Piezoelectric laminate and piezoelectric chip

Abstract

To provide a piezoelectric laminate having high piezoelectric characteristics and comprising a piezoelectric film with high long-term stability, and a piezoelectric element.SOLUTION: There are provided a piezoelectric laminate and a piezoelectric element. The piezoelectric laminate comprises a lower electrode layer and a piezoelectric film in this order on a substrate. The piezoelectric laminate includes a seed layer consisting of a piezoelectric oxide between the lower electrode layer and the piezoelectric film, and the piezoelectric film contains a perovskite oxide expressed by the general formula I of Pb1-y2+αAy2{(Ti,Zr)1-x-y1 NbxB1y1}O3, where A is A site elements and one or more elements containing at least La; B1 is B site elements and one or more divalent or trivalent elements; O is an oxygen element; and x, y1, y2 and α satisfy 0.05<x≤0.3, 0.2x≤y1+y2≤0.5x, 0≤y1≤0.15, 0≤y2≤0.15 and 0≤α≤0.2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] It is applied as a piezoelectric film in a piezoelectric element comprising a lower electrode, a piezoelectric film, and an upper electrode on a substrate. This piezoelectric element is being developed for 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] Lead zirconate titanate (Pb(Zr,Ti)O3, hereinafter referred to as PZT)-based perovskite oxides are known to possess excellent piezoelectric properties. In PZT-based perovskite oxide piezoelectric films, the piezoelectric constant is highest and the film is known to be suitable for actuator applications when it has a morphotropic phase boundary (MPB) composition where Zr:Ti is near 52:48.

[0004] Conventionally, piezoelectric elements have utilized piezoelectric strain (ordinary field-induced strain) that extends along the spontaneous polarization axis by applying an electric field in the direction of the spontaneous polarization axis. Furthermore, in order to obtain a large displacement, piezoelectric elements have been proposed in which the polarization axis of the piezoelectric material can be reversibly rotated non-180° by increasing or decreasing the electric field applied in a direction different from the spontaneous polarization direction. Piezoelectric strain is generated when the domains in the piezoelectric material rotate non-180°, such as 90° domain rotation. By utilizing reversible non-180° domain rotation, a much larger piezoelectric strain can be obtained than by using only the ordinary field-induced strain that extends along the polarization axis when an electric field is applied in the direction of the spontaneous polarization of the piezoelectric material.

[0005] In PZT-based perovskite oxides, it is known that adding Nb (niobium) as the B-site element can induce non-180° domain rotation, resulting in large piezoelectric strain accompanied by non-180° domain rotation. It is also known that the amount of piezoelectric strain associated with non-180° domain rotation tends to increase with increasing Nb content. However, increasing the amount of Nb added to PZT beyond a certain point leads to the formation of a non-piezoelectric phase called a pyrochlore phase, or increases film stress (internal stress of the film), causing cracks in the piezoelectric film, making it unsuitable for use as a piezoelectric element.

[0006] In contrast, Patent Document 1 or Patent Document 2 proposes adding Nb to PZT, along with Ni (nickel) or Sc (scandium). Patent Document 1 or Patent Document 2 shows that by adding Ni or Sc, the pyrochlore phase that occurs when the amount of Nb added is increased can be suppressed.

[0007] On the other hand, Patent Document 3 discloses a method for achieving a piezoelectric film with high piezoelectric properties in Nb-doped PZT (hereinafter referred to as Nb-PZT) by setting the molar ratio of Nb in the range of 10 to 20%, adjusting the ratio of Zr, and setting the film stress within an appropriate range. In other words, Patent Document 3 achieves an increase in the amount of Nb added by suppressing the increase in film stress by adjusting the ratio of Zr. Furthermore, Patent Document 3 states that a piezoelectric film with high crystallineity can be obtained by providing a seed layer between the substrate and the piezoelectric film, thereby obtaining high piezoelectric properties. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2017 / 085924 [Patent Document 2] International Publication No. 2018 / 042946 [Patent Document 3] International Publication No. 2015 / 174265 [Overview of the project] [Problems that the invention aims to solve]

[0009] Patent documents 1 and 2 show that high piezoelectric constants can be obtained for PZT films doped with Nb and Sc or Ni, but their dielectric strength and long-term stability have not been evaluated. According to the inventors' research, piezoelectric elements equipped with PZT films doped with Nb and Sc or Ni sometimes lacked sufficient long-term stability.

[0010] According to the results of the examples and comparative examples in Patent Document 3, piezoelectric elements with high piezoelectric properties have been obtained. On the other hand, the dielectric strength of the examples and comparative examples in Patent Document 3 is not sufficient. Furthermore, long-term stability has not been investigated, and given the low dielectric strength, it is highly likely that the long-term stability is insufficient.

[0011] When applied to actual devices, high piezoelectric properties alone are not sufficient; high long-term stability is also required.

[0012] The technology disclosed herein has been made in view of the above circumstances, and aims to provide a piezoelectric laminate and piezoelectric element equipped with a piezoelectric film that has high piezoelectric properties and high long-term stability. [Means for solving the problem]

[0013] The following embodiments are included as specific means for solving the above problems.

