Tag Crucible

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.

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.

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