Piezoelectric actuator

Abstract

A piezoelectric actuator comprises a stack of a plurality of piezoelectric films, a piezoelectric element having electrodes disposed between the piezoelectric films and at both end surfaces, and a drive circuit. Each of the plurality of piezoelectric films has asymmetric polarization-voltage hysteresis characteristics with two different absolute values of coercive voltage. The drive circuit applies a unipolar voltage waveform that generates an electric field in the same direction as the direction of spontaneous polarization in at least one layer from among the plurality of piezoelectric films and generates an electric field in the direction opposite to the direction of spontaneous polarization in the other piezoelectric films. A minimum voltage value V1 having a minimum absolute value in the unipolar voltage waveform is set such that the ratio of the slope T0 at 0 [V] to the slope T1 at the minimum voltage value V1 [V] in a hysteresis curve indicating bipolar polarization-hysteresis characteristics in another piezoelectric film satisfies |T0 / T1| ≥ 1.5.

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 stacked alternately 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 is applied to one piezoelectric film in the same direction as the spontaneous polarization, and an electric field is applied to the other piezoelectric film in the opposite direction to the spontaneous polarization. Japanese Patent Application Publication No. 2024-052272 includes two piezoelectric films with different polarization-voltage hysteresis characteristics, and applies an electric field in the same direction as the spontaneous polarization to the piezoelectric film with the larger difference in coercive voltage, and applies an electric field in the opposite direction to the spontaneous polarization to the piezoelectric film with the smaller difference in coercive voltage.

[0007] The piezoelectric element of JP 2024-052272 A has good piezoelectric performance in the low voltage range. However, JP 2024-052272 A does not mention the current consumption when driving the piezoelectric element, and does not fully consider how to suppress the current consumption.

[0008] In view of the above circumstances, an object of the present disclosure is to provide a piezoelectric actuator including a stacked piezoelectric element, in which current consumption is suppressed.

[0009] The piezoelectric actuator of the present disclosure comprises a piezoelectric element having electrodes arranged between each of the piezoelectric films in the stacking direction of the piezoelectric films and on both end faces of the piezoelectric films, and a drive circuit connected to the electrodes and applying a drive voltage to the piezoelectric element, wherein in the piezoelectric element, the plurality of piezoelectric films each have an asymmetric bipolar polarization-voltage hysteresis characteristic in which the absolute values ​​of two coercive voltages are different, and the drive circuit applies a unipolar voltage waveform that generates an electric field in the same direction as the spontaneous polarization in at least one layer of the plurality of piezoelectric films and generates an electric field in the opposite direction to the spontaneous polarization in the other piezoelectric films, and a minimum voltage value V1, which has the smallest absolute value in the unipolar voltage waveform, is set under the condition that the ratio of the slope T0 at 0 [V] to the slope T1 at the minimum voltage value V1 [V] in the hysteresis curve showing the bipolar polarization-voltage hysteresis characteristic of the other piezoelectric films satisfies |T0 / T1|≧1.5.

[0010] Preferably, the piezoelectric film is two-layered.

[0011] The piezoelectric film is made of a material having 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] It is preferred that the metal element M is Nb and y is greater than 0.1.

[0013] It is preferable that |T0 / T1|≧1.9 is satisfied, it is more preferable that |T0 / T1|≧3.4 is satisfied, and it is even more preferable that |T0 / T1|≧7.0 is satisfied.

[0014] According to the technique of the present disclosure, it is possible to provide a piezoelectric actuator including a stacked piezoelectric element, which has reduced current consumption.

[0015] 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. 3A and FIG. 3B are diagrams showing examples of drive waveforms; FIG. 4A and FIG. 4B are diagrams showing examples of drive waveforms; FIG. 3A is a diagram showing a schematic configuration of a piezoelectric element actuator of a modified example; FIG. 4B is a diagram showing a schematic configuration of a piezoelectric element actuator of a modified example; FIG. 4C is a cross-sectional view showing a schematic configuration of a piezoelectric element for evaluation; FIG. 9A is a diagram showing the P-V hysteresis characteristic of the first piezoelectric film of Sample A, and FIG. 9B is a diagram showing the P-V hysteresis characteristic of the second piezoelectric film of Sample A; FIG. 5A is a diagram showing the relationship between T0 / T1 and the amount of current; and FIG. 5B is a diagram showing the relationship between T0 / T1 and the amount of displacement. FIG. 12A is a diagram showing the drive waveform of Comparative Example 1 in Test 4, and FIG. 12B is a diagram showing the drive waveform of Example 12.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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 is a silicon substrate having a SiO ２A 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.

