Patent Publication Number: US-2015070444-A1

Title: Piezoelectric actuator, fluid discharge head, and image forming device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority from Japanese Patent Application No. 2013-187818, filed on Sep. 11, 2013 and No. 2014-137441, filed on Jul. 3, 2014, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piezoelectric actuator, a fluid discharge head incorporating the piezoelectric actuator, and an image forming device incorporating the fluid discharge head. 
     2. Description of the Related Art 
     In general an image recording device such as printer, facsimile machine, and photocopier comprises an ink jet head as a fluid discharge head. The ink jet head includes a nozzle to discharge drops of ink, a pressure chamber in which ink is accumulated, a vibrating plate forming a part of the walls of the pressure chamber, and an electromechanical transducer to apply pressure to the ink in the pressure chamber. The electromechanical transducer is driven to pressurize the pressure chamber via the vibrating plate and discharge ink drops from the nozzle. 
     For example, two kinds of a piezoelectric ink jet head are known. One uses a piezoelectric actuator with a vertical vibration mode in which the actuator is extended/shrunk along the axis of the electromechanical transducer and the other uses a piezoelectric actuator with a bend mode. A thin-film piezoelectric actuator, which is the latter piezoelectric actuator reduced in thickness, is a thin-film device comprised of laminated membranes formed by repeatedly forming various thin layers and patterning on a substrate. 
     The crystallization of the piezoelectric thin film is affected by crystal orientation of a substrate as a base or a lower electrode. The interface between the lower electrode and a PZT (lead zirconate titanate) film is particularly susceptible. In order to achieve a high piezoelectric constant, it is important that the piezoelectric thin film exerts plane orientation of (100). 
     Japanese Laid-open Patent Application Publication No. H11-191646 discloses a piezoelectric actuator comprising a silicon oxide film, a titanium oxide film, a lower electrode, a piezoelectric thin film, and an upper electrode laminated on a silicon substrate from bottom to top in this order. In this actuator the lower electrode is made from platinum, and titanium to be the crystal of a piezoelectric material is formed in an island shape in the crystal grain boundary of the platinum. 
     Further, the lower electrode is comprised of a titanium oxide film, platinum, and titanium laminated on the vibrating plate from bottom to top. The titanium oxide film is used to enhance the adherence between the vibrating plate and the lower electrode and it is an adherent layer. There is a problem with using a titanium oxide film as an adherent layer in that minute holes of 100 nm or less occur in the lower electrode film and on the surface of the lower electrode after thermal oxidation or PZT calcination. That is, in the thermal oxidation or PZT calcination titanium is diffused in a platinum film, causing the formation of holes in the platinum film. 
     The holes in the platinum film hinder continuity of platinum crystal so that it is hard to enlarge the mean particle diameter of the piezoelectric thin film formed thereon. The crystallinity of platinum and titanium oxide affects the quality of the piezoelectric thin film or piezoelectric material formed thereon. Because of this, the piezoelectric actuator containing the piezoelectric material cannot exert sufficient piezoelectric properties, which results in lowering manufacturing reproducibility thereof. 
     Further, lead components of the PZT film are excessively trapped in the holes on the lower electrode surface and the platinum film. Excessively diffused lead in the lower electrode causes the formation of a leak path or electric field concentration, which leads to decreasing the voltage holding capabilities of the piezoelectric actuator. This causes another problem that a decrease in a drop speed of the fluid discharge head is accelerated. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a piezoelectric actuator with good piezoelectric properties including a dense pillar-structured film with no holes in a lower electrode. 
     According to one embodiment, a piezoelectric actuator comprises a vibrating plate, a lower electrode provided on the vibrating plate, including a platinum film and a titanium oxide film formed on the platinum film, a piezoelectric thin film provided on the lower electrode, and an upper electrode provided on the piezoelectric thin film, wherein the titanium oxide film is provided between the platinum film and the piezoelectric thin film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, embodiments, and advantages of the present invention will become apparent from the following detailed description with reference to the accompanying drawings: 
         FIG. 1  is a schematic cross section view of a piezoelectric actuator according to a first embodiment; 
         FIG. 2  is a cross section view of the piezoelectric actuator with a lower electrode of an electromechanical transducer shown in detail; 
         FIG. 3  is a cross section view of an ink jet head according to a second embodiment; 
         FIG. 4  is a photographic view of a cross section of the periphery of the lower electrode of the piezoelectric actuator; 
         FIG. 5  is a perspective view of a fluid cartridge according to a third embodiment; 
         FIG. 6  is a perspective view of an image forming device; and 
         FIG. 7  is a vertical cross section view of the image forming device in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     First Embodiment 
