Patent Publication Number: US-2011048486-A1

Title: Thermoelectric module

Description:
The entire disclosure of Japanese Patent Application No. 2009-198979 filed Aug. 28, 2009 is expressly incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermoelectric module with use of Peltier effect. 
     2. DESCRIPTION OF RELATED ART 
     There has been conventionally known a thermoelectric module in which a plurality of thermoelectric elements (a P-type thermoelectric element and an N-type thermoelectric element) are held between a pair of substrates and bonded thereto, the thermoelectric module cooling or heating an object by supplying electricity between the thermoelectric elements. The thermoelectric elements are disposed so as to be electrically series-connected to each other via electrodes provided on each of the substrates. When electricity is supplied to respective thermoelectric elements, a unidirectional heat transport is generated between the substrates by Peltier effect. 
     An endothermic reaction is generated on one of the pair of substrates and a heat-release reaction is generated on the other of the pair of substrates. Based on this, an object to be cooled is attached to the substrate having the endothermic reaction and a heat sink is attached to the substrate having the heat-release reaction, whereby the object to be cooled is temperature-controlled. At this time, generally, a metallization layer is formed in advance on surfaces of the substrates, which are formed of alumina and the like, in order to facilitate direct-soldering of the object to the substrates. The object and the heat sink are installed on the surfaces of the metallization layer through a solder layer or an adhesive. 
     A comparison between a coefficient of thermal expansion of the substrates and that of the solder layer reveals that the coefficient of thermal expansion of the solder layer is larger than that of the substrates by three times or more. Accordingly, when both temperatures of the substrates and the solder layer are lowered, the solder layer is more shrunk than the substrates, so that the substrates are warped. At this time, each of the substrates is warped from thermoelectric elements disposed at the center of the substrates. Accordingly, a displacement of warpage of respective substrates becomes larger in accordance with the distance from the center and stress generated on the elements on an outer circumference is enlarged, so that the thermoelectric elements may be damaged by the displacement and stress. 
     Accordingly, it has been known to prevent damage of the theimoelectric elements by eliminating the thermoelectric elements disposed at a predetermined area at the center of the substrates and suppressing stress due to warpage generated on the substrates (see, for instance, Patent Literature 1: JP-A-2009-129968). 
     In Patent Literature 1, since the thermoelectric elements disposed at the predetennined area at the center of the substrates are eliminated, an elongated and different-shaped electrode spanning over the area eliminated with the thermoelectric elements, is formed on each of the substrates holding the thermoelectric elements, thereby interconnecting a pair of thermoelectric elements largely separated from each other. 
     However, since a size and a shape of the area in which the thermoelectric elements are eliminated, in other words, the number of pairs of the thermoelectric elements to be disposed between the substrates (herein, “the number of pairs” means a total number counted for each of a pair of P-type thermoelectric element and N-type thermoelectric element which are bonded to a single electrode) and layout thereof differ depending on a design of the thermoelectric module, substrates having various electrode patterns for different designs are required as an upper substrate and a lower substrate, which increases manufacturing cost. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a thermoelectric module whose manufacturing cost is reduced while stress generation caused by warpage of a substrate being suppressed. 
     A thermoelectric module according to an aspect of the invention includes: a plurality of thermoelectric elements that is electrically connected to each other via a plurality of electrodes; and a pair of substrates including a first substrate and a second substrate on which the plurality of electrodes are formed on facing surfaces, the pair of substrates being provided perpendicularly to a heat transfer direction with the plurality of thermoelectric elements being interposed, in which the plurality of electrodes formed on the first substrate of the pair of substrates include a bypass electrode having a size enough to electrically connect a pair of the thermoelectric elements that are spaced apart by a distance corresponding to an area of one of the thermoelectric elements or more, and the plurality of electrodes foimed on the second substrate of the pair of substrates are provided correspondingly to a maximum placement number of the plurality of thermoelectric elements interposed between the pair of substrates and have a size enough to electrically connect an adjacent pair of the thermoelectric elements. 
