Patent Application
20240244979
The present invention relates to a piezoelectric element, a droplet discharge head, a ferroelectric memory, and a piezoelectric actuator.
More particularly, the present invention relates to a piezoelectric element, and the like, with less degradation of piezoelectric characteristics in association with use.
It is known that piezoelectric characteristics of a piezoelectric element degrade by a leak current slightly flowing when a voltage is applied. A leak current that becomes a cause of degradation of piezoelectric characteristics tends to increase as a voltage and a temperature upon application are higher.
Further, even if the voltage and the temperature are the same, continuous or intermittent application of a voltage makes it easier for this leak current to flow, and the leak current increases over time. Increase of the leak current over time is one cause of degradation of piezoelectric characteristics in association with use.
As a technique for preventing degradation of piezoelectric characteristics, for example, a technique of improving durability in a high-humidity and high-temperature environment by preventing concentration of stress by alleviating high stress generated locally and finely in a piezoelectric film upon application of a voltage is disclosed (see Patent Literature 1).
Further, as another technique, a technique of preventing generation of a tunnel current or a Poole-Frenkel current when a piezoelectric element is driven using a drive system that drives the piezoelectric element with a maximum voltage lower than a voltage at which the tunnel current or the Poole-Frenkel current starts to be generated in the piezoelectric element is disclosed (see Patent Literature 2).
As described above, some techniques of preventing degradation of piezoelectric characteristics are disclosed. However, to extend the life of a piezoelectric element, further prevention of degradation is desired, and solutions with different viewpoints are desired.
The present invention has been made in view of the above-described problem and circumstances, and a problem to be solved by the present invention is to provide a piezoelectric element with less degradation of piezoelectric characteristics in association with use, a droplet discharge head. a ferroelectric memory, and a piezoelectric actuator including the piezoelectric element.
The present inventors have studied causes, and the like, of the above-described problem to solve the above-described problem, have found that a piezoelectric element, and the like, with less degradation of piezoelectric characteristics in association with use can be provided, the piezoelectric element including a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode by making a degree of degradation due to aging smaller under specific conditions of a Schottky barrier height between the second electrode and the piezoelectric film, and have achieved the present invention.
In other words, the problem according to the present invention can be solved by the following means.
1. A piezoelectric element including a first electrode, a second electrode, and a piezoelectric film located between the first electrode and the second electrode, in which
Φ2=A×In(t)+B
2. The piezoelectric element according to the first item, in which the coefficient A is equal to or greater than −1.000×10−2.
3. The piezoelectric element according to the first item or the second item, in which a crystal structure of a material of the piezoelectric film is a perovskite structure, and
4. The piezoelectric element according to any one of the first to the third items, in which a material of the piezoelectric film is lead zirconate titanate.
5. The piezoelectric element according to any one of the first to the fourth items, in which a material of the piezoelectric film is lead zirconate titanate represented by PbX(ZrY, Ti1-Y)O3 [0.5≤X≤1.5, 0.1≤Y≤0.9] in a whole of the piezoelectric film, and
6. The piezoelectric element according to any one of the first to the fifth items, including a dielectric film on the second electrode side between the second electrode and the piezoelectric film.
7. The piezoelectric element according to the sixth item, in which a crystal structure of the material of the dielectric film on the second electrode side is a perovskite structure, and
8. The piezoelectric element according to the sixth or the seventh item, including a dielectric film on the first electrode side between the first electrode and the piezoelectric film.
9. The piezoelectric element according to any one of the sixth to the eighth items, in which the material of the dielectric film on the second electrode side is lead lanthanum titanate.
10. The piezoelectric element according to any one of the first to the ninth items, in which a 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 an aging test is equal to or greater than 0.5 eV.
11. A droplet discharge head including a piezoelectric element.
12. A ferroelectric memory including a piezoelectric element.
13. A piezoelectric actuator including a piezoelectric element.
According to the above-described means of the present invention, it is possible to provide a piezoelectric element with less degradation of piezoelectric characteristics in association with use, a droplet discharge head. a ferroelectric memory, and a piezoelectric actuator including the piezoelectric element.
A mechanism of expression or a mechanism of action of effects of the present invention is inferred as follows.
