With the expansion of polysilicon production capacity, improving the quality of polysilicon products is an important way to improve the market competitiveness. In this paper, carbon pollution caused by graphite parts used in polysilicon production is chuckussed. Graphite can react with argon to form methane and other hydrocarbons at high temperature. Methane decomposes into carbon and hydrogen when it meets red hot silicon rod. Carbon and silicon are deposited together, resulting in the increase of carbon content in polysilicon and affecting the quality of polysilicon. There must be strict quality standards for selecting graphite parts, and the graphite parts purchased from abroad must be pretreated before they can be put into use. In order to reduce the carbon content of polysilicon to the lowest level, graphite parts should be coated with surface treatment, such as glassy carbon surface treatment, to reduce the direct contact of graphite parts with hydrogen and reaction, and ultimately reduce the carbon content of polysilicon, improve product quality.
The use of modified Siemens Process to produce polysilicon, the factors affecting the purity of polysilicon are mainly from raw materials, equipment and consumables used in polysilicon production systems, as well as external environmental factors. The resistivity and minority carrier lifetime of polysilicon are mainly related to the purity of the trichlorosilane rectification product. By increasing the number of rectification stages and using a new rectification equipment, the purity of trichlorosilane can be achieved to produce high resistivity and high minority carrier lifetime polysilicon; carbon is mainly derived from trichlorosilane rectification products, hydrogen and graphite consumables used in reduction furnaces. The dissolved carbon in trichlorosilane can be reduced by improved distillation. The carbon in the hydrogen is derived from the activated carbon adsorption column in the dry recovery system. As long as the amount of treated gas in the activated carbon adsorption column is not exceeded, the carbon in the hydrogen does not increase; the graphite consumable mainly refers to the graphite chucks and graphite caps which used for fixed silicon core in the reduction furnace. Graphite chucks and graphite caps, are all made of high-purity graphite produced in China. After entering the plant, they are put into the reduction furnace after being subjected to series pretreatment to reduce the ash. As for the degree of influence on the carbon content of polysilicon, It is a blind spot in the production of polysilicon. In this paper, the mechanism of the influence of graphite consumables on polysilicon carbon content, the selection criteria and pretreatment methods of graphite consumables, and how to improve the graphite parts to reduce carbon content of polycrystalline silicon are described.
The mechanism of graphite affecting carbon content in polysilicon
Because carbon graphite material has very good density, hardness and pressure resistance, it has the advantages of high temperature resistance, high pressure resistance, corrosion resistance, good electrical conductivity and stable performance. It is widely used as an accessory in the production of polysilicon by Siemens Process. The silicon core used for depositing polycrystalline silicon in the reduction furnace is fixed by a graphite parts such as a graphite chuck or a graphite cap, and is connected to the electrode, so that the current can be well transferred from the electrode to the silicon core through the graphite parts.
Carbon is generally considered to be a non-electroactive medium in silicon. However, carbon has been considered a harmful substance for both single crystal silicon and epitaxial silicon. Carbon has an important influence on the generation of vortex defects in single crystal silicon. When the carbon content in silicon is greater than 5 ppma, the P-N junction leakage current increases sharply. Therefore, as a raw material for preparing single crystal silicon, the carbon content of polycrystalline silicon will directly affect the quality of downstream materials. Production practice shows that by improving the quality of the rectified product and controlling the purity of hydrogen, the N, P type resistivity, minority carrier lifetime and oxygen concentration of polycrystalline silicon can reach the electron level 2 or even the zone melting level, but the carbon content in the polysilicon is difficult to reach 1.5×1016 atoms/cm3 or less, that is to say, to produce polysilicon of the electronic level 1 and above, it is necessary to solve the bottleneck problem of the carbon content.