[0014] The piezoelectric laminate of this disclosure is a piezoelectric laminate comprising a lower electrode layer and a piezoelectric film on a substrate in this order, A seed layer made of a conductive oxide is provided between the lower electrode layer and the piezoelectric film. The seed layer is made of a conductive oxide, The piezoelectric film contains a perovskite-type oxide represented by the following general formula I. Pb

[0022] , , , , , ,

[0021] , A y2 {(Ti,Zr) 1-x-y1 Nb x B1 y1 (Ti,Zr)ₓNb₁₋ₓ₋(y₁ + y₂)(B1)₍y₁₊y₂₎O₃ General formula I Here, A is an A-site element and is one or more elements including at least La,[[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​According to this disclosure, it is possible to obtain piezoelectric laminates and piezoelectric elements that have high piezoelectric properties and piezoelectric films with high long-term stability. [Brief explanation of the drawing]

[0023] [Figure 1] This is a cross-sectional view showing the configuration of a piezoelectric laminate and piezoelectric element in one embodiment. [Figure 2] (a) is a diagram showing the behavior of piezoelectric strain under normal conditions, and (b) is a diagram showing the behavior of piezoelectric strain due to reversible non-180° domain rotation. [Figure 3] This figure schematically illustrates the non-180° domain rotation and electric field-strain characteristics of an example piezoelectric film. [Figure 4] This figure schematically illustrates the non-180° domain rotation and electric field-strain characteristics of another example of a piezoelectric film. [Figure 5] This diagram schematically shows an example of the electric field-strain characteristics in the case of only normal piezoelectric strain. [Figure 6] This graph shows the measured values ​​of the electric field-strain (displacement) characteristics for Example 4 and Comparative Example 6. [Modes for carrying out the invention]

[0024] Embodiments of the present invention 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 this specification, numerical ranges represented by "~" 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.

[0025] "Piezoelectric laminate 5 and piezoelectric element 1" Figure 1 is a schematic cross-sectional view showing the layer configuration of a piezoelectric laminate 5 and a piezoelectric element 1 equipped with the piezoelectric laminate 5 according to the first embodiment. As shown in Figure 1, the piezoelectric element 1 comprises a piezoelectric laminate 5 and an upper electrode layer 18. The piezoelectric laminate 5 comprises a substrate 10 and a lower electrode layer 12, a seed layer 14, and a piezoelectric film 15 laminated on the substrate 10. The piezoelectric element 1 is configured such that an electric field is applied to the piezoelectric film 15 in the film thickness direction by the lower electrode layer 12 and the upper electrode layer 18. Here, "lower" and "upper" do not mean up and down in the vertical direction, but rather the electrodes positioned on the substrate 10 side with respect to the piezoelectric film 15 are called the lower electrode layer 12 and the electrodes positioned on the side opposite to the substrate 10 with respect to the piezoelectric film 15 are called the upper electrode layer 18.

[0026] There are no particular limitations on the substrate 10, and examples include silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. As the substrate 10, a laminated substrate such as a silicon substrate with a thermal oxide film formed on the surface of a silicon substrate may also be used.

[0027] The lower electrode layer 12 is an electrode for applying a voltage to the piezoelectric film 15. The main component of the lower electrode layer 12 is not particularly limited and includes metals such as gold (Au), platinum (Pt), iridium (Ir), ruthenium (Ru), titanium (Ti), molybdenum (Mo), tantalum (Ta), aluminum (Al), copper (Cu), silver (Ag), and combinations thereof. Platinum group metals such as Pt (platinum) and Ir (iridium) are particularly preferred for the lower electrode layer 12.

[0028] Preferably, an adhesion layer, not shown in the figure, is provided between the lower electrode layer 12 and the substrate 10, and the adhesion layer is mainly composed of a Ti (titanium) or TiW (titanium-tungsten) alloy.

[0029] The upper electrode layer 18 forms a pair with the lower electrode layer 12 and is an electrode for applying a voltage to the piezoelectric film 15. The main component of the upper electrode layer 18 is not particularly limited. In addition to the materials exemplified for the lower electrode layer 12, electrode materials generally used in semiconductor processes such as chromium (Cr), and conductive oxides such as indium tin oxide (ITO), iridium oxide (IrO2), ruthenium oxide (RuO2), lanthanum nickelate (LaNiO3), and strontium ruthenate (SrRuO3), and combinations thereof can be used.

[0030] The thicknesses of the lower electrode layer 12 and the upper electrode layer 18 are not particularly limited, and are preferably about 50 nm to 300 nm, more preferably 100 nm to 300 nm.

[0031] The seed layer 14 is a layer provided to enhance the crystallinity of the piezoelectric film 15 provided on the seed layer 14. The seed layer 14 is a conductive oxide and can be used without limitation as long as it is a material having a function of enhancing the crystallinity of the piezoelectric film 15 provided on the upper layer. The seed layer 14 is cubic or pseudo-cubic, and preferably has a lattice constant equal to or less than that of the Nb-PZT system. Specifically, the lattice constant of the seed layer 14 is preferably 0.4 nm (= 4.0 Å) or less. Specific examples of the material of the seed layer 14 include LaNiO3, SrRuO3, and lanthanum strontium cobaltate (La,Sr)CoO3. The lattice constant of LaNiO3 is 3.84 Å, the lattice constant of SrRuO3 is 3.905 Å, and the lattice constant of (La,Sr)CoO3 is 3.84. As the seed layer 14, LaNiO3 and SrRuO3 are particularly preferred.

[0032] The piezoelectric film 15 contains a perovskite-type oxide represented by the following general formula I. Pb 1-y2+α A y2 {(Ti,Zr) 1-x-y1 Nb x B1 y1}O3 General formula I Here, A is an A-site element and is at least one element containing La. B1 is a B-site element excluding Ti, Zr, and Nb, and is one or more divalent or trivalent elements. O is an oxygen element. x, y1, y2, and α are 0.05 < x ≤ 0.3, 0.2x ≤ y1 + y2 ≤ 0.5x, 0 ≤ y1 ≤ 0.15, 0 ≤ y2 ≤ 0.15, 0 ≤ α ≤ 0.2 satisfy.

[0033] x represents the addition amount of Nb at the B-site. That 0.05 < x ≤ 0.3 means that the molar ratio of Nb at the B-site is 5% or more and 30% or less. Note that x is preferably 0.1 ≤ x ≤ 0.2, and more preferably 0.15 ≤ x ≤ 0.2.

