Abstract:
A piezoelectric device includes a piezoelectric film and an electrode film. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less, the piezoelectric film having a tension stress. The electrode film applies a voltage to the piezoelectric film.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application JP 2007-297321 filed in the Japanese Patent Office on Nov. 15, 2007, Japanese Patent Application JP 2007-297323 filed in the Japanese Patent Office on Nov. 15, 2007, and Japanese Patent Application JP 2007-297325 filed in the Japanese Patent Office on Nov. 15, 2007, the entire contents of which are being incorporated herein by reference. 
     BACKGROUND 
     The present application relates to a piezoelectric device used in a piezoelectric sensor, a piezoelectric actuator, and a pyroelectric infrared ray sensor, an angular velocity sensor including the piezoelectric device, and a method of manufacturing a piezoelectric device. 
     From the past, lead zirconium titanate (Pb 1+X (Zr Y T 1−Y )O 3+X ) (hereinafter, referred to as PZT) is used as a piezoelectric material of a piezoelectric thin film used for a piezoelectric sensor such as an angular velocity sensor, an ink jet head, and the like. Various techniques are proposed for improving piezoelectric characteristics, ferromagnetic material characteristics, pyroelectric characteristics, and the like of the PZT (see, for example, Japanese Patent Application Laid-open No. Hei 06-350154 (paragraphs (0030) to (0044), (0060) to (0073),  FIGS. 3 ,  4 , etc.) and Japanese Patent Application Laid-open No. Hei 09-298324 (paragraphs (0007) to (0009),  FIG. 5 ); hereinafter, will respectively be referred to as Patent Document 1 and Patent Document 2). 
     Patent Document 1 discloses a PZT thin film whose crystalline structure is rhombohedral, in which, when lead zirconium titanate is represented by Pb 1+Y (Zr X T 1−X )O 3+Y , a PbO excessive composition ratio Y is within a range of 0≦Y≦0.5, and a Zr composition ratio X is within a range of 0≦Y≦0.55. The PZT thin film of Patent Document 1 exhibits favorable piezoelectric characteristics. Moreover, there is also disclosed a PZT thin film whose crystalline structure is tetragonal, in which the PbO excessive composition ratio Y is within a range of 0≦Y&lt;0.5, and the Zr composition ratio X is within a range of 0.55≦X&lt;1. 
     Patent Document 2 discloses a piezoelectric thin film having a thickness of 1 μm or more and 10 μm or less, a crystal grain size of 0.55 μm or less, and surface roughness of 1 μm or less at R MAX . The piezoelectric thin film is useful as a piezoelectric thin film for an ink-jet-type storage apparatus that requires a predetermined film thickness or more. 
     SUMMARY 
     Incidentally, when heated, the piezoelectric material is known to deteriorate in piezoelectric performance, which is called depolarization. However, because heating processing by solder reflow and the like is generally carried out in a process of manufacturing an electronic apparatus that includes the piezoelectric material, there is a problem that the piezoelectric performance of the piezoelectric material deteriorates due to the heat. 
     Particularly in recent years, a solder reflow temperature is increasing due to lead-free soldering in consideration of environmental problems, and heat caused by the solder reflow causes the piezoelectric performance of the piezoelectric material to deteriorate, which is problematic. However, Patent Documents 1 and 2 above give no consideration to the effect of heat. 
     Further, there is a problem that when the piezoelectric member is to have a film thickness of 1 μm or more as described in Patent Document 2, a possibility of cracks being caused or crystallinity being deteriorated increases. The deterioration of the crystallinity may also become a cause of the depolarization due to the heating processing. 
     In view of the above circumstances, there is a need for a piezoelectric device excellent in piezoelectric characteristics and heat resistance, an angular velocity sensor including the piezoelectric device, and a method of manufacturing a piezoelectric device. 
     According to an embodiment, there is provided a piezoelectric device including a piezoelectric film and an electrode film. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less, the piezoelectric film having a tension stress. The electrode film applies a voltage to the piezoelectric film. 
     By setting a PbO excessive composition ratio X of the PZT to be 0 or more and 0.3 or less and a Zr composition ratio Y to be 0 or more and 0.55 or less, a piezoelectric device excellent in piezoelectric characteristics can be obtained. If the Zr composition ratio Y is 0 or more and 0.55 or less, depolarization hardly occurs and excellent heat resistance can be obtained. 
     In addition, by providing the tension stress to the piezoelectric film, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the tension stress of the piezoelectric film may be 50 MPa or more and 500 MPa or less. Accordingly, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may have a film thickness of 400 nm or more and 1,000 nm or less. 
     Accordingly, a piezoelectric device with additionally-improved piezoelectric characteristics can be obtained. 
     In the piezoelectric device according to an embodiment, the electrode film may have a tension stress of 500 MPa or more and 1,500 MPa or less. 
     Accordingly, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may have an orientation of 80% or more in a &lt;111&gt; direction. 
     Accordingly, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may include at least one of additive elements selected from the group consisting of Cr, Mn, Fe, Ni, Mg, Sn, Cu, Ag, Nb, Sb, and N. 
     In the piezoelectric device according to an embodiment, the electrode film may be formed of at least one of Ti and Pt. The electrode film may also be formed of Ir, Au, and Ru, or oxides of Ti, Pt, Ir, Au, and Ru. 
     According to another embodiment, there is provided a piezoelectric device including a piezoelectric film and an electrode film. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less. The electrode film has a tension stress of 500 MPa or more and 1,500 MPa or less and applies a voltage to the piezoelectric film. 
