Patent Publication Number: US-7591542-B2

Title: Piezoelectric actuator, method for producing the same and ink-jet head

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing piezoelectric actuator, a piezoelectric actuator, and an ink-jet head which includes the piezoelectric actuator. 
     2. Description of the Related Art 
     A piezoelectric actuator which drives an object by deforming a piezoelectric layer by an action of an electric field on the piezoelectric layer has been hitherto known. An actuator disclosed in Japanese Patent Application Laid-open Publication No. 2003-154646 ( FIG. 3 ) which is used in an ink-jet recording apparatus is one of such type. The piezoelectric actuator according to this patent document includes a piezoelectric layer which is formed by lead zirconate titanate (PZT), a plurality of lower electrodes which are provided on an under surface of the piezoelectric layer to correspond to a plurality of pressure chambers, respectively of an ink-jet head, and a plurality of upper electrodes which are provided on a top surface of the piezoelectric layer to correspond to a plurality of pressure chambers respectively. When drive voltage is supplied to the piezoelectric actuator, there is a difference in electric potential of the lower electrode and that of the upper electrode. Due to the difference in the electric potentials, the electric field acts on the piezoelectric layer which is sandwiched between the two electrodes, thereby deforming the piezoelectric layer, and pressure is applied to ink in the pressure chambers via the lower electrodes which also serves as a vibration plate. 
     This piezoelectric actuator is manufactured as described below. To start with, the upper electrode is formed on an MgO (magnesium oxide) single crystal substrate by sputtering method. Then, a layer of PZT is formed on a surface of the upper electrode by the sputtering method and the piezoelectric layer is formed by performing a heat treatment on the PZT layer. Furthermore, the lower electrode is formed on the surface of the piezoelectric layer by using the sputter method. 
     However, in the piezoelectric actuator disclosed in this patent document, there is a substantial difference in a coefficient of thermal expansion between a conductive material of the electrode and PZT of the piezoelectric layer which is in contact with the electrode. As a result, there is a substantial difference in amount of deformation between the PZT and the electrode during a process of forming the piezoelectric layer by means of a heat treatment of PZT. Therefore, interface stress between the piezoelectric layer and the electrode increases, so that the piezoelectric layer tends to exfoliate easily from the electrode. This results in deterioration of durability of the piezoelectric actuator and there is a decline in the yield at the manufacturing stage. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of manufacturing piezoelectric actuator and a piezoelectric actuator capable of improving an adhesion between the piezoelectric layer and the electrode. 
     According to a first aspect of the present invention, there is provided a method of manufacturing a piezoelectric actuator comprising:
         a first step of providing, on an insulating layer, a first electrode which has a recess formed on a surface of the first electrode;   a second step of forming a piezoelectric layer on the surface of the first electrode; and   a third step of forming a second electrode on a surface of the piezoelectric layer. In this method, the recess may be formed as a through hole which penetrates the first electrode. The through hole may include a plurality of holes. Moreover, in the first step, an electrode may be formed on the surface of the insulating layer and a plurality of through holes which penetrate the electrode may be formed.       

     According to the method of manufacturing piezoelectric actuator, since a part of the piezoelectric layer enters into the recess or the through hole and the piezoelectric layer is formed with recesses and projections in an interface between the first electrode and the piezoelectric layer, the piezoelectric layer is constrained by a plurality of recesses or through holes. Therefore, the piezoelectric layer hardly exfoliates from the surface of the first electrode, thereby improving the adhesion between the piezoelectric layer and the first electrode. Consequently, there is an improvement in durability of the piezoelectric actuator and a better yield during the manufacturing stage. 
     Moreover, when the piezoelectric layer is formed of a material which requires heat treatment such as lead zirconate titanate (PZT) or the like, there is a difference in an amount of deformation during the heat treatment due to a difference in coefficient of thermal expansion between the material forming the piezoelectric layer and a material forming the first electrode, which results in generating an interface stress between the first electrode and the piezoelectric layer. When the interface stress is high, the piezoelectric layer tends to easily exfoliate from the first electrode. However, when the recess is a through hole, a contact surface between the first electrode and the piezoelectric layer is fragmented in places by the through hole. This lowers the interface stress between the first electrode and the piezoelectric layer, and even in a case where the heat treatment is performed on such piezoelectric layer, there is an improvement in the adhesion between the piezoelectric layer and the first electrode. 
