JP2021104912A - Single crystal production apparatus - Google Patents

Abstract

To provide a single crystal production apparatus that can efficiently heat the raw material stored in a crucible.SOLUTION: A single crystal production apparatus 100 has a crucible 1 that can store raw material M on an inner bottom 3 and allows seed crystal SD to be installed on a lid 2, and heating means H surrounding the crucible 1. The crucible 1 has a soaking member 5 that erects from the inner bottom 3 to the seed crystal SD, and the soaking member 5 has a core rod 51 and an outer shell 52 that covers the core rod 51.SELECTED DRAWING: Figure 1

Description

The present invention relates to a single crystal manufacturing apparatus.

Silicon carbide (SiC) has characteristics such as a dielectric breakdown electric field that is an order of magnitude larger, a band gap that is three times larger, and a thermal conductivity that is about three times higher than that of silicon (Si). Since silicon carbide has these characteristics, it is expected to be applied to power devices, high-frequency devices, high-temperature operation devices, and the like.

Silicon carbide has many polymorphs (polytypes) that have the same stoichiometric composition but different atomic stacking periods (only in the C-axis direction). Typical polytypes are 3C, 4H, 6H, etc., but 4H-SiC single crystals have good bandgap and saturated electron velocity characteristics, so they can be used as the main substrate material for optical devices and electronic devices. It has become.

The sublimation method is widely known as one of the methods for producing a SiC single crystal. In the sublimation method, a seed crystal composed of a SiC single crystal is placed on a pedestal placed in a graphite crucible. Then, by heating the crucible, the sublimation gas sublimated from the raw material powder in the crucible is supplied to the seed crystal, and the seed crystal is grown into a larger SiC single crystal. In the sublimation method, it is required to efficiently grow a high-quality SiC single crystal.

For example, Patent Document 1 describes a method of growing a SiC single crystal on a seed crystal by attaching a seed crystal to a pedestal of a crucible, arranging a SiC raw material at a position facing the pedestal, and sublimating the SiC raw material. Has been done.

Patent Document 2 describes a silicon carbide single crystal growth apparatus container in which a seed crystal is placed on the upper side of the crucible and a rod-shaped thermal conductor is placed on the bottom surface of the crucible. In Patent Document 2, by installing a rod-shaped heat conductor on the bottom surface of the crucible, heat is conducted to the center of the crucible, so that the raw material near the inner wall of the crucible is sublimated and at the same time, the raw material in the center is sublimated. Is also stated to sublimate.

Japanese Patent No. 4450118 Japanese Unexamined Patent Publication No. 5-587774

However, in the method described in Patent Document 1, the heating means is arranged so as to cover the outer periphery of the crucible. Therefore, the temperature near the wall of the crucible tends to be high, and the temperature distribution in the center of the crucible tends to be low. Due to this temperature distribution, the sublimation gas generated near the wall of the crucible heated to a high temperature may crystallize in the center of the low temperature. That is, it was difficult to make effective use of raw materials, especially near the center of the crucible.

Further, in the method described in Patent Document 2, the heat of the raw material escapes to the growth space or the crucible through the heat conductor installed at the inner bottom of the crucible. Therefore, the raw material stored in the crucible may become cold, and the heat may not be sufficiently transferred to the raw material stored in the vicinity of the heat conductor. In particular, in a large crucible used for growing a large-diameter / long-length SiC single crystal of 6 inches or more, heat may not be sufficiently transferred to the raw material. Therefore, in the sublimation method, a method of efficiently heating the raw material stored in the vicinity of the center of the crucible without dissipating the heat of the raw material stored in the crucible is being studied.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a single crystal manufacturing apparatus capable of efficiently heating raw materials contained in a crucible.

The present invention provides the following means for solving the above problems.

(1) The single crystal manufacturing apparatus according to the first aspect of the present invention includes a crucible capable of accommodating a raw material in an inner bottom portion and a seed crystal can be placed on a lid, and a heating means surrounding the crucible. The heat equalizing member has a heat equalizing member that rises from the inner bottom portion toward the seed crystal, and the heat equalizing member includes a core rod and an outer shell that covers the core rod, and the core rod is made of a heat insulating material. be.

