Metal silicide

There exist a number of metal silicides, in which silicon anions formally occur. The structural types of the silicides in the solid state are diverse  

Molybdenum disilicide is utilized for the manufacture of electrical heating elements, which can be operated in air at temperatures up to 16()0°C 

The silicides of the metals of the subgroups of the IVth, Vth and Vlth groups are, in principle, manufactured using the same processes as those utilized for the corresponding borides. 

The silicides exhibit the lowest melting points and hardness values of the metallic hard materials (see Table 5.6-5). Their brittleness makes them unsuitable for utilization in hard metal alloys. Silicides have only been utilized industrially in metallurgical fields in which their scaling resistance and chemical resistance are important. They are also deposited using the CVD process e.g. as protective layers on high melting metallic surfaces. 

---

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. 

As with borides (p. 145) and carbides (p. 297) the formulae of metal silicides cannot be rationalized by the application of simple valency rules, and... 

The product, Si4(SiBu 3)4, forms intensely orange crystals that are stable to heat, light, water and air, and do not melt below 350°. The Si-Si distances within the closo-Si4 cluster are 232-234 pm and the exo Si-Si distances are slightly longer, 235-237 pm (cf. Si-Si 235.17 in crystalline Si). Comparison with the closo-amon Si4 , which occurs in several metal silicides (p. 337) and is isoelectronic with the P4 molecule, is also appropriate. 

The general understanding of the electronic structure and the bonding properties of transition-metal silicides is in terms of low-lying Si(3.s) and metal-d silicon-p hybridization. There are two dominant contributions to the bonding in transition-metal compounds, the decrease of the d band width and the covalent hybridization of atomic states. The former is caused by the increase in the distance between the transition-metal atoms due to the insertion of the silicon atoms, which decreases the d band broadening contribution to the stability of the lattice. 

Bernard, C., Madar, R., andPauleau, Y., Chemical Vapor Deposit on ofRefractory Metal Silicides for VLSIMetallizaton, 5o//J State Tech., pp. 79-84 (Feb. 1989)... 

Deposition of Refractory Metal Silicides for VLSI Metallization, Technology, pp. 79-84 (Feb. 1989)... 

Aylett, B. J., and Tannahill, A. A., Chemical Vapour Deposition of Metal Silicides from Organometallic Compounds with Silicon-Metal Bonds, SIRA Int. Seminar on Thin Film Preparation and Processing Technolgy, Brighton, UK (March 1985)... 

The trend in CVD metallization is toward greater use of copper, and the refractory metals and their silicides in multilayered metallization designs, typically consisting of metal-silicide contacts, refractory-metal barriers, and copper or an aluminum alloy as the principal interconnect metal. Other metals deposited by CVD such as chromium, molybdenum, platinum, rhodium, and ruthenium are also actively considered for use as conductors. 

Among metal borides of the formula MjM B or (Mj, M/r)2B, the competing structural units are (a) the antiprism and (b) the trigonal metal prism. In many cases the CUAI2 structure with BMg-antiprismatic B coordination is adopted in close resemblance to transition-metal silicides, but no boron-carbon substitution is ob-served - " . 

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. 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

Contact with liquid hydrogen fluoride causes violent evolution of silicon tetraflu-oride. (The same is probably true of metal silicides and other silicon compounds generally.)... 

Transition metal-silicon compounds are thought to be precursors in metal silicide CVD processes. The main advantage compared with common precursors is the defined silicon to metal ratio. 

An impurity that has formed a precipitate, for example a metal silicide. 

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. 

We further suggest that silane is produced and becomes airborne, which then reacts with metal or metal silicide nanoparticles that have not reacted with silicon or with Co silicide nanostructures to produce SiNW, as shown in Scheme 1. We are looking into how to directly prove the existence of silane. 

C. Li, N. Wang, S. Wong, C. Lee, and S. Lee, Metal silicide/silicon nanowires from metal vapor vacuum arc implantation, Adv. Mater. 14, 218-221 (2002). 

Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new silicon nitride-bonded type exhibits improved resistance to cryolite. 

Link. B. Field-Qualified Silicon Accelerometers, Sensors, 28 (March 1993). Maugh, T,H., 11 A New Route to lnlennetallics (Metal Silicides), Science. 225, 403 (1984),... 

Silicides and Titanium-Tungsten. The determination of the ratio of metals to silicon in metal silicides or of titanium to tungsten in tita-... 

Silicobromoform is usually prepared by passing hydrogen bromide over heated silicon112,4,5 or a silicide such as copper silicide.3 The product, consisting of a mixture of silicon tetrabromide with a few per cent of tri- and dibromosilanes, is purified by shaking with mercury, if necessary, to remove any free bromine, and by fractional distillation. The use of metal silicides instead of silicon does not add appreciably to the yield of the bromoform and is not recommended in the following procedure. 

---