     In the method of manufacturing according to the present invention, the insulating layer may be formed of a material which has a coefficient of thermal expansion closer to a coefficient of thermal expansion of the piezoelectric layer than to a coefficient of thermal expansion of the first electrode. Even when the piezoelectric layer is formed of a material which requires heat treatment such as the lead zirconate titanate (PZT), the coefficient of thermal expansion of the insulating layer is closer to that of the piezoelectric layer than to that of the first electrode. Accordingly, when the heat treatment is performed on the piezoelectric layer, the difference in the amount of deformation between the piezoelectric layer and the insulating layer is smaller than the difference in the amount of deformation between the piezoelectric layer and the first electrode. Therefore, when the recess is a through hole, the piezoelectric layer is adhered easily to the insulating layer and by adhering of the piezoelectric layer partially to the insulating layer via the plurality of through holes formed on the first electrode, the adhesion of the piezoelectric layer and the first electrode is further improved. 
     In the method of manufacturing according to the present invention, a diameter of the through hole may be greater than a thickness of the first electrode. Therefore, the piezoelectric layer is adhered easily to the insulating layer through the through hole and the adhesion of the piezoelectric layer and the first electrode is improved. 
     In the method of manufacturing according to the present invention, the diameter of the through hole may be smaller than the thickness of the piezoelectric layer. Accordingly, it is possible to minimize the decline in deformation efficiency of the piezoelectric layer which would be otherwise caused by a disturbance in an electric field applied to the piezoelectric layer due to the presence of through holes. 
     In the method of manufacturing according to the present invention, when the through hole includes a plurality of holes and the holes are respectively formed in a central portion and an outer peripheral portion of the first electrode, a density of allocation of the holes in the outer peripheral portion of the first electrode may be greater than a density of allocation of the holes in the central portion of the first electrode. At the interface between the piezoelectric layer and the first electrode, as the position is nearer to the outer peripheral portion of the first electrode, the interface stress becomes greater, and thus the piezoelectric layer tends to easily exfoliate from the outer peripheral portion. However, by making the density of allocation of the holes in the outer peripheral portion greater than that in the central portion, the adhesion in the outer peripheral portion can be improved. Because of this, the piezoelectric level hardly exfoliates from the first electrode. 
     In the method of manufacturing according to the present invention, in the first step, a wire which extends from the first electrode and which applies a drive voltage to the first electrode may be formed. A plurality of through holes may be formed in this wire. Thus, by forming the plurality of through holes also in the wire which extends from the first electrode, it is possible to improve the adhesion between the first electrode, the wire, and the piezoelectric layer. 
     In the method of manufacturing according to the present invention, in the second step, the piezoelectric layer may be formed by any one of aerosol deposition method, sputtering method, CVD (Chemical Vapor Deposition) method, and sol-gel method. When the piezoelectric layer is formed by these methods, particles of the material which forms the piezoelectric layer can enter easily into the through holes. Therefore, the piezoelectric layer can be adhered partially to the insulating layer via the through holes and the adhesion of the piezoelectric layer and the first electrode is thus improved. 
     The method of manufacturing according to the present invention may further comprise forming the insulating layer on a metallic vibration plate formed of metal. This enables to form the vibration plate of a metallic material which has a high elasticity and to improve the response of the piezoelectric actuator. Furthermore, the insulating layer enables to electrically insulate the metallic vibration plate and the first electrode. 
     According to a second aspect of the present invention, there is provided a piezoelectric actuator comprising:
         an insulating layer;   a first electrode which has a recess formed on a surface of the first electrode and which is provided on the insulating layer;   a piezoelectric layer which is formed on the surface of the first electrode; and   a second electrode which is formed on a surface of the piezoelectric layer. The recess may be formed as a through hole which penetrates the first electrode. The through hole may include a plurality of holes. In this piezoelectric actuator, the piezoelectric layer enters into the recess such as the through hole, the piezoelectric layer with recesses and projections is formed in the interface between the first electrode and the piezoelectric layer, and the piezoelectric layer is constrained by a recess such as the through hole. Therefore, the piezoelectric layer hardly exfoliates from the surface of the first electrode, thereby improving the adhesion between the piezoelectric layer and the first electrode. Consequently, the durability of the piezoelectric actuator can be improved and a better yield during the manufacturing stage can be realized.       