(2) In the single crystal manufacturing apparatus according to the above aspect, the outer diameter of the heat equalizing member may be 0.2 times or more and 0.7 times or less the inner diameter of the crucible.

(3) In the single crystal manufacturing apparatus according to the above aspect, the outer diameter of the core rod may be 0.5 times or more and 0.95 times or less the outer diameter of the heat equalizing member.

(4) In the single crystal manufacturing apparatus according to the above aspect, the outer shell may be made of graphite.

(5) In the single crystal manufacturing apparatus according to the above aspect, the thickness of the outer shell may be 2.5 mm or more.

(6) In the single crystal manufacturing apparatus according to the above aspect, the heat equalizing member has a parallel surface parallel to the inner bottom portion and spreading in the radial direction, and the heat equalizing member has an inner bottom portion via the parallel surface. It may be fixed to.

According to the single crystal manufacturing apparatus of the present invention, the raw material contained in the crucible can be efficiently heated.

It is sectional drawing of the single crystal manufacturing apparatus which concerns on one Embodiment of this invention. It is sectional drawing of the single crystal manufacturing apparatus which concerns on one Embodiment of this invention. It is sectional drawing which concerns on the modification of the heat equalizing member used in the single crystal manufacturing apparatus which concerns on one Embodiment of this invention.

Hereinafter, the present invention will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of the respective components may differ from the actual ones. be. The materials, numbers, arrangements, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and can be appropriately modified and carried out within the range in which the effects of the present invention are exhibited. Is.

[Single crystal manufacturing equipment]
FIG. 1 is a schematic cross-sectional view schematically showing the configuration of the single crystal manufacturing apparatus 100 according to the first embodiment. The single crystal manufacturing apparatus 100 includes, for example, a crucible 1, a heat insulating material 4 that covers the crucible 1, and a heating means H that surrounds the heat insulating material 4. The single crystal manufacturing apparatus 100 according to the present embodiment is used when the crucible 1 is heated by the heating means H and the SiC single crystal is grown by the sublimation method. FIG. 1 shows how a SiC single crystal S is grown using the single crystal manufacturing apparatus 100. In FIG. 1, the raw material M and the seed crystal SD are shown at the same time for easy understanding.

First, the direction is defined. In the crucible 1, the direction from the inner bottom portion 3 to the lid 2 is defined as the z direction (first direction). The direction perpendicular to the z direction and extending from the center of the crucible 1 is the radial direction.

The crucible 1 is, for example, a cylindrical member. The crucible 1 has a lid 2 and an inner bottom portion 3. The lid 2 may be separable from the crucible 1.

The lid 2 has a pedestal 2a for installing the seed crystal SD. The pedestal 2a is, for example, a cylindrical member. A seed crystal SD is installed on the pedestal 2a. The seed crystal SD is composed of a SiC single crystal.

The inner bottom portion 3 is the bottom surface of the crucible 1. The lid 2 and the inner bottom portion 3 are arranged at opposite positions. The raw material M for growing the SiC single crystal S is housed in the inner bottom portion 3. A heat equalizing member 5 is installed on the inner bottom portion 3. It is preferable that the raw material M and the heat equalizing member 5 are arranged so as not to overlap each other in a plan view from the z direction. That is, it is preferable that the heat equalizing member 5 is arranged so as to be higher in the z direction than the raw material M.

The heat equalizing member 5 stands up in the z direction. That is, the heat equalizing member 5 stands up toward the pedestal 2a and the seed crystal SD. The heat equalizing member 5 is, for example, a cylindrical member. The heat equalizing member 5 includes a core rod 51 and an outer shell 52. The outer shell 52 is made of graphite, for example, and covers the core rod 51. In FIG. 1, for convenience of explanation, the outer diameter w1 of the core rod 51, the outer diameter w2 of the heat equalizing member 5, the inner diameter w3 of the crucible 1, and the thickness T of the outer shell 52 are shown.

The heat equalizing member 5 is fixed to the inner bottom portion 3 by, for example, screwing or caulking. The height of the heat equalizing member 5 is, for example, 0.4 times the height of the crucible 1. The height of the heat equalizing member 5 is preferably higher in the z direction than the raw material M housed in the inner bottom portion 3. That is, it is preferable that a part of the heat equalizing member 5 projects in the z direction from the surface of the raw material M. Since a part of the heat equalizing member 5 protrudes from the surface of the raw material M, the heat equalizing member 5 is easily radiated.

