阅读说明：本技术 一种铸造单晶的热场结构及其方法 (Thermal field structure and method for casting single crystal ) 是由 史珺 于 2020-11-25 设计创作，主要内容包括：本发明公开了一种铸造单晶的热场结构及其方法。它包括保温体、加热体、热开关、石墨均热平台、坩埚本体、坩埚托板、坩埚护板、水冷盘和支架,保温体置于支架的上方,保温体的底部中间设有通孔,加热体置于保温体的内部,热开关置于支架的下方且与保温体上的通孔相对应,石墨均热平台、坩埚本体、坩埚托板和坩埚护板置于加热体的内部,坩埚托板置于石墨均热平台置于坩埚本体之间,坩埚护板置于坩埚本体的外侧面上,水冷盘置于热开关的下方,热开关的底部设有升降杆。本发明的有益效果是：铸锭单晶能够达到100%的单晶率,提高单晶的晶体质量,提高晶体的可切片率,缩短晶体冷却时间,节省工艺时间,提高设备产能。(The invention discloses a thermal field structure for casting single crystal and a method thereof. It includes the heat retainer, the heating member, the hot switch, graphite soaking platform, the crucible body, the crucible layer board, the crucible backplate, water-cooling dish and support, the top of support is arranged in to the heat retainer, be equipped with the through-hole in the middle of the bottom of heat retainer, the inside of heat retainer is arranged in to the heating member, the hot switch is arranged in the below of support and is corresponding with the through-hole on the heat retainer, graphite soaking platform, the crucible body, the inside of heating member is arranged in to crucible layer board and crucible backplate, graphite soaking platform is arranged in to the crucible layer board between the crucible body, the crucible backplate is arranged in on the lateral surface of crucible body, the below of hot switch is arranged in to the water-cooling dish, the. The invention has the beneficial effects that: the ingot casting single crystal can reach 100 percent of single crystal rate, the crystal quality of the single crystal is improved, the slicing rate of the crystal is improved, the cooling time of the crystal is shortened, the process time is saved, and the equipment productivity is improved.)

1. The utility model provides a thermal field structure of casting single crystal, characterized by, includes insulator, heating member, thermal switch (13), graphite soaking platform (3), crucible body (4), crucible layer board (9), crucible backplate (8), water-cooling dish (14) and support (2), the insulator arrange the top of support (2) in, be equipped with through-hole (17) in the middle of the bottom of insulator, the heating member arrange the inside of insulator in, thermal switch (13) arrange the below of support (2) in and corresponding with through-hole (17) on the insulator, graphite soaking platform (3), crucible body (4), crucible layer board (9) and crucible backplate (8) arrange the inside of heating member in, crucible layer board (9) arrange graphite soaking platform (3) in between crucible body (4), crucible backplate (8) arrange in on the lateral surface of crucible body (4), the water cooling disc (14) is arranged below the thermal switch (13), and a lifting rod (12) is arranged at the bottom of the thermal switch (13).

2. The thermal field structure of claim 1, wherein the thermal insulator comprises a top thermal insulator (6), a side thermal insulator (5) and a bottom thermal insulator (10), the through hole (17) is disposed at a middle position of the bottom thermal insulator (10), the side thermal insulator (5) is disposed between the top thermal insulator (6) and the bottom thermal insulator (10), the interior of the thermal insulator is carbon fiber felt, and the exterior of the thermal insulator is a heat-resistant metal frame.

3. The thermal field structure for casting single crystal according to claim 2, wherein the heating body comprises a top heating body (7) with adjustable power and a bottom heating body (11) with adjustable power, the top heating body (7) is arranged between the top heat insulator (6) and the crucible body (4), the bottom heating body (11) is arranged between the bottom heat insulator (10) and the graphite soaking platform (3), and the cross section of the top heating body (7) is U-shaped.

4. The thermal field structure for casting a single crystal according to claim 3, wherein the top heater (7) comprises a planar heater (15) and a side heater (16), the side heater (16) is disposed at the edge of the planar heater (15) and outside the crucible protective plate (8), and the minimum distance between the side heater (16) and the bottom of the crucible body (4) is 2/3 of the height of the crucible body (4).

