JP3132412B2 - Single crystal pulling method - Google Patents

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 cylindrical quartz crucible with a bottom, and the single crystal is rotated with its bottom surface in contact with the surface of the molten silicon liquid, and the rate of solidification growth on the bottom surface And a single crystal having a required size is obtained by growing the crystal. 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 possible such 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 for a silicon single crystal for a semiconductor to have 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,
These impurities must also be uniformly distributed throughout the single crystal without segregation.

Oxygen is mainly melted from quartz used for a crucible to increase its concentration in a melt, while at a temperature higher than the melting point of silicon, the vapor pressure of SiO gas is high.
Excluded from the melt surface under reduced pressure. Although oxygen in the melt moves by diffusion, most of the oxygen moves by heat convection of the melt. The large heat convection is that the surface of the crucible is washed, rises to the surface of the melt, and then reaches the interface between the single crystal being pulled and the melt. In this case, if the amount of oxygen supplied from the crucible surface, the amount of oxygen lost on the surface, and the amount of oxygen taken into the crystal are balanced, a single crystal having a uniform oxygen concentration distribution in the growth direction can be obtained. However, in reality, it is not easy to make the thermal convection steady and uniform due to the influence of the laminar flow accompanying the rotation of the crystal or the crucible, or the Marangoni convection of the surface layer. 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 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,
It utilizes the principle of receiving the opposite force (Lorentz force). When a horizontal magnetic field is applied to the crucible from the side, the vertical movement of the heat convection is hindered, and the flow along the surface of the crucible side wall is suppressed, so that the mixing of oxygen can be suppressed, and the horizontal flow is restricted. Therefore, oxygen is sufficiently removed from the melt surface, and the amount of oxygen taken into the crystal can be reduced.

In addition to the method of applying a magnetic field in the horizontal direction, for example, JP-A-57-149894 discloses a method of applying a magnetic field in a crystal pulling direction, that is, a vertical direction. The gazette discloses an invention in which a vertical magnetic field has a strong distribution around the center axis of the pulling up at the surface of the melt and a distribution of almost 0 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 symmetrical cusp magnetic field is generated in the melt, and a single crystal is pulled.

The application of such a magnetic field suppresses the agitation of the melt, and, like oxygen, a phenomenon caused by a specific place in the crucible, such as mixing from the crucible wall and discharge from the melt surface. If they are different, they can be used effectively. However, it does not always work effectively to make the temperature uniform and the concentration distribution of impurities such as dopants uniform.

When a liquid phase and a solid phase coexist, the impurity element serving as a dopant is different from the concentration in the solid phase (C _S ) and the concentration in the liquid phase (C _L ), and the concentration ratio is different. 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. The method of applying a magnetic field in the horizontal direction is effective for reducing the oxygen concentration and controlling to a specific concentration range, and is also said to be effective for uniformizing the impurity concentration distribution. A change in concentration along the growth direction is unavoidable. Further, it is said that there is a micro-uniformity of the impurity having the periodicity of rotation because it is asymmetric with respect to the rotation pulling shaft.

In the method of applying a vertical magnetic field, the convection along the crucible side wall and the effect of suppressing the upstream and downstream from the bottom of the crucible are small, and the horizontal flow on the surface of the melt is restrained and oxygen is discharged. Since the flow of the melt into the crystal growth interface is suppressed, the oxygen concentration increases, and the uniformity of the oxygen concentration of the obtained crystal is not good. However, since the melt flow due to the rotation of the crystal near the crystal growth surface is suppressed,
The effect that the effective segregation coefficient is closer to 1 is obtained, so that the change in the concentration of the dopant in the growth direction is small, the portion satisfying the target resistivity is increased, and the yield is improved.

In the method of applying a cusp magnetic field, the magnetic field in the vertical direction near the surface of the melt is brought close to zero so that the flow of the melt is not restricted, and at the same time, the flow such as thermal convection along the crucible wall is restricted. Therefore, the effects of oxygen reduction and crystal rotation can be fully utilized. However, the concentration change along the growth direction of the dopant is caused by the C
It seems to be comparable to the Z method.

