Category Preparation of crystalline silicon materials

Since the energy crisis occurred in the 1970s, people have begun to apply solar cells to general livelihood purposes. At present, in countries such as the United States, Japan and Israel, solar energy installations have been widely used and are moving towards the goal of commercialization. Among these countries, the United States established the world’s largest solar power plant in California in 1983, which can generate up to 16 MW of electricity. South Africa, Botswana, Namibia and other countries in southern Africa have also set up programs to encourage the installation of low-cost solar cell power systems in remote rural areas. The most active country in the implementation of solar power generation is Japan. In 1994, Japan implemented the subsidy and incentive method to promote the “mains parallel solar photovoltaic power system” of 3000 W per household. In the first year, the government subsidizes 49% of the funds, and the subsidy thereafter decreases year by year. “Mains parallel type solar photovoltaic power system” is when the sunshine is sufficient, the solar cell provides electricity to the load of the home, and if there is excess electricity, it will be stored separately. When the power generation is insufficient or does not generate electricity, the required power is provided by the power company. By 1996, 2,600 households in Japan had installed solar power generation systems, with a total installed capacity of 8 MW. A year later, there were 9,400 installations, with a total installed capacity of 32 MW. In recent years, due to the rising awareness of environmental protection and the government’s subsidy system, it is estimated that the demand for household solar cells in Japan will also increase rapidly. In terms of industry, the total output of solar cells in Japan in 1999 was 86 MW, and in 2000 it had increased to 120 MW, ranking first in the world for two consecutive years. Recently, many Japanese solar cell manufacturers, such as Sharp Corporation and Mitsubishi Heavy Industries, have expanded their production plants. In the United States, the “Million Roofs Solar Power” plan proposed by former President Clinton is planned to be completed before 2010. The construction of 1 million solar power generation systems has been completed. In addition to Japan and the United States, Detong also started in 1990. Begin to implement the home-based plan, each household’s installed capacity of solar power is 15 kW, and the government will subsidize 70% of the funds. By 1995. 2,250 households have installed solar energy systems, with a total installed capacity of 5.6 MW. In addition, the Dutch government expects that the total installed capacity of solar energy systems can reach 1,450 MW in 2020. Other countries, such as Switzerland, Norway and Australia, have also implemented plans to install thousands of solar cells each year. in Taiwan, China. At present, the main manufacturers of solar cells include Guanghua, Motech and Shilin Electric. Guanghua Development Technology Co., Ltd. has been mainly producing amorphous silicon solar cells since 1988. Mainly used in super-eliminating electronic products, such as sub-tables, calculators, etc.

In 1999, Moody Corporation began to set up a factory in the same district of Tainan Science and Industry. It mainly produces solar cells of polycrystalline silicon and monocrystalline silicon. Shihlin Electric has also sent a research and development team to the United States for training to learn the manufacturing and packaging technology of solar panels used by satellites. At the same time, after the successful launch of China Satellite 1 in 1999, it further invested in the research and development of solar cells for people’s livelihood. In addition, the Institute of Materials of Industrial Technology Research Institute has also successfully developed the manufacturing and packaging technology of solar cells, and transferred the technology to Moody Corporation and Shihlin Electric Company to promote the solar power generation business. In recent years, Taiwanese manufacturers have gradually become interested in investing in the solar cell business. The main reason is that in addition to the shortage of supply in the international market, another factor is that Taiwan has vigorously promoted solar cell power generation since 1999, and started to promote various incentives. As a result, the number of companies investing in this business has also increased significantly. At present, there are still some difficulties in the implementation of solar power generation in Taiwan. The main reason is that it is obviously much more convenient to apply for utility power compared to the application procedures for general utility power and solar power generation, and the installation of solar power generation must first be invested in a funds. Based on economic considerations, it is indeed difficult for the general public to accept. even so. Look at it from another angle. Taiwan has favorable conditions such as abundant sunshine, sound development of semiconductor and power electronics industries, and strong official promotion, plus possible tourism, crisis, and popularization of environmental awareness. The solar power generation business indeed has a very large space for development in Taiwan. I believe that as long as the manufacturing cost can be greatly reduced. It can occupy a place in the field of solar power generation in the world.

In addition, the development and use of solar energy. The inability to generate electricity at night is a major disadvantage of solar cells. But for this shortcoming, scientists use two ways to overcome. The first way is to convert sunlight energy during the day into other forms of energy for storage, such as batteries, flywheel devices, pumped storage power plants, etc., and release the stored energy at night. Another way is the “satellite solar power station” (SPSS) that the United States and Japan are working on Places, such as near the equator, launch satellites with solar cells or thermal power generation systems, and use artificial satellites to absorb solar energy in space to generate electricity. Due to the avoidance of factors such as day and night, temperature difference and climate, artificial satellites can continuously and stably receive solar energy, convert it into electrical energy, and then transmit it back to the earth in the form of microwaves. After being received by the earth’s microwave receiving station, it is converted back into electrical energy. delivered to various places. Currently. Because scientists continue to study. Coupled with the advancement of semiconductor industry technology, the efficiency of solar cells is gradually increasing, and the unit cost of power generation systems is also decreasing year by year. Therefore, as the efficiency of solar cells increases and the cost decreases, the use of solar cells will become more and more common.

