Silicon Silicon is a chemical element with symbol Si and atomic number 14. Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is most widely distributed in dusts , sands , planetoids , and planets as various forms of silicon dioxide (silica) or silicates . Over 90% of the Earth's crust is composed of silicate minerals , making silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen . [7] Silicon is a solid at room temperature, with relatively high melting and boiling points of 1414 and 3265 °C, respectively.
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SiliconSilicon is a chemical element with symbol Si and atomic number 14.
Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. Over 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen.[7]
Silicon is a solid at room temperature, with relatively high melting and boiling points of 1414 and 3265 °C, respectively.
Metallurgical grade 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 1,900 °C (3,450 °F), the carbon and the silicon undergo the chemical reaction SiO2 + 2 C → Si + 2 CO. Liquid silicon collects in the bottom of the furnace, which is then drained and cooled. The silicon produced in this manner is called metallurgical grade silicon and is at least 98% pure.
As of September 2008, metallurgical grade silicon costs about US$1.45 per pound ($3.20/kg),[29] up from $0.77 per pound ($1.70/kg) in 2005.
Electronic grade
The use of silicon in semiconductor devices demands a much greater purity than afforded by metallurgical grade silicon. Very pure silicon (>99.9%) can be extracted directly from solid silica or other silicon compounds by molten salt electrolysis. This method, known as early as 1854 (see also FFC Cambridge process, mentioned later), has the potential to directly produce solar-grade silicon without any carbon dioxide emission at much lower energy consumption.
Solar grade silicon cannot be used for microelectronics. To properly control the quantum mechanical properties, the purity of the silicon must be very high. Bulk silicon wafers used at the beginning of the integrated circuit making process must first be refined to a purity of 99.9999999% often referred to as "9N" for "9 nines", a process which requires repeated applications of refining technology.
The majority of silicon crystals grown for device production are produced by the Czochralski process, (CZ-Si) it was the cheapest method available. However, single crystals grown by the Czochralski process contain impurities because the crucible containing the melt often dissolves. Historically, a number of methods have been used to produce ultra-high-purity silicon.
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.
A polycrystalline silicon rod made by the Siemens process
Today's purification processes involve the conversion of silicon into volatile liquids, such as trichlorosilane (HSiCl3) and silicon tetrachloride (SiCl4) or into the gaseous silane (SiH4). These compounds are then separated by a distillation and transformed into high-purity silicon, either by a redox reaction or by chemical decomposition at high temperatures.
In the late 1950s, the American chemical company DuPont patented a method for the production of 99.99% pure silicon, using the metal zinc as a reductant to transform redistilled silicon tetrachloride into high-purity silicon by a vapor phase reaction at 900 °C. This technique, however, was plagued with practical problems, as the byproduct zinc chloride (ZnCl2) solidified and clogged lines, and was eventually abandoned in favor of more sophisticated processes.[35]
Schematic diagram of the traditional Siemens and the Fluidized bed reactor purification process.
The best known technique is the so-called Siemens process. This technique does not require a reductant such as zinc, as it grows high-purity silicon crystallites directly on the surface of (pre-existing) pure silicon seed rods by a chemical decomposition that takes place when the gaseous trichlorosilane is blown over the rod's surface at 1150 °C. This process produces polycrystalline silicon, also known as polysilicon, at typically 9N–11N purities, that is, it contains impurity levels of less than one part per billion (ppb).[36][37][38]
A more recent alternative for the production of polysilicon is the fluidized bed reactor (FBR) manufacturing technology. Compared to the traditional Siemens process, FBR features a number of advantages that lead to cheaper polysilicon demanded by the fast-growing photovoltaic industry.
Contrary to Siemens' batch process, FBR runs continuously, wasting fewer resources and requires less setup and downtime. It uses about 10 percent of the electricity consumed by a conventional rod reactor in the established Siemens process, as it does not waste energy by placing heated gas and silicon in contact with cold surfaces. In the FBR, silane (SiH4) is injected into the reactor from below and forms a fluidized bed together with the silicon seed particles that are fed from above. The gaseous silane then decomposes and deposits silicon on the seed particles. When the particle has grown to larger granules, they eventually sink to the bottom of the reactor where they are continuously withdrawn from the reaction process.
While the Siemens technology can produce polysilicon at purity levels at or above 9N to 11N, FBR outputs polysilicon at 6N to 9N, purity suited for the photovoltaic industry and still higher than the 5N to 6N of upgraded metallurgical silicon (UMG-Si). Currently most silicon for the photovoltaic market is produced by the Siemens process and only about 10 percent by the FBR technology, while UMG-Si accounts for about 2
percent. By 2020, however, IHS Technology predicts that market shares for FBR technology and UMG-Si will grow to 16.7 and 5.4 percent, respectively.
The company REC is one of the leading producers of silane and polysilicon using FBR technology. The three-step chemical reaction involves (last step occurs inside the FB-reactor): (1.) 3 SiCl4 + Si + 2 H2 → 4 HSiCl3, followed by (2.) 4 HSiCl3 → 3 SiCl4 + SiH4, and (3.) SiH4 → Si + 2 H2.[40] Other precursors such as tribromosilane had been used by other companies as well.
Most elemental silicon produced remains as ferrosilicon alloy, and only a relatively small amount (20%) of the elemental silicon produced is refined to metallurgical grade purity (a total of 1.3–1.5 million metric tons/year). The fraction of silicon metal which is further refined to semiconductor purity is estimated at only 15% of the world production of metallurgical grade silicon.[28] However, the economic importance of this small very high-purity fraction (especially the ~ 5% which is processed to monocrystalline silicon for use in integrated circuits) is disproportionately large.
Pure monocrystalline silicon is used to produce silicon wafers used in the semiconductor industry, in electronics and in some high-cost and high-efficiency photovoltaic applications. In terms of charge conduction, pure silicon is an intrinsic semiconductor which means that unlike metals it conducts electron
holes and electrons that may be released from atoms within the crystal by heat, and thus increase silicon's electrical conductivity with higher temperatures. Pure silicon has too low a conductivity (i.e., too high a resistivity) to be used as a circuit element in electronics. In practice, pure silicon is doped with small concentrations of certain other elements, a process that greatly increases its conductivity and adjusts its electrical response by controlling the number and charge (positive or negative) of activated carriers. Such control is necessary for transistors, solar cells, semiconductor detectors and other semiconductor devices, which are used in the computer industry and other technical applications. For example, in silicon photonics, silicon can be used as a continuous wave Raman laser medium to produce coherent light, though it is ineffective as an everyday light source.
In common integrated circuits, a wafer of monocrystalline silicon serves as a mechanical support for the circuits, which are created by doping, and insulated from each other by thin layers of silicon oxide, an insulator that is easily produced by exposing the element to oxygen under the proper conditions. Silicon has become the most popular material to build both high power semiconductors and integrated circuits. The reason is that silicon is the semiconductor that can withstand the highest temperatures and electrical powers without becoming dysfunctional due to avalanche breakdown (a process in which an electron avalanche is created by a chain reaction process whereby heat produces free electrons and holes, which in turn produce more current which produces more heat). In addition, the insulating oxide of silicon is not soluble in water, which gives it an advantage over germanium (an element with similar properties which can also be used in semiconductor devices) in certain type of fabrication techniques.[59]
Monocrystalline silicon is expensive to produce, and is usually only justified in production of integrated circuits, where tiny crystal imperfections can interfere with tiny circuit paths. For other uses, other types of
pure silicon which do not exist as single crystals may be employed. These include hydrogenated amorphous silicon and upgraded metallurgical-grade silicon (UMG-Si) which are used in the production of low-cost, large-area electronics in applications such as liquid crystal displays, and of large-area, low-cost, thin-film solar cells. Such semiconductor grades of silicon which are either slightly less pure than those used in integrated circuits, or which are produced in polycrystalline rather than monocrystalline form, make up roughly similar amount of silicon as are produced for the monocrystalline silicon semiconductor industry, or 75,000 to 150,000 metric tons per year. However, production of such materials is growing more quickly than silicon for the integrated circuit market. By 2013 polycrystalline silicon production, used mostly in solar cells, is projected to reach 200,000 metric tons per year, while monocrystalline semiconductor silicon production (used in computer microchips) remains below 50,000 tons/year.
FFC Cambridge processThe FFC Cambridge Process is an electrochemical method in which solid metal compounds, particularly oxides, are catholically reduced to the respective metals or alloys in molten salts. It is thought that this process will eventually be capable of producing metals or alloys more efficiently than by current conventional processes, such as titanium by the Kroll process.
The FFC Cambridge process was developed by George Z. Chen, Derek J. Fray and Tom W. Farthing between 1996 and 1997 in the University of Cambridge. (The name FFC derives from the first letters of their last names.) They reduced oxide scales on titanium foils, as well as small pellets of titanium dioxide powder, to the metal by molten salt electrochemistry.[1] The technology has since been commercialized in the form of an intellectual property company known as Metalizes, which is based in Sheffield, UK.[2] A similar process was patented in 1904 as German patent 150557.
Process
The process typically takes place between 900–1100 °C, with an anode (typically carbon) and a cathode (oxide being reduced) in a bath of molten CaCl2. Depending on the nature of the oxide it will exist at a particular potential relative to the anode, which is dependent on the quantity of CaO present in CaCl2. The cathode is then polarized to more negative voltages versus the anode. This is simply achieved by imposing a voltage between the anode and cathode. When polarized to more negative voltages the oxide releases oxygen ions into the CaCl2 salt, which exists as CaO. To maintain charge neutrality, as oxygen ions are released from the cathode into the salt, so oxygen ions must be released from the salt to the anode. This is observed as CO or CO2 being evolved at the carbon anode. In theory an inert anode could be used to produce oxygen.
