Monocrystalline silicon rod, preparation method thereof and silicon wafer

Abstract

The embodiment of the invention discloses a silicon single crystal rod, a preparation method thereof and a silicon wafer. The oxygen content of the silicon single crystal rod is lower than 4 ppma, the impurity center in the silicon single crystal rod can be reduced, and the electrical performance of a silicon wafer obtained by cutting the silicon single crystal rod is improved. In addition, oxygen deposits in the silicon single crystal rod can be reduced, and generation of defects is reduced.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to the field of semiconductor manufacturing technology, and in particular, to a single crystal silicon rod, a preparation method thereof, and a silicon wafer. Background Art

[0002] Most single crystal silicon rods are manufactured by the Czochralski (hereinafter referred to as "CZ") method, also known as the direct pulling method. This CZ method utilizes the principle of melt condensation crystallization drive. At the interface between the solid and the liquid, due to the decrease in the melt temperature, a phase change from liquid to solid occurs. During the process of pulling a single crystal silicon rod by the CZ method, solid polycrystalline silicon raw materials are placed in a quartz crucible and heated by a graphite heater to melt the polycrystalline silicon raw materials contained in the quartz crucible. After that, through processes such as temperature testing, seed crystal introduction, shoulder release, shoulder turning, equal diameter, and tailing, a dislocation-free single crystal silicon rod is finally pulled.

[0003] Currently, the oxygen content of the single crystal silicon rods prepared by the preparation method of single crystal silicon rods can be 10 ppma to 20 ppma. However, with the development of the semiconductor industry, preparing single crystal silicon rods with a low oxygen content will gradually become a new technical route and product standard. Summary of the Invention

[0004] In view of this, embodiments of the present disclosure are expected to provide a single crystal silicon rod, a preparation method thereof, and a silicon wafer; a single crystal silicon rod with an oxygen content lower than 4 ppma can be prepared.

[0005] The technical solution of the embodiments of the present disclosure is implemented as follows:

[0006] In a first aspect, embodiments of the present disclosure provide a single crystal silicon rod, and the oxygen content of the single crystal silicon rod is lower than 4 ppma.

[0007] In a second aspect, embodiments of the present disclosure provide a preparation method of a single crystal silicon rod, the preparation method is used to prepare the single crystal silicon rod described in the first aspect, and the preparation method includes:

[0008] Determine the rotation direction and rotation speed of the single crystal silicon rod according to the weaving direction of the outermost weaving layer in the seed crystal rope for pulling the single crystal silicon rod;

[0009] Determine the rotation direction and rotation speed of the graphite crucible according to the rotation direction and rotation speed of the single crystal silicon rod;

[0010] Determine the descending speed of the graphite crucible.

[0011] Optionally, in some examples, the rotation direction of the single-crystal silicon rod is opposite to the braiding direction of the outermost braiding layer in the seed crystal rope; wherein, the seed crystal rope is composed of a central part and at least one braiding layer arranged on the outer periphery of the central part.

[0012] Optionally, in some examples, the rotation speed of the single-crystal silicon rod is 15 rpm to 20 rpm.

[0013] Optionally, in some examples, the rotation direction of the graphite crucible is opposite to the rotation direction of the single-crystal silicon rod.

[0014] Optionally, in some examples, the rotation speed of the graphite crucible is 0.1 rpm to 0.2 rpm.

[0015] Optionally, in some examples, the descending speed of the graphite crucible is 0.1 mm / min to 1.0 mm / min.

[0016] Optionally, in some examples, the descending speed of the graphite crucible is 0.1 mm / min to 0.5 mm / min.

[0017] Optionally, in some examples, the height of the upper edge of the graphite crucible is lower than the height of the upper edge of the quartz crucible.

[0018] Optionally, in some examples, a fastening ring is arranged on the upper edge of the quartz crucible, and the fastening ring abuts against the upper edge of the graphite crucible.

