Piezoelectric element and actuator

Abstract

The invention relates to a piezoelectric element and an actuator. The present invention addresses the problem of providing, at low cost, a piezoelectric element and a piezoelectric actuator that can obtain a large driving force, have good responsiveness, and can obtain long-term reliability. In the piezoelectric element and the actuator, the piezoelectric element includes a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode in this order on a substrate, the first piezoelectric film and the second piezoelectric film each have a perovskite oxide as a main component, one of the first piezoelectric film and the second piezoelectric film is an epitaxial film, and the other is a uniaxially oriented film.

Description

Technical Field

[0001] The invention relates to a piezoelectric element and an actuator. Background Art

[0002] As materials with excellent piezoelectric properties and ferroelectricity, perovskite oxides such as lead zirconate titanate (Pb(Zr, Ti)O3, hereinafter referred to as PZT) are known. Piezoelectric bodies composed of perovskite oxides are used as piezoelectric films in piezoelectric elements having a lower electrode, a piezoelectric film, and an upper electrode on a substrate. The piezoelectric element is used in various devices such as memory, inkjet head (actuator), micromirror device, angular velocity sensor, gyro sensor, ultrasonic element (PMUT: Piezoelectric Micromachined Ultrasonic Transducer) and vibration power generation device.

[0003] As a piezoelectric element, a stacked piezoelectric element in which a plurality of piezoelectric films are stacked with electrodes interposed therebetween is proposed to obtain high piezoelectric characteristics (Patent Documents 1, 2, etc.).

[0004] Patent Document 1 discloses a piezoelectric element in which a plurality of layers of uniaxially oriented PZT films are stacked with electrodes interposed therebetween. Patent Document 2 discloses a piezoelectric element in which a plurality of layers of uniaxially oriented PZT films are stacked with electrodes interposed therebetween. a La b )(Zr c Ti d Nb e ) 3-δ The epitaxial film of the piezoelectric element is stacked with electrodes in between. The contents disclosed in any of the above cases are that a plurality of electrodes are connected to a positive terminal and a negative terminal in between, and the direction of the electric field applied to the stacked piezoelectric film is alternately reversed.

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 11-233844

[0006] Patent Document 2: International Publication No. 2017 / 018222 Summary of the invention

[0007] As disclosed in Patent Documents 1 and 2, a piezoelectric element including a multilayer piezoelectric film can obtain a larger driving force than a piezoelectric element including a single layer piezoelectric film.

[0008] However, the piezoelectric elements described in Patent Documents 1 and 2 have the following problems. In a stacked piezoelectric element formed by stacking multiple layers of uniaxially oriented PZT films, an electric field in the opposite direction to the self-polarization is applied to at least a portion of the multilayer piezoelectric film, thereby generating a nonlinear response component, resulting in low linearity and failure to achieve the displacement behavior envisioned as a device. In some cases, the long-term reliability of a stacked piezoelectric element formed by stacking piezoelectric films composed of epitaxial films is reduced.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a piezoelectric element and a piezoelectric actuator which can obtain a large driving force, good responsiveness, and long-term reliability at low cost.

[0010] The piezoelectric element of the present invention comprises a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film and a third electrode in this order on a substrate.

[0011] The first piezoelectric film and the second piezoelectric film each have a perovskite-type oxide as a main component.

[0012] One of the first piezoelectric film and the second piezoelectric film is an epitaxial film, and the other is a uniaxially oriented film.

[0013] Preferably, the first piezoelectric film is an epitaxial film, and the second piezoelectric film is a uniaxially oriented film.

[0014] Preferably, a surface of the first electrode in contact with the first piezoelectric film is an epitaxial film.

[0015] Preferably, the perovskite type oxide as a main component of the first piezoelectric film and the perovskite type oxide as a main component of the second piezoelectric film are composed of the same constituent elements.

[0016] Preferably, the perovskite-type oxide is lead zirconate titanate or lead zirconate titanate added with a metal element M, and the metal element M is at least one of vanadium, niobium, tantalum, antimony, molybdenum and tungsten.

[0017] Preferably, the metal element M in the perovskite type oxide is niobium, and at least the amount of the metal element M added in the composition ratio of the perovskite type oxides contained in the first piezoelectric film and the second piezoelectric film is the same.

[0018] Preferably, the first electrode, the second electrode, and the third electrode each contain at least one of iridium, platinum, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide.

[0019] The first electrode and the third electrode may be maintained at a ground potential, and the second electrode may be a driving electrode for applying a positive driving voltage to the first piezoelectric film and the second piezoelectric film.

[0020] The second electrode may be maintained at a ground potential, and the first electrode and the third electrode may be driving electrodes for applying a negative driving voltage to the first piezoelectric film and the second piezoelectric film.

[0021] The actuator of the present invention comprises the piezoelectric element of the present invention and a drive circuit for applying a drive voltage to the piezoelectric element, wherein:

[0022] The driving circuit applies an electric field in the same direction as the polarization, i.e., the self-polarization, which the piezoelectric film spontaneously possesses immediately after film formation, to the piezoelectric film as a uniaxially oriented film, and applies an electric field in the opposite direction to the electric field applied to the piezoelectric film as a uniaxially oriented film to the piezoelectric film as an epitaxial film.

