Piezoelectric actuator

Abstract

This piezoelectric actuator is provided with a laminated piezoelectric element and a pre-drive circuit. A first piezoelectric film and a second piezoelectric film both have spontaneous polarization aligned in a film thickness direction, and have the same orientation of spontaneous polarization. ΔVcf < ΔVcr - 1.0 is satisfied where ΔVcf is the absolute value |Vcf+ - Vcf-| of the difference between two coercive voltages in a hysteresis curve of one piezoelectric film of the first piezoelectric film and the second piezoelectric film, and ΔVcr is the absolute value |Vcr+ - Vcr-| of the difference between two coercive voltages in the hysteresis curve of the other piezoelectric film thereof. A drive circuit causes the one piezoelectric film to generate an electric field in the same orientation as the orientation of spontaneous polarization, and causes the other piezoelectric film to generate an electric field in the orientation opposite the orientation of spontaneous polarization.

Description

Piezoelectric Actuator

[0001] The present disclosure relates to piezoelectric actuators.

[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 type piezoelectric element in which a plurality of piezoelectric films are laminated with electrode layers interposed therebetween has been proposed in order to obtain high piezoelectric characteristics.

[0004] For example, Japanese Patent Application Laid-Open Publication No. 2013-80886 proposes a piezoelectric element in which a first electrode, an Nb-doped PZT film, a second electrode, an Nb-doped PZT film, and a third electrode are stacked in this order. It is known that the spontaneous polarization of an Nb-doped PZT film is aligned upward relative to the substrate during film formation. That is, the two Nb-doped PZT films in Japanese Patent Application Laid-Open Publication No. 2013-80886 both have spontaneous polarization aligned upward. Generally, for a piezoelectric film with aligned spontaneous polarization, applying an electric field in the same direction as the spontaneous polarization will result in higher piezoelectric performance. Therefore, in JP 2013-80886 A, an electric field is applied to each of the two Nb-doped PZT films in the same direction as the spontaneous polarization by using a first driving method in which the second electrode is grounded and a positive voltage (+V) is applied to the first electrode and a negative voltage (-V) is applied to the third electrode, or a second driving method in which the first electrode is grounded and a negative voltage (-V) is applied to the second electrode and a negative voltage (-2V) with an absolute value greater than that of the second electrode is applied to the third electrode. This achieves approximately twice the displacement amount compared to a piezoelectric element with only one layer.

[0005] In JP 2013-80886 A, to implement the first drive method in which voltages of different signs are applied to the first electrode and the third electrode, it is necessary to provide a positive drive circuit and a negative drive circuit, which results in high costs. Also, in the second drive method in which a voltage of a larger absolute value is applied to the third electrode than to the second electrode, in order to obtain piezoelectric performance equivalent to that of the first drive method, it becomes necessary to apply an extremely large voltage to the third electrode, and sufficient piezoelectric performance cannot be obtained with a low voltage.

[0006] Japanese Patent Application Publication No. 2024-052272, like Japanese Patent Application Publication No. 2013-80886, also discloses a piezoelectric element configured with two layers of piezoelectric films having spontaneous polarization alternately stacked with electrodes. With the aim of reducing costs and achieving high piezoelectric performance in low-voltage regions, Japanese Patent Application Publication No. 2024-052272 discloses a configuration in which an electric field in the same direction as the spontaneous polarization is applied to one piezoelectric film, and an electric field in the opposite direction to the spontaneous polarization is applied to the other piezoelectric film. In Japanese Patent Application Publication No. 2024-052272, two piezoelectric films with different polarization-voltage hysteresis characteristics are provided, and an electric field in the same direction as the spontaneous polarization is applied to the piezoelectric film with the larger difference in coercive field between the two, and an electric field in the opposite direction to the spontaneous polarization is applied to the piezoelectric film with the smaller difference in coercive field.

[0007] The piezoelectric element of JP 2024-052272 A exhibits good piezoelectric performance in the low voltage region. However, the present inventors have discovered that when a piezoelectric element is provided with two piezoelectric films with different polarization-voltage hysteresis characteristics, as in JP 2024-052272 A, and an electric field in the same direction as the spontaneous polarization is applied to the piezoelectric film with the larger difference in coercive field, and an electric field in the opposite direction to the spontaneous polarization is applied to the piezoelectric film with the smaller difference in coercive field, the response linearity decreases in the high voltage region.

[0008] In view of the above circumstances, an object of the present disclosure is to provide a piezoelectric actuator including a stacked piezoelectric element, which has good response linearity when driven at a high voltage.

[0009] A piezoelectric actuator according to the present disclosure is a piezoelectric actuator including a piezoelectric element and a drive circuit that applies a drive voltage to the piezoelectric element, wherein the piezoelectric element includes a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode arranged in this order on a substrate, wherein the first piezoelectric film and the second piezoelectric film both have spontaneous polarization aligned in the film thickness direction and the spontaneous polarization of the first piezoelectric film and the second piezoelectric film are in the same direction, and a positive coercive voltage in a hysteresis curve showing the polarization-voltage characteristics of one of the first piezoelectric film and the second piezoelectric film is defined as Vcf. ＋ , negative coercive voltage is Vcf － , the absolute value of the difference between them |Vcf ＋ -Vcf － | is ΔVcf, and the positive coercive voltage in the hysteresis curve showing the polarization-voltage characteristics of the other of the first and second piezoelectric films is Vcr. ＋ , negative coercive voltage is Vcr － , the absolute value of the difference between them |Vcr ＋ -Vcr － | is ΔVcr, the following condition is satisfied: ΔVcf<ΔVcr-1.0, where the units are all [V]. The drive circuit generates an electric field in the same direction as the spontaneous polarization in one piezoelectric film, and generates an electric field in the opposite direction to the spontaneous polarization in the other piezoelectric film.

