Abstract:
A method of manufacturing a heat pipe, including the steps of: forming in a substrate a cylindrical opening provided with a plurality of ring-shaped recessed radially extending around a central axis of the opening; arranging in the recesses separate ring-shaped strips made of a material catalyzing the growth of carbon nanotubes; and growing carbon nanotubes in the opening from said ring-shaped strips.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a heat pipe and to a method of manufacturing this heat pipe. It more specifically aims at a miniature heat pipe capable of performing heat transfers inside of an integrated circuit chip or of a stack of integrated circuit chips. 
         [0003]    2. Description of the Related Art 
         [0004]    A heat pipe is a device capable of transferring heat from a hot surface to a cold surface, by using liquid-vapor phase changes of a fluid. 
         [0005]    Conventionally, a heat pipe comprises a tight enclosure enclosing a fluid. In operation, one side of the enclosure is placed in the vicinity of a hot source, for example, an electronic device which is desired to be cooled, and an opposite side of the enclosure is placed in the vicinity of a cold source, for example, a heat sink. In the hot area of the heat pipe, the fluid vaporizes, thus storing heat. The vapor thus formed diffuses through the enclosure all the way to the cold area of the heat pipe, and then condenses in the cold area, thus releasing heat. Once in the liquid state, the fluid returns to the hot area of the heat pipe by capillary action along the enclosure walls, and the cycle resumes. 
         [0006]    Existing heat pipes are capable of substantially uniformly cooling a relatively large surface, for example, the whole surface of an integrated circuit chip or of a stack of integrated circuit chips. However, due to their rather significant bulk, existing heat pipes are not capable of transferring heat between specific local areas of an integrated circuit chip or of a stack of integrated circuit chips. 
         [0007]    A miniature heat pipe, which is easy to form and may for example be used to remove heat from local hot areas of an integrated circuit chip or of a stack of integrated circuit chips is thus needed. 
       BRIEF SUMMARY 
       [0008]    Thus, an embodiment provides a heat pipe manufacturing method, comprising the steps of: a) forming in a substrate a cylindrical opening provided with a plurality of ring-shaped recesses radially extending around a central axis of the opening; b) arranging in the recesses separate ring-shaped strips made of a material that catalyzes the growth of carbon nanotubes; and c) growing carbon nanotubes in the opening from said ring-shaped strips. 
         [0009]    According to an embodiment, step a) comprises alternating between anisotropic etch steps and of passivation steps. 
         [0010]    According to an embodiment, the anisotropic etch steps are carried out by means of an SF 6  plasma, and the passivation steps are carried out by means of a C 4 F 8  plasma. 
         [0011]    According to an embodiment, step b) comprises a step of depositing, on the walls of the opening, a continuous layer of a material catalyzing the growth of carbon nanotubes, and a step of focused isotropic etching of a portion of this layer located outside of the recesses. 
         [0012]    According to an embodiment, the catalyzing material comprises iron or aluminum. 
         [0013]    According to an embodiment, the method further comprises, before step b), a step of depositing, on the walls of the opening, an intermediate layer made of a bonding material for said catalyst material. 
         [0014]    According to an embodiment, the material of the intermediate layer is further capable of making the walls of the opening tighter. 
         [0015]    According to an embodiment, the intermediate layer is made of silicon oxide. 
         [0016]    According to an embodiment, the method further comprises, after step c), a step of densification of the carbon nanotubes. 
         [0017]    According to an embodiment, the densification step comprises a step of soaking the carbon nanotubes in a solution containing a solvent, followed by an evaporation of the solvent. 
         [0018]    According to an embodiment, the method further comprises, after step c), a step of partially filling the opening with a heat-transfer liquid, followed by a step of tight closing of the opening. 
         [0019]    Another embodiment provides a heat pipe comprising: a cylindrical opening arranged in a substrate, said opening being provided with a plurality of ring-shaped recessed radially extending around a central axis of the opening; separate ring-shaped strips made of a material catalyzing the growth of carbon nanotubes, located in the recesses; and carbon nanotubes extending in the opening from said ring-shaped strips. 
         [0020]    According to an embodiment, the heat pipe further comprises a heat-transfer liquid partially filling the opening. 
