Patent Publication Number: US-9417574-B2

Title: Heater and method of forming a heater

Description:
CLAIM OF PRIORITY 
     This application claims benefit of priority to Japanese Patent Application No. 2014-216010 filed on Oct. 23, 2014, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The present disclosure relates to a heater used as a heat source of a toner fixing device in a copying machine or the like. 
     2. Description of the Related Art 
     In recent years, a heater having a linear heat generator, having a low heat capacity, has been used as a heat source of a toner fixing device in a copying machine, an electro-photographic printer, or the like. A toner fixing device in which such a heater is incorporated has the advantage of achieving power saving and a reduction in wait time compared to a toner fixing device employing a heat roller system, a heat plate system, a flash fixing system, an oven fixing system, or the like. 
     Examples of such heaters include a heating element of a fixing device described in Japanese Unexamined Patent Application Publication No. 2-157886, and a heating element of a fixing device described in Japanese Unexamined Patent Application Publication No. 9-068877. 
     The heating element  820  described in Japanese Unexamined Patent Application Publication No. 2-157886 will be described below with reference to  FIG. 13 .  FIG. 13  is a plan view showing the configuration of the heating element  820 . 
     As shown in  FIG. 13 , the heating element  820  includes a heater body  821   a  and a heater body holding member  820   a . The heater body  821   a  includes an alumina substrate  821 , a heat generator  822  made of silver-palladium (Ag/Pd) that is formed substantially in the center of the surface of the alumina substrate  821  along the longitudinal direction by screen printing in a linear or band-like shape, and a wear-resistant material such as glass. The temperature of a fixing nip part corresponding to the heat generator  822  is maintained and controlled at a predetermined fixing temperature. Using a side wall surface  820   k  of a horizontally long slot  820   b  of the heater body holding member  820   a  as a surface reference K, the heater body  821   a  is fitted into the slot  820   b  with a longitudinal side wall surface  821   k  of the alumina substrate  821  in close contact with the surface reference K, and is integrally attached to and held by the heater body holding member  820   a . Owing to the surface reference K, the heat generator  822  can be positioned. 
     In the fixing device described in Japanese Unexamined Patent Application Publication No. 9-068877, in order to improve the positional precision, the following structure is discussed. The fixing device  911  described in Japanese Unexamined Patent Application Publication No. 9-068877 will be described below with reference to  FIGS. 14 and 15 .  FIG. 14  is a sectional view showing the schematic configuration of the fixing device  911 .  FIG. 15  is a bottom view showing a heater substrate  922  of the heating element  920  of the fixing device  911 . 
     As shown in  FIG. 14 , the fixing device  911  includes a fixing film  925 , a moving driving means for the fixing film  925 , the heating element  920 , and a pressure roller  928 . The heating element  920  includes a heater support  921  and the heater substrate  922 . The heater substrate  922  is a flat plate formed of alumina, aluminum nitride, or the like, and is embedded in the heating element  920 . The heating element  920  and the pressure roller  928  are disposed opposite to each other with the fixing film  925  interposed therebetween. As shown in  FIG. 15 , the heater substrate  922  is provided with a heat generator  923  formed integrally with electrodes  931 , and marks  930  for position adjustment of the heat generator. The heat generator  923  and the marks  930  for position adjustment of the heat generator are printed patterns formed by collective printing using the same printing plate. 
     However, if the positional precision of the linear heat generator is low, toner disposed on a recording material cannot be heated appropriately. In the case of the heating element  820  of the fixing device described in Japanese Unexamined Patent Application Publication No. 2-157886, the positional precision of the heat generator  822  may be deteriorated owing to the processing variation of the heater body holding member  820   a  and the processing variation of the alumina substrate  821 . In the case of the fixing device  911  described in Japanese Unexamined Patent Application Publication No. 9-068877, the heat generator  923  and the electrodes  931  are often covered with a protective layer in order to secure durability and insulation thereof. When a protective layer is formed over the marks  930  for position adjustment of the heat generator, the outlines of the marks  930  for position adjustment of the heat generator may not be clearly visible. In particular, when the protective layer is formed by screen printing, surface roughness due to a mesh of a printing plate is left on the surface, therefore light is scattered, the outlines of the marks  930  for position adjustment of the heat generator are not clearly visible, and highly precise positioning is difficult. Therefore, with such a structure of the heat generator, highly precise positioning cannot be performed easily. 