[0010] In the piezoelectric actuator of the present disclosure, the first electrode and the third electrode are preferably connected to each other.

[0011] In the piezoelectric actuator of the present disclosure, both the first piezoelectric film and the second piezoelectric film are made of a material represented by the general formula Pb{(Zr ｘ Ti １－ｘ ) １－ｙ M ｙ O ３ M is a metal element selected from the group consisting of V, Nb, Ta, Sb, Mo and W, and preferably contains a perovskite oxide represented by 0<x<1, 0<y<1.

[0012] In the piezoelectric actuator of the present disclosure, it is preferable that the metal element M is Nb and y is greater than 0.1.

[0013] In the piezoelectric actuator of the present disclosure, it is preferable that the absolute value of the drive voltage is 20 V or more.

[0014] In the piezoelectric actuator of the present disclosure, the coercive voltage Vcf on the positive side of one of the piezoelectric films ＋ and negative coercive voltage Vcf － It is preferable that the polarities of the electrodes are the same.

[0015] According to the technology of the present disclosure, it is possible to provide a piezoelectric actuator that includes a stacked piezoelectric element and that exhibits good response linearity when driven at a high voltage.

[0016] FIG. 1 is a diagram showing a schematic configuration of a piezoelectric actuator of one embodiment; FIG. 2 is a diagram showing a polarization-voltage hysteresis curve of a piezoelectric film; FIG. 3 is a diagram showing a schematic configuration of a piezoelectric element actuator of a modified example; FIG. 4 is a diagram showing a schematic configuration of a piezoelectric element actuator of a modified example; FIG. 5 is a diagram showing a schematic configuration of a piezoelectric element actuator of a modified example; FIG. 6 is a cross-sectional view showing a schematic configuration of a piezoelectric element for evaluation; FIG. 7 is a diagram showing the P-V hysteresis characteristics of the first piezoelectric film and the second piezoelectric film of Example 1; FIG. 8 is a diagram showing a drive waveform; and FIG. 9 is a graph showing the nonlinear response components of the displacement of Example 1 and Comparative Example 1.

[0017] Hereinafter, embodiments of the piezoelectric actuator of the present disclosure will be described with reference to the drawings. Note that in the drawings, the thicknesses and ratios of each layer are appropriately modified for ease of viewing, and do not necessarily reflect the actual thicknesses and ratios.

[0018] Fig. 1 is a diagram showing a schematic configuration of a piezoelectric actuator 1 according to one embodiment. The piezoelectric actuator 1 includes a piezoelectric element 2 and a drive circuit 30. In Fig. 1, the piezoelectric element 2 is shown in a cross-sectional schematic diagram showing the layer structure.

[0019] As shown in FIG. 1, the piezoelectric element 2 is formed by laminating a first electrode 12, a first piezoelectric film 14, a second electrode 16, a second piezoelectric film 18, and a third electrode 20 in this order on a substrate 10.

[0020] The substrate 10 is not particularly limited, and examples thereof include substrates of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, and silicon carbide. The substrate 10 may be a silicon substrate having a SiO ２ A laminated substrate such as a silicon substrate with a thermally oxidized film on which an oxide film is formed may also be used. Alternatively, a resin substrate such as PET (polyethylene terephthalate), PEN (polyethylene naphthalata), or polyimide may also be used as the substrate 10.

[0021] The first electrode 12 is formed on the substrate 10. The main component of the first electrode 12 is not particularly limited, and examples thereof 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, examples of the first electrode 12 include ITO (indium tin oxide), LaNiO ３ , and SRO (SrRuO ３ ) etc. may also be used.

[0022] The second electrode 16 is laminated on the first piezoelectric film 14, and the third electrode 20 is laminated on the second piezoelectric film 18. The first electrode 12 and the second electrode 16 form a pair to apply an electric field to the first piezoelectric film 14. The second electrode 16 and the third electrode 20 form a pair to apply an electric field to the second piezoelectric film 18.

[0023] The main components of the second electrode 16 and the third electrode 20 are not particularly limited, and include the materials exemplified for the first electrode 12, as well as electrode materials generally used in semiconductor processes, such as Cr, and combinations thereof. However, it is preferable to use an oxide conductor for the layer in contact with the first piezoelectric film 14 or the second piezoelectric film 18. Specific examples of oxide conductor layers include ITO (Indium Tin Oxide), Ir oxide, and SRO (SrRuO ３ ), as well as LaNiO ３ Examples of such a material include ZnO and doped ZnO.

[0024] There are no particular limitations on the thickness of the first electrode 12, the second electrode 16, and the third electrode 20, but it is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0025] The spontaneous polarization of both the first piezoelectric film 14 and the second piezoelectric film 18 is aligned in the film thickness direction, and the aligned directions of the spontaneous polarization are the same. In the example shown in FIG. 1 , the direction of the spontaneous polarization P1 of the first piezoelectric film 14 and the direction of the spontaneous polarization P2 of the second piezoelectric film 18 are both upward in the film thickness direction. In this specification, the substrate 10 is used as the reference, and the direction away from the substrate 10 is defined as up, and the substrate side as down. Whether the spontaneous polarization in the piezoelectric film is aligned and the direction in which the spontaneous polarization is aligned can be confirmed by measuring the P-V hysteresis curve (or P-E hysteresis curve) that indicates the polarization-voltage characteristics (or polarization-electric field characteristics) of the piezoelectric film.

