Abstract:
A thermoelectric conversion element is provided as an element module with improved utility having an enhanced performance index and utilizing Fe 2 VAl type alloy thin-film under the condition of the drop in thermal conductivity. The structure of thermoelectric conversion element is comprised of a conductive buffer layer and plural repeating stages of single structures including thermoelectric conversion material layer and a conductive buffer layer, over a buffer layer formed on a substrate; and each of the thermoelectric conversion material layers is comprised of Full-Heusler alloy or Full-Heusler alloy thin film in a thickness range between 1 nm to 200 nm.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2011-152460 filed on Jul. 11, 2011, the content of which is hereby incorporated by reference into this application. 
       CROSS-REFERENCE TO RELATED APPLICATION 
       [0002]    This application is related to U.S. application Ser. No. 13/338,740 filed on Dec. 28, 2011, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0003]    The present invention relates to a thin-film thermoelectric conversion element and a manufacturing method for that thin-film thermoelectric conversion element. 
         [0004]    In recent years there has been increasing international concern over the issue of reducing carbon dioxide which is a substance causing the global warming phenomenon. Continuous progress is being made in technical innovations for shifting from resource energy that discharges large quantities of carbon dioxide, to reusable next generation energy such as natural energy and thermal energy. Next generation energy technology candidates includes technology utilizing natural energy such as solar power and wind power, and reusable technology for reutilizing the lost portion of primary energy such as heat and vibration emitted from using resource energy. Though conventional resource energy is a centralized energy mainly in the form of large-scale electrical generating facilities; next generation energy is featured by an uneven distribution of both natural energy and reusable energy. In current energy utilization, the energy that is waste-discharged without being used amounts to approximately 60% of primary energy and that amount is mainly in the form of waste heat. Therefore, what is needed besides increasing the proportion of next generation energy among primary energy is improved energy reutilization technology and in particular, better power conversion technology for waste heat energy. Waste heat is generated in all manner of situations so reutilizing waste heat energy requires an electrical generating system with a high degree of universality in all types of installation formats. Contriving such a generating system requires developing thermoelectric conversion materials possessing high electromotive force and in a space-saving format such as film. 
         [0005]    This thermoelectric conversion material is an element utilized for thermoelectric cooling by using the Peltier effect and generating thermoelectric power by way of the Seebeck effect. These elements generally possess a structure where plural P-type thermoelectric material and plural N-type thermoelectric material that is alternately arrayed and coupled in series. 
         [0006]    The currently used thermoelectric conversion material for actual applications is bismuth telluride (Bi 2 Te 3 ). The conversion efficiency of bismuth telluride is high but both bismuth and telluride are expensive, and tellurium is toxic so bismuth telluride is not a suitable choice in terms of the goals of mass production, low-cost, and reducing the load on the environment. So a substitute high-efficiency thermoelectric conversion material is needed as a substitute for bismuth telluride (Bi 2 Te 3 ). These circumstances have focused attention on Fe 2 VAl-based alloy as a potential thermoelectric conversion material that is both non-toxic and inexpensive. 
         [0007]    Methods for producing these thermoelectric conversion materials involved fusing or sintering by heating the raw material, and mechanically processing (cutting out) the material into a block shape. The advantage provided by this method is that the crystalline structure and elemental composition of the crystal can be controlled. However, most thermoelectric conversion materials have low mechanical strength so that intricate and precise processing is difficult, and achieving a thin and compact material was impossible. Moreover another problem was that the processing to cut-out the block had a low product yield. These circumstances served to focus attention on methods for manufacturing thermoelectric conversion material into thin film. A thermoelectric conversion material formed into a thin film can be formed into a thin-film thermoelectric conversion material possessing a fine and intricate structure, and extremely tiny and thin thermoelectric conversion elements can be made. These tiny and thin thermoelectric conversion elements could be mounted even in narrow spaces impossible for block-shaped elements to fit. A high-efficiency, thin-film thermoelectric conversion element suitable for practical use is therefore needed. 
         [0008]    Japanese Unexamined Patent Application Publication No. 2005-277343 discloses a thermoelectric conversion element utilizing an Fe 2 VAl-based thermoelectric material thin film deposited over a heated substrate. The disclosed element is a 5 μm thick N-type thermoelectric material sections and P-type thermoelectric material sections alternately arrayed in a zigzag pattern over a flat substrate. The thickness of the thermoelectric material thin film is preferably between 0.1 to 100 μm. 
       SUMMARY 
       [0009]    The performance index for thermoelectric conversion material is typically a dimensionless quantity called XT, and is given as follows. 
         [0000]    
       
