METHOD OF MANUFACTURING POLYCRYSTALLINE SILICON ROD

Abstract

There is provided a method of manufacturing a polycrystalline silicon rod suitable as a raw material for manufacturing monocrystalline silicon by a FZ process. The method of manufacturing a polycrystalline silicon rod according to the present invention is a method of manufacturing a polycrystalline silicon rod by Siemens process, and includes a post-deposition energization step of, after an end of a deposition step of polycrystalline silicon, performing energization under a condition that provides a skin depth D shallower than a skin depth D.sub.0 provided at a time when the deposition step ends. For example, the post-deposition energization step is performed by passage of current at a frequency f higher than a frequency f.sub.0 of current that is passed at a time when the deposition step ends.

Claims

1. A method of manufacturing a polycrystalline silicon rod by Siemens process, comprising a post-deposition energization step of, after an end of a deposition step of polycrystalline silicon, performing energization under a condition that provides a skin depth D shallower than a skin depth D.sub.0 provided at a time when the deposition step ends.

2. The method of manufacturing a polycrystalline silicon rod according to claim 1, wherein the post-deposition energization step is performed by passage of current at a frequency f higher than a frequency f.sub.0 of current that is passed at a time when the deposition step ends.

3. The method of manufacturing a polycrystalline silicon rod according to claim 2, wherein the post-deposition energization step is included as a step performed during a period in which the polycrystalline silicon rod is cooled to a room temperature after the deposition step ends, and a period, in which the frequency f for energization in the post-deposition energization step is set to be higher in response to lowering of a crystal temperature of the polycrystalline silicon rod, is provided.

4. The method of manufacturing a polycrystalline silicon rod according to claim 1, wherein the skin depth D is shallower than a radius R of the polycrystalline silicon rod provided after the deposition step ends.

5. A method of manufacturing a polycrystalline silicon rod by Siemens process, comprising a post-deposition heat treatment step of, after an end of a deposition step of polycrystalline silicon, treating the polycrystalline silicon rod at a temperature T that is higher than a crystal temperature T.sub.0 at a time when the deposition step ends and is lower than a melting temperature of polycrystalline silicon, wherein the post-deposition heat treatment step is performed while the polycrystalline silicon rod is energized under a condition that provides a skin depth D shallower than a radius R of the polycrystalline silicon rod provided after the deposition step ends.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a view for conceptually explaining an outline of a process in a first example;

[0025] FIG. 2 is a view for conceptually explaining an outline of a process in a second example;

[0026] FIG. 3 is a view for conceptually explaining an outline of a process in a third example;

[0027] FIG. 4 is a view for conceptually explaining an outline of a process in a fourth example;

[0028] FIG. 5 is a view for conceptually explaining an outline of a process in a first comparative example;

[0029] FIG. 6 is a view for conceptually explaining an outline of a process in a second comparative example; and

[0030] FIG. 7 is a view for conceptually explaining an outline of a process in a third comparative example.

DETAILED DESCRIPTION

[0031] Hereinafter, an embodiment for carrying out the present invention will be described.

[0032] The present inventors, in pursuit of studies on a method of manufacturing a polycrystalline silicon rod that is long enough to be suitably used as a raw material for manufacturing monocrystalline silicon by a FZ process, have concluded that it is difficult to satisfactorily suppress occurrence of a crack with the methods ever proposed.

[0033] Thus, the inventors, having further worked on the studies, have concluded that the probability of crack occurrence is reduced by passage of high-frequency current through a polycrystalline silicon rod under a condition that provides a skin depth shallower than a skin depth provided at the end of a deposition step of polycrystalline silicon, in a cooling step after the deposition step.

[0034] A skin depth (m) is given by =[/(.Math.f.Math.)].sup.1/2. In this equation, represents resistivity (m), f represents a frequency (Hz), and represents permeability (H/m) of a conductor.

[0035] Hence, a skin depth can be reduced by increase of a frequency of current to be supplied. Further, the electrical resistivity of a silicon crystal becomes lower as a temperature rises. Thus, a skin depth can be made smaller (made shallower) also by rise of a crystal temperature. In other words, for reduction of a skin depth, there can be provided two options (conditions) of increasing a frequency of current and of raising a crystal temperature. Additionally, in the case of the latter option, a crystal temperature is set at a temperature lower than a melting temperature of polycrystalline silicon to prevent melting of silicon.

[0036] The present inventors have found that energization under the above-described two conditions are effective for suppressing occurrence of a crack not only in a deposition step of silicon, but also in a step after the deposition step (the step will be referred to as a cooling step for the sake of convenience), and have devised the present invention.

[0037] The shallower a skin depth is, the larger amount of current flows near a surface of a polycrystalline silicon rod, and no current flows in an inside region of the rod. Thus, a difference in temperature between a surface and a central part of the polycrystalline silicon rod is reduced. This results in suppression of occurrence of a crack.

