Piezoelectric laminate and piezoelectric element

Abstract

This piezoelectric laminate and this piezoelectric element comprise, on a substrate, a lower electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film in the indicated sequence. The first piezoelectric film, the intermediate electrode, and the second piezoelectric film are all epitaxial films. The intermediate electrode includes at least one metal oxide layer. A main component of the first piezoelectric film is a first perovskite-type oxide. A main component of the second piezoelectric film is a second perovskite-type oxide. The polarization-field characteristics of the second piezoelectric film exhibit a polarization-field hysteresis that is asymmetric with respect to the origin and has two coercive fields with different absolute values.

Description

Piezoelectric laminate and piezoelectric chip

[0001] This disclosure relates to piezoelectric laminates and piezoelectric elements.

[0002] As a material with excellent piezoelectric and ferroelectric properties, lead zirconate titanate (Pb(Zr,Ti)O ３ Perovskite-type oxides such as (hereinafter referred to as PZT) are known. Piezoelectric materials made of perovskite-type oxides are used as piezoelectric films in piezoelectric elements that have a lower electrode, a piezoelectric film, and an upper electrode on a substrate. These piezoelectric elements are being developed into various devices such as memory, inkjet heads (actuators), micromirror devices, angular velocity sensors, gyro sensors, ultrasonic elements (PMUT: Piezoelectric Micromachined Ultrasonic Transducer), and vibration power generation devices.

[0003] To obtain higher piezoelectric properties, stacked piezoelectric elements have been proposed in which multiple piezoelectric films are stacked via electrodes (see Japanese Patent Publication No. 2013-80886, Japanese Patent Publication No. 2014-82464, and Japanese Patent Publication No. 2016-219603, etc.).

[0004] Research is underway to improve multilayer piezoelectric elements to achieve even higher piezoelectric properties and longer-term durability.

[0005] This disclosure is made in view of the above circumstances and aims to provide a piezoelectric laminate for obtaining a piezoelectric element with higher piezoelectric properties and longer-term durability than conventional elements.

[0006] The piezoelectric laminate of the present disclosure includes a lower electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film on a substrate in this order. The first piezoelectric film, the intermediate electrode, and the second piezoelectric film are all epitaxial films. The intermediate electrode includes at least one metal oxide layer. The main component of the first piezoelectric film is a first perovskite-type oxide, and the main component of the second piezoelectric film is a second perovskite-type oxide. The second piezoelectric film has polarization-electric field characteristics showing a polarization-electric field hysteresis that is asymmetric with respect to the origin and has two coercive electric fields with different absolute values.

[0007] Among the two coercive electric fields in the polarization-electric field hysteresis of the second piezoelectric film, the coercive electric field on the relatively positive electric field side is Ec2 ＋ , and the coercive electric field on the relatively negative electric field side is Ec2 － . When expressed as such, Ec2 ＋ ≧0, Ec2 － ≦0, Max{|Ec2 ＋ |, |Ec2 － |}≧|Ec2 ＋ | - |Ec2 － |>5. Here, it is preferable that the units of Ec2 ＋ and Ec2 － satisfy [V / cm].

[0008] It is preferable that the second perovskite-type oxide is lead zirconate titanate added with niobium.

[0009] It is preferable that the first perovskite-type oxide is lead zirconate titanate added with niobium.

[0010] It is preferable that the intermediate electrode further includes a platinum group layer.

[0011] More preferably, the intermediate electrode includes two metal oxide layers and a platinum group layer, and is arranged in the order of the metal oxide layer, the platinum group layer, and the metal oxide layer from the side of the first piezoelectric film.

[0012] The platinum group layer is preferably (100)-oriented.

[0013] It is preferable that the surface roughness of the surface of the second piezoelectric film opposite to the intermediate electrode is 0.5 μm or less.

[0014] The metal oxide layer is an oxide containing a first metal M and a second metal N as metal oxides, and the metal oxide M is represented by the following general formula (1). ｄ N １－ｄ O ｅ (1) M consists of one or more metal elements that can be substituted at the A site of the first perovskite-type oxide and has an electronegativity of less than 0.95, N is mainly composed of at least one selected from the group Sc, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, Ni, Cu, Zn, Ga, Sn, In and Sb, O is the element oxygen, d and e indicate the composition ratio, 0 < d < 1, and when the electronegativity is X, 1.41X - 1.05 ≤ d ≤ A1・exp(-X / t1) + y0 A1 = 1.68 × 10 １２ It is preferable that t1 = 0.0306 and y0 = 0.59958 are included.

[0015] The piezoelectric element of this disclosure comprises a piezoelectric laminate of this disclosure and an upper electrode provided on a second piezoelectric film of the piezoelectric laminate.

[0016] According to the technology disclosed herein, piezoelectric elements with higher piezoelectric properties and longer-term durability than conventional elements, and piezoelectric laminates for obtaining such piezoelectric elements can be obtained.

[0017] This is a schematic cross-sectional view of a piezoelectric element according to an embodiment. This is a schematic cross-sectional view of a modified piezoelectric element. This is a schematic cross-sectional view of a modified piezoelectric element. This is a diagram showing the polarization-electric field characteristics of the second piezoelectric film. This is a diagram showing the Q-V characteristics of the epitaxial film and the polycrystalline film. This is a diagram showing the displacement-voltage characteristics of the epitaxial film and the polycrystalline film. This is a schematic configuration diagram of the actuator. This is a schematic configuration diagram of a modified actuator. This is a schematic configuration diagram of a modified actuator. This is a diagram showing the φ scan of Example 3. This is a diagram showing the hysteresis curves of the second piezoelectric film of Example 1 and the second piezoelectric film of Comparative Example 3.

[0018] Embodiments of this disclosure will be described below with reference to the drawings. In the following drawings, for ease of viewing, the layer thicknesses and their ratios have been appropriately modified and do not necessarily reflect the actual layer thicknesses and ratios. In each figure, equivalent components are denoted by the same reference numerals. In this specification, numerical ranges expressed using "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits. In numerical ranges described in stages in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Also, in numerical ranges described in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the values ​​shown in the examples.

[0019] Figure 1 is a schematic cross-sectional view showing the layer structure of a piezoelectric element 1 including a piezoelectric laminate 5 according to one embodiment. As shown in Figure 1, the piezoelectric element 1 comprises a piezoelectric laminate 5 and an upper electrode 20. The piezoelectric laminate 5 comprises a substrate 10 and, sequentially laminated on the substrate 10, a lower electrode 12, a first piezoelectric film 14, an intermediate electrode 16, and a second piezoelectric film 18. The terms "lower" and "upper" in the lower electrode 12 and upper electrode 20 do not refer to the upper and lower in the vertical direction, but rather the electrode positioned on the substrate 10 side with the first piezoelectric film 14, intermediate electrode 16, and second piezoelectric film 18 in between is called the lower electrode 12, and the electrode positioned on the side opposite to the substrate 10 is called the upper electrode 20.