     By setting the PbO excessive composition ratio X of the PZT to be 0 or more and 0.3 or less and the Zr composition ratio Y to be 0 or more and 0.55 or less, a piezoelectric device excellent in piezoelectric characteristics can be obtained. If the Zr composition ratio Y is 0 or more and 0.55 or less, depolarization hardly occurs and excellent heat resistance can be obtained. 
     In addition, by providing the tension stress of 500 MPa or more and 1,500 MPa or less to the electrode film, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may have a film thickness of 400 nm or more and 1,000 nm or less. 
     Accordingly, a piezoelectric device with additionally-improved piezoelectric characteristics can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may have a tension stress of 50 MPa or more and 500 MPa or less. 
     Accordingly, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may have an orientation of 80% or more in a &lt;111&gt; direction. 
     Accordingly, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may include at least one of additive elements selected from the group consisting of Cr, Mn, Fe, Ni, Mg, Sn, Cu, Ag, Nb, Sb, and N. 
     In the piezoelectric device according to an embodiment, the electrode film may be formed of at least one of Ti and Pt. The electrode film may also be formed of Ir, Au, and Ru, or oxides of Ti, Pt, Ir, Au, and Ru. 
     According to another embodiment, there is provided a piezoelectric device including a piezoelectric film and an electrode film. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less, the piezoelectric film having a film thickness of 400 nm or more and 1,000 nm or less. The electrode film applies a voltage to the piezoelectric film. 
     By setting the PbO excessive composition ratio X of the PZT to be 0 or more and 0.3 or less and the Zr composition ratio Y to be 0 or more and 0.55 or less, a piezoelectric device excellent in piezoelectric characteristics can be obtained. If the Zr composition ratio Y is 0 or more and 0.55 or less, depolarization hardly occurs and excellent heat resistance can be obtained. 
     In addition, by setting the film thickness to be 400 nm or more and 1,000 nm or less, a piezoelectric device with additionally-improved piezoelectric characteristics can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may have an orientation of 80% or more in a &lt;111&gt; direction. 
     Accordingly, a piezoelectric device with additionally-improved heat resistance can be obtained. 
     In the piezoelectric device according to an embodiment, the piezoelectric film may include at least one of additive elements selected from the group consisting of Cr, Mn, Fe, Ni, Mg, Sn, Cu, Ag, Nb, Sb, and N. 
     In the piezoelectric device according to an embodiment, the electrode film may be formed of at least one of Ti and Pt. The electrode film may also be formed of Ir, Au, and Ru, or oxides of Ti, Pt, Ir, Au, and Ru. 
     According to another embodiment, there is provided an angular velocity sensor including a substrate, a first electrode film, a piezoelectric film, and a second electrode film. The first electrode film is formed on the substrate. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less, the piezoelectric film having a tension stress and formed on the first electrode film. The second electrode film is formed on the piezoelectric film. 
     According to another embodiment, there is provided an angular velocity sensor including a substrate, a first electrode film, a piezoelectric film, and a second electrode film. The first electrode film has a tension stress of 500 MPa or more and 1,500 MPa or less and is formed on the substrate. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less, the piezoelectric film formed on the first electrode film. The second electrode film is formed on the piezoelectric film. 
     According to another embodiment, there is provided an angular velocity sensor including a substrate, a first electrode film, a piezoelectric film, and a second electrode film. The first electrode film is formed on the substrate. The piezoelectric film is constituted of lead zirconium titanate represented by Pb 1+X (Zr Y Ti 1−Y )O 3+X , where X is 0 or more and 0.3 or less and Y is 0 or more and 0.55 or less, the piezoelectric film having a film thickness of 400 nm or more and 1,000 nm or less and formed on the first electrode film. The second electrode film is formed on the piezoelectric film. 
     According to an embodiment, a piezoelectric device excellent in piezoelectric characteristics and heat resistance, an angular velocity sensor including the piezoelectric device, and a method of manufacturing a piezoelectric device can be provided. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  are diagrams showing a piezoelectric device and an angular velocity sensor including the piezoelectric device according to a first embodiment; 
         FIG. 2  is a diagram showing an XRD (X-ray diffraction) pattern of a PZT thin film; 
         FIG. 3  is a diagram showing a relationship between a film thickness (100 nm to 1,400 nm) of the PZT thin film and a piezoelectric constant d31; 
         FIG. 4  is a diagram showing relationships of a PbO excessive composition ratio X (−0.1 to 0.5) of the PZT thin film with the piezoelectric constant d31 and a loss rate tan δ; 
         FIG. 5  is a diagram showing a relationship between a Zr composition ratio Y (0.35 to 0.65) of the PZT thin film and the piezoelectric constant d31; 
         FIG. 6  is a diagram showing a relationship between the Zr composition ratio Y (0.35 to 0.7) of the PZT thin film and an attenuation rate of a vibration arm after application of heat; 
         FIG. 7  is a diagram showing a relationship between a heating time and the amplitude attenuation rate after application of heat in a case where a heating temperature is set to 240°; 
         FIG. 8  is a diagram showing a relationship between the heating temperature and the amplitude attenuation rate after application of heat; 
         FIG. 9  is a diagram showing a relationship between a stress (−100 MPa to 600 MPa) of the PZT thin film and the amplitude attenuation rate after application of heat; 
         FIG. 10  is a diagram showing a relationship between a stress (−500 MPa to 2,000 MPa) of a first electrode film and the amplitude attenuation rate after application of heat; 
         FIG. 11  is a diagram showing a relationship between an orientation degree of the PZT in a &lt;111&gt; surface direction and the amplitude attenuation rate of the vibration arm after application of heat; 
         FIG. 12  is a diagram showing a relationship between a ratio of a polarization voltage in polarization processing with respect to a coercive electric field (1 to 20-times the coercive electric field E c ) and the amplitude attenuation rate of the vibration arm after application of heat; 
         FIG. 13  is a diagram showing a relationship between a ratio of a withstanding voltage of the PZT thin film with respect to the coercive electric field and a polarization temperature; 
         FIG. 14  is a diagram showing a relationship between a ratio of the polarization temperature in the polarization processing with respect to the Curie temperature ( 1/16 to 5/4-times the Curie temperature T c ) and the amplitude attenuation rate after application of heat; 
         FIG. 15  is a diagram showing relationships of a ratio of a prebake temperature with respect to the Curie temperature (½ to 5/4 the Curie temperature T c ), with the amplitude attenuation rate after application of heat and a post-baking/post-polarization amplitude attenuation rate; 
         FIG. 16  is a plan diagram showing an angular velocity sensor according to a second embodiment; 
         FIG. 17  is a schematic diagram of the angular velocity sensor shown in  FIG. 16 ; and 
         FIG. 18  is a cross-sectional diagram taken along the line A-A of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment will be described with reference to the drawings. 