     Herein, “adjacent” means that, when the then ioelectric elements are located at the maximum placement number, the thermoelectric elements are positioned next to each other, irrespective of an alignment direction. Accordingly, in the aspect of the invention, when the thermoelectric elements are located at the maximum placement number, the electrode that electrically connects the pair of the thermoelectric elements is provided as an electrode having the smallest shape. 
     In the thermoelectric module according to the aspect of the invention, the bypass electrode includes a plurality of adjacent bypass electrodes, and bypass-electrode aggregation portion formed by the plurality of adjacent bypass electrodes is surrounded by an electrode having a size enough to electrically connect an adjacent pair of the thermoelectric elements. 
     Herein, “bypass-electrode aggregation portion” means a portion on which a plurality of the bypass electrodes are aggregated, irrespective of a size and shape thereof. 
     In the thermoelectric module according to the aspect of the invention, an object to be cooled is installed on a side opposite to a side of the first or second substrate which the plurality of electrodes are formed, the plurality of thermoelectric elements are sparsely o densely located in accordance with installation layouts of the object to be cooled, and the bypass electrode has a size enough to connect the pair of theillioelectric elements at dense parts that are located over a sparse part. 
     According to the aspect of the invention, the bypass electrodes are only provided on the first substrate and the electrodes corresponding to the maximum placement number of the thermoelectric elements are provided on the second substrate. Accordingly, when various designs, in which warpage of the substrates, endothermic efficiency and the like are taken into consideration, are present for disposing the thermoelectric elements, it is just necessary to change a shape and the like of the bypass electrodes. Specifically, it is just necessary to only change the first substrate on which the bypass electrodes are formed. 
     Accordingly, the second substrate, on which electrodes corresponding to the maximum placement number of the thermoelectric elements are provided, can be used in common with a thermoelectric module having a different design, thereby reducing the kind of parts to save the manufacturing cost. Depending on the number, shape and location of the bypass electrodes, i.e., the number and location of the thermoelectric elements to be eliminated, stress generation caused by warpage of the substrate can be suppressed. 
     In the aspect of the invention, when the plurality of the bypass electrodes are aggregated in a predetermined area on the first substrate, and thus obtained bypass-electrode aggregation portion is surrounded by other small electrodes, the thermoelectric elements located, for instance, substantially at the center of the substrate as the starting point of warpage thereof are not necessary. Accordingly, even when the substrate is warped from the starting point, i.e., the eccentrically arranged thermoelectric elements, a distance between the starting point of the warpage and an outer circumference of the substrate is shorter than that between the outer circumference and the starting point of warpage located at the center. Accordingly, warpage displacement and stress can be further reduced. 
     Incidentally, the thermoelectric module according to the aspect of the invention can be provided as one capable of transporting heat from the first substrate to the second substrate by Peltier effect, thereby temperature-controlling a heat-generating object to be cooled by installing the object on the first substrate. In such a thermoelectric module, by densely locating the thermoelectric elements in an area where the object to be cooled generates a large amount of heat and by sparsely locating the thermoelectric elements in an area where the object to be cooled generates a small amount of heat and locating the bypass electrodes corresponding to the sparse area, the necessary number of the thermoelectric elements can be located in the necessary area and endothermic efficiency can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view showing a thermoelectric module and a semiconductor laser unit according to a first exemplary embodiment of the invention. 
         FIG. 2  is a plan view seen from II-II line of  FIG. 1 . 
         FIG. 3  is a plan view seen from line of  FIG. 1 . 
         FIG. 4  is an overall plan view of a semiconductor laser unit. 
         FIG. 5  is a plan view when a maximum number of thermoelectric elements are disposed between the substrates. 
         FIG. 6  is a cross sectional view showing a thermoelectric module according to a second exemplary embodiment of the invention. 
         FIG. 7  is a plan view seen from VII-VII line of  FIG. 6 . 
         FIG. 8  is a cross sectional view showing a thermoelectric module according to a third exemplary embodiment of the invention. 
         FIG. 9  is a plan view seen from IX-IX line of  FIG. 8 . 
         FIG. 10  is a cross sectional view showing a thermoelectric module according to a fourth exemplary embodiment of the invention. 
