Abstract:
The object of the present invention is to provide a thermal head wherein a satisfactory image free of unevenness in printing density can be acquired by suitably correcting voltage drop in the common electrode of the thermal head and heating an individual heater element at uniform temperature. The thermal head according to the invention is characterized in that a common electrode comprised of a conductive layer and a base electrode layer is formed on most of the surface of a heat insulating layer, a separate electrode and a heater element are provided on the common electrode via a layer insulating layer and the edge of the base electrode layer is suitably overlapped with the conductive layer. Also, the thermal head according to the invention is characterized in that the conductive layer is divided every heater element and suitable electric resistance is applied to divided each conductive layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal head, particularly relates to a thermal head wherein a conductive layer electrically connected to a common electrode is formed on a heat insulating layer and a heater element is laminated on the conductive layer via a layer insulating layer. 
     2. Description of the Prior Art 
     Heretofore, a general thermal head as a recording head mounted in a thermal printer and others records on thermosensitive recording paper in colors by arraying plural heater elements composed of a heating resistor in one or plural columns on a substrate and selectively energizing and heating each heater element according to record information or prints on various record media by melting ink of an ink ribbon and transferring it on a record medium such as plain paper and paper for an overhead projector (OHP). 
     Such a conventional type thermal head is provided with a substrate  124  made of a ceramic or silicon and others and having an insulating layer as shown in FIGS. 24 and 25, and a convex portion  124   a  the section of which is approximately trapezoidal is formed in the vicinity of the right end of the upper surface of the substrate  124  shown in FIG.  25 . 
     Also, a heat insulating layer  125  is formed on the upper surface of the substrate  124  and plural heater elements  126  formed in a desired pattern by executing plasma etching by gas acquired by mixing fluoric gas and gaseous oxygen after a heating resistor made of cermet such as Ta—SiO 2  is formed by sputtering and others and hereby, arrayed in a row according to the number of dots corresponding to desired resolution are provided on the upper surface of the heat insulating layer  125  on the convex portion  124   a.    
     Further, a common electrode  127  is formed on the right side of each heater element  126  shown in FIG.  25  and is formed in a U shape along the outside margin of a thermal head  123  as shown in FIG.  24 . 
     A common electrode terminal  127   a  is formed at two ends of the right and left outside margins of the common electrode  127 . A terminal plating layer  127   b  is applied to the upper surface of the common electrode terminal  127   a.    
     In the meantime, plural separate electrodes  128  for independently energizing each heater element  126  are patterned above the left side of the heater element  126  shown in FIG. 25. A connecting electrode  129  on the separate electrode side for electrically connecting the separate electrode  128  and the heater element  126  is formed between the separate electrode  128  and the heater element  126 . The left end of the separate electrode  128  shown in FIG. 25 is equivalent to a separate electrode terminal  128   a  and a terminal plating layer  128   b  is applied to the upper surface of the separate electrode terminal  128   a.    
     The common electrode  127  and the separate electrode  128  are made of Al or Cu and others, are formed in a predetermined pattern by etching and others after Al or Cu is deposited on the upper surface of the heater element  126  and the heat insulating layer  125  by sputtering and others until Al or Cu is approximately 2 μm thick, and a part between the common electrode  127  and the separate electrode  128  of each heater element  126  is equivalent to a heating part  126   a.    
     Also, a passivation layer  130  for preventing oxidation and abrasion is formed on/over the upper surface of the heat insulating layer  125 , the heater element  126 , the common electrode  127  and the separate electrode  128 . The passivation layer  130  is made of SIALON (Si—Al—O—N compound solid solution) excellent in oxidation resistance and abrasion resistance and is laminated by sputtering so that any upper surface except the separate electrode terminal  128   a  and the common electrode terminal  127   a  is coated. 
     In the conventional type thermal head  123  having configuration described above, predetermined recording is performed by selectively heating the heater element  126  by selectively energizing between the common electrode  127  and the separate electrode  128 , melting ink of an ink ribbon touched to the part and transferring it on recording paper or directly coloring thermosensitive recording paper. 
     Also, in the conventional type thermal head  123 , as described above, the convex heat insulating layer  125  is provided near to the end of the substrate  124  with the layer biased, the heating part  126   a  is formed on the convex heat insulating layer  125  and the quality of recording is enhanced by using such a thermal head  123 . 
     However, the conventional type thermal head  123  has a problem that as the common electrode  127  is extremely elongated in a narrow part between the end of the substrate  124  and the heater element  126  because the respective heater elements  126  are formed near the end of the substrate  124  as shown in FIG. 24, the resistance value of the electrode is increased even if the common electrode  127  is made of copper or aluminum, so-called voltage drop occurs in the common electrode  127  and dispersion occurs in exoergic temperature among the heater elements  126 . 
     Further, the length of an energizing path from the heater element  126  to the common electrode terminal  127   a  differs every heater element  126  and the heater element  126  near the center of the row is the farthest from the common electrode terminal  127   a . Therefore, the quantity of voltage drop in the common electrode  127  connected to the heater element  126  near the center of the row is increased and the exoergic temperature of the heater element  126  near the center of the row sometimes lowers. 
     In thermal imprint printing for example when voltage drop occurs in such a thermal head and the exoergic temperature of the heater element  126  lowers, there is a problem that the quality of printing is deteriorated such as the unevenness of density in printing is increased. 
     For a countermeasure against voltage drop, it is conceivable to reduce the quantity of voltage drop by thickening the thickness of the common electrode  127 , however, when the thickness of the common electrode  127  is thickened, it becomes difficult to laminate the passivation layer  130  on the common electrode  127 , in addition, in printing, the pressure of the thermal head  123  and a platen of a printer not shown concentrates at the heating part  126   a  and the common electrode  127  adjacent to the heating part  126   a  and large shearing stress is applied to the common electrode  127 . 
     Therefore, there is a defect that material fatigue (metal fatigue) occurs in the common electrode  127  made of soft metallic material such as Al and Cu, the passivation layer  130  is peeled or destroyed early, disconnection due to abrasion and corrosion occurs in the common electrode  127  and the life is short. 
     Also, voltage drop is inhibited and the difference of exoergic temperature in the position of each heater element  126  can be removed by using a well-known thermal head called an alternate lead head adjacent to the row of each heater element  126  and provided with plural adjacent common electrodes, however, in this case, there is also a defect that as the adjacent common electrode is formed between adjacent heater elements, a dimension between heater elements is increased, a dimension between dots of a printed image is increased and high definition printing is difficult. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a thermal head wherein voltage drop in a common electrode of the thermal head is properly corrected, a separate heater element is heated at uniform temperature and a satisfactory image free of unevenness in printing density can be acquired. 
