Patent Publication Number: US-2023140231-A1

Title: Thermal print head, thermal printer, and method for manufacturing thermal print head

Description:
TECHNICAL FIELD 
     The present disclosure relates to a thermal print head and a thermal printer. The present disclosure also relates to a method for manufacturing a thermal print head. 
     BACKGROUND ART 
     Patent document 1 discloses a conventional thermal print head. The thermal print head includes a main substrate having a conductive layer and a resistive layer, and a circuit board having a driver IC mounted thereon. The resistive layer includes a plurality of heat generating parts arranged side by side in the main scanning direction. The conductive layer forms a conductive path for passing electrical current to the heat generating parts. 
     For printing by the thermal print head, electric current is passed to the resistive layer to cause the heat generating parts to generate heat. The heat is transferred to a print medium (e.g., a thermal recording paper), so that the color of the print medium changes to form an image. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-A-2017-65021 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     One object of the present disclosure is to provide a thermal print head and a thermal printer having higher durability and reliability than conventional designs. Another object of the present disclosure is to provide a method for manufacturing such a thermal print head. 
     Means to Solve the Problem 
     A first aspect of the present disclosure provides a thermal print head that includes: a substrate made of a single crystal semiconductor and including an obverse surface facing in one sense of a thickness direction; a resistive layer supported by the substrate and including a plurality of heat generating parts arranged side by side in a main scanning direction; and a wiring layer supported by the substrate and forming a conductive path to the plurality of heat generating parts. The wiring layer includes a conductive part and a heat generating sub-part for each of the plurality of heat generating parts, where the conductive part has a lower resistance value per unit length in a sub-scanning direction than the heat generating part, and where the heat generating sub-part has a resistance value per unit length in the sub-scanning direction that falls between the respective resistance values of the heat generating part and the conductive part. The substrate includes a ridge raised from the obverse surface and extending in the main scanning direction. The heat generating part, the heat generating sub-part and the conductive part are disposed on the ridge. The heat generating sub-part is located between the heat generating part and the conductive part in the sub-scanning direction. 
     A second aspect of the present disclosure provides a thermal printer that includes the thermal print head of the first aspect, and a platen directly opposite the thermal print head. 
     A third aspect of the present disclosure provides a method for manufacturing a thermal print heat, the method including: a substrate preparing step of preparing a substrate made of a single crystal semiconductor; a substrate processing step of processing the substrate to form an obverse surface facing in one sense of a thickness direction and a ridge that is raised from the obverse surface and extends in a main scanning direction; a resistive layer forming step of forming a resistive layer that is supported by the substrate and includes a plurality of heat generating parts arranged side by side in the main scanning direction; and a wiring layer forming step of forming a wiring layer that is supported by the substrate and forms a conductive path to the plurality of heat generating parts. The wiring layer includes a conductive part and a heat generating sub-part for each of the plurality of heat generating parts, where the conductive part has a lower resistance value per unit length in a sub-scanning direction than the heat generating part, and where the heat generating sub-part has a resistance value per unit length in the sub-scanning direction that falls between the respective resistance values of the heat generating part and the conductive part. The heat generating part, the heat generating sub-part and the conductive part are formed on the ridge. The heat generating sub-part is located between the heat generating part and the conductive part in the sub-scanning direction. 
     Advantages of Invention 
     The present disclosure provides a thermal print head (and a thermal printer) having higher durability and reliability. Additionally, the present disclosure can provide a method for manufacturing a thermal print head having higher durability and reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a plan view of a thermal print head according to a first embodiment. 
         FIG.  2    is an enlarged fragmentary plan view of  FIG.  1   . 
         FIG.  3    is an enlarged fragmentary plan view of  FIG.  2   . 
         FIG.  4    an enlarged fragmentary sectional view taken along line IV-IV of  FIG.  1    and showing a thermal printer that includes the thermal print head of the first embodiment. 
         FIG.  5    is an enlarged sectional view showing a part of  FIG.  4   . 
         FIG.  6    is an enlarged fragmentary sectional view of  FIG.  5   . 
         FIG.  7    is a fragmentary sectional view illustrating a step of a method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  8    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  9    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  10    is an enlarged fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  11    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  12    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  13    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  14    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  15    is a fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  16    is an enlarged fragmentary sectional view illustrating a step of the method for manufacturing the thermal print head according to the first embodiment. 
         FIG.  17    is an enlarged fragmentary sectional view showing a thermal printer that includes a thermal print head according to a second embodiment. 
         FIG.  18    is an enlarged fragmentary sectional view of  FIG.  17   . 
         FIG.  19    is an enlarged fragmentary plan view of the thermal print head according to the second embodiment. 
         FIG.  20    is a sectional view taken along line XX-XX of FIG.  19 . 
         FIG.  21    is an enlarged fragmentary plan view of a thermal print head according to a variation of the second embodiment. 
         FIG.  22    is a fragmentary sectional view of a thermal print head according to a third embodiment. 
         FIG.  23    is an enlarged fragmentary sectional view of the thermal print head according to the third embodiment. 
         FIG.  24    is a fragmentary sectional view of a thermal print head according to a variation of the third embodiment. 
         FIG.  25    is an enlarged fragmentary sectional view of a thermal print head according to the variation of the third embodiment. 
         FIG.  26    is an enlarged fragmentary plan view of a thermal print head according to a fourth embodiment. 
         FIG.  27    is an enlarged fragmentary sectional view of the thermal print head according to the fourth embodiment. 
         FIG.  28    is an enlarged fragmentary sectional view of a thermal print head according to a variation of the fourth embodiment. 
         FIG.  29    is an enlarged fragmentary sectional view of a thermal print head according to a variation of the fourth embodiment. 
         FIG.  30    is an enlarged fragmentary sectional view of the thermal print head according to the variation of the fourth embodiment. 
         FIG.  31    is an enlarged fragmentary sectional view of a thermal print head according to a fifth embodiment. 
         FIG.  32    is an enlarged fragmentary plan view of the thermal print head according to the fifth embodiment. 
         FIG.  33    is an enlarged fragmentary sectional view of a thermal print head according to a variation of the fifth embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present disclosure will be described below with reference to the drawings. In the following description, the same or slimier components are denoted by the same reference numerals, and a description of such a component is omitted. 
       FIGS.  1  to  6    show a thermal print head A 1  according to a first embodiment. The thermal print head A 1  includes a head substrate  1 , an insulating layer  19 , a protective layer  2 , a wiring layer  3 , a resistive layer  4 , a connecting substrate  5 , a plurality of wires  61  and  62 , a plurality of driver ICs  7 , a protective resin  78  and a heat dissipating member  8 . The thermal print head A 1  is a component installed into a thermal printer Pr (see  FIG.  4   ), which is for printing an image on a print medium (not shown). The thermal printer Pr includes the thermal print head A 1  and a platen roller  91 . The platen roller  91  is disposed directly opposite the thermal print head A 1 . The platen roller  91  forwards a print medium inserted between the thermal print head A 1  and the platen roller  91  in a sub-scanning direction. Examples of print media include thermal recording paper, such as for thermal barcode labels and thermal receipts. The platen roller  91  may alternatively be a planar platen made of rubber. The planar platen may have an arc shape in cross section and may be a part of a cylindrical rubber member having a relatively large radius of curvature. The platen roller  91  and a planar platen are both examples of the “platen” according to the present disclosure. 
       FIG.  1    is a plan view of the thermal print head A 1 .  FIG.  2    is a fragmentary plan view of the thermal print head A 1 .  FIG.  3    is a fragmentary enlarged plan view of the thermal print head A 1 .  FIG.  4    is a fragmentary enlarged sectional view of a thermal printer Pr installed with the thermal print head A 1 . The section shown in this figure corresponds to a section taken along line IV-IV of  FIG.  1   .  FIG.  5    is a fragmentary sectional view of the thermal print head A 1 .  FIG.  6    is a fragmentary enlarged sectional view of the thermal print head A 1 . In  FIGS.  1  to  3   , the protective layer  2  is omitted. In  FIGS.  1  and  2   , the protective resin  78  is omitted. In  FIG.  2   , the wires  61  are omitted. In  FIGS.  1  to  3   , the lower side corresponds to the upstream in the sub-scanning direction y, and the upper side to the downstream. In  FIGS.  4  to  6   , the right side corresponds to the upstream in the sub-scanning direction y, and the left side to the downstream. 
     The head substrate  1  supports the wiring layer  3  and the resistive layer  4 . The head substrate  1  has a rectangular shape elongated in the main scanning direction x. In the following description, the thickness direction of the head substrate  1  is designated as a thickness direction z. The head substrate  1  is not limited to specific dimensions. In one example, the head substrate  1  measures 725 μm in thickness (a dimension in the thickness direction z), from 50 to 150 mm in the main scanning direction x, and from 2.0 to 5.0 mm in the sub-scanning direction y. 
