Polycrystalline silicon
Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.
Polysilicon is produced from metallurgical grade silicon by a chemical purification process, called the Siemens process. This process involves distillation of volatile silicon compounds, and their decomposition into silicon at high temperatures. An emerging, alternative process of refinement uses a fluidized bed reactor, which is lower cost. The photovoltaic industry also produces upgraded metallurgical-grade silicon, using metallurgical instead of chemical purification processes for lower cost at the expense of purity. When produced for the electronics industry, polysilicon contains impurity levels of less than one part per billion, while polycrystalline solar grade silicon is generally less pure.
In the 2010's, production shifted toward China, with China-based companies accounting for seven of the top ten producers and around 90% of total worldwide production capacity of approximately 1,400,000 MT. German, US and South Korea companies account for the remainder.
The polysilicon feedstock – large rods from the Siemens process, usually broken into chunks of specific sizes and packaged in clean rooms before shipment – is directly cast into multicrystalline ingots which are large square blocks weighing around 800 kg for making solar wafers or submitted as-is to a recrystallization process to grow single crystal boules usually with the Czochralski method. The boules are then sliced into thin silicon wafers and used for the production of solar cells, integrated circuits and other semiconductor devices.
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 crystals larger than one millimetre. Multicrystalline solar cells are the most common type of solar cells in the fast-growing PV market and consume most of the worldwide produced polysilicon. About 5 tons of polysilicon is required to manufacture one 1 megawatt of conventional solar modules. Polysilicon is distinct from monocrystalline silicon and amorphous silicon.
Vs monocrystalline silicon
In single-crystal silicon, also known as monocrystalline silicon, the crystalline framework is homogeneous, which can be recognized by an even external colouring. 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. 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 is a material consisting of multiple small silicon crystals. Polycrystalline cells can be recognized by a visible grain, a "metal flake effect". Semiconductor 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. Ultra-pure poly is used in the semiconductor industry, starting from poly rods that are two to three meters in length resulting from the Siemens process. In the microelectronics industry, poly is used at both the macro and micro scales. Single crystals are grown using the Czochralski, zone melting and Bridgman–Stockbarger methods. The Czochralski method often uses polysilicon as the starting material.
Components
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 reactors at high temperatures and is usually heavily doped n-type or p-type.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, or solid-phase crystallization 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 deposition of polycrystalline silicon on plastic substrates is motivated 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 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.
Image:Silicon poly 640x480.jpg|thumb|left|Polycrystalline silicon
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 nanometers to 1 micrometer 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 150 °C if annealed while in contact of another metal film such as aluminium, gold, or silver.
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 or a metal 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.
One major difference between polysilicon and a-Si is that the mobility of the charge carriers of the polysilicon 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 circuitry 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.
Feedstock for PV industry
Polycrystalline silicon is the key feedstock in the crystalline silicon based photovoltaic industry and used for the production of conventional solar cells. For the first time, in 2006, over half of the world's supply of polysilicon was being used by PV manufacturers. The solar industry was severely hindered by a shortage in supply of polysilicon feedstock and was forced to idle about a quarter of its cell and module manufacturing capacity in 2007. Only twelve factories were known to produce solar-grade polysilicon in 2008; however, by 2013 the number increased to over 100 manufacturers. Monocrystalline silicon is higher priced and a more efficient semiconductor than polycrystalline as it has undergone additional recrystallization via the Czochralski method. Trichlorosilane is used to create high purity electronic grade silicon from metallurgical grade silicon which is produced from quartzite. EGS is used for silicon photovoltaics and is necessary for the Czochralski method of wafer production used in integrated circuits.Deposition methods
Polysilicon deposition, or the process of depositing a layer of polycrystalline silicon on a semiconductor wafer, is achieved by the chemical decomposition of silane at high temperatures of 580 to 650 °C. This pyrolysis process releases hydrogen.Polysilicon layers can be deposited using 100% silane at a pressure of or with 20–30% silane 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%. 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 Arrhenius behavior, that is deposition rate = A·exp where q is electron charge and k is the Boltzmann 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 °C is too slow to be practical. Above 650 °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. Recent investigations have shown that e-beam evaporation, followed by SPC can be a cost-effective and faster alternative for producing solar-grade poly-Si thin films. Modules produced by such method are shown to have a photovoltaic efficiency of ~6%.
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.