     First Embodiment 
     A first embodiment will be described.  FIG. 1  are diagrams showing a piezoelectric device and an angular velocity sensor including the piezoelectric device according to the first embodiment. 
     An angular velocity sensor  31  includes a base body  130  and a vibration arm  132  that extends from the base body  130  and is capable of vibrating.  FIG. 1B  is a cross-sectional diagram of a surface vertical to a longitudinal axis (Z axis) of the vibration arm  132 . 
     The angular velocity sensor  31  includes a semiconductor arm base  133  made of, for example, silicon, and a piezoelectric device  139  disposed on the arm base  133 . As shown in  FIG. 1B , for example, a first electrode film  34   a  as a common electrode is laminated on a silicon substrate, and a piezoelectric film  33  is laminated on the first electrode film  34   a . On a first surface  33   a  as an upper surface of the piezoelectric film  33 , a second electrode film  34   b , a first detection electrode  34   c , and a second detection electrode  34   d  each having a predetermined elongated shape are formed. 
     Also on the base body  130 , a lead electrode including lead wires  136 , electrode pads  138 , bumps  134   a  to  134   d , and the like is formed. The bump  134   a  is connected to the second electrode film  34   b , the bumps  134   b  and  134   c  are respectively connected to the first detection electrode  34   c  and the second detection electrode  34   d , and the bump  134   d  is connected to the first electrode film  34   a . An external connection to a control circuit (not shown) such as an IC is made via the bumps  134   a  to  134   d . The bumps  134   a  to  134   d  are each formed of metal, for example, but are not limited thereto. 
     After the first electrode film  34   a , the second electrode film  34   b , the first detection electrode  34   c , the second detection electrode  34   d , the lead wires  136 , and the like are formed as described above, the angular velocity sensor  31  of a shape as shown in  FIG. 1A  is cut out from a silicon wafer. 
     Next, a typical example of an operation of the angular velocity sensor  31  will be described. 
     The first electrode film  34   a  of the piezoelectric device  139  is connected to a DC power supply, and an AC power supply is connected between a first electrode film  34   a  and the second electrode film  34   b . Accordingly, a voltage is applied to the piezoelectric film  33  disposed between the first electrode film  34   a  and the second electrode film  34   b  so that the vibration arm  132  is caused of a flexion movement in a vertical direction (Y direction). 
     When an angular velocity ω 0  is applied to the vibration arm  132  during the flexion movement, Coriolis force is generated in the vibration arm  132 . The Coriolis force is generated in a direction vertical (X direction) to a direction of the flexion movement of the vibration arm  132  (Y direction), a magnitude of which is proportional to a value of the applied angular velocity ω 0 . The Coriolis force is converted into an electric signal by the piezoelectric film  33 , and the converted signal is detected by the first detection electrode  34   c  and the second detection electrode  34   d.    
     Next, piezoelectric performance and heat resistance performance of the piezoelectric device  139  will be described while describing a method of manufacturing the angular velocity sensor  31 . It should be noted that descriptions will mainly be given on a method of forming the piezoelectric device  139  formed on the arm base  133 . 
     First, a silicon wafer is prepared. An oxidation protection film may be formed on the silicon wafer by thermal oxidation processing. 
     The first electrode film  34   a  is formed by depositing Ti of 30 nm on the silicon wafer by a sputtering method, and then depositing Pt of 100 nm thereon, for example. The deposition method of the first electrode film  34   a  is not limited to the sputtering method, and a vacuum vapor deposition method or other deposition methods may be used. Moreover, the metal materials that constitute the first electrode film  34   a  are not limited to Ti and Pt, and examples thereof include Ir, Au, and Ru, or oxides of Ti, Pt, Ir, Au, and Ru. The second electrode film  34   b  may also be constituted of those metal materials. 