         FIG. 11  is a plan view seen from XI-XI line of  FIG. 10 . 
         FIG. 12  is a cross sectional view showing a thermoelectric module according to a fifth exemplary embodiment of the invention. 
         FIG. 13  is a plan view seen from XIII-XIII line of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     First Exemplary Embodiment 
     A first exemplary embodiment of the invention will be described below with reference to the attached drawings. 
       FIG. 1  is an overall plan view of a thermoelectric module  1  and a semiconductor laser unit  10 .  FIG. 2  is a plan view seen from an arrow of II-II of  FIG. 1 , in which an upper substrate  4 A and a metallization layer  6  (described below) are eliminated from the thermoelectric module  1  shown in  FIG. 1 .  FIG. 3  is a plan view seen from an arrow of III-III of  FIG. 1 .  FIG. 4  is an overall plan view of the semiconductor laser unit  10 . 
     The thermoelectric module  1  substantially includes, as shown in  FIGS. 1 and 2 , a plurality of P-type thermoelectric elements  2  and N-type thermoelectric elements  3  and a pair of substrates  4  that are disposed perpendicularly to a heat transfer direction (described below) and interposed by the thermoelectric elements  2  and  3 . 
     Each of P-type thermoelectric elements  2  includes: a P-type thermoelectric material containing bismuth (Bi) and tellurium (Te); and a diffusion prevention layer containing any one of molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta) and Nickel (Ni). 
     Each of N-type thermoelectric elements  3  includes: an N-type thermolectric material containing bismuth (Bi) and tellurium (Te); and the same diffusion prevention layer as that of the P-type thermoelectric element  2 . 
     As shown in  FIG. 1 , the substrates  4  include an upper substrate  4 A that is disposed at an upper side of  FIG. 1 , serving as a first substrate; and a lower substrate  4 B that is disposed at a lower side of  FIG. 1 , serving as a second substrate. The substrates  4  are made of ceramics which is typically an insulating material, and formed of alumina (Al 2 O 3 ), alumina nitride (AN), silicon carbide (SiC) and the like. Alternatively, the substrates  4  may be made of a resin such as polyimide. 
     A metallization layer  6  is respectively formed on an upper surface of the upper substrate  4 A and a lower surface of the lower substrate  4 B. The metallization layer  6  is provided by laminating copper (Cu), nickel (Ni) and gold (Au). The semiconductor laser unit  10  (described below) is bonded to the metallization layer  6  of the upper substrate  4 A through a solder layer (not shown). A heat sink (not shown) provided with a plurality of heat release fins or a metal member such as a package is bonded to a substantially whole surface of the metallization layer  6  of the lower substrate  4 B through a solder layer (not shown) or an adhesive. 
     In such a thermoelectric module  1 , when electricity is supplied to each of the thermoelectric elements  2  and  3 , heat is unidirectionally transferred between the substrates  4  by Peltier effect, in which an endothermic reaction is generated on the upper substrate  4 A and a heat-release reaction is generated on the lower substrate  4 B. It should be noted that, depending on a direction in which electricity is supplied to each of the thermoelectric elements  2  and  3 , a heat release reaction is generated on the upper substrate  4 A and an endothermic reaction is generated on the lower substrate  4 B. 
     Accordingly, in this exemplary embodiment in which the thermoelectric elements  2  and  3  are interposed between the substrates  4 A and  4 B aligned in a heat transfer direction thereof, heat of the semiconductor laser unit  10  is absorbed through the upper substrate  4 A by attaching the semiconductor laser unit  10  to the upper substrate  4 A having an endothermic reaction. At the same time, the absorbed heat is transported to the lower substrate  4 B through each of the thermoelectric elements  2  and  3 , and then released outward by a heat sink and the like, thereby cooling the semiconductor laser unit  10 . 
     A plurality of first electrodes  51  to third electrodes  53 , to which an upper end of each of the thermoelectric elements  2  and  3  is bonded, are provided on a lower surface of the upper substrate  4 A. A plurality of fourth electrodes  54 , to which a lower end of each of the thermoelectric elements  2  and  3  is bonded, are provided on an upper surface of the lower substrate  4 B. Among the first electrodes  51  to fourth electrodes  54 , the first substrates  51  provided on the upper substrate  4 A are bypass electrodes according to the invention. 
