Silicon-metal Compounds Silicides

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

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. 

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. 

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 fuels are finely powdered metals (2.0-10.0 g) among which titanium, zirconium, manganese, tungsten, molybdenum and antimony are very common. Sometimes, non-metal powders such as boron and silicon (for fast burning delays), binary alloy powders such as ferrosilicon, zirconium-nickel, aluminum-palladium and metal compounds such as antimony sulfide, calcium silicide etc. are also used. 

In this article the intention is to place the main emphasis on studies of molecular silicon-transition-metal compounds, in particular those from the author s laboratory and those that have been reported since the comprehensive reviews noted above. As, however, it is now known that these molecular compounds can act as convenient precursors for certain transition-metal silicides, aspects of the properties of silicides will also be outlined, and relationships between the two classes of compounds examined. Compounds with bonds between transition metals... 

Silicides represent the transition from intermetallics with predominantly metallic bonding to non-metallic compounds since silicon is no longer a metal, but a semiconductor. Nevertheless the silicides are often comprised within the intermetallics. Silicides were selected for high-temperature applications already in the past because of their potentially high oxidation resistance at highest temperatures [89-91]. Presently... 

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. 

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. 

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

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. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

This is a very fundamental hypothesis (very well verified indeed within the accuracy limits +0,02 A) of EXAFS and allows the analysis of an unknown system, say an interface between a transition metal and silicon, by using the amplitudes and phase shifts from a model compound of known crystallography, say a silicide. 

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. 

Silicon is usually found in oxides and silicates. With very electropositive metals it forms silicides. The compound CaSi2 can be viewed as containing Ca2+ and Si ions. The crystal structure is trigonal, Dgft, R3m, a = 3.8259, and c = 15.904 A with three molecules in the unit cell. The Si ion has the same valence shell as As, and layers of... 

Compounds with silicon-transition-metal bonds fall into two distinct classes, which until recently showed no points of connection. The first class comprises the metal silicides typical binary examples are FeSi,... 

The sole example of a silicon-platinum cluster is the compound in entry 24 its structure has been noted in Section IV,A. It seems very likely that many further cluster systems await discovery, particularly with iridium, platinum, and gold, and that this represents an important future area of research. One obvious application is as precursors to metal silicides with high metal silicon ratios using c.v.d. techniques (compare Section V,A). 

Note that the silicide layer may grow not only between silicon and a transition metal, but also between a silicon-containing phase and a transition metal or an intermetallic compound. Such layers are known to occur in the process of brazing the transition metals by their own melts with Si3N4-base ceramics239 and also during the interaction of transition metals with silicon carbide.238 240 245... 

Improvements in the performance of integrated circuits and the trend towards VLSI-technology require the replacement of polycrystalline silicon by materials with a lower resistivity for use as gate electrodes. Transition metal silicides appear to be valuable possibilities for these applications. Timgsten-silicon compounds could be suitable precursors for the precipitation of tungsten-silicide thin films. Moreover tungsten-silicon compounds are nearly unknown and of scientific interest. 

A variety of metallic silicides are obtained by melting silicon with a transition metal. Condensation of a transition-metal vapor with a silicon compound, or silicon vapor with a transition-metal complex has not so far yielded a species with a discrete silicon-transition-metal bond. 

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