[0026] In piezoelectric films that have not been poled, the spontaneous polarization is aligned in the absence of an external electric field. This is thought to be due to the generation of an electric field (hereinafter referred to as the spontaneous internal electric field) within the piezoelectric film due to distortions or defects in the crystalline structure. For piezoelectric films that do not generate a spontaneous internal electric field in the absence of an external electric field, the P-E hysteresis curve (or P-V hysteresis curve) has a shape whose center coincides with the origin. On the other hand, for piezoelectric films that generate a spontaneous internal electric field, i.e., for piezoelectric films in which the spontaneous polarization is aligned in the absence of an external electric field, the center of the hysteresis curve shifts from the origin along the voltage axis because the spontaneous polarization is aligned relative to the spontaneous internal electric field. In the case of a P-E hysteresis curve, if the spontaneous internal electric field is Ei and the external electric field Eo is applied, the applied electric field is Ei + Eo, so the center of the hysteresis curve shifts from the origin by the spontaneous internal electric field Ei. In the case of a P-V hysteresis curve, the center of the hysteresis curve shifts from the origin by the product of Ei and film thickness. Therefore, if the center of the measured hysteresis curve shifts from the origin in the voltage axis direction, it can be assumed that a spontaneous internal electric field is generated and the spontaneous polarization is aligned. The amount of shift of the center of the hysteresis curve from the origin is proportional to the degree of alignment of the spontaneous polarization, and the greater the shift, the higher the degree of alignment of the spontaneous polarization (the stronger the spontaneous internal electric field). Furthermore, the direction of aligned spontaneous polarization can be determined by whether the hysteresis curve shifts from the origin in the voltage axis direction to the positive or negative side. In the case of a P-V hysteresis curve, the center of hysteresis is defined as the midpoint between two coercive voltages, as described below. Hereinafter, the term "direction of spontaneous polarization" refers to the direction of spontaneous polarization aligned by the spontaneous internal electric field.

[0027] FIG. 2 shows the hysteresis curves of one of the first piezoelectric film 14 and the second piezoelectric film 18 and the other piezoelectric film. Here, the hysteresis curve shown by the solid line is that of one piezoelectric film, and the hysteresis curve shown by the dashed line is that of the other piezoelectric film. In a hysteresis curve, the voltage at which polarization becomes zero is called the coercive voltage. As shown in FIG. 2, one hysteresis curve has two coercive voltages. The positive coercive voltage refers to the coercive voltage on the relatively positive voltage side (right side in the figure) of the two coercive voltages, and the negative coercive voltage refers to the coercive voltage on the relatively negative voltage side (left side in the figure).

[0028] Here, the coercive voltage of the positive side of one piezoelectric film is Vcf ＋ , negative coercive voltage is Vcf － , the absolute value of the difference between them |Vcf ＋ -Vcf － | is set to ΔVcf.

[0029] The coercive voltage of the positive side of the other piezoelectric film is Vcr ＋ , negative coercive voltage is Vcr － , the absolute value of the difference between them |Vcr ＋ -Vcr － | is set to ΔVcr.

[0030] The piezoelectric element 2 satisfies the following condition: ΔVcf<ΔVcr−1.0 (1).

[0031] 2, ΔVcf corresponds to the distance between two coercive voltages (hereinafter referred to as "hysteresis width") of the hysteresis curve of one piezoelectric film, and ΔVcr corresponds to the hysteresis width of the other piezoelectric film. Therefore, the above formula (1) means that the hysteresis width ΔVcf of one piezoelectric film is smaller than the hysteresis width ΔVcr of the other piezoelectric film, and the difference between them exceeds 1.0 V.

[0032] In the example shown in FIG. 2, the two coercive voltages Vcf of one of the piezoelectric films ＋ , Vcf － The coercive voltage with the higher absolute value (here, Vcf ＋ ) is the value of the two coercive voltages Vcr of the other piezoelectric film ＋ , Vcr － The coercive voltage in the direction with the higher absolute value (here, Vcr ＋ ) is smaller than

[0033] In this example, the first piezoelectric film 14 corresponds to one of the piezoelectric films and has the hysteresis characteristic shown by the solid line in Fig. 2, and the second piezoelectric film 18 corresponds to the other piezoelectric film and has the hysteresis characteristic shown by the dashed line in Fig. 2. The hysteresis curves shown in Fig. 2 are obtained by grounding the lower electrodes 12, 16 of each piezoelectric film 14, 18 and using the upper electrodes 16, 20 as driving electrodes and applying a sweep voltage to the piezoelectric films 14, 18. The hysteresis curves of the first piezoelectric film 14 and the second piezoelectric film 18 both shift in the same polarity direction (positive direction in the example of Fig. 2) in the voltage direction from the origin of the hysteresis curve. The hysteresis curve obtained by grounding the upper electrodes 16, 20 of the piezoelectric films 14, 18, using the lower electrodes 12, 16 as driving electrodes, and applying a sweep voltage to the piezoelectric films 14, 18 is the hysteresis curve shown in FIG. 2 rotated 180° around the origin. When measuring the P-V characteristics of the first piezoelectric film 14 and the second piezoelectric film 18, it does not matter whether the upper electrode or the lower electrode is used as the driving electrode. The positive coercive voltage and the negative coercive voltage may have opposite polarities or the same polarity. It is preferable that the two coercive voltages of one piezoelectric film (the first piezoelectric film 14 in this example) have the same polarity.

