Piezoelectric laminate and piezoelectric element

Abstract

This piezoelectric laminate and piezoelectric element include, on a substrate, a lower electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film in this order. The first piezoelectric film consists of a first perovskite oxide as a main component and has a first columnar structure including a plurality of columnar bodies extending in a non-parallel direction with respect to a substrate surface of the substrate. The second piezoelectric film consists of a second perovskite oxide as a main component and has a second columnar structure including a plurality of columnar bodies extending in a non-parallel direction with respect to the substrate surface of the substrate. The difference between a first column diameter, which is the average column diameter of the first columnar structure, and a second column diameter, which is the average column diameter of the second columnar structure, is within 10% of the larger of the first column diameter and the second column diameter.

Description

Piezoelectric laminate and piezoelectric element

[0001] The present disclosure relates to a piezoelectric stack and a piezoelectric element.

[0002] Lead zirconate titanate (Pb(Zr,Ti)O) is a material with excellent piezoelectric and ferroelectric properties. ３ Perovskite oxides such as PZT (Piezoelectric Crystalline Zinc Oxide, hereafter referred to as PZT) are known. Piezoelectric materials made of perovskite 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 have been applied to a variety of devices, including memories, inkjet heads (actuators), micromirror devices, angular velocity sensors, gyro sensors, ultrasonic elements (PMUT: Piezoelectric Micromachined Ultrasonic Transducers), and vibration-powered harvesting devices.

[0003] As a piezoelectric element, a laminated piezoelectric element in which a plurality of piezoelectric films are laminated with electrodes interposed therebetween has been proposed in order to obtain higher piezoelectric characteristics (Japanese Patent Laid-Open Nos. 2009-54994 and 2013-80886, etc.).

[0004] In a laminated piezoelectric element obtained by sputtering a piezoelectric film and an electrode, peeling may occur at or near the interface between the laminated piezoelectric film and the intermediate electrode after long-term use. For example, when a piezoelectric element is used as a sensor or actuator for an automobile, durability that allows for long-term use is required.

[0005] The present disclosure has been made in view of the above circumstances, and has an object to provide a piezoelectric laminate and a piezoelectric element that have higher durability than conventional ones.

[0006] The piezoelectric laminate of the present disclosure comprises a substrate and, in this order, a lower electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film; the first piezoelectric film has a first columnar structure containing a first perovskite oxide as a main component and including a plurality of columns extending in a direction non-parallel to the substrate surface of the substrate; the second piezoelectric film has a second columnar structure containing a second perovskite oxide as a main component and including a plurality of columns extending in a direction non-parallel to the substrate surface of the substrate; and the difference between the first column diameter, which is the average column diameter of the first columnar structure, and the second column diameter, which is the average column diameter of the second columnar structure, is within 10% of the larger of the first column diameter and the second column diameter.

[0007] The first and second column diameters are preferably 200 nm or more, and more preferably 250 nm or more.

[0008] The first perovskite oxide and the second perovskite oxide may have the same constituent elements, but may have different composition ratios.

[0009] The first perovskite oxide and the second perovskite oxide may differ in at least one of their constituent elements.

[0010] The first perovskite type oxide and the second perovskite type oxide are preferably lead zirconate titanate doped with a metal element M, and the metal element M is preferably at least one of vanadium, niobium, tantalum, antimony, molybdenum, and tungsten.

[0011] The thickness of the intermediate electrode may be 150 nm or more, the thickness of the intermediate electrode may be 200 nm or more, or further, the thickness of the intermediate electrode may be 250 nm or more.

[0012] The intermediate electrode preferably contains at least one of iridium, platinum, ruthenium, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide.

[0013] The piezoelectric element of the present disclosure includes the piezoelectric laminate of the present disclosure and an upper electrode provided on the second piezoelectric film of the piezoelectric laminate.

[0014] The upper and lower electrodes may be maintained at ground potential, and the middle electrode may be a drive electrode to which a negative drive potential is applied.

[0015] Alternatively, the middle electrode may be maintained at ground potential, and the bottom and top electrodes may be drive electrodes to which a positive drive potential is applied.

[0016] According to the technology of the present disclosure, a piezoelectric laminate and a piezoelectric element having higher durability than conventional ones can be obtained.

