Electrostatic chuck

Abstract

The present invention provides an electrostatic chuck capable of making the in-plane temperature distribution of a substrate during processing more uniform than that in the prior art. An electrostatic chuck (10) is provided with: a dielectric substrate (100) having a placement surface (110); and a base plate (200) which is bonded to the dielectric substrate (100) and in which a cooling medium flow path (250) through which a cooling medium passes is formed. In plan view, the base plate (200) has: a first portion (P1) that is a portion overlapping an end (E) on the outer peripheral side of the surface (110); and a second portion (P2) that is a portion closer to the outer peripheral side than the first portion (P1). A cooling medium flow path (250) is formed in each of the first portion (P1) and the second portion (P2). The cooling medium flow path (250) is formed such that the cooling performance with respect to the first portion (P1) is higher than the cooling performance with respect to the second portion (P2).

Description

Technical Field

[0001] The present invention relates to an electrostatic chuck. Background Art

[0002] For example, in a semiconductor manufacturing apparatus such as an etching apparatus, an electrostatic chuck is provided as an apparatus for adsorbing and holding a substrate such as a silicon wafer that is an object to be processed. The electrostatic chuck includes: a dielectric substrate provided with an adsorption electrode; and a base plate that supports the dielectric substrate, having a structure in which these are joined to each other. When a voltage is applied to the adsorption electrode, an electrostatic force is generated to adsorb and hold the substrate placed on the dielectric substrate.

[0003] In a process such as etching, it is necessary to maintain the temperature of the substrate at an appropriate temperature. Therefore, as described in Patent Document 1 below, a cooling medium flow path for allowing a cooling medium to pass through is formed inside the base plate. Heat from the substrate is transferred to the cooling medium through the dielectric substrate and the base plate, and is discharged to the outside together with the cooling medium.

[0004] Patent Document Patent Document 1: Japanese Patent Application Laid-Open No. 2011-151055 Summary of the Invention

[0005] In a process such as etching, the temperature of the outer peripheral side portion of the substrate in particular tends to increase. For the purpose of suppressing the temperature rise of such an outer peripheral side portion and making the in-plane temperature distribution of the substrate as uniform as possible, the surrounding arrangement of the optimal cooling medium flow path in the base plate has been studied. However, conventionally, the surrounding arrangement of the cooling medium flow path within the range overlapping the substrate when viewed from above has been mainly studied, and there is still room for improvement in the surrounding arrangement of the cooling medium flow path in other portions.

[0006] The present invention has been made in view of such problems, and the technical problem to be solved is to provide an electrostatic chuck capable of making the in-plane temperature distribution of the substrate during processing more uniform than before.

[0007] To solve the above problems, the electrostatic chuck according to the present invention includes: a dielectric substrate having a placement surface for placing an object to be adsorbed; and a base plate joined to the dielectric substrate and having a cooling medium flow path formed therein through which a cooling medium passes. When viewed from a direction perpendicular to the placement surface, the base plate has: a first portion that overlaps with the outer peripheral side end portion of the placement surface; and a second portion that is closer to the outer peripheral side than the first portion, and cooling medium flow paths are respectively formed in the first portion and the second portion. The cooling medium flow path is formed such that the cooling performance of the first portion is higher than that of the second portion.

[0008] In the electrostatic chuck having the above-described structure, a coolant flow path is formed not only in a portion directly below the substrate in the base plate, but also in a second portion outside thereof. By forming the coolant flow path within such a range, it is possible to suppress the temperature rise of the outer peripheral portion of the substrate. Further, in the electrostatic chuck having the above-described structure, the coolant flow path is formed such that the cooling performance of the first portion is higher than that of the second portion. By providing a difference in cooling performance between the first portion and the second portion, it is possible to sufficiently suppress the temperature rise of the outer peripheral portion of the substrate while also preventing members (such as a focus ring, etc.) disposed at a position more outside than the outer peripheral portion of the substrate from being over-cooled more than necessary. As a result, it is possible to make the in-plane temperature distribution of the substrate being processed more uniform than in the past.