     Subsequently, the piezoelectric film  33  is formed by forming a PZT thin film on the first electrode film  34   a  by, for example, the sputtering method. The deposition method of the piezoelectric film  33  is not limited to the sputtering method, and deposition methods such as a vacuum vapor deposition method, a PLD (pulsed laser deposition) method, a sol-gel method, an aerosol deposition method, and the like may be used. A substrate temperature when depositing the PZT thin film  33  may either be at room temperature or at a high temperature. 
     In the deposition of the PZT thin film  33 , a PbO excessive composition ratio X is set to be −0.1 or more and 0.5 or less, and a Zr composition ratio Y is set to be 0.35 or more and 0.65 or less. For realizing such a PZT composition ratio, a target composition, sputtering conditions, annealing conditions, and the like are set appropriately. For increasing a perovskite structure of the PZT after the PZT thin film  33  is formed on the first electrode film  34   a , heating processing at 700°, for example, may be carried out on the PZT thin film  33 . A crystalline structure of the PZT thin film  33  in this case is tetragonal. 
     A film thickness of the PZT thin film  33  formed as described above is 100 nm to 1,400 nm. 
     After the PZT thin film  33  is formed, Pt of 200 nm is deposited on the PZT thin film  33  by the sputtering method to thus form the second electrode film  34   b . The deposition method of the second electrode film  34   b  in this case is not limited to the sputtering method, and a vacuum vapor deposition method or other deposition methods may be used. 
     Next, a voltage is applied between the first electrode film  34   a  and the second electrode film  34   b  in an atmosphere heated to 240°, and polarization processing is carried out on the PZT thin film  33 . The voltage applied between the first electrode film  34   a  and the second electrode film  34   b  is 1 to 20-times as large as a coercive electric field E c . Moreover, a polarization temperature in the polarization processing is, compared to a Curie temperature, 1/16 to 5/4 the Curie temperature. It should be noted that the polarization processing may be carried out in any of an atmosphere, an oxygen atmosphere, and a nitrogen atmosphere. 
     After the polarization processing, prebake processing is carried out on the deposited PZT thin film  33 . A prebake temperature of the prebake processing is ½ to 5/4 the Curie temperature. 
     The PZT thin film  33  described above may have a tension stress. For providing the tension stress to the PZT thin film  33 , the PZT thin film  33  may be subjected to heating processing at, for example, 650° C. to 750° C. after being formed on the first electrode film  34   a . Accordingly, crystallization of the PZT thin film  33  is accelerated, thus obtaining the tension stress. In addition, in this case, the target composition, the sputtering conditions, the annealing conditions, and the like are set appropriately such that the PbO excessive composition ratio X of the PZT becomes 0.04, the Zr composition ratio Y thereof becomes 0.35 to 0.65, and the tension stress becomes −100 MPa to 600 MPa, for example. 
     Further, the first electrode film  34   a  described above may also have the tension stress. For providing the tension stress to the first electrode film  34   a , the first electrode film  34   a  may be subjected to the heating processing at, for example, 100° C. to 800° C. after the PZT thin film  33  is formed thereon. Alternatively, it is also possible to provide the tension stress to the first electrode film  34   a  by the heating processing carried out during deposition instead of after the deposition of the first electrode film  34   a  and the PZT thin film  33 . By changing deposition conditions, heating processing conditions, and the like, a tension stress of a wide range can be provided to the first electrode film  34   a . The tension stress of the first electrode film  34   a  formed as described above is −200 MPa to 2,000 MPa. 
     (Evaluation of Piezoelectric Characteristics) 
     Next, descriptions will be given on piezoelectric characteristics of the piezoelectric device  139  thus formed on the silicon wafer. 
       FIG. 2  is a diagram showing an XRD (X-ray diffraction) pattern of the PZT thin film  33 . The PZT is oriented to a &lt;111&gt; surface and has an orientation degree of 97%. In  FIG. 2 , the film thickness of the PZT thin film  33  whose XRD pattern has been measured is 900 nm, the voltage used in the polarization processing is 6-times the coercive electric field, and the polarization temperature is 240° C. In addition, the prebake temperature is 200° C. for 100 s. 
     It should be noted that in the following descriptions made on the figures, unless specified otherwise, the film thickness of the PZT thin film  33  is 900 nm. 
       FIG. 3  is a diagram showing a relationship between the film thickness (100 nm to 1,400 nm) of the PZT thin film  33  and a piezoelectric constant d31. As shown in  FIG. 3 , the PZT thin film  33  shows favorable piezoelectric characteristics when the film thickness is 400 nm to 1,000 nm. Consequently, piezoelectric characteristics sufficient for the piezoelectric device  139  of the angular velocity sensor  31  can be obtained when the film thickness is within the range of 400 nm to 1,000 nm. 
     The piezoelectric constant d31 decreases when the film thickness of the PZT thin film  33  is 1,000 nm or more. This is probably because when the film thickness is 1,000 nm or more, crystals grow in a direction other than the &lt;111&gt; surface direction, such as a &lt;001&gt; surface direction, and thus a peak intensity in the &lt;111&gt; surface direction is saturated. Therefore, by setting the film thickness of the PZT thin film  33  to be less than 1,000 nm, peak growths in directions other than the &lt;111&gt; surface direction can be suppressed. It should be noted that a main peak of a crystal mainly contributes to the piezoelectric characteristics. 
     Meanwhile, the film thickness of less than 400 nm leads to an increase in leak current, whereby it becomes difficult to obtain piezoelectric characteristics sufficient for the piezoelectric device  139 . 
