Piezoelectric element and liquid discharge head

Abstract

To suppress leakage current.SOLUTION: A piezoelectric element 36 includes: a top electrode TE; a bottom electrode BE; a piezoelectric layer PL provided between the top electrode TE and the bottom electrode BE; and an insulating layer IL1 provided between the top electrode TE and the piezoelectric layer PL and having insulation properties. When EAp represents the electron affinity of a piezoelectric material included in the piezoelectric layer PL, IEp represents the ionization energy of the piezoelectric material, WF represents the work function of an electrode material included in the top electrode TE, and IEi represents the ionization energy of an insulating material included in the insulating layer IL1, both of WF>(EAp+IEp) / 2 and IEi>IEp are satisfied.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a piezoelectric element and a liquid ejection head. [Background technology]

[0002] A liquid ejection device that prints an image by ejecting liquid such as ink from a nozzle using a piezoelectric element is known. For example, the liquid ejection device has a liquid ejection head that ejects liquid from a nozzle communicating with a pressure chamber by applying pressure fluctuations to the liquid in the pressure chamber using a piezoelectric element. The piezoelectric element has, for example, a first electrode, a piezoelectric layer provided on the first electrode, and a second electrode provided on the piezoelectric layer. Patent Document 1 discloses a piezoelectric element in which platinum is used as the material for the second electrode in order to suppress leakage current occurring between the first electrode and the second electrode. In the manufacturing method of a piezoelectric element described in Patent Document 1, a first platinum layer is formed on a piezoelectric layer by a liquid phase method, and a second platinum layer is formed on the first platinum layer by a gas phase method, thereby improving the adhesion of each layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-050422 A Summary of the Invention [Problem to be solved by the invention]

[0004] In order to suppress the leakage current occurring between the first electrode and the second electrode, it is possible to provide an insulating layer between the first electrode and the piezoelectric layer and / or between the second electrode and the piezoelectric layer, but depending on the insulating layer, it may not be possible to suppress the leakage current. For this reason, it is desirable to suppress the leakage current more reliably. [Means for solving the problem]

[0005] In order to solve the above problems, the piezoelectric element according to the present invention includes a first electrode, a second electrode, a piezoelectric layer provided between the first electrode and the second electrode, and a first insulating layer provided between the first electrode and the piezoelectric layer and having insulating properties. When the electron affinity of the piezoelectric material contained in the piezoelectric layer is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material contained in the first electrode is WF1, and the ionization energy of the insulating material contained in the first insulating layer is IEi1, both WF1 > (EAp + IEp) / 2 and IEi1 > IEp are satisfied.

[0006] Further, another piezoelectric element according to the present invention includes a first electrode, a second electrode, a piezoelectric layer provided between the first electrode and the second electrode, and a first insulating layer provided between the first electrode and the piezoelectric layer and having insulating properties. When the electron affinity of the piezoelectric material contained in the piezoelectric layer is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material contained in the first electrode is WF1, and the electron affinity of the insulating material contained in the first insulating layer is EAi1, both WF1 < (EAp + IEp) / 2 and EAi1 < EAp are satisfied.

[0007] Moreover, the liquid ejection head according to the present invention includes any one of the above-described piezoelectric elements, a pressure chamber filled with a liquid, the pressure of which changes when the piezoelectric element is driven, and a nozzle that ejects the liquid filled in the pressure chamber when the piezoelectric element is driven.

Brief Description of the Drawings

[0008] [Figure 1] It is an explanatory diagram for explaining an example of a liquid ejection device according to an embodiment of the present invention. [Diagram 2] It is an exploded perspective view of a liquid ejection head. [Diagram 3] It is a cross-sectional view taken along line a-a shown in FIG. 2. [Figure 4] It is an explanatory diagram for explaining a specific configuration of a piezoelectric element. [Diagram 5]5 is an explanatory diagram for explaining the relationship of the energy gap between an upper electrode, an insulating layer, and a piezoelectric layer. FIG. [Figure 6] FIG. 11 is an explanatory diagram for explaining a piezoelectric element according to a first modified example. [Figure 7] 13 is an explanatory diagram for explaining a piezoelectric element according to a second modified example. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the embodiments for carrying out the present invention will be described with reference to the drawings. However, in each drawing, the dimensions and scale of each part are appropriately different from the actual ones. In addition, since the embodiments described below are preferred specific examples of the present invention, various technically preferable limitations are attached, but the scope of the present invention is not limited to these embodiments unless otherwise stated in the following description to the effect that the present invention is particularly limited.

[0010] [1. Embodiment] First, an overview of a liquid ejection device 1 according to this embodiment will be described with reference to Fig. 1. In this embodiment, it is assumed, as an example, that the liquid ejection device 1 is a serial printer.

