Patent Publication Number: US-2006012649-A1

Title: Inkjet head unit

Description:
INCORPORATION BY REFERENCE  
      The present application is based on Japanese Patent Application No. 2004-209848, filed on Jul. 16, 2004, the content of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to an inkjet head unit including a printhead for ejecting droplets of ink onto a recording medium.  
      2. Description of the Related Art  
      As an inkjet head unit including a printhead which ejects ink droplets onto a sheet of paper or other recording media, there is known one which comprises a flow-path unit and a piezoelectric actuator unit. The flow-path unit has a plurality of pressure chambers and a plurality of nozzles respectively in communication with the pressure chambers so as to eject ink droplets therethrough. The piezoelectric actuator unit functions to pressurize ink in the pressure chambers by varying the inner volume of the pressure chambers. Such an inkjet head unit is disclosed in JP-A-11-334061 (see FIG. 2(a)), and JP-A-9-156099, for instance. A typical piezoelectric actuator unit includes a piezoelectric sheet extending across the pressure chambers, a plurality of individual electrodes disposed at respective positions corresponding to the pressure chambers, and a common electrode on the side of the piezoelectric sheet opposite to the individual electrodes. Upon application of a drive voltage to one of the individual electrodes, a portion of the piezoelectric sheet interposed between the individual electrode and the common electrode contracts by being affected by the electric field in the direction of the thickness of the piezoelectric sheet. Hence, the inner volume of the pressure chamber corresponding to the individual electrode changes, pressurizing the ink in the pressure chamber.  
      The individual electrodes are connected to wires so that the drive voltage is applied to the individual electrodes therethrough. In the inkjet head unit of the above-mentioned publication JP-A-11-334061, for instance, a plurality of upper electrodes arranged in matrix are respectively connected to a plurality of connecting terminals of a printed wiring board in which a pattern of wires is formed. In the inkjet head unit of the other publication JP-A-9-156099, a plurality of drive electrodes comprising a plurality of upper drive electrodes (individual electrodes) and a lower drive electrode (common electrode) are disposed on a piezoelectric sheet in a deformation region of the piezoelectric sheet, and a voltage is applied to the drive electrodes. A plurality of wires extend from the respective drive electrodes in a same direction into a wiring region on the piezoelectric sheet, to be connected to the printed wiring board there. The wiring region is adjacent to the deformation region where the piezoelectric sheet is interposed between the upper and lower drive electrodes. In order to prevent, creation of an unnecessary electrostatic capacity, upon application of voltage to an upper drive electrode, in the piezoelectric sheet at a portion interposed between a wire connected to that upper drive electrode and the lower drive electrode, a dielectric film having a relatively low dielectric constant is formed on the piezoelectric sheet across the wiring region and the wires are formed on a surface of the dielectric film opposite to the piezoelectric sheet.  
      In the inkjet head unit of the publication JP-A-11-334061, the printed wiring board is disposed to cover a matrix of the upper electrodes so that the upper electrodes are respectively connected to the connecting terminals of the printed wiring board. In this arrangement, when subjected to an external force, the printed wiring board tends to be separated from the upper electrodes. Thus, a reliability in the electrical connection between the printed wiring board and the upper electrodes is low.  
      According to the technique of the publication JP-A-9-156099, meanwhile, as long as the number of the pressure chambers are small, it is easy to form only within the wiring region the wires which extend from the individual electrodes disposed in the deformation region. However, where the number of the pressure chambers is increased, particularly where the pressure chambers are arranged in matrix, a part of the wires are inevitably formed in the deformation region as well as the wiring region. Since the dielectric film having the low dielectric constant is not provided in the deformation region, an unnecessary electrostatic capacity is created in the deformation region between the wire supplied with the voltage and the lower drive electrode or common electrode. The unnecessary electrostatic capacity deforms the piezoelectric sheet at the portion interposed between the wire to which the voltage is applied and the common electrode, leading to unintended deformation of the piezoelectric sheet at a place corresponding to a pressure chamber or chambers in the vicinity of that wire. That is, a crosstalk occurs, which varies the characteristics of ejection of ink droplets among the pressure chambers and accordingly among the nozzles, deteriorating the print quality. Further, in a case where a part of a wire connected to an individual electrode corresponding to a pressure chamber is disposed over another pressure chamber in order to ensure a sufficient spacing between each two adjacent wires in the deformation region, the electric field created around the wire to which the voltage is applied, directly affects a corresponding portion of the piezoelectric sheet over the another pressure chamber. In this case, the adverse influence of the crosstalk becomes serious at the another pressure chamber.  
     SUMMARY OF THE INVENTION  
      The prevent invention has been developed in view of the above-described situations and it is an object of the invention to provide an inkjet head unit including a printhead capable of preventing creation of an unnecessary electrostatic capacity and occurrence of a crosstalk.  
      To obtain the above object, this invention provides an inkjet head unit including a printhead comprising:  
      a flow-path unit having a plurality of nozzles and a plurality of pressure chambers respectively in communication with the nozzles;  
      an actuator unit having a piezoelectric sheet extending across the pressure chambers, a plurality of individual electrodes disposed on the piezoelectric sheet at respective positions corresponding to the pressure chambers, and a common electrode which is disposed on a surface of the piezoelectric sheet opposite to the individual electrodes, the actuator unit being fixed on a surface of the flow-path unit to vary the inner volume of each of the pressure chambers;  
      a dielectric film continuously formed on a surface of the actuator unit opposite to the flow-path unit, to extend over a first region not corresponding positionally to the pressure chambers as well as over a second region corresponding positionally to the pressure chambers, the dielectric film having a plurality of through-holes, and a dielectric constant of the dielectric film being lower than that of the piezoelectric sheet;  
      a plurality of first wires formed on a surface of the dielectric film opposite to the actuator unit, to extend in a substantially same direction; and  
      a plurality of second wires extending through the through-holes of the dielectric film, and connecting the respective individual electrodes to the first wires.  
      In this inkjet head unit, upon selective application of the voltage to one of the individual electrodes of the actuator unit of the printhead, a portion of the piezoelectric sheet interposed between the common electrode and the individual electrode to which the voltage is applied is affected by the electric field in the direction of the thickness of the piezoelectric sheet, and deformed. This deformation changes the inner volume of the corresponding pressure chamber to pressurize the ink therein, thereby ejecting an ink droplet from the nozzle in communication with the pressure chamber.  