5. The thermal field structure for casting the single crystal according to claim 1, wherein the area of the thermal switch (13) is larger than or equal to that of the through hole (17), the upper part of the thermal switch (13) is made of carbon fiber felt, the bottom of the thermal switch (13) is a heat-resistant metal tray, the lifting rod (12) is arranged in the middle of the bottom of the thermal switch (13), one end of the lifting rod (12) is connected with the thermal switch (13), and the other end of the lifting rod (12) penetrates through the water cooling disc (14) and then is connected with a lifting motor.

6. The thermal field structure for casting the single crystal according to claim 5, wherein guide rods (1) are arranged on the left side and the right side of the bottom of the graphite soaking platform (3), guide holes matched with the guide rods (1) are formed in the thermal switch (13), the thermal switch (13) is connected with the guide rods (1) in a sliding mode through the guide holes, the guide rods (1) are installed on a support (2), and the water cooling disc (14) is arranged below the guide rods (1).

7. A method for casting single crystal is characterized by comprising the following steps:

(1) melting: the crucible body (4) is heated after being charged, when most of the silicon material is melted, the unmelted silicon material at the bottom continuously floats, and the melting of most of the silicon material is completed;

(2) inoculating seed crystals: after the silicon material is melted, properly increasing the power of a top heating body (7), increasing the top temperature, and simultaneously keeping the bottom temperature, so that the top of the seed crystal can be completely melted, and the lower part of the seed crystal is not completely melted;

(3) building a foundation: the lateral growth of the seed crystal when the seed crystal just starts to grow after the guiding is finished is carried out, the seed crystal grows along the direction from the bottom of the crucible body (4) to the peripheral crucible wall until the crucible bottom of the whole crucible is fully paved and the solid-liquid interface has the same size as the inner plane of the bottom of the crucible body (4);

(4) crystal upward growth: in the whole crystal growth process, proper temperature fields in the crystal and the silicon melt are kept, and the temperature fields are controlled to change along with time, so that various stresses in the crystal are smaller than critical stress required for dislocation nucleation, and no new dislocation growth is ensured in the ingot single crystal growth process;

(5) and (3) crystallization ending: when the crystal grows to be less than 50mm away from the surface of the silicon liquid, the crystal enters the ending stage of the crystallization, because the crystal is close to the top of the silicon liquid, the influence of the latent heat of the crystallization on the temperature rise of the top of the silicon liquid is large, the power of a heating body (7) at the top is controlled, and the silicon liquid layer at the top is ensured to be cooled after being less than 5-10 mm;

(6) and (3) annealing stage: after the crystallization is finished, annealing is carried out in a high-temperature area, the thermal stress generated by the high temperature and the low temperature in the crystal is slowly released, at the moment, the top heating body (7) is gradually closed, the bottom heating body (11) determines whether the power needs to be increased or not according to a bottom temperature rising curve, and the opened bottom thermal switch (13) is closed again at a certain speed;

(7) and (3) cooling: and after the annealing stage is finished, entering a cooling stage, descending the bottom thermal switch (13) to the bottom at the moment, and contacting with the water cooling disc (14) to enable the silicon ingot to be cooled more quickly so as to shorten the process time, wherein when the temperature is reduced to below 200 ℃, the whole crystal growth process of the ingot casting single crystal is finished.

8. A method of casting a single crystal according to claim 7, wherein in the step (3), at the start of setting, the temperature of the bottom of the crucible body (4) is lowered from the central region, so that the solid-liquid interface in the seed crystal is raised slowly, and when the solid-liquid interface is raised to be flush with the upper edge of the seed crystal pit at the bottom of the crucible body (4), the crystal starts to grow laterally.

9. A method of casting a single crystal according to claim 8, wherein in the step (3), since the bottom of the crucible body (4) is a slope with a low center and a high periphery, the temperature is also the case of the low center and the high periphery, when the crystal is grown in a lateral direction, the bottom of the crucible does not nucleate, only the single crystal grown from the seed crystal can grow, and finally the single crystal spreads the bottom while the solid-liquid interface is kept horizontal, forming a large single crystal of a square truncated pyramid shape with a large top and a small bottom.

10. A method for casting a single crystal according to claim 7, wherein in step (4), the temperature field is such as to keep the bottom of the crucible body (4) cool down at a constant rate, keeping the temperature at the top substantially constant, while the side heating bodies (16) suspended from the sides of the top heat insulator (6) maintain the temperature at the walls of the crucible.

Technical Field

The invention relates to the technical field related to ingot single crystal processing, in particular to a thermal field structure and a method for casting single crystals.