[0009] According to the present invention, it is possible to provide an electrostatic chuck capable of making the in-plane temperature distribution of the substrate being processed more uniform than in the past. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a cross-sectional view schematically showing the structure of the electrostatic chuck according to the first embodiment. Figure 2 is a view schematically showing the structure of the coolant flow path formed in the base plate of the first embodiment. Figure 3 is a cross-sectional view schematically showing the structure of the electrostatic chuck according to the second embodiment. Figure 4 is a view schematically showing the structure of the coolant flow path formed in the base plate of the third embodiment. REFERENCE SIGNS 10 - electrostatic chuck; 100 - dielectric substrate; 110 - surface; E - end; 200 - base plate; 250, 251 - coolant flow path; P1 - first portion; P2 - second portion. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0011] Hereinafter, the present embodiment will be described with reference to the drawings. For ease of explanation, in each drawing, the same reference signs are given to the same components as much as possible and redundant explanations are omitted.

[0012] The first embodiment will be described. The electrostatic chuck 10 according to the present embodiment adsorbs and holds a substrate W to be processed by electrostatic force inside a semiconductor manufacturing apparatus (not shown) such as an etching apparatus. The substrate W as the object to be adsorbed is, for example, a silicon wafer. The electrostatic chuck 10 can also be used in apparatuses other than semiconductor manufacturing apparatuses.

[0013] Figure 1In [the figure], the structure of the electrostatic chuck 10 in the state of adsorbing and holding the substrate W is shown as a schematic cross-sectional view. The electrostatic chuck 10 includes a dielectric substrate 100 and a base plate 200.

[0014] The dielectric substrate 100 is a substantially disk-shaped member made of a ceramic sintered body. Although the dielectric substrate 100 contains, for example, high-purity alumina (Al2O3), it may also contain other materials. Considering the plasma resistance and other requirements for the dielectric substrate 100 in semiconductor manufacturing equipment, the ceramic purity, type, additives, etc. in the dielectric substrate 100 can be appropriately set. The diameter of the dielectric substrate 100 is, for example, 290 to 300 mm. The thickness of the dielectric substrate 100 is, for example, 0.5 to 3.0 mm.

[0015] In the dielectric substrate 100, Figure 1 the upper-side surface 110 in [it] becomes the "placement surface" for placing the substrate W. Additionally, in the dielectric substrate 100, Figure 1 the lower-side surface 120 in [it] becomes the "surface to be joined" that is joined to the base plate 200 through the joining layer 300. Hereinafter, the viewpoint when observing the electrostatic chuck 10 from the side of the surface 110 along the direction perpendicular to the surface 110 will also be referred to as "top-down observation".

[0016] An adsorption electrode 130 is embedded inside the dielectric substrate 100. The adsorption electrode 130 is, for example, a thin plate-shaped layer formed of a metal material such as tungsten and is arranged parallel to the surface 110. As the material of the adsorption electrode 130, in addition to tungsten, molybdenum, platinum, palladium, etc. can also be used. If a voltage is externally applied to the adsorption electrode 130 through a power supply circuit (not shown), an electrostatic force is generated between the surface 110 and the substrate W, and thereby the substrate W is adsorbed and held. As the structure of the above-mentioned power supply circuit, various known structures can be adopted. The adsorption electrode 130 can be provided as a so-called "single-pole" electrode with only one as in this embodiment, or can be provided as a so-called "bipolar" electrode with two. The depth of the position where the adsorption electrode 130 is arranged, that is, the distance from the bottom surface 116 (described later) to the adsorption electrode 130 is, for example, 0.1 to 0.5 mm.

[0017] As Figure 1 shown, a space SP is formed between the dielectric substrate 100 and the substrate W. When performing processes such as etching in a semiconductor manufacturing equipment, helium for temperature adjustment is supplied from the outside to the space SP through pores (not shown). By making helium exist between the dielectric substrate 100 and the substrate W, the thermal resistance between the two is adjusted, and thereby the temperature of the substrate W is maintained at an appropriate temperature. And the gas for temperature adjustment supplied to the space SP can also be a gas of a different type from helium.

[0018] On the placement surface, i.e., surface 110, a sealing ring 111 and points 112 are provided, and the above-mentioned space SP is formed around these.

[0019] The sealing ring 111 is a wall that divides the space SP at the outermost peripheral position. The upper end of the sealing ring 111 becomes a part of the surface 110 and abuts against the substrate W. Also, a plurality of sealing rings 111 can be provided in a manner that divides the space SP. With such a structure, the pressure of helium gas in each space SP can be adjusted individually, and the surface temperature distribution of the substrate W can be made uniform during processing.

