Polysilicon

Polysilicon is a form of elemental silicon. When the melted elemental silicon solidifies under supercooling conditions, the silicon atoms are arranged in a diamond lattice shape into a plurality of crystal nuclei. If these crystal nuclei grow into crystal grains with different crystal orientations, these crystal grains combine to crystallize into polysilicon. . Utilization value: From the current development of international solar cells, it can be seen that the development trend is monocrystalline silicon, polycrystalline silicon, ribbon silicon, thin film materials (including microcrystalline silicon-based thin films, compound-based thin films, and dye thin films).

nature

Gray metallic sheen. Density 2.32~2.34. Melting point 1410°C. Boiling point 2355 °C. Soluble in hydrofluoric acid and ** mixed acid, insoluble in water, ** and **. The hardness is between the crucible and the quartz, and it is brittle at room temperature and easily cracked when cut. It is ductile when heated above 800°C and shows significant deformation at 1300°C. It is inactive at room temperature and reacts with oxygen, nitrogen, and so on at high temperatures. In the high-temperature molten state, it has a large chemical activity and can work with almost any material. With semiconductor properties, it is an extremely important and excellent semiconductor material, but trace impurities can greatly affect its conductivity. The electronics industry is widely used in the manufacture of basic materials such as semiconductor radios, tape recorders, refrigerators, color TVs, video recorders, and electronic computers. The dry silicon powder and the dry hydrogen chloride gas are chlorinated under certain conditions, and then condensed, distilled, and reduced.

Polysilicon can be used as a raw material for drawing monocrystalline silicon. The difference between polycrystalline silicon and monocrystalline silicon is mainly manifested in the physical properties. For example, the anisotropy of mechanical properties, optical properties, and thermal properties is far less pronounced than monocrystalline silicon; in terms of electrical properties, the polysilicon crystals are also far less conductive than single crystal silicon, and even have almost no conductivity. In terms of chemical activity, the difference between the two is minimal. Polysilicon and monocrystalline silicon can be distinguished from each other in appearance, but the actual identification must be determined by analysis of crystal orientation, conductivity type, and resistivity.

Polysilicon is a direct raw material for the production of monocrystalline silicon, and it is the basic electronic information material for contemporary semiconductor devices such as artificial intelligence, automatic control, information processing, and photoelectric conversion. It is called "the cornerstone of the microelectronics building." Industrial production methods Polysilicon production technology is mainly modified Siemens method and silane method. The Siemens process produces columnar polysilicon by vapor deposition. To improve raw material utilization and environmental friendliness, a closed-loop production process, the modified Siemens process, is used on the basis of the former. This process processes industrial silicon into SiHCI, and then SiHCl3 is reductively deposited in a reducing furnace in an H2 atmosphere to obtain polysilicon. The tail gas H2, SiHCl3 and HCl discharged from the reduction furnace are separated and recycled. The silane method is to pass silane into a fluidized bed with polycrystalline silicon seeds as the fluidized particles, so that the silane is cracked and deposited on the seed crystals to obtain granular polycrystalline silicon. The modified Siemens process and the silane process mainly produce electronic grade crystalline silicon, and solar grade polysilicon can also be produced.

Siemens method

The Siemens law was invented by the German company Siemens and applied for a patent in 1954. Industrialization was achieved around 1965. After several decades of application and exhibition, the Siemens method has been continuously improved. The first generation, second generation and third generation have appeared successively. The third generation polysilicon production process is the modified Siemens method, which was added on the basis of the second generation. Recovered tail gas dry recovery system, SiCl4 recovery hydrogenation process, to achieve a complete closed-loop production, Siemens production of high-tech polysilicon technology is the latest technology, the specific process shown in Figure 1. Silicon is recycled inside the Siemens process polysilicon production process.