[0017] It is a schematic cross-sectional view of a piezoelectric element. It is a schematic configuration diagram of an actuator. It is a schematic configuration diagram of an actuator. It is an EBSD image of a first piezoelectric film and a second piezoelectric film of Example 1.

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the thicknesses and ratios of each layer have been appropriately modified for ease of visualization and do not necessarily reflect the actual thicknesses and ratios. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in this disclosure, the upper or lower limit stated in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit stated in a certain numerical range may be replaced with a value shown in the examples.

[0019] Fig. 1 is a cross-sectional schematic diagram showing the layer configuration of a piezoelectric laminate 5 and a piezoelectric element 1 according to one embodiment. As shown in Fig. 1, the piezoelectric element 1 includes a piezoelectric laminate 5 and an upper electrode 20. The piezoelectric laminate 5 includes a substrate 10 and a lower electrode 12, a first piezoelectric film 14, an intermediate electrode 16, and a second piezoelectric film 18, which are laminated in this order on the substrate 10. The terms "lower" and "upper" in the lower electrode 12 and upper electrode 20 do not refer to the top and bottom in the vertical direction; rather, the electrode located on the substrate 10 side, with the first piezoelectric film 14, intermediate electrode 16, and second piezoelectric film 18 sandwiched therebetween, is simply referred to as the lower electrode 12, and the electrode located on the opposite side from the substrate 10 is simply referred to as the upper electrode 20.

[0020] The first piezoelectric film 14 is a first columnar structure film that contains a first perovskite oxide as a main component and includes a plurality of columns 14 a that extend in a direction non-parallel to the substrate surface 10 a that is the surface of the substrate 10 .

[0021] The second piezoelectric film 18 is a second columnar structure film that contains a second perovskite oxide as a main component and includes a plurality of columns 18 a that extend in a direction non-parallel to the substrate surface 10 a of the substrate 10 .

[0022] The columns 14a of the first piezoelectric film 14 and the columns 18a of the second piezoelectric film 18 extending in a direction non-parallel to the substrate surface 10a means that the longitudinal directions (long axes) of the columns 14a and 18a are not parallel to the substrate surface 10a but are tilted relative to the substrate surface 10a. In the example of Fig. 1, the columns 14a and 18a extend along the normal to the substrate surface 10a or in a direction tilted at an angle of about 10° or less from the normal to the substrate surface 10a.

[0023] The difference between the first column diameter D1, which is the average column diameter of the first columnar structure film, and the second column diameter D2, which is the average column diameter of the second columnar structure film, is within 10% of the larger of the first column diameter D1 and the second column diameter D2. The difference between the first column diameter D1 and the second column diameter D2 being within 10% of the larger of the first column diameter D1 and the second column diameter D2 means that the first column diameter D1 and the second column diameter D2 are the same or equivalent. When D1>D2, D1-D2≦D1×0.1, and when D1<D2, D2-D1≦D2×0.1. In summary, this can be expressed by the following formula: 0≦|D1-D2|≦MAX{D1, D2}×0.1

[0024] The "columnar structure film" referred to in this specification is a columnar structure film described in Movchan and Demchishin, Phys. Met. Metallogr., 28, 83 (1969) or Thonton, J. Vac. Sci. Technol., 11, 666 (1974), etc. The columnar structure film may have a crystalline structure or an amorphous structure, and preferably has a crystalline structure. When it has a crystalline structure, the individual columns become columnar crystal grains.

[0025] The columnar structure film is preferably a sputtered film formed by, for example, a sputtering method. The column diameter of the columnar structure film can be adjusted by changing the composition of the piezoelectric film and / or the film formation conditions.

[0026] In this specification, the column diameter of the columnar structure film is determined as follows: An electron backscatter diffraction (EBSD) image is obtained for a cross section of the piezoelectric element 1 (or the piezoelectric laminate 5), and the horizontal width of each column in the EBSD image or band contrast image is determined at approximately the center of the thickness direction of the piezoelectric film, and the average value is taken as the average column diameter of the columnar structure film.