[0020] 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.

[0021] 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 a voltage to the first piezoelectric film 14. The second electrode 16 and the third electrode 20 form a pair to apply a voltage to the second piezoelectric film 18.

[0022] 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.

[0023] 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.

[0024] The first piezoelectric film 14 and the second piezoelectric film 18 each have an asymmetric bipolar polarization (polarization density)-voltage hysteresis (hereinafter referred to as P-V hysteresis) characteristic in which the absolute values ​​of the coercive voltages are different (see FIG. 2). Here, P-V hysteresis is defined as the charge density P [μC / cm ２ ] and voltage V [V]. In this specification, the substrate 10 is used as the reference, and the direction away from the substrate 10 is defined as the top, and the substrate side is defined as the bottom. Therefore, here, the P-V hysteresis characteristics of the first piezoelectric film 14 are measured by grounding the first electrode 12 and applying a bipolar voltage to the second electrode 16, and the P-V hysteresis characteristics of the second piezoelectric film 18 are measured by grounding the second electrode 16 and applying a bipolar voltage to the third electrode 20.

[0025] The asymmetric polarization-voltage hysteresis characteristic means that the two coercive voltages Vc in the hysteresis curve are asymmetric. ＋ , Vc － The absolute values ​​of the two coercive voltages Vc are different. ＋ , Vc － The midpoint of the curve is shifted from the origin to the positive or negative side. ＋ , Vc － is the voltage at which polarization becomes zero in the hysteresis curve, and as shown in Figure 2, there are two coercive voltages Vc ＋ , Vc － Positive coercive voltage Vc ＋ indicates the coercive voltage on the relatively positive voltage side (right side in the figure) of the two coercive voltages, and the coercive voltage on the negative side Vc － indicates the coercive voltage on the relatively negative voltage side (left side in the figure). The two coercive voltages may be of the same polarity or different polarities. ＋ , Vc － The midpoint between (Vc ＋ -Vc －) / 2 is defined as the center of the P-V hysteresis curve. In the example shown in Figure 2, the center of the hysteresis curve is shifted from the origin to the positive side. A piezoelectric film having an asymmetric hysteresis curve as shown in Figure 2 means that the spontaneous polarization is uniform within the piezoelectric film. Note that "absolute values ​​differ" means that the difference in absolute values ​​is 1.0 V or more.

[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 have 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 have a spontaneous internal electric field, i.e., a piezoelectric film in which the spontaneous polarization is aligned in the absence of an external electric field, the center of the P-V hysteresis curve shifts from the origin toward the positive voltage side, as shown in Figure 2. In the case of a P-E hysteresis curve, if the spontaneous internal electric field is Ei and the externally applied electric field Eo is Ei + Eo, the center of the hysteresis curve shifts from the origin by the amount of 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, it can be assumed that a spontaneous internal electric field is generated and the spontaneous polarization is uniform. The amount of shift of the center of the hysteresis curve from the origin is proportional to the degree of uniformity of the spontaneous polarization; the greater the shift, the higher the degree of uniformity of the spontaneous polarization (the stronger the spontaneous internal electric field). Furthermore, the direction of the uniform spontaneous polarization can be determined by the shift direction of the hysteresis curve from the origin. When the center of the hysteresis curve obtained by grounding the lower electrode in the film thickness direction and applying a bipolar voltage to the upper side of the piezoelectric film being measured shifts positively from the origin, as shown in Figure 2, it indicates that the spontaneous polarization is directed from the bottom to the top in the film thickness direction (upward in the film thickness direction). Hereinafter, the term "direction of spontaneous polarization" refers to the direction of spontaneous polarization aligned by the spontaneous internal electric field.

[0027] In the example shown in FIG. 1, the direction of the spontaneous polarization P of the first piezoelectric film 14 and the direction of the spontaneous polarization P of the second piezoelectric film 18 are both upward in the film thickness direction.

[0028] 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).