[0020] Figure 1 In the figure, the portion marked with the symbol "116" is the bottom surface of the space SP. Hereinafter, this portion will also be referred to as the "bottom surface 116". The sealing ring 111, together with the points 112 described below, is formed as a result of digging a part of the surface 110 to the position of the bottom surface 116.

[0021] The points 112 are circular protrusions protruding from the bottom surface 116. A plurality of points 112 are provided and are disposed approximately evenly on the placement surface of the dielectric substrate 100. The upper end of each point 112 becomes a part of the surface 110 and abuts against the substrate W. By providing a plurality of such points 112, the flexure of the substrate W is suppressed.

[0022] The base plate 200 is a substantially disk-shaped member that supports the dielectric substrate 100. The base plate 200 is formed of a metal material such as aluminum, for example. The base plate 200 is joined to the surface 120 of the dielectric substrate 100 by a joining layer 300. In the base plate 200, Figure 1 the upper surface 210 on the upper side becomes the "joined surface" joined to the dielectric substrate 100.

[0023] The joining layer 300 is a layer provided between the dielectric substrate 100 and the base plate 200 and joins the two. The joining layer 300 is a layer formed by curing an adhesive made of an insulating material. In the present embodiment, a silicone adhesive is used as the above-mentioned adhesive. However, the joining layer 300 can also be a layer formed by curing other types of adhesives. In any case, it is preferable to use a material with a relatively high thermal conductivity as the material of the joining layer 300 so that the thermal resistance between the dielectric substrate 100 and the base plate 200 is small.

[0024] An insulating film can also be formed on the surface of the base plate 200. As the insulating film, for example, a film of alumina formed by sputtering can be used. By covering the surface of the base plate 200 with the insulating film, the insulation breakdown voltage of the base plate 200 can be improved. The thickness of the insulating film is, for example, 1 mm or less.

[0025] The base plate 200 has a support portion 201 and a flange portion 202. The support portion 201 is the upper side portion of the base plate 200 and is a substantially cylindrical portion that directly supports the dielectric substrate 100 from below. Although the diameter of the support portion 201, i.e., the diameter of the surface 210, may be the same as the diameter of the dielectric substrate 100, it may also be slightly smaller than the diameter of the dielectric substrate 100. The diameter of the support portion 201 is, for example, 290 to 300 mm. In addition, the thickness of the support portion 201, i.e., the protruding amount of the support portion 201 toward the upper side (the protruding amount from the flange portion 202), is, for example, 3 to 15 mm. Figure 1 Among them, it is the upper side portion and is a substantially cylindrical portion that directly supports the dielectric substrate 100 from below. Although the diameter of the support portion 201, that is, the diameter of the surface 210, can be the same as the diameter of the dielectric substrate 100, it can also be slightly smaller than the diameter of the dielectric substrate 100. The diameter of the support portion 201 is, for example, 290 to 300 mm. In addition, the thickness of the support portion 201, that is, the protruding amount of the support portion 201 toward the upper side (the protruding amount from the flange portion 202), is, for example, 3 to 15 mm. Figure 1 the upper side of the support portion 201 (the protruding amount from the flange portion 202) is, for example, 3 to 15 mm.

[0026] The flange portion 202 is the lower side portion of the base plate 200. Figure 1 Among them, it is the lower side portion. The shape of the flange portion 202 is substantially cylindrical, and its central axis coincides with the central axis of the support portion 201. The diameter of the flange portion 202 is larger than the diameter of the support portion 201. The protruding amount of the flange portion 202 starting from the outer side surface of the support portion 201 (that is, the protruding amount toward the radial direction) is, for example, 20 to 30 mm. The thickness of the flange portion 202 is, for example, 25 to 40 mm. The overall thickness of the base plate 200 including the support portion 201 and the flange portion 202 is, for example, 30 to 40 mm.

[0027] When processing the substrate W in the semiconductor manufacturing apparatus, a focusing ring (not shown) is provided on the upper surface 203 of the flange portion 202. The focusing ring is a circular and plate-like member formed of an insulating material such as quartz, and is provided for the purpose of adjusting the distribution of the plasma during processing. Substantially the entire dielectric substrate 100 and the support portion 201 are in a state of being surrounded by the focusing ring from the outer peripheral side.

