JPS6243958B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for heating a conductive substance to form a melt and pulling crystals from the melt. More specifically, the present invention relates to a new crystal pulling device that allows such a device to be made smaller and lighter. [Prior Art] A typical method for pulling crystals from a melt is the Czyochralski method. In the Czyochralski method, as shown in Fig. 3, a cylindrical heating element 2 is placed concentrically with the crucible 1 on the outside of the crucible 1 with a circular cross section, and, for example, an electric current is passed through the heating element 2. A melt 3 of a predetermined substance is created in the crucible 1 by Juur heat generated thereby, and a seed crystal 5 made of a single crystal with an aligned crystal orientation is positioned on the rotation center line 4 of the melt 3. By gradually pulling up the support 6 which is brought into contact with the surface of the melt 3 and connected to the seed crystal 5, a single crystal 7 having the same crystal orientation as the seed crystal 5 is grown.
In this case, since the molten body 3 is heated mainly from the sides by the heating body 2, the temperature at the center of the molten body 3 is lower than the temperature at the outer circumference. As a result, thermal convection occurs within the melt 3 as roughly indicated by the arrow 20. This thermal convection brings about temperature fluctuations at the interface 9 where the single crystal 7 grows, and as a result, it has an adverse effect such as causing non-uniformity of characteristics and crystal defects inside the grown single crystal 7. Therefore, when the melt 3 is a conductive substance such as silicon, which is an important crystalline material in the semiconductor industry, by applying a strong DC magnetic field of 1000 oersted or more to the melt 3, the above-mentioned It suppresses heat convection such as That is, as shown in FIG. 4, two huge magnets 38 are arranged so as to sandwich the heating element 2,
The horizontal direct current magnetic field 58 generated from the melt 3
is applied to. Note that the heating element 2 is a means for generating heat as described above, and even if a DC magnetic field is generated based on the current flowing there, the strength of the DC magnetic field is limited in terms of suppressing thermal convection. was so weak that it could be ignored. [Problems to be Solved by the Invention] However, such conventional devices have the following problems. Problem 1: The conventional device requires a huge magnet 38 that weighs several tons.
There were problems in that the entire device became larger and heavier, and the planar dimension of the device (indicated by _W1 in FIG. 4) became larger. That is, there is a drawback in that it is difficult to reduce the size and weight of the device, which is an extremely important and realistic requirement for accommodating the device. Problem 2: In conventional devices of this type, the cross-sectional shape of the melt 3 is usually circular and symmetrical with respect to the rotation center line 4, and it is desirable that the cross-sectional shape of the single crystal 7 is also usually circular. In contrast, the DC magnetic field 58 generated by the magnet 38 is asymmetrical with respect to the rotation center line 4. [Note that asymmetry in this case means that there is no isotropy in all directions when the rotation center line is the center. ] Therefore, asymmetry occurs in the temperature conditions and other aspects within the cross section of the melt 3, resulting in problems related to crystal growth, such as non-uniformity of properties within the deformed cross section of the single crystal 7. The drawback was that problems occurred as side effects. A supplementary explanation will be given regarding this phenomenon. When the molten body is uniformly heated from the outside by a heating element and no magnetic field is applied to the molten body, as shown schematically in FIG. Heat convection occurs. By the way, in the conventional apparatus shown in FIG. 4, the magnetic field is applied as a horizontal magnetic field orthogonal to the rotation center line of the melt (single crystal).
In other words, an asymmetric transverse magnetic field is applied to the melt, and in this case, the thermal convection a~ shown in Figure 7
The longitudinal flow of f is suppressed, and the thermal convection a, d
The lateral flow of is not suppressed, but the thermal convection b,
Since the components of the lateral flows c, e, and f in the direction perpendicular to the lines of magnetic force are suppressed, the lateral flows of the thermal convection b, c, e, and f become parallel to the lines of magnetic force. (Fig. 8a) Therefore, the heat flow due to thermal convection a to f becomes as shown in Fig. 8b, and the isotherm of the melt becomes elliptical as shown in Fig. 8c. If the crystal is pulled up without rotation, the cross-sectional shape of the crystal becomes elliptical. In order to prevent the cross-sectional shape of the crystal from becoming elliptical, when the crystal is rotated, one growth point of the crystal passes through points A to D shown in FIG. 8c, for example.