[0034] The first piezoelectric film 14 and the second piezoelectric film 18 are preferably films in which the direction of spontaneous polarization is aligned in the film thickness direction immediately after film formation. The first piezoelectric film 14 and the second piezoelectric film 18 are preferably composed primarily of perovskite oxide. Here, "main component" refers to a component that accounts for 80 mol % or more. The first piezoelectric film 14 and the second piezoelectric film 18 are preferably each composed of 90 mol % or more of perovskite oxide, and more preferably the first piezoelectric film 14 and the second piezoelectric film 18 are composed of perovskite oxide (however, unavoidable impurities are contained).

[0035] The perovskite oxide is preferably a lead zirconate titanate (PZT) type oxide containing Pb (lead), Zr (zirconium), Ti (titanium) and O (oxygen).

[0036] In particular, the perovskite oxide is preferably a compound represented by the following general formula (2), which contains a metal element M as an additive in the B site of PZT: Pb{(Zr ｘ Ti １－ｘ ) １－ｙ M ｙ O ３ (2) Here, the metal element M is preferably one or more elements selected from V (vanadium), Nb (niobium), Ta (tantalum), Sb (antimony), Mo (molybdenum), and W (tungsten). Here, 0<x<1, 0<y<1. In the general formula (2), Pb:{(Zr ｘ Ti １＋ｘ ) １－ｙ M ｙ}:O is based on a ratio of 1:1:3, but may deviate within a range in which a perovskite structure can be formed. ｘ Ti １－ｘ ) １－ｙ M ｙ O ３ is called M-doped PZT. For example, when the metal element M is Nb, it is called Nb-doped PZT.

[0037] The metal element M may be a single element such as only V or only Nb, 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 the metal element M is one of these elements, a very high piezoelectric constant can be achieved in combination with the A-site element Pb.

[0038] In particular, Pb{(Zr ｘ Ti １－ｘ ) １－ｙ Nb ｙ O ３ is optimal. In this case, a higher piezoelectric constant can be obtained when y>0.1. When a piezoelectric film is formed by vapor deposition such as sputtering using Nb-doped PZT where M is Nb, a piezoelectric film with a very high piezoelectric constant and with spontaneous polarization more uniformly oriented upward from the substrate in the film thickness direction can be obtained.

[0039] It is preferable that the perovskite oxide of the first piezoelectric film 14 and the perovskite oxide of the second piezoelectric film 18 have the same composition. Here, assuming that each element symbol indicates the respective molar ratio, the Pb composition ratio in the perovskite oxide is Pb / (Zr+Ti+M), the Zr composition ratio in the B site is Zr / (Zr+Ti), the Ti composition ratio is Ti / (Zr+Ti), and the M composition ratio is M / (Zr+Ti+M), which is the composition ratio of the metal element M. The perovskite oxide of the first piezoelectric film 14 and the perovskite oxide of the second piezoelectric film 18 having the same composition means that the Pb composition ratios, Zr composition ratios, Ti composition ratios, and M composition ratios are equal within the range of measurement error.

[0040] The film thickness of the first piezoelectric film 14 and the second piezoelectric film 18 is preferably 0.2 μm or more and 5 μm or less, and more preferably 1 μm or more.

[0041] The drive circuit 30 is a means for supplying a drive voltage to the first piezoelectric film 14 and the second piezoelectric film 18. In the piezoelectric actuator 1 of this embodiment shown in FIG. 1, the first electrode 12 and the third electrode 20 are connected to the ground terminal of the drive circuit 30 and are at ground potential. The second electrode 16 is connected to the drive voltage output terminal of the drive circuit 30 and functions as a drive electrode. The drive circuit 30 is a negative drive circuit that performs negative drive by applying a negative potential to the drive electrode. The drive circuit 30 in FIG. 1 is configured to apply a negative unipolar voltage waveform to the second electrode 16. When the drive circuit 30 applies a negative unipolar voltage waveform to the second electrode 16, an electric field Ef is formed in the first piezoelectric film 14 in the same direction as the spontaneous polarization P, and the first piezoelectric film 14 is displaced by forward drive. Meanwhile, an electric field Er is formed in the second piezoelectric film 18 in the opposite direction to the spontaneous polarization P, and the second piezoelectric film 18 is displaced by reverse drive.

[0042] As described above, the present piezoelectric actuator 1 comprises two piezoelectric films, each having a layered structure sandwiching an electrode, each having a spontaneous polarization aligned in the same direction along the film thickness direction. The piezoelectric element 2 has a smaller hysteresis width and a smaller coercive voltage than the other piezoelectric film, and a drive circuit 30. The drive circuit 30 is configured to drive the piezoelectric film with a hysteresis width smaller than the other by more than 1 V in the forward direction and drive the piezoelectric film with a larger hysteresis width in the reverse direction. The inventors' research has revealed that driving the piezoelectric film with a larger hysteresis width in the reverse direction results in good response linearity (see Examples below). It is believed that good response linearity can be achieved by reducing the number of polarization reversals during drive. Here, response linearity refers to the linearity in the relationship between drive voltage and displacement. It is believed that high linearity can be achieved, particularly when driven at a high voltage of 20 V or more, because the polarization direction is fixed in the reverse direction. Furthermore, by driving a piezoelectric film with a small hysteresis width in the forward direction, polarization reversal can be facilitated, and polarization fixation and a decrease in the amount of displacement during long-term driving can be suppressed.

[0043] Furthermore, when the piezoelectric actuator 1 is driven at a high voltage of 20 V or more, good linearity can be obtained and a sufficient piezoelectric constant can be ensured.

[0044] If the two coercive voltages of one of the piezoelectric films with a small hysteresis width have the same polarity, the effect of suppressing polarization fixation and reduction in displacement during long-term operation is high.