     The head substrate  1  is made of a single crystal semiconductor, such as silicon (Si). As shown in  FIGS.  4  and  5   , the head substrate  1  has a first obverse surface  11  and a first reverse surface  12 . The first obverse surface  11  and the first reverse surface  12  are spaced apart in the thickness direction z and face away from each other in the thickness direction z. The wiring layer  3  and the resistive layer  4  are disposed on the side of the first obverse surface  11 . The head substrate  1  is an example of the “substrate”, and the first obverse surface  11  is an example of the “obverse surface”. 
     The head substrate  1  has a ridge  13 . The ridge  13  is raised from the first obverse surface  11  in the thickness direction z and elongated in the main scanning direction x. In the illustrated example, the ridge  13  is offset in the sub-scanning direction y toward the downstream end of the head substrate  1 . The ridge  13 , which is a part of the head substrate  1 , is made of the single crystal semiconductor, such as Si. 
     The ridge  13  has a top part  130 , a pair of first slopes  131 A and  131 B, and a pair of second slopes  132 A and  132 B. 
     The top part  130  is where the distance from the first obverse surface  11  is largest within the ridge  13 . The top part  130  may be a flat surface substantially parallel to the first obverse surface  11 , for example. In view of the thickness direction z, the top part  130  has the shape of a long narrow rectangle extending in the main scanning direction x. 
     As shown in  FIG.  6   , the first slopes  131 A and  131 B are connected to the opposite ends of the top part  130  in the sub-scanning direction y. The first slope  131 A is connected to the top part  130  on the upstream side in the sub-scanning direction y. The first slope  131 B is connected to the top part  130  on the downstream side in the sub-scanning direction y. The first slope  131 A is an example of “upstream-side first slope”, whereas the first slope  131 B is an example of “downstream-side first slope”. Each of the first slopes  131 A and  131 B is inclined at an angle α 1  to the first obverse surface  11  (forms a first inclination angle of α 1 ). As viewed in the thickness direction z, each of the first slopes  131 A and  131 B is a flat surface having the shape of a long narrow rectangle extending in the main scanning direction x. The ridge  13  may also have slopes (not shown) connected to the first slopes  131 A and  131 B at the respective ends of the ridge  13  in the main scanning direction x. 
     As shown in  FIG.  6   , the second slopes  132 A and  132 B are respectively connected to the first slopes  131 A and  131 B on the sides away from the top part  130  in the sub-scanning direction y. The second slope  132 A is located between the first slope  131 A and the first obverse surface  11  in the sub-scanning direction y. The second slope  132 A connects to the first slope  131 A from the upstream side in the sub-scanning direction y, and to the first obverse surface  11  from the downstream side in the sub-scanning direction y. The second slope  132 B is located between the first slope  131 B and the first obverse surface  11  in the sub-scanning direction y. The second slope  132 B connects to the first slope  131 B from the downstream side in the sub-scanning direction y and to the first obverse surface  11  from the upstream side in the sub-scanning direction y. The second slope  132 A is an example of “upstream-side second slope”, whereas the second slope  132 B is an example of “downstream-side second slope”. Each of the second slopes  132 A and  132 B is inclined at an angle α 2  to the first obverse surface  11  (forms a second inclination angle of α 2 ). The angle α 2  is greater than the angle α 1 . As viewed in the thickness direction z, each of the second slopes  132 A and  132 B is a flat surface having the shape of a long narrow rectangle extending in the main scanning direction x. The second slopes  132 A and  132 B are both connected to the first obverse surface  11 . The ridge  13  may also have slopes (not shown) connected to the second slopes  132 A and  132 B at the respective ends of the ridge  13  in the main scanning direction x. 
     The first obverse surface  11  of the head substrate  1  has a ( 100 ) plane (by the Miller Indices). According to the manufacturing method described below, the angle α 1  (see  FIG.  6   ) formed by the first slopes  131 A and  131 B relative to the first obverse surface  11  is 30.1 degrees, for example. The angle α 2  (see  FIG.  6   ) formed by the second slopes  132 A and  132 B relative to the first obverse surface  11  is 54.7 degrees, for example. The dimension of the ridge  13  in the thickness direction z is from 150 to 300 μm, for example. 
     As shown in  FIGS.  5  and  6   , the insulating layer  19  covers the first obverse surface  11  and the ridge  13 . The insulating layer  19  is provided for more reliably insulating the first obverse surface  11  of the head substrate  1 . The insulating layer  19  is made of an insulating material. For example, the insulating layer  19  may be made of a SiO 2  film deposited by using tetraethyl orthosilicate (TEOS) as a material gas (TEOS—SiO 2  film). Instead of a TEOS—SiO 2  film, the insulating layer  19  may be made of a film of SiO 2  formed by a different process or a SiN film. The thickness of the insulating layer  19  is not limited. In one example, the thickness of the insulating layer  19  is from 5 to 15 μm (preferably from 5 to 10 μm). 
     The resistive layer  4  is supported by the head substrate  1 . As shown in  FIGS.  5  and  6   , the resistive layer  4  of this embodiment is supported on the head substrate  1  via the insulating layer  19 . The resistive layer  4  has a plurality of heat generating parts  41 . The heat generating parts  41  are selectively energized to heat the desired parts of a print medium. The heat generating parts  41  are regions of the resistive layer  4  not covered with the wiring layer  3 . The heat generating parts  41  are arranged side by side in the main scanning direction x at spaced intervals. The shape of the heat generating parts  41  may be, but not limited to, a rectangle elongated in the sub-scanning direction y as viewed in the thickness direction z. The resistive layer  4  is made of a material having higher resistivity than the wiring layer  3 . Preferably, the electrical resistivity of the resistive layer  4  is 10 −6  Ωm or higher. The resistive layer  4  may be made of TaN in one example, and other suitable materials include TaSiO 2 , TION, PolySi, Ta 2 O 5 , RuO 2 , RuTiO and TaSiN. The resistive layer  4  can be formed by any suitable process, such as sputtering, CVD and plating, depending on the material used. For example, when the material is TaN, the resistive layer  4  can be formed by sputtering. The thickness of the resistive layer  4  is not limited. In one example, the thickness of the resistive layer  4  is from 0.02 to 0.1 μm (preferably around 0.08 μm). 
     As shown in  FIG.  6   , each heat generating part  41  extends from the first slope  131 B to the top part  130 . The upstream end of each heat generating part  41  in the sub-scanning direction y is located on the top part  130 , and the downstream end on the first slope  131 B. In one example, around 10% to 30% of the overall dimension of each heat generating part  41  in the sub-scanning direction y is located on the top part  130 . 
     The wiring layer  3  forms a conductive path for passing electric current to the heat generating parts  41 . The wiring layer  3  is supported by the head substrate  1 . As shown in  FIGS.  5  and  6   , the wiring layer  3  of this embodiment is stacked on the resistive layer  4 . 
     As shown in  FIGS.  1  to  3 ,  5  and  6   , the wiring layer  3  includes a plurality of individual electrodes  31  and a common electrode  32 . 
     As shown in  FIGS.  2 ,  3  and  6   , each individual electrode  31  has the shape of a strip generally extending in the sub-scanning direction y. The individual electrodes  31  are located upstream from the respective heat generating parts  41  in the sub-scanning direction y. The downstream end of each individual electrode  31  in the sub-scanning direction y overlaps with the upstream end of the top part  130  of the ridge  13  in the sub-scanning direction y. As shown in  FIGS.  2  and  5   , each individual electrode  31  has an individual-electrode pad  311 . The individual-electrode pad  311  is where a wire  61  is bonded for electrical connection to a driver IC  7 . 
     As shown in  FIGS.  2 ,  3 ,  5  and  6   , the common electrode  32  has a connecting part  323  and a plurality of strip parts  324 . The strip parts  324  are located downstream from the respective heat generating parts  41  in the sub-scanning direction y. The upstream end of each strip part  324  in the sub-scanning direction y is located opposite the downstream end of the corresponding individual electrode  31  in the sub-scanning direction y across the corresponding heat generating part  41 . The upstream end of each strip part  324  in the sub-scanning direction y overlaps with the first slope  131 B of the ridge  13 . The connecting part  323  that connects the strip parts  324  is located downstream from the strip parts  324  in the sub-scanning direction y. The connecting part  323  is elongated in the main scanning direction x, and has a dimension in the sub-scanning direction y which is greater, in other words wider, than the dimension of each strip part  324  in the main scanning direction x. As shown in  FIG.  1   , the connecting part  323  includes a pair of opposite end portions spaced apart from each other in the main scanning direction x, where each of the opposite end portions extends in the sub-scanning direction y from a downstream side to an upstream side of the heat generating parts  41 . According to this embodiment, the downstream ends of the strip parts  324  and the connecting part  323  of the common electrodes  32  are disposed on the first obverse surface  11  of the head substrate  1  (in other words, above the first obverse surface  11  of the head substrate  1 ). 