     SUMMARY 
     In an aspect of the present invention, a heater includes a heater unit and a holder. The heater unit includes a flat and smooth substrate, a linear heat generator provided on a first surface of the substrate, a plurality of electrodes configured to supply power to the heat generator, and a protective layer disposed to cover a part of each of the electrodes and the heat generator. A second surface of the substrate is bonded to the holder. The holder is attached to an external device. The substrate is formed of a transparent material. A plurality of positioning marks for determining relative positions of the heater unit and the holder are disposed on the first surface. The holder has through-holes formed at positions opposite to the positioning marks. 
     This configuration makes it possible to observe the positioning marks provided on the first surface of the substrate formed of a transparent material through the through-holes of the holder. In addition, since the substrate is flat and smooth, and the interface between each positioning mark and the substrate is flat and smooth, the positioning marks are clearly visible without being affected by the scattering of light when the positioning marks formed on the first surface of the substrate is viewed from the second surface. The precision of positioning when incorporating the heater into the external device is thereby improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing the configuration of an external device to which a heater of a first embodiment of the present invention is attached; 
         FIG. 2  is an exploded perspective view schematically showing the heater of the first embodiment of the present invention; 
         FIG. 3  is a schematic plan view showing the heater as viewed from the Z1 direction shown in  FIG. 2 ; 
         FIG. 4  is a schematic bottom view illustrating the arrangement of a heating element, electrodes, and a protective layer that are visible through a substrate when a heater unit is viewed from the Z2 direction shown in  FIG. 2 ; 
         FIG. 5  is a schematic sectional view showing the heater unit taken along line V-V of  FIG. 4 ; 
         FIG. 6  is a schematic sectional view showing the heater unit taken along line VI-VI of  FIG. 4 ; 
         FIG. 7  is a schematic plan view showing a holder as viewed from the Z1 direction shown in  FIG. 2 ; 
         FIG. 8  is a schematic bottom view showing the holder as viewed from the Z2 direction shown in  FIG. 2 ; 
         FIG. 9  is a schematic sectional view showing the heater taken along line IX-IX of  FIG. 3 ; 
         FIG. 10  is a schematic sectional view showing the heater taken along line X-X of  FIG. 3 ; 
         FIG. 11  is a schematic plan view of a heater of a modification as viewed from the Z1 side shown in  FIG. 2 ; 
         FIG. 12  is a sectional view at the same position as  FIG. 5  in the modification; 
         FIG. 13  is a plan view showing the configuration of a heating element described in Japanese Unexamined Patent Application Publication No. 2-157886; 
         FIG. 14  is a sectional view showing the schematic configuration of a fixing device described in Japanese Unexamined Patent Application Publication No. 9-068877; and 
         FIG. 15  is a bottom view showing a heater substrate of a heating element described in Japanese Unexamined Patent Application Publication No. 9-068877. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     First Embodiment 
     Embodiments of the present invention will now be described in detail with reference to the drawings. For ease of understanding, dimensions are changed in the drawings. 