[0029] The perovskite oxide is preferably a lead zirconate titanate (PZT) type oxide containing Pb (lead), Zr (zirconium), Ti (titanium) and O (oxygen).

[0030] 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. ａ {(Zr 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.

[0031] 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.

[0032] 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.

[0033] The perovskite oxide of the first piezoelectric film 14 and the perovskite oxide of the second piezoelectric film 18 may have different compositions, but preferably 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.

[0034] The 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. The thickness of the first piezoelectric film 14 and the second piezoelectric film 18 may be the same or different.

[0035] The drive circuit 30 is a means for supplying a drive voltage to the piezoelectric film sandwiched between the electrodes. 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 applies a negative potential to the drive electrode. The drive circuit 30 in FIG. 1 is configured to apply a negative unipolar voltage waveform that varies between V1 and V2 to the second electrode 16. In this drive configuration, 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. As a result, the first piezoelectric film 14 is driven (forward driven) by an electric field that is aligned with the direction of the spontaneous polarization, resulting in a forward displacement. On the other hand, an electric field Er is formed in the second piezoelectric film 18 in a direction opposite to the direction of the spontaneous polarization P, and the second piezoelectric film 18 is displaced in the opposite direction.

[0036] In the case of the first piezoelectric film 14 and the second piezoelectric film 18 having the hysteresis characteristics shown in Fig. 2, the first piezoelectric film 14, which is driven in the forward direction, is driven substantially in the range of -V1 to -V2 in the third quadrant shown in Fig. 2. On the other hand, the second piezoelectric film 18, which is driven in the reverse direction, is driven substantially in the range of V1 to V2 in the first quadrant shown in Fig. 2.

[0037] The drive circuit 30 is configured so that the minimum voltage value V1, which is the smallest absolute value in the unipolar voltage waveform, satisfies the following equation: |T0 / T1|≧1.5 (1) where the ratio of the slope T0 at 0 [V] in the hysteresis curve of the second piezoelectric film 18 to the slope T1 at V1 [V].

[0038] The drive circuit 30 outputs a desired drive waveform, for example, by setting an arbitrary drive voltage waveform, digitally outputting it using a processor capable of outputting a digital signal, and then converting it into an analog waveform using a digital-analog converter (DAC). The drive waveform can be a sine wave, a pulse wave, or a combination thereof. A sufficient amount of displacement can be obtained by amplifying this voltage waveform to a desired voltage level using an amplifier (analog amplifier).

[0039] The drive circuit 30 preferably satisfies |T0 / T1|≧1.9, more preferably |T0 / T1|≧3.4, and even more preferably |T0 / T1|≧7.0. The numerical values ​​on the left side of each of the above inequalities are evaluated by rounding off the last two digits. Therefore, if T0 / T1 is 1.45 or more, it means that the above formula (1) is satisfied.

[0040] As shown in FIG. 2, the gradient T0 is the gradient [μC / cm ] of the tangent at the point where the hysteresis curve intersects with 0 [V] on the polarity side of the voltage of the same polarity as the polarity of the voltage in the direction of the shift. ２ Specifically, when the shift direction of the hysteresis curve is toward the positive voltage side, the slope T0 is the slope [μC / cm ２ ·V], and when the shift direction of the hysteresis curve is toward the negative voltage side, the slope T0 is the slope of the tangent at the point where the curve on the negative polarization side intersects with 0 V. Generally, the slope T0 is the slope of the tangent with the greater slope between the two tangents at the points where the hysteresis curve intersects with 0 V.

[0041] The slope T1 is the slope of the tangent line b at the minimum voltage value V1 [V] in the hysteresis curve. The minimum voltage value V1 is selected so that the slope T1 satisfies the above formula (1), and the drive waveform in the drive circuit 30 is set. The maximum voltage value V2 is determined according to the required amount of displacement. In this specification, the minimum and maximum voltage values ​​V1 and V2 are determined by the absolute values ​​of the voltage values.