[0038] Additionally, in the present invention, high-frequency current is not necessarily required to flow at all times in a step of cooling a polycrystalline silicon rod to a room temperature (that is, a cooling step referred to in the present specification) after a deposition step. High-frequency current may be caused to flow under a condition that reduces a skin depth only in a part of a period for cooling a polycrystalline silicon rod to a room temperature.

[0039] For example, it may be so designed that current at a frequency of 50 Hz or 60 Hz, for example, is passed during deposition reaction and high-frequency current flows under a condition that reduces a skin depth in a predetermined period after the deposition reaction.

[0040] When current at a frequency of 50 Hz or 60 Hz flows, a skin depth becomes equal to or larger than a radius of a polycrystalline silicon rod. Thus, current flows through not only a surface of the polycrystalline silicon rod, but also a central part thereof, which increases a difference in temperature between the surface and the central part of the polycrystalline silicon rod. This then increases the likelihood of crack occurrence. However, by causing high-frequency current to flow under a condition that reduces a skin depth in at least a predetermined period after the end of deposition reaction, it is possible to produce a state in which a difference in temperature between a surface and a central part of a polycrystalline silicon rod is small. This relieves thermal distortion, thereby suppressing occurrence of a crack.

[0041] As described above, the electric resistivity of a silicon crystal becomes lower as a temperature rises, and conversely, it becomes higher as a temperature decreases. Further, a skin depth becomes larger (deeper) as the electrical resistivity becomes higher.

[0042] Thus, in passing high-frequency current in a cooling step, it is preferable to increase a frequency of the current to be passed, in response to lowering of a crystal temperature. Further, it is also preferable to heat a polycrystalline silicon rod obtained through crystal growth at a temperature lower than a melting temperature and reduce the electrical resistivity of a surface of the polycrystalline silicon rod without changing a frequency of current to be passed, to energize the polycrystalline silicon rod under a condition that provides a skin depth shallower than a skin depth D.sub.0 provided at the end of a deposition step. Moreover, it is also preferable to increase a frequency of current to be passed and heat a polycrystalline silicon rod obtained through crystal growth at a temperature lower than a melting temperature.

[0043] The above-described method of manufacturing a polycrystalline silicon rod can be summarized as follows.

[0044] That is, the method of manufacturing a polycrystalline silicon rod according to the present invention is a method of manufacturing a polycrystalline silicon rod by Siemens process, and includes a post-deposition energization step of, after an end of a deposition step of polycrystalline silicon, performing energization under a condition that provides a skin depth D shallower than a skin depth D.sub.0 provided at a time when the deposition step ends.

[0045] For example, the post-deposition energization step is performed by passage of current at a frequency f higher than a frequency f.sub.0 of current that is passed at a time when the deposition step ends.

[0046] Further, in a certain aspect, the post-deposition energization step is included as a step that is performed until the polycrystalline silicon rod is cooled to a room temperature after the deposition step ends, and there is provided a period in which a frequency f for energization in the post-deposition energization step is set to be higher in response to lowering of a crystal temperature of the polycrystalline silicon rod.

[0047] Preferably, the skin depth D is shallower than a radius R of the polycrystalline silicon rod provided after the deposition step ends.

[0048] A method of manufacturing a polycrystalline silicon rod in another aspect according to the present invention is a method of manufacturing a polycrystalline silicon rod by Siemens process, and includes a post-deposition heat treatment step of, after an end of a deposition step of polycrystalline silicon, treating the polycrystalline silicon rod at a temperature T that is higher than a crystal temperature T.sub.0 at a time when the deposition step ends and is lower than a melting temperature of polycrystalline silicon. Further, while the post-deposition heat treatment step is performed, the polycrystalline silicon rod is energized under a condition that provides a skin depth D shallower than a radius R of the polycrystalline silicon rod provided after the deposition step ends.

EXAMPLES

First Example

[0049] FIG. 1 is a view for conceptually explaining an outline of a process in a first example. A mixture of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz until the diameter of a crystal became equal to 80 mm, and subsequently the frequency was changed to a high frequency of 80 kHz. After a deposition step of polycrystalline silicon ended, the polycrystalline silicon rod was energized with the frequency being set at 100 kHz for one hour in order to achieve energization under a condition that provides a skin depth D shallower than a skin depth D.sub.0 provided at the end of the deposition step. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature.

Second Example

[0050] FIG. 2 is a view for conceptually explaining an outline of a process in a second example. A mixture of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz until the diameter of a crystal became equal to 80 mm, and subsequently the frequency was changed to a high frequency of 80 kHz. After a deposition step of polycrystalline silicon ended, while the frequency was kept at 80 kHz, the polycrystalline silicon rod was heated so that the surface temperature thereof might reach 1020 C. in order to achieve energization under a condition that provides a skin depth D shallower than a skin depth D.sub.0 provided at the end of the deposition step. The polycrystalline silicon rod was maintained in that state for one hour. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature.