     In the piezoelectric actuator according to the present invention, when the insulating layer is formed of a material which has a coefficient of thermal expansion closer to a coefficient of thermal expansion of the piezoelectric layer than to a coefficient of thermal expansion of the first electrode and when the recess is a through hole, the piezoelectric layer may be tightly and partially adhered to the insulating layer via one or the plurality of through holes. Even when the piezoelectric layer is formed of the material which requires heat treatment such as the lead zirconate titanate (PZT), the coefficient of thermal expansion of the insulating layer is closer to that of the piezoelectric layer than to that of the first electrode. Accordingly, when the heat treatment is performed on the piezoelectric layer, the difference in the amount of deformation between the piezoelectric layer and the insulating layer is smaller than the difference in the amount of deformation between the piezoelectric layer and the first electrode. Therefore, the piezoelectric layer is adhered easily to the insulating layer. In addition, by adhering the piezoelectric layer partially to the insulating layer via one or the plurality of through holes which are formed on the first electrode, the adhesion of the piezoelectric layer and the first electrode is improved. 
     According to a third aspect of the present invention, there is provided an ink-jet head comprising:
         nozzles which discharge ink;   a channel unit which has a plurality of pressure chambers communicating with the nozzles, respectively; and   a piezoelectric actuator which selectively changes a volume of the plurality of pressure chambers,   wherein the piezoelectric actuator includes:   an insulating layer;   a plurality of individual electrodes provided on the insulating layer corresponding to the pressure chambers respectively, each of the individual electrodes being formed with a recess provided on a surface thereof;   a piezoelectric layer provided on surfaces of the plurality of individual electrodes; and   a common electrode provided on the piezoelectric layer. The recess may be a through hole which penetrates each of the individual electrodes. The through hole may include a plurality of holes.       

     In the ink-jet head according to the present invention, when a drive voltage is supplied selectively to the plurality of individual electrodes of the piezoelectric actuator, an electric field is generated in the piezoelectric layer between the individual electrode and the common electrode to deform the piezoelectric layer. Due to the deformation of the piezoelectric layer, the pressure is applied to the ink in the pressure chamber and the ink is discharged from the nozzle. In this case, in the piezoelectric actuator, since one or a plurality of recesses such as a through hole is provided on each of the individual electrodes, the piezoelectric layer hardly exfoliates from the individual electrode and the adhesion between the individual electrode and the piezoelectric layer is improved. 
     In the ink-jet head according to the present invention, the insulating layer may be formed of a material which has a coefficient of thermal expansion closer to a coefficient of thermal expansion of the piezoelectric layer than to a coefficient of thermal expansion of the individual electrodes, and when the recess is a through hole, the piezoelectric layer may be tightly and partially adhered to the insulating layer via the through hole. Therefore, since the piezoelectric layer is tightly adhered to the insulating layer via the through holes, the adhesion of the piezoelectric layer and the individual electrode is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an ink-jet head according to an embodiment of the present invention; 
         FIG. 2  is a plan view of a right half of the ink-jet head as shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along a line III-III shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along a line IV-IV shown in  FIG. 2 ; 
         FIG. 5  is a partly enlarged view of  FIG. 2 ; 
         FIG. 6  is a diagram showing a step of joining a channel unit and a vibration plate; 
         FIG. 7  is a diagram showing a step of forming an insulating layer; 
         FIG. 8  is a diagram showing a step of forming an individual electrode; 
         FIG. 9  is a diagram showing a step of forming a plurality of through holes; 
         FIG. 10  is a diagram showing a step of forming a piezoelectric layer; 
         FIG. 11  is a diagram showing a step of forming a common electrode; 
         FIG. 12  is a diagram showing a step of joining a nozzle plate; 
         FIG. 13  is a diagram showing a modified embodiment of an actuator according to the present invention shown in  FIG. 5 ; 
         FIG. 14  is a diagram showing a modified embodiment of the actuator according to the present invention shown in  FIG. 5 ; 
         FIG. 15  is a diagram showing a modified embodiment of the ink-jet head according to the present invention shown in  FIG. 3 ; 
         FIGS. 16A to 16D  are conceptual diagrams showing other patterns of the through hole of the individual electrode; 
         FIG. 17A  is an enlarged cross-sectional view of the actuator shown in  FIG. 3 ; and 
         FIG. 17B  is an enlarged cross-sectional view showing a modification  5  according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described below. The present embodiment is an example in which the present invention is applied to a piezoelectric actuator which is used in an ink-jet head. 
     As shown in  FIG. 1 , an ink-jet head  1  includes a channel unit  2  which has an ink channel formed therein, and a piezoelectric actuator  3  which is laminated on an upper surface of this channel unit  2 . 