       FIG. 1  is a schematic sectional view showing the configuration of an external device  100  to which a heater  1  of a first embodiment of the present invention is attached.  FIG. 2  is an exploded perspective view schematically showing the heater  1  of the first embodiment of the present invention.  FIG. 3  is a schematic plan view showing the heater  1  as viewed from the Z1 direction shown in  FIG. 2 .  FIG. 4  is a schematic bottom view illustrating the arrangement of a heating element  16 , electrodes  15 , and a protective layer  17  that are visible through a substrate  11  when a heater unit  10  is viewed from the Z2 direction shown in  FIG. 2 .  FIG. 5  is a schematic sectional view showing the heater unit  10  taken along line V-V of  FIG. 4 .  FIG. 6  is a schematic sectional view showing the heater unit  10  taken along line VI-VI of  FIG. 4 .  FIG. 7  is a schematic plan view showing a holder  30  as viewed from the Z1 direction shown in  FIG. 2 .  FIG. 8  is a schematic bottom view showing the holder  30  as viewed from the Z2 direction shown in  FIG. 2 .  FIG. 9  is a schematic sectional view showing the heater  1  taken along line IX-IX of  FIG. 3 .  FIG. 10  is a schematic sectional view showing the heater  1  taken along line X-X of  FIG. 3 . In these figures, a connecting part with an external power source is omitted. 
     As shown in  FIG. 1 , the heater  1  of the first embodiment of the present invention is attached for use to the external device  100  that is a toner fixing device. The external device  100  includes the heater  1 , a belt  110 , and a pressure roller  120 . As shown in  FIGS. 2 and 3 , the heater  1  includes the heater unit  10  and the holder  30 . The toner fixing device applies heat and pressure to toner disposed on recording paper, thereby fixing the toner on the recording paper. The external device  100  is configured such that the belt  110  moves with the rotation of the pressure roller  120 . By being pressed against the heater unit  10 , the belt  110  is heated, and fixes the toner on the recording paper. 
     By controlling the heat generation of the heater unit  10  in a desired state just before the toner disposed on the recording paper is nipped between the pressure roller  120  and the belt  110 , power can be saved. 
     The heater unit  10  of the heater  1  is housed in a slot  31  formed in the holder  30 , is positioned as described later, and is then fixed. The planar dimension of the slot  31  is larger than the dimension of the heater unit  10  so that the position adjustment of the heater unit  10  can be performed. For example, adhesive is used for fixing the heater unit  10 , and the Z2 side surface of the heater unit  10  and the opposite surface of the slot  31  of the holder  30  are bonded together with the adhesive. Only part of the heater unit  10  may be bonded. So, for example, part of the opposite surface along the outer periphery of the heater unit  10  may be bonded without bonding the central part of the heater unit  10 . In this case, a recess may be provided in the central part of the slot  31  so that a space is provided between the central part of the slot  31  and the Z2 side surface of the heater unit  10 . In the following description, the Z1 side of  FIG. 2  will be referred to as “the upper side,” and the Z2 side of  FIG. 2  will be referred to as “the lower side.” This is only for ease of understanding of the description, and “the upper side” and “the lower side” in the following description are different from those in the usage state shown in  FIG. 1 . 
     As shown in  FIGS. 4 to 6 , the heater unit  10  includes the flat and smooth substrate  11 , the plurality of electrodes  15 , the linear heat generator  16 , the protective layer  17 , and positioning marks  18 . 
     The substrate  11  is a flat plate-like insulator formed of a transparent material, and has heat resistance. The substrate  11  is preferably formed of glass. 
     The electrodes  15  are conductors formed by firing a conductive coating film patterned by screen printing on a first surface  11   a  of the substrate  11 . The electrodes  15  are preferably formed of a highly conductive material such as silver, gold, or copper. The electrodes  15  include a comb-shaped electrode  12  and L-shaped electrodes  13 . The electrodes  15  are connected to a power source (not shown) at the ends thereof, and supply power supplied from the power source to the heat generator  16 . 
     The heat generator  16  is a resistor that generates heat when supplied with power. The heat generator  16  has a rectangular shape. The heat generator  16  is formed by forming a resistor coating film by screen printing on the first surface  11   a  of the substrate  11  on which the electrodes  15  are formed, and firing the resistor film. The heat generator  16  is preferably formed of an electric resistance material such as RuO2 or Ag/Pd. 
