Float-zone silicon
Encyclopedia
Float-zone silicon is very pure silicon
obtained by vertical zone melting
. The process was developed at Bell Labs
by Henry Theuerer in 1955 as a modification of a method developed by William Gardner Pfann
for germanium
. In the vertical configuration molten silicon has sufficient surface tension
to keep the charge from separating. Avoidance of the necessity of a containment vessel prevents contamination of the silicon.
Float-zone silicon is a high-purity alternative to crystals grown by the Czochralski process
. The concentrations of light impurities, such as carbon
and oxygen
, are extremely low. Another light impurity, nitrogen
, helps to control microdefects and also brings about an improvement in 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 150mm due to the surface tension
limitations during growth. A polycrystalline rod of ultra-pure 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 in order 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 impurity.
and detector
applications. It is highly transparent to terahertz radiation, and is usually used to fabricate optical components, such as lenses and windows, for terahertz applications.
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
obtained by vertical zone melting
Zone melting
Zone melting is a group of similar methods of purifying crystals, in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal...
. The process was developed at Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
by Henry Theuerer in 1955 as a modification of a method developed by William Gardner Pfann
William Gardner Pfann
William Gardner Pfann 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 history of Bell Labs, "Timely invention of zone refining by W.G.Pfann .....
for germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
. In the vertical configuration molten silicon has sufficient surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
to keep the charge from separating. Avoidance of the necessity of a containment vessel prevents contamination of the silicon.
Float-zone silicon is a high-purity alternative to crystals grown by the Czochralski process
Czochralski process
The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors , metals , salts, and synthetic gemstones...
. The concentrations of light impurities, such as carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, are extremely low. Another light impurity, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, helps to control microdefects and also brings about an improvement in 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 150mm due to the surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
limitations during growth. A polycrystalline rod of ultra-pure 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
Seed crystal
A seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...
is used at one end in order 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 impurity.
Application
Float-zone silicon is typically used for power devicesPower semiconductor device
Power semiconductor devices are semiconductor devices used as switches or rectifiers in power electronic circuits . They are also called power devices or when used in integrated circuits, called power ICs....
and detector
Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
applications. It is highly transparent to terahertz radiation, and is usually used to fabricate optical components, such as lenses and windows, for terahertz applications.
See also
- Bridgman technique
- Czochralski processCzochralski processThe Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors , metals , salts, and synthetic gemstones...
- Micro-Pulling-DownMicro-pulling-down-Basics:The micro-pulling-down method is a crystal growth technique based on continuous transport of the melted substance through micro-channel made in a crucible bottom. Continuous solidification of the melt is progressed on a liquid/solid interface positioned under the crucible...
- Laser-heated pedestal growthLaser-heated pedestal growthLaser-heated pedestal growth is a crystal growth technique. The technique can be viewed as a miniature floating zone, where the heat source is replaced by a powerful CO2 or YAG laser...