[0027] As described above, in the piezoelectric element 1 and piezoelectric stack 5 of this embodiment, the difference between the first column diameter D1 and the second column diameter D2 is within 10% of the larger column diameter. This configuration achieves high durability. When the first column diameter D1 of the first columnar structure film and the second column diameter D2 of the second columnar structure film are equal, the stresses of the first piezoelectric film 14 and the second piezoelectric film 18 relative to the intermediate electrode 16 can be made substantially equal. When the difference between the stress of the first piezoelectric film 14 relative to the intermediate electrode 16 and the stress of the second piezoelectric film 18 relative to the intermediate electrode 16 is large, durability (i.e., long-term reliability) is low. However, by making the two stresses equal, peeling that occurs during long-term use can be suppressed and durability can be improved.

[0028] Although not limited, the first column diameter D1 and the second column diameter D2 are preferably 200 nm or more, and more preferably 250 nm or more. In a columnar structure film constituting a piezoelectric film, the larger the column diameter, the better the piezoelectric characteristics, i.e., the larger the piezoelectric constant tends to be obtained. Therefore, if the first column diameter D1 of the first piezoelectric film 14 and the second column diameter D2 of the second piezoelectric film 18 are both 200 nm or more, or even 250 nm or more, good piezoelectric characteristics can be obtained. Although there are no particular upper limits for the first column diameter D1 and the second column diameter D2, they are preferably about 3 μm or less, more preferably 1 μm or less, and particularly preferably 500 nm or less.

[0029] The substrate 10 is not particularly limited, and examples thereof include substrates of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. The substrate 10 may be a laminated substrate in which a thermally oxidized silicon film is formed on the surface of a silicon substrate.

[0030] The lower electrode 12 and the intermediate electrode 16 form a pair to apply a voltage to the first piezoelectric film 14. The intermediate electrode 16 and the upper electrode 20 form a pair to apply a voltage to the second piezoelectric film 18.

[0031] There are no particular limitations on the materials that make up the lower electrode 12 and the upper electrode 20, but examples of the main components include metals or metal oxides such as Au (gold), Pt (platinum), Ir (iridium), Ru (ruthenium), Ti (titanium), Mo (molybdenum), Ta (tantalum), and Al (aluminum), as well as combinations thereof. Also, ITO (indium tin oxide) and the like may be used. Also, ITO (indium tin oxide), LaNiO ３ , and SrRuO ３ The lower electrode 12 and the upper electrode 20 may be a single layer or may have a multi-layer structure. When the lower electrode 12 has a multi-layer structure, in addition to the conductive layer made of the above materials, Ti, TiW or ZrO may be used on the substrate 10 side. ２ It is also preferable to have a structure including an adhesive layer such as the above.

[0032] There are no particular limitations on the thickness of the lower electrode 12 and the upper electrode 20, but it is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0033] The intermediate electrode 16 preferably contains at least one of Pt, Ir, Ru, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide. The intermediate electrode 16 may also be a single layer or a multi-layer structure. In particular, the intermediate electrode 16 preferably has a multi-layer structure of an iridium oxide (IrOx) layer and an iridium layer arranged in this order from the first piezoelectric film 14 side.

[0034] The thickness of the intermediate electrode 16 is not particularly limited, but from the viewpoint of stress, it is preferably about 50 nm to 500 nm, more preferably 150 nm or more, and even more preferably greater than 150 nm, 200 nm or more, or greater 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, more preferably 250 nm or more, variation in etching performed in the manufacturing process of the piezoelectric element can be suppressed, and a decrease in yield caused by variation in etching can be suppressed.

[0035] In this specification, the thickness of the intermediate electrode 16 is measured as follows: For example, a cross section of a cleaved surface is processed, and a SEM observation and length measurement are performed using a combined FIB-SEM (Focused Ion Beam Scanning Electron Microscope) (for example, Helios-400S manufactured by TFS Corporation).

[0036] As described above, the first piezoelectric film 14 is primarily composed of a first perovskite oxide, and the second piezoelectric film 18 is primarily composed of a second perovskite oxide. In this specification, "primary component" refers to a component that accounts for 80 mol % or more. The first piezoelectric film 14 is preferably composed of the first perovskite oxide for 90 mol % or more, and more preferably the first piezoelectric film 14 is composed of the first perovskite oxide (but including inevitable impurities). The second piezoelectric film 18 is preferably composed of the second perovskite oxide for 90 mol % or more, and more preferably the second piezoelectric film 18 is composed of the second perovskite oxide (but including inevitable impurities). The first perovskite oxide that is the primary component of the first piezoelectric film 14 and the second perovskite oxide that is the primary component of the second piezoelectric film 18 may be composed of different constituent elements or the same constituent elements. The phrase "having the same constituent elements" means that the elements contained are the same, but the composition ratios may be different.

