Tag Crystal growth

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.

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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