Patent Publication Number: US-11384993-B2

Title: Heat pipe

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 15/807,797 filed on Nov. 9, 2017, which is based upon and claims priority to Japanese Patent Application No. 2016-242730, filed on Dec. 14, 2016, and No. 2017-112587 filed on Jun. 7, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Certain aspects of the embodiments discussed herein are related to heat pipes. 
     BACKGROUND 
     A heat pipe is a known device for cooling a heat-generating component, such as a CPU (Central Processing Unit) or the like, that is provided in electronic devices. The heat pipe utilizes a phase change of a working fluid to transfer heat. 
     One example of the heat pipe includes plates that are mutually arranged at 90-degree crossing angles in a lattice, where each plate has a meander groove formed on one surface thereof. The working fluid is sealed in a tunnel of the meander groove. This heat pipe has a structure in which a vapor pipe and a liquid pipe are not separate, as proposed in Japanese Laid-Open Patent Publication No. 2001-165582, for example. 
     However, according to the proposed heat pipe described above, the working fluid that is condensed and returned and the vapor diffusion from an evaporation part pass through the same tunnel. For this reason, the working fluid evaporates in a vicinity of the evaporation part and spreads along the tunnel of the groove, but the vapor can be prevented from spreading due to the working fluid existing in the tunnel. In addition, when the working fluid that is cooled, condensed, and liquefied returns to the evaporation part after the vapor spreads, the liquefied working fluid collides with the vapor. Accordingly, heat dissipation of the proposed heat pipe is poor because the evaporation and the condensation do not occur cyclically. 
     SUMMARY 
     Accordingly, it is an object in one aspect of the embodiments to provide a heat pipe that can improve the heat dissipation, and a method of manufacturing such a heat pipe. 
     According to one aspect of the embodiments, a heat pipe includes a first metal layer forming a liquid layer configured to move a working fluid that is liquefied from vapor; and a second metal layer forming a vapor layer configured to move the vapor of the working fluid that is vaporized, wherein the first metal layer includes a plurality of first cavities that cave in from a first surface of the first metal layer and are arranged apart from each other, a plurality of second cavities that cave in from a second surface of the first metal layer opposite to the first surface of the first metal layer, a plurality of first pores partially communicating with the plurality of first cavities and the plurality of second cavities, respectively, and a plurality of second pores partially communicating side surfaces of the plurality of second cavities that are adjacent to each other, and wherein the second metal layer is provided on the first surface of the first metal layer and includes an opening exposing the plurality of first cavities. 
     The object and advantages of the embodiments will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are diagrams illustrating an example of the heat pipe in a first embodiment; 
         FIG. 2  is a diagram for explaining functions of parts of the heat pipe in the first embodiment; 
         FIGS. 3A and 3B  are diagrams for explaining the functions of the parts of the heat pipe in the first embodiment; 
         FIGS. 4A, 4B, 4C, and 4D  are diagrams for explaining examples of manufacturing processes of the heat pipe in the first embodiment; 
         FIGS. 5A and 5B  are diagrams illustrating an example of the heat pipe in a first modification of the first embodiment; 
         FIGS. 6A and 6B  are diagrams illustrating an example of the heat pipe in a second modification of the first embodiment; 
         FIG. 7  is a diagram illustrating an example of the heat pipe in a third modification of the first embodiment; 
         FIG. 8  is a diagram illustrating an example of the heat pipe in a fourth modification of the first embodiment; 
         FIGS. 9A and 9B  are diagrams illustrating an example of the heat pipe in a second embodiment; 
         FIGS. 10A and 10B  are diagrams for explaining examples of the manufacturing processes of the heat pipe in the second embodiment; and 
         FIGS. 11A and 11B  are diagrams illustrating an example of the heat pipe in a first modification of the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, those parts that are the same are designated by the same reference numerals, and a repeated description of the same parts may be omitted. 
     A description will now be given of the heat pipe and the method of manufacturing the heat pipe in each embodiment according to the present invention. 
     First Embodiment 
     [Structure of Heat Pipe in First Embodiment] 
     First, a description will be given of a structure of the heat pipe in a first embodiment.  FIGS. 1A and 1B  are diagrams illustrating an example of the heat pipe in the first embodiment.  FIG. 1B  illustrates a plan view of the heat pipe, and  FIG. 1A  illustrates a cross sectional view of the heat pipe along a line A-A in  FIG. 1B . 
     As illustrated in  FIGS. 1A and 1B , a heat pipe  1  is an omnidirectional heat pipe having a stacked structure including 4 metal layers  11  through  14 . The metal layers  11  through  14  are made of copper having a sufficiently high thermal conductivity, for example, and are mutually bonded directly by solid-phase (or solid-state) welding. Each of the metal layers  11  through  14  may have a thickness in a range of approximately 50 μm to approximately 200 μm, for example. A material forming the metal layers  11  through  14  is not limited to copper, and the metal layers  11  through  14  may be made of any suitable material having the sufficiently high thermal conductivity, such as stainless steel, aluminum, magnesium alloys, or the like. In this example, a planar shape of the heat pipe  1 , in a plan view viewed from above a top surface  14   a  of the metal layer  14  in  FIG. 1A  in a normal direction to the top surface  14   a , is a rectangular shape. 
