Silicon carbide single crystal manufacturing apparatus and silicon carbide single crystal manufacturing method

Abstract

To provides a silicon carbide single crystal manufacturing apparatus that can reduce poor bonding between a silicon carbide seed crystal and a graphite base.SOLUTION: A silicon carbide single crystal manufacturing apparatus includes: a crucible constituted by a crucible body and a crucible lid; and a base 20 that is placed on an underside of the crucible lid and holds a silicon carbide single crystal. The base 20 has a structure in which a plurality of graphite plates 20a, 20b, 20c, 20d having anisotropy in a linear expansion coefficient is laminated and bonded. When viewed in a plan view from the lamination direction, in the plurality of graphite plates, maximum directional axes of linear expansion coefficients between adjacent graphite plates are orthogonal to each other or the maximum directional axes intersect within an angle range of ±15° from the orthogonal state.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a silicon carbide single crystal growth apparatus and a method for producing a silicon carbide single crystal. [Background technology]

[0002] Silicon carbide (SiC) has a dielectric breakdown field that is one order of magnitude larger than that of silicon (Si) and a band gap that is three times larger. Silicon carbide (SiC) also has properties such as a thermal conductivity that is about three times higher than that of silicon (Si). Therefore, silicon carbide (SiC) is expected to be applied to power devices, high-frequency devices, high-temperature operating devices, etc. For this reason, SiC epitaxial wafers have recently come to be used in these types of semiconductor devices.

[0003] SiC epitaxial wafers are manufactured by growing an SiC epitaxial film, which will become the active region of a SiC semiconductor device, on a silicon carbide single crystal substrate by chemical vapor deposition (CVD).

[0004] Silicon carbide single crystal substrates are produced by cutting out silicon carbide single crystals. These silicon carbide single crystals are generally obtained by sublimation. Sublimation is a method in which a seed crystal made of silicon carbide single crystal is placed on a pedestal placed in a graphite crucible, and the crucible is heated to supply sublimation gas sublimated from the raw material powder in the crucible to the seed crystal, causing the seed crystal to grow into a larger silicon carbide single crystal.

[0005] In the sublimation method, the seed crystal needs to be held on the pedestal, which is generally done using an adhesive. In addition, a stress buffer material may be used to alleviate the stress (shear stress) that occurs parallel to the adhesive surface due to the difference in thermal expansion between the seed crystal and the graphite pedestal (Patent Document 1).

[0006] The stresses described above when holding the seed crystal can cause poor adhesion or cracks in the stress buffer (hereinafter collectively referred to as "poor adhesion"), resulting in localized temperature distribution within the seed crystal surface. If this temperature distribution is large, macro-defects are likely to occur in the single crystal, leading to a problem of reduced quality of the single crystal. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-269297 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-88036 [Patent Document 3] Japanese Patent Application Publication No. 59-182213 [Patent Document 4] Japanese Patent Application Laid-Open No. 2016-13949 Summary of the Invention [Problem to be solved by the invention]

[0008] To address this issue, Patent Document 2 proposes a method of suppressing the shear stress generated between the seed crystal and the graphite pedestal by using a member for holding the seed crystal that has a linear expansion coefficient close to that of silicon carbide at room temperature. In Patent Document 3, an isotropic carbon material with an anisotropy ratio of 1.01 is obtained by devising a manufacturing method (see Table 2). However, as Patent Document 3 defines isotropy as anything between 1.0 and 1.1 (see the paragraph at the bottom left of page (3)), measurements of commercially available isotropic graphite used in sublimation methods show an anisotropy of about 1.0 to 1.1. If the graphite base is anisotropic, large shear stress occurs at the bonding surface between the base and the seed crystal in the direction of the larger linear expansion coefficient of the graphite used for the base. Taking into account the anisotropy of isotropic graphite, a method has also been proposed (Patent Document 4) of suppressing the generated stress by aligning the anisotropy with that of silicon carbide.

