Oxidation silicides

Comparison of the consequences following from the physicochemical and purely diffusional approaches is given to show that the latter is one of the limiting cases of the former. Theoretical conclusions are illustrated by the available experimental data on the formation of intermetallics, silicides, oxides, salts and other chemical compounds. 

Numerous types of materials have been made by CVD techniques, such as metal carbides, borides, silicides, oxides, nitrides, and chalcogenides, as well as films of pure metals.27 For example, thermolysis of Cp PtMe3 deposits films of platinum on glass or silicon wafer substrates,28 while Pd(hfacac)2 (hfacac = 1,1,1,5,5,5-... 

The data in Table 3.5 show the trends one expects for an inverse dependence on bulk modulus, i.e., thermal expansion coefficients increase in the order carbides, silicides, oxides and halides. The main exception appears to be the silicates, which often have quite low thermal expansion coeffiecients. Silicate structures are often open and coupled transverse modes of vibration can lead to a decrease... 

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. 

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. 

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. 

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

Clearly the best protection from oxidation by a silicide as a coating on a reactive substrate would be the disilicide, which has the highest silicon content, and could be expected to provide a relatively protective silica coating. 

Silicides are usually prepared by direct fusion of the elements hut coreduction of Si02 and a metal oxide with C or A1 is sometimes used. Heats of formation are similar to those of borides and carbides but mps arc substantially lower e.g. TiC 3140°, TiBj 2980°, TiSij 1540° and TaC 3800°, TaB2 3100°, TaSi2 1560°C. Few silicides melt as high as 2000-2500°, and above this temperature only SiC is solid (decomp 2700°C). 

Protection of niobium and its alloys from oxidation in air is accomplished by coating, e.g. with zinc deposited by holding in zinc vapour at 865°C or coating with a layer of chemically stable oxide, nitride or silicide. Silicide coatings applied by pack cementation, fused slurry or by electrolytic methods have been found to be one of the most effective means of preventing oxidation of the metal. 

Priceman, C. and Soma, L., Development of Fused Slurry Silicide Coatings for the Elevated Temperature Oxidation Protection of Niobium and Tantalum Alloys, Report AFML-TR-68-210, Sylvania Electric Products Inc., Dec. (1968)... 

In general, many metals and alloys (e.g. of Al, Ta and Mo) can be deposited on metallic and some non-metallic substrates. M may also be a metal compound having special useful properties (e.g. borides, nitrides, oxides, silicides and carbides), or even a non-metal such as Si (as in Ihrigising ). 

The deposition of a binary compound can be achieved by a coreduction reaction. In this manner, ceramic materials such as oxides, carbides, nitrides, borides, and silicides can be produced readily and usually more readily than the parent metal. A common example is the deposition of titanium diboride ... 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

An older process to form silicides is siliconizing which is a relatively simple CVD process used to provide oxidation and chemical resistance to refractory metals. The siliconizing reaction uses the substrate itself, (such as Mo or Ti), as the metal source. Silicon diffuses readily in... 

The nozzle of original design was fabricated from a niobium alloy coated with niobium silicide and could not operate above 1320°C. This was replaced by a thin shell of rhenium protected on the inside by a thin layer of iridium. The iridium was deposited first on a disposable mandrel, from iridium acetylacetonate (pentadionate) (see Ch. 6). The rhenium was then deposited over the iridium by hydrogen reduction of the chloride. The mandrel was then chemically removed. Iridium has a high melting point (2410°C) and provides good corrosion protection for the rhenium. The nozzle was tested at 2000°C and survived 400 cycles in a high oxidizer to fuel ratio with no measurable corrosion.O l... 

Compounds isotypic with the k phases arc found among intcrmetallics, borides, carbides and oxides and also with silicides, germanides, arsenides, sulfides and sclcnides no nitrides, however, are found. The mode of filling the various voids in the metal host lattice of the k phases follows the schemein Ref. 4 and is presented in Table 1 for all those compounds for which the atom distribution is well known from x-ray or neutron diffraction. Accordingly, B atoms in tc-borides, Zr, Mo, W, Re)4B and Hfy(Mo, W, Re, Os)4B , occupy the trigonal prismatic interstices within the parent metal framework of a Mn, Aln,-type structure (see Table 1 see also ref. 48). Extended solid solutions are found for (Hf, Al)[Pg.140]

Globars" (silicon carbide rods) are the next most frequently used furnace heating elements. They will operate continuously at 1450 °C. and intermittently at 1500 C. A newer t3q>e of element. Mo wire coated with silicide, i.e.- MoSi2(to protect the Mo wire against oxidation), heis become... 

Sources" are formed in the first step and then "Gates" are formed. A silicide is then used to lower the contact resistance between the silicon of the gate, source and drain and the contacting plug. The thickness of gate oxides is only about 40-50 A. They are thermally grown rather than... 

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. 

Barium sulphate is not a strong oxidant yet it can give rise to a very violent thermite reaction when it is heated in the presence of a very electropositive metal. This applies to aluminium for which an accident was described. In addition, there was an accident which involved a mixture of this sulphate with red phosphorus although the latter had been previously treated with potassium nitrate and calcium silicide. 

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