[0020] The first piezoelectric film 14, the intermediate electrode 16, and the second piezoelectric film 18 are all epitaxial films. The intermediate electrode 16 includes at least one metal oxide layer. The main component of the first piezoelectric film 14 is a first perovskite-type oxide. The main component of the second piezoelectric film 18 is a second perovskite-type oxide. The first perovskite-type oxide and the second perovskite-type oxide may be the same or different. The second piezoelectric film 18 has two coelectric fields with different absolute values ​​and has bipolar polarization-field hysteresis characteristics asymmetric with respect to the origin.

[0021] The epitaxial growth and single crystal formation of each layer can be confirmed by φ scanning, in-plane measurement, TEM (Transmission Electron Microscope) observation of sections prepared by FIB (Focused Ion Beam) processing, or electron diffraction.

[0022] The details of each layer constituting the piezoelectric element 1 are described below.

[0023] There are no particular limitations on the substrate 10, and examples include substrates made of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. A substrate made of single-crystal silicon is particularly preferred as the substrate 10. When a single-crystal silicon substrate is used as the substrate 10, it is preferable that there is no thermal oxide film on the surface. It is also preferable that there is no native oxide film, but it is acceptable for a native oxide film to be present.

[0024] The lower electrode 12 and the intermediate electrode 16 work together to apply a voltage to the first piezoelectric film 14. The intermediate electrode 16 and the upper electrode 20 work together to apply a voltage to the second piezoelectric film 18.

[0025] The main components of the lower electrode 12 and the upper electrode 20 are not particularly limited, but examples include metals or metal oxides such as gold (Au), platinum (Pt), iridium (Ir), ruthenium (Ru), titanium (Ti), molybdenum (Mo), tantalum (Ta), and aluminum (Al), and combinations thereof. Also, ITO (Indium Tin Oxide) and lanthanum nickelate (LNO: LaNiO) are also possible. ３ ), and strontium ruthenate (SRO: SrRuO ３ ) etc. may be used. The lower electrode 12 and the upper electrode 20 may both be single layers, but they may also have a multi-layered structure.

[0026] The lower electrode 12 is particularly preferably a laminated structure of platinum group layers 12a and metal oxide layers 12b stacked sequentially from the substrate 10 side, as shown in Figure 1. Pt or Ir are particularly preferred as the elements constituting the platinum group layers. The piezoelectric element 1 shown in Figure 1 further includes a buffer layer 11 between the substrate 10 and the lower electrode 12. The buffer layer 11 is preferably made of zirconium oxide (ZrO). ２ ), hafnium oxide (HfO ２ ) or zirconium hafnium oxide ((Zr,Hf)O ２ ) is preferable. For example, a silicon substrate is used as the substrate 10, and ZrO is placed on the substrate 10. ２ , HfO ２ Or (Zr, Hf)O ２ It is preferable to provide a buffer layer 11 made of an epitaxial layer, on which the lower electrode 12 is provided. It is particularly preferable that the lower electrode 12 has a two-layer structure of an epitaxial Pt layer and an epitaxial SRO layer, in which a Pt layer as a platinum group layer 12a and an SRO layer as a metal oxide layer 12b are sequentially epitaxially grown on the buffer layer 11.

[0027] The thickness of the lower electrode 12 and the upper electrode 20 is not particularly limited, but is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0028] As described above, the intermediate electrode 16 is composed of an epitaxial film and includes at least one metal oxide layer 16a. That is, the intermediate electrode 16 includes at least one metal oxide layer 16a which is an epitaxial film.

[0029] The metal oxide layer 16a is an oxide containing a first metal M and a second metal N as metal oxides, and contains a metal oxide represented by the following general formula (1). M ｄ N １－ｄ O ｅ(1) M consists of one or more metal elements that can be substituted at the A site of the first perovskite-type oxide and has an electronegativity of less than 0.95, N is mainly composed of at least one selected from the group Sc, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, Ni, Cu, Zn, Ga, Sn, In and Sb, O is the element oxygen, d and e indicate the composition ratio, where 0 < d < 1, and when the electronegativity is X, then 1.41X - 1.05 ≤ d ≤ A1・exp(-X / t1) + y0 A1 = 1.68 × 10 １２ , t1=0.0306, y0=0.59958

[0030] In particular, the metal oxide layer 16a is SrRuO ３ , BaRuO ３ , or (Sr,Ba)RuO ３ These are preferable. SrRuO ３ In equation (1) above, M is Sr, N is Ru, d = 0.5, and e = 1.5.

[0031] The metal oxide layer 16a is composed of any metal oxide represented by the general formula (1) above, and does not have to be a perovskite-type oxide. Because the metal oxide layer 16a is an epitaxial film, the second piezoelectric film 18 provided on the upper layer can be epitaxially grown. However, if the metal oxide layer 16a is an epitaxially grown film of a perovskite-type oxide, the second piezoelectric film 18 will grow more easily epitaxially.

[0032] In the piezoelectric element 1 shown in Figure 1, the intermediate electrode 16 has a configuration comprising only one metal oxide layer 16a. On the other hand, the intermediate electrode 16 may include multiple layers, for example, as in the piezoelectric element 2 in Figure 2 or the piezoelectric element 3 in Figure 3, it may have a laminated structure further comprising a metal layer 16b in addition to the metal oxide layer 16a. It is particularly preferable that the metal layer 16b be a platinum group layer (hereinafter sometimes referred to as a platinum group layer 16b). By including a metal layer (particularly a platinum group layer) 16b in the intermediate electrode 16, the overall resistivity of the intermediate electrode 16 can be reduced compared to the case where only a metal oxide layer 16a is present.

[0033] When the intermediate electrode 16 has a two-layer structure consisting of a metal oxide layer 16a and a metal layer 16b, the metal layer 16b may be positioned on the side of the metal oxide layer 16a that has the first piezoelectric film 14, or on the side of the metal oxide layer 16a that has the second piezoelectric film 18. When the intermediate electrode 16 has a two-layer structure consisting of a metal oxide layer 16a and a metal layer 16b, it is more preferable that the metal layer 16b is positioned on the side of the metal oxide layer 16a that has the second piezoelectric film 18, as shown in Figure 2.

[0034] Furthermore, the intermediate electrode 16 may have a three-layer structure, as shown in the piezoelectric element 3 of Figure 3, with a metal layer 16b in between, and a metal oxide layer 16a on both the first piezoelectric film 14 side and the second piezoelectric film 18 side. It is particularly preferable that the intermediate electrode 16 has a laminated structure of metal oxide layer 16a, platinum group layer 16b, and metal oxide layer 16a from the first piezoelectric film 14 side.