[0034] y1 represents the addition amount of the B1 element at the B-site, and y2 represents the addition amount of the A element at the A-site. That 0.2x ≤ y1 + y2 ≤ 0.5x means that the A element and / or the B1 element are added to the A-site and / or the B-site in the range of 0.2x to 0.5x with respect to the addition amount x of Nb. It is preferable that 0.2x ≤ y1 + y2 ≤ 0.4x, 0 ≤ y1 ≤ 0.12, and 0 ≤ y2 ≤ 0.12. Note that only the A element may be added, or only the B1 element may be added. Only the addition of the A element to the A-site may be present, or only the addition of the B1 element to the B-site may be present.

[0035] In General Formula I, Pb 1-y2+α A y2 :(Ti,Zr) 1-x-y1 Nb x B1 y1 :O is based on 1:1:3, but may deviate within the range where a perovskite structure can be taken.

[0036] Pb 1-y2+α A y2 :(Ti,Zr) 1-x-y1 [[ID=4l]]Nb x B1 y1When the ratio of :O is 1:1:3, α=0. Pb is an element that is easily reverse-sputtered, and if Pb is removed from the deposited piezoelectric film, it will adversely affect crystal growth. Therefore, the amount of target Pb is often made greater than the stoichiometric composition when the film is deposited. In that case, depending on the reverse sputtering rate of Pb, the deposited film may also be Pb-rich (i.e., α>0). It is preferable from the viewpoint of obtaining a high-quality perovskite-type oxide film that the molar ratio of Pb in the piezoelectric film 15 is greater than or equal to the stoichiometric composition.

[0037] Generally, B-site elements in perovskite oxides represented as ABO3 include Sc (scandium), Ti (titanium), Zr (zirconium), V (vanadium), Nb (niobium), Ta (tantalum), Cr (chromium), Mo (molybdenum), W (tungsten), Mn (manganese), Fe (iron), Ru (ruthenium), Co (cobalt), Ir (iridium), Ni (nickel), Cu (copper), Zn (zinc), Ga (gallium), In (indium), Sn (tin), and Sb (antimony).

[0038] B1 contains one or more divalent or trivalent elements from among the B-site elements, excluding Ti, Zr, and Nb. In particular, B1 is preferably one or more of Sc, Co, and Ni.

[0039] PZT-based perovskite oxides are said to exhibit high piezoelectric properties at and near the morphotropic phase boundary (MPB). In PZT systems, the MPB is around a Zr / Ti molar ratio of 55 / 45. In the above general formula, it is preferable that the composition is at or near the MPB. "MPB or near the MPB" 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 within the range of 45:55 to 55:45.

[0040] Examples of A-site elements in perovskite oxides represented as ABO3 include Pb (lead), Ba (barium), Sr (strontium), Bi (bismuth), Li (lithium), Na (sodium), Ca (calcium), Cd (cadmium), Mg (magnesium), K (potassium), and lanthanides.

[0041] In general formula I, A is one or more elements that include at least La (lanthanum) among the A-site elements excluding Pb.

[0042] In particular, it is preferable that A is La and B1 is one or more of Sc, Co, and Ni. Only one of La, Sc, Co, and Ni may be added, or a combination of two or more may be added. For example, if only Sc is added from among La, Sc, Co, and Ni, then B1 should be the element Sc, satisfying y2=0 and 0.2x≦y1≦0.5x.

[0043] Thus, the perovskite-type oxide contained in the piezoelectric film 15 is a PZT-based perovskite-type oxide obtained by adding a predetermined amount of element A and / or element B1 as additional elements to Nb-doped PZT (hereinafter referred to as Nb-PZT).

[0044] Furthermore, the content of the perovskite-type oxide satisfying the above general formula I in the piezoelectric film 15 is preferably 80 mol% or more, and more preferably 90 mol% or more. Moreover, it is particularly preferable that the piezoelectric film 15 is a perovskite-type oxide satisfying the above general formula I, excluding unavoidable impurities.

[0045] The thickness of the piezoelectric film 15 is usually 200 nm or more, for example 0.2 μm to 5 μm, but 1 μm or more is preferable.

[0046] In the piezoelectric film 15, it is preferable that the perovskite-type oxide crystals are preferentially oriented in the film thickness direction. Preferably, the tetragonal c-axis is oriented in the (001) plane along the film thickness direction, the tetragonal a-axis is oriented in the (100) plane along the film thickness direction, or the rhombohedral crystal is oriented in the (111) plane, and a mixture of these may also be used.

[0047] The presence or absence of crystal orientation can be confirmed by conventional θ-2θ X-ray diffraction. However, in conventional θ-2θ X-ray diffraction, in the case of morphotropic Nb-PZT-based perovskite oxides, the reflection peak positions of the tetragonal (001) plane, tetragonal (100) plane, and rhombohedral (111) plane overlap and are almost indistinguishable. In this specification, "having crystal orientation in the film thickness direction" means that the degree of orientation is 80% or more, when the ratio of the sum of the tetragonal (001) plane peaks, tetragonal (100) plane peaks, and rhombohedral (111) plane peaks to the peaks from all reflection planes obtained by θ-2θ X-ray diffraction is defined as the degree of orientation. Preferably, the degree of orientation is 90% or more.

[0048] Furthermore, the piezoelectric film 15 preferably has an internal stress in the range of 50 MPa to 250 MPa, and more preferably in the range of 110 MPa to 220 MPa. If the internal stress is within this range, the occurrence of cracks in the piezoelectric film 15 can be effectively suppressed.

