Ultrapure silicon

Large-scale synthesis of materials Ultrapure single-crystal silicon Ultrapure glass... 

The semiconductor industry would have been impossible had not the process of zone refining been invented first. It is the standard way of producing ultrapure materials, both for research and for making silicon and germanium-based devices. 

Silicon shows a rich variety of chemical properties and it lies at the heart of much modern technology/ Indeed, it ranges from such bulk commodities as concrete, clays and ceramics, through more chemically modified systems such as soluble silicates, glasses and glazes to the recent industries based on silicone polymers and solid-state electronics devices. The refined technology of ultrapure silicon itself is perhaps the most elegant example of the close relation between chemistry and solid-state physics and has led to numerous developments such as the transistor, printed circuits and microelectronics (p. 332). 

Some elements come in and out of fashion, so to speak. Sixty years ago, elemental silicon was a chemical curiosity. Today, ultrapure silicon has become the basis for the multibillion-dollar semiconductor industry. Lead, on the other hand, is an element moving in the other direction. A generation ago it was widely used to make paint pigments, plumbing connections, and gasoline additives. Today, because of the toxicity of lead compounds, all of these applications have been banned in the United States. 

The silicon used for making solid-state semiconductor devices such as transistors, computer chips, and solar cells must be ultrapure, with impurities at a level of less than 10 7% (1 ppb). For electronic applications, silicon is purified by converting it to SiCl4, a volatile liquid (bp 58°C) that can be separated from impurities by fractional distillation and then converted back to elemental silicon by reduction with hydrogen ... 

The silicon is purified further by a process called zone refining (Figure 19.4a), in which a heater melts a narrow zone of a silicon rod. Because the impurities are more soluble in the liquid phase than in the solid, they concentrate in the molten zone. As the heater sweeps slowly down the rod, ultrapure silicon crystallizes at the trailing edge of the molten zone, and the impurities are dragged to the rod s lower end. Figure 19.4b shows some samples of ultrapure silicon. 

FIGURE 19.4 (a) Purifica- tion of silicon by zone refining. The heater coil sweeps the molten zone and the impurities to the lower end of the rod. After the rod has cooled, the impurities are removed by cutting off the rod s lower end. (b) A rod of ultrapure silicon and silicon wafers cut from the rod. Silicon wafers are used to produce the integrated-circuit chips found in solid-state electronic devices. 

Finally, Vycor devitrifies far less than fused silica. Therefore, if you do not require ultrapure baking environments (similar to those demanded in the silicon industry), furnace tubes made from Vycor may be cheaper in the long run than those made from less expensive fused silica. 

The manufacture of silicon integrated circuits involves a sequence of inter-related steps, each designed to purify, modify, deposit, or pattern materials. Figure 3 illustrates the principal steps from silicon purification to packaging. A good review of the entire process can be found in Wolf and Tauber [6]. The requirement for ultrapure starting reagents (contaminants... 

The next step is to produce nearly perfect single-crystal boules of silicon from the ultrapure polycrystalline silicon. Many techniques have been developed to accomplish this, and they all rely on a similar set of concepts that describe the transport process, thermodynamically controlled solubility, and kinetics [8]. Three important methods are the vertical Bridgman-Stockbarger, Czochralski, and floating zone processes, fully described in Fundamentals of Crystal Growth by Rosenberger [9]. 

Siemens A method for making ultrapure silicon for semiconductors by thermally decomposing trichlorosilane. Invented in 1954 by F. Bischof at Siemens-Halska. It was the major process used worldwide in 1993. 

Since the electrical properties of semiconductors are strongly influenced by the incorporation of heteroelements, ultrapure semiconductor grade silicon is obtained by the reduction of SiCL, or SiHCh with very pure Mg or Zn, followed by the growth from the molten silicon of a columnar single crystal which is further purified... 

An existing industrial application of the ultrapure hydrogen separated by dense Pd alloy membranes is for electronics industry. In the fabrication of silicon chips, hydrogen acts as a carrier to transport small quantities of vaporized chemical compounds required to "dope" the chip to the surface of the silicon wafer [Philpott and Coupland, 1988]. The hydrogen used must be of a very high purity. The membrane units are used not only for gas purification but also for hydrogen recovery from hydrogen>rich gas streams. 

Si(Li) detector can be stored for years, without losing its properties. When it is cooled again, it will display the same characteristics as when it was new. Therefore, the search for ultrapure silicon material does not have the same significance as in the case of germanium. 

M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, and K. Suma, Control factor of native oxide growth on silicon in air or in ultrapure water, Appl. Phys. Lett. 55(6), 562, 1989. 

Whereas ca. 3000 t of ultrapure silicon ( electronic grade ) was produced in 1980 for the manufacture of electronic components markets, the booming electronic industry in the meantime has led to an explosive expansion in production capacity to ca. 20 10 t/a, of which 40% is in the USA, 30% is in Japan and ca. 30% is in Europe. Due to the strongly growing electronics market and the emerging photovoltaic market (solar cells on the basis of crystalline silicon), a strongly expanding demand for ultrapure silicon is expected in the future. 

Ultrapure silicon is the product of a very expensive multistage purification process (see Section 3.4.1.1.2). The price for this material therefore increases strongly with the degree of refining. 1 kg of polycrystalline ultrapure silicon ( polysilicon ) from the pyrolysis of SiHCly cost ca. 80 DEM in 1997, silicon single crystals ca. 600 DEM/kg and silicon wafers used in semiconductor technology ca. 1700 DEM/kg. 

Silicon only exhibits semiconducting properties when ultrapure. 

The specific resistance of ultrapure silicon single crystals of up to 150 000 12 cm decreases upon doping with 1 ppb of phosphorus to 100 Q cm. Therefore the purity... 

Manufacture of ultrapure silicon by pyrolysis of SiHCli or SiH4... 

The Na AlFg (cryolite) produced as a byproduct in this process is utilized in the aluminum industry and the SiH4, after ultrapurification, is decomposed in a fluidized bed reactor to hydrogen and ultrapure silicon on nuclei of elemental silicon already present there (see Fig. 3.4-3) ... 

This process supplies ultrapure silicon in the form of ca. 1-3 mm, easily flowing and easily dosable beads. Compared with SiHCl3-pyrolysis, this process is characterized by low process temperatures and non-corrosive byproducts, but, due to the spontaneous inflammability of SiH4, it requires extensive safety measures. 1500 t of ultrapure silicon was produced by this process in 1997. 

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