Patent Publication Number: US-2023142065-A1

Title: Piezoelectric laminate, production method for piezoelectric laminate, and piezoelectric element

Description:
TECHNICAL FIELD 
     The present disclosure relates to a piezoelectric stack, a method of manufacturing a piezoelectric stack, and a piezoelectric element. 
     DESCRIPTION OF RELATED ART 
     Piezoelectric materials are used in an ultrasonic sensor of a p-MUT (Piezoelectric Micromachined Ultrasonic Transducer), for example. Widely used piezoelectric materials include a lead-zirconium-titanium oxide-based (PZT-based) ferroelectric (see Patent Document 1, for example). A potassium sodium niobium oxide-based (KNN-based) ferroelectric or aluminum nitride (AlN) is also sometimes used as a piezoelectric material used in ultrasonic sensors or the like (see Patent Document 2, for example). In recent years, there is a demand to further improve a performance in ultrasonic sensors including a piezoelectric material. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Laid Open Publication No.2019-146020 
     Patent Document 2: Japanese Patent Laid Open Publication No. 2019-165307 
     SUMMARY 
     Problem to be Solved by Disclosure 
     According to the present disclosure, there is provided a piezoelectric stack and the like that enable providing an ultrasonic sensor having a high performance. 
     Means for Solving Problem 
     According to an aspect of the present disclosure, there is provided a piezoelectric stack, a piezoelectric element, and an ultrasonic sensor, each including:
         a substrate;   an output-side bottom electrode film on the substrate;   an output-side piezoelectric film, being an oxide film, on the output-side bottom electrode film;   an output-side top electrode film on the output-side piezoelectric film;   an input-side bottom electrode film on the substrate;   an input-side piezoelectric film, being a nitride film, on the input-side bottom electrode film;   an input-side top electrode film on the input-side piezoelectric film; and   an ultrasonic output part and an ultrasonic input part placed in such a manner as not overlapping each other when viewed from a top surface of the substrate,
           the ultrasonic output part including a stacked part of the output-side bottom electrode film, the output-side piezoelectric film, and the output-side top electrode film,   the ultrasonic input part including a stacked part of the input-side bottom electrode film, the input-side piezoelectric film, and the input-side top electrode film.   
               

     According to another aspect of the prevention disclosure, there is provided a method of manufacturing a piezoelectric stack, including:
         depositing an output-side bottom electrode film and an input-side bottom electrode film on a substrate;   depositing an output-side piezoelectric film, being an oxide film, on the output-side bottom electrode film;   depositing a protective film that protects the output-side piezoelectric film;   depositing an input-side piezoelectric film, being a nitride film, on the input-side bottom electrode film;   exposing the output-side piezoelectric film by removing the protective film by etching; and   depositing an output-side top electrode film on the output-side piezoelectric film and depositing an input-side top electrode film on the input-side piezoelectric film, thereby   producing a stack in which an ultrasonic output part and an ultrasonic input part are placed in such a manner as not overlapping each other when viewed from a top surface of the substrate,
           the ultrasonic output part including a stacked part of the output-side bottom electrode film, the output-side piezoelectric film, and the output-side top electrode film,   the ultrasonic input part including a stacked part of the input-side bottom electrode film, the input-side piezoelectric film, and the input-side top electrode film.   
               

     Advantage of Disclosure 
     According to the present disclosure, there is provided a piezoelectric stack and a piezoelectric element that enable providing an ultrasonic sensor having a high performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a view illustrating an example of a cross-sectional structure of a piezoelectric stack according to an embodiment of the present disclosure. 
         FIG.  2    is a view illustrating a modified example of the cross-sectional structure of the piezoelectric stack according to the embodiment of the present disclosure. 
         FIG.  3    is a view illustrating an example of a schematic configuration of an ultrasonic sensor according to the embodiment of the present disclosure. 
         FIG.  4 ( a )  is a view illustrating an example of a cross-sectional structure of a stack  10   a  before depositing an output-side piezoelectric film,  FIG.  4 ( b )  is a view illustrating an example of a cross-sectional structure of a stack  10   b  after depositing the output-side piezoelectric film,  FIG.  4 ( c )  is a view illustrating an example of a cross-sectional structure of a stack  10   c  after shaping the output-side piezoelectric film into a predetermined form,  FIG.  4 ( d )  is a view illustrating an example of a cross-sectional structure of a stack  10   d  after depositing a protective film,  FIG.  4 ( e )  is a view illustrating an example of a cross-sectional structure of a stack  10   e  after depositing an input-side piezoelectric film, and  FIG.  4 ( f )  is a view illustrating an example of a cross-sectional structure of a stack  10   f  after removing the protective film. 
         FIG.  5    is a view illustrating a cross-sectional structure of a piezoelectric stack according to a modified example of an embodiment of the present disclosure. 
         FIG.  6 ( a )  is a view illustrating an example of a cross-sectional structure of a stack  40   a  before depositing an output-side piezoelectric film,  FIG.  6 ( b )  is a view illustrating an example of a cross-sectional structure of a stack  40   b  after depositing the output-side piezoelectric film and shaping the output-side piezoelectric film into a predetermined form,  FIG.  6 ( c )  is a view illustrating an example of a cross-sectional structure of a stack  40   c  after depositing a protective film,  FIG.  6 ( d )  is a view illustrating an example of a cross-sectional structure of a stack  40   d  after depositing an input-side bottom electrode film,  FIG.  6 ( e )  is a view illustrating an example of a cross-sectional structure of a stack  40   e  after depositing an input-side piezoelectric film, and  FIG.  6 ( f )  is a view illustrating an example of a cross-sectional structure of a stack  40   f  after removing the protective film. 
         FIG.  7    is a view illustrating a cross-sectional structure of a piezoelectric stack according to a modified example of an embodiment of the present disclosure. 
         FIG.  8    is a view illustrating a cross-sectional structure of a piezoelectric stack according to a modified example of an embodiment of the present disclosure. 
         FIG.  9 ( a )  is a view illustrating an example of a cross-sectional structure of a stack  42   a  before depositing an output-side piezoelectric film,  FIG.  9 ( b )  is a view illustrating an example of a cross-sectional structure of a stack  42   b  after depositing the output-side piezoelectric film,  FIG.  9 ( c )  is a view illustrating an example of a cross-sectional structure of a stack  42   c  after depositing a protective film,  FIG.  9 ( d )  is a view illustrating an example of a cross-sectional structure of a stack  42   d  after depositing an input-side piezoelectric film, and  FIG.  9 ( e )  is a view illustrating an example of a cross-sectional structure of a stack  42   e  after removing the protective film. 
         FIG.  10    is a view illustrating a cross-sectional structure of a piezoelectric stack according to a modified example of an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Embodiment of the Present Disclosure 
     An embodiment of the present disclosure will be described hereinafter, with reference to  FIGS.  1  to  4   . 
     (1) CONFIGURATION OF PIEZOELECTRIC STACK 
     As illustrated in  FIG.  1   , a stack (stacked substrate)  10  (hereinafter also referred to as a piezoelectric stack  10 ) having a piezoelectric film according to the present embodiment includes a substrate  1 , a bottom electrode film  2  provided (deposited) on the substrate  1 , an output-side piezoelectric film  3 A provided (deposited) on the bottom electrode film  2 , an output-side top electrode film  4 A provided (deposited) on the output-side piezoelectric film  3 A, an input-side piezoelectric film  3 B provided (deposited) on the bottom electrode film  2 , and an input-side top electrode film  4 B provided (deposited) on the input-side piezoelectric film  3 B. In the present embodiment, the bottom electrode film  2  functions as the output-side bottom electrode film  2 A and also functions as the input-side bottom electrode film  2 B. 
     The substrate  1  can suitably be a single-crystal silicon (Si) substrate  1   a  on which a surface oxide film (SiO 2 -film)  1   b  such as a thermal oxide film or a CVD (Chemical Vapor Deposition) oxide film is formed, i.e., a Si-substrate having the surface oxide film. Alternatively, the substrate  1  can be a Si-substrate  1   a  having an insulating film  1   d  formed on a surface thereof, the insulating film  1   d  containing an insulating material other than SiO 2 , as illustrated in  FIG.  2   . Alternatively, the substrate  1  can be the Si-substrate  1   a  in which Si-(100), Si-(111), or the like, is exposed on the surface thereof, i.e., a Si-substrate not having the surface oxide film  1   b  or the insulating film  1   d . Alternatively, the substrate  1  can be an SOI (Silicon-On-Insulator) substrate, a quartz glass (SiO 2 ) substrate, a gallium arsenide (GaAs) substrate, a sapphire (Al 2 O 3 ) substrate, or a metal substrate containing a metallic material such as stainless steel (SUS). A thickness of the single-crystal Si-substrate  1   a  can be, for example, 300 μm or more and 1000 μm or less, and a thickness of the surface oxide film  1   b  can be, for example, 1 nm or more and 4000 nm or less. 