     A piezoelectric actuator according to the present embodiment comprises an electromechanical transducer comprised of a lower electrode, a piezoelectric thin film, and an upper electrode. The lower electrode is laminated on a titanium oxide adherent layer formed on the top side of the vibrating plate. 
     The lower electrode is a dense film free from minute holes of 100 am or less and comprises a platinum film formed on the titanium oxide adherent layer and a titanium oxide film on the platinum film. 
       FIGS. 1 and 2  show a piezoelectric actuator  10  according to the present embodiment.  FIG. 1  is a schematic cross section view of the piezoelectric actuator and  FIG. 2  is the same showing the lower electrode of the electromechanical transducer in detail 
     In  FIG. 1  the piezoelectric actuator  10  comprises a substrate  11  and a piezoelectric element  12  formed on the substrate  11 . 
     The piezoelectric element  12  comprises a vibrating plate  13  laminated on the substrate  11  and an electromechanical transducer  14  provided on the vibrating plate  13 . The electromechanical transducer  14  comprises a lower electrode  15 , a piezoelectric thin film  16 , and an upper electrode  17  laminated on the vibrating plate  13  in this order from bottom to top. 
     In the present embodiment a titanium oxide adherent layer  18  is formed on the vibrating plate  13  and the electromechanical transducer  14  is formed on the titanium oxide adherent layer  18 , as shown in  FIG. 2 . The lower electrode  15  of the electromechanical transducer  14  includes a platinum film or platinum electrode  15   a  and a titanium oxide film  15   b  on the platinum film  15   a . The upper electrode  17  is formed on the piezoelectric thin film  16 . 
     As described above, the lower electrode  15  of the piezoelectric actuator  10  according to the present embodiment includes the platinum film  15   a  and the titanium oxide film  15   b , and the titanium oxide film  15   b  is provided between the platinum film  15   a  and the piezoelectric thin film  16 . 
     The platinum film  15   a  includes Pt (111) crystal. By providing the titanium oxide film  15   b  on this platinum film  15   a  as a buffer layer, the piezoelectric thin film  16  acquires PZT (100) crystallinity. As a result, PZT (100) preferred crystal orientation can be 90% or more. 
     Meanwhile, generally, a film with PZT (111) preferred orientation is formed by directly laminating the piezoelectric thin film  16  on the platinum film  15   a  formed on the titanium oxide adherent layer  18 . Accordingly, the PZT (100) preferred crystal orientation cannot be increased. 
     Next, a manufacturing method of the piezoelectric actuator  10  as configured above will be described, referring to  FIG. 2 . 
     First, a silicon substrate made from silicon (Si) is prepared for the substrate  11 . Then, the surface of the substrate  11  is thermally oxidized to create a SiO 2  insulating film thereon. The SiO 2  insulating film is the vibrating plate  13 . The thickness of the SiO 2  insulating film is 2 μm. 
     Next, a titanium film is formed on the SiO 2  insulating film by sputtering and subjected to thermal oxidation in O 2  atmosphere for 1 to 10 minutes at temperature of 650 to 800 C by use of a RTA (Rapid Thermal Annealing) device to form a titanium oxide film. Thereby, the titanium oxide adherent layer  18  is formed on the vibrating plate  13 . Preferably, the thickness of the titanium oxide film should be 20 to 50 nm. The titanium oxide film can be created by reactive sputtering, however, thermal oxidation of a titanium film at a high temperature is more preferable. 
     The platinum film (platinum electrode)  15   a  of the lower electrode  15  is formed on the titanium oxide adherent layer  18  by sputtering. 
     Then, the titanium oxide film  15   b  is formed on the platinum film  15   a  by forming a titanium film by sputtering and subjected to thermal oxidation in O 2  atmosphere for 1 to 5 minutes at temperature of 650 to 800 C (particularly, 700 to 750 C) by use of the RTA device. By thermal oxidation at 700 to 750 C, the titanium film can be oxidized to the interface with the platinum film  15   a  so that a good titanium oxide film can be obtained. The thickness of the titanium oxide film  15   b  should be preferably 50 to 100 angstrom (A) to form a PZT film (PZT(100) film) having crystallinity (100) with good piezoelectric properties. The titanium oxide film  15   b  can be created by reactive sputtering, however, thermal oxidation of a titanium film at a high temperature is more preferable. Further, the crystallinity of the titanium oxide film (TiO 2  film) is enhanced by thermal oxidation with the RTA device than with a general heating furnace. This is because by oxidization with a heating furnace, an easily-oxidized titanium film forms a number of crystal structures at a low temperature. Thus, by use of the RTA device which can rapidly raise temperature, good crystal can be obtained. 
     Next, to create the piezoelectric thin film  16 , a solution mixed at a composition ratio of Pb (lead):Zr (zirconium):Ti (titanium)=110:53:47 is prepared. For synthesizing precursor embrocation of this solution, specifically, lead (II) acetate rehydrate, isopropoxide titanate, and zirconium (IV) isopropoxide are used as a starting material. Crystal water of the lead acetate is dissolved in methoxyethanol and dehydrated. Herein, the amount of the lead is excessively prepared relative to stoichiometric composition for the purpose of preventing a deterioration in the crystallinity due to evaporation of lead during a thermal process. 
