JPH10279380A - Method for pulling single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for increasing the speed of pulling a single crystal without impairing the soundness of its quality. SOLUTION: In the method for growing the single crystal by bringing a seed crystal 3 into contact with the surface of the melt 4 of a raw material and pulling the melt; the upper part of the melt 4 in a crucible 1a is formed as a molten layer and the lower part as a solid layer 12 and the melt is pulled while impressing a magnetic field axisymmetrical with respect to the revolving shaft 11 of the crucible 1a on the melt in such a manner that the perpendicular component in its direction attains 0.05 to 0.2T on the melt surface, by which the pulling speed is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing a single crystal by rotation pulling, that is, a method for producing a silicon single crystal used as a semiconductor material.

[0002]

2. Description of the Related Art The rotational pulling method, that is, the Czochralski method (CZ method), as a means for producing a single crystal of silicon used for a semiconductor material, has various improvements and improvements.
Widely used in industrial mass production.

FIG. 1 schematically shows the principle of an apparatus for producing a single crystal by the CZ method. Here, silicon as a raw material is in a molten state in a bottomed cylindrical quartz crucible, and the single crystal is rotated with its bottom surface in contact with the surface of the molten silicon liquid, and the speed of solidification growth on the bottom surface is increased. At the same time, it is pulled up and grown to obtain a single crystal of a required size. The quartz crucible for holding the melt is fitted inside the bottomed cylindrical outer supporting graphite crucible, and the whole crucible can be rotated around the central axis and moved up and down. Can be. Above the central axis of the crucible is a lifting device consisting of a wire that can rotate about the central axis and can be raised upward. On the outside of the crucible, an electric heater for heating, and further on the outside, a heat insulating material are arranged concentrically, all of which are installed in a chamber capable of shutting off outside air, and constitute a rotary pulling device of the CZ method. .

[0004] A usual method for producing a single crystal using this apparatus is as follows.
First, a required amount of high-purity polycrystalline silicon as a raw material is loaded in a crucible, and is heated to a high temperature by an electric heater and melted in an inert atmosphere such as argon under reduced pressure. After adjusting the surface temperature of the melt, the seed crystal attached to the tip of the pulling device is brought into contact with the surface of the melt, and is pulled up while rotating to first form an elongated neck portion. Next, the pulling speed and temperature are adjusted to increase the diameter to a constant diameter portion having a predetermined diameter, and thereafter, the single crystal having a constant diameter is grown by pulling upward while rotating in accordance with the crystal growth.
The single crystal that has reached the predetermined weight is gradually reduced in crystal diameter from the constant diameter portion, and finally has a diameter of zero to be separated from the melt.

[0005] In the CZ method, various contrivances have been made to stably produce a uniform and large single crystal having as few crystal defects as vacancies and dislocations.
For example, slowly rotate the crystal to be pulled around the central axis, and at the same time rotate the crucible filled with the melt in the opposite direction to the crystal, use a wire for pulling, or generate a furnace atmosphere under reduced pressure of inert gas. Or to eliminate the SiO gas.

It is very important that a silicon single crystal for a semiconductor has no crystal defects and no segregation of impurities. In particular, oxygen, which is one of the impurities, causes crystal defects if it is too much. However, in the process of manufacturing a device from a wafer, a certain level of inclusion is required to suppress distortion due to heat treatment and to create a normal surface without defects. In addition, it is necessary to add a specific impurity element (dopant) to the single crystal in order to determine the electrical resistivity and conductivity type of the silicon wafer, and this impurity is uniformly distributed throughout the single crystal without segregation. There must be.

Oxygen melts out of the quartz used for the crucible and enters the melt, and its concentration is increased near the crucible wall. However, at a temperature higher than the melting point of silicon, the vapor pressure of SiO gas is high. Excluded from the melt surface,
Its concentration is reduced. On the other hand, the melt flows due to thermal convection caused by a temperature difference and forced flow caused by rotation of the crystal and the crucible, and its movement is greatly affected by the liquid amount, the temperature distribution, the number of rotations of the crystal and the crucible. During the pulling of the single crystal, it is not easy to constantly control the flow of these melts, and the amount of oxygen taken into the crystal varies variously. For this reason, it is difficult to obtain a single crystal having a uniform concentration distribution of oxygen, and the oxygen level of the crystal fluctuates in a longitudinal direction or a cross-sectional direction, and may be out of a desired range.