         
           
             
               
                 
                   ZT 
                   = 
                   
                     
                       
                         S 
                         2 
                       
                       
                         κ 
                          
                         
                             
                         
                          
                         ρ 
                       
                     
                      
                     T 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0010]    Here, S denotes the Seebeck coefficient, κ is the thermal conductivity, ρ is the resistivity, and T equals the room temperature (300K). The larger the Seebeck coefficient, and the smaller the thermal conductivity and electrical resistivity, the larger the performance index becomes. The Seebeck coefficient and the electrical resistivity are physical quantities determined by the electron state of the material. According to Mott&#39;s formula, the Seebeck coefficient has a relation as shown next. 
         [0000]    
       
         
           
             
               
                 
                   
                     S 
                     ∝ 
                     
                       
                         1 
                         
                           N 
                            
                           
                             ( 
                             
                               E 
                               F 
                             
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                        
                       
                         
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                               ∂ 
                               
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                                  
                                 
                                   ( 
                                   E 
                                   ) 
                                 
                               
                             
                             
                               ∂ 
                               E 
                             
                           
                           ) 
                         
                         
                           E 
                           ∼ 
                           
                             E 
                             F 
                           
                         
                       
                     
                   
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                      
                     
                       : 
                     
                      
                     
                         
                     
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                     binding 
                      
                     
                         
                     
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                     energy 
                   
                    
                   
                     
 
                   
                    
                   
                     N 
                      
                     
                       : 
                     
                      
                     
                         
                     
                      
                     density 
                      
                     
                         
                     
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                     of 
                      
                     
                         
                     
                      
                     states 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
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                     2 
                   
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         [0011]    According to formula 2, the Seebeck coefficient is inversely proportional to the absolute value of the density of states in the Fermi level, and is proportional to that energy gradient. Therefore, a material with a small density of states (hereafter DOS) in the Fermi level and whose DOS rise fluctuates drastically signifies a material with a high Seebeck coefficient. Moreover in regards to electrical resistivity has the following relation. 
         [0000]    
       
         
           
             
               
                 
                   
                     1 
                     ρ 
                   
                   ∝ 
                   
                     
                       λ 
                       F 
                     
                      
                     
                       v 
                       F 
                     
                      
                     
                       N 
                        
                       
                         ( 
                         
                           E 
                           F 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
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                     3 
                   
