Silicon, zone refining

Now for some practical examples of how phase diagrams are used. In the first, a typical design problem, we find out how solders are chosen for different uses. In the second we look at the high-technology area of microchip fabrication and study the production, by zone refining, of ultra-pure silicon. And lastly, for some light-hearted relief, we find out how bubble-free ice is made for up-market cocktails. 

Fig. 4.4. Stages in zone refining o bar of impure silicon (a) We start with a bar that has a uniform concentration of impurity, Q. (b) The left-hand end of the bar is melted by o small electric tube furnace, making a liquid zone. The bar is encapsulated in a ceramic tube to stop the liquid running away. ( ) The furnace is moved off to the right, pulling the zone with it. (d) As the zone moves, it takes in more impurity from the melted solid on the right than it leaves behind in the freshly frozen solid on the left. The surplus pushes up the concentration of impurity in the zone, which in turn pushes up the concentration of impurity in the next layer of solid frozen from it. (e) Eventually we reach steady state, (f) When the zone gets to the end of the bar the concentrations in both solid and liquid increase rapidly, (g) How we set up eqn. (4.1).

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. 

Float zone crystal growth, silicon purification via, 22 496 Float zone refining, of silicon, 22 492-493 Flocculant addition point, 11 639 Flocculant chemistry, selection of, 11 638 Flocculants, 8 709-710 16 659. See also Flocculating agents dispersants contrasted, 8 687 organic, 11 627-631... 

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. 

Silicon, the second most abundant element in the earth s crust, is obtained by reducing silica sand (Si02) with coke. It is purified for use in the semiconductor industry by zone refining. In the silicates, SiC>4 tetrahedra share common O atoms to give silicon oxoanions with ring, chain, layer, and extended three-dimensional structures. In aluminosilicates, such as KAlSi30g, Al3+ replaces some of the Si4+. 

The primary application for floating-zone melting is crystal growth rather than purification. Semiconductor-grade sflicon is not purified by zone refining silicon chlorides are distilled and then reduced with hydrogen. 

The equilibrium distribution coefficient close to the melting point is also known as the partition coefficient. Since the partition coefficient controls the incorporation of impurities in the crystal during crystal growth and zone refining, it is one of the most important parameters that can be obtained from the thermochemical database. It is worth noting that the distribution coefficient determined by the ratio of volume concentrations, cm 3, can be related to the distribution coefficient by introducing the density ratio of liquid and solid silicon ... 

The minimum energy gap is also the important factor for other properties of a solid which depend on the electrons in the conduction band. These include the Pauli spin paramagnetism, and the (small) contribution of the electrons to thermal conductivity. All of these properties are due to extremely small concentrations of free electrons. Thus for silicon, where El = 1.1 eV, the number of conduction electrons is only 2 x 10 /cm, compared with an atom concentration of 5 X 10 /cm. This is for a sample where impurity concentrations have been reduced to 1 part in 10 by zone refining. 

This crude silicon is made ultrapure through zone refining for use in the manufacture of computer chips. 

Pure silicon is then made super-pure (impurities <10-9 atom percent) by zone refining. In this process a rod of metal is heated near one end so that a cross-sectional wafer of molten silicon is produced. Since impurities are more soluble in the melt than they are in the solid they concentrate in the melt, and the melted zone is then caused to move slowly along the rod by moving the heat source. This carries impurities to the end. The process may be repeated. The impure end is then removed. 

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