       FIG. 4  is a diagram showing relationships of the PbO excessive composition ratio X (−0.1 to 0.5) of the PZT thin film  33  with the piezoelectric constant d31 and a loss rate tan δ. The orientation degree of the PZT thin film  33  in the &lt;111&gt; surface direction is 80% or more and less than 100%, and the Zr composition ratio Y is 0.5. 
     It can be seen from  FIG. 4  that the piezoelectric constant d31 and the loss rate tan δ are both favorable when the PbO excessive composition ratio X is within the range of 0 to 0.3. The piezoelectric characteristics deteriorate when the PbO excessive composition ratio X is less than 0. This is probably because PZT crystallinity deteriorates when the PbO excessive composition ratio X is small. On the other hand, the loss rate tan δ increases and the piezoelectric characteristics deteriorate when the PbO excessive composition ratio X is 0.3 or more. This is probably because an insulation property of the PZT thin film  33  deteriorates when the PbO excessive composition ratio X is large, thus resulting in a decrease in piezoelectric characteristics. 
       FIG. 5  is a diagram showing a relationship between the Zr composition ratio Y (0.35 to 0.65) of the PZT thin film  33  and the piezoelectric constant d31. As shown in  FIG. 5 , the PZT thin film  33  shows maximum piezoelectric characteristics when the Zr composition ratio Y is 0.51 and favorable piezoelectric characteristics when the Zr composition ratio Y is 0.4 or more and 0.55 or less. As long as the Zr composition ratio Y is 0.4 or more and 0.55 or less, piezoelectric characteristics sufficient for the piezoelectric device  139  of the angular velocity sensor  31  can be obtained. 
     Incidentally, it is known that a bulk PZT shows favorable piezoelectric characteristics when the Zr composition ratio Y thereof is 0.5 or more and 0.53 or less. However, the piezoelectric characteristics of the bulk PZT deteriorate precipitously when the Zr composition ratio Y becomes less than 0.5. On the other hand, as shown in  FIG. 5 , the PZT thin film deposited by, for example, the sputtering method shows favorable piezoelectric characteristics even when the Zr composition ratio Y is 0.4 or more and 0.5 or less. 
     (Evaluation of Heat Resistance) 
     Next, an evaluation on the heat resistance will be described, but first, descriptions will be given on a method of evaluating heat resistance. 
     The angular velocity sensor  31  of a shape as shown in  FIG. 1A  is cut out from the silicon wafer on which the piezoelectric device  139  including the first electrode film  34   a , the PZT thin film  33 , and second electrode film  34   b , the lead wires  136 , and the like are formed. An MEMS (Micro Electro Mechanical Systems) technique is typically used for the cutout from the silicon wafer. It should be noted that length, width, and thickness of the vibration arm  132  are, for example, 2,000 μm, 150 μm, and 150 μm, respectively. 
     The heat resistance is evaluated by measuring an amplitude of the vibration arm  132  of the thus-formed angular velocity sensor  31  in the Y direction. Specifically, the heat resistance of the piezoelectric device  139  is evaluated by measuring the amplitude of the vibration arm  132  in the Y direction, applying to the PZT thin film  33  heat that takes into account the heating processing carried out at the time of manufacturing the device, such as solder reflow, and re-measuring the amplitude of the vibration arm  132  in the Y direction thereafter. It should be noted that the heat applied to the PZT thin film  33  is, considering the heating processing at the time of manufacturing the device, 180° C. to 300° C., and a heating time thereof is 30 s to 300 s. In addition, the voltage applied between the first electrode film  34   a  and second electrode film  34   b  is an AC voltage of, for example, 1 kHz, 1V. 
       FIG. 6  is a diagram showing a relationship between the Zr composition ratio Y (0.35 to 0.7) of the PZT thin film  33  and an attenuation rate of the vibration arm  132  after application of heat. The heating temperature and the heating time are 240° C. and 90 s, respectively. 
     It can be seen from  FIG. 6  that attenuation of the amplitude of the vibration arm  132  after application of heat increases when the Zr composition ratio Y exceeds 0.55, whereas the attenuation thereof is hardly observed when the Zr composition ratio Y is 0.55 or less. In other words, the PZT thin film  33  with the Zr composition ratio Y of 0.55 or less has excellent heat resistance. 
       FIG. 7  is a diagram showing a relationship between the heating time and the amplitude attenuation rate after application of heat in a case where the heating temperature is set to 240°. The Zr composition ratio Y of the PZT thin film  33  is within the range of 0.35 to 0.60. It can be seen from  FIG. 7  that when the Zr composition ratio Y is 0.55 or less, the amplitude attenuation after application of heat hardly occurs even when the heating time is extended, which implies excellent heat resistance. 
       FIG. 8  is a diagram showing a relationship between the heating temperature and the amplitude attenuation rate after application of heat. The Zr composition ratio Y of the PZT thin film  33  is within the range of 0.35 to 0.60. It can be seen from  FIG. 8  that when the Zr composition ratio Y is 0.55 or less, the amplitude attenuation after application of heat hardly occurs even when the heating temperature is increased, which implies excellent heat resistance. 
       FIG. 9  is a diagram showing a relationship between a stress (−100 MPa to 600 MPa) of the PZT thin film  33  and the amplitude attenuation rate after application of heat. In this case, the tension stress of the first electrode film  34   a  is 1,000 MPa. In addition, the heating temperature and heating time of the PZT thin film  33  are 240° C. and 90 s, respectively. It should be noted that in  FIG. 9 , a stress of a positive value represents a tension stress, and a stress of a negative value represents a compression stress. 