[0014]

The crystal growth method by applying a magnetic field with the primary purpose of controlling the oxygen concentration in the pulled crystal is as follows.
As described above, there are several methods, each of which has its own characteristics. It is an object of the present invention to provide a method for pulling up a healthy and uniform single crystal at a higher yield at a higher speed by utilizing a magnetic field application method for improving the quality of the single crystal. By increasing the pulling speed, productivity is improved, and high-quality single crystals can be manufactured at lower cost.

[0015]

Means for Solving the Problems The present inventors have studied means for improving the yield and increasing the pulling speed without losing the soundness of the quality of the single crystal. First, to increase the pulling speed, it is necessary to lower the temperature of the melt. However, when the temperature of the melt is lowered, non-axially symmetric growth of the crystal is induced, and the crystal tends to be deformed. 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.

In order to solve these problems, the temperature near the growth interface of the single crystal at the center is lowered to increase the growth rate, but the temperature gradient on the surface of the melt becomes higher as it approaches the crucible wall. It was confirmed that it could be dealt with by attaching. On the surface of the disk-shaped molten liquid surrounded by the crucible wall, a temperature gradient that increases the temperature in the vicinity of the periphery and lowers the temperature in the center can be generated in the process of increasing the temperature. However, the temperature of the melt rises due to heat input from the outside of the crucible, and there is further heat convection at the surface, so that the temperature gradient is secured while keeping the temperature in the center constant throughout the crystal pulling period. Maintaining a stable steady state was not considered easy.

In the CZ method, the purpose of applying a magnetic field is to suppress convection. If convection is suppressed, heat transfer in the melt is mainly conducted, and the temperature gradient can be increased. Therefore, the use of a vertical magnetic field application, which can suppress the melt flow in the horizontal direction, was studied. As a result, it has been clarified that, by applying a vertical magnetic field, a temperature gradient can be realized in which the temperature near the peripheral crucible wall becomes higher while maintaining the surface temperature of the melt at a low and constant temperature at the central portion. This is because the horizontal convection of the melt is suppressed, and the heat transfer from the heater installed near the outer periphery of the crucible,
It was thought that it became the main body of conduction.

Further, the application of the vertical magnetic field has the effect of making the concentration distribution of impurities such as dopants in the crystal growth direction uniform. This is because the vertical magnetic field restricts the flow of the melt in the horizontal direction due to the rotation of the crystal, and the melt near the growth interface where impurities are concentrated due to solidification is not easily removed. This is because the effective segregation coefficient approaches 1 significantly. As a result, a change in resistivity in the crystal growth direction is reduced, and the yield in a portion falling within a required resistivity range is improved.

As described above, application of the vertical magnetic field realizes a condition capable of increasing the pulling speed, and also improves the yield by making the resistivity uniform, which is significant for significantly improving the productivity. However, the oxygen concentration in the crystal tends to be higher than in the CZ method in which a normal magnetic field is not applied, and the concentration of impurities such as oxygen and dopant is not uniform in a crystal cross section perpendicular to the growth direction. There are difficulties.
Therefore, various methods for utilizing the characteristics of the application of the perpendicular magnetic field, controlling the oxygen concentration on the basis of the characteristics, and improving the uniformity of the dopant or the like in the crystal cross section have been studied.

As a result, apart from the coil for applying the vertical magnetic field, a coil having the same axis as the pull-up axis may be installed above the surface of the melt, and a current may be passed through the coil to generate a magnetic field in the opposite direction. Proved to be extremely effective. In this case, on the surface of the melt, the magnetic field component in the vertical direction is set to a magnetic field state such that the central portion is small and the peripheral portion near the crucible wall is high. As a result, the oxygen concentration can be controlled, and the uniformity of oxygen and the dopant in the cross section can be improved.

