Method for manufacturing silicon single crystals

Abstract

To grow a silicon single crystal having small defect density over a crystal full length and a high defect-free area rate by controlling a variation in a diameter within a fixed range even when constantly maintaining a value v / G when pulling the silicon single crystal from a silicon melt by the Czochralski method to cause a periodic variation in a crystal diameter.SOLUTION: A method for manufacturing a silicon single crystal includes forming a silicon melt M in a crucible by the heating of a heater 4 to grow a silicon single crystal C by the Czochralski method. The pulling of a straight body part when manufacturing the silicon single crystal comprises the steps of: setting a pulling rate of the crystal pulled while rotating around the axis to a constant and controlling output of the heater so that a temperature gradient of a solid-liquid interface is constant; and controlling a crystal rotation frequency so as to maintain -0.0335<(a diameter variation / a period of time (mm / min))<0.0335 during the step of controlling the output of the heater.SELECTED DRAWING: Figure 2

Description

Technical Field

[0004]

[0001] The present invention relates to a method for manufacturing a silicon single crystal, and particularly to a method for manufacturing a silicon single crystal that grows a silicon single crystal with a high defect-free region ratio by the Czochralski method (CZ method).

Background Art

[0002] The growth of a silicon single crystal by the CZ method is carried out by filling a quartz crucible 51 installed in a chamber 50 as shown in FIG. 4 with polysilicon as a raw material, heating and melting the polysilicon by a heater 52 provided around the quartz crucible 51 to obtain a silicon melt M, immersing a seed crystal (seed) P attached to a seed chuck in the silicon melt, and pulling up the seed chuck while rotating the seed chuck and the quartz crucible 51 in the same or opposite directions.

[0003] Generally, prior to the start of pulling, after the temperature of the silicon melt M has stabilized, necking is performed by bringing the seed crystal P into contact with the silicon melt M to dissolve the tip of the seed crystal P. Necking is an essential process for removing dislocations generated in the silicon single crystal due to the thermal shock generated by the contact between the seed crystal P and the silicon melt M. A neck portion P1 is formed by this necking. Also, this neck portion P1 generally has a diameter of 3 to 4 mm, and its length requires at least 30 mm or more, and depending on conditions, a length of 100 to 500 mm is required.

[0004] Also, as a process after the start of pulling, after the necking is completed, a process of forming a shoulder portion C1 that expands the crystal to the diameter of the straight body portion, a process of forming a straight body portion C2 that grows the single crystal to be a product, and a process of forming a tail portion (not shown) that gradually reduces the diameter of the single crystal after the straight body portion forming process are performed.

[0005] Incidentally, crystals grown by the CZ method incorporate point defects during crystallization. These point defects form grown-in defects, but generally, to grow crystals known as defect-free crystals, the pulling rate v and the temperature gradient at the solid-liquid interface G are controlled so that v / G is a certain value, and this is used as an indicator to grow crystals with a very low defect density.

[0006] However, in single crystal growth, the margin of the v / G value required to create a defect-free region is very small, making it difficult to obtain a defect-free crystal throughout its entire length. To address these challenges, Patent Document 1 discloses a method for predicting the change in the temperature gradient G in the crystal length direction in advance and changing the pulling rate v in accordance with this change in temperature gradient G. With this method, if the prediction of the change in temperature gradient G is accurate, the value of v / G can be kept constant. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-15297 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The single crystal pulling method disclosed in Patent Document 1 is based on the premise that the crystal diameter of the single crystal is always constant. However, in typical single crystal pulling, the pulling rate v is kept constant, and the output of the heater heating the quartz crucible is varied to control the temperature gradient G at the solid-liquid interface to be constant. As a result, the molten convection inside the crucible changes in response to the amount of heating, and, as schematically shown in Figure 5, small but periodic diameter fluctuations occur during the growth of the straight cylinder C2. When such periodic fluctuations in crystal diameter occur, the defect density also changes periodically along the crystal length. When the fluctuations are large, the region partially changes to one dominated by vacancies or interstitial silicon, resulting in the growth of crystals containing high-density defects and a decrease in yield.

