JPH0234915B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a single crystal pulling apparatus that applies a magnetic field to a melt when pulling a semiconductor single crystal using the CZ (Cyochralski) method.

[Technical background and problems]

When pulling a single crystal using the Czyochralski method, a horizontal static magnetic field is applied to suppress thermal convection, which stabilizes the temperature of the melt, reduces contamination from the crucible, and improves the quality of the single crystal. Improvements have recently been demonstrated.

The conventional magnetic field application single crystal pulling apparatus has normally conducting coils 1a, 1b and 1, as shown in the cross-sectional view in FIG.
It applies a DC magnetic field to the raw material melt 5 in the crucible 4 inside the chamber 3 of the pulling machine. In this device, by DC exciting the coils 1a and 1b, N and S magnetic poles are generated at both ends of the iron core 2, as shown in FIG. 7, and a magnetic flux as indicated by the dotted arrow 10 is generated within the magnetic poles. The uniformity of the magnetic flux density inside the crucible 4 is due to the presence of the iron core, so there is little leakage magnetic flux, and the magnetic flux uniformity is 5% or less, which is quite uniform. Bmin x 100).

The conventional thinking is that in order to maintain the uniformity of the temperature of the melt in the crucible, the magnetic flux density must also be maintained uniformly. It was composed of electromagnets. Figure 8 is an example of analyzing the magnetic flux distribution of an electromagnet with an iron core (only the left half of the electromagnet is shown in the figure). Here, the space distance between iron cores is 480 mm, and the iron core diameter is 500 mm.
mm, crucible diameter 150 mm placed at the center of iron core 2
This is an example showing a magnetic flux uniformity of 4.8% inside the crucible (analysis using the finite element method). As shown in the example here, in order to ensure the uniformity of magnetic flux, a coil having a diameter sufficiently larger than that of the target crucible is required, and an iron core is required to reduce leakage magnetic flux.

Therefore, as an electromagnet, it is large in size and heavy, and there are problems in attaching it to a pulling machine and in operability. In particular, in the case of silicon single crystals, etc., the diameter of the single crystal is large and the pulling machine is also large, so it may be necessary to provide a space between poles of 900 mm or more. The resulting electromagnet would be huge, weighing over 30 tons.

The effect of applying a magnetic field to a single-crystal raw material melt is based on the principle that a braking force is generated according to Lenz's law on a conductor (melt flow in this case) moving perpendicular to the direction of the magnetic field. be.

As shown in FIG. 7, the single crystal raw material 5 is placed in a crucible 4 and melted by electric heat from a heater 7 installed on the outer periphery. Heat convection occurs in the melt as it is heated by the heater.

Thermal convection generally occurs when the balance between the buoyant force due to thermal expansion of the fluid and the viscous force of the fluid is broken. The dimensionless quantity that expresses the balance between ^the buoyant force and the viscous force is _the Grashof number ^N _Gr . Thermal expansion coefficient ΔT; Temperature difference R; Crucible radius ν; Kinematic viscosity coefficient of the melt Generally, when the Grashof number N _Gr exceeds a critical value determined by the geometric dimensions of the melt, thermal boundary conditions, etc. Heat convection occurs within the melt. Usually N _Gr
When N Gr >10 ⁵ , the thermal convection of the melt becomes turbulent, and when N _Gr >10 ⁹ , it becomes turbulent.

Under the current melt conditions for pulling single crystals with a diameter of 3 inches or more, N _Gr >10 ⁹ and the inside of the melt is in a disturbed state (as shown in Figure 2), and the solid-liquid interface boundary layer 52 near the end crystals is The state becomes violently turbulent.

When such disturbed thermal convection exists, the temperature fluctuations within the melt, especially at the solid-liquid interface, become severe, and the microscopic re-dissolution of the growing crystal becomes significant, causing dislocation loops in the growing single crystal. , stacking faults, etc. occur.

In addition, as shown in FIG. 3, a large flow of thermal convection 51 also occurs at the crucible inner wall, and due to chemical changes between the melt and the crucible, impurities dissolved in the melt are transported from the crucible inner wall to the thermal convection 51. It diffuses into the melt, causing defects in the single crystal and deteriorating the quality of the single crystal.