[0008] In order to control the amount of oxygen in the crystal and to make the concentration distribution uniform, for example, as disclosed in JP-A-56-104791 and JP-A-56-45889, There is a method of applying a magnetic field to the melt. This is because the conductor moving in the direction perpendicular to the magnetic field, due to the induced current generated,
Utilizing the principle of receiving a reverse force (Lorentz force), applying a horizontal magnetic field to the crucible from the side will hinder the vertical movement of thermal convection and the movement of oxygen in the melt. Can be suppressed, and the amount of oxygen taken into the crystal can be controlled.

In addition to the method of applying a magnetic field in the horizontal direction, for example, Japanese Patent Application Laid-Open No. 57-149894 discloses a method of applying a magnetic field in the crystal pulling direction, that is, in the vertical direction. Japanese Patent Application Laid-Open Publication No. H11-15064 discloses an invention in which a vertical magnetic field has a strong distribution around the central axis of pulling up at a position of a melt surface and almost zero near the crucible wall.
Japanese Patent Publication No. 2-12920 discloses a method in which magnets of the same polarity are placed above and below a crucible, an axially symmetric and radial cusp magnetic field is formed in the melt, and a single crystal is pulled. By changing the method of applying the magnetic field in this way, not only the level and distribution of the oxygen concentration are improved, but also the concentration distribution in the growth direction of the dopant impurity and the concentration distribution in the plane perpendicular to the growth. Non-uniformity is also said to be improved.

When a liquid phase and a solid phase coexist, the impurity element serving as a dopant has a different concentration (C _S ) in the solid phase from a concentration (C _L ) in the liquid phase. If the temperature and the concentration in the liquid phase are determined, they show a constant value in the equilibrium state. In actual single crystal pulling, there is a slight deviation from the equilibrium state. However, in the case of a dopant used for silicon, the effective segregation coefficient K _E = (C _S / C _L ) is usually smaller than 1.
That is, the concentration in the growing single crystal is lower than the concentration in the melt. For this reason, as the crystal grows and the melt decreases, the concentration of the dopant in the melt tends to increase, and the dopant concentration tends to increase toward the rear of the grown crystal. In order to collect more single crystal portions having the required electrical characteristics, it is necessary to minimize the change in concentration of the dopant along the growth direction. As a method for suppressing such segregation, a molten layer method (DLCZ method) is known.

[0011] In the melt layer method, the lower layer of the melt in the crucible is left as an undissolved solid layer, and the upper layer melt is pulled up as a single crystal. This is a method of dissolving the layer and replenishing the melt. JP-B-34-8242 or JP-B-62-880,
As the single crystal is pulled up, the solid layer with a low impurity concentration in the crucible is dissolved and the thickness of the molten layer is kept constant, thereby suppressing the rise in the impurity concentration in the melt, A method for keeping the dopant concentration in the growth direction constant has been proposed. Japanese Patent Application Laid-Open No. 61-205691 and the like also disclose a method of changing the amount of the melt to keep the impurity concentration in the liquid phase constant. In the molten layer method, first, a polycrystalline piece serving as a raw material is loaded in a crucible, and the upper portion of the crucible is heated more strongly to form a molten liquid in the upper layer. However, bubbles may be released from the raw material that melts later, and the crystals may be cut off by the neck part at the start of pulling, so once the whole is melted, the temperature below the crucible is lowered. An improved method of forming a solid layer is also disclosed in
It is disclosed in JP-A-1591.

By applying such a molten layer method, the heat convection in the melt is reduced, and if the rotation speed of the crucible is further increased, the oxygen concentration of the obtained single crystal can be reduced as compared with the ordinary CZ method. Is disclosed in Japanese Patent Application Laid-Open No. Hei 52-4972. However, increasing the rotation speed of the crucible increases the fluctuation of the temperature of the melt, and the influence is further increased in the case of the melt layer method with a small amount of the melt.
In particular, when forming a neck for the purpose of removing dislocations of a single crystal due to thermal shock caused by seed crystal contact, if the liquid temperature fluctuation is large, the dislocations will not completely disappear and the resulting single crystal will contain dislocation defects. Become. In order to avoid this, the rotation speed of the crucible may be changed. However, the change in the rotation speed of the crucible causes instability of the temperature of the melt, and there is a problem that the control of crystal pulling does not operate stably. As a countermeasure for this, an invention in which a magnetic field is applied to the molten layer is disclosed in Japanese Patent Application Laid-Open No. 7-267776. This is because by applying a magnetic field in the horizontal direction, oxygen can be reduced without increasing the number of rotations of the crucible, the oxygen level in the single crystal can be controlled low and uniformly, and the conventional CZ method using magnetic field control Compared to the case, much lower magnetic field is effective enough.