[0049] In the piezoelectric film 15, when the polarization direction of spontaneous polarization coincides with the direction of electric field application, the effects of both normal piezoelectric strain, which expands and contracts in the direction of electric field application due to increases and decreases in electric field application strength, and piezoelectric strain due to reversible non-180° domain rotation are obtained. In the piezoelectric strain due to reversible non-180° domain rotation, the crystal lattice rotates and a large strain is obtained, so the combination of normal piezoelectric strain and piezoelectric strain due to reversible non-180° domain rotation results in a greater piezoelectric strain than normal piezoelectric strain alone. Therefore, the piezoelectric element 1 has high piezoelectric properties (high piezoelectric constant).

[0050] Here, with reference to Figure 2, the piezoelectric strain associated with non-180° domain rotation occurring in the piezoelectric film 15 will be explained. Figure 2(a) shows the typical piezoelectric strain. In Figure 2, arrow E indicates the direction of electric field application, arrow P0 indicates the polarization direction when no electric field is applied, and arrow PE indicates the polarization direction when an electric field is applied.

[0051] When an electric field is applied in the direction of polarization (E>0) to a c-domain whose c-axis is oriented in the direction of electric field application (i.e., the direction of the piezoelectric film thickness), i.e., oriented in the (001) plane, the crystal lattice elongates in the direction of the applied electric field. This is normal piezoelectric strain.

[0052] In contrast, Figure 2(b) shows the piezoelectric strain caused by non-180° domain rotation. As shown in Figure 2(b), when an a-domain, whose a-axis is oriented in the direction of the applied electric field (i.e., 100-oriented), is rotated by 90° by the application of an electric field to become a c-domain, whose c-axis is oriented in the direction of the applied electric field (E>0), the major axis of the crystal lattice rotates by 90°, resulting in a piezoelectric strain greater than that shown in Figure 2(a).

[0053] As previously described, the piezoelectric film 15 in this example exhibits piezoelectric strain that combines piezoelectric strain due to non-180° domain rotation and normal piezoelectric strain when an electric field is applied in the film thickness direction. Therefore, the piezoelectric film 15 in this example has a characteristic in which the slope of the electric field-strain characteristic changes when an electric field is applied in the film thickness direction, as shown in Figure 3 or Figure 4.

[0054] Figure 3 shows a schematic graph illustrating an example of the electric field-strain characteristics of the piezoelectric film 15, and a schematic diagram illustrating non-180° domain rotation. Figure 4 shows a schematic graph illustrating another example of the electric field-strain characteristics of the piezoelectric film 15, and a schematic diagram illustrating non-180° domain rotation. For comparison, Figure 5 shows a schematic diagram illustrating the electric field-strain characteristics of a piezoelectric film that does not produce piezoelectric strain due to non-180° domain rotation. In Figures 3 to 5, arrow E indicates the direction of electric field application, and arrow P indicates the direction of polarization within each domain.

[0055] As shown in Figure 5, when a piezoelectric film is composed of c-domains Dc in which the c-axis is uniformly oriented in the direction of electric field application, an electric field E is applied to the c-domains Dc in the direction of polarization P, resulting in only normal piezoelectric strain (see Figure 2(a)). As shown in the graph in Figure 5, normal piezoelectric strain is proportional to the electric field, and its slope is constant.

[0056] In this example, the piezoelectric film 15 has elements A and / or B1 added to PZT in addition to Nb, which causes disorder in the (001) plane orientation of the crystallinity, resulting in the inclusion of a domain Da in addition to c domain Dc. In this case, the relationship between the electric field and the amount of strain changes in three stages: the slope from no electric field to E1, the slope from E1 to E2, and the slope from E2 onward.

[0057] When the piezoelectric film 15 contains both a domain Da and a domain Dc, applying an electric field E in the thickness direction generates piezoelectric strain in addition to the normal piezoelectric strain in the c domain Dc, due to the non-180° domain rotation of the a domain Da. Specifically, when an electric field E is applied perpendicular to the polarization direction in the a domain Da, the a domain Da rotates by 90°, generating piezoelectric strain associated with the non-180° domain rotation. Furthermore, after the a domain Da rotates to become the c domain Dc due to the application of the electric field, the electric field is applied in the polarization direction, resulting in normal piezoelectric strain. Note that when the a domain Da rotates by 90°, it rotates through the rhombohedral domain Db to the c domain Dc. Since the amount of change until it rotates to the rhombohedral domain Db is different from the amount of change from the rhombohedral domain Db to the c domain Dc, in Figure 3, the amount of change (slope) of the strain with respect to the electric field changes in three stages. At E1, domain rotation occurs towards the rhombohedral domain Db, and then domain rotation occurs towards the c domain Dc up to E2. E2 is the electric field strength at which the rotation of the rotatable domains within the piezoelectric film 15 is completed.

[0058] Furthermore, the piezoelectric film 15 in this example also includes a rhombohedral domain Db in addition to the c domain Dc, as shown in Figure 4. In this case, the relationship between the electric field and the amount of strain changes in two stages: the slope from no electric field to E3, and the slope from E3 onward.

[0059] When the piezoelectric film 15 contains both c-domains Dc and rhombohedral domains Db, applying an electric field E in the thickness direction generates piezoelectric strain not only in the c-domains Dc but also due to non-180° domain rotation of the rhombohedral domains Db. Specifically, when an electric field is applied in a direction intersecting the polarization direction P within the rhombohedral domains Db, the rhombohedral domains Db rotate non-180° to become c-domains Dc, generating piezoelectric strain associated with this non-180° domain rotation. Furthermore, after the rhombohedral domains Db rotate to become c-domains Dc due to the application of the electric field, the electric field is applied in a direction along the polarization direction, resulting in normal piezoelectric strain. As a result, in the example shown in Figure 3, the change in the amount of strain with respect to the electric field (slope) changes in two stages. Increasing the electric field from no field causes domain rotation from rhombohedral domains Db to c-domains Dc. In this case, E3 is the electric field strength at which the rotation of the rotatable domains within the piezoelectric film 15 is completed, and in the region beyond E3, strain displacement associated with normal piezoelectric strain occurs.