The outer diameter w2 of the heat equalizing member 5 is, for example, 0.2 times or more and 0.7 times or less, preferably 0.3 times or more and 0.6 times or less, and more preferably 0. It is 35 times or more and 0.5 times or less. When the outer diameter w2 of the heat equalizing member is 0.3 times or more the inner diameter w3 of the crucible, the temperature of the raw material M arranged near the center of the crucible 1 in a plan view can be further improved, and the raw material M to be accommodated can be efficiently stored. Can be sublimated. Further, when the outer diameter w2 of the heat equalizing member is 0.7 times or less the inner diameter w3 of the crucible, a sufficient amount of the raw material M can be accommodated for growing the long SiC single crystal S.

The core rod 51 is a heat insulating material that insulates the heat of the raw material M. For the core rod 51, for example, a heat insulating material having a thermal conductivity of 1.0 W / mK or less at a high temperature of 2000 ° C. or higher is used. Specifically, as the core rod 51, graphite felt, a heat insulating material obtained by solidifying and molding graphite felt (molded heat insulating material) or the like can be used.

The thickness T of the outer shell 52 is, for example, 0.025 times or more and 0.25 times or less, and more preferably 0.05 times or more and 0.15 times or less of the outer diameter w2 of the heat equalizing member 5. That is, the outer diameter w1 of the core rod is, for example, 0.5 times or more and 0.95 times or less, and more preferably 0.7 times or more and 0.9 times or less the outer diameter w2 of the heat equalizing member 5. When the outer diameter w1 of the core rod 51 is 0.5 times or more the outer diameter w2 of the heat equalizing member 5, the heat insulating effect can be further improved, and the raw material M to be accommodated can be efficiently sublimated. Further, when the outer diameter w1 of the core rod 51 is 0.95 times or less of the outer diameter w2 of the heat equalizing member 5, the deterioration of the heat insulating effect due to the permeation of the sublimation gas into the outer shell 52 is suppressed, and the raw material M Can be sublimated efficiently. In addition, the frequency of replacement of the core rod 51 can be reduced, and the throughput is improved.

Further, the thickness T of the outer shell 52 is preferably 2.5 mm or more, and more preferably 4 mm or more. When the thickness T of the outer shell 52 is 2.5 mm or more, it is possible to suppress a decrease in the heat insulating effect due to the permeation of the sublimation gas into the outer shell 52 and efficiently sublimate the raw material M. Further, the thickness T of the outer shell 52 is preferably 0.25 times or less the outer diameter w2 of the heat equalizing member 5.

The heating means H is, for example, a high frequency coil. In this case, a magnetic field can be generated by passing a high-frequency current through the heating means H, and the crucible 1 can be heated by electromagnetic induction. The structure of the heating means H is not limited to this, and a heat generating body may be provided between the crucible 1 and the heat insulating material 4 to heat the crucible 1 by an indirect heating method.

The heat insulating material 4 is arranged so as to cover the crucible 1. The heat insulating material 4 is preferably arranged in close contact with the crucible 1. The heat insulating material is arranged to stably maintain the crucible 1 in a high temperature state. As the heat insulating material 4, a material whose thickness and thermal conductivity are appropriately adjusted so that the crucible 1 can be stably maintained at a high temperature to a required degree can be used. For example, graphite felt, a molded heat insulating material, or the like is used. be able to. Further, in the single crystal manufacturing apparatus 100, since the raw material M is housed in the region sandwiched between the heat insulating material 4 and the core rod 51 provided with the heat insulating material, the heat of the raw material M is more difficult to escape.

According to the single crystal manufacturing apparatus 100 according to the first embodiment, the raw material M is not accommodated in the vicinity of the center of the crucible 1 which tends to have a low temperature. Therefore, crystallization of the raw material near the center can be suppressed. Further, a heat equalizing member 5 provided with a heat insulating material is arranged near the center of the crucible 1. Therefore, it is possible to suppress the heat of the raw material M from being transferred to the heat equalizing member 5. Further, it is possible to prevent the heat transferred to the heat equalizing member 5 from escaping to the growth space R. Therefore, it is possible to suppress the heat applied to the raw material M from being transferred to the outside of the raw material M, and to efficiently heat the raw material M housed in the crucible 1.