As described above, increase of a leak current over time causes degradation of piezoelectric characteristics. While this increase of the leak current over time occurs by continuous or intermittent application of a voltage, as one cause of this, lowering of a Schottky barrier height can be considered.
In the present invention, the inventors have focused attention on lowering of the Schottky barrier height between the second electrode and the piezoelectric film by continuous or intermittent application of a voltage to the piezoelectric element and increase of a leak current due to the lowering of the Schottky barrier height and have found that increase of a leak current and degradation of piezoelectric characteristics due to the increase of the leak current can be prevented by reducing a degree of lowering of the Schottky barrier height due to aging within a specific range.
Specifically, the inventors have found that by setting the coefficient A in the above-described logarithmic approximation formula obtained in the aging test under the above-described conditions at equal to or greater than −4.200×10−2, increase of the leak current and degradation of the piezoelectric characteristics due to the increase of the leak current can be prevented.
Here, the coefficient A becomes an index of a degree of prevention of lowering of the Schottky barrier height. The coefficient A is a value equal to or less than 0, and the greater value, that is, the value closer to 0 indicates a smaller degree of lowering of the Schottky barrier height.
According to the mechanism of expression or mechanism of action, it is possible to provide a piezoelectric element with less degradation of piezoelectric characteristics in association with use.
A piezoelectric element of the present invention includes 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 the first electrode is driven, the second electrode is an electrode to which a relatively negative voltage is applied when the second electrode is driven, and a coefficient A obtained by 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 equal to or greater than −4.200×10−2.
This feature is a technical feature common to or corresponding to the following embodiment.
As the embodiment of the piezoelectric element of the present invention, the coefficient A is preferably equal to or greater than −1.000×10−2. This makes it possible to further reduce degradation of the piezoelectric characteristics in association with use.
As the embodiment of the piezoelectric element of the present invention, a crystal structure of a material of the piezoelectric film is preferably a perovskite structure, and a thickness of the piezoelectric film preferably falls within a range from 0.1 to 5 μm. By this means, displacement generation force required for the piezoelectric element can be obtained.
As the embodiment of the piezoelectric element of the present invention, a material of the piezoelectric film is preferably lead zirconate titanate. This makes it possible to form a piezoelectric element having favorable performance.
As the embodiment of the piezoelectric element of the present invention, the material of the piezoelectric film is preferably lead zirconate titanate represented by PbX(ZrY, Ti1-Y)O3 [0.5≤X≤1.5, 0.1≤Y≤0.9], and in a case where, when the piezoelectric film is divided in half in a thickness direction, an atomic composition ratio X of lead on the first electrode side is set as X1, and an atomic composition ratio X of lead on the second electrode side is set as X2, a value of a ratio X1/X2 is preferably equal to or greater than 1.04. This makes it possible to further prevent lowering of the Schottky barrier height in association with use.
As the 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 a crystal lattice volume of a material of the dielectric film on the second electrode side is preferably smaller than a crystal lattice volume of the material of the piezoelectric film. This increases a bandgap, so that the Schottky barrier height at an interface can be increased.
As the embodiment of the piezoelectric element of the present invention, a crystal structure of the material of the dielectric film on the second electrode side is preferably a perovskite structure, and a total thickness of the piezoelectric film and the dielectric film on the second electrode side preferably falls within a range from 0.1 to 5 μm. By this means, displacement generation force required for the piezoelectric element can be obtained.
As the embodiment of the piezoelectric element of the present invention, a dielectric film on the first electrode side is preferably provided between the first electrode and the piezoelectric film, and a total thickness of the dielectric film on the second electrode side and the dielectric film on the first electrode side preferably falls within a range from 5 to 15% of a total thickness of the dielectric film on the second electrode side, the dielectric film on the first electrode side and the piezoelectric film. By this means, decrease of an effective piezoelectric constant of the piezoelectric element by insertion of the dielectric body can be kept small, and a Schottky junction can be formed.
As the 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. By this means, a leak current can be prevented by forming a Schottky junction.
As the 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 equal to or greater than 0.5 eV. This can further reduce a leak current.
