Patent Publication Number: US-2022212469-A1

Title: Piezoelectric film utilization device

Description:
TECHNICAL FIELD 
     The present invention relates to a piezoelectric film utilization device, such as an actuator or sensor, etc., that uses a piezoelectric film. 
     BACKGROUND ART 
     An inkjet printing head is known as an actuator that uses a piezoelectric film. An example of such an inkjet printing head is disclosed in Patent Literature 1. The inkjet printing head disclosed in Patent Literature 1 includes a nozzle substrate, a pressure chamber substrate, a movable film (vibrating film), and a piezoelectric element bonded to the movable film. A pressure chamber, into which ink is introduced, is formed in the pressure chamber substrate and the movable film faces the pressure chamber. The piezoelectric element is arranged by laminating a lower electrode, a piezoelectric film, and an upper electrode in that order from the movable film side. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Publication No. 2013-215930 
     SUMMARY OF INVENTION 
     Technical Problem 
     With the arrangement described in Patent Literature 1, the upper electrode is formed to have a uniform thickness. To make a displacement amount of the movable film large, it is preferable for the upper electrode to be thinner in thickness. However, if the upper electrode is made thin in thickness, it increases in electrical resistance and it may not be possible to apply an electric field uniformly to the piezoelectric film. If an applied voltage is then increased, the piezoelectric film may be put in a so-called fatigue state in which the displacement amount decreases or response becomes slow. 
     An object of the present invention is to provide a piezoelectric film utilization device with which a normal electric field can be applied to a piezoelectric film and a movable film can be made large in displacement amount. 
     Solution to Problem 
     A piezoelectric film utilization device according to the present invention includes a cavity, a movable film formation layer that includes a movable film disposed on the cavity and defining a top surface portion of the cavity, and a piezoelectric element that is formed to contact a surface of the movable film at an opposite side from the cavity and having a peripheral edge receded further toward an interior of the cavity than the movable film in plan view, and the piezoelectric element includes a lower electrode formed on a surface of the movable film formation layer at the opposite side from the cavity, an upper electrode disposed at an opposite side from the movable film formation layer with respect to the lower electrode, and a piezoelectric film provided between the upper electrode and the lower electrode, and the upper electrode has a thin portion at least at a portion of a peripheral edge portion. 
     With this arrangement, a central portion of the upper electrode besides the peripheral edge portion and a portion of the peripheral edge portion besides the thin portion are thick portions. The thick portions are low in electrical resistance value in comparison to the thin portion and therefore, charges can be injected into the piezoelectric film without causing a large voltage drop. On the other hand, in comparison to the thick portions, the thin portion is high in electrical resistance value but is short in distance to a peripheral edge of the upper electrode and short in distance through which charges pass and therefore, an amount of current flowing through the peripheral edge portion is low. A large voltage drop thus does not occur in the thin portion as well. 
     Consequently, in comparison to a case where an entirety of the upper electrode is of the same thickness as the thick portions, a normal electric field can be applied to the piezoelectric film even if an average thickness is decreased. Since the average thickness can be decreased in comparison to the case where the entirety of the upper electrode is of the same thickness as the thick portions, a displacement amount of the movable film can be increased. 
     In a preferred embodiment of the present invention, the upper electrode has the thin portion across an entirety of the peripheral edge portion. 
     In the preferred embodiment of the present invention, the upper electrode has a rectangular shape that is long in one direction in plan view and the upper electrode has thin portions at both side portions. 
     In the preferred embodiment of the present invention, the upper electrode also has thin portions at both end portions. 
     In the preferred embodiment of the present invention, the thin portion has a tapered upper surface with which a distance to an upper surface of the piezoelectric film decreases gradually toward the outside. 
     In the preferred embodiment of the present invention, a length in an inside/outside direction of the thin portion is not less than 0.5 μm in plan view. 
     In the preferred embodiment of the present invention, an inclination angle of the tapered upper surface with respect to the upper surface of the piezoelectric film is not less than 1 degree and within 8 degrees. 
     In the preferred embodiment of the present invention, an outer edge portion of the tapered upper surface is formed to a curved surface that is outwardly convex. 