      On the surface of the actuator unit remote from the flow-path unit, there is formed the dielectric film having a dielectric constant lower than that of the piezoelectric sheet. On the surface of the dielectric film opposite to the actuator unit, there are formed the first wires for the respective individual electrodes. Each of the first wires and the corresponding individual electrode are connected to each other via the second wire extending through one of the through-holes formed through the dielectric film.  
      In this way, the dielectric film having the dielectric constant lower than that of the piezoelectric sheet is interposed between the piezoelectric sheet and the first wires. Hence, when the voltage is applied to one of the individual electrodes via the corresponding first and second wires, an unnecessary electrostatic capacity is not created, thereby improving the driving efficiency of the actuator unit. Since there is minimized the deformation due to the unnecessary electrostatic capacity, in the piezoelectric sheet at a portion between the common electrode and the first wire to which the voltage is applied, occurrence of the crosstalk with the pressure chamber(s) in the vicinity of the first wire to which the voltage is applied is prevented.  
      The dielectric film is continuously formed on the actuator unit over the first region not corresponding positionally to the pressure chambers, as well as over the second region corresponding positionally to the pressure chambers. Hence, even where a first wire for an individual electrode corresponding to one of the pressure chambers is disposed over another pressure chamber, the deformation in the piezoelectric sheet at the portion over the another pressure chamber due to the application of the voltage to the first wire is minimized, preventing the crosstalk due to presence of the first wire over the another pressure chamber. As a consequence, it is enabled to dispose the first wires in the second region positionally corresponding to the pressure chambers also, thereby increasing the space for arranging the first wires. This enables to widen the spacing between each adjacent two of the first wires, facilitating the formation of the first wires, in turn enabling reduction in the manufacturing cost of the actuator unit or the printhead. The first wires extend in a substantially same direction so that ends of the first wires on a same side are collected in an area at which the first wires are connected to a wiring member such as a flexible printed wiring board, which may be referred to as a flexible printed circuit (FPC). According to this arrangement, the first wires are easily connected to the wiring member, thereby improving the reliability of the connection therebetween.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features, advantages and technical and industrial significance of the present invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:  
       FIG. 1  is a perspective view showing an inkjet head unit to which a principle of the present invention is applied;  
       FIG. 2  is a cross sectional view taken along line  2 - 2  in  FIG. 1 ;  
       FIG. 3  is a plan view showing an inkjet printhead of the inkjet head unit of  FIG. 1 ;  
       FIG. 4  is an enlarged view of an area enclosed with one-dot chain line in  FIG. 3 ;  
       FIG. 5  is an enlarged view of an area enclosed with one-dot chain line in  FIG. 4 ;  
       FIG. 6  is a cross sectional view taken along line  6 - 6  in  FIG. 5 ;  
       FIG. 7  is an enlarged plan view schematically showing a part of an actuator unit of the printhead;  
       FIG. 8  is a cross sectional view taken along line  8 - 8  in  FIG. 7 ;  
       FIG. 9  is an enlarged plan view schematically showing a part of an actuator unit of a printhead of an inkjet head unit according to a second embodiment of the invention;  
       FIG. 10  is an enlarged plan view schematically showing a part of an actuator unit of a printhead of an inkjet head unit according to a third embodiment of the invention; and  
       FIG. 11  is an enlarged plan view schematically showing a part of an actuator unit of a printhead of an inkjet head unit according to a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Hereinafter, there will be described presently preferred embodiments of the invention, by referring to the accompanying drawings.  
      Referring to FIGS.  1  to  8 , there will be described an inkjet head unit according to a first embodiment of the invention. In  FIGS. 1 and 2 , reference numeral  1  generally denotes the inkjet head unit which is disposed in an inkjet printer (not shown) for ejecting ink droplets onto a recording medium in the form of a sheet of paper so as to. record information or an image thereon. The inkjet head unit  1  includes: an inkjet printhead  70  which has, in plan view, a rectangular shape extending in a main scanning direction and a plurality of nozzles  8  ( FIGS. 4 and 5 ) through which ink is ejected toward the sheet of paper; and a base block  71  which is disposed above the printhead  70  and in which are formed two ink reservoirs  3 ,  3  each functioning as a flow passage of ink to be supplied to the printhead  70 .  
      The printhead  70  includes: a flow-path unit  4  in which ink paths are formed; and a plurality of actuator units  21  bonded to an upper surface of the flow-path unit  4 . Each of the ink-flow path unit  4  and a plurality of actuator units  21  is formed such that a plurality of thin plates are stacked on and bonded to one another. As shown in  FIG. 2 , an end portion of each of the actuator units  21  is bonded to one of two flexible printed wiring boards  50  (hereinafter referred to as “FPCs  50 ” as explained in the “Description of the Related Art”) which are drawn out to both sides. The base block  71  is formed of a metal such as stainless steel, for instance. Each of the ink reservoirs  3 ,  3  formed in the base block  71  is a substantially rectangular parallelepiped hollow region extending in a longitudinal direction of the base block  71 .  
      A lower surface  73  of the base block  71  protrudes downward at portions  73   a  thereof in the vicinity of openings  3   b . The portions  73   a  may be hereinafter referred to as the “opening-vicinity portions  73   a ”. The base block  71  is in contact with the flow-path unit  4  only at the opening-vicinity portions  73   a  of its lower surface  73 . Thus, regions of the lower surface  73  of the base block  71  other than the opening-vicinity portions  73   a  are spaced apart from the printhead  70 , and the actuator units  21  are disposed within the space between the printhead  70  and the lower surface  73  of the base block  71  at the above-indicated spaced regions thereof.  
      The base block  71  is accommodated in a recess formed in a lower surface of a holding portion  72   a  of a holder  72  and is bonded and fixed to the holding portion  72   a . The holder  72  includes the holding portion  72   a  and a pair of tabular projecting portions  72   b  extending from an upper surface of the holding portion  72   a  in an upward direction perpendicular to the upper surface, so as to be opposed to each other with a predetermined distance therebetween. Each of the two FPCs  50  bonded to the actuator units  21  extends along the outer surface of the projecting portion  72   b  with an elastic member  83  such as a sponge interposed therebetween. Driver ICs  80  are mounted on the respective FPCs  50  disposed along the surfaces of the projecting portions  72   b  of the holder  72 . Each FPC  50  is electrically connected by soldering to both of the driver IC  80  and the actuator units  21  (described later) of the printhead  70  in order to transmit operating signals outputted from the driver IC  80  to the actuator units  21 .  
