Abstract:
A tunnel-effect power converter including first and second electrodes having opposite surfaces, wherein the first electrode includes protrusions extending towards the second electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 12/50497, filed on Jan. 18, 2012, which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to power conversion devices, and more specifically to devices enabling to convert heat into electric power. 
     Discussion of the Related Art 
       FIG. 1  is a perspective view schematically showing an example of a device  1  enabling to convert heat into electric power. Device  1  comprises two electrodes  3  and  5  having opposite surfaces separated by a distance which is on the order of atomic dimensions, for example, on the order of a few nanometers. Here and in the following description, “opposite surfaces” means surfaces facing each other and between which no solid material is interposed. Electrodes  3  and  5  are for example made of metal or of a semiconductor material such as silicon. Electrodes  3  and  5  may further comprise, on the side of their opposite surfaces, a thin coating (respectively  4  for electrode  3  and  6  for electrode  5 ) of an electrically conductive material of low work function, for example a metal such as cesium, or a metal oxide such as potassium peroxide (K 2 O 2 ) or a cesium oxide (Cs 2 O). To maintain the opposite surfaces at the desired distance, spacers  7  made of an insulating material are arranged between electrodes  3  and  5  in certain regions of device  1 . The free space between the opposite surfaces of electrodes  3  and  5  and spacers  7  may be placed under vacuum or filled with an inert gas. 
     In operation, electrode  3 , also called emitter, is heated up to a temperature T H , and electrode  5 , also called collector, is maintained at a temperature T C  lower than temperature T H . By thermionic barrier effect, electrons are extracted from hot electrode  3  and cross the potential barrier which separates them from cold electrode  5 . The short distance separating opposite electrode surfaces makes the electron transport from electrode  3  to electrode  5  by tunnel effect easier. There thus is an electron flow between hot electrode  3  and cold electrode  5  and, when a load  9  (LOAD) is connected between electrodes  3  and  5 , a current flows through the load going from cold electrode  5  (positive electrode) to hot electrode  3  (negative electrode). 
     Power conversion devices of this type, exploiting both thermionic emission and tunnel-effect conduction phenomena, are generally called tunnel-effect power converters, or thermionic power converters, or tunnel-effect thermionic power converters. 
     It would be desirable to be able to improve the performance, and especially the power conversion efficiency, of tunnel-effect converters. 
     SUMMARY 
     An embodiment provides a tunnel-effect power converter overcoming at least some of the disadvantages of known converters. 
     An embodiment provides a tunnel-effect power converter which has a better conversion efficiency than known converters. 
     An embodiment provides a method for manufacturing a tunnel-effect power converter. 
     Thus, an embodiment provides a tunnel-effect power converter comprising first and second electrodes having opposite surfaces, wherein the first electrode comprises protrusions extending towards the second electrode. 
     According to an embodiment, the second electrode comprises protrusions extending towards the protrusions of the first electrode. 
     According to an embodiment, the protrusions of the second electrode face the protrusions of the first electrode. 
     According to an embodiment, the converter comprises at least one third electrode, the second and third electrodes having opposite surfaces. 
     According to an embodiment, the second electrode comprises protrusions extending towards the third electrode. 
     According to an embodiment, the third electrode comprises protrusions extending towards the protrusions of the second electrode. 
     According to an embodiment, said protrusions are point-shaped. 
     According to an embodiment, the points have a height ranging between 5 and 25 nm. 
     According to an embodiment, the minimum distance separating said opposite surfaces ranges between 1 and 30 nm. 
     According to an embodiment, the electrodes comprise silicon. 
     According to an embodiment, the surfaces are coated with a material from the group comprising cesium, cesium oxides, and potassium peroxide. 
     Another embodiment provides a method for manufacturing a tunnel-effect power converter of the above-mentioned type, comprising: forming resin or oxide islands masking regions of a single-crystal silicon layer; partially thinning the single-crystal silicon layer by means of a solution preferentially etching oblique crystal planes of said layer. 