The heat equalizing member 5 is arranged at the center of the crucible 1 in a plan view, for example. Although FIG. 1 shows an example in which only one heat equalizing member 5 is arranged at the center of the crucible 1 in a plan view, the SiC single crystal manufacturing apparatus according to the present embodiment is not limited to this example. For example, a plurality of heat equalizing members 5 may be arranged symmetrically with respect to the center of the crucible 1 in a plan view according to the size of the SiC single crystal to be grown.

Although FIG. 1 illustrates a configuration in which one heat insulating material 4 covers the entire outer surface of the crucible 1, the present embodiment is not limited to this example. For example, the heat insulating material may be separated into a portion covering the lid 2 and a portion covering other than the lid 2. Further, an opening may be formed at any position of the heat insulating material 4. By forming an opening at any position of the heat insulating material 4, the temperature of the crucible 1 can be measured using, for example, a radiation thermometer.

<Modification example 1>
FIG. 2 is a schematic cross-sectional view schematically showing the configuration of the single crystal manufacturing apparatus according to the first modification. In the single crystal manufacturing apparatus 200, the arrangement of the heat equalizing member 5 is different from that of the single crystal manufacturing apparatus 100. In this modification, the same configuration as the single crystal manufacturing apparatus 100 may be designated by the same reference numerals and the description thereof may be omitted.

In this modification, the heat equalizing member 5 has an embedded structure embedded in the crucible 1. The embedded structure is realized, for example, by embedding the heat equalizing member 5 in the inner bottom portion 3 having an opening formed in the center in a plan view. In the single crystal manufacturing apparatus 200, the core rod 51 and the heat insulating material 4 are in contact with each other, and the outer shell 52 is fixed to the inner bottom portion 3.

The embedded structure is not limited to the above example, and is realized by embedding at least one of the core rod 51 and the outer shell 52 in the inner bottom portion 3 of the crucible 1. The heat equalizing member 5 may be embedded in the inner bottom portion 3 by, for example, screwing or fitting. Further, the core rod 51 and the heat insulating material 4, and the outer shell 52 and the crucible 1 may be arranged so as to be integrated with each other. The position of the heat equalizing member 5 in the first direction is arbitrarily selected.

Even when the single crystal manufacturing apparatus 200 according to the first modification is used, the same effect as that of the single crystal manufacturing apparatus 100 according to the first embodiment can be obtained.

<Modification 2>
FIG. 3 is an example of modification of the heat equalizing member used in the single crystal manufacturing apparatus according to the present embodiment. The heat equalizing member according to the present embodiment is not limited to the shape shown in FIGS. 1 and 2, and may have the shape shown in FIG. The heat equalizing member illustrated below may have the above-mentioned embedded structure.

FIG. 3A is a schematic cross-sectional view of the heat equalizing member 5a. The structure of the outer shell 52a of the heat equalizing member 5a is different from that of the heat equalizing member 5. The outer shell 52a has a parallel surface 520 parallel to the inner bottom portion 3. The parallel surface 520 extends in the radial direction. The same configurations as those of the heat equalizing member 5 are designated by the same reference numerals, and the description thereof will be omitted. The size of the parallel plane 520 can be arbitrarily selected. For example, the length may be the same as the inner diameter w3 of the crucible 1 when viewed in a plan view from the z direction. The heat equalizing member 5a may be fixed to the inner bottom portion 3 via the parallel surface 520. In the heat equalizing member 5a, the outer diameter w2 is the outer diameter of the portion of the heat equalizing member 5 that does not include the parallel surface 520.

Even when the heat equalizing member 5a is used, the same effect as when the heat equalizing member 5 is used can be obtained. Further, when the outer shell 52a has the parallel surface 520, it is possible to further suppress the sublimation gas produced by the raw material M from entering the inside of the heat equalizing member 5.

