Patent Publication Number: US-9427966-B2

Title: Inkjet head, method for manufacturing same, and inkjet printer

Description:
The present U.S. patent application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP2014/055302 filed on Mar. 3, 2014. This application claims a priority under the Paris Convention of Japanese patent application No. 2013-053097 filed on Mar. 15, 2013, the entirety of which is incorporated herein by reference. 
     This application claims a priority under the Paris Convention of Japanese patent application No. 2013-053097 filed on Mar. 15, 2013, the entirety of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an inkjet head that discharges ink to outside itself, a method for producing the same, and an inkjet printer including the inkjet head. 
     BACKGROUND ART 
     There have conventionally been known inkjet printers that include an inkjet head having a plurality of channels that discharge ink. Such inkjet printers are capable of outputting a two-dimensional image onto a recording medium such as a sheet of paper, cloth, etc. by controlling discharging of ink while moving the inkjet head relatively with respect to the recording medium. Discharging of ink can be performed by using an actuator (a piezoelectric actuator, an electrostatic actuator, a thermal actuator, or the like), or by generating air bubbles in ink in a tube by means of heat. In particular, piezoelectric actuators have recently been widely used for their advantages of large output, modifiability, high responsiveness, adaptability to any type of ink, etc. 
     Piezoelectric actuators are classified into two types: one using a bulk-state piezoelectric body and the other using a thin-film piezoelectric body (piezoelectric thin film). The former type has a large output and thus is capable of discharging ink droplets of a large size, but it is large-sized and thus is high in cost unfortunately. In contrast, the latter type has a small output and thus is not capable of forming ink droplets of a large size, but is compact and thus is low in cost. Consequently, it can be said that forming an actuator with a piezoelectric thin film is suitable to realize high-resolution printers (which can be achieved with small ink droplets) at low cost. 
     Reference is now made to  FIG. 8 , which presents a plan view schematically showing a configuration of a conventional actuator  100  using a piezoelectric thin film, and a sectional view taken along line A-A′ of the plan view and viewed in the direction indicated by the arrows. The actuator  100  is configured by stacking, on a substrate  101  having a pressure chamber  101   a , an insulation layer  102 , a lower electrode  103 , a piezoelectric film  104  as a piezoelectric thin film, and an upper electrode  105  in this order. An upper wall  101   b  of the pressure chamber  101   a  in the substrate  101  constitutes a driven film operable to be displaced according as the piezoelectric film  104  expands and contracts. 
     Specifically, when a voltage is applied from a drive circuit  106  to the lower electrode  103  and the upper electrode  105  and the piezoelectric film  104  is caused to expand and contract in a direction perpendicular to its thickness direction (a direction parallel to a face of the substrate  101 ), curvature is generated in the driven film due to difference in length between the piezoelectric film  104  and the driven film, the curvature causing the driven film to be displaced (curved) in its thickness direction. 
     A configuration of a channel  200  including the actuator  100  shown in  FIG. 8  is schematically shown in  FIG. 9 , which is a sectional view. As shown in the figure, an ink chamber is formed by closing a space (the pressure chamber  101   a ) in a lower portion of the actuator  100  with a nozzle plate  201 . With ink held in the pressure chamber  101   a , by making use of the above-described displacement of the driven film caused by the expansion and contraction of the piezoelectric film  104 , it is possible to apply pressure to the ink held in the pressure chamber  101   a  to thereby discharge the ink as ink droplets through a nozzle hole  201   a  to outside the pressure chamber  101   a . An inkjet head is formed by arranging a plurality of such piezoelectric actuators  100  (channels  200 ) in a lateral direction. 
     Piezoelectric bodies widely used in such piezoelectric actuators as described above are perovskite metal oxides such as BaTiO 3  and Pb(Ti/Zr)O 3  which is called PZT. As for actuators using, a piezoelectric thin film, the piezoelectric thin film is produced by forming on a substrate a film of PZT, for example. The PZT film can be formed by means of various methods, such as a sputtering method, a CVD (chemical vapor deposition) method, a sol-gel method, and the like. Incidentally, since it requires a high temperature to crystalize piezoelectric materials, Si substrates are often used as the substrate. 
     Performance indices of an inkjet head include droplet amount, injection speed, drive frequency, etc., and output and responsiveness of each actuator serve as factors that determine these indices. The output of an actuator depends on the applied voltage, the piezoelectric constant, and the volume of the piezoelectric body, while the responsiveness of an actuator depends on the weight, the stiffness, etc. of the actuator. 
     The drive frequency of a head is also affected by weight and elasticity of ink. Specifically, with a large-capacity pressure chamber (ink chamber), which holds ink of a large weight, the ink as a whole becomes more elastically deformed, as a result of which the responsiveness of the actuator is degraded. Accordingly, to improve the responsiveness of the actuator so as to improve (increase) the drive frequency of the head, it is necessary to reduce the capacity of the ink chamber. 
     Methods for reducing capacity of an ink chamber include the following two methods. One is to polish a substrate, on which a piezoelectric body is supported, to reduce the height of an ink chamber formed in the substrate. The other is to transfer onto a thin substrate, in which a small-capacity ink chamber is formed in advance, a piezoelectric film formed on another substrate, thereafter removing the another substrate. Although adopted for different purposes, polishing a substrate as in the former method is disclosed in Patent Literature 1, for example, and transferring a piezoelectric film as in the latter method is disclosed in Patent Literature 2, for example. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 5013025 (claim 1, paragraph [0012], FIG. 1, etc.) 