[0037] The first perovskite oxide and the second perovskite oxide preferably have the same constituent elements but different composition ratios, or preferably have at least one constituent element different from the first perovskite oxide and the second perovskite oxide.

[0038] When two piezoelectric films are stacked as in the piezoelectric element 1 of this embodiment, and the two layers are formed with the same composition, the second layer may have weaker piezoelectric characteristics than the first layer. In response to this, by adjusting the film formation conditions for the first piezoelectric film 14 and the second piezoelectric film 18, and by differentiating at least one of the composition ratio and the constituent elements, the first piezoelectric film 14 and the second piezoelectric film 18 can have the same amount of displacement in response to an applied voltage. Compared to when the first piezoelectric film 14 and the second piezoelectric film 18 have different amounts of displacement, the stress difference generated inside the element during displacement can be suppressed, improving durability.

[0039] The constituent elements of the first piezoelectric film 14 and the second piezoelectric film 18 can be identified by a variety of analytical methods, such as ICP (Inductively Coupled Plasma) emission spectroscopy or XRF (X-ray Fluorescence). The term "same composition ratio" means that the composition ratios are equal within the measurement error range. For example, when composition analysis is performed using XRF, the measurement error is approximately 0.005.

[0040] The first perovskite oxide and the second perovskite oxide are preferably lead zirconate titanate (PZT) oxides containing Pb (lead), Zr (zirconium), Ti (titanium) and O (oxygen).

[0041] In particular, a compound represented by the following general formula (1) containing an additive M at the B site of PZT is preferred. ａ {(Zr ｘ Ti 1-ｘ ) 1-ｙ M ｙ O ３(1) Here, M is preferably one or more elements selected from V (vanadium), Nb (niobium), Ta (tantalum), Sb (antimony), Mo (molybdenum), and W (tungsten). Here, it is preferable that 0<x<1, 0<y<1, and 0.9≦a≦1.2, and more preferably 0.08≦y≦0.30. In the general formula (1), Pb:{(Zr ｘ Ti 1-ｘ ) 1-ｙ M ｙ}:O is based on a 1:1:3 ratio, but may deviate within a range that allows for a perovskite structure. M may be a single element such as V alone or Nb alone, or may be a combination of two or more elements, such as a mixture of V and Nb, or a mixture of V, Nb, and Ta. When M is one of these elements, a very high piezoelectric constant can be achieved in combination with the A-site element Pb.

[0042] In particular, Pb in which the metal element M is Nb ａ {(Zr ｘ Ti １－ｘ ) １－ｙ Nb ｙ O ３ In this case, a higher piezoelectric constant can be obtained when 0.08≦y≦0.30.

[0043] It is said that PZT-based perovskite oxides exhibit high piezoelectric properties at or near the morphotropic phase boundary (MPB). The MPB composition is near a Zr:Ti (molar ratio) of 52:48, and in the above general formula, the MPB composition or its vicinity is preferable. "At or near the MPB" refers to the region where a phase transition occurs when an electric field is applied to the piezoelectric film. Specifically, the Zr:Ti (molar ratio) is preferably in the range of 45:55 to 55:45, i.e., x = 0.45 to 0.55 in the above general formula (1).

[0044] The 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 thicknesses of the first piezoelectric film 14 and the second piezoelectric film 18 may be the same or different. The thickness of the first piezoelectric film 14 and the second piezoelectric film 18 is preferably 2 μm or less.

[0045] Furthermore, both the first piezoelectric film 14 and the second piezoelectric film 18 are preferably uniaxially oriented films. The term "uniaxially oriented film" for the first piezoelectric film 14 means that the first perovskite oxide, which is the main component of the first piezoelectric film 14, is uniaxially oriented. Similarly, the term "uniaxially oriented film" for the second piezoelectric film 18 means that the second perovskite oxide, which is the main component of the second piezoelectric film 18, is uniaxially oriented. A uniaxially oriented perovskite oxide is spontaneously polarized immediately after deposition without poling treatment, i.e., the spontaneous polarization is aligned in the film thickness direction. The reason why a piezoelectric film that has not been poled exhibits aligned spontaneous polarization in the absence of an applied external electric field is thought to be due to the generation of an electric field (hereinafter referred to as a spontaneous internal electric field) within the piezoelectric film due to distortion or defects in the crystalline structure. The first piezoelectric film 14 and the second piezoelectric film 18 preferably have spontaneous polarization aligned in the film thickness direction due to such spontaneous internal electric field. The first piezoelectric film 14 and the second piezoelectric film 18 in this embodiment are piezoelectric films that are polarized in the film thickness direction when no external electric field is applied, as the spontaneous polarizations P1 and P2 (see Figure 2) are aligned in the film thickness direction.

