Patent Publication Number: US-2017372910-A1

Title: Reinforcing structure, vacuum chamber and plasma processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2016-123367 filed on Jun. 22, 2016, the entire contents of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The disclosure relates to a reinforcing structure for reinforcing a cover of a vacuum chamber, the vacuum chamber having the reinforcing structure, and a plasma processing apparatus. 
     BACKGROUND OF THE INVENTION 
     In manufacturing a flat panel display (FPD) represented by a liquid crystal display (LCD), plasma processing such as plasma etching, sputtering, plasma CVD or the like is performed on a glass substrate for use in FPD. 
     In a plasma processing apparatus for performing the plasma processing, vacuum processing is required and, thus, a vacuum chamber, which can be evacuated, is used as a processing chamber. The vacuum chamber includes a main body and a cover having thicknesses enough to endure a pressure difference between the inside and the outside of the vacuum chamber. 
     Recently, however, a FPD substrate is considerably scaled up and a large FPD substrate has a side length of more than 2 meters. A large vacuum chamber corresponding to the large FPD substrate requires an extremely large thickness in order to ensure strength against an atmospheric pressure. As a result, a weight is increased and a material cost or a manufacturing cost is considerably increased. 
     As a technique for solving the above drawbacks, in Japanese Patent No. 5285403, a reinforcing structure including beam members is provided at an outer side of an upper part (cover) of a vacuum chamber. Accordingly, it is possible to realize weight reduction and reduction of the material cost and the manufacturing cost while maintaining sufficient strength against an atmospheric pressure. In addition, in Japanese Patent Application Publication No. 2015-22806, a reinforcing structure including arch-shaped ribs for suppressing deformation of a ceiling plate of a vacuum chamber is provided at an outer side of the ceiling plate. 
     In a conventional plasma processing apparatus, an opening/closing mechanism for opening/closing the cover of the vacuum chamber is provided. In a large vacuum chamber corresponding to a large substrate having a side length of more than 2 meters, the strength is ensured by the reinforcing structures disclosed in Japanese Patent No. 5285403 and Japanese Patent Application Publication No. 2015-22806. However, the effect of weight reduction is not sufficient because the reinforcing structures have weights of about 1.5 tons and about 2.0 tons, respectively. Also, it is required to scale up the opening/closing mechanism. Recently, in view of cost reduction, the cover is opened/closed by a ceiling crane installed in a user&#39;s factory without using the opening/closing mechanism. However, in the case of the reinforcing structures disclosed in Japanese Patent No. 5285403 and Japanese Patent Application Publication No. 2015-22806, the weight of the cover exceeds a tolerable range of the ceiling crane due to the heavy weights of the reinforcing structures. 
     SUMMARY OF THE INVENTION 
     In view of the above, the disclosure provides a reinforcing structure capable of realizing desired weight reduction, a vacuum chamber having the reinforcing structure, and a plasma processing apparatus. 
     In accordance with an aspect, there is provided a reinforcing structure in which a plurality of beam members provided on a top surface of a cover of a vacuum chamber for performing predetermined processing on a substrate is combined to reinforce the cover, including: a ring-shaped portion formed by arranging beam members in a ring shape at a central region of the top surface of the cover; and a radial portion formed by radially extending beam members from the ring-shaped portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the disclosure will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross sectional view showing a plasma processing apparatus including a reinforcing structure according to an embodiment; 
         FIG. 2  is a perspective view showing an external appearance of a vacuum chamber of the plasma processing apparatus shown in  FIG. 1 ; 
         FIG. 3  is a top view showing the reinforcing structure according to the embodiment; 
         FIG. 4  explains a state at the time of opening/closing a cover of the vacuum chamber by a crane; 
         FIG. 5  is a top view showing a reinforcing structure according to another embodiment; and 
         FIG. 6  is a top view showing a reinforcing structure according to still another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. 
       FIG. 1  is a cross sectional view showing a plasma processing apparatus including a reinforcing structure according to an embodiment.  FIG. 2  is a perspective view showing an external appearance of a vacuum chamber of the plasma processing apparatus shown in  FIG. 1 .  FIG. 3  is a top view showing the reinforcing structure according to the embodiment. 