[0011] FIG. 1 is an explanatory diagram for explaining an example of a liquid ejection device 1 according to an embodiment of the present invention. In the following, for convenience of explanation, an X-axis, a Y-axis, and a Z-axis that are perpendicular to each other will be appropriately used. In addition, in the following, one direction along the X-axis will be referred to as the X1 direction, and the direction opposite to the X1 direction will be referred to as the X2 direction. Similarly, one direction along the Y-axis will be referred to as the Y1 direction, and the direction opposite to the Y1 direction will be referred to as the Y2 direction. Furthermore, one direction along the Z-axis will be referred to as the Z1 direction, and the direction opposite to the Z1 direction will be referred to as the Z2 direction.

[0012] The liquid ejection device 1 in Fig. 1 is an inkjet printing device that ejects ink onto a medium PP. Note that ink is an example of a "liquid." The medium PP is typically printing paper, but a printing target made of any material, such as a resin film or fabric, can be used as the medium PP.

[0013] 1, a liquid container 14 for storing ink is installed in the liquid ejection device 1. As the liquid container 14, for example, a cartridge that is detachable from the liquid ejection device 1, a bag-shaped ink pack made of a flexible film, or an ink tank that can be refilled with ink can be used. The type of ink stored in the liquid container 14 is not particularly limited and can be any type.

[0014] The liquid ejection device 1 also has a control unit 2, a medium transport mechanism 71, a carriage transport mechanism 72, and a liquid ejection head 3. The control unit 2 includes, for example, one or more processing circuits such as a CPU (Central Processing Unit) or an FPGA (Field Programmable Gate Array), and one or more storage circuits such as a semiconductor memory, and performs overall control of each element of the liquid ejection device 1.

[0015] The medium transport mechanism 71 transports the medium PP in the Y2 direction along the Y axis under the control of the control unit 2. The carriage transport mechanism 72 reciprocates the liquid ejection head 3 along the X axis under the control of the control unit 2. The carriage transport mechanism 72 has, for example, a substantially box-shaped carriage 721 that houses the liquid ejection head 3, and an endless belt 722 to which the carriage 721 is fixed. Note that a configuration in which multiple liquid ejection heads 3 are mounted on the carriage 721 may also be employed. The liquid container 14 may also be stored in the carriage 721 together with the liquid ejection head 3.

[0016] The liquid ejection head 3 ejects ink onto the medium PP from a plurality of nozzles N shown in Fig. 2 (described later) under the control of the control unit 2. For example, the liquid ejection head 3 ejects ink supplied from a liquid container 14 onto the medium PP from a plurality of nozzles N. An image is formed on the surface of the medium PP by each liquid ejection head 3 ejecting ink onto the medium PP in parallel with the transport of the medium PP by the medium transport mechanism 71 and the repeated reciprocating movement of the carriage 721.

[0017] Next, the schematic structure of the liquid ejection head 3 will be described with reference to FIGS.

[0018] Fig. 2 is an exploded perspective view of the liquid ejection head 3. Fig. 3 is a cross-sectional view taken along line aa shown in Fig. 2. The aa cross section is parallel to the XZ plane and passes through an introduction port 384, which will be described later.

[0019] 2 and 3, the liquid ejection head 3 includes a wiring board 20, a nozzle board 31, a vibration absorber 32, a flow path board 33, a pressure chamber board 34, a vibration plate 35, a plurality of piezoelectric elements 36, a sealing plate 37, and a case 38. The wiring board 20 is a mounting component on which a plurality of wirings are formed for electrically connecting the control unit 2 and the piezoelectric elements 36. For example, a flexible wiring board 20 such as an FPC (Flexible Printed Circuit) or an FFC (Flexible Flat Cable) is preferably used. A drive signal and a reference voltage for driving the piezoelectric elements 36 are supplied to each piezoelectric element 36 from the wiring board 20.

[0020] Here, the nozzle substrate 31 and the vibration absorber 32 are disposed in an area located in the Z2 direction from the flow path substrate 33. On the other hand, the pressure chamber substrate 34, the vibration plate 35, the plurality of piezoelectric elements 36, the sealing plate 37, and the case 38 are disposed in an area located in the Z1 direction from the flow path substrate 33. Also, the wiring substrate 20 is provided, for example, on the Z1 direction surface of the sealing plate 37. Each element of the liquid ejection head 3 is roughly a plate-like member that is elongated in the direction along the Y axis, and is joined to one another, for example, by an adhesive.