In production, by periodically detecting the carbon content of hydrogen entering the reduction furnace, it is found that the carbon content in the polycrystalline silicon is substantially proportional to the carbon content in the hydrogen entering the reduction furnace. It is generally believed that the carbon in hydrogen is mainly derived from the activated carbon adsorption column of the dry recovery system and the trichlorosilane rectification product. After the improvement process, the carbon content in the hydrogen can be reduced to less than 10 ppm, but the carbon content of the produced polysilicon remains difficult to reach the level above the electronic level 1. Studies have shown that in hydrogen, graphite is not a chemically stable substance at high temperatures. Graphite can react with hydrogen at high temperatures to form hydrocarbons such as methane. U.S. Patent (Patent No. 456730, December 26, 1989: Patent No. 544611, June 27, 1990) also considers graphite as one of the sources of polysilicon carbon contamination. At high temperatures, graphite can react with hydrogen to form methane. After contact with the red hot silicon rod, it will continue to decompose into carbon and hydrogen. The carbon in the graphite will enter the polysilicon to form pollution through this way.
Studies have shown that in the temperature range of 1050-1250 °C, the dependence of CH4 concentration in H2 on the pyrolysis graphite temperature T is that the CH4 concentration in H2 increases with the increase of T, and the graphite is coated after surface coating (such as silicon or silicon carbide), the CH4 concentration in H2 is significantly reduced, but compared with silicon carbide coating, the carbon content of silicon coating decreases more obviously., mainly because SiC is not a highly chemically stable substance in H2. At high temperatures, H2 and SiC still undergo chemical reactions, producing hydrocarbons such as methane. Therefore, as long as the graphite material can chemically react with hydrogen, carbon pollution will be formed on the polycrystalline silicon. To reducing the carbon content of the polycrystalline silicon, In addition to reducing the carbon content in the raw materials (such as trichlorosilane and hydrogen), processing such as surface coating for the graphite material is also necessary.
Selection criteria and pretreatment methods for high-purity graphite materials for polysilicon production
Since graphite is a porous structural material, the total internal surface area of the pores is much larger than the geometrical area of the outer surface. According to the literature, 0.22 g of graphite can have an internal surface area of 14,000 cm 2 and an external surface area of only 1.18 cm 2 . Of course, graphite made by different processes has different densities, and the internal surface area is obviously different, and the concentration of the reaction product CH4 is also different. In addition, the graphite materials used by major manufacturers are different. Generally, graphite materials purchased from different manufacturers will have certain quality differences, such as mechanical strength and ash. At the same time, because of the lack of certain testing methods and standards, the general graphite parts are only put into use after the series of pretreatments after being purchased into the factory. It is not known whether there are quality differences in each batch of graphite parts. At present, large graphite manufacturers will provide the physical characteristics of the products to the customers as a reference basis, and polysilicon manufacturers can choose according to production needs. Graphite fittings in polysilicon production generally require ash content less than 10ppm, resistivity less than 10Ω*m, compressive strength greater than 100MPa, flexural strength 50MPa, bulk density greater than 1.78g/cm3, in order to reduce other pollution caused by graphite parts to polysilicon, generally high purity graphite or ultra high purity graphite with low impurity content is required.
In view of the limitations of the detection means, the graphite parts‘ quality may not be guaranteed the processing technology of the manufacturer, or the mechanical friction during transportation may increase the ash. Therefore, further processing is required before being put into the reduction furnace. The treatment method is generally: purging the surface particle dust of the graphite parts with nitrogen and then placing it in an ultrasonic cleaning machine, injecting 18M high-purity water, boiling for about 15-30 minutes, and then ultrasonically cleaning for 15 to 30 minutes, after the water temperature is lowered to room temperature, the graphite is removed. The pieces are taken out and dried in an oven at 150~200 °C for 8~12h. Then the graphite parts are placed in a vacuum graphite calciner and vacuum calcined. It is required to be calcined at a constant temperature of 1550~1750 °C for about 3~5h. After the water temperature is lowered to room temperature, put the graphite pieces into a high-purity plastic bag and put them into use.