     The bottom electrode film  2  can be deposited using platinum (Pt), for example. The bottom electrode film  2  is a single-crystal film or a polycrystalline film (hereinafter collectively also referred to as a Pt-film). Preferably, crystals constituting the Pt-film are preferentially oriented in (111) direction with respect to the surface of the substrate  1 . That is, a surface of the bottom electrode film (Pt-film)  2  (a surface to be a base of the output-side piezoelectric film  3 A) is preferably mainly constituted of Pt-(111). The Pt-film can be deposited using a method such as a sputtering method or a vapor deposition method. The bottom electrode film  2  can also be deposited using, in place of Pt, various metals such as gold (Au), ruthenium (Ru), or iridium (Ir); alloys containing these as main components; metal oxides such as strontium ruthenium oxide (SrRuO 3 ; abbreviated as SRO) or lanthanum nickel oxide (LaNiO 3 ; abbreviated as LNO); or the like. The bottom electrode film  2  may also be a single-layer film deposited using the above-listed various metals or metal oxides; a stack including a Pt-film and a film containing LNO provided on the Pt-film; a stack including a Pt-film and a film containing SRO provided on the Pt-film; or the like. A thickness of the bottom electrode film  2  can be, for example, 100 nm or more and 400 nm or less. 
     An adhesion layer  6  mainly containing, for example, titanium (Ti), tantalum (Ta), titanium oxide (TiO 2 ), nickel (Ni), ruthenium oxide (RuO 2 ), iridium oxide (IrO 2 ), or the like may also be provided between the substrate  1  and the bottom electrode film  2  in order to enhance an adhesiveness between them. The adhesion layer  6  can be deposited using a method such as a sputtering method or a vapor deposition method. A thickness of the adhesion layer  6  can be, for example, 1 nm or more and 200 nm or less. 
     The output-side piezoelectric film  3 A is a film containing oxygen (O) and not containing nitrogen (N). That is, the output-side piezoelectric film  3 A is an oxide film. The term “film not containing N” used herein includes not only a film containing no N at all, but also a film containing a small amount of N as unavoidable impurities. The output-side piezoelectric film  3 A is preferably a film having a high piezoelectric constant. Thereby, the output-side piezoelectric film  3 A can be largely deformed when applying a predetermined voltage using a later-described voltage applicator  11   a , which in turn largely vibrate a later-described output-side vibration part. As a result, an intensity of ultrasonic waves transmitted from a later-described ultrasonic output part can be increased, and a vibration depth of ultrasonic waves in a test object can be increased. The piezoelectric constant of the output-side piezoelectric film  3 A can be, for example, 100 pm/V or more, and preferably 170 pm/V or more. An upper limit of the piezoelectric constant of the output-side piezoelectric film  3 A is not particularly limited, but the upper limit thereof is about 200 pm/V in a current technology. Also, the output-side piezoelectric film  3 A preferably has a higher piezoelectric constant than that of the input-side piezoelectric film  3 B. 
     The output-side piezoelectric film  3 A can be deposited using, for example, an alkali niobium oxide which contains potassium (K), sodium (Na), and niobium (Nb), and which is represented by a composition formula (K 1-x Na x ) y NbO 3 . That is, the output-side piezoelectric film  3 A can be deposited using potassium sodium niobium oxide (KNN). A coefficient x[=Na/(K+Na)] in the above composition formula is a value in a range of 0&lt;x&lt;1. Preferably, the coefficient x in the above composition formula is in the range of 0&lt;x&lt;1, and a coefficient y[=(K+Na)/Nb] is a value in a range of 0.7≤y≤1.50. The output-side piezoelectric film  3 A is a polycrystalline film of KNN (hereinafter also referred to as a KNN-film  3 A). A crystal structure of KNN is a perovskite structure. The KNN-film  3 A can be deposited using a method such as a sputtering method, a PLD (Pulsed Laser Deposition) method, or a sol-gel method. A thickness of the KNN-film  3 A can be, for example, 0.5 μm or more and 5 μm or less. 
     Preferably, crystals constituting the KNN-film  3 A are preferentially oriented in (001) direction with respect to the surface of the substrate  1  (a surface of the Si-substrate  1   a , in a case where the substrate  1  is, for example, the Si-substrate  1   a  having the surface oxide film  1   b , the insulating film  1   d , or the like, hereinafter the same). That is, a surface of the KNN-film  3 A (a surface to be a base of the output-side top electrode film  4 A) is preferably mainly constituted of KNN-(001). By depositing the KNN-film  3 A directly on the Pt-film preferentially oriented in (111) direction with respect to the surface of the substrate  1 , the crystals constituting the KNN-film  3 A can be easily preferentially oriented in (001) direction with respect to the surface of the substrate  1 . For example, 80% or more crystals in a crystal group constituting the KNN-film  3 A can be oriented in (001) direction with respect to the surface of the substrate, and 80% or more regions of the surface of the KNN-film  3  can be easily KNN-(001). 
     It is preferable that half or more crystals in the crystal group constituting the KNN-film  3 A have a columnar structure. Also, boundaries between the crystals constituting the KNN-film  3 A, i.e., crystal grain boundaries present in the KNN-film  3 A preferably penetrate the KNN-film  3 A in a thickness direction of the KNN-film  3 A. For example, the KNN-film  3 A has more crystal grain boundaries that penetrate the KNN-film  3 A in the thickness direction thereof than crystal grain boundaries (e.g., crystal grain boundaries parallel to a planar direction of the substrate  1 ) that do not penetrate the KNN-film  3 A in the thickness direction thereof. 
     Preferably, the KNN-film  3 A contains at least one metal element (hereinafter also referred to simply as “metal element”) selected from a group consisting of copper (Cu), manganese (Mn), iron (Fe), and vanadium (V). More preferably, the KNN-film  3 A contains at least one metal element selected from a group of Cu and Mn. “Contains at least one metal element selected from the group of Cu and Mn” may include a case of containing only Cu, a case of containing only Mn, and a case of containing both Cu and Mn. 
     Preferably, the KNN-film  3 A contains the above metal element at a concentration in a range of, for example, 0.2 at % or more and 2.0 at % or less relative to an amount of niobium (Nb) in the KNN-film  3 A. That is, a concentration of the above metal element in the KNN-film  3 A is preferably, for example, 0.2 at % or more and 2.0 at % or less. When the KNN-film  3 A contains a plurality of the metal elements of Cu, Mn, Fe, and V, the concentration of the metal element indicates a total concentration of the plurality of the metal elements. 
     Since the concentration of the metal element in the KNN-film  3 A is 0.2 at % or more, a resistance (etching resistance) to fluorine-based etchants can be improved while also improving an insulating property (leakage resistance) of the KNN-film  3 A. Since the insulating property of the KNN-film  3 A is improved, a higher voltage can be applied to the KNN-film  3 A, using the later-described voltage applicator  11   a . As a result, an amount of the deformation of the KNN-film  3 A can be increased. Since the etching resistance of the KNN-film  3 A is improved, i.e., since the KNN-film  3 A is less likely to be etched, deteriorations in a piezoelectric performance and a quality of the KNN-film  3 A can be suppressed in a production process of the piezoelectric stack  10 . 
     Since the concentration of the metal element in the KNN-film  3 A is 2.0 at % or less, a dielectric constant of the KNN-film  3 A can be a value suitable for applications such as a vibrator for generating ultrasonic waves, and an increase in a power consumption can be suppressed when the KNN-film  3 A is applied to the vibrator for generating ultrasonic waves, as described later. 
     The KNN-film  3 A may also contain a secondary component other than K, Na, Nb, and the above metal element in a range where the effect obtained by adding the above metal element in a predetermined range is not impaired, e.g., in a range of 5 at % or less (in a case where a plurality of the secondary components are added, a total concentration is 5 at % or less). Examples of the secondary components include lithium (Li), Ta, and antimony (Sb). 
     The output-side top electrode film  4 A (hereinafter also referred to as a top electrode film  4 A) can be deposited using, for example, various metals such as Pt, Au, aluminum (Al), or Cu; or an alloy containing these metals. The top electrode film  4 A can be deposited using a method such as a sputtering method, a vapor deposition method, a plating method, or a metal paste method. The top electrode film  4 A does not greatly affect the crystal structure of the KNN-film  3 A, unlike the bottom electrode film  2 A. Therefore, there are no particular limitations on a material and a crystal structure of the top electrode film  4 A, and the method of depositing the top electrode film  4 A. An adhesion layer mainly containing, for example, Ti, Ta, TiO 2 , Ni, RuO 2 , IrO 2 , or the like may also be provided between the KNN-film  3 A and the top electrode film  4 A in order to enhance an adhesiveness between them. A thickness of the top electrode film  4 A can be, for example, 10 nm or more and 5000 nm or less. When the adhesion layer is provided, a thickness of the adhesion layer can be, for example, 1 nm or more and 200 nm or less. 
