JPS621357B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a crystal growth method in which a conductive substance is heated to form a melt and a crystal is pulled from the melt. More specifically, the present invention relates to a technique for suppressing thermal convection occurring in a molten liquid by applying a magnetic field.

[Conventional technology]

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 element 2, the temperature at the center of the molten body 3 is lower than the temperature at the outer periphery. Heat convection occurs as shown. FIG. 5 shows a more three-dimensional explanation of such thermal convection. 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 A technology to suppress heat convection has been developed. That is, as shown in FIG.
Two huge magnets 38 are arranged to sandwich the
8 is applied to the melt 3.

Such a direct current magnetic field is effective in suppressing thermal convection in a molten material, and furthermore, the principle of suppressing convection in a molten material is based on the suppressing force when an electrically conductive fluid crosses lines of magnetic force (in other words, This suppresses the flow of the melt perpendicular to the magnetic field), as shown in, for example, Electronic Materials 19 , [10], 1980, P107-113. In addition, in these conventional technologies, the heating body 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 DC magnetic field is The strength of the magnetic field was negligible in terms of suppressing thermal convection, and the magnetic field intended to control the convection of the molten material was a lateral (horizontal) magnetic field generated by a huge electromagnet. .

[Problem that the invention seeks to solve]

However, such conventional crystal growth methods have the following problems.

In the prior art, as shown in FIG. 4, it was assumed that a huge electromagnet was used as the magnetic field generating means, so the magnetic field was directed horizontally, that is, along the crystal pulling axis (or the rotation of the melt). The magnetic field is applied in a direction perpendicular to the center line (center line), and there is no technique that applies a magnetic field in the vertical direction as in the present invention or a technique that suggests it.

This is because in the crystal pulling technology represented by the Czyochralski method, the ratio of the width and height of the equipment is 5 to 10 times, and in order to apply a magnetic field to suppress thermal convection to the melt, it is necessary to If the electromagnets were arranged vertically, it would be necessary to arrange the electromagnets vertically, which would be much larger than if they were arranged horizontally, in order to obtain the same magnetic field strength. This is because there was no sense of purpose in applying a magnetic field in the vertical direction.

However, studies by the inventors of the present application have revealed that such conventional methods of applying a magnetic field in the horizontal direction have various serious drawbacks. That is, in the conventional method of applying a magnetic field in the horizontal direction, asymmetry occurs in temperature conditions and other factors within the cross section of the melt 3, resulting in non-uniformity of properties within the cross section of the single crystal 7, etc. It has become clear that serious problems related to growth will arise. This point will be explained in more detail. 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 schematically shown in FIG. Heat convection occurs that is symmetrical about the center line of rotation.

When a horizontal magnetic field is applied as in the conventional example, the vertical flow of the individual closed curves of thermal convection a to f is suppressed, and the horizontal direction ( The flow in the horizontal direction (in the horizontal plane) is not suppressed, but the component of the flow in the horizontal direction (in the horizontal plane) of the thermal convection b, c, e, f in the direction perpendicular to the magnetic field lines is suppressed, so the thermal convection b, c, e, The lateral flow of f is parallel to the magnetic field lines.

The results are shown in Figure 6a. Here, 58 is a horizontal magnetic field. Furthermore, the dotted arrow conceptually illustrates that this portion of the convection is suppressed by the applied magnetic field. (b, c, e,
(The suppression component in the horizontal plane of f is omitted) Therefore, the heat flow due to thermal convection a to f is as shown in Figure 6b, and the isothermal line of the melt is elliptical as shown in Figure 6c. Therefore, if the crystal is pulled without rotation, the cross-sectional shape of the crystal will be 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 is, for example, A to A shown in FIG. 6c.
Because it passes through point D, temperature fluctuations occur at the interface where the crystal grows, causing changes in the crystal growth rate, resulting in uneven impurity concentration in the cross section of the crystal, and crystal defects. There were flaws.

Furthermore, in the conventional crystal growth method, since a transverse magnetic field is applied to the melt, the flow in the longitudinal direction of the closed thermal convection curve in the melt is suppressed, as described above.

FIG. 8 shows this state in side cross section. A shows a case where a large amount of the molten liquid remains immediately after the crystal pulling is started, and b shows a case where only a small amount of the molten liquid remains immediately after the crystal pulling is completed. As is clear from comparing the two, in the conventional example, 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. As a result, there was a drawback that the concentration of impurities could not be made constant in the length direction of the crystal.

[Means for solving problems]

In order to solve the above problems, in the present invention,
In a crystal growth method in which a conductive substance is heated to form a molten substance and a crystal is pulled from the molten substance, the crystal is symmetrical with respect to the center line of rotation of the molten substance, and
A DC magnetic field along the crystal pulling direction (so-called longitudinal magnetic field), or a DC magnetic field having a longitudinal magnetic field and a component radially radially from a position on the center line of rotation, is applied to the melt. It is designed to pull the crystal while suppressing thermal convection.