     In  FIGS. 1A and 1B , a Z-direction denotes a stacking direction (or thickness direction) in which the metal layers  11  through  14  are stacked (or the thickness of the metal layers  11  through  14  is measured). An X-direction denotes a direction parallel to one side forming a geometrical shape of the top surface  14   a  of the metal layer, and a Y-direction denotes a direction perpendicular to the X-direction within the top surface  14   a  of the metal layer  14 . Definitions of the X-direction, the Y-direction, and the Z-direction in  FIGS. 1A and 1B  are the same for similar figures described hereinafter. In addition, in this embodiment, it is assumed for the sake of convenience that a top side or one side of the heat pipe  1  refers to a side provided with the metal layer  14 , and that a bottom side or the other side of the heat pipe  1  refers to a side provided with the metal layer  11 . Further, it is also assumed for the sake of convenience that a top surface or one surface of each part refers to a surface facing towards the metal layer  14 , and a bottom surface or the other surface of each part refers to a surface facing towards the metal layer  11 . 
     In the heat pipe  1 , the metal layer  14  and the metal layer  11 , respectively forming outermost layers, are continuous metal layers having no holes or grooves. 
     The metal layer  12  is stacked on a top surface of the metal layer  11 . The metal layer  12  includes a plurality of cavities  121  extending in the Z-direction from the side of the metal layer  13  (the top surface of the metal layer  12 ), and a plurality of cavities  122  extending in the Z-direction from the side of the metal layer  11  (the bottom surface of the metal layer  12 ). Each cavity  121  caves in from the top surface of the metal layer  12  towards an approximate center part of the metal layer  12  along the Z-direction, and each cavity  122  caves in from the bottom surface of the metal layer  12  towards an approximate center part of the metal layer  12  along the Z-direction. In addition, the cavity  121  and the cavity  122 , that correspond to each other, partially communicate to a pore  123 . 
     The metal layer  12  includes a plurality of through-holes  12   x  that penetrate in the Z-direction. Each through-hole  12   x  is formed by the corresponding cavities  121  and  122  and the pore  123 . 
     The plurality of cavities  121  are arranged in a matrix arrangement. For example, the plurality of cavities  121  includes rows of cavities arranged at predetermined intervals in the X-direction, and columns of cavities arranged at predetermined intervals in the Y-direction. However, the rows of the cavities do not necessarily have to be arranged in the X-direction, and the columns of the cavities do not necessarily have to be arranged in the Y-direction. 
     In addition, the rows and the columns of the cavities do not necessarily have to be perpendicular to each other. For example, the columns of the cavities may be arranged obliquely to the rows of the cavities, and an entire planar shape of a region in which the plurality of cavities  121  are arranged may be a parallelogram shape. Further, the number of cavities  121  included each row and the number of cavities  121  included in each column may be the same, or may be different. For example, in a case in which the number of cavities  121  included each row and the number of cavities  121  included in each column are different, the entire planar shape of the region in which the plurality of cavities  121  are arranged may be a trapezoidal shape. Moreover, the plurality of cavities  121  may be arranged in a staggered pattern. 
     One cavity  122  is provided in correspondence with each cavity  121 . The corresponding cavities  121  and  122  are arranged to overlap in the plan view, and bottom surfaces of the corresponding cavities  121  and  122  partially communicate with each other to form the pore  123 . In other words, the plurality of cavities  122  are arranged in a matrix arrangement, in correspondence with the plurality of cavities  121 , and the bottom surfaces of the cavities  121  and  122  that overlap in the plan view connect with each other and communicate in the Z-direction. The cavities  121  and  122  do not need to be arranged to perfectly overlap each other in the plan view, as long as the bottom surfaces of the cavities  121  and  122  are arranged to communicate with each other through the pore  123 . 
     The cavities  121  are arranged apart from each other. In other words, the cavities  121  that are adjacent to each other in the X-direction and the Y-direction do not communicate with each other. On the other hand, side surfaces defining the cavities  122  that are adjacent to each other in the X-direction and the Y-direction partially communicate with each other in the X-direction and the Y-direction through corresponding pores  125 . In other words, all of the cavities  122  that are arranged in the matrix arrangement communicate through the pores  125 . 
     An area of a part of each of the plurality of cavities  121  opening at the top surface of the metal layer  12  is smaller than an area of a part of each of the plurality of cavities  122  opening at the bottom surface of the metal layer  12 . Each cavity  121  is formed in an approximate hemispherical shape, and the planar shape of the cavity  121  is a circular shape. In this case, a diameter Ø 1  of the part of the cavity  121  opening on the side of the metal layer  13  may be approximately 25 μm, for example. 
     Each cavity  122  is formed to an approximate hemispherical shape, and the planar shape of the cavity  122  is a circular shape. In this case, a diameter Ø 2  of the part of the cavity  122  opening at the bottom surface of the metal layer  12  is greater than the diameter Ø 1  of the part of the cavity  121  opening at the top surface of the metal layer  12 , and may be approximately 50 μm, for example. 
     A position where the corresponding cavities  121  and  122  communicate with each other (that is, a position of the pore  123 ) is located closer to the top surface of the metal layer  12  than a center along the thickness direction of the metal layer  12 , and a ratio D 1 :D 2  in  FIG. 1A  may be approximately 3:7, for example. A diameter Ø 3  of the pore  123  is smaller than the diameter Ø 1  of the cavity  121  and the diameter Ø 2  of the cavity  122 , and may be approximately 15 μm, for example. 
     The planar shape of each of the cavities  121  and  122  is not limited to the circular shape, and may be an arbitrary shape, such as an oval shape, a polygonal shape, or the like. In addition, the cavity  121  is not limited to the approximate hemispherical shape, and may have an arbitrary tapered shape defined by inner walls that widen from the pore  123  towards the top surface of the metal layer  12 . Similarly, the cavity  122  is not limited to the approximate hemispherical shape, and may have an arbitrary tapered shape defined by inner walls that widen from the pore  123  towards the bottom surface of the metal layer  12 . 