     To start with, the channel unit  2  is described below.  FIG. 2  is a schematic plan view of a right half of the ink-jet head  1  shown in  FIG. 1 .  FIG. 3  is a cross-sectional view taken along a line III-III shown in  FIG. 2  and  FIG. 4  is a cross-sectional view taken along a line IV-IV shown in  FIG. 2 . As shown in  FIG. 2  to  FIG. 4 , the channel unit  2  includes a cavity plate  10 , a base plate  11 , a manifold plate  12 , and a nozzle plate  13 , and these four plates  10  to  13  are joined in stacked layers. Among these, the cavity plate  10 , the base plate  11 , and the manifold plate  12  are stainless steel plates having a substantially rectangular shape. Therefore, ink channels such as a manifold  17  and a pressure chamber  14  which will be described later, can be formed easily by etching or the like on these three plates  10  to  12 . Moreover, the nozzle plate  13  is formed of a high molecular synthetic resin material such as polyimide and is joined on a lower surface of the manifold plate  12 . Or, the nozzle plate  13  may be formed of a metallic material such as stainless steel, similar to the three plates  10  to  12 . 
     As shown in  FIG. 2 , in the cavity plate  10  a plurality of pressure chambers  14  arranged along a plane surface is formed. These pressure chambers  14  are open in a surface (an upper surface of the cavity plate  10  to which a vibration plate  30  which will be described later is joined).  FIG. 2  shows a part (eight) of the plurality of pressure chambers  14 . Each pressure chamber  14  is formed substantially elliptical in a plan view and is arranged such that the long axis direction of the ellipse is parallel to the longitudinal direction of the cavity plate  10 . 
     Communication holes  15  and  16  are formed respectively in the base plate  11  at positions which overlap in a plan view with one end of the associated chamber  14  in the long axis direction thereof. In addition, a manifold  17  which overlaps in a plan view with the right halves of the pressure chambers  14  in  FIG. 2 , is formed in the manifold plate  12  such that portions of the manifold  17  extend in two rows in the direction of width (vertical direction in  FIG. 1 ) of the manifold plate  12 . Ink is supplied to the manifold  17  from an ink tank (not shown) via an ink-supply port  18  which is formed in the cavity plate  10 . Moreover, a communication hole  19  is formed at a position which overlaps in a plan view with a left-end portion of the pressure chamber  14  in  FIG. 2 . Furthermore, a plurality of nozzles  20  is formed in the nozzle plate  13  at positions which overlap in a plan view with the left-end portion of the plurality of pressure chambers  14  respectively. The nozzle  20  is formed, for example, by means of excimer laser process on a substrate of a high-molecular synthetic resin such as polyimide. 
     As shown in  FIG. 3 , the manifold  17  communicates with the pressure chamber  14  via the communication hole  15 , and the pressure chambers  14  communicates with the nozzle  20  via the communication holes  16  and  19 . Thus, an individual ink channel from the manifold  17  leading to the nozzle  20  via the pressure chamber  14  is formed in the channel unit  2 . 
     Next, the piezoelectric actuator  3  will be described below. As shown in  FIG. 1  to  FIG. 5 , the piezoelectric actuator  3  includes the vibration plate  30 , an insulating layer  31 , a plurality of individual electrodes  32  (first electrode), a piezoelectric layer  33 , and a common electrode  34  (second electrode). The vibration plate  30  is arranged on a surface of the channel unit  2 . The insulating layer  31  is formed on a surface of the vibration pate  30 . The plurality of individual electrodes  32  is formed on a surface of the insulating layer  31  corresponding to the plurality of pressure chambers  14  respectively. The piezoelectric layer  33  is formed on surfaces of plurality of individual electrodes  32  so as to spread over the individual electrodes  32 . The common electrode  34  is formed on a surface of the piezoelectric layer  33  and is provided in common for the plurality of individual electrodes  32 . 
     The vibration plate  30  is a stainless steel plate having substantially rectangular shape in a plan view and is joined in a laminated state to the upper surface of the cavity plate  10  such that openings of the plurality of pressure chambers  14  are closed. Since the vibration plate  30  is formed of stainless steel which has comparatively high coefficient of elasticity, as will be described later on, when the piezoelectric layer  33  is deformed during discharge operation of the ink, the high rigidity of the vibration plate  30  makes the piezoelectric actuator  3  highly responsive. Moreover, the vibration plate  30  is joined to the surface of the cavity plate  10  which is also formed of the stainless steel material. Therefore, the coefficient of thermal expansion of the vibration plate  30  and that of the cavity plate  10  become equal, thereby improving the joining strength between the vibration plate  30  and the cavity plate  10 . Furthermore, the ink in the channel unit  2  comes in contact with the channel unit  2  and the vibration plate  30  which is formed of stainless steel. Since the stainless steel is highly resistant to the corrosion by the ink, there is no likelihood that any local cells are formed in the channel unit  2  or in the vibration plate  30  regardless of the selection of ink type. Thus, since the selection of the ink is not restricted due to the corrosion related aspect, there is an increased degree of freedom of ink selection. 