     In the preferred embodiment of the present invention, the upper electrode is constituted of a laminated film of an IrO 2  film that is formed on the upper surface of the piezoelectric film and an Ir film that is laminated on the IrO 2  film and the Ir film is present on the IrO 2  film at an outer edge portion of the thin portion. 
     In the preferred embodiment of the present invention, the upper electrode is constituted of a laminated film of an IrO 2  film that is formed on the upper surface of the piezoelectric film and an Ir film that is laminated on the IrO 2  film and the Ir film is not present on the IrO 2  film at an outer edge portion of the thin portion. 
     In the preferred embodiment of the present invention, the upper electrode is constituted of a laminated film of an IrO 2  film that is formed on an upper surface of the piezoelectric film and an Ir film that is laminated on the IrO 2  film. 
     In the preferred embodiment of the present invention, the piezoelectric film is constituted of a material having PZT as a main component. 
     In the preferred embodiment of the present invention, the lower electrode is constituted of a material having Pt as a main component. 
     In the preferred embodiment of the present invention, a thickness of the thin portion is constant and the thin portion has an upper surface that is parallel to an upper surface of the piezoelectric film. 
     In the preferred embodiment of the present invention, a length in an inside/outside direction of the thin portion is not less than 0.5 μm in plan view. 
     The aforementioned as well as yet other objects, features, and effects of the present invention will be made clear by the following description of the preferred embodiments made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic plan view of an inkjet printing head to which a piezoelectric film utilization device according to a preferred embodiment of the present invention is applied. 
         FIG. 2  is a schematic enlarged sectional view taken along line II-II in  FIG. 1 . 
         FIG. 3  is a schematic enlarged sectional view taken along line in  FIG. 1 . 
         FIG. 4A  is a partially enlarged view of an actual shape of mainly a peripheral edge portion of an upper electrode of  FIG. 2 . 
         FIG. 4B  is a partially enlarged view of a modification example of the shape of the peripheral edge portion of the upper electrode. 
         FIG. 5A  is a sectional view of an example of a manufacturing process of the inkjet printing head. 
         FIG. 5B  is a sectional view of a step subsequent to that of  FIG. 5A . 
         FIG. 5C  is a sectional view of a step subsequent to that of  FIG. 5B . 
         FIG. 5D  is a sectional view of a step subsequent to that of  FIG. 5C . 
         FIG. 5E  is a sectional view of a step subsequent to that of  FIG. 5D . 
         FIG. 5F  is a sectional view of a step subsequent to that of  FIG. 5E . 
         FIG. 5G  is a sectional view of a step subsequent to that of  FIG. 5F . 
         FIG. 5H  is a sectional view of a step subsequent to that of  FIG. 5G . 
         FIG. 5I  is a sectional view of a step subsequent to that of  FIG. 5H . 
         FIG. 5J  is a sectional view of a step subsequent to that of  FIG. 5I . 
         FIG. 5K  is a sectional view of a step subsequent to that of  FIG. 5J . 
         FIG. 5L  is a sectional view of a step subsequent to that of  FIG. 5K . 
         FIG. 5M  is a sectional view of a step subsequent to that of  FIG. 5L . 
         FIG. 6  is a sectional view for describing the arrangement of an inkjet printing head according to another preferred embodiment and is a sectional view corresponding to  FIG. 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a schematic plan view of an inkjet printing head to which a piezoelectric film utilization device according to a preferred embodiment of the present invention is applied.  FIG. 2  is a schematic enlarged sectional view taken along line II-II in  FIG. 1 .  FIG. 3  is a schematic enlarged sectional view taken along line III-III in  FIG. 1 .  FIG. 4A  is a partially enlarged view of an actual shape of mainly a peripheral edge portion of an upper electrode of  FIG. 2 . A hydrogen barrier film and an insulating film indicated by the reference signs  11  and  12  in  FIG. 2  and  FIG. 3  are omitted in  FIG. 1  and  FIG. 4A . 
     Referring to  FIG. 2  and  FIG. 3 , the inkjet printing head  1  includes a silicon substrate  2  and a nozzle substrate  3  having discharge ports  31   a  that discharge ink. A movable film formation layer  10  is laminated on the silicon substrate  2 . In the laminate of the silicon substrate  2  and the movable film formation layer  10 , pressure chambers (cavities)  5  are formed as ink flow passages (ink reservoirs). 
