Piezoelectric element, droplet ejection head, ferroelectric memory, and piezoelectric actuator

Abstract

This piezoelectric element is provided with a first electrode, a second electrode, and a piezoelectric film positioned between the first electrode and the second electrode, the piezoelectric element being characterized in that: the first electrode is an electrode to which a relatively positive voltage is applied when the piezoelectric element is driven; the second electrode is an electrode to which a relatively negative voltage is applied when the piezoelectric element is driven; and, a coefficient A obtained from the following logarithmic approximation equation in an aging test in which an electric field of 10 V / μm is applied at an ambient temperature of 80°C is equal to or greater than -4.200×10-2.　Φ2 = A × ln(t) + B Φ2: Schottky barrier height [eV] between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode t: Aging time [h] A: Coefficient B: Coefficient

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric element, a droplet ejection head, a ferroelectric memory, and a piezoelectric actuator. More specifically, the present invention relates to a piezoelectric element or the like whose piezoelectric characteristics are little deteriorated with use. [Background technology]

[0002] It is known that the piezoelectric properties of a piezoelectric element are degraded by a small amount of leakage current that flows when a voltage is applied. The leakage current that causes the degradation of the piezoelectric properties tends to increase as the applied voltage and temperature increase.

[0003] Furthermore, even if the voltage and temperature are the same, this leakage current tends to flow more easily when a voltage is applied continuously or intermittently, and increases over time. This increase in leakage current over time is one of the causes of the deterioration of piezoelectric properties with use.

[0004] As a technology for suppressing the deterioration of piezoelectric characteristics, for example, a technology has been disclosed that improves durability in high humidity and high temperature environments by alleviating the high minute stress that occurs locally in the piezoelectric film when voltage is applied and suppressing stress concentration (see Patent Document 1). Another technique has been disclosed in which a driving system is used to drive a piezoelectric element at a maximum voltage lower than the voltage at which a tunneling current or a Poole-Frenkel current begins to be generated in the piezoelectric element, thereby suppressing the generation of a tunneling current or a Poole-Frenkel current when the piezoelectric element is driven (see Patent Document 2).

[0005] As described above, several techniques for suppressing the deterioration of piezoelectric characteristics have been disclosed. However, further suppression of the deterioration is desired to extend the life of piezoelectric elements, and a solution from a different perspective is required. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-70394 [Patent Document 2] Japanese Patent Application Publication No. 2017-71082 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in consideration of the above problems and circumstances, and its problem to be solved is to provide a piezoelectric element whose piezoelectric characteristics deteriorate little with use, as well as a droplet ejection head, a ferroelectric memory, and a piezoelectric actuator that are equipped with such a piezoelectric element. [Means for solving the problem]

[0008] In order to solve the above problems, the inventors have investigated the causes of the above problems and have found that it is possible to provide a piezoelectric element having a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode, in which the degree of decrease in the Schottky barrier height between the second electrode and the piezoelectric film due to aging under specific conditions is reduced, thereby achieving the present invention. That is, the above-mentioned problems of the present invention are solved by the following means.

[0009] 1. A piezoelectric element comprising a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode, the first electrode is an electrode to which a relatively positive voltage is applied when driven, the second electrode is an electrode to which a relatively negative voltage is applied when driven, and In an aging test in which an electric field of 10 V / μm is applied at an ambient temperature of 80°C, the coefficient A obtained from the following logarithmic approximation formula is -4.200×10 -2 That's all. the law of nature, The material of the piezoelectric film is Pb X (Zr Y ,Ti 1-Y)O 3 Lead zirconate titanate represented by the formula [0.5≦X≦1.5, 0.1≦Y≦0.9], and When the piezoelectric film is divided in half in the thickness direction, the atomic composition ratio X of lead on the first electrode side is X1, and the atomic composition ratio X of lead on the second electrode side is X2. The value of the ratio X1 / X2 is 1.04 or more. A piezoelectric element characterized by: Φ2=A×ln(t)+B Φ2: Schottky barrier height [eV] between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode t: Aging time [h] A: Coefficient B: Coefficient

[0010] 2. The coefficient A is -1.000 x 10 -2 2. The piezoelectric element according to claim 1, characterized in that:

[0011] 3. The crystal structure of the material of the piezoelectric film is a perovskite structure, and 3. The piezoelectric element according to item 1 or 2, wherein the thickness of the piezoelectric film is within a range of 0.1 to 5 μm.

[0012] 4. The piezoelectric film is made of lead zirconate titanate. Section 1 The piezoelectric element according to claim 1.

[0014] 5 A dielectric film is provided on the second electrode side between the second electrode and the piezoelectric film, and 2. The piezoelectric element according to claim 1, wherein the crystal lattice volume of the material of the dielectric film on the second electrode side is smaller than the crystal lattice volume of the material of the piezoelectric film.

[0015] 6 The crystal structure of the material of the dielectric film on the second electrode side is a perovskite structure, and The total thickness of the piezoelectric film and the dielectric film on the second electrode side is in the range of 0.1 to 5 μm. 5 Item 1. The piezoelectric element according to item 1.

[0016] 7 A dielectric film is provided on the first electrode side between the first electrode and the piezoelectric film, and The total thickness of the dielectric film on the second electrode side and the dielectric film on the first electrode side is within a range of 5 to 15% of the total thickness of the dielectric film on the second electrode side, the dielectric film on the first electrode side, and the piezoelectric film. 5 Section or Article 6 Item 1. The piezoelectric element according to item 1.

[0017] 8 The material of the dielectric film on the second electrode side is lead lanthanum titanate. 5 Item 1. The piezoelectric element according to item 1.

[0018] 9 .A piezoelectric element described in item 1, characterized in that before an aging test, the Schottky barrier height Φ2 between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode is 0.5 eV or more.

[0019] 1 0 A droplet ejection head equipped with a piezoelectric element, 2. A droplet ejection head, wherein the piezoelectric element is the piezoelectric element described in item 1.