The solar cell technology has been transferred from the space technology application in the 1950s to the general commercial use of people’s livelihood. With the reduction of cost and environmental protection considerations, the use of monocrystalline silicon solar cells has become more and more common. The main applications are as follows.

(1) Household power generation system 5 from 20W to tens of thousands of watts. Depends on demand.

Located in California, USA, it was built in 1983 and completed in 1986. It is a 6 MW PV power plant; there are more than 20 relatively small-scale PV systems. It has also been successively adopted by many power companies in the past 10 years, as an experimental auxiliary device or installed on residential roofs to provide household electricity. A 6-year PV experiment program of the New England Electric Power Company (NEES) selected some residential buildings to install 2.2 kW C 10 pieces of 220 W) PV photovoltaic panels. The result is an average saving of about 50% of summer electricity bills. And users respond well. The Sacramento Electric Power Company in California) installed and tried two 1000 kW PV systems from 1984 to 1986 in accordance with the requirements of local residents to pay attention to ambient air quality; since 1993, a large number of medium-scale PV systems have been installed, and the total installed capacity has now over 3.7 MW .

The application of solar photovoltaic power generation system is quite extensive, with different application occasions. The system architecture is also different. For example, systems used in remote areas without electricity are stand-alone systems. In areas where there is electricity, the utility power parallel system can be used. When the power generated by the solar power generation system is greater than the load power, the excess power can be sent back.

The independent solar power generation system mainly includes solar cell modules, charge controllers, batteries, converters and lighting loads. The solar cell first converts light energy into direct current, then charges the battery through the charge controller, and finally converts the direct current into alternating current through the converter to supply the lighting load. The solar power independent system has three possible operation modes: ① When the output power of the solar cell is greater than the load power, the excess power will be stored in the storage battery; otherwise. When the output power of the solar cell is less than the load power, the insufficient power will be provided by the battery. ② Add ATS between the converter and the load. There are two power sources for the load, one is the solar photovoltaic power generation system. One is the power system. When the power of the photovoltaic power generation system is sufficient to supply the load, the photovoltaic power generation system supplies the load power; when the power of the photovoltaic photovoltaic power generation system is insufficient, the ATS switches to the power system instead of the power system. This ensures that the load has an adequate source of power. ③ Integrate the charge controller with the converter.

(2) Traffic: electric vehicles, charging systems, road lighting systems and traffic signals.

(3) High-power solar power generation system.

(4) Agriculture: Power systems such as irrigation and pumping

(5) Telecommunications and communications: wireless power, wireless communications.

(6) Backup power: disaster recovery.

(7) Power supply for low-power commodities.

(8) Outdoor positioning monitoring system: electronic bus stop signs, billboards, etc.

The traditional energy sources that people mainly rely on today are limited. It is estimated that the remaining oil reserves are 1,033.8 billion barrels, which can be used for 43 years; the natural gas reserves are 146M, which can be used for 62 years; the coal reserves are 9,841.2 billion tons, which can be used for 230 years; the uranium reserves are 395 million. 10,000 tons can be used for 64 years. In addition, in recent years, as the issue of global warming has been paid attention to by countries all over the world, major countries in the world have actively developed clean renewable energy such as solar photovoltaic energy to replace fuel power generation in recent years, in order to alleviate the pollution problems caused by traditional power generation methods. Therefore, increasing the development and use of solar photovoltaic energy is an important direction for human life and survival. The solar photovoltaic industry is one of the most important energy technologies in the 21st century. Many companies have vigorously developed and promoted them. Many companies have also expanded their production capacity. In the past five years, the global solar cell production has grown at an average annual growth rate of more than 30%. , showing its unlimited development potential in the field of renewable clean energy. Taiwan has a complete semiconductor industry base and excellent conditions for the development of solar cells. Due to the promotion of policies and the vigorous development of the world market. At present, the solar cell industry has gradually emerged. Integrate related capital, technology, equipment and other manufacturers and research units, and cooperate with each other to develop competitive product technology. in order to enhance international competitiveness. To build a sustainable energy business.

What is Edge-defined film fed growth method？

The preparation methods of polysilicon include crucible descending method, casting method, heat exchange method and edge-defined film fed growth method.

Although the growth technology of monocrystalline and polycrystalline silicon, including the Czochralski method and the floating zone method, is quite mature, the complex growth system, the growth furnace price is too high, the chip is too thick, and the silicon block material is in the cutting and polishing process. , The material loss of more than 50% is its disadvantage. In addition, the diced silicon chip requires proper chemical corrosion to remove surface scratches, and a large amount of chemical solution will cause environmental pollution.

Compared with the traditional crystal growth method, the edge-defined film fed growth method has the following characteristics:

① The edge-defined film fed growth method uses the capillary principle to directly separate the silicon chip from the molten silicon, so the silicon chip does not need to be cut and polished, and the loss of raw materials is less than 20%.