Third Example

[0051] FIG. 3 is a view for conceptually explaining an outline of a process in a third example. A mixture gas of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz at all times in a deposition step of polycrystalline silicon. After the deposition step ended, high-frequency current at 100 kHz was passed through the polycrystalline silicon rod for one hour. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature.

Fourth Example

[0052] FIG. 4 is a view for conceptually explaining an outline of a process in a fourth example. A mixture gas of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz until the diameter of a crystal became equal to 80 mm, and subsequently the frequency was changed to a high frequency of 15 kHz. After a deposition step of polycrystalline silicon ended, the frequency for energization was gradually changed from 15 kHz to 35 kHz during a period in which a surface temperature of the polycrystalline silicon rod was lowered from 970 C. to 700 C. in order to achieve energization under a condition that provides a skin depth D shallower than a skin depth D.sub.0 provided at the end of the deposition step. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature.

First Comparative Example

[0053] FIG. 5 is a view for conceptually explaining an outline of a process in a first comparative example. A mixture gas of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz at all times in a deposition step of polycrystalline silicon. After the deposition step of polycrystalline silicon ended, while the frequency for energization was kept at 50 Hz, the polycrystalline silicon rod was heated so that the surface temperature of the polycrystalline silicon rod might reach 1020 C. Then, the polycrystalline silicon rod was maintained in that state for one hour. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature. Under the foregoing condition, a part of the polycrystalline silicon rod collapsed. It is considered that the reason is that a difference in temperature between the surface and the center of the polycrystalline silicon rod is increased due to rise in a crystal temperature during passage of low-frequency current at 50 Hz.

Second Comparative Example

[0054] FIG. 6 is a view for conceptually explaining an outline of a process in a second comparative example. A mixture gas of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz at all times in a deposition step of polycrystalline silicon. After the deposition step of polycrystalline silicon ended, while the frequency for energization was kept at 50 Hz, the polycrystalline silicon rod was maintained in that state for one hour. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature. Under this condition, cracks occurred considerably in the polycrystalline silicon rod. It is considered that the reason is that the polycrystalline silicon rod was cooled with a large difference in temperature between a surface and a central part thereof as a result of current flow in, and heating of, a central region of the polycrystalline silicon rod due to a start of the cooling during passage of low-frequency current at 50 Hz.

Third Comparative Example

[0055] FIG. 7 is a view for conceptually explaining an outline of a process in a third comparative example. A mixture gas of a trichlorosilane gas as a raw-material gas and a hydrogen gas was supplied and the temperature was kept at 970 C., so that a polycrystalline silicon rod having a diameter of 160 mm was grown by Siemens process. A frequency of current to be passed was set at a low frequency of 50 Hz until the diameter of a crystal became equal to 80 mm. Subsequently, the frequency was changed to a high frequency of 15 kHz. After a deposition step of polycrystalline silicon ended, the frequency for energization was kept at 15 kHz during a period in which the surface temperature of the polycrystalline silicon rod was lowered from 970 C. to 700 C. Thereafter, energization was stopped and the polycrystalline silicon rod was cooled to a room temperature.

[0056] Relative yields of obtained lengths of the polycrystalline silicon rods (relative yields of crystal lengths) in the above-described examples are tabulated in Table 1. The relative yield of crystal length referred to in this specification is a ratio of a length of a polycrystalline silicon rod that could be obtained in the form of a region having no crack in each example, to that (1.00) in the first example used as a reference.

[0057] In addition, each of the relative yields of crystal lengths tabulated in Table 1 is an average value of yields obtained from ten pairs of samples in each example.

TABLE-US-00001 TABLE 1 Example/Comparative example Example Comparative example First Second Third Fourth First Second Third Relative yield 1.00 0.99 0.83 1.20 0.35 0.58 0.70 of crystal length

[0058] As shown in Table 1, a yield higher than 0.8 could be obtained in each of the examples. In contrast, in the comparative examples, a yield of 0.7 could be obtained at the highest.

[0059] Additionally, in the second example, a high yield of 0.99 was obtained. This is because the polycrystalline silicon rod was heated so that the surface temperature having been kept at 970 C. might reach 1020 C. at the end of the deposition step, which reduced a resistance value of the surface of the polycrystalline silicon rod, resulting in the skin depth D shallower than the skin depth D.sub.0 provided at the end of the deposition step.

[0060] Further, even the yield that was obtained in the third comparative example and is the highest among the yields obtained in the comparative examples is still as low as 0.70. This is because in the third comparative example, the surface temperature of the polycrystalline silicon rod was lowered from 970 C. to 700 C. while the frequency for energization was kept at 15 kHz after the end of the deposition step of polycrystalline silicon, which increased a resistance value of the surface of the polycrystalline silicon rod, resulting in the skin depth D larger than the radius R of the polycrystalline silicon rod.

[0061] The present invention provides a method of manufacturing a polycrystalline silicon rod suitable as a raw material for manufacturing monocrystalline silicon by a FZ process, while effectively preventing occurrence of a crack or breakage.