Background

With the development of the photovoltaic industry, the productivity of silicon crystal growth is rapidly developed. The crystal growth mainly comprises two modes of Czochralski single crystal and polycrystal ingot casting. Since 2018, the solar cell efficiency of monocrystalline silicon wafers has breakthrough progress, the czochralski silicon growing mode adopting the CZ method gradually occupies the mainstream, and polycrystalline silicon ingot manufacturers face a large amount of production halt. For this reason, many polysilicon ingot manufacturers attempt to perform single crystal growth by ingot, called ingot single crystal.

At present, manufacturers for ingot single crystal growth all adopt an ingot furnace and a quartz ceramic crucible which are the same as those of a polycrystalline silicon ingot, seed crystals are fully paved at the bottom, the size of the seed crystals is the same as or similar to that of a silicon wafer, and then silicon materials are put into the crucible. By bottom cooling, the crystal grows upward from the seed at the bottom. When the polycrystalline silicon ingot furnace is adopted for ingot single crystal production, the directional solidification mode of the polycrystalline silicon ingot furnace is basically not changed by the manufacturers. Or the bottom cooling is realized by adopting a mode of lifting a heat insulator or a mode of descending a crucible (which is gradually reduced). Although this approach can achieve bottom cooling, the periphery is often cooled first, and then the heat in the middle of the silicon melt or crystal is gradually cooled by dissipating heat to the bottom and periphery. The bottom cooling mode necessarily brings that the isothermal surface is concave to the melt, namely the temperature around the crucible is low and the temperature in the center is high. This will cause the following problems:

1) the periphery of the bottom of the crucible is easy to form nucleus by polycrystal and grows inwards.

2) Dislocations within the crystal will concentrate at the center of the crystal, causing the dislocation density in the middle of the ingot to increase substantially.

3) Polycrystalline nucleation can be continuously generated on the crucible wall, and the polycrystalline nucleation carried by the multi-seed crystal is combined, so that the polycrystalline silicon continuously grows inwards, the yield of the ingot single crystal is too low, and even the single crystal growth fails.

These three problems of these manufacturers have resulted in ingot single crystals having a large amount of polycrystals around and in the middle, which have long been called "single crystal-like" and have therefore not been able to be mass-produced due to low yield and slicing rate.

In order to enable the ingot casting single crystal to reach a perfect standard so as to be capable of resisting the crystal quality of the Czochralski single crystal, the thermal field of the traditional polycrystalline silicon ingot casting furnace must be upgraded.

There are two main types of conventional polysilicon ingot furnaces in the world, one is the furnace of GT corporation in the united states and the other is the furnace of ALD corporation in germany. These furnaces have fatal disadvantages in the casting of single crystals, resulting in the failure to achieve the minimum yield and single crystal rate. In short, in all these furnaces, when the growth starts after the completion of the melting, the temperature is lowered by raising the heating body and the heat retaining body, which inevitably results in a low temperature around the bottom of the crucible and a high temperature in the center. Thus, the isothermal surface is concave toward the melt, which not only causes easy polycrystalline nucleation at the edge of the silicon ingot, but also causes the polycrystalline silicon at the periphery to grow obliquely inward and upward, and causes several dislocations to be generated at the central single crystal portion, resulting in an increase in dislocation density. The german ALD furnace also has the problem that the temperature drop at the bottom suddenly leads to a temperature jump, thereby generating strain and causing dislocation.

Disclosure of Invention

The invention provides a thermal field structure for casting single crystals and a method thereof, which aim to overcome the defects in the prior art and improve the single crystal rate.

In order to achieve the purpose, the invention adopts the following technical scheme:

the utility model provides a thermal field structure of casting single crystal, includes insulator, heating member, heat switch, graphite soaking platform, crucible body, crucible layer board, crucible backplate, water-cooling dish and support, the insulator arrange the top of support in, be equipped with the through-hole in the middle of the bottom of insulator, the heating member arrange the inside of insulator in, the heat switch arrange the below of support in and corresponding with the through-hole on the insulator in, graphite soaking platform, crucible body, crucible layer board and crucible backplate arrange the inside of heating member in, the crucible layer board arrange graphite soaking platform in between the crucible body, the crucible backplate arrange the lateral surface of crucible body in on, the water-cooling dish arrange the below of heat switch in, the bottom of heat switch is equipped with the lifter.