② The crystal shape can be controlled by the geometry of the top of the mold.

③ Fast growth rate.

④ Easy to fill continuously.

In 1972, TFCiszek first proposed to grow solar silicon chips using the edge-defined film fed growth method. At that time, only one silicon chip could be grown at a time, with an area of ​​about 2.5cm × 2.5cm. The growth rate was 6 cm²/min. 1971 In 1999, HELaBelle used the edge-defined film fed growth method to grow a hollow column with a diameter of 0.95 cm, a wall thickness of 0.005 to 0.1 cm, and a length of 140 cm. The growth rate was 12cm/min. In 1990, JP Kalejs and others could grow 15 cm in diameter. Hollow column. After years of hard work, the current industrial edge-defined film fed growth method can grow 5-6m octahedral hollow silicon pillars, which are then cut into 12.5cm × 12.5cm wafers by laser. The chip thickness is about 300µm, and the growth rate is 150cm²/min. D. Garcia et al. proposed an improved edge-defined film fed growth method in 2001 to grow cylindrical hollow silicon crystals. The purpose is to reduce the defect density and excessive thermal stress; at the same time, the cylindrical edge-defined film fed growth method also allows the mold to rotate. Therefore, silicon chips can become thinner. The current silicon full-core column has a diameter of up to 50cm, a chip thickness of 100 to 150µm, and a growth rate of 1.5 to 3cm/min. The current research direction focuses on how to increase the size of the chip, reduce the thickness of the chip and improve the quality of the crystal.

Figure is a cross-sectional schematic diagram of the edge-limited flake crystal growth method, in which the mold is made of a material infiltrated with liquid silicon (such as graphite), and the bottom has a porous structure that leads to the top. The meniscus is a thin layer of liquid connected between the upper surface of the mold and the crystal interface. The mold is the core component of the entire system, which controls the shape of the crystal, the crystal interface and the transfer of heat and silicon from the crucible to the meniscus. The crystal growth process is briefly described as follows: heating makes the silicon raw material in the graphite crucible melt, and the liquid silicon enters the mold from the pores at the bottom of the mold under capillary action and rises to the top. Move the seed crystal down so that the bottom edge of the seed crystal is in contact with the molten silicon at the top of the mold. Under the action of surface tension, the crystalline silicon and the molten silicon will be connected together to form a curved surface. Lower the temperature of the system until the seed crystal can move freely, then increase the pulling speed while reducing the system temperature. Pull the seed crystal upward to pull out the molten silicon, and the pulled molten silicon will solidify at the solid-liquid interface to form silicon crystals. . At the same time, the molten silicon in the crucible will continuously rise to the top of the mold under capillary action to replenish the pulled molten silicon. In order to enable the continuous growth of silicon crystals, molten silicon can be continuously added to the crucible. During the crystal growth process, the system is placed in an inert gas or vacuum atmosphere to prevent silicon oxidation.

The silicon chip of the edge-defined film fed growth method is usually composed of many large columnar crystal grains that are parallel to the growth direction and penetrate the entire chip thickness, and the chip surface is in a (110) preferred orientation. The formation of this orientation has nothing to do with the seed crystal. Even if the single crystal silicon seed crystal starts to grow, during the growth process, external crystal grains will be produced at the contact point and quickly grow across the surface to form a multi-product structure. Since the size of the component particles is greater than the thickness of the silicon chip and the diffusion length of minority carriers, the efficiency of solar cells made of edge-limited flaky crystalline silicon chips is basically not affected by the grain boundaries.

The shape of the crystal grown by the edge-defined film fed growth method is determined by the meniscus. The pulling speed and system temperature can be changed to control the shape and position of the meniscus, so that it can continue to grow from simple columnar or filamentary to almost arbitrarily complex geometry Shape crystals. For the growth of silicon crystals by the edge-defined film fed growth method, it is mainly to control the thickness of the growth of the silicon chip, which is determined by the height and shape of the meniscus. The height and shape of the meniscus can be changed by controlling the temperature of the mold top, the pulling speed, the temperature gradient in the vertical direction of the system, and the height of the mold from the molten silicon, so that the thickness of the silicon chip can be controlled. The thickness of the silicon chip can be adjusted in the following ways.

(1) Increase the pulling speed when the mold top temperature is constant, the height of the meniscus is getting higher and higher, and the thickness of the silicon chip will become thinner and thinner, until a cavity appears, and then the growth is stopped.

(2) Reduce the pulling speed when the mold top temperature is low, the height of the meniscus will become lower and lower, and the thickness of the silicon chip will become thicker and thicker until the solid-liquid interface moves down to the top position of the mold. Growth is interrupted.

(3) When the pulling speed is constant, lower the mold top temperature, the meniscus height becomes lower and lower, and the thickness of the silicon chip will become thicker and thicker.

