Silicon (IPA: /?s?l?k?n/ or /?s?l??kɑn/, Latin: silicium) is the chemical
element that has the symbol Si and atomic number 14. A tetravalent metalloid,
silicon is less reactive than its chemical analog carbon. As the eighth most
common element in the universe by mass, silicon occasionally occurs as the pure
free element in nature, but is more widely distributed in dusts, planetoids and
planets as various forms of silicon dioxide or silicate. On Earth, silicon is
the second most abundant element (after oxygen) in the crust, making up 25.7% of
the crust by mass.
Silicon has many industrial uses. Elemental silicon is the principal component
of most semiconductor devices, most importantly integrated circuits or
microchips. Silicon is widely used in semiconductors because it remains a
semiconductor at higher temperatures than the semiconductor germanium and
because its native oxide is easily grown in a furnace and forms a better
semiconductor/dielectric interface than almost all other material combinations.
In the form of silica and silicates, silicon forms useful glasses, cements, and
ceramics. It is also a component of silicones, a class-name for various
synthetic plastic substances made of silicon, oxygen, carbon and hydrogen, often
confused with silicon itself.
Silicon is an essential element in biology, although only tiny traces of it
appear to be required by animals. It is much more important to the metabolism of
plants, particularly many grasses, and silicic acid (a type of silica) forms the
basis of the striking array of protective shells of the microscopic diatoms.
Notable characteristics
Having the same structure to the outer electron orbitals (half filled subshell
holding up to eight electrons) as carbon, the two elements are very similar
chemically and both are semiconductors readily either donating or sharing their
four outer electrons allowing many different forms of chemical bonding. Pure
silicon has a negative temperature coefficient of resistance, since the number
of free charge carriers increases with temperature. The electrical resistance of
single crystal silicon significantly changes under the application of mechanical
stress due to the piezoresistive effect.
In its elemental crystalline form, silicon has a gray color and a metallic
luster which increases with the size of the crystal. It is similar to glass in
that it is rather strong, very brittle, and prone to chipping. Even though it is
a relatively inert element, silicon still reacts with halogens and dilute
alkalis, but most acids (except for some hyper-reactive combinations of nitric
acid and hydrofluoric acid) do not affect it. Having four bonding electrons
however gives it, like carbon, many opportunities to combine with other elements
or compounds under the right circumstances.
Applications
As the second most common element on earth, silicon is a very useful element
that is vital to many human industries, and impacts much of modern life as a
principal component in glass, concrete and cements of many kinds. Outside of the
many modern world features its construction uses enable, perhaps silicon's most
lifestyle affecting application is its use as the fundamental substrate in
manufacturing electronics integrated circuits such as computer chips, and
discrete active devices such as power transistors. Further, the element and its
compounds find widespread use in explosives and pyrotechnics and further uses in
mechanical seals, high temperature silicon based greases, caulking compounds and
so forth.
Alloys
* The largest application of pure silicon (metallurgical grade silicon) is in
aluminium-silicon alloys, often called "light alloys", to produce cast parts,
mainly for automotive industry. (This represents about 55% of the world
consumption of pure silicon.)
* The second largest application of pure silicon is as a raw material in the
production of silicones (about 40% of the world consumption of silicon)
* Pure silicon is also used to produce ultra-pure silicon for electronic and
photovoltaic applications:
* Semiconductor: Ultrapure silicon can be doped with other elements to adjust
its electrical response by controlling the number and charge (positive or
negative) of current carriers. Such control is necessary for transistors, solar
cells, microprocessors, semiconductor detectors and other semiconductor devices
which are used in electronics and other high-tech applications.
* Photonics: Silicon can be used as a continuous wave Raman laser to produce
coherent light. (Though it is ineffective as a light source.)
* LCDs and solar cells: Hydrogenated amorphous silicon is widely used in the
production of low-cost, large-area electronics in applications such as LCDs. It
has also shown promise for large-area, low-cost thin-film solar cells.
* Steel and cast iron: Silicon is an important constituent of some steels, and
it is used in the production process of cast iron. It is introduced as
ferrosilicon or silicocalcium alloys.
Compounds
* Construction: Silicon dioxide or silica in the form of sand and clay is an
important ingredient of concrete and brick and is also used to produce Portland
cement.
* Pottery/Enamel is a refractory material used in high-temperature material
production and its silicates are used in making enamels and pottery.
* Glass: Silica from sand is a principal component of glass. Glass can be made
into a great variety of shapes and with a many different physical properties.
Silica is used as a base material to make window glass, containers, insulators,
and many other useful objects.
* Abrasives: Silicon carbide is one of the most important abrasives.
* Medical materials: Silicones are flexible compounds containing silicon-oxygen
and silicon-carbon bonds; they are widely used in applications such as
artificial breast implants and contact lenses. Silicones are also used in many
other applications.
* Silly Putty was originally made by adding boric acid to silicone oil. Now
name-brand Silly Putty also contains significant amounts of elemental silicon.
(Silicon binds to the silicone and allows the material to bounce 20% higher.)
See also
History
Silicon was first identified by Antoine Lavoisier in 1787 (as a component of the
Latin , or silicis (meaning what were more generally termed "the flints" or
"Hard Rocks" during the Early Modern era where nowadays as we would say "silica"
or "silicates"), and was later mistaken by Humphry Davy in 1800 for a compound.
