Patent Application
20240114797
This application claims priority from Japanese Application No. 2022-158870, filed on Sep. 30, 2022, the entire disclosure of which is incorporated herein by reference.
The present disclosure relates to a piezoelectric element and an actuator.
As a material having excellent piezoelectric characteristics and excellent ferroelectricity, there is known a perovskite-type oxide such as lead zirconate titanate (Pb(Zr,Ti)O3, hereinafter referred to as PZT). A piezoelectric body consisting of a perovskite-type oxide is applied as a piezoelectric film in a piezoelectric element comprising a lower electrode, a piezoelectric film, and an upper electrode on a substrate. This piezoelectric element has been developed into various devices such as a memory, an inkjet head (an actuator), a micromirror device, an angular velocity sensor, a gyro sensor, a piezoelectric micromachined ultrasonic transducer (PMUT), and an oscillation power generation device.
As the piezoelectric element, in order to obtain high piezoelectric characteristics, a multilayered piezoelectric element in which a plurality of piezoelectric films are laminated via an electrode layer has been proposed.
For example, JP2013-80886A proposes a piezoelectric element in which a first electrode, a Nb-doped PZT film, a second electrode, a Nb-doped PZT film, and a third electrode are laminated in this order. It is known that the Nb-doped PZT film has a direction of a spontaneous polarization that is aligned upward with respect to the substrate during film formation. That is, both of two layers of Nb-doped PZT films in JP2013-80886A have a spontaneous polarization of which a direction is aligned upward. In general, for the piezoelectric film having the spontaneous polarization of which a direction is aligned, higher piezoelectric performance can be obtained by applying an electric field in the same direction as the direction of the spontaneous polarization. Therefore, in JP2013-80886A, the electric field in the same direction as the direction of the spontaneous polarization is applied to each of two Nb-doped PZT films by a first driving method in which a second electrode is grounded, a positive voltage (+V) is applied to a first electrode, and a negative voltage (−V) is applied to a third electrode, a second driving method in which the first electrode is grounded, a negative voltage (−V) is applied to the second electrode, and a negative voltage (−2V) that is larger in absolute value than that of the second electrode is applied to the third electrode, or the like. As a result, a displacement amount that is substantially twice that of the piezoelectric element having only one layer is realized.
Further, JP2013-80887A proposes a piezoelectric element in which a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode are laminated in this order, and a direction in which a spontaneous polarization of the first piezoelectric film is aligned and a direction in which a spontaneous polarization of the second piezoelectric film is aligned are different from each other. Further, as a specific example, a case where the first piezoelectric film is a Nb-doped PZT film and the second piezoelectric film is a PZT film without Nb addition (hereinafter, referred to as pure PZT) is illustrated. The Nb-doped PZT film has a direction of the spontaneous polarization which is aligned in a state in which a poling treatment is not performed, and the pure PZT film has a direction of the spontaneous polarization which is not aligned in a state in which the poling treatment is not performed. Therefore, by performing the poling treatment on the pure PZT film such that the spontaneous polarization is aligned in a direction opposite to the direction of the spontaneous polarization of the Nb-doped PZT film, the direction of the spontaneous polarization of the first piezoelectric film and the direction of the spontaneous polarization of the second piezoelectric film are made different. In JP2013-80887A, by setting the first electrode and the third electrode of the piezoelectric element to have the same potential and using the second electrode as a drive electrode, an electric field in the same direction as the direction of the spontaneous polarization is applied to each of the first piezoelectric film and the second piezoelectric film. Accordingly, piezoelectric performance corresponding to two layers of piezoelectric films can be obtained with a voltage large enough to drive one layer of piezoelectric film, so that high piezoelectric performance can be obtained at a low voltage.
However, in JP2013-80886A, in order to obtain piezoelectric performance equivalent to that of the first driving method in the second driving method in which a voltage having an absolute value larger than that of the second electrode is applied to the third electrode, it is necessary to apply a very large voltage to the third electrode and sufficient piezoelectric performance cannot be obtained at a low voltage. In order to carry out the first driving 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 leads to high costs.