     As shown in  FIG. 4 , the comb-shaped electrode  12  includes a comb-shaped electrode  12   a  provided parallel to the long side of the heat generator  16  and comb-shaped electrodes  12   b  provided perpendicularly to the long side of the heat generator  16 . The comb-shaped electrode  12   a  is disposed a predetermined distance apart from the heat generator  16  in the Y1 direction of the heat generator  16 . The comb-shaped electrodes  12   b  are disposed at regular intervals in the longitudinal direction of the heat generator  16 . The L-shaped electrodes  13  each include an L-shaped electrode  13   a  provided parallel to the long side of the heat generator  16  and an L-shaped electrode  13   b  provided perpendicularly to the long side of the heat generator  16 . The L-shaped electrodes  13   a  are disposed apart from the heat generator  16  in the Y2 direction of the heat generator  16 . The L-shaped electrodes  13   b  are disposed at regular intervals in the longitudinal direction of the heat generator  16  between the comb-shaped electrodes  12   b . In the example shown in  FIG. 4 , the heat generator  16  is divided into six parts. However, the present invention is not limited to this, and the heat generator  16  may be divided into any number of parts. 
     Thus, current when power is supplied from the power source flows between the comb-shaped electrodes  12   b  and the L-shaped electrodes  13   b , and causes the heat generator  16  to generate heat. In other words, the heat generating area of the heat generator  16  is determined by the arrangement of the electrodes  15  and which of the L-shaped electrodes  13   b  are supplied with power. For example, only part of the heating area can be caused to generate heat according to the width of the recording paper, or according to the area where toner is disposed. 
     The positioning marks  18  each include a first positioning mark  18   a  formed in the same step as the heat generator  16  and a second positioning mark  18   b  formed in the same step as the electrodes  15 . As shown in  FIG. 4 , the positioning marks  18  are disposed apart from the comb-shaped electrodes  12   b  and the L-shaped electrode  13   b  located at both longitudinal ends of the heat generator  16 , one on each side of the heat generator  16 . The first positioning mark  18   a  and the second positioning mark  18   b  are provided so as to crisscross in directions parallel to and perpendicular to the longitudinal direction of the heat generator  16 . 
     In this embodiment, the step of forming the electrodes  15  and the second positioning marks  18   b  on the first surface  11   a  of the substrate  11  is performed, and then, the step of forming the heat generator  16  and the first positioning marks  18   a  is performed. As shown in  FIG. 5 , at positions where the electrodes  15  are formed on the first surface  11   a  of the substrate  11 , the heat generator  16  is formed on the upper side (Z1 side) of the electrodes  15 . As shown in  FIG. 6 , at positions where the second positioning marks  18   b  are formed, the first positioning marks  18   a  are formed on the upper side (Z1 side) of the second positioning marks  18   b.    
     The protective layer  17  is a thick film of insulator formed by firing a coating film formed of inorganic paste containing particles by screen printing. The protective layer  17  is formed, for example, of a glass material, and is a thick film containing 0.1 wt % to 30 wt % of hard glass particles having a particle diameter of 0.1 μm to 10 μm. The main component of the protective layer  17  and the size and amount of particles included in the protective layer  17  are not limited to this. As shown in  FIGS. 5 and 6 , the protective layer  17  is formed on substantially the entire first surface  11   a  of the substrate  11  so as to cover the electrodes  15 , the heat generator  16 , and the positioning marks  18 . The ends of the electrodes  15  are exposed from the protective layer  17  so that they can be electrically connected with the power source (not shown). 
     Since part of the protective layer  17  on the heat generator  16  is heated to a high temperature by the heat generation of the heat generator  16 , the protective layer  17  has to be formed of a highly heat-resistant material. In addition, since temperature distribution is produced, the protective layer  17  preferably has a low thermal expansion coefficient and resistance to thermal stress. Further, since the heater  1  is attached for use to the external device  100  that is a toner fixing device as shown in  FIG. 1 , the protective layer  17  is preferably has wear resistance in contact with the belt  110 . 