In 1811 Gay-Lussac and Thénard probably prepared impure amorphous silicon
through the heating of potassium with silicon tetrafluoride. It was first
discovered as an element by Berzelius in 1823. In 1824, Berzelius prepared
amorphous silicon using approximately the same method as Lussac. Berzelius also
purified the product by repeatedly washing it.
Because silicon is an important element in semiconductors and high-tech devices,
the high-tech region of Silicon Valley, California, is named after this element.
Occurrence
Measured by mass, silicon makes up 25.7% of the Earth's crust and is the second
most abundant element on Earth, after oxygen. Pure silicon crystals are only
occasionally found in nature; they can be found as inclusions with gold and in
volcanic exhalations. Silicon is usually found in the form of silicon dioxide
(also known as silica), and silicate.
Silica occurs in minerals consisting of (practically) pure silicon dioxide in
different crystalline forms. Sand, amethyst, agate, quartz, rock crystal,
chalcedony, flint, jasper, and opal are some of the forms in which silicon
dioxide appears. (They are known as "lithogenic", as opposed to "biogenic",
silicas.)
Silicon also occurs as silicates (various minerals containing silicon, oxygen
and one or another metal), for example feldspar. These minerals occur in clay,
sand and various types of rock such as granite and sandstone. Asbestos,
feldspar, clay, hornblende, and mica are a few of the many silicate minerals.
Silicon is a principal component of aerolites, which are a class of meteoroids,
and also is a component of tektites, which are a natural form of glass.
See also
Production
Silicon is commercially prepared by the reaction of high-purity silica with
wood, charcoal, and coal, in an electric arc furnace using carbon electrodes. At
temperatures over 1900 °C, the carbon reduces the silica to silicon according to
the chemical equation
SiO2 + C → Si + CO2.
Liquid silicon collects in the bottom of the furnace, and is then drained and
cooled. The silicon produced via this process is called metallurgical grade
silicon and is at least 98% pure. Using this method, silicon carbide, SiC, can
form. However, provided the amount of SiO2 is kept high, silicon carbide may be
eliminated, as explained by this equation:
2 SiC + SiO2 → 3 Si + 2 CO.
In 2005, metallurgical grade silicon cost about $ 0.77 per pound ($1.70/kg).
Purification
The use of silicon in semiconductor devices demands a much greater purity than
afforded by metallurgical grade silicon. Historically, a number of methods have
been used to produce high-purity silicon.
Physical methods
Silicon wafer with mirror finish (NASA)
Early silicon purification techniques were based on the fact that if silicon is
melted and re-solidified, the last parts of the mass to solidify contain most of
the impurities. The earliest method of silicon purification, first described in
1919 and used on a limited basis to make radar components during World War II,
involved crushing metallurgical grade silicon and then partially dissolving the
silicon powder in an acid. When crushed, the silicon cracked so that the weaker
impurity-rich regions were on the outside of the resulting grains of silicon. As
a result, the impurity-rich silicon was the first to be dissolved when treated
with acid, leaving behind a more pure product.
In zone melting, also called zone refining, the first silicon purification
method to be widely used industrially, rods of metallurgical grade silicon are
heated to melt at one end. Then, the heater is slowly moved down the length of
the rod, keeping a small length of the rod molten as the silicon cools and
re-solidifies behind it. Since most impurities tend to remain in the molten
region rather than re-solidify, when the process is complete, most of the
impurities in the rod will have been moved into the end that was the last to be
melted. This end is then cut off and discarded, and the process repeated if a
still higher purity is desired.
Chemical methods
Today, silicon is purified by converting it to a silicon compound that can be
more easily purified than in its original state, and then converting that
silicon element back into pure silicon. Trichlorosilane is the silicon compound
most commonly used as the intermediate, although silicon tetrachloride and
silane are also used. When these gases are blown over silicon at high
temperature, they decompose to high-purity silicon.
At one time, DuPont produced ultra-pure silicon by reacting silicon
tetrachloride with high-purity zinc vapors at 950 °C, producing silicon
according to the chemical equation
SiCl4 + 2 Zn → Si + 2 ZnCl2.
However, this technique was plagued with practical problems (such as the zinc
chloride byproduct solidifying and clogging lines) and was eventually abandoned
in favor of the Siemens process.
In the Siemens process, high-purity silicon rods are exposed to trichlorosilane
at 1150 °C. The trichlorosilane gas decomposes and deposits additional silicon
onto the rods, enlarging them according to chemical reactions like
2 HSiCl3 → Si + 2 HCl + SiCl4.
Silicon produced from this and similar processes is called polycrystalline
silicon. Polycrystalline silicon typically has impurity levels of less than
10?9.
In 2006 REC announced construction of a plant based on fluidized bed technology
using silane .
3SiCl4 + Si + 2H2 → 4HSiCl3
4HSiCl3 → 3SiCl4 + SiH4
SiH4 → Si + 2H2
Crystallization
The majority of silicon crystals grown for device production are produced by the
Czochralski process, (CZ-Si) since it is the cheapest method available and it is
capable of producing large size crystals. However, silicon single-crystals grown
by the Czochralski method contain impurities since the crucible which contains
the melt dissolves. For certain electronic devices, particularly those required
for high power applications, silicon grown by the Czochralski method is not pure
enough. For these applications, float-zone silicon (FZ-Si) can be used instead.