In addition, in the piezoelectric element of JP2013-80887A, very good piezoelectric performance can be obtained. However, in order to manufacture the piezoelectric element of JP2013-80887A, for example, the first piezoelectric film and the second piezoelectric film need to be composed of piezoelectric films containing different materials such that the first piezoelectric film is a Nb-doped PZT film and the second piezoelectric film is a pure PZT film. Therefore, in order to form the first piezoelectric film and the second piezoelectric film, two different targets are required, and at least one of the piezoelectric films needs to be subjected to the poling treatment, so that sufficient cost reduction cannot be achieved.
As described above, the piezoelectric element that can obtain high piezoelectric performance is expensive or requires application of a high voltage, and a low-cost piezoelectric element that can obtain high piezoelectric performance at a low voltage has not been realized.
An object of the present disclosure is to provide a piezoelectric element and a piezoelectric actuator capable of obtaining high piezoelectric performance at a low voltage at low costs.
A piezoelectric element according to an aspect of the present disclosure comprises a substrate; and a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode which are provided on the substrate in this order, in which both the first piezoelectric film and the second piezoelectric film have spontaneous polarizations aligned in a film thickness direction and directions of the spontaneous polarizations of the first piezoelectric film and the second piezoelectric film are the same, and in a case where in a hysteresis curve showing polarization-voltage characteristics of one piezoelectric film of the first piezoelectric film and the second piezoelectric film, a coercive voltage on a positive side is denoted by Vcf+, a coercive voltage on a negative side is denoted by Vcr−, an absolute value |Vcf+−Vcf−| of a difference between both coercive voltages is denoted by ΔVcf, and the larger of an absolute value of the coercive voltage Vcf+ and an absolute value of the coercive voltage Vcf− is denoted by Vcf, and in a hysteresis curve showing polarization-voltage characteristics of the other piezoelectric film of the first piezoelectric film and the second piezoelectric film, a coercive voltage on a positive side is denoted by Vcr+, a coercive voltage on a negative side is denoted by Vcr−, an absolute value |Vcr+−Vcr−| of a difference between both coercive voltages is denoted by ΔVcr, and the larger of an absolute value of the coercive voltage Vcr+ and an absolute value of the coercive voltage Vcr− is denoted by Vcr, ΔVcr<ΔVcf−0.2 and Vcr<Vcf−0.2, where, units are all [V], are satisfied.
It is preferable that |Vcr++Vcr−|≤ΔVcr is further satisfied.
It is preferable that an electric field in the same direction as a direction of the spontaneous polarization is applied to the one piezoelectric film, and an electric field in a direction opposite to the direction of the spontaneous polarization is applied to the other piezoelectric film.
It is preferable that the first electrode and the third electrode are connected to each other.
It is preferable that both the first piezoelectric film and the second piezoelectric film contain a perovskite-type oxide represented by General Formula Pb{(ZrxTi1-x)1-yMy}O3, where M is a metal element selected from the group consisting of V, Nb, Ta, Sb, Mo, and W, and 0<x<1, and 0<y<1 are satisfied.
It is preferable that the metal element M is Nb and y is greater than 0.1.
The first piezoelectric film and the second piezoelectric film may have the same composition.
In a case where the first piezoelectric film and the second piezoelectric film have the same composition, it is preferable that a film thickness of the other piezoelectric film is thinner than a film thickness of the one piezoelectric film.
An actuator according to another aspect of the present disclosure is an actuator comprising: the piezoelectric element according to the aspect of the present disclosure; and a drive circuit that applies a drive voltage to the piezoelectric element, in which the drive circuit applies an electric field in the same direction as a direction of the spontaneous polarization to the one piezoelectric film, and applies an electric field in a direction opposite to the direction of the spontaneous polarization to the other piezoelectric film.
According to a technology of the present disclosure, it is possible to provide a piezoelectric element and a piezoelectric actuator capable of obtaining high piezoelectric performance at a low voltage at low costs.
Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In the drawings below, a layer thickness of each of layers and a ratio therebetween are depicted with appropriate changes for easy visibility, and thus they do not necessarily reflect the actual layer thickness and ratio.
The substrate 10 is not particularly limited, and examples thereof include silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, or other substrates. As the substrate 10, a laminated substrate such as a thermal oxide film-attached silicon substrate having a SiO2 oxide film formed on a surface of a silicon substrate may be used. Further, as the substrate 10, a resin substrate such as polyethylene terephthalate (PET), polyethylene naphthalata (PEN), polyimide, and the like may be used.
