Czochralski process

The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors, metals (e.g. palladium, platinum, silver, gold) and salts.

The most important application may be the growth of large cylindrical ingots, or boules, of single-crystal silicon. High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted down in a crucible. Dopant impurity atoms such as boron or phosphorus can be added to the molten intrinsic silicon in precise amounts in order to dope the silicon, thus changing it into n-type or p-type extrinsic silicon. This influences the electrical conductivity of the silicon. A seed crystal, mounted on a rod, is dipped into the molten silicon. The seed crystal's rod is pulled upwards and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. This process is normally performed in an inert atmosphere, such as argon, and in an inert chamber, such as quartz.

The largest silicon ingots produced today are 400mm in diameter and 1 to 2 metres in length. Thin silicon wafers are cut from these ingots and polished to a very high flatness to be used for creating integrated circuits. Other semiconductors, such as gallium arsenide, can also be grown by this method, although lower defect densities in this case can be obtained using variants of the Bridgeman technique.

The process is named after Jan Czochralski, who discovered the method in 1916 while investigating the crystallization rates of metals.