Patent Application
20240114796
This application claims priority from Japanese Application No. 2022-158868, 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 potential (+V) is applied to a first electrode, and a negative potential (−V) is applied to a third electrode for driving, a second driving method in which the first electrode is grounded, a negative potential (−V) is applied to the second electrode, and a negative potential (−2V) that is larger in absolute value than that of the second electrode is applied to the third electrode for driving, 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 contain a perovskite-type oxide containing Pb at an A site and containing Zr, Ti, and M at a B site as a main component, where M is a metal element selected from V, Nb, Ta, Sb, Mo, and W, in a case where a Pb composition ratio in the perovskite-type oxide is defined by Pb/(Zr+Ti+M), where element symbols each represent a molar ratio, the Pb composition ratio in the first piezoelectric film is different from the Pb composition ratio in the second piezoelectric film, 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 polarization-electric field hysteresis measured for the first piezoelectric film with the first electrode grounded and the second electrode as a drive electrode, and polarization-electric field hysteresis measured for the second piezoelectric film with the second electrode grounded and the third electrode as a drive electrode are shifted in the same electric field direction with respect to origins thereof.
It is preferable that at the B sites of the perovskite-type oxides contained in the first piezoelectric film and the second piezoelectric film, Zr composition ratios represented by Zr/(Zr+Ti), Ti composition ratios represented by Ti/(Zr+Ti), and M composition ratios represented by M/(Zr+Ti+M) are the same, respectively.
It is preferable that in a case where the metal element M is Nb and the M composition ratio at the B site is y, 0.08≤y≤0.15 is satisfied.
It is preferable that the Pb composition ratio of the perovskite-type oxide contained in the first piezoelectric film and the Pb composition ratio of the perovskite-type oxide contained in the second piezoelectric film differ from each other by 0.01 or more.
It is preferable that, out of the first piezoelectric film and the second piezoelectric film, a film thickness of the piezoelectric film having a relatively small Pb composition ratio is thinner than a film thickness of the piezoelectric film having a relatively large Pb composition ratio by 100 nm or more.
It is preferable that the film thickness of the piezoelectric film having a relatively small Pb composition ratio is 1.5 μm or less.
It is preferable that the film thickness of the piezoelectric film having a relatively large Pb composition ratio is 2 μm or less.
It is preferable that the first electrode and the third electrode are connected to each other.
It is preferable that, out of the first piezoelectric film and the second piezoelectric film, an electric field in the same direction as the direction of the spontaneous polarization is applied to the piezoelectric film having a relatively large Pb composition ratio, and an electric field in a direction opposite to the direction of the spontaneous polarization is applied to the piezoelectric film having a relatively small Pb composition ratio.
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 the direction of the spontaneous polarization to the piezoelectric film having a relatively large Pb composition ratio, and applies an electric field in a direction opposite to the direction of the spontaneous polarization to the piezoelectric film having a relatively small Pb composition ratio, out of the first piezoelectric film and the second piezoelectric film.
According to a technology of the present disclosure, it is possible to provide a piezoelectric element and an 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.
The first piezoelectric film 14 and the second piezoelectric film 18 are piezoelectric films in which a direction of a spontaneous polarization is aligned in a film thickness direction immediately after film formation and have, as main components, perovskite-type oxides which are represented by General Formula ABO3 and contain lead (Pb) at an A site, and zirconium (Zr), titanium (Ti), and a metal element M at a B site. This perovskite-type oxide is represented by the following general formula.
Pba{(ZrxTi1-x)1-yMy}O3
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). 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, Pba{(ZrxTi1-x)1-yNby}O3 in which the metal element M is Nb, that is, Nb-doped PZT is optimal. Here, it is preferable that 0.08≤y≤0.15. A higher piezoelectric constant can be obtained with 0.08≤y. 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 having more aligned spontaneous polarization upward from the substrate 10 can be obtained. In addition, in this specification, with reference to the substrate 10, a direction away from the substrate 10 is defined as upward, and a substrate side is defined as downward.
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).
A Pb composition ratio a in the perovskite-type oxide is defined by a=Pb/(Zr+Ti+M) where element symbols each represent a molar ratio.