(4) Different heights from the top of the mold to the molten silicon will affect the curvature of the meniscus, which in turn affects the thickness of the growing silicon chip. When the pulling speed is relatively low, the height of the top of the mold from the molten silicon has a significant effect on the thickness of the silicon chip. When the pulling speed is constant, the higher the height of the top of the mold from the molten silicon, the greater the curvature of the meniscus, and the thinner the grown silicon chip.

(5) With the temperature gradient in the vertical direction of the system, the lower the height of the meniscus, the thicker the grown silicon chip. During crystal growth, choosing a suitable mold can prevent silicon from solidifying at the top of the mold. And when the pulling speed is very high, it is necessary to let the top temperature of the mold pass the ridge to absorb the heat that cannot be completely taken away by the crystal. The thickness of silicon chips is becoming thinner and thinner. This is because the reduction in the thickness of the silicon chip can improve the efficiency of the solar cell and can save the loss of silicon raw materials. At present, the main problem of reducing the thickness of EFG silicon chips is that as the growth of silicon chips becomes thinner, the thickness uniformity along the width direction will become worse, and bending deformation is likely to occur.

The growth of edge-limited flake crystals is a non-equilibrium process, which contains a variety of complex physical phenomena, such as the formation mechanism of the curved surface, the movement of the crystal interface and the interaction of crystal-liquid-gas. The geometry of the mold, the growth rate, the stability of the solid-liquid interface and the thermal field are important factors that affect the growth and crystal quality. Common defects are micro twins, grain boundaries, inclusions, dislocations, and stacking faults parallel to the growth direction. These defects can be observed by chemical corrosion and transmission electron microscopy. Different growth rates will have different defects. Too high a growth rate will not affect the crystal surface, but it is easy to cause high-angle grain boundaries.

The corrosion of the mold material is another main cause of crystal defects. The mold material is mainly porous carbon fiber. When the silicon chip grows, the mold is immersed in the molten silicon, and the silicon melt will penetrate into the black through the pores of the carbon fiber; At the same time, the carbon yuan ping will diffuse into the silicon melt in the opposite direction, and the graphite crucible and the molten silicon are infiltrated and corroded, resulting in the carbon content in the edge-limited flaky crystalline silicon chip almost at a saturation level. These diffusion processes will stop when the mold surface is completely covered by silicon carbide and the broken fiber holes are filled with silicon melt. When the silicon carbide particles adhere to the growth gate of the edge-limited flake crystals, it will cause defects such as double products and large-angle grain boundaries.

When growing silicon chips with the edge-defined film fed growth method, the segregation of impurities at the solid-liquid interface causes the concentration of impurities in the ficus liquid to accumulate, which in turn affects the quality of senior silicon chips.

What is the heat exchange method?

Schmid and Viechnicki proposed a method of growing sapphire in 1970, called the Schmid-Viechnicki method, compared with other crystal growth methods. The biggest difference in this method is the addition of a heat exchanger. Therefore, in 1972, this method was renamed the heat exchange method. In 1974, C.P.Khattak and F.Schmid applied for the first time to grow silicon crystals.

The growth method of the heat exchange method is to control the heating power so that the solid-liquid interface gradually moves upward from the bottom of the crucible. The crystallization process is as follows.

Place the aluminum crucible filled with silicon raw material above a small diameter heat exchanger, and the seed crystal is placed between the bottom of the crucible and the heat exchanger. During the process of melting and growing, the ammonia gas is continuously passed through the heat exchanger Inside to ensure that the seed crystal will not be melted. After the silicon raw material is melted into a liquid, it needs to stand still for a period of time to reach a stable heat balance. After that, the temperature of the heat exchanger is gradually reduced to start crystal growth, and the temperature of the heat exchanger and the furnace body needs to be reduced at the same time during the final crystallization process. During the growth process, the heat exchanger always plays the role of controlling the temperature gradient. The growth atmosphere of the furnace body needs to be low oxygen and low carbon to prevent silicon crystals from being contaminated by both, and the solidification method is directional solidification. After the crystallization is completed, the silicon crystal is still placed in the thermal field. At this time, the furnace temperature is adjusted to slightly lower than the solidification temperature and annealed to eliminate residual stress, reduce defect density and make the silicon crystal have better uniformity.

Figure 1 Silicon crystallization process in the heat exchange method

T_M is the melting point; T_i, T₂, and T₃ are the temperatures at different positions on the crucible wall; T_DF is the temperature of directional solidification

Figure 1 is a schematic diagram of the silicon crystallization process in the heat exchange method. The average temperature of molten silicon is 5~10°C higher than the melting point of silicon. Figure 1(a) is the crucible, silicon raw material and seed crystal; increase the temperature to melt the silicon raw material It becomes liquid, as shown in Figure 1(b); part of the seed crystal is melted and crystallized from there, as shown in Figure 1(b)~(d); The crystal gradually covers the bottom of the crucible, as shown in Figure 1(e) As shown; the solid-coated interface expands to the liquid surface in a nearly ellipsoidal manner and completes the crystallization, as shown in Figure 1(f)~(h). Figure 1(c) is the most critical part of the entire crystallization process. The temperature of the molten silicon and the seed crystal must be measured carefully to ensure that the seed crystal does not melt.

