Silicide

Silicides. The M-Si (M = Mo or W) phase diagrams in argon plasmas have been investigated using X-ray and other techniques to characterize the quenched products. 

Metal silicides are widely used in microelectronic devices for the increase of the potential barrier height in metal-sUicon Schottky contact. Titanium silicide is favored for its low resistivity and thermal stability. IR spectroscopy is not typically used for investigation of metal silicides, because they do not have a characteristic and selective absorption in the IR range. Nevertheless, FTIR spectroscopy has been used to measure the thickness of Ti and titanium silicide films [274] as well as to monitor titanium silicide formation during the reaction of Ti films on a Si wafer [275]. 

Only the as-deposited and/or annealed (T = 300°C) Ti films show wide absorption bands near 3000 cm [275]. At different stages of the silicidation reaction, the films reveal different nonselective absorption curves in the 400-4000-cm range. Thus, it was shown that the absorption at 4000 cm increases monotonically as the annealing temperature of the sample increases and the absorption at 400 cm increases monotonically with increasing sample conductivity. Thus the composition of a silicide may be deduced however, in calculations of absorbance it is necessary to take into account the strong dependence of the reflectance on both the frequency (/ = 95% at 400 cm , R = 60% at 4000 cm ) and the silicide composition [275]. 

INFRARED SPECTROSCOPY OF THIN LAYERS IN SILICON MICROELECTRONICS 

The SrTi03 films have even larger dielectric constants than Ta20s (up to 270 [280]) and are also of interest for use as storage capacitors in DRAM and for silicon VLSI applications. The SrTiOs directly deposited on a Si substrate reveals a much lower effective dielectric constant dne to the formation of a Si02 

Carbides and Silicides. MC (M = Zr or HI) is deposited from the heated vapour mixture MCI4-H2-CH4 the rate constant for ZrC formation was determined.  

The heats of formation have been measured for a range of non-stoicheiometric zirconium carbides ZrC, (x = 0.716—0.990). Differences in details of the bands in the X-ray emission spectra of the zirconium silicides Zr3Si, Zr5Si3, Zr3Si2, and ZrSi2 have been related to the type of crystal lattice of the solid silicide.  

The transition metal silicides show close similarities to the intermetallics and thus they are frequently classed with the intermetallics though silicon is no longer a metal, but a semiconductor. Silicides are generally hard and brittle with a metallic luster, high electrical and thermal conductivities, a positive temperature coefficient of 

which is a candidate phase for high-temperature applications, and VjSi, which is superconducting at low temperatures with a comparatively high transition temperature, both crystallize with the topologically close-packed cubic, A15 structure and have been discussed in Secs. 7.7 and 7.2, respectively. 

A metal-rich silicide with a very simple crystal structure is NijSi with the ordered f.c.c. LI 2 structure. It has a very high potential for structural applications because of its advantageous mechanical properties and its outstanding corrosion resistance. Its deformation behavior is similar to that of other LI 2 phases, in particular NisAl, and thus NijSi has been discussed in Sec. 4.2.2 together with other LI 2 phases. 

The partial substitution of the transition element M in MjSi by a second transition metal M leads to ternary silicides of approximate composition MM Si corresponding to (M,M )2Si. Such ternaries are primarily the Si-containing E phases and V phases (Jeitschko etal., 1969 Jeitschko, 1970) and the ternary Si-containing Laves phases (Bardos etal., 1961), which were discussed in Sec. 8, as well as many other phases, which all differ by composition and crystal structure (Nowotny, 1972 a). This is exemplified by the Fe-Nb-Si system with the ternary silicides E, V, Xj, Xj, Xj and the Laves phase Nb(Fe,Si)2 with up to 25 at.% Si (Raghavan, 1987), or the Co-Nb-Si system with the ternary silicides E, T, v, Ti, v i, and the ternary Laves phase Nb(Co,Si)2 with Si contents between about 10 and 20 at.% (Argent, 1984). Finally, it is noted that other phases - in particular a phases and A13 Mn-base phases - dissolve large amounts of Si by which these phases are stabilized (Gupta et al., 1960 Bardos et al., 1966). 

Other MjSis phases are less attractive for applications, and thus they have been studied less. MjSij phases can be synthesized by mechanical alloying, which leads to amorphization in the case of TajSij (Kumar and Mannan, 1989). 5813 and the ternary Y5(Si,Ge)3 are of interest for use as hydrogen storage materials (McColm and Ward, 1992). 

It is interesting that the silicides of U provide examples of all the main types of silicide structure/ for in addition to those listed there is U3Si which has a distorted version of the CusAu structure (p. 842), in which there are discrete Si atoms entirely surrounded by U atoms. They also show that some silicides adopt the same structures as certain borides which contain extended systems of linked B atoms, though because of the greater size of Si and its different electronic structure borides and silicides are not generally isostructural. 