     A width W 1  of the horizontally oriented pores  125  along the X-direction, a width W 2  of the vertically oriented pores  125  along the Y-direction, and a height H 1  of the horizontally and vertically oriented pores  125  along the Z-direction respectively are smaller than the diameter Ø 2  of the cavity  122 . The width W 1  of the horizontally oriented pores  125  along the X-direction may be approximately 20 μm, for example. The width W 2  of the vertically oriented pores  125  along the Y-direction may be approximately 20 μm, for example. The height H 1  of the horizontally and vertically oriented pores  125  along the Z-direction may be approximately 10 μm, for example. 
     The metal layer  13  is stacked on the top surface of the metal layer  12 . The metal layer  13  is frame-shaped, and includes an opening  13   x  that exposes the plurality of through-holes  12   x  arranged in the matrix arrangement. The metal layer  14  is stacked on the metal layer  13 , to form a lid on the frame-shaped metal layer  13 . 
       FIG. 2  is a diagram for explaining functions of parts of the heat pipe in the first embodiment, and illustrates a cross section corresponding to that of  FIG. 1A . 
     As illustrated in  FIG. 2 , the metal layer  11  and the metal layer  14  form outer walls of the heat pipe  1 . In addition, the frame-shaped metal layer  13  forms a vapor layer of the heat pipe  1 . More particularly, the metal layer (or vapor layer)  13  includes a vapor-phase part  21  that is surrounded by the top surface of the metal layer  12  and the bottom surface of the metal layer  14 , within the opening  13   x  of the metal layer  13 . The vapor-phase part  21  forms a region in which vapor C v , obtained by vaporizing a working fluid C, is moved (or transferred) from a high-temperature end to a low-temperature end. 
     The metal layer  12  forms a liquid layer of the heat pipe  1 . More particularly, the metal layer (or liquid layer)  12  includes a liquid passage part  22  and a vent part  23 . The liquid passage part  22  is formed by the cavities  122  communicating in the X-direction and the Y-direction at the metal layer  12 . The liquid passage part  22  (or the cavities  122 ) forms a region in which the working fluid C, liquefied at the low-temperature end, is moved to the high-temperature end. 
     The vent part  23  is famed by each of the cavities  121  communicating to the cavities  122 , and the pores  123 , at the metal layer  12 . The vent part  23  partitions the vapor-phase part  21  with respect to the liquid passage part  22 , and forms a region in which the working fluid C generated by the vapor-phase part  21  is moved to the liquid passage part  22 . 
     In an initial state in which the heat pipe  1  is not in contact with heat-generating components, the liquid passage part  22  is filled by the working fluid C. The working fluid C is not limited to a particular kind of fluid. From a viewpoint of efficiently cooling the heat-generating components by evaporative latent heat, it is preferable to use, as the working fluid C, a fluid having a high vapor pressure and a high evaporative latent heat. Examples of such a fluid having the high vapor pressure and the high evaporative latent heat include ammonia, water, freon, alcohol, acetone, or the like, for example. 
       FIGS. 3A and 3B  are diagrams for explaining the functions of the parts of the heat pipe in the first embodiment.  FIG. 3B  illustrates a plan view of the heat pipe, and  FIG. 3A  illustrates a cross sectional view along a line B-B in  FIG. 3B . 
     As illustrated in  FIGS. 3A and 3B , the through-holes  12   x  (the cavities  121  and  122 , and the pores  123 ) of the heat pipe  1  are uniformly arranged in the plan view viewed from above the top surface  14   a  of the metal layer  14  in  FIG. 3A  in the normal direction to the top surface  14   a . For this reason, it is possible to arrange the heat-generating components, such as semiconductor devices or the like, at arbitrary positions on the outer wall formed by the metal layer  11 . A position where the heat-generating component is arranged, becomes a heat-generating part. In the example illustrated in  FIG. 3A , a heat-generating part (or evaporation part) H is located at the bottom left of the metal layer  11 , as encircled by dotted lines. 
     In  FIGS. 3A and 3B , when a temperature of the metal layers  11  and  12  in a vicinity of the heat-generating part H rises due to heat generation, the working fluid C within the liquid passage part  22  in the vicinity of the heat-generating part H vaporizes (or evaporates) to generate the vapor C v . The generated vapor C v  moves to the vapor-phase part  21  through the vent part  23  as indicted by arrows, to spread within the entire vapor-phase part  21 . A condensation part G is located at a position separated from the heat-generating part H, as encircled by dotted lines. The vapor C v  is liquefied at the condensation part G due to heat dissipation. 
     Accordingly, the heat generated from the heat-generating part H moves to the condensation part G and is dissipated from the condensation part G. The working fluid C that is liquefied at the condensation part G is attracted into the liquid passage part  22  through the vent part  23  due to capillary attraction of the pores  123 . The working fluid C attracted into the liquid passage part  22  passes through the liquid passage part  22  due to capillary attraction of the pores  125 , to move to a location lacking the working fluid C, that is, to the heat-generating part H. Thereafter, the evaporation and the condensation are cyclically repeated in a similar manner, to control and limit the temperature rise of the heat-generating part H. 
     [Method of Manufacturing Heat Pipe in First Embodiment] 
     Next, a description will be given of the method of manufacturing the heat pipe in the first embodiment.  FIGS. 4A, 4B, 4C, and 4D  are diagrams for explaining examples of manufacturing processes of the heat pipe in the first embodiment, and respectively illustrate cross sectional views corresponding to the cross sectional view of  FIG. 1A . 