The ammonia flow in the heat exchanger is related to the following factors.
①The size and shape of the furnace body;
②The size and shape of the crucible and the wall thickness of the crucible;
③The relative position of the heat exchanger and the crucible;
④The temperature of molten silicon, etc., and the appropriate ammonia flow rate must be obtained by repeated experiments.

The growth environment of the heat exchange method is close to isothermal. During the process of heating and melting, the temperature gradient of the silicon raw material does not change significantly; during the growth process, the slight temperature gradient change is controlled by the inflow heat exchange The nitrogen flow rate of the reactor, the crystallization process starts from the seed crystal at the bottom of the crucible,

The solid-liquid interface gradually moves to the crucible wall and the top of the crucible. The hot spot is located at the top of the crucible and the cold spot is located at the bottom of the crucible. The stable temperature gradient and small natural convection in the molten silicon are the characteristics of HEM. Since most of the time the silicon crystals are located below the liquid surface, the crystals can be prevented from being affected by mechanical and temperature fluctuations. The stability of the solid-liquid interface is extremely high, so neither the crystal nor the Yutong need to be rotated.

Compared with the general crystal growth method, during the heat exchange method growth process, the positions of the crucible, the crystal and the thermal field remain fixed, and there is no need for specially designed temperature gradients or different heating zones to drive the crystal growth. Its growth is mainly driven by fine-tuning the ammonia flow of the heat exchanger and the temperature of the furnace itself. Most of the heat energy generated by the crucible and crystals can be taken away by the heat exchanger.

Generally speaking, the cycle from feeding to completion of growth is about 50h. Polysilicon grown by the heat exchange method has the following characteristics: better uniformity, small grain boundaries (only up to centimeters), low oxygen pollution, vertical columnar grain boundary growth, and narrower resistance value range, etc. . At present, the solar polysilicon with the highest conversion efficiency (18.6 %, 1 cm 2 area) developed in the laboratory is grown by the heat exchange method. It can grow silicon cubes up to 200kg in length, width and height of about 60 cm each.

Figure 2 shows the heat exchange equipment and furnace structure used by Crystal System Inc. in the United States. It can grow silicon cubes weighing 200kg and each having a length, width, and height of about 60 cm. Swiss Wafer AG in Switzerland and GT Solar Technologies in the United States also use similar growth methods.

The main difference between the crucible descending method and the casting method is that the melting and crystallization of the raw materials of the former are carried out in the same crucible; the melting process of the latter is carried out in the first crucible, and the crystallization is carried out in the second crucible, as shown in Figure 1. Show. The silicon raw material is melted in the first uncoated quartz crucible, and then the molten silicon is poured into the second quartz crucible coated with silicon nitride (Si 3 N 4) film on the inner wall to crystallize the molten silicon. In addition, the crucible descending method is to pass the crucible containing molten silicon through the hot zone of the heating coil to crystallize; while the casting method is to control the temperature by adjusting the power of the heating wire. During the crystallization process, the crucible itself does not move. The liquid interface is buried under the melt, so the influence of temperature fluctuations and mechanical instability can be minimized. These two solidification methods from liquid to solid are called directional solidification (directional solidification), which is easy to cause columnar crystal growth. Therefore, chips cut from the same polysilicon crystal block will Have the same defect structure, such as grain boundaries and dislocations.

Figure 1 Polysilicon is manufactured by casting method. The silicon raw material is melted in the first quartz crucible, and then the molten silicon is poured into the second quartz crucible coated with silicon nitride (Si3N4) film on the inner wall. Compared with the crucible descending method, the casting method has shorter crystallization and cooling time.

Figure 2 is a schematic diagram of the front and cross-section of the molten silicon crystallization process in the casting method. Adjusting the power of the heating coil can change the temperature gradient of the molten silicon. The bottom gradually solidifies upwards; then the grain boundaries will grow laterally in a dendritic crystalline manner (lateral growth), forming a polysilicon layer (see Figure 2(b)). Eventually, the product grain boundaries will gradually grow from the polysilicon film to the liquid surface, forming polysilicon bulk materials, and the dendritic crystals will also become a wide range of grain boundaries, as shown in Figure 2(c).

At present, Deutsche Solar GmbH in Germany and Куocerа in Japan both use the casting method to grow polycrystalline silicon bulk materials, the mass can reach 250~400 kg, and the length, width and height are about 70 cm × 70 cm × 30 cm. German Deutsche Solar GmbH uses the casting method to grow Polycrystalline silicon bulk material, from 1997 to 2004, the mass of the bulk material has increased from 180 kg to 330 kg. Compared with the traditional method, the new type of casting method developed by Deutsche Solar GmbH can save about 30% of the growth time, and the mass of polysilicon bulk material can reach 400kg.

What are the characteristics of the crucible descending method?

