Detailed Description
The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.
Referring to fig. 1, an embodiment of the present invention provides a thermal field control device 10 for a crystal pulling furnace 1, wherein for the crystal pulling furnace 1 only a crucible 20 and a guide shell 11 thereof are shown in fig. 1, while for other components of the crystal pulling furnace 1 such as a furnace body, a heater, etc., which are known to those skilled in the art and therefore not shown in fig. 1, the thermal field control device 10 may comprise:
a guide cylinder 11 of the crystal pulling furnace 1, the guide cylinder 11 being fixedly disposed in the crystal pulling furnace 1, as known to those skilled in the art, the guide cylinder 11 functions to guide a protective gas such as argon gas to the liquid level L of the silicon melt SM shown in fig. 1, for example, to prevent unnecessary chemical reaction of the silicon melt SM, and during the pulling of the single crystal silicon rod R, the single crystal silicon rod R moves through the guide cylinder 11 and the guide cylinder 11 may function to shield heat radiated from the outside thereof to the single crystal silicon rod R, and further, the guide cylinder 11 is made of a brittle graphite material and is thus not suitable for use as a mechanically driven part, since it is easily broken to cause damage in case of being frequently driven;
a heat insulator 12 disposed between the silicon melt SM and the silicon single crystal rod R drawn from the silicon melt SM to constitute a heat shield 10A for blocking heat radiated from the silicon melt SM to the silicon single crystal rod R together with the guide cylinder 11, for example, in fig. 1, a heat radiation path schematically shown by a dotted arrow is blocked by the heat shield 10A so that heat cannot be radiated to the silicon single crystal rod R, and a heat radiation path schematically shown by a solid arrow below the dotted arrow is not blocked so that heat from the silicon melt SM is radiated to the silicon single crystal rod R, wherein a material of the heat insulator 12 is different from a material of the guide cylinder 11, and the heat insulator 12 is made of a material suitable for mechanical transmission;
a heat shield driver 13 for driving the heat shield 12 to move to vary the distance D1 between the bottom of the heat shield 10A and the liquid level L of the silicon melt SM and correspondingly vary the amount of heat radiated from the silicon melt SM to the single crystal silicon rod R to obtain a desired axial temperature gradient in the single crystal silicon rod R, to which it can be readily understood with reference to fig. 1 that when the heat shield driver 13 drives the heat shield 12 to move upwardly, the distance D1 increases, and thus more heat is radiated from the silicon melt SM to the single crystal silicon rod R, and when the heat shield driver 13 drives the heat shield 12 to move downwardly, the distance D1 decreases, and thus less heat is radiated from the silicon melt SM to the single crystal silicon rod R.
According to the solution of the above embodiment of the present invention, the heat shield 10A is disposed between the silicon melt SM and the silicon single crystal rod R, and the amount of heat radiated from the silicon melt SM to the silicon single crystal rod R is changed by changing the distance D1 between the bottom of the heat shield 10A and the liquid level L of the silicon melt SM, or equivalently, by moving the heat shield 10A, so that the control of the thermal field around the silicon single crystal rod R is achieved in a simple and effective manner, thereby satisfying the requirement of the axial temperature gradient of the silicon single crystal rod R, and the guide cylinder of the crystal pulling furnace 1 is fixedly disposed, so that the change of the distance D1 by the movement of the guide cylinder 11 made of a brittle graphite material and thus not suitable for mechanical transmission or otherwise prone to break is avoided.
As mentioned above, the heat insulating element 12 is made of a material suitable for mechanical transmission, for which, in a preferred embodiment of the invention, the heat insulating element 12 can be made of stainless steel, which is a tough material that is inexpensive and therefore different from graphite or suitable for mechanical transmission without breaking even in the case of frequent driving, but on the other hand, the presence of steel in the crystal pulling furnace 1 can lead to the introduction of metal contamination, which affects the crystal pulling process or can degrade the quality of the pulled monocrystalline silicon rod R, for which the surface area of the heat insulating element 12 can be smaller than the surface area of the guide shell 11, which reduces the possibility of metal contamination compared to the above-mentioned variation of the distance D1 with the guide shell 11, which is likewise made of steel, as a moving part.