[0060] In Figures 3 and 4, all changes are shown as straight lines. However, when measuring an actual piezoelectric element, the slope changes gradually, and the relationship between the electric field and the amount of strain is represented by an electric field-strain curve (see Example 4 in Figure 6). Therefore, the slope is shown as an approximation line. The measurement method for the electric field and the amount of strain displacement will be explained in detail in the Examples.

[0061] Furthermore, from the viewpoint of obtaining large piezoelectric strain in a practical electric field range, it is preferable that the piezoelectric film 15 has a characteristic in which the slope of the electric field-strain curve, which shows the relationship between the electric field and the amount of strain when an electric field from 0 kV / cm to 200 kV / cm is applied in the direction of film thickness, changes.

[0062] As previously described, the piezoelectric element 1 and piezoelectric laminate 5 in this example are provided on a substrate 10 in the following order: a lower electrode layer 12, a seed layer 14, and a piezoelectric film 15. The seed layer 14 is made of a conductive oxide, and the piezoelectric film 15 contains a perovskite-type oxide represented by the general formula I described above. In this example, in the perovskite-type oxide satisfying general formula I, the amount of Nb added is set to 0.2 to 0.5 times the sum of the amounts of element A added to site A and element B1 added to site B (y1 + y2). This makes it possible to suppress the increase in film stress that occurs when the amount of Nb added in Nb-PZT is increased, so that the amount of Nb added can be increased compared to when element A and / or element B1 are not added, and a piezoelectric film with high piezoelectric properties can be realized.

[0063] Furthermore, in this example, a seed layer 14 is provided between the lower electrode layer 12 and the piezoelectric film 15, and the piezoelectric film 15 is formed on the seed layer 14, so the generation of the pyrochlore phase, which tends to occur in the early stages of piezoelectric film formation, can be effectively suppressed. As described in the "Background Art" section, Patent Document 1, etc., discloses that the pyrochlore phase can be suppressed by adding Ni or Sc to Nb-PZT. However, as described in the "Problems to be Solved by the Invention" section, according to the inventors' studies, the long-term reliability of piezoelectric elements with Ni or Sc added to Nb-PZT was sometimes insufficient. The inventors found that the reason for the insufficient long-term reliability was the pyrochlore phase formed at the interface with the lower electrode during piezoelectric film formation. As described in Patent Documents 1 or 2, by adding Ni or Sc, the pyrochlore phase is suppressed to the extent that it is hardly observed by X-ray diffraction. However, the inventors have found that when observing a cross-sectional TEM image of the piezoelectric film, a pyrochlore phase is formed at the film deposition interface, and that dielectric breakdown occurs with the pyrochlore phase as the starting point when long-term operation is performed. By providing a seed layer 14, the piezoelectric film 15 can be grown epitaxially with respect to the seed layer 14, thereby suppressing the generation of the pyrochlore phase.

[0064] As described in the "Background Technology" section, Nb-PZT is a configuration that improves the piezoelectric constant by adding Nb to PZT, thereby generating a large piezoelectric strain associated with non-180° domain rotation. In Nb-PZT, the piezoelectric properties are enhanced by adding Nb because the addition of Nb relatively increases the a domain Da or rhombohedral domain Db in the piezoelectric film 15, making it easier to generate reversible non-180° domain rotation and to generate non-180° domain rotation with a small electric field strength. In contrast, the seed layer 14 increases the crystallinity of the piezoelectric film 15, and relatively increases the number of (001) plane-oriented c domains. Therefore, while the presence of the seed layer 14 can suppress the generation of the pyrochlore phase, it is possible that non-180° domain rotation will be suppressed, thus suppressing the piezoelectric strain due to non-180° domain rotation. In response to this, the inventors have found that by adding element A and / or element B1 in an amount of 0.2 to 0.5 times the amount of Nb added x, it is possible to obtain high piezoelectric properties without suppressing non-180° domain rotation due to Nb addition, even with the presence of a seed layer 14 (see the examples below). The piezoelectric element 1 and piezoelectric laminate 5 of this example satisfy general formula I and, with the configuration including a seed layer 14, can achieve both an increase in piezoelectric strain associated with reversible non-180° domain rotation by increasing the a domain Da or rhombohedral domain Db of the piezoelectric film 15, and suppression of the pyrochlore phase by improving crystallinity. Therefore, the piezoelectric element 1 and piezoelectric laminate 5 have high piezoelectric properties and high long-term stability.

[0065] Furthermore, if the seed layer 14, which improves the crystallinity of the piezoelectric film 15, is a dielectric or insulator with lower piezoelectric properties than the piezoelectric film 15, the piezoelectric properties of the piezoelectric element will decrease compared to the case without the seed layer 14. However, in this example, since the seed layer 14 is a conductive oxide, in the piezoelectric element, the seed layer 14 functions as an electrode when an electric field is applied to the piezoelectric film 15 together with the lower electrode layer 12, thus suppressing the decrease in piezoelectric properties.

[0066] The piezoelectric element 1 or piezoelectric laminate 5 in each of the above embodiments can be applied to ultrasonic devices, mirror devices, sensors, and memories. [Examples]

[0067] The following describes specific examples and comparative examples of the piezoelectric elements of this disclosure. First, the manufacturing method of each piezoelectric element will be described. An RF (Radio Frequency) sputtering apparatus was used to deposit each layer.

[0068] (Adhesion layer formation) A silicon substrate with a thermal oxide film was used as the substrate. A 50 nm thick TiW layer was deposited on the substrate as an adhesion layer under the following deposition conditions. The substrate and adhesion layer were the same in all examples and comparative examples.