     Patent Literature 2: Japanese Patent Application Publication No. 2005-169965 (claim 1, paragraph [0019], FIGS. 3(a) and (b)) 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the case of polishing a substrate to reduce the capacity of an ink chamber, there is a concern that the substrate may crack or break during the process (polishing), or may warp while a film is being formed or during the process, lowering the yield and degrading the performance as an actuator. On the other hand, the case of transferring a piezoelectric film onto a thin substrate suffers damage of the piezoelectric film occurring at the time of film transfer, degradation of performance due to the damage, and increase in cost resulting from the use of two substrates. Thus, it is desirable that an inkjet head be so configured as to allow the capacity of an ink chamber to be reduced without substrate polishing or film transfer. 
     The present invention has been made to solve the above problems, and its object is to provide an inkjet head capable of reducing the capacity of an ink chamber without performing substrate polishing or film transfer, to thereby improve the drive frequency of the head, a method for producing such an inkjet head, and an inkjet printer including such an inkjet head. 
     Solution to Problem 
     To achieve the above object, according to one aspect of the present invention, an inkjet head includes a displacement film that includes a driving film operable to expand and contract in a direction perpendicular to a thickness direction of the driving film, the displacement film being operable to undergo curving deformation in a thickness direction of the displacement film, a substrate that includes a hole portion formed therein in a thickness direction thereof and that supports the displacement film such that the displacement, film covers the hole portion so as to allow expansion and contraction of the driving film to cause the curving deformation of the displacement film in the thickness direction of the displacement film in an area of the displacement film corresponding to the hole portion, and an ink discharge portion that includes a ink chamber holding ink therein and that discharges the ink to outside the ink discharge portion by having pressure resulting from the curving deformation of the displacement film applied to the ink. Here, the ink discharge portion is disposed on a side opposite to the hole portion of the substrate with respect to the displacement film. 
     According to another aspect of the present invention, a method for producing an inkjet head includes the steps of forming a driving film at a substrate, forming a hole portion in the substrate on a side opposite to a side where the driving film is formed and supporting a displacement film including the driving film such that the displacement film covers the hole portion so as to allow expansion and contraction of the driving film in a direction perpendicular to a thickness direction of the driving film to cause curving deformation of the displacement film in a thickness direction of the displacement film in an area of the displacement film corresponding to the hole portion, and forming an ink discharge portion, through which ink held in the ink chamber is discharged to outside the ink chamber by the curving deformation of the displacement film, on a side opposite to the hole portion of the substrate with respect to the displacement film. 
     Advantageous Effects of Invention 
     An ink discharge portion is provided on a side opposite to a substrate (a hole portion) with respect to a displacement film, independently of the substrate, and this makes it possible to achieve a design for reducing the capacity of an ink chamber by working on the design of the ink discharge portion independently and regardless of the substrate. This helps improve the drive frequency of a head by reducing the capacity of an ink chamber without performing substrate polishing or film transfer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing part of an inkjet printer according to one embodiment of the present invention in an enlarged manner; 
         FIG. 2  presents a plan view schematically showing a configuration of one channel of an inkjet head incorporated in the inkjet printer, and a sectional view taken along line A-A′ of the plan view and viewed in the direction indicated by the arrows; 
         FIG. 3  presents a plan view showing a configuration of a plurality of channels of the inkjet head, and a sectional view taken along line A-A′ of the plan view and viewed in the direction indicated by the arrows; 
         FIG. 4  is a sectional view showing another configuration of the channel; 
         FIG. 5  is a sectional view showing still another configuration of the channel; 
         FIG. 6  is a sectional view showing a production process of the inkjet head configured as shown in  FIG. 2 ; 
         FIG. 7  is a sectional view showing a production process of the inkjet head configured as shown in  FIG. 5 ; 
         FIG. 8  presents a plan view schematically showing a configuration of a conventional actuator using a piezoelectric thin film and a sectional view taken along line A-A′ of the plan view and viewed in the direction indicated by the arrows; and 
         FIG. 9  is a sectional view schematically showing a configuration of a channel including the conventional actuator. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Presented below is a description of an embodiment of the present invention with reference to the accompanying drawings. 
     [Configuration of Inkjet Printer] 
       FIG. 1  is a perspective view showing part of an inkjet printer according to the present embodiment in an enlarged manner. An inkjet printer  1  includes a carriage  1   b  movable in a right-left direction (direction B in the figure) and disposed inside a cabinet  1   a  part of which is open. On the carriage  1   b , a plurality of inkjet treads  10  are mounted in an array each corresponding to one of a plurality of colors (such as four colors of yellow, magenta, cyan, and black). The inkjet printer  1  is capable of forming a color image on a recording medium (unillustrated) by making the inkjet heads  10  discharge ink of each corresponding color while moving the carriage  1   b  in the right-left direction and conveying the recording medium frontward (in direction A in the figure) from a rear side. 
     The inkjet printer  1  may be configured such that the inkjet heads  10  are arranged all along a width direction of the recording medium, with a plurality of inkjet heads  10  for each color arranged in the recording medium conveyance direction. In this case, a color image can be formed on the recording medium while moving only the recording medium and keeping the inkjet heads  10  stationary. 