FIG. 3B is a schematic cross-sectional view of the heat equalizing member 5b. The heat equalizing member 5b has a larger radial size at the middle 5b3 than a radial size at the upper end 5b1 and a radial size at the lower end 5b2. The middle portion 5b3 is a portion that bisects the size of the heat equalizing member 5 in the z direction. Even when the heat equalizing member 5b is used, the same effect as when the heat equalizing member 5 is used can be obtained. Further, the heat equalizing member 5b has a small area in contact with the crucible 1 of the core rod 51b, and a small area facing the growth space R. Therefore, it is possible to more efficiently suppress the escape of heat through the heat equalizing member 5b.

[Manufacturing method of SiC single crystal]
In the method for producing a SiC single crystal according to the present embodiment, for example, the SiC single crystal S is produced using a single crystal production apparatus 100. The method for producing a SiC single crystal according to the present embodiment includes a preparation step and a heating step.

In the preparatory step, the raw material M is housed in the inner bottom portion 3 of the crucible 1. It is preferable that the raw material M is housed in a plan view from a direction perpendicular to the lid 2 so as not to overlap the heat equalizing member 5. The SiC used in the raw material M is, for example, a crystal containing any one of 2H, 3C, 4H, 6H, and 15R. The average particle size of the powder particles constituting the raw material M is, for example, 1000 μm or less.

In the heating step, the crucible 1 is heated by the heating means H to sublimate the raw material M. In the heating step, the crucible 1 may be heated by direct heating, or the crucible 1 may be heated by indirect heating.

In the method for producing a SiC single crystal of the present embodiment, it is possible to prevent the heat of the raw material M housed in the crucible 1 from escaping by the heat equalizing member 5. That is, the raw material M can be kept at a high temperature, and the raw material M can be heated efficiently.

<Example 1>
When the SiC single crystal S was produced by accommodating the SiC raw material M in the single crystal manufacturing apparatus 100 having the configuration shown in FIG. 1, the ratio of the sublimated SiC raw material was determined. Example 1 was carried out under the following conditions. In addition, the reference numerals in the following conditions are described for convenience in order to make it easier to understand the correspondence between the configuration of FIG. 1 and the conditions of the examples, and the present embodiment is not limited to the following numerical values. do not have.

Inner diameter w3 of crucible 1: 200 mm
Outer diameter of heat equalizing member 5: 60 mm
Outer diameter of core rod 51: 30 mm
Thickness of outer shell 52 T: 15 mm
Height of crucible 1: 300 mm
Height of heat equalizing member 5: 110 mm
Height of contained raw material M: 100 mm

As described above, in the first embodiment, the outer diameter w2 of the heat equalizing member 5 is set to be 0.3 times the inner diameter w3 of the crucible 1, and the outer diameter w1 of the core rod 51 is the outer diameter w2 of the heat equalizing member 5. It was set to be 0.5 times that of. Graphite felt was used as the core rod 51. In Example 1, 91% of the SiC raw material was accommodated in terms of weight ratio, as compared with the case where the SiC raw material was accommodated without arranging the heat equalizing member 5. A SiC single crystal S was produced using this single crystal production apparatus, and the weight of the SiC raw material after production was measured.

As a result, the weight ratio of the sublimated SiC raw material to the filled SiC raw material was 37%. That is, assuming that the amount of the SiC raw material that can be filled when the heat equalizing member 5 is not installed is 100 g, the weight of the SiC raw material M that can be accommodated in Example 1 is 91 g, and the weight of the sublimated SiC raw material is 33.67 g. Met.

<Examples 2 to 6, Comparative Examples 1 to 4>
In the configuration shown in FIG. 1, the ratio of the outer diameter w1 of the core rod 51 to the outer diameter w2 of the heat equalizing member 5 (w1 / w2) and the ratio of the outer diameter w2 of the heat equalizing member 5 to the inner diameter w3 of the crucible 1 (w2 / The conditions shown in Table 1 were changed to w3) to produce a SiC single crystal S. Other conditions were the same as in Example 1. Comparative Example 1 is a condition in which the heat equalizing member 5 is not arranged, and Comparative Examples 2 to 4 are conditions in which a rod (graphite rod) formed only of graphite is arranged as the heat equalizing member.

The weight ratio of the filled SiC raw material is the ratio of the weight of the filled SiC raw material to the weight of the filled SiC raw material in Comparative Example 1. The ratio of the sublimated SiC raw material is the ratio of the weight of the sublimated SiC raw material to the weight of the filled SiC raw material. The amount of the sublimated SiC raw material is the weight (g) of the sublimated SiC raw material when the weight of the SiC raw material contained in Comparative Example 1 is 100 g.

