Crystalline silicon
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Crystalline silicon, also called wafer silicon, is a material consisting of one or more small silicon crystals. It is different from amorphous silicon, used for thin films (thin-film silicon cell).
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[edit] Monocrystalline
Silicon that is pulled as a single crystal. The internal crystalline structure is completely homogenous, which can be recognized by an even external coloring [1].
A single crystal, also called monocrystal, is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The opposite of a single crystal sample is an amorphous structure where the atomic position is limited to short range order only. In between the two extremes exist polycrystalline and paracrystalline phases, which are made up of a number of smaller crystals known as crystallites. Because of a variety of entropic effects on the microstructure of solids, including the distorting effects of impurities and the mobility of crystallographic defects and dislocations, single crystals of meaningful size are exceedingly rare in nature, and can also be difficult to produce in the laboratory under controlled conditions (see also recrystallisation).
Semiconductor grade (also solar grade) polycrystalline silicon is converted to "single crystal" silicon - meaning that the randomly associated grains of silicon in "polycrystalline silicon" are converted to large "single" crystals of silicon. Single crystal silicon is used to manufacture 99% of all electronic devices.[citation needed] The devices are used in watches, refrigerators, microwaves, televisions, radios, communications equipment such as cell phones, and controls for cars, ships, aircraft, missiles, and atomic weapons.
[edit] Polycrystalline silicon
Polycrystalline silicon (or semicrystalline silicon, polysilicon, poly-Si, or simply poly in context) is a material consisting of multiple small silicon crystals. Polycrystalline cells can be recognized by a visible grain, a “metal flake effect” [2].
Polycrystalline silicon can be as much as 99.9999999% pure.[citation needed] Silicon is most often companioned with oxygen to form silica. When the oxygen is stripped from the silicon, crude polycrystalline silicon remains. Ultra-pure poly is used in the semiconductor industry, starting from poly rods that are five to eight feet in length.
In microelectronic industry (semiconductor industry), poly is used both at the macro-scale and micro-scale (component) level.
At the macro scale, polysilicon is used as a raw material entering a process in which single crystals are grown (see Czochralski process, Bridgman technique, Float-zone silicon).
At the component level, polysilicon has long been used as the conducting gate material in MOSFET and CMOS processing technologies. For these technologies it is deposited using low-pressure chemical-vapour deposition (LPCVD) reactors at high temperatures and is usually heavily N or P-doped.
More recently, intrinsic and doped polysilicon is being used in large-area electronics as the active and/or doped layers in thin-film transistors. Although it can be deposited by LPCVD, plasma-enhanced chemical vapour deposition (PECVD), or solid-phase crystallization (SPC) of amorphous silicon in certain processing regimes, these processes still require relatively high temperatures of at least 300°C. These temperatures make deposition of polysilicon possible for glass substrates but not for plastic substrates. The drive to deposit Polycrystalline silicon or poly-Si on plastic substrates is powered by the desire to be able to manufacture digital displays on flexible screens. Therefore, a relatively new technique called laser crystallization has been devised to crystallize a precursor amorphous silicon (a-Si) material on a plastic substrate without melting or damaging the plastic. Short, high-intensity ultraviolet laser pulses are used to heat the deposited a-Si material to above the melting point of silicon, without melting the entire substrate. The molten silicon will then crystallize as it cools. By precisely controlling the temperature gradients, researchers have been able to grow very large grains, of up to hundreds of micrometers in size in the extreme case, although grain sizes of 10 nanometres to 1 micrometre are also common. In order to create devices on polysilicon over large-areas however, a crystal grain size smaller than the device feature size is needed for homogeneity of the devices. Another method to produce poly-Si at low temperatures is metal-induced crystallization where an amorphous-Si thin film can be crystallized at temperatures as low as 150C if annealed while in contact of another metal film such as aluminium, gold, or silver
One major difference between polysilicon and a-Si is that the mobility of the charge carriers can be orders of magnitude larger and the material also shows greater stability under electric field and light-induced stress. This allows more complex, high-speed circuity to be created on the glass substrate along with the a-Si devices, which are still needed for their low-leakage characteristics. When polysilicon and a-Si devices are used in the same process this is called hybrid processing. A complete polysilicon active layer process is also used in some cases where a small pixel size is required, such as in projection displays.