[0023] Effects of the Invention

[0024] According to the technology of the present invention, a piezoelectric element and a piezoelectric actuator that can obtain a large driving force, have good responsiveness, and obtain long-term reliability can be provided at low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a cross-sectional view of a piezoelectric element according to one embodiment.

[0026] Figure 2 It is a diagram showing a schematic structure of an actuator.

[0027] Figure 3 It is a diagram showing a schematic structure of an actuator.

[0028] Figure 4 These are the measured data of PZT epitaxial films based on XRD-φ scanning.

[0029] Figure 5 It is a diagram showing a driving signal and a velocity waveform for the piezoelectric element of Example 1.

[0030] Figure 6 3 is a diagram showing a driving signal and a velocity waveform for the piezoelectric element of Comparative Example 1. DETAILED DESCRIPTION

[0031] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following drawings, the thickness of each layer and their ratio are appropriately changed and drawn for easy visual recognition, and do not necessarily reflect the actual layer thickness and ratio.

[0032] Figure 1 1 is a schematic cross-sectional view showing a layer structure of a piezoelectric element 1 according to an embodiment. Figure 1 As shown, the piezoelectric element 1 includes a first electrode 12 , a first piezoelectric film 14 , a second electrode 16 , a second piezoelectric film 18 , and a third electrode 20 in this order on a substrate 10 .

[0033] The substrate 10 is not particularly limited, and examples thereof include substrates made of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. As the substrate 10 , a substrate made of single crystal silicon is particularly preferred.

[0034] The first electrode 12 and the second electrode 16 form a pair and apply a voltage to the first piezoelectric film 14 . Furthermore, the second electrode 16 and the third electrode 20 form a pair and apply a voltage to the second piezoelectric film 18 .

[0035] The main components of the first electrode 12, the second electrode 16, and the third electrode 20 are not particularly limited, but preferably include iridium (Ir), platinum (Pt), strontium ruthenate (SrRuO3), barium ruthenate (BaRuO3), zirconium oxide (ZrO2), hafnium oxide (HfO2), ruthenium (Ru), ruthenium oxide (RuO x ), iridium oxide (IrO x ), platinum oxide (PtO x ) and rhenium oxide (ReO x The first electrode 12, the second electrode 16 and the third electrode 20 may be a single layer or may have a multi-layer stacked structure.

[0036] The thickness of the first electrode 12 , the second electrode 16 , and the third electrode 20 is not particularly limited, but is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0037] The first piezoelectric film 14 and the second piezoelectric film 18 each have a perovskite oxide represented by the general formula ABO3 as a main component. In this specification, the main component refers to a component that occupies more than 80 mol%. The first piezoelectric film 14 and the second piezoelectric film 18 preferably have a perovskite oxide that occupies more than 90 mol%, and more preferably the first piezoelectric film 14 and the second piezoelectric film 18 are composed of a perovskite oxide (including inevitable impurities). The perovskite oxide as the main component of the first piezoelectric film 14 and the perovskite oxide as the main component of the second piezoelectric film 18 may be composed of different constituent elements, or the constituent elements may be the same as each other. However, from the viewpoint of cost reduction, it is preferred that the constituent elements of the perovskite oxide of the first piezoelectric film 14 and the second piezoelectric film 18 are the same. In addition, "the same constituent elements" means that the elements contained are the same, but the composition ratio may be different.

[0038] The first piezoelectric film 14 and the second piezoelectric film 18 are preferably so-called PZT-based perovskite oxides containing Pb (lead) at the A position and Zr (zirconium) and Ti (titanium) at the B position, and are particularly preferably M-added PZT in which a metal element M is added at the B position. Here, the metal element M is at least one of vanadium (V), niobium (Nb), tantalum (Ta), antimony (Sb), molybdenum (Mo) and tungsten (W). The M-added PZT is represented by the following general formula.

[0039] Pb a {(Zr x Ti 1-x ) 1-y M y}O3

[0040] Here, 0<x<1, 0<y<1, and 0.9≤a≤1.2. The reference of the molar ratio a of Pb is 1, but a perovskite structure can be formed as long as it is in the range of 0.9≤a≤1.2.

[0041] The metal element M may be a single element such as V or Nb, or a combination of two or more elements such as a mixture of V and Nb or a mixture of V, Nb and Ta. When the metal element M is these elements, it can be combined with Pb, the A-site element, to achieve a very high piezoelectric constant.

[0042] The most preferred metal element is Pb, wherein the metal element M is Nb. a {(Zr x Ti 1-x ) 1-y Nb y}O3. At this time, 0.1≤y≤0.3, and a higher piezoelectric constant can be obtained.