     The isopropoxide titanate and zirconium (IV) isopropoxide are dissolved in methoxyethanol, subjected to alcohol exchange and etherification, and mixed with the methoxyethanol solution in which the lead(II) acetate is dissolved. Thereby a PZT precursor solution is synthesized. The PZT concentration of this solution is 0.5 mol/liter. A film is formed from this solution by spin coating, dried at 120 C, and subjected to thermal decomposition at 500 C. Then, after the third layer is thermally discomposed, the film is subjected to crystallization heat treatment by RTA at the temperature of 750 C. The thickness of the PZT is 250 nm. By repeating the above process eight times (24 layers), a PZT film in thickness of about 2 μm is acquired. 
     Then, a platinum film is formed as the upper electrode  17  by sputtering. The material of the upper electrode  17  is not limited to platinum. Alternatively, a metal electrode such as iridium or gold can be used or a laminated film of oxide electrode layer and metal electrode layer can be used. IrO 2 , LaNiO 3 , RuO 2 , SrO, and SrRuO 3  are used for the oxide electrode layer and platinum, iridium, and gold are used for the metal electrode layer. 
     The piezoelectric actuator  10  in  FIG. 2  is manufactured in the above manner. The piezoelectric actuator  10  is comprised of the substrate  11 , vibrating plate  13 , titanium oxide adherent layer  18 , platinum film  15   a , titanium oxide film, piezoelectric thin film  16 , and upper electrode  17  laminated in this order. 
     As described above, minute holes are prevented from occurring on the platinum film  15   a , titanium oxide film  15   b , and piezoelectric thin film  16  by oxidizing titanium (Ti) by rapid thermal annealing. 
     Meanwhile, without the titanium oxide film  15   b , the minute holes in the platinum film  15   a  make it hard to increase the mean particle diameter of the piezoelectric thin film  16  provided on the platinum film  15   a.    
     According to the present embodiment, by forming the titanium oxide adherent layer  18  on the vibrating plate  13 , the lower electrode  15  (platinum film  15   a  and titanium oxide film  15   b ) in a dense pillar structural film with no minute holes can be formed. Note that the minute holes are ones of 100 nm or less. 
     Further, the inventors of the present application have found that provided with the titanium oxide film  15   b  on the side of the lower electrode  15  close to the piezoelectric thin film  16 , the crystal orientation of the piezoelectric thin film  16  becomes (100). Also, the PZT (100) orientation can be improved by thermally oxidizing the titanium film at 650 C or more. 
     Thus, by thermally oxidizing the titanium film at 650 C or more, it is able to realize a dense pillar-structured film having no minute holes in the lower electrode  15  with a good, stable reproducibility even after the thermal oxidation. 
     Diffusion of titanium in the platinum film  15   a  during the thermal oxidation and PZT calcination causes the creation of holes therein. This can be prevented by completely oxidizing the titanium film (that is, titanium oxide adherent layer  18 ) on the vibrating plate  13 . Owing to the titanium oxide adherent layer  18 , in forming the side of the titanium oxide film  15   b  close to the piezoelectric thin film  16 , titanium is prevented from dispersing in the platinum film  15   a . Specifically, the titanium oxide adherent layer  18  is formed by forming the titanium film by sputtering. Then, the titanium film is thermally oxidized in O 2  atmosphere at 650 C or more with the RTA device, thereby improving the crystallinity of the platinum film  15   a  while maintaining the adherence. 
     Further, without the holes, the continuity of platinum crystal in the platinum film  15   a  is not hindered so that the mean particle diameter of the piezoelectric thin film  16  formed thereon can be increased from that of a conventional piezoelectric thin film of titanium oxide, platinum, and titanium layers. Accordingly, the piezoelectric actuator  10  can exert a good manufacturing reproducibility and sufficient piezoelectric properties. 
     Further, according to the present embodiment since the piezoelectric thin film  16  has crystallinity of 100, the piezoelectric constant of the piezoelectric element  12  can be enhanced. 
     Moreover, an excellent PZT(100) film can be produced by forming the titanium oxide film  15   b  by thermal oxidation to the titanium film formed on the platinum film  15   a  by RTA. Thus, the properties of the piezoelectric thin film  16  can be improved. 
     Second Embodiment 
       FIG. 3  shows an example of an ink jet head as a fluid discharge head incorporating the piezoelectric actuator  10  according to the first embodiment. 
     An ink jet head  20  according to a second embodiment includes partition walls  11 ′ on the four sides at the bottom although front and back walls are not shown. The four partition walls  11 ′ form a barrel shape with a rectangular transverse cross section. The vibrating plate  13  and a nozzle plate  19  including a nozzle  19   a  are secured on the top and bottom end faces of the partition walls, respectively. The nozzle plate  19  encloses the opening ends of the partitioning walls  11 ′. A pressure chamber  11   a  in which ink is temporarily stored is formed in a space surrounded by the partitioning walls  11 ′, vibrating plate  13 , and nozzle plate  19 . By deforming the vibrating plate  13  with pressure, the ink is discharged from the pressure chamber  11   a  through the nozzle  19   a  of the nozzle plate  19 . 
     In the ink jet head  20  in  FIG. 3  the partitioning walls  11 ′ and nozzle plate  19  are provided below the piezoelectric element  12  while in the piezoelectric actuator in  FIG. 2  the substrate  11  is provided therebelow. The partitioning walls  11 ′ and nozzle plate  19  are produced by processing the substrate  11 , as described below. 