Polysilicon is a key component for integrated circuit and central processing unit manufacturers such as AMD and Intel.
[edit] Efficiency
Mitsubishi Electric Corporation announced on February 2009 that it has improved its world's highest conversion efficiency rate for a 150 x 150 millimeter practical-size multi-crystalline silicon photovoltaic (PV) cell by 0.3 points from 18.6 percent to achieve a new world record of 18.9 percent [3].
[edit] Solar panel
Polycrystalline silicon is also a key component of solar panel construction. The photovoltaic solar industry is growing rapidly but is likely going to be very limited in 2006-2008 due to severe shortages and allocations of the polysilicon material.[4]
For the first time in 2006, over half of the world's supply of polysilicon is being used for production of renewable electricity solar power panels.[5] There are only twelve factories of solar grade polysilicon in the world (in 2008).
Monocrystalline silicon is slightly higher priced and slightly more efficient than multicrystalline.
[edit] PolySilicon in VLSI
Polysilicon Deposition, or the process of depositing a layer of polycrystalline silicon on a semiconductor wafer, is achieved by pyrolyzing (decomposing thermally) silane, SiH4, inside a low-pressure reactor at a temperature of 580 to 650 deg C. This pyrolysis process involves the following basic reaction: SiH4 --> Si + 2H2.
Polysilicon has many applications in VLSI manufacturing. One of its primary uses is as gate electrode material for MOS devices. A polysilicon gate's electrical conductivity may be increased by depositing a metal (such as tungsten) or a metal silicide (such as tungsten silicide) over the gate. Polysilicon may also be employed as a resistor, a conductor, or as an ohmic contact for shallow junctions, with the desired electrical conductivity attained by doping the polysilicon material.
There are two common low-pressure processes for depositing polysilicon layers: 1) using 100% silane at a pressure of 25-130 Pa (0.2 to 1.0 Torr); and 2) using 20-30% silane (diluted in nitrogen) at the same total pressure. Both of these processes can deposit polysilicon on 10-200 wafers per run, at a rate of 10-20 nm/min and with thickness uniformities of +/- 5%.
The critical process variables for polysilicon deposition include temperature, pressure, silane concentration, and dopant concentration. Wafer spacing and load size have been shown to have only minor effects on the deposition process.
The rate of polysilicon deposition increases rapidly with temperature, since it follows the Arrhenius equation: R=A*exp(-qEa/kT) where R is the deposition rate, Ea is the activation energy in electron volts, T is the absolute temperature in kelvins, k is the Boltzmann constant, q is the electron charge, and A is a constant. The activation energy for polysilicon deposition is about 1.7 eV.
Based on this equation, the rate of polysilicon deposition increases as the deposition temperature increases. There will be a minimum temperature, however, wherein the rate of deposition becomes faster than the rate at which unreacted silane arrives at the surface. Beyond this temperature, the deposition rate can no longer increase with temperature, since it is now being hampered by lack of silane from which the polysilicon will be generated. Such a reaction is then said to be 'mass-transport-limited.' When a polysilicon deposition process becomes mass-transport-limited, the reaction rate becomes dependent primarily on reactant concentration, reactor geometry, and gas flow.
When the rate at which polysilicon deposition occurs is slower than the rate at which unreacted silane arrives, then it is said to be surface-reaction-limited. A deposition process that is surface-reaction-limited is primarily dependent on reactant concentration and reaction temperature. Deposition processes must be surface-reaction-limited because they result in excellent thickness uniformity and step coverage. A plot of the logarithm of the deposition rate against the reciprocal of the absolute temperature in the surface-reaction-limited region results in a straight line whose slope is equal to -qEa/k.