When negative voltages are reached, it is possible that the cathode would begin to produce Ca (which is soluble in CaCl2). Ca is highly reductive and would further strip oxygen from the cathode, resulting in calciothermic reduction. However, Ca dissolved into CaCl2 results in a more conductive salt leading to reduced current efficiencies.
Cathode reaction mechanism
The electro-calciothermic reduction mechanism may be represented by the following sequence of reactions.
When this reaction takes place on its own, it is referred to as the "calciothermic reduction" (or, more generally, an example of metallothermic reduction). For example, if the cathode was primarily made from TiO then calciothermic reduction would appear as:
TiO + Ca → Ti + CaO
Whilst the cathode reaction can be written as above it is in fact a gradual removal of oxygen from the oxide. For example, it has been shown that TiO2 does not simply reduce to Ti. It, in fact, reduces through the lower oxides (Ti3O5, Ti2O3, TiO etc.) to Ti.
The calcium oxide produced is then electrolyzed:
(2a) x CaO → x Ca2+ + x O2–
(2b) x Ca2+ + 2x e– → x Ca
And
(2c) x O2– → x/2 O2 + 2x e–
Reaction (2b) describes the production of Ca metal from Ca2+ ions within the salt, at the cathode. The Ca would then proceed to reduce the cathode.
The net result of reactions (1) and (2) is simply the reduction of the oxide into metal plus oxygen:
The use of molten CaCl2 is important because this molten salt can dissolve and transport the O2– ions to the anode to be discharged. The anode reaction depends on the material of the anode. Depending on the system it is possible to produce either CO or CO2 or a mixture at the carbon anode.
C + 2O2– → CO2 +4 e−
C + O2– → CO +2 e−
However, if an inert anode is used, such as that of high density SnO2, the discharge of the O2– ions leads to the evolution of oxygen gas. However the use of an inert anode has disadvantages. Firstly, when the concentration of CaO is low, Cl2 evolution at the anode becomes more favorable. In addition, when compared to a carbon anode, more energy is required to achieve the same reduced phase at the cathode. Inert anodes suffer from stability issues.[citation needed]
Inert anode: 2O2– → O2 + 4 e−
Czochralski processFrom Wikipedia, the free encyclopediaJump to: navigation, search
The 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, gold), salts and synthetic gemstones. The process is named after Polish scientist Jan Czochralski,[1] who invented the method in 1916 while investigating the crystallization rates of metals.[2]
The most important application may be the growth of large cylindrical ingots, or boules, of single crystal silicon. Other semiconductors, such as gallium arsenide, can also be grown by this method, although lower defect densities in this case can be obtained using variants of the Bridgman-Stockbarger technique.
A puller rod with seed crystal for growing single-crystal silicon by the Czochralski process.
High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted in a crucible at 1425 degree Celsius, usually made of quartz. Dopant impurity atoms such as boron or phosphorus can be added to the molten silicon in precise amounts to dope the silicon, thus changing it into p-type or n-type silicon, with different electronic properties. A precisely oriented rod-mounted seed crystal is dipped into the molten silicon. The seed crystal's rod is slowly pulled upwards and rotated simultaneously. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. Occurrence of unwanted instabilities in the melt can be avoided by investigating and visualizing the temperature and velocity fields during the crystal growth process.[3] This process is normally performed in an inert atmosphere, such as argon, in an inert chamber, such as quartz.
Silicon crystal grown by Czochralski process at Raytheon, 1956. The induction heating coil is visible, and the end of the crystal just emerging from the melt. The technician is measuring the temperature with an optical pyrometer. One of the earliest Si plants, the crystals produced by this early apparatus were only one inch in diameter.
Due to the efficiencies of common wafer specifications, the semiconductor industry has used wafers with standardized dimensions. In the early days, the boules were smaller, only a few inches wide. With advanced technology, high-end device manufacturers use 200 mm and 300 mm
diameter wafers. The width is controlled by precise control of the temperature, the speeds of rotation and the speed the seed holder is withdrawn. The crystal ingots from which these wafers are sliced can be up to 2 meters in length, weighing several hundred kilograms. Larger wafers allow improvements in manufacturing efficiency, as more chips can be fabricated on each wafer, so there has been a steady drive to increase silicon wafer sizes. The next step up, 450 mm, is currently scheduled for introduction in 2018.[4] Silicon wafers are typically about 0.2–0.75 mm thick, and can be polished to great flatness for making integrated circuits or textured for making solar cells.
The process begins when the chamber is heated to approximately 1500 degrees Celsius, melting the silicon. When the silicon is fully melted, a small seed crystal mounted on the end of a rotating shaft is slowly lowered until it just dips below the surface of the molten silicon. The shaft rotates counterclockwise and the crucible rotates clockwise. The rotating rod is then drawn upwards very slowly, allowing a roughly cylindrical boule to be formed. The boule can be from one to two meters, depending on the amount of silicon in the crucible.
The electrical characteristics of the silicon are controlled by adding material like phosphorus or boron to the silicon before it is melted. The added material is called dopant and the process is called doping. This method is also used with semiconductor materials other than silicon, such as gallium arsenide.
Monocrystalline silicon grown by the Czochralski process is the basic material in the production of the large-scale integrated circuit chips used in computers, TVs, mobile phones and all types of electronic equipment.[5] Monocrystalline silicon is also used in large quantity for producing photovoltaic solar cells. The almost perfect crystal structure yields the highest light-to-electricity conversion efficiency for silicon.
Impurity incorporation
When silicon is grown by the Czochralski method, the melt is contained in a silica (quartz) crucible. During growth, the walls of the crucible dissolve into the melt and Czochralski silicon therefore contains oxygen at a typical concentration of 1018 cm−3. Oxygen impurities can have beneficial effects. Carefully chosen annealing conditions can allow the formation of oxygen precipitates. These have the effect of trapping unwanted transition metal impurities in a process known as gettering. Additionally, oxygen impurities can improve the mechanical strength of silicon wafers by immobilizing any dislocations which may be introduced during device processing. It was experimentally shown in the 1990s that the high oxygen concentration is also beneficial for radiation hardness of silicon particle detectors used in harsh radiation environment (such as CERN's LHC/S-LHC projects).[6][7] Therefore, radiation detectors made of Czochralski- and Magnetic
Czochralski-silicon are considered to be promising candidates for many future high-energy physics experiments.[8][9] It has also been shown that presence of oxygen in silicon increases impurity trapping during post-implantation annealing processes.[10]
However, oxygen impurities can react with boron in an illuminated environment, such as experienced by solar cells. This results in the formation of an electrically active boron–oxygen complex that detracts from cell performance. Module output drops by approximately 3 % during the first few hours of light exposure.
The Czochralski method is a technique for growing single-crystal silicon ingots for use in manufacturing semiconductor devices. The finished crystals are called boules. The boules are later sliced into very thin, circular wafers and then diced into the little silicon chips from which all silicon semiconductor LSI1 chips are made.
Growing large monocrystalline silicon boules is a tricky business. The Czochralski method (abbreviated as CZ in the technical literature) begins with silicon powder that is 99.999999999% pure2. A lot of that ridiculously pure silicon is placed in a large crucible that is made of fused quartz. The crucible is then put in a vacuum chamber, which is sealed and then filled with some conveniently inert gas, usually argon.
The chamber is then heated up to 1500° Centigrade or so, to melt the silicon. When the silicon is nicely melted, a small seed crystal mounted on the end of a rotating shaft is slowly lowered until it just dips below the surface of the red-hot silicon melt. The shaft rotates counterclockwise and the crucible rotates clockwise. Now, the rotating rod is drawn upwards very slowly, allowing a roughly cylindrical boule to form by some technical magic as it does so3. The boule can be from one to two metres or so long, depending on how much silicon there is in the crucible.
In the early days of the technology, the boules were quite thin, only a few inches wide. However, the crystal growers have had a lot of practice, and nowadays they can make nice, fat 300mm (12-inch) wide boules. The thickness is controlled by precise control of the temperature, the speeds of rotation and how fast the seed holder is withdrawn. Widths of 400mm (16 inches) are expected in the next several years. This is one reason for the rapidly decreasing cost of chips that we have enjoyed over the years, because more LSI chips can be created from a single wafer with the same number of fabrication process steps.
The electrical characteristics of the silicon are controlled by adding stuff like phosphorus or boron to the silicon before it is melted. The stuff added is called dopant and the process is called doping. This method is also used with semiconductor materials other than silicon, such as gallium arsenide.
As a necessary step in the production of large-scale integrated circuit chips, the Czochralski method is a basic technique in the making of computers, TVs, cell phones and the advanced electronic equipment of all kinds that shape modern life as we know it at the beginning of the 21st Century.
1Large-scale Integrated Circuit.2Yes, that's no joke. This stuff contains less than a few parts per billion of anything that is not silicon. That purity earns it a special name: electronic grade silicon. It has to be that pure for use in making semiconductor devices, because the electrical characteristics of the silicon are controlled by the intentional addition of special kinds of impurities at very low, precisely controlled concentrations, a process called doping. Those special impurities are called dopants and include things like boron, arsenic and phosphorus.3OK, it's not really magic, of course. When the seed crystal is dipped into the melt and then pulled up slightly, surface tension causes the liquid very near the seed to rise up above its normal level to meet the withdrawing seed, forming a meniscus. (Just like water does around the edges of a tube.) The silicon in the meniscus cools down and solidifies as crystalline silicon, thus forming the boule.