The advantage of polysilicon is that it is cheap, and the bulk shape is mostly cube or cuboid, while monocrystalline silicon is mainly round or nearly square. Therefore, for general manufacturing, polycrystalline silicon has a better material usage rate than monocrystalline silicon. The disadvantage is that the conversion efficiency is slightly lower than that of monocrystalline silicon. The commonly used polycrystalline silicon bulk growth methods include: crucible descending method, casting method, heat exchange method and edge-limited flake crystal growth method. The following mainly introduces the characteristics of the crucible descending method.

The crucible descending method is also called the Bridgman-Stockbarger method. The characteristic of this method is to let the melt cool and solidify in the crucible. The solidification process starts from one end of the crucible and gradually expands to the entire melt. The crucible can be placed vertically or horizontally. The solidification process is completed through the solid-liquid interface. The interface can be moved by moving the crucible or moving the heating coil. The crucible descending method can be used to grow optical crystals (such as LiF, MgF₂, CaF₂, etc.), scintillation crystals (such as NaI(TI), Bi₄Ge₃O₁₂, BaF₂, etc.), laser crystals (such as Ni ^2- :MgF₂ , V²⁺: MgF₂ etc.).

Since 2004, companies have begun to use the crucible descending method to grow polycrystalline silicon bulk materials, with a growth rate of up to 10 kg/h. The growth method is as follows: the silicon raw material is placed in a quartz crucible, and the inner wall of the crucible is coated with a layer of nitride For the silicon (Si₃N₄) film, the melting and crystallization of the raw materials are carried out in the crucible. The main purpose of plating silicon nitride film is to prevent polysilicon from sticking to the quartz crucible during the crystallization process, causing the crucible or silicon crystal to crack. Increase the temperature to melt the silicon raw material, and gradually move the crucible down so that the bottom of the crucible passes through the area of ​​higher temperature gradient. The whole crystallization process will begin to crystallize from the bottom of the crucible and gradually extend upward. The solid-liquid interface will gradually move upward as the crucible position drops to complete crystallization. Graphite resistance heating is generally used, and the insulation system is graphite tube and molybdenum tube.

Figure 1.1 The crucible descending method, the melting and crystallization process simultaneously exist in a quartz crucible coated with silicon nitride (Si₃N₄) film on the inner wall. The crystallization process is to slowly move the molten silicon and crucible below the heating coil to leave the coil. The crystallization process is The report is complete.

Figure 1.2 The relationship between the temperature of the crucible descending method and the crucible moving position, the crystal crystallization process occurs in the region of the temperature gradient in the figure

The silicon crystal grown by the crucible descending method needs to have an appropriate thermal field, including the resistance heater, insulation material, the size of the crucible and the relative position of the heater, adjust these factors to make the temperature gradient of the melting zone smaller, and the crystallization zone The temperature gradient becomes larger. The shape and position of the solid-liquid interface of the crucible descending method are closely related to the integrity and defects of the crystal. Generally speaking, the solid-liquid interface can be divided into three shapes: convex, flat, and concave. From the perspective of reducing dislocations and other defects and avoiding internal stress, the flat interface is the most ideal situation, but in the actual crystal growth process Middle; The concave interface will cause crystals to grow from the edge of the crucible to the center, easy to form polycrystalline, and impurities and bubbles will form inclusions. Therefore, a convex solid-liquid interface is usually maintained, but the convex interface is likely to cause uneven radial temperature distribution and generate internal stress. During the crystal growth process, as the crucible gradually drops, the part in the high temperature zone decreases, and the part in the low temperature zone gradually increases, which will cause the solid-liquid interface to move to the high temperature zone, and the temperature gradient of the interface will become smaller. At this time, it is easy to appear that the crystallization speed is greater than the crucible falling speed, causing bubbles or inclusions inside the crystal. Generally, it can be solved by increasing the control temperature or reducing the crucible falling speed.

Overall, the crucible descending method has these characteristics.

Advantages of the crucible descending method :

(1) The shape of the crystal can be controlled by the shape of the crucible.

(2) The growth direction of the crystal can be determined by the seed crystal, if there is no seed crystal. The crystal will grow along the optimal direction (preference).

(3) Because the growth environment is closed or semi-closed, the volatilization of melt and dopants can be avoided.

(4) The molten silicon starts to crystallize from the crucible wall, which can prevent the molten silicon from being further contaminated by the quartz crucible.

(5) The operation is simple, and large-size crystals can be grown. A single growth furnace can grow several crystals at the same time, which is suitable for industrial mass production.

Disadvantages of the crucible descending method:

(l) The process of crystal growth takes place inside the crucible, so it cannot be observed.

(2) The high growth rate easily makes the temperature gradient of the crystal too large, causing the crystal to break.

(3) The thermal stress of silicon crystal rods is relatively large, which leads to high dislocation density and uneven grain boundary distribution.

(4) The inner wall of the crucible must be specially coated to prevent the crystal from sticking to the crucible, causing the crucible or crystal to break.

(5) During the crystallization process, internal stress is easily introduced into the crystal from the crucible, so the thermal expansion coefficient of the crucible material should be smaller than that of the crystal, and the inner wall of the crucible must be very smooth to prevent stress.