     The nozzle substrate  3  is constituted, for example, of a silicon plate, is adhered to a rear surface of the silicon substrate  2 , and, together with the silicon substrate  2  and the movable film formation layer  10 , defines the pressure chambers  5 . Ink discharge passages  31  are formed in the nozzle substrate  3 . Each ink discharge passage  31  penetrates through the nozzle substrate  3  and has the discharge port  31   a  at an opposite side from the pressure chamber  5 . Therefore, when a volume change occurs in the pressure chamber  5 , the ink retained in the pressure chamber  5  passes through the ink discharge passage  31  and is discharged from the discharge port  31   a.    
     Each pressure chamber  5  is formed by digging into the silicon substrate  2  (or the silicon substrate  2  and the movable film formation layer  10 ) from the rear surface side of the silicon substrate  2 . Ink supply passages  4  (see  FIG. 1  and  FIG. 3  together), in communication with the pressure chambers  5 , are further formed in the silicon substrate  2  (or the silicon substrate  2  and the movable film formation layer  10 ). The ink supply passages  4  are in communication with the pressure chambers  5  and are formed to guide ink from an ink tank (for example, an ink cartridge) that is an ink supply source to the pressure chambers  5 . 
     Each pressure chamber  5  is formed to be elongate along an ink flow direction  21 , which is a right/left direction in  FIG. 3 . A portion of the movable film formation layer  10  that is a top roof portion of the pressure chamber  5  constitutes a movable film  10 A. The movable film  10 A (movable film formation layer  10 ) is constituted, for example, of a silicon oxide (SiO 2 ) film formed on the silicon substrate  2 . The movable film  10 A (movable film formation layer  10 ) may instead be constituted of a laminate, for example, of a silicon (Si) layer formed on the silicon substrate  2 , a silicon oxide (SiO 2 ) layer formed on the silicon layer, and a silicon nitride (SiN) layer formed on the silicon oxide layer. In the present description, the movable film  10 A refers to the portion of the movable film formation layer  10  that is the top roof portion defining the pressure chamber  5 . Therefore, portions of the movable film formation layer  10  besides the top roof portion of the pressure chamber  5  do not constitute the movable film  10 A. 
     The movable film  10 A has a thickness of, for example, approximately 0.4 μm to 3 μm. In the present preferred embodiment, the thickness of the movable film  10 A is approximately 2 μm. 
     The pressure chamber  5  is defined by the movable film  10 A, the silicon substrate  2 , and the nozzle substrate and is formed to a substantially rectangular parallelepiped shape in the present preferred embodiment. A length of the pressure chamber  5  is, for example, approximately 600 μm, a width thereof is approximately 100 μm, and a depth thereof is approximately 100 μm. The ink supply passages  4  are formed to be in communication with one end portion (an end portion positioned at an opposite side from the discharge port  31   a  in the present preferred embodiment) in a long direction of the pressure chamber  5 . In the present preferred embodiment, the discharge port  31   a  of the nozzle substrate  3  is disposed near another end portion related to the long direction of the pressure chamber  5 . 
     The pressure chamber  5  has the rectangular parallelepiped shape and therefore, the movable film  10 A has a rectangular shape that is long in the ink flow direction  21  in plan view. The movable film  10 A has a length in a long direction of approximately 600 μm and a length in a short direction of approximately 100 μm. 
     A piezoelectric element  6  is disposed on a front surface of each movable film  10 A. The piezoelectric element  6  includes a lower electrode  7  formed on the movable film formation layer  10 , a piezoelectric film  8  formed on the lower electrode  7 , and an upper electrode  9  formed on the piezoelectric film  8 . In other words, the piezoelectric element  6  is arranged by sandwiching the piezoelectric film from above and below by the upper electrode  9  and the lower electrode  7 . 
     The lower electrode  7  is constituted, for example, of a single film that is a Pt (platinum) layer. Besides this, the lower electrode  7  can instead be formed of a single film that is an Au (gold) film, a Cr (chromium) layer, or an Ni (nickel) layer, etc. Also, the lower electrode  7  may instead be formed of a laminated film in which a Ti (titanium) layer and a Pt (platinum) layer are laminated in that order from the movable film  10 A side. A film thickness of the lower electrode  7  is, for example, approximately 200 μm. 