A single-crystal melt is generally a conductor with an electrical conductivity σ, and a braking force of f per unit volume acts on a fluid moving in a direction perpendicular to a magnetic field.

f=K・σ・V _R・B ² _Z ...(2) where K ; Constant σ ; Electrical conductivity of the melt V _R ; Flow velocity component in the direction perpendicular to the magnetic flux B _Z ; Magnetic flux density The braking force due to the magnetic field is Because it is proportional to the flow velocity V _R , the violent wave tip state caused by the thermal convection in the disturbed state mentioned earlier can be suppressed with a relatively low magnetic flux density of about 1000 Gauss, but the thermal convection that transports and diffuses impurities from the crucible wall51 Since the flow is relatively slow, a relatively large magnetic flux density of several thousand Gauss is required.

Conventional thinking required that a relatively large magnetic field of several thousand gauss be uniformly applied to the entire melt in the crucible in order to suppress thermal vibrations caused by disturbed conditions as well as impurity contamination from the crucible wall.

As described above, an electromagnet that produces a uniform magnetic field in this manner is extremely large in size and heavy, and there are problems in attaching it to a pulling device and in its operability.

[Purpose of the invention]

The present invention was made in view of these problems, and in order to make the electromagnet smaller and lighter, it does not use an iron core.
The present invention provides a magnetic field application device for pulling a superconducting single crystal.

[Summary of the invention]

The present invention aims to make the electromagnet superconducting to make it smaller and lighter, improve operability, and also provide a difference in magnetic flux distribution by eliminating an iron core.
A magnetic flux distribution in which the magnetic flux density BA at the crucible wall is increased relative to the magnetic flux density BC at the center of the crucible, and the ratio of braking force due to the magnetic field K = (Ba/Bc) ² is 1.5 to 3.5 under conditions of constant flow velocity. As a superconducting electromagnet, the effective radius of the coil is a, the distance between the coils is b, and the relationship between a and b is 0.8b>a>0.3b.
That is.

〔Example〕

FIG. 1 shows an embodiment of the present invention, which is comprised of a superconducting electromagnet and an excitation power supply (not shown). A superconducting electromagnet consists of a cryogenic container 11 called a cryostat filled with liquid helium and containing a superconducting coil 12. The detailed structure of the superconducting magnet, the mechanism for attaching it to the pulling machine, etc. are not directly related to the present invention and will therefore be omitted. Superconducting wire has zero electrical resistance, so a large current can flow through a thin wire, so even if it is small,
An electromagnet that can obtain a high magnetic field without an iron core can be realized.

When there is an iron core, the relative magnetic permeability of the iron core is larger than that of space, and the magnetic flux density in the iron core can be considered to be almost constant, so the magnetic flux distribution on the magnetic pole surface is also almost uniform, and the magnetic flux distribution between the magnetic poles is also uniform. Become. On the other hand, when two coils without iron cores are arranged as shown in FIG. 1, the magnetic flux density changes at each spatial position. The magnetic flux density at each position can be found by ampere circuit integration. For example, the magnetic flux density B _Z at a position a distance Z away from the coaxial center of the coil is B _Z = μ ₀ a ² I/2 [1 / {a ² + (b + z) ² } 3/2 + 1/{a ² + (b - z) ² } 3/2] ...(3) It is given as follows.

Here, μ ₀ ; Magnetic permeability (=4π×10 ^-7 ) a ; Coil effective radius 2b ; Inter-coil distance I ; Ampere-turn Z ; On-axis distance from the center of the coils As shown in the graph in Figure 1 (3 ) The magnetic flux density B _Z shown by the formula increases from the center to the outer periphery. Further, as shown in equation (2), the braking force f is fαB ² _Z when the flow velocity V _R is constant, and similarly increases toward the outer circumference.