[0046] 2 and 3, the actuators 2 and 3 each including the piezoelectric element 1 will be described. In the cross-sectional views of the piezoelectric element 1 in Figures 2 and 3, the pillars 14a and 18a shown in Figure 1 are omitted.

[0047] The actuator 2 shown in Fig. 2 includes a piezoelectric element 1 and a drive circuit 30. In the actuator 2 shown in Fig. 2, the lower electrode 12 and upper electrode 20 are connected to the ground terminal of the drive circuit 30, and the intermediate electrode 16 is connected to the drive voltage output terminal of the drive circuit 30. In other words, the lower electrode 12 and upper electrode 20 are at ground potential, and the intermediate electrode 16 functions as a drive electrode.

[0048] The drive circuit 30 is a means for supplying a drive voltage to the first piezoelectric film 14 and the second piezoelectric film 18 sandwiched between the electrodes to drive the piezoelectric element 1. 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. This allows the drive circuit 30 to apply electric fields 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 to the first piezoelectric film 14 in the same direction as the direction of the aligned spontaneous polarization (polarization direction) P1, and applies an electric field Er to the second piezoelectric film 18 in the opposite direction to the electric field Ef applied to the first piezoelectric film 14. The drive circuit 30 is a negative drive circuit that applies a negative potential to the drive electrode (here, the intermediate electrode 16).

[0049] 3, a modified actuator 3 may be provided with a positive drive circuit 32 that applies a positive potential to the drive electrodes instead of the drive circuit 30. In the actuator 3 shown in FIG. 3, 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, a positive potential is applied to the drive electrodes (here, the lower electrode 12 and upper electrode 20) of the piezoelectric element 1 when it is driven. As a result, as in the case of actuator 2, in actuator 3 as well, the drive circuit 32 applies electric fields in opposite directions to the first piezoelectric film 14 and the second piezoelectric film 18.

[0050] It is preferable that the lower electrode 12 and the upper electrode 20 are connected in the piezoelectric element 1. If the lower electrode 12 and the upper electrode 20 are connected, drive control is easy.

[0051] The actuators 2 and 3 are provided with only one polarity drive circuit as the drive circuits 30 and 32, and can be realized at low cost.

[0052] Specific examples and comparative examples of the piezoelectric element of the present disclosure will be described below. First, the configuration and manufacturing method of the piezoelectric element of each example will be described.

[0053] Piezoelectric elements of Examples and Comparative Examples were fabricated and evaluated for durability. The structure and manufacturing method of the piezoelectric elements of the following samples will be described with reference to the reference numerals of the layers of the piezoelectric element 1 shown in FIG.

[0054] "Method of Manufacturing Piezoelectric Elements" Methods of manufacturing the piezoelectric elements of the examples and comparative examples will be described.

[0055] (Substrate with Lower Electrode) A substrate with an electrode was prepared, which was provided with a lower electrode formed by sequentially laminating a 20 nm thick TiW film and a 230 nm thick Ir film on a substrate 10 made of a silicon wafer with a thermal oxide film.

[0056] (First Piezoelectric Film) An Nb-doped PZT film was formed on the lower electrode 12 as the first piezoelectric film 14. The thickness of the Nb-doped PZT film was 2 μm. In Example 1, an Nb-doped PZT target was used in which the amount of Nb added to the B site was 12%, the Pb composition ratio a was 1.3, and the Zr / Ti molar ratio was an MPB composition (Zr / Ti=52 / 48). The sputtering conditions were as follows:

[0057] -Sputtering conditions for first piezoelectric film- Target input power: 3 kW Vacuum degree: 0.5 Pa, Ar and O ２ Mixed atmosphere (O ２ Substrate set temperature: In order to control the diameter of the columnar structures, the temperature was set in the range of 500°C to 650°C for each example and comparative example. Note that the higher the temperature, the larger the diameter of the columnar structures obtained. In Example 1, the temperature was set to 600°C.

