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Crystal cultivation

The most widespread production method for silicon wafers begins with the drawing of single silicon crystals using the Czochralski technique (CZ). A single silicon crystal cylinder (ingot) is drawn from ultra-pure molten silicon. The starting point is a tiny single silicon crystal that is immersed in the melt as a seed medium. The single crystal obtains its crystal orientation through exact alignment of the seed crystal in one of its crystallographic directions. The seed crystal is continually drawn out of the melt on a metal rod while being rotated constantly at low speed. Drawing produces ingots with a length of over 2 metres and a diameter of up to 300 mm. The diameter of the ingots has been continually increased in recent years, as more circuits can be produced on a substrate during chip production, and waste is reduced as a result. The silicon melt is contained in quartz crucibles, these in turn being held in CFC or graphite crucibles. Ultra-pure graphite heating elements fixed with CFC screws are used to heat the melt. Decisive in the selection of material for all these components are the extreme purity requirements governing single crystal drawing. Even impurities in the ppm (parts per million) range have a negative effect on the process and the characteristics of the single crystal.

The ingots are then sawn into 0.5–1.5 mm thick single crystal slices (the so-called wafers), the silicon cylinder being glued to a graphite strip for this purpose. These wafers are subject to different etching, grinding and polishing processes, acquiring an almost perfectly level surface with unevenness within the range of a few nanometres. The wafers obtain their typical mirror-like surface after this treatment.

An alternative technique for manufacturing single silicon crystals is the zone melting technique. This involves the inductive heating of a small zone of a polycrystalline silicon rod, forming a liquid melt zone. This melt zone is brought into contact with a seed crystal, causing the silicon to solidify in the same crystal orientation during cooling. The melt zone is subsequently moved slowly and continually further and further away from the seed crystal, leading to the formation of increasingly more single crystal silicon. The melt is simultaneously enriched with impurities, so that ultra-pure single Si crystals are obtained through zone melting. However, the process is comparatively expensive. Other options for cultivating crystals include vertical drawing (Bridgman-Stockbarger method) and the vertical gradient freeze technique.

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| CZ Heater

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