     The lower electrode  7  has a rectangular portion  71  that is disposed on the movable film  10 A and has substantially the same shape and size as the movable film  10 A, a lead-out electrode portion  72  that is led out from one end side of the rectangular portion  71 , and a common connection portion  73  that connects the lower electrodes  7  of a plurality of piezoelectric elements  6  in common. A portion of the rectangular portion  71  that is in contact with a lower surface of the piezoelectric film  8  is a main electrode portion  71 A that constitutes the piezoelectric element  6 . 
     Referring to  FIG. 3  and  FIG. 1 , each lead-out electrode portion  72  is led out to a downstream side from a width central portion of a downstream side end in the ink flow direction  21  of the rectangular portion  71 . At a further downstream side than the rectangular portions  71 , the common connection portion  73  extends in a direction orthogonal to the ink flow direction  21 . Downstream side ends of a plurality of lead-out electrode portions  72  are connected to the common connection portion  73 . 
     Returning to  FIG. 2  and  FIG. 3 , as the piezoelectric film  8 , for example, a PZT (PbZr x Ti 1-x O 3 : lead zirconate titanate) film formed by a sol-gel method or a sputtering method can be applied. Such a piezoelectric film  8  is constituted of a sintered body of a metal oxide crystal. The piezoelectric film  8  preferably has a thickness of 1 μm to 5 μm. In the present preferred embodiment, the thickness of the piezoelectric film  8  is approximately 2 μm. An overall thickness of the movable film  10 A is preferably approximately the same as the thickness of the piezoelectric film  8  or approximately ⅔ the thickness of the piezoelectric film  8 . 
     In plan view, the piezoelectric film  8  has a rectangular shape that is long in the ink flow direction  21 . Each side surface of the piezoelectric film  8  is formed to an inclined surface that spreads outward toward a lower side. The piezoelectric film  8  is formed with a length in a long direction thereof being shorter than the length in the long direction of the movable film  10 A (pressure chamber  5 ) and both end edges thereof are disposed inwardly at predetermined first intervals from corresponding both end edges of the movable film  10 A. A length in a long direction of an upper surface of the piezoelectric film  8  is approximately 500 μm. 
     The piezoelectric film  8  is formed with a length in a short direction thereof being shorter than the length in the short direction of the movable film  10 A (pressure chamber  5 ) and both side edges thereof are disposed inwardly at predetermined second intervals from corresponding both side edges of the movable film  10 A. A length in a short direction of the upper surface of the piezoelectric film  8  is approximately 33 μm. 
     The upper electrodes  9  are formed to be of substantially the same pattern as the upper surfaces of the piezoelectric films  8 . That is, each upper electrode  9  has a rectangular shape that is long in the ink flow direction  21 . The upper electrode  9  has, for example, a two-layer structure in which an IrO 2  (iridium oxide) film  9 A and an Ir (iridium) film  9 B are laminated in that order from the piezoelectric film  8  side as shown in  FIG. 4A . 
     In plan view, the upper electrode  9  is constituted of a thick central portion  91  and a thin peripheral edge portion  92  at outer sides of the central portion  91 . In plan view, the central portion  91  has a rectangular shape that is long in the ink flow direction  21 . A film thickness of the central portion  91  is constant and an upper surface of the central portion  91  is formed to a flat surface that is substantially parallel to the upper surface of the piezoelectric film  8 . 
     The peripheral edge portion  92  has a rectangular annular shape in plan view. In the present preferred embodiment, the peripheral edge portion  92  is formed to a thin portion of tapered shape with which a film thickness decreases gradually towards the outside (outer peripheral edges). In other words, the peripheral edge portion  92  has a tapered upper surface  92   a  with which a distance to the piezoelectric film  8  upper surface decreases gradually toward the outside. 
     In the present preferred embodiment, a length in a long direction of the central portion  91  is approximately 497 μm and a length in a short direction of the central portion  91  is approximately 30 μm. Also, in plan view, a length in an inside/outside direction of the peripheral edge portion  92  (width of the peripheral edge portion) is approximately 1.5 μm. The length in the inside/outside direction of the peripheral edge portion  92  is preferably not less than 0.5 μm. In the present preferred embodiment, an inclination angle of the tapered upper surface  92   a  with respect to the upper surface of the piezoelectric film  8  is approximately 4 degrees. The inclination angle of the tapered upper surface  92   a  with respect to the upper surface of the piezoelectric film  8  is preferably not less than 1 degree and not more than 8 degrees. 