     [Configuration of Inkjet Head] 
     Next, a description will be given of a configuration of the inkjet head  10 .  FIG. 2  presents a plan view schematically showing a configuration of one channel of the inkjet head  10  together with a sectional view taken along line A-A′ of the plan view and viewed in the direction indicated by the arrows, for convenience&#39; sake, the nozzle substrate  23  is not illustrated in the plan view of  FIG. 2 . This way of illustration applies also to the other plan views which will be referred to later. 
     The inkjet head  10  includes a thermally oxidized film  12 , a lower electrode  13 , a piezoelectric thin film  14 , the upper electrode  15 , and an ink discharge portion  21  provided on a substrate  11  in this order. 
     The substrate  11  is composed of a semiconductor substrate made of a single crystal Si (silicon) alone with a thickness of, for example, 200 to 700 μm (preferably 300 μm or more, in view of its susceptibility to breakage during processing) or an SOI (silicon on insulator) substrate. Note that  FIG. 2  shows a case where the substrate  11  is composed of an SOI substrate. An SOI substrate is made of two Si substrates joined together via an oxidized film. 
     The substrate  11  includes a dug portion  11   a  as a hole or concave portion formed (dug) in its thickness direction, and a driven film  11   b  a part of which in its thickness direction constitutes an upper wall of the dug portion  11   a , the upper wall being located to a piezoelectric-thin-film- 14  side of the dug portion  11   a . The driven film  11   b  is composed of one of the two Si substrates constituting the SOI substrate, and is connected, at its peripheral portion, with a side wall  11   c  (the other Si substrate constituting the SOI substrate) of the dug portion  11   a  via an oxidized film. The driven film  11   b , the lower electrode  13 , and the thermally oxidized film  12  are operable to be deformed to curve in their thickness directions along with expansion and contraction of the piezoelectric thin film  14  in a direction perpendicular to its thickness direction (that is, a direction parallel to a face of the substrate  11 ). Along with such curving deformation of the driven film  11   b , the lower electrode  13 , and the thermally oxidized film  12 , the piezoelectric thin film  14  also curves in its thickness direction. Thus, it can be said that a displacement film  17  operable to be deformed to curve in its thickness direction is formed by including the piezoelectric thin film  14 , the lower electrode  13 , the thermally oxidized film  12 , and the driven film  11   b . The substrate  11  supports the displacement film  17  such that the displacement film  17  covers the dug portion  11   a  to allow the displacement film  17  to be operable to be deformed to curve in its thickness direction at an area thereof corresponding to the dug portion  11   a  (an area thereof located over the dug portion  11   a ). 
     The thermally oxidized film  12  is formed of SiO 2  (silicon oxide) having a thickness of about 0.1 μm, for example, for the sake of protection and insulation of the substrate  11 . 
     The lower electrode  13  is composed by stacking a Ti (titanium) layer and a Pt (platinum) layer. The Ti layer is formed to enhance adhesion between the thermally oxidized film  12  and the Pt layer. The Ti layer is about 0.02 μm thick, for example, and the Pt layer is about 0.1 μm thick, for example. The lower electrode  13  is connected to a circuit board  16 . 
     As has been described above, the piezoelectric thin film  14  is a driving film operable to expand and contract in a direction perpendicular to its thickness direction, and is composed of a thin film of PZT (lead zirconate titanate), which is a solid solution of PTO (PbTiO 3 ; lead titanate) and PZO (PbZrO 3 ; lead zirconate). The piezoelectric thin film  14  is 3 to 5 μm thick, for example. 
     The upper electrode  15  is composed by stacking a Ti layer and a Pt layer. The Ti layer is formed to enhance adhesion between the piezoelectric thin film  14  and the Pt layer. The Ti layer is about 0.02 μm thick, for example, and the Pt layer is about 0.1 to 0.2 μm thick, for example. The upper electrode  15  is formed to be smaller than the piezoelectric thin film  14  in size, and a part of the upper electrode  15  is drawn out along a top surface of the piezoelectric thin film  14  to outside the ink discharge portion  21  to be connected to the circuit board  16 . The lower electrode  13  and the upper electrode  15  are disposed so as to sandwich the piezoelectric thin film  14  in its thickness direction. 
     The ink discharge portion  21  discharges ink to outside itself by having pressure resulting from the curving deformation of the displacement film  17  applied to the ink. The ink discharge portion  21  is disposed on a side opposite to the substrate  11  (the dug portion  11   a ) with respect to the displacement film  17  (in particular, the piezoelectric thin film  14 ), and the ink discharge portion  21  includes a partition portion  22  and a nozzle substrate  23 . 
     The partition portion  22  is located more to the piezoelectric-thin-film- 14  side than the nozzle substrate  23  is, and forms a side wall of an ink chamber  21   a . That is, the ink chamber  21   a  is formed as a space located inward from the partition portion  22  and closer to the piezoelectric thin film  14  than the nozzle substrate  23  is (a space sandwiched by the nozzle substrate  23  and the piezoelectric thin film  14 ). In  FIG. 2 , an opening width B (mm) of the partition portion  22  is illustrated as wider than an opening width C (mm) of the dug portion  11   a  of the substrate  11 , but the opening width B and a height (thickness) of the partition portion  22  may be set to arbitrary valises. The nozzle substrate  23  includes a nozzle hole  23   a  through which to discharge ink held inside the ink chamber  21   a  to outside the ink chamber  21   a.    