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         [0012]    Here. λ F  and ν F  denote the mean free path and velocity of electrons at the Fermi level. This relation is inversely proportional to DOS of formula 3 so the electrical resistivity is small when there is a Fermi level where DOS of the absolute value is at a large energy position. 
         [0013]    The thermoelectric conversion material of Fe 2 VAl-based alloy possesses a pseudogap band structure. A pseudogap band structure is a matter or material system with an electronic state where the DOS in the vicinity of the Fermi level has drastically dropped. One feature of the Fe 2 VAl-based alloy band structure is said to be behavior as a rigid band model where only the Fermi level energy position changes and also that no significant fluctuations in the band structure occur when the composition ratio of the compound is changed. Therefore, by changing the composition ratio of the compound or changing the composition of the compound for hole doping or electron doping, the Fe 2 VAl-based alloy can control the Fermi level at the energy position to make steep changes in DOS and moreover attain an optimal absolute value for DOS for optimizing the relation between the Seebeck coefficient and resistivity. The above DOS changes and optimal values can be achieved in both a P-type and N-type matter system. 
         [0014]    Under current circumstances however, the FeiVAl-based compound has large thermal conductivity near that of metal at room temperatures and higher and is still far away from attaining a practical performance index figure. 
         [0015]    In view of the above problems and from results of extensive research, the present invention has the object of providing a thermoelectric conversion element as an element module with improved utility that possesses an enhanced performance index by utilizing the drop in thermal conductivity in Fe 2 VAl-besed alloy thin-film as an operating condition. 
         [0016]    A representative example of the thermoelectric conversion element of the present invention is featured in including a buffer layer, a thermoelectric conversion material layer, and an electrode layer laminated over a substrate, and in which the thermoelectric conversion material layer is a thin film in a range from 1 nm to 200 nm in film thick comprised of Full-Heusler alloy or an alloy with elements replaced from Full-Heusler alloy. 
         [0017]    Another feature of the present invention is a thermoelectric conversion element comprised of a plurality of layers of the above described thermoelectric conversion material layer and possessing a structure to obtain a total electromotive force that is the sum of the electromotive force in each layer. Specific features are the points that a plurality of single-unit structures comprised of laminated thermoelectric conversion material layer and a conductive buffer layer, are repeatedly deposited (formed); and that a lower electrode is coupled to the lowermost buffer layer for extracting the summed output of the electromotive force for each of the thermoelectric conversion material layers when a temperature gradient is applied in a direction perpendicular to the film plane, and that an upper electrode is deposited over the uppermost thermoelectric conversion material layer. 
         [0018]    Another specific feature is the point that the invention is comprised of multilayer structure layer comprised of a plurality of N-type thermoelectric conversion material layers and insulator layers alternately and repetitively formed with an insulator layer interposed between them, over a buffer layer deposited over the substrate; and a lower electrode coupled to one end of an N-type thermoelectric conversion material layer within the single-unit structure of the lowermost section; a plurality of coupling electrodes are coupled successively and moreover at alternative positions at both ends of the thermoelectric conversion material layers laminated adjacent to the upper section, a coupling electrode for example couples the other end of the N-type thermoelectric conversion material layer to one end of the P-type thermoelectric conversion material layers; and a coupling electrode couples the other end of that P-type thermoelectric conversion material layer (however the end side where the above described lower electrode is coupled) to one end of the P-type thermoelectric conversion material layer above that coupled P-type thermoelectric conversion material layer; and an upper electrode is then coupled to the P-type thermoelectric conversion material layer within the single-unit structure of the uppermost section. 
         [0019]    The material for the above described thermoelectric conversion material layer was Full-Heusler alloy or an alloy with elements replaced from Full-Heusler alloy, however the term Fe 2 VAl-based alloy may also be utilized. Besides Fe 2 VAl-based alloy other typical compounds for the material may include Fe 2 TiSn. Fe 2 TiSi, or Fe 2 NbAl; and more specifically an alloy whose composition is Fe 2 N 1-x M x X 1-x Y x  (however, N or M=V, Nb, Ti, Mo, W, Zr, and also X or Y═Al, Si, Sn, Ge). 
         [0020]    The Seebeck coefficient and the electrical conductivity of the Fe 2 VAl-based alloy can be optimally controlled by replacing the elements to alter the electron state. However, at room temperatures, the Fe 2 VAl-based alloy has properties resembling those of metal so that the thermal conductivity becomes large. Attaining ZT=2 which is said to be the boundary for practical use requires lowering this thermal conductivity. 
         [0021]    The thermal conductivity κ is expressed as follows. 
         [0000]    
       
         
           
             
               