[0020] 1 1 A ferroelectric memory having a piezoelectric element, 2. A ferroelectric memory, wherein the piezoelectric element is the piezoelectric element described in claim 1.

[0021] 1 2 A piezoelectric actuator having a piezoelectric element, 2. A piezoelectric actuator, wherein the piezoelectric element is the piezoelectric element described in item 1. [Effects of the Invention]

[0022] The above-described means of the present invention can provide a piezoelectric element whose piezoelectric characteristics are less likely to deteriorate with use, as well as a droplet ejection head, a ferroelectric memory, and a piezoelectric actuator each including the piezoelectric element.

[0023] The mechanism by which the effects of the present invention are exhibited or the mechanism of action is assumed to be as follows.

[0024] As mentioned above, the increase in leakage current over time causes the deterioration of piezoelectric properties. This increase in leakage current over time occurs due to continuous or intermittent voltage application, and one of the causes is thought to be a decrease in the Schottky barrier height.

[0025] In the present invention, the inventors have focused on the fact that continuous or intermittent application of voltage to a piezoelectric element reduces the Schottky barrier height between the second electrode and the piezoelectric film, thereby increasing the leakage current, and have found that by suppressing the degree of reduction in the Schottky barrier height due to aging within a specific range, it is possible to suppress the increase in leakage current and the resulting deterioration of the piezoelectric characteristics.

[0026] Specifically, the coefficient A in the logarithmic approximation formula obtained in the aging test under the above conditions is -4.200 × 10 -2 It has been found that by doing so, it is possible to suppress an increase in leakage current and the resulting deterioration of piezoelectric characteristics.

[0027] Here, the coefficient A is an index of the degree of suppression of the reduction in the Schottky barrier height. The coefficient A is a value equal to or less than 0, and the larger the value, i.e., the closer the value is to 0, the smaller the degree of reduction in the Schottky barrier height.

[0028] These mechanisms of expression or action make it possible to provide a piezoelectric element whose piezoelectric properties are less likely to deteriorate with use. [Brief explanation of the drawings]

[0029] [Figure 1] Schematic cross-sectional view of an example of the configuration of a piezoelectric element [Figure 2] Schematic diagram showing the change in band alignment due to lead diffusion and charge injection [Figure 3] Graph plotting aging time t and Schottky barrier height Φ2 in the example [Figure 4] Graph plotting In(E) and In(J / T2) for Example (Droplet Discharge Head No. 1) [Figure 5] Graph showing changes in PE loop with aging in Example (droplet ejection head No. 1) [Figure 6] Graph showing changes in PE loop with aging in Example (droplet ejection head No. 2) [Figure 7] Graph showing the change in coercive electric field with aging in an example (droplet ejection head No. 1) [Figure 8] Graph showing the change in coercive electric field with aging in an example (droplet ejection head No. 2) [Figure 9] Graph showing the change in remanent polarization with aging in an example (droplet ejection head No. 1) [Figure 10] Graph showing the change in remanent polarization with aging in an example (droplet ejection head No. 2) DETAILED DESCRIPTION OF THE INVENTION

[0030] The piezoelectric element of the present invention is a piezoelectric element comprising a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode, wherein the first electrode is an electrode to which a relatively positive voltage is applied when driven, and the second electrode is an electrode to which a relatively negative voltage is applied when driven, and wherein a coefficient A obtained from the following logarithmic approximation formula in an aging test in which an electric field of 10 V / μm is applied at an ambient temperature of 80° C. is −4.200×10 -2 The present invention is characterized in that: This feature is a technical feature common to or corresponding to the following embodiments.

[0031] In an embodiment of the piezoelectric element of the present invention, the coefficient A is -1.000 × 10 -2 This makes it possible to further reduce the deterioration of the piezoelectric properties that occurs with use.

[0032] In a preferred embodiment of the piezoelectric element of the present invention, the crystal structure of the material of the piezoelectric film is a perovskite structure, and the thickness of the piezoelectric film is preferably within a range of 0.1 to 5 μm, thereby achieving the displacement generating force required for the piezoelectric element.

[0033] In an embodiment of the piezoelectric element of the present invention, the material of the piezoelectric film is preferably lead zirconate titanate, which makes it possible to form a piezoelectric element with good performance.

[0034] In an embodiment of the piezoelectric element of the present invention, the material of the piezoelectric film is Pb X (Zr Y ,Ti 1-Y )O3 [0.5≦X≦1.5, 0.1≦Y≦0.9], and when the piezoelectric film is divided in half in the thickness direction, the atomic composition ratio X of lead on the first electrode side is X1, and the atomic composition ratio X of lead on the second electrode side is X2, the ratio X1 / X2 is preferably 1.04 or more. This makes it possible to further suppress the reduction in Schottky barrier height with use.

[0035] In an embodiment of the piezoelectric element of the present invention, a dielectric film on the second electrode side is preferably provided between the second electrode and the piezoelectric film, and the crystal lattice volume of the material of the dielectric film on the second electrode side is preferably smaller than the crystal lattice volume of the material of the piezoelectric film, thereby increasing the band gap and thereby increasing the Schottky barrier height at the interface.

[0036] In an embodiment of the piezoelectric element of the present invention, it is preferable that the crystal structure of the material of the dielectric film on the second electrode side is a perovskite structure, and the total thickness of the piezoelectric film and the dielectric film on the second electrode side is within a range of 0.1 to 5 μm, thereby obtaining the displacement generating force required for the piezoelectric element.

[0037] In an embodiment of the piezoelectric element of the present invention, a first electrode-side dielectric film is preferably provided between the first electrode and the piezoelectric film, and the total thickness of the second electrode-side dielectric film and the first electrode-side dielectric film is preferably within a range of 5 to 15% of the total thickness of the second electrode-side dielectric film, the first electrode-side dielectric film, and the piezoelectric film. This minimizes the decrease in the effective piezoelectric constant of the piezoelectric element due to the insertion of the dielectric, and enables the formation of a Schottky junction.