      Heat sinks  82  each having a substantially rectangular parallelepiped shape are disposed to be in close contact with the respective driver ICs  80 , whereby heat generated at the driver ICs  80  is dissipated through the heat sinks  82 . At each side, above the driver IC  80  and the heat sink  82  and on the outer side of the FPC  50 , there is disposed a substrate  81 . A seal member  84  is provided between the upper surface of each heat sink  82  and the substrate  81  on the same side, and between the lower surface of each heat sink  82  and the FPC  50  on the same side.  
       FIG. 3  is a plan view of the printhead  70  shown in  FIG. 1 . In  FIG. 3 , each ink reservoir  3  formed in the base block  71  is virtually illustrated by broken line. The two ink reservoirs  3 ,  3  extend in a longitudinal direction of the printhead  70  so as to be in parallel to each other with a predetermined spacing therebetween. At one of opposite ends of each ink reservoir  3 , there is formed an end opening  3   a  through which the ink reservoir  3  communicates with an ink tank (not shown) so as to be filled with ink. Each ink reservoir  3  is formed with a plurality of openings  3   b  which align in the longitudinal direction of the printhead  70 . The ink reservoirs  3 ,  3  and the flow-path unit  4  are connected to each other through the openings  3   b . The openings  3   b  are formed in a plurality of pairs, and two openings  3   b  of each pair are disposed adjacent to each other along the longitudinal direction of the printhead  70 . The pairs of openings  3   b  communicating with one of the two ink reservoirs  3  and the pairs of openings  3   b  communicating with the other ink reservoir  3  are arranged in a zigzag pattern in plan view.  
      The actuator units  21  each having a trapezoidal shape in plan view are provided on regions of the upper surface of the flow-path unit  4  which do not correspond to the openings  3   b , such that the actuator units  21  are arranged in a zigzag pattern opposite to that of the pairs of openings  3   b . Parallel opposed sides (short and long sides) of each actuator unit  21  are parallel to the longitudinal direction of the printhead  70 , and oblique sides of adjacent actuator units  21  partially overlap as viewed in the main scanning direction. As shown in  FIG. 3 , the left end portions of the respective actuator units  21  disposed at the left-hand side are bonded to one of the two FPCs  50 , and the right end portions of the respective actuator units  21  disposed at the right-hand side are bonded to the other FPC  50 .  
       FIG. 4  is a view showing in enlargement an area enclosed with one-dot chain line in  FIG. 3 . As shown in  FIG. 4 , the openings  3   b  provided for each of the ink reservoirs  3  communicate with respective manifolds  5 . Each manifold  5  is branched into two sub manifolds  5   a  each as a common ink chamber. Two branched sub manifolds  5   a  extend from one of two openings  3   b  which are located on opposite sides of the two oblique sides of each actuator unit  21 , and another two branched sub manifolds  5   a  extend from the other of the two openings  3   b . Thus, in plan view, four sub manifolds  5   a  in total extend below each actuator unit  21  along the two parallel sides of the same  21  so as to be spaced apart from one another.  
      On the lower surface of the flow-path unit  4 , there are formed a plurality of ink ejection regions in each of which a multiplicity of nozzles  8  are arranged in matrix as described below. While only some of the nozzles  8  are illustrated in  FIG. 4  in the interest of brevity, the nozzles  8  are actually arranged all over each ink ejection region.  
       FIG. 5  is an enlarged view of an area enclosed with one-dot chain line in  FIG. 4 .  FIGS. 4 and 5  show a state wherein a plane on which a multiplicity of pressure chambers  10  of the flow-path unit  4  are arranged in matrix is viewed in a direction perpendicular to the ink ejection surface of the printhead  70 . Each of the pressure chambers  10  has, in plan view, a substantially rhombic shape having rounded corners. The long diagonal line of each rhombic pressure chamber  10  is parallel to a widthwise direction of the flow-path unit  4 . As shown in  FIG. 6 , one end of each pressure chamber  10  communicates with a corresponding nozzle  8  and the other end thereof communicates, via a corresponding aperture  12 , with a corresponding sub manifold  5   a  as a common ink chamber. A plurality of individual electrodes  35  are formed on each actuator unit  21  at positions which overlap the respective pressure chambers  10  in plan view. Each individual electrode  35   a  has, in plan view, a shape which is similar to that of the pressure chamber  10 , and whose size is slightly smaller than that of the pressure chamber  10 . For the sake of simplicity, only some of the individual electrodes  35  are illustrated in  FIG. 5 . Further, the pressure chambers  10 , the apertures  12 , etc., which are in the actuator units  21  or the flow-path unit  4  and which should be expressed by broken line are illustrated in solid line in  FIGS. 4 and 5 .  
      In  FIG. 5 , a plurality of imaginary rhombic areas  10   x  in which the respective pressure chambers  10  ( 10   a ,  10   b ,  10   c ,  10   d ) are accommodated are arranged adjacent to one another in matrix in two directions, i.e., an arrangement direction A and an arrangement direction B, such that adjacent rhombic areas  10   x  do not overlap with one another and have respective sides in common. The arrangement direction A is a longitudinal direction of the printhead  70 , i.e., a direction of extension of each sub manifold  5   a , and parallel to a short diagonal line of each rhombic area  10   x . The arrangement direction B is a direction of one oblique side of each rhombic area  10   x  that forms an obtuse angle θ with respect to the arrangement direction A. The center position of each pressure chamber  10  is common to that of the corresponding rhombic area  10   x , and the contour line of each pressure chamber  10  is separated from that of the corresponding rhombic area  10   x  in plan view.  
      The pressure chambers  10  arranged adjacent to one another in matrix in the two arrangement directions A and B are spaced apart from each other by a distance R corresponding to 37.5 dpi in the arrangement direction A. In the present embodiment, eighteen pressure chambers  10  are arranged in one row in the arrangement direction B in one ink ejection region. The pressure chambers  10  located at opposite ends in the arrangement direction B are dummy chambers that do not contribute to ink ejection.  