[0035] The intermediate electrode 16 can have a low resistance value by including a platinum group layer 16b, and if the surface in contact with the first piezoelectric film 14 and the surface in contact with the second piezoelectric film 18 are metal oxide layers 16a, oxygen leakage from the first piezoelectric film 14 and the second piezoelectric film 18 can be suppressed. In this sense, it is particularly preferable for the intermediate electrode 16 to have the three-layer structure shown in Figure 3.

[0036] Furthermore, "the intermediate electrode 16 is an epitaxial film" means that, if the intermediate electrode 16 has a stacked structure of multiple layers, all layers constituting the intermediate electrode 16 are epitaxial films.

[0037] If the intermediate electrode 16 is equipped with a platinum group layer 16b, it is preferable that the platinum group layer 16b is (100) oriented. This facilitates lattice matching with the perovskite-type oxide, making it easier to epitaxially grow the metal oxide layer 16a or the second piezoelectric film 18 formed on the platinum group layer 16b.

[0038] The thickness of the intermediate electrode 16 is not particularly limited, but from the viewpoint of stress and other factors, it is preferably about 50 nm to 500 nm, more preferably 150 nm or more, even more preferably more than 150 nm, 200 nm or more, or more than 200 nm, and particularly preferably 250 nm or more. By making the intermediate electrode 16 150 nm or more, preferably 200 nm or more, and more preferably 250 nm or more, variations in etching performed in the piezoelectric element manufacturing process can be suppressed, and the decrease in yield caused by etching variations can be suppressed. Note that the thickness of the intermediate electrode 16 refers to the total thickness of the multiple layers constituting the intermediate electrode 16 when the intermediate electrode 16 has a multilayer structure.

[0039] In this specification, the thickness of the intermediate electrode 16 is to be measured as follows: For example, the cleaved cross section is observed with an SEM (Scanning Electron Microscope), or the cross section is processed and observed with an SEM using a FIB-SEM (Focused Ion Beam Scanning Electron Microscope) combination device (for example, a TFS Helios-400S model), and the thickness of the intermediate electrode 16 portion is measured in the SEM image.

[0040] The first piezoelectric film 14 and the second piezoelectric film 18 are epitaxial films as previously described, and each has the general formula ABO ３The main component is a perovskite-type oxide represented by . In this specification, the main component refers to a component that accounts for 80 mol% or more. Preferably, the first piezoelectric film 14 and the second piezoelectric film 18 each consist of 90 mol% or more of perovskite-type oxide, and more preferably, the first piezoelectric film 14 and the second piezoelectric film 18 consist of perovskite-type oxide (however, including unavoidable impurities). The first perovskite-type oxide, which is the main component of the first piezoelectric film 14, and the second perovskite-type oxide, which is the main component of the second piezoelectric film 18, may be composed of different constituent elements, or their constituent elements may be the same. From the viewpoint of cost reduction, it is preferable that the constituent elements of the first perovskite-type oxide and the second perovskite-type oxide are the same. Note that "constituent elements are the same" means that the elements contained are the same, and includes cases where the composition ratio is the same or different.

[0041] The first and second perovskite-type oxides are preferably so-called PZT-type perovskite oxides represented by the following general formula (2), containing Pb (lead) at the A site and Zr (zirconium) and Ti (titanium) at the B site. (Pb ａ１ α ａ２ ) (Zr ｂ１ Ti ｂ２ β ｂ３ ) O ｃ General formula (2) Pb and α are A-site elements, Zr, Ti, and β are B-site elements, O is the oxygen element, β is at least one selected from niobium (Nb), tantalum (Ta), vanadium (V), antimony (Sb), and scandium (Sc), and a1, a2, b1, b2, b3, and c each represent the composition ratio. 0.5 ≤ a1, 0 ≤ a2 < 0.5, 0 < b1 < 1, 0 < b2 < 1, 0.14 ≤ b3 / (b1 + b2 + b3) ≤ 0.4. The ratio a1 + a2:b1 + b2 + b3:c is based on 1:1:3, but may deviate within the range that allows for a perovskite-type structure.

[0042] It is preferable that both the first perovskite-type oxide and the second perovskite-type oxide are Nb-doped PZT. Nb-doped PZT is Pb(Zr ｂ１Ti ｂ２ Nb ｂ３ ) is represented by Oc.

[0043] Furthermore, PZT-based perovskite oxides are said to exhibit high piezoelectric properties at and near the morphotropic phase boundary (MPB). The MPB composition is around Zr:Ti (molar ratio) = 52:48, and in the above general formula (2), it is preferable that the composition is the MPB composition or near it. "MPB or near it" refers to the region where a phase transition occurs when an electric field is applied to the piezoelectric film. Specifically, it is preferable that the Zr:Ti (molar ratio) is in the range of 45:55 to 55:45.

[0044] The film thickness of the first piezoelectric film 14 and the second piezoelectric film 18 is 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 3 μm or less.

[0045] The polarization-electric field characteristics of the second piezoelectric film 18 exhibit a polarization-electric field hysteresis that is asymmetrical with respect to the origin, with two coercive fields having different absolute values. Figure 4 shows the hysteresis curve illustrating the polarization-electric field characteristics of the second piezoelectric film 18. Hysteresis asymmetrical with respect to the origin means that the center of the hysteresis curve does not coincide with the origin, and that the center of the hysteresis curve is shifted from the origin to either the positive or negative electric field side. The hysteresis curve shown in Figure 4 is obtained by grounding the intermediate electrode 16 and sweeping the potential applied to the upper electrode 20. In this specification, the polarization-electric field hysteresis of the piezoelectric film is measured by grounding the electrode on the substrate 10 side of the two electrodes sandwiching the piezoelectric film and applying a potential to the electrode on the opposite side of the substrate 10. Specifically, the polarization-field hysteresis of the first piezoelectric film 14 is measured by grounding the lower electrode 12 and changing the potential applied to the intermediate electrode 14, while the polarization-field hysteresis of the second piezoelectric film 18 is measured by grounding the intermediate electrode 14 and changing the potential applied to the upper electrode 20.

[0046] The point where the polarization value of the hysteresis curve shown in Figure 4 is zero is the coercive field. In the hysteresis curve, the coercive field on the relatively negative field side (left side in the figure) is Ec²－ The coercive field on the relatively positive electric field side (right side in the diagram) is Ec² ＋ In this example, the coercive field Ec2 － This is a negative electric field, and the coercive electric field Ec² ＋ This is a positive electric field, but the coarse electric field Ec² － and coercive field Ec2 ＋ These may have the same sign. In this specification, two coercive fields having different absolute values ​​means |Ec2 － | and | Ec2 ＋ | is defined as a difference of 5 [V / cm] or more. That is, |Ec² ＋ |-|Ec2 － When the amount of hysteresis shift is defined by |, |Ec2 ＋ |-|Ec2 － If | is less than or equal to ±5V / cm, then there is no hysteresis shift, and the hysteresis is considered symmetric with respect to the origin, and |Ec2 ＋ |-|Ec2 － If | exceeds ±5V / cm, a hysteresis shift is present, and the hysteresis is considered asymmetric with respect to the origin.