[0009] The inventors of this invention have diligently conducted research to grow single crystals with a high defect-free region ratio, based on the premise that the pulling speed v is kept constant and the output of the heater heating the quartz crucible is varied to keep the value of the temperature gradient G at the solid-liquid interface constant (maintaining a constant value of v / G) (i.e., assuming that the above-mentioned periodic fluctuations in crystal diameter occur), and have arrived at the present invention. The object of the present invention is to provide a method for producing silicon single crystals that, when pulling up silicon single crystals from a silicon molten liquid by the Czochralski method, maintains a constant value for v / G and controls the amount of diameter change within a certain range even if periodic crystal diameter fluctuations occur, thereby enabling the growth of silicon single crystals with a low defect density and a high defect-free region ratio throughout the entire crystal length. [Means for solving the problem]

[0010] To solve the aforementioned problems, the present invention provides a method for manufacturing a silicon single crystal, comprising: forming a silicon melt in a crucible by heating with a heater and growing a silicon single crystal by the Czochralski method, characterized in that, in the pulling of the straight body portion in the manufacturing of the silicon single crystal, the pulling speed of the silicon single crystal pulled while rotating it around its axis is kept constant, and the output of the heater is controlled so that the temperature gradient at the solid-liquid interface and the diameter of the silicon single crystal are kept constant; and during the step of controlling the heater output, the crystal rotation speed is controlled so that -0.0335 < (change in silicon single crystal diameter / time (mm / min)) < 0.0335.

[0011] Furthermore, during the process of controlling the output of the heater, it is desirable to set the initial value of the crystal rotation speed based on the crystal rotation speed during the pulling of the previous lot in the process of controlling the crystal rotation speed so as to maintain -0.0335 < (change in silicon single crystal diameter per time (mm / min)) < 0.0335. Furthermore, during the process of controlling the output of the heater, it is desirable to control the crystal rotation speed in the process of controlling the crystal rotation speed so as to maintain -0.0335 < (change in diameter of silicon single crystal per time (mm / min)) < 0.0335, based on the relationship between the change in diameter per time (mm / min) and the crystal rotation speed until at least one period of change in crystal diameter has elapsed in the crystal length direction. Furthermore, during the process of controlling the output of the heater, in the process of controlling the crystal rotation speed so as to maintain -0.0335 < (change in diameter of silicon single crystal per time (mm / min)) < 0.0335, the crystal rotation speed may be controlled based on the relationship y = 0.0049x, where y is the change in diameter per time (mm / min) and x is the adjustment range from the reference value of the crystal rotation speed (rpm).

[0012] Thus, according to the present invention, in the pulling of a straight single crystal body, the pulling speed is kept constant, and the heater output is controlled so that the temperature gradient at the solid-liquid interface and the diameter of the silicon single crystal remain constant. In addition, the crystal rotation speed is controlled so as to maintain -0.0335 < (diameter change / time (mm / min)) < 0.0335. This minimizes the periodic change in crystal diameter, preventing the occurrence of regions with excessively high defect density (vacancies, interstitial silicon-dominant regions) in the crystal length direction of the single crystal, thereby suppressing a decrease in the yield of the grown single crystal. [Effects of the Invention]

[0013] According to the present invention, when pulling a silicon single crystal from a silicon molten liquid using the Czochralski method, even if periodic crystal diameter fluctuations occur, the amount of diameter change can be controlled to stay within a certain range by maintaining a constant v / G value, thereby providing a method for growing a silicon single crystal with a low defect density and a high defect-free region ratio throughout the entire crystal length. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a cross-sectional view of a single crystal pulling apparatus in which the silicon single crystal manufacturing method according to the present invention is carried out. [Figure 2] Figure 2 is a flowchart of the method for manufacturing a silicon single crystal according to the present invention. [Figure 3] Figure 3 is a graph showing the results of Experiment 1 in an embodiment of the present invention. [Figure 4] Figure 4 is a cross-sectional view showing the schematic configuration of a conventional single crystal pulling apparatus. [Figure 5] Figure 5 is a schematic cross-sectional view showing the periodic change in crystal diameter. [Modes for carrying out the invention]