It is worth mentioning though, in contrast with CZ-Si method in which the seed
is dipped into the silicon melt and the growing crystal is pulled upward, the
thin seed crystal in the FZ-Si method sustains the growing crystal as well as
the polysilicon rod from the bottom. As a result, it is difficult to grow large
size crystals using the float-zone method. Today, all the dislocation-free
silicon crystals used in semiconductor industry with diameter 300mm or larger
are grown by the Czochralski method with purity level significantly improved.
Different forms of silicon
Granular silicon
Polycrystal silicon
Silicon monocrystal
Nanocrystalline silicon
Silicon Ingot
One can notice the color change in silicon nanopowder. This is caused by the
quantum effects which occur in particles of nanometric dimensions. See also
Potential well, Quantum dot, and Nanoparticle.
Isotopes
M isotopes of silicon
Silicon has numerous known isotopes, with mass numbers ranging from 22 to 44.
28Si (the most abundant isotope, at 92.23%), 29Si (4.67%), and 30Si (3.1%) are
stable; 32Si is a radioactive isotope produced by argon decay. Its half-life has
been determined to be approximately 170 years (0.21 MeV), and it decays by beta
- emission to 32P (which has a 14.28 day half-life ) and then to 32S.
Silicon-based life
Since silicon is similar to carbon, particularly in its valency, some people
have proposed the possibility of silicon-based life. One main detraction for
silicon-based life is that unlike carbon, silicon does not have the tendency to
form double and triple bonds.
Although there are no known forms of life that rely entirely on silicon-based
chemistry, there are some that rely on silicon minerals for specific functions.
Some bacteria and other forms of life, such as the protozoa radiolaria, have
silicon dioxide skeletons, and the sea urchin has spines made of silicon
dioxide. These forms of silicon dioxide are known as biogenic silica. Silicate
bacteria use silicates in their metabolism.
Life as we know it could not have developed based on a silicon biochemistry. The
main reason for this fact is that life on Earth depends on the carbon cycle:
autotrophic entities use carbon dioxide to synthesize organic compounds with
carbon, which is then used as food by heterotrophic entities, which produce
energy and carbon dioxide from these compounds. If carbon was to be replaced
with silicon, there would be a need for a silicon cycle. However, silicon
dioxide precipitates in aqueous systems, and cannot be transported among living
beings by common biological means.
As such, another solvent would be necessary to sustain silicon-based life forms;
it would be difficult (if not impossible) to find another common compound with
the unusual properties of water which make it an ideal solvent for carbon-based
life. Larger silicon compounds analogous to common hydrocarbon chains (silanes)
are also generally unstable owing to the larger atomic radius of silicon and the
correspondingly weaker silicon-silicon bond; silanes decompose readily and often
violently in the presence of oxygen making them unsuitable for an oxidizing
atmosphere such as our own. Silicon also does not readily participate in
pi-bonding (the second and third bonds in triple bonds and double bonds are
pi-bonds) as its p-orbital electrons experience greater shielding and are less
able to take on the necessary geometry. Furthermore, although some silicon rings
(cyclosilanes) analogous to common the cycloalkanes formed by carbon have been
synthesized, these are largely unknown. Their synthesis suffers from the
difficulties inherent in producing any silane compound, whereas carbon will
readily form five-, six-, and seven-membered rings by a variety of pathways (the
Diels-Alder reaction is one naturally-occurring example), even in the presence
of oxygen. Silicon's inability to readily form long silane chains, multiple
bonds, and rings severely limits the diversity of compounds that can be
synthesized from it. Under known conditions, silicon chemistry simply cannot
begin to approach the diversity of organic chemistry, a crucial factor in
carbon's role in biology.
However, silicon-based life could be construed as being life which exists under
a computational substrate. This concept is yet to be explored in mainstream
technology but receives ample coverage by sci-fi authors.
A. G. Cairns-Smith has proposed that the first living organisms to exist were
forms of clay minerals—which were probably based around the silicon atom.
Compounds
For examples of silicon compounds see silicate, silane (SiH4), silicic acid
(H4SiO4), silicon carbide (SiC), silicon dioxide (SiO2), silicon tetrachloride
(SiCl4), silicon tetrafluoride (SiF4), and trichlorosilane (HSiCl3)
A semiconductor is a solid that has electrical conductivity in between that of a
conductor and that of an insulator, and can be controlled over a wide range,
either permanently or dynamically. Semiconductors are tremendously important in
technology. Semiconductor devices, electronic components made of semiconductor
materials, are essential in modern electrical devices. Examples range from
computers to cellular phones to digital audio players. Silicon is used to create
most semiconductors commercially, but dozens of other materials are used as
well.
Overview
Semiconductors are very similar to insulators. The two categories of solids
differ primarily in that insulators have larger band gaps — energies that
electrons must acquire to be free to move from atom to atom. In semiconductors
at room temperature, just as in insulators, very few electrons gain enough
thermal energy to leap the band gap from the valence band to the conduction
band, which is necessary for electrons to be available for electric current
conduction. For this reason, pure semiconductors and insulators in the absence
of applied electric fields, have roughly similar resistance. The smaller
bandgaps of semiconductors, however, allow for other means besides temperature
to control their electrical properties.