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 such as gold (Au), platinum (Pt), iridium (Ir), ruthenium (Ru), titanium (Ti), molybdenum (Mo), tantalum (Ta), and aluminum (Al) or metal oxides, and combinations thereof. In addition, indium tin oxide (ITO), LaNiO3, SrRuO3 (SRO), or the like may be used.
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 are paired with each other to apply an electric field to the first piezoelectric film 14. In addition, the second electrode 16 and the third electrode 20 are paired with each other to apply an electric field to the second piezoelectric film 18.
The main components of the second electrode 16 and the third electrode 20 are not particularly limited, and examples thereof include, in addition to the materials exemplified for the first electrode 12, electrode materials that are generally used in a semiconductor process such as chromium (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 an oxide conductor layer include indium tin oxide (ITO), Ir oxide, and SrRuO3 (SRO), as well as LaNiO3 and a doped ZnO.
The thicknesses of the first electrode 12, the second electrode 16, and the third electrode 20 are not particularly limited and are preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.
Both the first piezoelectric film 14 and the second piezoelectric film 18 have spontaneous polarizations aligned in the film thickness direction, and the directions in which the spontaneous polarizations are aligned are the same. In the example shown in
In the piezoelectric film that has not been subjected to the poling treatment, the reason why the direction of the spontaneous polarization is aligned in a state in which an external electric field is not applied is thought to be due to generation of an electric field caused by distortion or defect of a crystal structure (hereinafter referred to as a spontaneous internal electric field) in the piezoelectric film. In a case of a piezoelectric film in which a spontaneous internal electric field is not generated in a state in which an external electric field is not applied, the P-E hysteresis curve (or P-V hysteresis curve) depicts a shape whose center coincides with an origin. On the other hand, in a case of a piezoelectric film in which a spontaneous internal electric field is generated, that is, a piezoelectric film in which the direction of the spontaneous polarization is aligned in a state in which the external electric field is not applied, the center of the hysteresis curve is deviated (shifted) from the origin because the direction of the spontaneous polarization is aligned with respect to the spontaneous internal electric field. In a case of the P-E hysteresis curve, assuming that the spontaneous internal electric field is Ei and the external electric field applied from the outside is Eo, an electric field of Ei+Eo is applied, so that the center of the hysteresis curve is shifted from the origin by an amount of the spontaneous internal electric field Ei. In a case of the P-V hysteresis curve, the center of the hysteresis curve is shifted from the origin by the product of Ei and the film thickness. Therefore, in a case where the center of the measured hysteresis curve is shifted from the origin, it can be considered that the 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 an alignment degree of the spontaneous polarization, and a larger shift amount means that the alignment degree of the spontaneous polarization is higher (the spontaneous internal electric field is larger). In addition, it is possible to specify the direction of the spontaneous polarization that is aligned by a shift direction of the hysteresis curve from the origin. In a case of the P-V hysteresis curve, the center of the hysteresis is defined as a midpoint between two coercive voltages described below.
Hereinafter, the term “direction of the spontaneous polarization” means the direction of the spontaneous polarization aligned by the spontaneous internal electric field.
The piezoelectric element 1 satisfies ΔVcr<ΔVcf−0.2 and Vcr<Vcf−0.2 (1).
It is preferable that ΔVcf×0.5<ΔVcr.
The first piezoelectric film 14 may be the one piezoelectric film showing the hysteresis curve shown in
The coercive voltage is a voltage at which the polarization is zero in the hysteresis curve, and as shown in
As shown in
Vcr<Vcf−0.2 indicates that the coercive voltage Vcr of the other piezoelectric film is smaller than the coercive voltage Vcf of the one piezoelectric film. This means that a voltage at which polarization reversal occurs in a case where an electric field in a direction opposite to the direction of the spontaneous polarization is applied to the other piezoelectric film is smaller than a voltage at which polarization reversal occurs in a case where an electric field in a direction opposite to the direction of the spontaneous polarization is applied to the one piezoelectric film. Again, “−0.2” on the right side takes the measurement error into account. The range of Vcf±0.2 [V] is considered to be equivalent to Vcf.
The coercive voltage Vc is represented by the product of a coercive electric field Ec and a film thickness t. Therefore, in a case where a difference in the coercive electric field of the one piezoelectric film is denoted by ΔEcf, a film thickness of the one piezoelectric film is denoted by tf, a difference in the coercive electric field of the other piezoelectric film is denoted by ΔEcr. and a film thickness of the other piezoelectric film is denoted by tr, ΔVcf=ΔEcf×tf, and ΔVcr=ΔEcr×tr are established.