In the case of materials suitable for mechanical transmission, such as the steels mentioned above, which tend to introduce undesirable contamination in the crystal pulling furnace 1 because of their presence in the crystal pulling furnace 1, in a preferred embodiment of the invention, with reference to fig. 2, the thermal insulation 12 may comprise a body 120 and a coating 121 covering the body 120, the coating 121 being intended to prevent the escape of contaminating impurities of the body 120. In this way, while ensuring that the insulation 12 is suitable for mechanical transmission, the introduction of contamination due to its material is avoided.
In a preferred embodiment of the invention, said thermal insulation 12 can be made of molybdenum, which, as known to those skilled in the art, is a conventional material present in the crystal pulling furnace 1, not only does not introduce contamination but also has a high thermal radiation reflectivity, enabling a more effective thermal insulation from the silicon melt SM to be achieved.
In a preferred embodiment of the present invention, referring back to fig. 1, the heat insulator 12 may be cylindrical like the guide shell 11, thereby more efficiently achieving heat insulation against the single crystal silicon rod R, and the inner circumferential wall 12W of the heat insulator 12 extends vertically. As previously mentioned, guide cylinder 11 functions to direct a protective gas, for example argon, at level L of silicon melt SM shown in fig. 1, in order to prevent, for example, unnecessary chemical reactions of silicon melt SM, therefore, the guide cylinder 11 needs to have a specific shape in order to form a passage for effectively guiding the protective gas, and particularly, as shown in fig. 1, the inner circumferential wall 11W of the guide cylinder 11 has a portion tapered from top to bottom, thereby being easily subjected to a large impact force of the protective gas guided to flow, under the condition that the guide cylinder 11 is driven to move, the guide cylinder can shake due to the action of the impact force of the airflow, so that the connection part of the guide cylinder and the driving device is further forced to be broken under stress, and even when the shaking amplitude is large, the guide cylinder can collide with the silicon single crystal rod R, and the silicon single crystal rod R or the guide cylinder 11 can fall off. The heat insulator 12 does not need to play a role of guiding the gas flow, and therefore, the inner peripheral wall 12W thereof can be extended vertically as described above, thereby avoiding the impact force of the flowing protective gas, and the heat insulator 12 is not shaken even if it is driven to move, and the stability of the parts is improved and the production safety is ensured compared to the case where the guide cylinder 11 having the passage for guiding the protective gas is used as a moving member to realize the change of the distance D1.
To further avoid the thermal insulation 12 from shaking, in a preferred embodiment of the present invention, still referring to fig. 1, the height of the thermal insulation 12 may be less than the height of the draft tube 11. In this way, when the protective gas flows, the "windward side" of the heat insulator 12 is further reduced, and therefore the force of the flowing protective gas received by the heat insulator 12 is further reduced, and the occurrence of rattling can be further avoided.
Preferably, in the case where the diameter of the silicon single crystal rod R is 300mm to 308mm, referring to fig. 1, the distance D2 between the inner circumferential wall 12W of the heat shield 12 and the outer circumferential wall of the silicon single crystal rod R may be between 20mm to 50 mm.
Preferably, in the above case, still referring to fig. 1, the distance D3 between the bottom of the guide shell 11 and the liquid level of the silicon melt may be between 20mm and 60mm, it being understood that this distance D3 determines the maximum value of the distance D1 between the bottom of the thermal shield 10A and the liquid level L of the silicon melt SM, while the minimum value of this distance D1 is determined by the movement of the thermal shield 12 and may be 10 mm. With the guide shell 11 fixed, changing this distance D3 changes the distance D1 between the bottom of the heat shield 10A and the liquid level L of the silicon melt SM.
In order to achieve the heat insulation effect of the guide shell 11, in a preferred embodiment of the present invention, referring to fig. 3, the guide shell 11 may include a housing 110 and a thermal insulation material 111 disposed inside the housing. The housing 110 may be made of high purity graphite and may be coated with a silicon carbide coating on the outer surface, and the insulation material 111 may be insulation graphite felt.