[0043] It is preferred that both the first piezoelectric film 14 and the second piezoelectric film 18 have Nb-doped PZT as the main component. In this case, it is preferred that the composition ratios of the Nb-doped PZT as the main component of the first piezoelectric film 14 and the Nb-doped PZT as the main component of the second piezoelectric film 18 are the same. That is, when the Pb composition ratio in the first piezoelectric film 14 is set to a1, the Zr composition ratio is set to x1, and the M composition ratio is set to y1, and when the Pb composition ratio of the perovskite-type oxide in the second piezoelectric film 18 is set to a2, the Zr composition ratio is set to x2, and the M composition ratio is set to y2, it is preferred that y1=y2. Moreover, it is more preferred that y1=y2 and x1 and x2 are the same. Here, the same composition ratio means that it is equal within the range of measurement error. As a method for measuring the composition, there are various analysis methods such as ICP (Inductively Coupled Plasma: Inductively Coupled Plasma) emission spectroscopy or XRF (X-ray Fluorescence: X-ray fluorescence). For example, when the composition analysis is performed by XRF, the measurement error is about 0.005.

[0044] In addition, it is believed that in PZT-based perovskite oxides, piezoelectric properties are shown at the morphotropic phase boundary (MPB) and its vicinity. The MPB composition is near Zr:Ti (molar ratio) = 52:48, and in the above general formula, it is preferably the MPB composition or its vicinity. "MPB or its vicinity" is the region where a phase change occurs when an electric field is applied to the piezoelectric film. Specifically, Zr:Ti (molar ratio) is preferably in the range of 45:55 to 55:45, that is, in the range of x = 0.45 to 0.55.

[0045] The film thickness t1 of the first piezoelectric film 14 and the film thickness t2 of the second piezoelectric film 18 are preferably 0.1 μm or more and 5 μm or less, and more preferably 1 μm or more and less than 5 μm. The film thicknesses of the first piezoelectric film 14 and the second piezoelectric film 18 may be the same or different. The first piezoelectric film 14 and the second piezoelectric film 18 are preferably 2 μm or less.

[0046] One of the first piezoelectric film 14 and the second piezoelectric film 18 is an epitaxial film, and the other is a uniaxially oriented film. It is particularly preferred that the first piezoelectric film 14 disposed on the substrate 10 side is an epitaxial film, and the second piezoelectric film 18 is a uniaxially oriented film. Hereinafter, the case where the first piezoelectric film 14 is an epitaxial film and the second piezoelectric film 18 is a uniaxially oriented film will be described.

[0047] In order to epitaxially grow the first piezoelectric film 14, it is particularly preferred that a single crystal silicon substrate is used as the substrate 10, and a stacked structure having a first film 12A composed of a ZrO2 or HfO2 film, a second film 12B composed of a Pt film or an Ir film, and a third film 12C composed of a SrRuO3 or BaRuO3 film is provided on the single crystal silicon substrate as the first electrode 12. In order to epitaxially grow the first piezoelectric film 14, it is preferred that the surface of the first electrode 12 that contacts the first piezoelectric film 14 is an epitaxial film. That is, when the first electrode 12 is a three-layer structure as described above, it is preferred that the third film 12C that is closest to the first piezoelectric film 14 is an epitaxial film.

[0048] The second piezoelectric film 18 , which is a uniaxially oriented film, has its spontaneous polarization aligned upward in the film thickness direction when no external electric field is applied. Figure 1 The arrow P2 in represents the direction of polarization. Hereinafter, the direction of polarization is referred to as P2. In addition, whether the spontaneous polarization in the piezoelectric film is consistent and the direction in which the spontaneous polarization is consistent can be confirmed by measuring the PV hysteresis loop (or PE hysteresis loop) representing the polarization-voltage characteristic (or polarization-electric field characteristic) of the piezoelectric film.

[0049] The second piezoelectric film 18 is a uniaxially oriented film, which means that the perovskite oxide as the main component of the second piezoelectric film 18 is uniaxially oriented. The uniaxially oriented perovskite oxide is a film that is polarized in the thickness direction of the film in a state where the film is just formed, i.e., the direction of spontaneous polarization is consistent without polarization treatment. The piezoelectric film that has not been subjected to polarization treatment has a consistent direction of spontaneous polarization in a state where no external electric field is applied. It is believed that this is because an electric field (hereinafter referred to as spontaneous internal electric field) caused by strain, defects, etc. of the crystal structure is generated in the piezoelectric film. The second piezoelectric film 18 has a spontaneous polarization that is consistent in the thickness direction of the film caused by such a spontaneous internal electric field. The second piezoelectric film 18 is a piezoelectric film that is polarized in the thickness direction of the film when no external electric field is applied because the spontaneous polarization is consistent in the thickness direction of the film. The piezoelectric film is composed of multiple partitions (domains) of spontaneous polarization. In this specification, "spontaneous polarization is consistent along a specific direction" refers to a state in which the spontaneous polarization component in a specific direction is relatively more than the spontaneous polarization component in other directions. In a piezoelectric film, if the spontaneous polarization component in a specific direction is relatively larger than the spontaneous polarization components in other directions, the film can be regarded as being polarized in the specific direction.

[0050] On the other hand, the first piezoelectric film 14 is an epitaxial film, and its spontaneous polarization is isotropic in a state where no polarization treatment is performed. Therefore, it is polarized in the direction of applied electric field, and a stable polarization state can be obtained.