     The wiring layer  3  (the individual electrodes  31  and the common electrode  32 ) is composed of a first conductive layer  301  and a second conductive layer  302  stacked in the thickness direction z. 
     The first conductive layer  301  is disposed on the resistive layer  4 . The first conductive layer  301  is made of a material having a resistivity that is lower than the resistive layer  4  and higher than the second conductive layer  302 . Preferably, the first conductive layer  301  has an electrical resistivity from 10 −6  to 10 −7  Ωm, for example. Preferably, in addition, the first conductive layer  301  has a heat conductivity lower than 100 W/m, for example. The first conductive layer  301  may be made of titanium (Ti) in one example, and other suitable materials include Ta, Ga, Sn, PtIr, Pt, thallium (TI), vanadium (V) and Cr. The first conductive layer  301  can be formed by any suitable process, such as sputtering, CVD, and plating, depending on the material used. For example, when the material is Ti, the first conductive layer  301  can be formed by sputtering. The thickness of the first conductive layer  301  is not specifically limited. In one example, the thickness of the first conductive layer  301  is from 0.1 to 0.2 μm. 
     The second conductive layer  302  is disposed on the first conductive layer  301 . The second conductive layer  302  covers a part of the first conductive layer  301 . As such, a part of the first conductive layer  301  is exposed from the second conductive layer  302 . The second conductive layer  302  is made of a material having a lower resistivity than the resistive layer  4  and the first conductive layer  301 . Preferably, the second conductive layer  302  has an electrical resistivity of 10 −7  Ωm or lower. In addition, the second conductive layer  302  is made of a material that is more heat conductive than the first conductive layer  301 . Preferably, the second conductive layer  302  has a heat conductivity of 100 W/m or higher, for example. The second conductive layer  302  may be made of Cu in one example, and other suitable materials include alloys of Cu, Al, alloys of Al, Au, Ag, Ni and tungsten (W). The second conductive layer  302  can be formed by any suitable process, such as sputtering, CVD, and plating, selected depending on the material used. For example, when the material is Cu, the second conductive layer  302  can be formed by sputtering. When the material is Au, Ag or Ni, the second conductive layer  302  is typically formed by plating. In this case, the second conductive layer  302  may include a seed layer (of Cu, for example). The second conductive layer  302  is thicker than the first conductive layer  301 . The thickness of the second conductive layer  302  depends on the material used, the magnitude of current passed to the wiring layer  3 , and so on. In one example, the thickness of the second conductive layer  302  is from 0.5 to 5 μm. 
     The wiring layer  3  includes a pair of heat generating sub-parts  35 A and  35 B and a pair of conductive parts  36 A and  36 B for each heat generating part  41 . 
     Each pair of heat generating sub-parts  35 A and  35 B are formed by the parts of the first conductive layer  301  exposed from the second conductive layer  302 . In other words, the heat generating sub-parts  35 A and  35 B are the parts of the wiring layer  3  where the first conductive layer  301  is not covered with the second conductive layer  302 . The heat generating sub-parts  35 A and  35 B in each pair are adjacent to the opposite ends of the corresponding heat generating part  41  in the sub-scanning direction y. The heat generating sub-part  35 A is adjacent to the heat generating part  41  on the upstream side in the sub-scanning direction y, and the heat generating sub-part  35 B is adjacent to the heat generating part  41  on the downstream side in the sub-scanning direction y. The heat generating sub-part  35 A is an example of “upstream-side heat generating sub-part”, whereas the heat generating sub-part  35 B an example of “downstream-side heat generating sub-part”. 
     The heat generating sub-part  35 A is located on the top part  130 . The opposite ends of the heat generating sub-part  35 A in the sub-scanning direction y are both located on the top part  130 . The heat generating sub-part  35 B extends from the first slope  131 B to the second slope  132 B. The upstream end of the heat generating sub-part  35 B in the sub-scanning direction y is located on the first slope  131 B, and the downstream end of the heat generating sub-part  35 B in the sub-scanning direction y is located on the second slope  132 B. 
     Each pair of conductive parts  36 A and  36 B is formed by the first conductive layer  301  and the second conductive layer  302 . In other words, the conductive parts  36 A and  36 B are the parts of the wiring layer  3  where the second conductive layer  302  is stacked on the first conductive layer  301 . The conductive parts  36 A and  36 B in each pair are respectively located on the sides of the heat generating sub-part  35 A and  35 B away from the corresponding heat generating part  41 . The conductive part  36 A is adjacent to the heat generating sub-part  35 A on the upstream side in the sub-scanning direction y, and the conductive part  36 B is adjacent to the heat generating sub-part  35 B on the downstream side in the sub-scanning direction y. The conductive part  36 A is an example of “upstream side conductive part”, whereas the conductive part  36 B is an example of “downstream conductive part”. 
     The conductive part  36 A extends from the top part  130  along the first slope  131 A and the second slope  132 A to reach a part of the first obverse surface  11  located upstream from the ridge  13  in the sub-scanning direction y. The downstream end of the conductive part  36 A in the sub-scanning direction y is located on the top part  130 . The conductive part  36 B extends from the second slope  132 B to a part of the first obverse surface  11  located downstream from the ridge  13  in the sub-scanning direction y. The upstream end of the conductive part  36 B in the sub-scanning direction y is located on the second slope  132 B. 
     Since the first conductive layer  301 , the second conductive layer  302  and the resistive layer  4  have the resistance values satisfying the relation described above, the resistance value of the conductive parts  36 A and  36 B per unit length in the sub-scanning direction is lower than that of the heat generating parts  41 . In addition, the resistance value of the heat generating sub-parts  35 A and  35 B per unit length in the sub-scanning direction falls between the resistance value of the heat generating part  41  and the resistance value of the conductive parts  36 A and  36 B. Consequently, when electric current is passed to each heat generating part  41 , the amount of heat generated by each of the heat generating sub-parts  35 A and  35 B is smaller than the amount of heat generated by the heat generating part  41  and greater than the amount heat generated by each of the conductive parts  36 A and  36 B. For example, under the energization condition where the heat generating part  41  generates heat of around 300° C., each of the heat generating sub-parts  35 A and  35 B will generate heat of around 150 to 200° C. 
     The protective layer  2  covers and protects the wiring layer  3  and the resistive layer  4 . The protective layer  2  is made of an insulating material. For example, the protective layer  2  may be made of silicon nitride (SiN), and other examples of the insulating material include silicon oxide (SiO 2 ), silicon carbide (SiC) aluminum nitride (AlN). The protective layer  2  may be composed of a single layer or two or more layers containing the insulating material. The thickness of the protective layer  2  is not specifically limited. In one example, the thickness of the protective layer  2  is from 0.1 to 10 μm. 
     The protective layer  2  has a plurality of pad openings  21  as shown in  FIG.  5   . Each pad opening  21  penetrates through the protective layer  2  in the thickness direction z. Through the pad openings  21 , the individual-electrode pads  311  of the individual electrodes  31  are exposed. Unlike the illustrated example, the pad openings  21  may be filled with a conductive material. In this case, a plating layer may be disposed on the conductive material. The configuration of the plating layer is not limited. In one example, the plating layer is formed by laminating Ni, palladium (Pd) and Au on the surface of the conductive material in the stated order. 
     As shown in  FIGS.  1  and  4   , the connecting substrate  5  is located upstream from the head substrate  1  in the sub-scanning direction y. The connecting substrate  5  may be a printed circuit board for mounting the driver ICs  7  and a connector  59  (described later) thereon. The connecting substrate  5  is not limited to a specific shape. In this embodiment, the connecting substrate  5  has the shape of a rectangle elongated in the main scanning direction x. The connecting substrate  5  has a second obverse surface  51  and a second reverse surface  52 . The second obverse surface  51  faces in the same direction as the first obverse surface  11  of the head substrate  1 , and the second reverse surface  52  faces in the same direction as the first reverse surface  12  of the head substrate  1 . In this embodiment, the second obverse surface  51  is located below the first obverse surface  11  in the thickness direction z in the figure. 
     The driver ICs  7  are mounted on the second obverse surface  51  of the connecting substrate  5  and selectively energize the heat generating parts  41 . The driver ICs  7  are connected to the individual electrodes  31  with the wires  61 . The driver ICs  7  controls energization of the heat generating parts  41  according to an external command signal provided to the thermal print head A 1  through the connecting substrate  5 . The driver ICs  7  are connected to the wiring pattern (not shown) of the connecting substrate  5  with a plurality of wires  62 . The driver ICs  7  are provided as many as necessary for the number of heat generating parts  41 . 
     The driver ICs  7  and the wires  61  and  62  are covered with the protective resin  78 . The protective resin  78  is made of an insulating resin, which may be black. The protective resin  78  extends from the head substrate  1  to the connecting substrate  5 . 