[0042] Examples of drive waveforms are shown in Figures 3A, 3B, 4A, and 4B. As shown in Figure 3A, the drive waveform may be a sine wave, or as shown in Figure 3B, a pulse wave. The drive waveforms shown in Figures 3A and 3B have a minimum voltage value V1, which has the smallest absolute value, and a maximum voltage value V2, which has the largest absolute value. As shown in Figures 4A and 4B, the drive waveform may include multiple pulses with different minimum values, maximum values, and / or amplitudes within one period T. When multiple pulses with different amplitudes are included within one period T, as in Figures 4A and 4B, the minimum voltage value |V1| can be calculated by considering the average of the minimum values ​​of the pulses included within one period T as the minimum voltage value V1 of the second unipolar voltage waveform, and taking into account T0 / T1 in the above equation (1). For example, the drive waveform shown in Figure 4A includes three pulses within one period T, each with a minimum value of V11, V12, and V13. Therefore, the minimum voltage value V1 of the drive waveform is (V11 + V12 + V13) / 3. In addition, in the case of a drive waveform including four pulses in one period T as shown in Figure 4B, the minimum voltage value V1 is (V11 + V12 + V13 + V14) / 4. Note that in Figure 4A or 4B, the average value of each maximum value is similarly regarded as the maximum voltage value V2.

[0043] As described above, the piezoelectric actuator 1 of this embodiment includes a piezoelectric element having a plurality of stacked piezoelectric films (here, the first piezoelectric film 14 and the second piezoelectric film 18) and electrodes (here, the first electrode 12, the second electrode 16, and the third electrode 20) arranged between each of the piezoelectric films in the stacking direction and on both end faces of the plurality of piezoelectric films, and a drive circuit 30. Each of the piezoelectric films has an asymmetric polarization-voltage hysteresis characteristic. The drive circuit 30 is a drive circuit that applies a unipolar voltage waveform that generates an electric field Ef in the same direction as the spontaneous polarization P in at least one layer of the plurality of piezoelectric films (here, the first piezoelectric film 14), and generates an electric field Er in the opposite direction to the spontaneous polarization P in the other piezoelectric film (here, the second piezoelectric film 18). The minimum voltage value V1 in the unipolar voltage waveform is set so that the ratio of the slope T0 at 0 [V] in the hysteresis curve showing the bipolar polarization-voltage hysteresis characteristics of the other piezoelectric film to the slope T1 at the minimum voltage value V1 [V] satisfies |T0 / T1|≧1.5.

[0044] As shown in FIG. 2 , the piezoelectric films (here, the first piezoelectric film 14 and the second piezoelectric film 18) in the piezoelectric actuator 1 have asymmetric P-V hysteresis characteristics. In this example, the first piezoelectric film 14 and the second piezoelectric film 18 have an upward spontaneous polarization P in the film thickness direction. Applying a second unipolar voltage waveform to the second piezoelectric film 18, which generates an electric field opposite to the spontaneous polarization P, means driving the second piezoelectric film 18 within the first quadrant of the hysteresis curve shown in FIG. 2 . The hysteresis curve has a shape that rises sharply from a voltage of 0 V toward the positive side. This steeply sloping region is where polarization reversal occurs when the voltage applied to the piezoelectric film is changed. Therefore, when the second piezoelectric film 18 is driven in this region, excessive current is generated due to polarization reversal, increasing current consumption and potentially placing a load on the drive circuit 30. However, by applying the second unipolar voltage waveform that satisfies the above formula (1) in the drive circuit 30, the power consumption during driving can be significantly reduced without compromising the large displacement amount (see Examples below).

[0045] The driving circuit 30 preferably satisfies |T0 / T1| ≧ 1.9, more preferably |T0 / T1| ≧ 3.4, and even more preferably |T0 / T1| ≧ 7.0, because the greater the |T0 / T1|, the greater the effect of suppressing power consumption during driving. ２ In other words, if the hysteresis curve is steeper (more angular) and the coercive voltage is more asymmetric, the steep parts intersect at the point of zero voltage, so T0 becomes larger, and the effect of reducing power consumption becomes greater.

[0046] Even if the second unipolar voltage waveform includes multiple different pulses in one period T, as long as the average value of the minimum values ​​of the multiple pulses is the above-mentioned minimum voltage value V1 and the above is satisfied, the power consumption for one period as a whole can be effectively suppressed.