Semiconductors' intrinsic electrical properties are often permanently modified
by introducing impurities by a process known as doping. Usually, it is
sufficient to approximate that each impurity atom adds one electron or one
"hole" (a concept to be discussed later) that may flow freely. Upon the addition
of a sufficiently large proportion of impurity dopants, semiconductors will
conduct electricity nearly as well as metals. Depending on the kind of impurity,
a doped region of semiconductor can have more electrons or holes, and is named
N-type or P-type semiconductor material, respectively. Junctions between regions
of N- and P-type semiconductors create electric fields, which cause electrons
and holes to be available to move away from them, and this effect is critical to
semiconductor device operation. Also, a density difference in the amount of
impurities produces a small electric field in the region which is used to
accelerate non-equilibrium electrons or holes.
In addition to permanent modification through doping, the resistance of
semiconductors is normally modified dynamically by applying electric fields. The
ability to control resistance/conductivity in regions of semiconductor material
dynamically through the application of electric fields is the feature that makes
semiconductors useful. It has led to the development of a broad range of
semiconductor devices, like transistors and diodes. Semiconductor devices that
have dynamically controllable conductivity, such as transistors, are the
building blocks of integrated circuits devices like the microprocessor. These
"active" semiconductor devices (transistors) are combined with passive
components implemented from semiconductor material such as capacitors and
resistors, to produce complete electronic circuits.
In most semiconductors, when electrons lose enough energy to fall from the
conduction band to the valence band (the energy levels above and below the band
gap), they often emit light, a quantum of energy in the visible electromagnetic
spectrum. This photoemission process underlies the light-emitting diode (LED)
and the semiconductor laser, both of which are very important commercially.
Conversely, semiconductor absorption of light in photodetectors excites
electrons to move from the valence band to the higher energy conduction band,
thus facilitating detection of light and vary with its intensity. This is useful
for fiber optic communications, and providing the basis for energy from solar
cells.
Semiconductors may be elemental materials such as silicon and germanium, or
compound semiconductors such as gallium arsenide and indium phosphide, or alloys
such as silicon germanium or aluminium gallium arsenide.
Band structure
Band structure of a semiconductor showing a full valence band and an empty
conduction band.
There are three popular ways to describe the electronic structure of a crystal.
The first starts from single atoms. An atom has discrete energy levels. When two
atoms come close each energy levels splits (coupled pendulum) into an upper and
a lower level, whereby they delocalize across the two atoms. With more atoms the
number of levels increase more and more and groups of levels forming bands. In
semiconductors multiple bands exist. If the atoms have occupied states, then a
large distance, and then unoccupied states, it is likely that even after band
formation a gap between occupied and unoccupied bands exists. A second way
starts with free electrons waves. When fading in an electrostatic potential due
to the cores, due to Bragg reflection some waves are reflected and cannot
penetrate the bulk, that is a band gap opens. In this description it is not
clear, while the number of electrons fills up exactly all states below the gap.
A third description starts with two atoms. The split states form a covalent bond
where two electrons with spin up an spin down are mostly in between the two
atoms. Adding more atoms now is supposed not to lead to splitting, but to more
bonds. This is the way silicon is typically drawn. The band gap is now formed by
lifting one electron from the lower electron level into the upper level. This
level is known to be anti-bonding, but bulk silicon has not been seen to lose
atoms as easy as electrons are wandering through it. Also this model is most
unsuitable to explain how in graded hetero-junction the band gap can vary
smoothly.
Like in other solids, the electrons in semiconductors can have energies only
within certain bands (ie. ranges of levels of energy) between the energy of the
ground state, corresponding to electrons tightly bound to the atomic nuclei of
the material, and the free electron energy, which is the energy required for an
electron to escape entirely from the material. The energy bands each correspond
to a large number of discrete quantum states of the electrons, and most of the
states with low energy (closer to the nucleus) are full, up to a particular band
called the valence band. Semiconductors and insulators are distinguished from
metals because the valence band in the semiconductor materials is very nearly
full under usual operating conditions, thus causing more electrons to be
available in the conduction band.
The ease with which electrons in a semiconductor can be excited from the valence
band to the conduction band depends on the band gap between the bands, and it is
the size of this energy bandgap that serves as an arbitrary dividing line
(roughly 4 eV) between semiconductors and insulators.
The electrons must move between states to conduct electric current, and so due
to the Pauli exclusion principle full bands do not contribute to the electrical
conductivity. However, as the temperature of a semiconductor rises above
absolute zero, the range of energy values of the electrons in a given band are
increased, and some electrons are likely to be found in with energy states of
the conduction band, which is the band immediately above the valence band. The
current-carrying electrons in the conduction band are known as "free electrons",
although they are often simply called "electrons" if context allows this usage
to be clear.
Electrons excited to the conduction band also leave behind electron holes, or
unoccupied states in the valence band. Both the conduction band electrons and
the valence band holes contribute to electrical conductivity. The holes
themselves don't actually move, but a neighbouring electron can move to fill the
hole, leaving a hole at the place it has just come from, and in this way the
holes appear to move, and the holes behave as if they were actual positively
charged particles.
One covalent bond between neighboring atoms in the solid is ten times stronger
than the binding of the single electron to the atom, so freeing the electron
does not imply to destroy the crystal structure.
The notion of holes, which was introduced for semiconductors, can also be
applied to metals, where the Fermi level lies within the conduction band. With
most metals the Hall effect reveals electrons to be the charge carriers, but
some metals have a mostly filled conduction band, and the Hall effect reveals
positive charge carriers, which are not the ion-cores, but holes. Contrast this
to some conductors like solutions of salts, or plasma. In the case of a metal,
only a small amount of energy is needed for the electrons to find other
unoccupied states to move into, and hence for current to flow. Sometimes even in
this case it may be said that a hole was left behind, to explain why the
electron does not fall back to lower energies: It cannot find a hole. In the end
in both materials electron-phonon scattering and defects are the dominant causes
for resistance.