     That is, the thermal head according to the invention is characterized in that it is provided with a heat insulating layer formed on the upper surface of a substrate, plural heater elements formed in a row over the heat insulating layer, plural separate electrodes respectively electrically connected to one and the other of the plural heater elements, and a common electrode, the common electrode is composed of a base electrode layer connected to a common electrode terminal provided on both outsides of the sequence of the separate electrodes and a conductive layer connected to the base electrode layer and formed on the side of the heat element, the common electrode is formed on the surface of most of the heat insulating layer, a layer insulating layer for electrically insulating the common electrode and the separate electrode is formed on the common electrode, the separate electrode and the heater element are formed on the layer insulating layer and the edge of the base electrode layer is provided opposite to the edge of the heater element in a direction of the surface of the substrate. 
     The object of the invention is to increase the area of the common electrode from the common electrode terminal to the heater element, to inhibit voltage drop in the common electrode and to uniformly heat each heater element and acquire a satisfactory printed image free of unevenness in printing density. 
     Further, the thermal head according to the invention is characterized in that the edge of the base electrode layer opposed to the heater element is linearly formed in parallel to the row of the heater elements. 
     The object of the invention is to effectively reduce the electrical resistance of the common electrode and to suitably inhibit voltage drop in the common electrode. 
     Further, the thermal head according to the invention is characterized in that the edge opposed to the heater element of the base electrode layer is put close to the side of the heater element in the vicinity of the center of the row of the heater elements. 
     The object of the invention is to reduce distance from the edge of the base electrode layer to the vicinity of the center of the row of the heater elements and to suitably inhibit voltage drop in the common electrode. 
     Also, further, the thermal head according to the invention is characterized in that the edge of the base electrode layer is protruded in a triangle or in an arc. 
     The object of the invention is to facilitate protrusion to put the edge of the base electrode layer close to the side of the heater element. 
     Also, the thermal head according to the invention is characterized in that the edge opposed to the heater element of the base electrode layer is put far from the heater element in the vicinity of the center of the row of the heater elements. 
     The object of the invention is to reduce heat reserve in the vicinity of the center of the row of the heater elements and to enable high quality of printing even if continuous printing is executed for a long time. 
     Further, the thermal head according to the invention is characterized in that the edge of the base electrode layer is formed in a triangle or with the edge concave in an arc. 
     The object of the invention is to hollow the edge of the base electrode layer so that it is put far from the heater element. 
     Also, the thermal head according to the invention is characterized in that it is provided with a heat insulating layer formed on the upper surface of a substrate, plural heater elements formed in a row over the heat insulating layer and plural separate electrodes electrically connected to one of the heater element, plural conductive layers formed in parallel in the same direction as the separate electrode are provided on the upper surface of the heat insulating layer under the separate electrodes, a layer insulating layer for insulating the conductive layer and the separate electrode is provided between the separate electrode and the conductive layer, the conductive layer is formed so that the resistance value of the conductive layer on the side of the end is larger than that of the conductive layer in the vicinity of the center of the row of the heater elements and a base electrode layer and the heater element are respectively electrically connected to one and the other of the conductive layer and the heater element is energized. 
     The object of the invention is to correct voltage drop in the conductive layer, to apply constant voltage to each heater element, to heat each heater element at uniform temperature and to acquire a satisfactory printed image free of unevenness in printing density. 
     Further, the thermal head according to the invention is characterized in that the conductive layer is composed of a pair of good conductors mutually opposite with clearance having a predetermined dimension between them and a resistor formed in the clearance and the resistance value of the conductive layer on the side of the end is increased, compared with that in the vicinity of the center by differentiating the length of the clearance. 
     The object of the invention is to correct voltage drop in the conductive layer and to uniform the heat of each heater element in the vicinity of the center of the row of the heater elements and on the side of the end. 
     Furthermore, the thermal head according to the invention is characterized in that the clearance is extended from the vicinity of the center to the side of the end. 
     The object of the invention is to sequentially increase the resistance value of the conductive layer from the vicinity of the center of the row of the heater elements to the side of the end and to further securely correct voltage drop in the conductive layer. 
     Also, the thermal head according to the invention is characterized in that the conductive layer is provided with plural groups of good conductors each of which is composed of adjacent plural good conductors on the side of the end and adjacent plural good conductors in the group of good conductors are connected via the resistor. 
     The object of the invention is to facilitate the manufacture by collecting adjacent plural good conductors and forming a resistor with every group of collected good conductors. 
     Further, the thermal head according to the invention is characterized in that plural groups of good conductors are formed from the vicinity of the center to the side of the end, in each group of good conductors, the length of the clearance between good conductors is equalized and the clearance between groups of good conductors is elongated from the vicinity of the center to the side of the end. 
     The object of the invention is to facilitate the manufacture without forming a resistor every conductive layer. 
     Also, the thermal head according to the invention is characterized in that the conductive layer on the side of the end is composed of a narrow part and a wide part and the resistance value of the conductive layer on the side of the end is increased, compared with that in the vicinity of the center by differentiating the length of the narrow part. 
     The object of the invention is to facilitate the manufacture without forming a resistor on the conductive layer. 
     Further, the thermal head according to the invention is characterized in that the narrow part is elongated from the vicinity of the center to the side of the end. 
     The object of the invention is to sequentially increase the resistance value of the conductive layer from the vicinity of the center of the row of the heater elements to the side of the end and to further securely correct voltage drop in the conductive layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan showing an embodiment of a thermal head according to the invention; 
     FIGS. 2A and 2B are sectional views showing the thermal head viewed along a line  2 — 2  in FIG. 1; 
     FIG. 3 is a sectional view showing the thermal head viewed along a line  3 — 3  in FIG. 1; 
     FIG. 4 is a schematic drawing for explaining a method of manufacturing the thermal head according to the invention; 
     FIG. 5 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 6 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 7 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 8 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 9 is a schematic drawing for explaining another embodiment of the invention; 
     FIG. 10 is a schematic drawing for explaining another embodiment of the invention; 
     FIG. 11 is a schematic drawing for explaining another embodiment of the invention; 
     FIG. 12 is a schematic drawing for explaining another embodiment of the invention; 
     FIG. 13 is a schematic drawing for explaining another embodiment of the invention; 
     FIG. 14 is a plan showing another embodiment of the thermal head according to the invention; 
     FIG. 15 is a sectional view viewed along a line  15 — 15  in FIG. 14; 
     FIG. 16 is a sectional view viewed along a line  16 — 16  in FIG. 14; 
     FIG. 17 is a schematic drawing for explaining a method of manufacturing the thermal head according to the invention; 
     FIG. 18 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 19 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 20 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 21 is a schematic drawing for explaining the method of manufacturing the thermal head according to the invention; 
     FIG. 22 is a plan showing another embodiment of the thermal head according to the invention; 
     FIG. 23 is a plan showing the other embodiment of the thermal head according to the invention; 
     FIG. 24 is a plan showing a conventional type thermal head; and 
     FIG. 25 is a sectional view viewed along a line  25 — 25  in FIG.  24 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 to  9 , an embodiment of the invention will be described below. FIGS. 1 to  3  are schematic drawings for explaining a thermal head according to the invention, FIGS. 4 to  8  are schematic drawings for explaining a manufacturing method according to the invention and FIGS. 9 to  13  are schematic drawings showing other embodiments of the invention. 