As shown in Table 1, in comparison with the case where neither the core rod nor the outer shell is arranged (Comparative Example 1), in Comparative Examples 2 to 4 in which the graphite rod is arranged and Examples 1 to 6 in which the heat equalizing member 5 is arranged. , The ratio of sublimated SiC raw material can be increased. When the remaining SiC raw material was confirmed in Comparative Example 1, it was confirmed that the SiC raw material was crystallized near the central portion.

The outer diameter w2 of the heat equalizing member 5 of Examples 1 and 2 and the outer diameter of the graphite rod of Comparative Example 2 are the same. Therefore, the weights of the filled SiC raw materials are the same in Examples 1 and 2 and Comparative Example 2. Comparing the results of Examples 1 and 2 and Comparative Example 2, Examples 1 and 2 have a higher ratio of sublimated SiC raw materials than Comparative Example 2.

Similarly, the outer diameter w2 of the heat equalizing member 5 of Examples 3 and 4 and the outer diameter of the graphite rod of Comparative Example 3, and the outer diameter w2 of the heat equalizing member 5 of Examples 5 and 6 and the graphite rod of Comparative Example 4 The outer diameters of are the same. Comparing the results of Examples 3 and 4 and Comparative Example 3, Examples 3 and 4 have a higher ratio of sublimated SiC raw materials than Comparative Example 3, and Examples 5 and 6 have a higher ratio of Sublimated SiC raw materials than Comparative Example 4. The ratio of sublimated SiC raw material is higher than that.

Therefore, as shown in Examples 1 to 6 and Comparative Examples 2 to 4, by arranging the heat equalizing member 5 including the outer shell 52 and the core rod 51, the SiC raw material is compared with the case where the graphite rod is arranged. Can be effectively used.

As shown in the above embodiment, when the outer diameter w2 of the heat equalizing member 5 is 0.3 times or more and 0.7 times or less of the inner diameter w3 of the crucible 1, the raw material can be efficiently heated and the vicinity of the heat equalizing member. The temperature of the crucible can be increased. Further, when the outer diameter of the core rod is 0.5 times or more and 0.95 times or less the outer diameter of the heat equalizing member, the raw material can be efficiently heated and the temperature in the vicinity of the heat equalizing member can be raised. Further, the heat equalizing member can efficiently heat the raw material by having a heat insulating material as a core rod inside the outer shell. That is, it is possible to achieve both effective utilization of the SiC raw material and improvement of the amount of growth in one growth.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and modifications are made within the scope of the gist of the present invention described in the claims. It can be changed.

1: Crucible, 2: Lid, 2a pedestal, 3: Inner bottom, 4: Insulation material, 5: Heat equalizing member, 51: Core rod,
52: Outer shell, 100: Single crystal production equipment, H: Heating means, M: Raw material, S: SiC single crystal,
SD: Seed crystal, R: Growth space

Claims (6)

A crucible that can store raw materials in the inner bottom and can place seed crystals on the lid,
A heating means that surrounds the crucible is provided.
The crucible has a heat equalizing member that rises from the inner bottom toward the seed crystal.
The heat equalizing member includes a core rod and an outer shell that covers the core rod.
The core rod is a single crystal manufacturing apparatus that is a heat insulating material. The single crystal manufacturing apparatus according to claim 1, wherein the outer diameter of the heat equalizing member is 0.2 times or more and 0.7 times or less the inner diameter of the crucible. The single crystal manufacturing apparatus according to claim 1 or 2, wherein the outer diameter of the core rod is 0.5 times or more and 0.95 times or less the outer diameter of the heat equalizing member. The single crystal manufacturing apparatus according to any one of claims 1 to 3, wherein the outer shell is made of graphite. The single crystal manufacturing apparatus according to any one of claims 1 to 4, wherein the outer shell has a thickness of 2.5 mm or more. The heat equalizing member has a parallel surface that is parallel to the inner bottom portion and extends in the radial direction.
The single crystal manufacturing apparatus according to any one of claims 1 to 5, wherein the heat equalizing member is fixed to the inner bottom portion via the parallel surface.

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