         [0021]    According to an embodiment, the heat pipe comprises at least one cap tightly closing the opening. 
         [0022]    Another embodiment provides an integrated circuit chip comprising at least one heat pipe of the above-mentioned type. 
         [0023]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0024]      FIGS. 1A to 1H  are cross-section views schematically illustrating steps of a manufacturing method of an embodiment of a heat pipe; 
           [0025]      FIG. 2  is a cross-section view schematically and partially illustrating an embodiment of an integrated circuit chip comprising a heat pipe; and 
           [0026]      FIG. 3  is a cross-section view schematically and partially illustrating an embodiment of a system comprising an integrated circuit chip and a cooling system comprising a heat pipe. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. Further, in the following description, unless otherwise indicated, terms “approximately”, “substantially”, “around”, “in the order of”, and “almost” mean “to within 10%”, and terms referring to directions, such as “vertical”, “horizontal”, “lateral”, “under”, “above”, “upper”, “lower”, etc., apply to devices arranged as illustrated in the cross-section views of the corresponding drawings, it being understood that, in practice, the devices may have different orientations. 
         [0028]      FIGS. 1A to 1H  are cross-section views schematically illustrating steps of an example of a heat pipe manufacturing method. 
         [0029]      FIG. 1A  illustrates a step of depositing, on an upper surface of a substrate  100  where the heat pipe is desired to be formed, a first mask  103  defining an opening leaving the upper surface of substrate  100  apparent and a second mask  105  covering mask  103  as well as the periphery of the upper surface region of substrate  100  which is not coated with mask  103 . The apparent region of substrate  100  which is not coated with mask  105  for example has a substantially circular shape. As an example, mask  103  defines a circular opening, and mask  105  defines a circular opening centered on the opening of mask  103  but having a smaller diameter. Other shapes of opening of masks  103  and  105  may however be provided. As a non-limiting example, substrate  100  is a substrate made of a semiconductor material, for example, mask  103  is a metal mask, and mask  105  is a resin mask. 
         [0030]      FIG. 1B  illustrates a step of forming, in an upper portion of substrate  100 , an opening  107  extending from the upper surface region of substrate  100  which is not coated with masks  103  and  105  towards the lower surface of the substrate, for example, along an approximately vertical general direction. As appears in  FIG. 1B , opening  107  has corrugated or wavy lateral walls, defining in substrate  100  a plurality of substantially parallel ring-shaped recesses  109 , radially extending around a central axis X of the opening. To obtain this wavy shape of opening  107 , alternating between anisotropic etch steps and of steps of passivation of the walls of the already etched portion of the opening may for example be carried out. As an example, the anisotropic etch steps are performed by DRIE (Deep Reactive Ion Etching), by means of a plasma based on SF 6 , and the passivation steps are carried out by means of a plasma based on C 4 F 8 . Any other etch method enabling to obtain the desired corrugations may however be used. 
         [0031]    As a non-limiting example, opening  107  has a total height or depth in the range from 10 to 1,000 μm and an average diameter in the range from 2 to 250 μm, and each recess  109  has a height in the range from 100 to 500 nm and a width or thickness (distance from the bottom of the recess to the outlet of the recess on the cylindrical central portion of opening  107 ) in the range from 30 to 300 nm. The described embodiments are however not limited to these specific examples of dimensions. 
         [0032]    In the shown example, the etch conditions are selected so that, as shown in  FIG. 1B , ring-shaped recesses  109  emerge onto the cylindrical central portion of opening  107  at the level of the inner edge of mask  105  or between the inner edge of mask  105  and the inner edge of mask  103 . In other words, in this example, the etch conditions are selected so that the smallest diameter of ring-shaped recesses  109  is smaller than the diameter of the opening defined by mask  103 , and greater than or equal to the diameter of the opening defined by mask  105 . As a non-limiting example, the smallest diameter of ring-shaped recesses  109  is smaller by from 10 to 500 nm than the diameter of the opening defined by mask  103 . 