A droplet discharge head, a ferroelectric memory, and a piezoelectric actuator of the present invention includes the piezoelectric element of the present invention.
The present invention, components of the present invention, and modes/aspects for implementing the present invention will be described in detail below. Note that in the present application. “-” is used to indicate that numerical values before and after the “-” are included as a lower limit and an upper limit.
The piezoelectric element of the present invention includes 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 the first electrode is driven, the second electrode is an electrode to which a relatively negative voltage is applied when the second electrode is driven, and a coefficient A obtained by 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 equal to or greater than −4.200×10−2.
A schematic cross-sectional view of a configuration example of the piezoelectric element of the present invention is illustrated in
The respective components will be described in detail below.
The first electrode 10 is an electrode to which a relatively positive voltage is applied when the first electrode 10 is driven. A material of the first electrode 10 is not particularly limited, and Cr, Ni, Cu, Pt, Ir, Ti, an Ir—Ti alloy, LaNiO3, SrRuO3, or the like, can be used. A thickness of the first electrode 10 preferably falls within a range from 0.1 to 1 μm.
The second electrode 50 is an electrode to which a relatively negative voltage is applied when the second electrode 50 is driven. A material of the second electrode 50 is not particularly limited, and Cr, Ni, Cu, Pt, Ir, Ti, an Ir—Ti alloy, LaNiO3, SrRuO3, or the like, can be used. A thickness of the second electrode 50 preferably falls within a range from 0.5 to 5 μm.
A “piezoelectric film” is a film formed with a piezoelectric body. A crystal structure of a piezoelectric body that becomes a material of the piezoelectric film 30 according to the present invention is preferably a perovskite structure.
The “perovskite structure” refers to a crystal structure similar to perovskite (CaTiO3). Normally, a composition of the perovskite crystal structure is expressed as ABX3, and in the perovskite crystal structure. A. B and X exist as component ions of A cation. B cation and X anion, respectively. Further, a B cation defect-type perovskite compound, an A cation defect-type perovskite compound, and an X anion defect-type perovskite compound are also defined as compounds having a perovskite crystal structure in the present invention.
Examples of the piezoelectric body having a perovskite structure as a crystal structure can include lead zirconate titanate (PZT: Pb(Zr, Ti)O3), lead titanate (PbTiO3), lead zirconate (PbZrO3), lead lanthanum titanate (PLT: (Pb, La)TiO3), barium titanate (BaTiO3), and the like. Among these, lead zirconate titanate represented by PbX(ZrY, Ti1-Y)O3 [0.5≤X≤1.5, 0.1≤Y≤0.9] is particularly preferable.
Lead zirconate titanate preferably has a nonstoichiometric composition. Specifically, in a case where the composition is represented by PbX(ZrY, Ti1-Y)O3 [0.5≤X≤1.5, 0.1≤Y≤0.9], X is preferably X>1.
Further, in a case where the composition is represented by PbX(ZrY, Ti1-Y)O3 [0.5≤X≤1.5, 0.1≤Y≤0.9], and, when the piezoelectric film 30 is divided in half in a thickness direction, an atomic composition ratio X of lead on the first electrode side is set as X1 and an atomic composition ratio X of lead on the second electrode side is set as X2, a value of the ratio X1/X2 is preferably equal to or greater than 1.04 and more preferably equal to or greater than 1.11. This can further prevent lowering of the Schottky barrier height in association with use. Further, in terms of prevention of lowering of the Schottky barrier height. X2 is preferably equal to or less than 1.2, and a value of the ratio X1/X2 is preferably equal to or less than 1.14.
The composition of the piezoelectric film 30 can be analyzed by examining the composition in a depth direction of the piezoelectric film by alternately performing ion sputtering using Auger electron spectroscopy.
The atomic composition ratio X of lead can be adjusted by adjusting a partial pressure of oxygen of a sputtering gas when the piezoelectric film 30 is formed. As is known in related art, the atomic composition ratio X of lead can be increased by setting a higher partial pressure of oxygen of the sputtering gas, and the atomic composition ratio X of lead can be reduced by setting a lower partial pressure of oxygen of the sputtering gas. Further, Y is derived from a sputtering target and does not change in the whole piezoelectric film.