[0038] In an embodiment of the piezoelectric element of the present invention, the material of the dielectric film on the second electrode side is preferably lead lanthanum titanate, which makes it possible to form a Schottky junction and suppress leakage current.

[0039] In an embodiment of the piezoelectric element of the present invention, the Schottky barrier height Φ2 between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode is preferably 0.5 eV or more, thereby further suppressing leakage current.

[0040] The droplet ejection head, ferroelectric memory, and piezoelectric actuator of the present invention are characterized by including the piezoelectric element of the present invention.

[0041] The present invention, its components, and embodiments for carrying out the present invention will be described in detail below. In this application, the symbol "to" is used to mean that the numerical values ​​before and after it are included as lower and upper limits.

[0042] <1. Overview of the Piezoelectric Element of the Present Invention> The piezoelectric element of the present invention is a piezoelectric element comprising a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode, wherein the first electrode is an electrode to which a relatively positive voltage is applied when driven, and the second electrode is an electrode to which a relatively negative voltage is applied when driven, and wherein a coefficient A obtained from the following logarithmic approximation formula in an aging test in which an electric field of 10 V / μm is applied at an ambient temperature of 80° C. is −4.200×10 -2The present invention is characterized in that:

[0043] Φ2=A×ln(t)+B Φ2: Schottky barrier height [eV] between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode t: Aging time [h] A: Coefficient B: Coefficient

[0044] <2. Piezoelectric element configuration> A schematic cross-sectional view of an example of the configuration of a piezoelectric element of the present invention is shown in Fig. 1. The piezoelectric element of the present invention is characterized by comprising a first electrode 10, a second electrode 50, and a piezoelectric film 30 located between the first electrode 10 and the second electrode 50. It is also preferable to provide a dielectric film 40 on the second electrode side between the second electrode 50 and the piezoelectric film 30, and it is more preferable to provide a dielectric film 20 on the first electrode side between the first electrode 10 and the piezoelectric film 30.

[0045] Each component will be described in detail below.

[0046] <2.1 Electrode> A relatively positive voltage is applied to the first electrode 10 when the device is driven. There are no particular limitations on the material of the first electrode 10, and Cr, Ni, Cu, Pt, Ir, Ti, an Ir-Ti alloy, LaNiO3, SrRuO3, etc. can be used. The thickness of the first electrode 10 is preferably within the range of 0.1 to 1 μm.

[0047] A relatively negative voltage is applied to the second electrode 50 when the device is driven. There are no particular restrictions on the material of the second electrode 50, and Cr, Ni, Cu, Pt, Ir, Ti, an Ir-Ti alloy, LaNiO3, SrRuO3, etc. can be used. The thickness of the second electrode 50 is preferably within a range of 0.1 to 5 μm.

[0048] 2.2 Piezoelectric film The term "piezoelectric film" refers to a film formed of a piezoelectric material. The piezoelectric material that is the material for the piezoelectric film 30 according to the present invention preferably has a perovskite crystal structure. The term "perovskite structure" refers to a crystal structure similar to perovskite (perovskite, CaTiO). The composition of a perovskite crystal structure is usually expressed as ABX, where A, B, and X exist as constituent ions of A cations, B cations, and X anions, respectively. In the present invention, B cation-deficient perovskite compounds, A cation-deficient perovskite compounds, and X anion-deficient perovskite compounds are also defined as compounds having a perovskite crystal structure.

[0049] Examples of piezoelectric materials with a perovskite crystal structure include lead zirconate titanate (PZT: Pb(Zr,Ti)O3), lead titanate (PbTiO3), lead zirconate (PbZrO3), lead lanthanum titanate (PLT: (Pb,La)TiO3), and barium titanate (BaTiO3). X (Zr Y ,Ti 1-Y Lead zirconate titanate represented by the formula: 0.5≦X≦1.5, 0.1≦Y≦0.9 is particularly preferred.

[0050] The lead zirconate titanate preferably has a non-stoichiometric composition. X (Zr Y ,Ti 1-Y )O3 [0.5≦X≦1.5, 0.1≦Y≦0.9], it is preferable that X>1.

[0051] In addition, the composition is Pb X (Zr Y ,Ti 1-Y)O3 [0.5≦X≦1.5, 0.1≦Y≦0.9], where X is the atomic composition ratio of lead on the first electrode side when the piezoelectric film 30 is divided in half in the thickness direction, and X is the atomic composition ratio of lead on the second electrode side when the piezoelectric film 30 is divided in half in the thickness direction. The ratio X1 / X2 is preferably 1.04 or greater, and more preferably 1.11 or greater. This further suppresses the reduction in Schottky barrier height with use. Furthermore, in order to suppress the reduction in Schottky barrier height, X2 is preferably 1.2 or less, and the ratio X1 / X2 is preferably 1.14 or less.

[0052] The composition of the piezoelectric film 30 can be analyzed by examining the composition in the depth direction of the piezoelectric film by alternately performing Auger electron spectroscopy and ion sputtering.

[0053] The atomic composition ratio X of lead can be adjusted by adjusting the oxygen partial pressure of the sputtering gas when depositing the piezoelectric film 30. As is conventionally known, the atomic composition ratio X of lead can be increased by setting the oxygen partial pressure of the sputtering gas high, and the atomic composition ratio X of lead can be decreased by setting the oxygen partial pressure of the sputtering gas low. Furthermore, Y originates from the sputtering target and does not change throughout the piezoelectric film.

[0054] When the piezoelectric film 30 is divided in half in the thickness direction, if the atomic composition ratio X of lead is to be changed between the piezoelectric film on the first electrode side and the piezoelectric film on the second electrode side, the oxygen partial pressure of the sputtering gas can be changed when depositing one half.