     The input-side piezoelectric film  3 B is a film containing nitrogen (N) and not containing oxygen (O), i.e., a nitride film. The term “film not containing O” as used herein includes not only a film containing no O at all, but also a film containing a small amount of O as unavoidable impurities. The input-side piezoelectric film  3 B is preferably a film having a low dielectric constant. Thereby, a reception sensitivity of ultrasonic waves can be increased. Specifically, even when a vibration of the later-described input-side vibration part is small, the input-side piezoelectric film  3 B can be deformed, and a voltage can be generated due to the deformation. The dielectric constant of the input-side piezoelectric film  3 B can be, for example, 25 or less, and preferably 15 or less. A lower limit of the dielectric constant of the input-side piezoelectric film  3 B is not particularly limited, but the lower limit thereof is about 8 in a current technology. Also, the input-side piezoelectric film  3 B preferably has a lower dielectric constant than that of the output-side piezoelectric film  3 A. 
     The input-side piezoelectric film  3 B can be deposited using, for example, a nitride which contains aluminum (Al). That is, the input-side piezoelectric film  3 B can be deposited using, for example, aluminum nitride (AlN). The input-side piezoelectric film  3 B is a polycrystalline film of AlN (hereinafter also referred to as an AlN-film  3 B). The AlN-film  3 B can be deposited using a method such as a sputtering method, a PLD (Pulsed Laser Deposition) method, an MOCVD (Metal Organic Chemical Vapor Deposition) method, a HYPE (Hydride Vapor Phase Epitaxy) method, or the like. A thickness of the AlN-film  3 B can be, for example, 0.3 μm or more and 5 μm or less. The thickness of the AlN-film  3 B is preferably as thick as possible within the above range. Thereby, the reception sensitivity of ultrasonic waves can be increased. 
     Preferably, crystals constituting the AlN-film  3 B are preferentially oriented in (001) direction with respect to the surface of the substrate  1 . That is, a surface of the AlN-film  3 B (a surface to be a base of the input-side top electrode film  4 B) is preferably mainly constituted of AlN-(001). By depositing the AlN-film  3 B directly on the bottom electrode film  2 B preferentially oriented in (111) direction with respect to the surface of the substrate  1 , the crystals constituting the AlN-film  3 B can be easily preferentially oriented in (001) direction with respect to the surface of the substrate  1 . For example, 80% or more crystals in a crystal group constituting the AlN-film  3 B can be oriented in (001) direction with respect to the surface of the substrate  1 , and 80% or more regions of the surface of the AlN-film  3 B can be easily AlN-(001). 
     It is preferable that half or more crystals in the crystal group constituting the AlN-film  3 B have a columnar structure. Also, boundaries between the crystals constituting the AlN-film  3 B, i.e., crystal grain boundaries present in the AlN-film  3 B preferably penetrate the AlN-film  3 B in a thickness direction of the AlN-film  3 B. For example, the AlN-film  3 B has more crystal grain boundaries that penetrate the AlN-film  3 B in the thickness direction thereof than crystal grain boundaries (e.g., crystal grain boundaries parallel to the planar direction of the substrate  1 ) that do not penetrate the AlN-film  3 B in the thickness direction thereof. 
     The AlN-film  3 B may be an AlN-film containing scandium (Sc) (i.e., a Sc—AlN-film), may be an AlN-film containing magnesium (Mg) and zirconium (Zr) (i.e., a MgZr—AlN-film), or may be an AlN-film containing Mg and hafnium (Hf) (i.e., a MgHf—AlN-film). Thereby, a piezoelectric constant of the AlN-film  3 B can be increased. As a result, the reception sensitivity of ultrasonic waves can be reliably increased. 
     The input-side top electrode film  4 B (hereinafter also referred to as a top electrode film  4 B) can have configurations similar to those of the above top electrode film  4 A. That is, the top electrode film  4 B can be deposited using, for example, various metals such as Pt, Au, Al, or Cu; or alloy containing these metals. The top electrode film  4 B can also be deposited using various metals such as molybdenum (Mo) or Ru; or alloy containing these metals. For example, the top electrode film  4 B can be deposited using a method such as a sputtering method, a vapor deposition method, a plating method, or a metal paste method. The top electrode film  4 B does not greatly affect a crystal structure of the AlN-film  3 B, unlike the bottom electrode film  2 . Therefore, there are no particular limitations on a material and a crystal structure of the top electrode film  4 B, and the method of depositing the top electrode film  4 B. An adhesion layer mainly containing, for example, Ti, Ta, TiO 2 , Ni, or the like may also be provided between the AlN-film  3 B and the top electrode film  4 B in order to enhance an adhesiveness between them. A thickness of the top electrode film  4 B can be, for example, 10 nm or more and 5000 nm or less. When the adhesion layer is provided, a thickness of the adhesion layer can be, for example, 1 nm or more and 200 nm or less. 
     An ultrasonic output part (hereinafter also referred to as an “output part”) includes a stacked part of the bottom electrode film  2 , the KNN-film  3 A, and the top electrode film  4 A. The output part may include the adhesion layer  6 , the later-described output-side vibration part formed on the substrate  1 , and the like. The output part is a part that generates and transmits (outputs) ultrasonic waves. The output part is configured such that the KNN-film  3 A deforms under an electric field (voltage) application from a later-described voltage applicator  11   a  connected between the bottom electrode film  2  and the top electrode film  4 A, and the output-side vibration part vibrates due do this deformation of the KNN-film  3 A, and ultrasonic waves generated due to this vibration of the output-side vibration part are output. 
     An ultrasonic input part (hereinafter also referred to as an “input part”) includes a stacked part of the bottom electrode film  2 , the AlN-film  3 B, and the top electrode film  4 B. The input part may include the adhesion layer  6 , the later-described input-side vibration part formed on the substrate  1 , and the like. The input part is a part that receives (inputs) ultrasonic waves which are output from the output part and are reflected by the test object. The input part is configured such that when receiving ultrasonic waves, the input-side vibration part vibrates, and thus the AlN-film  3 B deforms. A voltage is generated between the bottom electrode film  2  and the top electrode film  4 B due to this deformation of the AlN-film  3 B. 
     The output part and the input part are placed in such a manner as not overlapping each other when viewed from a top surface of the substrate  1  (piezoelectric stack  10 ). Thereby, it is possible to prevent the output portion and the input portion from interfering with each other. As a result, a sensing accuracy of a later-described ultrasonic sensor  30  produced using the piezoelectric stack  10 , can be improved. The term “when viewed from the top surface of the substrate  1 ” used herein indicates “when viewed a main surface of the substrate  1  on which the KNN-film  3 A, the AlN-film  3 B, and the like are provided, from above in a vertical direction”. The term “vertical direction” used herein is a direction which coincides with at least either a propagation direction of ultrasonic waves transmitted from the output part or a propagation direction of ultrasonic waves received by the input part. 
     A distance d between the output part and the input part is preferably the shortest possible distance at which interference (contact) does not occur between them. That is, although the output part and the input part are not in contact with each other, the distance d between the output part and the input part is preferably as short as possible. For example, the distance (maximum distance) d between the output part and the input part is preferably 500 μm or less. More preferably, the distance between the output part and the input part is brought as close as possible with a MEMS manufacturing technology. Thereby, the later-described ultrasonic sensor  30  can be reduced in size by increasing a degree of an integration of the output part and the input part while also having a high performance. 
     (2) CONFIGURATION OF PIEZOELECTRIC ELEMENT AND ULTRASONIC SENSOR 
       FIG.  3    illustrates a schematic configuration view of an ultrasonic sensor  30  according to the present embodiment. The ultrasonic sensor  30  includes at least one piezoelectric element  20 , as well as a voltage applicator  11   a  and a voltage detector  11   b  connected to the piezoelectric element  20 . Examples of applications of the ultrasonic sensor  30  include a p-MUT. 
     The piezoelectric element  20  indicates an element including the output-side piezoelectric film  3 A and the input-side piezoelectric film  3 B, and is obtained by shaping the above piezoelectric stack  10  into a predetermined form. When shaping the piezoelectric stack  10  into a predetermined form, an output-side vibration part is formed on the substrate  1  at a position corresponding to the output part, and an input-side vibration part is formed on the substrate  1  at a position corresponding to the input part, for example. The output-side vibration part and the input-side vibration part can be formed by forming, for example, a membrane structure or a cantilever structure into the substrate  1  included in the piezoelectric stack  10 .  FIG.  3    illustrates an example of the piezoelectric element  20  in which a membrane structure is formed into the substrate  1 . 