Fermi-Dirac distribution. States with energy math:9/32E6E4BB17815B1D16232D45.gif
below the Fermi energy, here math:4/1DC018B23B88B5F8.gif, have higher
probability math:2/E02854F.gif to be occupied, and those above are less likely
to be occupied. Smearing of the distribution increases with temperature.
The energy distribution of the electrons determines which of the states are
filled and which are empty. This distribution is described by Fermi-Dirac
statistics. The distribution is characterized by the temperature of the
electrons, and the Fermi energy or Fermi level. Under absolute zero conditions
the Fermi energy can be thought of as the energy up to which available electron
states are occupied. At higher temperatures, the Fermi energy is the energy at
which the probability of a state being occupied has fallen to 0.5.
The dependence of the electron energy distribution on temperature also explains
why the conductivity of a semiconductor has a strong temperature dependency, as
a semiconductor operating at lower temperatures will have fewer available free
electrons and holes able to do the work.
Energy–momentum dispersion
In the preceding description an important fact is ignored for the sake of
simplicity: the dispersion of the energy. The reason that the energies of the
states are broadened into a band is that the energy depends on the value of the
wave vector, or k-vector, of the electron. The k-vector, in quantum mechanics,
is the representation of the momentum of a particle.
The dispersion relationship determines the effective mass, , of electrons or
holes in the semiconductor, according to the formula:
The effective mass is important as it affects many of the electrical properties
of the semiconductor, such as the electron or hole mobility, which in turn
influences the diffusivity of the charge carriers and the electrical
conductivity of the semiconductor.
Typically the effective mass of electrons and holes are different. This affects
the relative performance of p-channel and n-channel IGFETs, for example (Muller
& Kamins 1986:427).
The top of the valence band and the bottom of the conduction band might not
occur at that same value of k. Materials with this situation, such as silicon
and germanium, are known as indirect bandgap materials. Materials in which the
band extrema are aligned in k, for example gallium arsenide, are called direct
bandgap semiconductors. Direct gap semiconductors are particularly important in
optoelectronics because they are much more efficient as light emitters than
indirect gap materials.
Carrier generation and recombination
For more details on this topic, see Carrier generation and recombination.
When ionizing radiation strikes a semiconductor, it may excite an electron out
of its energy level and consequently leave a hole. This process is known as
electron–hole pair generation. Electron-hole pairs are constantly generated from
thermal energy as well, in the absence of any external energy source.
Electron-hole pairs are also apt to recombine. Conservation of energy demands
that these recombination events, in which an electron loses an amount of energy
larger than the band gap, be accompanied by the emission of thermal energy (in
the form of phonons) or radiation (in the form of photons).
In the steady state, the generation and recombination of electron–hole pairs are
in equipoise. The number of electron-hole pairs in the steady state at a given
temperature is determined by quantum statistical mechanics. The precise quantum
mechanical mechanisms of generation and recombination are governed by
conservation of energy and conservation of momentum.
As probability that electrons and holes meet together is proportional to the
product of their amounts, the product is in steady state nearly constant at a
given temperature, providing that there is no significant electric field (which
might "flush" carriers of both types, or move them from neighbour regions
containing more of them to meet together) or externally driven pair generation.
The product is a function of the temperature, as the probability of getting
enough thermal energy to produce a pair increases with temperature, being
approximately 1/exp(band gap / kT), where k is Boltzmann's constant and T is
absolute temperature.
The probability of meeting is increased by carrier traps – impurities or
dislocations which can trap an electron or hole and hold it until a pair is
completed. Such carrier traps are sometimes purposely added to reduce the time
needed to reach the steady state.
Doping
For more details on this topic, see Doping (semiconductor).
The property of semiconductors that makes them most useful for constructing
electronic devices is that their conductivity may easily be modified by
introducing impurities into their crystal lattice. The process of adding
controlled impurities to a semiconductor is known as doping. The amount of
impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level
of conductivity. Doped semiconductors are often referred to as extrinsic.
Dopants
The materials chosen as suitable dopants depend on the atomic properties of both
the dopant and the material to be doped. In general, dopants that produce the
desired controlled changes are classified as either electron acceptors or
donors. A donor atom that activates (that is, becomes incorporated into the
crystal lattice) donates weakly-bound valence electrons to the material,
creating excess negative charge carriers. These weakly-bound electrons can move
about in the crystal lattice relatively freely and can facilitate conduction in
the presence of an electric field. (The donor atoms introduce some states under,
but very close to the conduction band edge. Electrons at these states can be
easily excited to conduction band, becoming free electrons, at room
temperature.) Conversely, an activated acceptor produces a hole. Semiconductors
doped with donor impurities are called n-type, while those doped with acceptor
impurities are known as p-type. The n and p type designations indicate which
charge carrier acts as the material's majority carrier. The opposite carrier is
called the minority carrier, which exists due to thermal excitation at a much
lower concentration compared to the majority carrier.