     The insulating layer  31 , which is formed of a ceramic material having a high coefficient of elasticity such as alumina, zirconia, or silicon nitride, and which surface is flat, is formed on the surface of the vibration plate  30 . Because the insulating layer  31  is formed of the ceramic material which has a high coefficient of elasticity, the rigidity and the response of the actuator are improved. 
     Furthermore, the plurality of individual electrodes  32  which are elliptic, flat, and smaller in size than the pressure chamber  14  to a certain extent is formed on the surface of the insulating layer  31 . Each of the individual electrodes  32  is formed at a position which overlaps in a plan view with a central portion of the corresponding pressure chamber  14 . The individual electrode  32  is made of a conductive material such as gold. The adjacent individual electrode  32  is insulated electrically from each other by the insulating layer  31 . Moreover, as shown in  FIG. 3  to  FIG. 5 , a plurality of through holes  32   a  is formed in each of the individual electrodes  32 . These through holes  32   a  are provided in the surface of the individual electrode  32  to improve the joining between the individual electrodes  32  and the piezoelectric layer  33  as will be described later on. The details of these through holes will be described later. 
     On the surface of the insulating layer  31 , a plurality of wires  35  extends from one end portion (right-end portion in  FIG. 2 ) of each of the plurality of individual electrodes  32  in parallel to a longitudinal direction of the individual electrode  32 . A terminal  36  is formed at an end portion of each of the plurality of wires  35 . Output terminals  37   a  of a driver IC  37 , which selectively supplies drive voltage to the individual electrodes  32 , are joined, via a bump (not shown) composed of a conductive brazing filler material such as solder, to the terminals  36  corresponding to the plurality of individual electrodes  32  respectively. The driver IC  37  is disposed on the surface of the insulating layer  31 . 
     Furthermore, a plurality of connecting terminals  40  is formed on the insulating layer  31 . These connecting terminals  40  and input terminals  37   b  of the driver IC  37  are joined respectively via a bump  39  made of solder or the like. The driver IC  37  and a control device (which is omitted in the drawing) which controls the driver IC  37  are connected via the connecting terminals  40 . 
     The piezoelectric layer  33  which is principally composed of lead zirconate titanate (PZT), which is a solid solution of lead titanate and lead zirconate, and which is a ferroelectric substance, is formed on the surface of the plurality of individual electrodes  32 . This piezoelectric layer  33  is formed as one continuous layer spreading over the plurality of individual electrodes  32  so as to cover the entire surfaces of the plurality of individual electrodes  32 . 
     As described above, the plurality of through holes  32   a  is formed in each of the individual electrodes  32 . As shown in  FIG. 3 ,  FIG. 4 , and  FIG. 17A , and particularly as seen in an enlarged cross-sectional view in  FIG. 17A , a part of the piezoelectric layer  33  has entered into the through hole  32   a , and the under surface of the piezoelectric layer  33  is formed with recesses and projections. Therefore, the piezoelectric layer  33  is constrained by the through holes  32   a , and the adhesion between the individual electrode  32  and the piezoelectric layer  33  is improved. As shown in  FIG. 17A , although diameter d of the through hole  32   a  is not restricted to any particular diameter, the diameter d may be greater than thickness t 1  of the individual electrode  32  and may be smaller than thickness t 2  of the piezoelectric layer  33 . Moreover, as it is described later, the piezoelectric layer  33  formed on the surface of the individual electrode  32  is subjected to the heat treatment. In this case, while the coefficient of thermal expansion for PZT which forms the piezoelectric layer  33  is about 3×10 −6 , the coefficient of thermal expansion of the conductive material such as gold, which forms the individual electrode  32 , is about 14×10 −6 . Therefore, after performing the heat treatment on the piezoelectric layer  33 , the interface stress is developed between the individual electrode  32  and the piezoelectric layer  33  due to the difference in the amount of thermal deformation between the individual electrode  32  and PZT which forms the piezoelectric layer  33 . When this interface stress is high, the piezoelectric layer  33  tends to exfoliate easily from the individual electrode  32 . However, since a contact surface between the individual electrode  32  and the piezoelectric layer  33  is fragmented in places by the plurality of through holes  32   a , the interface stress between the individual electrode  32  and the piezoelectric layer  33  is low. Therefore, the piezoelectric layer  33  hardly exfoliates from the individual electrode  32 . Furthermore, the coefficient of thermal expansion of the insulating layer  31  under the individual electrode  32  such as alumina, for example, is about 7×10 −6  which is closer to the coefficient of thermal expansion of the piezoelectric layer  33  than to that of the individual electrode  32 . Therefore, the piezoelectric layer  33  tends to easily adhere to the insulating layer  31  via the plurality of through holes  32   a . Thus, by adhering the piezoelectric layer  33  partially to the insulating layer  31  via the plurality of through holes  32   a , it is even more unlikely for the piezoelectric layer  33  to exfoliate from the individual electrode  32 . 