[0055] In addition, from the viewpoint of the dielectric constant and the magnitude of the piezoelectric constant, the composition is Pb X (Zr Y ,Ti 1-Y When expressed as 0.5≦X≦1.5, 0.1≦Y≦0.9, Y is preferably in the range of 0.50 to 0.58, and particularly preferably 0.52.

[0056] The relationship between the value of the ratio X1 / X2 and the decrease in Schottky barrier height with use will be explained. When depositing a piezoelectric film using a lead-containing material such as PZT, excess lead is added during film deposition due to the high volatility of lead. This excess lead, which is greater than the stoichiometric composition, exists in the film with a positive charge and is called a lead defect. Because these lead defects have a positive charge, applying a positive voltage to the first electrode promotes their diffusion toward the second electrode. The lead defects that diffuse toward the second electrode lower the Schottky barrier height between the second electrode and the piezoelectric film. Therefore, if the lead composition ratios X1 and X2 are adjusted so that the ratio X1 / X2 is 1.04 or more, that is, so that the lead content is higher on the first electrode side, when a positive voltage is applied to the first electrode, the time it takes for lead defects to diffuse to the interface with the second electrode is lengthened, thereby suppressing a decrease in the Schottky barrier height.

[0057] Figure 2 is a schematic diagram showing the change in band alignment due to the diffusion of lead defects and charge injection when the piezoelectric material is PZT. Positively charged lead defects originating from excess lead and segregating on the first electrode side diffuse to the second electrode side when voltage is applied, reaching the interface between the second electrode and the piezoelectric film and lowering the Schottky barrier height between the second electrode and the piezoelectric film. Meanwhile, charge injection at the interface between the first electrode and the piezoelectric film also lowers the Schottky barrier height between the first electrode and the piezoelectric film.

[0058] In addition, differences in the ratio X1 / X2 result in differences in the way the coercive field and remanent polarization change with use. When a positive voltage is applied to the first electrode, lead defects diffuse toward the second electrode, reducing the internal bias. As a result, the coercive field shifts toward the positive side, and the remanent polarization increases due to the relaxation of pinning. After that, an inflection point is reached, the coercive field shifts toward the negative side, the remanent polarization decreases, and pinning progresses. This is thought to be due to charge injection at the interface between the first electrode and the piezoelectric film. The larger the ratio X1 / X2, that is, the greater the degree of segregation of lead defects toward the first electrode, the later the inflection point is reached. Therefore, the larger the ratio X1 / X2, the more effectively the piezoelectric properties are suppressed from deteriorating.

[0059] The thickness of the piezoelectric film 30 is preferably within the range of 0.1 to 5 μm, and more preferably within the range of 2.0 to 3.5 μm, which allows the piezoelectric element to obtain the displacement generating force required.

[0060] 2.3 Dielectric film on the second electrode side The "dielectric film on the second electrode side" refers to a film formed of a dielectric between the second electrode and the piezoelectric film. In the present invention, the crystal lattice volume of the material of the dielectric film 40 on the second electrode side is preferably smaller than the crystal lattice volume of the material of the piezoelectric film. This increases the band gap, thereby increasing the Schottky barrier height at the interface.

[0061] The crystal lattice volume can be measured by X-ray diffraction (XRD). By out-of-plane 2θ-ω scan and in-plane 2θ-ω scan, The interplanar spacing of the (001) plane and the interplanar spacing of the (100) plane of the crystal can be determined as c and a, respectively, and the crystal lattice volume can be calculated as a × a × c.

[0062] The dielectric film 40 on the second electrode side preferably has a perovskite crystal structure. Examples of piezoelectric materials with a perovskite crystal structure include lead titanate (PbTiO3), lead lanthanum titanate (PLT:(Pb,La)TiO3), and barium titanate (BaTiO3). Among these, those containing lead are preferred, and lead lanthanum titanate is particularly preferred. This allows a Schottky barrier to be formed at the interface with PZT due to the difference in crystal lattice capacitance.

[0063] In the piezoelectric element of the present invention, the sum of the thickness of the piezoelectric film 30 and the thickness of the dielectric film 40 on the second electrode side is preferably within the range of 0.1 to 5 μm, which allows the piezoelectric element to obtain the displacement generating force required.

[0064] 2.4 Dielectric film on the first electrode side The "dielectric film on the first electrode side" refers to a film made of a dielectric material and formed between the first electrode and the piezoelectric film. In the present invention, the dielectric film 20 on the first electrode side can be the same as the dielectric film 40 on the second electrode side described above.

[0065] The total thickness of the dielectric film on the second electrode side and the dielectric film on the first electrode side is preferably within a range of 5 to 15% of the total thickness of the dielectric film on the second electrode side, the dielectric film on the first electrode side, and the piezoelectric film, which minimizes the decrease in the effective piezoelectric constant of the piezoelectric element due to the insertion of the dielectric and enables the formation of a Schottky junction.

[0066] <3 Coefficient A> In an aging test in which an electric field of 10 V / μm is applied at an ambient temperature of 80° C., the coefficient A obtained from the following logarithmic approximation formula is −4.200×10 -2 The present invention is characterized in that:

[0067] Φ2=A×ln(t)+B Φ2: Schottky barrier height [eV] between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode t: Aging time [h] A: Coefficient B: Coefficient

[0068] The above logarithmic approximation formula can be expressed as a graph, for example, as shown in Figure 3. Figure 3 is a graph of an example described later. Because the change in the early stage of aging is important, the aging time t is set to a maximum of 20 hours.

[0069] The coefficient A is an index of the degree of suppression of the reduction in Schottky barrier height. The coefficient A is a value less than 0, and the larger the value, i.e., the closer to 0, the smaller the degree of reduction in Schottky barrier height due to aging.

[0070] In the present invention, the coefficient A is −4.200×10 -2 Furthermore, from the viewpoint of the effects of the present invention, it is characterized in that the value is −1.000×10 -2It is preferable that this is equal to or greater than this.