[0051] Figure 2 The schematic structure of the actuator 5 including the piezoelectric element 1 is shown. The actuator 5 includes the piezoelectric element 1 and a drive circuit 30 .

[0052] The driving circuit 30 is a mechanism for supplying a driving voltage to the first piezoelectric film 14 and the second piezoelectric film 18 sandwiched between electrodes in order to drive the piezoelectric element 1. In this example, the second electrode 16 is connected to the ground terminal (GND) of the driving circuit 30, and the first electrode 12 and the third electrode 20 are connected to the driving voltage output terminal (-V) of the driving circuit 30. Thus, the driving circuit 30 applies electric fields in opposite directions to the first piezoelectric film 14 and the second piezoelectric film 18. In this example, the driving circuit 30 applies an electric field Ef in the same direction as the direction (polarization direction) P2 consistent with the spontaneous polarization to the second piezoelectric film 18, which is one of the piezoelectric films, and applies an electric field Er in the opposite direction to the electric field Ef applied to the second piezoelectric film 18 to the first piezoelectric film 14, which is the other piezoelectric film. The driving circuit 30 is a negative driving circuit that applies a negative potential to the driving electrode (here, the second electrode 16).

[0053] exist Figure 2 In the actuator 5 shown, regarding the piezoelectric element 1, the second electrode 16 is set to the ground potential, and the first electrode 12 and the third electrode 20 are used as driving electrodes. In this example, when the piezoelectric element 1 is driven, a negative potential (-V) is applied to the driving electrodes (here, the first electrode 12 and the third electrode 20). As a result, an electric field Ef in the same direction as the direction (polarization direction) P2 consistent with the spontaneous polarization is applied to the second piezoelectric film 18 as one of the piezoelectric films, and an electric field Er in the opposite direction to the electric field Ef applied to the second piezoelectric film 18 is applied to the first piezoelectric film 14 as the other piezoelectric film.

[0054] In addition, the piezoelectric element 1 preferably connects the first electrode 12 and the third electrode 20. If the first electrode 12 and the third electrode 20 are connected, driving control is easy.

[0055] In contrast, Figure 3 As shown in the modified example of the actuator 6, a positive drive circuit for applying a positive potential to the drive electrode may be provided as the drive circuit 32. Figure 3 In the example shown, the first electrode 12 and the third electrode 20 are connected to the ground terminal of the driving circuit 32, and the second electrode 16 is connected to the driving voltage output terminal of the driving circuit 32. That is, the first electrode 12 and the third electrode 20 are set to the ground potential, and the second electrode 16 functions as a driving electrode. At this time, the driving circuit 32 can also apply an electric field Ef in the same direction as the direction (polarization direction) P2 consistent with the spontaneous polarization to the second piezoelectric film 18, and an electric field Er in the opposite direction to the electric field Ef applied to the second piezoelectric film 18 can be applied to the first piezoelectric film 14.

[0056] In actuators 5 and 6 (ref. Figure 2 , Figure 3), an electric field in the opposite direction to the electric field applied to the second piezoelectric film 18 is applied to the first piezoelectric film 14. Figure 2 and Figure 3 As shown, the first piezoelectric film 14 is polarized in a direction P1 opposite to a direction P2 of polarization of the second piezoelectric film 18 .

[0057] The actuators 5 and 6 include only one polarity driving circuit as the driving circuits 30 and 32 , and can be realized at low cost.

[0058] The inventors of the present invention have discovered that, according to the piezoelectric element 1 shown in the present invention, a first electrode 12, a first piezoelectric film 14, a second electrode 16, a second piezoelectric film 18 and a third electrode 20 are sequentially provided on a substrate 10, the first piezoelectric film 14 and the second piezoelectric film 18 each contain a perovskite-type oxide as a main component, one of the first piezoelectric film 14 and the second piezoelectric film 18 is an epitaxial film, and the other is a uniaxially oriented film structure, which has good responsiveness during driving and can obtain very high long-term reliability (refer to the embodiments described later).

[0059] Since the piezoelectric element has a multilayer piezoelectric film, a larger driving force can be obtained compared to a piezoelectric element having a single layer of piezoelectric film. Figure 2 or Figure 3 As shown, by setting the first electrode 12 and the third electrode 20 to the same potential and applying a voltage in a manner to generate an electric field between the first electrode 12 and the third electrode 20 and the second electrode 16, the driving circuit can be composed of one, so that when applied to an actuator, the cost can be reduced, and an actuator with a large driving force can be realized at a low cost.

[0060] Example

[0061] Hereinafter, specific examples and comparative examples of the piezoelectric element of the present invention will be described. First, the structure and production method of the piezoelectric element of each example will be described.

[0062] As an example and a comparative example, a piezoelectric element having a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode in this order on a substrate was produced (refer to Figure 1 The layer structures of the embodiments and comparative examples are summarized in Table 1 described below.

[0063] Example 1

[0064] A method for manufacturing the piezoelectric element of Example 1 is described.

[0065] (Substrate with first electrode)

[0066] A substrate with an electrode (SiZrO2 / Pt / SrRuO3) having a first electrode layer formed by laminating a 40 nm thick ZrO2 film, a 150 nm thick Pt film, and a 60 nm thick SrRuO3 film in sequence on a substrate composed of a silicon wafer was prepared.