     As shown in  FIG. 4A , an outer edge portion  92   b  of the tapered upper surface  92   a  of the peripheral edge portion  92  is formed to a curved surface that is outwardly convex. In  FIG. 4A , the Ir film  9 B is present on the IrO 2  film  9 A at the outer edge portion  92   b  of the peripheral edge portion  92 . However, as shown in  FIG. 4B , the Ir film  9 B does not have to be present on the IrO 2  film  9 A at the outer edge portion  92   b  of the peripheral edge portion  92 . 
     A front surface of the movable film formation layer  10 , front surfaces of the piezoelectric elements  6 , and front surfaces of portions of the lower electrodes  7  besides the main electrode portions  71 A are covered with a hydrogen barrier film  11 . The hydrogen barrier film  11  is constituted, for example, of Al 2 O 3 (alumina). Degradation of characteristics of the piezoelectric films  8  due to hydrogen reduction can thereby be prevented. A film thickness of the hydrogen barrier film  11  is approximately 100 nm. 
     An insulating film  12  is laminated on the hydrogen barrier film  11 . The insulating film  12  is constituted, for example, of SiO 2 . Wirings  13  are formed on the insulating film  12 . The wirings  13  are constituted of a metal material that includes Al (aluminum). 
     One end portion of each wiring  13  is disposed above one end portion of the upper electrode  9 . A penetrating hole  14  penetrating continuously through the hydrogen barrier film  11  and the insulating film  12  is formed between the wiring  13  and the upper electrode  9 . The one end portion of the wiring  13  enters into the penetrating hole  14  and is connected to the upper electrode  9  inside the penetrating hole  14 . Also, the hydrogen barrier film  11  and the insulating film  12  have a cutout portion  15  at a position corresponding to a region surrounded by a peripheral edge portion of a front surface of the central portion  91  of each upper electrode  9 . The cutout portions  15  are portions at which the hydrogen barrier film  11  and the insulating film  12  are cut out. 
     Also, although not illustrated, an opening that penetrates continuously through the hydrogen barrier film and the insulating film  12  is formed at a position corresponding to a predetermined region on the common connection portion  73  of the lower electrodes  7  and a front surface of the common connection portion  73  is exposed via the opening. The exposed portion constitutes a pad portion that is arranged to connect the lower electrode  7  to the exterior. 
     Each piezoelectric element  6  is formed at a position facing the pressure chamber  5  across the movable film  10 A. That is, the piezoelectric element  6  is formed to contact a surface of the movable film  10 A at the opposite side from the pressure chamber  5 . The pressure chamber  5  is filled with ink supplied from an unillustrated ink tank through the ink supply passages  4 . The movable film  10 A defines a top surface portion of the pressure chamber  5  and faces the pressure chamber  5 . The movable film  10 A is supported by portions of the laminate of the movable film formation layer  10  and the silicon substrate  2  at a periphery of the pressure chamber  5  and has flexibility enabling deformation in a direction facing the pressure chamber  5  (in other words, in a thickness direction of the movable film  10 A). 
     The wirings  13  and the common connection portion  73  of the lower electrodes  7  are connected to a drive circuit  20 . The drive circuit  20  may be formed in a region of the silicon substrate  2  separate from the pressure chambers  5  or may be formed outside the silicon substrate  2 . When a drive voltage is applied from the drive circuit  20  to a piezoelectric element  6 , the piezoelectric film  8  deforms due to an inverse piezoelectric effect. The movable film  10 A is thereby made to deform together with the piezoelectric element  6  to bring about a volume change of the pressure chamber  5  and the ink inside the pressure chamber  5  is pressurized. The pressurized ink passes through the ink discharge passage  31  and is discharged as microdroplets from the discharge port  31   a.    