At reduced pressure levels for VLSI manufacturing, polysilicon deposition rate below 575 deg C is too slow to be practical. Above 650 deg C, poor deposition uniformity and excessive roughness will be encountered due to unwanted gas-phase reactions and silane depletion. Pressure can be varied inside a low-pressure reactor either by changing the pumping speed or changing the inlet gas flow into the reactor. If the inlet gas is composed of both silane and nitrogen, the inlet gas flow, and hence the reactor pressure, may be varied either by changing the nitrogen flow at constant silane flow, or changing both the nitrogen and silane flow to change the total gas flow while keeping the gas ratio constant.
Polysilicon doping, if needed, is also done during the deposition process, usually by adding phosphine, arsine, or diborane. Adding phosphine or arsine results in slower deposition, while adding diborane increases the deposition rate. The deposition thickness uniformity usually degrades when dopants are added during deposition.
[edit] Upgraded metallurgical-grade silicon
Upgraded metallurgical-grade (UMG) silicon (also known as UMG Si) solar cell was created to close the efficiency gap between industrial multicrystalline and high-efficiency monocrystalline silicon cell. UMG silicon, which is three orders of magnitude less pure than polysilicon is being researched and considered as a cost-effective alternative to polysilicon. [6].
A project is targeting 18-22% efficient cells (upgraded metallurgical silicon could potentially reach efficiencies of only 0.5 percent less than polysilicon), at manufacturing costs of less than $1 per peak watt.
A project is also attempting to build 1,000 tons of UMG silicon capacity for $15 million in six months. [7].
SolarWorld has established a joint venture with Scheuten Solarholding to turn dirty metallurgical-grade silicon into high-purity solar-grade silicon. [8].
[edit] Manufacturers
[edit] Polysilicon
Major polysilicon manufacturers include Hemlock Semiconductor Corporation [9], Wacker Chemie, REC, Tokuyama, MEMC, Mitsubishi (Japan and America) and Sumitomo Corporation, as well as several small sites in Kyrgyzstan, China and Russia.
The first 7 companies cover over 75% of the worldwide production capacity of polysilicon (2006).[citation needed][10]
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[edit] Printed electronics
Micro-tec Company has introduced an automated multi-layer print system for crystalline solar cells [11].
[edit] See also
- Low-cost solar cell
- Metallurgical grade silicon
- Monocrystalline silicon
- Nanocrystalline silicon
- Polycrystal
- Photovoltaic cells
- Photovoltaic module
- Wafer (electronics)
[edit] References
- ^ http://www.solarworld.de/Solar-ABC.123.0.html?&L=1
- ^ http://www.solarworld.de/Solar-ABC.123.0.html?&L=1
- ^ http://www.renewableenergyworld.com/rea/partner/mitsubishi-electric-electronics-usa-inc-3971/news/article/2009/02/mitsubishi-electric-breaks-own-record-with-worlds-highest-conversion-efficiency-rate-of-18-9-for-multi-crystalline-silicon-photovoltaic-cells?cmpid=WNL-Wednesday-February25-2009
- ^ The Wall Street Journal, A Shortage Hits Solar Power. April 29, 2006.
- ^ Photovoltaics: Getting Cheaper
- ^ Calisolar - Home
- ^ Greentech Media | Charting a Path to Low-Cost Solar
- ^ Solar: Doing the Dirty
- ^ Hemlock Semiconductor Corporation
- ^ [1]
- ^ http://www.solarindustrymag.com/e107_plugins/content/content.php?content.2181
[edit] External links
- Crystalline silicon (EERE).
- The US Display Consortium, promoting the development of polycrystalline silicon flat-panel display technologies
- The Coming Boom in Photovoltaic Power
- Alan Joch (November 10, 2006). "Sand Trap: Will the silicon shortage stunt the solar industry’s growth?". Plenty Magazine. http://plentymag.com/features/2006/11/sand_trap.php.
- Polycrystalline Silicon Technology - A company that offers technology for polysilicon production
- Polysilicon Plant in India.