[0021] 2, the nozzle substrate 31 is a plate-like member on which a plurality of nozzles N are formed and arranged along the Y axis. Each nozzle N is a through hole through which ink passes. The nozzle substrate 31, the flow path substrate 33, and the pressure chamber substrate 34 are formed, for example, by processing a silicon single crystal substrate using semiconductor manufacturing techniques such as etching. However, the materials and manufacturing methods of each element of the liquid ejection head 3 are arbitrary.

[0022] The flow path substrate 33 is a plate-like member for forming a flow path of ink. As shown in FIG. 2 and FIG. 3, an opening 332, a plurality of supply flow paths 334, and a plurality of communication flow paths 336 are formed in the flow path substrate 33. The opening 332 is a through hole that is continuous across the plurality of nozzles N along the Y axis in a plan view from the direction along the Z axis. That is, the opening 332 is an elongated through hole that extends in the direction along the Y axis. The supply flow path 334 and the communication flow path 336 are through holes that are individually formed for each nozzle N. In addition, as shown in FIG. 3, a relay flow path 338 that spans the plurality of supply flow paths 334 is formed on the surface of the flow path substrate 33 in the Z2 direction. The relay flow path 338 is a flow path that communicates the opening 332 and the plurality of supply flow paths 334.

[0023] The pressure chamber substrate 34 is a plate-like member in which a plurality of pressure chambers CV corresponding to the plurality of nozzles N are formed. The pressure chambers CV are spaces called cavities for applying pressure to the ink filled in the pressure chambers CV. For example, the pressure chambers CV are spaces located between the flow path substrate 33 and the vibration plate 35 and partitioned by the partition walls 342 of the pressure chamber substrate 34. The plurality of pressure chambers CV are arranged in the direction along the Y axis. Each pressure chamber CV is formed of a hole that opens on both sides of the pressure chamber substrate 34 and has an elongated shape extending in the direction along the X axis. In other words, the direction along the X axis corresponds to the longitudinal direction of the pressure chambers CV. The end of each pressure chamber CV in the X2 direction communicates with the corresponding supply flow path 334 among the plurality of supply flow paths 334. On the other hand, the end of each pressure chamber CV in the X1 direction communicates with the corresponding communication flow path 336 among the plurality of communication flow paths 336.

[0024] A vibration plate 35 is provided on the surface of the pressure chamber substrate 34 opposite to the surface facing the flow channel substrate 33. The vibration plate 35 is an elastically deformable plate-like member. As shown in FIG. 3, the vibration plate 35 includes an elastic film 351 and an insulating film 352 laminated in a direction along the Z axis. In this specification, the expression "element A and element B are laminated" does not mean to limit the configuration in which element A and element B are in direct contact with each other. In other words, a configuration in which another element C is interposed between element A and element B is also included in the concept of "element A and element B are laminated." Similarly, the expression "element B is formed on the surface of element A" is not limited to a configuration in which element A and element B are in direct contact with each other. In other words, even if an element C is formed on the surface of element A and an element B is formed on the surface of element C, the concept of "element B is formed on the surface of element A" is included in the concept of "element B is formed on the surface of element A" as long as at least a part of element A and element B overlap in a plan view. The insulating film 352 is located on the opposite side to the pressure chamber substrate 34 as viewed from the elastic film 351. The elastic film 351 is made of, for example, silicon oxide. The insulating film 352 is made of, for example, zirconium oxide.

[0025] 2 and 3, the flow path substrate 33 and the diaphragm 35 face each other with a gap between them inside each pressure chamber CV. The diaphragm 35 forms part of the wall surface of the pressure chamber CV. For example, when the pressure chamber substrate 34 is provided on the first surface SF1 side of the diaphragm 35, the second surface SF2 of the diaphragm 35 opposite the first surface SF1 forms part of the wall surface of the pressure chamber CV. As shown in FIG. 3, the ink stored in the liquid storage chamber RS ​​branches from the relay flow path 338 to each supply flow path 334 and is supplied to and filled in parallel into the multiple pressure chambers CV.

[0026] As shown in FIG. 2 and FIG. 3, a surface of the vibration plate 35 facing away from the pressure chamber substrate 34, i.e., the second surface SF2, is provided with a plurality of piezoelectric elements 36 corresponding to the plurality of nozzles N. The nozzle N corresponding to the piezoelectric element 36 is a nozzle N communicating with the pressure chamber CV that partially or completely overlaps with the piezoelectric element 36 in a plan view in the Z2 direction. Each piezoelectric element 36 is an actuator that deforms when a drive signal is supplied, and has an elongated shape extending in the direction along the X axis. The plurality of piezoelectric elements 36 are arranged in the direction along the Y axis so as to correspond to the plurality of pressure chambers CV. When the drive signal is supplied to the piezoelectric element 36, the vibration plate 35 vibrates in conjunction with the deformation of the piezoelectric element 36, and the pressure in the pressure chamber CV fluctuates. When the pressure in the pressure chamber CV fluctuates, the pressure of the ink filled in the pressure chamber CV changes, and the ink filled in the pressure chamber CV passes through the communication flow path 336 and the nozzle N and is ejected. That is, the piezoelectric element 36 is a drive element that causes the ink in the pressure chamber CV to be ejected from the nozzle N by vibrating the vibration plate 35.

