JP7616035B2 - Single crystal pulling equipment - Google Patents

Description

The present invention relates to a pulling device for pulling silicon single crystals used, for example, as semiconductor substrates, and more specifically, to a single crystal pulling device having a lower heater for heating the bottom of a crucible that stores the raw material melt.

Semiconductors such as silicon and gallium arsenide are made of single crystals and are used in computer memory, from small to large, and there is a demand for storage devices that have larger capacities, lower costs, and higher quality.

The Czochralski method, the main method for producing silicon single crystals, involves melting silicon raw material in a quartz crucible to form a melt (raw material melt), contacting a seed crystal with it, and pulling it up while rotating to obtain a single crystal. Currently, the mainstream method for producing large-diameter crystals with a diameter of 300 mm or more is the magnetic field applied CZ method (MCZ method), which applies a magnetic field to the melt to suppress convection.

In a typical silicon single crystal pulling device, the raw material melt is stored inside a quartz crucible, which is held by a graphite crucible. The graphite crucible is further held by a pedestal that can be raised and lowered in the vertical direction. A graphite resistance heater is used to melt the polysilicon raw material and keep the raw material melt warm. Generally, a cylindrical heater is used that is placed on the side of the graphite crucible, but in addition, a lower heater that is placed below the graphite crucible may be used to assist in melting and to control the heat distribution inside the furnace when pulling the single crystal.

Heater materials used in silicon single crystal pulling devices include isotropic graphite material (CIP material) formed by cold isostatic pressing (CIP) and carbon fiber reinforced carbon composite material (C/C composite material). Carbon fiber reinforced carbon composite material has higher strength and higher thermal conductivity than isotropic graphite material, and has a lower thermal expansion coefficient (thermal expansion coefficient), so it has better characteristics as a heater material. In general, the linear thermal expansion coefficient of isotropic graphite material is about 5×10 ⁻⁶ /K, while that of carbon fiber reinforced carbon composite material is less than 1×10 ⁻⁶ /K in the direction parallel to the layered surface. However, carbon fiber reinforced carbon composite material is very expensive compared to isotropic graphite material, and it is difficult to process complex shapes, so isotropic graphite material is used for general heaters.

A conventional lower heater is shown in Figure 5. As shown in Figure 5, the lower heater 99 is structured to consist of a circular heating part 991 and two electrode clamps 992 extending from the heating part 991. The central axis of the pedestal passes through the inside of the ring of the heating part 991. In addition, holes are formed at the tips of the two electrode clamps 992, which are connected and fixed to electrodes extending from the bottom of the single crystal pulling device. The heating part is resistively heated by power supplied from a power supply unit, thereby heating the inside of the furnace.

In the structure shown in Figure 5, the ends of the two electrode clamps are fixed by connecting them to the electrodes, so the lower heater cannot freely expand when it becomes hot, and thermal stress occurs. Graphite material has the tendency to retain distortion even after the stress is removed, and once deformation occurs due to thermal stress at high temperatures, the deformation does not completely return to normal even when cold. Furthermore, creep deformation also progresses when stress is applied continuously at high temperatures. Due to these factors, the lower heater gradually deforms with use. As the deformation progresses, the dimensions no longer match and it becomes impossible to connect the lower heater to the electrode connection, and the lower heater becomes unusable in a shorter time than its original lifespan, which leads to increased costs.

In order to reduce power consumption during operation, molded insulation made of carbon fiber is often placed on the bottom of the graphite crucible opposite the lower heater, but this makes the lower heater prone to high temperatures. Also, in order to precisely control the heating range of the lower heater, the heat generating part may be limited to near the center of the bottom of the crucible. In this case, spatial constraints are imposed on the placement of the lower heater, making it difficult to increase the surface area of the heat generating part compared to a cylindrical heater placed opposite the side of the graphite crucible, and the lower heater is prone to high temperatures due to the increased amount of heat generated per unit area (heat flow rate). When the lower heater becomes hot due to these factors, greater thermal stress is generated, making deformation more likely to progress.