[0019] In a third aspect, embodiments of the present disclosure provide a silicon wafer, which is obtained by cutting the single-crystal silicon rod according to the first aspect.

[0020] Optionally, in some examples, the oxygen content in the silicon wafer is less than 4 ppma.

[0021] Embodiments of the present disclosure provide a single-crystal silicon rod, a preparation method thereof, and a silicon wafer; by using the single-crystal silicon rod with an oxygen content less than 4 ppma provided by the embodiments of the present disclosure, the impurity centers in the single-crystal silicon rod can be reduced, and the electrical performance of the silicon wafer cut from the single-crystal silicon rod can be improved. In addition, the oxygen deposits in the single-crystal silicon rod can be reduced, and the generation of defects can be reduced. Description of the Drawings

[0022] Figure 1 It is a schematic structural diagram of a crystal pulling furnace 1 in the related art.

[0023] Figure 2 It is a schematic flow diagram of a preparation method of a single-crystal silicon rod provided by an embodiment of the present disclosure.

[0024] Figure 3Schematic diagram of the seed crystal rope provided by the embodiments of the present disclosure.

[0025] Figure 4 Schematic diagram of the structures of the graphite crucible and the quartz crucible provided by the embodiments of the present disclosure. Detailed implementation manners

[0026] Next, the technical solutions in the embodiments of the present disclosure will be clearly and completely described with reference to the accompanying drawings in the embodiments of the present disclosure.

[0027] Refer to Figure 1 , which shows a schematic diagram of the structure of the crystal pulling furnace 1 in the related art. As Figure 1 shown, the crystal pulling furnace 1 includes: a furnace body 10, a quartz crucible 20, a graphite heater 30, a deflector 40, a cover plate 50, a support frame 60, and a water-cooled jacket 70.

[0028] The above-mentioned furnace body 10 is used to define a furnace chamber 101 for pulling a single crystal silicon rod S. The furnace chamber 101 includes an upper furnace chamber 1011 ( Figure 1 the chamber shown above the center dotted line) and a lower furnace chamber 1012 ( Figure 1 the chamber shown below the center dotted line), and the upper furnace chamber 1011 and the lower furnace chamber 1012 are communicated.

[0029] In some examples, the above-mentioned crystal pulling furnace 1 is usually divided into a cold zone and a hot zone. The upper furnace chamber 1011 is the cold zone, and its temperature is about 100°C. The lower furnace chamber 1012 is the hot zone, and the temperature can reach above 1000°C. Usually, the quartz crucible 20, the graphite heater 30, the deflector 40, the support frame 50, and the water-cooled jacket 60 are all arranged in the lower furnace chamber 1012.

[0030] The above-mentioned quartz crucible 20 is arranged at the bottom of the lower furnace chamber 1012 and is used to hold the solid polycrystalline silicon raw material (or can be called "polycrystalline silicon melt") at the initial stage of pulling the single crystal silicon rod S.

[0031] The above-mentioned graphite heater 30 is distributed around the above-mentioned quartz crucible 20.

[0032] During the specific process of pulling the single crystal silicon rod S, after a set mass of polycrystalline silicon raw material is loaded into the above-mentioned quartz crucible 20, the polycrystalline silicon raw material in the quartz crucible 20 is heated by the graphite heater 30 to melt the polycrystalline silicon raw material to form a silicon melt MS.

[0033] The above-mentioned deflector 40 is in the shape of an inverted conical cylinder.

[0034] On the one hand, the draft tube 40 is used to isolate the thermal radiation generated by the graphite heater 30 on the single crystal silicon rod S during the drawing process of the single crystal silicon rod S, so as to ensure the temperature gradient required for the growth of the single crystal silicon rod S, and further ensure the crystallization rate of the single crystal silicon rod S. On the other hand, the draft tube 40 is used to drain an inert protective gas such as argon from top to bottom to the upper part of the silicon melt in the quartz crucible 20, so as to accelerate the gas flow rate on the liquid surface of the silicon melt and quickly remove volatile impurities.

