Czochralski process

The Czochralski process is a procedure of materials technology for the production of single-crystal materials. It is also known under the terms crucible pulling process, or drawing from the melt. In the crucible, the substance to be crystallized is held slightly above the melting point ( within the Ostwald Miers - range in which no spontaneous nucleation takes place). Immersed in the seed (for example, less of the single crystal to be grown substance). By turning and slow up- pull - without any contact with the melt breaks off - growing the solidifying material to a single crystal, which continues the crystal lattice of the seed.

History

The Czochralski method was discovered in 1916 in the laboratory of metal AEG from Polish chemist, Jan Czochralski ( 1885-1953, 1904-1929 in Germany ) by mistake: he dipped his pen into a crucible with molten tin rather than the inkwell. Subsequently, he developed and improved the method showed that this single crystals can be prepared and used it to estimate rates of crystallization.

Although Hans von Wartenberg in 1918 recognized that the method can be used for crystal growth, it only came in 1950 for practical use on a large scale.

Technology

In a pot, there is a already cleaned melt of the desired material (eg, silicon ). Instead of high-purity material can be used depending on the desired use also predoped material, for example, with elements of III. or V of the periodic table, so it can be used directly as a base material for Integrated Circuits.

A fixed on a slowly rotating metal rod seed crystal is dipped from the top with the tip in the melt. The seed crystal must be aligned precisely with the desired crystal orientation on the metal bar, since it specifies the crystal orientation of the resulting crystal. The dipped by only a few millimeters end of the seed crystal must melt until a completely homogeneous boundary layer between the melt and the solid parts of the seed yields. The staff with the single crystal is slowly pulled upward while the melt solidifies at the forming interface. By varying the pulling rate and temperature of the growing crystal has reached the desired diameter. By means of an appropriate regime, the crystal diameter are maintained very well by the end of the drawing process.

The rotation of the seed crystal returns to the Konvektionsrichtung directly under the seed crystal, and only made possible by the directional growth of the crystal. Without rotation, would a " crystal plate " form on the cooler melt surface.

In a refinement of the method is directly based on the approach to the first seed crystal pulled an even thinner piece to only then go to the desired final diameter. On the resulting constriction should dislocations could still exist in the seed crystal, wander out to the side. Dislocations represent disorders of the single crystal structure, and are therefore especially not exactly aligned parallel to the axis of symmetry. So you drag them walk diagonally to the side, so that the remaining crystal is free of dislocations at a narrow point then even completely out of the crystal addition.

Known as the ingot crystal pillar can be up to two meters long. The current standard in the semiconductor industry is 30 cm diameter, from which 300 mm wafers are produced. Since 2010, the crystal growth of wafer is tested with a diameter of 450 mm for the silicon - Einkristallherstellern.

Application

With this method, the preparation of pure, single crystal materials are possible. It does not quite reach the quality of the zone melting method, but is less expensive. There are, inter alia, single crystals of semiconductors such as silicon, metals such as palladium, platinum, gold and silver salts such as alkali metal halides, oxides, silicates, such as yttrium aluminum garnets and yttrium - iron garnets with numerous applications especially for optical purposes (laser and sensor technology ) produced by this method.

Single crystal of silicon are manufactured in this manner in large quantities. After the crystal pulling they are cut into thin slices, the wafers are called. Use to find the so-called CZ wafers mainly in the manufacture of integrated circuits, microelectronics and microsystems technology.

For use in the photovoltaic the ingots are first cut to a pseudo- square in shape. In contrast to the circular wafers of microelectronics so formed after sawing the wafer in the form of a square with rounded corners. The solar modules produced therefrom may be denser equipped with solar cells, is so lost less floor space. The pseudo square solar wafers thus represent an economic compromise between space utilization and optimum utilization of the originally round ingots, obtained in the relatively little waste.