[0028] Inside the base plate 200, a cooling medium flow path 250 for allowing the cooling medium to pass through is formed. When performing processing such as etching in the semiconductor manufacturing apparatus, the cooling medium is supplied from the outside to the cooling medium flow path 250, thereby cooling the base plate 200. During processing, the heat generated on the substrate W is transferred to the cooling medium through the helium gas in the space SP, the dielectric substrate 100, and the base plate 200, and is discharged to the outside together with the cooling medium. In the base plate 200, the supply and discharge of the cooling medium to the cooling medium flow path 250 are performed through the openings 255, 256 ( Figure 1 not shown in, refer to Figure 2 ).

[0029] For the convenience of explanation, hereinafter, the outer peripheral side end portion of the placement surface, that is, the surface 110, is also referred to as "end portion E". When viewed from above, the end portion E forms a circular ridge line, but in a part (for example, the positioning edge portion), it can also form a non-circular ridge line.

[0030] Hereinafter, the portion of the base plate 200 that overlaps with the end portion E when viewed from above is also referred to as the "first portion P1". When viewed from above, the first portion P1 is, for example, a portion having a certain width so as to include the entire end portion E, but the manner of obtaining its width is not limited.

[0031] Hereinafter, the portion of the base plate 200 that is closer to the outer peripheral side than the first portion P1 when viewed from above is also referred to as the "second portion P2". The second portion P2 is the portion of the base plate 200 that is adjacent to the first portion P1 from the outer peripheral side, and refers to the entire portion that is closer to the outer peripheral side than the first portion P1.

[0032] However, in a process such as etching, it is well known that the temperature of the portion particularly on the outer peripheral side of the substrate W tends to increase. In the electrostatic chuck 10 of the present embodiment, in order to suppress such a local temperature rise and make the in-plane temperature distribution of the substrate W during the process as uniform as possible, the following various improvements have been implemented.

[0033] As Figure 1 shown, the cooling medium flow path 250 is disposed not only around the first portion P1 and its inner side in the base plate 200, but also around the outer second portion P2. That is, a cooling medium flow path 250 is also formed over the span of the second portion P2 that is closer to the outer side than the portion directly below the substrate W and the support portion 201 (that is, the first portion P1 and the like). The cooling medium flowing through the cooling medium flow path 250 in the second portion P2 cools a focusing ring (not shown) located directly above the second portion P2, and the outer peripheral side portion of the substrate W is also cooled by the focusing ring.

[0034] In the present embodiment, the diameter of the flange portion 202 is large, and accordingly the second portion P2 also becomes large. By increasing the second portion P2 and circulating the cooling medium on the basis of forming the cooling medium flow path 250 over substantially the entire span thereof, it is possible to suppress the temperature rise of the outer peripheral side portion of the substrate W.

[0035] Figure 2 The structure of the cooling medium flow path 250 formed inside the base plate 200 when viewed from above is schematically depicted. As described above, openings 255 and 256 are provided on the surface 220 of the base plate 200. A cooling medium flow path 250 is formed along the path connecting the opening 255 and the opening 256. For example, the opening 255 is used as the inlet of the cooling medium, and the opening 256 is used as the outlet of the cooling medium. Figure 2 The circular dashed line shown represents the end portion E on the outer peripheral side of the placement surface, that is, the surface 110.

[0036] In the present embodiment, an opening 255 is formed at a position near the end portion on the outer peripheral side in the base plate 200, and an opening 256 is formed at the position that is the center of the base plate 200. The cooling medium flow path 250 is connected in a spiral shape between the opening 255 and the opening 256 and is disposed so as to surround a path through substantially the entire base plate 200.

[0037] Hereinafter, in the cooling medium flow path 250, the portion surrounded in the first part P1 (that is, directly below the end portion E) is also referred to as "cooling medium flow path 251". As Figure 2 shown, when viewed from above, the cooling medium flow path 251 is disposed so as to extend in an arc shape toward the end portion E.