In a vertical magnetic field, when a reverse magnetic field is applied so as to reduce the vertical component of the magnetic field in the central portion at the position of the melt surface, that is, the growth interface of the single crystal and its vicinity, and to increase the periphery, And the majority of the entire melt is governed by the vertical magnetic field, which is preferable for increasing the pulling speed, and the effect of making the resistivity uniform can be maintained to some extent. In addition, it is considered that the flow of the molten liquid in the horizontal direction becomes easy at the growth interface and in the vicinity thereof, so that oxygen was reduced and impurities such as dopants were made uniform at the growth interface.

As described above, the gist of the present invention is to provide a single crystal manufacturing apparatus by a CZ method in which a magnetic field is applied in the vertical direction while the magnetization direction of a coil for generating a magnetic field coincides with the central axis. By arranging another coil coaxial with the lifting axis and generating a magnetic field in the opposite direction, the strength distribution of the vertical component of the magnetic field on the melt surface is reduced near the solid-liquid interface of the melt near the crucible center, and the crucible wall This is a single crystal pulling method characterized by making the periphery high.

When a silicon single crystal is pulled by the CZ method by this method, a crystal having a uniform concentration distribution of impurities such as oxygen and dopant in a cross section perpendicular to the growth direction and a small variation in resistivity in the growth direction is obtained. Can be manufactured at a higher speed than before.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention is based on the CZ method by applying a vertical magnetic field. The method of applying a magnetic field in the vertical direction to the melt at the time of pulling a silicon single crystal is such that one or a plurality of cylindrically wound coils for generating a magnetic field are rotated around the center axis of the single crystal pulling axis. Alternatively, it is arranged at the crucible position or at a vertical position with the crucible as the center so as to coincide with the crucible rotation axis, and a direct current is passed to generate a magnetic field. In the case of a plurality of coils, the current flows so that the magnetization directions of all the coils match.

FIG. 2 is a schematic view showing an example of the embodiment of the present invention. The method of the present invention will be described with reference to FIG.
If an electric current is applied so that a magnetic field in the same direction is generated in the coils 6a and 6b installed above and below the crucible, the magnetic flux between the coils in a cross section parallel to the central axis and perpendicular to the central axis is generated. A relatively uniform density magnetic field is generated. In this state, the crucible containing the melt and the obtained single crystal are pulled up while rotating, and the CZ method by applying a vertical magnetic field. On the other hand, in the present invention, a coil 12 having the same central axis is arranged in the vertical magnetic field region above the melt, separately from the coil that generates the vertical magnetic field, and generates a magnetic field opposite to the surroundings.

The strength of the reverse magnetic field generated by the coil 12 is such that the vertical component of the magnetic field is lower at the center of the liquid surface of the melt and higher as it approaches the peripheral crucible wall. The distribution state of the vertical component of the magnetic field varies in various ways depending on the relationship between the position, the size of the coil 12, the generated reverse magnetic field, and the strength of the original vertical magnetic field. Parts must be high. By setting the vertical magnetic field at the liquid surface position in this way, the state where the vertical magnetic field is applied is maintained for most of the melt. As a result, the temperature near the crucible wall on the melt surface can be high, and the center can be lowered. Even if the melt temperature is lowered, deformation of single crystals and precipitation of fine crystals are suppressed, so the pulling speed can be increased. It is. On the other hand, the surface of the melt around the pulled single crystal and the vicinity of the growth interface have a low perpendicular component of the magnetic field, and the flow of the melt due to the rotation of the crystal causes deoxidation and the cross section perpendicular to the growth direction of oxygen and dopants. Uniformization can be realized.

[0027]

FIG. 2 shows a schematic diagram of the apparatus shown in FIG.
Using a crucible, a charge amount was set to 70 kg, P was added as a dopant, and a single crystal having a target resistance value of 10 Ωcm, a diameter of 6 inches and a length of 1300 mm was pulled up and grown.
At that time, in the case of the CZ method without a normal magnetic field, when a vertical magnetic field is applied, and when a reverse magnetic field is applied to the vertical magnetic field of the present invention from above the melt, and the vertical magnetic field at the center of the liquid surface is lower than the surroundings, Were compared.

