Current Czochralski MonoCrystalline Growth

Institution Course Tutor Date Current Czochralski MonoCrystalline Growth Abstract The purpose of this report is to discuss current crystal growth process. There are several Silicon growth processes commonly used in the industry, the Czochralski MonoCrystalline Growth seems to be a popular choice among other ingot grow processes. The Silicon Growth process is considered as one of the most time consuming and expensive on the development of silicon cell (Tom P. Hough, 49). The process requires state of the art equipments to meet the critical requirements. The growing Process first start with high purity semiconductor-grade silicon, this is the primary source of the silicon wafer. The material has been process to at least 99.99 purity from one of the silicon grade refining process. The substance will be placed in a crucible, which is usually made of Quart. The material will be heated up to 1500C and dissolve in a vacuum into its liquid form. Then a small piece of seed crystal will be dipped into the melted silicon, it is the leading source for the ingot grow. The melted silicon will be attracted to the seed and the ingot begins to grow. The seed will be slowly pulled upward and rotate at the same time. In the meantime, the crucible will be rotated in an opposite direction to minimize the convection (Vasileska, 114). The ingot will first grow neck and the diameter increase in a constant rate until it reaches to the desired diameter also known as the body. When the body has reached to the desired length or when the crucible is low on silicon material, the tail diameter of the ingot will reduce to the seed diameter to exit the process. The process will take around two days to produce high purity MonoCrystalline ingot (Hough, 76). Introduction The Czochralski process is a method used in crystal growth and it produces single crystals of semiconductors like gallium arsenide, silicon and germanium. The name of this process is derived from a polish scientist who discovered it in 1916 Jan Czochralski. Jan discovered this new method of crystal growth while studying the crystallization features of metals. The Czochralski process is mostly used in the growth of solitary crystal silicon as well as cylindrical ingots and boules. The process can also be used to grow other semiconductors like gallium arsenide although it requires the use of alternatives of the Bridgman-Stockbarger technique. Process requirements The first major requirement for the crystal growth using the Czochralski process is a vacuum pump. In dusty environments, the Czochralski process can produce a lot of silicon monoxide particles which can lower the quality of the crystals grown. The vacuum pump is used to absorb such particles therefore reducing their impact on the final crystal. The second requirement for the Czochralski process to be successful is an iridium crucible. The iridium crucible is the most preferred material mainly because of its superior compatibility to chemicals as well as their durability. Iridium is also used because it has a higher boiling temperature compared to the aluminum mixture used to grow the crystals (Vasileska, 28). The second requirement for the Czochralski process is a cooling system. After the aluminum mixture is melted, there is need to maintain it in a constant temperature of above 2050 degrees. The cooling system also known as the kiln contains nitrogen which keeps the iridium crucible oxidizing. The kiln also helps the iridium crucible react with the melt. To produce prefect crystals, the process requires a computer system is required for maintaining measurements while the crystal is being pulled from the mixture. The Czochralski process involves slowly withdrawing a solitary crystal from a molten mixture of oxide. The melt hardens and turns into a seed which is later turned into a crystal of required diameter. This is achieved through controlling the amount of heat entering into the crucible. The most common methods used in heating the melt that is put into the crucible are resistance radio frequency heating (Geilker, Kwapil and Rein, 3). Iridium cubicles used in the Czochralski process are of different sizes and capacities and they are cylindrical. The crucibles are made from molten and rolled iridium sheets through welding. They can also be manufactured from iridium powder which is hot pressed. During crystal growth, the crucible undergoes several processes including charging, growing the crystal during the oxide melting point and cooling both the residue and the crucible charge after the bond between the melt and the crystal is broken. The main steps of the Czochralski process In the first step of the Czochralski process, a fused silica crucible is charged with undoped electronic grade polycrystalline silicon (EGS). After this, the charged silica