[0035] The cover plate 50 is in the shape of a circular ring flat plate. The cover plate 50 is a single-layer cover plate made of, for example, graphite. The cover plate 50 is horizontally arranged in such a way that its outer ring edge contacts the side wall of the furnace body 10 and its inner ring edge contacts the draft tube 40, so as to prevent the heat generated by the graphite heater 30 from being dissipated through the top of the furnace body 10.

[0036] The above-mentioned support frame 60 is connected to the cover plate 50 and is used to support the draft tube 40.

[0037] The above-mentioned water-cooling jacket 70 is in a cylindrical shape and is used to cool the drawn single crystal silicon rod S.

[0038] The radial dimension of the water-cooling jacket 70 is smaller than the radial dimension of the top of the draft tube 40, so that the water-cooling jacket 70 is arranged above the draft tube 40 in an overlapping manner in the vertical direction.

[0039] In some examples, the structure of the above-mentioned crystal pulling furnace 1 further includes: a seed crystal rope 80.

[0040] It should be noted that at the uppermost part of the furnace body 10 of the crystal pulling furnace 1, there is also a pulling head 90 connected to the seed crystal rope 80. The pulling head 90 is mainly used to realize the rotation and lifting of the seed crystal, and can record data such as the displacement of the seed crystal.

[0041] Understandably, Figure 1 The shown crystal pulling furnace 1 may also include other Figure 1 structures not shown in the figure, such as a crucible lifting device, etc. The embodiments of the present disclosure will not be specifically described herein.

[0042] Specifically, after the quartz crucible 20 is filled with a set mass of polysilicon raw material, the quartz crucible 20 is heated by the graphite heater 30 to melt the polysilicon raw material in the quartz crucible 20 to form a silicon melt MS. When the temperature of the liquid surface of the silicon melt is stable, a seed crystal (not shown in the figure) is lowered to the solid-liquid interface of the silicon melt MS through the seed crystal rope 70, and processes such as seeding, necking, shoulder opening, equal diameter growth, and tailing are started, and finally a single crystal silicon rod S with a set length is drawn.

[0043] For the above method for preparing a single-crystal silicon rod, as the single-crystal silicon rod is continuously pulled and grown, the liquid level of the silicon melt MS will continuously decrease, and the crucible lifting device (not shown in the figure) drives the quartz crucible 20 to continuously rise, so that the position of the liquid level of the silicon melt MS relative to the graphite heater 30 remains substantially unchanged in the horizontal direction. The oxygen content of the single-crystal silicon rod S prepared by this preparation method shows a trend of first decreasing and then increasing from the head to the tail of the single-crystal silicon rod in the axial direction, and the oxygen content is generally relatively high, approximately 10 ppma to 20 ppma. These oxygen atoms form impurity centers in the single-crystal silicon rod, and these impurity centers will capture or scatter carriers, thereby reducing the electrical performance of the silicon wafer prepared from the single-crystal silicon rod. In addition, these oxygen atoms form oxygen deposits in the single-crystal silicon rod, affecting the performance of the single-crystal silicon rod. With the development of the semiconductor industry, lower oxygen content requirements are put forward for single-crystal silicon rods, but the above preparation method cannot prepare single-crystal silicon rods with low oxygen content.

[0044] It should be noted that in the embodiments of the present disclosure, the single-crystal silicon rod S has a central axis ( Figure 1 shown by the dashed line X in), and the axial direction of the single-crystal silicon rod S is the extension direction of the central axis of the single-crystal silicon rod S.

[0045] Based on the above description, the embodiments of the present disclosure are expected to provide a method for obtaining a single-crystal silicon rod with low oxygen content and its preparation method. Specifically, referring to Figure 2 , which shows a method for preparing a single-crystal silicon rod provided by the embodiments of the present disclosure. This preparation method specifically includes the following steps.

[0046] In step S201, according to the weaving direction of the outermost woven layer in the seed crystal rope for pulling the single-crystal silicon rod, the rotation direction and rotation speed of the single-crystal silicon rod are determined.