[0047] In the piezoelectric actuator 1 of the above embodiment, the drive circuit 30 is a single drive circuit of negative polarity, but in the piezoelectric actuator of the present disclosure, the drive circuit may include a drive circuit of positive polarity and a drive circuit of negative polarity. That is, the drive circuit may be configured to apply drive voltages independently to the first piezoelectric film 14 and the second piezoelectric film 18 by the drive circuits of positive polarity and negative polarity. However, as in the present embodiment, if a voltage is applied such that 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 achieving lower costs compared to when two drive circuits of different polarities are provided.

[0048] 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. 5. In the example shown in Fig. 5, 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, an electric field Ef in the same direction as the spontaneous polarization P can be generated in the first piezoelectric film 14, and an electric field Er in the opposite direction to the spontaneous polarization P can be generated in the second piezoelectric film 18.

[0049] 6, the first electrode 12 and the third electrode 20 are connected to the ground terminal of the drive circuit 32 and are set to the ground potential. The second electrode 16 is connected to the drive voltage output terminal of the drive circuit 32 and functions as a drive electrode. In this case, since the drive circuit 32 is a positive drive circuit, an electric field Er in the opposite direction to the spontaneous polarization P can be generated in the first piezoelectric film 14, and an electric field Ef in the same direction as the spontaneous polarization P can be generated in the second piezoelectric film 18.

[0050] In the above embodiment, the first electrode 12 and the third electrode 20 of the piezoelectric element 2 are connected, but they may be individually connected to the drive circuit. However, if the first electrode 12 and the third electrode 20 are connected, drive control is easier.

[0051] As described above, for example, when the first piezoelectric film 14 and the second piezoelectric film 18 are made of Nb-doped PZT, the spontaneous polarization is aligned in the same direction, so poling is not necessary. The first piezoelectric film 14 and the second piezoelectric film 18 can be formed using the same material and film formation method, which reduces costs.

[0052] 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. 7.

[0053] 7 includes a piezoelectric element 3 having four layers of piezoelectric films 14, 18 and electrodes 12, 16, and 20 arranged between and at both ends of the piezoelectric films 14, 18. More specifically, 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. The first piezoelectric film 14 and the second piezoelectric film 18 are substantially the same and do not need to be distinguished from each other, but here they are referred to as the first piezoelectric film 14 and the second piezoelectric film 18 in order to distinguish between a piezoelectric film to which an electric field in the same direction as the spontaneous polarization P is applied and a piezoelectric film to which an electric field in the opposite direction is applied.

[0054] 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 to generate an electric field Ef in the same direction as the spontaneous polarization P in 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 in the second piezoelectric films 18, which are arranged in the second and fourth layers.

[0055] 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.

[0056] (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.

[0057] (First Piezoelectric Film) The substrate 10 with the first electrode 12 was placed in an RF sputtering device, the substrate temperature was set to 700°C, and 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).

[0058] (Second Electrode) As the second electrode 16, a 50 nm IrO ｚ (Z≦2) and 200 nm of Ir were laminated in this order.

[0059] (Second Piezoelectric Film) A Nb-doped PZT film was formed as the second piezoelectric film under the same film formation conditions as those for the first piezoelectric film.

[0060] (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.

[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 8 is a diagram showing a schematic cross-sectional structure of the piezoelectric element for evaluation 101. In Figure 8, 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, to enable displacement. Using the above procedure, samples A and B of the piezoelectric element 101 for evaluation were fabricated. Samples A and B were fabricated using the same procedure, but there were differences in the P-V hysteresis characteristics due to individual differences.

[0063] <Measurement of P-V hysteresis characteristics> For each of the piezoelectric films of Sample A and Sample B, the lower side in the film thickness direction was grounded, and a bipolar voltage (triangular wave with a frequency of 5 Hz) was applied to the upper 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. 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] FIG. 9A shows the measurement results of the PV hysteresis characteristics of the first piezoelectric film 14 of sample A, and FIG. 9B shows the measurement results of the PV hysteresis characteristics of the second piezoelectric film 18.

[0065] 9A and 9B, the hysteresis curves of both the first piezoelectric film 14 and the second piezoelectric film 18 had asymmetric shapes biased toward the positive side. As described above, since the hysteresis curves were biased toward the positive side, it can be said that the spontaneous polarization was aligned in the film thickness direction from the substrate side toward the film deposition surface side.