     In FIG. 1, a thermal head  1  provided with heater elements  10  for five dots extremely smaller in the number for the convenience of explanation, compared with the actual number is shown as an example. 
     First, the thermal head  1  equivalent to an embodiment of the invention is provided with an approximately flat substrate  4  on the lower side as shown in FIG. 2. A convex portion  4   a  having an approximately trapezoidal section is formed in the vicinity of the right end of the substrate  4  as shown in FIG.  2 B. 
     A heat insulating layer  5  is formed on the upper surface of the substrate  4 . The heat insulating layer  5  is mainly made of high melting point glass as heretofore. 
     A common electrode  6  is formed on the upper surface of the heat insulating layer  5 . The common electrode  6  is formed in a range from the end (hereinafter called the end on the side of the heater element) on the side where the heater element  10  shown in FIG. 2B is formed to the vicinity of the end (hereinafter called the end on the side of an electrode terminal) on the side where a common electrode terminal  14  shown in FIG. 3 and a separate electrode terminal  18  shown in FIG. 2A are formed via over the convex portion  4   a.    
     Also, the common electrode  6  is composed of the common electrode terminal  14  placed side by side with plural separate electrodes  16  described later formed in parallel and formed on both outsides of the separate electrodes  16 , a base electrode layer  8  connected to the common electrode terminal  14  and a conductive layer  7  formed on the side of the heater element  10  described later and connected to the base electrode layer  8 . The base electrode layer  8  and the conductive layer  7  occupy most of the surface of the heat insulating layer  5 . 
     The conductive layer  7  is formed so that it has uniform thickness in a range from the end on the side of the heater element before the end on the side of the electrode terminal on the upper surface of the heat insulating layer  5  as shown in FIG.  2 . The conductive layer  7  is formed thinly to reduce an effect upon a temperature dependence characteristic and it is desirable that the conductive layer has the thickness of approximately 0.15 μm. Also, as the conductive layer  7  is arranged under the heater element  10  that emits high temperature, it is formed by high melting point metal not melted and not damaged by the exoergic temperature of the heater element  10  such as Cr, Mo, Ti, Zr, Ta, Nb, W and Hf respectively having the melting point of 1500° C. or higher. Further, the width of the conductive layer  7  in a direction in which the heater elements  10  are arrayed (in a lateral direction in FIG. 1) is formed so that it is slightly wider than the width of the row of the heater elements  10  as shown in FIG.  8 . 
     Also, the base electrode layer  8  is formed rectangularly having an edge  8   a  which is the front end and a rear edge  8   b  respectively formed linearly as shown in FIG. 8, the edge  8   a  is laminated on the edge of the conductive layer  7 , and the conductive layer  7  and the base electrode layer  8  are electrically connected. 
     The base electrode layer  8  is formed on the upper surface of the heat insulating layer  5  in a range including a terminal connection  12 . 
     Also, the linear edge  8   a  of the base electrode layer  8  is opposed to the respective edges  10   a  of the plural heater elements  10  in a horizontal direction via a layer insulating layer  9  described later as shown in FIG.  8 . 
     The base electrode layer  8  is formed by metal having low resistivity and it is desirable that the base electrode layer is formed by metal having the resistivity of 1×10 −7  Ωm or lower such as Al, Cu, Au and Ni so that the thickness is approximately 1.0 to 3.0 μm and is thicker than that of the conductive layer  7 . The width in a lateral direction shown in FIG. 8 of the base electrode layer  8  is formed so that it is wider than that of the conductive layer  7  in the same direction, the base electrode layer  8  is formed up to the terminal connection  12  formed along each end face of both right and left sides of the substrate  4  and is connected to the common electrode terminal  14  described later. 
     A layer insulating layer  9  is formed on the upper surface of the conductive layer  7  and the base electrode layer  8 . The layer insulating layer  9  is formed by the oxide of silicon or nitrogen oxide to secure insulation so that the thickness is approximately 1.5 μm. 
     The heater element  10  made of Ta—SiO 2  is laminated in a position corresponding to the convex portion  4   a  on the upper surface of the layer insulating layer  9 . The heater elements  10  are connected to a connecting electrode  15  shown in FIG. 1 according to the number of dots corresponding to the desired resolution of the thermal head  1  and the size and are arrayed in a lateral direction. 
     The connecting electrode  15  is formed by conductive high melting point metal material such as Mo, Cr and Ti on the upper surface of the conductive layer  7  exposed from the end on the side of the heater element of the layer insulating layer  9 . 
     One end of the connecting electrode  15  is connected to the conductive layer  7  of the common electrode  6  on the lower side, the other end is connected to the heater element  10  on the upper side, and the conductive layer  7  and the heater element  10  are electrically connected with the layer insulating layer  9  between them. 
     A range in the lateral direction shown in FIG. 8 in which the connecting electrode  15  is formed is substantially equal to a range in the same direction in which the conductive layer  7  is formed. 
     Further, the terminal connection  12  shown in FIG. 3 is laminated on the base electrode layer  8  on the side of the common electrode terminal of the layer insulating layer  9 , placed side by side with plural separate electrodes  16  described later formed in parallel and on both outsides of the separate electrodes  16 . A side electrode  13  made of conductive satisfactory metal such as Al is formed on the upper surface of the base electrode layer  8  exposed by the terminal connection  12 . 
     The left end shown in FIG. 3 of the side electrode  13  is equivalent to the common electrode terminal  14 , a terminal plating layer  14   a  is applied on the common electrode terminal  14  and the terminal plating layer  14   a  is exposed from the passivation layer  19  described later. 
     Therefore, the base electrode layer  8  and the common electrode terminal  14  are electrically connected by the terminal connection  12 . 
     As described above, the common electrode  6  is formed on the surface of most of the heat insulating layer  5 , however, the common electrode is not limited to this and has only to be in such a shape as a conductive path on which distance between each heater element  10  and the common electrode terminal  14  is substantially uniform is formed. 
     The separate electrodes  16  made of conductive satisfactory metal such as Al and Cu are formed in a range on the upper surface of the layer insulating layer  9  from the vicinity of the left end of the heater elements  10  shown in FIG. 2B to the left end shown in FIG.  2 A. 