     The first electrodes  51  to fourth electrode  54  are, for instance, nickel (Ni)-plated by a copper plating method, or made of copper plated with nickel (Ni) and gold (Au). The first electrodes  51  to fourth electrodes  54  all are rectangular, but each of the first electrodes  51  has a different length in a longitudinal direction from each of the second electrodes  52  to fourth electrodes  54 . The each of the second electrodes  52  to fourth electrodes  54  has the same shape and length. The second electrodes  52  and the fourth electrodes  54  are directed differently from the third electrodes  53  by 90 degrees. 
     In  FIGS. 2 and 3 , it should be noted that a size of each of the fourth electrodes  54  is drawn in a slightly smaller size than that of the second and third electrodes  52  and  53  in order to clarify positions of the fourth electrodes  54  against those of the first to third electrodes  51  to  53 . 
     Moreover, the longitudinal direction and a transversal direction of each of the first, second and fourth electrodes  51 ,  52  and  54  are the same as those of the substrates  4 . A longitudinal dimension of each of the first electrodes  51  is represented by P w ′ including gaps between adjacent electrodes. A longitudinal dimension of each of the second and fourth electrodes  52  and  54  is represented by  2 P w  including gaps between adjacent electrodes. Similarly, a transversal dimension of each of the first, second and fourth electrodes  51 ,  52  and  54  is represented by P L . 
     A longitudinal dimension of each of the third electrodes  53  is represented by  2 P L  including gaps between adjacent electrodes. Similarly, a transversal dimension thereof is represented by P. The longitudinal dimension of each of the first electrodes  51  is larger than those of the second and fourth electrodes  52  and  54 . 
     As described above, in this exemplary embodiment, the second to fourth electrodes  52  to  54  have the same shape and size. When the dimension P w  or P L  is altered, the second to fourth electrodes  52  to  54  are different from the third electrodes  53  in size. 
     In the above thermoelectric module  1 , as shown in  FIG. 1 , the first to the third electrodes  51  to  53  on the upper substrate  4 A (not shown in  FIG. 1 ) are disposed in a manner to be alternated with the fourth electrodes  54  on the lower substrate  4 B. With this arrangement, the thermoelectric elements  2  and  3  are electrically series-connected through the first to fourth electrodes  51  to  54 . A lead wire L is connected to the fourth electrodes  54  disposed on both ends in a connection direction. Instead of the lead wire L of this exemplary embodiment, a wire bonding method by using an electrode pad and a post electrode may be used for connection. 
     Numerals W 1  to W 14  and L 1  to L 6  in  FIG. 2  respectively represent a row number (W 1  to W 14 ) and a line number (L 1  to L 6 ) of positions of thermoelectric elements  2  and  3 . The same applies for numerical numbers shown in the drawings to be used for explanation of the following exemplary embodiments. Moreover, as shown in  FIG. 5 , the thermoelectric elements of 84 at the maximum, i.e., 42 pairs at the maximum can be provided in principle on the upper and lower substrates by using all the lines and the rows. In  FIG. 5 , actually-used thermoelectric elements are  41  pairs since electricity is supplied by the lead wire. However, a maximum placement number of the pairs on the substrates is  42  pairs. 
     However, 6 pairs (corresponding to W 7  and W 8  rows) at the center and 6 pairs (corresponding to W 11  and W 12  rows) on the right, i.e., 12 pairs in total of the thermoelectric elements  2  and  3  are eliminated in this exemplary embodiment, as shown in  FIG. 2 . Consequently, 29 pairs of the thermoelectric elements  2  and  3  are provided between the substrates  4 . On the other hand, as shown in  FIG. 3 , the fourth electrodes  54  on the lower substrate  4 B are provided correspondingly to the maximum placement number of the pairs, as is the case in which 41 pairs of the thermoelectric elements  2  and  3  are located in  FIG. 5 . 