     Referring to  FIG. 1  to  FIG. 3 , a plurality of the pressure chambers  5  are formed as stripes extending parallel to each other in the silicon substrate  2  (or the laminate of the silicon substrate  2  and the movable film formation layer  10 ). The plurality of pressure chambers  5  are formed at equal intervals that are minute intervals (for example, of approximately 30 μm to 350 μm) in a width direction thereof. 
     In plan view, each pressure chamber  5  has an oblong shape that is elongate along the ink flow direction directed from the ink supply passages  4  to the ink discharge passage  31 . That is, the top surface portion of the pressure chamber  5  has two side edges  5   c  and  5   d  along the ink flow direction  21  and two end edges  5   a  and  5   b  along the direction orthogonal to the ink flow direction  21 . 
     At the one end portion of each pressure chamber  5 , the ink supply passages  4  are divided and formed as two passages and are in communication with a common ink passage  22 . The common ink passage  22  is in communication with the ink supply passages  4  corresponding to the plurality of pressure chambers  5  and is formed to supply the ink from the ink tank to the ink supply passages  4 . 
     Each wiring  13  is constituted of a lead-out portion  13 A having one end portion connected to one end portion of the upper electrode  9  at an upstream side in the ink flow direction  21  and extending in a direction opposite to the ink flow direction  21  and a pad portion  13 B of rectangular shape in plan view that is made integral to the lead-out portion  13 A and connected to a tip of the lead-out portion  13 A. With the exception of the portion connected to the upper electrode  9 , the lead-out portion  13 A is formed on a front surface of the insulating film  12  that covers one end portion of the upper surface of the piezoelectric element  6 , an end surface of the piezoelectric element  6  continuous thereto, and the front surface of the movable film formation layer  10 . The pad portion  13 B is formed on the insulating layer  12  that covers the front surface of the movable film formation layer  10 . 
     An annular region (a rectangular annular region that is long in the ink flow direction  21  in the present preferred embodiment) of each movable film  10 A between peripheral edges of the movable film  10 A and peripheral edges of the piezoelectric element  6  is a region that is not constrained by the piezoelectric element  6  or a peripheral wall of the pressure chamber  5  and is a region in which a large deformation occurs. That is, a peripheral edge portion of the movable film  10 A is a region in which a large deformation occurs. Therefore, when the piezoelectric element  6  is driven, the peripheral edge portion of the movable film  10 A bends such that an inner peripheral edge side of the peripheral edge portion of the movable film  10 A is displaced in a thickness direction of the pressure chamber  5  (downward in the present preferred embodiment) and an entirety of a central portion surrounded by the peripheral edge portion of the movable film  10 A is thereby displaced in the thickness direction of the pressure chamber  5  (downward in the present preferred embodiment). 
     A method for manufacturing the inkjet printing head  1  shall now be described specifically. 
       FIG. 5A to 5M  are sectional views of a manufacturing process of the inkjet printing head  1  and are sectional views corresponding to the section plane of  FIG. 2 . 
     First, as shown in  FIG. 5A , the silicon substrate  2  is prepared. Here, as the silicon substrate  2 , that which is thicker in thickness than the silicon substrate  2  at the final stage is prepared. 
     Next, as shown in  FIG. 5B , the movable film formation layer  10  is formed on the front surface of the silicon substrate  2 . Specifically, a silicon film (for example, of 2 μm thickness) is formed on the front surface of the silicon substrate  2 . If the movable film formation layer  10  is constituted of a laminated film of a silicon film, a silicon oxide film, and a silicon nitride film, the silicon film (for example, of 0.6 μm thickness) is formed on the front surface of the silicon substrate  2 , the silicon oxide film (for example, of 0.6 μm thickness) is formed on the silicon film, and the silicon nitride film (for example, of 0.6 μm thickness) is formed on the silicon oxide film. 
     Next, as shown in  FIG. 5C , a lower electrode film  107  that is a material layer of the lower electrodes  7  is formed on the movable film formation layer  10 . The lower electrode film  107  is constituted, for example, of a Pt film (for example, of 200 nm thickness). Such a lower electrode film  107  is formed, for example, by a sputtering method. 
     Next, as shown in  FIG. 5D , a piezoelectric material film  108  that is a material of the piezoelectric films  8  is formed on an entire surface of the lower electrode film  107 . Specifically, the piezoelectric material film  108  of, for example, 2 μm thickness is formed by, for example, a sol-gel method. Such a piezoelectric material film  108  is constituted of a sintered body of metal oxide crystal grains. 