As a prior art related to heater deformation, Patent Document 1 discloses a CZ crystal manufacturing apparatus characterized in that a graphite heater is supported by a heater support made of a heat-resistant member and an electrically insulating member.
Moreover, Patent Document 2 discloses a graphite heater characterized in that a groove is formed along the longitudinal direction of the resistance heating portion.

Japanese Patent Application Publication No. 10-167876 JP 2014-062004 A

The aim of Patent Document 1 is to suppress the vertical deformation caused by the weight of the heater, but it is not capable of dealing with the horizontal deformation caused by thermal expansion as described above. In addition, there are problems such as the heat generated by the heater being easily diffused through the heater support, so this is not a desirable solution.
In Patent Document 2, the number of resistive heating parts can be reduced and each resistive heating part can be made thicker, thereby increasing the strength of the heater against deflection due to increased weight. However, since the magnitude of thermal stress generated by the inhibition of thermal expansion in the horizontal direction is the same regardless of whether or not there is a groove, it is believed that the technology in Patent Document 2 cannot prevent the above-mentioned deformation of the lower heater.

As such, the reality is that there is little prior art that attempts to solve the problem of horizontal deformation of the lower heater. This is thought to be a new issue that has arisen in recent years due to the larger size of pulling equipment, as well as improvements in furnace internal structures with strengthened insulation to save energy and in the control of heating distribution.

The present invention has been made in consideration of the above, and aims to provide a single crystal pulling device that can suppress deformation of the lower heater.

In order to achieve the above object, the present invention provides a single crystal pulling apparatus including a graphite crucible for holding a quartz crucible for storing a raw material melt, a pedestal for holding the graphite crucible so as to be movable up and down, and a lower heater disposed below the graphite crucible,
The lower heater is
an electrode clamp connected to and fixed to an electrode extending from a lower portion of the single crystal pulling apparatus;
a heating portion having a circular member surrounding a central axis of the pedestal connected and fixed to the electrode clamp, and a heater pattern formed on the circular member;
The present invention provides a single crystal pulling apparatus, characterized in that the electrode clamp is made of a material having a smaller thermal expansion coefficient than a material of the heat generating portion.

In the single crystal pulling apparatus of the present invention, the thermal expansion of the electrode clamp can be mitigated, and the thermal stress of the entire lower heater can be mitigated, compared to when the electrode clamp is made of the same material as the heat generating part, which prevents the lower heater from being deformed, thereby improving the service life of the lower heater and reducing costs.
Moreover, the use of more expensive materials with low thermal expansion coefficients can be minimized by limiting it to the electrode clamps, so that increases in material costs can also be kept to a minimum.

Here, the annular member of the heat generating portion may be in the form of a flat plate.
Alternatively, the annular member of the heat generating portion may be cylindrical.
Alternatively, the annular member of the heat generating portion may have a cylindrical portion and a flat ring portion located on the opposite side of the cylindrical portion to the side connected and fixed to the electrode clamp.

Even if the annular member of the heat generating part is a lower heater of these shapes, the relationship between the materials of the heat generating part and the electrode clamp can be as described above, and the effect of preventing deformation of the lower heater can be obtained.

The heat generating portion may also be connected and fixed to the electrode clamp with a bolt.

This allows the heat generating portion and the electrode clamp to be firmly connected, preventing heat generation due to contact resistance at these connections.

The electrode clamp may be made of a carbon fiber reinforced carbon composite material.

By forming the electrode clamp, which is easy to process, from a carbon fiber reinforced carbon composite material in this way, it is possible to efficiently prevent deformation of the lower heater while keeping manufacturing costs low.

The material of the heat generating portion can also be isotropic graphite material.

By using isotropic graphite material for the heat generating part with the heater pattern, it is relatively easy to process and the heater pattern can be easily formed.