For example, in a case where the first piezoelectric film 14 and the second piezoelectric film 18 are composed of piezoelectric bodies having exactly the same composition, a configuration satisfying the above expression (1) can be made by making the film thickness tr of the other piezoelectric film thinner than the film thickness tf of the one piezoelectric film (tr<tf). In a case where the first piezoelectric film 14 and the second piezoelectric film 18 have the same composition, it is preferable that the film thickness tr of the other piezoelectric film is 0.4 to 0.9 times the film thickness tf of one piezoelectric film.
In the example shown in
In a case where the first piezoelectric film 14 and the second piezoelectric film 18 are composed of different piezoelectric bodies, and the difference ΔEcf in the coercive electric field of the one piezoelectric film is larger than the difference ΔEcr in the coercive electric field of the other piezoelectric film, the equation (1) can be satisfied even in a case where the film thickness tf of the one piezoelectric film and the film thickness tr of the other piezoelectric film have a relationship of tf≤tr. However, it is preferable that the first piezoelectric film 14 and the second piezoelectric film 18 are composed of piezoelectric bodies having the same composition, because materials can be shared and manufacturing costs can be suppressed.
It is preferable that the piezoelectric element 1 further satisfies |Vcr++Vcr−|≤ΔVcr. The center of the hysteresis curve of the other piezoelectric film is represented by (Vcr++Vcr−)/2. Therefore, |Vcr++Vcr−|≤ΔVcr means that a distance between the center of the hysteresis curve and the origin is less than or equal to half the hysteresis width, that is, the hysteresis curve includes the origin. In a case where the shift amount of the hysteresis curve of the other piezoelectric film is smaller, the coercive voltage Vcr can be suppressed to be relatively small.
There are no particular restrictions on the piezoelectric bodies constituting the first piezoelectric film 14 and the second piezoelectric film 18, as long as a film in which the direction of the spontaneous polarization is aligned in the film thickness direction immediately after film formation can be obtained, and the above expression (1) is satisfied. It is preferable that the first piezoelectric film 14 and the second piezoelectric film 18 contain a perovskite-type oxide as a main component. Here, the main component means a component that occupies 80 mol % or more. It is preferable that 90 mol % or more of each of the first piezoelectric film 14 and the second piezoelectric film 18 is occupied by the perovskite-type oxide, and it is more preferable that the first piezoelectric film 14 and the second piezoelectric film 18 are composed of the perovskite-type oxide (however, containing unavoidable impurities).
The perovskite-type oxide is preferably a lead zirconate titanate (PZT) type that contains lead (Pb), zirconium (Zr), titanium (Ti), and oxygen (O).
In particular, it is preferable that the perovskite-type oxide is a compound represented by General Formula (2), which contains the metal element M, as an additive, at the B site of PZT.
Pb{(ZrxTi1-x)1-yMy}O3 (2)
Here, the metal element M is preferably one or more elements selected from vanadium (V), niobium (Nb), tantalum (Ta), antimony (Sb), molybdenum (Mo), and tungsten (W). Here, 0<x<1 and 0<y<1 are satisfied. Further, in General Formula (2), regarding Pb:{(ZrxTi1+x)1-yMy}:O, a reference ratio is 1:1:3; however, the ratio may deviate within a range in which a perovskite structure can be obtained. Hereinafter, Pba{(ZrxTi1-x)1-yMy}O3 will be referred to as M-doped PZT. In addition, for example, in a case where the metal element M is Nb, it is referred to as Nb-doped PZT.
The metal element M may be a single element such as V only or Nb only, or it may be a combination of two or three or more elements, such as a mixture of V and Nb or a mixture of V, Nb, and Ta. In a case where the metal element M is these elements, a very high piezoelectric constant can be realized in combination with Pb of the A site element.
In particular, Pb{(ZrxTi1-x)1-yNby}O3 in which the metal element M is Nb is optimal. Here, in a case where y>0.1, a higher piezoelectric constant can be obtained. In a case of forming a piezoelectric film by a vapor phase growth method such as sputtering using Nb-doped PZT in which M is Nb, a piezoelectric film having a very high piezoelectric constant in which the spontaneous polarization is more aligned upward in the film thickness direction from the substrate can be obtained.