      The plurality of pressure chambers  10  formed in matrix constitute a plurality of pressure-chamber rows along the arrangement direction A, as shown in  FIG. 5 . The direction in which each pressure-chamber row extends is perpendicular to the long diagonal line of the rhombic shape of the pressure chambers. The plurality of pressure-chamber rows are classified into a first pressure-chamber row  11   a , a second pressure-chamber row  11   b , a third pressure-chamber row  11   c , and a fourth pressure-chamber row  11   d  in accordance with their positions relative to the sub manifolds  5   a , as viewed in a direction perpendicular to the sheet surface of  FIG. 5 . Each of the first through fourth pressure-chamber rows  11   a - 11   d  are disposed periodically four times in order of  11   c ,  11   d ,  11   a ,  11   b ,  11   c ,  11   d ,  11   b , from the short side of the parallel opposed sides (hereinafter referred to as “the shorter base”) of the actuator unit  21  toward the long side (hereinafter referred to as “the longer base”).  
      In the pressure chambers  10   a  constituting the first pressure-chamber row  11   a  and the pressure chambers  10   b  constituting the second pressure-chamber row  11   b , the nozzles  8  are located at a lower end of each pressure chamber  10   a ,  10   b  nearer to the lower side of the sheet surface of  FIG. 5 , with respect to a vertical direction in  FIG. 5  perpendicular to the arrangement direction A, as viewed in the direction perpendicular to the sheet surface of  FIG. 5 . Namely, the nozzles  8  of the pressure chambers  10   a ,  10   b  are located at respective lower ends of the corresponding rhombic areas  10   x , as seen in the vertical direction in  FIG. 5 . On the other hand, in the pressure chambers  10   c  constituting the third pressure-chamber row  11   c  and the pressure chambers  10   d  constituting the fourth pressure-chamber row  11   d , the nozzles  8  are located at an upper end of each pressure chamber  10   c ,  10   d  nearer to the upper side of the sheet surface of  FIG. 5 , with respect to the vertical direction in  FIG. 5  perpendicular to the arrangement direction A, as viewed in the direction perpendicular to the sheet surface of  FIG. 5 . Namely, the nozzles  8  of the pressure chambers  10   c ,  10   d  are located at respective upper ends of the corresponding rhombic areas  10   x , as seen in the vertical direction in  FIG. 5 . In the first and fourth pressure-chamber rows  11   a ,  11   d , half or more of the region of each pressure chamber  10   a ,  10   d  overlaps the corresponding sub manifold  5   a , as viewed in the direction perpendicular to the sheet surface of  FIG. 5 . In the second and third pressure-chamber rows  11   b ,  11   c , the entire region of each pressure chamber  11   c ,  11   d  does not overlap any sub manifolds  5   a . Accordingly, the pressure chambers  10  belonging to any of the pressure-chamber rows  11   a - 11   d  can be formed such that the nozzles  8  communicating with the corresponding pressure chambers  10  do not overlap the sub manifolds  5   a  while making the width of the sub manifolds  5   a  as large as possible, whereby the ink can be smoothly supplied to the pressure chambers  10 .  
      Referring next to  FIG. 6 , which is a cross sectional view taken along line  6 - 6  in  FIG. 5 , there will be explained a sectional structure of the printhead  70 . As shown in  FIG. 6 , each nozzle  8  communicates with the corresponding sub manifold  5   a  through the corresponding pressure chamber  10  and the corresponding aperture  12 . Thus, in the printhead  70 , there is formed, for each pressure chamber  10 , an individual ink path  32  extending from an outlet of the sub manifold  5   a  to the nozzle  8  through the aperture  12  and the pressure chamber  10 .  
      The printhead  70  has a laminar structure in which ten plates in total are stacked or superposed on one another. The ten plates consist of the actuator unit  21  and nine plates constituting the flow-path unit  4 , namely, a cavity plate  22 , a base plate  23 , an aperture plate  24 , a supply plate  25 , manifold plates  26   27 ,  28 , a cover plate  29 , and a nozzle plate  30 .  
      Each actuator unit  21  includes four piezoelectric sheets  41 - 44  ( FIG. 8 ) which are stacked on one another and is provided with electrodes, so that only an uppermost sheet among the four piezoelectric sheets  41 - 44  serves as an active layer including active portions each of which becomes active at the time of application of an electric field, and the rest of three layers or sheets are non-active layers. The cavity plate  22  is a metal plate in which are formed a multiplicity of openings in a substantially rhombic shape in plan view that give the respective pressure chambers  10 . The base plate  23  is a metal plate in which are formed, for one pressure chamber  10  of the cavity plate  22 , a communication hole between the pressure chamber  10  and the corresponding aperture  12  and a communication hole for communication between the pressure chamber  10  and the corresponding nozzle  8 . The aperture plate  24  is a metal plate in which are formed, for one pressure chamber  10  of the cavity plate  22 , the aperture  12  constituted by two holes and a half-etched part connecting the two holes, and a communication hole for communication between the pressure chamber  10  and the corresponding nozzle  8 . The supply plate  25  is a metal plate in which are formed, for one pressure chambers  10  of the cavity plate  22 , a communication hole between the corresponding aperture  12  and the corresponding sub manifold  5   a  and a communication hole for communication between the pressure chamber  10  and the corresponding nozzle  8 . The manifold plates  26 ,  27 ,  28  are metal plates and have, for one pressure chamber  10 , respective communication holes for communication between the pressure chamber  10  and the corresponding nozzle  8 , in addition to cutouts which cooperate to form the sub manifolds  5   a  when these manifold plates  26 - 28  are stacked. The cover plate  29  is a metal plate in which is formed, for one pressure chamber  10 , a communication hole for communication between the pressure chamber  10  and the corresponding nozzle  8 . The nozzle plate  30  is a metal plate in which is formed, for one pressure chamber  10 , a hole which gives the corresponding nozzle  8 .  
      These nine plates  22 - 30  of the flow-path unit are stacked on one another while being positioned relative to one another so as to define the individual ink paths  32  one of which is shown in  FIG. 6 . Each individual ink path  32  extends first upward from the sub manifold  5   a , then extends horizontally at the aperture  12 , further extends upward, then again extends horizontally at the pressure chamber  10 , extends obliquely in a downward direction so as to be away from the aperture  12 , and extends vertically downward toward the nozzle  8 .  