     The connector  59  connects the thermal print head A 1  to the thermal printer Pr. The connector  59  is attached to the connecting substrate  5  and connected to the wiring pattern (not shown) of the connecting substrate  5 . 
     The heat dissipating member  8  supports the head substrate  1  and the connecting substrate  5  and dissipates heat from the heat generating parts  41  to the outside via the head substrate  1 . The heat dissipating member  8  may be a block of metal, such as A 1 . The heat dissipating member  8  has a first support surface  81  and a second support surface  82 . The first support surface  81  and the second support surface  82  face upward in the thickness direction z and are arranged side by side in the sub-scanning direction y. The first support surface  81  is bonded to the first reverse surface  12  of the head substrate  1 . The second support surface  82  is bonded to the second reverse surface  52  of the connecting substrate  5 . 
     Next, an example of a method for manufacturing the thermal print head A 1  is described below with reference to  FIGS.  7  to  16   . 
     First, a material substrate  1 K is prepared as shown in  FIG.  7   . The material substrate  1 K is made of a single crystal semiconductor. For example, the material substrate  1 K is a part of a substantially circular Si wafer. That is, a single Si wafer includes a plurality of material substrates  1 K. The figures mentioned below show one material substrate  1 K (a head substrate  1 ) for manufacturing one thermal print head A 1 , out of the plurality of material substrates  1 K included in the Si wafer. The thickness of the material substrate  1 K (i.e., the thickness of the Si wafer) is not limited and may be about 725 μm in this embodiment. The material substrate  1 K has a first obverse surface  11 K and a first reverse surface  12 K facing away from each other. The first obverse surface  11 K has a ( 100 ) plane. 
     Next, the first obverse surface  11 K is covered with a mask layer and then anisotropically etched using KOH, for example. This provides the material substrate  1 K with a ridge  13 K as shown in  FIG.  8   . The ridge  13 K is raised from the first obverse surface  11 K and elongated in the main scanning direction x. The ridge  13 K has a top part  130 K and a pair of slopes  132 K. The top part  130 K is a surface parallel to the first obverse surface  11 K and has a ( 100 ) plane as with the first obverse surface  11 K. The pair of slopes  132 K are located on the opposite sides of the top part  130 K and connect the top part  130 K to the first obverse surface  11 K. Each slope  132 K is a flat surface inclined relative to the top part  130 K and the first obverse surface  11 K. Each slope  132 K forms an angle of 54.7 degrees with the first obverse surface  11 K and also with the top part  130 K. 
     Next, the mask layer is removed, followed by anisotropic etching using KOH, for example. Processing the material substrate  1 K in this way provides a head substrate  1  having a first obverse surface  11 , a first reverse surface  12  and a ridge  13  as shown in  FIGS.  9  and  10   . The ridge  13  has a top part  130 , a pair of first slopes  131 A and  131 B, and a pair of second slopes  132 A and  132 B. The top part  130  is formed from the top part  130 K, and the pair of second slopes  132 A and  132 B are formed from the slopes  132 K. The first slopes  131 A and  131 B are formed by etching away the edges between the top part  130 K and each slope  132 K using KOH. The first slopes  131 A and  131 B each form an angle α 1  (see  FIG.  10   ) of 30.1 degrees to the first obverse surface  11 . The second slopes  132 A and  132 B form an angle α 2  (see  FIG.  10   ) of 54.7 degrees to the first obverse surface  11 . The step of forming the head substrate  1  from the material substrate  1 K as described above (see  FIGS.  8  to  10   ) is an example of “substrate processing step”. The first obverse surface  11  and the ridge  13  are formed through the substrate processing step. 
     Subsequently, an insulating layer  19  is formed as shown in  FIG.  11   . The insulating layer  19  is formed by depositing SiO 2  on the head substrate  1  by CVD using tetraethyl orthosilicate (TEOS) as a material gas. This process of forming the insulating layer  19  is merely an example, and a different process may be used. 
     Subsequently, a resistive film  4 K is formed as shown in  FIG.  12   . The step of forming the resistive film  4 K (the resistive film deposition step) may include depositing a thin TaN film on the insulating layer  19  by sputtering, for example. This process of forming the resistive film  4 K is merely an example, and a different process may be used. 
     Subsequently, a wiring film  3 K is formed as shown in  FIGS.  13  and  14   . The step of forming the wiring film  3 K includes two steps, one for forming a first conductive film  301 K as shown in  FIG.  13   , and another for forming a second conductive film  302 K as shown in  FIG.  14   . The step of forming the first conductive film  301 K (the first deposition step) includes depositing a thin film of Ti on the resistive film  4 K by sputtering, for example. At this stage, the conductive film  301 K covers substantially the entire surface of the resistive film  4 K. The step of forming the second conductive film  302 K (the second deposition step) includes depositing a Cu film on the first conductive film  301 K by sputtering or plating, for example. At this stage, the second conductive film  302 K covers substantially the entire surface of the first conductive film  301 K. 
     Subsequently, as shown in  FIGS.  15  and  16   , a part of the second conductive film  302 K is removed, followed by removing a part of the first conductive film  301 K and then a part of the resistive film  4 K. Each of the step of removing a part of the first conductive film  301 K (the first partial removal step), the step of removing a part of the second conductive film  302 K (the second partial removal step), and the step of removing a part of the resistive film  4 K (the resistive film partial removal step) is done by etching, for example. By the first partial removal step, a first conductive layer  301  is formed. By the second deposition step, a second conductive layer  302  is formed. By the resistive film partial removal step, a resistive layer  4  is formed. That is, the step of forming the wiring layer  3  (the wiring layer forming step) includes the first deposition step, the second deposition step, the first partial removal step and the second partial removal step. The step of forming the resistive layer  4  (the resistive layer forming step) includes the resistive film deposition step and the resistive film partial removal step. Note that the resistive film partial removal step may be performed before the first and second deposition steps. The first conductive layer  301  and the second conductive layer  302  formed in this way together constitute the wiring layer  3  described above, and the wiring layer  3  includes a plurality of individual electrodes  31  and a common electrode  32 . The wiring layer  3  also includes a plurality of heat generating sub-parts  35 A and  35 B and a plurality of conductive parts  36 A and  36 B. The resistive layer  4  formed in this way includes a plurality of heat generating parts  41 . 
     Next, a protective layer  2  is formed. The protective layer  2  is formed by, for example CVD to deposit SiN on the insulating layer  19 , the wiring layer  3  (the first conductive layer  301  and the second conductive layer  302 ) and the resistive layer  4 . A plurality of pad openings  21  are formed by removing parts of the protective layer  2  by etching, for example. Subsequently, the head substrate  1  ( FIGS.  1 ,  4 , and  5   ), as well as other head substrates  1 , is separated from the Si wafer by using a discing device, for example. 
     Subsequently, the head substrate  1  is subjected to assembling steps. The assembling steps may include attaching the head substrate  1  and a connecting substrate  5  to a heat dissipating member  8 , mounting driver ICs  7  to the connecting substrate  5 , and bonding a plurality of wires  61  and  62 , and forming a protective resin  78 . Then, the thermal print head A 1  is completed as described above. 
     The thermal print head A 1  described above has the following advantages. 
     According to the thermal print head A 1 , each of the heat generating sub-parts  35 A and  35 B is located between a heat generating part  41  and a conductive part  36 A or  36 B. When electric current is supplied, the temperature of the heat generating sub-parts  35 A and  35 B rises to a temperature lower than the temperature of the heat generating parts  41  and higher than the temperature of the conductive parts  36 A and  36 B. Consequently, the temperature gradient in the sub-scanning direction y is reduced, as compared with the case where the heat generating parts  41  are immediately adjacent to the conductive parts  36 A and  36 B. In the case where the heat generating parts  41  are immediately adjacent to the conductive parts  36 A and  36 B, the temperature difference around their boundaries can induce the thermal stress that would cause a break. In contrast, the thermal print head A 1  is configured to prevent damage or breakage resulting from thermal stress, so that durability and reliability of the thermal print head A 1  can be improved. In particular, providing a pair of heat generating sub-parts  35 A and  35 B at the opposite ends of each heat generating part  41  in the sub-scanning direction y is effective in reducing the temperature gradient and thus for improving reliability and durability. 