[0009] In the sublimation method, components are exposed to a wide temperature range, from room temperature to over 2400°C. It is not easy to accurately grasp the linear expansion coefficients of components, which are temperature-dependent within this range, and to match the linear expansion coefficients of the silicon carbide single crystal and the graphite pedestal across each temperature range. In addition, isotropic graphite generally tolerates an anisotropy of approximately 1.0 to 1.1, which is generally greater than the anisotropy in the plane direction of the silicon carbide seed crystal. Therefore, it is difficult to adequately suppress the shear stress generated between the graphite pedestal and the silicon carbide seed crystal across the entire surface.

[0010] The present invention has been made in view of the above circumstances, and provides a silicon carbide single crystal manufacturing apparatus and a silicon carbide single crystal manufacturing method that can reduce bonding defects between a silicon carbide seed crystal and a graphite pedestal. [Means for solving the problem]

[0011] In order to solve the above problems, the present invention provides the following means.

[0012] A silicon carbide single crystal manufacturing apparatus according to a first aspect of the present invention includes a crucible consisting of a crucible body and a lid, and a pedestal disposed on the underside of the lid and holding a silicon carbide seed crystal, wherein the pedestal is configured by stacking and bonding a plurality of graphite plates having anisotropic linear expansion coefficients, and when viewed in a plan view from the stacking direction, the axes of maximum linear expansion coefficients of adjacent graphite plates are perpendicular to each other or intersect within an angle range of ±15° from perpendicular.

[0013] In the silicon carbide single crystal manufacturing apparatus according to the above aspect, the anisotropy of the linear expansion coefficient may be 1.02 or more and 1.20 or less.

[0014] In the silicon carbide single crystal manufacturing apparatus according to the above aspect, the number of the plurality of graphite plates may be 2 to 8.

[0015] In the silicon carbide single crystal manufacturing apparatus according to the above aspect, the plurality of graphite plates may have a total thickness of 20 mm or more and 100 mm or less.

[0016] In the silicon carbide single crystal manufacturing apparatus according to the above aspect, each of the graphite plates constituting the base may have a thickness of 5 mm or more and 20 mm or less.

[0017] In the silicon carbide single crystal manufacturing apparatus according to the above aspect, the graphite plates constituting the base may have the same thickness.

[0018] A second aspect of the present invention provides a method for producing a silicon carbide single crystal, which comprises using a crucible comprising a crucible body and a lid, and a pedestal disposed on the underside of the lid for holding a silicon carbide seed crystal, arranging the seed crystal of the silicon carbide single crystal and a silicon carbide raw material in the crucible, and causing sublimation gas sublimated from the silicon carbide raw material to deposit on the seed crystal to grow the silicon carbide single crystal, the pedestal comprising a plurality of graphite plates, each having an anisotropic linear expansion coefficient, stacked and bonded together, and wherein the axes of the maximum direction of the linear expansion coefficient of adjacent graphite plates are orthogonal to each other or intersect within an angle range of ±15° from orthogonal.

[0019] In the method for producing a silicon carbide single crystal according to the above aspect, a stress buffer member may be disposed between the seed crystal and the pedestal. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a silicon carbide single crystal manufacturing apparatus capable of reducing bonding defects between a silicon carbide seed crystal and a graphite pedestal. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a cross-sectional view schematically illustrating an example of an apparatus for producing a silicon carbide single crystal according to an embodiment of the present invention. [Figure 2] 2(a) is a schematic perspective view of only the base part of the silicon carbide single crystal manufacturing apparatus shown in FIG. 1, and FIG. 2(b) is an exploded schematic perspective view of four graphite plates arranged apart from each other to illustrate the maximum directional axes of the linear expansion coefficients of the four graphite plates constituting the base part. [Figure 3]FIG. 2 is a schematic diagram showing the arrangement relationship of the axes of maximum linear expansion coefficients of adjacent graphite materials. [Figure 4] FIG. 10 is a cross-sectional view schematically illustrating a configuration in which a stress buffer member is disposed between a seed crystal and a pedestal. [Figure 5] The figure shows the results of a simulation of shear stress when a conventional base and a base made of multiple graphite plates are used. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that in the following embodiments, identical or equivalent parts may be designated by the same reference numerals in the drawings. Furthermore, the drawings used in the following description may show characteristic parts enlarged for the sake of clarity, and the dimensional ratios of each component may not necessarily be the same as in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications can be made within the scope of the effects of the present invention. A configuration shown in one embodiment may also be applied to other embodiments.