     First, in the process illustrated in  FIG. 4A , a metal sheet  120  is prepared, a resist layer  310  having openings  310   x  is formed on a top surface of the metal sheet  120 , and a resist layer  320  having openings  320   x  is formed on a bottom surface of the metal sheet  120 . The openings  310   x  are formed to expose the top surface of the metal sheet  120  at positions corresponding to the cavities  121  illustrated in  FIG. 1B . In addition, the openings  320   x  are formed to expose the bottom surface of the metal sheet  120  at positions corresponding to the cavities  122  illustrated in  FIG. 1B . 
     The metal sheet  120  is a member that finally becomes the metal layer  12 , and may be made of a material such as copper, stainless steel, aluminum, magnesium alloys, or the like, for example. The metal sheet  120  may have a thickness in a range of approximately 50 μm to approximately 200 μm, for example. The resist layers  310  and  320  may be made of a photosensitive dry film resist or the like, for example. The openings  310   x  and  320   x  may be formed by exposing and developing the resist layers  310  and  320 . 
     Next, in the process illustrated in  FIG. 4B , the metal sheet  120  exposed within the openings  310   x  is subjected to half-etching from the top surface of the metal sheet  120 , and the metal sheet exposed within the openings  320   x  is subjected to half-etching from the bottom surface of the metal sheet  120 . As a result, the cavities  121  are formed at the top surface of the metal sheet  120 , and the cavities  122  are formed at the bottom surface of the metal sheet  120 . In addition, the pores  123  are formed by partially communicating the bottom surfaces of the corresponding cavities  121  and  122  in the Z-direction, to form the through-holes  12   x  by the cavities  121  and  122  and the pores  123 . The pores  125  are formed by partially communicating the side surfaces of the cavities  122  that are adjacent to each other in the X-direction and the Y-direction. The half-etching of the metal sheet  120  may use an etchant such as a ferric chloride solution, for example. Thereafter, the resist layers  310  and  320  are stripped (or removed) by a stripping liquid (or remover), to complete the metal layer  12  in which the through-holes  12   x  are arranged in the matrix arrangement. 
     Next, in the process illustrated in  FIG. 4C , the frame-shaped metal layer  13 , having the opening  13   x , is formed. More particularly, a metal sheet may be prepared, and an unwanted part of the metal sheet may be removed by etching, to form the metal layer  13 . Alternatively, the metal sheet may be prepared, and the unwanted part of the metal sheet may be removed by pressing or laser machining, to form the metal layer  13 . 
     Next, in the process illustrated in  FIG. 4D , the metal layer  11  and the metal layer  14 , which are continuous metal layers having no holes or grooves, are prepared. Then, the metal layers  11 ,  12 ,  13 , and  14  are successively stacked, pressed, and heated, to be bonded by solid-phase (or solid-state) welding. Hence, the mutually adjacent metal layers are directly bonded to each other, to thereby complete the heat pipe  1  having the vapor-phase part  21 , the liquid passage part  22 , and the vent part  23 . Thereafter, a vacuum pump or the like is used to exhaust or purge the inside of the liquid passage part  22 , the working fluid C is injected into the liquid passage part  22  from an injection port (not illustrated), and the injection port is sealed. 
     The solid-phase (or solid-state) welding refers to a method of bonding two welding targets together in the solid-phase (or solid-state), without melting the two welding targets, by heating, softening, and pressing the welding targets to cause plastic deformation. Preferably, the metal layers  11  through  14  are all made of the same material, so that the mutually adjacent metal layers can be satisfactorily bonded by the solid-phase (or solid-state) welding. 
     In the heat pipe  1  described above, the vapor-phase part  21  through which the vapor flows, and the liquid passage part  22  through which the working fluid C flows, are provided separately. For this reason, diffusion of the vapor C v  from the heat-generating part (or evaporation part) H, and return of the working fluid C condensed at the condensation part G, occur in different layers and do not collide with each other, to prevent mutual interference. As a result, the evaporation and the condensation are cyclically repeated, to improve the heat dissipation. 
     In addition, the through-holes  12   x  (the cavities  121  and  122 , and the pores  123 ) of the heat pipe  1  are uniformly arranged in the plan view viewed from above the top surface  14   a  of the metal layer  14  in the normal direction to the top surface  14   a . For this reason, there is no distinction between the heat-generating part (or evaporation part) H and the condensation part G. In other words, the heat-generating part H and the condensation part G can be arranged at random, and it is possible to arrange the heat-generating components, such as the semiconductor devices or the like, at arbitrary positions on the outer wall formed by the metal layer  11 , such that the position where the heat-generating component is arranged becomes the heat-generating part H. Further, the vapor C v  evaporated in the vicinity of the heat-generating part H spreads in all directions, and a low-temperature part becomes the condensation part G that condenses the vapor. According to such a configuration, it is possible to provide a heat pipe that exhibits a uniform thermal diffusion performance in all directions and is not dependent on orientation of the heat pipe. 
     In addition, according to the heat pipe  1 , the liquid passage part  22  and the vent  23  are formed in a single metal layer. For this reason, it is possible to reduce the thickness of the heat pipe  1  and provide a thin heat pipe. 
     First Modification of First Embodiment 
     In a first modification of the first embodiment, an example of the heat pipe is provided with pillars (or supports). In this first modification of the first embodiment, a repeated description of those parts that are the same as those of the first embodiment may be omitted. 
       FIGS. 5A and 5B  are diagrams illustrating the example of the heat pipe in the first modification of the first embodiment.  FIG. 5B  illustrates a plan view of the heat pipe, and  FIG. 5A  illustrates a cross sectional view of the heat pipe along a line A-A in  FIG. 5B . 