Referring to fig. 4, the embodiment of the invention also provides a crystal pulling furnace 1, and the crystal pulling furnace 1 can comprise a thermal field control device 10 according to the previous embodiments of the invention.
As can be easily understood by referring to fig. 4, as silicon single crystal rod R continues to grow, the volume of silicon melt SM in crucible 20 gradually decreases, on one hand, the drop in liquid level L causes the distance D1 to increase, thereby changing the amount of heat radiated from silicon melt SM to silicon single crystal rod R, on the other hand, because there is less silicon melt SM, the amount of heat that silicon melt SM itself can radiate decreases, and also causes the amount of heat radiated from silicon melt SM to silicon single crystal rod R to change, the combined effect of which causes the axial temperature gradient of silicon single crystal rod R to change, thereby causing crystal growth defects. In the prior art, the distance between the bottom of the guide cylinder and the liquid level of the silicon melt is monitored, and the axial temperature gradient required for the single crystal silicon rod is obtained through the following steps: for the corresponding monitoring, a quartz lifting hook is hung at the bottom of the guide cylinder, a camera is used for capturing the reflection of the quartz lifting hook on the liquid level, the distance between the quartz lifting hook and the reflection is measured, and for the control, the crucible is lifted so as to ensure that the distance between the bottom of the guide cylinder and the liquid level of the silicon melt meets the requirement of defect-free growth of the silicon single crystal rod. However, due to high-temperature radiation, liquid level fluctuation and the like, the reflection of the quartz lifting hook captured by the camera on the liquid level is very unstable, the monitoring accuracy is greatly influenced, the accuracy of the control under the condition cannot meet the requirement, and the defect of crystal growth of the silicon single crystal rod is difficult to avoid. Moreover, the adjustment of the distance between the bottom of the guide cylinder and the liquid level of the silicon melt is in consideration of the coordination relationship between the crucible rising and the pulling speed of the silicon single crystal rod, otherwise, the silicon single crystal rod is easy to melt back or break, so that the adjustment capability of the above manner is limited, particularly in the silicon single crystal rod subjected to nitrogen doping treatment for ensuring the BMD density at present, along with the increase of the nitrogen concentration, the Δ G of the silicon single crystal rod is gradually increased, so that the defect-free pulling speed area of the silicon single crystal rod is reduced, and the adjustment of the distance between the bottom of the guide cylinder and the liquid level of the silicon melt in the actual production process is insufficient for improving the defect distribution in the silicon single crystal rod, so that the silicon rod has crystal growth defects.
In this regard, referring to fig. 4, in a preferred embodiment of the present invention, the crystal pulling furnace 1 may further include:
a crucible 20 for containing the silicon melt SM;
a crucible driver 30 for driving the crucible 20 to move, as schematically shown by an open arrow in FIG. 4, to keep the height of the liquid level L of the silicon melt SM constant during the process in which the amount of silicon melt SM contained in the crucible 20 is continuously reduced during the pulling of the single crystal silicon rod R,
wherein, the thermal field control device 10 may further include:
a measuring unit 14, the measuring unit 14 being used for measuring a moving distance of the heat insulating member 12;
a determination unit 15, the determination unit 15 being configured to determine a distance D1 between the bottom of the heat shield 10A and the liquid level L of the silicon melt SM solely from the movement distance.
In the above embodiment, instead of using the quartz hook and its reflection as in the prior art, the distance D1 is accurately obtained simply by measuring the moving distance of the thermal insulator 12, and the control accuracy can be ensured in the case where the measurement accuracy can be ensured, and therefore the generation of crystal growth defects can be avoided. Moreover, the adjustment mode of moving the heat insulation piece 12 can realize a larger adjustment range of the axial temperature gradient of the single crystal silicon rod S, can effectively control the crystal growth defect, and is beneficial to the growth of the single crystal silicon rod in a mode without the growth defect.
It should be noted that: the technical schemes described in the embodiments of the present invention can be combined arbitrarily without conflict.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.