[0047] Note: The coercive field Ec2 － and Ec2 ＋ It is more preferable that the following relationship is satisfied: Ec2 ＋ ≥ 0, Ec² － ≤ 0, and Max {|Ec2 ＋ |, |Ec2 － |} ≥ |Ec² ＋ |-|Ec2 － |>5 Here, Ec2 ＋ Ec2 － The unit is [V / cm]. The above formula is for the coercive field Ec². － If it is 0 or negative, Ec2 ＋ The coercive field Ec² is 0 or positive. ＋ The absolute value of the coercive field Ec² － Larger than the absolute value of, coercive field Ec² － The absolute value of and the coercive field Ec² ＋ The difference in absolute values ​​(shift amount) is the coercive field Ec² － and Ec2 ＋ This means that the absolute value is less than or equal to the larger of the two values.

[0048] Note that Max{|Ec2 ＋ |, |Ec2 － |}×2 / 3≧|Ec2 ＋ |-|Ec2 － It is preferable to be |

[0049] The above equation means that the center of the hysteresis curve is positively shifted. The hysteresis curve shown in Figure 4 satisfies the above equation. The shift of the hysteresis curve from the origin means that the spontaneous polarization of the second piezoelectric film 18 is aligned in a specific direction. Furthermore, in this measurement method, the positive shift of the center of the hysteresis curve means that the direction of the spontaneous polarization is aligned upward in the film thickness direction. A piezoelectric film is composed of numerous spontaneously polarized domains. In this specification, "spontaneous polarization aligned in a specific direction" means a state in which the spontaneous polarization component in a specific direction is relatively greater than the spontaneous polarization component in other directions. If the spontaneous polarization component in a specific direction is relatively greater than the spontaneous polarization component in other directions within a piezoelectric film, it can be considered a film polarized in that specific direction. Hereinafter, this specific direction in which the spontaneous polarization is aligned will be referred to as the "direction of spontaneous polarization". Arrow P2 in Figure 1 indicates the direction of polarization of the second piezoelectric film 18. The second piezoelectric film 18 is spontaneously polarized immediately after deposition without undergoing poling treatment; that is, the direction of spontaneous polarization is aligned, and it is a film polarized in the film thickness direction. The fact that a piezoelectric film that has not undergone poling treatment and has aligned spontaneous polarization in the absence of an applied external electric field is thought to be due to an electric field (hereinafter referred to as a spontaneous internal electric field) generated within the piezoelectric film due to distortions or defects in the crystal structure. The second piezoelectric film 18 has spontaneous polarization aligned in the film thickness direction due to such a spontaneous internal electric field. The second piezoelectric film 18 is a piezoelectric film that is polarized in the film thickness direction in the absence of an applied external electric field.

[0050] Furthermore, it is preferable that the second piezoelectric film 18 does not contain columnar structures or the like in the epitaxial film, and is uniform in the backscatter electron diffraction (EBD) image in the film thickness direction and in-plane direction. Uniformity in the EBD image means that the EBD image, in which the orientation of the orientation-mapped crystal grains is displayed in different colors by EBD analysis, is represented in a single color, and no crystal grains are observed in the EBD image.

[0051] Furthermore, it is preferable that the surface roughness of the surface 18S of the second piezoelectric film 18 opposite to the intermediate electrode 16 be 0.5 μm or less. In this specification, surface roughness Ra is the arithmetic mean surface roughness as shown in JIS B 0601-2001.

[0052] If the surface roughness Ra is 0.5 μm or less, the dielectric strength and durability can be improved, and the degradation of the piezoelectric element can be suppressed. When the surface roughness of the second piezoelectric film 18 is large, that is, when the surface is uneven, it is presumed that electric charge accumulates on the protrusions, and abnormal discharge occurs due to the influence of this charge, causing the element to degrade. If the surface roughness Ra is 0.5 μm or less and the unevenness is small, it is thought that the accumulation of electric charge on the protrusions can be suppressed, and thus the degradation of the piezoelectric element can be suppressed.

[0053] The first piezoelectric film 14 is an epitaxial film, but there are no particular restrictions on its polarization-electric field characteristics. The first piezoelectric film 14 may be a film with isotropic spontaneous polarization, where the center of the polarization-electric field hysteresis curve coincides with the origin, or, like the second piezoelectric film 18, it may have polarization-electric field characteristics that exhibit asymmetric polarization-electric field hysteresis with respect to the origin, having two coercive fields with different absolute values ​​as shown in Figure 4. That is, in the hysteresis curve of the first piezoelectric film 14, the coercive field on the relatively negative electric field side (left side in the figure) is Ec1 － The coercive field on the relatively positive electric field side (right side in the diagram) is Ec1 ＋ In this example, the coercive field Ec1 － This is a negative electric field, and the coercive electric field Ec1 ＋ This is a positive electric field, but the coarse electric field Ec1 － and coercive field Ec1 ＋ The same sign may be used. However, the coercive field Ec1 －and Ec1 ＋ more preferably satisfies the following relationship. Ec1 ＋ ≧0, Ec1 － ≦0, and Max{|Ec1 ＋ |, |Ec1 － |}≧|Ec1 ＋ |−|Ec1 － |>5 Here, Ec1 ＋ , Ec1 － The unit of is [V / cm]. In FIG. 1, as an example, the case where the spontaneous polarization of the first piezoelectric film 14 is aligned in the same film thickness direction as the second piezoelectric film 18 is shown. In FIG. 1, the arrow P1 in the first piezoelectric film 14 is the direction of the spontaneous polarization of the first piezoelectric film 14.

[0054] As described above, in this embodiment, the piezoelectric element 1 has an epitaxial film for the first piezoelectric film 14, the intermediate electrode 16, and the second piezoelectric film 18, and the intermediate electrode 16 includes at least one metal oxide layer 16a. The main component of the first piezoelectric film 14 is a first perovskite-type oxide, and the main component of the second piezoelectric film 18 is a second perovskite-type oxide. The polarization-electric field characteristics of the second piezoelectric film exhibit an asymmetric polarization-electric field hysteresis with respect to the origin, having two coercive fields with different absolute values. It has been shown that this configuration makes it possible to realize a piezoelectric element with higher piezoelectricity and longer long-term durability than conventional multilayer piezoelectric elements (see the examples below). The first piezoelectric film 14 and the second piezoelectric film 18 are epitaxial films, that is, single crystals. Although the mechanism is not clear, according to the inventors' research, single-crystal piezoelectric films have a lower dielectric constant compared to conventional columnar piezoelectric films, and the driving current can be reduced. Furthermore, high dielectric strength (dielectric strength) can be achieved, resulting in high long-term durability. Because such single-crystal piezoelectric films are stacked, higher piezoelectric characteristics and higher long-term durability can be obtained compared to conventional multilayer piezoelectric elements that consist of multiple stacked piezoelectric films made of polycrystalline films. In addition, single crystallization has the effect of reducing power consumption during operation. Moreover, because the second piezoelectric film 18 of piezoelectric element 1 has a hysteresis shift, the degree of freedom in the drive circuit configuration when it is made into a device is improved. Depending on the method of applying the electric field to the device, in the case of a device that continuously applies an electric field in a constant direction, such as a piezoelectric actuator, a phenomenon called imprinting is likely to occur in piezoelectric films that do not have a hysteresis shift, where the coercive field shifts in the direction of the electric field. In contrast, hysteresis shift is caused by an internal electric field due to defect dipoles that occur inside the crystal and is in a very stable state, so imprinting is less likely to occur even when applied to a device that continuously applies an electric field in a constant direction. Therefore, an improvement in the long-term reliability of the entire device can be expected. Furthermore, compared to conventional symmetrical hysteresis curves, imprinting does not occur, eliminating the need for individual control of drive power supplies in applications requiring high-precision position control, thus improving the flexibility of drive circuit configuration. It is also estimated that the presence of hysteresis shift reduces variability in displacement data.