Silane method

The silane method is a method in which silane is introduced into a fluidized bed in which polysilicon seed crystals are fluidized particles, and the silane is cracked and deposited on the seed crystals to obtain granular polycrystalline silicon. Different silane preparation methods include the magnesium silicide method invented by Komatsu in Japan. The specific procedures are shown in Fig. 2, the disproportionation method invented by Union Carbide in the United States, and the NaAlH4 and SiF4 reaction methods in U.S. MEMC.

Magnesium silicide method uses Mg2Si and NH C1 to react in ** to form silane. Due to the large amount of raw materials consumed, the cost is high, and the risk is high, this method has not been promoted. At present, only Japan Komatsu uses this method. The preparation of modern silanes uses a disproportionation method, which uses metallurgical grade silicon and SiC14 as raw materials to synthesize silanes. SiCl3 is firstly reacted with SiCl4, Si, and H2 to form SiHCl3. SiHCl3 is then disproportionated to produce SiH2Cl2, and finally SiH4 is produced by SiH2Cl2 to disproportionate it. 3SiCl4+Si+2H2=4SiHCl3, 2SiHC13=SiH2Cl2+ SiC14, 3SiH2C12=SiH4+2SiHC13. Since the conversion efficiency of each step above is relatively low, the material needs to be recycled many times, and the entire process must be repeatedly heated and cooled, resulting in relatively high energy consumption. After the purified silane is purified by distillation, it is thermally decomposed at 800°C, similar to the Siemens fixed bed reactor. The reaction is as follows: SiH4=Si+2H2.

Silane gas is a toxic flammable gas with a low boiling point, and the reaction equipment must be sealed. It should have safety measures such as fire prevention, antifreeze, and explosion proof. Silane is also known for its unique spontaneous combustion and explosiveness. Silane has a very wide spontaneous ignition range and strong combustion energy, which determines it is a high-risk gas. The application and promotion of silanes are largely limited due to their high-risk characteristics in engineering or experiments involving silanes. Improper design, operation, or management can cause serious accidents or even disasters. However, practice has shown that excessive fear and improper prevention do not provide the safety of silane application. Therefore, how to use silane safely and effectively has always been a matter of high concern for production lines and laboratories.

Compared with the Siemens process, the main advantages of silane thermal decomposition method are: silane is easier to purify, the silicon content is higher (87.5%, the decomposition rate is fast, the decomposition rate is as high as 99%), the decomposition temperature is low, and the generated polysilicon energy consumption Only 40 kW ·h/kg and high product purity. However, the disadvantages are also highlighted: silane is not only expensive to manufacture, but also flammable, explosive, and unsafe. There has been a serious explosion of silane plants in foreign countries. Therefore, in the industrial production, the application of the silane thermal decomposition method is not as good as the Siemens method. Although the modified Siemens method currently has the largest market share, it has the greatest operating risk because of its inherent technical weaknesses—low productivity, high energy consumption, high costs, large capital investment, and slow fund recovery. Only by introducing advanced technologies such as plasma enhancement and fluidized beds, and strengthening technological innovation can it be possible to increase market competitiveness. The advantage of the silane method is conducive to serving the chip industry. At present, its production safety has been gradually improved, and its production scale may rapidly expand, even replacing the modified Siemens method. Although the modified Siemens method is widely used, the silane method is promising.

Similar to the Siemens process, in order to reduce production costs, the fluidized bed technology has also been introduced into the thermal decomposition process of silanes, and the fluidized bed decomposition furnace can greatly increase the decomposition rate of SiH4 and the deposition rate of Si. However, the purity of the obtained product is not as good as that of the fixed-bed decomposer, but it can completely meet the quality requirements of solar grade silicon. In addition, the safety of silane still exists.