     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 DRAWINGS 
         FIG. 1 , previously described, is a perspective view schematically showing an example of a tunnel-effect power converter; 
         FIG. 2  is a cross-section view schematically illustrating an embodiment of a tunnel-effect power converter; 
         FIG. 3  is a cross-section view schematically illustrating another embodiment of a tunnel-effect power converter; and 
         FIGS. 4A to 4H  are cross-section views schematically illustrating steps of a method for forming a tunnel-effect power converter. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. Further, only those elements which are useful to the understanding of the embodiments have been shown and will be described. In particular, the various possible uses of power converters described hereafter have not been detailed, the provided converters being compatible with current uses of power converters. 
       FIG. 2  is a cross-section view schematically illustrating an embodiment of a tunnel-effect power converter  11 . Converter  11  comprises two electrodes  13  and  17  having opposite surfaces. A difference between converter  11  of  FIG. 2  and converter  1  of  FIG. 1  is that, in converter  11 , the opposite electrode surfaces are not planar. In the shown example, the surface of electrode  13  opposite to electrode  17  comprises protrusions  14  extending towards electrode  17 , and the surface of electrode  17  opposite to electrode  13  comprises protrusions  18  extending towards electrode  13 . In this example, protrusions  18  of electrode  17  face protrusions  14  of electrode  13 . In a preferred embodiment, the protrusions are point-shaped. As an example, the points have a base diameter ranging from 1 to 20 nm and a height ranging between 5 and 25 nm. The spacing step between points for example ranges between 5 and 25 nm, and the total surface area of the converter (in top view) for example has the shape of a square having a side length ranging from 500 nm to 50 μm. Opposite protrusions  14  and  18  are separated by a distance selected to enable a conduction by tunnel-effect between electrodes, for example, a distance ranging between 1 and 30 nm, and preferably ranging between 3 and 10 nm. 
     Electrodes  13  and  17  are, for example, made of a semiconductor material such as N-type doped single-crystal silicon, P-type doped single-crystal silicon, or undoped single-crystal silicon, and may comprise, on the side of their opposite surfaces, a thin coating (respectively  15  for electrode  13  and  19  for electrode  17 ) of a low work function material, for example, a metal, such as cesium, or a metal oxide, such as potassium peroxide (K 2 O 2 ) or a cesium oxide (Cs 2 O). The thickness of coatings  15  and  19  for example ranges between 1 and 10 nm, and preferably between 3 and 7 nm. To maintain the opposite surfaces at the desired distance, spacers  21  made of an electrically insulating material, for example, silicon oxide, are arranged between electrodes  13  and  17  in certain regions of converter  11 , for example in peripheral regions of the converter. 
     In the shown example, lower electrode  13  is formed in the upper part of a portion of a semiconductor wafer  22 , and upper electrode  17  is topped with a protection wafer  23 , for example, made of metal. Semiconductor wafer portion  22  and protection wafer  23  may form elements of an encapsulation package of the converter. The free space within the package, and in particular between the opposite surfaces of electrodes  13  and  17  and spacers  21 , may be placed in vacuum or filled with an inert gas. In this example, protection wafer  23  is electrically insulated from electrode  17 , and contacts are taken on electrodes  13  and  17 , for example, via the conductive coating made of a low work function material (respectively  15  for electrode  13  and  19  for electrode  17 ), forming electric power supply terminals of the converter. 
     In operation, electrode  13  (emitter) is heated, and electrode  17  (collector) is maintained at a temperature lower than that of electrode  13 . Under the effect of heat, electrons are extracted from hot electrode  13  and cross the potential barrier which separates them from cold electrode  17  by thermionic emission effect. The short distance separating the opposite surfaces of electrodes  13  and  17  eases the transport of electrons from electrode  13  to electrode  17  by tunnel effect. There thus is an electron flow between hot and cold electrodes  13  and  17  and, when a load (not shown) is connected between electrodes  13  and  17 , a current flows through the load going from cold electrode  17  (positive electrode) to hot electrode  13  (negative electrode). 
     As an example, converter  11  may be used at temperatures ranging from approximately 20 to 600° C. on the hot surface side, and from approximately −50 to 500° C. on the cold surface side, with a temperature gradient approximately ranging from 1 to 150° C. between the cold surface and the hot surface. The described embodiments are, however, not limited to such specific operating temperature ranges. 