[0071] Furthermore, in the piezoelectric element of the present invention, it is preferable that the Schottky barrier height Φ2 between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode before the aging test is 0.5 eV or more, thereby further suppressing leakage current.

[0072] <4. Schottky barrier height measurement method> In the present invention, the Schottky barrier height Φ2 between the second electrode and the piezoelectric film and the Schottky barrier height Φ1 between the first electrode and the piezoelectric film are included. S That's what they say. Hereinafter, the Schottky barrier height Φ S The measurement method is the same whether the measurement is performed independently of the aging test or during the aging test.

[0073] In the present invention, the Schottky barrier height Φ S [eV] is the leakage current density J [A / cm 2 ] at a given temperature T [K] using the following method. 2 ], the horizontal axis is ln(1000 / T) and the vertical axis is ln(J / T 2 ) is created, and the slope α is used to calculate the temperature T [K] using the following formula. Note that at least four predetermined temperatures T [K] are required to create an Arrhenius plot. α=qΦ S / k B α: Slope of the Arrhenius plot Φ S : Schottky barrier height [eV] q: Charge [C] k B :Boltzmann constant [J / K]

[0074] Hereafter, leakage current density J [A / cm 2 The measurement method for [] will be explained.

[0075] The piezoelectric element is placed in an electric furnace, sealed, and dry air is introduced. The furnace is then heated and the ambient temperature is adjusted to a predetermined temperature T [K]. To avoid the effects of temperature fluctuations, a waiting period is set (for example, about 45 minutes until the temperature stabilizes). The ambient temperature is measured using a thermocouple thermometer installed near the piezoelectric element.

[0076] With the ambient temperature adjusted to a predetermined temperature T [K], an electric field is applied between the first and second electrodes, gradually increasing from 0 V / μm to 12.86 V / μm, and the current density of the leakage current that flows when the electric field is 12.86 V / μm is measured. Here, in order to obtain the net leakage current excluding the influence of the polarization reversal current, the voltage is swept twice in the same direction, and the current density at the second sweep is defined as the leakage current density J [A / cm 2 ] will be treated as

[0077] The electric field is applied such that the second electrode is grounded, and when measuring the Schottky barrier height Φ1 between the first electrode and the piezoelectric film, the first electrode side is at a positive potential, and when measuring the Schottky barrier height Φ2 between the second electrode and the piezoelectric film, the first electrode side is at a negative potential. The leakage current density is measured using, for example, a semiconductor parameter analyzer (Agilent B1500A).

[0078] The leakage current density at each temperature is measured at an electric field of 12.86 V / μm in order to measure in an electric field where Schottky emission current is dominant. Figure 4 shows the measurement results for droplet discharge head No. 1 of an example described later, plotted on the horizontal axis as In(E) and on the vertical axis as In(J / T) according to the theoretical formula for Schottky emission current (formula (1) below). 2 ) is plotted as In(E) and In(J / T). This graph shows the results of measurements at temperatures of 40°C, 52°C, 65°C, and 80°C, and the symbol E represents the electric field [V / μm]. 2) plotted, the electric field region where the line connecting the plotted points is linear is the electric field region where the Schottky emission current is considered to be predominantly flowing. In the case of Figure 4, the electric field region where the Schottky emission current is considered to be predominantly flowing at any temperature is 7.714 V / μm or higher. Furthermore, when similar confirmation was performed for other samples, a linear plot was obtained at an electric field of 12.68 V / μm, so in the present invention, the leakage current density used to evaluate the Schottky barrier height was an electric field of 12.68 V / μm.

[0079]

number

[0080] J: Leakage current density [A / cm 2 ] A: any constant T: Predetermined temperature [K] q: Charge [C] Φ S : Schottky barrier height [eV] ε: permittivity [F / m] E: Electric field [V / μm] k B :Boltzmann constant [J / K]

[0081] In the present application, temperature [K] and temperature [°C] can be converted using the following formula. Temperature [K: Kelvin] = Temperature [℃: Celsius] + 273.15

[0082] <5. Aging test> In the present invention, the aging test of the piezoelectric element is carried out by applying an electric field of 10 V / μm for a predetermined period of time while heating the element so that the ambient temperature reaches 80° C. The aging test will be described in detail below.

[0083] First, the piezoelectric element is placed in an electric furnace and sealed. Dry air is poured into the electric furnace and the furnace is left to stand by until the dew point drops below -50°C. The electric furnace is then heated and the ambient temperature is adjusted to 80°C. Here, to avoid the effects of temperature fluctuations, a certain waiting time is provided (for example, about 45 minutes until the temperature stabilizes). The ambient temperature is measured using a thermocouple thermometer installed near the piezoelectric element. When the ambient temperature reaches 80°C, the second electrode is grounded and a positive electric field of 10 V / μm is applied to the first electrode. The electric field is applied using, for example, a DC stabilized power supply (KX-100L). Aging is performed by continuing to apply a 10 V / μm electric field while maintaining the ambient temperature at 80°C.

[0084] The aging time t, which is the time elapsed from the start of application, is set to a maximum of 20 hours. The Schottky barrier height Φ2 between the second electrode and the piezoelectric film is measured several times during the aging process, enough to identify the trend in changes associated with aging. It is preferable to perform this measurement at least four times by changing the aging time t. For example, measurements are taken at aging times t of 2, 5, 10, and 20 hours.

[0085] <6. Applications of piezoelectric elements> The droplet ejection head, ferroelectric memory, and piezoelectric actuator of the present invention are characterized by including the piezoelectric element of the present invention. Since the piezoelectric element of the present invention exhibits little deterioration in piezoelectric properties with use, the droplet ejection head, ferroelectric memory, and piezoelectric actuator including the piezoelectric element can be used stably for a long period of time.

[0086] As long as the droplet ejection head, ferroelectric memory, and piezoelectric actuator of the present invention are equipped with the piezoelectric element of the present invention, other configurations are not particularly limited, and they can be constructed using commonly used materials.