         [0033]      FIG. 1C  illustrates an optional step of forming a layer  111  on the lateral walls and on the bottom of opening  107 . A function of layer  111  may be to be used as a bonding layer for the subsequent deposition of a catalyzing material for the growth of carbon nanotubes, in the case where substrate  100  would not be itself made of a material capable of receiving such a catalyst. Further, opening  107  being intended to contain a heat-transfer liquid of the heat pipe, layer  111  may have the function of sealing the walls of opening  107  in the case where substrate  100  would not be itself made of a sufficiently fluid tight material. As a non-limiting example, layer  111  is made of silicon oxide. Layer  111  is for example formed by oxidation of the lateral walls and of the bottom of opening  107 , or by any other adapted deposition method, for example, by PVD (Physical Vapor Deposition). Layer  111  for example has a thickness in the range from 10 to 150 nm. The thickness of layer  111  is selected so that the material of layer  111  does not totally fill recesses  109 . 
         [0034]      FIG. 1D  illustrates a step, subsequent to the forming of layer  111 , if present, of depositing a layer  113  made of a material for catalyzing the growth of carbon nanotubes on the lateral walls of opening  107 . As an example, layer  113  contains iron, aluminum, or any other material capable of being used as a basis for the growth of carbon nanotubes. Layer  113  may be deposited by sputtering of a source of the catalyzing material by means of an electron beam, or by any other adapted deposition method. The thickness of layer  113  is preferably selected to be sufficiently large for the material of layer  113  to totally fill recesses  109  or the portion of recesses  109  which is not filled with layer  111 , when present. 
         [0035]      FIG. 1E  illustrates a step subsequent to the deposition of layer  113 , during which mask  105  is removed, while mask  103 , which has a wider opening, is kept. After the removal of mask  105 , an isotropic etch step, substantially vertical in this example, is implemented to remove, from the lateral walls of opening  115 , the portions of layer  113 , of layer  111 , and/or of substrate  100 , which are not coated with mask  103 . The portion of layer  113  covering the bottom of opening  107  is also removed. The isotropic etching of  FIG. 1E  may be implemented by means of a focused plasma, or by any other adapted etch method. At the end of this etching, as appears in  FIG. 1E , the material catalyzing the growth of carbon nanotubes forms approximately parallel separate rings  115 , arranged in recesses  109 . Each ring or ring portion or ring-shaped strip  115  of the material catalyzing the growth of carbon nanotubes is insulated from the adjacent rings  115  by a portion of layer  111 . After this step, mask  103  may be removed. 
         [0036]      FIG. 1F  illustrates a step during which carbon nanotubes are grown from the surface of rings  115  facing the inside of opening  107 . The carbon nanotube growth may be performed by CVD (Chemical Vapor Deposition) at a temperature in the range from 550 to 700° C., or by any other adapted method. As schematically shown in  FIG. 1F , the carbon nanotubes grow along a general direction approximately orthogonal to the surface of the catalyzing material, that is, along an approximately horizontal general direction in this example. Thus, from the inner surface of each ring  115 , there forms a ring  117  made of a cluster of carbon nanotubes. The length of the carbon nanotubes, which defines the width of rings  117 , is selected so that rings  117  do not totally close the openings of rings  115 . As a non-limiting example, the width of rings  117  is smaller than half the radius of the opening of rings  115 . The width of rings  117  is for example in the range from 1 to 50 μm. 
         [0037]    As will be explained in further detail hereafter, carbon nanotube rings  117  are intended to enable a heat-transfer fluid in liquid form to displace by capillarity along the lateral walls of opening  107 . 
         [0038]      FIG. 1G  illustrates an optional step of densification of rings  117  of carbon nanotubes, particularly aiming at improving the capacity of rings  117  to convey a liquid by capillarity. During this step, rings  117  may be soaked, for example, by dipping, with a solution containing a solvent, for example, isopropanol. The evaporation of the solvent after the dipping causes a densification of rings  117 . 
         [0039]      FIG. 1H  illustrates a step subsequent to the forming of carbon nanotubes, during which a heat-transfer liquid  119 , for example, water or any other adapted liquid, is placed in opening  107  to partially, but not totally fill opening  107 . As an example, substrate  100  is dipped into a bath containing liquid  119 , and then taken out. As it comes out of the bath, substrate  100  is placed in such a way that the open surface of the substrate faces downwards, so that opening  107  empties part of liquid  119  that it contains. Only part of liquid  119  trapped by carbon nanotubes  117  then remains confined in opening  117 . 