[0015] The method for manufacturing silicon single crystals according to the present invention will be described below with reference to the drawings. Figure 1 is a cross-sectional view of a single crystal pulling apparatus in which the method for manufacturing silicon single crystals according to the present invention is carried out. This single crystal pulling apparatus 1 comprises a furnace body 10 formed by stacking a pull chamber 10b on top of a cylindrical main chamber 10a, and a carbon crucible (or graphite crucible) 2 provided within the furnace body 10 that is rotatable around a vertical axis and can be raised and lowered, and a quartz glass crucible 3 (hereinafter simply referred to as crucible 3) held by the carbon crucible 2. This crucible 3 is rotatable around a vertical axis in conjunction with the rotation of the carbon crucible 2.

[0016] Also, below the carbon crucible 2, there are provided a rotation drive unit 14 such as a rotation motor for rotating the carbon crucible 2 around the vertical axis, and a lifting drive unit 15 for moving the carbon crucible 2 up and down. In addition, a rotation drive control unit 14a is connected to the rotation drive unit 14, and a lifting drive control unit 15a is connected to the lifting drive unit 15.

[0017] The single crystal pulling apparatus 1 further includes a side heater 4 for melting a semiconductor raw material (raw material polysilicon) loaded in the crucible 3 by resistance heating to obtain a silicon melt M (hereinafter simply referred to as melt M), and a pulling mechanism 9 for winding up the wire 6 and pulling up the grown single crystal C. A seed crystal P is attached to the tip of the wire 6 included in the pulling mechanism 9.

[0018] In addition, a heater control unit 4a for controlling the amount of supplied power is connected to the side heater 4, and a rotation drive control unit 9a for controlling the rotation drive of the pulling mechanism 9 is connected to the pulling mechanism 9. In this embodiment of the present invention, in this single crystal pulling apparatus 1, for example, a magnetic field applying electromagnetic coil 8 is installed outside the furnace body 10. When a predetermined current is applied to the magnetic field applying electromagnetic coil 8, a horizontal magnetic field of a predetermined intensity is applied to the silicon melt M in the crucible 3. An electromagnetic coil control unit 8a for controlling the operation of the magnetic field applying electromagnetic coil 8 is connected to the magnetic field applying electromagnetic coil 8. That is, in this embodiment, the MCZ method (Magnetic field applied CZ method) for growing a single crystal by applying a magnetic field in the melt M is implemented, thereby controlling the convection of the silicon melt M and achieving the stability of single crystallization.

[0019] Also, above the melt M formed in the crucible 3, a radiation shield 7 surrounding the periphery of the single crystal C is disposed. The radiation shield 7 has openings formed at the upper and lower portions, shields unnecessary radiant heat from the side heater 4, the melt M, etc. to the growing single crystal C, and rectifies the gas flow in the furnace. Furthermore, the gap between the lower end of the radiation shield 7 and the molten liquid surface is controlled to maintain a predetermined distance (e.g., 50 mm) according to the desired properties of the single crystal being grown.

[0020] Furthermore, the single crystal pulling apparatus 1 is equipped with an optical diameter measuring sensor 16, such as a CCD camera, for measuring the diameter of the single crystal being grown. A small observation window 10a1 is provided on the upper surface of the main chamber 10a, and the positional change of the crystal edge (position indicated by the dashed arrow) at the solid-liquid interface is detected from outside this small window 10a1.

[0021] Furthermore, the single crystal pulling apparatus 1 is equipped with a radiation thermometer 17 for measuring the temperature of the molten surface M1. A small window 10a2, separate from the small window 10a1, is provided on the upper surface of the main chamber 10a, and the temperature of the molten surface is measured from outside this small window 10a2.

[0022] Furthermore, this single crystal pulling apparatus 1 is equipped with a computer 11 having a memory device 11a and an arithmetic control device 11b, and the rotation drive control unit 14a, the lifting drive control unit 15a, the electromagnetic coil control unit 8a, the rotation drive control unit 9a, the diameter measuring sensor 16, and the radiation thermometer 17 are each connected to the arithmetic control device 11b.