     The common electrode  34  which is common to the plurality of individual electrodes  32  is formed on the surface of the piezoelectric layer  33  so as to spread over the entire surface of the piezoelectric layer  33 . As shown in  FIG. 2 , a wire  41  extends from the common electrode  34 . The wire  41  is formed to extend over the surface of the insulating layer  31  and the surface of the piezoelectric layer  33 . Moreover, a terminal  42  which is provided at an end of the wire  41  is connected to a terminal (not shown) of the driver IC  37 . Accordingly, the common electrode  34  is grounded via the wire  41  and the driver IC  37 , and is kept at a ground potential. This common electrode  34  is also made of a conductive material such as gold. 
     Next, an action of the piezoelectric actuator  3  during the ink discharge is described below. 
     When a drive voltage is selectively supplied from the driver IC  37  to the plurality of individual electrodes  32  respectively connected to the driver IC  37  via the plurality of wires  35 , the electric potential of the common electrode  34  on the piezoelectric layer  33  held at the ground potential differs from that of the individual electrode  32  under the piezoelectric layer  33  to which the drive voltage is supplied, and thus an electric field is generated in a vertical direction of the piezoelectric layer  33  sandwiched between the two electrodes  32  and  34 . Apart of the piezoelectric layer  33  which is disposed directly above the individual electrode  32 , to which the drive voltage is applied, contracts in a horizontal direction perpendicular to the vertical direction in which the piezoelectric layer  33  is polarized. In this case, since the vibration plate  30  and the insulating layer  31  under the piezoelectric layer  33  are fixed to the cavity plate  10 , the part of the piezoelectric layer  33  sandwiched between the two electrodes  32  and  34  is deformed due to the horizontal deformation so as to project toward the associated pressure chamber  14 . The partial deformation of the piezoelectric layer  33  also causes a part of the vibration plate  30  which covers the associated pressure chamber  14  to deform so as to project toward the pressure chamber  14 . This leads to a decrease in volume inside the pressure chamber  14 , thereby increasing the pressure of the ink, and the ink is discharged from the nozzle  20  communicating with the pressure chamber  14 . 
     Next, a method of manufacturing piezoelectric actuator  3  will be described below. 
     To start with, as shown in  FIG. 6 , the three stainless steel plates  10  to  12  are joined to the vibration plate  30  made of stainless steel by diffusion joining or the like. The communication holes  16  and  19  are formed in advance on these plates. In this case, since the cavity plate  10  and the vibration plate  30  are both made of the stainless steel material, their coefficient of thermal expansion is the same and the residual stress at a joining surface is low. Therefore, the joining strength in these plates is extremely satisfactory. 
     Next, as shown in  FIG. 7 , the insulating layer  31  is formed of a ceramic material such as alumina, zirconia, or silicon nitride on the surface of the vibration plate  30 . As a method for forming the insulating layer  31 , aerosol deposition method may be used in which ultra fine particles are deposited by colliding the particles onto the vibration plate at a high speed, so that a very thin layer can be formed. The insulating layer  31  may also be formed by a method such as sol-gel method, sputtering method, or CVD (Chemical Vapor Deposition) method. 
     Next, the individual electrodes  32  are formed on the surface of the insulating layer  31 , and the plurality of through holes  32   a  is formed in each of the individual electrodes  32  (first step). As shown in  FIG. 8 , after forming a conductive layer  140  on the entire surface of the insulating layer  31  by a method such as plating method, sputtering method, or vapor deposition method, the conductive layer  140  is partially removed as shown in  FIG. 9  by a method such as laser method, mask method, and resist method, thereby forming the individual electrodes  32  and the plurality of through holes  32   a . Alternatively, the individual electrodes  32  and the plurality of through holes  32   a  may be formed at one time by screen-printing a conductive paste on the surface of the insulating layer  31 . Accordingly, a first step, in the method according to the present invention, of providing, on the insulating layer, a first electrode in which the holes (recesses) are formed on the surface thereof can be carried out. 
