Abstract:
A thin wafer comprising through holes filled at least partially with conductive carbon nanotubes generally oriented transversally to the wafer. A fuel cell comprising, in a thin wafer, a through hole filled with an electrolyte surrounded with barriers of carbon nanotubes generally oriented transversally to the wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the manufacturing of microcomponents. It more specifically relates to the forming of through vias in microcomponents and to a specific application to the forming of a fuel cell. 
     2. Description of the Related Art 
       FIG. 1  shows an example of the forming of a fuel cell using microelectronic techniques. This cell is formed on a silicon wafer  1  coated with a first thin insulating layer  2  and with a second thicker insulating layer  3 . An opening is formed in a portion of insulating layer  3 . In this opening are successively deposited a support  4 , a lower catalyst layer  5 , an electrolyte  6 , and a upper catalyst layer  7 . An electrode  10  enables taking a contact on the lower surface of the cell, more specifically on the catalyst support in the shown example. An upper electrode  11  enables taking a contact on upper catalyst layer  7 . Electrodes  10  and  11  are provided with openings, and channels  12  are formed in silicon wafer  1  opposite to the openings in the lower surface metallization. 
     To operate the fuel cell, hydrogen is injected along arrow H 2  on the lower surface side and air (carrying oxygen) is injected on the upper surface side. A hydrogen flow and an air flow may be ensured to have a good contact between the hydrogen and lower catalyst  5  and between the air and upper catalyst  7 . In known fashion, with such a structure, a positive voltage is obtained on the upper electrode (on the oxygen side) and a negative voltage is obtained on the lower electrode (on the hydrogen side). 
       FIG. 1  shows a single fuel cell. In practice, on a same wafer  1 , a large number of cells that can be assembled in series/parallel according to the desired use will be formed. Electrolyte  7  for example is a polymeric acid such as solid Nafion and the catalyst layers are for example carbon- and platinum-based layers. This is an example only. Various types of fuel batteries that can be formed as illustrated in  FIG. 1  are known in the art. 
     In such a fuel cell, the power that can be provided is especially proportional to the surface area taken up by the cell in the silicon wafer plane. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, the present invention provides a novel type of fuel cell of greater power per surface area unit than the shown cell and which is simpler to manufacture. 
     In one embodiment, the present invention provides the forming of conductive channels in a wafer. 
     In particular, the present invention provides the forming of conductive channels in silicon wafers, for example, for the forming of vias. 
     In one embodiment, the present invention provides a thin wafer comprising through holes filled at least partially with conductive carbon nanotubes generally oriented transversally to the wafer. 
     According to an embodiment, the nanotubes are multiple-wall nanotubes. 
     According to an embodiment, the thin wafer is made of silicon, and an insulating layer is formed between the carbon nanotubes and the walls of the through holes. 
     The present invention also provides a fuel cell comprising in a thin wafer a through hole filled with an electrolyte surrounded with barriers of carbon nanotubes generally oriented transversally to the wafer. 
     According to an embodiment, a first gas is injected through a barrier of carbon nanotubes on one side of the electrolyte and a second gas is injected through the nanotube barrier on the other side of the electrolyte. 
     According to an embodiment, a porous material, such as porous silicon, is arranged on the side of the carbon nanotube barriers opposite to the electrolyte. 
     The present invention also provides a method for manufacturing a fuel cell, comprising the steps of forming through openings in a silicon wafer; making the silicon porous close to the openings; growing carbon nanotubes from a surface of the wafer towards the opposite surface; and hollowing portions arranged between two carbon nanotube barriers and filling the hollowed portions with an electrolyte. 
     The foregoing features, and advantages of the present invention 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 
         FIG. 1  is a simplified cross-section view of a fuel cell according to prior art; 
         FIG. 2A  is a simplified cross-section view of an embodiment of a fuel cell according to the present invention; 
         FIG. 2B  shows an example of a top view of the cell of  FIG. 2A ; and 
         FIGS. 3A to 3F  are simplified cross-section views illustrating the successive manufacturing steps of the fuel cell of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For clarity, same elements have been designated with same reference numerals in the different drawings and, further, as usual in the representation of integrated components, the various drawings are out of scale. 
       FIG. 2A  is a cross-section view of two symmetrical fuel cells according to an embodiment of the present invention. Other cells may be formed by repetition of this elementary pattern. The structure of these cells will be better understood by also referring to the simplified top view of  FIG. 2B . A silicon wafer  20  is thoroughly crossed by a porous silicon strip  21  encircled by through vertical barriers of carbon nanotubes  22  and  23 , themselves surrounded with electrolyte strips ( 24 ,  25 ) surrounded with carbon nanotube strips  26  and  27 , then with porous silicon strips  28  and  29 . The carbon nanotubes generally extend in the direction transverse to the silicon wafer and are coated or impregnated with a catalyst such as platinum. The electrolyte for example is Nafion or another polymer acid. 
     An upper electrode M 1  is in contact with the upper portion of the carbon nanotube barriers  26  and  27 . A lower electrode M 2  is in contact with the lower portion of carbon nanotube barriers  22  and  23 . Upper and lower tight insulating layers  30  and  31  are deposited to ensure the necessary insulations. Upper insulating layer  30  comprises openings above the upper portion of porous silicon strips  28  and  29 . Lower insulating layer  31  comprises an opening at the level of the lower portion of porous silicon layer  21 . In operation, hydrogen is injected on the lower surface side into central porous silicon layer  21  and oxygen (air) is injected on the upper surface side into external porous silicon strips  28 ,  29 . Thus, a potential difference develops across the electrolyte and a voltage is sampled between metallizations M 1  and M 2  which respectively form the positive terminal and the negative terminal of the fuel cell. 