FIG. 2 is a detailed calculation of the braking force graph in FIG. 1. As a specific example, 6 inches (152mm)
For ~8 inch (203 mm) single crystal pulling equipment,
The diameter of the crucible will be approximately 400mm, and the outer diameter of the chamber of the pulling device will be slightly less than 900mm. Therefore, the distance between the coils is 1040 mm (b = 540 mm), and the braking force ratio to the distance when changing the coil effective radius a (standardized with the center point C as 1, K = (Ba / Bc) ² ) is shown. According to Figure 2, when a = 0.3b, the braking force ratio K = 3.55 at the crucible wall position A point.
Therefore, when a=0.8b, K=1.58.

When the size of the single crystal is small, the thickness of the heater etc. may not be very proportional to the size of the single crystal, so there is a tendency for the distance between the coils to be larger compared to the size of the single crystal. The ratio of the distance to the crucible diameter is approximately 1:0.2 to 0.4, which can be roughly represented by the calculation conditions shown in Figure 2. Therefore, in order to make the braking force ratio K at point A to point C 1.5 to 3.5, the effective radius of the coil for the distance 2b between the coils as an electromagnet should be 0.8b>a>0.3b.

Experiments have shown that setting the braking force at point A (crucible wall) to 1.5 to 3.5 times that at point C is more effective than in the case of uniform magnetic flux distribution. Judging from the results, etc., the following can be considered.

That is, as shown in FIG. 3, the single crystal raw material 5 is heated from the side by the heater 7 and melted. To make the temperature of the melt in the crucible uniform, there are two ways to transfer heat: heat transfer by thermal conduction and heat transfer by convection stirring, but if stirring by convection etc. is suppressed, heat transfer can only be carried out by heat conduction. This is not sufficient, and there is a large temperature difference of several tens of degrees between the periphery and the center.

Therefore, in order to obtain a stable and uniform temperature gradient, sufficient heat transfer is required, and it is necessary to stir the melt by rotating the crystal, as is conventionally done.

FIG. 4 shows an example in which the flow of melt when the crystal is rotated is analyzed using the finite element method. FIG. 4 shows the right half of the cross-sectional view of the crucible and single crystal, and the arrows indicate streamlines within the cross-section. The flow velocity in the rotational direction is maximum near the outer periphery of the crystal, forming a bundle of concentric belt-shaped flows. In FIG. 4, the flow near the interface is a concentric flow, but it can be seen that a vortex, ie, a radial flow component, is generated in the lower part due to the influence of the crucible wall.

When a magnetic field is applied to the melt, the braking force shown in equation (2) above is applied to the flow of the component perpendicular to the direction of the magnetic flux,
The orthogonal component becomes difficult to flow. Therefore, in the case of a horizontal magnetic field, the flow in the vertical direction becomes difficult, and the radial eddy current as seen in the lower part of FIG. 4 is difficult to form, and the entire flow becomes many concentric band-shaped flows as shown in the upper part.

The graph in Figure 5b shows the rotational flow velocity at a location in the radial direction of the crucible, and as shown here, when stirred by crystal rotation, many concentric band-shaped flows 55 are formed, and each flow band has a Because of the difference in speed, the heat given from the crucible wall is well exchanged in each band and transmitted to the center even without radial flow, creating a stable thermal gradient.
Also, as long as the concentric flow continues, 5 in Figure 3
1, no radial vortex or convection occurs,
The melt from the contaminated portion of the crucible wall is not transferred to the vicinity of the crystal, and a single crystal with few impurities can be obtained. However, in the case of a horizontal magnetic field, as seen in the plan view of FIG. 5b, the rotating flow 55 has a part perpendicular to the magnetic flux 10 (on the coil side) and a part parallel to the magnetic flux 10 (in a direction perpendicular to the coil surface). A braking force is applied to the rotating flow in the orthogonal parts, but no braking force is applied in the parallel parts, which is a partial imbalance. However, since the melt has inertia, it maintains a concentric flow as a whole. Also, rotational flow 5
5 is maximum at the edge of the rotating crystal as shown in FIG. 6, and becomes smaller toward the crucible wall. The fact that this flow velocity differs in the radial direction has important implications for heat transfer. (60 indicates the rotational flow velocity distribution when no magnetic field is applied.) As shown in equation (2) above, the braking force f is proportional to the flow velocity V _R , so in the high flow velocity section as shown in Figure 6, When a large braking force is applied and a uniform magnetic field is applied, the flow velocity distribution becomes uniform as shown in 61 compared to 60 when no magnetic flux is applied. Due to the difference in flow velocity in the concentric circles, heat exchange is performed and a stable temperature gradient is obtained. However, if the flow velocity difference disappears, heat transfer deteriorates and a stable temperature gradient cannot be obtained.