The thermal field structure of the invention adopts a bottom center cooling mode during cooling while keeping a positive vertical temperature gradient, and simultaneously combines a graphite soaking platform to soak, so that a solid-liquid interface is always kept in a horizontal or slightly convex shape towards a melt in the whole crystal growth process; and then, by placing a single seed crystal in the groove in the center of the crucible body, the nucleation free energy coming out of the seed crystal is the lowest, and the nucleation free energy of the polycrystalline silicon at the bottom and the four walls of the crucible is higher than that required by the growth of the seed crystal, so that the nucleation of the polycrystalline silicon is stopped when the ingot is cast for single crystal, and the single crystal rate of 100 percent is realized. In conclusion, the thermal field structure of the invention enables the bottom temperature to be reduced from the center when ingot single crystal growth is carried out, so that the solid-liquid interface during crystal growth is planar or slightly convex to a melt, thereby eliminating the possibility of polycrystalline silicon nucleation during single crystal growth, enabling the ingot single crystal to reach 100% single crystal rate, improving the crystal quality of the single crystal and further improving the slicing rate of the crystal.

Preferably, the heat preservation body include top heat preservation body, side heat preservation body and bottom heat preservation body, the through-hole put in bottom heat preservation body's intermediate position department, side heat preservation body arrange in between top heat preservation body and the bottom heat preservation body, the inside of heat preservation body is the carbon fiber felt, the outside of heat preservation body is heat-resisting metal frame. The periphery of the thermal field is provided with a heat insulator which consists of a top heat insulator, a side heat insulator and a bottom heat insulator, the center of the bottom heat insulator is hollow and only has the periphery, the center of the bottom heat insulator is replaced by a thermal switch, and the thermal switch can move up and down; when melting, the thermal switch moves upwards to the top to completely seal the heat-insulating body, so as to realize the optimal heat-insulating effect; during crystallization, the thermal switch can descend to realize heat dissipation at the bottom of the crucible.

Preferably, the heating body comprises a top heating body with adjustable power and a bottom heating body with adjustable power, the top heating body is arranged between the top heat insulator and the crucible body, the bottom heating body is arranged between the bottom heat insulator and the graphite soaking platform, and the cross section of the top heating body is U-shaped.

Preferably, the top heating body comprises a plane heating body and a side heating body, the side heating body is arranged at the edge of the plane heating body and is arranged at the outer side of the crucible protection plate, and the minimum distance between the side heating body and the bottom of the crucible body is 2/3 of the height of the crucible body. The top heating body is not a conventional plane structure, but is in a 'crown shape', a drooping 'cap peak' is arranged at the periphery, the height of the cap peak is about 2/3 of the crucible, and the 'cap peak' is designed to provide a positive vertical temperature gradient with the same height as the inner part on the four walls of the crucible, so that the solid-liquid interface in the silicon liquid is kept horizontal or slightly convex to the melt during crystallization.

Preferably, the area of the thermal switch is larger than or equal to that of the through hole, the upper part of the thermal switch is made of carbon fiber felt, the bottom of the thermal switch is made of a heat-resistant metal tray, the lifting rod is arranged in the middle of the bottom of the thermal switch, one end of the lifting rod is connected with the thermal switch, and the other end of the lifting rod penetrates through the water cooling disc and then is connected with the lifting motor.

Preferably, guide rods are arranged on the left side and the right side of the bottom of the graphite soaking platform, guide holes matched with the guide rods are formed in the thermal switch, the thermal switch is connected with the guide rods in a sliding mode through the guide holes, the guide rods are installed on the support, and the water cooling disc is arranged below the guide rods. The graphite soaking platform is used for smoothing horizontal temperature distribution at the bottom of the crucible body, the crucible supporting plate is arranged above the graphite soaking platform, the quartz ceramic crucible body for bearing silicon materials is arranged on the crucible supporting plate, and the crucible body is provided with the crucible protecting plate at the periphery, so that the crucible is protected from being damaged by pressure when high-temperature silicon liquid is melted.