crucible is also loaded with an exact amount of a watered down silicon alloy. During the crystallization process, there are gases that are contained in the growth chamber. After the first process, the second process involves releasing the gas in the growth chamber so as to avoid lowering the quality of the crystal produced. After releasing the gas in the growth chamber, the third step involves refilling the chamber with an inert gas. This process is important as it holds back or inhibits the entry of atmospheric gases into growth chamber. Entry of atmospheric gases into the melt is dangerous as it could lower the quality of the final crystals during crystal growing. The silicon charge contained in the chamber is them melted a temperature of 1421 degrees Celsius. After silicon is molten into the required temperatures, a thin seed of crystal silicon with exact orientation tolerance is lowered in. After a short time the seed crystal is then removed from the molten silicon at a much regulated rate. During the removal process, the seed crystal is rotated in opposite direction with the crucible (Xakalashe and Tangstad, 89). Electrical features of silicon are managed by incorporating materials like boron or phosphorous to silicon before its melted. These materials that are incorporated into silicon to control electrical characteristics of silicon are known as dopants and the process of adding them is known as doping. Other than silicon, the doping is also used with semiconductor materials like gallium arsenide. Growing silicon using the Czochralski method Challenges of the Czochralski process The Czochralski process presents several challenges and its necessary that great care should be taken to ensure that end product is of the desired quality. The first challenge of the process is temperature should be carefully monitored. Temperature monitoring is important because if it rises too high, there is a danger that the seed crystal may melt. If the temperatures are too low and the crystals are lowered into it, there is a risk that all the contents in the crucible may crystallize prematurely (Castellano, 43). When this happens, the resultant crystal is of a very low quality and this can be exhibited through fracture lines and cracks. The withdrawal of the crystal can also be challenging because one needs to be very careful and ensure that the withdrawal is correctly measured. Production cost The product cost of the Czochralski process is mainly dependant on the required raw materials as well as the size of the crystal being manufactured. Compared to small ones, large crystals are more expensive and difficult to grow. They also require higher degrees of control to ensure that the resultant crystals are of high quality. When choosing raw materials for use in production of crystals in the Czochralski process, one should consider different options since some are more expensive than others (Geilker, Kwapil and Rein, 5). Crystals grown using the Czochralski process are chemically the same to those of natural formations but they are more reliable and of higher quality. In normal circumstances, the production cost of the Czochralski process MonoCrystalline is about 24.55 per kilogram while that of polycrystalline is about 9.97 per kilogram. Conclusion The Czochralski process is the most popular method for growing crystals of high quality. A crucible is heated in a furnace after which the material to be melted put in. The crystal only grows when its pulled out vertically and this is due to phase change. After the melt has heated up, a seed crystal is lowered into it. The temperature of the melt in the crucible is constantly adjusted such that the temperature at the middle of the melt attains the freezing point of material. After this, the seed crystal is removed by being rotated as the crystal begins to grow. Compared to other methods, the Czochralski process has several advantages (Vasileska, 59). First, the Czochralski process makes it possible to observe both seeding and growth. Secondly, crucible walls do not restrict the crystals during cooling down and growth. Lastly, it is easy to control the level of forced convention in the melt. Works Cited B.S. Xakalashe and M. Tangstad. Silicon Processing From Quartz to Crystalline Solar Cells. Sothern African Pyrometallurgy, 2011. Dragica Vasileska. Nano-Electronic Devices Semiclassical and Quantum Transport Modeling. Springer, 2011. Print. Juliane Geilker, Wolfram Kwapil and Stefan Rein. Light Induced Degradation in Compensated P- and N- Type Czochralski Silicon Waters. Journal of Applied Physics, Vol 109, 053718, 1-6, 2011. Robert Castellano. Solar Panel Processing. Archives contemporaines, 2010. Tom P. Hough. Recent Developments in Solar Energy. Nova drafters, 2007. Print Surname PAGE MERGEFORMAT 6 Y, 4IsNXp
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