     Next, as shown in  FIG. 10 , the piezoelectric layer  33  which is made of PZT is formed on the surfaces of the individual electrodes  32  by using a method such as aerosol deposition method, sol-gel method, sputtering method, or CVD method (second step). By forming the piezoelectric layer  33  by any of these methods, particles of PZT forming the piezoelectric layer  33  tend to enter easily into the plurality of through holes  32   a  formed in the individual electrode  32  and thus the piezoelectric layer  33  can easily adhere partially to the insulating layer  31  via the plurality of through holes  32   a . Then, the heat treatment at about 600° C. is performed on the piezoelectric layer  33  to refine the structure of the piezoelectric layer  33 . 
     Further, as shown in  FIG. 11 , the common electrode  34  is formed on the entire surface of the piezoelectric layer  33  by using a method such as screen printing, vapor deposition method, or sputtering method (third step). Finally, as shown in  FIG. 12 , the nozzle plate  13  having the nozzles  20  formed therein in advance is joined on an under surface of the manifold plate  12 . 
     It is desirable that the hole-diameter d of the plurality of through holes  32   a  is greater than the thickness t 1  of the individual electrode  32  as shown in  FIG. 17  so that the piezoelectric layer  33  easily adheres to the insulating layer  31  via the plurality of through holes  32 . When the diameter of the through hole  32   a  is greater than the thickness of the individual electrode  32 , the part of the piezoelectric layer  33  which is formed in one of the through holes  32   a  has a sufficient diameter with respect to the depth thereof. Therefore, there is little possibility that the part of the piezoelectric layer  33  disposed in and near to one of the through holes  32   a  is ruptured. On the other hand, when the diameter of the through holes  32   a  is too big, there is a disturbance in an electric field applied to the piezoelectric layer  33  between the individual electrode  32  and the common electrode  34 . This leads to a possibility of decline in efficiency of deformation of the piezoelectric layer  33 . Therefore, it is desirable that the diameter d of the through holes  32   a  is smaller than the thickness t 2  of the piezoelectric layer  33  as shown in  FIG. 17A . For example, when the thickness of the individual electrode  32  is about 2 μm and the thickness of the piezoelectric layer  33  is about 10 μm, it is desirable that the diameter of the plurality of through holes  32   a  is about 2 μm to 10 μm. The planar shape of the through hole is not restricted to a circular shape shown in  FIG. 5  and may be any shape such as rectangular, elliptical, and slot shape. 
     Moreover, when the nozzle plate  13  is made of a metallic material such as stainless steel, after forming the channel unit  2  by joining in advance the nozzle plate  13  with the other three plates simultaneously, the vibration plate  30 , the insulating layer  31 , the individual electrode  32 , and the piezoelectric layer  33  may be laminated in sequence on the surface of the channel unit  2 , and the heat treatment may be performed for the piezoelectric layer  33 . 
     According to the ink-jet head  1 , the piezoelectric actuator  3 , and the method of manufacturing the piezoelectric actuator  3 , the following effects are achieved. 
     By forming the plurality of through holes  32   a  in the individual electrode  32 , a part of the piezoelectric layer  33  on the surface of the individual electrode  32  enters into the through holes  32   a  and that part is constrained by the through holes  32   a , thereby making it difficult for the piezoelectric layer  33  to exfoliate from the individual electrode  32 . Moreover, since the contact surface between the individual electrode  32  and the piezoelectric layer  33  is fragmented by the plurality of through holes  32   a , the interface stress between the individual electrode  32  and the piezoelectric layer  33  which is generated after the heat treatment of the piezoelectric layer  33  becomes low and the adhesion between the individual electrode  32  and the piezoelectric layer  33  is improved. Furthermore, since the insulating layer  31  under the individual electrode  32  has the coefficient of thermal expansion closer to that of the piezoelectric layer  33  than to that of the individual electrode  32 , and since the part of the piezoelectric layer  33  which has entered into the plurality of through holes  32   a  tends to be adhered easily to the insulating layer  31 , the adhesion between the individual electrode  32  and the piezoelectric layer  33  is improved further. 
     Next, modified embodiments in which various changes are made in the embodiment as described above are described below. Components or the like in the modified embodiments which have a structure same as those in the embodiment are assigned the same reference numerals and their description is omitted. 