Fluidized bed reactor
A fluidized bed reactor (FBR) is a type of reactor device that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material (usually a catalyst possibly shaped as tiny spheres) at high enough velocities to suspend the solid and cause it to behave as though it were a fluid. This process, known as fluidization, imparts many important advantages to the FBR. As a result, the fluidized bed reactor is now used in many industrial applications.
The solid substrate (the catalytic material upon which chemical species react) material in the fluidized bed reactor is typically supported by a porous plate, known as a distributor.[1] The fluid is then forced through the distributor up through the solid material. At lower fluid velocities, the solids remain in place as the fluid
passes through the voids in the material. This is known as a packed bed reactor. As the fluid velocity is increased, the reactor will reach a stage where the force of the fluid on the solids is enough to balance the weight of the solid material. This stage is known as incipient fluidization and occurs at this minimum fluidization velocity. Once this minimum velocity is surpassed, the contents of the reactor bed begin to expand and swirl around much like an agitated tank or boiling pot of water. The reactor is now a fluidized bed. Depending on the operating conditions and properties of solid phase various flow regimes can be observed in this reactor.
Monocrystalline silicon (or "single-crystal silicon", "single-crystal Si", "mono c-Si", or just mono-Si) is the base material for silicon chips used in virtually all electronic equipment today. Mono-Si also serves as photovoltaic, light-absorbing material in the manufacture of solar cells.
It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges, and free of any grain boundaries. Mono-Si can be prepared intrinsic, consisting only of exceedingly pure silicon, or doped, containing very small quantities of other elements added to change its semiconducting properties. Most silicon monocrystals are grown by the Czochralski process into ingots of up to 2 meters in length and weighing several hundred kilogrammes. These cylinders are then sliced into thin wafers of a few hundred microns for further processing.
Single-crystal silicon is perhaps the most important technological material of the last few decades—the "silicon era", [1] because its availability at an affordable cost has been essential for the development of the electronic devices on which the present day electronic and informatic revolution is based.
Monocrystalline silicon differs from other allotropic forms, such as the non-crystalline amorphous silicon—used in thin-film solar cells, and polycrystalline silicon, that consists of small crystals, also known as crystallites.
Global market-share in terms of annual production by PV technology since 1990
Monocrystalline silicon is used in the manufacturing of high performance solar cells. Since, however, solar cells are less demanding than microelectronics for as concerns structural imperfections, monocrystaline solar grade (Sog-Si) is often used.
Market-share
In 2013, monocrystalline solar cells had a market-share of 36 percent, that translated into the production of 12,600 megawatts of photovoltaic capacity,[3] and ranked second behind the somewhat cheaper sister-technology of polycrystalline silicon.[4]
Efficiency
Lab efficiencies of 25.0 percent for mono-Si cells are the highest in the commercial PV market, ahead of polysilicon with 20.4 percent and all established thin-film technologies namely, CIGS cells (19.8%), CdTe cells (19.6%), and a-Si cells (13.4%).[5]
Solar module efficiencies—which are always lower than those of their corresponding cells—crossed the 20 percent mark for mono-Si in 2012; an improvement of 5.5 percent over a period of ten years. The thickness of a silicon waver used to produce a solar cell also decreased significantly, requiring less raw material and therefore less energy for its manufacture. Increased efficiency combined with economic usage of resources and materials was the main driver for the price decline over the last decade.[6]
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VLSI devices (Intel) fabricated on a single-crystal silicon wafer)
Wafer (electronics)From Wikipedia, the free encyclopediaJump to: navigation, search
Polished 12" and 6" silicon wafers. The notch in the left wafer and the flat cut into the right wafer indicates its crystallographic orientation (see below)
VLSI microcircuits fabricated on a 12-inch (300 mm) silicon wafer, before dicing and packaging
A wafer, also called a slice or substrate,[1] is a thin slice of semiconductor material, such as a crystalline silicon, used in electronics for the fabrication of integrated circuits and in photovoltaics for conventional, wafer-based solar cells. The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many microfabrication process steps such as doping or ion implantation, etching, deposition of various materials, and photolithographic patterning. Finally the individual microcircuits are separated (dicing) and packaged.
On the 13th of December 1965, American-Engineers Eric O Ernst, Donald J. Hurd and Gerard Seeley filed Patent US3423629A[2] while working under IBM. This Patent was for the first High-Capacity Epitaxial Apparatus and the methods to create the, now, Silicon Wafer.
Wafers are formed of highly pure (99.9999999% purity),[3] nearly defect-free single crystalline material.[4] One process for forming crystalline wafers is known as Czochralski growth invented by the Polish chemist Jan Czochralski. In this process, a cylindrical ingot of high purity monocrystalline semiconductor, such as silicon or germanium, is formed by pulling a seed crystal from a 'melt'.[5][6] Donor impurity atoms, such as boron or phosphorus in the case of silicon, can be added to the molten intrinsic material in precise amounts in order to dope the crystal, thus changing it into n-type or p-type extrinsic semiconductor.
The ingot is then sliced with a wafer saw (wire saw) and polished to form wafers.[7] The size of wafers for photovoltaics is 100–200 mm square and the thickness is 200–300 μm. In the future, 160 μm will be the standard.[8] Electronics use wafer sizes from 100–450 mm diameter. (The largest wafers made have a diameter of 450 mm but are not yet in general use.)
Cleaning, texturing and etching
Wafers are cleaned with weak acids to remove unwanted particles, or repair damage caused during the sawing process. When used for solar cells, the wafers are textured to create a rough surface to increase their efficiency. The generated PSG (phosphosilicate glass) is removed from the edge of the wafer in the etching .
Silicon wafers are available in a variety of diameters from 25.4 mm (1 inch) to 300 mm (11.8 inches).[10] Semiconductor fabrication plants (also known as fabs) are defined by the diameter of wafers that they are tooled to produce. The diameter has gradually increased to improve throughput and reduce cost with the current state-of-the-art fab considered to be 300 mm (12 inch), with the next standard projected to be 450 mm (18 inch).[11][12] Intel, TSMC and Samsung are separately conducting research to the advent of 450 mm "prototype" (research) fabs, though serious hurdles remain.
5-inch (130 mm) or 125 mm (4.9 inch). Thickness 625 µm.
150 mm (5.9 inch, usually referred to as "6 inch"). Thickness 675 µm.
200 mm (7.9 inch, usually referred to as "8 inch"). Thickness 725 µm.
300 mm (11.8 inch, usually referred to as "12 inch"). Thickness 775 µm.
450 mm (17.7 inch, usually referred to as "18 inch"). Thickness 925 µm (expected).[13]
Wafers grown using materials other than silicon will have different thicknesses than a silicon wafer of the same diameter. Wafer thickness is determined by the mechanical strength of the material used; the wafer must be thick enough to support its own weight without cracking during handling.
A unit wafer fabrication step, such as an etch step or a lithography step, can be performed on more chips per wafer as roughly the square of the increase in wafer diameter, while the cost of the unit fabrication step goes up more slowly than the square of the wafer diameter. This is the cost basis for shifting to larger and larger wafer sizes. Conversion to 300 mm wafers from 200 mm wafers began in earnest in 2000, and reduced the price per die about 30-40%.[14] However, this was not without significant problems for the industry.
There is considerable resistance to moving up to 450 mm despite the expected productivity improvement, mainly because companies feel it would take too long to recoup their investment.[15] Machinery needed to handle and process larger wafers results in increased investment costs to build a single factory. Lithographer Chris Mack claimed in 2012 that the overall price per die for 450 mm wafers would be reduced by only 10-20% compared to 300 mm wafers, because presently over
50% of total wafer processing costs are lithography-related. Converting to larger 450 mm wafers would reduce price per die only for process operations such as etch where cost is related to wafer count, not wafer area. Cost for processes such as lithography is proportional to wafer area, and larger wafers would not reduce the lithography contribution to die cost.[16] Nikon plans to deliver 450-mm lithography equipment in 2015, with volume production in 2017.[17][18] In November 2013 ASML paused development of 450-mm lithography equipment, citing uncertain timing of chipmaker demand.[19]
The time-line for 450 mm has not been fixed as of 2014. Mark Durcan, CEO of Micron Technology, said in February 2014 that he expects 450 mm adoption to be delayed indefinitely or discontinued. “I am not convinced that 450mm will ever happen but, to the extent that it does, it’s a long way out in the future. There is not a lot of necessity for Micron, at least over the next five years, to be spending a lot of money on 450mm. There is a lot of investment that needs to go on in the equipment community to make that happen. And the value at the end of the day – so that customers would buy that equipment – I think is dubious.”[20] As of March 2014, Intel Corporation expects 450 mm deployment by 2020 (by the end of this decade).[21] Mark LaPedus of semiengineering.com reported in mid-2014 that chipmakers had previously set 2016 to 2018, though this has been delayed “for the foreseeable future.” According to this report some observers expect 2018 to 2020, while “G. Dan Hutcheson, chief executive of VLSI Research, doesn’t see 450mm fabs moving into production until 2020 to 2025.”[22]
The step up to 300 mm required a major change from the past, with fully automated factories using 300 mm wafers versus barely automated factories for the 200 mm wafers. These major investments were undertaken in the economic downturn following the dot-com bubble, resulting in huge resistance to upgrading to 450 mm by the original timeframe. Other initial technical problems in the ramp up to 300 mm included vibration effects, gravitational bending (sag), and problems with flatness. Among the new problems in the ramp up to 450 mm are that the crystal ingots will be 3 times heavier (total weight a metric ton) and take 2-4 times longer to cool, and the process time will be double.[23] All told, the development of 450 mm wafers requires significant engineering, time, and cost to overcome.