     A silicon single-crystal substrate in thickness of 100 to 650 μm is preferable for the substrate  11 . To create the pressure chamber  11   a  in  FIG. 3 , the substrate  11  is processed by anisotropic etching. 
     To apply the piezoelectric element  12  to the ink jet head  20  in  FIG. 3 , the pressure chamber  11   a  is formed in the substrate  11 . Also, the vibrating plate  13  preferably has certain strength since it is deformed to discharge ink drops in the pressure chamber  11   a , upon receiving a force from the piezoelectric thin film  16 . 
     A material of the vibrating plate as a base can be one made from Si, SiO 2 , Si 3 N 4  by chemical vapor deposition (CVD) method. Moreover, it is preferable to use a material with a linear expansion coefficient close to that of the lower electrode  15  and piezoelectric thin film  16 . In particular the use of a material with a linear expansion coefficient of 5×10 −6  to 10×10 −6 [1/K] close to 8×10 −6 [1/K] of PZT is preferable since the piezoelectric thin film  16  is generally made from PZT. A material with a linear expansion coefficient of 7×10 −6  to 9×10 −6  [1/K] is more preferable. Specifically, aluminum oxide, zirconium oxide, iridium oxide, ruthenium oxide, tantalum oxide, hafnium oxide, osmium oxide, rhenium oxide, rhodium oxide, palladium oxide and compounds of these oxides are exemplified. The vibrating plate  13  can be created from these materials with a spin coater by sputtering or sol-gel method. The thickness thereof should be preferably 0.1 to 10 μm, more preferably 0.5 to 3 μm. With a thickness below these ranges, it is difficult to process the pressure chamber  11   a  while with a thickness exceeding the ranges, the base is not easily deformed so that ink drops cannot be stably discharged. 
     Next, a titanium film in thickness of 20 to 50 nm is formed on the vibrating plate  13  before the lower electrode  15 . 
     Then, the titanium film is subjected to thermal oxidation in O 2  atmosphere for 1 to 10 minutes at 650 to 800 C by use of the RTA device to create the titanium oxide adherent layer  18  or titanium oxide film on the vibrating plate  13 . 
     The titanium oxide film can be created by reactive sputtering, however, the thermal oxidation of the titanium film at a high temperature is more preferable. To create the titanium oxide film by reactive sputtering, a special sputtering chamber structure is required because a silicon substrate needs to be heated at a high temperature. Further, the crystallinity of the titanium oxide film is enhanced by the thermal oxidation with the RTA device than with a general heating furnace. This is because an easily-oxidized titanium film forms a number of crystal structures at a low temperature by oxidization with a heating furnace. Thus, by use of the RTA device which can rapidly raise temperature, good crystal can be obtained. 
     Next, the platinum film or platinum electrode  15   a  ( FIG. 2 ) of the lower electrode  15  is formed in thickness of 200 am or less on the titanium oxide adherent layer  18  by sputtering. 
     Then, the titanium oxide film  15   b  ( FIG. 2 ) is formed on the platinum film  15   a . That is, a titanium film is formed thereon by sputtering and subjected to thermal oxidation in 02 atmosphere for 1 to 5 minutes at 650 to 800 C by use of the RTA device. The thickness of the titanium oxide film should be preferably in the range of 30 to 70 angstrom (A), to realize a PZT (100) film with good piezoelectric properties. It is preferable to produce the titanium oxide film by high-temperature thermal oxidation. Further, the crystallinity of the titanium oxide film is enhanced by the thermal oxidation with the RTA device than with a general heating furnace. This is because an easily-oxidized titanium film forms a number of crystal structures at a low temperature by oxidization with a heating furnace. Thus, by use of the RTA device which can rapidly raise temperature, good crystal can be obtained. 
     The piezoelectric thin film  16  is chiefly made from PZT (Lead Titanate Zirconate) at a ratio of zirconium (Zr) and titanium (Ti),  52  to  48 , due to its stable and good piezoelectric performance and properties. Lead titanate zirconate is, however, not limited to the above compound. Alternatively, various oxides containing lead, zirconium, and titanium at different ratios or mixed with or replaced with an additive can be used. Also, perovskite oxides with a general formula of ABO 3  (A including Pb, B including Zr and Ti) are suitably used, for example. Niobic acid titanic acid zirconic acid lead using (PZTN) Nb is well known. Moreover. In view of environmental concerns, barium titanate (BaTiO 3 ) with no use of Pb, a complex oxide (BST) of barium, strontium and titanium, and a complex oxide of strontium, bismuth and tantalum can be used. 
     The piezoelectric thin film  16  is manufactured by sol-gel method, that is, spin coating of sol-gel. For example, in the case of PZT, organic metal compounds containing Pb, Zr, Ti are dissolved in a solution and coated on the film of the lower electrode  15 . The coated piezoelectric film is subjected to calcinations for solidifying and crystallization. Thereby, the piezoelectric thin film  16  can be created. The calcination for solidifying is in general conducted for every coated layer while that for crystallization is collectively conducted for every several solidified layers. A piezoelectric layer in a desired thickness can be obtained through repetition of a series of the calcinations for solidification and crystallization. It is dried at temperature of 350 to 550 C and heated at about 650 to 800 C for crystallization. By use of the RTA device, heating time is several seconds to several minutes. The thickness of the piezoelectric layer is for example several dozen nm to several μm. 
