SILICON (CZOCHRALSKI METHOD)

Silicon is the most abundant solid element on earth, being second only to oxygen and it makes up more than 25% of the earth’s crust. However, it rarely occurs in elemental form, virtually all of it is existing as compounds.

Very pure sand (SiO2) is converted into mono-crystalline silicon and later on into silicon detectors. After a description of the different growth techniques for mono-crystalline silicon with special interest in the material used.

The material requirements for the manufacturing of silicon particle detectors used for high energy physics applications have to meet two basic demands: high resistivity and high minority carrier lifetime. A very high resistivity (> l Kohm/cm) is needed in order to fully deplete the detector bulk with a thickness of about 200 – 300 um by an adequate voltage below about 300 V.

Together with the demand for a reasonable price and a homogeneous resistivity distribution, not only over a single wafer but also over the whole ingot, Float Zone silicon is the best choice of material and is therefore exclusively used for detector applications today. Further requirements for detector grade silicon are often a high minority carrier lifetime and a very low bulk generation current in order to avoid detector noise.

However, these requirements should not be taken too strictly for particle detectors that will be exposed to severe radiation levels since already after small radiation fluences the lifetimes are reduced by orders of magnitude and therefore the good initial lifetime qualities are of no use any more.

Czochralski Silicon (Cz)

The vast majority of the commercially grown silicon is Czochralski silicon due to the better resistance of the wafers to thermal stress, the speed of production, the low cost and the high oxygen concentration that offers the possibility of Internal Gettering. The industrial standard crystals range in diameter from 75 to 200 mm, are typically l m long and of < 100> orientation.

Standard CZ

The Czochralski method is named after J. Czochralski, who determined the crystallisation velocity of metals by pulling mono- and polycrystals against gravity out of a melt which is held in a crucible. The pull-from-melt method widely employed today was developed by Teal and Little in 1950.

It is estimated that 99% of all semiconductor devices are made of monocrystalline silicon. Crystal silicon is an incredibly important part of modern life due to the large dependency users have on technological devices. There are different methods used in growing the necessary crystals for silicon wafers. The process in which crystalline materials are grown is incredibly important for the end use of the wafer in terms of its purity. One of the processes used for growing this crystalline material used in silicon wafers is called the Czochralski Crystal Growth Process. History of Czochralski Method It was in 1916 that a Polish metallurgist published a method for measuring maximum crystallization rates of metals. He was pulling metal wires vertically from melts with increasing velocities, and in these single crystalline wires occurred. He realized that with this pulling technique, single crystals could be grown successfully if single crystal seeds are used. First, single geranium crystals were grown using this method in 1948. In 1949, it was recognized that silicon was a better semiconductor material and so in 1951 Silicon crystals were grown using the Czochralski Method. Crystals grown using this method are often referred to as monocrystalline Czochralski silicon (Cz-Si).Czochralski Process OverviewTo begin, high purity silicon is melted in a crucible. At this point, dopants can be added in, depending on the end result of the wafer. Boron or phosphorus is often added to change the silicon into p-type or n-type and thus changing the silicon’s electronic properties. Dipped into the molten silicon is a rod mounted seed crystal which is slowly pulled upwards and rotated. By controlling the speed of rotation and pulling rate, a large, single crystal, cylindrical ingot can be extracted. The pull rate and temperature profile determines the diameter of the crystal. The Czochralski process is the preferred method for high volume production of silicon single crystals. After the crystals are produced, they can be cut into slices and polished and the wafers can be used as starting materials for chip production. Depending on the purpose of the silicon wafers, its diameter and thickness need to be precisely measured. Thanks to this process, the technology has been made possible. Electronic devices today are all dependent on silicon wafer technology, and the Czochralski Process is just one of the many processes behind the production of these incredible technological feats!For any questions you may have regarding the production of silicon wafers or for more information about obtaining silicon wafers for your business, you can connect with Wafer World online today! We are a leader in silicon wafer manufacturing and serve customers in over 45 countries! Contact us about your wafer needs today!