     The separate electrodes  16  are respectively electrically connected to the heater elements  10  via a connecting electrode on the separate electrode side  17  made of high melting point metal such as Mo, Cr and Ti. Further, the left end shown in FIG. 2A of the separate electrodes  16  is equivalent to the separate electrode terminal  18 . The terminal plating layer  18   a  is applied on the separate electrode terminal  18  and is exposed from the passivation layer  19 . 
     The passivation layer  19  for protecting the heater element  10 , the separate electrode  16 , the connecting electrode  15 , the connecting electrode on the separate electrode side  17 , the layer insulating layer  9 , the side electrode  13 , the conductive layer  7  and the heat insulating layer  5  from oxidation or abrasion is formed on/over them. The passivation layer  19  is made of SIALON (Si—Al—O—N compound solid solution) satisfactory in oxidation resistance and abrasion resistance or Si—O—N compound and others. 
     Next, referring to FIGS. 4 to  8 , a method of manufacturing the thermal head  1  according to the invention will be described. 
     In the method of manufacturing the thermal head  1  equivalent to the embodiment of the invention, plural thermal heads  1  are simultaneously formed on one large substrate  4  as heretofore though it is not shown and the plural thermal heads  1  can be simultaneously acquired by dividing the large substrate  4  in desired positions. 
     In such a method of manufacturing the thermal head  1 , first in a process for forming a substrate, silicon formed flatly is etched as heretofore to form a substrate  4  having a convex portion  4   a , next in a process for forming a heat insulating layer, a heat insulating layer  5  having approximately similar thickness shown in FIG. 4 made of the oxide of silicon, transition metals and oxygen is formed on the upper surface of the substrate  4  as heretofore. 
     Next, first in a process for forming a conductive layer in a process for forming a common electrode, after high melting point metal having the melting point of 1500° C. or higher such as Cr, Mo, Ti, Zr, Ta, No, W and Hf is deposited by sputtering and others until the metal has the uniform thickness of approximately 0.15 μm on/over the substrate  4  and the heat insulating layer  5 , the conductive layer  7  having a substantially rectangular shape shown in FIG. 5 as a part of a common electrode  6  of a desired pattern is formed by etching. At this time, as an unnecessary short circuit is prevented by forming the peripheral edge of the conductive layer  7  so that it is smaller than the peripheral edge of the substrate  4 , it is desirable. 
     Next, in a process for forming a base electrode layer included in the process for forming the common electrode, after metal satisfactory in conductivity having the resistivity of 1×10 −7  Ωm or lower such as Al, Cu, Au and Ni is deposited on the heat insulating layer  5  and as a part of the conductive layer  7  by sputtering and others until the metal has the uniform thickness of approximately 1 to 3 μm, the base electrode layer  8  shown in FIG. 6 as a part of the common electrode  6  is formed by etching. 
     As for the base electrode layer  8 , the shape having the upper edge  8   a  on the front side shown in FIG.  6  and the lower rear edge  8   b  is formed in a rectangle and the width in the lateral direction is formed so that it is wider than the width of the conductive layer  7 . 
     Next, in a process for forming a layer insulating layer, the layer insulating layer  9  having a connection on a common electrode side  15   a  and a terminal connection  12  is formed on the upper surface of the conductive layer  7  and the base electrode layer  8 . 
     To explain the process for forming the layer insulating layer further detailedly, first, in a film forming process, material such as SiO 2  or Si—O—N compound is deposited on the overall upper surface of the common electrode  6  composed of the conductive layer  7  and the base electrode layer  8  by well-known sputtering or CVD and others until the material has the uniform thickness of approximately 1 to 3 μm so as to form the layer insulating layer  9  shown in FIG.  7 . 
     Next, in an etching process, after photoresist is applied to the overall upper surface of the layer insulating layer  9  by a spinner and others to form a resist film, the layer insulating layer  9  is etched using buffered hydrofluoric acid (BHF) and others to form the connection on the common electrode side  15   a  and the terminal connection  12  shown in FIG. 8 in respective desired positions. 
     Next, in a process for forming a heater element, after a desired heating resistor made of Ta 2  N or Ta—SiO 2  and others is formed by sputtering and others as heretofore so that the heating resistor has the uniform thickness of approximately 0.3 μm, etching by photolithography is applied and plural heater elements  10  in a predetermined shape arrayed according to the number of predetermined dots corresponding to resolution are laminated. 
     At this time, each heater element  10  is insulated from the conductive layer  7  because of the layer insulating layer  9 . 
     Next, in a process for forming a connecting electrode, high melting point metal such as Mo, Cr and Ti is deposited by sputtering and others, and the connecting electrode  15  in a desired pattern for electrically connecting one side of the heater element  10  and the conductive layer  7  is formed by etching and others. 
     Next, in a process for forming a connecting electrode on the separate electrode side, high melting point metal made of the same material as the connecting electrode  15  is deposited by sputtering and others on the other side of the heater elements  10  and the layer insulating layer  9  in the vicinity of the other side and the connecting electrode on the separate electrode side  17  in a desired pattern for electrically connecting the heater element  10  and a separate electrode  16  formed in a process for forming a separate electrode described later is formed by etching and others. 
     Next, in a process for forming the separate electrode, after Al and others are deposited on the layer insulating layer  9  by sputtering and others until they have the thickness of approximately 1 to 3 μm for example, the separate electrode  16  in a desired pattern connected to each heater element  10  for independently energizing it is formed on one side (the left side shown in FIG. 2B) of the connecting electrode on the separate electrode side  17 . 
     Next, in a process for forming a side electrode, after metal having satisfactory conductivity such as Al is deposited by sputtering and others until the metal has the uniform thickness of approximately 1 to 3 μm for example, the side electrode  13  in a desired pattern electrically connected to the base electrode layer  8  inside the terminal connection  12  is formed by well-known etching. 
     Next, in a process for forming a passivation layer, the passivation layer  19  is formed by depositing material such as SIALON (Si—Al—O—N compound solid solution) or Si—O—N compound on a part except the common electrode terminal  14  and the separate electrode terminal  18  by sputtering, CVD and others until the material has the substantially even thickness of approximately 5 to 10 μm for example and next, in a dicing process, the manufacture of the thermal head  1  equivalent to this embodiment is completed by simultaneously acquiring chipped plural thermal heads  1  by dividing the large substrate  4  in each desired position. 
     A mask layer such a heat-resistant pressure sensitive adhesive tape is applied to the common electrode terminal  14  and the separate electrode terminal  18  when the passivation layer  19  is formed, and the common electrode terminal  14  and the separate electrode terminal  18  are exposed by peeling the mask layer after the passivation layer  19  is formed. 
     Terminal plating layers  14   a  and  18   a  easy in solderability such as Ni—Sn compound are respectively applied to the upper surface of the common electrode terminal  14  and the separate electrode terminal  18 . 