[0045] In the piezoelectric actuator 1 of the above embodiment, the drive circuit 30 is a single negative drive circuit. However, in the piezoelectric actuator of the present disclosure, the drive circuit may include a positive drive circuit and a negative drive circuit. That is, the drive circuits may be configured to apply drive voltages independently to the first piezoelectric film 14 and the second piezoelectric film 18 using positive and negative drive circuits. However, as in the present embodiment, if the first electrode 12 and the third electrode 20 have the same polarity and the second electrode 16 has a polarity opposite to that of the first electrode 12 and the third electrode 20, only a drive circuit of one polarity is required, thereby reducing costs compared to when two drive circuits of different polarities are required. Note that it is preferable that the first electrode 12 and the third electrode 20 of the piezoelectric element 2 are connected. If the first electrode 12 and the third electrode 20 are connected, drive control is easier.

[0046] Furthermore, instead of the drive circuit 30, a positive drive circuit 32 that applies a positive potential to the drive electrodes may be provided, as in the modified piezoelectric actuator 6 shown in Fig. 3. In the example shown in Fig. 3, the second electrode 16 is connected to the ground terminal of the drive circuit 32 and is at ground potential. The first electrode 12 and the third electrode 20 are connected to the drive voltage output terminals of the drive circuit 32 and function as drive electrodes. In this case, since the drive circuit 32 is a positive drive circuit, it is possible to apply an electric field Ef in the same direction as the spontaneous polarization P1 to the first piezoelectric film 14, and to apply an electric field Er in the opposite direction to the spontaneous polarization P2 to the second piezoelectric film 18, as in the case of the piezoelectric actuator 1 shown in Fig. 1.

[0047] In the piezoelectric element 2 shown in Fig. 1, the first piezoelectric film 14 is one piezoelectric film having a hysteresis curve with a relatively small hysteresis width, as shown by the solid line in Fig. 2, and the second piezoelectric film 18 is the other piezoelectric film having a hysteresis curve with a relatively large hysteresis width, as shown by the dashed line in Fig. 2. However, in the piezoelectric element of the present disclosure, the first piezoelectric film 14 may be the other piezoelectric film with a large hysteresis width, as shown by the dashed line in Fig. 2, and the second piezoelectric film 18 may be one piezoelectric film with a small hysteresis width, as shown by the solid line in Fig. 2.

[0048] In the piezoelectric element 2A of the modified piezoelectric actuator 7 shown in Figure 4, the first piezoelectric film 14r is the other piezoelectric film having a hysteresis curve with a small hysteresis width, and the second piezoelectric film 18f is one piezoelectric film having a hysteresis curve with a large hysteresis width. The piezoelectric actuator 7 is equipped with a positive drive circuit 32, as in Figure 3. The first electrode 12 and the third electrode 20 of the piezoelectric element 2A are connected to the ground terminal of the drive circuit 32 and are at ground potential, and the second electrode 16 is connected to the drive voltage output terminal of the drive circuit 32. In this way, the drive circuit 32 is configured to generate an electric field Ef in the second piezoelectric film 18f in the same direction as the spontaneous polarization P2, thereby driving the second piezoelectric film 18f in the forward direction, and to generate an electric field Er in the opposite direction to the spontaneous polarization P1, thereby driving the first piezoelectric film 14r in the reverse direction.

[0049] In the modified piezoelectric actuators 6 and 7 shown in FIG. 3 or 4, the drive circuit 32 also drives the piezoelectric film with a small hysteresis width in the forward direction and drives the piezoelectric film with a large hysteresis width in the reverse direction, so that driving can be performed with good response linearity.

[0050] Furthermore, the piezoelectric element 2 shown in FIG. 1 is a two-layer laminated piezoelectric element in which two piezoelectric films, each having one first piezoelectric film 14 and one second piezoelectric film 18, are laminated. However, the piezoelectric element of the present disclosure is not limited to two layers, and may have three or more piezoelectric film layers, as in the piezoelectric element 3 shown in FIG. 5.

[0051] A piezoelectric actuator 8 of a modified example shown in Fig. 5 includes a piezoelectric element 3 having a plurality of alternating first piezoelectric films 14 and second piezoelectric films 18. The piezoelectric element 3 includes a first electrode 12, a first piezoelectric film 14, a second electrode 16, a second piezoelectric film 18, a third electrode 20, a first piezoelectric film 14, a second electrode 16, a second piezoelectric film 18, and a third electrode 20 stacked in this order on a substrate 10. As with the piezoelectric element 2 of Fig. 1, the hysteresis width of the first piezoelectric film 14 is smaller than the hysteresis width of the second piezoelectric film 18 in the P-V hysteresis characteristic. In this way, one piezoelectric film having the hysteresis curve shown by the solid line in Fig. 2 and the other piezoelectric film having the hysteresis curve shown by the dashed line in Fig. 2 may be provided in multiple layers, alternately arranged with electrodes interposed therebetween.

[0052] The piezoelectric actuator 8 includes a drive circuit 30 similar to that of the piezoelectric actuator 1 shown in Fig. 1. The electrodes 12, 16, and 20 are connected so that an electric field Ef in the same direction as the spontaneous polarization P is applied to the first piezoelectric films 14, which are arranged in the first and third layers counting from the substrate 10 side, and an electric field Er in the opposite direction to the spontaneous polarization P is applied to the second piezoelectric films 18, which are arranged in the second and fourth layers. As a result, the drive circuit 30 drives the first piezoelectric films 14, which have a small hysteresis width, in the forward direction and drives the second piezoelectric films 18, which have a large hysteresis width, in the reverse direction, thereby enabling drive with good response linearity.