[0067] (First piezoelectric film)

[0068] On the first electrode, a PZT film doped with Nb is formed as the first piezoelectric film, in which the amount of Nb added to the B site is set to 12 at%. The film thickness of the PZT film doped with Nb is set to 2 μm. PZT doped with Nb is used as a target, and the sputtering conditions are as follows. In addition, the PZT target doped with Nb is set to have a Pb composition ratio of 1.3, a Zr / Ti molar ratio of MPB composition (Zr / Ti=52 / 48), i.e., x=0.52, and a Nb composition ratio y=0.12. In Table 1, the case where the Nb composition ratio y=0.12, i.e., the case where the amount of Nb added to the B site is 12%, is recorded as Nb-PZT as the default case.

[0069] -Sputtering conditions of the first piezoelectric film-

[0070] Distance between target and substrate: 100mm

[0071] Target input power: 3kW

[0072] Vacuum degree: 0.5Pa, Ar and O2 mixed atmosphere (O2 volume fraction 2.5%)

[0073] Substrate setting temperature: 600℃

[0074] Substrate bias voltage: +40V

[0075] (Second electrode)

[0076] 50nm of IrO is stacked on the first piezoelectric film. x (x≤2) and 200nm of Ir as the second electrode. The sputtering conditions are as follows.

[0077] -IrO x , Ir sputtering conditions -

[0078] Distance between target and substrate: 100mm

[0079] Target input power: 200W

[0080] Vacuum degree: 0.3Pa, Ar atmosphere during Ir film formation, IrO x The film formation was carried out in a mixed atmosphere of Ar and O2 (O2 volume fraction 5%)

[0081] Substrate setting temperature: room temperature

[0082] In addition, after the above-mentioned IrO2 and Ir films were formed, annealing was performed at 350°C for 15 minutes.

[0083] (Second piezoelectric film)

[0084] On the second electrode, a PZT film doped with Nb was formed as the second piezoelectric film, with the amount of Nb added to the B site set to 12 at % similarly to the first piezoelectric film. The target material and sputtering conditions were set the same as those for the first piezoelectric film.

[0085] (Third electrode)

[0086] 50nm of IrO is stacked on the second piezoelectric film. x The sputtering conditions were the same as those for the second electrode.

[0087] (Formation of Evaluation Electrode Pattern)

[0088] In order to form the voltage applying electrode pads on the first electrode, the second electrode, and the third electrode, the third electrode, the second piezoelectric film, the second electrode, and the first piezoelectric film were patterned in this order by photolithography and dry etching.

[0089] The piezoelectric laminate of Example 1 was produced through the above steps.

[0090] "Examples 2 to 8, Comparative Examples 1 and 2"

[0091] For Examples 2 to 8 and Comparative Examples 1 and 2, as substrates with electrodes, substrates with electrodes having a different structure of the first electrode from the substrate with electrodes in Example 1 were prepared. The structure of each first electrode is shown in Table 1. For example, in Example 2, a substrate with a first electrode (Si / ZrO2 / Pt / BaRuO3) was prepared in which the third film on the outermost surface of the first electrode in Example 1 was changed from SrRuO3 to BaRuO3, and in Example 3, a substrate with a first electrode (Si / HfO2 / Pt / SrRuO3) was prepared in which the first film on the substrate side of the first electrode in Example 1 was changed from ZrO2 to HfO2.

[0092] Regarding Comparative Example 1, a substrate with an electrode (Si / SiO2 / ZrO2 / Pt / SrRuO3) was prepared, which had a first electrode layer formed by stacking a 40nm thick ZrO2 film, a 150nm thick Pt film, and a 60nm thick SrRuO3 film in sequence on a substrate composed of a silicon wafer with an oxide film (SiO2 film) as in Example 1. Furthermore, regarding Comparative Example 2, a substrate with an electrode (Si / SiO2 / TiW / Ir) was prepared, which had a first electrode layer formed by stacking a 20nm thick TiW film and a 150nm thick Ir film in sequence on a substrate composed of a silicon wafer with an oxide film (SiO2 film).

[0093] Piezoelectric laminates of Examples 2 to 8 and Comparative Examples 1 and 2 were obtained in the same manner as in Example 1 except for the structure of the substrate with the first electrode.

[0094] In Examples 1 to 8, the first piezoelectric film is an epitaxial film (marked as "epi" in the Orientation State column in Tables 1 and 2), and the second piezoelectric film is a uniaxially oriented film (marked as "uniaxially oriented" in the Orientation State column in Tables 1 and 2). Furthermore, in Examples 1 to 8 in Table 1, the orientation state of the substrate with the first electrode refers to the orientation state of the outermost surface of the first electrode. For example, in Example 1, the SrRuO3 that becomes the outermost surface of the first electrode is an epitaxial film. In Comparative Examples 1 and 2, the first piezoelectric film and the second piezoelectric film are both uniaxially oriented films. Furthermore, the SrRuO3 on the outermost surface of the first electrode of Comparative Example 1 and the Ir on the outermost surface of the first electrode of Comparative Example 2 are uniaxially oriented films.