     Now, a method of measuring a stress of the PZT thin film  33  will be described. An X-ray reciprocal lattice map measurement method is used as the method of measuring a stress of the PZT thin film  33 , and an X-ray diffraction apparatus X&#39;pert PRO MRD (registered trademark) from PANalytical (registered trademark) is used as a measurement apparatus therefor. In the reciprocal lattice map technique, a measurement target sample is tilted about a φ axis orthogonal to a θ axis, and diffraction from a crystal surface of the sample is detected. Identification of the measurement target sample is made based on the detected diffraction peak. 
     For example, in a case where there is no distortion or stress in the crystal of the measurement target sample, no change in diffraction angle occurs at any φ angle regarding a main orientation peak of the PZT &lt;111&gt; diffraction (in the vicinity of (2θ, φ)=(38°, 0°), (2θ, φ)=(38°, 70°)). However, in a case where there is a tension stress in the measurement target sample, the diffraction angle when φ=70° shifts more on a low-angle side than the diffraction angle when φ=0. On the other hand, in a case where there is a compression stress in the measurement target sample, the diffraction angle when φ=70° shifts more on a wide-angle side than the diffraction angle when φ=0. By evaluating a magnitude of the shift, it is possible to measure the stress of the PZT thin film  33 . 
     The method of measuring a stress of the PZT thin film  33  is not limited to the X-ray reciprocal lattice map measurement method, and other methods may be used instead. For example, as described in the following reference, a value of the stress may be evaluated by using Stoney Expression after measuring a warpage of a substrate on which a film is deposited (reference: “Basics and Application of Thin Films by a Plasma Process”, Hiroshi Ichimura, Masaru Ikenaga, THE NIKKAN KOGYO SHIMBUN, LTD., 2005). For measuring Young&#39;s modulus necessary for derivation of a stress, a nanoindentation method is used as described in the reference, for example. The X-ray reciprocal lattice map measurement method, the measurement method described in the reference, or the like is also used as a measurement method of the first electrode film  34   a  to be described later. 
     As shown in  FIG. 9 , the amplitude after application of heat is not attenuated when the tension stress of the PZT thin film  33  is 50 MPa or more and 500 MPa or less. In other words, the PZT thin film  33  with the tension stress of 50 MPa to 500 MPa has favorable heat resistance. In particular, the PZT thin film  33  with the tension stress of 100 MPa to 300 MPa has favorable heat resistance. 
     The reason why the PZT thin film  33  has favorable heat resistance when provided with the tension stress that is within the range described above is that a crystal lattice of the PZT is distorted to thus suppress a movement of domains. 
     As shown in  FIG. 9 , the amplitude after application of heat is attenuated when the tension stress of the PZT thin film  33  exceeds 500 MPa. This is probably because cracks increase due to the stress of the PZT thin film  33 , and the distortion of the crystal lattice is thus eliminated. On the other hand, the amplitude after application of heat is attenuated when the tension stress is less than 50 MPa. This is probably because the movement of domains is facilitated since there is no distortion in crystal lattice due to a low stress of the PZT thin film  33 . 
       FIG. 10  is a diagram showing a relationship between a stress (−500 MPa to 2,000 MPa) of the first electrode film  34   a  and the amplitude attenuation rate after application of heat. In this case, the tension stress of the PZT thin film  33  is 200 MPa. In addition, the heating temperature and heating time of the PZT thin film  33  are 240° C. and 90 s, respectively. 
     As shown in  FIG. 10 , the amplitude after application of heat is not attenuated when the tension stress of the first electrode film  34   a  is 500 MPa or more and 1,500 MPa or less. In other words, it can be seen that the piezoelectric device  139  including the first electrode film  34   a  with the tension stress of 500 MPa to 1,500 MPa has favorable heat resistance. In particular, it can be seen that when the first electrode film  34   a  has a tension stress of 700 MPa to 1,200 MPa, the PZT thin film  33  formed on the first electrode film  34   a  has favorable heat resistance. 
     The reason why the piezoelectric device  139  has favorable heat resistance when the first electrode film  34   a  is provided with the tension stress that is within the range described above is that a crystal lattice of the PZT thin film  33  is distorted to an appropriate degree by the tension stress of the first electrode film  34   a , to thus suppress the movement of domains. 
     The amplitude after application of heat is attenuated when the tension stress of the first electrode film  34   a  exceeds 1,500 MPa. This is probably because cracks of the PZT thin film  33  increase by the tension stress of first electrode film  34   a , and the distortion of the crystal lattice is thus eliminated. In this case, cracks have actually been observed on the surface of the PZT thin film  33 . Moreover, a peeling has been observed between the first electrode film  34   a  having the tension stress of more than 1,500 MPa and the arm base  133 . 
     On the other hand, the amplitude after application of heat is attenuated also when the tension stress of the first electrode film  34   a  is less than 500 MPa. This is probably because the movement of domains is facilitated since there is no distortion in crystal lattice due to a low stress of the first electrode film  34   a.    
       FIG. 11  is a diagram showing a relationship between an orientation degree of the PZT in the &lt;111&gt; surface direction and the amplitude attenuation rate of the vibration arm  132  after application of heat. The PbO excessive composition ratio X and Zr composition ratio Y of the PZT thin film  33  are 0.04 and 0.48, respectively. It can be seen from  FIG. 11  that when the orientation degree of the PZT in the &lt;111&gt; surface direction is 80% or more, the amplitude attenuation after application of heat hardly occurs, which implies excellent heat resistance. On the other hand, it can be seen that when the orientation degree of the PZT in the &lt;111&gt; surface direction is less than 80%, the amplitude attenuation after application of heat is apt to occur. 