     Resonance frequencies of the output-side vibration part and the input-side vibration part may be the same, or may be different. For example, the output-side vibration part and the input-side vibration part may be given different widths according to the resonance frequency, such as making the width of the output-side vibration part larger than the width of the input-side vibration part. As another example, a portion of the substrate  1  constituting the output-side vibration part and a portion of the substrate  1  constituting the input-side vibration part may be given different thicknesses according to the resonance frequency, such as making the thickness of the substrate  1  at a position corresponding to the output part larger than the thickness of the substrate  1  at a position corresponding to the input part. 
     The voltage applicator  11   a  is a means that applies a voltage between the bottom electrode film  2  (output-side bottom electrode film  2 A) and the top electrode film  4 A, and the voltage detector  11   b  is a means thar detects a voltage generated between the bottom electrode film  2  (input-side bottom electrode film  2 B) and the top electrode film  4 B. Various known means can be used as the voltage applicator  11   a  and the voltage detector  11   b.    
     The above output part can function as an vibrator for generating ultrasonic waves by connecting the voltage applicator  11   a  between the bottom electrode film  2  and the top electrode film  4 A of the piezoelectric element  20 . The KNN-film  3 A can deform under a voltage application from the voltage applicator  11   a  between the bottom electrode film  2  and the top electrode film  4 A. Due to this deformation motion, the output-side vibration part can vibrate, and ultrasonic waves can be generated by this vibration. 
     The input part can function as a sensor by connecting the voltage detector  11   b  between the bottom electrode film  2  and the top electrode film  4 B of the piezoelectric element  20 . The input part receives ultrasonic waves, the input-side vibration part vibrates, which in turn deforming the AlN-film  3 B. Due to the deformation of the AlN-film  3 B, a voltage is generated between the bottom electrode film  2  and the top electrode film  4 B. A magnitude of ultrasonic waves received by the input part can be measured by detecting the voltage using the voltage detector  11   b.    
     The ultrasonic sensor  30  is configured such that ultrasonic waves are transmitted from the output part toward a test object, and the input part receives ultrasonic waves reflected by the test object. Accordingly, for example, the presence or absence of the test object can be determined by detecting a magnitude of the voltage detected using the voltage detector  11   b  a plurality of times, and observing changes in the voltage. Also, for example, a distance to the test object can be known by measuring the time from a start of a voltage application using the voltage applicator  11   a  to a voltage detection using the voltage detector  11   b  a plurality of times, and observing changes in that time. 
     (3) METHOD OF MANUFACTURING PIEZOELECTRIC STACK, PIEZOELECTRIC ELEMENT, AND ULTRASONIC SENSOR 
     A method of manufacturing the piezoelectric stack  10 , the piezoelectric element  20 , and the ultrasonic sensor  30  will be described hereinafter, with reference to  FIGS.  4 ( a ) to  4 ( f ) . 
     Deposition of Bottom Electrode Film 
     The substrate  1  is firstly prepared, and the adhesion layer  6  (Ti-layer) and the bottom electrode film  2  (Pt-film) are deposited in this order on any one of main surfaces of the substrate  1  using, for example, the sputtering method. As a result, a stack  10   a  as illustrated in  FIG.  4 ( a )  is obtained. It is also acceptable to prepare the substrate  1  (i.e., the stack  10   a ) with the adhesion layer  6  or the bottom electrode film  2  deposited on any one of the main surfaces of the substrate  1  in advance. 
     The following conditions are exemplified as the conditions for forming the adhesion layer  6 .
         Temperature (substrate temperature): 100° C. or more and 500° C. or less, and preferably 200° C. or more and 400° C. or less   RF-power: 1000 W or more and 1500 W or less, and preferably 1100 W or more and 1300 W or less   Atmosphere: Argon (Ar) gas atmosphere   Atmospheric pressure: 0.1 Pa or more and 0.5 Pa and less, and preferably 0.2 Pa or more and 0.4 Pa or less   Time: 30 seconds or more and 3 minutes or less, and preferably 30 seconds or more and 2 minutes or less       

     The following conditions are exemplified as the conditions for depositing the bottom electrode film  2 . A metal target containing, for example, Pt can be used as a target used in the sputtering deposition.
         Deposition temperature (substrate temperature): 100° C. or more and 500° C. or less, and preferably 200° C. or more and 400° C. or less   RF-power: 1000 W or more and 1500 W or less, and preferably 1100 W or more and 1300 W or less   Deposition atmosphere: Ar-gas atmosphere   Atmospheric pressure: 0.1 Pa or more and 0.5 Pa and less, and preferably 0.2 Pa or more and 0.4 Pa or less   Deposition time: 3 minutes or more and 10 minutes or less, and preferably 4 minutes or more and 7 minutes or less       

     Deposition of Output-Side Piezoelectric Film 
     After the deposition of the bottom electrode film  2  is complete, the KNN-film  3 A is deposited on the stack  10   a  (on the bottom electrode film  2 ) using, for example, the sputtering method. A composition ratio of the KNN-film  3 A can be adjusted by controlling a composition of a target material used in the sputtering deposition, for example. The target material can be produced by mixing and calcining K 2 CO 3 -powder, Na 2 CO 3 -powder, Nb 2 O 5 -powder, Cu-powder (or CuO-powder or Cu 2 O-powder), MnO-powder, and the like. The composition of the target material can be controlled by adjusting a mixing ratio of K 2 CO 3 -powder, Na 2 CO 3 -powder, Nb 2 O 5 -powder, Cu-powder (or CuO-powder or Cu 2 O-powder), MnO-powder, and the like. 
     The following conditions are exemplified as the conditions for depositing the KNN-film  3 A. A deposition time is appropriately set according to the thickness of the KNN-film  3 A to be deposited.
         Deposition temperature (substrate temperature): more than 350° C. and 700° C. or less, and preferably 550° C. or more and 650° C. or less   RF-power: 2000 W or more and 2400 W or less, and preferably 2100 W or more and 2300 W or less   Deposition atmosphere: mixed gas of Ar gas+oxygen (O 2 ) gas   Atmosphere (oxygen (O)-containing atmosphere) pressure: 0.2 Pa or more and 0.5 Pa or less, and preferably 0.2 Pa or more and 0.4 Pa or less   Partial pressure of Ar-gas relative to O 2 -gas (partial pressure ratio of Ar/O 2 ): 30/1 to 20/1, and preferably 27/1 to 23/1   Deposition rate: 0.5 μm/hr or more and 2 μm/hr or less, and preferably 0.5 μm/hr or more and 1.5 μm/hr or less       

     As a result, a stack  10   b  as illustrated in  FIG.  4 ( b )  is obtained. Also, the KNN-film  3 A having a high quality, for example, the KNN-film  3 A having the piezoelectric constant of 100 pm/V or more can be deposited by depositing the KNN-film  3 A under the above conditions. 
     After the deposition of the KNN-film  3 A is complete, the KNN-film  3 A is shaped into a predetermined form (predetermined pattern) by etching or the like. As a result, a stack  10   c  as illustrated in  FIG.  4 I  is obtained. 
     Deposition of Protective Film 
     Subsequently, a protective film  12  that protects the KNN-film  3 A is deposited. The protective film  12  is provided in such a manner as protecting (covering) the KNN-film  3 A. 
     The protective film  12  can be deposited using a material which is not reduced (i.e., which does not deteriorate) in a deposition atmosphere (N-containing atmosphere) of the AlN-film  3 B, which can protect the KNN-film  3 A, and which can be easily removed by wet etching or the like using, for example, an etchant containing fluorine (F). The protective film  12  can be deposited using, for example, silicon oxide (SiO 2 ). The protective film  12  can be deposited using a CVD (Chemical Vapor Deposition) method, a sputtering method, a vapor deposition method, or the like. A thickness of the protective film  12  can be a thickness which can continuously cover a surface of the KNN-film  3 A, i.e., a thickness which allows the protective film  12  to be a continuous film. Hereinafter, the “deposition atmosphere of the AlN-film  3 B” is also referred to as an “N-containing atmosphere”. 
     The following conditions are exemplified as cthe onditions for depositing the protective film  12  using a plasma CVD method.
         Deposition temperature (substrate temperature): 200° C. or more and 400° C. or less, preferably 300° C. or more and 400° C. or less, and more preferably 350° C.   RF-power: 70 W or more and 150 W or less, preferably 70 W or more and 120 W or less, and more preferably 100 W   Raw material: Tetraethoxysilane (abbreviation: TEOS)   Deposition atmosphere: O 2 -gas atmosphere   Atmospheric pressure: 50 Pa or more and 100 Pa or less, preferably 60 Pa or more and 70 Pa or less, and more preferably 67 Pa       

     A deposition time is appropriately set according to the thickness of the protective film  12  to be deposited. A deposition time can be, for example, 5 minutes or more and 15 minutes or less. Specifically, the deposition time can be 11 minutes when depositing the protective film  12  having the thickness of 400 nm. 