[0023] In the single crystal pulling apparatus 1 configured in this way, for example, when growing a single crystal C with a diameter of 310 mm, the pulling is performed as follows. Specifically, the raw material polysilicon (for example, 350 kg) is first loaded into the crucible 3, and the crystal growth process is started based on a program stored in the memory device 11a of the computer 11.

[0024] First, the inside of the furnace body 10 is set to a predetermined atmosphere (mainly an inert gas such as argon gas). For example, a furnace atmosphere with a furnace pressure of 65 torr and an argon gas flow rate of 90 l / min is formed. Then, with the crucible 3 rotating in a predetermined direction at a predetermined rotational speed (rpm), the raw polysilicon loaded into the crucible 3 is melted by heating with the side heater 4 and becomes molten liquid M (step S1 in Figure 2).

[0025] Next, a predetermined current is passed through the electromagnetic coil 8 for applying the magnetic field, and a horizontal magnetic field is applied to the molten liquid M with a magnetic flux density set within the range of 1000 to 4000 Gauss (for example, 3000 Gauss) (step S2 in Figure 2). Furthermore, the pulling conditions are adjusted using parameters such as the power supplied to the side heater 4, the pulling speed, and the applied magnetic field strength, and the seed crystal P starts rotating around its axis at a predetermined rotational speed. The direction of rotation is opposite to the direction of rotation of the crucible 3. Then, the wire 6 is lowered and the seed crystal P comes into contact with the molten liquid M, and after the tip of the seed crystal P is melted, necking is performed and the neck portion P1 is formed (step S3 in Figure 2).

[0026] Then, the crystal diameter gradually expands to form the shoulder portion C1 (step S4 in Figure 2), and the process moves on to forming the straight body portion C2, which will become the product (step S5 in Figure 2). Here, the computer 11 controls the lifting drive unit 15 with the lifting drive control unit 15a to keep the lifting speed v constant at, for example, 0.55 mm / min. Furthermore, the computer 11 instructs the heater control unit 4a to control the amount of power supplied to the side heater 4 so that the temperature gradient G at the solid-liquid interface detected by the radiation thermometer 17 remains constant, i.e., the value of v / G remains constant, and so that the position change of the crystal edge at the solid-liquid interface detected by the diameter measuring sensor 16 is converted into a crystal diameter, and this crystal diameter value remains constant. This control allows the crystal diameter to approach a constant value (310 mm in this embodiment), but in reality, the molten convection in the crucible changes, causing periodic small diameter fluctuations in the single crystal C.

[0027] In order to suppress this periodic diameter variation, the crystal rotation speed is controlled in this embodiment. Specifically, the computer 11 sets an initial value for the crystal rotation speed based on the crystal rotation speed (for example, the average value) in the straight body formation process of the previous lot and starts the control. Since the crystal diameter in the direction of the crystal length changes periodically, the computer 11 monitors the relationship between the diameter change per time (mm / min) and the crystal rotation speed (rpm) until at least one period of change in crystal diameter has elapsed (for example, until the straight body length becomes 250 mm), and adjusts the crystal rotation speed in subsequent straight body formation based on the results. More specifically, the computer 11 monitors the rate of change in crystal diameter per minute (diameter change / time (mm / min)) and determines whether -0.0335 < (diameter change / time (mm / min)) < 0.0335 is maintained until one cycle of change in crystal diameter has elapsed.

[0028] Here, the computer 11 controls the rotation speed of the crystal by the rotation drive control unit 9a if the rate of change in crystal diameter per minute (rate of change in diameter per hour (mm / min)) is not -0.0335 < (rate of change in diameter per hour (mm / min)) < 0.0335. The lower the crystal rotation speed per unit time, the smaller the change in crystal diameter; and the higher the crystal rotation speed, the larger the change in crystal diameter. Specifically, the computer 11 controls the crystal rotation speed based on the relationship equation 1: y = 0.0049x, where y is the diameter change per unit time (mm / min) and x is the adjustment range (rpm) from the reference value of the crystal rotation speed. When the diameter change per unit time (mm / min) becomes y = 0.0335 or higher (step S6 in Figure 2), the adjustment range of the crystal rotation speed x is determined from equation 1 (step S7 in Figure 2).