         [0040]    A cap  121  is then placed on the open surface (the upper surface in the orientation of the drawing) of substrate  100 , to tightly close opening  107 . As a non-limiting example, cap  121  may be a rigid plate or a polymer, glued to the upper surface of the substrate, a filling paste spread on the upper surface of the substrate, a tight film glued to the upper surface of the substrate, etc. As a variation, cap  121  may directly be a heat source, for example, an integrated circuit chip or any other element to be cooled, or a cold source, for example, a heat sink. 
         [0041]    A heat pipe  130  having the following operation is thus obtained. One side of the enclosure formed by opening  107 , for example, the upper portion of the enclosure, is placed in the vicinity of or in contact with a hot source, and an opposite side of the enclosure, for example, the lower portion of the enclosure, is placed in the vicinity of or in contact with a cold source. In the hot portion of the heat pipe, that is, in its upper portion in this example, fluid  119  vaporizes, thus storing heat. The vapor thus formed diffuses through enclosure  107 , mainly via the enclosure area free of carbon nanotubes, all the way to the cold portion of the heat pipe, that is, its lower portion in this example. In the cold area of the heat pipe, the fluid condenses, thus releasing heat. Once in the liquid state, the fluid returns to the hot area by capillarity along the enclosure walls, due to the presence of carbon nanotube rings  117 , after which the cycle resumes. 
         [0042]    An advantage of the described embodiments is that they enable to form a heat pipe in a particularly simple way, and in particular in a way compatible with usual integrated circuit manufacturing techniques and tools. 
         [0043]    Another advantage is that the heat pipes thus formed may easily be integrated in an integrated circuit chip or in a stack of integrated circuit chips, for example, next to, under, or above components capable of dissipating heat. As a non-limiting example, a plurality of heat pipes of the above-described type may be integrated in a same substrate to form a cooling device intended to be arranged in contact with an integrated circuit chip capable of dissipating heat, or between two chips of a stack of integrated circuit chips. 
         [0044]      FIG. 2  schematically and partially illustrates an example of an integrated circuit chip  200  comprising a substrate  100 , for example, made of silicon. Chip  200  comprises, inside and/or on top of a region  202  of the substrate, components (not shown) capable of generating heat. Chip  200  further comprises, integrated in substrate  100  in the vicinity of active region  202 , heat pipes  130  of the type described in relation with  FIGS. 1A to 1H . An advantage is that heat pipes  130  may be placed as close as possible to active region  202  of the substrate, and thus efficiently cool the heat generation components. 
         [0045]      FIG. 3  schematically and partially illustrates an example of a system comprising an integrated circuit chip  300  and, placed against a surface of chip  300 , a cooling device  303  comprising a substrate  100 , for example, made of silicon, having a large number of heat pipes  130  of the type described in relation with  FIGS. 1A to 1H  formed therein. 
         [0046]    Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
         [0047]    In particular, the described embodiments are not limited to the above-described method example enabling to arrange the separate ring-shaped strips of a material catalyzing the growth of carbon nanotubes, in ring-shaped recesses, along the lateral walls of an opening formed in a substrate. More generally, any other method providing a structure of the type illustrated in  FIG. 1E  may be used. 
         [0048]    Further, the described embodiments are not limited to the above-mentioned examples where opening  107  has a generally cylindrical shape with a circular cross-section. Other opening shapes may be provided, for example, cylindrical shapes with a hexagonal cross-section, a square cross-section, an oval cross-section, etc. 
         [0049]    Further, in the above-describe examples, opening  107  formed at the step of  FIG. 1B  does not entirely cross substrate  100 . The lower portion of substrate  100  then closes the lower portion of the heat pipe. The described embodiments are however not limited to this specific case. As a variation, it will be within the abilities of those skilled in the art to adapt the described method to the case where opening  107  formed at the step of  FIG. 1B  is a through opening. In this case, an additional step of closing the lower surface of the heat pipe with a cap may particularly be provided. 
         [0050]    Further, it will readily occur to those skilled in the art that the described embodiments may be combined with other known cooling or heat transport devices. 
         [0051]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. 
         [0052]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.