     By depositing the protective film  12  under the above conditions, the protective film  12  being a continuous film and covering the KNN-film  3 A, can be deposited. The protective film  12  acts as a film suppressing a reduction of the KNN-film  3 A in the N-containing atmosphere, and protects the KNN-film  3 A. This protective action can prevent the KNN-film  3 A from being exposed to the N-containing atmosphere during the deposition of the AlN-film  3 B. Thereby, it is possible to prevent the KNN-film  3 A from being reduced and deteriorating by N-element during the deposition of the AlN-film  3 B, i.e., to prevent the KNN-film  3 A from deteriorating due to a removal of oxygen from the KNN-film  3 A by the N-element. As a result, changes in the piezoelectric performance and the quality of the KNN-film  3 A (e.g., a reduction in the insulating property) can be suppressed. 
     After the deposition of the protective film  12  is complete, the protective film  12  is shaped into a predetermined pattern. For example, the protective film  12  is removed by etching or the like from a position on the substrate  1  where the AlN-film  3 B will be to be deposited. As a result, a stack  10   d  as illustrated in  FIG.  4 ( d )  is obtained. 
     Deposition of Input-Side Piezoelectric Film 
     Subsequently, the AlN-film  3 B is deposited using, for example, the sputtering method. A metal target containing, for example, Al can be used as a target material. 
     The following conditions are exemplified as cthe onditions for depositing the AlN-film  3 B. A deposition time is appropriately set according to the thickness of the AlN-film  3 B to be deposited.
         Deposition temperature (substrate temperature): 300° C. or more and 350° C. or less   RF-power: 600 W or more and 900 W or less, and preferably 700 W or more and 800 W or less   Deposition atmosphere: mixed gas atmosphere of Ar-gas+nitrogen (N 2 ) gas (N-containing atmosphere)   Atmospheric pressure: 0.5 Pa or more and 2.0 Pa or less, and preferably 0.7 Pa or more and 1.0 Pa or less   Partial pressure of Ar-gas to N 2 -gas (partial pressure ratio of Ar/N 2 ): 1/10 to 1/2, preferably 1/7 to 1/3, and more preferably 1/6 to 1/4   Deposition rate: 0.3 μm/hr or more and 2 μm/hr or less, preferably 0.3 μm/hr or more and 1 μm/hr or less, and more preferably 0.3 μm/hr or more and 0.5 μm/hr or less   The deposition atmosphere may be a mixed gas atmosphere of Ar-gas and ammonia (NH 3 ) gas.       

     As a result, a stack  10   e  as illustrated in  FIG.  4 II  is obtained. Also, by depositing the AlN-film  3 B under the above conditions, the AlN-film  3 B having a high quality, for example, the AlN-film  3 B having the dielectric constant of 25 or less can be deposited. Since the KNN-film  3 A is covered with the protective film  12  as described above, almost none of the KNN-film  3 A is reduced by the N-element present in the N-containing atmosphere. 
     After the deposition of the AlN-film  3 B is complete, the protective film  12  is removed by wet etching using, for example, an etchant containing fluorine (F). As a result, the KNN-film  3 A is exposed. Also, using the protective film  12  as a lift-off layer, an unnecessary AlN-film  3 B formed on the protective film  12  is removed. That is, the AlN-film  3 B is left only in a region where the AlN-film  3 B is to be deposited. As a result, a stack  10   f  as illustrated in  FIG.  4 ( f )  is obtained. The etchant containing F can be, for example, a buffered hydrofluoric acid (BHF) solution containing hydrogen fluoride (HF) at a concentration of 4.32 mol/L and ammonium fluoride (NH 4 F) at a concentration of 10.67 mol/L. 
     Deposition of Top Clectrode Film 
     Then, Pt-films as the top electrode films  4 A and  4 B are deposited on the KNN-film  3 A and the AlN-film  3 B using, for example, the sputtering method. Conditions for depositing the top electrode films  4 A and  4 B can be similar to the above conditions for depositing the bottom electrode film  2 . As a result, the piezoelectric stack  10  as illustrated in  FIG.  1    is obtained. 
     Then, the piezoelectric element  20  as illustrated in  FIG.  3    is obtained by shaping the piezoelectric stack  10  into a predetermined form by etching or the like, and the ultrasonic sensor  30  is obtained by connecting at least either the voltage applicator  11   a  or the voltage detector  11   b  to the piezoelectric element  20 . 
     (4) EFFECTS OBTAINED BY PRESENT EMBODIMENT 
     According to the present embodiment, the following one or more effects are obtained. 
     (a) The piezoelectric stack  10  includes the output-side piezoelectric film  3 A (KNN-film  3 A) and the input-side piezoelectric film  3 B (AlN-film  3 B), and the output part and the input part do not overlap each other when viewed from the top surface of the substrate  1 , the output part being configured by the stacked part including the KNN-film  3 A, and the input part being configured by the stacked part including the AlN-film  3 B. Thereby, the KNN-film  3 A and the AlN-film  3 B can be driven independently. Also, in the piezoelectric stack  10 , the output-side piezoelectric film  3 A is the oxide film having a relatively high piezoelectric constant (i.e., the KNN-film  3 A), and the input-side piezoelectric film  3 B is the nitride film having a relatively low dielectric constant (i.e., the AlN-film  3 B). By using such a piezoelectric stack  10 , the ultrasonic sensor  30  having a deep penetration depth of ultrasonic waves in a test object, and having a high resolution (high reception sensitivity), can be obtained. That is, the ultrasonic sensor  30  having the high performance can be obtained. 
     (b) According to the present embodiment, the piezoelectric stack  10  including the output part and the input part, the piezoelectric element  20  obtained by processing the piezoelectric stack  10 , and the ultrasonic sensor  30  produced using the piezoelectric element  20  can be collectively formed in the MEMS manufacturing process. That is, the ultrasonic sensor  30  having the high performance can be produced without causing a complicated manufacturing process. 
     (c) Since the output part and the input part are not in contact with each other, it is possible to prevent the output part and the input part from interfering with each other. For example, it is possible to prevent the vibration of the output-side vibration part from transmitting to the input-side vibration part. Thereby, the sensing accuracy of the ultrasonic sensor  30  can be improved. That is, the ultrasonic sensor  30  can have a higher performance. 
     (d) In the piezoelectric stack  10 , the distance between the output part and the input part is, for example, 500 μm or less, and preferably 300 μm or less. Preferably, the distance between the output part and the input part is brought as close as possible with the MEMS manufacturing technology. Thereby, the degree of the integration of the output part and the input part can be increased while also improving the sensing accuracy of the ultrasonic sensor  30 . That is, the ultrasonic sensor  30  having the high performance and a small size can be obtained. 
     (e) Since the protective film  12  that protects the KNN-film  3 A, is provided after depositing the KNN-film  3 A and before depositing the AlN-film  3 B, it is possible to prevent the KNN-film  3 A from being exposed to the N-containing atmosphere during the deposition of the AlN-film  3 B. Thereby, it is possible to prevent the KNN-film from being reduced the deteriorating by the N element in the N-containing atmosphere. As a result, the deterioration of the piezoelectric performance and the quality of the KNN-film  3 A can be suppressed. That is, the piezoelectric stack  10  including the KNN-film  3 A having the high quality and the high performance, and the AlN-film  3 B having the high quality and the high performance, can be obtained. For example, the piezoelectric stack  10  including the KNN-film  3 A having the high piezoelectric constant and the AlN-film  3 B having the low dielectric constant, can be obtained. 
     (f) Since the KNN-film  3 A contains at least one metal element selected from the group consisting of Cu, Mn, Fe, and V, the etching resistance of the KNN-film  3 A to the fluorine-based etchant such as a BHF solution, can be improved. Thereby, it is possible to prevent the KNN-film  3 A from being etched when removing the protective film  12  by wet etching using the BHF solution or the like. As a result, the deterioration in the piezoelectric performance and the quality of the KNN-film  3 A can be reliably suppressed. The inventors have confirmed that the effect of improving the etching resistance of the KNN-film  3 A can be particularly improved when, among the above metal elements, Cu is added in the KNN-film  3 A. 