It is preferable that the perovskite-type oxide of the first piezoelectric film 14 and the perovskite-type oxide of the second piezoelectric film 18 have the same composition. Here, in a case where each element symbol indicates a molar ratio of each element, it is assumed that in the perovskite-type oxide, the Pb composition ratio is Pb/(Zr+Ti+M), the Zr composition ratio at the B site is Zr/(Zr+Ti), the Ti composition ratio at the B site is Ti/(Zr+Ti), and the M composition ratio which is a composition ratio of the metal element M at the B site is M/(Zr+Ti+M). The fact that the perovskite-type oxide of the first piezoelectric film 14 and the perovskite-type oxide of the second piezoelectric film 18 have the same composition means that the Pb composition ratios, the Zr composition ratios, the Ti composition ratios, and the M composition ratios are equal to each other, respectively, within the measurement error.
The film thicknesses t1 and t2 of the first piezoelectric film 14 and the second piezoelectric film 18 are preferably 0.2 μm or more and 5 μm or less, and more preferably 1 μm or more. As described above, in a case where the first piezoelectric film 14 and the second piezoelectric film 18 are composed of a perovskite-type oxide having the same composition and the first piezoelectric film 14 is the one piezoelectric film, the film thickness t1 of the first piezoelectric film 14 may be made thicker than the film thickness t2 of the second piezoelectric film 18. On the contrary, in a case where the second piezoelectric film 18 is the one piezoelectric film, the film thickness t2 of the second piezoelectric film 18 may be made thicker than the film thickness t1 of the first piezoelectric film 14.
As described above, in the piezoelectric element 1, two layers of piezoelectric films having spontaneous polarizations aligned in the same direction along the film thickness direction are laminated with the electrode interposed therebetween, the hysteresis width of the other piezoelectric film is smaller than the hysteresis width of the one piezoelectric film, and the coercive voltage is small. With this configuration, as compared with a piezoelectric element comprising two layers of piezoelectric films having the same hysteresis curve, high piezoelectric performance can be obtained in a low voltage region in a case where an electric field in the same direction as the direction of the spontaneous polarization is applied to the one piezoelectric film, and an electric field in a direction opposite to the direction of the spontaneous polarization is applied to the other piezoelectric film to perform driving. Here, the low voltage region refers to a voltage region suitable for a case where the piezoelectric element is assumed to be incorporated in a consumer device, and specifically, a voltage region with an absolute value of 12 V or less. It is preferable that high piezoelectric performance can be obtained at a voltage of 7 V or less, and further 5 V or less.
The first piezoelectric film 14 and the second piezoelectric film 18 have the spontaneous polarizations P1 and P2 aligned in the same direction, and in a case where an electric field in the same direction as the direction of the spontaneous polarization P1 is applied to the first piezoelectric film 14 and an electric field in a direction opposite to the direction of the spontaneous polarization P2 is applied to the second piezoelectric film 18, the electric field may be applied by using the first electrode 12 and the third electrode 20 as the same pole, and using the second electrode 16 as a different pole from the first electrode 12 and the third electrode 20. Therefore, since a drive circuit having one polarity can be used for driving, it is possible to reduce the cost as compared with a case where two drive circuits having different polarities are provided.
It is preferable that the piezoelectric element 1 is configured such that the first electrode 12 and the third electrode 20 are connected. In a case where the first electrode 12 and the third electrode 20 are connected, drive control is easy.
The first piezoelectric film 14 and the second piezoelectric film 18 have the spontaneous polarizations aligned in the same direction and do not require a poling treatment. The first piezoelectric film 14 and the second piezoelectric film 18 can be formed using the same material and film formation method, so that cost reduction can be achieved.
The drive circuit 30 is means for supplying a drive voltage to the piezoelectric film sandwiched between the electrodes. In this example, the first electrode 12 and the third electrode 20 are connected to a ground terminal of the drive circuit 30 and are set to a ground potential. The second electrode 16 is connected to a drive voltage output terminal of the drive circuit 30 and functions as a drive electrode. Accordingly, the drive circuit 30 applies a drive voltage to the first piezoelectric film 14f and the second piezoelectric film 18r. In this example, the drive circuit 30 applies an electric field Ef in the same direction as the direction of the spontaneous polarization P1 to the first piezoelectric film 14f, and applies an electric field Er in a direction opposite to the direction of the spontaneous polarization P2 to the second piezoelectric film 18r. That is, the drive circuit 30 is a negative drive circuit that executes a negative drive for applying a negative potential to the drive electrode.
