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
TM is the melting point; Ti, T2, and T3 are the temperatures at different positions on the crucible wall; TDF 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.