     According to the thermal print head A 1 , each heat generating parts  41  has a heat generating sub-part  35 A on the upstream side in the sub-scanning direction y. Thus, a print medium fed in the sub-scanning direction y is first heated by the heat generating sub-parts  35 A and then by the heat generating parts  41  that is elevated to a higher temperature. Although the heat generating sub-parts  35 A generate higher-temperature heat than the conductive parts  36 A, the temperature is about 150 to 200° C. under the energization conditions where the heat generating parts  41  generates heat of about 300° C. The temperature of this level is not enough to clearly change the color of thermal paper with standard sensitivity, considering the length of time (which is short) taken for the thermal paper to pass over the heat generating sub-parts  35 A in the sub-scanning direction y. Yet, the thermal paper having been heated in advance by the heat generating sub-parts  35 A undergoes change of color more promptly and clearly upon heating by the heat generating parts  41 . This serves to improve print quality and print speed. In addition, the temperature of the heat generating parts  41  required for causing the color of a print medium to change can be lower than the temperature required when the heat generating sub-parts  35 A are not present. The thermal print head A 1  of this embodiment can therefore enhance energy efficiency, reduce the temperature gradient described above, reduce power consumption, and improve durability and reliability. This means that the energy load is not concentrated on the heat generating parts  41  but distributed to the heat generating sub-parts  35 A. Consequently, degradation or deterioration of the heat generating parts  41  is reduced. In addition, since the temperature gradient described above is reduced, the thermal print head A 1  can improve durability and reliability without decreasing printing efficiency. The thermal print head A 1  can therefore achieve energy saving and longevity. 
     According to the thermal print head A 1 , the first conductive layer  301  is made of a material with lower thermally conductivity than the second conductive layer  302 . This means that the heat generating sub-parts  35 A can block the transfer of heat from the heat generating parts  41  to the conductive part  36 A. Consequently, loss of heat generated by the heat generating parts  41  is reduced, so that energy efficiency and thermal response of the thermal print head A 1  can be improved. 
     According to the thermal print head A 1 , the ridge  13  is composed of the top part  130 , the first slopes  131 A and  131 B, and the second slopes  132 A and  132 B, where the first slopes  131 A,  131 B and the second slopes  132 A,  132 B are arranged in the sub-scanning directions y, with the top part  130  located in the middle. Thus, the ridge  13  has a configuration that slopes in two stages with respect to the top part  130  (the first obverse surface  11 ). With this configuration, the first slopes  131 A and  131 B can be inclined at a smaller angle α 1  relative to the top part  130 , which is preferable for improving print quality. A smaller angle α 1  is also preferable for reducing wear of the protective layer  2  caused by a print medium passing over the protective layer  2 . The thermal print head A 1  can therefore improve print quality and longevity. 
     According to the thermal print head A 1 , the heat generating parts  41  are located on the first slope  131 B. Consequently, the platen roller  91  can be arranged such that the center of contact  910  (see  FIG.  4   ) with the heat generating parts  41  is offset downstream in the sub-scanning direction y from the ridge  13 , without degrading print quality. With this arrangement, it is easier to avoid interference between the platen roller  91  and the protective resin  78 , so that the dimension of the head substrate  1  in the sub-scanning direction y can be reduced. 
     According to the thermal print head A 1 , each heat generating part  41  extends from the first slope  131 B to the top part  130 . This arrangement allows for misalignment of the platen roller  91  in the sub-scanning direction y without degrading print quality. 
     According to the thermal print head A 1 , the heat generating sub-parts  35 A are located on the top part  130  but not on the first slope  131 A. In a configuration different from the thermal print head A 1 , the heat generating sub-parts  35  may be disposed to extend from the top part  130  to the first slope  131 A. Such a configuration aims to allow for misalignment of the platen roller  91  in the sub-scanning direction y. With recent improvements in manufacturing accuracy, however, the possibility is minimized that the center of contact  910  deviates to a position upstream from the top part  130  even if the platen roller  91  is misaligned in the sub-scanning direction y. In addition, the heat generating sub-parts  35 A do not contribute much to printing, and energy loss increases with the size of heat generating sub-parts  35 A. That is, the thermal print head A 1  is configured to reduce energy loss and prevent the reduction of printing efficiency resulting from the energy loss, as compared with the configuration in which the heat generating sub-parts  35 A extend from the top part  130  to the first slope  131 A. In other words, the thermal print head A 1  is provided with the heat generating sub-parts  35 A to reduce the temperature gradient, and yet the size (formation areas) of the heat generating sub-parts  35 A is arranged to reduce or minimize reduction of printing efficiency resulting from energy loss. 
     According to the thermal print head A 1 , since the common electrode  32  is located on the downstream side of the heat generating parts  41  in the sub-scanning direction y, the individual electrodes  31  are located on the upstream side separately from the common electrode  32 . As such, the pitch of the individual electrodes  31  in the main scanning direction x can be reduced to increase he printing resolution. 
     According to one example of the thermal print head A 1 , the first conductive layer  301  is made of Ti, and the second conductive layer  302  is made of Cu. This means that the resistance value per unit length in the sub-scanning direction y is higher at the heat generating sub-parts  35 A and  35 B where the first conductive layer  301  is not covered with the second conductive layer  302  than at the conductive parts  36 A and  36 B where the first conductive layer  301  and the second conductive layer  302  are stacked. In addition, the first conductive layer  301  is thinner than the second conductive layer  302 , and thus the cross section of the wiring layer  3  is smaller at the heat generating sub-parts  35 A and  35 B than at the conductive parts  36 A and  36 B. This also contribute to the configuration that the resistance value per unit length in the sub-scanning direction y is higher at the heat generating sub-parts  35 A and  35 B than at the conductive parts  36 A and  36 B. 
       FIGS.  17  to  20    show a thermal print head B 1  according to a second embodiment. 
       FIG.  17    is an enlarged fragmentary sectional view showing a thermal printer Pr installed with the thermal print head B 1 . This figure corresponds to  FIG.  4    showing the sectional view of the first embodiment.  FIG.  18    is a fragmentary sectional view showing the thermal print head B 1  and corresponds to  FIG.  5    showing the sectional view of the first embodiment.  FIG.  19    is a fragmentary enlarged plan view of the thermal print head B 1 .  FIG.  20    is an enlarged sectional view taken along line XX-XX of  FIG.  19   . 
     The thermal print head B 1  includes the ridge  13  along the downstream edge of the head substrate  1  in the sub-scanning direction y. That is, no part of the first obverse surface  11  is located downstream from the ridge  13  in the sub-scanning direction y. Thus, the wiring layer  3  of this embodiment is arranged as shown in  FIG.  20    to have the downstream end of each conductive part  36 B overlapping with the second slope  132 B. 
     As shown in  FIG.  19   , the wiring layer  3  of this embodiment includes a plurality of individual electrodes  31 , a plurality of common electrodes  32  and a plurality of relay electrodes  33 . 
     As shown in  FIG.  19   , the individual electrodes  31  and the common electrodes  32  are located on the upstream side of the heat generating parts  41  in the sub-scanning direction y. The relay electrodes  33  are located on the downstream side of the heat generating parts  41  in the sub-scanning direction y. The individual electrodes  31  and the common electrodes  32  are arranged substantially parallel to each other at predetermined pitch in the main scanning direction x. The relay electrodes  33  are arranged at a predetermined pitch in the main scanning direction x. Each relay electrode  33  is shaped to form a conductive path that is reversely bent in the sub-scanning direction y. Each relay electrode  33  extends from the first slope  131 B to the second slope  132 B of the ridge  13 . 
     With reference to  FIG.  19   , the common electrodes  32  are described below referring to, as an exemplary example, the left one of the two common electrodes indicated by reference numeral  32 ( 3 ). As shown in  FIG.  19   , the common electrode  32  has a branching part  325  and two adjacent strip parts  324 . The two strip parts  324  are located at the downstream end of the common electrode  32  in the sub-scanning direction y. The branching part  325  is also a downstream part of the common electrode  32  as a whole and connected to the two strip parts  324 . The branching part  325  is connected, via the two strip parts  324 , to a pair of mutually adjacent heat generating parts  41  (the fourth and fifth ones from the left in  FIG.  19   ) on the upstream side in the sub-scanning direction y. The two heat generating parts  41  are each connected to a part of a corresponding one of the two mutually adjacent relay electrodes  33  (i.e., to the part of each relay electrode  33  that is closer to the other relay electrode  33 ) on the downstream side in the sub-scanning direction y. The other part of each relay electrode  33  (the part of each relay electrode  33  that is away from the other relay electrode  33 ) is connected to a corresponding one of two other heat generating parts  41  (the third and sixth ones from the left in  FIG.  19   ) on the downstream side in the sub-scanning direction y. That is, the common electrode  32  is connected to a first pair of mutually adjacent heat generating parts  41  (the fourth and fifth ones from the left in  FIG.  19   ) and further to a second pair of heat generating parts  41  (the third and sixth ones from the left) flanking the first pair in the main scanning direction x (on the right and left in  FIG.  19   ). The second pair of heat generating parts  41  are adjacent to two individual electrodes  31  (the two individual electrodes  31  flanking the exemplary common electrode  32 ). 
     According to the arrangement described above, one common electrode  32  forms two adjacent conductive paths. Each of the two conductive paths includes, in order of connection, the common electrode  32 , i.e., one branching part  325  and one of two strip parts  324 ), a first heat generating part  41 , a relay electrode  33  and a second heat generating part  41  adjacent to the first heat generating part  41 , and an individual electrode  31 . Energizing one individual electrode  31  will energize the two heat generating parts  41  that are adjacent to each other in the main scanning direction and electrically connected between the one individual electrode  31  and a common electrode  32 . Such two adjacent heat generating parts  41  correspond to one dot on a print medium. 