(a) Close-packed MSij layer in some disilicides. (b) Three succesave layers in CrSii. (c) The crystal structure of MoSij. (d) Projection of the structure of MoSij on (110) showing the relative positions of atoms in two adjacent layers (full and dotted circles). Metal atoms are 

In this silicide the Ca and Si atoms are arranged in alternate layers, as shown in Fig. 23.3. A silicon layer might be described as a puckered graphite layer 

The structures of the metal silicides (prepared by direct combination of the elements at high temperatures) are diverse, and a full discussion of the structures is beyond the scope of this book. Some examples of their solid state structural types are  

The structures of the metal silicides (prepared by direct combination of the elements at high temperatures) are 

The thermodynamic characteristics of the chromium silicides Cr3Si, Cr5Si3, CrSi, and CrSi2 have been determined electrochemically.  

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 

Structural Inorganic Chemistry, 5th edn., Oxford University Press, Oxford, pp. 987-91 (1984). 

Holtmann, D. Kanne, C. Kruger, R. Blom, R. Gleiter and I. Hyla-Kryspin Chem. Ber. 122, 1629-39 (1989). 

Berezhoi, Silicon and its Binary Systems, Consultants Bureau, New York, 1960, 275 pp. 

Aronsson, T. Lundstrom and S. Rvndqvist, Borides, Silicides, and Phosphides, Methuen, London, 1965, 120 pp. 

Other five Co atoms has one, and there are eight bridging CO groups. The coordination number 10 is found in the structures of several transition metal silicides and in decamethylsilicocene 

Experimental TiB2 coatings for cemented carbide cutting tools and other wear- and erosion-resistant applications (pumps, valves, etc.). ] 

TiB2 coatings for electrodes for aluminum production (Hall-cell cathodes). TiB2 has high resistance to molten aluminum yet it is readily wetted by the molten metal and good electrical contact is assured. 

Silicides are useful compounds characterized by their refractoriness and high electrical conductivity. There are many silicides, since silicon reacts with most metals and often more than one silicide is formed. For instance, there are five known tantalum silicides. 

The silicides of major industrial importance are the disilicides of the refractory metals molybdenum, tantalum, titanium, tungsten, vanadium, and zirconium.pl] These compounds are of great interest par- 

Property MoSi2 TaSi2 TiSi2 WSi2 ZrSi2 

Both NiSi2 and CoSii crystallize in the bulk fluorite crystal structure. The (111) surface of these solids is a stack of Si-Co-Si or Si—Ni—Si trilayers. Consequently, these surfaces are structurally similar to the disulfides and diselenides discussed in sect. 5.3.2. There have been several studies of the (111) surfaces of NiSi2 and CoSi2, although the overall structural trends 

Young s modulus ( ,GPa) Flexural strength (x,MPa) Compressivi strength (a, MPa) Vickers hardness HV (Mohs hardness HM) Other physicochemical properties, corrosion resistance and uses lUPAC name 

Corrosion-resistant to molten metals such as Zn, Pd, Ag, Bi, and disilicide Rb. It is corroded by the following liquid metals Mg, Al, Si, V, Cr, 

Toth Transition Metals, Carbides and Nitrides 2.6 (Academic Press, New York 1971) 

Freer The Physics and Chemistry of Carbides, Nitrides and Borides (Kluwer, Boston 2.7 1989) 

Combustion of Metal-Fluorocarbon Pyrolants with Fuels Other than Magnesium 

At stoichiometries 75wt% Mg2Si, the combustion phenomenology changes. Below that, hnear combustion rate occurs with significant gas-phase combustion. Above the point, fast glowing of the pyrolant strand with eventual terminal sparkling or explosion is encountered. TTius, the determination of the bum rate for values 

Zirconium disilicide ZrSi2 [12039-90-6] 147.395 Orthorhombic a = 372pm b= 1469pm 4880 161 1604 8.6 

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Higher chlorides, Si2Cle to Si6Cl,4 (highly branched - some cyclic) are formed from SiCU plus Si or a silicide or by amine catalysed disproportionations of Si2Cl,5, etc. Partial hydrolysis gives oxide chlorides, e.g. CUSiOSiCla. SiCU is used for preparation of silicones. 

The discussion of Rutherford backscattering spectrometry starts with an overview of the experimental target chamber, proceeds to the particle kinematics that detennine mass identification and depth resolution, and then provides an example of the analysis of a silicide. 