     The upper electrode  17  is formed on the piezoelectric layer or piezoelectric thin film  16 . The upper electrode  17  can be made from the same material as that of the lower electrode  15 . The material of the upper electrode layer  17  can be chosen from a larger number of materials than that of the lower electrode  15  since a high-temperature process and lattice constant matching necessary for forming the piezoelectric layer are unneeded. A platinum film and a metal electrode such as iridium or gold are usable for the upper electrode  17 . A laminated layer of an oxide electrode layer and a metal electrode layer is also usable. For the oxide electrode layer, IrO 2 , LaNiO 3 , RuO 2 , SrO, or SrRuO 3  can be used, for instance. 
     The upper electrode is chiefly formed by sputtering, however, it can be created by a known method such vacuum deposition and CVD (Chemical Vapor Deposition). The thickness of the upper electrode  17  can be in the range of about 50 to 300 nm. In general the piezoelectric thin film  16  is formed after the lower electrode  15  and the upper electrode  17  is formed on the piezoelectric thin film  16 . 
     The piezoelectric thin film  16  is generally formed on the lower electrode  15  by dry or wet etching after an etching mask layer is formed thereon by patterning of a photosensitive resist. 
     Then, another piezoelectric element is formed after the upper electrode  17 . It is formed on the upper electrode  17  by dry or wet etching after an etching mask layer is formed thereon by patterning of a photosensitive resist. The photosensitive resist is patterned by a known photo-lithography. That is, a photosensitive resist is coated on a specimen substrate with a spin coater or roll coater and exposed to an ultraviolet ray by a glass photo mask having a desired pattern. Thereafter, the pattern is developed, and the substrate is washed with water and dried to form a photosensitive resist mask layer. The end of the patterning of the photosensitive resist mask layer is tilted, which affects an inclined section thereof at the time of etching. It is selected in accordance with a desired tilt angle with a resist selection ratio (ratio of etching rates between etched material and masking material) taken into account. A residual photosensitive resist on the film after etching can be removed by a dedicated peeling fluid or oxygen plasma ashing. 
     Because of a shape stability, dry etching using reactive gas is adopted. However, halogenous gas including chlorine or fluorine gas, or halogenous gas mixed with Ar or oxygen can be also used. By changing etching gas or etching condition, the upper electrode  17  and piezoelectric element can be continuously etched or resist patterning can be repeated to separately conduct etching at several times. 
     Although not shown, a protective layer is disposed to shield the piezoelectric thin film  16  placed between the electrodes and a cross section thereof forming the shape of the film from a driving condition such as humidity. The protective layer is made from oxide by atomic layer deposition (ALD) in view of required density. Specifically, an ALD film of Al 2 O 3  in thickness of about 30 to 100 nm is used. 
     Further, although not shown, an insulating layer is formed on the upper electrode  17  to insulate between a wired electrode laminated in the next process and the upper and lower electrodes  17 ,  15 . It is made from a silicon dioxide film, a silicon nitride film, or a mixed film of the two. The thickness thereof is 300 to 700 nm. 
     A through hole is formed by photo lithography and etching in the wired electrode, upper electrode  17 , and lower electrode  15  for contact. Residual resist is removed by oxygen plasma ashing, for example. 
     The wired electrode layer is used to fetch an individual or common electrode of a ferroelectric element and made from a material in ohmic contact with the upper and lower electrodes. Specifically, a wiring material containing pure Al or Al containing a hillock preventing component such as Si of several atomic % can be used. In view of conductivity, a semiconductor wiring material consisting mainly of Cu can be also used. The thickness of the layer is set to a value such that a wiring resistance including one due to a wiring distance does not trouble the driving of the piezoelectric element. Specifically, the thickness of an Al wiring should be set to about 1 μm. The wired electrode layer can be shaped as desired by photo lithography and residual resist is removed by oxygen plasma ashing, for example. 
     The wired electrode layer is covered with a protective layer of oxide or nitrade except for a portion for electric connection, in order to secure resistance to environment. 
     Lastly, an ink chamber is created by deeply cutting the Si substrate till the vibrating plate by inductively coupled plasma (ICP) etching. Thus, the substrate on which the piezoelectric actuator is formed is completed. 
     In the subsequent process the nozzle plate, a driver circuit, and an ink supplier are assembled to form the ink jet head. 
     Comparison Example 
     For the purpose of comparison with the piezoelectric actuator according to the present embodiment, a piezoelectric actuator as a first example was created by forming the titanium film as the titanium oxide adherent layer by sputtering but without the RTA process. The particle diameter, withstand voltage and else were measured in the following manner. The results of measurement are described in the following. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 PARTICLE  
               