[0013]

As described above, the single crystal growth method by the rotation pulling has been described as a main example thereof.
Numerous improvements have been made. Many of the improvements involved means for obtaining single crystals of uniform composition without defects.

An object of the present invention is to provide a method for pulling a sound and homogeneous single crystal at a higher speed by utilizing a magnetic field application method and a molten layer method for improving the quality of the single crystal. . By increasing the pulling speed, productivity is improved, and a high-quality single crystal can be manufactured at lower cost.

[0015]

Means for Solving the Problems The present inventors have conducted various studies on means for increasing the pulling speed without losing the soundness of the quality of a single crystal. The temperature gradient in the single crystal pulling direction at the solid-liquid interface has the greatest influence on the increase in the pulling speed. In order to increase this temperature gradient, heat must be efficiently removed from the single crystal. In addition, it is desirable that radiation from the surface of the melt and from the heater be shielded to reduce heat input to the single crystal. In order to make this temperature gradient uniform and sufficiently large over the entire solid-liquid interface, it was considered most necessary to lower the temperature of the melt as much as possible from the viewpoint of heat removal after solidification. However, when the temperature of the melt is lowered, non-axially symmetric growth of the crystal is induced, and the crystal tends to become a deformed single crystal.
At the same time, fine crystals are crystallized on the surface of the melt, which makes it difficult to pull up a healthy single crystal. Further, depending on the position of the heater, solidification may occur from the surface in contact with the crucible wall, which may grow and make it impossible to pull up the crystal. These problems can be solved by providing a temperature gradient such that the temperature of the single crystal at the center of the crucible on the surface of the melt is low and the temperature increases as the temperature approaches the crucible wall.

This temperature gradient is not feasible to some extent even in a normal case, because the heat convection due to heating or the flow of the melt due to the rotation of the crucible is directed from the crucible wall to the center of the melt surface. Not enough for purpose. Also, even if a sufficient thermal gradient can be realized in the process of flowing heat from outside the crucible during the heating process, this will cause the temperature of the entire melt to rise, and the temperature of the melt to increase the pulling speed will increase. Makes implementation difficult. That is, in the entire process of pulling a single crystal, it is impossible to use a conventional method to constantly give a large gradient to the melt surface temperature.

The transfer of heat in a fluid is mainly performed by convection. Therefore, if the technology of applying a magnetic field applied by the CZ method is utilized, convection can be suppressed, and there is a possibility that the temperature gradient can be increased. The magnetic field is generally applied in a direction parallel to the melt surface for the purpose of suppressing vertical convection of the melt. On the other hand, when a magnetic field perpendicular to the melt surface is applied, the flow of the melt in the horizontal direction can be suppressed. Then, the effect of applying a magnetic field in the vertical direction that is almost axisymmetric to the rotation axis of the crucible was examined, and it was found that a temperature gradient considered to be sufficient for increasing the pulling speed due to a decrease in the temperature of the melt was obtained. Further, if the crucible is simultaneously rotated, the temperature gradient can be further increased. However, application of a vertical magnetic field has the effect of stabilizing the average impurity concentration in the growth direction of the single crystal, but increases the oxygen concentration, and tends to make the impurity concentration distribution in a plane perpendicular to the growth direction non-uniform. There is a problem. This is because the melt containing a large amount of oxygen mixed in from the crucible wall has a reduced chance of being deoxygenated on the surface, and the stirring by the horizontal convection is hindered and the uniformity of the impurity distribution is insufficient. is there.

Although the vertical magnetic field suppresses horizontal convection, the effect of suppressing vertical convection is small. Therefore, in the molten layer method in which thermal convection in the vertical direction is assumed to be small, an attempt was made to utilize the application of a vertical magnetic field. as a result,
The ideal oxygen concentration can be reduced, the non-uniformity in the cross section perpendicular to the growth direction of the single crystal is eliminated, and the crystal is not deformed due to the temperature drop and the dislocation is not accompanied by it. It was found that a molten state was obtained.