Wafers are grown from crystal having a regular crystal structure, with silicon having a diamond cubic structure with a lattice spacing of 5.430710 Å (0.5430710 nm).[25] When cut into wafers, the surface is aligned in one of several relative directions known as crystal orientations. Orientation is defined by the Miller index with (100) or (111) faces being the most common for silicon.[25] Orientation is important since many of a single crystal's structural and electronic properties are highly anisotropic. Ion implantation depths depend on the wafer's crystal orientation, since each direction offers distinct paths for transport.[26] Wafer cleavage typically occurs only in a few well-defined directions. Scoring the wafer along cleavage planes allows it to be easily diced into individual chips ("dies") so that the billions of individual circuit elements on an average wafer can be separated into many individual circuits.
Left side: solar cells made of multicrystalline silicon Right side: polysilicon rod (top) and chunks (bottom)
Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity, multicrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Large polysilicon rods, usually broken into chunks of specific sizes and packaged in clean rooms before shipment, are either directly cast into multicrystalline ingots or submitted to a recrystallization process to grow monocrystalline silicon. The resulting ingots are then sliced into thin silicon wafers and used for the production of solar cells, integrated circuits and other semiconductor devices
Polysilicon is produced from metallurgical grade silicon by a chemical purification process. This process involves distillation of volatile silicon compounds, decomposition at high temperatures, or refinement in fluid phase. When produced for the electronics industry, polysilicon contains impurity levels of less than one part per billion (ppb), while polycrystalline solar grade silicon (SoG-Si) is generally less pure. The photovoltaic industry also produces upgraded metallurgical-grade silicon (UMG-Si), using metallurgical instead of chemical purification processes.
Polysilicon consists of small crystals, also known as crystallites, giving the material its typical metal flake effect. While polysilicon and multisilicon are often used as synonyms, multicrystalline usually refers to crystalls larger than 1 mm. Multicrystalline solar cells are the most common type of solar cells used in photovoltaics and consume most of the worldwide produced polysilicon. Polysilicon is distinct from monocrystalline silicon and amorphous silicon.
Polycrystalline vs monocrystalline silicon
Comparing polycrystalline (left) to monocrystalline (right) solar cells
In single crystal silicon, also known as monocrystalline silicon, the crystalline framework is homogenous, which can be recognized by an even external colouring.[1] The entire sample is one single, continuous and unbroken crystal as its structure contains no grain boundaries. Large single crystals are rare in nature and can also be difficult to produce in the laboratory (see also recrystallisation). In contrast, in an amorphous structure the order in atomic positions is limited to short range.
Polycrystalline and paracrystalline phases are composed of a number of smaller crystals or crystallites. Polycrystalline silicon (or semi-crystalline silicon, polysilicon, poly-Si, or simply "poly") is a material consisting of multiple small silicon crystals. Polycrystalline cells can be recognized by a visible grain, a "metal flake effect". Semiconductor grade (also solar grade) polycrystalline silicon is converted to "single crystal" silicon – meaning that the randomly associated crystallites of silicon in "polycrystalline silicon" are converted to a large "single" crystal. Single crystal silicon is used to manufacture most Si-based microelectronic devices. Polycrystalline silicon can be as much as 99.9999% pure.[2] Ultra-pure poly is used in the semiconductor industry, starting from poly rods that are two to three meters in length. In microelectronic industry (semiconductor industry), poly is used both at the macro-scale and micro-scale (component) level. Single crystals are grown using the Czochralski process, float-zone and Bridgman techniques.
PolySim is a powerful tool for design and optimization of reactors for polycrystalline silicon deposition from chlorosilanes by Siemens process. The tool is largely oriented to engineers and researchers and does not require expertise in Computational Fluid Dynamics from the users. The unique features of the tool are built-in chemical and heat reactor models combined with simple and convenient Graphical User Interface (GUI), which works in "grower-friendly" terms similar to those used in real reactor operation, minimizing the possible errors and saving user time needed to set-up the problem.
The growth simulation includes modeling of numerous physicochemical processes such as
turbulent heat and mass transfer, radiative heat transfer, gas-phase and surface chemical reactions, and electrical heating of the silicon rods. The software employs chemical models involving gas-phase precursor decomposition and interaction and an original surface chemistry model. The approach suggested allows description of the chemical processes at the gas-solid interfaces in a wide range of operating conditions.
The PolySim GUI includes everything required for the problem specification, solution control, and visualization of the results. The program allows the user to find almost all the reactor
Fig. 2. PolySim results: dependence of energy cost on temperature and pressure
characteristics depending on varying operating conditions and main characteristics of reactor design. It is also possible to evaluate the effect of the conditions non-uniformity on the growth of the different rod sections. For modeling the whole growth process, a number of separate points for different rod diameters are computed, and then the instantaneous reactor characteristics are time-integrated. The solution residuals are visualized by the GUI, allowing easy convergence control. The computed characteristics include reactor productivity, energy consumption per 1 kg of silicon, silicon conversion, electrical current parameters, gas flow rates, rod center temperature, gas depletion at the growth surface, the flow criteria, the energy consumption content. Run-time and post-processing visualization is available within the GUI, presenting dependencies of the input parameters and computed characteristics.
Hot-line support can be provided on request. The support includes free of charge supply of updated versions released during the license period and technical consulting on the PolySim operation.
PV Manufacturing
Some of the manufacturing processes and resources for photovoltaics are shared with other applications, especially electronic chips for computers, mobile phones
Fig. 3. PolySim results: dependence of Silicon output on temperature and pressure
and any other electronic device. This competition has caused a shortage in supply of crystalline cells.
Manufacturing Silicon
The raw material of most solar cells today is crystalline silicon. Luckily, silicon is one of the most widely available elements in the form of sand. Before silicon can be cut into thin wafers, however, it has to be purified, as otherwise the photoeffect will not be very efficient. Purity levels for solar cells do not have to be as high as in chip applications. Solar-grade purity is 99.999% (5N) as opposed to electronic-grade silicon purity of up to 99.9999999% (9N).
There are three main categories of manufacturing processes, resulting in different
purity levels:
Electronic-grade Silicon: 9N
There are three main steps to produce high-purity polycrystalline silicon.
1. Coke reduction: Metallurgical-grade silicon with 98.5% purity is produced from quartz sand in an arc furnace at very high temperatures.
2. Distillation: In a second step, the metallurgical grade silicon powder is disolved in hydrogen chloride and subsequently distilled to form a silane gas. In most instances, this is the trichlorosilane, but could be others.
3. Siemens Process: In the so-called Siemens Process the polycrystalline silicon is grown at very high temperatures. It requires hydrogen and produces more hydrogen-chloride as a by-product.
Medium-grade Silicon: 6-7N
The big drawback of the standard process as above is that a Siemens reactor is very expensive and the Siemens process itself requires a lot of energy. A number of new proprietary processes reduce the energy consumption and the capital costs for silicon production, though they are still similar to the traditional Siemens process.
Fluidized Bed Reactor (Renewable Energy Corporation) : operates at much lower temperatures and does not produce by-products.
Vapour to liquid deposition (Tokuyama) : similar to Siemens, but faster extraction.
Upgraded Metallurgical-grade Silicon (UMG): > 5N
In an altogether different process, metallurgical-grade silicon is chemically refined.
By blowing gasses through the silicon melt, the boron and phosphorous impurities are removed, followed by directional solidification. Companies like Timminco, Arise or RSI Silicon all have their own proprietary processes. However, they all have in common that by avoiding high purification, manufacturing costs are reduced significantly.
There are mainly three different silicon wafer types of different qualities:
Monocrystalline wafer: Silicon with a single, continuous crystal structure is grown from a small seed crystal that is slowly pulled out of a polysilicon melt into a cylindrical shaped ingot (Czochralski process). The ingot is cut into wafers using a diamond saw. Silicon waste from the sawing process can be re-cycled into polysilicon.
Polycrystalline wafer:Polycrystalline silicon consists of small grains of monocrystalline silicon. Cube-shaped ingots can be made directly by casting molten polysilicon, which are then cut into wafers similar to monocrystalline wafers.
Silicon ribbons: This is a continuous process whereby thin ribbons or sheets of multicrystalline silicon are drawn from a polysilicon melt. The subsequent cutting into wafers does not produce waste, as the drawn sheets are already wafer-thin. Silicon ribbons require around 5g of silicon per Watt rather than 8g/W using crystalline wafers.
Manufacturing Cells and Modules
Crystalline
Crystalline cells are made from silicon wafers by cleaning and doping the wafer. In
a separate manufacturing process, a number of cells are wired up to form a module. As such the manufacturing process of crystalline modules consists of four distinct processes: Polysilicon production, Ingot & Wafer manufacturing, cell manufacturing and module manufacturing.
Thin-Film
"Thin" means that the semiconductor layer is about 1/100th times "thinner" than in crystalline cells. The manufacturing process starts by depositing the thin photoactive film on the substrate, which could be either glass or a transparent film. Afterwards, the film is structured into cells similarly to the crystalline module. Unlike crystalline modules, the manufacturing process of thin-film modules is a single process that can not be split up..
For CdTe - thin-film, 220kg are required for 1MWp, which is 36 times less weight
per kWp than crystalline silicon.
Polysilicon Production
Polysilicon is a key ingredient in the manufacture of semiconductors and solar cells. Polysilicon is grown into ingots and then sliced into wafers, which are then used either for solar cells or for semiconductor chips. The most common manufacturing process of polysilicon is the Siemens process, but other methods are also being developed. This blog post will discuss the Siemens method, the fluidized bed reactor method, and UMG silicon production method.