In a case where the atomic composition ratio X of lead is changed between the piezoelectric film on the first electrode side and the piezoelectric film on the second electrode side when the piezoelectric film 30 is divided in half in a thickness direction, it is only necessary to change the partial pressure of oxygen of the sputtering gas when half of the film is formed.
Further, in terms of magnitudes of a permittivity and a piezoelectric constant. Y when the composition is represented by PbX(ZrY, Ti1-Y)O3 [0.5≤X≤1.5, 0.1≤Y≤0.9] preferably falls within a range from 0.50 to 0.58 and is more preferably 0.52.
A relationship between a value of the ratio X1/X2 and lowering of the Schottky barrier height in association with use will be described.
In a case where the piezoelectric film is formed using a material including lead like PZT, volatility of lead is high, and thus, the film is formed while lead is excessively added. Excessive lead compared to a stoichiometric composition due to this exists with a positive charge within the film and is called a lead defect. This lead defect has a positive charge, and thus, is encouraged to be diffused to the second electrode side by a positive voltage being applied to the first electrode. The lead defect diffused toward the second electrode side causes lowering of the Schottky barrier height between the second electrode and the piezoelectric film.
Thus, if the composition ratios X1 and X2 of lead are adjusted so that the value of the ratio X1/X2 becomes equal to or greater than 1.04, that is, more lead exists on the first electrode side, when a positive voltage is applied to the first electrode, a period until the lead defect is diffused to an interface of the second electrode becomes long, so that lowering of the Schottky barrier height can be prevented.
Further, by a difference in the value of the ratio X1/X2, a difference also occurs in an aspect of change of a coercive electric field and residual polarization in association with use. If a positive voltage is applied to the first electrode, diffusion of the lead defect to the second electrode side proceeds, which makes an internal bias small. As a result, the coercive electric field is shifted to a positive side, and the residual polarization becomes greater by alleviation of pinning. Thereafter, the coercive electric field reaches an inflection point and is shifted on a negative side, the residual polarization becomes smaller, and pinning proceeds. It can be considered that this is caused by injection of a charge to an interface of the first electrode and the piezoelectric film. As the value of the ratio X1/X2 is greater, that is, as a degree of segregation of the lead defect on the first electrode side is larger, the coercive electric field reaches the inflection point later. It is therefore possible to prevent degradation of the piezoelectric characteristics as the value of the ratio X1/X2 is greater.
A thickness of the piezoelectric film 30 preferably falls within a range from 0.1 to 5 μm and more preferably falls within a range from 2.0 to 3.5 μm. By this means, displacement generation force required for the piezoelectric element can be obtained.
The “dielectric film on the second electrode side” refers to a film formed between the second electrode and the piezoelectric film with a dielectric body. In the present invention, a crystal lattice volume of a material of the dielectric film 40 on the second electrode side is preferably smaller than a crystal lattice volume of the material of the piezoelectric film. This increases a bandgap, and thus, the Schottky barrier height at the interface is increased.
The crystal lattice volume can be measured using an X-ray diffraction (XRD) method. An interval between (001) planes and an interval between (100) planes of the crystal can be respectively obtained as c and a through out-of-plane 20-ω scanning and in-plane 20-ω scanning, and the crystal lattice volume can be calculated by a×a×c.
A crystal structure of the dielectric film 40 on the second electrode side is preferably a perovskite structure. Examples of the piezoelectric body having a perovskite crystal structure can include lead titanate (PbTiO3), lead lanthanum titanate (PLT: (Pb, La)TiO3), barium titanate (BaTiO3), and the like. Among these, the piezoelectric body containing lead is preferable, and particularly, lead lanthanum titanate is preferable. By this means, a Schottky barrier can be formed at the interface with the PZT by a difference in a crystal lattice volume.
In the piezoelectric element of the present invention, a total thickness of the piezoelectric film 30 and the dielectric film 40 on the second electrode side preferably falls within a range from 0.1 to 5 μm. By this means, displacement generation force required for the piezoelectric element can be obtained.
The “dielectric film on the first electrode side” refers to a film formed between the first electrode and the piezoelectric film with a dielectric body. In the present invention, as the dielectric film 20 on the first electrode side, a dielectric film similar to the dielectric film 40 on the second electrode side described above can be used.