[0023] (Silicon carbide single crystal manufacturing equipment) Fig. 1 is a cross-sectional schematic diagram showing an example of a silicon carbide single crystal manufacturing apparatus according to one embodiment of the present invention. Fig. 2(a) is a perspective schematic diagram showing only the base unit of the silicon carbide single crystal manufacturing apparatus shown in Fig. 1, and Fig. 2(b) is an exploded perspective schematic diagram showing the four graphite plates constituting the base unit spaced apart from each other to show the maximum directional axes of the linear expansion coefficients of the four graphite plates. The silicon carbide single crystal manufacturing apparatus 100 shown in Figure 1 includes a crucible 10 consisting of a crucible body 1 and a lid 2, and a pedestal 20 having a lid-side surface (lid side surface) 20A supported by the underside 2A of the lid 2 and a seed crystal mounting surface 20B on the opposite side of the lid side surface 20A to which a seed crystal S is attached.The pedestal 20 is configured by stacking and bonding multiple graphite plates having anisotropic linear expansion coefficients, and the maximum directional axes of the linear expansion coefficients of adjacent graphite plates are perpendicular to each other. Here, the "axis in the direction of the maximum linear expansion coefficient" refers to the axis indicating the direction in which the multiple graphite plates constituting the base portion 20 exhibit the maximum linear expansion coefficient among the multiple linear expansion coefficients, since each of the multiple graphite plates is not isotropic in terms of thermal expansion but has anisotropy in terms of the linear expansion coefficient, i.e., has a different linear expansion coefficient for each direction. The crucible body 1 is provided on its outer periphery with a heat insulating material (not shown) for keeping the crucible 10 warm, and a heating means (not shown). In Fig. 1, a raw material (raw material powder) G for single crystal growth and a seed crystal S are also shown to aid understanding.

[0024] When producing a silicon carbide single crystal, raw material powder G is filled into the bottom of crucible 10, and a seed crystal S made of silicon carbide is placed on pedestal 20. Pedestal 20 is positioned opposite raw material powder G. Next, crucible 10 is heated to approximately 2100 to 2400°C in a reduced pressure atmosphere, and raw material powder G is sublimated to supply sublimation gas (raw material gas) onto seed crystal S. The raw material gas sublimated from raw material powder G recrystallizes on the surface of seed crystal S, resulting in crystal growth of a silicon carbide single crystal.

[0025] <Crucible> Crucible 10 is a crucible for producing silicon carbide single crystals by sublimation, and is composed of crucible body 1 and lid 2. There are no limitations on the shape of crucible body 1 and lid 2 as long as they can together form a crystal growth space. The crucible 10 can be made of, for example, graphite. The crucible 10 becomes very hot during growth. Therefore, it must be made of a material that can withstand high temperatures. Graphite has an extremely high sublimation temperature of 3550°C, and can withstand the high temperatures that occur during growth. When the crucible 10 is made of graphite (graphite material), the surface thereof may be coated with TaC or SiC.

[0026] <Base> 2(a) shows a base 20 made up of four graphite plates 20a, 20b, 20c, and 20d, each of which has anisotropic linear expansion coefficients, stacked and bonded together. Fig. 2(b) shows the maximum linear expansion coefficient axes of the four graphite plates 20a, 20b, 20c, and 20d. The arrows shown on the four graphite plates 20a, 20b, 20c, and 20d indicate the maximum direction axis of the linear expansion coefficient of each graphite plate. As shown in FIG. 2, the arrows of the four graphite plates 20a, 20b, 20c, and 20d of adjacent graphite plates, that is, the axes of maximum linear expansion coefficients, are perpendicular to each other.

[0027] In the base portion 20 shown in FIG. 2, the axes of maximum linear expansion coefficients of adjacent graphite plates are perpendicular to each other, but as shown in FIG. 3, they may be arranged so that they intersect within an angle range of ±15° from perpendicular. Compared to a configuration in which the axes of the maximum linear expansion coefficients of adjacent graphite plates are perpendicular to each other, if the axes are deviated from this perpendicularity, the stress relaxation effect is reduced, but if the deviation is within an angle range of 15°, a sufficient stress relaxation effect can be obtained, although it depends on the number of graphite plates constituting base 20. For example, under the conditions of Example 1 described below, when base 20 is made up of two or four graphite plates and the deviation between adjacent graphite plates is 15°, the stress relaxation effect was 9% or 12%, respectively, compared to when the base was constituted as a single unit. The deviation from the perpendicular direction is more preferably within an angular range of ±10°, and even more preferably within an angular range of ±5°.