     On the upper substrate  4 A, the first electrodes  51  are provided correspondingly to  12  positions from which the thermoelectric elements  2  and  3  are eliminated. The first electrodes  51  are provided over the area corresponding to the eliminated thermoelectric elements  2  and  3  and are formed longer than the second electrodes  52  along the longitudinal direction of the upper substrate  4 A. In other words, the first electrodes  51  serving as the bypass electrodes are electrically connected to the thermoelectric elements  2  and  3  in W 6  and W 9  rows, which are spaced apart by W 7  and W 8  rows. The first electrodes  51  are also electrically connected to the thermoelectric elements  2  and  3  in W 10  and W 13  rows, which are spaced apart by W 11  and W 12  rows. 
     For a detailed explanation of other electrodes, at a left end of  FIG. 2 , three third electrodes  53  are located in a single row on the upper substrate  4 A while longitudinal directions of the three third electrodes  53  are aligned along a transversal direction of the upper substrate  4 A. In other words, in W 1  row, the third electrodes electrically connect the thermoelectric elements  2  and  3  in L 1  and L 2  lines, those in L 3  and L 4  lines and those in L 5  and L 6  lines to each other. 
     On an inner side (a right side) of the third electrodes  53 , the second electrodes  52  are located with longitudinal directions thereof aligned along a longitudinal direction of the upper substrate  4 A, the number of the second electrodes  52  being 12 in total in two rows of six lines. Accordingly, in L 1  to L 6  lines, the second electrodes connect the thermoelectric elements  2  and  3  in W 2  and W 3  rows as well as those in W 4  and W 5  rows. 
     The first electrodes  51  are located on a right side of the second electrodes  52 , the number of the first electrodes  51  being  12  in total in two rows of six lines in the same manner as described above. At a right end of the upper substrate  4 A, two third electrodes  53  are located in a single row with longitudinal directions thereof aligned along a transversal direction of the upper substrate  4 A. The two third electrodes  53  are located around a center of the transversal direction of the upper substrate  4 A and connect the thermoelectric elements  2  and  3  in L 2  and L 3  lines and those in L 4  and L 5  lines to each other in W 14  row. Accordingly, no electrode is provided at corners on the right end of the upper substrate  4 A. Consequently, a half of each of the fourth electrodes  54  at the corners on the lower substrate  4 B is exposed for connecting the lead wire L. 
     On the other hand, as shown in  FIG. 3 , on the lower substrate  4 B, the fourth electrodes  54  are provided with longitudinal directions thereof aligned along the longitudinal direction of the lower substrate  4 B in seven rows of six lines, which corresponds to the maximum placement number of the thermoelectric elements  2  and  3 . 
     Such a lower substrate  4 B is used in common for a thermoelectric module of alternative design in which the number of thermoelectric elements  2  and  3  to be practically provided is changed from 29 pairs, as long as the maximum placement number of the thermoelectric elements  2  and  3  is not changed. In addition, the lower substrate  4 B is used in common as a lower substrate  4 B of a second and other exemplary embodiments (see  FIGS. 6 to 13 ) in which a layout of the thermoelectric elements  2  and  3  is altered though the number of thermoelectric elements  2  and  3  to be practically provided stays the same (29 pairs). 
     In  FIGS. 1 and 4 , the semiconductor laser unit  10  exemplarily includes a semiconductor laser diode (LD)  11  and an optical modulator  12  as objects to be cooled and other parts (not shown) which are installed by soldering and the like on a submount  13  formed on the upper surface of the upper substrate  4 A. However, the arrangement of the semiconductor laser unit  10  is not limited to one shown in  FIGS. 1 and 4 . 
     The LD  11  is primarily used for optical communication, and additionally for an optical disc player and the like. The LD  11  oscillates and outputs a light. The light is intensity-modulated by the optical modulator  12  and is output from an emitting portion (not shown). A heat generation amount of the LD  11  is larger than that of the optical modulator  12 . The LD  11  is installed at a center of an area corresponding to W 1  to W 6  rows of the thermoelectric module  1 . The optical modulator  12 , which generates less heat than the LD  11 , is installed correspondingly to W 9  to W 14  rows of the thermoelectric module  1 . 