A total thickness of the dielectric film on the second electrode side and the dielectric film on the first electrode side preferably falls within a range from 5 to 15% of a total thickness of the dielectric film on the second electrode side, the dielectric film on the first electrode side and the piezoelectric film. By this means, decrease of an effective piezoelectric constant of the piezoelectric element by insertion of the dielectric body can be kept small, and a Schottky junction can be formed.
In the piezoelectric element of the present invention, a coefficient A obtained by 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 equal to or greater than −4.200×10−2.
The above-described logarithmic approximation formula can be expressed as a graph, for example as indicated in
The coefficient A becomes an index of a degree of prevention of lowering of the Schottky barrier height. The coefficient A is a value equal to or less than 0, and a greater value, that is, a value closer to 0 indicates a smaller degree of lowering of the Schottky barrier height in association with aging.
In the present invention, the coefficient A is equal to or greater than −4.200×10−2. Further, in terms of effects of the present invention, the coefficient A is preferably equal to or greater than −1.000×10−2.
Further, in 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 before the aging test is preferably equal to or greater than 0.5 eV. This can further reduce a leak current.
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 collectively referred to as a Schottky barrier height ΦS.
A method for measuring the Schottky barrier height ΦS in the present invention will be described below. Note that the measurement method is similar between in a case where measurement is performed independent of the aging test and in a case where measurement is performed in the aging test.
In the present invention, the Schottky barrier height ΦS [eV] is obtained using the following expression from a slope α by creating an Arrhenius plot that indicates In (1000/T) on a horizontal axis and In (J/T2) on a vertical axis after measuring leak current density J [A/cm2] at a predetermined temperature T [K] using the following method. Note that at least four kinds of the predetermined temperature T [K] are required to create the Arrhenius plot.
α=qΦS/kB
A method for measuring the leak current density J [A/cm2] will be described below.
The piezoelectric element is put into an electric furnace and sealed, and dry air is introduced. It is waited until a dew point becomes equal to or less than −50° C. Then, a temperature of the electric furnace is increased, and an ambient temperature is adjusted at the predetermined temperature T [K]. Here, to avoid influence of fluctuation of the temperature, a fixed period (a period until the temperature becomes stable, for example, approximately 45 minutes) is provided as a waiting period. A temperature measured with a thermocouple thermometer provided near the piezoelectric element is set as the ambient temperature.
An electric field applied between the first electrode and the second electrode is gradually increased from 0 V/μm to 12.86 V/μm in a state where the ambient temperature is adjusted at the predetermined temperature T [K].
and current density of the leak current flowing when the electric field is 12.86 V/μm is measured. Here, to obtain a net leak current excluding influence of a polarization inversion current, application is swept in the same direction twice, and current density at the second sweeping is dealt with as the leak current density J [A/cm2].
The electric field is applied so that the first electrode side becomes a positive potential in a case where the Schottky barrier height Φ1 between the first electrode and the piezoelectric film is measured while the second electrode is grounded, and is applied so that the first electrode side becomes a negative potential in a case where the Schottky barrier height Φ2 between the second electrode and the piezoelectric film is measured.
The leak current density is measured using, for example, a semiconductor parameter analyzer (Agilent B1500A).
The reason why the leak current density is measured at each temperature when the electric field is 12.86 V/μm is to perform measurement in an electric field in which a Schottky emission current is dominant.
In the present application, the temperature [K] and the temperature [° C.] can be converted using the following expression.
Temperature [K: kelvin]=temperature [° C.: Celsius]+273.15
In the present invention, the aging test of the piezoelectric element is performed by continuously applying an electric field of 10 V/μm for a predetermined period while heating is performed so that the ambient temperature becomes 80° C.
The aging test will be described in detail below.
First, the piezoelectric element is put into an electric furnace and sealed. Dry air is introduced into the electric furnace, and it is waited until a dew point becomes equal to or lower than −50° C. Then, a temperature of the electric furnace is increased, and the ambient temperature is adjusted to 80° C. Here, to avoid influence of fluctuation of the temperature, a fixed period (for example, approximately 45 minutes until the temperature becomes stable) is provided as a waiting period. A temperature measured with a thermocouple thermometer provided near the piezoelectric element is set as the ambient temperature.