     As illustrated in  FIGS. 5A and 5B , a heat pipe  1 A includes pillars (or supports)  15  that are provided on the inner side of the frame-shaped metal layer  13 . In the example illustrated in  FIGS. 5A and 5B , 4 pillars  15  are provided, however, it is possible to provide 1 to 3 pillars  15 , or to provide 5 or more pillars  15 . 
     By providing the pillars  15  on the inner side of the frame-shaped metal layer  13 , it is possible to prevent the metal layer  14  from collapsing during the manufacture of the heat pipe  1 A at the process illustrated in  FIG. 4D  when the metal layers  11 ,  12 ,  13 , and  14  are successively stacked and pressed. In addition, it is possible to prevent the vapor-phase part  21  from collapsing due to deformation of the metal layer  14  while the heat pipe  1 A operates. 
     Second Modification of First Embodiment 
     In a second modification of the first embodiment, an example of the heat pipe is provided with a plurality of cavities at the top surface of the metal layer  12  with respect to a single cavity  122 . In this second modification of the first embodiment, a repeated description of those parts that are the same as those of the first embodiment may be omitted. 
       FIGS. 6A and 6B  are diagrams illustrating the example of the heat pipe in the second modification of the first embodiment.  FIG. 6B  illustrates a partial plan view of the heat pipe, and  FIG. 6A  illustrates a partial cross sectional view of the heat pipe along a line C-C in  FIG. 6B . 
     In a heat pipe  1 B illustrated in  FIGS. 6A and 6B , each of cavities  121   a  and  121   b  caves in from the top surface of the metal layer  12  towards the approximate center part of the metal layer  12  along the Z-direction, and each cavity  122  caves in from the bottom surface of the metal layer  12  towards an approximate center part of the metal layer  12  along the Z-direction. In addition, the cavities  121   a  and  121   b  and the cavity  122 , that correspond to each other, partially communicate with each other to form pores  123   a  and  123   b.    
     The metal layer  12  includes through-holes  12   y  that penetrate the metal layer  12  in the Z-direction. Each through-hole  12   y  is formed by the cavities  121   a  and  121   b , the cavity  122 , and the pores  123   a  and  123   b , that correspond to each other. 
     In other words, in each through-hole  12   y , the cavities  121   a  and  121   b  are provided with respect to one cavity  122 . The cavities  121   a  and  121   b  and the cavity  122 , that correspond to each other, are arranged to overlap each other in the plan view. Bottom surfaces of the cavities  121   a  and  122 , that correspond to each other, partially communicate with each other to form the pore  123   a . In addition, bottom surfaces of the cavity  121   b  and  122 , that correspond to each other, partially communicate with each other to form the pore  123   b.    
     The cavity  121   a  and the cavity  121   b , that are adjacent to each other in the X-direction, are arranged apart from each other. Further, the cavities  121   a  that are adjacent to each other in the Y-direction, and the cavities  121   b  that are adjacent to each other in the Y-direction, are arranged apart from each other. 
     Areas of the cavities  121   a  and  121   b  opening at the top surface of the metal layer  12  are smaller than an area of the cavity  122  opening at the bottom surface of the metal layer  12 . The cavities  121   a  and  121   b  are formed to an approximately hemispherical shape, and have a planar shape that is a circular shape, for example. Positions where the corresponding cavities  121   a  and  121   b  and the cavity  122  communicate with each other (that is, positions of the pores  123   a  and  123   b ) are located closer to the top surface of the metal layer  12  than the center along the thickness direction of the metal layer  12 . 
     The planar shape of the cavities  121   a  and  121   b  is not limited to the circular shape, and may be an arbitrary shape, such as an oval shape, a polygonal shape, or the like. In addition, the cavities  121   a  and  121   b  are not limited to the approximate hemispherical shape, and may have an arbitrary tapered shape defined by inner walls that widen from the pores  123   a  and  123   b  towards the top surface of the metal layer  12 . 
     Accordingly, in each through-hole  12   y ,  2  cavities  121   a  and  121   b  may be provided with respect to one cavity  122  at the top surface of the metal layer  12 . In this case, the size of the pores  123   a  and  123   b  can be made smaller than the size of the pore  123  of the first embodiment, to thereby increase the capillary attraction when the working fluid C is attracted into the liquid passage part  22  from the vapor-phase part  21 . 
     Of course, 3 or more cavities  121  may be provided with respect to one cavity  122 . In addition, the plurality of cavities  121  provided with respect to one cavity  122  at the top surface of the metal layer  12  may have different sizes (for example, different diameters). 
     Third Modification of First Embodiment 
     In a third modification of the first embodiment, a density (or denseness) of the cavities is varied. In this third modification of the first embodiment, a repeated description of those parts that are the same as those of the first embodiment may be omitted. 
       FIG. 7  is a diagram illustrating an example of the heat pipe in the third modification of the first embodiment, and is a plan view corresponding to  FIG. 1B . However,  FIG. 7  only illustrates the cavities  121  provided at the top surface of the metal layer  12  with respect to the cavities  122  of the metal layer  12 , and the illustration of the cavities  122  and the pores is omitted. 
     In a heat pipe  1 C illustrated in  FIG. 7 , a high-density region H d  and a low-density region L d  are alternately arranged in the X-direction and the Y-direction. The cavities  121  are arranged at a high density in the high-density region H d , while the cavities  121  are arranged at a low density in the low-density region L d . In the high-density region H d , a plurality of cavities  121  may be provided with respect to one cavity  122 . 