     As described above, the LD  11  has the larger generation amount in spite of the smaller installation area. Accordingly, by densely providing 18 pairs of the thermoelectric elements  2  and  3  in the area corresponding to W 1  to W 6  rows, the thermoelectric elements  2  and  3  can reliably absorb the heat from the LD  11  to favorably cool the LD  11 . 
     In contrast, since the optical modulator  12  generates less heat in spite of the large installation area in W 9  to W 14  rows, the heat from the optical modulator  12  can be sufficiently absorbed even in a sparse layout in which thermoelectric elements  2  and  3  are not provided in W 11  and W 12  rows. Further, since no object to be cooled is present in an area between the LD  11  and the optical modulator  12 , the thermoelectric elements  2  and  3  are not provided also in W 7  and W 8  rows corresponding to such an area, resulting in a sparse layout. 
     As described above, in the thermoelectric module  1  of this exemplary embodiment, the thermoelectric elements  2  and  3  are densely or sparsely provided in accordance with installation layouts of the LD  11  and the optical modulator  12  which are to be cooled. Accordingly, endothermic efficiency of the installed thermoelectric elements  2  and  3  can be improved and the thermoelectric module  1  can be reduced in cost by eliminating excessive thermoelectric elements  2  and  3 . 
     Because the thermoelectric elements  2  and  3  are not present in W 7  and W 8  rows at the center of the substrates  4 A and  4 B, the substrates  4 A and  4 B are warped starting from a position offset from the center thereof toward an outside of the longitudinal direction thereof Accordingly, displacements at both ends and stress at the center can be lowered, thereby suppressing damage on the substrates  4 A and  4 B. 
     In this exemplary embodiment, the first electrodes  51  are applied in a manner to bridge over the area where the thermoelectric elements  2  and  3  are sparse. However, an electrode pattern having the above first electrodes  51  is applied only for the upper substrate  4 A. On the lower substrate  4 B, the fourth electrodes  54  are provided correspondingly to the maximum placement number of the thermoelectric elements  2  and  3 . 
     In other words, in a thermoelectric module of a different design in which the thermoelectric elements  2  and  3  are located in consideration of warpage of the substrates  4  and the like as described in the second and other exemplary embodiments, by providing such different-shaped first electrodes  41  only on the upper substrate  4 A, it is only necessary to alter the electrode pattern of the upper substrate  4 A according to the design and to constantly use the lower substrate  4 B having the same electrode pattern. Accordingly, even if the thermoelectric module has the different design, the substrate  4 B can be used in common by changing the substrate  4 A, thereby further reducing a manufacturing cost. 
     Second Exemplary Embodiment 
     A second exemplary embodiment of the invention will be described below with reference to  FIGS. 6 and 7 . 
     In the following description, the same structure and components as those in the first exemplary embodiment are indicated by the same reference symbols or numerals for omitting or simplifying the detailed description thereof. The electrode pattern of the fourth electrodes  54  formed on the lower substrate  4 B is the same as that in the first exemplary embodiment The same applies for other exemplary embodiments described below. 
     In the first exemplary embodiment, the first electrodes  51  are provided at 12 positions on the upper substrate  4 A in a manner to bridge over an area corresponding to a pair of the thermoelectric elements  2  and  3 . However, in the second exemplary embodiment, the fifth electrodes  55  are formed at four positions as a bypass electrode to bridge over an area occupied by three pairs of the thermoelectric elements  2  and  3  as shown in  FIGS. 6 and 7 . This is a difference between the first and the second exemplary embodiments. 
     Specifically, in the second exemplary embodiment, the thermoelectric elements  2  and  3  are eliminated by 12 pairs (in L 2  to L 5  lines of W 7  to W 12 ) and 29 pairs of the thermoelectric elements  2  and  3  are provided between the substrates  4 A and  4 B. The area where the thermoelectric elements  2  and  3  are eliminated is closer to the optical modulator  12  than the substrates  4 A and  4 B and requires relatively gentle absorption of heat. 
