Float-zone silicon is very pure

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...

obtained by vertical

zone melting Zone melting (or zone refining, or floating-zone method, or floating-zone technique) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved along the crystal. The molte ...

. The process was developed at

Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial Research and development, research and scientific developm ...

by Henry Theuerer in 1955 as a modification of a method developed by

William Gardner Pfann William Gardner Pfann (October 27, 1917 – October 22, 1982) was an inventor and materials scientist with Bell Labs. Pfann is known for his development of zone melting which is essential to the semiconductor industry. As stated in an offici ...

for germanium. In the vertical configuration molten silicon has sufficient surface tension to keep the charge from separating. The major advantages is crucibleless growth that prevents contamination of the silicon from the vessel itself and therefore an inherently high-purity alternative to boule crystals grown by the

Czochralski method The Czochralski method, also Czochralski technique or Czochralski process, is a method of crystal growth used to obtain single crystals of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, platinum, silver, ...

. The concentrations of light impurities, such as carbon (C) and oxygen (O₂) elements, are extremely low. Another light impurity, nitrogen (N₂), helps to control microdefects and also brings about an improvement in mechanical strength of the

wafers A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...

, and is now being intentionally added during the growth stages. The diameters of float-zone wafers are generally not greater than 200 mm due to the surface tension limitations during growth. A

polycrystalline A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...

rod of ultrapure electronic-grade silicon is passed through an RF heating coil, which creates a localized molten zone from which the crystal ingot grows. A seed crystal is used at one end to start the growth. The whole process is carried out in an evacuated chamber or in an inert gas purge. The molten zone carries the impurities away with it and hence reduces impurity concentration (most impurities are more soluble in the melt than the crystal). Specialized doping techniques like core doping, pill doping, gas doping and neutron transmutation doping are used to incorporate a uniform concentration of desirable impurity. Float-zone silicon wafers may be irradiated by neutrons to turn it into a n-doped semiconductor.

Application

Float-zone silicon is typically used for

power devices A power semiconductor device is a semiconductor device used as a switch or rectifier in power electronics (for example in a switch-mode power supply). Such a device is also called a power device or, when used in an integrated circuit, a power IC. ...

and

detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

applications, where high-resistivity is required. It is highly transparent to

terahertz radiation Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of fre ...

, and is usually used to fabricate optical components, such as

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

es and windows, for terahertz applications. It is also used in

solar arrays A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and c ...

of satellites as it has higher conversion efficiency.

References

* Michael Riordan & Lillian Hoddeson (1997) ''Crystal Fire: the birth of the information age'', page 230, W. W. Norton & Company . {{DEFAULTSORT:Float-Zone Silicon Industrial processes Semiconductor growth Silicon, Float-zone Methods of crystal growth

Application

See also

References