[0058] After the first piezoelectric film 14 was deposited, annealing was performed by maintaining the substrate at the set temperature for 0 to 1 hour. The annealing time was adjusted for each example and comparative example. Holding for 0 hour means that no annealing was performed. For example, in Example 1, annealing was performed after the first piezoelectric film 14 was deposited, while annealing was not performed after the second piezoelectric film 18 (described below) was deposited. The longer the annealing time, the larger the column diameter of the resulting columnar structure. The column diameter can also be changed by varying the amount of lead in the target. The larger the lead content, the larger the column diameter, and the smaller the lead content, the smaller the column diameter. In each example and comparative example, the composition ratio, deposition temperature, and post-deposition annealing time were adjusted to obtain a first piezoelectric film 14 with a columnar structure having the column diameters shown in Table 1.

[0059] (Intermediate Electrode) In Examples 1, 2, and 5 and Comparative Examples 1 to 3, a 50 nm thick IrO ｘ In Example 3, 50 nm of IrOx and 100 nm of Ir were stacked in this order on the first piezoelectric film 14 to form the intermediate electrode 16 with a thickness of 150 nm. In Example 4, 50 nm of IrOx and 150 nm of Ir were stacked in this order on the first piezoelectric film 14 to form the intermediate electrode 16 with a thickness of 200 nm.

[0060] The deposition of IrOx and Ir films was performed using Ir as a target, with a vacuum of 0.5 Pa, an Ar atmosphere for Ir deposition, and an Ar and O atmosphere for IrOx deposition. ２ Mixed atmosphere (O ２ The volume fraction was 0.5-10%), and the substrate temperature was set to about 400°C.

[0061] (Second Piezoelectric Film) As in the first piezoelectric film 14, an Nb-doped PZT film was formed on the intermediate electrode 16 as the second piezoelectric film 18. To control the diameter of the columnar structure, targets with different Pb and Nb contents and with or without additives were used as necessary. The sputtering conditions were the same as those for the first piezoelectric film 14. The substrate setting temperature and annealing time were adjusted for each example and comparative example to control the columnar structure.

[0062] (Upper electrode) A 50 nm thick IrO film was formed on the second piezoelectric film 18 as the upper electrode 20. ｘ and 100 nm of Ir were laminated in this order. The sputtering conditions were the same as those for the intermediate electrode 16.

[0063] (Formation of electrode pattern for evaluation) In order to form electrode pads for applying voltage to the lower electrode 12, the intermediate electrode 16, and the upper electrode 20, patterning was performed sequentially on the upper electrode 20, the second piezoelectric film 18, the intermediate electrode 16, and the first piezoelectric film 14 by photolithography and dry etching.

[0064] The column diameters D1 and D2 for each example and comparative example were determined from cross-sectional EBSD images. Figure 4 shows cross-sectional EBSD images of the first piezoelectric film 14 and the second piezoelectric film 18 of Example 1. As shown in Figure 4, horizontal lines V1 and V2 were drawn at the centers of the first piezoelectric film 14 and the second piezoelectric film 18 in the thickness direction. For the columns present on the horizontal lines V1 and V2, partition lines were drawn between adjacent columns. The column diameters D1 and D2 were determined by dividing the length between the two horizontal partition lines by the number of columns present between the two partition lines. The space between adjacent partition lines on the horizontal lines V1 and V2 in Figure 4 corresponds to one column. The column diameters D1 and D2 for each example and comparative example are summarized in Table 1. As shown in Table 1, Examples 1 to 5 satisfied the condition that the difference |D1-D2| between the first column diameter D1 and the second column diameter D2 was within 10% of the larger of the first column diameter D1 and the second column diameter D2, MAX{D1, D2}. In contrast, Comparative Examples 1 to 3 did not satisfy the above condition.

[0065] <Measurement of Piezoelectric Properties> The piezoelectric properties of each example and comparative example were evaluated by measuring the pseudo piezoelectric constant d ３１ The piezoelectric constant d ３１ For the measurement, a cantilever was used which was prepared by cutting out a strip of 2 mm x 25 mm from a wafer on which the layers were stacked as described above.