[0028] The graphite plates that make up the base 20 can be bonded together using a carbon adhesive or the like. Carbon adhesive is made by dispersing carbon powder in an organic solvent, and by volatilizing the solvent, the plates can be bonded together without damaging the properties of the carbon material.

[0029] The base 20 shown in FIG. 2 is made up of four graphite plates, but four plates is merely an example and any number of plates may be used. For example, the base 20 can be made up of two to eight graphite plates. Based on the simulations described below, compared to the shear stress when the base was made of a single graphite member, the shear stress was smaller for bases made of two graphite plates, four graphite plates, five graphite plates, and eight graphite plates in that order. From the viewpoint of cost etc., the number of graphite plates constituting the base portion 20 is preferably two to four.

[0030] It is preferable that all of the graphite plates constituting the base portion 20 are fabricated by processing from a single graphite block, because in this case, the variation in the linear expansion coefficient between the graphite plates is reduced. A method for producing a base from a single graphite block includes, for example, (i) a step of hollowing out a single graphite block into a cylindrical shape, (ii) a step of slicing the hollowed-out cylindrical graphite member into a plurality of disk-like pieces, and (iii) a step of bonding the plurality of disk-like graphite members so that adjacent graphite plates are aligned perpendicularly to each other in the direction of the maximum linear expansion coefficient (maximum axis), as shown in FIG. 2. The graphite material for the graphite plate is commercially available with various linear expansion coefficients, and the base can be made from an appropriate material.

[0031] The graphite material of the graphite plate is preferably a CIP (Cold Isostatic Press) material, because the anisotropy of the linear expansion coefficient can be reduced by the CIP method. Although anisotropy can be reduced using methods such as CIP, raw materials are limited and it is difficult to produce graphite material (blocks) that are completely free of anisotropy.

[0032] The anisotropy of the linear expansion coefficient is preferably 1.02 or more and 1.20 or less. Here, the "anisotropy of linear expansion coefficient" refers to the ratio of the maximum linear expansion coefficient to the minimum linear expansion coefficient exhibited by each graphite plate (maximum linear expansion coefficient / minimum linear expansion coefficient). Although the effects of the present invention can be obtained even when the anisotropy is small, it is preferable that the anisotropy is within the above range because the generated stress is small if the anisotropy is small to begin with. On the other hand, if the anisotropy is large, the stress applied between the graphite particles during heating becomes large, increasing the possibility of poor adhesion.

[0033] The thickness of each graphite plate constituting the base is preferably 5 mm or more and 20 mm or less. If the thickness of the graphite plate is large, the effect of the present invention will be reduced (see Example 1). On the other hand, if the thickness of the graphite plate is small, the manufacturing of the base member will require a lot of labor and cost, so it is preferable that the thickness be within this range.

[0034] It is preferable that the thickness of each graphite plate constituting the base portion is the same. This is because the thermal expansion of adjacent graphite plates cancels out each other, suppressing shear stress.

[0035] The total thickness of the plurality of graphite plates that make up the base is preferably 20 mm or more and 100 mm or less. If the total thickness is thin, shear stress is difficult to apply, and the effect is small. On the other hand, if the total thickness is thick, especially if each graphite plate is thick, the effect of the present invention is small. Also, if each graphite plate is small, the number of plates increases, which increases the effort and cost. Therefore, the above range is preferable.

[0036] The graphite material is preferably made of a graphite material having a Young's modulus of 5 GPa or more at room temperature, because this provides the rigidity necessary to stably support the silicon carbide single crystal.

