Czochralski method
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The Czochralski process is a material technology process for the production of single-crystal materials ( crystal growth ). It is also known as the crucible pulling process or pulling from the melt . In the crucible, the substance to be crystallized is kept a few degrees above the melting point (within the Ostwald-Miers range , in which no spontaneous nucleation takes place). A nucleus (e.g. small single crystal) of the substance to be grown is immersed in its surface. By turning and slowly pulling it upwards - without breaking the contact with the melt - the solidifying material grows to a single crystal, which continues the crystal lattice of the nucleus.
history
The Czochralski process was discovered by mistake in the AEG metal laboratory in 1916 by the Polish chemist Jan Czochralski (1885–1953, 1904–1929 in Germany): he dipped his pen into a melting pot filled with liquid tin instead of in an inkwell. He then developed and improved the process, demonstrated that single crystals can be produced with it and used it to estimate crystallization rates.
technology
In a crucible there is an already cleaned melt of the desired material (e.g. silicon ). Instead of high-purity material, depending on the intended use, predoped material can also be used, for example with elements of III. or V. main group of the periodic table so that it can be used directly as a base material for integrated circuits .
A seed crystal attached to a slowly rotating metal rod is dipped into the melt from above with the tip. The seed crystal must be precisely aligned with the desired crystal orientation on the metal rod , as it specifies the crystal orientation of the single crystal being produced. The end of the seed crystal, which is only a few millimeters submerged, must melt until a very homogeneous boundary layer is created between the melt and the solid part of the seed crystal. The rod with the single crystal is slowly pulled up again, while the melt solidifies at the interface that forms. By varying the pulling speed and temperature, the growing crystal achieves the desired diameter. By means of a suitable control, the crystal diameter can be maintained very precisely until the end of the pulling process.
The rotation of the seed crystal reverses the convection direction directly under the seed crystal and only then enables the crystal to grow in a directional manner. Without rotation, a "crystal plate" would form on the cooler melt surface.
In a refinement of the process, an even thinner piece is first drawn directly after the seed crystal has been applied, and only then is it reached the desired final diameter. At the resulting bottleneck, dislocations that could still exist in the seed crystal should migrate to the side. Dislocations represent disturbances in the monocrystalline structure and are therefore precisely not aligned exactly parallel to the axis of symmetry. When pulled, they move diagonally to the side, at a narrow point then even completely out of the crystal, so that the remaining crystal is free of dislocations.
The crystal column known as an ingot can be up to two meters long. The current standard in the semiconductor industry is 30 cm in diameter, from which 300 mm wafers are made. Since 2010, silicon single crystal manufacturers have been testing crystal growth for wafers with a diameter of 450 mm.
application
This process enables the production of pure, monocrystalline materials. It does not quite match the quality of the zone melting process , but it is cheaper. There are, among other things, single crystals made of semiconductors such. B. silicon , metals such. B. palladium , platinum , gold and silver , salts such as. B. alkali metal halides, oxides and silicates such as. B. Yttrium-aluminum garnets and yttrium-iron garnets with numerous possible uses, especially for optical purposes (laser technology and sensor technology), are manufactured using this method.
Single crystals of silicon are produced in large quantities in this way. After crystal pulling, they are cut into thin slices called wafers . These so-called CZ wafers are primarily used in the production of integrated circuits in microelectronics and in microsystem technology .
For use in photovoltaics , the ingots are first cut to a pseudo-square cross-section. This results in wafers with the shape of a square with rounded corners by sawing. The solar modules produced from this can be equipped with more solar cells, so that less usable area is lost. The pseudo-square solar wafers thus represent an economic compromise between the use of space and the best possible use of the originally round ingot, with which there is relatively little waste.
literature
- Jürgen Evers, Peter Klüfers, Rudolf Staudigl, Peter Stallhofer: Czochralski's creative mistake: a milestone on the way to the gigabit era . In: Angewandte Chemie . tape 115 , 2003, ISSN 0044-8249 , p. 5862-5877 , doi : 10.1002 / anie.200300587 .
Web links
Individual evidence
- ↑ Jan Czochralski : A new method for measuring the crystallization rate of metals. In: Journal of physical chemistry . Vol. 92, 1918, pp. 219-221 .
- ↑ Swisswafers AG: Mono-silicon wafers (monocrystalline / Czochralski / CZ wafers) ( Memento from July 20, 2012 in the Internet Archive ), accessed on March 31, 2010 (description of manufacture).