[0055] Figure 5 shows the Q-V hysteresis curve Epi (solid line), which shows the charge-voltage characteristics of an epitaxial piezoelectric film made of the same Nb-doped PZT, and the Q-V hysteresis curve Poly (dashed line), which shows the charge-voltage characteristics of a polycrystalline film (uniaxially oriented film in this case). The thickness of each film is 3 μm. The epitaxial film has characteristics equivalent to the polarization-electric field characteristics of the second piezoelectric film 18 in the piezoelectric element 1 of this disclosure. In Figure 5, the horizontal axis is voltage V (V), and the vertical axis is charge Q (nC). The voltage V on the horizontal axis is related to the electric field E by E = V / d (where d is the film thickness), and the charge Q on the vertical axis is related to the polarization P by P = Q / S (where S is the area of ​​the electrode). Therefore, the Q-V hysteresis can be converted to P-E hysteresis.

[0056] Compared to polycrystalline films, epitaxial films exhibit a larger slope at the onset of polarization reversal. That is, while polarization reversal in polycrystalline films progresses gradually with changes in the electric field, epitaxial films have a high responsiveness to changes in the electric field. Furthermore, epitaxial films exhibit higher linearity compared to polycrystalline films when the electric field is changed toward zero after saturation polarization is reached. Here, high linearity means that the range of electric fields in which the relationship between the electric field and polarization is linear is wide. Polycrystalline films exhibit high linearity when a negative electric field is applied and saturation polarization is reached, and then the electric field is changed toward zero, but their linearity is low when a positive electric field is applied and saturation polarization is reached, and then the electric field is changed toward zero.

[0057] Figure 6 shows the piezoelectric characteristics of a piezoelectric film that is an epitaxial film having the characteristics shown in FIG. 5 and a piezoelectric film that is a polycrystalline film. The horizontal axis represents the applied voltage (V), and the vertical axis represents the displacement amount (μm). As shown in FIG. 6, in both the epitaxial film and the polycrystalline film, the linearity of the voltage change of the displacement amount is equally high during negative driving. In the polycrystalline film, the linearity of the displacement amount during positive driving is lower than that during negative driving. Here, in FIG. 6, the region where the voltage is negative is referred to as negative driving, and the region where the voltage is positive is referred to as positive driving. As shown in the lower diagram in FIG. 6, in the element in which the first electrode, the piezoelectric film, and the second electrode are laminated in this order, the direction P of the spontaneous polarization of the piezoelectric film is from the first electrode toward the second electrode. In this case, the first electrode is grounded, and a driving potential is applied to the second electrode. Therefore, when the driving potential is negative as shown in the lower left diagram in FIG. 6, the direction of the electric field Ef generated in the piezoelectric film coincides with the direction P of the spontaneous polarization, and when the driving potential is positive as shown in the lower right diagram in FIG. 6, the direction of the electric field Er related to the piezoelectric film is opposite to the direction P of the spontaneous polarization.

[0058] When driving a laminated piezoelectric element in which piezoelectric films are laminated, the driving voltages applied to the first layer and the second layer may be in opposite directions (see FIG. 7). When both the first layer and the second layer are the polycrystalline films shown in FIGS. 5 and 6, one is driven negatively and the other is driven positively. In the case of a two-layer structure, an output twice that in the case of one layer (the displacement amount in the case of an actuator) is expected, but since the piezoelectric characteristics on the positive driving side are small, a displacement amount twice that in the case of one layer is not obtained. On the other hand, in the piezoelectric element 1 of this example, since the second piezoelectric film 18 is an epitaxial film having the characteristics shown in FIGS. 5 and 6, the second piezoelectric film 18 can obtain piezoelectric characteristics equal to or better than those during negative driving even during positive driving. Therefore, the piezoelectric element 1 can be expected to have a displacement amount about twice that in the case of a one-layer structure.

[0059] As described above, the hysteresis showing the polarization-electric field characteristics of the second piezoelectric film 18 is Ec2 ＋ ≧0, Ec2 － ≦0, and Max{|Ec2 ＋ |, |Ec2 － |} ≧ |Ec2 ＋ | - |Ec2 －The condition |> 5 is satisfied. In addition to the above, the maximum polarization value on the positive electric field side in the hysteresis showing the polarization-electric field characteristics of the second piezoelectric film 18 is Pmax. ＋ , the residual polarization value Pr ＋ In that case, hysteresis is (Pmax ＋ -Pr ＋ ) / Pmax ＋ It is preferable that the value <0.5 be satisfied. Also, the maximum polarization value on the negative electric field side of the hysteresis showing the polarization-electric field characteristics of the second piezoelectric film 18 is Pmax. － , the residual polarization value Pr － In that case, hysteresis is (Pmax － -Pr － ) / Pmax － It is preferable that the value < 0.5 be satisfied.

[0060] Figure 7 shows a schematic configuration of an actuator 6 equipped with a piezoelectric element 1. The actuator 6 comprises a piezoelectric element 1 and a drive circuit 30 for driving the piezoelectric element 1.

[0061] The drive circuit 30 is a means for supplying a drive voltage to the first piezoelectric film 14 and the second piezoelectric film 18, which are sandwiched between electrodes to drive the piezoelectric element 1. The drive circuit 30 is a negative drive circuit that applies a negative potential to the drive electrode (in this case, the intermediate electrode 16). In this example, the intermediate electrode 16 is connected to the drive voltage output terminal -V of the drive circuit 30, and the lower electrode 12 and the upper electrode 20 are connected to the ground terminal GND of the drive circuit 30. That is, the lower electrode 12 and the upper electrode 20 are at ground potential, and the intermediate electrode 16 functions as the drive electrode. With this configuration, the drive circuit 30 applies electric fields Er and Ef in opposite directions to the first piezoelectric film 14 and the second piezoelectric film 18. In this example, the drive circuit 30 applies an electric field Ef in the same direction as the direction of spontaneous polarization P1 to the first piezoelectric film 14, and applies an electric field Er in the opposite direction to the direction of spontaneous polarization P2 to the second piezoelectric film 18. During such driving, as previously described, the second piezoelectric film 18 has the hysteresis characteristics of the Epi film shown in Figure 6. Therefore, when the second piezoelectric film 18 is driven with an electric field Er opposite to the direction of spontaneous polarization P2, it can obtain the same amount of displacement as when driven with an electric field Ef in the same direction as the direction of spontaneous polarization P1, and a large amount of displacement can be obtained for the entire element.