                 
                   κ 
                   = 
                   
                     
                       k 
                       f 
                     
                     × 
                     
                       C 
                       p 
                     
                     × 
                     ς 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     4 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     k 
                     f 
                   
                   = 
                   
                     
                       d 
                       2 
                     
                     
                       τ 
                       f 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
         [0022]    Here, ζ or zeta is the density of the material, d is the sample film thickness, C p  is the sample specific heat at constant pressure, if is the time for the heat to propagate from the rear side of the thin film sample with a thickness d to the front side. As can be understood from formula 4 and formula 5, the thinner the sample film thickness, the smaller the thermal conductivity becomes. Heat is conveyed within a substance by way of electrons or lattice vibrations. Heat propagation by way of electrons is determined by the electron density within the substance. Heat propagation by way of lattice vibration is determined by the type of element and the crystalline structure. In other words, the change in thermal conductivity induced by controlling the thickness of the thin film is a property unique to the particular substance. The Physical Review B, 82, 075418, for example reported a change in thermal conductivity characteristics relative to the film thickness of copper (Cu). In this example, one can understand that at a film thickness of 100 nm or less, the thermal conductivity is proportional to the film thickness. However, as the film thickness approaches 200 nm, this proportional relation has already been lost and that thermal conductivity relative to increased film thickness is asymptotic to bulk thermal conductivity. In other words in copper (Cu) no clear effect in reducing thermal conductivity appears even if the film thickness was reduced at the vicinity of 200 nm. 
         [0023]    Whereupon the present inventors, sought to ascertain the thermal conductivity characteristics of Fe 2 VAl-based alloy relative to changes in film thickness and verified the correct film thickness for obtaining a clear reduction in thermal conductivity.  FIG. 1  shows the results derived from those thermal conductivity characteristics. These results confirmed that in Fe 2 VAl-based alloy, the film thickness is proportional to thermal conductivity in a film thickness range to 200 nm, or namely that the effect from a drop in thermal conductivity relative to film thickness is clearly evident in this range. The above description therefore shows that the method for lowering thermal conductivity in the representative structure of the present invention is producing the thermoelectric conversion material Fe 2 VAl-based alloy as a thin film, and that a suitable film thickness is 1 nm to 200 nm. The film thickness lower threshold value of 1 nm is equivalent to a few molecules of Fe 2 Val, and is a lower threshold value that allows forming a stable and uniform alloy film. 
         [0024]    If the film thickness range of the Fe 2 VAl-based alloy could be further narrowed to 100 nm or lower, then a thin film will possess a thermal conductivity less than one-fourth that of the bulk thermal conductivity, and the performance index as a thermoelectric conversion element can be increased to a higher level. Moreover, at a film thickness of 50 nm, the value for a performance index XT was confirmed as approximately 10 times that of bulk thermal conductivity. 
         [0025]    The present invention therefore achieves a thermoelectric conversion element possessing a high performance index by utilizing material having a small environmental load and moreover by selecting film thickness conditions and contriving a suitable structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a cross-sectional correlation diagram of the film thickness and thermal conductivity; 
           [0027]      FIG. 2  is a cross-sectional schematic diagram of the thin-film thermoelectric conversion element of a first embodiment of the present invention; 