[0053] For example, if the first piezoelectric film 14 and the second piezoelectric film 18 have identical characteristics and the same polarization-electric field hysteresis, a piezoelectric element 2 satisfying the above formula (1) can be obtained by making the thickness of the first piezoelectric film 14 smaller than that of the second piezoelectric film 18. Alternatively, by varying the composition of the first piezoelectric film 14 and the second piezoelectric film 18, a piezoelectric element having a piezoelectric film with a relatively large hysteresis width and a piezoelectric film with a relatively small hysteresis width can be obtained. Furthermore, even if multiple piezoelectric films are formed using the same composition and under the same conditions, the piezoelectric films do not necessarily have the same hysteresis characteristics. In some cases, the hysteresis characteristics may differ depending on the composition or orientation of the underlying electrodes. In such cases, a drive circuit can be configured to drive the piezoelectric film with a relatively small hysteresis width in the forward direction and the piezoelectric film with a large hysteresis width in the reverse direction, depending on the hysteresis characteristics.

[0054] Specific examples and comparative examples of the piezoelectric element of the present disclosure will be described below. First, a method for manufacturing the piezoelectric element of each example will be described. A radio frequency (RF) sputtering device was used to deposit each layer. In describing the manufacturing method, the reference numerals of each layer of the piezoelectric element 2 shown in FIG. 1 will be used.

[0055] (First Electrode Formation) An SOI (Silicon on Insulator) substrate with a thermal oxide film was used as the substrate 10. The first electrode 12 was formed on the substrate 10 by RF (radio-frequency) sputtering. Specifically, the substrate temperature was set to 350° C., and a 50 nm TiW layer and a 200 nm Ir layer were stacked in this order on the substrate 10 to form the first electrode 12.

[0056] (First Piezoelectric Film) The substrate 10 with the first electrode 12 was placed in an RF sputtering device, and the substrate temperature was set to 700°C immediately after the start of film formation. An Nb-doped PZT film with 12 at% Nb added to the B site was deposited as the first piezoelectric film 14. Nb-doped PZT was used as the target, and the amount of Pb in the target was set to be higher than the stoichiometric composition. The Ti / Zr molar ratio in the target was set to an MPB composition (Ti / Zr = 52 / 48).

[0057] (Second Electrode) As the second electrode 16, a 50 nm IrO ｚ (Z≦2) and 200 nm of Ir were laminated in this order.

[0058] (Second Piezoelectric Film) A Nb-doped PZT film was formed as a second piezoelectric film under the same film formation conditions as those for the first piezoelectric film, except for the film formation temperature.

[0059] (Third Electrode) A 50 nm thick IrO film was deposited on the second piezoelectric film 18 at room temperature as the third electrode 20. ｚ and 100 nm of Ir were laminated in this order.

[0060] In the above film formation recipe, in the film formation process of the first piezoelectric film and the second piezoelectric film, the substrate temperature was increased after the start of film formation (temperature change: 0°C to 50°C) to change the amount of defects in each of the first and second piezoelectric films, thereby controlling the hysteresis shape. If the increase in film formation temperature is large, the hysteresis width tends to become smaller. Piezoelectric stacks were produced using film formation recipes #1 to #4, which have different increase in film formation temperature, and used for the following evaluation.

[0061] (Fabrication of Piezoelectric Element for Evaluation) A piezoelectric element for evaluation 101 was fabricated using a piezoelectric laminate formed by laminating electrodes and piezoelectric films on a substrate as described above. Figure 6 is a diagram showing a schematic cross-sectional structure of the piezoelectric element for evaluation 101. In Figure 6, layers equivalent to those shown in Figure 1 are assigned the same reference numerals.

[0062] In order to connect the first electrode 12, the second electrode 16, and the third electrode 20 to the electrode pads for applying voltage, the third electrode 20, the second piezoelectric film 18, the second electrode 16, and the first piezoelectric film 14 were patterned in this order by photolithography and dry etching. After that, an insulating film 105 was formed, and patterned by photolithography and dry etching to insulate each layer. The insulating film was fabricated by the CVD (Chemical Vapor Deposition) method. Here, SiO ２ The insulating film was made of Al. ２ O ３ Alternatively, an organic insulating film such as photosensitive polyimide may be used. Then, wiring 106 for applying a drive signal to the second electrode 16 and wiring 107 for grounding the first electrode 12 and the third electrode 20 were formed. Finally, the silicon of the rear handle layer was removed by vertical etching only in the actuator displacement portion, leaving behind the SOI device layer (here, 10 μm) that would become the diaphragm, thereby forming a displaceable configuration.

[0063] <Measurement of P-V hysteresis characteristics> For the piezoelectric elements fabricated using film formation recipes No. 1 to 4, the bottom side of each piezoelectric film in the film thickness direction was grounded, and a bipolar voltage (triangular wave with a frequency of 5 Hz) was applied to the top side to measure the polarization (charge density) P [μC / cm ２ The relationship between the voltage V [V] and the applied current [V] was measured. Specifically, the P-V hysteresis characteristics of the first piezoelectric film 14 were measured by grounding the first electrode 12 and applying a voltage to the second electrode 16, and the P-V hysteresis characteristics of the second piezoelectric film 18 were measured by grounding the second electrode 16 and applying a voltage to the third electrode 20.

[0064] As an example, Fig. 7 shows the hysteresis characteristics of a piezoelectric element fabricated using film formation recipe No. 1. In Fig. 7, "first layer" refers to the first piezoelectric film 14, and "second layer" refers to the second piezoelectric film 18.