[0069] -TiW layer sputtering conditions- Target-to-substrate distance: 100mm Target input power: 600W Ar gas pressure: 0.5 Pa Board setting temperature: 350℃

[0070] (Bottom electrode layer deposition) A 150 nm thick Ir or Pt layer was deposited on the TiW layer as a lower electrode layer under the following sputtering conditions. The configuration of the lower electrode layer in each example and comparative example is shown in Table 1 below.

[0071] -Sputtering conditions for the Ir and Pt layers- Target-to-substrate distance: 100mm Target input power: 600W Ar gas pressure: 0.1 Pa Board setting temperature: 350℃

[0072] (Seed layer formation) A 200 nm thick SrRuO3 (SRO) or LaNiO3 (LNO) layer was deposited as a seed layer continuously on the lower electrode layer. An SRO target was used for depositing the SRO layer, and an LNO target was used for depositing the LNO layer, both under the sputtering conditions described below. The seed layers for each example and comparative example are shown in Table 1 below. In comparative examples 1 to 5, no seed layer was formed.

[0073] -SRO layer and LNO layer sputtering conditions- Target-substrate distance: 100nm Target input power: 200W Vacuum level: 0.3 Pa, Ar / O2 mixed atmosphere (O2 volume fraction 10%) Board setting temperature: 450℃

[0074] (Piezoelectric film deposition) A substrate with the lower electrode layer and seed layer, or a substrate with only the lower electrode layer, was placed in an RF sputtering apparatus, and a piezoelectric film with a thickness of 2 μm was deposited under the following sputtering conditions. For the piezoelectric film, the amount of Nb added to the PZT, and which of Ni, Sc, Co, and La were added as additional elements, and the amounts added, are as shown in Table 1 below. PZT sintered targets were prepared to form piezoelectric films with the Nb addition amounts and additional element addition amounts shown in each example and comparative example in Table 1. The amount of Pb in the target was set to be greater than the stoichiometric composition, and the Ti / Zr molar ratio was set to the MPB composition (Ti / Zr = 52 / 48). The Nb addition amounts and additional element addition amounts in Table 1 were approximately equivalent to the content in the target.

[0075] - Piezoelectric film sputtering conditions - Target-to-substrate distance: 60mm Target input power: 500W Vacuum level: 0.3 Pa, Ar / O2 mixed atmosphere (O2 volume fraction 10.0%) Board setting temperature: 700℃

[0076] (Top electrode layer formation) Next, the substrate 10 after the piezoelectric film 15 was deposited was placed in the deposition chamber of the RF sputtering apparatus, and an ITO (Indium Tin Oxide) target was used to deposit a 200 nm thick ITO layer as the upper electrode layer 18. Before depositing the upper electrode layer 18, a lift-off pattern for the evaluation sample was created on the piezoelectric film 15, and the upper electrode layer 18 was formed on the lift-off pattern. The deposition conditions for the upper electrode layer 18 were as follows.

[0077] -Upper electrode layer sputtering conditions- Target-to-substrate distance: 100mm Target input power: 200W Vacuum level: 0.3 Pa, Ar / O2 mixed gas (O2 volume fraction 5%) Board setting temperature: RT (room temperature)

[0078] (Formation of evaluation electrode patterns) After the formation of the upper electrode layer 18, the upper electrode layer 18 was patterned by lifting it off along the lift-off pattern using the lift-off method.

[0079] Through the above process, piezoelectric laminated substrates for each example were fabricated, each comprising a lower electrode layer, a piezoelectric film, and a patterned upper electrode layer on a substrate.

[0080] (Preparation of evaluation samples) -Evaluation Sample 1- A cantilever was fabricated as evaluation sample 1 by cutting out a 2mm x 25mm strip from a piezoelectric multilayer substrate.

[0081] -Evaluation Sample 2- A 10mm x 25mm portion with a top electrode layer patterned in a circle with a diameter of 400μm at the center of the piezoelectric film surface was cut from the piezoelectric multilayer substrate and designated as evaluation sample 2.

[0082] <Evaluation of piezoelectric properties> The piezoelectric properties of each embodiment and comparative example are evaluated using the piezoelectric constant d 31 We measured it. Piezoelectric constant d31 The measurement was performed using the cantilever of evaluation sample 1, following the method described in I. Kanno et. al. Sensor and Actuator A 107 (2003) 68., with the lower electrode layer grounded and a sinusoidal voltage of -10V ± 10V applied to the upper electrode layer. The measurement results for each example and comparative example are shown in Table 2.

[0083] <Measurement of electric field-distortion characteristics> For each example and comparative example, the electric field-strain characteristics were measured. A voltage was applied between the lower electrode layer and the upper electrode layer using the same method as for evaluating piezoelectric properties, and the displacement of the cantilever in response to the applied voltage was measured. The relationship between the electric field (=voltage / thickness of the piezoelectric film) and the displacement of the cantilever was graphed as the electric field-strain characteristic. The presence or absence of a change in slope was checked from the acquired electric field-displacement graph. The evaluation results are shown in Table 2.

[0084] The measurement data for Example 4 and Comparative Example 6 are shown in Figure 6. In Example 4, the relationship between the electric field and displacement is represented by a slope a1 on the low electric field side and a2 on the high electric field side. That is, the electric field-strain (displacement) characteristic has a characteristic where the slope changes. In this way, when the slope of the electric field-strain characteristic changes in the electric field range of 0kV / cm to 200kV / cm, it was evaluated as "change present". In Comparative Example 6, the relationship between the electric field and displacement is shown by a single slope a over the electric field range of 0kV / cm to 200kV / cm. In this case, the presence or absence of a change in the slope of the electric field-strain characteristic was evaluated as "no".