[0095] "Examples 11 to 15, Comparative Example 3"

[0096] Examples 11 to 15 and Comparative Example 3 have different compositions of the second electrode from Example 1. For example, the second electrode of Example 11 is formed by sequentially stacking PtO x and Pt stacked structure (PtO x / Pt). In Example 14, TaO x , Ta and Pt stacked structure (TaO x / Ta / Pt).

[0097] In Examples 11 to 15, the first piezoelectric film is an epitaxial film, and the second piezoelectric film is a uniaxially oriented film. In Comparative Example 3, both the first piezoelectric film and the second piezoelectric film are epitaxial films.

[0098] "Examples 21 to 26"

[0099] As shown in Table 2, in Examples 21 to 26, the combination of the composition of the first piezoelectric film and the second piezoelectric film is different from that in Example 1. In Example 21, both the first piezoelectric film and the second piezoelectric film are PZT films not doped with the metal element M. The first piezoelectric film of Example 21 is a PZT film doped with 12% Nb as in Example 1, and the second piezoelectric film is a PZT film doped with 10% Nb. The first piezoelectric film and the second piezoelectric film of Examples 24 to 26 are shown in Table 2, respectively.

[0100] In Examples 21 to 26, the first piezoelectric film is an epitaxial film and the second piezoelectric film is a uniaxially oriented film. However, the crystallinity of the epitaxial film of the first piezoelectric film of Example 26 is lower than that of the other examples. In addition, the uniaxially oriented film of the second piezoelectric film of Examples 25 and 26 is lower than that of the uniaxially oriented films of the other examples.

[0101] "Examples 31 to 35"

[0102] In Examples 31 to 35, the composition of the first piezoelectric film and the second piezoelectric film is different from that of Example 1. The first piezoelectric film and the second piezoelectric film of Example 31 are set to be PZT films added with 1% vanadium. Similarly, Examples 32 to 35 are set to be PZT films added with 1% Ta, Sb, Mo, and W, respectively.

[0103] In Examples 31 to 35, the first piezoelectric film is an epitaxial film, and the second piezoelectric film is a uniaxially oriented film.

[0104] <Crystallinity>

[0105] The crystallinity of the first piezoelectric film and the second piezoelectric film was measured by observing the (111) peak of each piezoelectric film by φ scanning of XRD (X-Ray Diffraction). The measurement was performed under the following conditions using RINT3000 manufactured by Rigaku Corporation as a measuring apparatus.

[0106] Radiation source: CuKα

[0107] Optical system used: Parallel optical system

[0108] X-ray incident angle: set to an angle at which the (111) peak of the PZT-based perovskite oxide can be observed (θ=near 38 degrees, χ=near 55 degrees)

[0109] φ scanning conditions: within the range of -180deg to 180deg, 0.1deg / step, 100deg / min

[0110] In the case of epitaxial films, it was confirmed that Figure 4In the case of a uniaxially oriented film, data of a substantially straight line with a specific intensity are obtained.

[0111] The characteristics of each example and comparative example were evaluated.

[0112] (Preparation of samples for evaluation)

[0113] -Evaluation Samples-

[0114] A strip-shaped portion of 2 mm×25 mm was cut out from the laminate to prepare a cantilever as a sample 1 for evaluation.

[0115] <Measurement of piezoelectric characteristics>

[0116] As an evaluation of the piezoelectric characteristics of each example and comparative example, the piezoelectric constant d 31 .

[0117] The piezoelectric constant d was measured using the evaluation samples. 31 The piezoelectric constant d was measured according to the method described in I. Kanno et al. Sensor and Actuator A 107 (2003) 68. 31 Specifically, the second electrode 16 was grounded, the first electrode 12 and the third electrode 20 were used as driving electrodes, and the piezoelectric constant d was measured using a sine wave applied voltage of +10V±10V, i.e., a bias voltage of 10V, and a sine wave applied voltage with an amplitude of 10V. 31 [pm / V] The measurement results are shown in Tables 1 and 2. In addition, Tables 1 and 2 also show the evaluation results based on the following standards.

[0118] a: 360pm / V or above

[0119] c: less than 360pm / V

[0120] <Measurement of displacement waveform>

[0121] The displacement waveform was obtained by measuring the velocity waveform using the evaluation sample and integrating the velocity waveform. The second electrode was set to the ground potential, the first electrode and the third electrode were set to the drive electrode, and a 200Hz drive signal (positive potential) was applied. The drive signal was set to a sine wave with an amplitude of 15V. At this time, the velocity waveform of the evaluation sample was measured using Ono Tester LV-1720A.

[0122] Figure 5 The drive signal (dashed line) and the speed waveform (solid line) for Example 1 are shown. Figure 6 The driving signal (dashed line) and the speed waveform (solid line) for Comparative Example 1 are shown. Figure 5 and Figure 6In the figure, the drive signal is expressed as voltage and the velocity waveform is expressed in arbitrary units (au). Since the displacement waveform is obtained by integrating the velocity waveform, if there is a nonlinear component in the velocity waveform, it means that there is a nonlinear component in the displacement waveform.