     The partition portion  22  and the nozzle substrate  23  are in direct contact with the ink held in the ink chamber  21   a , and thus are preferably composed of materials that are highly ink-resistant and also easy to process. Usable as such materials are resin materials such as epoxy-based photosensitive materials, acrylic-based materials, and polyimide-based materials, for example. Besides these materials, metal materials such as iron, copper, nickel. SUS, and the like, glass, ceramic, etc. may be used to form the partition portion  22  and the nozzle substrate  23 . 
     In the above configuration, when a voltage is applied from the circuit board  16  to the lower electrode  13  and the upper electrode  15 , the piezoelectric thin film  14  expands and contracts in the direction perpendicular to its thickness direction. Then, curvature is generated in the driven film  11   b  due to the difference in length between the piezoelectric film  14  and the driven film  11   b , such that the driven film  11   b  is deformed to curve in its thickness direction, and this in turn causes the piezoelectric thin film  14  to be deformed to curve in its thickness direction. Such curving deformation of the displacement film  17  (including the piezoelectric thin film  14  and the driven film  11   b ) generates pressure to be applied to the ink held in the ink chamber  21   a , and thereby the ink is discharged through the nozzle hole  23   a  to outside the ink chamber  21   a.    
     The present embodiment is configured such that the ink discharge portion  21  is provided on a side opposite to the dug portion  11   a  of the substrate  11  with respect to the displacement film  17 , and thus is provided independent of the substrate  11 , and this configuration makes it possible to design the ink discharge portion  21  independently and regardless of the substrate  11 , and to reduce the capacity of the ink chamber  21   a  through such a design. 
     A substrate is necessary to form a piezoelectric thin film, and in conventional configurations, an ink chamber is formed in such a substrate. To achieve reduced capacity of an ink chamber in such a conventional configuration, it is inevitable to adopt methods such as polishing the substrate in which the ink chamber is formed or transferring a piezoelectric thin film onto the thin substrate in which the ink chamber is formed. However, with the present embodiment where no ink chamber is formed in the substrate  11 , it is possible to reduce the capacity of the ink chamber with ease through the independent design of the ink discharge portion  21 , without performing substrate polishing or film transfer. This makes it possible to improve the drive frequency of the head to thereby give the inkjet head  10  a high performance. Furthermore, in reducing the capacity of the ink chamber  21   a , there is no need of performing substrate polishing or film transfer, and thus, the present embodiment is free from such problems (reduction in yield, degradation of performance, damage to films, increase in cost) as have been experienced in cases where substrate polishing or film transfer is performed. 
     In particular, in configurations where the ink discharge portion  21  includes the nozzle substrate  23  and the partition portion  22  as in the present embodiment, the capacity of the ink chamber  21   a  depends on the opening width B and the thickness (height) of the partition portion  22 , because the upper electrode  15  is sufficiently thin. Accordingly, the capacity of the ink chamber  21   a  can be easily reduced by designing to reduce at least one of the opening width B and the height of the partition portion  22 . 
     For example, in the conventional configuration, the ink chamber is sized to have a diameter of 200 μm and a height of 500 μm, but according to the configuration of the present embodiment, it is possible for the ink chamber to be sized to have a diameter of about 250 μm and a height of about 50 μm that is, the capacity of the ink chamber can be reduced to about one-sixth of that of the conventional configuration. 
     The inkjet head  10  of the present embodiment, which includes the piezoelectric thin film  14  functioning as a driving film, further includes the driven film  11   b  that is operable to be curved along with the expansion and contraction of the piezoelectric thin film. Even with this configuration provided with the driven film  11   b , it is nonetheless possible to achieve a design for reducing the capacity of the ink chamber  21   a  in the design of the ink discharge portion  21  alone. Thus, even with the configuration provided with the driven film  11   b , it is possible to reduce the capacity of the ink chamber  21   a  without performing substrate polishing or film transfer, thereby improving the drive frequency of a head. In particular, with the configuration where a part of the substrate  11  in its thickness direction functions as the driven film  11   b  as in the case shown in  FIG. 2 , there is no need of providing (forming) a driven film aside from the substrate  11 . This helps achieve a simple configuration, and with such a simple configuration, the above-described advantages can be achieved. 
     The present embodiment also employs the piezoelectric thin film  14  as the driving film for discharging ink, and this makes it possible to achieve the above-described advantages with a more compact and lower-cost configuration as compared with cases where ink is discharged by means of the other methods such as the electrostatic method. 
     The present embodiment is also provided with the upper electrode  15  and the lower electrode  13  disposed so as to sandwich the piezoelectric thin film  14  in its thickness direction, and this makes it possible to cause the piezoelectric thin film  14  to expand and contract in the direction perpendicular to its thickness direction by applying a voltage across the piezoelectric thin film  14  in its thickness direction. Thus, with the configuration where the piezoelectric thin film  14  is driven in this manner, it is possible to achieve the above-described advantages. 