[0085] <Evaluation of voltage resistance> The dielectric breakdown voltage (dielectric breakdown voltage) of each example and comparative example was measured. Using evaluation sample 2, the lower electrode layer was grounded, and a negative voltage was applied to the upper electrode layer at a rate of change of -1 V / second. The voltage at which a current of 1 mA or more flowed was considered the dielectric breakdown voltage. Ten samples were prepared for each example, and a total of ten measurements were taken. The average value (absolute value) is shown in Table 2 as the dielectric breakdown voltage [V].

[0086] <Evaluation of driving stability (long-term reliability)> Time-dependent dielectric breakdown (TDDB) tests were performed to evaluate the driving stability of each example and comparative example. Using evaluation sample 2, the lower electrode layer was grounded and a voltage of -40V was applied to the upper electrode layer in an environment of 120°C. The time (hr) from the start of voltage application until dielectric breakdown occurred was measured. The measurement results are shown in Table 2. The TDDB test was performed for 1000 hours, and samples in which dielectric breakdown did not occur up to 1000 hours are indicated as 1000 in Table 2.

[0087] <Pyrochlore layer thickness evaluation> For the examples and comparative examples, TEM (Transmission Electron Microscope) images were taken, and the thickness of the pyrochlore phase was determined from the TEM images. In piezoelectric films, the contrast between the pyrochlore phase and the perovskite phase differs in the TEM image, allowing for the identification of the pyrochlore phase region and calculation of its thickness. It was observed that columnar crystals of perovskite-type oxides were formed in the parts of the piezoelectric film other than the pyrochlore phase. The thickness of the pyrochlore phase was calculated as the average thickness because the pyrochlore phase is not uniformly formed on the surface of the lower electrode layer.

[0088] Specifically, the contrast adjustment function of the image processing software is used to binarize the original image at a predetermined threshold, and the pyrochlore phase is extracted using the edge extraction function of the image processing software. The threshold in this case is set to remove as much noise as possible while extracting only those that are clearly identifiable as the pyrochlore phase. If the outline of the pyrochlore-type oxide layer is unclear in the binarized image, an outline is drawn empirically while viewing the binarized image, and the inside is filled in. The area of ​​the extracted pyrochlore phase is calculated from the number of pixels in the image processing software and divided by the field of view width of the TEM image to obtain the average layer thickness. In this case, Photoshop® was used as the image processing software. The thickness of the pyrochlore phase obtained as described above is shown in Table 1.

[0089] <Stress evaluation of substrates> Film stress measurements were performed using the FLX thin-film stress measurement device manufactured by Toho Technology Co., Ltd. The amount of warpage of the laminate before and after piezoelectric film deposition was measured, and the film stress of the piezoelectric film was calculated from the change in warpage.

[0090] Table 1 shows the layer configuration of the piezoelectric elements in each example and comparative example, and Table 2 shows the evaluation results for the piezoelectric elements in each example and comparative example. [Table 1]

[0091] [Table 2]

[0092] As shown in Table 1, Examples 1 to 24 are piezoelectric elements comprising a lower electrode layer, a conductive oxide seed layer, and a piezoelectric film on a substrate in that order, and the piezoelectric film is composed of a perovskite-type oxide represented by general formula I. In contrast, Comparative Examples 1 to 12 do not satisfy the conditions for the piezoelectric film in the piezoelectric laminate of this disclosure, as they lack a seed layer, do not contain any additional elements other than Nb added to PZT, and the amount of Nb added or the amount of additional elements added is outside the specified range.

[0093] For practical application, the target values ​​were set as a piezoelectric constant of 200 pm / V or higher, a TDDB test result indicating long-term reliability of 600 hours or higher, and a withstand voltage of 80V or higher. Examples 1 to 24 all met the target values, while Comparative Examples 1 to 12 did not meet the target values ​​in at least one evaluation. By satisfying the conditions for the piezoelectric laminate of this disclosure, a piezoelectric element with a high piezoelectric constant and high long-term reliability suitable for practical application was obtained.

[0094] As shown in Tables 1 and 2, Comparative Examples 1-4 have Nb-PZT piezoelectric films with no additional elements added and no seed layer. Comparative Examples 1-3, with the addition of Nb, show a change in the slope of the electric field-strain characteristics, suggesting that piezoelectric strain occurs due to non-180° domain rotation. Furthermore, Comparative Examples 1-3 clearly show that the piezoelectric constant tends to increase with increasing Nb content. In Comparative Example 4, where the Nb content was 20 mol%, too much Nb resulted in cracks forming in the piezoelectric film, preventing it from functioning as a piezoelectric element. In Table 2, "-" in the evaluation items means that evaluation was not possible.

[0095] Comparative Example 4 and Comparative Example 5 differ only in the presence or absence of Sc added to Nb-PZT. A comparison of the two shows that when high concentrations of Nb are added, adding Sc results in a piezoelectric film that functions as a piezoelectric element. Comparative Example 5 obtained a relatively high piezoelectric constant. On the other hand, Comparative Example 5 had low dielectric strength and long-term reliability.

[0096] Comparative Example 6 has a piezoelectric film made of Nb-PZT and is equipped with a seed layer. Comparative Example 6 differs from Comparative Example 1 only in the presence or absence of a seed layer. Comparative Example 6 had a lower piezoelectric constant than Comparative Example 1 and showed no change in the slope of the electric field-strain characteristics. From the results of Comparative Example 6, it can be inferred that the non-180° domain inversion of the domain was suppressed by providing a seed layer.

[0097] Comparative Examples 7-10 were obtained by adding Sc, Ni, or La to Comparative Example 6, respectively. The piezoelectric constants were the same as in Comparative Example 6, and no piezoelectric strain was obtained due to non-180° domain inversion. No effect was observed from the addition of Sc, etc.

[0098] In Comparative Example 11, when the Nb content was 20 mol% and the Sc content was 2 mol%, both the piezoelectric constant and long-term reliability were low.