      Referring next to  FIGS. 7 and 8 , there will be explained a structure of each actuator unit  21  superposed on the cavity plate  22  which is the uppermost plate of the flow-path unit  4 .  FIG. 7  is a fragmentary plan view showing in enlargement the end portion of the actuator unit  21  at which the actuator unit  21  is bonded to the FPC  50 , and  FIG. 8  is a cross sectional view taken along line  8 - 8  in  FIG. 7 . As shown in  FIGS. 7 and 8 , the actuator unit  21  comprises four piezoelectric sheets  41 - 44  extending across the pressure chambers  10 , the individual electrodes  35  disposed on the uppermost piezoelectric sheet  41  at respective positions corresponding to the pressure chambers  10 , and a common electrode  34  disposed on the side of the uppermost piezoelectric sheet  41  opposite to the individual electrodes  35 .  
      The four piezoelectric sheets  41 ,  42 ,  43 ,  44  have a substantially same thickness of about 15 μm. These piezoelectric sheets  41 - 44  are formed as a layered flat plate (consisting of continuous flat layers) which continuously extends over the multiplicity of pressure chambers  10  formed in one ink ejection region in the printhead  70 . Since the piezoelectric sheets  41 - 44  extend over the multiplicity of pressure chambers  10  as the continuous flat layers, the individual electrodes  35  can be disposed at high density on the piezoelectric sheet  41  by screen printing, for instance. Further, the pressure chambers  10  formed at positions corresponding to the respective individual electrodes  35  can also be disposed at high density, whereby high-resolution image printing can be achieved. The piezoelectric sheets  41 - 44  are formed of a ceramic material of lead zirconate titanate (PZT) having ferroelectricity.  
      As shown in  FIG. 7 , each individual electrode  35  has, in plan view, a rhombic shape similar to, but slightly smaller than, that of the pressure chamber  10 . The individual electrodes  35  are formed on the uppermost one  41  of the piezoelectric sheets  41 - 44  such that each individual electrode  35  is located within an outline of the corresponding one of the pressure chambers  10 , in plan view. Accordingly, the individual electrodes  35  are arranged in matrix on the upper surface of the piezoelectric sheet  41 , in a fashion similar to the pressure chambers  10 . The thickness of the individual electrodes  35  is about 1 μm, for instance.  
      The common electrode  34  is formed between the uppermost piezoelectric sheet  41  and the second uppermost piezoelectric sheet  42 , to extend over the entire areas of the piezoelectric sheets  41 ,  42 . The thickness of the common electrode  34  is about 2 μm, for instance. The common electrode  34  is grounded at a place not shown, and thus maintained at the ground potential at every place corresponding to any pressure chamber  10 .  
      Both the individual and common electrodes  35 ,  34  are made of a metal material such as Ag—Pd based metal material, for instance.  
      In this inkjet printhead  70 , a dielectric film  60  is formed over the entire area of an upper surface of the actuator unit  21  which is the surface thereof opposite to the flow-path unit  4 . That is, the dielectric film  60  is formed across the individual electrodes  35 . The dielectric film  60  has a dielectric constant lower than that of the piezoelectric sheets  41 - 44 , and is preferably made of a low-k (low dielectric constant) material having a dielectric constant which is not larger than 1/100 of that of the piezoelectric sheets  41 - 44 . In the present embodiment, the relative dielectric constant of the piezoelectric sheets  41 - 44  is about 3500. Hence, the relative dielectric constant of the material forming the dielectric film  60  should be about a few dozen. By the provision of the dielectric film  60  having such a dielectric constant, the driving efficiency of the actuator unit  21  is enhanced, while the crosstalk is effectively prevented.  
      The dielectric film  60  may be formed by a known method with a relatively inexpensive material. For instance, the dielectric film  60  may be formed of glass material which is deposited by chemical vapor deposition (CVD), or formed of fluoro resin by printing. By employing a relatively inexpensive material such as glass and resin as the material of the insulating film or the dielectric film  60 , the manufacturing cost of the printhead is reduced.  
      The thickness of the dielectric film  60  is about 0.5 to 2 μm, for instance.  
      At a part in the dielectric film  60  corresponding to a lower side one, as seen in  FIG. 7 , of two acute portions of each rhombic pressure chamber  10 , there is formed a through-hole  60 a extending through the thickness of the dielectric film  60 .  
      A connecting wire  61  extends from a lower end portion, as seen in  FIG. 7 , of each substantially rhombic individual electrode  35 , and this connecting wire  61  is connected to a vertical wire  62  (constituting a second wire) disposed inside the through-hole  60   a  to extend through the thickness of the dielectric film  60 .  
      The through-hole  60   a  is formed at the position as described above, since upon application of the voltage to an individual electrode  35 , a portion of the piezoelectric sheet  41  positionally corresponding to each of two longitudinal end portions of the pressure chamber  10  corresponding to that individual electrode does not easily deform. That is, the through-hole  60   a  is formed at a position where the piezoelectric sheet  41  is the least deformable in an area of the pressure chamber  10 , so that the individual electrode  35  is connected to the vertical wire  62  at a place where the piezoelectric sheet  41  is the least deformable. According to this arrangement, deformation of the piezoelectric sheet  41  is not inhibited by presence of the connection between the individual electrode  35  and the vertical wire  62 . In order to obtain such an effect, this arrangement is equally applicable to any cases where the shape of the pressure chamber in plan view is elongate, that is, not only where the pressure chamber has a rhombic shape or an elongate quadrilateral with two acute portions at its longitudinal ends, as in the present embodiment, but also where the pressure chamber has any other elongate shapes in plan view.  
      Each of the vertical wires  62  is connected to a surface wire  63  (constituting a first wire). As shown in  FIG. 7 , the surface wires  63  are disposed on the dielectric film  60  such that a part of each of the surface wires  63  (strictly, except the surface wires  63  extending from the individual electrodes  35  aligned at the edge of the actuator unit  21  on the side to be connected to the FPC  50 ) is located over one or more pressure chambers  10  different than the one from which the surface wire  63  extends from, but not over any individual electrode  35 . The surface wires  63  extend in the same direction, namely, the direction of the long diagonal line of the pressure chambers  10  toward the lower side in  FIG. 7 , and ends of the surface wires  63  on this side are connected to respective connecting terminals  64  for connection with the FPC  50 . The connecting terminals  64  are disposed in an area (constituting a second area) extending adjacent and along the longer base  21 a of the trapezoidal shape of the actuator unit  21 . This area extends along a side of another area (constituting a first area) in which a group of the individual electrodes are arranged. As shown in  FIGS. 7 and 8 , the FPC  50  is connected to the connecting terminals  64  through its connecting lands  50   a , with an edge of the FPC  50  parallel to the longer base  21   a  of the trapezoidal shape of the actuator unit  21 . Each individual electrode  35  is electrically connected to one of the drivers IC  80 , via the connecting, vertical and surface wires  61 ,  62 ,  63 , and a lead wire formed in the FPC  50 . When it is requested to eject an ink droplet from a nozzle  8 , the driver IC  80  selectively applies a voltage to one of the individual electrodes  35  which corresponds to the nozzle  8 .  