The single crystal pulling device of the present invention reduces the thermal expansion of the electrode clamp while suppressing increases in material costs, thereby reducing the thermal stress generated in the entire lower heater. This reduces the deformation that progresses with use of the lower heater, and extends the life of the lower heater.

1 is a schematic diagram showing an example of a single crystal pulling apparatus of the present invention. FIG. 2 is a structural diagram showing a first example of the structure of a lower heater. FIG. 11 is a structural diagram showing a second example of the structure of the lower heater. FIG. 11 is a structural diagram showing a third example of the structure of the lower heater. FIG. 1 is a top view showing an example of a conventional lower heater.

The present invention will be described in more detail below, but the present invention is not limited thereto.
An example of a single crystal pulling apparatus (hereinafter, also referred to as a pulling apparatus) of the present invention is shown in Fig. 1. This single crystal pulling apparatus 1 is provided with a quartz crucible 3 on the central axis of a pulling furnace 2, and further includes a graphite crucible 4 for holding the quartz crucible 3, and a pedestal 5 for holding the graphite crucible 4 so that it can be raised and lowered and rotated. A wire 7 having a seed crystal 6 held at its tip is suspended above the quartz crucible 3 so that it can be raised and lowered.
The cylindrical heater 8 and the lower heater 9 are each connected and fixed to an electrode 10 extending from the bottom of the pulling device 1 (more specifically, the pulling furnace 2), and resistance heating is performed by supplying power from a power supply (not shown). The raw material (here, a polysilicon raw material will be described as an example in order to pull a silicon single crystal) contained inside the quartz crucible 3 is heated by the cylindrical heater 8 and the lower heater 9, and a raw material melt 11 is formed. Furthermore, a magnetic field generator 12 having a superconducting coil is provided around the pulling furnace 2, and a magnetic field is applied to the raw material melt 11 by passing current through the superconducting coil, thereby suppressing melt convection.
In this state, the seed crystal 6 is gently immersed in the raw material melt 11 and then raised, so that the silicon single crystal 13 is grown while being pulled up.

(First form)
An example of the structure of the lower heater 9 according to the present invention is shown in Fig. 2. Fig. 2 is a top view. The lower heater 9 is mainly composed of a heat generating portion 91 and an electrode clamp 92.
First, the heating unit 91 will be described. The heating unit 91 has a circular ring member 91A, which is flat in the example shown in FIG. 2. A hole H1 is provided in the center of the circular ring member 91A to heat the bottom of the graphite crucible 4, and the central axis of the pedestal 5 is disposed through the hole H1. A heater pattern 91B is formed on the circular ring member 91A. This heater pattern 91B is designed by forming linear or curved slits in the horizontal direction so as to have an appropriate heater resistance and heat distribution. In FIG. 2, the heater pattern 91B is formed by forming linear slits in a zigzag (alternate) pattern from the inner and outer circumferential sides of the circular ring member 91A.

The electrode clamp 92 is a structure for connecting the heat generating part 91 and the electrode 10. Here, two linear, flat electrode clamps 92 are arranged facing each other across a hole H1 in the annular member 91A of the heat generating part 91. For connection to the electrode 10, various methods can be used, such as, for example, as shown in FIG. 2, a hole H2 for passing an electrode bolt is provided on one end of each electrode clamp 92, and the electrode clamp 92 is clamped and connected and fixed using a screw hole on the top surface of the electrode 10 and a matching electrode bolt.

On the other hand, the other end of each electrode clamp 92 and the annular member 91A of the heating part 91 are connected (fastened) by, for example, a bolt 93. In this case, the heating part 91 is firmly fixed to the electrode clamp 92, and heat generation due to contact resistance can be suppressed. Also, it is possible to fix it with adhesive without using fastening parts such as bolts, or to fasten it by combining fastening parts and adhesive. When fastening it with the bolt 93 as shown in FIG. 2, shear stress is applied when thermal expansion occurs, so it is necessary to determine the material and dimensions so that it has sufficient strength. It is desirable to use the same material for the bolt 93 as that of the heating part 91 or the electrode clamp 92 in order to match the thermal expansion coefficient with that of the heating part 91 or the electrode clamp 92.