[0047] The above rotation direction of the single-crystal silicon rod refers to the direction in which the single-crystal silicon rod rotates around its own central axis (such as Figure 1 shown by the dashed line X in).

[0048] For the technical solution shown in step S201, in some possible implementation manners, the rotation direction of the single-crystal silicon rod is opposite to the weaving direction of the outermost woven layer in the seed crystal rope; wherein, the seed crystal rope is composed of a central part and at least one woven layer arranged on the outer periphery of the central part.

[0049] Specifically, as shown in Figure 3As shown, a cross-sectional view of the seed crystal rope 301 is exemplarily shown. The seed crystal rope 301 includes a central portion 3011 and two layers of braided layers 3012 provided on the outer periphery of the central portion 3011. In an embodiment of the present disclosure, when the braided layer 3012 is multi-layered and is provided layer by layer on the outer periphery of the above-mentioned central portion 3011, the rotation direction of the single crystal silicon rod is opposite to the braiding direction of the outermost braided layer 3012 in the seed crystal rope 301.

[0050] In some examples, the outermost braided layer 3012 refers to the braided layer that is farthest from the central portion 3011 in the seed crystal rope 301.

[0051] In some examples, the braided layer 3012 is provided on the outer periphery of the above-mentioned central portion 3011 in a helical winding form. Of course, in the specific implementation process, the braided layer 3012 can also be provided on the outer periphery of the above-mentioned central portion through other braiding forms, and the embodiments of the present disclosure do not elaborate specifically on this.

[0052] In an embodiment of the present disclosure, the rotation direction of the single crystal silicon rod is opposite to the braiding direction of the outermost braided layer in the above-mentioned seed crystal rope 301, which can ensure that the crystal direction in the single crystal silicon rod is consistent with the crystal direction in the seed crystal. In addition, the rotation direction of the single crystal silicon rod being opposite to the braiding direction of the outermost braided layer in the above-mentioned seed crystal rope 301 can reduce the defects generated during the preparation of the single crystal silicon rod, such as dislocation defects.

[0053] For the above technical solution, in some possible implementation manners, the rotation speed of the single crystal silicon rod is 15 rpm to 20 rpm. It can be understood that when the rotation speed of the single crystal silicon rod is between 15 rpm and 20 rpm, it can drive the forced convection of the silicon melt during the preparation of the single crystal silicon rod, making the oxygen distribution in the silicon melt more uniform, and then causing the oxygen to uniformly penetrate into the single crystal silicon rod, thereby obtaining a single crystal silicon rod with uniform oxygen content.

[0054] In the specific implementation process, controlling the rotation speed of the single crystal silicon rod can make the contact area between the silicon melt and the seed crystal have a uniform heat distribution, so as to reduce the generation of defects such as dislocation defects in the single crystal silicon rod and ensure the crystal direction of the single crystal silicon rod. In addition, the rotation speed of the single crystal silicon rod will affect the growth speed and growth shape of the single crystal silicon rod. Controlling the rotation speed of the single crystal silicon rod can ensure that a single crystal silicon rod with the required size and shape is prepared.

[0055] In addition, the rotation speed of the single crystal silicon rod can reduce the influence on the surface tension of the silicon melt to ensure the stability of the liquid surface of the silicon melt during the preparation process.

[0056] In step S202, determine the rotation direction and rotation speed of the graphite crucible according to the rotation direction and rotation speed of the single crystal silicon rod.

[0057] For the technical solution shown in step S202 above, in some possible implementation manners, the rotation direction of the graphite crucible is opposite to the rotation direction of the single-crystal silicon rod.

[0058] For the above implementation manner, in some examples, the rotation speed of the graphite crucible is 0.1 rpm to 0.2 rpm.