(6) During the growth process, the crystal does not rotate, so the uniformity of the crystal, especially the doped silicon crystal, is worse than that of the Czochralski method.

Basic knowledge of semiconductor solar cells,You can learn more.

Silicon is currently the most widely used solar cell material, including single crystal silicon (sc-Si) and polycrystal silicon (polycrystal silicon), with a total solar market share of more than 95%. In the early days, solar cells mainly used Czochralski pulling technique (CZochralski pulling technique, CZ) to grow silicon crystals, but due to market price factors, more and more companies invested in the growth of large-scale polysiliconingot (polysiliconingot).

1.1 Growth of single crystal silicon

The growth methods of monocrystalline silicon mainly include the Czochralski method and the floating zone technique (FZ). In 1950, Teal and Little first applied the Czochralski method to the growth of silicon (Si) and germanium (Ge) single crystals. At present, about 80% of silicon single crystals are grown by the Czochralski method. The single crystal size can reach 12in. In the Czochralski method, because the molten silicon is in direct contact with the crucible and produces a chemical reaction, the silicon single crystal has serious oxygen and carbon pollution problems. Keck and Golay proposed the floating zone method in 1953 to grow silicon single crystals without oxygen and low metal pollution, which are mainly used in high-power transistor devices. However, due to the high cost of the floating zone method, the Czochralski method is still the main method for growing solar-grade silicon single crystals.

1.1.1 Chai-style lifting method

The Czochralski pulling technique was accidentally invented by Professor Jan Czochralski in 1916, originally to study the crystallization rate of metals such as tin, zinc and lead in solid-liquid contact. After the Czochralski method was invented, it was gradually forgotten. Until the end of the Second World War, the semiconductor industry was booming, which made the importance of semiconductor materials such as silicon and germanium increase. In 1950, Teal and Little of Bell Laboratories first applied the Czochralski method to grow silicon and germanium crystals, and obtained high-quality single crystals. In 1958, Dash proposed the use of necking technique to grow silicon single crystals with low dislocation density. At present, the size of silicon ingots has increased from 1in in the 1950s to 12in today. Dr. Lin Mingxian from Taiwan Sino-German Electronics Co., Ltd. once edited the book “Silicon Wafer Semiconductor Material Technology”, which is about the technology of silicon single crystal growth. Including the Czochralski method and the floating zone method, the growth defects of silicon crystals and the processing technology are all introduced in detail. It is a good reference book for those who are engaged in semiconductor devices and solar cell materials. In addition, the book “Crystal Growth Science and Technology” edited by Zhang Kecong and Zhang Leping has a very detailed explanation of the theory of melt growth and the thermodynamics involved.

Figure 1.1 Czochralski pulling technique

Figure 1.1 is a schematic diagram of the Czochralski method. The growth process is briefly described as follows: First, the polysilicon raw material is placed in a quartz crucible, and the crucible is placed in a graphite thermal insulation field; vacuum is drawn from the crystal growth furnace and a certain pressure range is maintained Use graphite resistance heater to melt silicon raw material into liquid (melting point is 1420℃), adjust the temperature so that the center of molten silicon becomes the cooling point in the whole thermal field. When the temperature of the molten silicon stabilizes, the positioned (100) or (111) direction seed crystal (seed) is gradually lowered until it contacts the surface of the molten silicon. Due to the seed crystal itself and the thermal stress when the seed crystal contacts the molten silicon Dislocation (dislocation), at this time, the temperature must be slightly increased to melt part of the seed crystal. At the same time, the seed crystal is rotated on one side and pulled up quickly at the same time, using the crystal neck technology
(Dashing technique or necking technique), to pull out the seed crystal with a smaller diameter (3 ~ 6mm) than the original seed crystal and low defect density. As long as the crystal neck is long enough, dislocations can be smoothly discharged from the crystal surface. After the crystal neck process is over, the pulling speed and temperature need to be reduced to gradually increase the crystal diameter to the required diameter. This step is called shoulder growth or crown growth. After the shoulders are placed, the cylindrical body of equal diameter is grown by adjusting the pulling speed and temperature. The most important work in this part is the diameter control. Finally, when the crystal grows to an appropriate length, the temperature must be increased or the pulling speed must be increased to gradually reduce the diameter of the crystal rod until it is completely separated from the liquid surface. Then the ingot is cooled for a period of time and then taken out to complete a life cycle.

The preparation equipment of the Czochralski method can be roughly divided into four parts.
(1) Furnace body. The water-cooled stainless steel furnace body can generally be divided into an upper chamber and a lower chamber.The upper furnace chamber is where the silicon crystal rods are cooled, and the lower furnace chamber is the place where crystals grow.