For example, the pure semiconductor silicon has four valence electrons. In
silicon, the most common dopants are IUPAC group 13 (commonly known as group
III) and group 15 (commonly known as group V) elements. Group 13 elements all
contain three valence electrons, causing them to function as acceptors when used
to dope silicon. Group 15 elements have five valence electrons, which allows
them to act as a donor. Therefore, a silicon crystal doped with boron creates a
p-type semiconductor whereas one doped with phosphorus results in an n-type
material.
Carrier concentration
The concentration of dopant introduced to an intrinsic semiconductor determines
its concentration and indirectly affects many of its electrical properties. The
most important factor that doping directly affects is the material's carrier
concentration. In an intrinsic semiconductor under thermal equilibrium, the
concentration of electrons and holes is equivalent. That is,
Where is the concentration of conducting electrons, is the electron hole
concentration, and is the material's intrinsic carrier concentration. Intrinsic
carrier concentration varies between materials and is dependent on temperature.
Silicon's , for example, is roughly 1.18×1010 cm-3 at 300 kelvins (room
temperature).
In general, an increase in doping concentration affords an increase in
conductivity due to the higher concentration of carriers available for
conduction. Degenerately (very highly) doped semiconductors have conductivity
levels comparable to metals and are often used in modern integrated circuits as
a replacement for metal. Often superscript plus and minus symbols are used to
denote relative doping concentration in semiconductors. For example, denotes an
n-type semiconductor with a high, often degenerate, doping concentration.
Similarly, would indicate a very lightly doped p-type material. It is useful to
note that even degenerate levels of doping imply low concentrations of
impurities with respect to the base semiconductor. In crystalline intrinsic
silicon, there are approximately 5×1022 atoms/cm3. Doping concentration for
silicon semiconductors may range anywhere from 1013 cm-3 to 1018 cm-3. Doping
concentration above about 1018 cm-3 is considered degenerate at room
temperature. Degenerately doped silicon contains a proportion of impurity to
silicon in the order of parts per thousand. This proportion may be reduced to
parts per billion in very lightly doped silicon. Typical concentration values
fall somewhere in this range and are tailored to produce the desired properties
in the device that the semiconductor is intended for.
Effect on band structure
Band diagram of a p+n junction. The band bending is a result of the positioning
of the Fermi levels in the p+ and n sides.
Doping a semiconductor crystal introduces allowed energy states within the band
gap but very close to the energy band that corresponds with the dopant type. In
other words, donor impurities create states near the conduction band while
acceptors create states near the valence band. The gap between these energy
states and the nearest energy band is usually referred to as dopant-site bonding
energy or and is relatively small. For example, the for boron in silicon bulk is
0.045 eV, compared with silicon's band gap of about 1.12 eV. Because is so
small, it takes little energy to ionize the dopant atoms and create free
carriers in the conduction or valence bands. Usually the thermal energy
available at room temperature is sufficient to ionize most of the dopant.
Dopants also have the important effect of shifting the material's Fermi level
towards the energy band that corresponds with the dopant with the greatest
concentration. Since the Fermi level must remain constant in a system in
thermodynamic equilibrium, stacking layers of materials with different
properties leads to many useful electrical properties. For example, the p-n
junction's properties are due to the energy band bending that happens as a
result of lining up the Fermi levels in contacting regions of p-type and n-type
material.
This effect is shown in a band diagram. The band diagram typically indicates the
variation in the valence band and conduction band edges versus some spatial
dimension, often denoted x. The Fermi energy is also usually indicated in the
diagram. Sometimes the intrinsic Fermi energy, Ei, which is the Fermi level in
the absence of doping, is shown. These diagrams are useful in explaining the
operation of many kinds of semiconductor devices.
Preparation of semiconductor materials
Semiconductors with predictable, reliable electronic properties are necessary
for mass production. The level of chemical purity needed is extremely high
because the presence of impurities even in very small proportions can have large
effects on the properties of the material. A high degree of crystalline
perfection is also required, since faults in crystal structure (such as
dislocations, twins, and stacking faults) interfere with the semiconducting
properties of the material. Crystalline faults are a major cause of defective
semiconductor devices. The larger the crystal, the more difficult it is to
achieve the necessary perfection. Current mass production processes use crystal
ingots between four and twelve inches (300 mm) in diameter which are grown as
cylinders and sliced into wafers.
Because of the required level of chemical purity and the perfection of the
crystal structure which are needed to make semiconductor devices, special
methods have been developed to produce the initial semiconductor material. A
technique for achieving high purity includes growing the crystal using the
Czochralski process. An additional step that can be used to further increase
purity is known as zone refining. In zone refining, part of a solid crystal is
melted. The impurities tend to concentrate in the melted region, while the
desired material recrystalizes leaving the solid material more pure and with
fewer crystalline faults.
In manufacturing semiconductor devices involving heterojunctions between
different semiconductor materials, the lattice constant, which is the length of
the repeating element of the crystal structure, is important for determining the
compatibility of materials.
Semiconductor device fabrication is the process used to create chips, the
integrated circuits that are present in everyday electrical and electronic
devices. It is a multiple-step sequence of photographic and chemical processing
steps during which electronic circuits are gradually created on a wafer made of
pure semiconducting material. Silicon is the most commonly used semiconductor
material today, along with various compound semiconductors.
The entire manufacturing process from start to packaged chips ready for shipment
takes six to eight weeks and is performed in highly specialized facilities
referred to as fabs.