[0037] The linear expansion coefficient of the graphite (graphite material) constituting the pedestal 20 does not match the linear expansion coefficient of the silicon carbide constituting the seed crystal over the entire wide temperature range used in sublimation, from room temperature to 2400°C or higher, and shear stress occurs at the bonding surface (adhesion surface) between the graphite pedestal and the silicon carbide seed crystal. The difference in expansion becomes particularly large at the outer periphery of the seed crystal. When this shear stress becomes large, a gap forms between the seed crystal and the pedestal due to poor bonding, leading to the generation of macroscopic defects in the crystal. In contrast, by configuring the pedestal by stacking and bonding a plurality of graphite plates having anisotropic linear expansion coefficients, and configuring the axes of maximum linear expansion coefficients of adjacent graphite plates to be perpendicular to each other or to intersect at an angle of ±15° from the perpendicular direction, the thermal expansion of the graphite plates in the stacked structure can be suppressed. As a result, the shear stress can be suppressed, and macro-defects in the silicon carbide seed crystal can be reduced.

[0038] (Method for producing silicon carbide single crystal) A method for producing a silicon carbide single crystal according to one embodiment of the present invention uses a crucible comprising a crucible body and a lid, and a pedestal disposed on the underside of the lid for holding a silicon carbide seed crystal, the method comprising arranging the seed crystal comprising the silicon carbide single crystal and a silicon carbide raw material in the crucible, and causing sublimation gas sublimated from the silicon carbide raw material to deposit on the seed crystal to grow the silicon carbide single crystal, the pedestal having a configuration in which a plurality of graphite plates having anisotropic linear expansion coefficients are stacked and bonded together, and the axes of the maximum direction of the linear expansion coefficients of adjacent graphite plates are orthogonal to each other or intersect within an angle range of ±15° from orthogonal.

[0039] The pedestal and seed crystal can be bonded (attached) using a carbon adhesive, etc. Carbon adhesive is made by dispersing carbon powder in an organic solvent, and by volatilizing the solvent, the adhesion (bonding) can be achieved without impairing the properties of the carbon material.

[0040] 4 shows an example of a configuration in which a stress buffering member 30 is disposed between the seed crystal S and the pedestal 20 when the pedestal 20 shown in FIG. 2 is used. The stress buffering member may also be disposed between the seed crystal S and a pedestal having another configuration.

[0041] In this method for producing a silicon carbide single crystal, a stress buffering member (stress buffering layer) may be used that is disposed between the seed crystal and the pedestal portion in order to reduce the stress applied to the seed crystal S during growth. By providing the stress buffering member 30 between the seed crystal S and the pedestal 20, it is possible to suppress such shear stress and reduce macro defects in the silicon carbide seed crystal.

[0042] The stress buffering member preferably has a Young's modulus of less than 5 GPa. An example of a stress buffering member having a Young's modulus of less than 5 GPa is a carbon sheet.

[0043] In this method for producing a silicon carbide single crystal, the seed crystal S can have an outer diameter of 150 mm or more. Also, the seed crystal S can have an outer diameter of 200 mm or more. [Example]

[0044] Example 1 The configuration shown in Figure 4 (a configuration using a stress buffer member between the silicon carbide seed crystal and the pedestal) was reproduced in a simulation, and the shear stress generated at the center height of the stress buffer layer when the temperature was increased was determined. The simulation was performed using the general-purpose FEM analysis software ANSYS Mechanical (ANSYS, Inc.). The simulation included the pedestal, silicon carbide seed crystal, and stress buffer member, and was performed on a 1 / 4 symmetrical portion to reduce the calculation load. The simulation conditions were as follows: Silicon carbide seed crystal thickness: 3 mm Silicon carbide seed crystal radius: 80mm Stress buffer layer thickness: 1 mm Radius of base: 80 mm Base thickness (overall): 40mm In addition, typical values ​​shown in Table 1 were used as the physical property values ​​of various materials.

[0045] [Table 1] In Table 1, direction A is the direction in which the linear expansion coefficient is greatest, and direction B is the direction perpendicular to direction A.

[0046] The shear stress generated in the stress buffering member when these structures were heated to 1000°C, which is the elastic range of the material, was evaluated at a position halfway along the height direction. It was assumed that the upper end of the pedestal was bonded to the crucible lid or was integral with the crucible, and the condition was that the displacement of the side of the pedestal opposite the seed crystal in the height direction was fixed.