[0065] As shown in FIG. 7, the coercive voltage of the first piezoelectric film 14 is Vc1 － , Vc1 ＋ , the hysteresis width is ΔVc1, and the coercive voltage of the second piezoelectric film 18 is Vc2. － , Vc2 ＋ , and the hysteresis width is ΔVc2.

[0066] 7, the hysteresis curves of both piezoelectric films 14 and 18 are biased toward the positive side of the voltage direction and are asymmetric, which means that the spontaneous polarization is aligned in the direction from the substrate side to the film deposition surface side in the film thickness direction.

[0067] <Measurement of Displacement Amount During Driving> In Examples 1 to 3 and Comparative Example 4, the circuit scheme of the piezoelectric element 101 and the drive circuit 30 was the same as the configuration of the piezoelectric element 2 and the drive circuit 30 shown in FIG. 1 . That is, the first electrode 12 and the third electrode 20 were grounded, and a negative unipolar voltage waveform was input to the second electrode 16. As a result, as shown in FIG. 1 , the first piezoelectric film 14 was driven in the forward direction, and the second piezoelectric film 18 was driven in the reverse direction. That is, one piezoelectric film with a small hysteresis width was driven in the forward direction, and the other piezoelectric film with a large hysteresis width was driven in the reverse direction. On the other hand, in Comparative Examples 1 to 3, the second electrode 16 was grounded, and a negative unipolar voltage waveform was input to the first electrode 12 and the third electrode 20. As a result, in Comparative Examples 1 to 3, the first piezoelectric film 14 was driven in the reverse direction, and the second piezoelectric film 18 was driven in the forward direction. That is, one piezoelectric film with a small hysteresis width was driven in the reverse direction, and the other piezoelectric film with a large hysteresis width was driven in the forward direction.

[0068] A sine wave with a frequency of 10 kHz and a peak-to-peak voltage of 0-Vp [V] was used as the drive waveform for applying the drive voltage (see Figure 8). In this example, the maximum voltage Vp was set to -40 V. The voltage was output using a combination of a function generator and a voltage amplifier, and the amount of displacement during driving was measured. The time waveform of the displacement velocity at the maximum displacement point of the actuator (the center point of the diaphragm) was obtained using a laser Doppler vibrometer, and this data was integrated over time to obtain the time waveform of the displacement amount.

[0069] When the absolute value of the maximum driving voltage is greater than 20 V, the actuator generates a large force, which is preferable. On the other hand, because the driving spans the coercive voltage of the reverse-direction driving piezoelectric film, polarization reversal occurs, resulting in a risk of high-order frequency components being mixed into the displacement response and causing unintended actuator operation. The smaller the high-order frequency components, the better the linearity and the more likely it is that the actuator will operate as designed, which is preferable. To evaluate the magnitude of this high-order mode, Fourier analysis was used to analyze the magnitude of the frequency components (fundamental frequency (= 10 kHz), higher-order frequencies (2nd: 20 kHz, 3rd: 30 kHz, etc.)) contained in the actuator's displacement-time waveform.

[0070] 9 shows the analysis results of the frequency components of Example 1 and Comparative Example 1. The 2nd, 3rd, etc. on the horizontal axis represent high-frequency components, and the vertical axis represents the ratio of each component to the fundamental frequency. As shown in FIG. 9, Example 1 had more second-order components than Comparative Example 1, but the 3rd and higher order components were much smaller than Comparative Example 1. In evaluating nonlinearity, the integral values ​​of the 2nd to 14th order frequency components shown in FIG. 9 were evaluated as good (A) when they were 10% or less of the intensity of the reference frequency, and as poor (B) when they exceeded 10%.

[0071] Furthermore, to evaluate the change in displacement due to long-term operation, a square wave with a frequency of 200 kHz and Vp = -40 V was input for 200 hours in an environment of 25°C, and the displacement immediately after the start of long-term operation and after 200 hours of operation were obtained. The displacement-time waveform obtained by the above-mentioned method was used to calculate the maximum displacement immediately after the start of long-term operation and after operation. The drive waveform used for measuring the displacement was a sine wave with a frequency of 10 kHz and a maximum voltage of Vp = -40 V.

[0072] Table 1 shows the evaluation of the hysteresis characteristics and nonlinearity of each piezoelectric film of the piezoelectric elements of each example and comparative example, as well as the rate of decrease in displacement after 200 hours of operation.

[0073] In Table 1, ΔVcf is the hysteresis width of the first or second piezoelectric film that is driven in the forward direction, in which an electric field is applied in the direction of the spontaneous polarization, and ΔVcr is the hysteresis width of the film that is driven in the reverse direction, in which an electric field in the opposite direction to the spontaneous polarization is applied. For example, in Example 1, the first piezoelectric film is driven in the forward direction, in which a forward voltage is applied, so the hysteresis width ΔVc1 of the first piezoelectric film is ΔVcf, and the hysteresis width ΔVc2 of the second piezoelectric film is ΔVcr.

[0074] As shown in Table 1, Comparative Examples 1 and 2 have a small rate of decrease in displacement after 200 hours of driving, but poor response nonlinearity. Comparative Examples 3 and 4 have good response nonlinearity, but the rate of decrease in displacement after 200 hours of driving exceeds 10%. In contrast, Examples 1 to 3 have good response nonlinearity and a small rate of decrease in displacement after 200 hours of driving. In all of Examples 1 to 3, the difference ΔVcr-Vcf between the hysteresis width ΔVcr of the piezoelectric film (here, the second piezoelectric film) driven in the reverse direction and the hysteresis width ΔVcf of the piezoelectric film (here, the first piezoelectric film) driven in the forward direction is greater than 1 V (1 < ΔVcr-ΔVcf). In other words, it is clear that high response nonlinearity and high long-term reliability are achieved when ΔVcf < ΔVcf-1) is satisfied. Furthermore, in Example 1, the difference between the two coercive voltages Vc1 of the first piezoelectric film driven in the forward direction is － , Vc1 ＋In this case, the decrease in displacement rate after 200 hours of operation was the smallest.