When the magnetic flux density is made different as in the present invention, the braking force in the radial direction becomes weaker toward the center.
As shown in 61, stable heat exchange can be performed by creating a difference in flow velocity in concentric circles.

As mentioned above, the effect of applying a magnetic field is to suppress thermal disturbance near the solidification interface and to suppress radial movement due to convection, thereby reducing contamination from the crucible wall. A relatively low magnetic flux density of about 1000 Gauss is sufficient to suppress thermal disturbances, but a relatively large magnetic flux density of 3000 to 4000 Gauss is required to suppress slow convection. The convection in question here is a large flow path that allows the contaminated melt on the crucible wall to flow to the vicinity of the center where crystals grow, as shown in Figure 5. The flow can be stopped by blocking part of the road. Therefore, as shown in Fig. 1 or 2, convection can be stopped even if the magnetic flux density is changed to weaken the center, and as shown in Fig. 6, the flow velocity distribution curve 61 when a uniform magnetic field is applied. Compared to , the flow velocity distribution curve 62 when the magnetic flux distribution is changed by making the center part weaker than the crucible wall part has a concentric flow velocity difference, so good heat exchange can be maintained and a good thermal gradient can be obtained. can get.

As a result of experiments and analysis, as shown in FIG. 2, the magnetic flux distribution that yields good results yields a braking force ratio of 1.5 to 3.5 times at the crucible wall position relative to the center position (1.24 to 1.88 times the magnetic flux distribution).

As described above, the lifting device according to the present invention is more compact than conventional devices and is easier to handle. Furthermore, by using the pulling apparatus according to the present invention, a high quality single crystal with few defects can be obtained.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the single crystal pulling device according to the present invention and its magnetic field distribution, Fig. 2 is a diagram of braking force characteristics in the crucible in the same device, and Fig. 3 is an explanatory diagram of the state of the melt in the crucible. , Figure 4 is a diagram showing the flow analysis results of the melt in the crucible when a single crystal is rotated and stirred, Figures 5 a and b are diagrams showing the relationship between thermal convection, rotational flow, and horizontal magnetic field, and Figure 6 is a diagram showing the relationship between thermal convection, rotational flow, and horizontal magnetic field. The figure shows the flow velocity distribution in the radial direction of the crucible when a single crystal is rotated and stirred, Figure 7 is an explanatory diagram of a conventional single crystal pulling device, and Figure 8 is an explanation of the state of the melt inside the crucible of the same device. It is a diagram. 1a, 1b... Normal conducting coil, 2... Iron core, 3... Pulling machine chamber, 4... Crucible, 5... Single crystal raw material melt, 6... Single crystal, 7... Heater, 10... Magnetic flux, 11
... Cryostat, 12 ... Superconducting coil, 51
... Thermal convection, 52... Solid-liquid interface boundary layer, 55... Stirring rotational flow, 60... Rotational flow velocity distribution (no magnetic field), 61... Rotational flow velocity distribution (uniform magnetic field), 62... Rotational flow velocity distribution (magnetic field of the present invention), a... Coil effective radius, 2b... Distance between coils, B _Z ... Magnetic flux density, f... Braking force, Point A... Crucible wall position, C point... Crucible center position, B _A ...
Magnetic flux density at point A, B _C ...magnetic flux density at point C.

Claims (1)

[Scope of Claims] 1. An apparatus for pulling a single crystal from a crucible while braking a semiconductor single crystal melt housed in a crucible by applying a horizontal magnetic field using an electromagnet, wherein the electromagnet has a coil effective radius of a. , a single crystal pulling device which is a superconducting electromagnet with 0.8b>a>0.3b when the distance between coils is 2b.