U.S. MEMC used fluidized bed technology to achieve mass production. It uses NaAlH4 and SiF4 as raw materials to prepare silane. The reaction formula is as follows: SiF4+NaAlH4=Sil4+4NaAlF4. The silane was purified and then decomposed in a fluidized bed decomposer. The reaction temperature was about 730°C to obtain granular polysilicon having a size of 1000 μm. The method has low energy consumption, and the decomposition power consumption of granular polysilicon production is about 12 kW·h/kg, which is about 1/10 of that of the modified Siemens method, and the primary conversion rate is as high as 98%. However, there is a large amount of dust in the micron scale in the product, and Granular polysilicon has a large surface area and is easily contaminated. The product contains high amounts of hydrogen and must be dehydrogenated.

Metallurgy

The preparation of solar grade polysilicon (SOG-Si) by metallurgical method refers to the use of metallurgical Grade Silicon (MG-Si) as raw material (98.5% to 99.5%). After the purification by metallurgy, a method for producing a polysilicon raw material for solar cells with a purity of 99.9999% or more is obtained. The metallurgical industry has the advantages of low cost, low energy consumption, high output rate, and low investment threshold for the solar photovoltaic power generation industry. By developing a new generation of energy-harvesting high vacuum metallurgy technology, the purity can reach over 6N, and In a few years, it gradually developed into a mainstream preparation technology for solar grade polysilicon.

Different metallurgical grade silicon contains different impurity elements, but the main impurities are basically the same, mainly including Al, Fe, Ti, C, P, B and other impurity elements. Moreover, some effective removal methods have also been studied for different impurities. Since Wacker's polysilicon material was made by casting method in 1975, the preparation of solar grade polysilicon by metallurgical method is considered as a production method that can effectively reduce the production cost and is specifically positioned in the solar multi-stage polysilicon, and can meet the rapid development demand of the photovoltaic industry. For different impurity properties, the technical route for preparing solar grade polysilicon is shown in Figure 3.

Utilization value

In solar energy utilization, monocrystalline silicon and polysilicon also play a huge role. Although from now on, to make solar power have a larger market, accepted by the majority of consumers, we must improve the photoelectric conversion efficiency of solar cells and reduce production costs. From the current international solar cell development process, we can see that its development trend is monocrystalline silicon, polysilicon, ribbon silicon, thin film materials (including microcrystalline silicon-based thin films, compound-based thin films, and dye thin films).

Industrial Development

From the point of view of industrialization, the center of gravity has been developed from single crystal to polycrystalline. The main reasons are as follows: [1] There are fewer and fewer head and tail materials available for solar cells; [2] For solar cells, square substrates are more Conveniently, the polysilicon obtained by the casting method and direct solidification method can directly obtain a square material; [3] The production process of polysilicon continues to progress, and a full-automatic casting furnace can produce silicon ingots of over 200 kg per production cycle (50 hours). The grain size reaches centimeters; [4] The research and development of monocrystalline silicon process in recent ten years is very fast, and the process is also applied to the production of polycrystalline silicon cells, such as selective etching of the emitter junction, back surface field, and corrosion of suede. , surface and body passivation, fine metal gate electrode, using screen printing technology can reduce the width of the gate electrode to 50 microns, height of 15 microns or more, rapid thermal annealing technology for polysilicon production can greatly shorten the process time, single The thermal process time can be completed within one minute, and the battery conversion efficiency of more than 14% is achieved on a 100 square centimeter polycrystalline silicon wafer using this process. According to reports, the efficiency of the current battery fabricated on a 50- to 60-micron polysilicon substrate exceeds 16%. The use of mechanical notching, screen printing technology on the 100 square cm polycrystalline efficiency of more than 17%, no mechanical groove in the same area efficiency of 16%, the use of buried grid structure, mechanical groove in the 130 square cm polycrystalline Battery efficiency reached 15.8%.

International polysilicon industry overview

At present, crystalline silicon materials (including polysilicon and monocrystalline silicon) are the most important photovoltaic materials, with a market share of over 90%, and they will continue to be the mainstream material for solar cells for quite a long time to come. The production technology of polysilicon materials has long been in the hands of 10 factories of 7 companies in 3 countries including the United States, Japan and Germany, forming a state of technological dominance and market monopoly.