     Moreover, the area where the thermoelectric elements  2  and  3  are eliminated is a bypass-electrode aggregation portion  55 A in which four fifth electrodes  55  are aggregated. A circumference of the fifth electrodes  55  is surrounded by the second electrodes. In other words, a circumference of the sparse area is surrounded by the thermoelectric elements  2  and  3 . The fifth electrodes  55  electrically connect the thermoelectric elements  2  and  3  provided in W 6  and W 13  rows to each other in L 2  to L 5 . 
     According to the thermoelectric module  1  of the second exemplary embodiment, the following advantages can be obtained in addition to the same advantages of the first exemplary embodiment. 
     In this exemplary embodiment, a plurality of different-shaped fifth electrodes  55  are aggregated in a predetermined area which requires gentle absorption of heat, while, in a manner to correspond to the area, an area where the thermoelectric elements  2  and  3  are eliminated is formed. The area and the bypass-electrode aggregation portion  55 A corresponding to the area are respectively surrounded by the second electrodes  52  and the thermoelectric elements  2  and  3 . Accordingly, although the number of the pairs of the thermoelectric elements  2  and  3  in use is the same as that of the first exemplary embodiment, increase in stress caused by warpage of the substrates  4 A and  4 B can be more reliably prevented than in the first exemplary embodiment. 
     In this exemplary embodiment, in case of warpage of the substrates  4 A and  4 B, starting points are W 6  and W 13  rows. A distance between the starting points and outer circumferences of the substrates  4 A and  4 B is shorter, particularly on the right, than a distance between a center and the outer circumference of the substrates  4 . Accordingly, a displacement by warpage on the right can be reliably reduced, thereby decreasing the stress at the center. Further, against warpage of the substrates  4 A and  4 B, the second electrodes  52  provided in W 6  to W 13  rows of L 1  line are favorably balanced with the second electrodes  52  provided in W 6  to W 13  rows of L 6  line, thereby significantly decreasing the displacement of warpage. 
     Third Exemplary Embodiment 
     A third exemplary embodiment of the invention will be described below with reference to  FIGS. 8 and 9 . 
     In the third exemplary embodiment, six sixth electrodes  56  are provided as a bypass electrode that bridges over an area corresponding to two pairs of the thermoelectric elements  2  and  3 . 
     Accordingly, in the third exemplary embodiment, the thermoelectric elements  2  and  3  are eliminated by 12 pairs (W 7  to W 10  rows) and 29 pairs of the thermoelectric elements  2  and  3  are provided between the substrates  4 A and  4 B. As in the first and second exemplary embodiments, the bypass electrodes are also provided correspondingly to the area where the thermoelectric elements  2  and  3  are eliminated. 
     The sixth electrodes  56  electrically connect a single pair of the thermoelectric elements  2  and  3  to each other which is provided in a manner to bridge over the eliminated thermoelectric elements  2  and  3 . Specifically, in L 1  to L 6 , the sixth electrodes  56  connect the thermoelectric elements  2  and  3  provided in W 6  and W 11  rows. 
     In the thermoelectric module  1  of the third exemplary embodiment, the thea inoelectric elements  2  and  3  are more densely provided in the area of W  11  to W 14  corresponding to the optical modulator  12  than in the second exemplary embodiment, so that an endothermic efficiency of the optical modulator  12  can be improved. Although not to the extent of the second exemplary embodiment, the substrate warpage can be reduced near the optical modulator  12 , thereby efficiently preventing warpage. 
     Fourth Exemplary Embodiment 
     A fourth exemplary embodiment of the invention will be described below with reference to  FIGS. 10 and 11 . 
     In the fourth exemplary embodiment, the first electrodes  51  used in the first exemplary embodiment are provided at six positions and the fifth electrodes  55  used in the second exemplary embodiment are provided at two positions. 
     Also in the fourth exemplary embodiment, 12 pairs (L 1  to L 6  of W 7  and W 8  rows, L 2  to L 5  of W 11  and W 12  rows and L 3  to LA of W 9  and W 10  rows) of the thermoelectric elements  2  and  3  are eliminated and 29 pairs of the thermoelectric elements  2  and  3  are used. The first electrodes  51  and the fifth electrodes  55  are provided correspondingly to the area where the thermoelectric elements  2  and  3  are eliminated. 