     As shown in  FIG. 1 , a plasma processing apparatus  100  is configured as an inductively coupled plasma processing apparatus for performing plasma processing, e.g., plasma etching, on a rectangular glass substrate for use in FPD (hereinafter, simply referred to as “substrate”) G. The FPD may be a liquid crystal display (LCD), an electro Luminescence (EL) display, a plasma display panel (PDP) or the like. 
     The plasma processing apparatus  100  includes a vacuum chamber  1  having an inner wall surface made of a conductive material, e.g., anodically oxidized aluminum. The vacuum chamber  1  has a substantially rectangular parallelepiped shape and a rectangular cross sectional shape. The vacuum chamber  1  is grounded by a ground line  1   a . The vacuum chamber  1  includes a chamber main body  2 , a cover  3 , and a reinforcing structure  4 . 
     The chamber main body  2  has a bottom wall  2   a  and a sidewall  2   b  and an opening at an upper portion thereof. The opening can be opened/closed by the cover  3 . By closing the opening by the cover  3 , a processing space  5  is formed inside the chamber main body  2 . 
     At a bottom portion in the processing space  5 , a substrate mounting table  10  for mounting thereon the substrate G is provided on the bottom wall  2   a  of the chamber main body  2  through an insulating member  9  made of resin or insulating ceramic such as alumina or the like. The substrate mounting table  10  includes a base  11  made of a metal, e.g., aluminum, and an insulating ring  12  provided around the base  11 . Although it is not illustrated, an electrostatic chuck for attracting and holding the substrate G is provided on a top surface of the substrate mounting table  10 , and elevating pins used for transferring the substrate G penetrate through the substrate mounting table  10 . Further, although it is not illustrated, the substrate mounting table  10  is provided with a temperature control unit for controlling a temperature of the substrate G and a temperature sensor. 
     A plurality of gas exhaust ports  13  is provided at the bottom wall  2   a  of the chamber main body  2 . Gas exhaust lines  14  are connected to the respective gas exhaust ports  13 . The gas exhaust lines  14  are connected to gas exhaust units  15 , each including an automatic pressure control valve and a vacuum pump. The processing space  5  is vacuum-evacuated by the gas exhaust units  15  and a pressure in the processing space  5  is controlled to a predetermined pressure. 
     Provided at the sidewall  2   b  of the chamber main body  2  is a loading/unloading port  16  for loading/unloading the substrate G into/from the processing space  5 . The loading/unloading port  16  can be opened/closed by a gate valve  17 . A transfer chamber (not shown) is provided near the chamber main body  2 . By opening the gate valve  17 , the substrate G can be loaded into and unloaded from the processing space  5  through the loading/unloading port  16  by a transfer unit (not shown) provided in the transfer chamber. The base  11  of the substrate mounting table  10  is connected to a high frequency bias power supply  19  for applying a high frequency bias power for ion attraction via a matching unit  18 . 
     The cover  3  includes a ceiling wall  3   a , a sidewall  3   b , and a dielectric wall  21  serving as a bottom wall. The dielectric wall  21  also serves as a ceiling wall of the chamber main body  2 . A space surrounded by those walls becomes an antenna space  6 . The dielectric wall  21  is made of quartz, ceramic such as Al 2 O 3  or the like. 
     A ring-shaped supporting member  22  having an inwardly protruding ring-shaped supporting portion  22   a  is provided below the sidewall  3   b . The dielectric wall  21  is supported by the ring-shaped supporting portion  22   a . The dielectric wall  21  and the ring-shaped supporting member  22  are sealed by a seal ring  23 . 
     A shower housing  24  for supplying a processing gas is made of a metal, e.g., aluminum. The shower housing  24  is fitted to a lower portion of the dielectric wall  21 . The shower housing  24  is formed in a cross shape and has a structure, e.g., a beam structure, for supporting the dielectric wall  21  from the bottom. The dielectric wall  21  is divided into a plurality of pieces. The shower housing  24 , as a beam, supports the divided pieces at contact portions between adjacent divided pieces. The shower housing  24  for supporting the dielectric wall  21  is suspended from the ceiling wall  3   a  by a plurality of suspenders  25 . The ring-shaped supporting member  22  and the shower housing  24  may be coated with a dielectric material. 