     As shown in  FIG.  17   , the center of contact  910  between the platen roller  91  and each heat generating part  41  is positioned downstream from the ridge  13  of the head substrate  1  in the sub-scanning direction y. That is, the platen roller  91  is pressed against the heat generating parts  41  disposed on the ridge  13  via the protective layer  2 , at an angle inclined toward the downstream in the sub-scanning direction y. 
     Similarly to the thermal print head A 1 , the thermal print head B 1  includes the heat generating sub-parts  35 A and  35 B each of which is located between a heat generating part  41  and a conductive part  36 A or  36 B. Consequently, the temperature gradient in the sub-scanning direction y is reduced, as compared within the case where the heat generating parts  41  are immediately adjacent to the conductive parts  36 A and  36 B. Similarly to the first embodiment, the thermal print head B 1  can therefore improve durability and reliability. 
     According to the thermal print head B 1 , no part of the first obverse surface  11  is located downstream from the ridge  13  in the sub-scanning direction y. With this configuration, the downstream part of the head substrate  1  in the sub-scanning direction y can be shorter. Consequently, the possibility is reduced that a print medium being transported makes contact with a part of the head substrate  1  that is downstream from the ridge  13  in the sub-scanning direction y. This means that a print medium P 1  can be fed through a straight path as shown in  FIG.  17    without being curved or bent. This is preferable for providing a straight-path feeding mechanism to the thermal printer Pr installed with the thermal print head B 1 . The thermal printer Pr with the straight-path feeding mechanism can print on such a print medium as a plastic card having a thermal layer. 
     According to the second embodiment, no part of the first obverse surface  11  is located downstream from the ridge  13  in the sub-scanning direction y. However, the present disclosure is not limited to this. In one variation, the first obverse surface  11  may have a relatively small part located downstream from the ridge  13  in the sub-scanning direction y, as compared with the thermal print head A 1 .  FIG.  21    is an enlarged fragmentary sectional view of a thermal print head B 2  according to the variation. This figure corresponds to the sectional view shown in  FIG.  20   . According to the thermal print head B 2 , the first obverse surface  11  has a small part located downstream from the ridge  13  in the sub-scanning direction y. The thermal print head B 2  can therefore achieve the same advantages as the thermal print head B 1 . That is, a print medium can be transported without making contact with a part of the head substrate  1  that is downstream from the ridge  13  in the sub-scanning direction y. Similarly to the thermal print head B 1 , the thermal print head B 2  shown in  FIG.  21    is preferable for providing a straight-path feeding mechanism. 
       FIGS.  22  and  23    show a thermal print head C 1  according to a third embodiment.  FIG.  22    is a fragmentary enlarged plan view of the thermal print head C 1  and corresponds to  FIG.  5   .  FIG.  23    is a fragmentary enlarged plan view of the thermal print head C 1  and corresponds to  FIG.  6   . 
     As shown in  FIGS.  22  and  23   , the resistive layer  4  and the wiring layer  3  of the thermal print head C 1  is stacked in a different order than those of the thermal print head A 1 . The thermal print head C 1  includes the wiring layer  3  (the first conductive layer  301  and the second conductive layer  302 ) disposed on the head substrate  1  (the first obverse surface  11  and the ridge  13 ) via the insulating layer  19 , and the resistive layer  4  disposed on the wiring layer  3 . 
     In the method for manufacturing the thermal print head C 1 , the resistive layer  4  is formed after the wiring layer  3 . Specifically, the method for manufacturing the thermal print head A 1  is modified such that the step of forming the insulating layer  19  (see  FIG.  11   ) is not followed by the resistive film deposition step but by the first deposition step and the second deposition step in the stated order. Then, the first partial removal step and the second partial removal step are performed. Through these steps, the wiring layer  3  (the first conductive layer  301  and the second conductive layer  302 ) is formed on the insulating layer  19 . In other words, the wiring layer forming step is performed before the resistive film deposition step. Subsequently, the resistive film deposition step and the resistive film partial removal step are performed in the stated order. Through these steps, the resistive layer  4  is formed on the wiring layer  3  and also on the parts of the insulating layer  19  exposed from the wiring layer  3 . Thereafter, the protective layer  2  is formed though the same steps as in the method for manufacturing the thermal print head A 1 . 
     Similarly to the thermal print head A 1 , the thermal print head C 1  includes the heat generating sub-parts  35 A and  35 B each of which is located between a heat generating part  41  and a conductive part  36 A or  36 B. Consequently, the temperature gradient in the sub-scanning direction y is reduced, as compared with the case where the heat generating parts  41  are immediately adjacent to the conductive parts  36 A and  36 B. Similarly to the first embodiment, the thermal print head C 1  can therefore improve durability and reliability. 
     According to the thermal print head C 1 , the wiring layer  3  (the first conductive layer  301  and the second conductive layer  302 ) and the resistive layer  4  are stacked on the insulating layer  19  in the stated order. That is, in the method for manufacturing the thermal print head C 1 , the resistive layer  4  is formed after the wiring layer  3  is formed on the insulating layer  19 . In the method for manufacturing the thermal print head A 1 , the resistive film  4 K, the first conductive film  301 K and the second conductive film  302 K are deposited in the stated order, and then parts of the first conductive film  301 K and the second conductive film  302 K are removed by etching, for example. Since the etching of the resistive film  4 K, the first conductive film  301 K and the second conductive film  302 K is sequentially performed after all of these films are deposited, the transportation work between the deposition apparatus and the etching apparatus is reduced. However, when each of the first conductive film  301 K and the second conductive film  302 K is etched, the resistive film  4 K is also placed in the environment for the etching. The resistive film  4 K may be damaged, depending on the material of the resistive film  4 K or the process used for etching each of the first conductive film  301 K and the second conductive film  302 K. In contrast, according to the thermal print head C 1 , the resistive film  4 K (the resistive layer  4 ) is formed after the first conductive layer  301  and the second conductive layer  302  are processed (in the first partial removal step and the second partial removal step) and thus without a risk of damaging the resistive film  4 K. The present embodiment can therefore reduce the risk of damaging the resistive layer  4  (the heat generating parts  41 ) during processing. 
     According to the third embodiment, the first conductive layer  301  and the second conductive layer  302  may be stacked in reverse of the order described above.  FIGS.  24  and  25    show a thermal print head C 2  according to such a variation.  FIG.  24    is a fragmentary enlarged plan view of the thermal print head C 2  and corresponds to  FIG.  22   .  FIG.  25    is a fragmentary enlarged plan view of the thermal print head C 2  and corresponds to  FIG.  23   . 
     As shown in  FIGS.  24  and  25   , the thermal print head C 2  includes the wiring layer  3  formed by stacking the second conductive layer  302  and the first conductive layer  301  on the insulating layer  19  in the state order, and the resistive layer  4  is stacked on the first conductive layer  301 . According to the thermal print head C 2 , the heat generating sub-parts  35 A and  35 B are formed by parts of the first conductive layer  301  not stacked on the second conductive layer  302 , in other words, by parts of the first conductive layer  301  that are in contact with the insulating layer  19 . 
     The thermal print head C 2  can achieve the same advantages as the thermal print head C 1 . 
     According to the thermal print heads C 1  and C 2  shown in  FIGS.  22  and  24   , the resistive layer  4  is interposed between the individual-electrode pads  311  and the wiring layer  3 . The resistive layer  4 , however, does not significantly affect the electrical continuity between each individual-electrode pad  311  and the wiring layer  3  due to the size of each individual-electrode pad  311  in plan view and the small thickness of the resistive layer  4 . Yet, for better electrical continuity, it is preferable not to place the resistive layer  4  between the wiring layer  3  and the individual-electrode pads  311 . 
     The thermal print heads C 1  and C 2  may also be modified such that no part of the first obverse surface  11  is located downstream from the ridge  13  in the sub-scanning direction y as in the second embodiment ( FIG.  20   ), or only a small part of the first obverse surface  11  is located downstream as in the variation of the second embodiment ( FIG.  21   ). 
       FIGS.  26  and  27    show a thermal print head D 1  according to a fourth embodiment.  FIG.  26    is a fragmentary sectional view showing the thermal print head D 1  and corresponds to  FIG.  3    of the first embodiment.  FIG.  27    is a fragmentary sectional view showing the thermal print head D 1  and corresponds to  FIG.  6    of the first embodiment. 