     According to the thermoelectric module  1  of the fourth exemplary embodiment as described above, an endothermic property and a warpage prevention property, which are substantially intermediate between those in the second and the third exemplary embodiments, are obtained by the electrode pattern applied on the upper substrate  4 A. 
     Fifth Exemplary Embodiment 
     A fifth exemplary embodiment of the invention will be described below with reference to  FIGS. 12 and 13 . 
     A distinctive feature of the fifth exemplary embodiment is that the first electrodes  51  of the first exemplary embodiment are provided at two positions, the sixth electrode  56  of the third exemplary embodiment are provided at four positions and seventh electrodes  57  are provided at two positions as an L-shaped bypass electrode to bridge over a single pair. 
     Also in the fifth exemplary embodiment, 12 pairs (L 1 , L 3 , L 4  and L 6  of W 9  and W 10  rows, L 1  to L 6  of W 7  and W 8  rows and  12  and L 5  of W 13  and W 14  rows) of the thermoelectric elements  2  and  3  are eliminated. In other words, 29 pairs of the thermoelectric elements  2  and  3  are provided between the substrates  4  in the same manner as in the first to fourth exemplary embodiments. 
     The seventh electrodes  57  connect the thermoelectric elements  2  and  3  in W 12  row of L 2  line and those in W 14  of L 3  line as well as the thermoelectric elements  2  and  3  in W 12  row of L 5  line and those in W 14  of L 4  line, the seventh electrodes  57  being formed in an L-shaped to bridge over the area where the theunoelectric elements  2  and  3  are eliminated. 
     According to the thermoelectric module  1  of the fifth exemplary embodiment, substantially the same advantages as in the fourth exemplary embodiment can be obtained in spite of the different electrode pattern of the upper substrate  4 A. 
     Although the best configuration, methods and the like for implementing the invention has been disclosed above, the invention is not limited thereto. 
     For instance, in the above exemplary embodiments, 12 pairs of the thermoelectric elements  2  and  3  are eliminated from the maximum placement number thereof, whereby 29 pairs of the thermoelectric elements  2  and  3  are provided irrespective of differences of the endothermic efficiency and the warpage property. However, the maximum placement number of the thermoelectric elements  2  and  3  and the number of the thermoelectric elements  2  and  3  to be eliminated (the actual placement number) can be defined as needed and not limited to the above exemplary embodiments. What is necessary is that a substrate including electrodes provided correspondingly to the maximum placement number of the thermoelectric elements  2  and  3  is used in common to the thermoelectric module having different designs. 
     Though the thermoelectric elements  2  and  3  are series-connected via respective electrodes  51  to  57  in the above exemplary embodiments, the thermoelectric elements  2  and  3  may be parallel-connected. 
     In the above exemplary embodiments, the first, fifth, sixth and seventh electrodes  51 ,  55 ,  56  and  57  serving as a bypass electrode are formed to bridge over the plurality of the theimoelectric elements  2  and  3 , but may be formed to bridge over a single pair of thermoelectric elements  2  and  3 . 
     In the above exemplary embodiments, the bypass electrodes are formed on the upper substrate  4 A and the fourth electrodes  54  of the maximum placement number are formed on the lower substrate  4 B. Alternatively, the bypass electrodes may be formed on the lower substrate  4 B and the fourth electrodes  54  of the maximum placement number may be formed on the upper substrate  4 A. 
     As shown in  FIG. 7 , in the second exemplary embodiment, a plurality (four in the second exemplary embodiment) of the fifth electrodes  55  serving as a bypass electrode are aggregated and the second and third electrodes  52  and  53  are provided in a manner to entirely surround the plurality of the fifth electrodes  55 . However, the number of the bypass electrodes surrounded by the second and third electrodes  52  and  53  may be defined as needed. In the aspect of the invention, the second and third electrodes  52  and  53  may surround an entirety of the plurality of the aggregated bypass electrodes or may surround even a single bypass electrode.