     A gas channel  26  extending horizontally is formed in the shower housing  24 . A plurality of gas injection holes  26   a  extending downward communicates with the gas channel  26 . A gas supply line  27  is provided at a central portion of a top surface of the dielectric wall  21  to communicate with the gas channel  26 . The gas supply line  27  penetrates through the ceiling wall  3   a  or the sidewall  3   b  and is connected to a processing gas supply unit  28  including a processing gas supply source, a valve system and the like. Therefore, when the plasma processing is performed, the processing gas is supplied from the processing gas supply unit  28  to the gas channel  26  in the shower housing  24  through the gas supply line  27  and then is injected into the processing space  5  through the gas injection holes  26   a  formed at the bottom surface of the shower housing  24 . 
     A high frequency (RF) antenna  30  is provided in the antenna space  6 . The RF antenna  30  is formed by arranging an antenna line  31  made of a highly conductive metal such as copper, aluminum or the like in a conventional shape such as a ring shape, a spiral shape or the like. The RF antenna  30  may be a multiplex antenna having a plurality of antenna sections. 
     A power feed member  33  extending to an upper portion in the antenna space  6  is connected to a terminal  32  of the antenna line  31 . A matching unit  34  is connected to the power feed member  33 . A high frequency power supply  36  is connected to the matching unit  34  through a power feed line  35 . The antenna line  31  of the RF antenna  30  is separated from the dielectric wall  21  by a spacer  38  made of an insulating material. 
     By supplying a high frequency power having a predetermined frequency, e.g., 13.56 MHz, from the high frequency power supply  36  to the RF antenna  30 , an induced electric field is generated in the processing space  5 . The processing gas supplied from the shower housing  24  is converted into a plasma by the induced electric field. As a consequence, an inductively coupled plasma is generated. 
     When the cover  3  is attached to the chamber main body  2 , the cover  3  is fixed by screws (not shown). A gap between the chamber main body  2  and the cover  3  is sealed by a seal ring  37 . 
     As shown in  FIGS. 2 and 3 , the reinforcing structure  4  is formed by combining a plurality of beam members made of H-shaped steel and provided on a rectangular top surface of the ceiling wall  3   a  of the cover  3 . Specifically, as shown in  FIG. 3 , the reinforcing structure  4  includes two first beam members  41  having a linear shape and disposed in parallel along the entire length of a pair of long sides  301  of the ceiling wall  3   a  of the cover  3  and two second beam members  42  having a linear shape and disposed in parallel along the entire length of a pair of short sides  302  of the ceiling wall  3   a  of the cover  3 . The first beam members  41  and the second beam members  42  are arranged in a parallel cross shape. The reinforcing structure  4  further includes four third beam members  43  extending in a diagonal direction from intersection points between the first beam members  41  and the second beam members  41 . These beam members are attached to the ceiling wall  3   a  by fastening units such as bolts or the like. 
     The first and the second beam members  41  and  42  are arranged such that the top surface of the ceiling wall  3   a  of the cover  3  is divided into (substantially) nine parts. Two central portions  41   a  of the two first beam members  41  and two central portions  42   a  of the two second beam members  42  form a ring-shaped portion  44  that is a rectangular frame body. End portions  41   b  disposed at both sides of the central portions  41   a  in the first beam members  41 , end portions  42   b  disposed at both sides of the central portions  42   a  in the second beam members  42 , and the third beam members  43  radially extend outward from the ring-shaped portion  44 . These beam members form a radial portion  45 . In other words, the reinforcing structure  4  includes the ring-shaped portion  44  formed by combining the beam members in a frame shape and provided at the central region of the top surface of the ceiling wall  3   a  of the cover  3 , and the radial portion  45  formed by radially extending the beam members outward from the ring-shaped portion  44 . 
     The end portions  41   b  of the first beam members  41  forming the radial portion  45  are perpendicular to the central portions  42   a  of the second beam members  42  forming the ring-shaped portion  44 . The end portions  42   b  of the second beam members  42  forming the radial portion  45  are perpendicular to the central portions  41   a  of the first beam members  41  forming the ring-shaped portion  44 . The third beam members  43  forming the radial portion extend in a diagonal direction from corners of the ring-shaped portion  44 . 