[0059] In the embodiment of the present disclosure, when the crucible lifting device ( Figure 1 not shown in the figure) drives the graphite crucible (not shown in the figure) to drive the quartz crucible 20 to rotate around the central axis X, the rotation direction of the graphite crucible around the central axis X is opposite to the rotation direction of the above single-crystal silicon rod, and the rotation speed of the graphite crucible is 0.1 rpm to 0.2 rpm.

[0060] During the preparation process, the rotation direction of the graphite crucible is opposite to the rotation direction of the single-crystal silicon rod, which can ensure the stability of the silicon melt surface to avoid the generation of vortices and turbulent flows, thereby causing the generation of dislocation defects and the like. Secondly, the rotation direction of the graphite crucible being opposite to the rotation direction of the single-crystal silicon rod can ensure uniform heat distribution to ensure uniform cooling of the single-crystal silicon rod and improve the quality of the single-crystal silicon rod. In some other examples, the rotation direction of the graphite crucible being opposite to the rotation direction of the single-crystal silicon rod causes the single-crystal silicon rod and the silicon melt to generate a relative rotational motion, and then forced convection is generated by the single-crystal silicon rod driving the silicon melt to prepare a single-crystal silicon rod with a more uniform oxygen content distribution.

[0061] In step S203, determine the descending speed of the graphite crucible.

[0062] For the technical solution shown in step S303 above, in some possible implementation manners, the descending speed of the graphite crucible is 0.1 mm / min to 1.0 min / min.

[0063] For the above implementation manner, in some examples, the descending speed of the graphite crucible is 0.1 mm / min to 0.5 mm / min.

[0064] During the preparation of a single-crystal silicon rod, oxygen in the single-crystal silicon rod mainly precipitates from the corner part of the quartz crucible (also known as the "R part"). At the initial moment of the equal-diameter stage, the heating zone formed by the graphite heater 30 is near the liquid surface of the silicon melt MS. As the length of the equal-diameter part of the single-crystal silicon rod increases, the liquid surface of the silicon melt MS gradually descends and moves away from the heating zone formed by the graphite heater 30, and the temperature of the corner part of the quartz crucible gradually decreases, which slows down the chemical reaction between the quartz crucible and the graphite crucible, thereby reducing the precipitation of oxygen at the corner part of the quartz crucible and suppressing the content of oxygen immersed in the single-crystal silicon rod. In addition, as the liquid surface of the silicon melt MS continuously descends, the distance between the deflector 40 and the liquid surface of the silicon melt MS gradually increases, increasing the volatilization of oxygen on the free surface of the liquid surface of the silicon melt MS, resulting in a reduction in the content of oxygen immersed in the single-crystal silicon rod.

[0065] It should be noted that the free surface of the liquid surface of the above-mentioned silicon melt MS refers to the liquid surface of the silicon melt except for the liquid surface where the single-crystal silicon rod grows ( Figure 1 as shown by the dashed oval in

[0066] For Figure 2 the technical solution shown, in some possible implementation manners, as Figure 4 shown, the height of the upper edge 1101 of the graphite crucible 110 is lower than the height of the upper edge 201A of the quartz crucible 20.

[0067] During the preparation of a single-crystal silicon rod, carbon powder will be generated at the upper edge 1101 of the graphite crucible 110 in a high-temperature environment. These carbon powders may fall into the silicon melt MS and become impurities in the silicon melt MS, affecting the quality of the single-crystal silicon rod. Therefore, in the embodiments of the present disclosure, the height of the upper edge 1101 of the graphite crucible 110 is set lower than the height of the upper edge 201A of the quartz crucible 20.

[0068] For the above implementation manner, in some examples, as Figure 4 shown, a fastening ring 401 is provided at the upper edge 201A of the quartz crucible 20, and the fastening ring 401 abuts against the upper edge 1101 of the graphite crucible 110.