     The measurements performed by the present inventors have shown that the efficiency of converter  11  is greater than the efficiency of a planar-electrode converter of the type described in relation with  FIG. 1 . As an example, for a given total converter surface area (in top view), the efficiency of converter  11  is approximately 600 times greater than the efficiency of a planar-electrode converter of the type described in relation with  FIG. 1 . The improvement of the conversion performance is especially due to the point effect which causes a local increase of the electric field at the free ends of protrusions  14  and  18 . This results in a decrease of the work function at the points, which causes an increase of the total electron flow between electrodes  13  and  17 , and thus an increase of the amount of the electric current capable of being generated by the converter. 
       FIG. 3  is a cross-section view schematically illustrating another embodiment of a tunnel-effect power converter. Converter  31  of  FIG. 3  differs from converter  11  of  FIG. 2  in that it comprises a stack of three electrodes  33 ,  37 , and  41  having opposite surfaces, instead of two. Electrodes  33  and  37  are arranged substantially in the same way as electrodes  13  and  17  of converter  11  of  FIG. 2 . Electrode  41  is arranged above (in the orientation of the drawing) electrode  37 , electrodes  37  and  41  having opposite surfaces. The surface of electrode  33  opposite to electrode  37  comprises protrusions  34  extending towards electrode  37 , and the surface of electrode  37  opposite to electrode  33  comprises protrusions  38  extending towards electrode  33 , protrusions  34  of electrode  33  facing protrusions  38  of electrode  37 . Further, the surface of electrode  37  opposite to electrode  41  comprises protrusions  39  extending towards electrode  41 , and the surface of electrode  41  opposite to electrode  37  comprises protrusions  42  extending towards electrode  37 , protrusions  39  of electrode  37  facing protrusions  42  of electrode  41 . As in the example of  FIG. 2 , electrodes  33 ,  37 , and  41  may comprise a coating made of a conductive material of low work function. In the shown example, contacts are taken on electrodes  33  and  41 , forming electric power supply terminals of the converter. 
     The device of  FIG. 3  corresponds to the placing in series of two tunnel-effect converters of the type described in relation with  FIG. 2 . An advantage of such a device is that the provided electric power is greater than in a single-stage converter. Another advantage is that the voltage under which the electric power generated by the converter is provided (between electrodes  33  and  41 ) is greater than in a single-stage converter. Another advantage is that in such a converter, the distance between the hot and cold surfaces of the converter is greater than in a single-stage converter, which enables to more easily maintain a significant temperature gradient between the hot surface and the cold surface. 
     More generally, it will be within the abilities of those skilled in the art to adapt the number of stages of the converter according to the targeted application, and in particular to provide a converter comprising more than three superposed electrodes. It will also be within the abilities of those skilled in the art to form a converter with several stages in which the stages are connected in parallel, rather than in series as in the example of  FIG. 3 . 
       FIGS. 4A to 4H  are cross-section views schematically illustrating steps of a method for forming a tunnel-effect power converter of the type described in relation with  FIG. 3 . 
       FIG. 4A  illustrates an initial structure comprising a portion  22  of an N-type doped silicon semiconductor substrate (Si(N + )), and a stack comprising, in the order from the surface of the substrate, a layer  51  made of a silicon-germanium alloy (SiGe), an N-type doped silicon layer  52  (Si(N + )), a layer  53  of a silicon-germanium alloy (SiGe), and an N-type doped silicon layer  54  ((Si(N − )). As a variation, substrate  22  and layers  52  and  54  may be made of P-type doped silicon, or of undoped silicon, or of other adapted materials. Layers  51 ,  52 ,  53 , and  54  are for example formed by epitaxy over the entire surface of substrate  22 , where a portion of the stack can then be delimited by etching, thus resulting in the structure of  FIG. 4A . In an embodiment, silicon-germanium layers  51  and  53  have a thickness on the order of 15 nm, and silicon layers  52  and  54  have a thickness on the order of 60 nm. 
       FIG. 4B  illustrates a step of partial removal of silicon-germanium layers  51  and  53 , by selective etching. A small portion only of layers  51  and  53  is kept in the central portion of the block, to avoid a collapsing of the block. As an example, the partial removal of layers  51  and  53  is performed by plasma etching or by chemical etching. 