     The density of the cavities  121  does not necessarily have to be uniform, and the high-density regions H d  and the low-density regions L d  may be provided as in the case of the heat pipe  1 C. In this case, it is possible to expect effects of improving a thermal diffusion efficiency from the heat-generating parts. In addition, it is also possible to expect effects of improving a vaporization efficiency of the working fluid, and improving an efficiency of returning the liquefied working fluid to the liquid layer. 
     A number of region types having mutually different densities of the cavities  121  is not limited to 2 region types, and it is of course possible to provide 3 or more region types in which the densities of the cavities  121  are mutually different. 
     Fourth Modification of First Embodiment 
     In a fourth modification of the first embodiment, the size of the cavities is varied. In this fourth modification of the first embodiment, a repeated description of those parts that are the same as those of the first embodiment may be omitted. 
       FIG. 8  is a diagram illustrating an example of the heat pipe in the fourth modification of the first embodiment, and is a plan view corresponding to  FIG. 1A . However,  FIG. 8  only illustrates cavities  121   c  and  121   d  opening on the side of the metal layer  13  with respect to the cavities  122  of the metal layer  12 , and the illustration of the cavities  122  and the pores  123  is omitted. 
     In a heat pipe  1 D illustrated in  FIG. 8 , an area of the cavities  121   c  opening at the top surface of the metal layer  12  is large (for example, the diameter is large), while an area of the cavities  121   d  opening at the top surface of the metal layer  12  is small (for example, the diameter is small). The cavity  121   c  and the cavity  121   d  are alternately arranged in the X-direction and the Y-direction. The area of the cavities  121   c  is larger than the area of the cavities  121   d . In other words, the area of the cavities  121   d  is smaller than the area of the cavities  121   c . Similarly to the cavities  121  described above, the cavities  121   c  and  121   d  may be formed to an approximate spherical shape or the like, for example. 
     The areas of the cavities opening at the top surface of the metal layer  12  do not necessarily have to be the same, and the cavities  121   c  opening with the large area and the cavities  121   d  opening with the small area may be provided as in the case of the heat pipe  1 D. In this case, it is possible to expect the effects of improving the vaporization efficiency of the working fluid, and improving the efficiency of returning the liquefied working fluid to the liquid layer. 
     A number of area types of the cavities opening on the side of the metal layer  12  is not limited to 2 area types having mutually different areas. The number of area types of the cavities opening on the side of the metal layer  12  may be 3 area types or more. 
     Second Embodiment 
     In a second embodiment, an example of the heat pipe is made even thinner. In this second embodiment, a repeated description of those parts that are the same as those of the first embodiment may be omitted. 
     [Structure of Heat Pipe in Second Embodiment] 
     First, a description will be given of a structure of the heat pipe in the second embodiment.  FIGS. 9A and 9B  are diagrams illustrating an example of the heat pipe in the second embodiment.  FIG. 9B  illustrates a plan view of the heat pipe, and  FIG. 9A  illustrates a cross sectional view along a line A-A in  FIG. 9B . 
     As illustrated in  FIGS. 9A and 9B , a heat pipe  2  differs from the heat pipe  1  illustrated in  FIGS. 1A and 1B , in that the metal layers  13  and  14  of the first embodiment are replaced by a single metal layer  25 . Otherwise, the heat pipe  2  is the same as the heat pipe  1  of the first embodiment. In other words, the heat pipe  2  is an omnidirectional heat pipe having a structure in which 3 metal layers, namely, the metal layers  11 ,  12 , and  25 , are stacked. The metal layers  11 ,  12 , and  25  are made of a material, such as stainless steel, aluminum, magnesium alloys, or the like, and are mutually bonded directly by solid-phase (or solid-state) welding. 
     The metal layer  25  includes a rectangular flat plate part  251  having a top surface  25   a  and a bottom surface  25   b , and a sidewall part  252  projecting towards the metal layer  12  from an outer peripheral part of the bottom surface  25   b  of the flat plate part  251 . The flat plate part  251  and the sidewall part  252  of the metal layer  25  are integrally formed to a concave shape corresponding to an opening  25   x . The opening  25   x  of the sidewall part  252  exposes the through-holes  12   x  that are arranged in the matrix arrangement, and is formed to a frame-shape on the outer peripheral part of the bottom surface  25   b  of the flat plate part  251 . A bottom surface of the sidewall part  252  of the metal layer  25  is directly bonded to an outer peripheral part of the top surface of the metal layer  12 . 
     A total thickness T 1  of the metal layer  25  may be in a range of approximately 50 μm to approximately 200 μm, for example. The total thickness T 1  of the metal layer  25  may be the same as the thickness of each of the metal layers  11  and  12 . A thickness T 2  of the sidewall part  252  of the metal layer  25 , measured from the bottom surface  25   b  of the metal layer  25 , may be approximately one-half the thickness T 1 , for example. 
     The sidewall part  252  of the metal layer  25  forms a vapor layer, and the vapor-phase part  21  (illustrated in  FIG. 2 , for example) is surrounded by the top surface of the metal layer  12  and the bottom surface  25   b  of the metal layer  25 . The vapor-phase part  21  is the region in which the vapor C v , obtained by vaporizing the working fluid C, is moved from the high-temperature end to the low-temperature end. 
     [Method of Manufacturing Heat Pipe in Second Embodiment] 
     Next, a description will be given of the method of manufacturing the heat pipe in the second embodiment.  FIGS. 10A and 10B  are diagrams for explaining examples of manufacturing processes of the heat pipe in the second embodiment, and respectively illustrate cross sectional views corresponding to the cross sectional view of  FIG. 9A . 