Similarly, a Zr composition ratio x at the B site of the perovskite-type oxide is x=Zr/(Zr+Ti), a Ti composition ratio 1-x is 1-x=Ti/(Zr+Ti), and an M composition ratio y which is a compositional ratio of the metal element M at the B site is y=M/(Zr+Ti+M). Here, element symbols in the formula each represent a molar ratio.
In the piezoelectric element 1, in a case where the Pb composition ratio of the perovskite-type oxide in the first piezoelectric film 14 is denoted by a1, and the Pb composition ratio of the perovskite-type oxide in the second piezoelectric film 18 is denoted by a2, a1 and a2 are different. The difference between a1 and a2 means that there is a difference greater than or equal to a measurement error. As a method for measuring the composition, there are a plurality of analysis methods such as inductively coupled plasma (ICP) emission spectroscopy or X-ray fluorescence (XRF). For example, in a case where composition analysis is performed by XRF, a measurement error is about 0.005. It is preferable that a1 and a2 differ by 0.01 or more, that is, |a1−a2|≥0.01.
In addition, it is preferable that the Zr composition ratios, the Ti composition ratios, and the M composition ratios of the perovskite-type oxides contained in the first piezoelectric film 14 and the second piezoelectric film 18 are the same, respectively. That is, in a case where, in the perovskite-type oxide in the first piezoelectric film 14, the Zr composition ratio at the B site is denoted by x1, the M composition ratio at the B site is denoted by y1, and in the perovskite-type oxide in the second piezoelectric film 18, the Zr composition ratio at the B site is dented by x2, and the M composition ratio at the B site is denoted by y2, it is preferable that x1 and x2 are the same, and y1 and y2 are the same. Here, “same” means that they are equal within a range of a measurement error.
It is said that, in the PZT-based perovskite-type oxide, high piezoelectric characteristics are exhibited at a morphotropic phase boundary (MPB) and its vicinity. A Zr/Ti molar ratio of around 55/45 is the MPB composition, and in the above general formula, the MPB composition or its vicinity is preferable. The “MPB or its vicinity” refers to a region in which a phase transition occurs in a case where an electric field is applied to the piezoelectric film. Specifically, Zr:Ti (molar ratio) is preferably in a range of 45:55 to 55:45, that is, in a range of x=0.45 to 0.55.
A film thickness t1 of the first piezoelectric film 14 and a film thickness t2 of the second piezoelectric film 18 are preferably 0.2 μm or more and 5 μm or less, and more preferably 1 μm or more. The film thicknesses of the first piezoelectric film 14 and the second piezoelectric film 18 may be the same as or different from each other. However, it is preferable that, out of the first piezoelectric film 14 and the second piezoelectric film 18, the film thickness of the piezoelectric film having a relatively small Pb composition ratio is thinner than the film thickness of the piezoelectric film having a relatively large Pb composition ratio by 100 nm or more. In this case, the film thickness of the piezoelectric film having a relatively small Pb composition ratio is preferably 1.5 μm or less. In addition, the film thickness of the piezoelectric film having a relatively large Pb composition ratio is preferably 2 μm or less.
Both the first piezoelectric film 14 and the second piezoelectric film 18 have the spontaneous polarization aligned in the film thickness direction, and a direction of a spontaneous polarization P1 of the first piezoelectric film 14 and a direction of a spontaneous polarization P2 of the second piezoelectric film 18 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 a degree of alignment of the spontaneous polarization, and a larger shift amount means that the degree of alignment 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 curve 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 present inventors have found that in the M-doped PZT film, the degree of alignment of the spontaneous polarization can be changed by changing the Pb composition ratio. Increasing the Pb composition ratio tends to increase the degree of alignment of the spontaneous polarization. Although a mechanism by which the degree of alignment of the spontaneous polarization changes depending on the Pb composition ratio is not clear, it is presumed that defects in Pb, which is the A site of the perovskite-type structure, are related to the degree of alignment of the spontaneous polarization. It is considered that the Pb composition ratio correlates with the amount of Pb defects, and the degree of alignment of the spontaneous polarization changes depending on the Pb composition ratio, and as a result, a coercive electric field changes.