     As shown in  FIGS.  26  and  27   , the thermal print head D 1  has the resistive layer  4  and the first conductive layer  301  covering the regions different from those in the thermal print head A 1 . Specifically, as shown in  FIG.  27   , the resistive layer  4  extends from the top part  130  to the first slope  131 B. That is, the resistive layer  4  is not disposed on the first slope  131 A, the second slopes  132 A and  132 B and the first obverse surface  11 . The first conductive layer  301  is partly disposed on the resistive layer  4 , and the other part is disposed directly on the insulating layer  19 . The first conductive layer  301  has a plurality of segments, including those disposed on the top part  130  and those extending from the first slope  131 B to the second slope  132 B. The second conductive layer  302  is partly disposed on the first conductive layer  301 , and the other part is disposed directly on the insulating layer  19 . The second conductive layer  302  has a plurality of segments, including those extending from the top part  130  along the first slope  131 A and the second slope  132 A to reach the first obverse surface  11  and those extending from the second slope  132 B to the first obverse surface  11 . As described above, the wiring layer  3  (the first conductive layer  301  and the second conductive layer  302 ) and the resistive layer  4  are disposed more locally in the thermal print head D 1  than in the thermal print head A 1 . According to the thermal print head D 1 , each the heat generating sub-parts  35 A and  35 B is formed by a part of the first conductive layer  301  exposed from the second conductive layer  302 , i.e., a part not overlapping with the second conductive layer  302  as viewed in the z direction. Each of the conductive parts  36 A and  36 B is formed by a part of the wiring layer  3  where the second conductive layer  302  is present. 
     A method for manufacturing the thermal print head D 1  includes, in sequence, the resistive film deposition step, the resistive film partial removal step, the first deposition step, the first partial removal step, the second deposition step and the second partial removal step. Through these steps, the resistive layer  4  and the wiring layer  3  (the first conductive layer  301  and the second conductive layer  302 ) are sequentially formed. In other words, the wiring layer forming step is performed after the resistive film deposition step. In this way, as shown in  FIGS.  26  and  27   , the resistive layer  4  and the first conductive layer  301  are deposited in more limited regions in the thermal print head D 1  than in the thermal print head A 1 . 
     Similarly to the thermal print head A 1 , the thermal print head D 1  includes the heat generating sub-parts  35 A and  35 B each of which is located between a heat generating part  41  and a conductive part  36 A or  36 B. Consequently, the temperature gradient in the sub-scanning direction y is reduced, as compared with the case where the heat generating parts  41  are immediately adjacent to the conductive parts  36 A and  36 B. Similarly to the first embodiment, the thermal print head D 1  can therefore improve durability and reliability. 
     As shown in  FIGS.  26  and  27   , the resistive layer  4  and the first conductive layer  301  are disposed more locally in the thermal print head D 1  than in the thermal print head A 1 . The thermal print head D 1  therefore allows greater design flexibility as to the sizes and locations of the heat generating parts  41 , the heat generating sub-parts  35 A and  35 B, and the conductive parts  36 A and  36 B. In addition, the material costs can be reduced as compared to the thermal print head A 1 . 
     The thermal print head D 1  is a variation of the thermal print head A 1  having the resistive layer  4  and the first conductive layer  301  disposed more locally. Such a modification may also be made to other configurations. For example, the thermal print heads C 1  and C 2  may be modified such that the resistive layer  4   a  and the first conductive layer  301  are disposed locally.  FIG.  28    is a fragmentary enlarged plan view of a thermal print head D 2  that is a variation of the thermal print head C 1  modified such that the resistive layer  4   a  and the first conductive layer  301  are disposed locally.  FIG.  29    is a fragmentary enlarged plan view of a thermal print head D 3  that is a variation of the thermal print head C 2  modified such that the resistive layer  4   a  and the first conductive layer  301  are disposed locally. 
     As shown in  FIGS.  28  and  29   , the thermal print heads D 2  and D 3  have the resistive layer  4  and the first conductive layer  301  disposed locally in limited regions. As with the thermal print head D 1 , this allows greater design flexibility as to the sizes and locations of the heat generating parts  41 , the heat generating sub-parts  35 A and  35 B, and the conductive parts  36 A and  36 B. In addition, the material costs can be reduced as compared to the thermal print head A 1 . Notably, in a method for manufacturing of the thermal print heads according to the variations shown in  FIGS.  28  and  29   , the resistive film deposition step is performed after the wiring layer forming step (that is, the resistive film deposition step is performed after the first partial removal step and the second partial removal step). Consequently, the risk of damaging the resistive layer  4  during processing is reduced, as with the thermal print heads C 1  and C 2  (see  FIGS.  22  to  25   ) according to the third embodiment and the variation thereof. 
     The thermal print heads D 1  to D 3  may also be modified such that no part of the first obverse surface  11  is located downstream of the ridge  13  in the sub-scanning direction y as in the second embodiment ( FIG.  20   ), or that only a small part of the first obverse surface  11  is located downstream as in the variation of the second embodiment ( FIG.  21   ).  FIG.  30    shows a variation of the thermal print head D 1  modified such that a small part of the first obverse surface  11  is located downstream as in the variation of the second embodiment.  FIG.  30    is a fragmentary enlarged sectional view of the thermal print head according to this variation. 
       FIGS.  31  and  32    show a thermal print head E 1  according to a fifth embodiment.  FIG.  31    is a fragmentary enlarged plan view of the thermal print head E 1  and corresponds to  FIG.  6    of the first embodiment.  FIG.  32    is a fragmentary enlarged plan view of the thermal print head E 1  and corresponds to  FIG.  3    of the first embodiment. 
     As shown in  FIGS.  31  and  32   , the thermal print head E 1  differs from the thermal print head A 1  in the configuration of the wiring layer  3 . The wiring layer  3  of the thermal print head E 1  is composed of a single conductive layer  300 . This embodiment is a variation of the thermal print head A 1  that includes the conductive layer  300  instead of the first conductive layer  301  and the second conductive layer  302 . The thermal print heads of other embodiments may also be modified by replacing the first conductive layer  301  and the second conductive layer  302  with the conductive layer  300 . 
     The conductive layer  300  may be made of Cu as with the second conductive layer  302 . As shown in  FIG.  31   , the conductive layer  300  has a thicker part  300   a  and a thinner part  300   b  of different thicknesses. The thicker part  300   a  is thicker than the thinner part  300   b . Since the thinner part  300   b  is smaller in cross section than the thicker part  300   a , the resistance value per unit length in the sub-scanning direction y is higher in the thinner part  300   b  than in the thicker part  300   a . In addition, the resistance value of the thinner part  300   b  per unit length in the sub-scanning direction y is lower than the resistance value of the resistive layer  4  (heat generating parts  41 ) per unit length in the sub-scanning direction y. The thinner parts  300   b  form the heat generating sub-parts  35 A and  35 B, and the thicker parts  300   a  form the conductive parts  36 A and  36 B. The thicknesses of the thicker parts  300   a  and the thinner parts  300   b  are not limited as long as the above-described relation of the resistance values per unit length in the sub-scanning direction y is satisfied. 
     Similarly to the thermal print head A 1 , the thermal print head E 1  includes the heat generating sub-parts  35 A and  35 B each of which is located between a heat generating part  41  and a conductive part  36 A or  36 B. Thus, the temperature gradient in the sub-scanning direction y is reduced, as compared with the case where the heat generating parts  41  are immediately adjacent to the conductive parts  36 A and  36 B. Similarly to the first embodiment, the thermal print head E 1  can therefore improve durability and reliability. 
     According the fifth embodiment, the heat generating sub-parts  35 A and  35 B (i.e., the thinner parts  300   b ) are rectangular as viewed in the thickness direction z. However, the shape of the heat generating sub-parts  35 A and  35 B is not limited to a rectangle. In one variation, patterning may be applied to the thinner parts  300   b .  FIG.  33    is an enlarged fragmentary sectional view of a thermal print head E 2  according to this variation and corresponds to  FIG.  32   . As shown in  FIG.  33   , each thinner part  300   b  of the thermal print head E 2  is patterned into a comb-like shape as viewed in the thickness direction z. The thinner part  300   b  may be patterned into a shape other than the comb-like shape shown in  FIG.  33   . Patterning the conductive layer  300  in this way can reduce the cross-sectional areas of the heat generating sub-parts  35 A and  35 B, thereby adjusting the resistance values of the heat generating sub-parts  35 A and  35 B per unit length in the sub-scanning direction y. According to the thermal print head E 2 , the heat generating sub-parts  35 A and  35 B are formed by thinning the conductive layer  300  (by providing the thinner parts  300   b ) and then patterning. However, the present disclosure is not limited to this. For example, patterning may be applied to the conductive layer  300  of a uniform thickness (that is not processed to form the thinner parts  300   b ). 
     The method for manufacturing a thermal print head, a thermal printer and a thermal print head according to the present disclosure is not limited to the foregoing embodiments. Further, the specific configuration of each part of the thermal print head, the thermal printer and the thermal print head according to the present disclosure may be modified in design in many ways. The present disclosure includes the configurations described in the following clauses. 
     Clause 1. 