     First, the processes of the first embodiment described above with reference to  FIGS. 4A and 4B  are performed to form the metal layer  12 . 
     Next, in a process illustrated in  FIG. 10A , a metal sheet  250  is prepared, a continuous resist layer  330  is formed on an entire top surface of the metal sheet  250 , and a frame-shaped resist layer  340  having a rectangular opening  340   x  is formed on a bottom surface of the metal sheet  250 . The resist layer  340  is formed to cover a region in which the sidewall part  252  is to be formed. 
     The metal sheet  250  is a member that finally becomes the metal layer  25 , and may be made of a material such as copper, stainless steel, aluminum, magnesium alloys, or the like, for example. The metal sheet  250  may have a thickness in a range of approximately 50 μm to approximately 200 μm, for example. The resist layers  330  and  340  may be made of a photosensitive dry film resist or the like, for example. The opening  340   x  may be formed by exposing and developing the resist layer  340 . 
     Next, in the process illustrated in  FIG. 10B , the metal sheet  250  exposed within the opening  340   x  is subjected to half-etching from the bottom surface of the metal sheet  250 , to form the opening  25   x  at a central part of the bottom surface of the metal sheet  250 , and to form the sidewall part  252  on the outer peripheral part of the bottom surface of the metal sheet  250  and surrounding the opening  25   x . The half-etching of the metal sheet  250  may use an etchant such as a ferric chloride solution, for example. Thereafter, the resist layers  330  and  340  are stripped (or removed) by a stripping liquid (or remover), to form the metal layer  25  having the frame-shaped sidewall part  252  that is formed on the outer peripheral part of the bottom surface  25   b  of the flat plate part  251  and surrounds the opening  25   x.    
     In  FIG. 10A , the continuous resist layer  330  may be formed on the entire bottom surface of the metal sheet  250 , and the frame-shaped resist layer  340  having the rectangular opening  340   x  may be formed on the top surface of the metal sheet  250 . In this case, in the process illustrated in  FIG. 10B , the metal sheet  250  exposed within the openings  340   x  is subjected to half-etching from the top surface of the metal sheet  250 , to form the opening  25   x  at a central part of the top surface of the metal sheet  250 . 
     Next, the metal layer  11 , which is a continuous metal layer having no holes or grooves, is prepared. Then, the metal layers  11 ,  12 , and  25  are successively stacked, pressed, and heated, to be bonded by solid-phase (or solid-state) welding, similarly to the process described above with reference to  FIG. 4D . Hence, the mutually adjacent metal layers are directly bonded to each other, to thereby complete the heat pipe  2  having the vapor-phase part  21 , the liquid passage part  22 , and the vent part  23 . Thereafter, a vacuum pump or the like is used to exhaust or purge the inside of the liquid passage part  22 , the working fluid C is injected into the liquid passage part  22  from an injection port (not illustrated), and the injection port is sealed. Preferably, the metal layers  11 ,  12 , and  25  are all made of the same material, so that the mutually adjacent metal layers can be satisfactorily bonded by the solid-phase (or solid-state) welding. 
     Accordingly, the metal layers  13  and  14  of the heat pipe  1  described above may be replaced by the single metal layer  25  in the case of the heat pipe  2 . Because the cavities and the opening of the heat pipe  2  can be formed without using a being process or a press-forming process, it is possible to reduce the thickness of the heat pipe  2 , that is the heat pipe  2  can be made thin. In a case in which each of the metal layers  11 ,  12 , and  25  of the heat pipe  2  is formed to a thickness of 50 μm, for example, it is possible to manufacture a thin heat pipe having a total thickness of 150 μm. Effects obtainable in the second embodiment are the same as the effects obtainable in the first embodiment described above. 
     First Modification of Second Embodiment 
     In a first modification of the second embodiment, an example of the heat pipe is provided with pillars (or supports). In this first modification of the second embodiment, a repeated description of those parts that are the same as those of the first embodiment may be omitted. 
       FIGS. 11A and 11B  are diagrams illustrating the example of the heat pipe in the first modification of the second embodiment.  FIG. 11B  illustrates a plan view of the heat pipe, and  FIG. 11A  illustrates a cross sectional view along a line A-A in  FIG. 11B . 
     As illustrated in  FIGS. 11A and 11B , a heat pipe  2 A differs from the heat pipe  2  illustrated in  FIGS. 9A and 9B , in that the metal layer  25  is replaced by a metal layer  25 A. Otherwise, the heat pipe  2 A is the same as the heat pipe  2  of the second embodiment. In other words, the heat pipe  2 A is an omnidirectional heat pipe having a structure in which 3 metal layers, namely, the metal layers  11 ,  12 , and  25 A, are stacked. The metal layers  11 ,  12 , and  25 A are made of a material, such as stainless steel, aluminum, magnesium alloys, or the like, and are mutually bonded directly by solid-phase (or solid-state) welding. 
     The metal layer  25 A includes a rectangular flat plate part  251  having a top surface  25   a  and a bottom surface  25   b , a sidewall part  252  projecting towards the metal layer  12  from an outer peripheral part of the bottom surface  25   b  of the flat plate part  251 , and pillars  253  provided on the bottom surface  25   b  of the flat plate part  251  in a region on the inner side of the sidewall part  252 . The flat plate part  251 , the sidewall part  252 , and the pillars  253  of the metal layer  25 A are integrally formed. The sidewall part  252  includes an opening  25   x  that exposes the through-holes  12   x  that are arranged in the matrix arrangement, and is formed to a frame-shape on the outer peripheral part of the bottom surface  25   b  of the flat plate part  251 . The pillars  253  project towards the metal layer  12  from the bottom surface  25   b  of the flat plate part  251  that is exposed within the opening  25   x . In the example illustrated in  FIGS. 11A and 11B , 4 pillars  253  are provided, however, the number of pillars  253  may be 1 to 3, or 5 or more. A bottom surface of the sidewall part  252  of the metal layer  25 A is directly bonded to an outer peripheral part of the top surface of the metal layer  12 . In addition, a bottom surface of each of the pillars  253  of the metal layer  25 A is directly bonded to the top surface of the metal layer  12  at predetermined positions on the top surface of the metal layer  12 . 