The invention also provides a method for casting the single crystal, which comprises the following steps:

(1) melting: heating up after charging in the crucible body, and when most of the silicon material is melted, the unmelted silicon material at the bottom continuously floats, which means that most of the silicon material is melted;

(2) inoculating seed crystals: after the silicon material is melted, properly increasing the power of a heating body at the top, increasing the temperature at the top, and simultaneously keeping the temperature at the bottom, so that the top of the seed crystal is completely melted, and the lower part of the seed crystal is not completely melted;

(3) building a foundation: the lateral growth of the seed crystal when the seed crystal just starts to grow after the guiding is finished is carried out, the seed crystal grows along the direction from the bottom of the crucible body to the peripheral crucible wall until the bottom of the whole crucible is fully paved and the solid-liquid interface has the same size as the inner plane of the bottom of the crucible body;

(4) crystal upward growth: in the whole crystal growth process, proper temperature fields in the crystal and the silicon melt are kept, and the temperature fields are controlled to change along with time, so that various stresses in the crystal are smaller than critical stress required for dislocation nucleation, and no new dislocation growth is ensured in the ingot single crystal growth process;

(5) and (3) crystallization ending: when the crystal grows to be less than 50mm away from the surface of the silicon liquid, entering the ending stage of crystallization, wherein because the crystal is close to the top of the silicon liquid, the influence of crystallization latent heat on the temperature rise of the top of the silicon liquid is large, controlling the power of a heating body on the top, and ensuring that the temperature is reduced after the silicon liquid layer on the top is less than 5-10 mm;

(6) and (3) annealing stage: after the crystallization is finished, annealing is carried out in a high-temperature area, the thermal stress generated by the high temperature and the low temperature in the crystal is slowly released, at the moment, the top heating body is gradually closed, the bottom heating body determines whether the power needs to be increased or not according to the bottom temperature rising curve, and the opened bottom thermal switch is closed again at a certain speed;

(7) and (3) cooling: and after the annealing stage is finished, entering a cooling stage, descending the bottom thermal switch to the bottom at the moment, contacting with the water-cooling disc to enable the silicon ingot to be cooled more quickly so as to shorten the process time, and finishing the crystal growth process of the whole ingot casting single crystal when the temperature is reduced to be below 200 ℃.

When the material starts to be melted, the bottom thermal switch ascends to the top to seal the bottom heat insulator, so that the optimal heat insulation effect is kept. When the melting is finished and the crystal growth process starts, the thermal switch starts to descend, the bottom of the crucible body starts to cool, and the seed crystal starts to grow. The bottom temperature gradually decreases with the gradual downward movement of the thermal switch. In the process of reducing the bottom temperature, the solid-liquid interface in the silicon melt is always kept slightly convex to the melt due to the power conditions of the graphite soaking platform and the top heating body, so that the optimal crystal growth effect is obtained. When the crystal growth is finished at the top, the bottom thermal switch can move up and down in coordination with the annealing process. After the annealing process is finished, the thermal switch descends to the bottom to be in contact with the bottom water-cooling disc, and the temperature of the upper surface of the thermal switch is reduced due to the bottom water-cooling disc, so that the crystal cooling time can be shortened, the process time can be saved, and the equipment capacity can be improved.

Preferably, in step (3), the temperature of the bottom of the crucible body is lowered from the central region at the start of setting, so that the solid-liquid interface in the seed crystal is gradually raised, and when the solid-liquid interface is raised to be flush with the upper edge of the seed crystal pit in the bottom of the crucible body, the crystal starts to grow in the lateral direction.

Preferably, in the step (3), since the bottom of the crucible body is a slope with a low middle part and a high periphery part, the temperature is also the case with a low middle part and a high periphery part, when the crystal is transversely grown, the bottom of the crucible does not nucleate, only the single crystal grown from the seed crystal can grow, and finally the single crystal spreads the bottom and maintains the solid-liquid interface to be horizontal, so that a large single crystal with a square truncated cone shape with a large top and a small bottom is formed.

Preferably, in step (4), the temperature field can keep the bottom of the crucible body uniformly cooled, the temperature of the top is kept substantially constant, and the side heating bodies hung on the side surfaces of the top heat insulating body can keep the temperature of the four walls of the crucible.

The invention has the beneficial effects that: the bottom temperature is reduced from the center when ingot single crystal growth is carried out, so that a solid-liquid interface during crystal growth is planar or slightly convex to a melt, the possibility of polycrystalline silicon nucleation during single crystal growth is eliminated, the ingot single crystal can reach 100% single crystal rate, the crystal quality of the single crystal is improved, the slicing rate of the crystal is improved, the crystal cooling time can be shortened, the process time is saved, and the equipment productivity is improved.

Drawings

FIG. 1 is a schematic structural diagram of a thermal field bottom thermal switch in an off state according to the present invention;

fig. 2 is a structural diagram of the thermal field bottom thermal switch in an open state according to the present invention.