If the ambient temperature becomes 80° C., the second electrode is grounded, and application of a positive electric field of 10 V/μm to the first electrode is started. The electric field is applied using, for example, a DC stabilized power supply (KX-100L). Aging is performed by continuing application of the electric field of 10 V/μm while keeping the ambient temperature at 80° C.
The aging period t that is an elapsed period since application has been started is set at up to 20 hours. The Schottky barrier height Φ2 between the second electrode and the piezoelectric film is measured in the middle of the aging several times so that tendency of change in association with aging can be grasped. The measurement is preferably performed at least four times while changing the aging period t. For example, measurement is performed at a time point at which the aging period t is 2, 5, 10 and 20 hours.
The droplet discharge head, the ferroelectric memory, and the piezoelectric actuator of the present invention includes the piezoelectric element of the present invention. Piezoelectric characteristics of the piezoelectric element of the present invention less degrade in association with use, and thus, the droplet discharge head, the ferroelectric memory and the piezoelectric actuator including the piezoelectric element can be stably used for a long period.
If the droplet discharge head, the ferroelectric memory, and the piezoelectric actuator of the present invention include the piezoelectric element of the present invention, other configurations, and the like, are not particularly limited, and they can be constituted using members that are typically used.
The piezoelectric element of the present invention can be used in, for example, a piezoelectric microphone, a vibration sensor, a displacement sensor, a sonar, an oscillation circuit, a resonator, a ceramic filter, a piezoelectric transformer, a piezoelectric buzzer, an ultrasonic motor, and the like, other than the above.
The present invention will be specifically described below using examples, but the present invention is not limited to these. Note that while the expressions “part” or “%” are used in the example, the expressions “part” or “%” indicate “parts by mass” or “mass %” unless otherwise specified.
In the examples, droplet discharge heads including the piezoelectric element of the present invention were manufactured, and degradation of the piezoelectric characteristics in association with use was evaluated using the droplet discharge heads.
A first electrode was formed on a Bare-Si wafer of 8 inches using an Ir—Ti alloy target through an RF magnetron sputtering method. A thickness and sputtering conditions are as follows.
Then, a dielectric film on the first electrode side was formed on the first electrode using a lead lanthanum titanate (PLT: (Pb1.125, La0.1)TiO3) ceramic target that is an excessive lead composition including Pb 25% more than a stoichiometric composition that is Pb:La=0.9:0.1 through an RF magnetron sputtering method. A thickness and sputtering conditions are as follows.
Further, an interval between planes and a crystal lattice volume of lead lanthanum titanate of the dielectric film on the first electrode side, measured using an X-ray diffraction method are as follows.
Then, a piezoelectric film was formed on the dielectric film on the first electrode side using a lead zirconate titanate (PZT: Pb1.25(Zr0.52, Ti0.48)O3) ceramic target that is an excessive lead composition including Pb 25% more than a stoichiometric composition through an RF magnetron sputtering method. A thickness and sputtering conditions are as follows.
In an initial layer of film formation that becomes the first electrode side, an amount of lead was adjusted by forming a film while increasing a partial pressure of oxygen.
A composition of the piezoelectric film on the first electrode side in a case where the piezoelectric film was divided in half in a thickness direction was PbX1(ZrY, Ti1-Y)O3 where X1 was 1.16 and Y was 0.52.
Further, a composition of the piezoelectric film on the second electrode side in a case where the piezoelectric film was divided in half in a thickness direction was PbX2(ZrY, Ti1-Y)O3 where X2 was 1.08 and Y was 0.52.
Note that a composition of the piezoelectric film was analyzed by examining a composition in a depth direction of the piezoelectric film by alternately performing ion sputtering using Auger electron spectroscopy.
Further, an interval between planes and a crystal lattice volume of lead zirconate titanate of the piezoelectric film, measured using an X-ray diffraction method are as follows.