[0062] Furthermore, as shown in the modified actuator 7 in Figure 8, a positive drive circuit 32 that applies a positive potential to the drive electrodes may be provided instead of the drive circuit 30. In the actuator 7 shown in Figure 8, the intermediate electrode 16 is at ground potential, and the lower electrode 12 and upper electrode 20 are used as drive electrodes. In this example, when the piezoelectric element 1 is driven, a positive potential is applied to the drive electrodes (here, the lower electrode 12 and upper electrode 20). As a result, similar to the actuator 6, the drive circuit 32 in the actuator 7 also applies electric fields in opposite directions to the first piezoelectric film 14 and the second piezoelectric film 18. Generally, a positive drive circuit 32 is more versatile and less expensive than a negative drive circuit 30, so providing a positive drive circuit 32 allows for a more inexpensive actuator configuration.

[0063] Furthermore, in the case of piezoelectric element 1, where both the first piezoelectric film 14 and the second piezoelectric film 18 are epitaxial films and have similar hysteresis characteristics, a positive drive circuit 32 that applies a positive potential to the drive electrode can be provided, as in the modified actuator 8 shown in Figure 9, and the intermediate electrode 16 can be used as the drive electrode. In the actuator 8 shown in Figure 9, the lower electrode 12 and the upper electrode 20 are at ground potential, and a positive drive potential is applied to the intermediate electrode 16. Similar to actuator 7, the actuator can be constructed inexpensively because it is a positive drive circuit 32.

[0064] In addition, it is preferable that the lower electrode 12 and the upper electrode 20 of the piezoelectric element 1 are connected. If the lower electrode 12 and the upper electrode 20 are connected, drive control is easier.

[0065] As shown in Figures 7-9, the piezoelectric element 1 has an epitaxial film 18 with a hysteresis shift, allowing the drive electrode and drive circuit to be freely configured. When applying a multilayer piezoelectric element to various devices, it is preferable that the drive circuit configuration for applying voltage to each piezoelectric film can be freely configured according to the device. By using the piezoelectric multilayer of this disclosure, a piezoelectric element with a high degree of freedom in drive circuit configuration can be obtained.

[0066] The following describes specific examples and comparative examples of the piezoelectric elements of this disclosure. First, the configuration and manufacturing method of each piezoelectric element will be described.

[0067] As examples and comparative examples, a piezoelectric element 1 (see Figure 1) was fabricated on a substrate 10, comprising a lower electrode 12, a first piezoelectric film 14, an intermediate electrode 16, a second piezoelectric film 18, and an upper electrode 20 in that order. The layer configurations of each example and comparative example are summarized in Table 1 below.

[0068] "Example 1" The method for manufacturing the piezoelectric element 1 of Example 1 will be described.

[0069] (Substrate) A single-crystal silicon wafer without a thermal oxide film was prepared as the substrate 10.

[0070] (Lower electrode) 40 nm thick ZrO as buffer layer 11 ２ After depositing the film onto the substrate 10, a 150 nm thick Pt film and a 60 nm thick SRO film were sequentially sputter-deposited and stacked as the lower electrode 12. In the lower electrode column of Table 1, the layer configuration of the lower electrode is described so that the left side of the Pt / SRO layer faces the substrate.

[0071] (First Piezoelectric Film) A first piezoelectric film 14 was formed on the lower electrode 12 as an Nb-doped PZT film with an Nb addition amount of 12 at% to the B site. The thickness of the Nb-doped PZT film was 2 μm. Nb-doped PZT was used as the target, and the sputtering conditions were as follows. The Nb-doped PZT target had a Pb composition ratio a1 = 1.3, a Zr / Ti molar ratio of MPB composition (Zr / Ti = 52 / 48), and an Nb composition ratio b3 = 0.12. In Table 1, it is written as Nb-PZT.

[0072] - Sputtering conditions for the first piezoelectric film - Target-substrate distance: 100 mm Target input power: 3 kW Vacuum level: 0.5 Pa, Ar and O ２ Mixed atmosphere (O ２ Volume fraction: 2.5% Board setting temperature: 620°C Board bias voltage: +40V

[0073] (Intermediate electrode) On the first piezoelectric film 14, a 60 nm thick SrRuO ３Film, 150 nm thick Pt film, 60 nm thick SrRuO ３ The films were deposited by sputtering in this order and then stacked. In the lower electrode column of Table 1, the layer configuration of the lower electrode is described so that the left side is the substrate side for Pt / SRO.

[0074] (Second piezoelectric film) A second piezoelectric film 18 was deposited on the intermediate electrode 16 as an Nb-doped PZT film, with an Nb addition amount of 12 at% to the B site, similar to the first piezoelectric film. The thickness of the Nb-doped PZT film was 2 μm. The target and sputtering conditions were the same as for the first piezoelectric film.

[0075] (Upper electrode) A 50 nm IrO layer is placed on the second piezoelectric film 18 as the upper electrode 20. ｘ Then, 100 nm Ir was deposited in this order using sputtering.

[0076] (Formation of evaluation electrode patterns) In order to form electrode pads for voltage application on the lower electrode, intermediate electrode, and upper electrode, the upper electrode, second piezoelectric film, intermediate electrode, and first piezoelectric film were sequentially patterned using photolithography and dry etching.

[0077] The piezoelectric laminates of Examples 1 to 10 and Comparative Examples 1 to 3 were fabricated using the same procedure as in Example 1. Only the differences from Example 1 will be described below.

[0078] "Example 2" In Example 2, the piezoelectric element was modified so that the Pt layers of the lower electrode and the intermediate electrode in Example 1 were replaced with Ir layers.

[0079] "Example 3" In Example 3, the piezoelectric element has a single layer of SRO layer as the intermediate electrode compared to Example 1.

[0080] "Example 4" The piezoelectric element in Example 4 has a two-layer structure consisting of a Pt layer and an SRO layer as the intermediate electrode in Example 1.

[0081] "Example 5" The piezoelectric element of Example 5 has the SRO layers of the lower electrode and intermediate electrode in Example 1 replaced with BaRuO ３ It was made into layers.

[0082] "Example 6" The piezoelectric element of Example 6 has the SRO layer of the lower electrode and intermediate electrode in Example 1 replaced with (SrBa)RuO ３ It was made into layers.