Temperature fluctuations occur at the interface where the crystal grows, causing a change in the crystal growth rate, resulting in non-uniform impurity concentrations in the cross section of the crystal and crystal defects. Furthermore, in the conventional apparatus, since a transverse magnetic field is applied to the melt, as mentioned above, the flow in the longitudinal direction of the closed thermal convection curve within the melt is suppressed. In FIG. 10, the unsuppressed lateral flow is omitted and only the lengths contributing to suppression are shown by arrows. In the figure, a shows a case in which a large amount of the molten liquid remains immediately after the start of crystal pulling, and b shows a case in which only a small amount of molten liquid remains just before the crystal pulling is completed. As is clear from comparing the two, in the conventional example shown in Figure 4, as the amount of residual melt decreases as the crystal grows, the vertical flow of thermal convection becomes shorter.
As the amount of the melt decreases, the effect of suppressing thermal convection by the transverse magnetic field decreases, and the agitation of the melt increases. A drawback is that the concentration of crystal impurities cannot be made constant in the length direction of the crystal. [Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention heats a conductive substance to form a molten body,
A crystal growth apparatus for pulling crystals from the melt is provided with at least a cylindrical solenoid centered on the rotation center line of the melt as means for applying a magnetic field for suppressing convection in the melt. Hereinafter, the functions and effects of the present invention will be explained with reference to Examples. [Embodiment] FIG. 1 is a diagram showing a first embodiment of a crystal growth apparatus according to the present invention. In this crystal growth apparatus, a cylindrical solenoid 8 concentric with the heating element 2 is arranged outside the heating element 2. When a direct current is applied to such a solenoid 8, a direct current magnetic field 18, which is mainly longitudinal, is generated at a location inside the solenoid 8 where the molten material 3 is present, with a strength proportional to the number of turns of the solenoid 8 and the magnitude of the current. It has been known for a long time that this occurs based on the Biot-Savart law. For example, if the number of turns of the solenoid 8 is 10 turns per 1 cm of the height of the solenoid 8, and the magnitude of the current is 100 amperes, the strength of the DC magnetic field 18 will be approximately 1300 oersted,
It is strong enough to suppress heat convection. As described above, the present invention has the same feature as the conventional device in that it can generate a magnetic field strong enough to suppress thermal convection, but it differs from the conventional device shown in FIG. 4 in its configuration. That is, in the present invention, a cylindrical solenoid is provided that is centered on the rotation center line of the molten material, whereas in the conventional device, an electromagnet (magnetic field device) is installed so that its magnetic field lines are horizontal. are doing. Due to this difference in configuration, the present invention and the conventional device also have different effects. That is, in a crystal growth apparatus for pulling crystals from a melt to which the present invention pertains, the height is generally 5 to 10 mm compared to the diameter.
It has a vertically elongated structure, and many of the crucibles and other components are nearly cylindrical in shape. In other words, the present invention focuses on the fact that the entire device can be regarded as a vertically elongated cylinder, and in the present invention, a similarly vertically elongated solenoid is provided on the outside of the solenoid, thereby generating a magnetic field for suppressing convection. In this way, an extremely excellent practical effect can be obtained in that the overall dimensions including the magnetic field applying means need only be slightly increased compared to the basic structure shown in FIG. 3 which does not have the magnetic field applying means. Figures 1 and 4 show the main parts of the actual device at almost the same scale, but it is clear from comparing W ₁ and W ₀ that the device can be made smaller. I understand. Furthermore, by using a solenoid, a strong magnetic field can be applied with less electric power, so that not only the size of the device can be reduced, but also the weight and cost of the device can be reduced. Furthermore, from the viewpoint of magnetic field strength distribution, solenoids are advantageous in suppressing agitation of the melt. That is, it is natural for the overall structure of the device to align the center of the magnetic field application means such as a solenoid with the center of the melt, but considering the closed thermal convection loop of the melt as shown in FIG. In the center of the melt, there is a component that descends along the rotation center line, but in this case, as shown in Figure 9a, in the configuration of the present invention that can apply a vertical magnetic field, the part shown by the solid line (convection) Convection suppression does not act on the area (where the direction of convection is parallel to the magnetic field), and the suppression force acts only on the area indicated by the dotted line (the area where the direction of convection is perpendicular to the magnetic field). The suppression effect does not work, and a transverse flow that interacts with the longitudinal magnetic field occurs in a portion radially away from the rotation center line. Therefore, in order to effectively suppress the thermal convection closed loop using a magnetic field, it is more effective to use a magnetic field application means that has a stronger magnetic field strength in the radially distant parts than in the center