     A thermal print head comprising: 
     a substrate made of a single crystal semiconductor and including an obverse surface facing in one sense of a thickness direction; 
     a resistive layer supported by the substrate and including a plurality of heat generating parts arranged side by side in a main scanning direction; and 
     a wiring layer supported by the substrate and forming a conductive path to the plurality of heat generating parts, 
     wherein the wiring layer includes a conductive part and a heat generating sub-part for each of the plurality of heat generating parts, the conductive part having a lower resistance value per unit length in a sub-scanning direction than the heat generating part, the heat generating sub-part having a resistance value per unit length in the sub-scanning direction that falls between the respective resistance values of the heat generating part and the conductive part, 
     the substrate includes a ridge raised from the obverse surface and extending in the main scanning direction, 
     the heat generating part, the heat generating sub-part and the conductive part are disposed on the ridge, and 
     the heat generating sub-part is located between the heat generating part and the conductive part in the sub-scanning direction. 
     Clause 2. 
     The thermal print head according to clause 1, 
     wherein the ridge includes a top part that is most distant from the obverse surface, an upstream-side first slope connected to the top part on an upstream side in the sub-scanning direction, and a downstream-side first slope connected to the top part on a downstream side in the sub-scanning direction, 
     the upstream-side first slope and the downstream-side first slope are inclined to the obverse surface at a first inclination angle, and 
     the heat generating part extends from the downstream-side first slope to the top part. 
     Clause 3. 
     The thermal print head according to clause 2, 
     wherein the ridge includes an upstream-side second slope connected to the upstream-side first slope on an opposite side from the top part in the sub-scanning direction, and a downstream-side second slope connected to the downstream-side first slope on an opposite side from the top part in the sub-scanning direction, 
     the upstream-side second slope and the downstream-side second slope are inclined to the obverse surface at a second inclination angle, and 
     the second inclination angle is greater than the first inclination angle. 
     Clause 4. 
     The thermal print head according to clause 3, wherein the heat generating sub-part includes an upstream-side heat generating sub-part located upstream from the heat generating part in the sub-scanning direction, and a downstream-side heat generating sub-part located downstream from the heat generating part in the sub-scanning direction. 
     Clause 5. 
     The thermal print head according to clause 4, wherein the conductive part includes an upstream-side conductive part adjacent to the upstream-side heat generating sub-part on an opposite side from the heat generating part in the sub-scanning direction, and a downstream-side conductive part adjacent to the downstream-side heat generating sub-part on an opposite side from the heat generating part in the sub-scanning direction. 
     Clause 6. 
     The thermal print head according to clause 5, wherein the upstream-side heat generating sub-part is disposed on the top part. 
     Clause 7. 
     The thermal print head according to clause 6, wherein the upstream-side conductive part extends from the top part along the upstream-side first slope and the upstream-side second slope to reach the obverse surface. 
     Clause 8. 
     The thermal print head according to any one of clauses 5 to 7, wherein the downstream-side heat generating sub-part extends from the downstream-side second slope to the downstream-side first slope. 
     Clause 9. 
     The thermal print head according to clause 8, wherein the downstream-side conductive part is disposed on the downstream-side second slope. 
     Clause 10. 
     The thermal print head according to any one of clauses 1 to 9, 
     wherein the wiring layer and the resistive layer overlap with each other at least in part as viewed in the thickness direction, and 
     each of the plurality of heat generating parts is formed by a part of the resistive layer not overlapping with the wiring layer as viewed in the thickness direction. 
     Clause 11. 
     The thermal print head according to clause 10, 
     wherein the wiring layer includes a first conductive layer and a second conductive layer stacked in the thickness direction, 
     the conductive part is formed by a part where the second conductive layer is present, and 
     the heat generating sub-part is formed by a part of the first conductive layer not overlapping with the second conductive layer as viewed in the thickness direction. 
     Clause 12. 
     The thermal print head according to clause 11, 
     wherein the resistive layer is disposed on the substrate, 
     the first conductive layer is disposed on the resistive layer such that a part of the resistive layer remains exposed, and 
     the second conductive layer is disposed on the first conductive layer such that a part of the first conductive layer remains exposed. 
     Clause 13. 
     The thermal print head according to clause 11, 
     wherein the first conductive layer is disposed on the substrate, 
     the second conductive layer is disposed on the first conductive layer such that a part of the first conductive layer remains exposed, and 
     the resistive layer is disposed on the substrate and at least overlaps with the part of the first conductive layer exposed from the second conductive layer as viewed in the thickness direction. 
     Clause 14. 
     The thermal print head according to any one of clauses 11 to 13, wherein the first conductive layer is thinner than the second conductive layer. 
     Clause 15. 
     The thermal print head according to any one of clauses 11 to 14, wherein the first conductive layer is made of a material having a lower heat conductivity than that of the second conductive layer. 
     Clause 16. 
     The thermal print head according to clause 10, 
     wherein the wiring layer includes a thicker part and a thinner part having mutually different dimensions in the thickness direction, 
     the heat generating sub-part is formed by the thinner part, and 
     the conductive part is formed by the thicker part. 
     Clause 17. 
     The thermal print head according to clause 16, wherein the thinner part is patterned as viewed in the thickness direction. 
     Clause 18. 
     The thermal print head according to any one of clauses 1 to 17, wherein the single crystal semiconductor is Si. 
     Clause 19. 
     A thermal printer comprising: 
     the thermal print head according to any one of clauses 1 to 18; and 
     a platen directly opposite the thermal print head. 
     Clause 20. 
     A method for manufacturing a thermal print head, comprising: 
     a substrate preparing step of preparing a substrate made of a single crystal semiconductor; 
     a substrate processing step of processing the substrate to form an obverse surface facing in one sense of a thickness direction and a ridge that is raised from the obverse surface and extends in a main scanning direction; 
     a resistive layer forming step of forming a resistive layer that is supported by the substrate and includes a plurality of heat generating parts arranged side by side in the main scanning direction; and 
     a wiring layer forming step of forming a wiring layer that is supported by the substrate and forms a conductive path to the plurality of heat generating parts, 
     wherein the wiring layer includes a conductive part and a heat generating sub-part for each of the plurality of heat generating parts, the conductive part having a lower resistance value per unit length in a sub-scanning direction than the heat generating part, the heat generating sub-part having a resistance value per unit length in the sub-scanning direction that falls between the respective resistance values of the heat generating part and the conductive part, 
     the heat generating part, the heat generating sub-part and the conductive part are formed on the ridge, and 
     the heat generating sub-part is located between the heat generating part and the conductive part in the sub-scanning direction. 
     Clause 21. 
     The method according to clause 20, 
     wherein the resistive layer forming step includes a resistive film deposition step of depositing a resistive film, 
     the wiring layer forming step includes a first deposition step of depositing a first conductive film, a first partial removal step of removing a part of the first conductive film to form a first conductive layer, and a second deposition step of depositing a second conductive film, and a second partial removal step of removing a part of the second conductive film to form a second conductive layer, 
     the first conductive layer and the second conductive layer are stacked in the thickness direction, 
     the conductive part is formed by a part where the second conductive layer is present, and 
     the heat generating sub-part is formed by a part of the first conductive layer not overlapping with the second conductive layer as viewed in the thickness direction. 
     Clause 22. 
     The method according to clause 21, wherein the resistive film deposition step is performed before the wiring layer forming step. 
     Clause 23. 
     The method according to clause 21, wherein the resistive film deposition step is performed after the wiring layer forming step. 
     REFERENCE NUMERALS 
     
         
         A 1 , B 1 , B 2 , C 1 , C 2 , D 1  to D 3 , E 1 , E 2 : thermal print head 
           1 : head substrate  1 K: material substrate 
           11 ,  11 K: first obverse surface  12 ,  12 K: first reverse surface 
           13 ,  13 K: ridge  130 ,  130 K: top part 
           131 A,  131 B: first slope  132 A,  132 B: second slope 
           132 K: slope  19 : insulating layer  2 : protective layer 
           21 : pad opening  3 : wiring layer  300 : conductive layer 
           300   a : thicker part  300   b : thinner part  301 : first conductive layer 
           302 : second conductive layer  3 K: wiring film  301 K: first conductive film 
           302 K: second conductive film  31 : individual electrode  311 : individual-electrode pad 
           32 : common electrode  323 : connecting part  324 : strip part 
           325 : branching part  33 : relay electrode  35 A,  35 B: heat generating sub-part 
           36 A,  36 B: conductive part  4 : resistive layer 
           4 K: resistive film  41 : heat generating part  5 : connecting substrate 
           51 : second obverse surface  52 : second reverse surface  59 : connector 
           61 : wire  62 : wire  7 : driver IC 
           78 : protective resin  8 : heat dissipating member  81 : first supporting surface 
           82 : second supporting surface Pr: thermal printer 
           91 : platen roller  910 : center of contact 
         x: main scanning direction y: sub-scanning direction z: thickness direction