       FIG. 4C  illustrates a step of filling of the spaces left free between layers  52  and  54  after the partial removal of layers  51  and  53 , with a resin  55 . In this example, the stack is totally embedded in resin  55 , that is, its lateral and upper surfaces are also covered with resin  55 . Resin  55  for example is hydrogen silsesquioxane (H 8 Si 8 O 12 ), generally designated as HSQ in the art, which is a negative resin sensitive to electrons having the property of turning into silicon oxide after exposure, development, and anneal. The described method is however not limited to the use of this specific resin. 
       FIG. 4D  illustrates a step of etching of a pattern in resin  55 . Resin strips forming a grid in top view are removed. The resin is for example exposed by means of an electron beam scanning the upper surface of the stack. The electrons cross the silicon of layers  52  and  54  so that the resin is exposed along the entire height of the stack. A development step is then provided to only keep islands  56  of non-exposed resin, vertically aligned between substrate  22  and layer  52 , between layer  52  and layer  54 , and at the surface of layer  54 . As an example, islands  56  have a width of approximately 10 nm, and neighboring islands  56  are separated by a distance approximately ranging from 10 to 20 nm, corresponding to the width of the removed resin strips. In the peripheral region of the stack, regions  21  of resin  55  are kept to form spacers between the converter electrodes. An anneal can then be provided to transform resin  55  into silicon oxide  57 . 
       FIG. 4E  illustrates a step of removal of the remaining portions of silicon-germanium layers  51  and  53 . At this stage, the remaining portions of layers  51  and  53  may be removed with no risk for the structure to collapse, due to the presence of the silicon oxide pattern which interposes between silicon layers  22 ,  52 , and  54 , and in particular due to the presence of spacers  21 . 
       FIG. 4F  illustrates a step during which silicon layers  22 ,  52 , and  54  are partially etched by means of a solution preferentially etching oblique crystal planes of the silicon, for example, a solution based on potassium hydroxide (KOH). This has resulted in thinning the silicon layer at the level of the regions unmasked by silicon oxide islands  56 . At the end of this step, the partially thinned silicon layer has point-shaped protrusions at the level of the regions masked by islands  56 . 
       FIG. 4G  illustrates a step of removal of islands  56 , for example, by means of a hydrofluoric acid solution. Silicon oxide regions  57  forming spacers  21  are at least partially kept. 
       FIG. 4H  illustrates a step of deposition, at the surface of silicon regions  22 ,  52 , and  54 , of a thin coating of an electrically conductive material of low work function, for example, a metal, such as cesium, or a metal oxide, such as potassium peroxide (K 2 O 2 ) or a cesium oxide (Cs 2 O). As an example, a cesium oxide coating is formed by sputtering, or by deposition in successive atomic layers according to a method presently called ALD in the art (Atomic Layer Deposition). 
     The converter is then encapsulated in a package, for example, comprising a protection wafer  23  topping upper electrode  41 . Contacts are taken on electrodes  33  and  41 , forming electric power supply terminals of the converter. 
     Specific embodiments have been described. Various alterations, modifications and improvements will readily occur to those skilled in the art. 
     In particular, embodiments in which electrodes of the tunnel-effect power converter are partly formed in silicon have been described hereabove. The present invention is not limited to this specific case. It will be within the abilities of those skilled in the art to adapt the provided structure by using other materials than those mentioned hereabove, for example, metals, since the selected materials are compatible with the forming of protrusions, preferably pointed, at the surface of the electrodes. 
     Further, in the above-described embodiments, the hot and cold electrodes comprise opposite protrusions. A structure in which only the hot electrode has protrusions extending towards the cold electrode may also be provided, the cold electrode having a planar surface. 
     Further, the above-described examples of converters comprise a single stack portion comprising electrodes having opposite surfaces. It will be within the abilities of those skilled in the art to form a converter comprising, on a same semiconductor substrate, a plurality of juxtaposed stack portions, each corresponding to a structure of the type described in relation with  FIG. 2 or 3 . 
     Further, a method for manufacturing tunnel-effect power converters has been described hereabove as an example ( FIGS. 4A to 4H ). It will be within the abilities of those skilled in the art to adapt the provided method and use any other known manufacturing method to manufacture converters of the type described in relation with  FIGS. 2 and 3 . 
     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.