of the melt. Since the magnetic field passing through the center is the weakest and becomes stronger toward the periphery near the solenoid, it meets the above purpose. Therefore, the solenoid has the advantage of being able to efficiently suppress thermal convection even with a small amount of electric power.Furthermore, the solenoid has the advantage that the disturbance in the magnetic field distribution is small and the heating element does not vibrate or break. On the other hand, when using electromagnets to apply a vertical magnetic field as in conventional devices, a huge electromagnet with an extremely long air gap between the two magnetic poles must be installed, making the device large. , it is heavy (approximately 20 times as much as when using a solenoid), expensive (approximately 10 times as much as when using a solenoid), and the strength of the magnetic field generated in the air gap is large. is approximately inversely proportional to the square of the length of the air gap, and contrary to the magnetic field distribution of the solenoid described above, the lines of magnetic force are the strongest at the center of the magnetic pole of the electromagnet, and the lines of magnetic force become weaker toward the outside, so the desired strength can be obtained. The power consumption to generate such a magnetic field is unavoidable, and furthermore, the disturbance in the magnetic field distribution is large, causing vibration and damage to the heating element. Next, another effect of the present invention will be explained. In the present invention, since a DC magnetic field is applied to the melt that is symmetrical in any radial direction with respect to the center line of rotation of the melt, as shown in FIGS.
There is no asymmetrical heat flow or elliptical isothermal lines at the solid-liquid interface, which are caused by the conventional apparatuses described in items 1 to 3 applying a horizontal magnetic field. That is, if the features of the present invention are illustrated in comparison with FIG. 8, it will be as shown in FIG. As shown in the figure, when a symmetrical longitudinal magnetic field is applied to the melt, the flow perpendicular to the lines of magnetic force is suppressed, but the flow parallel to the lines of magnetic force is not suppressed, so the longitudinal flow of thermal convection a to f is suppressed. However, the lateral flow of thermal convection a to f is suppressed, and since thermal convection a to f are closed flows, thermal convection a to f are weakened overall, and thermal convection a to f are Since they are suppressed symmetrically with respect to the center line of rotation of the melt, thermal convection a to f become symmetrical with respect to the center line of rotation of the melt, and thermal convection a to
Since the heat flow due to f is as shown in FIG. 9b, the isothermal line in the upper main surface of the melt, which is most important for crystal growth, becomes circular as shown in FIG. 9c. (As shown in Figure 9a, the part of the closed loop of thermal convection that is essentially weakened by the external magnetic field is shown by a broken line, and the part that is not weakened is shown by a solid line.)
Therefore, the cross-sectional shape of the crystal becomes circular regardless of whether it is pulled with or without rotation. In addition, even when pulling with rotation, a specific point on the outer periphery of the crystal always rotates while contacting the melt at the same temperature, so the growth rate of the crystal does not change, and impurities in the cross section of the crystal do not change. It has the advantage that the concentration is uniform and no crystal defects occur. Furthermore, when a vertical magnetic field is applied to the melt as in the present invention, the lateral flow of thermal convection is suppressed, and even if the amount of the melt decreases as shown in Figures 11a and b, , the length of the horizontal flow of thermal convection (indicated by solid arrows; the longitudinal direction of unsuppressed thermal convection is omitted) does not change, so even if the amount of melt decreases, the vertical The effect of suppressing thermal convection by the magnetic field is always constant. When the melt is strongly stirred, the concentration of impurities varies greatly along the length of the crystal, whereas when the melt is not stirred at all, the concentration of impurities changes along the length of the crystal. It is known that it is constant in the horizontal direction. Therefore, as in the present invention, if the effect of suppressing thermal convection is always constant regardless of the decrease in the amount of the melt, the melt can be kept in a constant stirring state or stirred under appropriate conditions. Since the impurity concentration can be maintained in a state where there is almost no impurity, there is an advantage that the concentration of impurities can be kept constant in the length direction of the crystal. As described above, applying a vertical magnetic field symmetrically with respect to the center line of rotation of the melt has many advantages, but it is not possible to simply use the conventional magnetic field applying means shown in Fig. 4 as is. I can't. Fifth
The figure and Figure 6 show the conventional apparatus shown in Figure 4, but with the same dimensions as the crucible, crystal pulling section, etc., and the electromagnets are arranged vertically so that the same strength of magnetic field can be applied to the melt. This shows the size. Both the width W and the height H are extremely large, and are 5 to 10 times larger in volume and 10 to 20 times larger in weight. On the other hand, in the present invention, as shown in FIGS. 1 and 2, the increase in size and weight of the entire device due to the addition of the magnetic field applying means can be significantly smaller than that in FIG. 6. Note that FIG. 2 shows a second embodiment of the present invention.