     Here, for reference, a conventional ultrasonic sensor will be described. In the conventional ultrasonic sensor, a transmission and a reception of ultrasonic waves are performed using a piezoelectric element including a piezoelectric film being an oxide film such as a PZT-film. However, in such a ultrasonic sensor, there is a problem as follows: although the penetration depth of ultrasonic waves is large, the dielectric constant of the piezoelectric film is high, and the resolution of the sensor is low (i.e., the reception sensitivity is low). On the other hand, it is also conceivable that in the conventional ultrasonic sensor, in order to increase the resolution of the ultrasonic sensor, the transmission and reception of ultrasonic waves are performed using a piezoelectric element including a piezoelectric film being a nitride film such as an AlN-film. However, in such a ultrasonic sensor, there is a problem as follows: although the resolution is high, the piezoelectric constant of the piezoelectric film is low, and the penetration depth of ultrasonic waves is shallow. In response to such a problem, in the present embodiment, the ultrasonic sensor  30  includes the output-side piezoelectric film  3 A (KNN-film  3 A) being the oxide film and the input-side piezoelectric film  3 B (AlN-film  3 B) being the nitride film, the transmission of ultrasonic waves is performed using the output part including the KNN-film  3 A, and the reception of ultrasonic waves is performed using the input part including the AlN-film  3 B. Thereby, according to the present embodiment, the high-performance ultrasonic sensor  30  having the deep penetration depth of ultrasonic waves and the high resolution, and the like can be obtained. 
     It is also conceivable that at least two separate piezoelectric elements are prepared, these at least two piezoelectric elements are provided on each of vibration parts of the substrate on which the vibration parts are formed, and thereby constituting the ultrasonic sensor. Note that the at least two piezoelectric elements are an element for the transmission of ultrasonic waves and an element for the reception of ultrasonic waves, the element for the transmission including a piezoelectric film being the oxygen film, and the element for the reception including a piezoelectric film being the nitride film. However, when producing such an ultrasonic sensor, a process of attaching (adhering) the piezoelectric elements onto the substrate, or the like, is required. Therefore, a manufacturing process of the ultrasonic sensor is complicated in some cases. Also, in a process of attaching the piezoelectric elements onto the substrate, the distance between the adjacent piezoelectric elements can only be about 1 mm, or about 500 μm at a minimum. Therefore, there is a problem that it is difficult to increase the degree of the integration of the piezoelectric elements. On the other hand, according to the present embodiment, the piezoelectric stack  10  (the piezoelectric element  20 , the ultrasonic sensor  30 ) which includes the output part including the KNN-film  3 A and the input part including the AlN-film  3 B can be collectively produced in the MEMS manufacturing process. That is, according to the present embodiment, the high-performance ultrasonic sensor  30  having the deep penetration depth of ultrasonic waves and the high resolution can be produced without causing the complicated manufacturing process. Also, in the present embodiment, since the piezoelectric stack  10  and the like are produced collectively in the MEMS manufacturing process, the distance between the output part and the input part can be brought as close as possible with the MEMS manufacturing technology. As described above, according to the present embodiment, the ultrasonic sensor  30  having the high performance and the small size can be produced collectively in the MEMS manufacturing process. 
     (5) MODIFIED EXAMPLES 
     The present embodiment can be modified as the following modified examples. In the following description of modified examples, the same reference numerals are given to components that are the same as those in the above embodiment, and descriptions thereof are omitted. 
     Modified Example 1 
     As illustrated in  FIG.  5   , a piezoelectric stack  40  may include an output-side bottom electrode film  2 A (hereinafter also referred to as a bottom electrode film  2 A) and an input-side bottom electrode film  2 B (hereinafter also referred to as a bottom electrode film  2 B) provided on the bottom electrode film  2 A. 
     The bottom electrode film  2 A can have configurations similar to those of the bottom electrode film  2  in the above embodiment. 
     The bottom electrode film  2 B can be deposited using, for example, at least either hafnium (Hf) or molybdenum (Mo). The bottom electrode film  2 B is a single-crystal film or a polycrystalline film. Preferably, crystals constituting the bottom electrode film  2 B are preferentially oriented in (111) direction with respect to the surface of the substrate  1 . That is, a surface of the bottom electrode film  2 B (a surface to be a base of the input-side piezoelectric film  3 B) is preferably mainly constituted of Hf-(111) or Mo-(111). The bottom electrode film  2 B can be deposited using a method such as a sputtering method or a vapor deposition method. The bottom electrode film  2 B can also have configurations similar to those of the above bottom electrode film  2 . The bottom electrode film  2 B can also be deposited using, for example, Pt. The bottom electrode film  2 B can also be deposited using, for example, Al, Cu, or silver (Ag). A thickness of the bottom electrode film  2 B can be for example, 100 nm or more and 400 nm or less. Although not illustrated in  FIG.  5   , an adhesion layer similar to the above adhesion layer  6  may be provided between the bottom electrode film  2 A and the bottom electrode film  2 B in order to enhance an adhesiveness between them. 
     In the present modified example, an output part includes a stacked part of the bottom electrode film  2 A, the KNN-film  3 A, and the top electrode film  4 A. Also, an input part includes a stacked part of the bottom electrode film  2 B, the AlN-film  3 B, and the top electrode film  4 B. The input part may include the bottom electrode film  2 A located under the AlN-film  3 B. 
     A method of manufacturing the piezoelectric stack  40  illustrated in  FIG.  5    will be described later, with reference to  FIGS.  6 ( a ) to  6 ( f ) . 
     The substrate  1  is firstly prepared, and the adhesion layer  6  (Ti-layer) and the bottom electrode film  2 A are deposited in this order on any one of the main surfaces of the substrate  1  using, for example, the sputtering method. Conditions for forming the adhesion layer  6  and the bottom electrode film  2 A can be conditions similar to the above conditions for forming the adhesion layer  6  and the bottom electrode film  2 . As a result, a stack  40   a  as illustrated in  FIG.  6 ( a )  is obtained. 
     Subsequently, the KNN-film  3 A is deposited using a procedure and conditions similar to those in the above embodiment, and the KNN-film  3 A is shaped into a predetermined pattern. As a result, a stack  40   b  as illustrated in  FIG.  6 ( b )  is obtained. After that, the protective film  12  is deposited using a procedure and conditions similar to those in the above embodiment, and is shaped into a predetermined pattern. As a result, a stack  40   c  as illustrated in  FIG.  6 ( c )  is obtained. 
     Then, the bottom electrode film  2 B is deposited using, for example, the sputtering method. As a result, a stack  40   d  as illustrated in  FIG.  6 ( d )  is obtained. In the present modified example, the bottom electrode film  2 B is also deposited on the protective film  12 , as illustrated in  FIG.  6 ( d ) . 
     The following conditions are exemplified as the conditions for depositing the bottom electrode film  2 B. A metal target containing, for example, Hf or Mo can be used as a target used in the sputtering deposition.
         Deposition temperature (substrate temperature): 100° C. or more and 500° C. or less, and preferably 200° C. or more and 400° C. or less   RF-power: 1000 W or more and 1500 W or less, and preferably 1100 W or more and 1300 W or less   Deposition atmosphere: Ar-gas atmosphere   Atmospheric pressure: 0.1 Pa or more and 0.5 Pa and less, and preferably 0.2 Pa or more and 0.4 Pa or less   Deposition time: 3 minutes or more and 10 minutes or less, and preferably 4 minutes or more and 7 minutes or less       

     Then, the AlN-film  3 B is deposited using a procedure and conditions similar to those in the above embodiment. As a result, a stack  40   e  as illustrated in  FIG.  6 ( e )  is obtained. After that, the protective film  12  is removed using a procedure and conditions similar to those in the above embodiment. As a result, the KNN-film  3 A is exposed. Also, using the protective film  12  as a lift-off layer, unnecessary bottom electrode film  2 B and AlN-film  3 B formed on the protective film  12  are also removed. That is, the bottom electrode film  2 B and the AlN-film  3 B are left only in regions where the bottom electrode film  2 B and the AlN-film  3 B are to be deposited. As a result, a stack  40   f  as illustrated in  FIG.  6 ( f )  is obtained. After that, the top electrode films  4 A and  4 B are deposited using a procedure and conditions similar to those in the above embodiment. As a result, the piezoelectric stack  40  as illustrated in  FIG.  5    is obtained. 
     Other configurations and method can be similar to those in the above embodiment. The present modified example can also provide effects similar to those of the above embodiment. That is, in the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. 
     Modified Example 2 
     As illustrated in  FIG.  7   , a piezoelectric stack  41  may be configured by providing the bottom electrode film  2 A on the substrate  1  only at a position facing the KNN-film  3 A, and providing the bottom electrode film  2 B on the substrate  1  only at a position facing the AlN-film  3 B. The bottom electrode film  2 A can have configurations similar to those of the bottom electrode film  2 , and the bottom electrode film  2 B can have configurations similar to those of the above modified example 1. 
     In the present modified example, the output part includes a stacked part of the bottom electrode film  2 A, the KNN-film  3 A, and the top electrode film  4 A. Also, the input part includes a stacked part of the bottom electrode film  2 B, the AlN-film  3 B, and the top electrode film  4 B. 
     Other configurations can be similar to those in the above embodiment and modified examples. The present modified example can also provide effects similar to those of the above embodiment and the like. That is, in the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. 