In FIG. 2, the number of electrode clamps 92 and the number of electrodes 10 connected and fixed thereto are two, but they may be configured with three for suitable use with a three-phase AC power source, or may be four or more. In addition, the shape of the electrode clamp 92 is preferably designed to have a larger cross-sectional area than the heat generating portion 91 in order to concentrate heat generation in the heat generating portion 91.

In conventional technology, the electrode clamp is often formed integrally with the heat generating part using the same material (see FIG. 5). However, in the present invention, the electrode clamp 92 is made of a material having a smaller thermal expansion coefficient than the material of the heat generating part 91. By using a material having a smaller thermal expansion coefficient than the material of the heat generating part 91 for the electrode clamp 92, the thermal stress applied to the heat generating part 91 can be reduced, and deformation of the lower heater 9 can be suppressed compared to when the electrode clamp is formed of the same material as the heat generating part.
By using a relatively expensive material with a lower thermal expansion coefficient only for the electrode clamp 92, it is possible to suppress an increase in the cost of the lower heater 9 while still achieving the excellent deformation prevention effect described above.
For example, an isotropic graphite material, which is advantageous in terms of processing and cost, can be used for the heat generating part 91, and a carbon fiber reinforced carbon composite material, which has a smaller thermal expansion coefficient and is less likely to deform, can be selected for the electrode clamp 92. Naturally, the materials of the heat generating part 91 and the electrode clamp 92 are not limited to this combination, and any combination in which the material of the electrode clamp 92 has a smaller thermal expansion coefficient can be used.

Incidentally, when a carbon fiber reinforced carbon composite material is used for the entire heater including the heating part and the electrode clamp as the material of the lower heater, the linear thermal expansion coefficient is less than 1/5 of that of isotropic graphite material. However, since carbon fiber reinforced carbon composite material is an expensive material, an increase in the cost of procuring parts is unavoidable. In addition, since it is difficult to process the carbon fiber reinforced carbon composite material in a complex and precise manner compared to isotropic graphite material, it is not possible to manufacture it in some shapes, or even if it can be manufactured, it will be a factor that causes the cost to further increase, so it is not a desirable solution. In particular, the heating part with the heater pattern is likely to have a complex shape.
Therefore, when considering feasibility and cost, it is extremely effective to use different materials for the heat generating portion 91 and the electrode clamp 92 as in the present invention.

When using a carbon fiber reinforced carbon composite for the electrode clamp 92, the problematic horizontal thermal expansion coefficient can be reduced by fabricating the electrode clamp 92 so that the thickness direction (vertical direction) of the electrode clamp 92 coincides with the lamination direction of the carbon fiber reinforced carbon composite.

It is also preferable that the electrode 10 connected to the electrode clamp 92 of the lower heater 9 can be raised and lowered vertically. In the manufacturing process of the silicon single crystal 13, the graphite crucible 4 is operated while its vertical position is finely adjusted by the pedestal 5. If the electrode 10 carrying the lower heater 9 can be raised and lowered in accordance with the graphite crucible 4, heat can be transferred to the graphite crucible 4 more efficiently, which is advantageous for controlling the heat distribution inside the furnace.

(Second Form)
Fig. 3 shows another example of the structure of the lower heater according to the present invention. The drawing on the left side of Fig. 3 is a top view. The drawing on the right side is a cross-sectional view taken along line A-A of the top view, but for reference, the inner peripheral surface of the cylindrical ring member at the location of the pedestal hole in the center is shown as a partial side view.
The heating part 191 of the lower heater 19 (heating part 191, electrode clamp 192) has a cylindrical ring member 191A on which a heater pattern 191B is formed by upper and lower slits. The pedestal 5 is disposed so that the central axis of the ring member 191A passes through the hole in the center of the ring member 191A.
This shape is effective in heating a narrower area near the center of the graphite crucible 4 .