     Next, to explain the action of the embodiment of the invention using a thermal printer as an example, the thermal head  1  equivalent to the embodiment of the invention having the configuration described above is mounted on a carriage of the thermal printer not shown. 
     That is, the thermal head  1  equivalent to the embodiment of the invention melts and transfers or sublimates and transfers ink of an ink ribbon on the heater element  10  on recording paper by pressing the ink ribbon of a ribbon cassette installed on the carriage on the recording paper and selectively energizing the heater elements  10  according to record information, moving the carriage and prints a desired image. 
     To explain further concretely, one end of a cable for energizing not shown is connected to the separate electrode terminal  18  and the common electrode terminal  14  of the thermal head  1  equivalent to the embodiment of the invention and the other end of the cable is connected to an energizing controller not shown in the body of the printer. The energizing controller energizes the separate electrode terminal  18  based upon record information via the cable. 
     Hereby, first, a signal is transmitted from the separate electrode terminal  18  to the separate electrode  16 , the separate electrode  16  energizes the connecting electrode on the separate electrode side  17  and current flows from the connecting electrode on the separate electrode side  17  to the heater element  10 . Current flows from the heater element  10  to the conductive layer  7  under the heater element  10  via the connecting electrode  15 . 
     Next, current flows from the conductive layer  7  to the base electrode layer  8 . At this time, as the conductive layer  7  is approximately 0.15 μm thick and thin though the conductive layer  7  is located immediately under the heater element  10 , an effect upon a temperature dependence characteristic is small and as the conductive layer is made of high melting point metal, energizing is not prevented by the heat of the heater element  10 . 
     Next, the base electrode layer  8  energizes the side electrode  13 , however, at this time, as the base electrode layer  8  is made of metal having low resistivity, is 1 to 3 μm thick and slightly thick, conductivity is satisfactory and the base electrode layer  8  energizes the side electrode  13  via the terminal connection  12  in a state in which the energizing resistance of the base electrode layer  8  in a lateral direction in FIG. 1 can be almost ignored. 
     Current flows to the common electrode terminal  14  via the side electrode  13  and current flows from the common electrode terminal  14  to the side of the printer outside the thermal head  1 . Hereby, as control current according to an instruction from the energizing controller on the side of the printer flows from the separate electrode terminal  18  to the common electrode terminal  14  via the heater element  10 , Joule heat is generated in the heater element  10  and heats up to predetermined temperature. 
     Therefore, according to the embodiment of the invention, as the common electrode  6  made of material different from that of the conductive layer  7  and the base electrode layer  8  is formed under the heater element  10  via the layer insulating layer  9 , an energizing path between the heater element  10  and the common electrode terminal  14  can be reduced, compared with that in a conventional type, in the base electrode layer  8  satisfactory in conductivity, energizing resistance in a direction of the array in the thermal head  1  shown in FIG. 1 can be substantially ignored, energizing resistance as the whole common electrode  6  can be remarkably reduced, compared with that in a conventional type common electrode and voltage drop (common drop) in the common electrode which has been a problem heretofore can be inhibited. 
     Hereby, the thermal head  1  according to the invention can energize the heater elements  10  uniformly without being influenced by the length of the energizing path between the common electrode terminal  14  and the heater element  10 . Therefore, each heater element  10  can be heated at uniform temperature and when printing is performed using the thermal head  1  according to the invention, a satisfactory printed image free of unevenness in printing density can be acquired. 
     Also, as to the thermal head  1  according to the invention, it is explained that the edge  8   a  of the base electrode layer  8  is linearly formed in parallel to the row of the heater elements  10 , however, in such a thermal head  1 , the edge  8   a  of the base electrode layer  8  may be also formed in a triangle shown in FIG. 9 or in an arc shown in FIG. 10 so that the edge  8   a  is put close to the vicinity of the center of the row of the heater elements  10  and the edge  8   a  may be also protruded opposite to the edge  10   a  of the heater element  10  via the layer insulating layer  9  in the direction of the surface of the substrate which is a horizontal direction. 
     In such a thermal head, distance between a base electrode layer  8  and the vicinity of the center of the row of heater elements  10  can be reduced and even if an energizing path between a common electrode terminal  14  and the vicinity of the center of the row of the heater elements  10  is long, the heater elements  10  free of voltage drop in the vicinity of the center of the row of the heater elements  10  can be uniformly energized. 
     Also, in the case of a thermal head used for a printer for continuously printing for a long time, the edge  8   a  of a base electrode layer  8  is put far from heater elements  10  in the vicinity of the center of the row of the heater elements  10  and may be also made hollow in a triangle shown in FIG. 11 or in an arc shown in FIG.  12 . Such a thermal head also enables high quality of printing in continuous printing for a long time because the heat reserve of a substrate in the vicinity of the center of the row of the heater elements  10  is small. 
     Also, a thermal head shown in FIG. 13 wherein the edge  8   a  of a base electrode layer  8  is protruded so that a part opposite to the vicinity of the center of the row of the heater elements  10  is put close to the heater elements, a projection  8   c  is formed and each edge  8   a  on both sides of the projection  8   c  is made hollow in an arc may be also used for a thermal head wherein voltage drop in the vicinity of the center of the row of heater elements  10  is prevented and the heat reserve of a substrate in the vicinity of the center of the row of the heater elements can be also reduced in continuous printing for a long time. 
     Also, referring to FIGS. 14 to  21 , a thermal head equivalent to another embodiment of the invention will be described below. FIGS. 14 to  16  are schematic drawings showing the thermal head equivalent to another embodiment of the invention, FIGS. 17 to  21  are schematic drawings for explaining its manufacturing method, FIG. 22 shows further another embodiment of the invention and FIG.  23  is a schematic drawing showing the other embodiment of the invention. 
     The respective drawings show a thermal head having heater elements for dots extremely smaller in the number for the convenience of explanation, compared with that in an actual thermal head as an example. 
     First, a thermal head  1  equivalent to another embodiment of the invention is provided with a heat insulating layer  35  formed on the upper surface of a substrate  34 , plural heater elements  40  arrayed in a row over the heat insulating layer  35 , plural separate electrodes  46  respectively electrically connected to one and the other of the heater element  40  and a connecting electrode  45  connected to a conductive layer  36  as shown in FIGS. 14 and 15, and plural conductive layers  36  formed in parallel in the same direction as the separate electrode  46  are further provided on the upper surface of the substrate  34  under the separate electrode  46  via a layer insulating layer  39  for electrically insulating from the separate electrode  46 . 
     As for the detailed configuration of the thermal head  1  having the schematic configuration described above, as shown in FIG. 15, the substrate  34  having predetermined thickness is arranged in the lowest part In the substrate  34 , a convex portion  34   a  protruded upward having the substantially trapezoidal section is formed in the vicinity of the right end shown in FIG.  15 . 