     Also, according to the present modified example, ultrasonic waves can be transmitted from the output part toward a test object, and the input part can receive ultrasonic waves reflected by the test object. Thereby, for example, the ultrasonic sensor  30  can continuously detect the magnitude of the voltage detected using the voltage detector  11   b . Also, for example, the ultrasonic sensor  30  can continuously measure the time from the start of the voltage application using the voltage applicator  11   a  to the voltage detection using the voltage detector  11   b . Thus, according to the present modified example, a degree of freedom in the transmission and reception of ultrasonic waves in the ultrasonic sensor  30  can also be increased. 
     Modified Example 3 
     As illustrated in  FIG.  8   , a piezoelectric stack  42  may be configured by providing the AlN-film  3 B on the KNN-film  3 A. 
     In the present modified example, the output part includes a stacked part of the bottom electrode film  2 , the KNN-film  3 A, and the top electrode film  4 A. Also, the input part includes a stacked part of the bottom electrode film  2 , the AlN-film  3 B, and the top electrode film  4 B. The input part may include the portion of the KNN-film  3 A located under the AlN-film  3 B. 
     A method of manufacturing the piezoelectric stack  42  illustrated in  FIG.  8    will be described, with reference to  FIGS.  9 ( a ) to  9 ( e ) . 
     The substrate  1  is firstly prepared, and an adhesion layer  6  (Ti-layer) and the bottom electrode film  2  are deposited in this order on any one of the main surfaces of the substrate  1  by, for example, the sputtering method, using a procedure and conditions similar to those in the above embodiment. As a result, a stack  42   a  as illustrated in  FIG.  9 ( a )  is obtained. 
     Subsequently, the KNN-film  3 A is deposited using a procedure and conditions similar to those in the above embodiment. As a result, a stack  42   b  as illustrated in  FIG.  9 ( b )  is obtained. After that, the protective film  12  is deposited on the KNN-film  3 A using a procedure and conditions similar to those in the above embodiment. Then, the protective film  12  is shaped into a predetermined pattern. For example, the protective film  12  is removed by etching or the like from a position on the substrate  1  where the AlN-film  3 B is to be deposited, i.e., a position on the KNN-film  3 A. As a result, a stack  42   c  as illustrated in  FIG.  9 ( c )  is obtained. Then, the AlN-film  3 B is deposited using a procedure and conditions similar to those in the above embodiment. As a result, a stack  42   d  as illustrated in  FIG.  9 ( d )  is obtained. After that, the protective film  12  is removed using a procedure and conditions similar to those in the above embodiment. As a result, a predetermined area of the KNN-film  3 A is exposed. Also, using the protective film  12  as a lift-off layer, an unnecessary AlN-film  3 B formed on the protective film  12  is also removed. That is, the AlN-film  3 B is left only in a region where the AlN-film  3 B is to be deposited. As a result, a stack  42   e  as illustrated in  FIG.  9 ( e )  is obtained. After that, the top electrode films  4 A and  4 B are deposited using a procedure and conditions similar to those in the above embodiment. As a result, the piezoelectric stack  42  as illustrated in  FIG.  8    is obtained. 
     Other configurations can be similar to those in the above embodiment and modified examples. The present modified example can also provide effects similar to those of the above embodiment and modified examples. That is, in the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. The inventors have confirmed that the input part can detect the vibration of the input-side vibration part with a high sensitivity even when the AlN-film  3 B is provided on the KNN-film  3 A as in the present modified example. 
     The present modified example is not limited to the aspect illustrated in  FIG.  8   . For example, the bottom electrode film  2  illustrated in  FIG.  8    may function as the bottom electrode film  2 A, and the bottom electrode film  2 B having configurations similar to those in the modified example  1  may be provided between the KNN-film  3 A and the AlN-film  3 B. 
     Modified Example 4 
     In the above embodiment, explanation has been given for an example of depositing the AlN-film  3 B after depositing the KNN-film  3 A, but the present disclosure is not limited to such an aspect. That is, the KNN-film  3 A may be deposited after depositing the AlN-film  3 B. 
     In the present modified example, the protective film  12  is deposited in such a manner as covering the AlN-film  3 B, after depositing the AlN-film  3 B and before depositing the KNN-film  3 A, using a procedure and conditions similar to those in the above embodiment. Then, the protective film  12  is shaped into a predetermined pattern by removing the protective film  12  by etching or the like from a position on the substrate  1  (bottom electrode film  2 ) where the KNN-film  3 Ais to be deposited. The KNN-film  3 A is then deposited using a procedure and conditions similar to those in the above embodiment. After the deposition of the KNN-film  3 A is complete, the protective film  12  is removed using a procedure and conditions similar to those in the above embodiment. As a result, the AlN-film  3 B is exposed and the unnecessary KNN-film  3 A formed on the protective film  12  is removed. Thus, in the present modified example, the protective film  12  functions as a lift-off layer for removing the unnecessary KNN-film  3 A. 
     Other configurations can be similar to those in the embodiment and modified examples. The present modified example can also provide effects similar to those of the above embodiment and the like. That is, in the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. 
     Modified Example 5 
     As illustrated in  FIG.  10   , a piezoelectric stack  43  may include the substrate  1 , the bottom electrode film  2 B provided on the substrate  1 , the input-side piezoelectric film (AlN-film)  3 B provided on the bottom electrode film  2 B, the output-side bottom electrode film  2 A provided on the input-side piezoelectric film  3 B, the output-side piezoelectric film (KNN-film)  3 A provided on the output-side bottom electrode film  2 A, the top electrode film  4 A provided on the output-side piezoelectric film  3 A, and the top electrode film  4 B provided on the input-side piezoelectric film  3 B. 
     In the present modified example, the output part includes a stacked part of the bottom electrode film  2 A, the KNN-film  3 A, and the top electrode film  4 A. The output part may include portions of the bottom electrode film  2 B and the AlN-film  3 B which are located under the KNN-film  3 A. Also, the input part includes a stacked part of the bottom electrode film  2 B, the AlN-film  3 B, and the top electrode film  4 B. 
     Other aspects can be similar to the aspects in the above embodiment and modified examples. The present modified example can also provide effects similar to those of the above embodiment and modified examples. That is, in the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. 
     Also, in the present modified example, the bottom electrode film  2 A is provided on the AlN-film  3 B, and the KNN-film  3 A is provided on the bottom electrode film  2 A. Thereby, in the present modified example as well, since the KNN-film  3 A is deposited directly on the bottom electrode film  2 A, i.e., directly on the Pt-film, the crystals constituting the KNN-film  3 A are easily preferentially oriented in (001) direction. However, the bottom electrode film  2 A may not be provided. That is, the configurations of the bottom electrode film  2 B may be similar to the configurations of the bottom electrode film  2  in the above embodiment, and the bottom electrode film  2  may function as the output-side bottom electrode film  2 A and also function as the input-side bottom electrode film  2 B. 
     Modified Example 6 
     A substrate on which a semiconductor circuit such as CMOS is deposited, can also be used as the substrate  1 . In a case where the KNN-film  3 A is deposited on the substrate  1  on which the circuit such as CMOS is formed, from a viewpoint of suppressing a breakdown of the semiconductor circuit such as CMOS, the KNN-film  3 A is preferably deposited under a condition of a temperature less than 500° C., and more preferably higher than or equal to the deposition temperature of the AlN-film  3 B and less than 500° C. By depositing the KNN-film  3 A under the condition of the temperature less than 500° C., the breakdown of the semiconductor circuit formed on the substrate  1  can be suppressed during the deposition of the KNN-film  3 A. By depositing the KNN-film  3 A under the condition of the temperature higher than or equal to the deposition temperature of the AlN-film  3 B, even in a case where the AlN-film  3 B is deposited after depositing the KNN-film  3 A as in the above embodiment, the reduction of the KNN-film  3 A can be reliably suppressed in the deposition atmosphere of the AlN-film  3 B. The inventors have confirmed that even when the KNN-film  3 A is deposited at a low temperature of less than 500° C., the KNN-film  3 A having the piezoelectric constant of 100 pm/V or more can be obtained. 
     Other aspects can be similar to the aspects in the above embodiment and modified examples. The present modified example can also provide effects similar to those of the above embodiment and the like. That is, in the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. 
     Modified Example 7 
     The output part and the input part may be in contact with each other. Since the output-side vibration part and the input-side vibration part are formed independently with each other, it is possible to prevent the output-side vibration part and the input-side vibration part from interfering with each other. In the present modified example as well, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be produced collectively in the MEMS manufacturing process. However, from a viewpoint of reliably preventing the output part and the input part from interfering, and reliably suppressing the deterioration in the sensor performance of the ultrasonic sensor  30 , the output part and the input part are preferably not in contact with each other. 