Hereinafter, the functions and effects of the present invention will be explained with reference to Examples.

〔Example〕

FIG. 1 is a diagram for explaining a first embodiment of the present invention. That is, in FIG. 1, the magnetic field 18
is essentially a longitudinal magnetic field symmetrical about the center line of rotation. b is a plan view of the crystal 7, the melt 3, and the applied magnetic field 18. (The concept that the magnetic field 18 is isotropic in all directions is illustrated using eight typical lines of magnetic force.) In figure a, 2 is a heating element for making a molten body, and 1 is a heating element for making a molten body. A crucible, 3 is a melt, 4 is a center line of rotation, 5 is a seed crystal, 6 is a support, 7 is a pulled single crystal, 8 is a means for applying a magnetic field in the longitudinal direction (vertical direction),
For example, a solenoid. 18 is a magnetic field, 30 is an airtight container, and 9 is a solid-liquid interface.

In the present invention, a magnetic field is applied to the melt as in the conventional example, so the magnitude of the DC magnetic field is
For example, if it is about 1300 oersted, it has the same thermal convection control function as the conventional example, but the manner in which convection is suppressed is essentially different.

In the present invention, since a DC magnetic field that is symmetrical in any radial direction with respect to the center line of rotation of the melt is applied to the melt, the lateral (horizontal) magnetic field as explained in FIGS. There are no disadvantages such as asymmetrical heat flow or elliptical isothermal lines at the solid-liquid interface due to the application of a directional magnetic field. That is, in contrast to FIG. 6, the features of the present invention are illustrated in FIG. 7. 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 not suppressed. (solid line arrow), the flow in the lateral direction (components in all directions in the horizontal plane) of thermal convection a to f is suppressed (dashed line arrow), and since thermal convection a to f are closed flows, thermal convection a to Since f is weakened overall and thermal convection a to f is suppressed symmetrically with respect to the center line of rotation of the melt, thermal convection a to f
is symmetrical with respect to the center line of rotation of the melt, and the heat flow due to thermal convection a to f becomes as shown in Figure 7b. The isothermal line of the interface 9) is circular as shown in FIG. 7c. (As shown in Figure 7a, 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.) Even if the withdrawal is made with or without
The cross-sectional shape of the crystal is circular. It goes without saying that the wafer, which is the semiconductor integrated circuit base material, needs to be circular rather than elliptical. In addition, even when pulling with rotation, a specific point on the outer periphery of the crystal always rotates while in contact with 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 as shown in FIGS. 9a and b, even if the amount of the melt decreases. Since the length of the lateral flow of thermal convection does not change, even if the amount of melt decreases, the effect of suppressing thermal convection by the longitudinal 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 melt,
Since the melt can be kept in a constant state of agitation, or even with almost no agitation under appropriate conditions, it has the advantage that the concentration of impurities can be kept constant in the length direction of the crystal. be.

The above embodiments showed a method of applying a vertical DC magnetic field to a melt, but the present invention
It is not limited to this.

FIG. 2 shows a second embodiment of the present invention, in which a shows a side cross section and b shows a plan view of the main part. The difference from the first embodiment (FIG. 1) is that the applied magnetic field includes not only a longitudinal component but also a lateral component, and one line of magnetic force is curved as a whole. However, even though it includes a horizontal magnetic field component, as is clear from comparing Figure 2 a and b,
In the present invention, only the components radiate isotropically in the radial direction from the position on the rotation center line. Therefore, as explained using Figure 6, the transverse magnetic field and the convection components in the horizontal plane are orthogonal, and what was originally distributed radially from the rotation center line in the horizontal plane becomes biased due to the influence. There isn't. Therefore, the same effect as the first embodiment of the present invention can be obtained in that the convection suppressing effect acts isotropically in all directions from the rotation center line with respect to the thermal convection of the melt. In other words, as shown in Figure 7, a concentric temperature distribution can be achieved, and even if the crystal is pulled while rotating, the growth temperature at each point does not change, the impurity concentration in the cross section of the crystal is uniform, and no crystal defects occur. There is an advantage.

Furthermore, even if there is a horizontal magnetic field component, it is smaller than the vertical component and radiates in the same direction, so as shown in Figure 9, most of the convection The effect that the suppressing component remains almost constant regardless of the amount of melt is essentially unchanged, and therefore, as in the first embodiment, regardless of the decrease in the amount of melt, the suppressing component remains almost constant regardless of the amount of melt. The first implementation also has the advantage that it is possible to maintain a constant stirring state, or in particular, to maintain almost no stirring under appropriate conditions, thereby making the concentration of impurities constant in the length direction of the crystal. Same as example.

In the above embodiments, a solenoid is preferable as the means for generating a longitudinal magnetic field and applying a predetermined magnetic field to the melt.