[0029] Furthermore, if the diameter change per time (mm / min) exceeds ±0.0335, the diameter change is too large, the defect density changes periodically in the crystal length direction, and regions with excessively high defect density occur, which is undesirable. By controlling the crystal rotation speed in this way and keeping the periodic change in crystal diameter within a small range, it is possible to prevent the occurrence of regions with excessively high defect density (vacancies, interstitial silicon-dominated regions) in the crystal length direction of the single crystal C, thereby suppressing a decrease in the yield of the grown single crystal.

[0030] Once the straight body section C2 is formed to a predetermined length, the process moves to the final tail section (step S8 in Figure 2). In this tail section, the contact area between the lower end of the crystal and the molten liquid M gradually decreases, separating the single crystal C from the molten liquid M, and a silicon single crystal is produced.

[0031] As described above, according to this embodiment, in pulling the straight single crystal body C2, the pulling speed v is kept constant, the output of the side heater 4 is controlled so that the temperature gradient G at the solid-liquid interface is constant and the diameter of the silicon single crystal is constant, and the crystal rotation speed is controlled so that -0.0335 < (diameter change amount / time (mm / min)) < 0.0335. This minimizes the periodic change in crystal diameter, preventing the occurrence of regions with excessively high defect density (vacancies, interstitial silicon-dominant regions) in the crystal length direction of the single crystal C, thereby suppressing a decrease in the yield of the grown single crystal.

[0032] In the above embodiment, the initial value of the crystal rotation speed was based on the setting value at the time of pulling up the previous lot, but the present invention is not limited to that example. That is, the most distinctive feature of the present invention is to control the crystal rotation speed so as to maintain -0.0335 < (diameter change amount / time (mm / min)) < 0.0335, regardless of the initial value of the crystal rotation speed. Furthermore, in the above embodiment, the MCZ method, in which a magnetic field is applied to grow a single crystal in the molten liquid M, was applied. However, the present invention is not limited to this, and can also be applied to the CZ method without applying a magnetic field. [Examples]

[0033] The method for producing a silicon single crystal according to the present invention will be further described based on examples. (Experiment 1) In Experiment 1, 350 kg of silicon raw material was packed into a quartz crucible and melted. The distance between the radiation shield and the molten surface was set to 50 mm, the furnace pressure was 65 torr, argon gas was flowed at a flow rate of 90 l / min, and a transverse magnetic field strength of 3000 Gauss was created inside the furnace. The crucible rotation speed was set to 0.5 rpm, the reference crystal rotation speed to 10.0 rpm (opposite to the crucible rotation), and single crystal growth was performed with a target crystal diameter of 310 mm at a pulling speed of 0.55 mm / min.

[0034] In this process, the crystal rotation speed (rpm) was changed in the negative direction from the reference value of 10.0 rpm, and the change in diameter per time (mm / min) was measured. The graph in Figure 3 shows the results of Experiment 1. In the graph in Figure 3, the horizontal axis x represents the adjustment range (rpm) from the reference value of 10.0 rpm for crystal rotation speed (rpm), and the vertical axis y represents the change in diameter per time (mm / min). As shown in the graph in Figure 3, a proportional relationship was observed between the crystal rotation speed and the change in diameter per time (relationship y = 0.0049x). Based on this relationship, we confirmed that the adjustment range of the crystal rotation speed (rpm) can be adjusted to control the value of the diameter change per time (mm / min).

[0035] (Experiment 2) In Experiment 2, the same furnace environment as in Experiment 1 was created, the crucible rotation speed was set to 0.5 rpm, and the reference crystal rotation speed was set to 10.0 rpm (opposite to the crucible rotation direction). The heater output was controlled so that the temperature gradient at the solid-liquid interface and the diameter of the silicon single crystal remained constant. In the example, single crystal growth was performed while adjusting the crystal rotation speed. In the comparative example, no adjustment of the crystal rotation speed was performed. Then, we investigated the relationship between the magnitude of the diameter change and the defect-free region rate (good quality rate) of the grown single crystal. In Example 1, a silicon single crystal with a periodic diameter change of ±3.1 mm over a crystal length of 51 mm was grown at a pulling speed of 0.55 mm / min to a straight body length of 1500 mm, and the defect-free region rate (good product rate) was determined. In Example 2, a silicon single crystal with a periodic diameter change of ±2.3 mm over a crystal length of 41 mm was grown at a pulling speed of 0.56 mm / min to a straight body length of 1500 mm, and the defect-free region rate (good product rate) was determined.