On the other hand, as in an actuator 6 of a modification example shown in
The actuators 5 and 6 comprising the piezoelectric element 1A need only be provided with a drive circuit having one polarity and can be realized at low costs. Since the piezoelectric element 1A is provided, large piezoelectric performance can be obtained in a low voltage region.
Further, as described above, the second piezoelectric film 18 in the piezoelectric element 1 shown in
In the piezoelectric element 1B shown in
In this case, by applying the electric field Ef in the same direction as the direction of the spontaneous polarization P2 to the second piezoelectric film 18f, and applying the electric field Er in a direction opposite to the direction of the spontaneous polarization P1 to the first piezoelectric film 14r, good piezoelectric performance can be obtained in a low voltage region by a drive circuit having one polarity.
In an actuator 7 shown in
In a case where the piezoelectric element 1B is used, a configuration may be adopted in which a negative drive circuit is provided, the second electrode 16 is connected to the ground terminal to be a ground potential, and the first electrode 12 and the third electrode 20 are connected to the drive voltage output terminal to function as a drive electrode.
In addition, the piezoelectric element 1 shown in
Here, a principle in which the piezoelectric element 1 exhibits large piezoelectric performance in a low voltage region will be described.
A piezoelectric element 101 shown in
A potential of 0 to −V is applied to the second electrode 16 such that the electric field Ef in the same direction as the direction of the spontaneous polarization P1 is applied to the first piezoelectric film 14f and the electric field Er in a direction opposite to the direction of the spontaneous polarization P2 is applied to the second piezoelectric film 18f. Here, the first piezoelectric film 14f and the second piezoelectric film 18f expand and contract in an in-plane direction in a d31 mode with the application of a voltage, and the substrate 10 bends with the expansion and contraction of these piezoelectric films.
In a case where the potential applied to the second electrode 16 in
On the other hand, for example, in the piezoelectric element 1A shown in
Hereinafter, specific examples and comparative examples of the piezoelectric element according to the present disclosure will be described. First, a production method for a piezoelectric element of each example will be described. A radio frequency (RF) sputtering device was used for the deposition of each layer. The description of the manufacturing method will be made with reference to the references of the respective layers of the piezoelectric element 1 shown in
Deposition of First Electrode
A thermal oxide film-attached silicon substrate was used as the substrate 10. The first electrode 12 was deposited on the substrate 10 by radio-frequency (RF) sputtering. Specifically, as the first electrode 12, a TiW layer and an Ir layer were laminated on the substrate 10 in this order. The sputtering conditions for respective layers were as follows.
Sputtering Conditions for TiW Layer
Sputtering Conditions for Ir Layer
First Piezoelectric Film
The substrate 10 with the first electrode 12 attached thereto was placed in the inside of an RF sputtering device, and an Nb-doped PZT film in which an Nb addition amount to the B site was set to 12 at % was formed as the first piezoelectric film 14. The Nb-doped PZT was used as a target, and the sputtering conditions were as follows. An amount of Pb in the target was set to be larger than a stoichiometric composition, and a Ti/Zr molar ratio was set to the MPB composition (Ti/Zr=52/48).
Sputtering Conditions for First Piezoelectric Film
The film thickness t1 of the first piezoelectric film 14 was as shown in Table 1 for each Example and Comparative Example. The film thickness t1 was adjusted by changing a film formation time.
Second Electrode
As the second electrode 16, IrOz (Z≤2) of 50 nm and Ir of 100 nm were laminated on the first piezoelectric film 14 in this order. The sputtering conditions were as follows.
Sputtering Conditions for IrOz and Ir
Second Piezoelectric Film
As the second piezoelectric film 18, the Nb-doped PZT film in which the Nb addition amount to the B site was 12 at % was formed on the second electrode 16. The same target was used for film formation of the first piezoelectric film 14 and the second piezoelectric film 18, and the film formation conditions were also the same. The film thickness t2 of the second piezoelectric film 18 was set as shown in Table 1 for each Example and Comparative Example. The film thickness t2 was adjusted by changing a film formation time. Comparative Example 1 was a piezoelectric element having a single layer of the first piezoelectric film 14 not provided with the second piezoelectric film 18 and the third electrode 20.