[0087] In addition to the above, the piezoelectric element of the present invention can be used in, for example, piezoelectric microphones, vibration sensors, displacement sensors, ultrasonic detectors, oscillation circuits, resonators, ceramic filters, piezoelectric transformers, piezoelectric buzzers, ultrasonic motors, etc. [Example]

[0088] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these. In the examples, the terms "parts" and "%" are used, but unless otherwise specified, they represent "parts by mass" or "% by mass."

[0089] In the examples, a droplet discharge head equipped with the piezoelectric element of the present invention was produced, and the droplet discharge head was used to evaluate the deterioration of piezoelectric characteristics with use.

[0090] <Fabrication of droplet ejection head No. 1> The first electrode was formed on an 8-inch bare-Si wafer using an Ir-Ti alloy target. The film was formed by F magnetron sputtering under the following conditions: thickness and sputtering conditions. Thickness: 0.12 μm RF power supply...0.75kW Gas flow rate: Ar:O2=38:2sccm Sputtering pressure: 0.2 Pa Substrate temperature setting: 350°C

[0091] Next, a dielectric film on the first electrode side was formed on the first electrode using lead lanthanum titanate (PLT:(Pb 1.125 ,La 0.1 The film was formed by RF magnetron sputtering using a TiO3 ceramic target. The thickness and sputtering conditions were as follows: Thickness: 0.12 μm RF power supply...2.0kW Gas flow rate: Ar:O2=39.5:0.5sccm Sputtering pressure: 0.2 Pa Substrate temperature setting: 560°C

[0092] The lattice spacing and crystal lattice volume of the lead lanthanum titanate in the dielectric film on the first electrode side, measured by X-ray diffraction, are as follows: The interplanar spacing of the (100) plane is 3.95 Å. Planar spacing c of (001) plane: 3.94 Å Crystal lattice volume a×a×c 61.47 Å 3

[0093] Next, a piezoelectric film was formed on the dielectric film on the first electrode side using lead zirconate titanate (PZT:Pb 1.25 (Zr 0.52 ,Ti 0. 48 )O3) It was formed by RF magnetron sputtering using a ceramic target. The thickness and sputtering conditions are as follows. Thickness: 3.26 μm RF power supply...3.0kW Gas flow rate: Ar:O2=39.5:0.5sccm Sputtering pressure: 0.2 Pa Substrate temperature setting: 550°C

[0094] In the initial film formation layer on the first electrode side, the oxygen partial pressure was increased during film formation to adjust the lead content.

[0095] When the piezoelectric film is divided in half in the thickness direction, the composition of the piezoelectric film on the first electrode side is Pb X1 (Zr Y ,Ti 1-Y ) When expressed in O3, X1 was 1.16 and Y was 0.52. When the piezoelectric film is divided in half in the thickness direction, the composition of the piezoelectric film on the second electrode side is Pb X2 (Zr Y ,Ti 1-Y ) When expressed in O3, X2 was 1.08 and Y was 0.52.

[0096] The composition of the piezoelectric film was analyzed by examining the composition in the depth direction of the piezoelectric film by alternately performing Auger electron spectroscopy and ion sputtering.

[0097] The lattice spacing and crystal lattice volume of the lead zirconate titanate of the piezoelectric film measured by X-ray diffraction are as follows: The interplanar spacing of the (100) plane is 4.09 Å. The interplanar spacing of the (001) plane is c 4.08 Å Crystal lattice volume a×a×c 68.25 Å 3

[0098] Next, a dielectric film on the second electrode side was formed on the piezoelectric film using lead lanthanum titanate (PLT:(Pb 1.125 ,La 0.1 The film was formed by RF magnetron sputtering using a TiO3 ceramic target. The thickness and sputtering conditions were as follows: Thickness: 0.12 μm RF power supply...2.0kW Gas flow rate: Ar:O2=39.5:0.5sccm Sputtering pressure: 0.2 Pa Substrate temperature setting: 560°C

[0099] The lattice spacing and crystal lattice volume of the lead lanthanum titanate in the dielectric film on the second electrode side, measured by X-ray diffraction, are as follows: The interplanar spacing of the (100) plane is 3.95 Å. Planar spacing c of (001) plane: 3.94 Å Crystal lattice volume a×a×c 61.47 Å 3

[0100] Next, a second electrode was formed on the dielectric film on the second electrode side by sputtering using a Cu target. The thickness and sputtering conditions were as follows. The second electrode also served as a vibration plate in the piezoelectric actuator. Thickness: 2.8 μm DC power supply...1kW Gas flow rate: Ar=50sccm Sputtering pressure: 0.15 Pa Board temperature setting: Room temperature

[0101] Next, a photosensitive polyimide resin was applied onto the second electrode by spin coating and cured by baking at 230°C to form an ink-blocking film of 1 μm.

[0102] Next, a 0.5 μm seed layer was formed on the ink-blocking film by sputtering using a Ni target in argon gas at a high-frequency power of 500 W and a gas pressure of 1 Pa for 15 minutes.

[0103] Next, a pressure chamber with a height of 150 μm was formed by laminating two layers of 80 μm-thick dry film resist (ORDYL MP108 manufactured by Tokyo Ohka Kogyo Co., Ltd.), and then depositing a pressure chamber material made of Ni using Ni electroforming. The dry film resist layer was then removed, and the substrate was washed and dried.

[0104] Next, an 8-inch glass support substrate was attached to the pressure chamber using a double-sided thermal release sheet manufactured by Nitto Denko Corporation.

[0105] Next, the Si substrate was ground to a thickness of about 50 μm, and then completely removed by dry etching using SF6.

[0106] Next, a Tokyo Ohka OMR resist was applied to the first electrode, and the mask pattern was transferred by exposure and developed to form a resist mask. Next, the first electrode in the area where the resist mask was not formed was removed by dry etching using a mixed gas of argon, oxygen, and CHF3. After rinsing, the resist mask was removed using a remover.