      According to this arrangement, the individual electrodes  35  and the FPC  50  (as a wiring member) are connected via the surface wires  63  (each constituting the first wire) such that the ends of the surface wires  63  are collected in an area so that the FPC  50  is connected to the surface wires  63  at this area. Compared with an arrangement where the wiring member is connected with the actuator unit or its electrodes, in a state such that the surface of the wiring member is parallel to the surface of the actuator unit, as in the above-mentioned publication JP-A-11-334061, the present arrangement makes it easier to connect the individual electrodes to the wiring member in the form of the FPC  50 , via the surface wires  63 , while enhancing the reliability of the connection therebetween.  
      It is noted that in  FIG. 7  only a part of all the surface wires  63  actually provided is shown. In other words, in an actual printhead, the number of the surface wires  63  and accordingly the number of the connecting terminals  64  are double the numbers of those represented in  FIG. 7 .  
      As described above, between the piezoelectric sheet  41  and each of the surface wires  63  connected to the individual electrodes  35 , there is interposed the dielectric film  60  made of a low-k material. Hence, upon application of a voltage to one of the individual electrodes  35  via its corresponding surface wire  63 , the electric field acting on a portion of the piezoelectric sheet  41  beneath that surface wire  63  is minimized. Further, the unnecessary electrostatic capacity, i.e. a parasitic capacity, created at the portion of the piezoelectric sheet  41  with respect to that surface wire  63  is also minimized.  
      In contrast to the present arrangement, in a case where the dielectric film  60  is not employed, and the pressure chambers  10  are arranged in matrix in a fashion similar to the present printhead  70  shown in  FIG. 5 , even when the surface wires  63  are disposed not to be located over any pressure chamber  10  in order to prevent deformation of the pressure chambers  10  due to presence of the surface wires  63  over the pressure chambers  10 , an electric field depending on the distance between the common electrode  34  and each individual electrode  35  is produced around each surface wire  63 , thereby deforming the piezoelectric sheet  41  at the place interposed between the common electrode  34  and the each individual electrode  35 . Thus, a crosstalk occurs. Further, parasitic capacities with respect to the surface wires  63  depending on the way of arrangement of the surface wires  63  are generated, and therefore a variation occurs in the phase and waveform of the voltage applied by the driver IC  80 . The degree of the variation in the phase and waveform of the voltage differs depending on the distance of extension of the surface wires  63 , for instance, and thus the printhead  70  suffers from an inconvenience that the ink ejection characteristics varies among the pressure chambers  10  respectively corresponding to the surface wires  63 .  
      In the printhead  70  of the preset embodiment, on the other hand, the electric field acting on the piezoelectric sheet  41  at the unintended portion upon voltage application to a surface wire  63  is extremely weak, and the crosstalk with the pressure chamber or chambers  10  in the vicinity of that surface wire  63  is prevented. Further, since the parasitic capacities created for the respective surface wires  63  are also extremely small, the ink ejection characteristics is uniform among the pressure chambers  10 .  
      The dielectric film  60  is formed at a region positionally corresponding to the pressure chambers  10 , as well as a region corresponding to the interspace between the pressure chambers  10 . Hence, even when a surface wire  63  connected to an individual electrode  35  corresponding to one of the pressure chambers  10  is disposed over another pressure chamber  10 , there is minimized the deformation of the piezoelectric sheet  41  at the portion corresponding to the another pressure chamber  10  upon voltage application to that surface wire  63 , thereby preventing the crosstalk which would be otherwise caused by presence of that surface wire  63  over the another pressure chamber  10 . Since the surface wires  63  are allowed to be located over the pressure chambers  10 , it is enabled to widen the spacing between the surface wires  63 . This facilitates formation of the surface wires  63 .  
      As shown in  FIG. 7 , the through-hole  60   a  is formed within an area of the pressure chamber  10 , in plan view. Accordingly, the connecting wire  61 , which is disposed on the piezoelectric sheet  41  to connect the individual electrode  35  to the vertical electrode  62  in the through-hole  60   a , is also located within the area of the pressure chamber  10 . Meanwhile, the surface wire  63  connected to the other end of the vertical wire  62  is disposed on the upper or exterior surface of the dielectric film  60 . Hence, in the region not corresponding to the pressure chambers  10 , the piezoelectric sheet  41  as well as the dielectric film  60  are interposed between the surface wires  63  and the common electrode  34 , making the piezoelectric sheet  41  not directly affected by the electric field, at this region. Thus, the piezoelectric sheet  41  does not deform at this region, and there does not occur a crosstalk with the pressure chamber(s)  10  in the vicinity of the surface wire  63  supplied with the voltage, due to the deformation of the sheet  41  at this unintended region. In this way, the ink ejection characteristics is prevented from varying among the pressure chambers  10 , or from nozzle to nozzle.  
      Meanwhile, a part of the piezoelectric sheet  41  is directly sandwiched between each connecting wire  61  and the common electrode  34 , with the connecting wire  61  located within the area of the corresponding pressure chamber  10  in plan view. This may contribute to the ejection of ink droplets, but virtually never deteriorates the ink ejection characteristics, and at least has nothing to do with a crosstalk.  
      The surface wires  63  extend on the dielectric film  60  from the connecting points with the vertical wires  62  into the area extending along the longer base  21   a  of the trapezoidal shape of the actuator unit  21 . In this area, the connecting terminals  64  are formed for the respective surface wires  63 , and are connected with the FPC  50 . This arrangement facilitates the working operation for connecting the FPC  50  to the connecting terminals  64  with reliability in the production process of the inkjet head unit  1 , thereby enhancing the electrical connection therebetween.  