Improve graphite parts and improve the quality of polysilicon products
Under the existing technical conditions, the application of graphite parts in the production of polycrystalline silicon is irreplaceable. Therefore, if the contamination of polycrystalline silicon by graphite parts is to be minimized, it is necessary to start from the quality of the graphite parts. First, high-purity or ultra-high-purity graphite with a low impurity content is selected. Then, using more advanced processing techniques, such as improved processing method, make the graphite has less ash and higher strength; in addition, with today’s advanced surface treatment technology, graphite parts can be coated, such as silicon coating and silicon carbide. However, as mentioned above, the graphite member must have good electrical conductivity, and the lower the probability of reacting with hydrogen at a high temperature, the better, so the silicon coating has certain limitations in electrical conductivity, although the silicon carbide coating can reducing graphite and hydrogen to some extent, but the effect is not ideal compared with silicon coating.
Graphite parts used for fixing silicon cores such as graphite discs and graphite caps made from high purity graphite, and then surface coating of graphite parts is an ideal method for reducing the content of impurities such as polysilicon and carbon, and the materials used for surface coating are essential. Nowadays, there is a new material that can be used as a coating material, namely glassy carbon, which is a new type of carbon formed by carbonization of an fluorenone resin or a phenolic resin to 1200 ° C or more in an inert atmosphere. It is called glassy carbon because it is as bright as glass, . Glass carbon has the characteristics of good air tightness and electrical conductivity, small thermal expansion coefficient, hard texture and easy to be polished into mirror surface, chemically inert, etc. It is mostly used in working electrode materials for electrochemical and electrical analysis, so we can also apply it to the graphite parts, as shown in Figure 1.
Figure 1 Schematic diagram of glass carbon coated graphite parts
1-silicon core, 2-silicon core slot, 3-glass carbon, 4-graphite (graphite chuck), 5-electrode slot, 6-electrode
It is known from a European patent (application number: 921145124) filed in the United States in 1993 that the technology is relatively mature. After the graphite electrode is vacuum dried, it is immersed in a 50% furfural phenol resin solution for 30 minutes, the viscosity is 20 cps (22 ° C); then it is taken out to dry, treated in an argon atmosphere at 150 ° C for 1 h, then immersed again in the solution, repeat the previous steps, after two immersion treatment, and then in argon The atmosphere was heated at 250 ° C for 1 h; finally, it was heated to 1600 ° C under a nitrogen atmosphere at a temperature rise rate of 250 ° C for heat treatment, while adding a certain amount of chlorine gas to nitrogen to further purify and remove the ash. After this series of treatments, the graphite coated with glassy carbon can be put into use. Experiments have shown that, under the same premise of other conditions, the polycrystalline silicon produced by using the glass-carbon-coated graphite material has a carbon content lower than 5 times that of the former compared with the graphite material which is not coated. It can be seen that the use of glass carbon coated graphite parts is an important way to reduce the polysilicon carbon content and improve the quality of polysilicon.
Graphite materials have important application value in the production of polycrystalline silicon. The graphite parts prepared by them are materials which are now fixed silicon core and irreplaceable current. However, the graphite material also has a certain influence on the quality of polysilicon, mainly because it may cause carbon pollution of polysilicon, so that the carbon content of polysilicon cannot be reduced to the minimum level.
(1) At high temperatures, graphite parts may react with hydrogen to form hydrocarbons such as methane. These hydrocarbons continue to decompose into carbon and hydrogen on the surface of red hot silicon rods, and carbon and silicon deposit together to make carbon content in polysilicon products raise.
(2) Graphite parts purchased from graphite manufacturers, due to the difference in processing technology and graphite raw materials, are preferably put into use after pretreatment to avoid unnecessary carbon pollution to polysilicon.
(3) Applying surface treatment technology to apply glass carbon on the surface of graphite parts, which can effectively prevent the graphite parts from directly reacting with hydrogen at high temperature, and effectively reduce the carbon pollution caused by graphite parts to polysilicon products. Improve the quality of polysilicon products.
Author: Sun Qiang, Tang Chuanbin