     Since, in this embodiment, the protective layer  17  contains particles, the physical properties of the protective layer  17  can be modified. For example, in order to improve wear resistance, hard glass particles or alumina particles can be included. By including particles, the thermal expansion coefficient can be adjusted. 
     Owing to surface roughness caused by a mesh of a printing plate when the protective layer  17  is formed by screen printing, light is scattered by the surface when the protective layer  17  is viewed from the Z1 direction, the protective layer  17  is visually white and opaque, and therefore the positioning marks  18  are less visible. In the case of the protective layer  17  formed of inorganic paste containing particles by screen printing, the surface roughness is significant, and the protective layer  17  is whiter and opaquer. 
     The holder  30  is a molding formed by molding synthetic resin. As shown in  FIG. 7 , the holder  30  has a slot  31  formed therein, and through-holes  32  extending from the bottom surface of the slot  31  to the lower side (Z2 side) surface of the holder  30 . As shown in  FIG. 8 , the holder  30  further has reference portions  33 . In this embodiment, the reference portions  33  are holes (see  FIG. 10 ). 
     The heater unit  10  is bonded into the slot  31  of the holder  30  with an adhesive. At this time, positioning can be performed in advance while viewing the positioning marks  18  of the heater unit  10  through the through-holes  32  from the second surface  11   b  side of the substrate  11 . 
     Since the first positioning marks  18   a  of the heater unit  10  are formed in the same step as the heat generator  16 , the positional relationship with the heat generator  16  is fixed. Therefore, highly precise positioning relative to the heat generator  16  is performed by aligning the first positioning marks  18   a  with the reference positions. Since the second positioning marks  18   b  of the heater unit  10  are formed in the same step as the electrodes  15 , the positional relationship with the electrodes  15  is fixed. Since the electrodes  15  are formed so as to be located at both longitudinal ends of the heat generator  16 , highly precise positioning relative to the electrodes  15  is performed by aligning the second positioning marks  18   b  with the reference positions. 
     The slot  31  of the holder  30  is formed larger than the outline of the heater unit  10  in plan view shown  FIG. 3 , and the position of the heater unit  10  can be fine-tuned in the X1-X2 direction and the Y1-Y2 direction. In addition, the position of the heater unit  10  can be fine-tuned in the rotation direction within that margin range. Thus, fine tuning is performed such that the first positioning marks  18   a  and the second positioning marks  18   b  viewed through the through-holes  32  shown in  FIGS. 9 and 10  are located at desired positions relative to the reference portions  33  of the holder  30 , and the relative positions of the heater unit  10  and the holder  30  is thereby determined. For example, the reference portions  33  of the holder  30  are fixed to a jig, and a worker performs fine tuning such that the heater unit  10  is located at a predetermined position relative to the reference portions  33  while observing from the lower side using a microscope. Then, the heater unit  10  is fixed with adhesive so that the heater unit  10  is not displaced. Although not shown in  FIGS. 9 and 10 , the adhesive is applied to the bonding surfaces of the heater unit  10  and the holder  30 . In the case where the adhesive is applied to the lower surface of the heater unit  10  and the upper surface of the holder  30 , position adjustment is performed, for example, after the adhesive is applied and before the adhesive cures. In the case of a thermosetting adhesive, the adhesive is hardened by heating after the position adjustment, with the heater unit  10  temporarily fixed so as not to be displaced. 
     Since the holder  30  has reference portions  33 , the heater  1  of this embodiment can be precisely attached to the external device  100 . Since the heater unit  10  is positioned using the reference portions  33  of the holder  30  as references, the heat generator  16  can be highly precisely positioned without performing position adjustment while checking the positions of the heat generator  16  and the positioning marks  18  when incorporating the heater  1  into the external device  100 . Alternatively, position adjustment may be performed, without using the reference portions  33 , directly relative to the reference positions of the external device  100  while viewing the positioning marks  18  through the through-holes  32  of the holder  30  when attaching the heater  1  to the external device  100 . 