     Next, as shown in  FIG. 5E , an upper electrode film  109  that is a material of the upper electrodes  9  is formed on an entire surface of the piezoelectric material film  108 . The upper electrode film  109  is constituted, for example, of an IrO 2 /Ir laminated film having an IrO 2  film (for example, of 50 nm thickness) as a lower layer and an Ir film (for example, of 50 nm thickness) as an upper layer. Such an upper electrode film  109  is formed by a sputtering method. 
     Next, as shown in  FIG. 5F , a resist mask  111  with a pattern of the upper electrodes  9  is formed by photolithography. Each side surface  111   a  of the resist mask  111  is formed to an inclined surface that spreads outward toward a lower side. 
     Next, as shown in  FIG. 5G  and  FIG. 5H , the upper electrode film  109  is etched using the resist mask  111  as a mask to form the upper electrodes  9  of a predetermined pattern. In the etching step, as shown in  FIG. 5H , a front surface and the side surfaces  111   a  of inclined shapes of the resist mask  111  are etched gradually such that an etching amount increases toward outer sides at upper surface peripheral edge portions of the upper electrodes  9 . Consequently, the upper electrode  9  each constituted of the central portion (thick portion)  91  that is constant in film thickness and the peripheral edge portion (thin portion)  92  with which the film thickness decreases gradually toward the outside is obtained. The peripheral edge portion  92  has the tapered upper surface  92   a  with which the distance to the piezoelectric film  8  upper surface decreases gradually toward the outside. The outer edge portion  92   b  of the peripheral edge portion  92  is formed to the curved surface that is outwardly convex. 
     Next, as shown in  FIG. 5I , the piezoelectric material film  108  is etched to form the piezoelectric films  8  of a predetermined pattern. Thereafter, the resist mask  111  is peeled off. 
     Next, as shown in  FIG. 5J , a resist mask  112  with a pattern of the lower electrodes  7  is formed by photolithography. 
     Next, as shown in  FIG. 5K , the lower electrode film  107  is etched using the resist mask  112  as a mask to form the lower electrodes  7  that are each constituted of the rectangular portion  71  including the main electrode portion  71 A, the lead-out electrode portion  72 , and the common connection portion  73 . Thereafter, the resist mask  112  is peeled off. 
     Next, the hydrogen barrier film  11  covering the whole surface is formed. Thereafter, the insulating film  12  is formed on an entire surface of the hydrogen barrier film  11 . Subsequently, the insulating film  12  and the hydrogen barrier film  11  are etched successively to form the penetrating holes  14 . A wiring film that constitutes the wirings  13  is formed on the insulating film  12 , including interiors of the penetrating holes  14 , by a sputtering method. Thereafter, the wiring film is patterned by photolithography and etching to form the wirings  13 . Thereafter, as shown in  FIG. 5L , the insulating film  12  and the hydrogen barrier film  11  are etched successively to form the cutout portions  15 . 
     Next, as shown in  FIG. 5M , rear surface grinding for thinning the silicon substrate  2  is performed. The silicon substrate  2  is polished from the rear surface to make the silicon substrate  2  into a thin film. For example, the silicon substrate  2  of approximately 670 μm thickness in an initial state is thinned to approximately 100 μm thickness. Thereafter, the common ink passage  22 , the ink supply passages  4 , and the pressure chambers  5  are formed in the silicon substrate  2  by photolithography and etching. 
     Lastly, the nozzle substrate  3  is adhered to the rear surface of the silicon substrate  2  and the inkjet printing head  1  such as shown in  FIG. 1  to  FIG. 4A  is thereby obtained. 
     With the preferred embodiment described above, each upper electrode  9  is constituted of the thick central portion  91  and the thin peripheral edge portion  92  at the outer sides of the central portion  91 . The film thickness of the central portion  91  is constant. The peripheral edge portion  92  is formed to the thin portion of tapered shape with which the film thickness decreases gradually towards the outside. In other words, the peripheral edge portion  92  has the tapered upper surface  92   a  with which the distance to the piezoelectric film  8  upper surface decreases gradually toward the outside. 