     A plate-shaped member  46  is provided between adjacent ones of the beam members (the end portions  41   b  and  42   b  and the third beam members  43 ) forming the radial portion  45  to correct the adjacent beam members. The plate-shaped member  46  is provided to improve the reinforcing effect of the reinforcing structure  4 . In the present embodiment, the plate-shaped member  46  is provided at the entire gap between the adjacent beam members of the radial portion  45 . However, the plate-shaped member  46  is not necessarily provided at the entire gap between the adjacent beam members and may be provided at at least a part of the gap. In the case of providing the plate-shaped member  46  at a part of the entire gap, it is ideal that the plate-shaped member  46  is provided symmetrically. However, when the cover  3  has strength variation in terms of structure, the plate-shaped members  46  may be provided at a mechanically weak portion without symmetry. A width of the plate-shaped member  46  is set while considering balance between the reinforcing effect and the weight increase. The width of the plate-shaped member  46  is preferably about 20% to 80% and more preferably about 40% to 60% of the length of the beam members of the radial portion  45  which are adjacent thereto. 
     An auxiliary beam member  47  is provided between the two facing end portions  41   b  of the two first beam members  41  and between the two facing end portions  42   b  of the two second beam members  42  at the outer side of the plate-shaped member  46 . 
     With the combination of the ring-shaped portion  44  and the radial portion  45 , the reinforcing structure  4  can maintain the high reinforcing effect while realizing the weight reduction. 
     As described above, the ring-shaped portion  44  is provided at the central region of the top surface of the ceiling wall  3   a . A length of each side of the ring-shaped portion  44  is preferably about 30% to 80% of the side length of the ceiling wall  3   a . Accordingly, the reinforcing effect of the cover  3  can be maintained at a high level. The ring-shaped portion  44  has therein a space and the matching unit  34  that is a large device can be inserted in the space. As a consequence, it is possible to realize space saving. A large device that can be located in the ring-shaped portion  44  is not limited to the matching unit  34 . 
     The plasma processing apparatus  100  further includes a control unit  50  having a microprocessor (computer) for controlling the respective components of the plasma processing apparatus  10 . 
     In the plasma processing apparatus  100  configured as described above, first, the processing space  5  is exhausted by the gas exhaust unit  15  so that a pressure in the processing space  5  can be set to a predetermined level. Next, the gate valve  17  is opened and the substrate G is loaded through the loading/unloading port  16  by a transfer unit (not shown). Then, the substrate G is mounted on the substrate mounting table  10 . After the transfer unit is retreated from the processing space  5 , the gate valve  17  is closed. 
     In that state, the processing space  5  is vacuum-evacuated and a pressure in the processing space  5  is controlled to a predetermined vacuum level by a pressure control valve (not shown). At the same time, a predetermined processing gas is supplied into the processing space  5  from the processing gas supply unit  28  through the gas supply line  27  and the shower housing  24 . 
     Next, a high frequency power having a predetermined frequency (e.g., 13.56 MHz) is applied at a predetermined power level from the high frequency power supply  36  to the RF antenna  30 . Accordingly, an induced electric field is uniformly generated in the processing space  5  through the dielectric wall  21 . The processing gas is converted into a plasma in the processing space  5  by the induced electric field thus generated, thereby generating a high-density inductively coupled plasma. The predetermined plasma processing, e.g., film formation or etching, is performed on the substrate G by the plasma thus generated. 
     When the substrate G is a large substrate having a side length of more than 2 meters, the vacuum chamber  1  is also scaled up. Therefore, the reinforcing structure  4  is provided to reduce the weight of the cover  3  while maintaining a sufficient strength against an atmospheric pressure at the time of setting the processing space  5  to a vacuum state. 
     The techniques for reinforcing the cover by the reinforcing structure are disclosed in Japanese Patent No. 5285403 and Japanese Patent Application Publication No. 2015-22806. In the case of considering application to a large apparatus, ensuring strength is important in the technique disclosed in Japanese Patent Application Publication No. 2015-22806 and, thus, the weight of the reinforcing structure is extremely increased to about 2 tons. The reinforcing structure disclosed in Japanese Patent No. 5285403 which is intended to ensure the strength and reduce the weight also has a weight of about 1.5 tons, which is insufficient to realize the weight reduction. 
     On the other hand, the reinforcing structure  4  of the present embodiment is formed by combining a plurality of beam members on the rectangular top surface of the ceiling wall  3   a  of the cover  3 . Further, the reinforcing structure  4  of the present embodiment includes the ring-shaped portion  44  formed by combining the beam members in a frame shape and provided at the central region of the top surface of the ceiling wall  3   a  and the radial portion  45  formed by radially extending the beam members outward from the ring-shaped portion  44 . At this time, a certain level of strength can be ensured by the ring-shaped portion  44  provided at the central region and a sufficient strength can be obtained by radially arranging the beam members from the ring-shaped portion  44 . 