[0066] According to the method described in I. Kanno et al. Sensor and Actuator A 107 (2003) 68., the pseudo piezoelectric constant d ３１Specifically, the lower electrode 12 and the upper electrode 20 were grounded, and the intermediate electrode 16 was used as the driving electrode. A sine wave voltage of −10 V±10 V was applied to the driving electrode, that is, a bias voltage of −10 V and a sine wave voltage with an amplitude of 10 V were applied to measure the pseudo piezoelectric constant d ３１ The results of evaluation based on the following criteria are shown in Table 1: a: 350 pm / V or more b: 300 pm / V or more, less than 350 pm / V c: less than 300 pm / V

[0067] <Durability Evaluation> A time-dependent dielectric breakdown (TDDB) test was conducted to evaluate durability, i.e., long-term reliability. For the TDDB test, an evaluation sample was prepared by cutting a 25 mm × 25 mm section from a wafer in which each layer was stacked as described above. The lower electrode 12 and upper electrode 20 were grounded, and the intermediate electrode 16 was used as the driving electrode. An accelerated test was conducted in which a voltage of −40 V was applied to the driving electrode in an environment of 120°C, and the time from the start of voltage application to the occurrence of dielectric breakdown was measured. Dielectric breakdown was defined as a current of 1 mA or more flowing between the lower electrode 12 and the intermediate electrode 16 or between the intermediate electrode 16 and the upper electrode 20.

[0068] The time until breakdown was evaluated according to the following criteria, and the results are shown in Table 1: a: 100 hours or more b: 30 hours or more but less than 100 hours c: Less than 30 hours

[0069] Table 1 shows the piezoelectric constant d ３１ The overall evaluation of the piezoelectric constant d and durability is also shown. ３１ The lowest evaluation of the durability and durability was used as the overall evaluation.

[0070]

[0071] As shown in Table 1, in Examples 1 to 5, the piezoelectric constant d ３１ On the other hand, Comparative Examples 1 and 2 have a high piezoelectric constant d ３１ Although the column diameters D1 and D2 of Comparative Example 3 did not satisfy the above-mentioned conditions, there was no problem with durability. However, the piezoelectric constant d３１ The piezoelectric constant d ３１ tended to increase as the column diameter of the columnar structure film increased. When the column diameters D1 and D2 of the columns were relatively small, at least 150 nm or less as shown in Comparative Example 3, high durability was obtained even when the difference between the first column diameter D1 and the second column diameter D2 was greater than 10% of the larger one. However, when the column diameters D1 and D2 were 200 nm or greater, as in Comparative Examples 1 and 2, the durability was thought to decrease when the difference between the column diameters D1 and D2 was greater than 10% of the larger one, due to the poor stress balance. As in Examples 1 to 5, when the column diameters D1 and D2 satisfied the above conditions, high durability was achieved even when the column diameter was 200 nm or greater. In other words, when the relationship between the first column diameter D1 and the second column diameter D2 satisfied the above conditions, high durability was achieved in a piezoelectric element with high piezoelectric properties.

[0072] The disclosure of Japanese Patent Application No. 2024-055003, filed on March 28, 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 indicated to be incorporated by reference.