In both the hysteresis shown in
The piezoelectric element 1 is two layers of piezoelectric films including the M-doped PZT, in which the two layers of piezoelectric films having the spontaneous polarizations which are aligned in the same direction in the film thickness direction are laminated with an electrode interposed therebetween, and the Pb composition ratios of the two layers of piezoelectric films are different from each other. With this configuration, higher piezoelectric performance can be obtained in a low voltage region as compared with a piezoelectric element comprising two layers of piezoelectric films having the same Pb composition ratio. 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.
By changing the Pb composition ratio as described above, the coercive electric field of the piezoelectric film to which the electric field in a direction opposite to the direction of the spontaneous polarization is applied can be made smaller than the coercive electric field of the piezoelectric film to which the electric field in the same direction as the direction of the spontaneous polarization is applied, and as a result, high piezoelectric performance can be obtained in a low voltage region, although details will be described later.
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 one of the piezoelectric films and an electric field in a direction opposite to the direction of the spontaneous polarization P2 is applied to the other of the piezoelectric films, 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 driver 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.
As described above, in the piezoelectric element 1 shown in
A piezoelectric element 1B shown in
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 of the present disclosure exhibits large piezoelectric performance in a low voltage region will be described.
A piezoelectric element 101 shown in
Since the first piezoelectric film 14f and the second piezoelectric film 18f have the same composition and film thickness, both have the same piezoelectric characteristics, and the P-V hysteresis curves showing the polarization-voltage characteristics are substantially the same. In general, the characteristics of the piezoelectric film are discussed in terms of the P-E hysteresis curve showing the polarization-electric field (P-E) characteristics, but the behavior of the piezoelectric film in the piezoelectric element needs to consider the film thickness. Therefore, in the following, the characteristics of each piezoelectric film will be described with reference to the P-V hysteresis curve.
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. 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
For Comparative Examples 1 to 3, the first electrode 12, the first piezoelectric film 14, and the second electrode 16 were sputter-deposited on the substrate 10 in this order to prepare a laminate. That is, Comparative Examples 1 to 3 are piezoelectric elements comprising a single layer of a piezoelectric film.
In Comparative Examples 4 to 6 and Examples, the first electrode 12, the first piezoelectric film 14, the second electrode 16, the second piezoelectric film 18, and the third electrode 20 were sputter-deposited on the substrate 10 in this order to prepare a laminate. That is, Comparative Examples 4 to 6 and Examples are multilayered piezoelectric elements comprising two layers of piezoelectric films.
A radio frequency (RF) sputtering device was used for the deposition of each layer. Materials and deposition conditions (sputtering conditions) of respective layers were as follows.
A thermal oxide film-attached silicon substrate was used as the substrate 10.
Deposition of First Electrode
As the first electrode 12, a Ti layer having a thickness of 20 nm and an Ir layer having a thickness of 150 nm were laminated on the substrate 10 in this order. The sputtering conditions for respective layers were as follows.
Sputtering Conditions for Ti Layer
Sputtering Conditions for Ir Layer
First Piezoelectric Film and Second Piezoelectric Film
A Nb-doped PZT in which a Nb addition amount to the B-site is 12 at % was used as a target, and a film was formed under the following sputtering conditions. An amount of Pb in the target was set to a=1.3, which is 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 Layer
The set temperature of the substrate was appropriately set for each piezoelectric film of each example in order to control the Pb composition ratio in the piezoelectric film after the film formation. A relationship between the Pb composition ratio in the piezoelectric film (see Table 1) and the set temperature of the substrate is as follows.
For example, the first piezoelectric film according to Comparative Example 1 in which the Pb composition ratio a=Pb/(Zr+Ti+Nb) after film formation is 1.1 was formed at the set temperature of the substrate of 600° C. and the first piezoelectric film according to Comparative Example 2 in which the Pb composition ratio a is 1.11 was formed at the set temperature of 575° C.
The film thicknesses of the first piezoelectric film 14 and the second piezoelectric film 18 in each example were as shown in Table 1. The film thickness was adjusted by changing a film formation time. In each example, it was confirmed with an optical interference film thickness meter that a desired film thickness was obtained.
In addition, a composition of each piezoelectric film was confirmed by X-ray fluorescence (XRF) analysis. The Pb composition ratio for each example is as shown in Table 1, and the relationship between the Pb composition ratio and the set temperature of the substrate was confirmed. In addition, the composition ratios x and y other than Pb were confirmed to coincide in an error range (here, a range of ±0.005) for each example.