(Third Form)
Fig. 4 shows another example of the structure of the lower heater according to the present invention. The drawing on the left side of Fig. 4 is a top view. The drawing on the right side is a cross-sectional view taken along line B-B of the top view, but for reference, the inner peripheral surface of the cylindrical part at the location of the pedestal hole in the center is shown as a partial side view.
The circular ring member 291A of the heating part 291 of the lower heater 29 (heating part 291, electrode clamp 292) has a cylindrical part 291C and a flat ring part 291D on which a heater pattern 291B is formed. The pedestal 5 is arranged so that the central axis of the pedestal 5 passes through a hole in the center of the cylindrical part 291C and a hole in the center of the ring part 291D. One end (lower end) of the cylindrical part 291C is connected and fixed to the electrode clamp 292, and the ring part 291D is located at the other end (upper end) of the cylindrical part 291C.
Slits are formed on the top and bottom of the cylindrical portion 291C, and a slit is formed in the horizontal direction in the ring portion 291D, thereby forming a three-dimensional heater pattern 291B between the cylindrical portion 291C and the ring portion 291D.

As mentioned above, the shape of the second form in FIG. 3 is effective when heating a narrower area near the center of the graphite crucible 4, but the surface area of the heat generating part tends to be smaller than that of the first form in FIG. 2. Therefore, in order to prevent the temperature of the lower heater from becoming excessively high, a third form as shown in FIG. 4 can be used. As shown in FIG. 4, the cylindrical part 291C may be integrally formed with a flat ring part 291D positioned at the upper end thereof. Alternatively, a separate cylindrical member and a flat ring member may be combined and integrated by gluing or the like.

In both the second and third configurations, the configuration of the electrode clamps 192, 292 is basically the same as the electrode clamp 92 in the first configuration of FIG. 2 described above. However, since the electrode clamps in FIG. 3 and FIG. 4 are longer than those in FIG. 2, the deformation suppression effect obtained by using a material for the electrode clamp with a small thermal expansion coefficient is higher than that in FIG. 2. In addition, the method of connecting the heat generating part and the electrode clamp is the same as in FIG. 2.

In the production of silicon single crystals using the single crystal pulling apparatus 1 of the present invention, the lower heater is used at an appropriate power and height position for each process.
For example, in the melting process of polysilicon raw material, it is desirable that the lower heaters 9, 19, and 29 are used with electric power equal to or greater than that of the cylindrical heater 8. If the melting process is performed under conditions where the electric power of the lower heater is significantly smaller than that of the cylindrical heater, the polysilicon raw material will preferentially melt from the position close to the side of the quartz crucible, and unmelted material will easily remain near the center. In such a state, the balance of unmelted raw material will be poor, so that the raw material may rotate in the melt and come into contact with the graphite member located directly above the melt, or the raw material located on the lower side may melt first, causing the upper raw material to fall into the melt when the lower center melts, resulting in the melt being splashed. In addition, it takes a long time to complete the melting, which causes a decrease in productivity. For the same reason as above, it is desirable to position the lower heater as close to the graphite crucible as possible during melting.
In the pulling process after melting, the power and height are appropriately determined according to the quality of the crystal to be pulled. For example, when producing a single crystal with a low oxygen concentration, it is desirable to use a low power in order to suppress melting of the quartz crucible and natural convection from the bottom of the crucible, and conversely, when producing a single crystal with a high oxygen concentration, it is desirable to use a relatively high power.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.
Example 1
Using the single crystal pulling apparatus of the present invention shown in FIG. 1, 400 kg of polysilicon melt was formed in a quartz crucible having a diameter of 32 inches (800 mm), and a silicon single crystal having a diameter of 300 mm was pulled.
The lower heater used had the structure shown in FIG. 2, with the heating portion made of isotropic graphite and the electrode clamp made of carbon fiber reinforced carbon composite material.
The heating part was a flat circular member with an inner diameter of 240 mm, an outer diameter of 620 mm, and a thickness of 25 mm, and the electrode clamps were two linear members with a length of 240 mm and a thickness of 30 mm. The bolts fastening the heating part and the electrode clamps were made of isotropic graphite material. The power of the lower heater was changed to an appropriate value for each process.
Under the above conditions, silicon single crystals were repeatedly pulled, and the lower heater was used until its cumulative current-carrying time reached about 800 hours. After that, when the electrode clamp was cold, the shortest distance between the circumferences of the two holes (holes H2 for electrode bolts) provided in the electrode clamp for connection to the electrode was measured, and it was found to have shrunk by 2.1 mm compared to before the lower heater was used.