[0038] As Figure 1 shown, the width W1 of the cooling medium flow path 251 in the first part P1 is smaller than the width W2 of the cooling medium flow path 250 in the second part P2. The width of the cooling medium flow path 250 in the portion closer to the inside than the first part P1 is the same as the width W2 of the cooling medium flow path 250 in the second part P2, but it may also be a different size. And, the "width" of the cooling medium flow path 250 described herein is the size of the cooling medium flow path 250 in the direction perpendicular to the flow direction of the cooling medium when viewed from above.

[0039] In such a structure, the flow velocity of the cooling medium flowing through the cooling medium flow path 251 is greater than the flow velocity of the cooling medium flowing through the other portions of the cooling medium flow path 250. Along with the increase in the flow velocity, the heat transfer efficiency in the cooling medium flow path 251 is improved, and thus the first part P1 is cooled more efficiently than the second part P2. That is to say, the cooling medium flow path 250 of the present embodiment is as follows. As a result of narrowing the width of the cooling medium flow path 251 directly below the end portion E, the cooling performance of the first part P1 is higher than the cooling performance of the second part P2.

[0040] By providing a difference in the cooling performance between the first part P1 and the second part P2, while sufficiently suppressing the temperature rise of the outer peripheral side portion of the substrate W, it is also possible to prevent members such as a focus ring disposed at a position more outside than the outer peripheral side portion of the substrate W from being overcooled more than necessary. Thereby, the in-plane temperature distribution of the substrate W during processing can be made more uniform than in the past.

[0041] The second embodiment will be described. Hereinafter, mainly the parts different from the first embodiment will be described, and the parts common to the first embodiment will be appropriately omitted.

[0042] Figure 3 In, as compared with Figure 1The same cross-sectional view shows the structure of the electrostatic chuck 10 according to the present embodiment. Similar to the first embodiment, also in the present embodiment, a cooling medium flow path 250 is formed inside the base plate 200, and a part thereof (the cooling medium flow path 251) is arranged to surround in such a way as to pass directly below the end E, i.e., the first part P1. In the whole including the cooling medium flow path 251, the width of the cooling medium flow path 250 in the present embodiment tends to be uniform.

[0043] Hereinafter, the distance along the direction perpendicular to the placement surface, i.e., the surface 110, and the distance between the cooling medium flow path 250 and the surface 110 will also be referred to as the "cooling medium distance". The cooling medium distance is defined for each part of the cooling medium flow path 250, and it can be said that it is an index indicating the height position of the cooling medium flow path 250. As Figure 3 shown, in the present embodiment, the cooling medium flow path 250 is arranged to surround in such a way that the cooling medium distance (H1) in the first part P1 is smaller than the cooling medium distance (H2) in the second part P2. In other words, compared with other parts, the cooling medium flow path 250 is located at a higher position in the first part P1, and by being close to the placement surface, i.e., the surface 110, the cooling medium flow path 250 is arranged in this way. Although the cooling medium distance of the cooling medium flow path 250 in the part closer to the inside than the first part P1 is the same as the cooling medium distance (H2) in the second part P2, the distance can also be different from this.

[0044] In such a structure, the thermal resistance between the cooling medium flow path 251 in the first part P1 and the surface 110 is smaller than the thermal resistance between the cooling medium flow path 250 in the second part P2 and the surface 110. As a result, the first part P1 is cooled more efficiently than the second part P2. That is to say, the cooling medium flow path 250 of the present embodiment is as follows. As a result of reducing the cooling medium distance directly below the end E, the cooling performance of the first part P1 is higher than that of the second part P2. Even in such a structure, the same effects as those described in the first embodiment can be achieved.

[0045] The third embodiment will be described. Hereinafter, mainly the parts different from the first embodiment will be described, and the parts common to the first embodiment will be appropriately omitted.

[0046] Figure 4 In, by with Figure 2The same method represents the structure of the cooling medium flow path 250 formed in the base plate 200 of the present embodiment. Similar to the first embodiment, even in the present embodiment, a cooling medium flow path 250 is formed inside the base plate 200, and a part thereof (cooling medium flow path 251) is arranged in a surrounding manner so as to pass directly below the end E, i.e., the first part P1. In the whole including the cooling medium flow path 251, the width of the cooling medium flow path 250 in the present embodiment tends to be uniform. In addition, in the whole including the cooling medium flow path 251, the distance of the cooling medium in the present embodiment also tends to be uniform.