Second Electrode and Third Electrode
As the second electrode 16 and the third electrode 20, IrOz (z≤2) of 50 nm and Ir of 100 nm were laminated in this order, respectively.
Sputtering Conditions for IrOz and Ir
Formation of Electrode Pattern for Evaluation
In Comparative Examples 1 to 3, 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 Examples 4 to 6 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.
Evaluation Sample
A strip-shaped portion of 2 mm×25 mm was cut out from the laminate of each example to prepare a cantilever as an evaluation sample.
Measurement of Piezoelectric Constant
For each Example and Comparative Example, a piezoelectric constant d31 was measured using the cantilever prepared as described above. According to a method described in I. Kanno et al., Sensor and Actuator A 107 (2003) 68, the measurement was performed with an application voltage of a sinusoidal wave of −α±αV (that is, Vpp=−2α). The sinusoidal wave of −α±αV is a drive signal obtained by adding a sinusoidal wave having an amplitude of αV to a bias voltage of −αV. For example, Vpp=−10 means that a drive signal obtained by adding a sinusoidal wave having an amplitude of 5 V to a bias voltage of −5 V is used. In this example, the first electrode 12 and the third electrode 20 were grounded, the second electrode 16 was used as a drive electrode, and a drive signal was applied to the second electrode 16 for measurement. The results are shown in Table 1. The piezoelectric constant d 31 shown in Table 1 is a value measured with a drive signal of Vpp=−10.
In this example, since the first electrode 12 and the third electrode 20 were grounded and the second electrode 16 was used as the drive electrode, in the piezoelectric element having a two-layer structure comprising the first piezoelectric film 14 and the second piezoelectric film 18, the electric field in the same direction as the direction of the spontaneous polarization was applied to the first piezoelectric film 14, and the electric field in a direction opposite to the direction of the spontaneous polarization was applied to the second piezoelectric film 18.
As shown in Comparative Examples 1 to 3, in the piezoelectric element in which the piezoelectric film is a single layer, a higher piezoelectric constant d31 was obtained as the Pb composition ratio increased from 1.10 to 1.12. On the other hand, in Comparative Examples 4 to 6 in which two layers having the same composition were laminated, a rate of increase of the piezoelectric constant d31 in a case of two layers was lower as the piezoelectric constant was higher with the single layer.
In Examples 1 to 10, higher piezoelectric constants were obtained than in Comparative Examples 4 to 6. In Examples 1 to 10, the Pb composition ratio of the first piezoelectric film 14 and the Pb composition ratio of the second piezoelectric film 18 are different from each other. In Examples 1 to 10, a difference between the Pb composition ratios is at least 0.01 or more. As shown in Examples 1 and 2, a higher piezoelectric constant was obtained as the difference in the Pb composition ratio between the first piezoelectric film 14 and the second piezoelectric film 18 was larger. In addition, as shown in Examples 2 and 3, a higher piezoelectric constant was obtained in a case where the film thickness of the second piezoelectric film 18 having a smaller Pb composition ratio was smaller than that of the film thickness of the first piezoelectric film 14.
As shown in Examples 1 and 4 to 6, a higher piezoelectric constant was obtained as the film thickness of the second piezoelectric film 18 having a smaller Pb composition ratio is thinner. In a case where the film thickness of the second piezoelectric film 18 is thinner than the film thickness of the first piezoelectric film 14 by 100 nm or more as in Examples 4 to 6, the piezoelectric constant which is at least 1.1 times that of a case where the film thicknesses of both are the same was obtained.
As shown in Examples 6 to 10, in a case where the film thickness of the second piezoelectric film 18 having a smaller Pb composition ratio is fixed, a higher piezoelectric constant was obtained in a case where the film thickness of the first piezoelectric film 14 is 1.5 to 2.5 times the film thickness of the second piezoelectric film 18, as compared with a case where the film thicknesses of both are the same.
For Comparative Example 1, Example 1, and Example 6, the voltage dependence of the piezoelectric constant d31 was measured. The results are shown in
As described above, it is clear that Examples 1 to 10 have higher piezoelectric performance in a low voltage region of 10 V or less than a configuration in which two piezoelectric films having the same composition are simply laminated.
Further,
As shown in
Further, as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-158868 | Sep 2022 | JP | national |