           [0028]      FIG. 3  is a cross-sectional schematic diagram of the multilayer film perpendicular series type thermoelectric conversion element of a second embodiment; 
           [0029]      FIG. 4  is a cross-sectional schematic diagram of a variation of the thermoelectric conversion element of the second embodiment; 
           [0030]      FIG. 5  is cross-sectional schematic diagram of the multilayer internal plane series type thermoelectric conversion element of a third embodiment; 
           [0031]      FIG. 6  is a cross-sectional schematic diagram of a variation of the thermoelectric conversion element of the third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0032]      FIG. 2  is a cross-sectional schematic diagram showing the thermoelectric conversion element of the first embodiment. A multilayer film comprised of a buffer layer  101 , a thermoelectric conversion material layer  102 , and an electrode layer  103  were deposited over a silicon substrate  100  formed with a thermal oxide film. Tantalum (Ta) may for example be utilized as the buffer layer  101  or Ta/MgO (film thickness of 3 nm) may be utilized. If MgO having tantalum (Ta) as an under-layer was utilized as the buffer layer then the MgO structure is a rock-salt structure. The crystalline structure is oriented towards (100). The thermoelectric conversion material layer  102  utilized Fe 2 VAl as the Full-Heusler alloy. 
         [0033]    Each layer was deposited over the silicon substrate  100  utilizing the sputtering method along with argon (Ar) gas. The tantalum (Ta) was formed as a film in an amorphous state over the heat-oxidized silicon substrate at room temperature. After forming the laminated film, the laminated film was stripped away to directly above the buffer layer  101 , then the thermoelectric conversion material layer  102  and the electrode layer  103  was cut out over the buffer layer by using electron beam (EB) lithography and ion beam etching. Silicon dioxide (SiO 2 ) was formed as a film over the upper surface, a resist coating was applied, and electron beam (EB) lithography and ion beam etching were used in the forming process. Measuring the voltage across die electrodes showed that an electromotive force was generated when the substrate was in contact with a high-temperature section and generated a temperature gradient perpendicular to the element. Needless to say, wiring was formed in order to extract the respective voltages from the lower electrode and upper electrode. 
         [0034]    The thermal conductivity for various Fe 2 Val thin-film thicknesses was found for the present embodiment. Those results are as shown in  FIG. 1 . Examining  FIG. 1  reveals that there is a proportional relationship between the film thickness and thermal conductivity in a film thickness range of 200 nm and lower. At a film thickness of 200 nm, a thermal conductivity that is one-half that of the bulk thermal conductivity has already been attained. These results confirm the effect of lowering thermal conductivity in order to extract ample electrical generating performance in a film thickness range from 1 nm to 200 nm. At a film thickness range below 100 nm, the Fe 2 Val thin-film thermal conductivity was one-fourth or less that of the bulk thermal conductivity, and the effect on the performance index as a thermoelectric conversion element increase to a still higher level. Moreover a Fe 2 Val thin-film with a film thickness of 50 nm attained a performance index ZT value 10 times that of the bulk thermal conductivity and a Fe 2 Val thin-film with a film thickness of 10 nm attained a XT value 50 times that of the bulk thermal conductivity. 
         [0035]    In the example of the present embodiment, Fe 2 VAl was utilized as the thermoelectric conversion material, however other material may be utilized if a Full-Heusler alloy. Namely, besides Fe 2 VAl, other material may include Fe 2 TiSn, Fe 2 TiSi, or Fe 2 NbAl, etc., or an alloy whose composition is Fe 2 N 1-x M x X 1-x Y x  (however N or M=V, Nb, Ti, Mo, W, Zr, and also X or Y═Al, Si, Sn, Ge) will render the same effect. 
       Second Embodiment 
       [0036]      FIG. 3  is a cross-sectional schematic diagram of the thermoelectric conversion element of the second embodiment. The thermoelectric conversion element of the second embodiment has a laminated structure that includes a first buffer layer  201 , a thermoelectric conversion material layer  202 , and a conductive second buffer layer  203  deposited in layers over a silicon substrate  200  formed with a thermal oxide film, and a laminated structure as the film-formed electrode layer  204  formed after repeatedly laminating the thermoelectric conversion material layer  202  and conductive second buffer layer  203  single unit structures multiple times. Tantalum (Ta) was utilized in the first buffer layer  201 . Silver (Ag) was utilized in the second buffer layer. 
