Tag Molten silicon

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.