It is considered that the application of a vertical magnetic field to the molten layer method facilitates an increase in the pulling speed for the following reasons. In the molten layer method, the lower solid layer is melted as the single crystal is pulled up, but the amount of heat input from the side of the crucible compared to the case where all the raw materials in the normal crucible are melted for that purpose Have to do more. When this is used in combination with the restriction of the horizontal flow by the magnetic field, it is particularly effective in increasing the horizontal temperature gradient of the melt surface. In addition, since the solid-liquid coexistence state of the single crystal to be pulled and the solid layer exists above and below the molten layer, it is easy to stably maintain the temperature of the molten layer at a crystal growth temperature, that is, a lower temperature near the solidification point. become. As described above, the greatest influence on the increase in the pulling speed is to increase the temperature gradient in the vertical direction of the solid-liquid interface, and therefore, the heat must be sufficiently removed from the solidified single crystal. . If the temperature of the melt can be kept low, the amount of heat input can be reduced accordingly, and the heat can be effectively removed and the temperature gradient can be increased. In particular, the fact that the temperature at the central portion of the single crystal can be lowered is also advantageous for increasing the pulling speed. Further, since the molten layer method has the effect of making the concentration distribution of impurities uniform, a single crystal having excellent uniformity can be pulled up at high speed.

In order to increase the pulling speed by combining the molten layer method and the application of a magnetic field, a practical experiment was conducted to investigate the practicality of the method.
The present invention has been completed by clarifying the limit conditions.

The gist of the present invention is a method of growing a single crystal by bringing a seed crystal into contact with the surface of a melt of a raw material and pulling the same, wherein the upper portion of the melt in the crucible is a molten layer, and the lower portion is a solid layer. And a magnetic field that is axisymmetric about the axis of rotation of the crucible,
The vertical component in the direction of the melt surface is 0.05-0.2T
The single crystal pulling method is characterized in that the pulling speed is increased by pulling while applying a voltage so as to be as follows.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The applied magnetic field is substantially axially symmetric with respect to the rotation axis of the crucible, and the vertical component in that direction is 0.05 to 0.2 T on the surface of the melt. When the direction of the magnetic field is not vertical, the vertical component in that direction may be within the above range. This is to suppress the horizontal movement of the melt, including Marangoni convection, on the melt surface and increase the temperature gradient due to heating from the outer peripheral surface of the crucible. If the strength of the vertical component of the applied magnetic field is less than 0.05 T, the required surface temperature gradient cannot be obtained because the binding force of the melt is weak, and even if a magnetic field exceeding 0.2 T is applied, the magnetic field is applied. Effect saturates.

The vertical magnetic field is applied by a coil for generating a magnetic field centered on the crystal pulling axis or the rotation axis of the crucible provided outside. As long as the above magnetic field can be applied, the form and method are not limited.

FIG. 2 shows, as an example, a case in which two coils having a common axis are provided above and below. If a current is passed so that the generated magnetic fluxes have the same direction, a uniform magnetic field parallel to the axial direction can be generated between both coils.

When currents in opposite directions are applied to the coils sharing the upper and lower central axes, a so-called cusp magnetic field in which the magnetic field component becomes zero at the center between the two coils is generated. Even in such a case, the effects of the present invention can be obtained if the vertical component of the magnetic field on the melt surface is within the above range. CZ
In the method, the cusp magnetic field is also expected to have the effect of melt flow near the surface due to the rotation of the crystal or crucible, so that the strength of the magnetic field in the vertical direction at the growth interface of the single crystal, that is, at the melt surface, is almost zero. Usually, the voltage is applied as a proper position. On the other hand, in the case of the present invention, since a vertical magnetic field component is required, the portion where the magnetic field component becomes 0 must be located above or below the melt surface.

When a cusp magnetic field is applied, in the present invention,
The position where the strength of the vertical magnetic field becomes zero is located particularly in the central part of the molten layer below the melt surface, that is, from the middle part between the growing single crystal and the solid layer, near the boundary between the molten layer and the solid layer. Is more desirable. The reason for this is that the vertical component of the magnetic field restricts the horizontal movement on the melt surface, which makes it possible to increase the temperature gradient, while the central part of the molten layer facilitates the horizontal movement of the melt. Therefore, uniformization of impurities and the like is promoted. Further, since the heat input from the side of the crucible is quickly transmitted to the central portion, the uniformization of the temperature and the dissolution of the solid layer proceed without delay, and the increase in the pulling speed is further facilitated. Although a temperature gradient is required on the surface of the melt, it is preferable that the temperature inside the melt layer be uniform.