Figure I: Polysilicon Manufacturing Steps (Source: Green Rhino Energy)
All these processes begin with metallurgical grade silicon (MG silicon): SiO2 sand is mixed with carbon and undergoes coke reduction in an arc furnace at 1,800 degrees C. This product is then either dissolved in HCl at 300 degrees C and distilled to produce high purity trichlorosilane (HSiCl3), or is chemically refined.
Figure II: MG Silicon Production (Source: Open Stax)
The Siemens process is most commonly used due to the high purity polysilicon that is produced through the process. The polysilicon produced is 9N quality, or 99.9999999% silicon. This is the current standard for the semiconductor industry, and is known as electronic grade polysilicon. After the MG silicon has been processed into HSiCl3, this is introduced to the Siemens deposition reactor. The reactors contain two Si slim rods. The reactor is heated to 1100 degrees C, which act as a nucleation point for the deposition of silicon. This results in a polycrystalline rod consisting of columnar grains of silicon.
The fluidized bed reactor (FBR) method of producing polysilicon produced medium grade silicon, 6-7N or 99.999999%-99.9999999% Si. This process was developed by REC Silicon, and the following information is from their website. FBR operates at much lower temperatures than the Siemens process, and does not produce by-products. Instead of Si-rods, the process uses silicon seed granules. These are fed into a chamber with heated silane gas, which fluidises the silicon granules. As the silane gas breaks down, it deposits more silicon on the granules, which grow larger and exit the chamber once they have reached a sufficient weight. This process uses much less energy than the Siemens process and produces more silicon per cubic meter of reactor space because the silicon crystals have a larger total surface area than the rods used in the Siemens process.
Figure IV: FBR Method vs. Siemens Method of Polysilicon Production (Source: REC Silicon)
Finally, UMG silicon is produced using chemical refining of MG silicon. This process produces slightly lower grade silicon than the previous two methods discussed, but still> 99.99999% Si product. UMG silicon is produced by blowing gasses through the silicon melt, removing boron and phosphorous impurities. The material is then directionally solidified, creating an ingot of polysilicon. There is no one method for producing UMG silicon: Each company producing this product have their own processes and chemical mixtures that they use. This method is becoming more popular as the costs associated with the Siemens process and FBR process are greatly reduced by avoiding high purification.
Each of these processes have their own strengths and weaknesses. The Siemens process is the most expensive method of polysilicon production, but also produces the purest material. FBR polysilicon production is between the Siemens process and UMG silicon in price and quality, making it a potential replacement within the semiconductor industry. UMG silicon cannot be
used in semiconductor manufacturing, but is becoming more popular in PV applications, due to reduced production cost and time.
Silicon Production ++++++
1.Quartz mining
Mined at quartz deposits, mostly in rocks.
2.Refining quartz grit to metallurgical silicon
Metallurgical Silicon (98–99 % Si) is produced in an arc furnace by mixing quartz grit, coal, chips and coke at a high temperature.
During this stage, an electrode is placed into an arc furnace. Then the furnace is loaded with SiO2 and chips, coke and carbon in the form of a coal. The mixture reacts at 1800°С, consuming 13 kWh. Some intermediate reactions also take place in the furnace. Resulting reaction can be expressed as follows:
Metallurgical silicon produced with such method contains 98–99 % Si, 1–2 % Fe, Аu, В, Р, Са, Cr, Cu, Mg, Mn, Ni, Ti, V, Zn and others.
3.Chlorination
Trichlorosilane is usually produced by hydrochlorination (chlorination) of silicon: metallurgical silicon interacting with hydrogen chloride or mixture of gases containing hydrogen chloride at 260–400 °C.
Trichlorosilane synthesis is followed by side reactions forming silicon tetrachloride and other chlorosilanes as well as metal halides i.e. АlСl3, ВСl3, FeCl3 etc. Reactions producing silicon chlorosilanes are reversible and exothermic:
At the temperature higher than 300 °C TCS is almost completely absent in the resultant products. To increase the TCS output the process temperature is decreased slowing down the reaction speed (3).
Catalysts (copper, iron, aluminum or others) are used for speeding up the reaction (2). For instance, by introducing up to 5 % of copper into initial silicon, the TCS content in the resultant products reaches 95 %. at 265 °C.
TCS synthesis takes place in a fluidized bed reactor (FBR) continuously fed from above with MgSi powder with the particle size of 0.01–1 mm. Fluidized bed of particles 200–600 mm thick is created by a counter flow of hydrogen chloride delivered to the reactor’s bottom part at a rate of 1–8 cm/s. This provides the transition of heterogeneous chemical engineering process from diffusion area to kinetic one. As the process is exothermic, in order to stabilize the mode in the specified interval, the heat is intensively removed with temperature strictly controlled at different levels of the fluidized bed. Hydrogen chloride consumption and pressure inside the reactor are monitored apart from temperature.
TCS output is significantly affected by water and oxygen content in raw materials. By oxidizing silicon powder, these admixtures cause the formation of dense SiO2 beds at its surface preventing silicon and hydrogen chloride interaction and thus decreasing TCS output. For instance, at increasing Н2О content in HCl from 0.3 to 0.4 % TCS output decreases from 90 to 65 %. Therefore, hydrogen chloride and silicon powder are thoroughly dried and purified from oxygen before TCS synthesis.
Steam-gas mixture formed during the TCS synthesis is delivered to a cooling area where it is quenched to 40–130 °C, forming solid particles of admixtures (iron and aluminum chlorides etc.,) in the form of a dust. These particles are then removed along with parts of unreacted silicon and polychlorides (SinCl2n + 2) by filters. After removing the dust (explosive material), steam-gas mixture at the temperature of 70 °C. is delivered to a condenser. SiHCl3 and SiCl4 (boiling temperatures 31.8 and 57.2 °C correspondingly) are separated from hydrogen and HCl (boiling temperature 84 °C). The mixture obtained after condensation consists mostly
of TCS (up to 90–95 %), the remainders are STC, which is then separated during the rectification process. STC, formed during the separation, is then used for producing silicones, quartz glass and TCS by additional hydrogenation in presence of a catalyst.
4.Distillation
TCS is purified from admixtures (boron, phosphorous, carbon) via rectification (separation) and transition to nonvolatile or complex compounds. Final testing of silicon’s quality is conducted by measuring its specific resistance.
Produced TCS contains many admixtures hard to remove. The most effective way is rectification; however, it is hard to conduct a deep and complete purification from admixtures having various physical and chemical natures by using rectification alone. Therefore, some additional measures are taken in order to improve purification efficiency.
For instance, admixtures, which are difficult to remove by crystallization (boron, phosphorous, carbon), require the most thorough TCS purification. That is why these trace substances are transitioned to nonvolatile and complex compounds in order to increase the purification efficiency. E.g. for boron removal, TCS steams are flowed through aluminum chip at 120 °C. Chip surface consumes boron and it is almost completely removed from TCS. Accessory aluminum chloride is then sublimated at 220–250 °С, and separated by fractional condensation.
Silver, copper or stibium can be used apart from aluminum. Adding copper to aluminum allows removing arsenic and stibium from TCS simultaneously. Introducing phosphorous pentachloride or oxychloride also increases efficiency of boron’s removal from TCS. Nonvolatile complex compounds of phosphorous with boron РСl5·ВСl3 or РОСl3·ВСl3, thus form, which are then removed during by rectification. Transition of boron to nonvolatile compounds can also be carried out by adding trityl chloride (or trimethylamine, ethane nitrile, amino acids, ketone etc.) to TCS, which leads to formation of a boron compound like (С6Н5)3С·ВСl3, which is then removed by rectification.
Removal of boron-containing admixtures is also conducted by adsorption in reactors filled with aluminogel and other gels (TiO2, Fe2O3, Mg(OH)2) with following TCS rectification.
For phosphorous removal, TCS is saturated with chloride, transitioning phoshourous trichloride to pentachloride. Upon adding aluminum chloride to the solution, nonvolatile compound РСl5·АlСl3 forms, which is then removed by rectification.
The purity of purified TCS is tested by infrared spectroscopy, chromatography, and measuring of conductivity types and rates of silicon test patterns, obtained from TCS samples.
The content of remaining trace substances in TCS after purification should not exceed, mass %: boron — 3·10-8, phosphorous — 1·10-7, arsenic — 5·10-10, carbon (in the form of hydrocarbon) — 5·10-7.
The residual content of donors according to electrical measurements should provide silicon’s specific resistance of n-type and 5000 Ohm·cm, and by р-type crystal acceptors — no less than 8000 Ohm·cm.
5.Polysilicon productionReduction
Polycrystalline silicon is produced by reducing purified TCS from gas into a solid substance by depositing TCS at heated rods — seeds made of monosilicon.
Reduction of purified TCS and resulting production of polysilicon are conducted in the hydrogen atmosphere
at the surface of heated silicon rods — seed rods with diameter of 4–10 mm (sometimes up to 30 mm), produced by growing from pedestal, cutting after Czochralski process or after special modes of reduction process, molding. Some technologies use tabular rods (1–5 mm thick and 30–100 mm wide) with greater deposition surface instead of cylindrical ones. High quality polysilicon is used to grow roods. Rod surface undergoes ultrasonic cleaning, etching in acids mixture (e.g., HF+ + HNO3), washing off and drying. Seed rods for producing high quality polysilicon should meet strict purity requirements: they should have donor resistance of > 700 Ohm·cm and boron resistance of > 5000 Ohm·cm.