     When forming the metal layer  25 A, a metal sheet  250  is prepared, for example, a continuous first resist layer is formed on an entire top surface of the metal sheet, and a second resist layer is selectively formed on a bottom surface of the metal sheet at positions where the sidewall part  252  is to be formed at the outer peripheral part and where the pillars  253  are to be formed in the region on the inner side of the sidewall part  252 . The bottom surface of the metal sheet, exposed at positions where the second resist layer is not formed, is subjected to half-etching from the bottom surface of the metal sheet. As a result, the opening  25   x  at a central part of the bottom surface of the metal sheet, the sidewall part  252  on the outer peripheral part of the bottom surface of the metal sheet and surrounding the opening  25   x , and the pillars  253  on the bottom surface of the metal sheet in the region on the inner side of the sidewall part  252 , are formed by the half-etching. The half-etching of the metal sheet, that is a member that finally becomes the metal layer  25 A, may use an etchant such as a ferric chloride solution, for example. Thereafter, the first and second resist layers are stripped (or removed) by a stripping liquid (or remover), to complete the metal layer  25 A in which the flat plate part  251 , the sidewall part  252 , and the pillars  253  are integrally formed. 
     By providing the pillars  253  on the inner side of the frame-shaped sidewall part  252  of the metal layer  25 A, it is possible to prevent the metal layer  25 A from collapsing during the manufacture of the heat pipe  2 A at the process illustrated in  FIG. 4D  when the metal layers  11 ,  12 , and  25 A are successively stacked and pressed. In addition, it is possible to prevent the vapor-phase part  21  from collapsing due to deformation of the metal layer  25 A while the heat pipe  2 A operates. Effects obtainable in the first modification of the second embodiment are the same as the effects obtainable in the first or second embodiment described above. 
     For example, each of the first embodiment and the first through fourth modifications of the first embodiment may be appropriately combined. In addition, each of the second embodiment and the first modification of the second embodiment may be appropriately combined with any of the second through fourth modifications of the first embodiment. 
     According to each of the embodiments described above, it is possible to provide a heat pipe that can improve the heat dissipation, and to provide a method of manufacturing such a heat pipe. 
     Various aspects of the subject-matter described herein may be set out non-exhaustively in the following numbered clauses: 
     1. A method of manufacturing a heat pipe, comprising: 
     forming a first metal layer forming a liquid layer configured to move a working fluid that is liquefied from vapor; 
     forming a second metal layer forming a vapor layer configured to move vapor of the working fluid that is vaporized; and 
     bonding the second metal layer on a first surface of the first metal layer, 
     wherein the forming the first metal layer includes
         half-etching a first metal sheet from a first surface of the first metal sheet to form a plurality of first cavities,   half-etching the first metal sheet from a second surface of the first metal sheet opposite from the first surface to form a plurality of second cavities,   forming first pores partially communicating with the plurality of first cavities and the plurality of second cavities, respectively, and   forming second pores partially communicating side surfaces of the plurality of second cavities that are adjacent to each other,       

     wherein the forming the second metal layer includes
         forming a plurality of through-holes that penetrate the second metal sheet in a direction taken along a thickness of the second metal sheet.       

     2. A method of manufacturing a heat pipe, comprising: 
     forming a first metal layer forming a liquid layer configured to move a working fluid that is liquefied from vapor; 
     forming a second metal layer forming a vapor layer configured to move the vapor of the working fluid that is vaporized; and 
     bonding the second metal layer on a first surface of the first metal layer, 
     wherein the forming the first metal layer includes
         half-etching a first metal sheet from a first surface of the first metal sheet to form a plurality of first cavities,   half-etching the first metal sheet from a second surface of the first metal sheet opposite from the first surface to form a plurality of second cavities,   forming first pores partially communicating with the plurality of first cavities and the plurality of second cavities, respectively, and   forming second pores partially communicating side surfaces of the plurality of second cavities that are adjacent to each other,       

     wherein the forming the second metal layer includes
         half-etching the second metal sheet from a first surface of the second metal sheet or a second surface of the second metal sheet opposite from the first surface of the second metal sheet, to form an opening, and a sidewall part provided on an outer peripheral part of the second metal sheet and surrounding the opening.       

     3. The method of manufacturing the heat pipe according to clause 1 or 2, wherein an area of a part of each of the plurality of first cavities opening at the first surface is smaller than an area of a part of each of the plurality of second cavities opening at the second surface. 
     4. The method of manufacturing the heat pipe according to any of clauses 1 to 3, wherein an inner wall defining each of the plurality of first cavities is tapered and widen towards the first surface, and an inner wall defining each of the plurality of second cavities is tapered and widen towards the second surface. 
     5. The method of manufacturing the heat pipe according to any of clauses 1 to 4, wherein two or more first cavities among the plurality of first cavities communicate to one of the plurality of second cavities. 
     Although the embodiments and modifications are numbered with, for example, “first,” “second,” etc., the ordinal numbers do not imply priorities of the embodiments and modifications. Many other variations and modifications will be apparent to those skilled in the art. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.