The rotation of the crystal and the crucible is effective for uniformizing the impurity distribution and reducing oxygen, and the effect is further enhanced when used together with the application of a magnetic field. In particular, the rotation of the crucible can increase the temperature gradient on the melt surface by combining it with the application of an axially symmetric vertical magnetic field. That is, the rotation of the crystal is preferably 10 rpm or more, and the rotation of the crucible is preferably 5 rpm or more. The rotation of the crystal is necessary for making the in-plane concentration distribution of impurities uniform, and if it is less than 10 rpm, not only the in-plane concentration distribution is deteriorated, but also the single crystal is deformed. The upper limit of the rotation speed of the crystal is not particularly limited, but is naturally limited in order to achieve stable crystal growth. The rotation of the crucible is performed for the purpose of damping the flow of the melt,
When the rotation speed is lower than 5 rpm, the flow is not stable. The upper limit of the number of revolutions is not particularly limited, but the effect is saturated even if it is increased.

[0028]

[Example] Using a 16 inch φ crucible, charge amount 70 kg
And the target resistance value is set to 1 using P as an n-type dopant.
A single crystal having a length of 1300 mm and a length of 6 inches φ was grown at 0 Ωcm. At this time, the rotation of the crystal is constant at 15 rpm and the rotation of the crucible is constant at 7 rpm, the rotations are in opposite directions, the CZ method without applying a normal magnetic field, the solid layer under the crucible and the molten layer above A magnetic field was applied by a method (DLCZ method) or a molten layer method, and a single crystal was produced by a method in which the magnetic field was a horizontal magnetic field, a vertical magnetic field, or a cusp magnetic field. The intensity of the applied magnetic field was set to 0.1 T at the solid-liquid interface at the center of the crucible, that is, near the center of the melt surface in the case of a magnetic field in the horizontal or vertical direction.
In the case of the present invention, the application of the cusp magnetic field is such that the position where the vertical component is 0 is below the melt surface, and the magnetic field of the vertical component is 0.1 T near the center of the melt surface. The magnetic field of the same strength was shifted upward to match the melt surface with the position where the vertical component was zero. With respect to the obtained single crystal, the oxygen level, the fluctuation width of oxygen in the single crystal growth direction, the fluctuation of resistivity, and the generation of defects (dislocation) in the single crystal were evaluated.

Table 1 shows the pulling rates of these single crystals and the results of investigations on the obtained single crystals. In this case, the oxygen concentration is an average over the entire crystal. The variation range of the oxygen concentration is the ratio of the difference between the maximum value and the minimum value in the wafer to the average value, and indicates the maximum value in the single crystal. Resistance value yield is ± 1.0Ωcm against target resistance value.
The ratio of the portion falling within the range to the entire length of a healthy single crystal. As can be seen from these results, the resistivity layer yield of the molten layer method (DLCZ method) is superior to that of the normal CZ method. Further, when a magnetic field is applied by the molten layer method, the resistivity yield is further improved, and the effect is particularly large in the case of a vertical magnetic field. What is more remarkable than such uniformization of the single crystal impurities is the improvement in the pulling speed, and it can be seen that the increase can be at least twice as large as that of the conventional method.

[0030]

[Table 1]

[0031]

According to the method of the present invention, a single crystal having a more uniform impurity concentration in the crystal growth direction and in the radial direction perpendicular to the crystal growth direction can be manufactured by greatly increasing the pulling speed.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating a method of producing a single crystal such as silicon by rotational pulling (CZ method).

FIG. 2 is a diagram schematically illustrating a single crystal rotation pulling method when a vertical magnetic field is applied to the molten layer method according to the present invention.

[Explanation of symbols]

1a… Crucible (quartz part), 1b… Crucible (graphite part), 2
... heater, 3 ... seed crystal, 4 ... melt, 5 ... single crystal, 6 ... coil for applying vertical magnetic field, 7 ... wire for pulling up, 8 ... chamber, 9 ... pull chamber, 10 ... heat insulator, 11 ... crucible support shaft , 12 ... solid layer

Claims (1)

[Claims] 1. A method for growing a single crystal, comprising bringing a seed crystal into contact with the surface of a melt of a raw material and pulling up the melt, wherein an upper portion of the melt in the crucible is formed as a molten layer, a lower portion is formed as a solid layer, and A method for pulling a single crystal, wherein a pulling speed is increased by applying a magnetic field that is axially symmetric with respect to a rotation axis while applying a magnetic field perpendicular to the direction of the melt at the surface of the melt at 0.05 to 0.2 T.