Rods are used to produce electric heaters (e.g. of U-shape) and they are heated by conducting electric current. Current strength is raised in proportion to rod’s diameter increase.
The choice of conditions for TCS hydrogen reduction is made based on optimal interrelation of the following process parameters:
Equilibrium degree of SiHCl3 transition to Si, crystal structure of produced rods;
Process temperature;
Energy demands;
Н2 : SiHCl3 molar ratio;
Silicon deposition rate.
Optimal conditions for reduction process are considered to be as follows: temperature of 1100–1150 °C, Н2 : SiHCl3 molar ratio within 5–15, TCS feed density of 0.004 mol/(h·cm2). When the rod temperature is lower than optimal, the degree of TCS turning to STC increases lowering silicon output. Increasing temperature leads to significant increase of energy consumption. At optimal molar ratio of Н2 : SiHCl3 = 5–15, rods have a dense fine-crystalline structure and relatively even surface. An uneven surface forms beyond the limits of this ratio; rods’ structure becomes coarse-crystalline with pinholes, which result in swirling and fusion spitting when polysilicon is melted in the process of crystal growing.
The quantity of rods installed in various commercial reactors ranges from 2 tо 80with the length of each rod to 3.2 m and finishing diameter 150–250 mm. Due to mutual heating of the rods, silicon deposition rate
in multirod units is higher than in 2-rod installations; rate of rod diameter growth reaches 0.5 mm/h, with energy consumption of 3000 kWh/kg.
In order to increase the purity of produced silicon, hydrogen is thoroughly purified, reactors are made of special steel and their surface is protected against the impact of gas environment by introducing additional quartz (silicon) jars, separating the reaction medium from reactor walls. Protective films like polychlorosilane are good for protecting reactor walls.
Producing polysilicon from monosilane SiH4
Producing polycrystalline silicon rods by thermal decomposition of monosilane SiH4 is conducted by similar method at 800–1000 °C. Hydrogen formed during the decomposition process SiH4(Gas) ⟶ Si(Solid) + 2Н2(Gas) has a high degree of purity and is used in auxiliary production. Polysilicon obtained with this technology is purer than silicon produced by TCS reduction.
Silicon is extracted from SiCl4 and SiJ4 by reducing STC with zinc of by thermal dissociation of tetraiodide.
Produced polycrystalline rods are broke into pieces for easier load of crucibles or cut into cut-to-length sections before using them for growing monocrystals by Czochralski technology. For crucibleless zone melting rods are grinded to the required diameter. Removing surface layers enriched with admixture and gases also prevents spitting of silicon from the molten zone.
Modern process flows of polysilicon production include regeneration and reutilization of all components and products of reduction reactions (pyrolysis) thus improving technical and economic parameters of the process, decreasing silicon production cost and making the process more eco-friendly.
This polysilicon deposition process is also used for producing polycrystalline pipes with carbon expanders. Due to high purity and durability quartz pipes are replaced with polycrystalline pipes in furnaces for high temperature processes (higher than 1200 °C) used in technologies for producing semiconductors and microelectronics. Silicon pipes are not subject to settling or any other deformation for some years of exploitation despite of continuous temperature circulation ranging between 900 and 1250 °C, while quartz pipes have a limited lifetime in the same conditions.
6.Monosilicon productionProducing monosilicon by non-crucible zone meltingPolysilicon is purified from volatile and nonvolatile admixtures by pulling polysilicon rods through melted zone in vacuum.
Polysilicon rods are exposed to crystal purification by vacuum zone melting for producing high quality silicon. In addition, apart from crystal purification of silicon from nonvolatile admixtures (mostly acceptors), it is significantly cleaned out of volatile donors due to their evaporation from the molten zone. E.g. 15 passes through the molten zone at a rate 3 mm/min, result in producing p-type conductivity monocrystals of silicon with residual concentration of the admixture of less than 1013 cm-3 and specific resistance (to boron) more than 104 Ohm·cm.
Producing monosilicon by Czochralski technology
Silicon monocrystals are mostly produced with Czochralski method (up to 80–90 % of electronic industry consumption) and, to a lesser extent, by non-crucible zone melting.
Czochralski method implies growing monocrystals due to atoms transitioning from liquid to gaseous state at the border of their division.
In regard to silicon, this process can be characterized as a single component liquid-solid growing system.
The growth rate (V) is determined by the number of points on the growing crystal’s surface for attachment of the atoms coming from the liquid phase and by specifics of transitioning at the division borders.
Equipment for growing monocrystalline silicon
The unit consists of the following blocks:
Furnace, including a crucible (8), crucible container (14), heater (15), power supply (12), high-temperature chamber (6) and insulation (3, 16);
Crystal pulling device including a seed rod (5), seed rotation (1) and fixing devices, crucible rotation and lifting device (11);
Device for controlling atmosphere composition (4 — gas inlet,, 9 — exhaustion, 10 — vacuum pump);
Control module, consisting of microprocessor, temperature detector and ingot’s diameter data transmitter (13, 19), input devices;;
High quality rotating seed monocrystal is sunk into a silicon melt. Melt polysilicon is produced in a crucible within the inert atmosphere (of argon at depression of ~104 Па.) at the temperature insignificantly exceeding silicon melting point Т = 1415 °C. The crucible is rotated in the direction opposite to that one of the monocrystal in order to mix the melt and minimize inhomogeneity of temperature allocation. Growing at depression allows partial removing of volatile admixtures from silicon melt due to their evaporation and decreasing the formation of silicon monoxide powder, which in case of getting into the melt results in defects in the crystal and may disturb monocrystalline growth.
In the beginning of the monocrystal growth, a part of seed monocrystal is melted to liquidate sections with increased density of mechanical tensions and defects in it. Then monocrystal is gradually pulled from the melt.
There is highly efficient automated equipment developed and widely used for production of silicon monocrystals with Czochralski technology. It provides reproductible production of dislocation free monocrystals with diameter of up to 200–300 mm. Upon increasing the load and the diameter of monocrystals, their production cost decreases. However, in the melts of larger mass (60–120 kg) the character of convection streams becomes more complicated, creating additional troubles for providing required parameters of the material. Apart from that, at larger masses of the melt the cost decrease becomes insignificant due to high costs of quartz crucibles and decrease of crystal growth rate because of troubles caused by removal of crystallization latent heat. Thus, in order to further increase the process efficiency and decrease the volume of the melt used for growing crystals, the units of semicontinuous growth began to be developed intensively. Additionally, crucibles in these units are continuously or periodically loaded with silicon without cooling the furnace, e.g. by feeding the melt with liquid phase from other crucible, which in its turn also can be periodically or continuously fed with solid phase. This improvement of the Czochralski method allows reducing the cost of grown crystals for tens of percent. Along with that, it is possible to conduct growing from the melts of small and permanent volume. This simplifies regulating and optimizing the convection streams in the melt and liquidates crystal’s segregative inhomogeneity caused by adjusting the melt’s volume during its growth.
7.Multisilicon productionMultisilicon is produced by bottom-up direct crystallization of ingots from a melt in round quartz crucibles due to cooling crucibles bottom.
Before each melting, the furnace is cleaned and prepared and polysilicon is loaded into the crucible. After the furnace is closed and air removed, argon is fed and the heater is turned to the mode of raw materials preheating. Then the heater is turned to the melting mode. After the fusion, the melt is soaked for some period in order to average the composition and temperature, and then crucible with the melt are cooled determining the process of crystal growth and being know-how of equipment suppliers.
During the crystallization, the temperature of the silicon melt in the crucible (container) gradually decreases making the crystallites grow in the same direction, gradually expanding and pushing out smaller crystallites. Such production provides producing ingots with columnar texture and of required seed size, and allows moving the gases dissolved in the melt to the upper (cut) section. The size of the seed grown with this method can reach 5–10 mm in a section perpendicular to the growth direction. Produced ingots (with mass of 120–200 kg) are cut into smaller ones, which are then wire-cut into wafers for solar elements.
Specific heat capacity800 j/(kg·К), or 0.191 cal/(g·deg).
Thermal expansion coefficient2.33·10-6 К-1, becomes negative when lower 120 К.
Magnetic behaviorSilicon is diamagnetic.
Dielectric permittivity= 11.7. Shows how much the electric field is weakened by dielectric, quantitatively characterizing the dielectric property to polarize in the electric field.
Electrical resistivityPure silicon scarcely carries electrical current (specific resistance equals to 2.3·105 Ohm·cm). Depending on admixture, it can have not only electron (n-type) but also hole conduction (p-type), that vary within wide limits.
Chemical valenceSilicon is 4-valent in compounds
Chemical activityCrystalline silicon is low active, but more active than carbon.
PROPERTIES DETAILS
Silicon types
(Monosilicon)(Multisilicon)(Polysilicon)The difference between silicon types lies within the degree of regular and proper order of silicon’s crystalline structure. Thus, silicon can be classified in accordance to size of crystals it consists of. For example, monosilicon ingot is a silicon crystal, whereas polysilicon ingot has more crystals than multisilicon ingot has.
Name Notation Seed size Growing technology
Mono- sc-Si > 100 mm Czochralski (CZ), floating zone
Monocrystalline silicon differs from the polycrystalline modification by its crystalline structure oriented in a certain crystallographic plane. Monosilicon has a homogeneous crystalline structure without grain line junctions (which is noticeable even in appearance).
Ordered arrangement of silicon atoms in the silicon’s monocrystalline latitude creates an accurate zone structure. Each atom of silicon has 4 electrons at the outer shell. Electrons of neighbor atoms form couples, belonging to both atoms simultaneously; thus, each atom has 4 bonds with neighbor atoms.