[0107] Next, a Tokyo Ohka OMR resist was applied, and the mask pattern was transferred by exposure and developed to form a resist mask. Next, the dielectric film and piezoelectric film in the areas where the resist mask was not formed were removed by dry etching using a mixed gas of chlorine and bromine. After cleaning, the resist mask was stripped using a stripping solution.

[0108] Next, a 1 μm protective film was formed by spin-coating a photosensitive polyimide resin and then patterning it. The patterning was performed by transferring a mask pattern by exposure and developing it. After patterning, the film was cured by baking at 210°C.

[0109] Next, the support substrate was heated to a temperature above which the thermal release sheet foamed, and the support substrate was removed, thereby obtaining piezoelectric actuator No. 1.

[0110] Next, an ink flow path member and a nozzle plate were bonded to the piezoelectric actuator No. 1 with an adhesive, to obtain the droplet ejection head No. 1.

[0111] <Fabrication of droplet ejection head No. 2> Droplet discharge head No. 2 was fabricated in the same manner as droplet discharge head No. 1, except that X1 and X2 of the piezoelectric film were adjusted as shown in Table I.

[0112] <Fabrication of droplet ejection head No. 3> Droplet discharge head No. 3 was fabricated in the same manner as droplet discharge head No. 1, except that X1 and X2 of the piezoelectric film were adjusted as shown in Table I.

[0113] <Fabrication of droplet ejection head No. 4> Droplet ejection head No. 4 was fabricated in the same manner as droplet ejection head No. 1, except that instead of forming a dielectric film on the second electrode side, the thicknesses of the piezoelectric film and the dielectric film on the first electrode side were set to 3.50 μm (piezoelectric film: 3.38 μm, dielectric film on the first electrode side: 0.12 μm), and further, X1 and X2 of the piezoelectric film were adjusted as shown in Table I.

[0114] <Measurement of Schottky barrier height and coefficient A> The Schottky barrier heights Φ1 and Φ2 were measured before the aging test and after each aging time t for droplet discharge heads No. 1 to 4. The measurement results are shown in Table I.

[0115] The aging test method and Schottky barrier height measurement method were as described above. Since the inter-electrode distance in droplet discharge heads No. 1 to 4 was 3.50 μm, the absolute value of the applied voltage was set to 35 V, resulting in an applied electric field of 10.00 V / μm, in the aging test. Furthermore, the absolute value of the applied voltage was set to 45 V, resulting in an applied electric field of 12.86 V / μm, in the Schottky barrier height measurement.

[0116] The aging time t was set to four points: 2h, 5h, 10h, and 20h. The predetermined temperature T in the measurement of the Schottky barrier height Φ2 at the heating time t is the set temperature of the electric furnace. The temperature was set to five points: 24°C, 40°C, 52°C, 65°C, and 80°C, and the temperature measured by the thermocouple was used as the ambient temperature for evaluation.

[0117] A graph plotting the Schottky barrier height Φ2 at each aging time t is shown below. Shown in Figure 3.

[0118] The coefficient A of droplet discharge heads No. 1 to 4 is calculated as the Schottky barrier height at each aging time t. The coefficient A was derived from the logarithmic approximation formula above using the length Φ2. The derived values ​​of coefficient A are shown in Table I.

[0119] <Measurement of injection speed reduction rate> The durability of droplet discharge heads Nos. 1 to 4 was evaluated by the rate of decrease in ejection speed determined by the following method.

[0120] The injection speed [m / s] of droplet discharge heads No. 1 to 4 before and after long-term injection was measured using a drop watcher, and the injection speed reduction rate [%] was calculated using the following formula. The calculated injection speed reduction rate [%] values ​​are shown in Table I. Injection speed reduction rate [%]=[(S1-S2) / S1]×100 S1: Injection speed before long-term injection [m / s] S2: Injection speed after long-term injection [m / s]

[0121] The long-term injection period was 1500 hours. In Table I, "at room temperature" refers to a test where long-term injection was carried out at room temperature, and "at high temperature" refers to a test where long-term injection was carried out at 50°C. A square wave was used as the waveform during injection, and a positive voltage of 30V was applied to the first electrode with the second electrode grounded.

[0122] [Table 1]

[0123] Droplet ejection head No. 3 became unable to eject before 1500 hours had elapsed, so the ejection speed reduction rate [%] could not be measured.

[0124] The evaluation results shown in Table I demonstrate that the droplet ejection head equipped with the piezoelectric element of the present invention has high durability for long-term use. The results also demonstrate that the piezoelectric characteristics of the piezoelectric element of the present invention are less susceptible to deterioration with use. In Table I, "present invention" in the "remarks" column for "droplet ejection head No. 2" should be read as "reference example."

[0125] <Coercive field and remanent polarization> Before aging and after 20 hours of aging, the electric field E [V / μm] was swept at a frequency of 1 kHz to measure the polarization P [μC / cm 2 The PE characteristics were measured by measuring the PE curve. The PE characteristics of droplet discharge head No. 1 are shown in Figure 5, and the PE characteristics of droplet discharge head No. 2 are shown in Figure 6. The electric field on the horizontal axis is the electric field when a positive voltage is applied to the first electrode side. The polarization on the vertical axis is positive when a positive charge accumulates on the first electrode. Both heads No. 1 and No. 2 have PE loops that show PE characteristics due to aging. It can be seen that the value has shifted in the negative direction of the horizontal axis. Furthermore, the rectangularity of the PE loop after aging has improved. Comparing the two droplet ejection heads, droplet ejection head No. 2 has a larger shift in the negative direction than droplet ejection head No. 1.

[0126] Figure 7 shows the change in the coercive field with aging measured for droplet ejection head No. 1, and Figure 8 shows the change in the coercive field with aging measured for droplet ejection head No. 2. The coercive field was measured for three different samples, and the plots represent the average, with error bars indicating the range of variation in the values. Of the two coercive fields obtained due to differences in the application direction, Vc+ indicates the larger coercive field, and Vc- indicates the smaller coercive field. For both droplet ejection head No. 1 and droplet ejection head No. 2, the coercive field shifts once to the positive side and then to the negative side. The inflection point for droplet ejection head No. 1 is near 5 hours, while the inflection point for droplet ejection head No. 2 is near 2 hours.