[0069] In the specific implementation process, when the height of the upper edge 1101 of the graphite crucible 110 is lower than the height of the upper edge 201A of the quartz crucible 20, the upper edge 1101 of the quartz crucible 20 is prone to collapse due to the lack of support from the graphite crucible 110 in a high-temperature environment. Therefore, in the embodiments of the present disclosure, the fastening ring 401 is disposed around the upper edge 201A of the quartz crucible 20 and the fastening ring 401 abuts against the upper edge 1101 of the graphite crucible 110 to support the upper edge 201A of the quartz crucible 20.

[0070] In some examples, the fastening ring 401 is made of quartz with a relatively high aluminum (Al) content. It can be understood that the relatively high Al content in the fastening ring 401 helps to improve the strength of the fastening ring 401 to play a supporting role.

[0071] Based on the above preparation method, embodiments of the present disclosure provide a single crystal silicon rod, which can be prepared according to the preparation method described in the foregoing technical solution. The oxygen content in the single crystal silicon rod is less than 4 ppma.

[0072] Finally, embodiments of the present disclosure provide a silicon wafer, which is obtained by cutting the single crystal silicon rod described in the foregoing technical solution.

[0073] In some examples, the oxygen content in the silicon wafer is less than 4 ppma.

[0074] The preparation method of the single crystal silicon rod provided by the embodiments of the present disclosure will be elaborated in detail through specific examples below.

[0075] Example 1

[0076] The single crystal silicon rod is prepared by the Czochralski method. The rotation direction of the single crystal silicon rod is set to be opposite to the braiding direction of the outermost braided layer in the seed crystal rope, and the rotation speed of the single crystal silicon rod is 15 rpm.

[0077] The rotation direction of the graphite crucible is set to be opposite to the rotation direction of the single crystal silicon rod, and the rotation speed of the graphite crucible is 0.1 rpm, and the graphite crucible descends at a speed of 0.1 mm / min.

[0078] The oxygen content of the prepared single crystal silicon rod is tested. The oxygen content of the single crystal silicon rod in this Example 1 is 2.2 ± 0.5 ppma.

[0079] Example 2

[0080] The single crystal silicon rod is prepared by the Czochralski method. The rotation direction of the single crystal silicon rod is set to be opposite to the braiding direction of the outermost braided layer in the seed crystal rope, and the rotation speed of the single crystal silicon rod is 20 rpm.

[0081] The rotation direction of the graphite crucible is set to be opposite to the rotation direction of the single crystal silicon rod, and the rotation speed of the graphite crucible is 0.1 rpm, and the graphite crucible descends at a speed of 0.5 mm / min.

[0082] The oxygen content of the prepared single crystal silicon rod is tested. The oxygen content of the single crystal silicon rod in this Example 2 is 2.4 ± 0.5 ppma.

[0083] Example 3

[0084] The single-crystal silicon rod is prepared by the Czochralski method. The rotation direction of the single-crystal silicon rod is set to be opposite to the braiding direction of the outermost braided layer in the seed crystal rope, and the rotation speed of the single-crystal silicon rod is 15 rpm.

[0085] The rotation direction of the graphite crucible is set to be opposite to the rotation direction of the single-crystal silicon rod, and the rotation speed of the graphite crucible is 0.1 rpm, and the graphite crucible descends at a speed of 1.0 mm / min.

[0086] The oxygen content of the prepared single-crystal silicon rod is tested. The oxygen content of the single-crystal silicon rod in this Example 3 is 2.6 ± 0.5 ppma.

[0087] Example 4

[0088] The single-crystal silicon rod is prepared by the Czochralski method. The rotation direction of the single-crystal silicon rod is set to be opposite to the braiding direction of the outermost braided layer in the seed crystal rope, and the rotation speed of the single-crystal silicon rod is 15 rpm.

[0089] The rotation direction of the graphite crucible is set to be opposite to the rotation direction of the single-crystal silicon rod, and the rotation speed of the graphite crucible is 0.2 rpm, and the graphite crucible descends at a speed of 0.1 mm / min.

[0090] The oxygen content of the prepared single-crystal silicon rod is tested. The oxygen content of the single-crystal silicon rod in this Example 4 is 3.1 ± 0.5 ppma.