     Now, let us refer to  FIG. 3 , which presents a plan view showing a configuration of a plurality of channels of the above-discussed inkjet head  10  together with a sectional view taken along line A-A′ of the plan view and viewed in the direction indicated by the arrows. The substrate  11  may have an ink flow path  31  formed therein through which to supply ink to the ink chamber  21   a . The ink flow path  31 , which communicates with the ink chamber  21   a  via a communication path  32 , is connected with an ink storage portion (unillustrated) at a peripheral portion of the head. The ink flow path  31  is shared by a plurality of channels such that ink is supplied through one ink flow path  31  to the ink chamber  21   a  of each of the plurality of channels. 
     Usually, in an inkjet head, forming an ink flow path on an ink-discharging side (a recording-medium side) becomes a factor that prevents high-density arrangement of ink discharging holes (nozzle holes). In contrast, forming the ink flow path  31  in the substrate  11  that is disposed on a side opposite to the ink discharge portion  21  with respect to the piezoelectric thin film  14  as in the present embodiment makes it possible to arrange nozzle holes  23   a  at a high density on an ink discharging side, and this makes it possible to perform high-resolution image rendering (image formation). 
     Besides, by forming the ink flow path  31  in the substrate  11  that supports the piezoelectric thin film  14  and the like, it is possible not only to make an effective use of the substrate, and further to form the ink flow path  31  with ease by processing (etching, for example) the substrate  11 . Furthermore, since the substrate  11  has a thickness of about 300 to 500 μm, a sufficient capacity of the ink flow path  31  can be secured, and thus, even with one ink flow path  31  formed to communicate with the ink chamber  21   a  of each of the plurality of channels, ink can be securely supplied to the ink chamber  21   a  of each of the plurality of channels. 
     Another configuration of one channel of the inkjet head  10  is shown in  FIG. 4 , which is a sectional view. As shown in the figure, it is preferable to form the piezoelectric thin film  14  to be located above the dug portion  11   a  of the substrate  11  (on the ink chamber  21   a  side), with a width D (mm) smaller than the opening width C (mm) of the dug portion  11   a . That is, it is preferable to remove such an area of the piezoelectric thin film  14  as is located over a border between the dug portion  11   a  and the side wall  11   c . In this case, the space inward from the partition portion  22  and closer to the piezoelectric thin film  14  than the nozzle substrate  23  is becomes a space sandwiched between the nozzle substrate  23  and the lower electrode  13 , and this space constitutes the ink chamber  21   a . Also, in this configuration, the driven film  11   b  included in the displacement film  17  is supported at the substrate  11  such that the driven film  11   b  covers the dug portion  11   a.    
     Here, for the purpose of preventing electrical contact between the upper electrode  15  and the lower electrode  13  from occurring when outwardly drawing out the upper electrode  15 , an unillustrated protection film may be formed on the lower electrode  13  at an area where the piezoelectric thin film  14  has been removed, so that the upper electrode  15  can be outwardly drawn out along the surface of the protection film. Alternatively, a part of the piezoelectric thin film  14  may be left so as to stretch over the border, so that the upper electrode  15  can be outwardly drawn out along the surface of the piezoelectric thin film  14 . 
     Thus, by forming the piezoelectric thin film  14  inward from the opening width of the dug portion  11   a , it is possible to reduce risk of the deformation of the piezoelectric film  14  over the dug portion  11   a  being restrained by the surroundings (for example, the piezoelectric thin film  14  over the side wall  11   c ). This helps increase the displacement of the piezoelectric thin film  14  to improve the output of the head. 
     Note that, in  FIG. 4 , the substrate  11  is composed of a single Si substrate, and the dug portion  11   a  is formed by digging the substrate  11  to such a depth that part of the substrate  11  in its thickness direction is left without being dug. In this configuration as well, the upper wall of the dug portion  11   a , that is, such a part of the substrate  11  in its thickness direction as is located to the piezoelectric thin film  14  side of the dug portion  11   a  constituting the driven film  11   b  that is operable to be curved along with the expansion and contraction of the piezoelectric thin film  14 . 
     Still another configuration of one channel of the inkjet head  10  is shown in  FIG. 5 , which is a sectional view. The inkjet head  10  may be configured without a driven film as shown in the figure. That is, the inkjet head  10  may be configured such that the displacement film  17  is composed of the piezoelectric thin film  14  as a driving film, the lower electrode  13 , and the thermally oxidized film  12 , and such that the dug portion  11   a  is formed through the substrate  11  in its thickness direction. With this configuration, an end portion of the piezoelectric thin film  14  is supported on and restrained by the substrate  11  via the thermally oxidized film  12  and the lower electrode  13 , and thus, when the piezoelectric thin film  14  is caused to expand and contract in a direction perpendicular to its thickness direction by application of a voltage thereto, the piezoelectric thin film  14  itself is deformed to curve in its thickness direction, and along therewith, the lower electrode  13  and the thermally oxidized film  12  are also deformed to curve, to apply pressure to the ink held in the ink chamber  21   a . That is, with this configuration, the displacement film  17  is displaced in its thickness direction by the curving deformation of the piezoelectric thin film  14  caused by the expansion and contraction of the piezoelectric thin film  14  itself as a driving film. 
     With any of the configurations shown in  FIGS. 2, 4, and 5 , which all make it possible to achieve a design for reducing the capacity of an ink chamber  21   a  by working on the design of the ink discharge portion  21  alone, it is possible to improve the drive frequency of the head by reducing the capacity of the ink chamber  21   a  without performing substrate polishing or film transfer. 