               
                   
                   
                 DIAMETER (nm) 
               
               
                   
                   
               
             
            
               
                   
                 FIRST EXAMPLE 
                 80~70 
               
               
                   
                 FIRST EMBODIMENT 
                 120~200 
               
               
                   
                   
               
            
           
         
       
     
     The particle diameters of the two piezoelectric actuators according to the first embodiment and first example were evaluated immediately after the formation of a PZT film by use of an AFM (Atomic Force Microscope) Nanoscope IIIa manufactured by Veeco Instruments Inc. In the first example the AFM film configuration and the particle diameter thereof (nm) were evaluated. 
     The piezoelectric thin film of the first example was created in the same manner as in the first embodiment except for forming a titanium film by sputtering and not performing RTA process. The AFM was used in a tapping mode in the range of 3 μm×3 μm and scanning speed was 1 Hz. The results are shown in Table 1. 
     No holes were observed in the piezoelectric actuator according to the first embodiment. Meanwhile, the piezoelectric actuator of the first example contained a large number of holes on the platinum electrode. The lower electrode including the platinum film and titanium oxide adherent layer in the first embodiment was free from any holes after completion of the piezoelectric actuator, as shown in  FIG. 4 . To check the presence of holes, the cross section thereof along the thickness was processed by FIB (Focused Ion Beam) process and observed with a scanning electron microscope (SEM) with a magnification of 100 k or more. 
     The resistance to voltage (V), residual polarization Pr(μC/cm 2 ), piezoelectric constant d31(pm/V), and degradation rate (%) of the piezoelectric thin films according to the first embodiment and first example were compared. The piezoelectric thin films were applied with electric field of 150 kV/cm and amounts of deformation were measured with a laser Doppler vibrometer. The measuring results were matched with simulation results for calculation. After evaluation of the initial properties, their durabilities (properties immediately after voltage was applied repeatedly at 10 10  times) were evaluated. Table 2 shows the results. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 WITHSTAND 
                 Pr (μC/cm 2 ) 
                 d31 (pm/V) 
                 DEGRADATION 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 VOLTAGE 
                 INITIAL 
                 AFTER 
                 INITIAL 
                 AFTER 
                 RATE 
               