Wafers
A typical wafer is made out of extremely pure silicon that is grown into
mono-crystalline cylindrical ingots (boules) up to 300 mm (slightly less than 12
inches) in diameter using the Czochralski process. These ingots are then sliced
into wafers about 0.75 mm thick and polished to obtain a very regular and flat
surface.
Once the wafers are prepared, many process steps are necessary to produce the
desired semiconductor integrated circuit. In general, the steps can be grouped
into four areas:
* Front end processing
* Back end processing
* Test
* Packaging
Processing
In semiconductor device fabrication, the various processing steps fall into four
general categories: deposition, removal, patterning, and modification of
electrical properties.
* Deposition is any process that grows, coats, or otherwise transfers a material
onto the wafer. Available technologies consist of physical vapor deposition (PVD),
chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular
beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among
others.
* Removal processes are any that remove material from the wafer either in bulk
or selective form and consist primarily of etch processes, both wet etching and
dry etching such as reactive ion etch (RIE). Chemical-mechanical planarization (CMP)
is also a removal process used between levels.
* Patterning covers the series of processes that shape or alter the existing
shape of the deposited materials and is generally referred to as lithography.
For example, in conventional lithography, the wafer is coated with a chemical
called a “photoresist”. The photoresist is exposed by a “stepper”, a machine
that focuses, aligns, and moves the mask, exposing select portions of the wafer
to short wavelength light. The unexposed regions are washed away by a developer
solution. After etching or other processing, the remaining photoresist is
removed by plasma ashing.
* Modification of electrical properties has historically consisted of doping
transistor sources and drains originally by diffusion furnaces and later by ion
implantation. These doping processes are followed by furnace anneal or in
advanced devices, by rapid thermal anneal (RTA) which serve to activate the
implanted dopants. Modification of electrical properties now also extends to
reduction of dielectric constant in low-k insulating materials via exposure to
ultraviolet light in UV processing (UVP).
Many modern chips have eight or more levels produced in over 300 sequenced
processing steps.
Front End Processing
"Front End Processing" refers to the formation of the transistors directly on
the silicon. The raw wafer is engineered by the growth of an ultrapure,
virtually defect-free silicon layer through epitaxy. In the most advanced logic
devices, prior to the silicon epitaxy step, tricks are performed to improve the
performance of the transistors to be built. One method involves introducing a
"straining step" wherein a silicon variant such as "silicon-germanium" (SiGe) is
deposited. Once the epitaxial silicon is deposited, the crystal lattice becomes
stretched somewhat, resulting in improved electronic mobility. Another method,
called "silicon on insulator" technology involves the insertion of an insulating
layer between the raw silicon wafer and the thin layer of subsequent silicon
epitaxy. This method results in the creation of transistors with reduced
parasitic effects.
Silicon dioxide
Front end surface engineering is followed by: growth of the gate dielectric,
traditionally silicon dioxide (SiO2), patterning of the gate, patterning of the
source and drain regions, and subsequent implantation or diffusion of dopants to
obtain the desired complementary electrical properties. In memory devices,
storage cells, conventionally capacitors, are also fabricated at this time,
either into the silicon surface or stacked above the transistor.
Metal layers
Once the various semiconductor devices have been created they must be
interconnected to form the desired electrical circuits. This "Back End Of Line"
(BEOL – the latter portion of the front end of wafer fabrication, not to be
confused with "back end" of chip fabrication which refers to the package and
test stages) involves creating metal interconnecting wires that are isolated by
insulating dielectrics. The insulating material was traditionally a form of SiO2
or a silicate glass, but recently new low dielectric constant materials are
being used. These dielectrics presently take the form of SiOC and have
dielectric constants around 2.7 (compared to 3.9 for SiO2), although materials
with constants as low as 2.2 are being offered to chipmakers.
Interconnect
Historically, the metal wires consisted of aluminum. In this approach to wiring
often called "subtractive aluminum", blanket films of aluminum are deposited
first , patterned, and then etched, leaving isolated wires. Dielectric material
is then deposited over the exposed wires. The various metal layers are
interconnected by etching holes, called "vias," in the insulating material and
depositing tungsten in them with a CVD technique. This approach is still used in
the fabrication of many memory chips such as dynamic random access memory (DRAM)
as the number of interconnect levels is small, currently no more than four.
More recently, as the number of interconnect levels for logic has substantially
increased due to the large number of transistors that are now interconnected in
a modern microprocessor, the timing delay in the wiring has become significant
prompting a change in wiring material from aluminum to copper and from the
aforementioned silicon dioxides to newer low-K material. This performance
enhancement also comes at a reduced cost via damascene processing that
eliminates processing steps. In damascene processing, in contrast to subtractive
aluminum technology, the dielectric material is deposited first as a blanket
film and is patterned and etched leaving holes or trenches. In "single
damascene" processing, copper is then deposited in the holes or trenches
surrounded by a thin barrier film resulting in filled vias or wire "lines"
respectively. In "dual damascene" technology, both the trench and via are
fabricated before the deposition of copper resulting in formation of both the
via and line simultaneously, further reducing the number of processing steps.
The thin barrier film, called Copper Barrier Seed (CBS), is necessary to prevent
copper diffusion into the dielectric. The ideal barrier film is effective, but
is barely there. As the presence of excessive barrier film competes with the
available copper wire cross section, formation of the thinnest yet continuous
barrier represents one of the greatest ongoing challenges in copper processing
today.