[0091] Example 5

[0092] The single-crystal silicon rod is prepared by the Czochralski method. The rotation direction of the single-crystal silicon rod is set to be opposite to the braiding direction of the outermost braided layer in the seed crystal rope, and the rotation speed of the single-crystal silicon rod is 20 rpm.

[0093] The rotation direction of the graphite crucible is set to be opposite to the rotation direction of the single-crystal silicon rod, and the rotation speed of the graphite crucible is 0.2 rpm, and the graphite crucible descends at a speed of 0.5 mm / min.

[0094] The oxygen content of the prepared single-crystal silicon rod is tested. The oxygen content of the single-crystal silicon rod in this Example 5 is 2.5 ± 0.5 ppma.

[0095] Example 6

[0096] The single-crystal silicon rod is prepared by the Czochralski method. The rotation direction of the single-crystal silicon rod is set to be opposite to the braiding direction of the outermost braided layer in the seed crystal rope, and the rotation speed of the single-crystal silicon rod is 20 rpm.

[0097] The rotation direction of the graphite crucible is set to be opposite to the rotation direction of the single-crystal silicon rod, and the rotation speed of the graphite crucible is 0.2 rpm, and the graphite crucible descends at a speed of 1.0 mm / min.

[0098] The oxygen content of the prepared single-crystal silicon rod was tested, and the oxygen content of the single-crystal silicon rod in this Example 6 was 3.3 ± 0.5 ppma.

[0099] It should be noted that: among the technical solutions described in the embodiments of the present disclosure, any combination can be made without conflict.

[0100] As mentioned above, the above are only specific embodiments of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present disclosure can easily think of changes or substitutions, which should be covered within the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be subject to the protection scope of the claims.

Claims

1. A single-crystalline silicon rod, characterized in that, the oxygen content of the single-crystalline silicon rod is less than 4 ppma.

2. A method for preparing a single-crystalline silicon rod, characterized in that, the preparation method is used to prepare the single-crystalline silicon rod according to claim 1, and the preparation method includes: determining the rotation direction and rotation speed of the single-crystalline silicon rod according to the weaving direction of the outermost weaving layer in the seed crystal rope for pulling the single-crystalline silicon rod; determining the rotation direction and rotation speed of the graphite crucible according to the rotation direction and rotation speed of the single-crystalline silicon rod; determining the descending speed of the graphite crucible.

3. According to the preparation method described in claim 2, characterized in that, the rotation direction of the single-crystalline silicon rod is opposite to the weaving direction of the outermost weaving layer in the seed crystal rope; wherein, the seed crystal rope is composed of a central part and at least one weaving layer arranged on the outer circumference of the central part.

4. According to the preparation method described in claim 2, characterized in that, the rotation speed of the single-crystalline silicon rod is 15 rpm to 20 rpm.

5. According to the preparation method described in claim 2, characterized in that, the rotation direction of the graphite crucible is opposite to the rotation direction of the single-crystalline silicon rod.

6. According to the preparation method described in claim 2, characterized in that, the rotation speed of the graphite crucible is 0.1 rpm to 0.2 rpm.

7. According to the preparation method described in claim 2, characterized in that, the descending speed of the graphite crucible is 0.1 mm / min to 1.0 min / min.

8. According to the preparation method described in claim 7, characterized in that, the descending speed of the graphite crucible is 0.1 mm / min to 0.5 mm / min.

9. According to the preparation method described in claim 8, characterized in that, the height of the upper edge of the graphite crucible is lower than the height of the upper edge of the quartz crucible.

10. According to the preparation method described in claim 9, characterized in that, a fastening ring is provided on the upper edge of the quartz crucible, and the fastening ring abuts against the upper edge of the graphite crucible.

11. A silicon wafer, characterized in that, the silicon wafer is cut from the single-crystalline silicon rod according to claim 1.

12. According to the silicon wafer described in claim 11, characterized in that, the oxygen content in the silicon wafer is less than 4 ppma.

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