               
                   
                 (V) 
                 PERIOD 
                 1E10 TIMES 
                 PERIOD 
                 IE10 TIMES 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 FIRST 
                 204 
                 50 
                 48 
                 −180 
                 −178 
                 2.7 
               
               
                 EMBODIMENT 
               
               
                 FIRST 
                 101 
                 44 
                 36 
                 −140 
                 −126 
                 11.2 
               
               
                 EXAMPLE 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, the obtained properties of the piezoelectric thin film according to the first embodiment are equivalent to those of a typical sintered ceramic element. That is, the resistance to voltage was 204V, residual polarization was 50 μC/cm 2 , piezoelectric constant was −180 pm/V, and degradation rate (%) of ink drop speed was 2.7%. 
     Meanwhile, regarding the first example, although the initial properties were sufficient, in terms of durability both the residual polarization and piezoelectric constant are degraded after applied voltage at 10 10  times. That is, the resistance to voltage was 101V, residual polarization was 44 μC/cm 2 , piezoelectric constant was −140 pm/V, and degradation rate (%) of ink drop speed was 11.2%. 
     Third Embodiment 
       FIG. 5  shows an ink cartridge  80  according to a third embodiment comprising the ink jet head (fluid discharge head) of the second embodiment. 
     The ink cartridge  80  comprises a fluid discharge head  100  with a nozzle  81  and an ink tank  82  integrated with each other. The ink tank  82  contains ink in advance and supplies the ink to the fluid discharge head  100 . 
     The fluid discharge head  100  is the ink jet head  20  of the second embodiment in  FIG. 3  and includes the piezoelectric actuator  10  of the first embodiment 10 in  FIG. 2  as a piezoelectric element. 
     In such a fluid discharge head  100  integrated with the ink tank  82 , the yield and reliability of the ink cartridge  80  can be improved by the precise, highly dense, and reliable actuator. Thus, the cost reduction of the ink cartridge  80  can be achieved. 
     According to the present embodiment, the ink cartridge  80  comprising the ink jet head excelling in durability and reliability can be realized. 
     Fourth Embodiment 
       FIGS. 6 and 7  show an image forming device  90  according to a fourth embodiment which comprises the ink jet head of the second embodiment and the ink cartridge of the third embodiment. 
     The image forming device  90  comprises a carriage  98  movable in a scanning direction inside a device body and a printing unit  91  including the fluid discharge head  100  mounted on the carriage  98  and an ink cartridge  99  to supply ink to the fluid discharge head  100 . 
     A paper cassette or tray in which a large number of paper sheets  92  are set from the front is detachably provided in the bottom part of the device body. A manual paper tray  94  on which paper sheets are manually set is also provided. The paper sheets  92  are fetched from the paper cassette  93  or manual paper tray  94 , a desired image is recorded thereon by the printing unit  91 , and discharged to a discharge tray  95  at the back. 
     The printing unit  91  includes a main guide rod  96  and a sub guide rod  97  extending between not-shown right and left side plates to slidably hold the carriage  98  in a main scanning direction. The carriage  98  is provided with the fluid discharge heads  100  to discharge ink drops of four colors, yellow (Y), cyan (C), magenta (M), and black (Bk), respectively. In the carriage  98  discharge ports (nozzles) to discharge the ink downward are arranged orthogonally to the main scanning direction. Ink cartridges  99  are detachably mounted on the carriage  98  to supply the four-color ink to the fluid discharge heads  100 . 
     The ink cartridges  99  each include an air port on the top in communication with the atmosphere and a supply port on the bottom to supply the ink to the fluid discharge heads  100 . The ink cartridges  99  contain a porous element filled with the ink so that it is able to maintain the ink at a very small negative pressure by a capillary force of the porous element. The present embodiment uses four fluid discharge heads to discharge the four-color ink drops. However, a single fluid discharge head with nozzles to discharge the four-color ink drops can be used. 
