Tag Floating zone method

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.

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.