Silicon purity

Finally, in 1985, the results of an extensive investigation in which adsorjDtion took place onto an aluminium oxide layer fonned on a film of aluminium deposited in vacuo onto a silicon wafer was published by Allara and Nuzzo 1127, 1281. Various carboxylic acids were dissolved in high-purity hexadecane and allowed to adsorb from this solution onto the prepared aluminium oxide surface. It was found that for chains with more than 12 carbon atoms, chains are nearly in a vertical orientation and are tightly packed. For shorter chains, however, no stable monolayers were found. The kinetic processes involved in layer fonnation can take up to several days. 

Silicon is used in many fonns, from high-purity tliin films to bulk material, which may be crystalline, multi- or poly crystalline and amorjDhous (usually hydrogenated). Silicon is the material discussed tire most in tliis article. Substitutional B and P are tire most common (of many) shallow acceptors and donors, respectively. 

Boron of 99.9999% purity has been produced and is available commercially. Elemental boron has an energy band gap of 1.50 to 1.56 eV, which is higher than that of either silicon or germanium. 

Both the Toth and Alcoa processes provide aluminum chloride for subsequent reduction to aluminum. Pilot-plant tests of these processes have shown difficulties exist in producing aluminum chloride of the purity needed. In the Toth process for the production of aluminum chloride, kaolin [1332-58-7] clay is used as the source of alumina (5). The clay is mixed with sulfur and carbon, and the mixture is ground together, pelletized, and calcined at 700°C. The calcined mixture is chlorinated at 800°C and gaseous aluminum chloride is evolved. The clay used contains considerable amounts of silica, titania, and iron oxides, which chlorinate and must be separated. Silicon tetrachloride and titanium tetrachloride are separated by distillation. Resublimation of aluminum chloride is requited to reduce contamination from iron chloride. 

Silicon reacts at elevated temperatures with the halogens, forming SiCl, Sil, SiBr, and SiF. There is also a series of halogen-substituted silanes such as trichlorosilane, SiH Cl, and dichlorosilane, S1H2CI2. Both SiCl and SiH Cl are relatively easy to make, purify, and reduce to silicon. These are the silicon compounds most often used as feedstocks in the manufacture of high purity silicon. 

The commercial production of silicon in the form of binary and ternary alloys began early in the twentieth century with the development of electric-arc and blast furnaces and the subsequent rise in iron (qv) and steel (qv) production (1). The most important and most widely used method for making silicon and silicon alloys is by the reduction of oxides or silicates using carbon (qv) in an electric arc furnace. Primary uses of silicon having a purity of greater than 98% ate in the chemical, aluminum, and electronics markets (for higher purity silicon, see Silicon AND SILICON ALLOYS, PURE SILICON). 

Silicate esters are used ia the production of coating and refractories and in some semiconductor manufacturing operations. A broad range of purity grades of silicon tetrachloride are available to meet the requirements of these different appHcations. 

The partially alkoxylated chlorotitanates, (RO) TiCl, can be prepared in high purity by reaction of TiCl with an organosilane ester, Si(OR)4 (see Silicon compounds). The degree of esterification of the titanium can be controlled by the amount of silane ester used. When is 3 or 4, the addition of the appropriate alcohol and an amine receptor is required (5). 

The red cake can be further purified by dissolving it in an aqueous solution of Na2C02- The iron, aluminum, and silicon impurities precipitate from the solution upon pH adjustment. Ammonium metavanadate then precipitates upon the addition of NH Cl and is calcined to give vanadium pentoxide of greater than 99.8% purity. 

Arsenic from the decomposition of high purity arsine gas may be used to produce epitaxial layers of III—V compounds, such as Tn As, GaAs, AlAs, etc, and as an n-ty e dopant in the production of germanium and silicon semiconductor devices. A group of low melting glasses based on the use of high purity arsenic (24—27) were developed for semiconductor and infrared appHcations. 

Chromium oxide is mixed with aluminum powder, placed in a refractory-lined vessel, and ignited with barium peroxide and magnesium powder. The reaction is exothermic and self-sustaining. Chromium metal of 97—99% purity is obtained, the chief impurities being aluminum, iron, and silicon (Table 4). Commercial chromium metal may also be produced from the oxide by reduction with silicon in an electric-arc furnace. 

Purification of Silicon. Chemical purity plays an equally important role in the bulk of materials as on the surface. To approach the goal of absolute stmctural perfection and chemical purity, semiconductor Si is purified by the distillation of trichlorosilane [10025-78-2] SiHCl, followed by chemical vapor deposition (CVD) of hulk polycrystalline siUcon. 

The purity of cyclobutanone was checked by gas chromatography on a 3.6-m. column containing 20% silicone SE 30 on chromosorb W at 65°. The infrared spectrum (neat) shows carbonyl absorption at 1779 cm. - the proton magnetic resonance spectrum (carbon tetrachloride) shows a multiplet at 8 2.00 and a triplet at S 3.05 in the ratio 1 2. 

The term direct TXRF refers to surface impurity analysis with no surface preparation, as described above, achieving detection Umits of 10 °—10 cm for heavy-metal atoms on the silicon surface. The increasit complexity of integrated circuits fabricated from silicon wafers will demand even greater surfrce purity in the future, with accordingly better detection limits in analytical techniques. Detection limits of less than 10 cm can be achieved, for example, for Fe, using a preconcentration technique known as Vapor Phase Decomposition (VPD). 

Figure 5 Effect of cleaning on the surface purity of silicon wafers, as measured by VPD-...

Figure 6 Typical secondary ion mass spectrum obtained from high-purity silicon using an oxygen ion beam. Major ion peaks are identified in the spectrum.

Of course, the most reliable and accurate method of quantitative analysis is to calibrate each element with standards prepared in matrices similar to the unknown being analyzed. For a survey technique that is used to examine such a wide variety of materials, however, standards are not available in many cases. When the technique is used mainly in one application (typing steels, specifying the purity of alloys for a selected group of elements, or identifying impurities in silicon boules and... 

Attempts by Kao and others to enhance transparency by chemically removing impurities from glass met with little success the level of purity required was indeed comparable with that needed in silicon for integrated circuits. In the event, the required purification was achieved in the same way in which semiconductor-grade silicon is now manufactured, by going through the gas phase (silicon tetrachloride), which can be separated from the halides of impurity species because of dilTerences in vapour pressures. This breakthrough was achieved by R.D. Maurer and his... 

Silicon (96-99% pure) is now invariably made by the reduction of quartzite or sand with high purity coke in an electric arc furnace the Si02 is kept in excess to prevent the accumulation of SiC (p. 334) ... 

With increasing purity of aluminium, greater resistance to corrosion is developed. On high-purity materials, however, any pits which develop are likely to be deeper though fewer in number than those formed in more impure metal. In some special applications, notably in contact with ammonia solutions or pure water at elevated temperatures and pressures, the iron and silicon present in commercial-purity metal are beneficial and retard corrosion. Up to about 5% magnesium improves the corrosion resistance to sea-water. 

The effect of commercial metal purity (impurities mainly iron and silicon) on corrosion by 40% hydrochloric acid is shown in Fig. 4.7. This curve is typical of that obtained with many acids. 

Alloys of aluminium with magnesium or magnesium and silicon are generally more resistant than other alloys to alkaline media. The corrosion rate in potassium and sodium hydroxide solutions decreases with increasing purity of the metal (Fig. 4.9), but with ammonium hydroxide the reverse occurs. 

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