[0075] The disclosure of Japanese Patent Application No. 2024-139702, filed on August 21, 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.

[0076] The following additional notes are further disclosed regarding the above embodiment.

[0077] <Supplementary Note 1> A piezoelectric actuator including a piezoelectric element and a drive circuit that applies a drive voltage to the piezoelectric element, wherein the piezoelectric element includes a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode on a substrate in this order, wherein the first piezoelectric film and the second piezoelectric film both have spontaneous polarization aligned in the film thickness direction, and the spontaneous polarization of the first piezoelectric film and the second piezoelectric film are oriented in the same direction, and a positive coercive voltage in a hysteresis curve showing the polarization-voltage characteristics of one of the first piezoelectric film and the second piezoelectric film is defined as Vcf. ＋ , negative coercive voltage is Vcf － , the absolute value of the difference between them |Vcf ＋ -Vcf － | is ΔVcf, and the positive coercive voltage in the hysteresis curve showing the polarization-voltage characteristics of the other of the first and second piezoelectric films is Vcr. ＋ , negative coercive voltage is Vcr － , the absolute value of the difference between them |Vcr ＋ -Vcr － | is ΔVcr, the following condition is satisfied: ΔVcf<ΔVcr-1.0, where the unit is [V], and the drive circuit generates an electric field in the same direction as the spontaneous polarization in one piezoelectric film, and generates an electric field in the opposite direction to the spontaneous polarization in the other piezoelectric film. <Appendix 2> The piezoelectric actuator according to Appendix 1, wherein the first electrode and the third electrode are connected. <Appendix 3> The first piezoelectric film and the second piezoelectric film are both made of a material represented by the general formula Pb{(Zr ｘ Ti１－ｘ ) １－ｙ M ｙ O ３ The piezoelectric actuator according to Supplementary Note 1 or 2, wherein M is a metal element selected from the group consisting of V, Nb, Ta, Sb, Mo, and W, and contains a perovskite oxide represented by 0<x<1, 0<y<1. <Supplementary Note 4> The piezoelectric actuator according to any one of Supplementary Note 1 to Supplementary Note 3, wherein the metal element M is Nb and y is greater than 0.1. <Supplementary Note 5> The piezoelectric actuator according to any one of Supplementary Note 1 to Supplementary Note 4, wherein the absolute value of the drive voltage is 20 V or more. <Supplementary Note 6> The positive coercive voltage Vcf of one of the piezoelectric films ＋ and negative coercive voltage Vcf － and have the same polarity.

[0078] 1, 6, 7, 8 Piezoelectric actuator 2, 2A, 3, 101 Piezoelectric element 10 Substrate 12 First electrode 14 First piezoelectric film (one piezoelectric film) 14r First piezoelectric film (other piezoelectric film) 16 Second electrode 18 Second piezoelectric film (other piezoelectric film) 18f Second piezoelectric film (one piezoelectric film) 20 Third electrode 30, 32 Drive circuit 105 Insulating film 106, 107 Wiring

Claims

1. A piezoelectric actuator comprising a piezoelectric element and a drive circuit that applies a drive voltage to the piezoelectric element, wherein the piezoelectric element comprises a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode in this order on a substrate, wherein the first piezoelectric film and the second piezoelectric film both have spontaneous polarization aligned in the film thickness direction, and the spontaneous polarization of the first piezoelectric film and the second piezoelectric film are in the same direction, and the positive coercive voltage in a hysteresis curve showing the polarization-voltage characteristics of one of the first piezoelectric film and the second piezoelectric film is Vcf. ＋ , negative coercive voltage is Vcf － , the absolute value of the difference between them |Vcf ＋ -Vcf － | is ΔVcf, and the positive coercive voltage in the hysteresis curve showing the polarization-voltage characteristics of the other of the first piezoelectric film and the second piezoelectric film is Vcr. ＋ , negative coercive voltage is Vcr － , the absolute value of the difference between the two |Vcr ＋ -Vcr － a piezoelectric actuator in which the following condition is satisfied: ΔVcf<ΔVcr-1.0, where | is ΔVcr, and the unit is [V]; and the drive circuit generates an electric field in the same direction as the spontaneous polarization in one of the piezoelectric films, and generates an electric field in the opposite direction to the spontaneous polarization in the other piezoelectric film.

2. The piezoelectric actuator according to claim 1, wherein the first electrode and the third electrode are connected.

3. The first piezoelectric film and the second piezoelectric film are both made of a material having the general formula Pb{(Zr ｘ Ti １－ｘ ) １－ｙ M ｙ O ３ 2. The piezoelectric actuator according to claim 1, wherein M is a metal element selected from the group consisting of V, Nb, Ta, Sb, Mo, and W, and the piezoelectric actuator comprises a perovskite oxide represented by 0<x<1, 0<y<1.

4. The piezoelectric actuator according to claim 3, wherein the metal element M is Nb and y is greater than 0.

1.

5. A piezoelectric actuator according to any one of claims 1 to 4, wherein the absolute value of the drive voltage is 20 V or more.

6. The positive coercive voltage Vcf of the one piezoelectric film ＋ and the negative coercive voltage Vcf － 5. The piezoelectric actuator according to claim 1, wherein the polarities of the first and second electrodes are the same.

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