     Modified Example 8 
     In the above embodiment, explanation has been given for a case where the AlN-film  3 B is a polycrystalline film of AlN, but the AlN-film  3 B may be a single-crystal film of AlN. The present modified example can also provide effects similar to those of the above embodiment and modified examples. 
     Other Embodiments 
     As described above, explanation has been given specifically for the embodiments of the present disclosure. However, the present disclosure is not limited thereto, and can be variously modified without departing from the gist of the disclosure. 
     In the above embodiment and modified examples, explanation has been given for a case where the output-side piezoelectric film  3 A is the KNN-film, but the present disclosure is not limited thereto. The output-side piezoelectric film  3 A may also be deposited using, in place of KNN, a compound which contains lead (Pb), zirconium (Zr), and titanium (Ti), and which is represented by a composition formula Pb(Zr 1-x Ti x )O 3  (0&lt;x&lt;1). That is, the output-side piezoelectric film  3 A can be deposited using lead-zirconium-titanium oxide (PZT). The output-side piezoelectric film  3 A can also be deposited using a compound which contains bismuth (Bi), Na, and Ti, and which is represented by a composition formula (Bi 1-x Na x )TiO 3  (0&lt;x&lt;1). That is, the output-side piezoelectric film  3 A can also be deposited using bismuth sodium titanium oxide (BNT). Moreover, the output-side piezoelectric film  3 A can also be deposited using a compound represented by a composition formula BiFeO 3 , i.e., using bismuth ferrite (BFO). Even when the output-side piezoelectric film  3 A is deposited using PZT, BNT, or BFO, effects similar to those of the above embodiment and the like can be obtained. That is, the ultrasonic sensor  30  having the high performance can be obtained, and such an ultrasonic sensor  30  can be collectively produced in the MEMS manufacturing process. 
     In the above embodiment and modified examples, explanation has been given for a case where the input-side piezoelectric film  3 B is the AlN-film, but the present disclosure is not limited thereto. The input-side piezoelectric film  3 B may be another nitride film exhibiting the piezoelectric performance equivalent to that of the AlN-film. 
     For example, the output-side piezoelectric film  3 A may contain, in addition to or in place of the above metal elements such as Cu and Mn, other metal element having effects equivalent to those of the above metal element, at a predetermined concentration. 
     In the above embodiment, explanation has been given for a case of using the piezoelectric stack  10  and the piezoelectric element  20  to obtain the ultrasonic sensor  30 , but the present disclosure is not limited thereto. The piezoelectric stack  10  and piezoelectric element  20  may be used to obtain a piezoelectric device module used for applications such as a head for an inkjet printer, a MEMS mirror for a scanner, an angular velocity sensor, a pressure sensor, or an acceleration sensor. 
     Preferable Aspects of Present Disclosure Preferable aspects of the present disclosure will be supplementarily described hereinafter. 
     Supplementary Description 1 
     According to an aspect of the present disclosure, there is provided a piezoelectric stack, including:
         a substrate;   an output-side bottom electrode film on the substrate;   an output-side piezoelectric film, being an oxide film, on the output-side bottom electrode film;   an output-side top electrode film on the output-side piezoelectric film;   an input-side bottom electrode film on the substrate;   an input-side piezoelectric film, being a nitride film, on the input-side bottom electrode film;   an input-side top electrode film on the input-side piezoelectric film; and   an ultrasonic output part and an ultrasonic input part are placed in such a manner as not overlapping each other when viewed from a top surface of the substrate,
           the ultrasonic output part including a stacked part of the output-side bottom electrode film, the output-side piezoelectric film, and the output-side top electrode film,   the ultrasonic input part including a stacked part of the input-side bottom electrode film, the input-side piezoelectric film, and the input-side top electrode film.   
               

     Supplementary Description 2 
     Preferably, there is provided the piezoelectric stack according to the supplementary description 1, wherein the ultrasonic output part and the ultrasonic input part are not in contact with each other. 
     Supplementary Description 3 
     Preferably, there is provided the piezoelectric stack according to the supplementary description 1 or 2, wherein the output-side piezoelectric film has a larger piezoelectric constant than that of the input-side piezoelectric film, and the input-side piezoelectric film has a lower dielectric constant than that of the output-side piezoelectric film. 
     Supplementary Description 4 
     Preferably, there is provided the piezoelectric stack according to any one of the supplementary descriptions 1 to 3, wherein the output-side piezoelectric film, being a deposited film, contains any one of potassium sodium niobium oxide, lead-zirconium-titanium oxide, bismuth sodium titanium oxide, or bismuth ferrite. 
     Supplementary Description 5 
     Preferably, there is provided the piezoelectric stack according to any one of the supplementary descriptions 1 to 4, wherein the output-side piezoelectric film, being a deposited film, contains potassium sodium niobium oxide, and contains at least one selected from a group of Cu and Mn at a concentration of 0.2 at % or more and 2.0 at % or less relative to an amount of niobium in the output-side piezoelectric film. 
     Supplementary Description 6 
     Preferably, there is provided the piezoelectric stack according to any one of the supplementary descriptions 1 to 5, wherein the input-side piezoelectric film, being a deposited film, contains aluminum nitride. 
     Supplementary Description 7 
     Preferably, there is provided the piezoelectric stack according to the supplementary description 6, wherein the input-side piezoelectric film contains scandium (Sc), contains magnesium (Mg) and zirconium (Zr), or contains magnesium (Mg) and hafnium (Hf). 
     Supplementary Description 8 
     Preferably, there is provided the piezoelectric stack according to any one of the supplementary descriptions 1 to 7, further including: a semiconductor circuit on the substrate. 
     Supplementary Description 9 
     According to another aspect of the present disclosure, there is provided a method of manufacturing a piezoelectric stack, including:
         depositing an output-side bottom electrode film and an input-side bottom electrode film on a substrate;   depositing an output-side piezoelectric film, being an oxide film, on the output-side bottom electrode film;   depositing a protective film that protects the output-side piezoelectric film;   depositing an input-side piezoelectric film, being a nitride film, on the input-side bottom electrode film;   exposing the output-side piezoelectric film by removing the protective film by etching; and   depositing an output-side top electrode film on the output-side piezoelectric film and depositing an input-side top electrode film on the input-side piezoelectric film, thereby   producing a stack in which an ultrasonic output part and an ultrasonic input part are placed in such a manner as not overlapping each other when viewed from a top surface of the substrate,
           the ultrasonic output part including a stacked part of the output-side bottom electrode film, the output-side piezoelectric film, and the output-side top electrode film,   the ultrasonic input part including a stacked part of the input-side bottom electrode film, the input-side piezoelectric film, and the input-side top electrode film.   
               

     Supplementary Description 10 
     Preferably, there is provided the method according to the supplementary description 9, wherein the protective film is a film containing silicon dioxide (SiO 2 ). 
     Supplementary Description 11 
     Preferably, there is provided the method according to the supplementary description 9 or 10,
         wherein a substrate including a semiconductor circuit is prepared as the substrate, and   in the deposition of the output-side piezoelectric film, the output-side piezoelectric film is deposited under a condition of a temperature less than 500° C. Preferably, in the deposition of the output-side piezoelectric film, the output-side piezoelectric film is deposited under a condition of a temperature higher than or equal to a deposition temperature of the input-side piezoelectric film in the deposition of the input-side piezoelectric film and less than 500° C.       

     Supplementary Description 12 
     According to further another aspect of the present disclosure, there is provided a piezoelectric element and an ultrasonic sensor, each including:
         the piezoelectric stack according to any one of the supplementary descriptions 1 to 8;   a voltage applicator connected between the output-side bottom electrode film and the output-side top electrode film; and   a voltage detector connected between the input-side bottom electrode film and the input-side top electrode film,   wherein the output-side piezoelectric film deforms under a voltage application from the voltage applicator applying a predetermined electric field between the output-side bottom electrode film and the output-side top electrode film, and ultrasonic waves generated due to a deformation of the output-side piezoelectric film are transmitted from the ultrasonic output part, and the ultrasonic input part receives the ultrasonic waves reflected by a test object, and a voltage generated between the input-side bottom electrode film and the input-side top electrode film due to a deformation of the input-side piezoelectric film is detected by the voltage detector.       

     Supplementary Description 13 
     Preferably, there is provided the element according to the supplementary description 12, further including: a vibration part (e.g., a membrane structure or a cantilever structure) on the substrate at each of positions corresponding to the ultrasonic output part and corresponding to the ultrasonic input part. 
     DESCRIPTION OF SIGNS AND NUMERALS 
       1  Substrate 
       2 A Output-side bottom electrode film
   2 B Input-side bottom electrode film
   3 A Output-side piezoelectric film (KNN-film)
   3 B Input-side piezoelectric film (AlN-film)
   4 A Output-side top electrode film
   4 B Input-side top electrode film
   10 ,  40  to  43  Piezoelectric stack