         [0037]      FIG. 4  shows a thermoelectric conversion element as a variation of the thermoelectric conversion element in  FIG. 3 . The variation in  FIG. 4  differs from the structure of the second embodiment in  FIG. 3 , in the point that a third buffer layer  209  is interposed between the first buffer layer  201  and the thermoelectric conversion material layer  202 . Here, the third buffer layer  209  utilized MgO (film thickness of 3 ran). This structure is a rock-salt structure and the crystalline structure is oriented towards (100). In both the structures in  FIG. 3  and  FIG. 4  the thermoelectric conversion material layer  102  utilized Fe 2 VAl as the Full-Heusler alloy. 
         [0038]    Each layer was deposited over the silicon substrate  200  utilizing the sputtering method along with argon (Ar) gas. The tantalum (Ta) was formed as a film in an amorphous state over the heat-oxidized silicon substrate at room temperature. After forming the laminated film, the laminated film was stripped away to directly above the first buffer layer  201  in  FIG. 3 , and stripped away to directly above the third buffer layer  209  in  FIG. 4  by electron beam (EB) lithography and ion beam etching. In this way, a structure comprised of a gigantic thermoelectric conversion element pillar was made. Silicon dioxide (SiO 2 ) was deposited as a film over the upper surface, a resist coating applied, and electron beam (EB) lithography and ion beam etching utilized to form an electrode  205  and an electrode  206 . The electrode  205  was formed coupled to the first buffer layer  201  in  FIG. 3 . An insulating third buffer layer  209  was interposed between the first buffer layer  201  and the thermoelectric conversion material layer  202  as shown in  FIG. 4 , so that the electrode  205  is formed to directly couple to the thermoelectric conversion material layer  202 . When the substrate contacts a high temperature section and generated a temperature gradient perpendicular to the element, an electromotive force occurs in each thermoelectric conversion layer, and the voltage across the electrode  205  and the electrode  206  is the sum of those voltages. This voltage can be extracted as the output. 
         [0039]    The second embodiment provides an improved performance index by lowering the thermal conductivity in thermoelectric conversion material with a film thickness in a range from 1 nm to 200 nm the same as in the first embodiment. This effect is drastically evident at film thicknesses below 100 nra. Moreover in this embodiment, the number of laminations of thermoelectric conversion material thin film can be changed to match the required voltage. 
         [0040]    The present embodiment utilized Fe 2 VAl as an example of the thermoelectric conversion material, however other material may be utilized if a Full-Heuslcr alloy. Namely, besides Fe 2 VAl, other material may include Fe 2 TiSn, Fe 2 TiSi, or Fe 2 NbAl, etc., or an alloy whose composition is Fe 2 N 1-x M x X 1-x Y x  (however N or M=V, Nb, Ti, Mo, W, Zr, and also X or Y═Al, Si, Sn, Ge) will render the same effect. 
         [0041]    Besides silver (Ag), the material utilized in the second buffer layer may include: Cu, Au, Pt, Pd, Ru, Rh. Ta, W, V, Ti, and Mg. 
       Third Embodiment 
       [0042]      FIG. 5  is cross-sectional schematic diagram of the thermoelectric conversion element of the third embodiment. The thermoelectric conversion element of the third embodiment has a laminated structure that includes a first buffer layer  301 , a thermoelectric conversion material layer  302   a , and an insulator layer  303  deposited in layers over a silicon substrate  300  formed with a thermal oxide film, and the film-formed electrode layer  304  over single-unit structures comprised of thermoelectric conversion material layer and the second insulator layer repeatedly laminated multiple times. Tantalum (Ta) was utilized in the first buffer layer  301  the same as in the second embodiment in  FIG. 3 , and MgO was utilized in the insulation layer. 