[0073] The following supplementary notes are further disclosed regarding the above-described embodiments. <Supplementary Note 1> A piezoelectric stack comprising, on a substrate, a bottom electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film, in this order; the first piezoelectric film having a first columnar structure mainly composed of a first perovskite oxide and including a plurality of columns extending in a direction non-parallel to the substrate surface; the second piezoelectric film having a second columnar structure mainly composed of a second perovskite oxide and including a plurality of columns extending in a direction non-parallel to the substrate surface; and the difference between the first column diameter, which is the average column diameter of the first columnar structure, and the second column diameter, which is the average column diameter of the second columnar structure, is within 10% of the larger of the first column diameter and the second column diameter. <Supplementary Note 2> The piezoelectric stack according to Supplementary Note 1, wherein the first column diameter and the second column diameter are 200 nm or more. <Supplementary Note 3> The piezoelectric stack according to Supplementary Note 1, wherein the first column diameter and the second column diameter are 250 nm or more. <Appendix 4> The piezoelectric stack according to any one of Appendices 1 to 3, wherein the first perovskite oxide and the second perovskite oxide have the same constituent elements but different composition ratios. <Appendix 5> The piezoelectric stack according to any one of Appendices 1 to 3, wherein the first perovskite oxide and the second perovskite oxide have at least one different constituent element. <Appendix 6> The piezoelectric stack according to Appendices 4 or 5, wherein the first perovskite oxide and the second perovskite oxide are lead zirconate titanate to which a metal element M has been added, and the metal element M is at least one of vanadium, niobium, tantalum, antimony, molybdenum, and tungsten. <Appendix 7> The piezoelectric stack according to any one of Appendices 1 to 6, wherein the intermediate electrode has a thickness of 150 nm or more. <Appendix 8> The piezoelectric stack according to any one of Appendices 1 to 6, wherein the intermediate electrode has a thickness of 200 nm or more. <Appendix 9> The piezoelectric stack according to any one of Appendices 1 to 6, wherein the intermediate electrode has a thickness of 250 nm or more. <Appendix 10> The piezoelectric stack according to any one of Appendices 1 to 9, wherein the intermediate electrode contains at least one of iridium, platinum, ruthenium, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide.<Supplementary Note 11> A piezoelectric element comprising: the piezoelectric stack according to any one of Supplementary Notes 1 to 10; and an upper electrode provided on a second piezoelectric film of the piezoelectric stack. <Supplementary Note 12> The piezoelectric element according to Supplementary Note 11, wherein the upper electrode and the lower electrode are maintained at ground potential, and the intermediate electrode is a drive electrode to which a negative drive potential is applied. <Supplementary Note 13> The piezoelectric element according to Supplementary Note 11, wherein the intermediate electrode is maintained at ground potential, and the lower electrode and the upper electrode are drive electrodes to which a positive drive potential is applied.

Claims

1. A piezoelectric laminate comprising a substrate and, in this order, a bottom electrode, a first piezoelectric film, an intermediate electrode, and a second piezoelectric film; the first piezoelectric film is composed primarily of a first perovskite oxide and has a first columnar structure including a plurality of columns extending in a direction non-parallel to the substrate surface of the substrate; the second piezoelectric film is composed primarily of a second perovskite oxide and has a second columnar structure including a plurality of columns extending in a direction non-parallel to the substrate surface of the substrate; and the difference between a first column diameter that is the average column diameter of the first columnar structure and a second column diameter that is the average column diameter of the second columnar structure is within 10% of the larger of the first column diameter and the second column diameter.

2. The piezoelectric stack according to claim 1, wherein the first column diameter and the second column diameter are 200 nm or more.

3. The piezoelectric stack according to claim 1, wherein the first column diameter and the second column diameter are 250 nm or more.

4. The piezoelectric stack according to claim 1, wherein the first perovskite oxide and the second perovskite oxide have the same constituent elements but different composition ratios.

5. The piezoelectric stack according to claim 1, wherein the first perovskite oxide and the second perovskite oxide differ in at least one constituent element.

6. The piezoelectric stack according to claim 4, wherein the first perovskite oxide and the second perovskite oxide are lead zirconate titanate doped with a metal element M, and the metal element M is at least one of vanadium, niobium, tantalum, antimony, molybdenum, and tungsten.

7. The piezoelectric stack according to any one of claims 1 to 3, wherein the thickness of the intermediate electrode is 150 nm or more.

8. The piezoelectric stack according to any one of claims 1 to 3, wherein the thickness of the intermediate electrode is 200 nm or more.

9. The piezoelectric stack according to any one of claims 1 to 3, wherein the thickness of the intermediate electrode is 250 nm or more.

10. The piezoelectric stack of claim 1, wherein the intermediate electrode comprises at least one of iridium, platinum, ruthenium, strontium ruthenate, barium ruthenate, zirconium oxide, hafnium oxide, ruthenium oxide, iridium oxide, platinum oxide, and rhenium oxide.

11. A piezoelectric element comprising: the piezoelectric laminate according to claim 1; and an upper electrode provided on the second piezoelectric film of the piezoelectric laminate.

12. The piezoelectric element of claim 11, wherein the upper and lower electrodes are maintained at ground potential, and the middle electrode is a drive electrode to which a negative drive potential is applied.

13. The piezoelectric element of claim 11, wherein the intermediate electrode is maintained at ground potential, and the lower electrode and the upper electrode are drive electrodes to which a positive drive potential is applied.

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