     The structure obtained by combining the ring-shaped portion  44  provided at the central region and the radial portion  45  provides a high reinforcing effect. In the case of employing such a structure, even if the thicknesses of the beam members are thinner than conventional ones, a desired strength can be ensured. In addition, the number of the beam members can be reduced due to a simple structure. Accordingly, the weight of the reinforcing structure  4  can be reduced. At this time, a length of each side of the ring-shaped portion  44  is preferably about 30% to 60% of the side length of the ceiling wall  3   a  in order to maintain the reinforcing effect of the cover  3  at a high level. 
     By providing the ring-shaped portion  44  at the central region, a space can be ensured at the central region on the top surface of the cover  3  and a large device such as the matching unit  34  or the like can be installed therein. As a result, the space saving can be realized. 
     The end portions  41   b  of the first beam members  41  which form the radial portion  45  are perpendicular to the central portions  42   a  of the second beam members  42  forming the ring-shaped portion  44 . The end portions  42   b  of the second beam members  42  which form the radial portion  45  are perpendicular to the central portions  41   a  of the first beam members  41  forming the ring-shaped portion  44 . Therefore, the reinforcing effect of the radial portion  45  can be improved. By providing, as the radial portion  45 , the four third beam members  43  extending in a diagonal direction from the intersection points between the first beam members  41  and the second beam members  42 , the inner portion of the ring-shaped portion  44  of the cover  3  is reinforced and, thus, the reinforcing effect can be further increased. With the above configuration, the weight reduction effect can be further improved. 
     Further, the reinforcing structure  4  includes the two first beam members  41  having a linear shape and disposed in parallel along the entire length of a pair of long sides  301  of the ceiling wall  3   a  of the cover  3  and the two second beam members  42  having a linear shape and disposed in parallel along the entire length of a pair of short sides  302  of the ceiling wall  3   a  of the cover  3 . These beam members are arranged in a parallel cross shape, thereby forming the ring-shaped portion  44  and the radial portion  45 . This is basically the combination of the long beam members. The combination of the long beam members provides a higher reinforcing effect compared to the combination of short beam members. Therefore, the reinforcing effect can be further improved and the weight reduction effect can be further improved. Further, by arranging the two first beam members  41  and the two second beam members  42  such that the top surface of the ceiling wall  3   a  of the cover  3  is divided into substantially nine parts, the reinforcing effect and the weight reduction effect can be further improved. 
     Furthermore, by providing the plate-shaped member  46  at the gap between adjacent ones of the beam members forming the radial portion  45 , the reinforcing effect to the reinforcing structure  4  can be improved. Since the plate-shaped member  46  has a plate shape, the reinforcing effect can be improved without a considerable increase in the weight, which is advantageous when it is required to further improve the reinforcing effect obtained by combining the ring-shaped portion  44  and the radial portion  45 . At this time, the width of the plate-shaped member  46  may be appropriately set while considering the balance between the reinforcing effect and the weight increase. In other words, when the width of the plate-shaped member  46  is increased, the reinforcing effect is improved. However, the reinforcing effect is saturated and an adverse effect due to the increase in the weight of the plate-shaped member  46  is increased when the width exceeds a certain level. From the above, the width of the plate-shaped member  46  is preferably about 20% to 80% and more preferably about 40% to 60% of the length of the beam members adjacent to the plate-shaped member  46 . The plate-shaped member  46  is not necessarily provided at the entire gap between the adjacent beam members. A certain effect can be obtained by providing the plate-shaped member  46  at at least a part of the gap between the adjacent beam members. 
     In the present embodiment, the reinforcing structure  4  capable of realizing desired weight reduction can be obtained. In the case of applying the reinforcing structure  4  of the present embodiment to a large plasma processing apparatus corresponding to a large substrate, it is possible to reduce the weight of the reinforcing structure which is about 1.5 tons in Japanese Patent No. 5285403 and about 2 tons in Japanese Patent Application Publication No. 2015-22806 to about 1 ton. 