[0066] <Measurement of current consumption and displacement during driving> A driving circuit 30 was connected to each of sample A and sample B of the piezoelectric element 101 shown in Figure 8 to prepare piezoelectric actuators for evaluation. The circuit scheme of the piezoelectric element 101 and driving circuit 30 was the same as the configuration of the piezoelectric element 2 and driving circuit 30 shown in Figure 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 Figure 1, the first piezoelectric film 14 was driven in the forward direction, and the second piezoelectric film 18 was driven in the reverse direction.

[0067] In Tests 1 to 4, the current value and displacement were measured when drive waveforms with different waveforms, peak-to-peak potential difference (pp value), minimum voltage value V1, or maximum voltage value V2 were applied. The current value was measured as the peak current value generated at the peak voltage. Here, the drive circuit 30 was equipped with a function generator and a voltage amplifier, and the drive voltage was output from the function generator and applied to Sample A or Sample B via the voltage amplifier. The current value was measured using a current probe. The displacement time waveform was obtained by acquiring the time waveform of the displacement velocity at the maximum displacement point of the actuator (the center point of the diaphragm) using a laser Doppler vibrometer and integrating this data over time. The maximum displacement was calculated using the displacement time waveform obtained in this way.

[0068] In Test 1, the piezoelectric element of Sample A was used, and the pulse waveform and pp value were fixed at 40 V, and the current value and displacement amount were measured for Comparative Example 1 and Examples 1 to 4, in which at least one of the minimum voltage value V1 and the maximum voltage value V2 was changed. The driving waveform conditions and results for Test 1 are shown in Table 1.

[0069] In Test 2, the piezoelectric element of Sample B was used, and the current value and displacement amount were measured for Comparative Example 2 and Examples 5 to 7, in which the sinusoidal waveform and the p-p value were fixed at 40 V, and at least one of the minimum voltage value V1 and the maximum voltage value V2 was changed. The drive waveform conditions and results for Test 2 are shown in Table 2.

[0070] In Test 3, the piezoelectric element of Sample B was used, and the current value and displacement amount were measured for Comparative Example 3 and Examples 8 to 10, in which the sinusoidal waveform and the p-p value were fixed at 20 V, and at least one of the minimum voltage value V1 and the maximum voltage value V2 was changed. The drive waveform conditions and results for Test 3 are shown in Table 3.

[0071] In Tables 1 to 3, the current values ​​and displacements of the examples are normalized by the current values ​​and displacements of the respective comparative examples. Furthermore, T0 / T1 is a value rounded to one decimal place.

[0072]

[0073] The relationship between T0 / T1 and the peak current value obtained in Tests 1 to 3 is shown in Figure 10. The relationship between T0 / T1 and the displacement is also shown in Figure 11. As shown in Figures 10 and 11, when T0 / T1 was 1.5 or greater, the current was reduced to 80% or less compared to the comparative example where T0 / T1 was 1. Furthermore, even though the current was reduced to 80%, the reduction in displacement was only about 1.5%. In other words, by setting the minimum voltage value V1 in the drive waveform so that T0 / T1 ≥ 1.5 is satisfied, it is possible to effectively suppress the reversal current of the reverse-driven piezoelectric film without significantly compromising the displacement, and it is clear that the drive current can be significantly reduced.

[0074] Furthermore, in Test 4, current values ​​were measured for Examples 11 to 14, which included four pulses per cycle and varied the minimum values ​​V11, V12, V13, and V14 and / or maximum values ​​V21, V22, V23, and V24 of the four pulses #1 to #4 in one cycle, using the piezoelectric element of Sample A. Figure 12A shows the drive waveform input to the drive electrode in Comparative Example 1. Figure 12B shows the drive waveform input to the drive electrode in Example 12. Table 4 shows the minimum and maximum values ​​within one cycle, as well as the minimum voltage value V1, T0 / T1, and power consumption for each drive waveform, expressed as the average of the minimum values. In Test 4, since the minimum and maximum voltage values ​​change during one cycle, the power consumption per cycle was used for evaluation instead of the current amount used in Tests 1 to 3. Note that T0 / T1 was calculated based on V1, which is the average value of V11, V12, V13, and V14.