Then, a dielectric film on the second electrode side was formed on the piezoelectric film using a lead lanthanum titanate (PLT: (Pb1.125, La0.1)TiO3) ceramic target that is an excessive lead composition including Pb 25% more than a stoichiometric composition in which Pb:La=0.9:0.1 through an RF magnetron sputtering method. A thickness and sputtering conditions are as follows.
Further, an interval between planes and a crystal lattice volume of lead lanthanum titanate of the dielectric film on the second electrode side, measured using an X-ray diffraction method are as follows.
Then, the second electrode was formed on the dielectric film on the second electrode side using a Cu target through a sputtering method. A thickness and sputtering conditions are as follows. Note that the second electrode also serves as a vibration plate in the piezoelectric actuator.
Then, an ink shielding film of 1 μm was formed by applying a photosensitive polyimide resin on the second electrode through a spin coat method and curing the resin by burning at 230° C.
Then, a seed layer of 0.5 μm was formed on the ink shielding film using an Ni target through a sputtering method. Sputtering was performed for 15 minutes at high-frequency power of 500 W and in an argon gas having a gas pressure of 1 Pa upon sputtering.
Then, a pressure chamber with a height of 150 μm was formed by depositing a pressure chamber member made of Ni using an Ni electroforming method after forming two laminated layers of a dry film resist having a thickness of 80 μm of ORDYL MP108 manufactured by Tokyo Ohka Kogyo Co., Ltd. Then, the dry film resist layers were removed, and cleaning and drying were performed.
Then, a support substrate of 8 inches made of glass was pasted on the pressure chamber with a double-sided thermal release sheet manufactured by Nitto Denko Corporation.
Then, the Si substrate was ground until the thickness becomes approximately 50 μm and completely removed by further performing dry etching using SF6.
Then, a resist mask was formed by applying an OMR resist manufactured by Tokyo Ohka Kogyo Co., Ltd. on the first electrode and transferring a mask pattern and developing the mask pattern through exposure. Then, the first electrode in a region where a resist mask was not formed was removed through dry etching using a mixture gas of argon, oxygen and CHF3. After cleaning, the resist mask was peeled using a stripping solution.
Then, a resist mask was formed by applying an OMR resist manufactured by Tokyo Ohka Kogyo Co., Ltd. and transferring a mask pattern and developing the mask pattern through exposure. Then, the dielectric film and the piezoelectric film in a region where the resist mask was not formed was removed through dry etching using a mixture gas of chlorine and bromine. After cleaning, the resist mask was peeled using a stripping solution.
Then, a protective film of 1 μm was formed by applying a photosensitive polyimide resin through a spin coat method and further performing patterning. The patterning was performed by transferring a mask pattern and developing the mask pattern through exposure. After patterning, the photosensitive polyimide resin was burned at 210° C. and cured.
Then, the support substrate was removed by being heated at a temperature equal to or higher than a temperature at which the thermal release sheet foams, to obtain the piezoelectric actuator No. 1.
Then, an ink channel member and a nozzle plate were bonded to the piezoelectric actuator No. 1 with an adhesive to obtain the droplet discharge head No. 1.
The droplet discharge head No. 2 was manufactured in a similar manner to the droplet discharge head No. 1 except that X1 and X2 of the piezoelectric film were adjusted as indicated in Table I.
The droplet discharge head No. 3 was manufactured in a similar manner to the droplet discharge head No. 1 except that X1 and X2 of the piezoelectric film were adjusted as indicated in Table I.
The droplet discharge head No. 4 was manufactured in a similar manner to the droplet discharge head No. 1 except that instead of forming the dielectric film on the second electrode side, the thickness of the piezoelectric film and the dielectric film on the first electrode side was made 3.50 μm (the piezoelectric film: 3.38 μm, the dielectric film on the first electrode side: 0.12 μm) and further. X1 and X2 of the piezoelectric film were adjusted as indicated in Table I.
The Schottky barrier heights Φ1 and Φ2 before the aging test and during each aging period t of the droplet discharge heads No. 1 to 4 were measured. The measurement results are as indicated in Table I.
A method of the aging test and a method for measuring the Schottky barrier height are as described above. Note that a distance between electrodes in the droplet discharge heads No. 1 to 4 is 3.50 μm, and thus, in the aging test, an applied electric field was set at 10.00 V/μm by setting an absolute value of the applied voltage at 35 V. Further, in measurement of the Schottky barrier height, by setting the absolute value of the applied voltage at 45 V, the applied electric field was set at 12.86 V/μm.