     The central portion (thick portion)  91  is low in electrical resistance value in comparison to the peripheral edge portion (thin portion)  92  and therefore, charges can be injected into the piezoelectric film  8  without causing a large voltage drop. On the other hand, in comparison to the central portion  91 , the peripheral edge portion  92  is high in electrical resistance value but is short in distance to the peripheral edge of the upper electrode  9  and short in distance through which charges pass and therefore, an amount of current flowing through the peripheral edge portion  92  is low. A large voltage drop thus does not occur in the peripheral edge portion  92  as well. 
     Consequently, in comparison to a case where an entirety of the upper electrode  9  is of the same thickness as the central portion  91 , a normal electric field can be applied to the piezoelectric film  8  even if an average thickness is decreased. Since the average thickness can be decreased in comparison to the case where the entirety of the upper electrode  9  is of the same thickness as the central portion  91 , a displacement amount of the movable film  10 A can be increased. 
     Although with the preferred embodiment described above, an entirety of the peripheral edge portion of each upper electrode  9  is formed to the thin portion of tapered shape, just a portion of the peripheral edge portion of the upper electrode  9  may be formed to a thin portion of tapered shape instead. For example, of the both side portions and the both end portions of the upper electrode  9 , just the both side portions may be formed to thin portions of tapered shapes. 
     Also, although with the preferred embodiment described above, each upper electrode  9  has the rectangular shape in plan view, a planar shape of the upper electrode  9  may instead be a circular shape, an elliptical shape, or any other shape. 
     Also, although with the preferred embodiment described above, the peripheral edge portion  92  of the upper electrode  9  is formed to the thin portion of tapered shape, the peripheral edge portion  92  of the upper electrode  9  may instead be formed to a thin portion that is thinner than the central portion  91  and is of a constant thickness as in an inkjet printing head  1 A shown in  FIG. 6 .  FIG. 6  is a sectional view corresponding to  FIG. 2 . Even in this case, the length in the inside/outside direction of the peripheral edge portion  92  in plan view is preferably not less than 0.5 μm. With the example of  FIG. 6 , the length in the inside/outside direction of the peripheral edge portion  92  is approximately 1.5 μm. 
     Also, although with the preferred embodiments described above, the insulating film  12  is formed on the hydrogen barrier film  11 , the insulating film  12  does not have to be formed on the hydrogen barrier film  11 . 
     Also, although with the preferred embodiments described above, cases where the present invention is applied to an inkjet printing head was described, the present invention can also be applied to a microphone, pressure sensor, acceleration sensor, angular velocity sensor, ultrasonic sensor, speaker, or IR sensor (heat sensor), etc., that uses a piezoelectric film. 
     While preferred embodiments of the present invention were described in detail above, these are merely specific examples used to clarify the technical contents of the present invention and the present invention should not be interpreted as being limited to these specific examples and the scope of the present invention is limited only by the appended claims. 
     The present application corresponds to Japanese Patent Application No. 2019-083070 filed on Apr. 24, 2019 in the Japan Patent Office, and the entire disclosure of this application is incorporated herein by reference. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  1 A Inkjet printing head 
               2  Silicon substrate 
               3  Nozzle substrate 
               31  Ink discharge passage 
               31   a  Discharge port 
               4  Ink supply passage 
               5  Pressure chamber (cavity) 
               6  Piezoelectric element 
               7  Lower electrode 
               71  Rectangular portion 
               71 A Main electrode portion 
               72  Lead-out electrode portion 
               73  Common connection portion 
               8  Piezoelectric film 
               9  Upper electrode 
               9 A IrO 2  film 
               9 B Ir film 
               91  Central portion 
               92  Peripheral edge portion 
               92   a  Tapered upper surface 
               92   b  Outer edge portion 
               10  Movable film formation layer 
               10 A Movable film 
               11  Hydrogen barrier film 
               12  Insulating film 
               13  Wiring 
               13 A Lead-out portion 
               13 B Pad portion 
               14  Penetrating hole 
               15  Cutout portion 
               20  Drive circuit 
               21  Ink flow direction 
               22  Common ink passage 
               107  Lower electrode film 
               108  Piezoelectric material film 
               109  Upper electrode film 
               111 ,  112  Resist mask