Float-zone silicon is very pure

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

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 through the crystal. The molt ...

. The process was developed at

Bell Labs Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...

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 official ...

for

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

. In the vertical configuration molten silicon has sufficient

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...

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 (monocrystals) of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, platin ...

. 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 the mechanical strength of the wafers, 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. S ...

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 A seed crystal is a small piece of single crystal or polycrystal material from which a large crystal of typically the same material is grown in a laboratory. Used to replicate material, the use of seed crystal to promote growth avoids the otherwi ...

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 and

detector A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

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 International Telecommunicat ...

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

lens A lens is a transmissive optical device that 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 called a 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 abs ...

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