[0083] "Example 7" The piezoelectric element in Example 7 has the buffer layer in Example 1 replaced with HfO ２ That's what I decided.

[0084] "Example 8" The piezoelectric element in Example 8 has a buffer layer in Example 1 that is made of ZrHfO ２ That's what I decided.

[0085] "Example 9" In Example 9, the piezoelectric element had a thickness of 4.9 μm for both the first piezoelectric film and the second piezoelectric film compared to Example 1.

[0086] "Example 10" The piezoelectric element in Example 10 has a two-layer structure consisting of an SRO layer and an Ir layer, as opposed to the intermediate electrode in Example 1.

[0087] "Comparative Example 1" In Comparative Example 1, the piezoelectric element was a PZT film without Nb added, instead of the first and second piezoelectric films in Example 1. The PZT target had a Pb composition ratio a1 = 1.3, and the Zr / Ti molar ratio was the MPB composition (Zr / Ti = 52 / 48).

[0088] "Comparative Example 2" The piezoelectric element of Comparative Example 2 has iridium oxide (IrO) as the intermediate electrode in Example 1. ２ The structure consists of two layers: a ) layer and an iridium (Ir) layer.

[0089] "Comparative Example 3" In Comparative Example 3, the piezoelectric element does not have a buffer layer as in Example 1, and the lower electrode has a two-layer structure consisting of a TiW layer and an Ir layer.

[0090] In Examples 1 to 10 obtained as described above, each layer from the buffer layer to the second piezoelectric film was composed of an epitaxial film. On the other hand, in Comparative Example 2, the first piezoelectric film was an epitaxial film, but the second piezoelectric film was a uniaxially oriented polycrystalline film. In Comparative Example 3, both the first and second piezoelectric films were uniaxially oriented polycrystalline films. Epitaxial films among the piezoelectric films are indicated with (epi) in Table 1.

[0091] The determination of whether each layer is an epitaxial film was performed as follows. The single crystallinity of the first piezoelectric film, intermediate electrode, and second piezoelectric film, which are composed of perovskite-type oxides, was observed by observing the (111) peak of each film using XRD (X-ray Diffraction) φ scan. A RIGAK RINT3000 was used as the measurement device, and the measurements were performed under the following conditions: Radiation source: CuKα Optical system: Parallel optical system X-ray incidence angle: Set to an angle at which the (111) peak of the perovskite-type oxide can be observed (θ = around 38 deg, χ = around 55 deg) φ scan conditions: -180 deg to 180 deg in the range of 0.1 deg / step, 100 deg / min

[0092] Figure 10 shows the φ scan results obtained for the first piezoelectric film, the SRO film which is an intermediate electrode, and the second piezoelectric film of Example 3. As shown in Figure 10, a four-fold symmetrical peak was observed. Such a four-fold symmetrical peak indicates that it is an epitaxial film. In the case of a uniaxially oriented film, data of a nearly linear shape with a specific intensity was obtained. Furthermore, the orientation of Pt(100) was confirmed by checking the intensity of the Pt(200) peak around 46° at θ-ω, in addition to the φ scan.

[0093] Table 1 shows the layer configuration of the piezoelectric laminates for each example and comparative example. Table 2 shows the evaluation results for each piezoelectric laminate.

[0094] The evaluation method for the evaluation results shown in Table 2 will be explained below.

[0095] (Preparation of evaluation samples) -Evaluation sample 1- A 2mm x 25mm strip was cut from the laminate to create a cantilever as evaluation sample 1.

[0096] -Evaluation Sample 2- A 25 mm x 25 mm portion was cut from the laminate, which had an upper electrode patterned in a circle with a diameter of 400 μm at the center of the surface of the second piezoelectric film, and this was designated as Evaluation Sample 2.

[0097] <Measurement of Polarization-Electric Field Characteristics> For each piezoelectric element of the embodiment and comparative example, the polarization-electric field (P-E) hysteresis curve was measured using evaluation sample 2. For each piezoelectric element of the embodiment and comparative example, the first piezoelectric film 14 and the second piezoelectric film 18 were measured by applying a voltage at a frequency of 1 kHz until saturation polarization was reached. When measuring the P-E characteristics of the first piezoelectric film 14, the lower electrode 12 was grounded and the sweep voltage was applied to the first piezoelectric film 14 using the intermediate electrode 16 as the driving electrode. When measuring the P-E characteristics of the second piezoelectric film 18, the intermediate electrode 16 was grounded and the sweep voltage was applied to the second piezoelectric film 18 using the upper electrode 20 as the driving electrode.

[0098] Figure 11 shows the P-E hysteresis of the second piezoelectric film of Example 1 and the second piezoelectric film of Comparative Example 3.

[0099] The piezoelectric elements of Examples 1 to 10 and Comparative Examples 2 and 3 exhibited a hysteresis shift (indicated as "hys-shift" in the table) in which the center of the polarization-field hysteresis curve of the second piezoelectric film was shifted from the origin. On the other hand, the piezoelectric element of Comparative Example 1 did not exhibit a hysteresis shift in the second piezoelectric film.

[0100] <Measurement of Piezoelectric Properties> For the evaluation of the piezoelectric properties of each example and comparative example, the piezoelectric constant d ３１ The piezoelectric constant d was measured. ３１ The measurement was performed using evaluation sample 1. The piezoelectric constant d was measured according to the method described in I. Kanno et. al. Sensor and Actuator A 107(2003)68. ３１ The following was measured. Specifically, the lower electrode 12 and the upper electrode 20 were grounded, and the intermediate electrode 16 was used as the driving electrode. A sinusoidal voltage of -10V ± 10V was applied to the driving electrode, i.e., a bias voltage of -10V and an applied sinusoidal voltage of amplitude 10V, to simulate the piezoelectric constant d. ３１ The [pm / V] value was measured.

[0101] The piezoelectric element of Comparative Example 1, which had a first piezoelectric film and a second piezoelectric film without hysteresis shift, had a low piezoelectric constant of less than 300 pm / V. However, the other comparative examples and examples all obtained good piezoelectric constants of 380 pm / V or higher. In particular, Examples 1, 2 and 9 obtained very high piezoelectric constants of 400 pm / V or higher.