(Comparative Example 1)
The lower heater used had a structure in which the heat generating part and the electrode clamp were integrally formed as shown in Fig. 5, and was made of isotropic graphite. Using a single crystal pulling apparatus with the same configuration as in Example 1 except for that, pulling of silicon single crystals was repeated under the same conditions as in Example 1. After the cumulative current application time of the lower heater reached about 800 hours, the shortest distance between the circumferences of the two holes in the electrode clamp was measured and found to have shrunk by 4.8 mm compared to before the lower heater was used.

The amount of shrinkage of the lower heater in Example 1 was about 44% of that in Comparative Example 1, and deformation of the lower heater was suppressed by using a carbon fiber reinforced carbon composite material with a lower thermal expansion coefficient for the electrode clamp as in Example 1, rather than using isotropic graphite material for the entire lower heater as in the Comparative Example. Since a carbon fiber reinforced carbon composite material was used only for the electrode clamp, cost increases were also suppressed.

The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention.

1... Single crystal pulling apparatus of the present invention; 2... Pulling furnace; 3... Quartz crucible;
4...graphite crucible, 5...pedestal, 6...seed crystal, 7...wire,
8... cylindrical heater; 9, 19, 29... lower heater; 10... electrode;
11 ... raw material melt, 12 ... magnetic field generating device, 13 ... silicon single crystal,
91, 191, 291...heat generating portion; 91A, 191A, 291A...circular member;
91B, 191B, 291B...Heater pattern,
92, 192, 292...electrode clamp, 93...volt,
291C: cylindrical portion; 291D: ring portion;
H1: Hole for pedestal; H2: Hole for electrode bolt.

Claims (7)

A single crystal pulling apparatus comprising: a graphite crucible for holding a quartz crucible for storing a raw material melt; a pedestal for holding the graphite crucible so that the graphite crucible can be raised and lowered; and a lower heater disposed below the graphite crucible,
The lower heater is
an electrode clamp connected to and fixed to an electrode extending from a lower portion of the single crystal pulling apparatus;
a heating portion having a circular member surrounding a central axis of the pedestal connected and fixed to the electrode clamp, and a heater pattern formed on the circular member;
4. A single crystal pulling apparatus, comprising: said electrode clamp made of a material having a smaller thermal expansion coefficient than a material of said heat generating portion. The single crystal pulling apparatus according to claim 1, characterized in that the annular member of the heat generating portion is flat. The single crystal pulling apparatus according to claim 1, characterized in that the annular member of the heat generating portion is cylindrical. The single crystal pulling device according to claim 1, characterized in that the annular member of the heat generating portion has a cylindrical portion and a flat ring portion located on the opposite side of the cylindrical portion to the side connected and fixed to the electrode clamp. The single crystal pulling device according to any one of claims 1 to 4, characterized in that the heating part is connected and fixed to the electrode clamp with a bolt. The single crystal pulling device according to any one of claims 1 to 5, characterized in that the material of the electrode clamp is a carbon fiber reinforced carbon composite material. A single crystal pulling apparatus according to any one of claims 1 to 6, characterized in that the material of the heating part is an isotropic graphite material.

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