(Note that the airtight container 30 and the like are omitted.) In this crystal growth apparatus, instead of the heating body 2 and solenoid 8 shown in FIG. A body 8 is arranged. (In other words, the heating element 2 is omitted.) This heating/magnetic field applying element 8 is a cylindrical solenoid, which is provided concentrically with the crucible 1, and the number of turns per 1 cm of height of the solenoid. For example, if the current 28 is 1000 amperes, it is possible to heat the melt 3 sufficiently and apply a magnetic field strong enough to suppress thermal convection. Furthermore, it is clear that the mainly longitudinal magnetic field generated by the heating/magnetic field applying body 8 is symmetrical with respect to the rotation center line 4 of the melt 3. [Effects of the Invention] As explained above, in the present invention, in a crystal growth apparatus that heats a conductive substance to form a melt and pulls a crystal from the melt,
By providing a cylindrical solenoid centered on the rotation center line of the molten body as a magnetic field applying means for suppressing thermal convection of the molten body, it is possible to make the device smaller, lighter, and lower in price. Furthermore, since a solenoid is used, disturbances in the magnetic field distribution are small and the heating element does not vibrate or break. Furthermore, since an isotropic DC magnetic field (longitudinal magnetic field) is applied to the melt with respect to the center line of rotation of the melt, the heat flow becomes symmetrical and the isothermal lines at the solid-liquid interface also become concentric. Therefore, even if the crystal is pulled up by rotation, the growth point of the crystal rotates at the same temperature point, so that it is not subject to temperature fluctuations, ensuring uniformity of impurity concentration, and suppressing the occurrence of defects. Furthermore, the effect of suppressing thermal convection is always constant regardless of the decrease in the amount of melt, so that the concentration of impurities can be kept constant in the length direction of the crystal. The table below shows a comparison with a conventional example when applied to a crystal pulling device with a diameter of 3 inches. 【table】

[Brief explanation of the drawing]

Figures 1 and 2 are examples of the present invention, Figure 3 is a schematic cross-sectional view showing a conventional basic crystal growth apparatus that does not apply a magnetic field, and Figure 4 is a conventional crystal growth apparatus that applies a magnetic field. The explanatory diagrams of the apparatus, Figures 5 and 6, are for comparison of the overall size of the apparatus when the design of the magnetic field application device of the conventional crystal growth apparatus shown in Figure 4 is changed to simply apply a magnetic field in the vertical direction. FIG. 7 is a diagram for explaining the convection in the melt; FIG. 8 is a diagram for explaining the relationship between the applied magnetic field and the convection in the melt in the conventional example shown in FIG. 4; The figure is a diagram for explaining the relationship between the applied magnetic field and the convection in the melt in the present invention, and FIGS. 10 and 11 respectively show the conventional example shown in FIG. FIG. 2 is an explanatory diagram comparing the size of a portion between the state immediately after crystal pulling and the state immediately before completion of crystal pulling. 1... Crucible, 2... Heating body, 3... Molten body,
4... Rotation center line, 5... Seed crystal, 6... Support, 7... Single crystal, 8... Solenoid, 9... Solid-liquid interface, 18... DC magnetic field created by solenoid, 2
0...Convection in the melt caused by the heating element, 28
... Current flowing through the solenoid, 30 ... Airtight container,
38, 48...electromagnet, 58...DC magnetic field created by the electromagnet.

Claims (1)

[Claims] 1. Heating a conductive substance to make it into a melt,
A crystal growth apparatus for pulling crystals from the melt, characterized in that a cylindrical solenoid centered on the rotation center line of the melt is provided as means for applying a magnetic field for suppressing thermal convection in the melt. growth equipment. 2. The crystal growth apparatus according to claim 1, wherein the solenoid serves both as a device for applying a magnetic field to the melt and as a device for heating the melt.