Silicon and Silicides

The use of poly-carbon monofluoride as an oxidizer with silicon and siUddes yields SiC again, however, in significantly lower yields than that with PTFE [24, 25], 

Cudzilo and co-workers continue to do extensive investigations on the identity and morphology of combustion products of a great variety of metal/halocarbon reactants. Table 15.1 lists the systems investigated so far and the identified products. 

Tabie 15.1 Pyrolants used for SHS purposes and obtained products. 

U2C2/PTFE Multi-walled carbon nanotube (MWCNT) [8] (R.B. Kaner, personal communication) 

Possibly by a similar mechanism, the reaction of TiSi2 with PTFE yields dendritic TiC spheres indicative of rapid growth of TiC via the deposition of a volatile specie such as Tip4 or TiFs. Again, the carbide precipitates at temperatures below 3100°C and, thus, may also provide a means of shifting the reaction towards TiC instead to the thermodynamically more stable TiF4. 

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Figure 14.2 Chemical reactions and interdiffusion in the metal/silicon and silicide/silicon interfaces. Panels (a)-(d) describe the normal formation process of the interfacial compound. Possible interdiffusion processes are shown in panels (e)-(h).

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. 

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. 

Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

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. 

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. 

T. Shibata, A. Wakita, T.W. Sigmon, and James F. Gibbons, Metal-Silicon Reactions and Silicide... 

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

Another problem with Schottky diodes is that at high temperatures, the metal contact can anneal to the semiconductor, forming a silicide in the case of silicon and SiC [72, 80-83]. This can destroy the diode characteristic of the device, thus producing an unstable sensor. Use of an interfacial insulating layer, such as the oxide layer already mentioned, can prevent this from occurring. 

The various rare earths are used in the foundry industry as rare earth silicides, in which the rare earth content is about 30%. Other alloys are used in which the level of rare earths is about 10% (10% cerium, 2% other rare earths) with silicon and iron comprising the bulk of the remaining elements. In the magnesium-ferrosilicon alloys, the rare earths are present in amounts from about 0.1% to 1.0%. These alloys are used differently by the various consumers. However, the effects of the rare earth elements, introduced by whatever means, are the same. 

The following metals have been suggested for this purpose magnesium, aluminium, zinc and also silicon sometimes ferro-silicon, alumino-silicon and calcium silicide are also employed. Deissler [54] was the first (1897) to recommend aluminium as a component of explosives. He was followed by Goldschmidt [55], Escales [56], von Dahmen [57] and Roth [58], In later years Kast [59] investigated military explosives which contained aluminium. 

Silicides. Crystn compds formed from silicon and various other elements of metallic, transitional or non-metallic character. A complete discussion of silicides is to be found in the work of Sansonov (Ref 9). Those silicides of ordn interest are presented below... 

Investigations in this field have mainly used calcium silicides. In the Ca/Si system there are two silicides with Si-Si bonds in polymeric systems. In calcium monosilicide the silicon atoms are arranged in form of chains, while the calcium disilicide CaSi2 has a layer lattice, consisting of silicon- and calcium layers. It is possible to form polymeric compounds (SiX) and (SiX2) from both silicides by replacing calcium. 

In the present chapter, we will review the nature of plasma-enhanced CVD (PECVD) films for a variety of applications. We will look at dielectrics (silicon nitride, silicon dioxide), semiconductors (polysilicon, epi silicon) and metals (refractory metals, refractory metal silicides, aluminum). There are many other important films (i.e., amorphous silicon for solar cells and TiN for tool harden-... 

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