In the case of lifting by a vertical magnetic field, a magnetic field was applied by the coils 6a and 6b in FIG. The diameter of the center position of the coil winding is 600 mm, and 6a is 4
00b was set above and 6b was set below 400mm. In the case of the method of the present invention, a reverse magnetic field is further introduced by the coil 12, but the center position of the coil winding is 400 mm in diameter and the coil 6a
And arranged on the same plane. By adjusting the exciting current and applying a magnetic field in the opposite direction, the distribution of the magnetic field in the vertical direction on the melt surface can be changed. First, single crystal pulling conditions were selected by a normal CZ method without applying a magnetic field. The crystal rotation was 15 rpm, and the crucible rotation in the reverse direction was 5 rpm, and the limit pulling speed at which a sound single crystal was obtained was determined, and the conditions at that time were used as a reference. Vertical magnetic field coil 6a
In the pull-up test when applying by 6b and 6b, the surface temperature of the melt was measured with a radiation thermometer, the temperature near the crucible wall was set to the same temperature as when no magnetic field was applied, and the temperature near the single crystal was reduced. Then, the increase in the lifting speed was considered. As a result, by applying a vertical magnetic field of 0.2 T in intensity, it becomes possible to lower the temperature near the single crystal by 5 to 10 ° C with respect to the temperature near the crucible wall, and to double the pulling speed as compared to the case without a magnetic field. It was confirmed that a sufficiently healthy single crystal could be obtained even if the number increased.

Next, the vertical magnetic field of the present invention is applied to the coils 6a and 6a.
The magnetic field was applied by 6b and a reverse magnetic field was introduced by the coil 12, and the strength of the vertical magnetic field at the surface of the melt was set to 0.002T at the center of the crucible and 0.2T at the periphery. In this state, it was found that the same surface temperature gradient as in the case of only the vertical magnetic field was obtained, so that the growth was performed with the pulling speed twice that in the case without the magnetic field, as in the case of the vertical magnetic field only, It was confirmed that a sufficiently sound single crystal was obtained.

Table 1 shows the results of examination of single crystals by these three methods of pulling. These numerical values were obtained by investigating the central part of the obtained single crystal, and were obtained six times in the case of the usual CZ method, twice in the case of applying a vertical magnetic field, and six times in the case of the method of the present invention. This is the average value of the results of the repetition.

[0031]

[Table 1]

As can be seen, the method of the present invention can provide a sufficiently sound single crystal even when the pulling speed is doubled, as compared with the case of the conventional CZ method. The in-plane uniformity of the quantities is approximately equal.
Further, looking at the effective segregation coefficient of P added as a dopant, the effective segregation coefficient is increased by 30% or more, and the yield of the resistance value is improved.

[0033]

According to the method of the present invention, a sound single crystal with excellent uniformity can be manufactured at a higher speed and with a higher yield.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a cross section of a single crystal manufacturing apparatus by a normal rotation pulling method (CZ method).

FIG. 2 is a diagram schematically showing a cross section of a single crystal manufacturing apparatus for performing the method of the present invention.

[Explanation of symbols]

1a… Crucible (quartz part), 1b… Crucible (graphite part), 2
... heater, 3 ... seed crystal, 4 ... melt, 5 ... crystal, 6a ... vertical magnetic field applying coil (upper), 6b ... vertical magnetic field applying coil (lower), 7 ... pulling wire, 8 ... chamber, 9 ... Pull chamber, 10 ... Insulation material, 11 ... Crucible support shaft, 12 ... Coil for applying reverse magnetic field

──────────────────────────────────────────────────続 き Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C30B 1/00-35/00

Claims (1)

(57) [Claims] An apparatus for manufacturing a single crystal by a CZ method in which a magnetization direction of a coil for generating a magnetic field is made coincident with a central axis and a magnetic field is applied in a vertical direction, another coil coaxial with an axis is pulled up above a melt surface. By arranging and generating a magnetic field in the opposite direction, the strength distribution of the vertical component of the magnetic field at the melt surface is low near the solid-liquid interface of the melt near the center of the crucible and high near the crucible wall. A single crystal pulling method.