Behavior of monosilicon is well predicted, however, because of low growing speed and production process complexity it is the most expensive silicon type. Monosilicon is the base for modern electronics. High requirements regarding purity and structure perfection are imposed to it. Concentration of electrically active alloying admixtures usually lies within 1013–1018 cm-3, electrically active background impurities — less than 1015 cm-3, electrically inactive impurities — less than 1018–1019 cm-3. The main types of structure defects are so called microdefects. As a rule, these are small dislocation loops and clusters of own and admixtures’ point defects.
Multicrystalline siliconMulticrystalline silicon holds the intermediate position between poly- and monocrystalline silicon by size and amount of crystals. Growing of silicon multicrystals is a lot easier than monocrystals, which makes their cost lower. However, the quality of multicrystals in comparison with monocrystal is lower as well because of lots of grain junction lines of monocrystal containing in multicrystal.
Grains line junctions create additional defect levels in a semiconductor band gap, being local centers with high speed of recombination, which leads to decrease in general lifetime of minority carriers. In addition, grain line junctions decrease productivity blocking carrier’s current and creating bridging ways for current going through p-n transition.
To prevent high recombination losses at grain line junctions, the grain size should be at least few millimeters. This condition also means that the measurements of a seed will exceed the thickness of the solar element, which will decrease
resistance to carriers’ current and general length of border zones in the solar element. Such multicrystalline silicon is widely used in commercial manufacture of solar elements.
How to produce polysilicon?
Polycrystalline siliconPolysilicon is high-purity silicon containing less than 0.0001 % of admixtures, which consists of significant number of small crystalline seeds chaotically oriented in respect to each other.
In essence, technical silicon is also polycrystalline; however, in order to prevent confusion, conception “polycrystalline silicon” is applied only to especially pure semiconductor silicon.
Polysilicon is the purest form of industrially produced silicon and the main material for microelectronics and solar energetics; it is an intermediate product obtained by purification of technical silicon with chlorides, which is used for producing mono- and multicrystalline silicon.
Now polysilicon is distinguished into “electronic” (semiconductor) grade (with less than 1·10–10 % of admixtures) and “solar” grade (with 1·10–5 % admixtures content).
Most of silicon in the world is produced in a form of grey-colored wires with grained dendritical surface. In the wire’s center, there is a mono- and polysilicon “fuse” of round or square section with diameter (by side) of 8–10 mm. Dense-packed crystallites in form of short needles with section of less than 1 mm grow from the fuse transversely to generatrix.
Polysilicon is a raw material for production of more advanced silicon types like multicrystalline silicon (multisilicon) and monocrystalline silicon (monosilicon), and in some application areas can be used in its pure form.
Amorphous siliconAmorphous silicon is a brown or brownish black moisture-retentive powder chemically more active than crystalline silicon. At usual temperature reacts with fluorine directly forming fluorine silicon SiF4; at high temperature reacts with almost all non-metals and with many metals.
Silicon’s coordination number equals four, and that is why each atom of silicon is bond with four neighbor atoms. In crystalline silicon, these tetrahedral structures last in a wide range forming well-arranged crystalline latitude. In contrast, in an amorphous structure the order in atomic positions is limited to short range. Most likely, atoms form continuous random chains. In addition, not all the atoms in amorphous silicon are coordinated 4 times. In connection with unarranged character of the material, some atoms have cutoff chains. Physically these cutoff chains are defects in the continuous random chain and change silicon properties significantly.
In semiconductor instruments, amorphous silicon is usually used in a form of thin films deposited on a base coat. Layers of hydrogenized amorphous silicon with most of the cutoff chains filled with atoms of hydrogen are used in solar elements. Such silicon shows the best coefficient of light conversion to electric energy in comparison with pure amorphous silicon.
Market overviewPolysilicon production volume> 5010—50< 10 K Tons
Throughout the duration of a long period of time semiconductor (SC) industry was almost the only consumer of high-purity silicon: as of the beginning of the century its consumption share was 60–80 % of global polysilicon market:
In 2006 the dynamic increase of demand for Photovoltaics (PV), driven by European governments subsidizing PV installations mainly in Germany, Spain and Italy led to increased necessity of new capacities construction. First results of the investment boom appeared in 2008, when the first factories were launched. At that moment, more than 60 projects were still on different development stages. Capacity utilization exceeded 90 %.
Capacities growth was happening via expanding already operating factories and launching of new manufacturers, using both traditional and innovative technologies.
Innovative technologies were following the trends of lowering production costs and producing higher quality polysilicon in order to increase its performance in PV.
In 2008-2009, financial and economic crisis had a bad impact on polysilicon investors. In 2008 capacity utilization has reached 75 %, in 2009 it has only reached 51 %. The year of 2010 was a record for PV: the previous year installations’ volume increased more than twice, which became a reason for consumption growth.
The world electronics market started to recover, and the wafers production in volume and value has grown by 40 %. Despite new factories were launched with total capacity of 56,000 tons, the capacity utilization rate has increased to 60 %.
Market volume and market value
History
Market Volume Dynamics, K Tons200620072008200920102011201220132014201520162017201820192020020040060080047627810513918324532743256974543464953576065Show all Solar Electronic
Polysilicone Market Value Dynamics, Bn $20062007200820092010201120122013201420152016201720182019202001020303.5510.48.89.79.710.210.711.312.112.913.714.515.116.22.97.84.63.3322.93.95.27.19.612.717.122.7Show all Solar Electronic
We can easily distinguish the growth rate of solar quality polysilicon market from the electronic one. The latter is quite stable as opposed to the solar grade polysilicon. Photovoltaics, driven by European feed-in tariffs was booming. The prices that solar companies were able to pay for raw materials were incredible. In 2006–2008 polysilicon production was a highly profitable and promising market. This period was exactly the one when new capacities were being built.
Solar installations sponsorships were not indefinite: they started drying in 2009 when the growth rates declined. Nevertheless, photovoltaic industry is still supported by European governments and is considered one of the most efficient renewable energy sources. The industry has to start being self-regulatory and find new points of balance being costs, prices
and contractual terms. Solar installations growth will stabilize at a rate of ~33 % per year. Electronic grade polysilicon production growth will stagnate at a rate of ~7 % focusing more on product quality.
We can see the boom in market value in 2008 that is a result of increased capacity. As we were saying before, market value will reach ~ $20 Bn in 2015 and reach ~ $40 Bn in 2020. We expect the solar market to repeat the situation of 2008–2010 in 2017–1018. The LTA will still exist on the market with flexible prices though. Spot prices will be negotiated monthly, following the same path as solar modules do.
ForecastBased on various forecasts and official information provided by worldwide governments we expect the polysilicon market to grow 24 % CAGR from 2012 to 2020. The growth will be driven by solar grade silicone demand increase which will be led mainly by PV installations growth announced by governments worldwide.
Increased attention to renewable energy in general and solar energy in particular is explained by the number of various factors:
Government goals to reduce carbon dioxide burst
Increased climate change concerns
High dependence on oil and gas energy provided by certain countries only
Fossil energy price growth
Necessity to meet future energy needs via new energy sources
Cost of renewable energy generation and solar energy in particular is becoming more compatible with traditional energy generation.
According to various data sources, in 30–40 years the word economy will need additional 5000 GW of electrical generators capacity, which is approximately twice more of the present capacity of all worlds’ energy stations.
At the present moment, despite global financial and economic crisis, photovoltaic industry is still developing. The main drivers for this development are governmental programs and federal financing. This development results in:
Increased number of self-sustainable private houses in metropolitan areas
Usage of solar batteries for street and traffic lighting in Europe
Solar plants are being built in order to meet energy need of nearby towns
According to global Industry analysts, global silicone market will reach $ 17.2 billion in 2017. While PV is the main driver for such a growth, we need to understand that the 20 GW installed in 2011 are just 1 % of the total amount of electricity that was generated in 2011 by all sources. The forecast for 2012 of long term growth estimates range from 20 % to 30 %.
As we can see from value graph above, the period of “chaotic” growth rates finishes after year 2012. The growth speed will slowly decrease. Nevertheless, solar market will experience double-digit growth until 2020. Electronic market, as mentioned previously, will be growing slowly at 7 % rate.
Prices
Polysilicon prices are normally distinguished into:
contract (LTA) prices which are stated in long-term contracts between producers and consumers;
spot prices that are used if polysilicon is sold without preliminary arrangements.
The main indicator of price dynamics are spot prices which represent polysilicon as a commodity and which are the main indicator of supply/demand balance.
To illustrate the historical price development below is the graph for spot and contract price development from 2006 until today.
Polysilicon spot prices, Dec 17th, $/kg
Polysilicon Max Min Av. ∆ ∆%
1st grade quality
22.00
14.50
15.550
-0.20
-1.27%
2nd grade quality
15.40
13.50
14.710
-0.21
-1.41%
As we can see from the graph, pricing boom affected both solar and electronic grade of polysilicon equally until the beginning of 2009. After 2009, solar grade silicone prices experienced a moderate decrease as opposed to electronic grade silicone. The process illustrated supply/demand balance in the industry, which became stable in the electronic industry as opposed to solar.
Considering the fact that that semiconductors (electronic) industry is quite stable at the moment we will consider prices for electronic grade silicone to balance around $ 250 per kg. From now onwards we will analyze solar grade silicone prices because:
the weight of electronic grade silicon in the overall polysilicon is small but stable;
the prices are considered to be stable;
as a result, the influence of electronic grade silicone on the market dynamics is negligible.