     The positioning marks  18  are formed between the protective layer  17  and the substrate  11 . The protective layer  17  looks white owing to the surface roughness of the upper side (Z1 side) thereof and the particles contained therein. Therefore, the positioning marks  18  are difficult to check from the upper side (Z1 side). Since the substrate  11  of the heater  1  of this embodiment is formed of a transparent material, the positioning marks  18  can be clearly checked from the second surface  11   b  of the substrate  11 . Since the through-holes  32  of the holder  30  of the heater  1  of this embodiment are formed at positions coincident with the positioning marks  18  in plan view, the positioning marks  18  are visible from the lower side (Z2 side) of the holder  30 . In addition, since the substrate  11  is flat and smooth, the positioning marks  18  looks flat and smooth when the positioning marks  18  formed on the first surface  11   a  of the substrate  11  is viewed from the second surface  11   b . Since the interface between each positioning mark  18  and the first surface  11   a  of the substrate  11  is flat and smooth, the outlines of the positioning marks  18  observed through the through-holes  32  from the lower side (Z2 side) of the holder  30  can be clearly distinguished without being affected by the scattering of light or the like. In addition, since the surface roughness of the upper side (Z1 side) of the protective layer  17  and the particles contained in the protective layer  17  render the background of the positioning marks  18  observed from the Z2 side of the holder  30  white and opaque, the outlines of the positioning marks  18  are rendered clearer. Therefore, the heater  1  can be precisely positioned. 
     The positioning marks  18  each include a first positioning mark  18   a  corresponding to the position of the heat generator  16  and a second positioning mark  18   b  corresponding to the positions of the electrodes  15 . This configuration makes it possible to more reliably position the heat generator  16  in the feed direction and the electrodes  15  in the longitudinal direction. 
     Advantageous effects of this embodiment will be described below. 
     The heater  1  of this embodiment includes a heater unit  10  and a holder  30  to be attached to an external device  100 . The heater unit  10  includes a flat and smooth substrate  11 , a linear heat generator  16  provided on a first surface  11   a  of the substrate  11 , a plurality of electrodes  15  that supply power to the heat generator  16 , and a protective layer  17  formed so as to cover the heat generator  16  and the electrodes  15 . A second surface  11   b  of the substrate  11  is bonded to the holder  30 . The substrate  11  is formed of a transparent material. A plurality of positioning marks  18  for determining the relative positions of the heater unit  10  and the holder  30  are formed on the first surface  11   a  of the substrate  11 . The holder  30  has through-holes  32  formed at positions opposite to the positioning marks  18 . 
     This configuration makes it possible to observe the positioning marks  18  provided on the first surface  11   a  of the substrate  11  formed of a transparent material through the through-holes  32  of the holder  30 . In addition, since the substrate  11  is flat and smooth, and the interface between each positioning mark  18  and the substrate  11  is flat and smooth, the positioning marks  18  are clearly visible without being affected by the scattering of light when the positioning marks  18  formed on the first surface  11   a  of the substrate  11  is viewed from the second surface  11   b . The precision of positioning when incorporating the heater  1  into the external device  100  is thereby improved. 
     In the heater  1  of this embodiment, the holder  30  has reference portions  33  used as references for attachment to the external device  100 . This configuration makes it possible to attach the heater  1  precisely by aligning the positioning marks  18  with the reference portions  33  of the holder  30  in advance and thereby aligning the reference portions  33  of the holder  30  with the external device  100 . 
     In the heater  1  of this embodiment, the positioning marks  18  are formed between the protective layer  17  and the substrate  11 . Owing to this configuration, the lower surfaces of the positioning marks  18  provided on the flat and smooth substrate  11  are flat and smooth surfaces, and therefore the outlines of the positioning marks  18  observed from the lower side of the holder  30  can be clearly distinguished without being affected by the scattering of light or the like. Therefore, the heater  1  can be precisely positioned. 