      A part of each of the surface wires  63  (except some of them) is located over the pressure chamber or chambers  10  but not over any individual electrode  35 , as described in detail in paragraph [0033] above. This increases the space for arranging the surface wires  63 , enabling to widen the spacing between the adjacent surface wires  63  and to enhance the density of the wiring on the actuator unit  21 . The portion of the pressure chamber  10  over which the surface wire  63  is allowed to be disposed is such that even when disposed there the surface wire  63  virtually does not contribute to the deformation of the pressure chamber  10  at all, structurally. That is, upon voltage application to the individual electrode  35 , the portion of the piezoelectric sheet  41  interposed between that individual electrode  35  and the common electrode  34  greatly deforms. Without any surface wire  63  in the region corresponding to the portions of the piezoelectric sheets  41 - 44  or of the pressure chambers  10  to be greatly deformed, the intended deformation of the pressure chambers  10  is not inhibited.  
      As described above, in the present embodiment, in addition to the presence of the dielectric film  60  beneath the surface wires  63 , the specific way of arranging the surface wires  63  also contributes to prevent the crosstalk. Thus, the crosstalk is prevented with an enhanced reliability.  
      There will be now described an operation of the actuator unit  21  upon pressurization of the ink in the pressure chamber  10 . The direction of the polarization at the piezoelectric sheet  41  of the actuator unit  21  is parallel to the direction of the thickness of the piezoelectric sheet  41 . That is, the actuator unit  21  is of the unimorph type, namely, the uppermost one  41  of the piezoelectric sheets  41 - 44  which is the most remote from the pressure chamber  10  among the sheets  41 - 44  is an active layer, while the lower three sheets  42 ,  43 ,  44  near the pressure chamber  10  are non-active layers. Hence, with the directions of the electric field and the polarization coincident, when the electric potential at the individual electrode  35  is made at a given positive or negative value, the portion of the piezoelectric sheet  41  interposed between the individual and common electrodes and applied with the electric field functions as the active portion, and contracts in a direction perpendicular to the polarization direction, by the piezoelectric transverse effect. Meanwhile, the piezoelectric sheets  42 - 44  are not affected by the electric field, and therefore do not contract by themselves. Hence, there occurs a difference in deformation in the direction perpendicular to the polarization direction between the uppermost sheet  41  and the other sheets  42 - 44 , causing the piezoelectric sheets  41 - 44  as a whole to become convex toward the side of the non-active layers  42 - 44 . Since the piezoelectric sheets  41 - 44  or the actuator  21  are fixed to the upper surface of the cavity plate  22  where the pressure chambers  10  are formed, the piezoelectric sheets  41 - 44  deforms to be convex toward the pressure chamber  10 . This decreases the inner volume of the pressure chamber  10 , pressurizes the ink therein, and results in. ejection of a droplet of the ink from the nozzle  8 . Thereafter, when the electric potential at the individual electrode  35  is changed back to the level the same as the common electrode  34 , the piezoelectric sheets  41 - 44  are restored to its original shape, restoring the inner volume of the pressure chamber  10 , too. At this time, the ink in the manifold  5  is sucked into the pressure chamber  10 .  
      In another driving method, all of the individual electrodes  35  are set in advance to have an electric potential different from that of the common electrode  34 . Every time when an ejection request is made, any one of the individual electrodes  35  in accordance with the ejection request is once set to have the same electric potential as that of the common electrode  34 . Then, at a predetermined timing, the individual electrode  35  is again set to have the electric potential different from that of the common electrode  34 . In this instance, since the piezoelectric sheets  41 - 44  return to the original shape at a timing when the individual electrode  35  is set to have the same electric potential as that of the common electrode  34 , the volume of the pressure chamber  10  corresponding to the individual electrode  35  is increased as compared with that in the initial state (in which the electric potentials of the individual electrode  35  and the common electrode  34  are different from each other), so that the ink is sucked into the pressure chamber  10  form the manifold  5 . Thereafter, the piezoelectric sheets  41 - 44  deform into a convex shape that protrudes toward the pressure chamber  10  at a timing when the individual electrode  35  is again set to have the electric potential different from that of the common electrode  34 . As a result, the volume of the pressure chamber  10  is decreased to increase the pressure of the ink, so that the ink is ejected from the nozzle  8  in communication with the pressure chamber  10 .  
      When the voltage is applied to the individual electrode  35 , the piezoelectric sheets  41 - 44  deforms the most greatly at a place corresponding to that individual electrode  35 . As described above, at this place the surface wire  63  is not formed, eliminating the inconvenience that presence of the surface wire(s)  63  inhibits the deformation of the piezoelectric sheets  41 - 44  at the place corresponding to that individual electrode  35 . The pressure chamber  10  has the rhombic shape with two acute portions, and the through-hole  60   a  is formed at the position corresponding to the lower one of the acute portions of the pressure chamber  10  as seen in  FIG. 7 . The piezoelectric sheets  41 - 44  is structurally the least deformable, upon voltage application to the individual electrode  35 , at the part corresponding to the acute portions of the pressure chamber  10 . Hence, the connecting wire  61  and the vertical wire  62  disposed at the position corresponding to this part virtually does not deform the piezoelectric sheets  41 - 44  at all. According to the physical principles, the connecting and vertical wires  61 ,  62  will deform the piezoelectric sheets  41 - 44  when the voltage is applied thereto, just as the individual electrode  35  does. However, since the connecting and vertical wires  61 ,  62  are located at the position where the piezoelectric sheets  41 - 44  is structurally less deformable, the deformation of the pressure chamber  10  almost solely depends on the electric field created around the individual electrode  35 . Thus, by forming the through-hole  60   a  at the specific position over the pressure chamber  10 , there can be obtained an inkjet printhead free from the crosstalk and exhibiting uniform ink ejection characteristics.  
      The inkjet head unit  1  as described above enjoys the following advantages.  
      The arrangement that the dielectric film  60  having a dielectric constant which is smaller than that of the piezoelectric sheets  41 - 44  is interposed between each surface wire  63  and the piezoelectric sheet  41 , minimizes the electrostatic capacity or parasitic capacity created for a surface wire  63  upon voltage application to an individual electrode  35  through the surface wire  63 , as well as the unnecessary electric field produced between the surface wire  63  and the common electrode  34 . Hence, the driving efficiency of the actuator unit  21  is enhanced, and the ink ejection characteristics is made uniform across the actuator unit  21 , at the same time. With the unnecessary electric field between the surface wire  63  and the common electrode  34  minimized, the deformation of the piezoelectric sheet  41  at the portion interposed between the surface wire  63  and the common electrode  34  is minimized, meaning that the deformation of the piezoelectric sheets  41 - 44  in the vicinity of that portion is minimized, preventing the crosstalk with a pressure chamber or chambers  10  in the vicinity of the portion. Thus, the print quality is improved.  