     Also, the heat insulating layer  35  is formed on the upper surface of the substrate  34 . The heat insulating layer  35  is mainly made of high melting point glass as heretofore. 
     The patterned plural conductive layers  36  arrayed in parallel corresponding to the number of plural heater elements  40  are formed on the upper surface of the heat insulating layer  35 . 
     As for the conductive layer  36 , as shown in FIG. 19, in the conductive layer  36  on the side of the end in a direction shown by an arrow X, a pair of good conductors  36   b  and  36   c  mutually opposite with each clearance  36   a  having predetermined dimensions A and B between the pair of good conductors are formed. 
     The clearance  36   a  is formed by etching and others so that the length is longer from the vicinity of the center of the columns of the conductive layers to the conductive layer  36  on the side of the end. That is, relationship between the dimensions A and B of the clearance is A&lt;B. 
     The pair of good conductors  36   b  and  36   c  are made of high melting point metal not melted and not damaged by the exoergic temperature of the heat element  40 , for example high melting point metal having the melting point of 1500° C. or higher such as Cr, Mo, Ti, Zr, Ta, Nb, W and Hf so that the thickness is approximately 0.15 μm for example and the pair of good conductors have uniform thickness. 
     In the clearance  36   a  between the pair of good conductors  36   b  and  36   c , a resistor  37  made of Ta—SiO 2  which is the same material as that of the heater element  40  for example is provided, the pair of good conductors  36   b  and  36   c  are electrically connected via the resistor  37  and the conductive layer  36  on the side of the end of the columns is composed. 
     Also, the conductive layer  36  in the vicinity of the center of the columns has no clearance  36   a , the whole is formed by the same material as that of the pair of good conductors  36   b  and  36   c  and the whole length is formed so that it is equal to the length of the conductive layer  36  on the side of the end of the columns having the clearance  36   a.    
     The conductive layer  36  in the vicinity of the center of the columns and the resistor  37  formed in the conductive layer  36  on the side of the end are formed so that they have the same thickness as that of the pair of good conductors  36   b  and  36   c , for example approximately 0.15 μm so as to reduce an effect upon a temperature dependence characteristic. 
     That is, each conductive layer  36  on the side of the end of the columns is composed the pair of good conductors  36   b  and  36   c  mutually opposite with the clearance  36   a  having a predetermined dimension between them and the resistor  37  formed in the clearance  36   a  and the resistance value of the conductive layer  36  on the side of the end is increased by differentiating the length of the clearance  36   a , compared with that of the conductive layer  36  in the vicinity of the center of the columns. 
     Therefore, when the heater element  40  is energized, voltage drop (common drop) in the conductive layer  36  is corrected and voltage drop in the conductive layer  36  can be removed. 
     In the thermal head  31  according to the invention, voltage drop in the conductive layer  36  is corrected, the heater element  40  is heated at uniform temperature and a high quality of printed image free of unevenness in printing density can be acquired. 
     As shown in FIG. 15, a base electrode layer  38  connected to the other end of the conductive layer  36  on the left side is formed on the heat insulating layer  35 , is located in a low part as shown in FIG. 21, the shape is rectangular, is provided with a linear front edge  38   a  and a rear edge  38   b , the front edge  38   a  is laminated on the other end of the conductive layer  36 , and the conductive layer  36  and the base electrode layer  38  are electrically connected. 
     Also, the linear edge  38   a  of the base electrode layer  38  is provided opposite to the respective edges  40   a  of the plural heater elements  40  in a direction of the surface of the substrate which is a horizontal direction via the layer insulating layer  39  as shown in FIG.  21 . 
     The base electrode layer  38  is made of metal having low resistivity and it is desirable that the base electrode layer is made of metal having the resistivity of 1×10 −7  Ωm or lower such as Al, Cu, Au and Ni so that the thickness is approximately 1.0 to 3.0 μm and is thicker than that of the conductive layer  36 . 
     Such a base electrode layer  38  is formed up to a terminal connection  42  formed on the layer insulating layer  39  near to the end face on the right and left both sides of the substrate  34  in a lateral direction in FIG.  21  and is connected to a common electrode terminal  44  via a side electrode  43  described later. 
     The layer insulating layer  39  is formed on the upper surface of the conductive layer  36  and the base electrode layer  38 . The layer insulating layer  39  is made of the oxide of Si or nitrogen oxide so that the thickness is approximately 1.5 μm so as to secure insulation among the conductive layer  36  under it, the heater element  40  on it and the separate electrode  46 . 
     The heater elements  40  made of Ta—SiO 2  are arrayed in the direction shown by the arrow X which is the direction of the row as shown in FIG.  14  and are laminated in a position corresponding to over the convex portion  34   a  on the upper surface of the layer insulating layer  39 . One side of the heater element  40  is electrically connected to the connecting electrode  45  and the other side is electrically connected to the separate electrode  46  via a connecting electrode on the separate electrode side  47  as shown in FIG. 15 according to the number of dots corresponding to the desired resolution and the size of the thermal head  1 . 
     Also, the connecting electrode  45  is made of the same material as the conductive layer  36  or the similar material, as shown in FIG. 15, the lower side is connected to one end of the lower conductive layer  36 , the upper side is connected to one of the upper heater element  40 , and the conductive layer  36  and the heater element  40  with the layer insulating layer  39  between them are electrically connected via the connecting electrode  45 . 
     The connecting electrode  45  is formed in a range in a lateral direction in FIG. 21 having a dimension to which the heater element  40  formed in a position at both ends of the row can be connected. 
     Further, the terminal connection  42  which is a square hole for forming the side electrode  43  described later connected to the base electrode layer  38  is formed on the layer insulating layer  39  at the right and left ends of the base electrode layer  38  shown in FIG.  21 . 
     The terminal connection  42  is provided on the right and left outsides of the parallel plural separate electrodes  46  formed on the layer insulating layer  39  and as shown in FIG. 16, the side electrode  43  made of metal satisfactory in conductivity such as Al is formed on the base electrode layer  38  inside the terminal connection  42 . 
     The side electrode  43  is pulled out on the side of the left end on the layer insulating layer  39  as shown in FIG.  16  and the part is equivalent to the common electrode terminal  44 . Conductive plating is applied on the common electrode terminal  44 , a terminal plating layer  44   a  is formed and is exposed from an upper passivation layer  49  described later. 
     Therefore, the base electrode layer  38  and the common electrode terminal  44  are electrically connected via the side electrode  43 . 
     The separate electrodes  46  are formed on the upper surface of the layer insulating layer  39  and are arrayed in parallel in the same direction as the conductive layers  36  in a range from the vicinity of the left end of the heater element  40  in the drawing on the right side in FIG. 15 to the left end of the substrate  34 . The separate electrode  46  is made of metal satisfactory in conductivity such as Al and Cu. 