[0127] Figure 9 shows the change in remanent polarization with aging measured for droplet ejection head No. 1, and Figure 10 shows the change in remanent polarization with aging measured for droplet ejection head No. 2. Remanent polarization was measured for three different samples, and the plots represent the average, with error bars indicating the range of variation. Of the two remanent polarizations resulting from different application directions, Pr+ ​​refers to the larger remanent polarization, and Pr- refers to the smaller remanent polarization. Pr is the difference between Pr+ and Pr- divided by 2, and this value is used as the remanent polarization of the sample. Comparing Pr, similar to the coercive field, Pr increases once with aging time for both droplet ejection heads No. 1 and No. 2, then begins to decrease. Similar to the coercive field, the inflection points occur around 5 hours and 2 hours for droplet ejection heads No. 1 and No. 2, respectively.

[0128] Comparing the coercive electric fields before aging, it can be seen that droplet ejection head No. 1 has a larger shift in the negative direction than droplet ejection head No. 2. Compared to droplet ejection head No. 1, droplet ejection head No. 2 was manufactured to have a greater amount of lead overall, so the degree of segregation of internal lead defects is relatively greater in droplet ejection head No. 1. Therefore, the correlation between the degree of segregation of lead defects and the internal bias indicates that excess lead has a positive charge and is present within the film.

[0129] As aging progresses, the electric field from the first electrode to the second electrode during aging causes the diffusion of positively charged lead defects, reducing the internal bias. As a result, the coercive field shifts to the positive side, and the residual polarization increases due to the relaxation of pinning. Droplet ejection head No. The reason why droplet ejection head No. 1 reaches the inflection point later than droplet ejection head No. 2 is because droplet ejection head No. 1 has a larger ratio X1 / X2, i.e., the degree of lead defect segregation toward the first electrode side is greater.

[0130] After the inflection point, the coercive field shifts to the negative side. This is thought to be due to charge injection into the interface between the first electrode and the piezoelectric film, and the remanent polarization also decreases, indicating that pinning is progressing. [Industrial Applicability]

[0131] The present invention can be used for a piezoelectric element whose piezoelectric characteristics are less likely to deteriorate with use, as well as for a droplet ejection head, a ferroelectric memory, and a piezoelectric actuator that include the piezoelectric element. [Explanation of symbols]

[0132] 10 1st electrode 20 Dielectric film on the first electrode side 30 Piezoelectric film 40 Dielectric film on the second electrode side 50 2nd electrode

Claims

1. A piezoelectric element including a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode, the first electrode is an electrode to which a relatively positive voltage is applied when driven, the second electrode is an electrode to which a relatively negative voltage is applied when driven, and In an aging test in which an electric field of 10 V / μm is applied at an ambient temperature of 80° C., the coefficient A obtained from the following logarithmic approximation formula is −4.200×10 －２ That's all, the material of the piezoelectric film as a whole is lead zirconate titanate expressed as Pb x (Zr y , Ti 1-y )O 3 [0.5≦X≦1.5, 0.1≦Y≦0.9], and a piezoelectric element characterized in that, when the piezoelectric film is divided in half in the thickness direction, the atomic composition ratio X of lead on the first electrode side is X1, and the atomic composition ratio X of lead on the second electrode side is X2, the value of the ratio X1 / X2 is 1.04 or more. F ２ =A×･n(t)+s Φ ２ : Schottky barrier height [eV] between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode t: aging time [h] A: Coefficient B: Coefficient

2. The coefficient A is −1.000×10 －２ 2. The piezoelectric element according to claim 1, wherein the piezoelectric element is a piezoelectric element having the above structure.

3. The crystal structure of the material of the piezoelectric film is a perovskite structure, and 3. The piezoelectric element according to claim 1, wherein the thickness of the piezoelectric film is within a range of 0.1 to 5 μm.

4. 2. The piezoelectric element according to claim 1, wherein the material of the piezoelectric film is lead zirconate titanate.

5. a dielectric film on the second electrode side between the second electrode and the piezoelectric film; and 2. The piezoelectric element according to claim 1, wherein the crystal lattice volume of the material of the dielectric film on the second electrode side is smaller than the crystal lattice volume of the material of the piezoelectric film.

6. The crystal structure of the material of the dielectric film on the second electrode side is a perovskite structure, and 6. The piezoelectric element according to claim 5, wherein the total thickness of the piezoelectric film and the dielectric film on the second electrode side is within a range of 0.1 to 5 μm.

7. a dielectric film on the first electrode side between the first electrode and the piezoelectric film; and The piezoelectric element according to claim 5 or 6, characterized in that the total thickness of the dielectric film on the second electrode side and the dielectric film on the first electrode side is within a range of 5 to 15% of the total thickness of the dielectric film on the second electrode side, the dielectric film on the first electrode side, and the piezoelectric film.

8. 6. The piezoelectric element according to claim 5, wherein the material of the dielectric film on the second electrode side is lead lanthanum titanate.

9. Before the aging test, the Schottky barrier height Φ between the second electrode and the piezoelectric film when a positive electric field of 12.68 V / μm is applied to the first electrode ２ 2. The piezoelectric element according to claim 1, wherein the .lambda.

10. A droplet ejection head equipped with a piezoelectric element, A droplet ejection head, wherein the piezoelectric element is the piezoelectric element according to claim 1.

11. A ferroelectric memory including a piezoelectric element, 2. A ferroelectric memory, wherein the piezoelectric element is the piezoelectric element according to claim 1.

12. A piezoelectric actuator including a piezoelectric element, A piezoelectric actuator, wherein the piezoelectric element is the piezoelectric element according to claim 1.

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