         [0043]      FIG. 6  shows a thermoelectric conversion element whose structure was formed as a variation of the thermoelectric conversion element in  FIG. 5 . In contrast to the third embodiment in  FIG. 5 , the variation shown in  FIG. 6  is a laminated structure comprised of a second buffer layer  309  over a first buffer layer. The second buffer layer  309  is MgO (film thickness of 3 nm) having a crystal orientation (110) the same as the variation in  FIG. 4 . The thermoelectric conversion material is Fe 2 VAl serving as the Full-Heusler alloy, the same as in the previous embodiments. However, a feature of the elements in  FIG. 5  and  FIG. 6  is that in the laminated structure of multiple repeating layers of single-unit structures, the thermoelectric conversion material layer  302  is N-type Fe 2 VAl and P-type Fe 2 VAl alternately arrayed layers. Moreover, an electrode  307  is coupled to one end of the lowest layer N-type Fe 2  VAl layer  302   a , and the other opposing facing side, a coupling electrode  306  is formed coupled to the end side surface of the P-type Fe 2 VAl  302   b  (above the N-type Fe 2 VAl layer  302   a ). Moreover, a coupling electrode  308  is formed coupled to the end side of the N-type Fe 2 VAl layer  302   c  (above P-type Fe 2 VAl  302   b ) at the other end (first end side of lowermost layer of N-type Fe 2 VAl layer  302   a ) of the P-type Fe 2 VAl layer  302   b . The coupling electrodes are in this way formed coupled to the Fe 2 VAl layers laminated adjacent to the upper section of the Fe 2 VAl layer successively and moreover alternately coupled to both opposing ends. The upper electrode  304  is deposited near the other end of the uppermost N-type Fe 2 VAl layer  320   n . In this embodiment also, each layer is deposited over the silicon substrate  300  utilizing the sputtering method using argon (Ar) gas. After forming the laminated film, the laminated film was stripped away to directly above the first buffer layer  302  by utilizing electron beam (EB) lithography and ion beam etching to form a laminated structure comprised of a gigantic thermoelectric conversion element pillar. After forming a coupling electrode on the side surface, silicon dioxide (SiO 26 ) was deposited as a film over the upper surface, a resist coating applied, and electron beam (EB) lithography and ion beam etching utilized to form an electrode  305  and an electrode  307   
         [0044]    In the structure of the above third embodiment and its variation, when the substrate  300  contacts a high temperature section, a temperature gradient is generated along the internal plane of each layer in the element causing an electromotive force to occur in each layer of the Fe 2 VAl. The voltages of the N-type Fe 2 VAl and P-type Fe 2 VAl attain opposite states. A voltage is obtained that is the sum of the electromotive forces of each Fe 2 VAl layer between the electrode  305  and electrode  307  sequentially coupled by the above described coupling electrodes. The third embodiment and variation of the third embodiment are in this way thermoelectric conversion elements that generate an electromotive force when a temperature gradient is applied along the internal plane in each layer and the utilization of the element differs from the second embodiment. The same points in the first embodiment and the second embodiment also apply to the film thickness of each thermoelectric conversion layers in the present embodiment. Namely, by controlling the film thickness to lower the thermal conductivity, a performance index ZT value is attained that is definitely improved compared to bulk material Fe 2 VAl which is exactly the same as previously described in the first embodiment and the second embodiment so that the practicality of the thermoelectric conversion element is improved. 
         [0045]    The present embodiment need not utilize only Fe 2 VAl as the thermoelectric conversion material and other material may be utilized if a Full-Heusler alloy. Other possibilities include Fe 2 TiSn, Fe 2 TiSi, or Fe 2 NbAl, etc. Still other possibilities are alloys whose composition is Fe 2 N 1-x M x X 1-x Y x  (however, N or M=V, Nb, Ti, Mo, W, Zr, and also X or Y═Al, Si, Sn, Ge) will render the same effect. 
         [0046]    Besides MgO, the insulator layer  303  may also utilize Al 2 O 3 , and SiO 2 , etc. 
         [0047]    The present invention therefore provides a thermoelectric conversion element posing a low environmental load, ideal for mass production and moreover compact and with high performance and capable of practical use in many areas.