In the figure: 1. the crucible heating device comprises a guide rod, 2 a support, 3 a graphite soaking platform, 4 a crucible body, 5 a side heat insulator, 6 a top heat insulator, 7 a top heating body, 8 a crucible protective plate, 9 a crucible supporting plate, 10 a bottom heat insulator, 11 a bottom heating body, 12 a lifting rod, 13 a thermal switch, 14 a water cooling disc, 15 a plane heating body, 16 a side heating body and 17 a through hole.

Detailed Description

The invention is further described with reference to the following figures and detailed description.

In the embodiment shown in fig. 1 and 2, a thermal field structure for casting single crystal comprises a heat insulator, a heating body, a thermal switch 13, a graphite soaking platform 3, a crucible body 4, a crucible supporting plate 9, a crucible protecting plate 8, a water cooling plate 14 and a support 2, wherein the heat insulator is arranged above the support 2, a through hole 17 is arranged in the middle of the bottom of the heat insulator, the heating body is arranged inside the heat insulator, the thermal switch 13 is arranged below the support 2 and corresponds to the through hole 17 on the heat insulator, the graphite soaking platform 3, the crucible body 4, the crucible supporting plate 9 and the crucible protecting plate 8 are arranged inside the heating body, the crucible supporting plate 9 is arranged between the graphite soaking platform 3 and the crucible body 4, the crucible protecting plate 8 is arranged on the outer side surface of the crucible body 4, the water cooling plate 14 is arranged below the thermal switch 13, and a lifting rod 12 is arranged at the bottom of.

The heat preservation body includes top heat preservation body 6, side heat preservation body 5 and bottom heat preservation body 10, and bottom heat preservation body 10's intermediate position department is arranged in to through-hole 17, and top heat preservation body 6 and bottom heat preservation body 10 are arranged in to side heat preservation body 5 between, and the inside of heat preservation body is the carbon fiber felt, and the outside of heat preservation body is heat-resisting metal frame. The area of the thermal switch 13 is larger than or equal to that of the through hole 17, the upper portion of the thermal switch 13 is made of carbon fiber felt, the bottom of the thermal switch 13 is a heat-resistant metal tray, the lifting rod 12 is arranged in the middle of the bottom of the thermal switch 13, one end of the lifting rod 12 is connected with the thermal switch 13, and the other end of the lifting rod 12 penetrates through the water cooling disc 14 and then is connected with the lifting motor. The left side and the right side of the bottom of the graphite soaking platform 3 are provided with guide rods 1, the thermal switch 13 is provided with guide holes matched with the guide rods 1, the thermal switch 13 is in sliding connection with the guide rods 1 through the guide holes, the guide rods 1 are installed on the support 2, and the water cooling disc 14 is arranged below the guide rods 1.

The heating body comprises a top heating body 7 with adjustable power and a bottom heating body 11 with adjustable power, the top heating body 7 is arranged between the top heat insulator 6 and the crucible body 4, the bottom heating body 11 is arranged between the bottom heat insulator 10 and the graphite soaking platform 3, and the cross section of the top heating body 7 is U-shaped. The top heating body 7 includes a planar heating body 15 and a side heating body 16, the side heating body 16 is disposed at the edge of the planar heating body 15 and outside the crucible protective plate 8, and the minimum distance between the side heating body 16 and the bottom of the crucible body 4 is 2/3 of the height of the crucible body 4.

If described in terms of hydrodynamics and thermodynamics, the ideal thermal field of a crystal growth system for ingot single crystals can be expressed as: in a temperature field, a vertical temperature gradient is positive, i.e., high and low, and in a melt, only a crystal growth driving force exists near a solid-liquid interface (i.e., a supercooling degree Δ T = T-Tm, where Tm is a melting point temperature of silicon), and crystallization is possible only if T < Tm, and crystallization is easier as the supercooling degree is more negative (the lower the T). The driving force of Δ T in the portion above the solid-liquid interface is positive, and the portion above the solid-liquid interface is not crystallized because Δ T is larger (higher temperature) as it moves upward in the melt away from the solid-liquid interface. In order to ensure good crystal growth and no crystal formation at the part except the seed crystal, the temperature field also meets the following requirements:

1) when the melt and the seed crystal are fused, a positive vertical temperature gradient is kept, so that the bottom of the seed crystal at the bottom of the crucible is lower than the melting point of silicon. Preserving the heat for a period of time at the temperature to eliminate the possible thermal stress in the seed crystal at the front end and prepare for the smooth fusion of the seed crystal; this process is referred to as a seeded "seeding" process.