[0123] The evaluation results based on the following criteria are shown in Tables 1 and 2.

[0124] a: There is nonlinear component

[0125] c: No nonlinear components

[0126] <Measurement of Sheet Resistance of Second Electrode>

[0127] In the film resistance measurement, a general high-precision flexible resistivity measurement system was used. Specifically, VR250 manufactured by Kokusai Electric Semiconductor Service Inc. was used. The film resistance was measured at 5 or 9 points in the surface of the second electrode, and the average value was calculated. As a probe, a JS-TC-200-EP-200HK4-terminal probe was used, with a needle diameter of 200μmR, a needle spacing of 1mm, a needle pressure / 1 of 200g, and a needle material of tungsten carbide.

[0128] For each example, the sheet resistance of the second electrode was evaluated as a, b, or c based on the following criteria.

[0129] a: 1Ω or less

[0130] b: More than 1Ω and less than 10Ω

[0131] c: more than 10Ω

[0132] Tables 1 and 2 show the layer structures and evaluation results of the respective Examples and Comparative Examples.

[0133] In addition, as comprehensive evaluation, three evaluation comprehensive values ​​of a: 2 points, b: 1 point, and c: 0 points were calculated for the three evaluations of piezoelectric characteristics, displacement waveform, and sheet resistance of the second electrode, and the comprehensive values ​​are shown in Tables 1 and 2 as comprehensive evaluations.

[0134]

[0135]

[0136] The comprehensive evaluation of the embodiments in which the first piezoelectric film is an epitaxial film and the second piezoelectric film is a uniaxially oriented film is 5 points or more. If the comprehensive evaluation is 5 points or more, the piezoelectric characteristics are high (that is, a large driving force can be obtained), the responsiveness is good, and long-term reliability can be obtained. If the comprehensive evaluation is 6 points or more, it is considered that more excellent long-term reliability can be obtained. In Comparative Examples 1 and 2, in which both the first piezoelectric film and the second piezoelectric film are uniaxially oriented films, there is a nonlinear component in the displacement waveform and the responsiveness is low. In Comparative Examples 1 and 2, the first piezoelectric film and the second piezoelectric film are both uniaxially oriented films. At this time, the directions of the spontaneous polarizations of the two layers of piezoelectric films become the same direction. As shown in the present embodiment, when electric fields in opposite directions are applied to the first piezoelectric film and the second piezoelectric film, in the case of Comparative Examples 1 and 2, an electric field in the opposite direction to the spontaneous polarization is applied to at least one of the piezoelectric films. It is believed that since the direction of the spontaneous polarization needs to be reversed, the response is delayed, such as Figure 6 As shown in FIG. 1 , a nonlinear component is generated in the displacement waveform represented by the solid line relative to the shape of the driving waveform represented by the dotted line. As shown in the embodiment, when at least one layer is an epitaxial film, since the spontaneous polarization of the epitaxial film is isotropic, by applying an electric field in the opposite direction to the uniaxially oriented film, polarization can be easily performed in the direction, and a stable polarization state can be obtained. Therefore, it can be considered that when electric fields in opposite directions are applied to the first piezoelectric film and the second piezoelectric film, no nonlinear component is generated in the displacement waveform.

[0137] Furthermore, in Comparative Example 3 where both the first piezoelectric film and the second piezoelectric film are epitaxial films, there is no problem with responsiveness, but the film resistance of the second electrode is large. If the film resistance is large, heat is generated due to driving, and the heat generation causes an increase in power consumption. Furthermore, if the film resistance is large, when driven for a long time, Pb is precipitated at the interface with the piezoelectric film and in the piezoelectric film due to the temperature rise accompanying the heat generation, which leads to a decrease in withstand voltage and durability, and low long-term reliability. Furthermore, it is speculated that as shown in Comparative Example 3, when both the first piezoelectric film and the second piezoelectric film are epitaxial films, the stress becomes greater compared to the case where at least one of the first piezoelectric film and the second piezoelectric film is a uniaxially oriented film. When driven for a long time, the greater the stress, the more likely it is that cracks, peeling and other defects will occur. Therefore, it is believed that by setting one layer of the two piezoelectric films as an epitaxial film and the other layer as a uniaxially oriented film, the long-term reliability can be further improved compared to the case where both piezoelectric films are epitaxial films.

[0138] All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as if each individual document, patent application, or technical standard was specifically and individually described as being incorporated by reference.

[0139] The following additional notes are further disclosed regarding the above-mentioned embodiment.

[0140] <Note 1>

[0141] A piezoelectric element comprises a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film and a third electrode in this order on a substrate.

[0142] The first piezoelectric film and the second piezoelectric film each have a perovskite-type oxide as a main component.

[0143] One of the first piezoelectric film and the second piezoelectric film is an epitaxial film, and the other is a uniaxially oriented film.

[0144] <Note 2>

[0145] The piezoelectric element according to Supplementary Note 1, wherein:

[0146] The first piezoelectric film is an epitaxial film, and the second piezoelectric film is a uniaxially oriented film.

[0147] <Note 3>

[0148] The piezoelectric element according to Supplementary Note 2, wherein:

[0149] The surface of the first electrode in contact with the first piezoelectric film is an epitaxial film.