The demand for polysilicon mainly comes from semiconductors and solar cells. According to different purity requirements, it is divided into electronic and solar energy grades. Among them, for the electronic grade polysilicon accounted for about 55%, solar grade polysilicon accounted for 45%, with the rapid development of the photovoltaic industry, solar cell demand for polysilicon growth rate is higher than the development of semiconductor polysilicon, is expected to 2008 solar polysilicon Demand will exceed electronic grade polysilicon.

In 1994, the total output of solar cells in the world was only 69MW, and in 2004 it was close to 1200MW, which increased by 17 times in just 10 years. Experts predict that the solar photovoltaic industry will exceed nuclear power in the first half of the 21st century as one of the most important basic energy sources.

It is reported that the US Department of Energy plans to accumulate 4,600 MW in cumulative capacity by 2010, Japan plans to reach 5,000 MW in 2010, and the EU plans to reach 6,900 MW. It is estimated that the cumulative installed capacity in the world in 2010 will be at least 18,000 MW. From the above analysis, the polysilicon for solar cells will be at least 30,000 tons in 2010. Table 2 shows the prediction of the global solar polysilicon process. According to foreign data analysis and reports, the world's polysilicon production in 2005 was 28,750 tons, of which the semiconductor grade was 20,250 tons, the solar grade was 8,500 tons, the semiconductor grade demand was about 19,000 tons, there was a slight surplus, and the solar grade demand was 15,000 tons. , Insufficient supply, from the beginning of 2006, the demand for both solar grade and semiconductor grade polysilicon has gaps, among which the solar grade production capacity gap is even greater.

According to the Japanese rare metal magazine reported on November 24th, 2005, the world’s demand for semiconductors and solar polysilicon was tight, mainly due to the rapid expansion of the solar market with Europe as the center. It is expected that the imbalance in supply of polysilicon in 2006 and 2007 will be intensified. The difference between the semiconductor grade and the solar grade in the polysilicon price will be gradually reduced or even eliminated. In 2005, the world's solar cell production will be about 1GW. If the 1MW polysilicon is used for 12 tons, the total required polysilicon is 12,000 tons. The world will be in 2005-2010. The average annual growth rate of solar cells is 25%. By 2010, the total global demand for semiconductors for solar cells will exceed 63,000 tons.

The world's major producers of polysilicon are Japan's Tokuyama, Mitsubishi, Sumitomo, Hemlock of America, Asimi, SGS, MEMC, and Wacker of Germany. Their annual production capacity is mostly over 1,000 tons, among which Tokuyama, Hemlock, and Wacker One company has the largest production scale, with an annual production capacity of 3000-5000 tons.

International polysilicon main technical features

(1) Coexistence of multiple production process routes, industrialization technology, and monopoly situation will not change. As the polysilicon production plants use different main and auxiliary raw materials, so the production process technology is different; then the corresponding polysilicon products, technical and economic indicators, product quality indicators, uses, product testing methods, process safety and other aspects are also different, have their own technical characteristics And technical secrets, in general, currently the main traditional techniques for polysilicon production in the world include: modified Siemens method, silane method, and fluidized bed method. Among them, the production capacity of polysilicon produced by the Siemens process will account for about 80% of the world's total production capacity. The situation of monopolization of industrialization technology in the short term will not change.

(2) The new generation of low-cost polysilicon process technology is unprecedentedly active. In addition to traditional processes (electronic grade and solar grade compatibility) and technology upgrades, several new process technologies have been developed specifically for the production of solar grade polysilicon, including: the low-cost process of improving the Siemens process; metallurgical extraction from metallic silicon High-purity silicon; high-purity SiO2 direct preparation; melt deposition method (VLD: Vaper to liquid deposition); reduction or thermal decomposition process; chlorine-free process technology, Al-Si solution low-temperature preparation of solar grade silicon; molten salt electrolysis method.