[0102] <Evaluation of Long-Term Durability> Time-dependent dielectric breakdown (TDDB) tests were conducted to evaluate the long-term durability of each example and comparative example. Using evaluation sample 2, the lower electrode 12 was grounded, and a voltage of +40V was applied to the upper electrode 20 in a 120°C environment. The time (hr) from the start of voltage application until dielectric breakdown occurred was measured. The measurement results were evaluated according to the following criteria, and the evaluation results are shown in Table 2. S: 3000hr or more; A: 1500hr or more, less than 3000hr; B: 1000hr or more, less than 1500hr; C: Less than 1000hr

[0103] As described above, Examples 1 to 10, which satisfy all the conditions that the first piezoelectric film, the intermediate electrode, and the second piezoelectric film are all epitaxial films, the intermediate electrode includes at least one metal oxide layer, the main component of the first piezoelectric film is a first perovskite-type oxide, the main component of the second piezoelectric film is a second perovskite-type oxide, and the polarization-electric field characteristics of the second piezoelectric film exhibit asymmetric polarization-electric field hysteresis with respect to the origin, having two coelectric fields with different absolute values, have a good piezoelectric constant of 380 pm / V or higher and high durability of evaluation A or higher were obtained. In particular, Examples 1, 2 and 5 to 9 obtained a piezoelectric constant of 390 pm / V or higher and extremely high durability of evaluation S.

[0104] The disclosure of Japanese Patent Application No. 2024-232980, filed on 27 December 2024, is incorporated herein by reference in its entirety. All documents, patent applications and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application and technical standard were specifically and individually noted to be incorporated by reference.

[0105] Regarding the above embodiment, the following additional notes are disclosed. <Note 1> The substrate is provided with a lower electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film in this order, the first piezoelectric film, the intermediate electrode, and the second piezoelectric film are all epitaxial films, the intermediate electrode includes at least one metal oxide layer, the main component of the first piezoelectric film is a first perovskite-type oxide, the main component of the second piezoelectric film is a second perovskite-type oxide, and the polarization-electric field characteristics of the second piezoelectric film exhibit a polarization-electric field hysteresis asymmetric with respect to the origin, having two coercive fields with different absolute values. <Note 2> Of the two coercive fields in the polarization-electric field hysteresis of the second piezoelectric film, the coercive field on the relatively positive electric field side is Ec2 ＋ The coercive field on the relatively negative field side is Ec2 － If expressed as, Ec2 ＋ ≥ 0, Ec² － ≦0, Max{|Ec2 ＋ |, |Ec2 － |} ≥ |Ec² ＋ |-|Ec2 － A piezoelectric laminate according to Appendix 1 that satisfies |>5. <Appendix 3> A piezoelectric laminate according to Appendix 1 or Appendix 2, wherein the second perovskite-type oxide is niobium-doped lead zirconate titanate. <Appendix 4> A piezoelectric laminate according to Appendix 3, wherein the first perovskite-type oxide is niobium-doped lead zirconate titanate. <Appendix 5> A piezoelectric laminate according to any one of Appendix 1 to Appendix 4, wherein the intermediate electrode further comprises a platinum group layer. <Appendix 6> A piezoelectric laminate according to Appendix 5, wherein the intermediate electrode includes two metal oxide layers and a platinum group layer, and is arranged in the order of metal oxide layer, platinum group layer, and metal oxide layer from the first piezoelectric film side. <Appendix 7> A piezoelectric laminate according to Appendix 5 or Appendix 6, wherein the platinum group layer is (100) oriented. <Appendix 8> A piezoelectric laminate according to any one of Appendix 1 to Appendix 7, wherein the surface roughness of the side of the second piezoelectric film opposite the intermediate electrode is 0.5 μm or less. <Note 9> The metal oxide layer is an oxide containing a first metal M and a second metal N as metal oxides, and the metal oxide M is represented by the following general formula (1). ｄ N １－ｄ O ｅ(1) M consists of one or more metal elements that can be substituted at the A site of the first perovskite-type oxide and has an electronegativity of less than 0.95, N is mainly composed of at least one selected from the group Sc, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, Ni, Cu, Zn, Ga, Sn, In and Sb, O is the element oxygen, d and e indicate the composition ratio, 0 < d < 1, and when the electronegativity is X, 1.41X - 1.05 ≤ d ≤ A1・exp(-X / t1) + y0 A1 = 1.68 × 10 １２ A piezoelectric laminate according to any one of the appendices 1 to 8, including t1 = 0.0306 and y0 = 0.59958. <Appendix 10> A piezoelectric element comprising a piezoelectric laminate according to any one of the appendices 1 to 9, and an upper electrode provided on the second piezoelectric film of the piezoelectric laminate.

Claims

1. A piezoelectric laminate comprising a lower electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film in this order on a substrate, wherein the first piezoelectric film, the intermediate electrode, and the second piezoelectric film are all epitaxial films, the intermediate electrode includes at least one metal oxide layer, the main component of the first piezoelectric film is a first perovskite-type oxide, the main component of the second piezoelectric film is a second perovskite-type oxide, and the second piezoelectric film has polarization-electric field characteristics exhibiting polarization-electric field hysteresis asymmetric with respect to the origin, with two coercive fields having different absolute values.

2. Among the two coercive fields in the polarization-electric field hysteresis of the second piezoelectric film, the coercive field on the relatively positive electric field side is Ec2 ＋ , and the coercive field on the relatively negative electric field side is Ec2 － . When expressed as such, Ec2 ＋ ≥0, Ec2 － ≤0, Max{|Ec2 ＋ |, |Ec2 － |} ≥ |Ec2 ＋ | - |Ec2 － | > 5. Here, the units of Ec2 ＋ and Ec2 － are [V / cm]. The piezoelectric laminate according to claim 1, which satisfies this condition.

3. The piezoelectric laminate according to claim 1, wherein the second perovskite-type oxide is niobium-doped lead zirconate titanate.

4. The piezoelectric laminate according to claim 3, wherein the first perovskite-type oxide is niobium-doped lead zirconate titanate.

5. The piezoelectric laminate according to claim 1, wherein the intermediate electrode further comprises a platinum group layer.

6. The piezoelectric laminate according to claim 5, wherein the intermediate electrode comprises two metal oxide layers and a platinum group layer, and the metal oxide layer, platinum group layer, and platinum group layer, are arranged in that order from the first piezoelectric film side.

7. The piezoelectric laminate according to claim 5 or 6, wherein the platinum group layer is (100) oriented.

8. The piezoelectric laminate according to claim 1, wherein the surface roughness of the second piezoelectric film opposite to the intermediate electrode is 0.5 μm or less.

9. The metal oxide layer is an oxide containing a first metal M and a second metal N as metal oxides, and the metal oxide M is represented by the following general formula (1). ｄ N １－ｄ O ｅ (1) M consists of one or more metal elements that can be substituted at the A site of the first perovskite-type oxide and has an electronegativity of less than 0.95, N is mainly composed of at least one selected from the group Sc, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, Ni, Cu, Zn, Ga, Sn, In and Sb, O is the element oxygen, d and e indicate the composition ratio, where 0 < d < 1, and when the electronegativity is X, 1.41X - 1.05 ≤ d ≤ A1・exp(-X / t1) + y0 A1 = 1.68 × 10 １２ A piezoelectric laminate according to claim 1, comprising t1 = 0.0306 and y0 = 0.59958.

10. A piezoelectric element comprising a piezoelectric laminate according to claim 1, and an upper electrode provided on the second piezoelectric film of the piezoelectric laminate.

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