[0075] The relationship between T0 / T1 and power consumption obtained in Test 4 is shown in Figure 13. As shown in Figure 13, even when pulses with different minimum and maximum values ​​are included in one cycle, power consumption could be reduced to approximately 80% or less as long as T0 / T1, calculated based on V1, which is the average of the minimum values, was 1.5 or more.

[0076] The disclosure of Japanese Patent Application No. 2024-139701, 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.

[0077] The following additional notes are further disclosed regarding the above embodiment.

[0078] <Supplementary Note 1> A piezoelectric actuator comprising: a piezoelectric element having electrodes arranged between each of the piezoelectric films in the stacking direction of the plurality of piezoelectric films and on both end faces of the plurality of piezoelectric films; and a drive circuit connected to the electrodes and applying a drive voltage to the piezoelectric element, wherein in the piezoelectric element, the plurality of piezoelectric films each have an asymmetric bipolar polarization-voltage hysteresis characteristic in which the absolute values ​​of two coercive voltages are different; the drive circuit applies a unipolar voltage waveform that generates an electric field in the same direction as the spontaneous polarization in at least one layer of the plurality of piezoelectric films and generates an electric field in the opposite direction to the spontaneous polarization in the other piezoelectric films, and a minimum voltage value V1 having the smallest absolute value in the unipolar voltage waveform is set under the condition that the ratio of a slope T0 at 0 [V] to a slope T1 at the minimum voltage value V1 [V] in a hysteresis curve showing the bipolar polarization-voltage hysteresis characteristic of the other piezoelectric films satisfies |T0 / T1|≧1.5. <Supplementary Note 2> The piezoelectric actuator according to Supplementary Note 1, wherein the piezoelectric film is two-layered. <Supplementary Note 3> The piezoelectric film is a material represented by the general formula Pb{(Zr ｘ Ti １－ｘ ) １－ｙ M ｙ O ３ The piezoelectric actuator according to Appendix 1 or Appendix 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. <Appendix 4> The piezoelectric actuator according to Appendix 3, wherein the metal element M is Nb, and y is greater than 0.1. <Appendix 5> The piezoelectric actuator according to any one of Appendix 1 to Appendix 4, which satisfies |T0 / T1|≧1.9. <Appendix 6> The piezoelectric actuator according to any one of Appendix 1 to Appendix 4, which satisfies |T0 / T1|≧3.4. <Appendix 7> The piezoelectric actuator according to any one of Appendix 1 to Appendix 4, which satisfies |T0 / T1|≧7.0.

[0079] 1, 6, 7, 8 Piezoelectric actuator 2, 3, 101 Piezoelectric element 10 Substrate 12 First electrode 14 First piezoelectric film 16 Second electrode 18 Second piezoelectric film 20 Third electrode 30, 32 Drive circuit 105 Insulating film 106, 107 Wiring

Claims

1. A piezoelectric actuator comprising: a piezoelectric element having electrodes arranged between each of the piezoelectric films in the stacking direction of the plurality of piezoelectric films and on both end faces of the plurality of piezoelectric films; and a drive circuit connected to the electrodes and applying a drive voltage to the piezoelectric element, wherein the plurality of piezoelectric films in the piezoelectric element each have an asymmetric bipolar polarization-voltage hysteresis characteristic in which the absolute values ​​of two coercive voltages are different; the drive circuit applies a unipolar voltage waveform that generates an electric field in the same direction as the spontaneous polarization in at least one of the plurality of piezoelectric films and generates an electric field in the opposite direction to the spontaneous polarization in the other piezoelectric films; and a minimum voltage value V1, which has the smallest absolute value in the unipolar voltage waveform, is set under the condition that the ratio of the slope T0 at 0 [V] in a hysteresis curve showing the bipolar polarization-voltage hysteresis characteristic of the other piezoelectric films to the slope T1 at the minimum voltage value V1 [V] satisfies |T0 / T1|≧1.

5.

2. The piezoelectric actuator of claim 1, wherein the piezoelectric film is two-layered.

3. The piezoelectric film has 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, which satisfies |T0 / T1| ≥ 1.

9.

6. A piezoelectric actuator according to any one of claims 1 to 4, which satisfies |T0 / T1| ≥ 3.

4.

7. A piezoelectric actuator according to any one of claims 1 to 4, which satisfies |T0 / T1| ≥ 7.0.

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