni2Si silicide catalyst, but the selectivity is less. Traces of H2SiCl2 are found. These are attributed to noncatalyzed subreactions in addition to HSiClj (Eq. 10, 11). 

The intermediate Cu3Si phase should be only one of some catalytical active siiicides under these conditions. The mechanism, which suggests the intermediate formation of surface fixed silylenes, requires that the silicide clusters alternately release and accept silicon atoms (see hydrodehalogenation). 

Rogachev, A. S., Shugaev, V. A., Khomenko, I. A., Varma, A., and Kachelmyer, C., On the mechanism of structure formation during combustion synthesis of titanium silicides. Combust. Sci. Tech., 109,53(1995). 

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