(2) Hot field. Including quartz crucible, graphite crucible (supporting quartz crucible), graphite resistance heater and thermal insulation materials. The problem with the quartz crucible is that it will react with molten silicon at high temperatures, causing oxygen contamination of the silicon crystal rod; the graphite crucible is used to fix the quartz crucible to prevent its softening and deformation. The thermal field, also known as the thermal gradient, can generally be divided into external thermal gradient and internal thermal gradient, the after-heater in the furnace or the radiation shield ) Belongs to the external temperature gradient, while the temperature distribution in the crystal and molten soup belongs to the internal temperature gradient. Because the heating method is resistance heating, heat energy is provided to the quartz crucible and the surrounding insulation materials at the same time, the heating effect on the crucible is more uniform, and it is easy to produce a small temperature gradient, and the position of the crucible has little effect on the temperature gradient.

(3) Ar atmosphere and pressure control system. Because the quartz crucible reacts with molten silicon to produce silicon monoxide (SiO), the reaction of SiO and graphite devices will produce carbon monoxide (CO), and Ar is to take away both SiO and CO gases.
(4) Growth control system. The control parameters can include the diameter of the crystal rod, the pulling speed, the rotation speed and the temperature. Generally, the change in the shape of the crystal aperture (meniscus) is used to adjust the temperature or the pulling speed to control the diameter of the crystal rod. The crucible or silicon crystal rotates at the same time, and its purpose is to cause forced convection in the molten silicon, make the dopant concentration uniform, and make the temperature distribution in the furnace more symmetrical. Generally speaking, when making a crystal growth furnace, the furnace body itself will always have slight asymmetries. These asymmetries will cause excessive facet growth, striation, and difficult crystal growth. Controlling equal diameter growth and other shortcomings, rotating crystals and crucibles can effectively reduce these effects.

It is necessary to rely on the matching of the above four parts to grow silicon single crystals with low defect density.

In addition to defects such as dislocation, vacancy and stacking fault in silicon crystals grown by the Czochralski method, the most important defects are non-metallic impurities such as oxygen and carbon. Pollution. The quartz crucible reacts with molten silicon to form SiO2, which will affect the resistance value, conversion efficiency and carrier lifetime of silicon. But when the oxygen concentration reaches a certain level, it will enhance the mechanical strength of the silicon crystal. Because the Si-O bond in SiO2 vibrates and has a strong absorption at the infrared wavelength of 900nm, the oxygen content in the silicon crystal can be measured with an infrared spectrometer. The carbon in the silicon crystal is reacted by SiO and graphite heat insulation material to generate CO and merge into the molten silicon. The carbon content accelerates the deposition of oxygen, which in turn causes microscopic defects. In order to reduce carbon pollution, the gap between the quartz crucible and the graphite can be minimized to make the CO concentration at the contact point higher and prevent the quartz crucible and graphite from continuing to react. In addition, the flow rate of Ar gas can be adjusted to take away the generated CO gas.

The advantage of the Czochralski method is that it is easier to grow large-size and high-doped silicon single crystals, but for the application of solar cells, the most important factor is the price. The price of a single chip can be reduced to the minimum by increasing the crystal size, continuous feeding, and improving the cutting, grinding, and polishing process to minimize the loss of silicon raw materials, so that it is possible to reduce the price of silicon chips.

1.1.2 Floating zone method
The floating zone method was proposed by Keck and Golay in 1953, and Theuerer applied it to the growth of high-purity silicon single crystals. The biggest advantage of the FZ method is that no crucible is needed. In the CZ pulling method, molten silicon inevitably comes into contact with the crucible and reacts. Therefore, only a few substances can be used as crucible materials, such as quartz (SiO2), Si3N4, silicon carbide (SiC), graphite (graphite), and so on. Even so, the carbon, oxygen and other metal impurities in the crucible will still flow into the molten silicon, causing pollution to the silicon single crystal.

Figure 1.2 Floating zone method

The growth device of the floating zone method is shown in Figure 1.2. The polycrystalline silicon raw material rod is fixed above the high frequency coil (RFcoils), and the silicon single crystal seed crystal is placed under the polycrystalline silicon raw material rod. When the polysilicon raw material rod is heated by the coil, the bottom will begin to melt. At this time, the raw material rod is lowered, so that the molten area is attached to the seed crystal, forming a solid-liquid phase equilibrium, and the molten area is supported by surface tension. Then, the seed crystal and the raw material rod are rotated in opposite directions to make the distribution of impurities in the molten zone uniform. The melting zone moves from top to bottom, or from bottom to top, so that the polysilicon raw material rod can completely pass through the heating coil to complete the crystallization process. In the floating zone method, the stability of the melt zone is maintained by the balance of surface tension and gravity. The advantage of the floating zone method is that it can grow high-purity and defect-free silicon single crystals. The content of oxygen, carbon and other transition metals can be less than 1011cm-3, and its resistance can easily reach 300Ω·m, which is suitable for high-efficiency solar materials. Although the defect density of the silicon crystal grown by the floating zone method is low, its lifetime is only 0.5ms, which is still far below the theoretical value. The main reason is that the high-purity silicon crystal has many microscopic defects caused by the growth and cooling process. Another disadvantage of the floating zone method is that because the melting zone is only at the top of the crystal rod, only crystals with a smaller diameter can be grown.