[0099] In Comparative Example 12, when the Nb content was set to 20 mol% and the Sc content to 15 mol%, a piezoelectric film with a very high piezoelectric constant was obtained. On the other hand, Comparative Example 12 had low dielectric strength and low long-term reliability. It is thought that if the amount of Sc added is too high, the balance of valence states is disrupted, leading to a decrease in dielectric strength and durability.

[0100] Examples 1-8 are examples where the amount of Nb added was 6-30 mol% and Sc was added at a rate of 2x molT or more. In Examples 1-5, 7, and 8, where the amount of Nb added was 10-30 mol%, piezoelectric constants exceeding 200 pm / V were obtained. In Examples 2-5, 7, and 8, where the amount of Nb added was 15-30 mol%, piezoelectric constants of 250 pm / V or higher were obtained. Furthermore, in Examples 2-4, where the amount of Nb added was 10-20 mol% and the amount of Sc added was 0.2x-0.3x, very high piezoelectric constants and long-term reliability were obtained. In Examples 3-5, the amount of Sc added was varied from 0.2x to 0.5x for a given amount of Nb added x. Of these, Example 5 obtained the highest piezoelectric constant, but on the other hand, its long-term reliability was lower compared to the other examples. In Example 6, where the amount of Nb added was 6 mol%, the piezoelectric constant was lower compared to Examples 1-5, 7, and 8. In Examples 7 and 8, where the Nb content was 25 mol% or higher, very high piezoelectric constants were obtained, but the dielectric strength and long-term reliability were lower compared to Examples 1-4 and 6.

[0101] Example 9 used Co instead of Sc compared to Example 4. Evaluation results nearly identical to those of Example 4 were obtained. It is believed that similar effects can be obtained with Co within the same addition range as with Sc.

[0102] Examples 10-12 are variations of Examples 1, 2, and 4 in which the seed layer was changed from SRO to LNO. Evaluation results nearly identical to those of Examples 1, 2, and 4 were obtained. In other words, similar effects were obtained whether the seed layer was SRO or LNO.

[0103] Examples 13-15 are variations of Examples 1, 2, and 4 in which the additional element was changed from Sc to Ni. Evaluation results nearly identical to those of Examples 1, 2, and 4 were obtained.

[0104] Examples 16-18 are variations of Examples 1, 2, and 4 in which the additional element was changed from Sc to La. Evaluation results nearly identical to those of Examples 1, 2, and 4 were obtained.

[0105] In Examples 1, 2, and 4, where the added element is Sc; Example 9, where the added element is Co; Examples 13-15, where the added element is Ni; and Examples 16-18, where the added element is La, substantially similar evaluation results were obtained depending on the amount of added element. From this, it can be concluded that although the added element is a single element in the examples, similar results can be obtained by combining two or more of Sc, Ni, Co, and La. For example, instead of adding 6 mol% Sc, it is thought that the same effect as adding 6 mol% Sc can be obtained by adding 2% Sc, 2% Ni, and 2% Co, totaling 6%.

[0106] Examples 19-21 are examples in which the lower electrode layer was changed from Ir to Pt in Examples 1, 2, and 4. In this case, evaluation results were obtained that were substantially the same as those in Examples 1, 2, and 4. In other words, substantially the same effect was obtained whether the lower electrode layer was Ir or Pt.

[0107] Examples 22-24 are variations of Examples 19-21 in which the seed layer was changed from SRO to LNO. In this case, evaluation results were obtained that were substantially the same as those for Examples 1, 2, and 4.

[0108] In the above examples, Sc, Ni, Co, or La were used as additional elements, but similar evaluation results can be expected if any element behaves similarly. [Explanation of symbols]

[0109] 1. Piezoelectric element 5 Piezoelectric laminate 10 circuit boards 12 Lower electrode layer 14 Seed Layer 15 Piezoelectric film 18 Upper electrode layer

Claims

1. A piezoelectric laminate comprising a lower electrode layer and a piezoelectric film on a substrate in this order, A seed layer made of a conductive oxide is provided between the lower electrode layer and the piezoelectric film. The lower electrode layer is made of metal. The piezoelectric film is a piezoelectric laminate containing a perovskite-type oxide represented by the following general formula I. Pb １－ｙ２＋α A ｙ２ {(Ti, Zr) １－ｘ－ｙ１ Nb ｘ B1 ｙ１ }O ３ General formula I Here, A is an A-site element, and is one or more elements including at least La. B1 is a B-site element, and is one or more elements that are divalent or trivalent. O is the element oxygen, x, y1, y2, α are, 0.05<x≦0.3, 0.2x≦y1+y2≦0.5x, 0≦y1≦0.15, 0≦y2≦0.15, 0≦α≦0.2 It satisfies the condition.

2. The piezoelectric laminate according to claim 1, wherein B1 is at least one of Ni, Co, and Sc.

3. The piezoelectric laminate according to claim 1, wherein the seed layer is cubic or pseudocubic and has a lattice constant of 0.4 nm or less.

4. The piezoelectric laminate according to claim 1, wherein the piezoelectric film has a crystalline orientation that is preferentially oriented in the film thickness direction.

5. The piezoelectric laminate according to claim 1, wherein the piezoelectric film has an internal stress in the range of 50 to 250 MPa.

6. The piezoelectric laminate according to claim 1, wherein the piezoelectric film has a characteristic in which the slope of the electric field-strain characteristic changes when an electric field of 0 kV / cm to 200 kV / cm is applied in the film thickness direction.

7. The aforementioned seed layer is LaNiO ３ or SrRuO ３ The piezoelectric laminate according to claim 1.

8. The piezoelectric laminate according to claim 1, wherein A is La and B1 is at least one of Ni, Co and Sc.

9. A piezoelectric laminate according to any one of claims 1 to 8, A piezoelectric element comprising an upper electrode layer formed on the piezoelectric film.

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