     Next, descriptions will be given on relationships of the amplitude attenuation rate of the vibration arm  132  after application of heat with polarization processing conditions and prebake conditions. It should be noted that in  FIGS. 12 to 15  in descriptions below, the PbO excessive composition ratio X and Zr composition ratio Y of the PZT thin film  33  are 0.04 and 0.48, respectively. Moreover, the heating temperature and heating time of the PZT thin film  33  are 240° C. and 90 s, respectively. 
       FIG. 12  is a diagram showing a relationship between a ratio of a polarization voltage in the polarization processing with respect to a coercive electric field (1 to 20-times the coercive electric field E c ) and the amplitude attenuation rate of the vibration arm  132  after application of heat. As shown in  FIG. 12 , the amplitude attenuation after application of heat hardly occurs when the polarization voltage is 2 to 20-times the coercive electric field E c , which implies excellent heat resistance.  FIG. 13  shows a relationship between a ratio of a withstanding voltage of the PZT thin film  33  with respect to the coercive electric field and the polarization temperature. As shown in  FIG. 13 , an increase in polarization temperature leads to a decrease in ratio of the withstanding voltage of the PZT with respect to the coercive electric field. A dielectric breakdown of the PZT occurs when a polarization voltage that is 20-times or more the coercive electric field is applied to the PZT at the polarization temperature of 180° C. or more. Therefore, it can be seen that applying the polarization voltage that is 20-times or more the coercive electric field E c  to the PZT is inappropriate, and an appropriate polarization voltage is a voltage that is 2 to 20-times the coercive electric field E c . 
       FIG. 14  is a diagram showing a relationship between a ratio of the polarization temperature in the polarization processing with respect to a Curie temperature ( 1/16 to 5/4 the Curie temperature T c ) and the amplitude attenuation rate after application of heat. The polarization voltage is 6-times the coercive electric field E c . As shown in  FIG. 14 , when the polarization temperature is ¼ or more and equal to or smaller than the Curie temperature T c , the amplitude attenuation after application of heat hardly occurs, which implies excellent heat resistance. The reason why the amplitude attenuation is large when the polarization temperature is less than ¼ the Curie temperature T c  is probably because, due to insufficient polarization processing, the movement of domains of the PZT thin film  33  is suppressed. On the other hand, the reason why the amplitude attenuation is large when the polarization temperature exceeds the Curie temperature T c  is probably because, due to a cubic crystalline structure of the PZT thin film  33 , the movement of domains is facilitated after the polarization processing. 
       FIG. 15  is a diagram showing relationships of a ratio of a prebake temperature Ta with respect to the Curie temperature (½ to 5/4 the Curie temperature T c ), with the amplitude attenuation rate after application of heat (abscissa axis and right-hand side ordinate axis) and a post-baking/post-polarization amplitude attenuation rate (abscissa axis and left-hand side ordinate axis). 
     Specifically, in  FIG. 15 , the amplitude attenuation rate in the prebake processing is evaluated by measuring the amplitude of the vibration arm  132  after the polarization processing, and re-measuring the amplitude of the vibration arm  132  after the prebake processing (½ to 5/4 the Curie temperature T c ) (abscissa axis and left-hand side ordinate axis). After that, heat that takes into account the heating processing at the time of manufacturing the device is applied to the PZT thin film  33  that has been subjected to the prebake processing, the amplitude of the vibration arm  132  after application of heat is measured, and the amplitude attenuation rate after application of heat is thus measured (abscissa axis and right-hand side ordinate axis). It should be noted that the polarization voltage is 6-times the coercive electric field E c , and the polarization temperature is 260° C. 
     As shown in  FIG. 15 , it can be seen that when the prebake temperature Ta is ¾ or less the Curie temperature T c , the amplitude of the vibration arm  132  after the prebake processing is not attenuated as much as that after the polarization processing. Moreover, it can be seen that when the prebake temperature Ta is ¼ or more the Curie temperature T c , the amplitude after application of heat is not attenuated as much as that after the polarization processing, meaning that the piezoelectric device  139  has excellent heat resistance. Therefore, by setting the prebake temperature Ta in the prebake processing to be ¼ or more and ¾ or less the Curie temperature T c , it becomes possible to obtain a piezoelectric device  139  with excellent heat resistance. 
     Second Embodiment 
     Next, a second embodiment will be described. 
       FIG. 16  is a plan diagram showing an angular velocity sensor according to this embodiment. In addition,  FIG. 17  is a schematic diagram of the angular velocity sensor according to this embodiment, and  FIG. 18  is a cross-sectional diagram taken along the line A-A of  FIG. 16 . 
     As shown in the figures, an angular velocity sensor  200  includes a base body  214 , an arm retention portion  215  provided on one side of the base body  214 , and a vibration arm portion  216  provided on a tip end side of the arm retention portion  215 . 
     The vibration arm portion  216  includes a first vibration arm  211 , and second and third vibration arms  212  and  213  sandwiching the first vibration arm  211 . The first vibration arm  211  is constituted of an arm base  210   a  and a piezoelectric device  239   a  formed thereon, the second vibration arm  212  is constituted of an arm base  210   b  and a piezoelectric device  239   b  formed thereon, and the third vibration arm  213  is constituted of an arm base  210   c  and a piezoelectric device  239   c  formed thereon. In other words, the angular velocity sensor  200  according to this embodiment is a so-called triple-branch tuning-fork type angular velocity sensor. 