     The back side (downstream of paper feed direction) of the carriage  98  is slidably supported by the main guide rod  96  and the front side (upstream of paper feed direction) thereof is slidably supported by the sub guide rod  97 . The carriage  98  is provided with a main scan motor  101 , a drive pulley  102 , a driven pulley  103 , and a timing belt  104  which extends between the drive pulley  102  and driven pulley  103 . The carriage  98  is secured on the timing belt  104 . The main scan motor  101  regularly or reversely rotates the drive pulley  102  to reciprocate the timing belt  104 , thereby moving the carriage  98  in the main scanning direction. 
     To feed the paper sheets  92  downward from the paper cassette  93  to below the fluid discharge heads  100 , a paper feed roller  105 , a friction pad  106 , a guide element  107 , a conveying roller  108 , a conveying roller  109 , and an end roller  110  are provided. The paper feed roller  105  and friction pad  106  separate the paper sheets  92  from the paper cassette  93 . The guide element  107  guides the separated paper sheets to the conveying roller  108  and the conveying roller  108  inversely rotates the paper sheets  92 . The convey roller  109  is pressed onto the circumference of the conveying roller  108  and the end roller  110  is provided to define the angle at which the papers is sent fourth from the conveying roller  108 . The conveying roller  108  is rotated by a not-shown sub scan motor via a gear train. 
     Further, a paper receiver Ill is provided below the fluid discharge head  100  to guide the paper sheets from the convey roller  108  in accordance with a moving range of the carriage  98  in the main scanning direction. A roller  112  and a gear  113  are provided at downstream of the paper receiver  111  in the paper feed direction to be rotated to convey the paper sheets  92  in the paper discharge direction. At downstream of the roller  112  and gear  113 , a discharge roller  114  and a gear  115  to discharge the paper sheets  92  to the paper discharge tray and guide elements  116 ,  117  forming a paper discharge path are disposed. 
     The image forming device  90  drives the fluid discharge heads  100  in accordance with an image signal while moving the carriage  98 , to discharge ink to a still paper sheet  92  and records one line of an image thereon. Then, it moves the paper sheet  92  by a certain amount and record a next line of the image. Upon receiving a recording stop signal or a signal indicating that the rear end of the paper sheet  92  has reached a recording area, it completes a recording operation and discharges the paper sheet  92 . 
     Moreover, a recovery unit  118  is provided outside the recording area at the right end of the moving direction of the carriage  98 , to resolve a discharge failure of the fluid discharge heads  100 . The recovery unit  118  comprises a cap, a cleaner, and a suction element. The carriage  98  is moved to the end near the recovery unit during a print standby and the fluid discharge heads  100  are covered with the cap to maintain the moistness of discharge ports and prevent a discharge failure due to dried ink. Also, it is configured to discharge ink irrelevant to recording so that ink viscosities of all the discharge ports become constant. Thereby, a stable ink discharge can be maintained. 
     When a discharge failure occurs, the outlets (nozzles) of the fluid discharge heads  100  are sealed with the cap and the suction element sucks air bubbles and the ink through a tube. The cleaner removes the ink, dust and the like attached to the discharge ports, thereby resolving a discharge failure. The suctioned ink is discharged to a not-shown used ink tank and absorbed into an ink absorber therein. 
     As described above, the image forming device  90  according to the present embodiment achieves stable ink discharge properties and improve image quality since it comprises the fluid discharge heads  100  of the first embodiment. 
     In addition to the image forming device  90 , the fluid discharge heads  100  are applicable to a device to discharge fluid other than ink, for example, fluid resist for patterning. 
     According to the present embodiment, a high-quality image forming device can be obtained owing to the piezoelectric thin film excelling in durability and reliability. 
     Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. It should be appreciated that variations or modifications may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. 
     For example, the image forming device  90  according to the fourth embodiment is applicable to a printer, a facsimile machine, a copier, and a multifunction peripheral. 
     Further, the present invention is applicable to a fluid discharge head or fluid discharge device to discharge fluid other than ink, for example, DNA specimen, resist, or patterning material as well as to an image forming device incorporating such a head or device.