[0150] <Note 4>

[0151] The piezoelectric element according to any one of Supplementary Notes 1 to 3, wherein:

[0152] The perovskite type oxide as a main component of the first piezoelectric film and the perovskite type oxide as a main component of the second piezoelectric film are composed of the same constituent elements.

[0153] <Note 5>

[0154] The piezoelectric element according to any one of Supplementary Notes 1 to 4, wherein:

[0155] The perovskite oxide is lead zirconate titanate or lead zirconate titanate with added metal element M.

[0156] The metal element M is at least one of vanadium, niobium, tantalum, antimony, molybdenum and tungsten.

[0157] <Note 6>

[0158] The piezoelectric element according to Supplementary Note 5, wherein:

[0159] The metal element M in the perovskite oxide is niobium.

[0160] The first piezoelectric film and the second piezoelectric film each have the same composition ratio of at least the amount of the metal element M added in the perovskite-type oxide.

[0161] <Note 7>

[0162] The piezoelectric element according to any one of Supplementary Notes 1 to 6, wherein:

[0163] The first electrode, the second electrode, and the third electrode each include at least one of iridium, platinum, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide.

[0164] <Note 8>

[0165] The piezoelectric element according to any one of Supplementary Notes 1 to 7, wherein:

[0166] The first electrode and the third electrode are maintained at a ground potential, and the second electrode is a driving electrode for applying a positive driving voltage to the first piezoelectric film and the second piezoelectric film.

[0167] <Note 9>

[0168] The piezoelectric element according to any one of Supplementary Notes 1 to 7, wherein:

[0169] The second electrode is maintained at a ground potential, and the first electrode and the third electrode are driving electrodes for applying a negative driving voltage to the first piezoelectric film and the second piezoelectric film.

[0170] <Note 10>

[0171] An actuator comprising a piezoelectric element as described in any one of Supplementary Notes 1 to 9 and a drive circuit for applying a drive voltage to the piezoelectric element, wherein:

[0172] The driving circuit applies an electric field in the same direction as the polarization, i.e., the self-polarization, which the other piezoelectric film spontaneously possesses immediately after film formation, to the other piezoelectric film which is a uniaxially oriented film, and applies an electric field in the opposite direction to the electric field applied to the other piezoelectric film to one of the piezoelectric films which is an epitaxial film.

[0173] Explanation of symbols

[0174] 1-piezoelectric element, 5, 6-actuator, 10-substrate, 12-first electrode, 12A-first film, 12B-second film, 12C-third film, 14-first piezoelectric film, 16-second electrode, 18-second piezoelectric film, 20-third electrode, 30, 32-driving circuit.

Claims

1. A piezoelectric element comprising a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film and a third electrode in this order on a substrate, The first piezoelectric film and the second piezoelectric film each have a perovskite-type oxide as a main component. One of the first piezoelectric film and the second piezoelectric film is an epitaxial film, and the other is a uniaxially oriented film.

2. The piezoelectric element according to claim 1, wherein The first piezoelectric film is the epitaxial film, and the second piezoelectric film is the uniaxially oriented film.

3. The piezoelectric element according to claim 2, wherein: A surface of the first electrode in contact with the first piezoelectric film is an epitaxial film.

4. The piezoelectric element according to claim 1 or 2, wherein: The perovskite type oxide as a main component of the first piezoelectric film and the perovskite type oxide as a main component of the second piezoelectric film are composed of the same constituent elements.

5. The piezoelectric element according to claim 3, wherein The perovskite oxide is lead zirconate titanate or lead zirconate titanate added with a metal element M, The metal element M is at least one of vanadium, niobium, tantalum, antimony, molybdenum and tungsten.

6. The piezoelectric element according to claim 5, wherein: The metal element M in the perovskite-type oxide is niobium, In the composition ratio of the perovskite type oxide contained in the first piezoelectric film and the second piezoelectric film, at least the amount of the metal element M added is the same.

7. The piezoelectric element according to claim 1 or 2, wherein: The first electrode, the second electrode, and the third electrode each include at least one of iridium, platinum, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide.

8. The piezoelectric element according to claim 1 or 2, wherein: The first electrode and the third electrode are maintained at a ground potential, and the second electrode is a driving electrode for applying a positive driving voltage to the first piezoelectric film and the second piezoelectric film.

9. The piezoelectric element according to claim 1 or 2, wherein: The second electrode is maintained at a ground potential, and the first electrode and the third electrode are driving electrodes for applying a negative driving voltage to the first piezoelectric film and the second piezoelectric film.

10. An actuator comprising the piezoelectric element according to any one of claims 1 to 9 and a drive circuit for applying a drive voltage to the piezoelectric element, wherein: The driving circuit applies an electric field to the uniaxially oriented film, i.e., the other piezoelectric film, and the direction of the electric field is the same as the direction of the polarization, i.e., the self-polarization, which the other piezoelectric film spontaneously possesses immediately after film formation. The driving circuit applies an electric field to the epitaxial film, i.e., the one piezoelectric film, in an opposite direction to the electric field applied to the other piezoelectric film.

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