     First Modified Embodiment 
     In general, as the position is nearer to the outer peripheral portion of the individual electrode  32 , the interface stress becomes greater between the individual electrode  32  and the piezoelectric layer  33 . Therefore, the piezoelectric layer  33  tends to easily exfoliate from the outer peripheral side of the individual electrode  32 . In order to deal with this, as shown in  FIG. 13 , the density of allocation of the through holes  32   a  in the outer peripheral portion is made to be greater to improve and the adhesion between the piezoelectric layer  33  and the individual electrode  32  in the outer peripheral portion. This enables to prevent more assuredly the piezoelectric layer  33  from exfoliating from the individual electrode  32 . 
     Second Modified Embodiment 
     As shown in  FIG. 14 , while forming the plurality of through holes  32   a  in the individual electrode  32 , a plurality of through holes  35   a  may be formed also in the wire  35  which extends from the individual electrode  32  in order to further improve the adhesion among the piezoelectric layer  33 , and the individual electrode  32  and the wire  35 . 
     Third Modified Embodiment 
     According to the embodiment of the method of manufacturing piezoelectric actuator, the insulating layer  31  is formed on the surface of the metallic vibration plate  30  and further the plurality of individual electrodes  32  is formed on the surface of the insulating layer  31 . However, as shown in  FIG. 15 , the plurality of individual electrodes  32  may be formed directly on a surface of a vibration plate  50  (corresponds to the insulating layer according to the present invention) which is formed of an insulating material, and the plurality of through holes  32   a  may further be formed on each individual electrode  32 . In a piezoelectric actuator  53  manufactured in this manner, the part of the piezoelectric layer  33  which is formed on the surface of the individual electrode  32  is adhered closely to the vibration plate  50  via the plurality of through holes  32   a . To make the piezoelectric actuator  53  highly responsive, it is desirable that the insulating material which forms the vibration plate  50  is a material having a high coefficient of elasticity such as a ceramic material like alumina, zirconia or a glass material like boro-silicated glass. 
     Fourth Modified Embodiment 
     According to the embodiments described above, the individual electrodes  32  are formed on the surface of the insulating layer  31  (under surface of the piezoelectric layer  33 ) and the common electrode  34  is formed on the surface of the piezoelectric layer  33 . However, the common electrode  34  may be formed on the surface of the insulating layer  31  and the individual electrodes  32  may be formed on the surface of the piezoelectric layer  33 . In this case, a plurality of through holes is to be formed on the common electrode  34  to improve the adhesion with the piezoelectric layer  33 . 
     Fifth Modified Embodiment 
     According to the embodiment described above, the plurality of through holes is formed on each of the individual electrodes  32 . However, only one through hole may be formed on each of the individual electrodes as shown conceptually in  FIG. 16A  to  FIG. 16D . In  FIG. 16A , a groove having a spiral shape is formed on an individual electrode  60 . Or, as shown in  FIG. 16C , the through hole may be a S-shaped groove  64   a  formed on an individual electrode  64 . The groove may be formed to have arbitrary shape. Considering a fact that the layer tends to exfoliate easily in the outer peripheral portion of the individual electrode than in the central portion, the through hole may be a U-shaped groove or a groove having a shape close to a ring which is formed only along an area near to the outer periphery of the individual electrode  62 , as shown in  FIG. 16B . Further, a plurality of grooves  66   a  and  66   b  may be provided on the individual electrode  66 , as shown in  FIG. 16D . 
     Sixth Modified Embodiment 
     According to the embodiments described above, the plurality of through holes is formed in each of the individual electrodes  32 . However, as shown in  FIG. 17B , recesses  132   a  may be provided on a surface of an individual electrode  132  on a side of the piezoelectric layer  33 . The individual electrode  132  which has the recesses  132   a  may be formed by a method such as plating method, sputtering method, and vapor deposition method. It is desirable that the diameter of the recess  132   a , similar to that of the through hole, is greater than the thickness of the individual electrode  132  and smaller than the thickness of the piezoelectric layer  33 . The depth of the recess  132   a  may be selected appropriately upon considering the exfoliation-resistant property of the piezoelectric layer  33  from the individual electrode  132 . For example, when the recesses  132   a  are provided on the central portion and the outer peripheral portion of the individual electrode  132 , the recess  132   a  on the outer peripheral portion may be deeper than the recess  132   a  on the central portion. In this case, a portion which is defined between the two recesses  132   a  formed on the individual electrode  132  can be viewed as a projection. In other words, when the individual electrode  132  is provided with projections which project or protrude from its surface, a recess is formed in a portion sandwiched between the two projections. Therefore, a case in which the projections are formed is considered to be also included in the present invention. The planar shape of the recess  132   a  (or projection) is not restricted to the circular shape and may be a rectangular shape or various shapes. Moreover, only one recess  132   a  may be formed on the individual electrode  132  as shown in the fifth modified embodiment.