     The carbon nanotubes preferably are multiple-wall nanotubes which are well conductive. It should be noted that such carbon nanotubes are used for their double property of electric conduction and of transverse permeability to gases. Indeed, such nanotubes enables the gases to cross them and to come in contact with the electrolyte on either side thereof. 
     The structure very schematically illustrated in  FIGS. 2A and 2B  is likely to have many variations which will occur to those skilled in the art as concerns the used materials and the cell topology, the essential features of the application of the present invention to the forming of a fuel cell lying in the vertical direction (transverse to a surface of the wafer) of the contacts and in the use of carbon nanotubes which are used both as conductive elements and as permeable walls. 
     As an example of an alternative topology, it should be noted that the various structures crossing the wafer, instead of having the shape of parallel strips, may be concentric or have any other desired shape intended to optimize the surface area. 
     An advantage of the structure with a vertical contact between the electrolyte and the electrodes lies in the possible increase in the power generated per surface area unit. In a conventional structure such as illustrated in  FIG. 1 , the active surface of an individual cell is limited to the surface area that it takes up in the plane of the wafer in or on which it is formed. Conversely, with a structure according to the present invention, if the wafer thickness is much greater than the width of the contact and electrolyte constitutive layers, an integration density much higher than in prior art may be obtained. This assumption is true in practice for a 400-μm wafer thickness and a repetition step of the base structure (porous silicon-nanotubes-electrolyte-nanotubes-porous silicon) on the order of from 100 to 200 μm. 
     As alternative materials, still keeping carbon nanotubes, it should be noted that the wafer in which the structure is formed is not necessarily a silicon wafer. It could for example be a glass pane. Further, it is not necessary to use porous silicon as a gas conductor, but any other porous material may be used. Especially, porous silicon portions  28  and  29  may be replaced with a widening of carbon nanotube structures  26  and  27 . Similarly, porous silicon  21  may be replaced with a carbon nanotube area. 
       FIGS. 3A to 3F  illustrate an example of a method for manufacturing the structure illustrated in  FIGS. 2A and 2B . 
     First, through trenches are dug into a silicon wafer  20  at the locations where the carbon nanotube barriers will be arranged. These through trenches may be formed by many known methods. For example, the step sequence illustrated in  FIGS. 3A and 3B  is carried out. As shown in  FIG. 3A , through P-type trenches  40  are first formed in an N-type silicon substrate. The oriented deep diffusion of a P-type dopant may for example be obtained by a so-called TGZM aluminum diffusion caused by temperature gradient (Temperature Gradient Zone Melting). Then, a selective etch of the P-type doped silicon with respect to the N-type silicon is performed to obtain through trenches  41  ( FIG. 3B ). 
     After this, the wafer is submitted to an electrolytic processing to turn the areas surrounding the through trenches  41  thus formed into porous silicon. Porous silicon areas  21 ,  28 ,  29  described in relation with  FIG. 2  and additional areas  43  and  44  surrounding central porous silicon area  21  are thus obtained. 
     At the step illustrated in  FIG. 3C , the wafer is laid on a support  45  for growing carbon nanotubes, and multiple-wall carbon nanotubes  23 ,  26 , and  27  are grown, which fill openings  41  formed in the wafer. The carbon nanotube growth support may for example be a wafer coated with many carbon nanotube growth seeds, for example, based on iron, nickel, and/or cobalt. During or after the carbon nanotube growth, a doping by platinum or any other appropriate catalyst which is used as a catalyst for the reaction between the gases (hydrogen, oxygen) and the electrolyte is performed. 
     At the step illustrated in  FIG. 3D , porous silicon regions  43  and  44  are suppressed by a selective etch method, the portions not to be etched being protected by a mask, and a wet filling with a solid polymer acid such as Nafion is performed. The structure of  FIG. 2A  has thus been obtained without the upper and lower surface insulations and metallizations. 
     At the step illustrated in  FIG. 3E , insulating upper surface and lower surface layers  30  and  31  are deposited and opened as described in relation with  FIGS. 2A and 2B . Then, at the step illustrated in  FIG. 3F , the deposition and etching of metallizations M 1  and M 2  is performed also according to the pattern described in relation with  FIGS. 2A and 2B . 
     The present invention has been described hereabove in the context of a specific application to the forming of fuel cells. 
     In its more general aspect, the present invention provides starting from a wafer made of any material, for example, silicon or glass, boring through openings and growing, from the lower surface, the multiple-wall carbon nanotubes in these openings, to obtain islands or walls of carbon nanotubes extending from the lower surface to the upper surface of a substrate. These islands or walls of carbon nanotubes may be used as in the case of the previously-described fuel cell for their properties of conductivity and of permeability to gases but also, only for their conductive properties to form conductive vias through a wafer. In the case where the wafer is made of glass, this will enable for example taking contacts from the lower surface to electronic components, formed on the upper surface, for example, passive components. In the case where the wafer is a silicon wafer, a local transformation of the silicon into porous silicon, possibly oxidized, may as described previously be performed, or a thermal oxidation of through openings formed in the silicon before the carbon nanotube growth may simply be performed. An advantage of such a use of carbon nanotubes is that, in their longitudinal direction, the carbon nanotubes have an extremely high conductivity, even greater than that that an aluminum mass taking up the same volume would have. 
     Of course, the present invention is likely to have various, alterations, improvements, and modifications which will readily occur to those skilled in the art. 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. The present invention is limited only as defined in the following claims and the equivalents thereto. 
     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.