     In particular, with the configurations shown in  FIGS. 4 and 5 , it is possible to form an inkjet head without using an SOI substrate as the substrate  11 , and the disuse of an SOI substrate results in a lower cost. Further, with the configuration shown in  FIG. 5  provided with no driven film, load is reduced due to the absence of the driven film, and the output of the head is accordingly increased. 
     Note that, in the configuration shown in  FIG. 5 , the thermally oxidized film  12 , which is provided for the sake of protection of the lower electrode  13 , is too thin to function as a driven film. However, it is also possible to form the thermally oxidized film  12  thick enough to function as a driven film. 
     [Method for Producing Inkjet Head] 
     Next, a description will be given below of an example of a method for producing the inkjet head  10  of the present embodiment. A production process of the inkjet head  10  configured as shown in  FIG. 2  is illustrated in  FIG. 6 , which is a sectional view. Note that  FIG. 6  shows a section at different stages in the production process, the section being perpendicular to the section taken along line A-A′ of  FIG. 2 , and thus the drawn-out portion of the upper electrode  15  does not appear in the figure. Note also that the production process proceeds in the following order: in  FIG. 6 , from the top of the left-most column downward to the bottom, then from the top of the second column from the left to the bottom, then from the top of the third column from the left to the bottom, and then from the top of the fourth column from the left to the bottom. 
     First, the substrate  11  is prepared. As the substrate  11 , there can be used a crystalline silicon (Si) substrate, which is widely used in micro electro mechanical systems (MEMS). Used here is a substrate of an SOI structure where two Si substrates  11   d  and  11   e  are joined together via an oxidized film  11   f . The thickness of the substrate  11   f  is determined by standards, etc., such that a six-inch substrate has a thickness of about 600 μm. 
     The substrate  11  is placed in a furnace, where temperature is maintained at about 1500° C. for a pre determined period of time, and thereby thermally oxidized films  12   a  and  12   b  made of SiO 2  are formed on surfaces of the Si substrates  11   d  and  11   e , respectively. The thermally oxidized film  12   a  corresponds to the thermally oxidized film  12  shown in  FIG. 2 . Next, a titanium layer and a platinum layer are formed on the thermally oxidized film  12   a  in this order by the sputtering method, to thereby form the lower electrode  13 . 
     Subsequently, the substrate  11  is heated again to about 600° C. and the piezoelectric thin film  14 , which is to function as the driving film, is formed of lead zirconate titanate (PZT) by the sputtering method. Then, a titanium layer and a platinum layer are formed in this order on the piezoelectric thin film  14  to thereby form a layer  15   a  front which the upper electrode  15  is to be formed. Next, a photosensitive resin  41  is applied onto the layer  15   a  by the spin coat method, the photosensitive resin  41  is exposed to light and etched via a mask to thereby remove an unnecessary part thereof, and then the shape of the upper electrode  15  to be formed is transferred onto the photosensitive resin  41 . Thereafter, the upper electrode  15  is formed by processing the shape of the layer  15   a  by the reactive ion etching method, using the photosensitive resin  41  as a mask. 
     Next, a resin film  22   a  (made of an epoxy resin, for example) for forming the partition portion  22  is attached onto the upper electrode  15 . The resin film  22   a  has a thickness of about 50 to 200 μm, for example, and the thickness can be selected according to required levels of responsiveness, ink flow-ability, etc. Then, a photosensitive resin  42  is applied to a top surface of the resin film  22   a  by the spin coat method, the photosensitive resin  42  is exposed to light and etched via a mask to thereby remove an unnecessary part thereof, and then the shape of the partition portion  22  to be formed is transferred onto the photosensitive resin  42 . Thereafter, the resin film  22   a  is subjected to removing processing using the solvent etching method, with the photosensitive resin  42  as a mask, and thereby the partition portion  22  is formed. 
     Next, a resin film  23   b  (made of an epoxy resin, for example) for forming the nozzle substrate  23  is attached to a top surface of the partition portion  22 . The resin film  23   b  has a thickness of about 5 to 20 μm, for example, and the thickness can be selected according to a required droplet amount and a required droplet speed. Then, a photosensitive resin  43  is applied to a top surface of the resin film  23   b  by the spin coat method, the photosensitive resin  43  is exposed to light and etched via a mask to thereby remove an unnecessary part thereof, and then the shape of the nozzle hole  23   a  to be formed is transferred onto the photosensitive resin  43 . Thereafter, the resin film  23   b  is subjected to removing processing using the solvent etching method, with the photosensitive resin  43  as a mask, and thereby the nozzle substrate  23  having the nozzle hole  23   a  is formed. A space inside the partition portion  22  and located closer to the piezoelectric film  14  than the nozzle substrate  23  is will function as the ink chamber  21   a , and through this production process, the ink discharge portion  21  including the ink chamber  21   a  as described above is formed on a side opposite to the substrate  11  with respect to the piezoelectric thin film  14 . 
     Here, it is also possible to use photosensitive resin films as materials of the partition portion  22  and the nozzle substrate  23  such that the photosensitive resin films serve also as the above-described photosensitive resins  42  and  43 . It is also possible to attach thin films made of metal, glass, ceramic, and the like besides the resin films and process the thin films into the shapes of the partition portion  22  and the nozzle substrate  23 . It is also possible to process thin films of resin, metal, glass, ceramic, and the like into the shapes of the partition portion  22  and the nozzle substrate  23  in advance, and attach the thus processed thin film. 