2) When the bottom is at the beginning of crystallization, the solid-liquid interface of the silicon melt keeps slightly convex to the melt; guarantee like this that the regional temperature seed crystal region in crucible bottom is minimum, and peripheral higher makes crucible bottom unable crystal nucleus that forms polycrystalline silicon in advance, and the single crystal can only follow the seed crystal and grow out to grow and cover at the bottom of whole crucible all around along the slope of crucible bottom. We refer to this process as the "founding" process of ingot single crystals. This is a very important process because it can reduce the dislocation of the seed crystal by half.

3) The crystal then begins to grow vertically upward. In the whole process of crystal growth, the solid-liquid interface and all isothermal surfaces of the silicon melt are still slightly convex towards the melt, so that in case of polycrystalline nucleation caused by supercooling of the crucible wall for some reason, the polycrystal can only grow vertically or outwards along the crucible wall, cannot grow inwards, and cannot influence the crystal quality.

4) During the crystal growth period, the bottom temperature and the top temperature are controllable, and the temperature of the bottom and the top is controlled to ensure that the upward moving speed of the solid-liquid interface is controllable, so that the crystal can grow at a proper speed. In particular, the temperature cannot be lowered at the end of the final stage of crystal growth.

The temperature field meeting the above conditions can avoid the formation of polysilicon at the place outside the seed crystal to the maximum extent, even if the formation of polysilicon is near the crucible wall, the polysilicon will grow outwards and upwards, will leave the effective area of the silicon ingot, and will not affect the final single crystal quality. Therefore, a high-quality ingot single crystal can be grown.

The thermal field structure of the invention adopts a bottom center cooling mode during cooling while keeping a positive vertical temperature gradient, and simultaneously combines a graphite platform with a special structure to carry out soaking, so that a solid-liquid interface is always kept in a horizontal or slightly convex shape towards a melt in the whole process of crystal growth; and then, by placing a single seed crystal in the groove in the center of the crucible, the nucleation free energy coming out of the seed crystal is the lowest, and the nucleation free energy of the polycrystalline silicon at the bottom and the four walls of the crucible is higher than that required by the growth of the seed crystal, so that the nucleation of the polycrystalline silicon is stopped when the single crystal is cast, and the single crystal rate of 100 percent is realized.

The thermal field structure described in the invention skillfully achieves the purpose that the bottom of the crucible radiates heat from the center when crystals grow through the thermal switch 13, the water-cooling disc 14, the graphite soaking platform 3 for supporting the crucible body 4 and the movement mode of the thermal switch 13. When the bottom is cooled, the central area of the bottom of the crucible can be ensured to be cooler, and the peripheral temperature is higher, so that when crystals grow, the isothermal surface can keep a micro-convex shape towards a melt, and crystals can only start to grow from seed crystals positioned in the center of the crucible, so that nucleation of polycrystal at the bottom and the four walls of the crucible is avoided. In addition, the thermal field structure can realize the micro-convex and horizontal isothermal surface and solid-liquid interface in the whole crystal growth process, and provides an ideal temperature field for the crystal growth of ingot single crystals.

The invention also provides a method for casting the single crystal, which comprises the following steps:

(1) melting: the crucible body 4 is heated after being charged, when most of the silicon material is melted, the unmelted silicon material at the bottom continuously floats, and the melting of most of the silicon material is completed;

(2) inoculating seed crystals: after the silicon material is melted, properly increasing the power of the top heating body 7, increasing the top temperature, and simultaneously keeping the bottom temperature, so that the top of the seed crystal can be completely melted, and the lower part of the seed crystal is not completely melted; and finishing the seed crystal leading process.

(3) Building a foundation: the lateral growth of the seed crystal when the seed crystal just starts to grow after the guiding is finished is along the bottom of the crucible body 4 to the peripheral crucible wall direction until the crucible bottom of the whole crucible is fully paved and the solid-liquid interface has the same size with the inner plane of the bottom of the crucible body 4;

at the beginning of foundation building, the temperature of the bottom of the crucible body 4 is reduced from the central region, so that the solid-liquid interface in the seed crystal is slowly raised, and when the solid-liquid interface is raised to be flush with the upper edge of the seed crystal pit at the bottom of the crucible body 4, the crystal starts to grow transversely. Because the bottom of the crucible body 4 is a slope with low middle part and high periphery, the temperature is also the condition of low middle part and high periphery, when the crystal grows transversely, the bottom of the crucible can not be nucleated, only the single crystal grown from the seed crystal can grow, and finally the single crystal is fully paved with the bottom and the solid-liquid interface is kept horizontal, so that a large square-table-shaped single crystal with a large upper part and a small lower part is formed. When the special seed crystal direction is selected, the dislocation density of the ingot single crystal can be reduced to 50% of the seed crystal. When the seed crystal founding process enables the whole single crystal to be paved at the bottom of the crucible and grow to the peripheral crucible walls, the seed crystal founding period is finished, and the process of crystal upward growth is started.