[0047] As Figure 4 shown, in the present embodiment, an opening 255 that serves as an inlet of the cooling medium for the cooling medium flow path 250 is formed at a position that overlaps with the end E when viewed from above. That is, the end on one side (inlet side) of the cooling medium flow path 250 is arranged at a position that overlaps with the end E when viewed from above.

[0048] When viewed from above, the cooling medium flow path 250 extends in an arc shape along the end E from the opening 255 and is arranged in a surrounding manner so as to pass through the first part P1. That is, the part that extends in an arc shape from the opening 255 as described above becomes the cooling medium flow path 251 in the present embodiment. As is clear from Figure 4 this, the cooling medium supplied to the opening 255 first passes through the cooling medium flow path 251 located directly below the end E to cool the first part P1. After that, the cooling medium passes through the second part P2 that is closer to the outside than the end E to cool the second part P2. Then, the cooling medium passes through the part that is closer to the inside than the end E to cool this part. Finally, the cooling medium is discharged to the outside from the opening 256. While passing through the path as described above, the temperature of the cooling medium gradually rises.

[0049] Since the low-temperature cooling medium first passes through the first part P1, the first part P1 is cooled with higher efficiency. On the other hand, since the cooling medium whose temperature has risen after passing through the first part P1 passes through the second part P2, the second part P2 is cooled with relatively low efficiency. That is, the cooling medium flow path 250 of the present embodiment is as follows. As a result of arranging the inlet-side end connected to the opening 255 directly below the end E, the cooling performance of the first part P1 is higher than that of the second part P2. Even in such a structure, the same effects as those described in the first embodiment can be achieved.

[0050] Also, at a position that overlaps with the end portion E when viewed from above, either only the end portion on one side of the cooling medium flow path 250 may be arranged, or the end portions on both sides may be arranged. That is to say, the openings 255 and 256 on both sides may also be arranged at a position directly below the end portion E. In such a structure, even when using either one of the openings 255 and 256 as an inlet for the cooling medium, the first part P1 can be cooled with high efficiency.

[0051] As a structure of the cooling medium flow path 250 for making the cooling performance of the first part P1 higher than that of the second part P2, it is not limited to the examples described above, but various other forms can be adopted. For example, depending on the location, the distance between adjacent cooling medium flow paths 250 when viewed from above, that is, the arrangement interval of the cooling medium flow paths 250, can also be changed. Specifically, the arrangement interval of the cooling medium flow paths 250 in the first part P1 can be made smaller than the arrangement interval of the cooling medium flow paths 250 in the second part P2. Even in such a form, the same effects as those of the embodiments described above can be achieved.

[0052] As described above, the present embodiment has been described with reference to specific examples. However, the present invention is not limited to these specific examples. Regarding these specific examples, as long as they have the features of the present invention, the technologies appropriately designed and changed by those skilled in the art are also included in the scope of the present invention. The elements, their arrangements, conditions, shapes, etc. of each of the foregoing specific examples are not limited to the illustrated content, but can be appropriately changed. As long as there is no technical contradiction, the combinations of the elements of each of the foregoing specific examples can be appropriately changed.

Claims

1. An electrostatic chuck, characterized in that, it includes: a dielectric substrate having a placement surface for placing an object to be adsorbed; and a base plate joined to the dielectric substrate and having a cooling medium flow path through which a cooling medium passes formed therein, when viewed from a direction perpendicular to the placement surface, the base plate has: a first part that overlaps with an outer peripheral side end portion of the placement surface; and a second part that is closer to the outer peripheral side than the first part, the cooling medium flow paths are respectively formed in the first part and the second part, the cooling medium flow path is formed such that the cooling performance of the first part is higher than that of the second part.

2. The electrostatic chuck according to claim 1, characterized in that, The width of the cooling medium flow path in the first part is smaller than the width of the cooling medium flow path in the second part.

3. The electrostatic chuck according to claim 1, characterized in that, when the distance in the direction perpendicular to the placement surface and the distance between the cooling medium flow path and the placement surface is defined as the cooling medium distance, the cooling medium distance in the first part is smaller than the cooling medium distance in the second part.

4. The electrostatic chuck according to claim 1, characterized in that, when viewed from a direction perpendicular to the placement surface, an end portion on at least one side of the cooling medium flow path is arranged at a position overlapping with an outer peripheral side end portion of the placement surface.

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