     Therefore, in the case where the plasma processing apparatus  100  has the opening/closing mechanism of the cover  3 , the scaling up of the opening/closing mechanism can be suppressed and a cost increase of the opening/closing mechanism can be prevented. 
     Since the weight of the reinforcing structure  4  can be reduced, the weight of the cover  3  can be within a tolerable range of a conventional ceiling crane. Accordingly, the cover can be opened/closed by the ceiling crane installed in a user&#39;s factory without using the opening/closing mechanism. As a result, the apparatus cost can be reduced. 
     In the case of opening/closing the cover  3  by the crane, a crane opening/closing operation is performed by providing a crane opening/closing jig  61  at the cover  3  and directly or indirectly engaging a hook  63  of the crane with the crane opening/closing jig  61 , as can be seen from  FIG. 4 . In this example, the crane opening/closing jigs  61  are provided at a plurality of positions of the cover and a rope is coupled to the crane opening/closing jigs  61 . The rope  62  is engaged with the hook  63  of the crane and the cover  3  is moved up and down by the crane. Accordingly, the opening/closing of the cover  3  is carried out. It is also possible to provide a crane opening/closing jig directly coupled to the hook of the crane at the cover  3 . 
     The present disclosure is not limited to the above embodiments and may be variously modified within the scope of the present disclosure. For example, in the above embodiments, the present disclosure is applied to the inductively coupled plasma processing apparatus in which the dielectric wall is used as the ceiling wall of the chamber main body defining the processing space. However, the present disclosure may be applied to an inductively coupled plasma processing apparatus using a metal wall instead of the dielectric wall and may also be applied to another plasma processing apparatus such as a capacitively coupled parallel plate plasma processing apparatus, a microwave plasma processing apparatus or the like. Further, the present disclosure is not limited to the plasma processing apparatus and may be applied to a vacuum chamber for performing vacuum processing using no plasma, such as thermal CVD or the like. 
     The above embodiments have described the example in which the third beam members  43  extend diagonally. In that case, it is assumed that the ceiling wall  3   a  and the ring-shaped portion  44  have similar shapes having the same aspect ratio. However, the ceiling wall  3   a  and the ring-shaped portion  44  do not necessarily have the similar shapes. In that case, the third beam members  43  may be arranged to connect the corners of the ring-shaped portion  44  and the corners of the ceiling wall  3   a.    
     The above embodiments have described the example in which the reinforcing structure includes the ring-shaped portion  44  and the radial portion  45  which are formed by arranging in a parallel cross shape the two first beam members  41  having a linear shape and disposed in parallel to the pair of long sides  301  of the ceiling wall  3   a  of the cover  3  and the two second beam members  42  having a linear shape and disposed in parallel to the pair of short sides  302  of the ceiling wall  3   a  of the cover  3 . However, the present disclosure is not limited thereto. 
     For example, as shown in  FIG. 5 , there may be provided a reinforcing structure  4 ′ including a ring-shaped portion  44 ′ formed by combining first beam members  81  corresponding to long sides and second beam members  82  corresponding to short sides and a radial portion  45 ′ formed by third beam members  83  perpendicularly extending outward from the second beam members  82  forming the ring-shaped portion  44 ′, fourth beam members  84  perpendicularly extending outward from the first beam members  81 , and fifth beam members  85  obliquely extending outward from corners of the ring-shaped portion  44 ′. 
     The above embodiments have described the example in which the present disclosure is applied to the vacuum chamber having a rectangular cross sectional shape and configured to process a rectangular substrate. However, it is not limited thereto and the present disclosure may also be applied to a vacuum chamber having a circular cross sectional shape and configured to process a circular substrate. In that case, there is provided a reinforcing structure  4 ″ including a cylindrical ring-shaped portion  44 ″ formed by a circular beam member  91  provided at a central region of a top surface of a circular cover and a radial portion  45 ″ formed by arranging a plurality of linear beam members  92  directed outward from the cylindrical ring-shaped portion  44 ″, as can be seen from  FIG. 6 . 
     The above embodiments have described the case of using H-shaped steel for the beam members forming the reinforcing structure. However, steel having other shapes such as an L-shaped cross section (angle), a C-shaped cross section (channel) and the like may be used instead of H-shaped steel. Further, other types of materials such as a timber, a hollow pipe, a plate and the like may also be used. 
     While the disclosure has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the disclosure as defined in the following claims.