The ingot single crystal is greatly different from the straight pulling single crystal, namely the straight pulling single crystal has a necking operation to reduce dislocation in seed crystal into zero dislocation again; the traditional ingot single crystal can not carry out the process, and the adopted mode is a multi-seed crystal plane laying mode according to the existing ingot growth mode of the similar single crystal, so that not only the primary dislocation of the seed crystal can not be eliminated at the stage, but also a large amount of new dislocations can be generated, the dislocation of the seed crystal can not be newly generated, and the dislocation of the seed crystal is reduced by about 50 percent.

(4) Crystal upward growth: in the whole crystal growth process, a proper temperature field (a horizontal slightly convex isothermal surface and a proper vertical temperature gradient) in the crystal and the silicon melt is kept, and the change of the temperature field along with time is controlled, so that various stresses in the crystal are smaller than critical stress required for generating dislocation nucleation, and therefore, no new dislocation growth is ensured in the ingot single crystal growth process, and the dislocation of the crystal is reduced to below 50% of that of a seed crystal;

according to the thermodynamic principles, the creation of new dislocations will increase the energy of the system, so that the system can withstand the varying impact of certain growth conditions while continuing to maintain a low dislocation state. Therefore, as long as the temperature field can keep the bottom of the crucible uniformly cooled, the temperature at the top is kept substantially constant, and the side-hung heating bodies of the top heat-insulating body 6 can keep the temperature of the four walls of the crucible.

(5) And (3) crystallization ending: when the crystal grows to be less than 50mm away from the surface of the silicon liquid, entering the ending stage of crystallization, strictly prohibiting the supercooling phenomenon of the top heating body 7, because the crystal is close to the top of the silicon liquid, the influence of crystallization latent heat on the temperature rise of the top of the silicon liquid is large, controlling the power of the top heating body 7, ensuring that the temperature is reduced after the silicon liquid layer at the top is less than 5-10 mm, and preventing the top of the silicon liquid from being solidified in advance;

(6) and (3) annealing stage: after the crystallization is finished, annealing is carried out in a high-temperature area, the thermal stress generated by the high temperature and the low temperature in the crystal is slowly released, at the moment, the top heating body 7 is gradually closed, the bottom heating body 11 determines whether the power needs to be increased or not according to the bottom temperature rising curve, and the opened bottom thermal switch 13 is closed again at a certain speed;

(7) and (3) cooling: and after the annealing stage is finished, the cooling stage is carried out, the bottom thermal switch 13 is descended to the bottom at the moment, and the silicon ingot is contacted with the water cooling disc 14 so that the temperature of the silicon ingot can be reduced more quickly, the process time is shortened, and when the temperature is reduced to be below 200 ℃, the whole crystal growth process of the ingot casting single crystal is finished.

As shown in fig. 1 and 2, at the start of melting, the bottom thermal switch 13 moves up to the top, closing the bottom insulation 10 for optimal insulation. When the melting process is finished and the crystal growth process is started, the thermal switch 13 starts to descend, the bottom of the crucible starts to cool, and the seed crystal starts to grow. With the thermal switch 13 going down gradually, the bottom temperature drops gradually. In the process of the temperature reduction at the bottom, the solid-liquid interface in the silicon melt is always kept slightly convex to the melt due to the power conditions of the soaking platform and the heating body 7 at the top, so that the optimal crystal growth effect is obtained. When the crystal growth is finished at the top, the bottom thermal switch 13 can move up and down in coordination with the annealing process. After the annealing process is finished, the thermal switch 13 descends to the bottom to be in contact with the bottom water-cooling disc 14, and the temperature of the upper surface of the thermal switch 13 is reduced due to the bottom water-cooling disc 14, so that the crystal cooling time can be shortened, the process time can be saved, and the equipment capacity can be improved.