      The arrangement that the dielectric film  60  is formed at the region positionally corresponding to the pressure chambers  10  prevents the crosstalk which would otherwise occur where a surface wire  63  connected to an individual electrode  35  for a pressure chamber is disposed over another pressure chamber. According to this arrangement, the space for arranging the surface wires  63  increases, enabling to widen the spacing between the adjacent surface wires  63 . Thus, the formation of the surface wires  63  is facilitated, making it possible to reduce the manufacturing cost of the actuator unit  21  or the printhead  70 .  
      Conventionally, in the case where a large number of pressure chambers are arranged in matrix for achieving printing of high quality at high speed, the crosstalk tended to occur. The inkjet head unit of the present embodiment is applicable to such a case in order to prevent the crosstalk with reliability.  
      There will be now described several other embodiments of the invention. In the description below, the same elements or parts as those in the first embodiment, or elements or parts at least similar to the counterparts in the first embodiment, will be denoted by the same reference numerals and description thereof is not provided where unnecessary.  
      By referring to  FIG. 9 , an inkjet head unit according to a second embodiment of the invention will be described.  
      In this embodiment, a surface wire  63  is not disposed over any individual electrode  35 , similarly to the first embodiment. The difference of the second embodiment from the first embodiment resides in that a part of the dielectric film  60  is removed in a region positionally corresponding to each individual electrode  35 , so as to expose the individual electrode  35  to the outside.  
      According to this embodiment, the piezoelectric sheets  41 - 44  is more easily deformable at the place corresponding to each individual electrode  35 , enhancing the efficiency of the deformation of the piezoelectric sheets  41 - 44  and accordingly of the pressure chambers  10 .  
      It is noted that in  FIG. 9  only a part of all the surface wires  63  actually provided is shown. That is, in an actual printhead, the number of the surface wires  63  and accordingly the number of the connecting terminals  64  are double the numbers of those represented in  FIG. 9 .  
      By referring to  FIG. 10 , there will be described an inkjet head unit according to a third embodiment of the invention.  
      In the present embodiment, a part of each surface wire  63  (strictly, except surface wires  63  extending from individual electrodes  35  aligned at an edge of an actuator unit  21  on a side to be connected to an FPC  50 ) is allowed to be disposed over an individual electrode or electrodes  35  as well as over a pressure chamber or chambers  10  different than the one from which the surface wire  63  extends from.  
      According to this embodiment, the space for arranging the surface wires  63  further increases, enabling to further widen the spacing between the adjacent surface wires  63 . Where the way in which the surface wires  63  are disposed over the individual electrode(s)  35  varies from wire  63  to wire  63 , the deformation amount of the pressure chambers  10  varies thereamong. Hence, in order to achieve uniform ink ejection characteristics, it is preferable that the positions of the surface wires  63  relative to the respectively corresponding individual electrodes  35 , and the spaces or areas occupied by the respective surface wires  63 , are uniform among all the surface wires  63 .  
      It is noted that in  FIG. 10  only a part of all the surface wires  63  actually provided is shown. In other words, in an actual printhead, the number of the surface wires  63  and accordingly the number of the connecting terminals  64  are double the numbers of those represented in  FIG. 10 .  
      Referring now to  FIG. 11 , there will be described an inkjet head unit according to a fourth embodiment of the invention.  
      In each of the above-described first through third embodiments, the surface wires  63  are allowed to extend over the pressure chamber or chambers  10 . However, in the present embodiment, the surface wires  63  are disposed to extend only over the interspace between the pressure chambers, and not over any pressure chamber  10 .  
      According to the fourth embodiment, when a voltage is applied to an individual electrode  35  positionally corresponding to a pressure chamber  10 , through a surface wire  63  connected to the individual electrode  35 , an electric field created around that surface wire  63  does not affect a portion of the piezoelectric sheet  41  positionally corresponding to another pressure chamber  10 . Thus, the crosstalk, which would be otherwise caused by presence of the surface wire  63  over the another pressure chamber  10 , is prevented further reliably.  
      As described above, in the present embodiment the surface wires as the first wires are not disposed over any pressure chamber  10 , in principle. Strictly, however, a portion of each surface wire (as the first wire) at its one end connected to the vertical wire (as the second wire) is disposed over the pressure chamber from which the surface wire itself extends from, but over a very short distance, as can be seen in  FIG. 7 . This distance may be zero, that is, each through-hole  60   a  may be formed on the outline of the corresponding pressure chamber  10  indicated by a broken line, or at a position outside the outline. When the through-hole  60   a  is formed outside the outline, it is preferable that the through-hole  60   a  is located as close to the outline as possible.  
      It is noted that in  FIG. 11  only a part of all the surface wires  63  actually provided is shown. In other words, in an actual printhead, the number of the surface wires  63  and accordingly the number of the connecting terminals  64  are double the numbers of those represented in  FIG. 11 .  
      In each of the above-described embodiments a through-hole is formed for each pressure chamber and disposed at the position corresponding to one of two opposite longitudinal ends of the pressure chamber. However, each embodiment may be modified such that two through-holes are formed for each pressure chamber and disposed at respective positions corresponding to both of the opposite longitudinal ends of the pressure chamber. This modification is advantageous over the above-described embodiments in terms of versatility. That is, a same dielectric film can be easily usable in both of two types of inkjet printhead units where the surface wires extend upward and downward, respectively, as seen in  FIGS. 7 and 9 - 11 , to be connected to the FPC  50 . In this modification, it may be arranged such that two connecting wires extend from each individual electrode to be connected to respective vertical wires vertically extending in the two through-holes, and the vertical wires are connected to respective surface wires, or alternatively the vertical wires are connected to a common, single surface wire via respective individual wires. This arrangement is advantageous over the above-described embodiments in terms of fail-safe. That is, where the vertical wires are connected to the respective surface wires, even if one of the two surface wires for a pressure chamber is disconnected or otherwise fails, the other surface wire can work. Where the vertical wires are connected to the common, single surface wire via the respective individual wires, even if one of the two individual wires are disconnected or otherwise fails, the individual electrode is kept connected to the surface wire via the other, normal individual wire.  
      In each of the above-described embodiments, the shape of the pressure chambers  10  in plan view is the rhomboid. However, the principle of the invention can be equally applicable to cases where the pressure chambers have other shapes in plan view, such as ellipse and rectangle, for instance.