ForecastAs we have mentioned in the paragraph above for forecast purposes we will use solar grade silicone prices. In order to do so we have to consider both spot and LTA prices.
LTA prices are based on long-term relationships between contractors and are often confidential. During the period of undersupply, contract prices were a good indicator of market dynamics. Contract prices were aimed to hedge volatility risk of spot prices enabling both contractors to forecast revenues and costs better.
However, currently all producers are cancelling their long term contracts as the market is becoming a better tool for price formation. Therefore, we can identify the trend that LTA prices will lean towards spot prices and polysilicon becoming a commodity.
Polysilicon market in Russia
Russia is by far not the leading importer of polysilicon. The majority of silicone imported to the country is metallurgical silicon of less than 99.99 % purity. Russian largest aluminum producer is the major importer of this type of product.
Nevertheless, as we can see from the graph, semiconductors industry owns 10 % share in value in overall silicon imports. High technology, space and military technology has no option besides importing, as Russia does not produce high purity silicon.
Besides, as can be seen from the graph, the volume/value ratio of imported product is very low, determined by high price, especially for electronic grade silicone.
Import of silicone to Russia
Country Volume, tons Price, $
Total 510.824,931.9
China 473.7 17,212.0
Germany 18.1 3,103.7
Country Volume, tons Price, $
ROTW 6.2 2,111.4
USA 5.9 992.1
Ukraine 3.2 398.2
Canada 1.5 61.7
Italy 0.9 1,001.1
Netherla 0.8 22.5
Country Volume, tons Price, $
nds
Norway 0.7 29.1
Imported silicon, 2011
Product Volume, % Price, $
MgSi 97.50 %
70 %
Solar grade 2.30 % 20 %
Product Volume, % Price, $
Electronic grade
0.20 % 10 %
In total, the amount of imported polysilicon of more then 99.99 % purity is 510 tons in 2011 out of which 473.7 is imported from China. The main reason for working with Chinese vendors is mainly price.
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.
In this lecture the question will be answered how 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 in this work it is shown which kind of detectors have been used and how they have been produced.
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. In the search for radiation harder material and in order to perform radiation tests on an as wide as possible range of material also silicon grown by the Czochralski has been investigated in this lecture. While for the epitaxial technique the price and the substrate problem might rule out largely its application as detector material the Czochralski method could become of interest for the production of radiation hard material if it is possible to make high resistivity (> 1 KOhm/cm) CZ commercially available.
In this section the production of silicon with the two growth techniques mentioned above will shortly be reviewed with special interest in the high resistivity silicon production and the possibilities of defect engineering respectively the controlled incorporation of impurities into the crystal.
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. In the following a short review is given.
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 . A schematic diagram of a Czochralski-Si grower, called puller, is shown in Fig. 2.1. The puller consists of three main components:
1. a furnace, which includes a fused-silica crucible, a graphite susceptor, a rotation mechanism (clockwise as shown), a heating element, and a power supply;
2. a crystal-pulling mechanism, which includes a seed holder and a rotation mechanism (counter-Clockwise); and
3. an ambient control, which includes a gas source (such as argon), a flow control and an exhaust system.
The Czochralski method begins by melting high purity polysilicon (SGS) with additional dopants as required for the final resistivity in the rotating quartz crucible. A single crystal silicon seed is placed on the surface and gradually drawn upwards while simultaneously being rotated. This draws the molten silicon after it which solidifies into a continuous crystal extending from the seed. Temperature and pulling speed are adjusted to first neck the crystal diameter down to several millimetres, which eliminates dislocations generated by the seed/melt contact shock, and then to widen the crystal to full diameter.
During the production process the quartz crucible (SiO2) gradually dissolves, releasing large quantities of oxygen into the melt. More than 99% of this is lost as SiO gas from the molten surface, but the rest stays in the melt and can dissolve into the single crystal silicon. Another impurity, however with smaller concentrations, that is also introduced into the melt by the production process itself is carbon. The silicon monoxide evaporating from the melt surface interacts with the hot graphite susceptor and forms carbon monoxide that re-enters the melt. As the
crystal is pulled from the melt, the impurity concentration incorporated into the crystal (solid) is usually different from the impurity concentration of the melt (liquid) at the interface. The ratio of these two concentrations is defined as the equilibrium segregation coefficient k0 =Cs/c1 where Cs and C1 are the equilibrium concentrations of the impurity in the solid and liquid near the interface, respectively.
Figure 2.1: Schematic setup of a Czochralski crystal puller
Oxygen is always the impurity with the highest concentration in CZ silicon. Typical oxygen and carbon concentrations are [O] ≈ 5 - 10 10^17cm-3 and [C] ≈ 5 - 10 10^15cm-3, respectively. The solubility of O in Si is ≈ 10^18 cm-3 at the melting point but drops by several orders of magnitude at room temperature, hence there is a driving force for oxygen precipitation. Furthermore the high oxygen concentration can lead to the formation of unwanted electrically active defects. These are oxygen related thermal double donors (TDD) and shallow thermal donors (STD) which can seriously change the resistivity of the material. However, oxygen has also good properties.
Oxygen acts as a gettering agent for trace metal impurities in the crystal (Internal Gettering) and it can pin dislocations which greatly strengthens the crystal. Oxygen precipitates in the wafer core suppress stacking faults, and oxygen makes the Si more resistant to thermal stress during processing. This is the reason why CZ-Si is used for integrated circuit production, where there are many thermal processing steps.
However, the most important property of a high oxygen concentration from the point of view of this work is the improved radiation hardness. The main problem for the application as detector grade material arises from the resistivity of CZ silicon. Due to contamination with boron, phosphorus and aluminum from the dissolving quartz Crucible the highest commercially available resistivity is about l00 Ohmcm for n-type and only slightly higher for p-type material. Therefore standard CZ silicon is not suitable for detector production. However, first experiments to compensate the natural p-type background doping by adding a small quantity of phosphorus to the melt have been performed.
Magnetic Field Applied Cz (MGZ)
MCZ may be the future standard CZ technology since today’s approaches to solve the challenge of the 300 mm and later on also the 400 mm crystal diameter are based on this technology The method is the same as the CZ method except that it is carried out within a strong horizontal (HMCZ) or vertical (VMCZ) magnetic field. This
serves to control the convection fluid flow, allowing e.g. with the HMCZ method to minimise the mixing between the liquid in the center of the bath with that at the edge. This effectively creates a liquid silicon crucible around the central silicon bath, which can trap much of the oxygen and slow its migration into the crystal. Compared to the standard CZ a lower oxygen concentration can be obtained and the impurity distribution is more homogeneous. This method offers also the possibility to produce detector grade silicon with a high oxygen concentration. Since the technology is still a very young one, it is hard to get such material with reproducible impurity concentrations on a commercial basis. However, a first test material of 4 KΩcm p-type with an oxygen concentration of 7 - 8 l017 cm-3 and a carbon concentration below 2xl016 cm-3 was obtained.
Continuous Cz (CCZ)
With the CCZ method a continuous supply of molten polycrystalline silicon is achieved by using a double quartz crucible. In the first one the crystal is grown and in the second one, connected to the first one, a reservoir of molten silicon is kept, that can be refilled by new polysilicon during the growth process. This allows for larger crystal length and improves the throughput and operational costs of the CZ grower. Furthermore the resulting single crystals have a uniform resistivity and oxygen concentration and identical thermal history. In combination with the magnetic field method the Continuous Magnetic Field Applied CZ technique (CMCZ) offers the possibility to grow long and large diameter CZ. However, silicon produced by this technology has so far not been used for radiation damage experiments.
Float zone silicon (FZ)
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 thewafers, and is now being intentionally added during the growth stages.
The float zone method
The float Zone (FZ) method is based on the zone-melting principle and was invented by Theuerer in 1962. A schematic setup of the process is shown in Fig. 2.2. The production takes place under vacuum or in an inert gaseous atmosphere. The process starts with a high-purity polycrystalline rod and a monocrystalline seed crystal that are held face to face in a vertical position and are rotated.
Figure 2.2: Schematic setup for the Float Zone (FZ) process
With a radio frequency field both are partially melted. The seed is brought up from below to make contact with the drop of melt formed at the tip of the poly rod. A necking process is carried out to establish a dislocation free crystal before the neck is allowed to increase in diameter to form a taper and reach the desired diameter for steady-state growth. As the molten zone is moved along the polysilicon rod, the molten silicon solidifies into a single Crystal and, simultaneously, the material is purified. Typical oxygen and carbon concentrations in FZ silicon are below 5 1015 cm-3. FZ crystals are doped by adding the doping gas phosphine (PH3) or diborane (B2H6) to the inert gas for n- and p-type, respectively. Unlike CZ growth, the silicon molten Zone is not in contact
with any substances except ambient gas, which may only contain doping gas. Therefore FZ silicon can easily achieve much higher purity and higher resistivity.
Additionally multiple zone refining can be performed on a rod to further reduce the impurity concentrations. Once again the effective segregation coefficient k plays an important role. Boron, for example, has an equilibrium segregation coefficient of k0 = 0.8. In contrast to this phosphorus cannot only be segregated (k0 = 0.35) but also evaporates from the melt at a fairly high rate. This is the reason why on the one hand it is easier to produce more homogeneous p-type FZ than n-type FZ and on the other hand high resistivity p-type silicon can only be obtained from polysilicon with low boron content. Dopants with a small k0 like Sn can be introduced by pill doping - holes are drilled into the ingot into which the dopant is incorporated - or by evaporating a dopant layer on the whole ingot before the float zoning process.