     The separate electrode  46  is electrically connected to the heater element  10  via the connecting electrode on the separate electrode side  47  made of high melting point metal such as Mo, Cr and Ti and formed on the other side of the heater element  40 . 
     Further, the separate electrode  46  is pulled out at the left end shown in FIG.  15  and the left end is equivalent to the separate electrode terminal  48 . Conductive plating is applied on the separate electrode terminal  48 , a terminal plating layer  48   a  is formed and the surface of the terminal plating layer  48   a  is exposed from the passivation layer  49 . 
     The passivation layer  49  for protecting the heater element  40 , the separate electrode  46 , the connecting electrode  45 , the connecting electrode on the separate electrode side  47 , the layer insulating layer  39 , the side electrode  43 , the conductive layer  36 , the base electrode layer  38  and the heat insulating layer  35  from oxidation or abrasion and others is formed on/over them as shown in FIG.  15 . The passivation layer  49  is made of SIALON (Si—Al—O—N compound solid solution) or Si—O—N compound satisfactory in oxidation resistance and abrasion resistance and others. 
     Next, referring to FIGS. 17 to  21 , a method of manufacturing a thermal head  31  equivalent to another embodiment of the invention will be described. 
     For the method of manufacturing the thermal head according to the invention, as heretofore, plural thermal heads  31  are simultaneously formed on one large substrate  34  not shown and plural thermal heads  31  are simultaneously acquired by dividing the large substrate  34  into desired size. 
     In the method of manufacturing such a thermal head  31 , first in a process for forming a substrate, silicon formed flatly is etched as heretofore, the substrate  34  having a convex portion  34   a  is formed and next, in a process for forming a heat insulating layer, a heat insulating layer  35  having substantially even thickness shown in FIG.  17  and made of the oxide of silicon, transition metal and oxygen is formed on the upper surface of the substrate  34  as heretofore. 
     Next, in a process for forming a conductive layer, after high melting point metal having the melting point of 1500° C. or higher such as Cr, Mo, Ti, Zr, Ta, No, W and Hf is deposited on the supper surface of the heat insulating layer  35  by sputtering and others so that the metal has the uniform thickness of approximately 0.15 μm, plural conductive layers  36  in a desired pattern arrayed in parallel as shown in FIG. 18 are formed by etching. 
     At this time, the conductive layer  36  located at the end of the columns is composed of a pair of good conductors  36   b  and  36   c  mutually opposite with each clearance  36   a  respectively having a predetermined dimension A and B between them and the resistor  37  made of Ta—SiO 2  which is the same material as that of the heater element  40  for example is formed in the clearance  36   a  by sputtering and etching. The clearance  36   a  is elongated from the vicinity of the center of the columns to the conductive layer  36  located at the end. 
     The resistor  37  made of Ta—SiO 2  which is the same material as that of the heater element  40  for example is formed in each clearance  36   a  by sputtering and etching and the pair of good conductors  36   b  and  36   c  mutually opposite are connected. 
     Next, in a process for forming a base electrode layer, as shown in FIG. 19, a base electrode layer  38  is formed on the heat insulating layer  35  and on the other end of the conductive layer  36 . As for the base electrode layer  38 , after metal satisfactory in conductivity having the resistivity of 1×10 −7  Ωm or lower such as Al, Cu, Au and Ni is uniformly deposited by sputtering and others so that the metal has the thickness of approximately 1 to 3 μm, the shape having a front edge  38   a  and a rear edge  38   b  is formed rectangularly by etching and the right and left both ends in a lateral direction are elongated up to the vicinity of the end of the substrate  34 . 
     Next, in a process for forming a layer insulating layer, a layer insulating layer  39  is formed so that the layer covers each upper surface of the heat insulating layer  35 , the conductive layer  36  and the base electrode layer  38 . 
     The layer insulating layer  39  is formed by depositing material such as SiO 2  or Si—O—N compound on the overall upper surface of the conductive layer  36  and the base electrode layer  38  by sputtering or CVD and others so that the material has the uniform thickness of approximately 1 to 3 μm. 
     Next, in an etching process in the process for forming the layer insulating layer, after photoresist is applied on the overall upper surface of the layer insulating layer  39  by a spinner and others and a resist film is formed, the layer insulating layer  39  in the upper part shown in FIG. 21 is etched by buffered hydrofluoric acid (BHF) and others, one end of the conductive layer  36  is exposed, a connection on the common electrode side  35   a  is formed, a terminal connection  42  which is a square hole is formed at the right and left both ends of the base electrode layer  38  and a part of the base electrode layer  38  is exposed. 
     Next, in a process for forming a heater element, as heretofore, after a desired heating resistor made of Ta 2  N or Ta—SiO 2  is formed by sputtering and others so that the resistor has the uniform thickness of approximately 0.3 μm, etching by photolithography is applied and plural heater elements  40  in a predetermined shape arrayed in a row according to the desired number of dots corresponding to resolution are laminated on the layer insulating layer  39 . 
     At this time, each heater element  40  is insulated from the conductive layer  36  by the layer insulating layer  39 . 
     Next, in a process for forming a common electrode, a connecting electrode  45  shown in the upper part in FIG. 21 is formed. The connecting electrode  45  is formed in a desired shape for electrically connecting one side of the heater element  40  and one end of the conductive layer  36  exposed by a connection on the common electrode side  45   a  by etching and others by depositing the same material as that of the conductive layer  36  by sputtering and others. 
     Next, in a process for forming a connecting electrode on the separate electrode side, high melting point metal which is the same material as that of the connecting electrode  45  is deposited on the other side of the heater element  40  and on the layer insulating layer  39  in the vicinity by sputtering and others and a connecting electrode on the separate electrode side  47  in a desired pattern for electrically connecting the heater element  40  and a separate electrode  46  formed in a process for forming a separate electrode described later is formed by etching and others. 
     Next, in a process for forming the separate electrode, separate electrodes  46  arrayed in parallel are formed. As for the separate electrode  46 , Al is deposited on the layer insulating layer  39  by sputtering and others so that it has the thickness of approximately 1 to 3 μm for example, is connected to each heater element  40  via the connecting electrode on the separate electrode side  47  and can independently energize the heater element  40 . 
     Next, in a process for forming a side electrode, after metal having satisfactory conductivity such as Al is deposited by sputtering and others so that the metal has the uniform thickness of approximately 1 to 3 μm for example, a side electrode  43  in a desired pattern electrically connected to the base electrode layer  38  is formed by etching inside the terminal connection  42 . 