[0047] Figure 5 shows the results of simulations of the maximum shear stress at the evaluation position under the above simulation conditions for the following cases: a single base, a base consisting of two graphite plates, a base consisting of four graphite plates, a base consisting of five graphite plates, and a base consisting of eight graphite plates.

[0048] The vertical axis shows the stress ratio, where the maximum shear stress at the evaluation position when the base is integral is set to 1. It can be seen that the shear stress at the evaluation position is reduced by 10% or more when the base is made up of two or more graphite plates. When the base portion is integral, the value of the shear stress is in the range of 1 to 3 MPa, and a 10% reduction in shear stress in this range corresponds to 0.1 to 0.3 MPa. The strength of graphite varies depending on the material, but when graphite sheets (carbon sheets) with a Young's modulus of less than 5 GPa are used as stress buffers, their tensile strength is generally on the order of a few MPa. Shear strength is smaller than tensile strength, and for materials such as anisotropic carbon sheets, the shear strength is even smaller. For this reason, stress suppression of a few MPa or less is thought to be sufficient to prevent cracks in stress buffers.

[0049] Example 2 Example 2 differs from Example 1 in that no stress buffering member (stress buffering layer) is provided between the silicon carbide seed crystal and the pedestal, but the other conditions are the same.

[0050] Simulations were performed on the maximum shear stress occurring between the silicon carbide seed crystal and the pedestal when the pedestal was made up of two graphite plates and when the pedestal was made up of four graphite plates. When the base was made up of two graphite plates and when it was made up of four graphite plates, a stress relaxation effect of 2% and 4%, respectively, was obtained, with the stress ratio being 1 when the maximum shear stress when the base was one piece. [Explanation of symbols]

[0051] 1 Crucible body 2 Lid 10 Crucible 20 Base 20a, 20b, 20c, 20d Graphite plates 20A Lid side 20B Seed crystal mounting surface 30 Stress buffer member 100 Silicon carbide single crystal manufacturing equipment

Claims

1. a crucible consisting of a crucible body and a lid; a pedestal portion disposed on the underside of the lid portion and holding a silicon carbide seed crystal; the base portion is configured by laminating and bonding a plurality of graphite plates having anisotropic linear expansion coefficients, When viewed in a plane from the stacking direction, in the plurality of graphite plates, axes of maximum linear expansion coefficients of adjacent graphite plates are perpendicular to each other or intersect within an angle range of ±15° from perpendicular, The plurality of graphite plates is 2 to 8, The thickness of each graphite plate constituting the base portion is 5 mm or more and 20 mm or less, The silicon carbide single crystal manufacturing apparatus, wherein the plurality of graphite plates have a total thickness of 20 mm or more and 100 mm or less.

2. 2. The silicon carbide single crystal manufacturing apparatus according to claim 1, wherein the anisotropy of the linear expansion coefficient is 1.02 or more and 1.20 or less.

3. 3. The silicon carbide single crystal manufacturing apparatus according to claim 1, wherein the thicknesses of the graphite plates constituting the pedestal are the same.

4. A method for producing a silicon carbide single crystal includes using a crucible comprising a crucible body and a lid, and a pedestal disposed on the underside of the lid for holding a silicon carbide seed crystal, and arranging a silicon carbide seed crystal and a silicon carbide raw material in the crucible, and growing a silicon carbide single crystal by depositing sublimation gas sublimated from the silicon carbide raw material on the seed crystal, the base portion is configured by laminating and bonding a plurality of graphite plates having anisotropic linear expansion coefficients, When viewed in a plane from the stacking direction, in the plurality of graphite plates, axes of maximum linear expansion coefficients of adjacent graphite plates are perpendicular to each other or intersect within an angle range of ±15° from perpendicular, The plurality of graphite plates is 2 to 8, The thickness of each graphite plate constituting the base portion is 5 mm or more and 20 mm or less, the plurality of graphite plates have a total thickness of 20 mm or more and 100 mm or less.

5. The method for producing a silicon carbide single crystal according to claim 4 , further comprising disposing a stress buffer member between the seed crystal and the pedestal.

6. 6. The method for producing a silicon carbide single crystal according to claim 4, wherein the seed crystal has an outer diameter of 150 mm or more.

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