     Then, a photosensitive resin  44  is applied to a rear surface of the substrate  11  (that is, on the thermally oxidized film  12   b ) by the spin coat method, the photosensitive resin  44  is exposed to light and etched via a mask to thereby remove an unnecessary part thereof, and then the shape of the dug portion  11   a  and the ink flow path to be formed are transferred onto the photosensitive resin  44 . Thereafter, the substrate  11  is subjected to removing processing using the reactive ion etching method, with the photosensitive resin  44  as a mask, and thereby the dug portion  11   a , etc. are formed. That is, the dug portion  11   a  is formed by digging the substrate  11  from a side opposite to the side where the piezoelectric thin film  14  is formed. At this time, by forming the dug portion  11   a  such that a part (the Si substrate  11   d ) of the substrate  11  in its thickness direction is left without being dug, the driven film  11   b  constituted by the Si substrate  11   d  is formed, and the displacement film  17  (including the piezoelectric thin film  14  and the driven film  11   b ), which is operable to be deformed to curve in its thickness direction at its area corresponding to the dug portion  11   a  by expansion and contraction of the piezoelectric thin film  14 , is supported at the substrate  11  so as to cover the dug portion  11   a . This completes the production of the inkjet head  10 . 
     A production process of the inkjet head  10  configured as shown in  FIG. 5  is illustrated in  FIG. 7 , which is a sectional view. As for production of the inkjet head  10  without a driven film, the inkjet head  10  of such a type can be produced through the same production process as shown in  FIG. 6 , except that a common (single) Si substrate is used as the substrate  11 . In the production process, the inkjet head  10  without a driven film can be obtained by forming the dug portion  11   a  by digging through the substrate  11  in its thickness direction in the last digging step. 
     As has been discussed above, the production method of the inkjet head  11  of the present embodiment includes the steps of forming the piezoelectric thin film  14  as the driving film at the substrate  11 ; forming the dug portion  11   a  by digging the substrate  11  from a side opposite to the side where the piezoelectric thin film  14  is formed, and supporting the displacement film  17  including the piezoelectric thin film  14  such that the displacement film  17  covers the dug portion  11   a  so as to allow expansion and contraction of the piezoelectric thin film  14  in a direction perpendicular to its thickness direction to cause curving deformation of the displacement film  17  in its thickness direction in its area corresponding to the dug portion  11   a ; and forming the ink discharge portion  21 , through which ink held in the ink chamber  21   a  is discharged to outside the ink chamber  21   a  by the curving deformation of the displacement film  17 , on a side opposite to the dug portion  11   a  of the substrate  11  with respect to the displacement film  17 . 
     Thus, by forming the ink discharge portion  21  independently of the substrate  11 , it is possible to design the ink discharge portion  21  alone so as to reduce the capacity of the ink chamber  21   a . Such a design makes it possible to reduce the capacity of the ink chamber  21   a  to thereby improve the drive frequency of the head, without performing substrate polishing or film transfer. 
     It can be said that the inkjet head, the method for producing the same, and the inkjet printer of the present embodiment discussed above may also be described as follows. 
     An inkjet head of the present embodiment includes a displacement film that includes a driving film operable to expand and contract in a direction perpendicular to its thickness direction, the displacement film being operable to undergo curving deformation in its thickness direction, a substrate that includes a hole portion formed therein in its thickness direction and that supports the displacement film such that the displacement film covers the hole portion so as to allow expansion and contraction of the driving film to cause the curving deformation of the displacement film in its thickness direction in its area corresponding to the hole portion, and an ink discharge portion that includes an ink chamber holding ink and that discharges the ink to outside the ink discharge portion by having pressure resulting from the curving deformation of the displacement film applied to the ink. Here, the ink discharge portion may be disposed on a side opposite to the hole portion of the substrate with respect to the displacement film. 
     Another inkjet head according, to the present embodiment includes a displacement film that includes a driving film operable to expand and contract in the direction perpendicular to its thickness direction, the displacement film being operable to undergo curving deformation in its thickness direction, a substrate that includes a dug portion dug therethrough in its thickness direction and that supports the displacement film such that the displacement film covers the dug portion so as to allow expansion and contraction of the driving film to cause the curving deformation of the displacement film in its thickness direction in its area corresponding to the dug portion, and an ink discharge portion that includes an ink chamber holding ink and that discharges the ink to outside the ink discharge portion by having pressure resulting from the curving deformation of the displacement film applied to the ink. Here, the ink discharge portion may be disposed on a side opposite to the dug portion of the substrate with respect to the displacement film. 
     According to the above configurations, the ink discharge portion having the ink chamber is disposed on the side opposite to the hole portion (dug portion) of the substrate with respect to the displacement film including the driving film. Ink is discharged to outside the ink discharge portion by having pressure resulting from the curving deformation of the displacement film applied to the ink. The curving deformation of the displacement film is achieved by the driving film expanding and contracting in the direction perpendicular to the thickness direction thereof in a state where the displacement film is supported at the substrate so as to cover the hole portion (the dug portion). 