Third Electrode
As the third electrode 20, IrOz of 50 nm and Ir of 100 nm were laminated on the second piezoelectric film 18 in this order. The sputtering conditions were the same as those of the second electrode 16.
Formation of Electrode Pattern for Evaluation
In Comparative Example 1, in order to form electrode pads for applying a voltage to the first electrode 12 and the second electrode 16, the second electrode 16 and the first piezoelectric film 14 were sequentially patterned by photolithography and dry etching.
In Comparative Example 2 and Examples, in order to form electrode pads for applying a voltage to the first electrode 12, the second electrode 16, and the third electrode 20, the third electrode 20, the second piezoelectric film 18, the second electrode 16, and the first piezoelectric film 14 were sequentially patterned by photolithography and dry etching.
By the above process, a laminate obtained by laminating electrodes and piezoelectric layers was produced.
Preparation of Evaluation Sample
Evaluation Sample 1
A strip-shaped portion of 2 mm×25 mm was cut out from the laminate to prepare a cantilever as an evaluation sample 1.
Evaluation Sample 2
A portion of 25 mm×25 mm having, at the center of the surface of the piezoelectric film, the third electrode that had been patterned in a circular shape having a diameter of 400 μm, was cut out from the laminate and used as an evaluation sample 2.
Measurement of Piezoelectric Characteristics
The piezoelectric constant d31 was measured for the evaluation of the piezoelectric characteristics for each of Examples and Comparative Examples.
The measurement of the piezoelectric constant d31 was carried out using the evaluation sample 1. According to a method described in I. Kanno et al., Sensor and Actuator A 107 (2003) 68, the piezoelectric constant d31 was measured by grounding the first electrode 12 and the third electrode 20 and applying a drive signal to the second electrode 16. For each example, the piezoelectric constant d31 was measured in a case where application voltages were −1, −3, −5, −7, −10, and −15 V, respectively. For example, the piezoelectric constant d31 in a case where the application voltage was −1 V was measured by applying a drive signal obtained by adding a sinusoidal wave having an amplitude of 0.5 V to a bias voltage of −0.5 V to the second electrode 16. The measurement results are shown in Table 2.
Measurement of Polarization-Voltage Characteristics
For the piezoelectric elements of each Example and Comparative Example, a polarization-voltage (P-V) hysteresis curve was measured using the evaluation sample 2. The measurement was carried out by applying a voltage until saturation polarization was reached under a condition of a frequency of 1 kHz for each of the first piezoelectric film 14 and the second piezoelectric film 18 of the piezoelectric element of each Example and Comparative Example. In a case of measuring the hysteresis of the first piezoelectric film 14, a sweep voltage was applied to the first piezoelectric film 14 with the first electrode 12 being grounded and the second electrode 16 as a drive electrode. Further, in a case of measuring the hysteresis of the second piezoelectric film 18, a sweep voltage was applied to the second piezoelectric film 18 with the second electrode 16 being grounded and the third electrode 20 as a drive electrode.
Table 1 shows Vcf−, Vcf+, and ΔVcf in the hysteresis curve of the first piezoelectric film, and Vcr−, Vcr+, and ΔVcr in the hysteresis curve of the second piezoelectric film of the piezoelectric elements of each Example and Comparative Example, which are obtained as described above.
Examples 1 to 10 all satisfy ΔVcr<ΔVcf−0.2, and Vcr<Vcf−0.2.
In particular, Examples 1 to 6 and Examples 9 to 10 showed a piezoelectric constant larger than that of Comparative Examples 1 and 2 at 7 V, and good results were obtained in that the element was not destroyed at −15 V.
Example 7 showed the maximum piezoelectric constant d31 at an application voltage of −5 V, and thereafter, the piezoelectric constant decreases as the voltage further increases, and it is considered that element destruction occurs at an application voltage of −15 V from a degree of decrease in numerical value. In addition, Example 8 showed the maximum piezoelectric constant d31 at an application voltage of −5 V, and the element was destroyed at −10 V, so that the piezoelectric constant d31 could not be measured. Since the second piezoelectric films of Examples 7 and 8 are thin, it is considered that a withstand voltage is low. However, at application voltages of −3 V to −5 V, a very high piezoelectric constant d31 can be obtained as compared with Comparative Example 1, and it is clear that high piezoelectric performance can be obtained in a low voltage region.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-158870 | Sep 2022 | JP | national |