     The first to third vibration arms  211  to  213  have the same width and thickness, for example. Moreover, a gap between the first and second vibration arms  211  and  212  and a gap between the first and third vibration arms  211  and  213  are the same. 
     As shown in  FIG. 18 , first electrode films  221  to  223  are respectively formed on the arm bases  210   a  to  210   c , and PZT thin films  231  to  233  each as a piezoelectric film are respectively formed on the first electrode films  221  to  223 . Further, second electrode films  241  to  243  each as a drive electrode are respectively formed on the PZT thin films  231  to  233 . Moreover, a first detection electrode  251  and a second detection electrode  252  are formed on the piezoelectric thin film  231  of the first vibration arm  211  in the middle of the vibration arm portion  216 . 
     A film thickness of each of the PZT thin films  231  to  233  and the PbO excessive composition ratio X and Zr composition ratio Y of the PZT are the same as that of the PZT thin film  33  according to the first embodiment. Further, the PZT thin films  231  to  233  each have a tension stress of the same level as the PZT thin film  33 . Furthermore, the first electrode films  221  to  223  also have a tension stress of the same level as the first electrode film  34   a  of the first embodiment. 
     The plurality of electrodes  221  to  223 ,  241  to  243 ,  251 , and  252  included in the respective piezoelectric devices  239  are respectively connected to lead wires  261  to  268 . The lead wires  261  to  268  pass through a surface of the arm retention portion  215  to be respectively connected to lead terminals  271  to  278  provided on a surface of the base body  214 . The lead terminals  271  to  278  are provided four each on both sides in an X direction on the surface of the base body  214 . 
     Next, an operation of the angular velocity sensor  200  according to this embodiment will be described. 
     The first vibration arm  211  is caused of a flexion movement in the vertical direction of  FIG. 18  when a voltage is applied to the first electrode film  221  and the second electrode film  241 . Meanwhile, the second and third vibration arms  212  and  213  are caused of a flexion movement in the vertical direction at a phase opposite to that of the first vibration arm  211 , when a voltage is applied to the first electrode films  222  and  223  and second electrode films  242  and  243 . 
     Specifically, the second and third vibration arms  212  and  213  move downward when the first vibration arm  211  move upward, and the second and third vibration arms  212  and  213  move upward when the first vibration arm  211  move downward. Moreover, by the second and third vibration arms  212  and  213  being caused of the flexion movement at an amplitude half the amplitude of the first vibration arm  211 , moments generated by the first to third vibration arms  211  to  213  are canceled out. 
     As a result of evaluating the piezoelectric devices  239  of the thus-structured angular velocity sensor  200  in the same manner as in  FIGS. 2 to 15 , it has been confirmed that each of the piezoelectric devices  239  has the same piezoelectric performance and heat resistance as the piezoelectric device  139  of the angular velocity sensor  31  according to the first embodiment. 
     It should be noted that although the second electrode films  241  to  243  for driving the respective vibration arms are provided to the respective vibration arms in this embodiment, it is also possible to form the second electrode film on only the first vibration arm  211 , for example. In this case, the second and third vibration arms  212  and  213  vibrate at phases opposite to that of the first vibration arm  211  by a counteraction of the vibration of the first vibration arm  211 . 
     Alternatively, it is also possible to form the second electrode films on only the second and third vibration arms  212  and  213 . In this case, the first vibration arm  211  vibrates at a phase opposite to that of the second and third vibration arms  212  and  213  by a counteraction of the vibration of the second and third vibration arms  212  and  213 . 
     The piezoelectric device and angular velocity sensor described above are not limited to the above embodiments, and various modifications can be made. 
     For example, in the deposition of the PZT thin film  33  above, although the PZT is formed so as to have an orientation in the &lt;111&gt; surface direction, the present application is not limited thereto, and the PZT may be deposited so as to have an orientation in a &lt;100&gt; surface direction or a &lt;001&gt; surface direction. Even when the PZT is deposited as described above, a piezoelectric device  139  with excellent piezoelectric characteristics and heat resistance can still be obtained. 
     In the above embodiments, descriptions have been given on the case where the crystalline structure of the PZT thin film  33  is tetragonal. However, the crystalline structure of the PZT thin film  33  may be rhombohedral, pseudo tetragonal, pseudo rhombohedral, or the like. Moreover, the PZT thin film  33  may include at least one of additive elements selected from the group consisting of Cr, Mn, Fe, Ni, Mg, Sn, Cu, Ag, Nb, Sb, and N. 
     Instead of the angular velocity sensor  31 , the piezoelectric device  139  can also be applied to, for example, a pyroelectric infrared ray sensor, a liquid injection apparatus, a semiconductor storage apparatus, and the like. It should be noted that in this case, the piezoelectric device  139  only needs to be provided with at least one of the first and second electrode films, and the first and second detection electrodes do not necessarily need to be provided thereto. 
     The above embodiments respectively illustrate the so-called single-branch tuning-fork type angular velocity sensor  31  and triple-branch tuning-fork type angular velocity sensor  200 . However, the number of vibration arms may be 2 or more than 3. Alternatively, although the angular velocity sensors  31  and  200  each have a cantilever structure, the sensors may each have a center impeller structure. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.