The reason why solenoids are preferable is as follows. That is, the apparatus used in the crystal growth method of pulling crystals from a melt as in the present invention generally has a vertical structure with a height 5 to 10 times larger than the diameter, and also has a structure such as a crucible. Many of the requirements are similar to a cylindrical shape. In other words, since the entire device can be regarded as a vertically elongated cylinder, if a similarly elongated solenoid is provided on the outside with its central axis aligned with the rotation center axis, the entire device including the magnetic field applying means (solenoid) can be An excellent practical effect can be obtained in that the size only needs to be slightly increased compared to the basic structure shown in FIG. 3 which does not have a magnetic field applying means. That is, a solenoid is preferable from the viewpoint of making the device smaller and lighter. In addition, the solenoid has small disturbance in the magnetic field distribution, and the heating element does not vibrate or break. Furthermore, the solenoid has the characteristic that the magnetic field passing through the center is weakest and becomes stronger toward the periphery, so there is almost no convection suppressing force acting on the center of rotation of the molten material, and the magnetic field is radial from the position on the rotation center line. This is consistent with the feature of the present invention that the convection suppressing force effectively acts on the components of the convection flowing toward the opposite direction. Therefore, it is preferable to use it in an apparatus for carrying out the present invention, also from the viewpoint that the thermal convection suppressing effect can be efficiently exerted even with a small amount of electric power.

However, the gist of the present invention is to apply to the melt a longitudinal magnetic field that is symmetrical about the center line of rotation and along the crystal pulling direction, or a radial magnetic field that has such a longitudinal magnetic field as its main component. The means for realizing this is not limited to solenoids. Even if other magnetic field application means are used,
It goes without saying that the main objective of the present invention can be achieved by using any magnetic field as long as the desired magnetic field described above can be obtained.

〔Effect of the invention〕

As explained above, in the present invention, in a crystal growth method in which a conductive substance is heated to form a melt and a crystal is pulled from the melt, the crystal is symmetrical with respect to the rotation center line of the melt and A direct current magnetic field (longitudinal magnetic field) along the direction of pulling the crystal, or a component that is symmetrical about the center line of rotation of the melt and along the direction of pulling the crystal, and a component that radiates in the radial direction from the position on the center line of rotation. We adopted a method of applying a DC magnetic field to the melt to pull up the crystal while suppressing thermal convection in the melt, thereby suppressing heat flow while maintaining isotropy with respect to the center line of rotation. Therefore, the temperature distribution at the solid-liquid interface, which is most important in crystal pulling, also has concentric isothermal characteristics.

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, since the effect of suppressing thermal convection is always constant regardless of the decrease in the amount of melt, the concentration of impurities can be kept constant in the length direction of the crystal.

In this way, it is possible to achieve remarkable effects that cannot be achieved with the conventional method (applying a transverse magnetic field).

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating a first embodiment of the present invention,
FIG. 2 is a diagram illustrating a second embodiment of the present invention, FIG. 3 is an explanatory diagram of a conventional basic technique in which no magnetic field is applied, and FIG. 4 is an explanatory diagram of a conventional technique in which a transverse magnetic field is applied. Figure 5 is a diagram explaining the thermal convection of the melt;
FIG. 6 is a diagram explaining the phenomenon that occurs when a conventional transverse magnetic field is applied to the melt, FIG. 7 is a diagram explaining the phenomenon that occurs when the longitudinal magnetic field of the present invention is applied to the melt, Figure 8 is a diagram explaining that when a conventional transverse magnetic field is applied to a melt, the suppressed component of convection decreases as the melt decreases, and Figure 9 is a diagram showing how the suppressed component of convection decreases as the melt decreases when a conventional transverse magnetic field is applied to the melt. FIG. 6 is a diagram illustrating that when the voltage is applied, the suppressed component of convection remains unchanged regardless of the decrease in the melt. 1... Crucible, 2... Heating body, 3... Molten body,
4...Rotation center line, 5...Seed crystal, 6...Support, 7...Single crystal, 8...Magnetic field application means, 9...
Solid-liquid interface, 18... Magnetic field of the present invention, 20... Convection in the melt produced by the heating body, 30... Airtight container, 38... Electromagnet, 58... Transverse magnetic field created by the electromagnet.

Claims (1)

[Claims] 1. Heating a conductive substance to make it into a melt,
In a crystal growth method for pulling a crystal from the melt, a direct current magnetic field is applied to the melt, which is symmetrical about the center line of rotation of the melt and along the direction in which the crystal is pulled. A crystal growth method characterized by pulling the crystal while suppressing thermal convection. 2. The crystal growth method according to claim 1, wherein the DC magnetic field has a strength of 1000 oersted or more at the center. 3 Heating a conductive substance to make it into a melt,
In a crystal growth method of pulling a crystal from the melt, a component that is symmetrical with respect to the center line of rotation of the melt and along the direction of pulling the crystal, and a component radially radially from a position on the center line of rotation. applying a direct current magnetic field having a component to the melt,
A method for growing a crystal, comprising pulling the crystal while suppressing thermal convection in the melt.