[0036] In Comparative Example 1, a silicon single crystal with a periodic diameter change of ±4.1 mm over a crystal length of 57 mm was grown at a pulling speed of 0.55 mm / min to a straight body length of 1500 mm, and the defect-free region rate (good product rate) was determined. In Comparative Example 2, a silicon single crystal with a periodic diameter change of ±9.8 mm over a crystal length of 60 mm was grown at a pulling speed of 0.55 mm / min to a straight body length of 1500 mm, and the defect-free region rate (good product rate) was determined. In Comparative Example 3, a silicon single crystal with a periodic diameter change of ±4.0 mm over a crystal length of 63 mm was grown at a pulling speed of 0.54 mm / min to a straight body length of 1500 mm, and the defect-free region rate (good product rate) was determined.

[0037] The results for Examples 1 and 2, and Comparative Examples 1, 2, and 3 are shown in Table 1. The results in Table 1 include the defect-free area rate (good product rate) and the range of variation in diameter change / time (mm / min) relative to a diameter change / time = 0.0000 (mm / min).

[0038] [Table 1]

[0039] As shown in Table 1, we confirmed that the defect-free area rate (good product rate) is sufficiently high when the diameter change per time (mm / min) is within the range of ±0.0335 (mm / min).

[0040] From the results of the above examples, it was confirmed that a single crystal with a high defect-free region can be obtained by controlling the crystal rotation speed (rpm) so that the diameter change per time (mm / min) is greater than -0.0335 and less than 0.0335. [Explanation of symbols]

[0041] 1. Single crystal pulling apparatus 2 carbon crucibles 3. Quartz glass crucible 4 Side heaters (heaters) 6 wires 7. Radiation Shield M Silicone melt M1 Melting surface C silicon single crystal C2 Straight body part

Claims

1. A method for producing a silicon single crystal, comprising forming a silicon melt in a crucible by heating with a heater and growing a silicon single crystal by the Czochralski method, In the pulling of the straight body portion in the manufacturing of the aforementioned silicon single crystal, A step of controlling the output of the heater so that the pulling speed of the silicon single crystal, which is pulled up while rotating it around its axis, is constant, and the temperature gradient at the solid-liquid interface and the diameter of the silicon single crystal are constant. During the process of controlling the output of the heater, it is determined whether the change in the diameter of the silicon single crystal per minute, which is the change in the crystal diameter of the silicon single crystal per hour (mm / min), is maintained at -0.0335 < (change in the diameter of the silicon single crystal per hour (mm / min)) < 0.0335. If not maintained, the process involves controlling the crystal rotation speed so that -0.0335 < (silicon single crystal diameter change / time (mm / min)) < 0.0335, based on the relationship between the change in silicon single crystal diameter change / time (mm / min) and the crystal rotation adjustment range (rpm), which is the adjustment range from the reference value of the crystal rotation speed. A method for manufacturing silicon single crystals, characterized by comprising the following features.

2. In the process of controlling the crystal rotation speed, A method for manufacturing a silicon single crystal according to claim 1, characterized in that the initial value of the crystal rotation speed is set based on the crystal rotation speed during the pulling of the previous lot.

3. In the process of controlling the crystal rotation speed, A method for manufacturing a silicon single crystal according to claim 1 or 2, characterized by controlling the subsequent crystal rotation speed based on the relationship between the amount of diameter change / time (mm / min) and the crystal rotation speed until at least one period of change in crystal diameter has elapsed in the crystal length direction.

4. In the process of controlling the crystal rotation speed, A method for manufacturing a silicon single crystal according to any one of claims 1 to 3, characterized in that the crystal rotation speed is controlled based on the relationship y = 0.0049x, where y is the amount of change in diameter change / time (mm / min) and x is the adjustment range from a reference value of the crystal rotation speed (rpm).

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