After 60 minutes of aimealing, all the Pt has reacted to fonn Pt2Si. Almost immediately thereafter the reaction between Pt2Si and Si to fonn PtSi starts and after a fiirther 60 minutes all the Pt2Si has reacted, resulting in a stable PtSi film on Si. The data of silicide thickness versus ramped temperature can be plotted in reduced fonn in an Arrhenius-like plot to give the activation energy [6, 14] ... 

Gettering is a black art. It consists in forcing selected impurities (typically, transition metals) to diffuse toward unimportant regions of tlie device. This is often done by creating precipitation sites and perfoniiing heat treatments. The precipitation sites range from small oxygen complexes to layers such as an A1 silicide. The foniiation of such a... 

If an excess of magnesium is used, magnesium silicide, Mg2Si, is also produced.) The silicon obtained is a light brown hygroscopic powder. Crystalline or metallic silicon is obtained industrially by the reduction of silica with carbon in an electric arc furnace ... 

Reactions of HCl and nitrides, borides, silicides, germanides, carbides, and sulfides take place at significant rates only at elevated (>650° C) temperatures. The products are the metal chlorides and the corresponding hydrides. The reactions most studied are those involving nitrides of aluminum, magnesium, calcium, and titanium, where ammonia (qv) is formed along with the corresponding metal chloride. 

Miscellaneous. Chloroplatinic acid is used in the production of automobile catalysts. Platino-type prints based on reduction of Pt(II) to Pt(0) by a photosensitive reducing agent such as iron(III) oxalate are used in art photography (261,262). Infrared imaging devices based on a platinum siLicide detector have been developed (263). 

Borides and Silicides. These materials do not show good resistance to oxidation. Some siUcides, however, form Si02 coatings upon heating which retards further oxidation. Molybdenum disiUcide [1317-33-5] MoSi2, is used widely, primarily as an electrical heating element. 

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

Silicon (3), which resembles metals in its chemical behavior, generally has a valence of +4. In a few compounds it exhibits a +2 valence, and in silicides it exists as a negative ion and largely violates the normal valency rules. Silicon, carbon, germanium, tin, and lead comprise the Group 14 (IVA) elements. Silicon and carbon form the carbide, SiC (see Carbides). Silicon and germanium are isomorphous and thus mutually soluble in all proportions. Neither tin nor lead reacts with silicon. Molten silicon is immiscible in both molten tin and molten lead. 

Silicon is soluble in aluminum in the solid state to a maximum of 1.62 wt % at 577°C (2). It is soluble in silver, gold, and 2inc at temperatures above their melting points. Phase diagrams of systems containing silicides are available (2,3). 

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. 

Rare-Earth Silicides. Rare-earth sihcides, in the form of a ferroalloy that contain up to 33% rare earths, are used increasingly by the iron and steel industries. Whereas the term sihcides is no longer used for alloys of this type, it is stih in common usage for these materials. Eor nodular iron, addition... 

Titanium Silicides. The titanium—silicon system includes Ti Si, Ti Si, TiSi, and TiSi (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkalifluorosiUcate (155) reaction of TiCl, SiCl, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiSi2 (156) and, for Ti Si, reaction between titanium dioxide and calcium siUcide at ca 1200°C, followed by dissolution of excess lime and calcium siUcate in acetic acid. 

Table 18. Structure and Physical Constants of Titanium Silicides ...

Copper—chromium and copper—nickel—silicon—chromium alloys are also precipitation hardenable. The precipitates are nickel sdicides, chromium silicides, and elemental chromium. If conductivity is critical, the chromium—silicon ratio should be held at 10 1 so that appreciable amounts of either element are not left in soHd solution in the copper after aging. Lithium can be used as a deoxidizer in copper alloys when conductivity is important. For a discussion of the principle of age- or precipitation-hardening copper alloys, see Copperalloys,wrought copperalloys. 

When a mismatch is inevitable, as in the combination Gej-Sii j. — Si, it is found that up to a value of jc = 0.4, there is a small mismatch which leads to a strained silicide lattice (known as commensurate epitaxy) and at higher values of jc there are misfit dislocations (incommensurate epitaxy) at the interface (see p. 35). From tlrese and other results, it can be concluded that up to about 10% difference in the lattice parameters can be accommodated by commensurately strained thin films. 

To return to die problem of die vaporization of die tantalum silicides, which could be transported as the tetraiodide of each element, but not as the elementary species. From these data it can be concluded that whatever die starting point in the composition range, the composition of the surface phase will tend towards Tag Sis, which is die most nearly congruently vaporizing composition. 

Figure 3.3 Chemical potential diagram for the transport of titanium silicides by chlorine, showing that only TaSi2 can provide the proper ratio of Ta and Si for stoichiometric transport...

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