     As described above, the ink discharge portion is disposed on the side opposite to the hole portion (the dug portion) of the substrate with respect to the displacement film, that is, the ink discharge portion is provided independently of the substrate. This helps achieve a design for reducing the capacity of the ink chamber regardless of the substrate (working on the design of the ink discharge portion alone), by reducing the height of the ink discharge portion, for example. This makes it possible to improve (increase) the drive frequency of the head by reducing the capacity of the ink chamber without polishing the substrate or transferring the films. Furthermore, in reducing the capacity of the ink chamber, there is no need of performing substrate polishing or film transfer, and thus, the present invention is free from such disadvantages (reduction in yield, degradation of performance, damage to films, increase in cost) as have been suffered in cases where substrate polishing or film transfer is performed. 
     The ink discharge portion may further include a nozzle substrate that includes a nozzle hole through which to discharge the ink and a partition portion that is located closer to the displacement film than the nozzle substrate is and forms the side wall of the ink chamber. 
     With this configuration, it is possible to reduce the capacity of the ink chamber located closer to the displacement film than the nozzle substrate is, by means of a design where the height or the opening width (inner diameter) of the partition portion is reduced. 
     The displacement film may further include a driven film that is operable to curve in its thickness direction along with the expansion and contraction of the driving film. 
     Even with such a configuration where the displacement film includes the driven film in addition to the driving film as described above, it is nonetheless possible to achieve a design for reducing the capacity of the ink chamber by working on the design of the ink discharge portion alone. Consequently, even with the configuration where the displacement film includes the driven film, it is possible to reduce the capacity of an ink chamber without performing substrate polishing or film transfer. 
     The driven film may be composed of such a part of the substrate in its thickness direction as constitutes a wall located to a driving-film side of the hole portion. Alternatively, the driven film may be composed of such a part of the substrate in its thickness direction as constitutes an upper wall of the dug portion. In either of these cases, as compared with a case where the driven film is provided aside from the substrate, it is possible to make the configuration simpler, and with such a simple configuration, it is possible to achieve the above-described advantages. 
     The displacement film may be operable to be displaced in its thickness direction by the driving film being deformed to curve in its thickness direction by the expansion and contraction of the driving film itself. Even with a configuration where the displacement film does not include a driven film, it is nonetheless possible to achieve a design for reducing the capacity of the ink chamber by working on the design of the ink discharge portion alone. Thus, even with the above configuration, it is possible to improve the drive frequency of the head by reducing the capacity of the ink chamber without performing substrate polishing or film transfer. 
     The displacement film is preferably a piezoelectric thin film. In this case, the above-described advantages can be achieved with a compact and low-cost configuration using the piezoelectric thin film. 
     The above-described inkjet head may further include upper and lower electrodes disposed so as to sandwich the piezoelectric thin film in its thickness direction to apply a voltage across the piezoelectric thin film. In this case, it is possible to apply a voltage across the piezoelectric thin film in its thickness direction to thereby cause the piezoelectric thin film to displace (expand and contract) in a direction perpendicular to its thickness direction, and with such a configuration, it is possible to achieve the above-described advantages. 
     The substrate preferably has an ink flow path formed therein through which to supply ink to the ink chamber. The formation of the ink flow path in the substrate that is disposed opposite to the ink discharge portion with respect to the displacement film makes it easy to form discharge holes on the ink discharging side at a high density, which makes it possible to perform high-resolution image rendering. 
     An inkjet printer of the present embodiment includes the inkjet head configured as described above. Thereby, a high-performance inkjet printer with improved printing speed and resolution can be realized. 
     A method for producing the inkjet head of the present embodiment may include the steps of: forming a driving film at a substrate; forming a dug portion by digging the substrate from a side opposite to the side where the driving film is formed, and supporting a displacement film including the driving film such that the displacement film covers the dug portion so as to allow expansion and contraction of the driving film in a direction perpendicular to its thickness direction to cause curving deformation of the displacement film in its thickness direction in its area corresponding to the dug portion; and forming an ink discharge portion, through which ink held in the ink chamber is discharged to outside the ink chamber by the curving deformation of the displacement film, on a side opposite to the dug portion of the substrate with respect to the displacement film. This makes it possible to improve the drive frequency of the head by reducing the capacity of the ink chamber by working on the design of the ink discharge portion alone, without performing substrate polishing or film transfer. 
     Alternatively, a method for producing the inkjet head of the present embodiment may include the steps of: forming a driving film on a substrate; forming a hole portion in the substrate on a side opposite to a side where the driving film is formed, and supporting a displacement film including the driving film such that the displacement film covers the hole portion so as to allow expansion and contraction of the driving film in a direction perpendicular to its thickness direction to cause curving deformation of the displacement film in its thickness direction in its area corresponding to the hole portion; and forming an ink discharge portion, through which ink held in the ink chamber is discharged to outside the ink chamber by the curving deformation of the displacement film, on a side opposite to the hole portion of the substrate with respect to the displacement film. In this case as well, the same advantages as described above can be obtained. 
     INDUSTRIAL APPLICABILITY 
     The inkjet head of the present invention is usable in inkjet printers. 
     LIST OF REFERENCE SIGNS 
       1  inkjet printer 
       10  inkjet head 
       11  substrate 
       11   a  dug portion (hole portion) 
       11   b  driven film 
       13  lower electrode 
       14  piezoelectric thin film (driving film) 
       15  upper electrode 
       17  displacement film 
       21  ink discharge portion 
       21   a  ink chamber 
       22  partition portion 
       23  nozzle substrate 
       23   a  nozzle hole 
       31  ink flow path