Zone Melting

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Related to Float-zone process: Czochralski process

Zone Melting

 

a crystallophysical method of refining materials that consists in the movement of a narrow fusion zone along a long solid rod made of the material that is being refined. Nearly all industrially important metals, semiconductors, dielectrics, and inorganic and organic compounds— more than 120 substances—may undergo zone melting.

The first mention of the use of zone melting dates to 1927, when the method was used to purify iron. Zone melting became widely known in 1952 as a result of the works of W. Pfann (USA), who used it to produce germanium of high purity in a special container (container zone melting).

To perform container zone melting on a solid load placed in a container, a small melted region called a zone, which is shifted along the load, is created. In the process crystallization of the material takes place on one surface of the interface between the solid and liquid phases (the crystallization front), and the raw material is supplied at the other surface (the melting front). Container zone melting is used to refine a material that does not interact with the material of which the container is made. In 1953, P. Keck and M. Golei (USA) proposed a crucibleless method for zone melting of a vertical rod (the so-called floating zone method) for purifying semiconducting silicon. In this method, the fusion zone is restrained mainly by the forces of surface tension; therefore, crucibleless zone melting is widely used for refractory and active materials with sufficiently high surface tension and not very great density in liquid state (silicon, germanium, molybdenum, tungsten, platinum, palladium, rhenium, and niobium). Since 1955 zone melting has been widely used in laboratory and plant practice to produce pure materials with an impurity content of as little as 10−7-10−9 percent (so-called zone refining), for alloying and uniform distribution of the impurity throughout the ingot, and for growing single crystals, concentrating impurities in analytical practice, creating high-purity standards, and studying phase diagrams.

Zone refining is based on the fact that under conditions of equilibrium between the liquid and solid phases, the solubility of the impurities is different in the two phases. To produce pure metals the fusion zone is usually shifted through the ingot several times, or several shifting fusion zones are created simultaneously in the ingot, with sections of solid material between them. The rate of shifting of the fusion zone is usually 0.1-10.0 mm/min, and the number of passes is 10-15 or more. Refining is concluded when the maximum (final) distribution of the impurity, which cannot be changed by subsequent shifts of the zones, is attained.

The effectiveness of zone removal of impurities from a material depends on the distribution ratio of the impurity (the ratio of the concentration of the impurity in the solid phase to the concentration in the liquid phase), the number of passes and the rate of shifting of the zone, and the ratio of the length of the ingot to the length of the zone. Zone equalization consists in the introduction into the first zone of an alloying additive, which is uniformly distributed throughout the ingot upon repeated shifting of the zone. Alternate motion of the zone from the head to the tail of the ingot and back is sometimes used to achieve uniform distribution of the impurity throughout the ingot.

Zone melting may be used simultaneously with refining and to produce single crystals; a seed—the nucleus of a single crystal, oriented in a predetermined crystallographic direction—is used for this purpose. The first fusion zone is created at the interface of the seed with the rod that is to undergo zone melting, and part of the rod and part of the seed are fused. Thermal conditions that, upon solidification of the liquid metal, ensure controlled crystallization of the seed in the direction induced by the seed, are created at the interface of the seed and liquid metal phases. A special type is zone melting with a temperature gradient (a method of producing p-n transitions and producing phosphides and arsenides of gallium and indium). In this case a temperature and concentration difference is created between the boundaries of the liquid zone. Because of the different solubility of the components of the system at different temperatures, a shift of the zone in the direction of the temperature gradient takes place. The rate of shifting of the zone is usually 0.1-1.0 mm/hr, and the temperature difference may be as high as 80 deg/mm.

Various equipment is used for zone melting, depending on the function, conditions, and output of the process. Depending on the method of accomplishment, a distinction is made between container and crucibleless units, which in turn are divided into periodic, methodical, and continuous types according to the character of the process; into horizontal and vertical types according to the attitude of the melting material; into units with a moving ingot or heater according to the method by which the zone is shifted; into units that use resistance heaters (for materials with a melting point of up to 1500°C), induction heating (for melting substances with high electrical conductivity in a vacuum or inert gaseous medium), electron-beam heating (for in vacuo melting of materials with a high melting point), radiation heating (for materials with a low melting point), heating by thermal conductance and Joule heat according to the method of heating the zone; and according to the method used to mix the zone (convective, mechanical, or electromagnetic) and the composition of the atmosphere (vacuum or inert or shielding gas).

The equipment for container zone melting is a horizontal . pipe, within which a container with the load to be refined travels. The heaters are mounted outside the tube and heat either the load or the container. Zone-refined tin ingots weigh up to 60 kg; germanium ingots, 10 kg, and gallium arsenide ingots, 1 kg. Crucibleless zone melting is performed in a vertical tube in which the rod to be refined is mounted. The heater is located around the rod on the outside or within the pipe. The diameter of zone-refined silicon ingots may be 35-50 mm; of beryllium and iron ingots, 25 mm; and of vanadium ingots, 15 mm.

Container zone melting is developing in the direction of continuous zone melting units and processes (the zone-space, zone-transport, and electrodynamic methods), increased intensity of refining, reduced heterogeneity of the resultant crystals, and increased purity. The development of crucibleless zone melting is progressing toward an increase in the size of single crystals (a diameter of 55-65 mm), intensification of the refining process, and achievement of homogeneous distribution of impurities and structural flaws. The general trend in the development of zone melting is characterized by work on optimum conditions, the creation of more advanced equipment, the automation of the process, and the use of programming methods.

REFERENCES

Parr, N.Zonnaia ochistka i ee tekhnika. Moscow, 1963. (Translated from English.)
Zonnaia plavka, sb. Edited by V. N. Vigdorovich. Moscow, 1966.
Romanenko, V. N.Poluchenie odnorodnykh poluprovodnikovykh kristallov. Moscow, 1966.
Vigdorovich, V. N.Ochistka metallov i poluprovodnikov kristallizatsiei, Moscow, 1969.
Pfann, W. G.Zonnaia plavka. Moscow, 1960. (Translated from English.)

K. N. NEIMARK