Zone refining, ultrapure

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. 

Atmospheric sensitivity renders the preparation of ultrapure samples difficult. Nevertheless, vacuum distillation ", ultra-high-vacuum reactive distillation " and crystal growth purification methods " are described zone-refining methods have been applied on a limited scale only - , presumably because of the high volatility of the metals and the unfavorable distribution coefficients. 

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. 

Derivation From bauxite by Bayer process and subsequent electrolytic reduction by Hall process. There are several processes for obtaining ultrapure aluminum (1) electrolytic (three-layer), (2) zone refining, and (3) chemical refining. Impurities as low as 0.2 ppm are possible. 

Some of the problems seen with the commercially available polyimides such as limited shelf life,gelation and high ionic contamination are traceable to the raw materials themselves. A zone refining technique has been perfected for use with organic materials and these precursors have been used to synthesize ultrapure polyamic acids for IC device applications. The key feature of the synthesis is the use of solid ingots of the dianhydrides. Materials prepared by this technique show low metallic impurities and have been shown to be excellent film formers for a variety of applications. In particular a polyimide derived from PMDA-ODA has been used to passivate magnetic bubble devices. IR techniques coupled with electrical measurements have been used to optimize the cure conditions and a simple resist process has been defined to passivate these devices. Device performance compares well with conventional inorganic insulators. 

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

A FIGURE 22.33 Zone-refining apparatus for production of ultrapure silicon. 

Zone refining, an extremely simple technique in principle, provides an excellent means of preparing ultrapure crystalline materials. Handling of the... 

Ultrapure silicon is not only important for transistors, it is also required for solar batteries. Zone refining of silicon using a quartz boat is unable to remove the last traces of boron except when a chemical purification is simultaneously applied. The reactivity of molten silicon creates problems of contamination with oxygen. For higher purity, the floating zone technique has been used to quantify the concentration of boron, an electron acceptor, and phosphorus, an electron donor. The electrical conductivity measured as a function of distance along the rod is found to be proportional to the difference in concentration of boron and phosphorus their concentration after refining is of the order of 10 atoms cm . ... 

The mtyor uses of zone refining are the production of silicon crystals for making integrated-circuit chips and for developing ultrapure research materials and precious metals. 

Figure 26-4 Ultrapure silicon is produced by zone refining as shown in the schematic at right.

For some specific applications, particularly for microelectronics, the purification of these monomers is sometimes so critical that the isolation of suitable reactants requires sophisticated purification methods. For instance, miniaturization and tougher processing requirements for advanced microelectronics have forced researchers to attain ultrapure poly(amic acid)s from monomers purified by zone refining, and dianhydrides isolated in solid ingot form [35]. 

Liquid-solid When all the constituents are present in both phases at equilibrium, we have the operation of fractional crystallization. Perhaps the most interesting examples of this are the special techniques of zone refining, used to obtain ultrapure metals and semiconductors, and adductioe crystallization, where a substance, such as urea, has a crystal lattice which will selectively entrap long straight-chain molecules like the paraffin hydrocarbons but will exclude branched molecules. 

To be used in semiconductors, silicon must be purified further by zone refining (see Fig. 16.29 in the text) to obtain ultrapure Si. 

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