Four kinds of 2, 5, 10 and 20 hours were set as the aging period t. Further, as the predetermined temperature T in measurement of the Schottky barrier height Φ2 in each aging period t, a setting temperature of the electric furnace was set at five kinds of 24° C. 40° C. 52° C. 65° C. and 80° C., and a temperature acquired with a thermocouple was employed as the ambient temperature and used in evaluation.
A graph in which each aging period t and the Schottky barrier height Φ2 at that time are plotted is indicated in
The coefficient A in the droplet discharge heads No. 1 to 4 was derived from the above-described logarithmic approximation formula using the Schottky barrier height Φ2 in each aging period t. The derived value of the coefficient A is as indicated in Table I.
Durability of the droplet discharge heads No. 1 to 4 was evaluated with a decrease rate of injection speed obtained using the following method.
The injection speed [m/s] before and after injection for a long period of the droplet discharge heads No. 1 to 4 was measured using a drop watcher, and a decrease rate [%] of the injection speed was obtained using the following expression. The obtained value of the decrease rate [%] of the injection speed is as indicated in Table I.
The decrease rate [%] of injection speed=[(S1−S2)/S1]×100
A period of injection for a long period was set at 1500 hours. “Under ordinary temperature” in Table I indicates a test in a case where injection was performed for a long period at a room temperature, and “under high temperature” indicates a test in a case where injection was performed for a long period at 50° C. A square wave was employed as an injection waveform, and injection was performed while the second electrode was grounded, and a positive voltage of 30 V was applied to the first electrode.
The droplet discharge head No. 3 became unable to perform discharging before 1500 hours had elapsed, and thus, the decrease rate [%] of injection speed could not be measured.
From the evaluation results indicated in Table I, it can be seen that the droplet discharge head including the piezoelectric element of the present invention has high durability in use for a long period. Further, it can be seen from the results that piezoelectric characteristics of the piezoelectric element of the present invention less degrade in association with use.
In the droplet discharge heads No. 1 and No. 2 before aging and after 20 hours of aging. P-E characteristics were measured by sweeping the electric field E [V/μm] at a frequency of 1 kHz and measuring polarization P [μC/cm2]. P-E characteristics of the droplet discharge head No. 1 are indicated in
Change of a coercive electric field in association with aging, measured in the droplet discharge head No. 1 is indicated in
Change of residual polarization in association with aging, measured in the droplet discharge head No. 1 is indicated in
Comparing the coercive electric fields before aging, it can be seen that a shift amount to the negative direction is greater for the droplet discharge head No. 1 than the droplet discharge head No. 2. Compared to the droplet discharge head No. 1, the droplet discharge head No. 2 is manufactured to include a more amount of lead as a whole, and thus, a degree of segregation of the lead defect inside is greater for the droplet discharge head No. 1 in a case of relative comparison. Thus, it can be seen from correlation between a degree of segregation of the lead defect and an internal bias that excessive lead has a positive charge and exists within the film.
If aging proceeds, diffusion of the lead defect having a positive charge proceeds by an electric field from the first electrode to the second electrode during aging, which reduces the internal bias. Thus, the coercive electric field is shifted on the positive side, and residual polarization increases by abbreviation of pinning. The inflection point for the droplet discharge head No. 1 comes later than the inflection point for the droplet discharge head No. 2 because a value of the ratio X1/X2 is greater for the droplet discharge head No. 1, that is, a degree of segregation of the lead defect on the first electrode side is greater.
After the coercive electric field reaches the inflection point, the coercive electric field is shifted on the negative side. It can be considered that this is caused by injection of a charge to an interface of the first electrode and the piezoelectric film, the residual polarization also decreases, and pinning proceeds.
The present invention can be utilized in a piezoelectric element with less degradation of piezoelectric characteristics in association with use, a droplet discharge head, a ferroelectric memory, and a piezoelectric actuator including the piezoelectric element.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-093593 | Jun 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/020869 | 5/19/2022 | WO |