     Next, in a process for forming a passivation layer, a passivation layer  49  is formed by depositing material such as SIALON (Si—Al—O—N compound solid solution) or Si—O—N compound on the overall surface except the common electrode terminal  44  and the separate electrode terminal  48  by sputtering, CVD and others so that the material has the substantially even thickness of approximately 5 to 10 μm for example and next, in a dicing process, the manufacture of the thermal head  31  equivalent to this embodiment is completed by dividing the large substrate  34  in desired positions and simultaneously acquiring chipped plural thermal heads  31 . 
     When the passivation layer  49  is formed, a mask layer such as a heat-resistant pressure sensitive adhesive tape is applied to the common electrode terminal  44  and the separate electrode terminal  48 , and the common electrode terminal  44  and the separate electrode terminal  48  are exposed by peeling the mask layer after the passivation layer  49  is formed. 
     Terminal plating layers  44   a  and  48   a  easy in solderability such as Ni—Sn compound are respectively formed on the upper surface of the common electrode terminal  44  and the separate electrode terminal  48 . 
     In the description of the embodiment of the invention, the conductive layer  36  in the vicinity of the center of the columns without clearance  36   a  and a resistor  37  is described, however, clearance  36   a  is provided to the conductive layer  36  in the vicinity of the center of the columns and a resistor  37  may be also formed in the clearance  36   a.    
     Next, to explain action in the embodiment of the invention using the thermal printer as an example, the thermal head  31  having the configuration described above and equivalent to another embodiment of the invention is mounted on a carriage of the thermal printer not shown. 
     That is, the thermal head  31  according to the invention presses an ink ribbon of a ribbon cassette installed on the carriage on recording paper, melts and transfers or sublimates and transfers ink of the ink ribbon on the heater element  40  on the recording paper by selectively energizing the heater elements  40  based upon record information, moving the carriage and executes desired image printing. 
     To explain further concretely, one end of a cable for energizing not shown is connected to the separate electrode terminal  48  and the common electrode terminal  44  of the thermal head  31  equivalent to the embodiment of the invention and the other end of the cable is connected to an energizing controller not shown in the body of the printer. The energizing controller supplies slightly higher voltage current than that in a conventional type to the separate electrode terminal  48  based upon record information via the cable. 
     Hereby, first, a signal is respectively transmitted to the separate electrodes  46  via the separate electrode terminal  48 , the heater elements  40  are energized and are selectively heated at predetermined temperature based upon the record information. 
     Current respectively flows to the conductive layer  36  under the heater element  40  from the heater element  40  via the connecting electrode  45 . 
     Next, current flows from the conductive layer  36  to the base electrode layer  38 . At this time, as the conductive layer  36  is thinned up to approximately 0.15 μm though the conductive layer  36  is located immediately under the heater element  40 , an effect upon a temperature dependence characteristic is small and also, as the conductive layer is made of high melting point metal, energizing is not prevented by the heat of the heater element  40 . 
     Next, current flows from the base electrode layer  38  to the common electrode terminal  44  via the side electrode  43  and current flows from the common electrode terminal  44  to the side outside the thermal head  31  of the printer. 
     As described above, as control current according to an instruction from the energizing controller on the side of the printer flows from the separate electrode terminal  48  to the common electrode terminal  44  via the heater element  40 , Joule heat is generated in the heater element  40  and the heater element is heated up to predetermined temperature. 
     As described above, as the resistance value of the conductive layer  36  on the side of the end of the columns is increased even if voltage drop occurs in the conductive layer  36  connected to the heater element  40  in the vicinity of the center of the columns located in a far position from the right and left two common electrode terminals  44  because in the thermal head  31  according to the invention, the resistance value of the conductive layer  36  on the side of the end is increased, compared with that of the conductive layer  36  formed in parallel and located in the vicinity of the center of the columns, the drop of voltage applied to the conductive layer  36  on the side of the end can be also equalized to the drop of voltage applied to the conductive layer  36  in the vicinity of the center and voltage applied to each conductive layer  36  in the vicinity of the center of the columns and on the side of the end can be corrected so that the voltage is substantially equal. 
     Therefore, the heater elements  40  can be uniformly energized and voltage drop in the common electrode which has been a problem heretofore can be inhibited. 
     In such a thermal head  31  according to the invention, the exoergic temperature of the heater element  40  in the vicinity of the center of the columns never lowers as heretofore, the heater element  40  can be heated at uniform temperature and in printing by a thermal printer for example, a satisfactory printed image free of unevenness in printing density can be acquired. 
     Also, in a thermal head  61  equivalent to further another embodiment of the invention, as shown in FIG. 22, plural groups E and F of good conductors each of which is composed of adjacent plural good conductors  66   b  and  66   c  having a slit between them and which are located at the end of the columns are formed. 
     The group G of good conductors composed of plural conductive layers  66  having a slit between them without clearance  66   a  the whole of which is formed by the same material as the good conductors  66   b  and  66   c  is formed in the vicinity of the center of the columns. 
     The adjacent plural good conductors  66   b  and  66   c  in the groups E and F of good conductors located on the side of the end are mutually connected via a resistor  67 . 
     Also, plural groups E and F of good conductors are formed on the side of the end from the vicinity of the center of the columns, the length of the clearance  66   a  between mutually opposite good conductors  66   b  and  66   c  in the respective groups E and F of good conductors is equalized and the clearance  66   a  in the respective groups E and F of good conductors is formed so that it is longer from the vicinity of the center of the columns to the groups E and F of good conductors located on the side of the end. 
     That is, relationship between the dimensions C and D of the clearance  66   a  is C&lt;D. Therefore, the resistance value of the group of good conductors E on the side of the end having the clearance  66   a  in the dimension D is made larger than that of the group of good conductors F in the vicinity of the center having the clearance  66   a  in the dimension C. 
     Also, in a transformed example of further another embodiment of the invention, the good conductors  66   b  and  66   c  in the groups of good conductors E and F and the good conductors  66  in the group of good conductors G which have a slit between them in the embodiment described above have no slit between them and good conductors may be also integrated every group E, F and G. 
     Such a thermal head  61  equivalent to further another embodiment can be easily manufactured because the resistor  67  is not required to be formed every conductor  66 . 
     Also, in a thermal head  71  equivalent to another embodiment of the invention, as shown in FIG. 23, a conductor layer  76  located at the end of columns is composed of a narrow part  76   a  and a wide part  76   b  respectively different in the dimension of the width. 
     The resistance value of the conductive layer  76  is increased from the conductive layer  76  in the vicinity of the center of the columns to the conductive layer  76  on the side of the end by differentiating the length of the narrow part  76   a  from dimensions J and K. 
     The narrow part  76   a  of such a conductive layer  76  is elongated from the vicinity of the center of the columns to the conductive layer  76  located on the side of the end. 
     In such a thermal head  71 , the resistor made of material different from that of the conductive layer is not required as in another embodiment and the manufacture is easy.