     In the heater  1  of this embodiment, the positioning marks  18  each include a first positioning mark  18   a  corresponding to the position of the heat generator  16  and a second positioning mark  18   b  corresponding to the positions of the electrodes  15 . This configuration makes it possible to more reliably position the heat generator  16  in the feed direction and the electrodes  15  in the longitudinal direction. 
     In the heater  1  of this embodiment, the first positioning marks  18   a  are formed in the same step as the heat generator  16 , and the second positioning marks  18   b  are formed in the same step as the electrodes  15 . Owing to this configuration, the positional relationship between the first positioning marks  18   a  and the heat generator  16  is accurate, and the positional relationship between the second positioning marks  18   b  and the electrodes  15  is accurate, and therefore positioning can be precisely performed. 
     In the heater  1  of this embodiment, the protective layer  17  is a thick film formed by screen printing. Owing to this configuration, surface roughness due to a mesh of a printing plate is left on the protective layer  17  formed by screen printing, therefore light is scattered, the background of the positioning marks  18  looks white and opaque when viewed from the second surface  11   b , and therefore the positioning marks  18  can be observed more clearly. 
     In the heater  1  of this embodiment, the protective layer  17  contains particles. This configuration renders the protective layer  17  whiter and opaquer, and therefore the positioning marks  18  can be observed much more clearly. 
     In the heater  1  of this embodiment, the external device  100  is a toner fixing device. Owing to this configuration, the heat generator  16  in the toner fixing device can be accurately positioned, and therefore heating efficiency is improved. 
     Although the heater  1  of the first embodiment of the present invention has been described specifically, the present invention is not limited to the above-described embodiment, and various changes may be made therein without departing from the spirit of the present invention. The embodiment of the present invention may be modified, for example, as follows, and these are also included in the technical scope of the present invention. 
     (1) In this embodiment, the protective layer  17  is the uppermost layer. However, as shown in  FIGS. 11 and 12 , a heat transfer layer  19  may be provided on the upper surface of the protective layer  17 .  FIG. 11  is a schematic plan view of a heater  2  of a modification as viewed from the upper side (Z1 side) shown in  FIG. 2 .  FIG. 12  is a sectional view at the same position as  FIG. 5 . The heater  2  of the modification is the same as the heater  1  except that the heat transfer layer  19  is provided on the upper surface of the heater unit  10 . The heat transfer layer  19  is formed so as to cover the heat generator  16  in plan view, in a larger area. The heat transfer layer  19  is preferably formed of a material having high heat conductivity and high impact resistance. The possibility that the protective layer  17  is damaged by thermal shock is reduced, and the durability of the heater  2  is thereby improved.
 
(2) In this embodiment, the electrodes  15  are conductive coating films collectively formed by screen printing. However, the electrodes  15  may be formed in two operations: a screen printing operation to form the comb-shaped electrodes  12   b  and the L-shaped electrodes  13   b , and a screen printing operation to form the comb-shaped electrode  12   a  and the L-shaped electrodes  13   a . The comb-shaped electrode  12   a  and the L-shaped electrodes  13   a  may be formed in separate screen printing operations. In these cases, the second positioning marks  18   b  may be collectively formed in the screen printing operation to form the comb-shaped electrodes  12   b  and the L-shaped electrodes  13   b.  
 
(3) In this embodiment, the positioning marks  18  are formed one on each of the X1 side and the X2 side. However, the positioning marks  18  may be formed at two locations on the Y1 side and the Y2 side. By performing position adjustment using positioning marks  18  at a total of four locations, positioning can be performed more precisely. Similarly, the reference portions  33  of the holder  30  may be provided at four locations for position adjustment. Although the positioning marks  18  are formed in a cross shape, the shape of the positioning marks  18  is not limited to this, and the positioning marks  18  may have any shape as long as positioning is possible. Similarly, although the reference portions  33  of the holder  30  are holes, the present invention is not limited to this, and the reference portions  33  may be slots or protrusions.