As the number of interconnect levels increases, planarization of the previous
layers is required to ensure a flat surface prior to subsequent lithography.
Without it, the levels would become increasingly crooked and extend outside the
depth of focus of available lithography, interfering with the ability to
pattern. CMP (Chemical Mechanical Polishing) is the primary processing method to
achieve such planarization although dry "etch back" is still sometimes employed
if the number of interconnect levels is no more than three.
Wafer test
The highly serialized nature of wafer processing has increased the demand for
metrology in between the various processing steps. Wafer test metrology
equipment is used to verify that the wafers are still good and haven't been
damaged by previous processing steps. If the number of dies—the integrated
circuits that will eventually become chips—on a wafer that measure as fails
exceed a predetermined threshold, the wafer is scrapped rather than investing in
further processing.
Device test
Once the Front End Process has been completed, the semiconductor devices are
subjected to a variety of electrical tests to determine if they function
properly. The proportion of devices on the wafer found to perform properly is
referred to as the yield.
The fab tests the chips on the wafer with an electronic tester that presses tiny
probes against the chip. The machine marks each bad chip with a drop of dye. The
fab charges for test time; the prices are on the order of cents per second.
Chips are often designed with “testability features” to speed testing, and
reduce test costs.
Good designs try to test and statistically manage corners: extremes of silicon
behavior caused by operating temperature combined with the extremes of fab
processing steps. Most designs cope with more than 64 corners.
Packaging
Once tested, the wafer is scored and then broken into individual dice. Only the
good, undyed chips go on to be packaged.
Plastic or ceramic packaging involves mounting the die, connecting the die pads
to the pins on the package, and sealing the die. Tiny wires are used to connect
pads to the pins. In the old days, wires were attached by hand, but now
purpose-built machines perform the task. Traditionally, the wires to the chips
were gold, leading to a “lead frame” (pronounced “leed frame”) of copper, that
had been plated with solder, a mixture of tin and lead. Lead is poisonous, so
lead-free “lead frames” are now the best practice.
Chip-scale package (CSP) is another packaging technology. Plastic packaged chips
are usually considerably larger than the actual die, whereas CSP chips are
nearly the size of the die. CSP can be constructed for each die before the wafer
is diced .
The packaged chips are retested to ensure that they were not damaged during
packaging and that the die-to-pin interconnect operation was performed
correctly. A laser etches the chip’s name and numbers on the package.
List of steps
This is a list of processing techniques that are employed numerous times in a
modern electronic device and do not necessarily imply a specific order.
* Wafer processing
* Wet cleans
* Photolithography
* Ion implantation (in which dopants are embedded in the wafer creating regions
of increased (or decreased) conductivity)
* Dry etching
* Wet etching
* Plasma ashing
* Thermal treatments
* Rapid thermal anneal
* Furnace anneals
* Thermal oxidation
* Chemical vapor deposition (CVD)
* Physical vapor deposition (PVD)
* Molecular beam epitaxy (MBE)
* Electrochemical Deposition (ECD). See Electroplating
* Chemical-mechanical planarization (CMP)
* Wafer testing (where the electrical performance is verified)
* Wafer backgrinding (to reduce the thickness of the wafer so the resulting chip
can be put into a thin device like a smartcard or PCMCIA card.)
* Die preparation
* Wafer mounting
* Die cutting
* IC packaging
* Die attachment
* IC Bonding
* Wire bonding
* Flip chip
* Tab bonding
* IC encapsulation
* Baking
* Plating
* Lasermarking
* Trim and form
* IC testing
Hazardous materials note
Many toxic materials are used in the fabrication process. These include:
* poisonous elemental dopants such as arsenic, boron, antimony and phosphorus
* poisonous compounds like arsine, phosphine and silane
* highly reactive liquids, such as hydrogen peroxide, fuming nitric acid,
sulfuric acid and hydrofluoric acid
It is vital that workers not be directly exposed to these dangerous substances.
The high degree of automation common in the IC fabrication industry helps to
reduce the risks of exposure of this sort.
History
When feature widths were far greater than about 10 micrometres, purity was not
the issue that it is today in device manufacturing. But as the devices became
more integrated the cleanrooms became even cleaner. Today, the fabs are
pressurized with filtered air to remove even the smallest particles, which could
come to rest on the wafers and contribute to defects. The workers in a
semiconductor fabrication facility are required to wear cleanroom suits to
protect the devices from human contamination.
In an effort to increase profits, semiconductor device manufacture spread from
Texas and California in the 1960s to the rest of the world, such as Ireland,
Israel, Japan, Taiwan, Korea, Singapore and China, and is a global business
today.
The leading semiconductor manufacturers typically have facilities all over the
world. Intel, the world's largest manufacturer, has facilities in Europe and
Asia as well as the U.S. Other top manufacturers include Freescale Semiconductor
(US), Samsung (Korea), Texas Instruments (US), Advanced Micro Devices (AMD) (US)
see , Toshiba (Japan), NEC Electronics (Japan), STMicroelectronics (Europe),
Infineon (Europe), Renesas (Japan), Taiwan Semiconductor Manufacturing Company
(Taiwan, see TSMC web site), Sony(Japan), and NXP Semiconductors (Europe).
In 2006, there are approximately 5,000 semi-conductor and electronic components
manufacturers in the United States, accounting for $165 billion, according to
the 2006 U.S. Industry & Market Outlook by Barnes Reports.
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