[0027] 2 and 3, the case 38 is a structure manufactured by, for example, injection molding of a resin material, and is fixed to the surface of the flow path substrate 33 in the Z1 direction. As shown in FIG. 3, a storage section 382 and an inlet 384 are formed in the case 38. The storage section 382 is a recess having an outer shape corresponding to the opening 332 of the flow path substrate 33. The inlet 384 is a through hole communicating with the storage section 382. The space formed by the opening 332 of the flow path substrate 33 and the storage section 382 of the case 38 functions as a liquid storage chamber RS, which is a reservoir that stores ink to be supplied to the multiple pressure chambers CV. Ink supplied from the liquid container 14 and passing through the inlet 384 is stored in the liquid storage chamber RS.

[0028] The sealing plate 37 is a structure that protects the multiple piezoelectric elements 36 from the outside air and reinforces the mechanical strength of the pressure chamber substrate 34 and the vibration plate 35. The sealing plate 37 is fixed to the second surface SF2 of the vibration plate 35, for example, by an adhesive. As shown in FIG. 3, the sealing plate 37 has a recess on the surface facing the vibration plate 35. A sealed space 372 is formed by fixing the sealing plate 37 to the second surface SF2 of the vibration plate 35. The sealed space 372 contains the multiple piezoelectric elements 36.

[0029] The vibration absorber 32 absorbs pressure fluctuations in the liquid storage chamber RS. That is, the vibration absorber 32 absorbs vibrations of the ink stored in the liquid storage chamber RS. For example, the vibration absorber 32 includes a flexible sheet member capable of elastic deformation. Specifically, the vibration absorber 32 is installed on the surface in the Z2 direction of the flow path substrate 33 so as to close the opening 332, the relay flow path 338, and the multiple supply flow paths 334 of the flow path substrate 33 and form the bottom surface of the liquid storage chamber RS.

[0030] Next, a specific configuration of the piezoelectric element 36 will be described with reference to FIG.

[0031] Fig. 4 is an explanatory diagram for explaining a specific configuration of the piezoelectric element 36. Fig. 4 also shows a schematic cross section of the piezoelectric element 36 when the liquid ejection head 3 is cut along the line aa shown in Fig. 2.

[0032] As shown in FIG. 4, the piezoelectric element 36 is generally composed of a lower electrode BE, a piezoelectric layer PL, and an upper electrode TE laminated together. However, as described above, the expression "element A and element B are laminated" is not limited to a configuration in which element A and element B are in direct contact with each other. For example, in this embodiment, an insulating layer IL1 having insulating properties is provided between the upper electrode TE and the piezoelectric layer PL. That is, in this embodiment, the piezoelectric element 36 has a lower electrode BE, an upper electrode TE, a piezoelectric layer PL provided between the lower electrode BE and the upper electrode TE, and an insulating layer IL1 provided between the upper electrode TE and the piezoelectric layer PL. The upper electrode TE is an example of a "first electrode", the lower electrode BE is an example of a "second electrode", and the insulating layer IL1 is an example of a "first insulating layer".

[0033] First, an overview of the piezoelectric element 36 will be described, focusing on the lower electrode BE, the piezoelectric layer PL, and the upper electrode TE.

[0034] The lower electrode BE is formed on the second surface SF2 of the vibration plate 35. The lower electrode BE is an individual electrode formed for each piezoelectric element 36. Specifically, a plurality of lower electrodes BE extending in the direction along the X-axis are arranged in the direction along the Y-axis at intervals from each other. A drive signal for controlling the ejection of ink from the nozzle N corresponding to the piezoelectric element 36 is applied to the lower electrode BE of each piezoelectric element 36 via the wiring board 20.

[0035] The piezoelectric layer PL is formed on the surface of the lower electrode BE. The piezoelectric layer PL is a strip-shaped dielectric film extending in the direction along the Y axis so as to be continuous across the piezoelectric elements 36. As shown in FIG. 4, in a plan view from the Z2 direction, a long notch G is formed in the direction along the X axis in an area of ​​the piezoelectric layer PL corresponding to the gap between adjacent pressure chambers CV. The notch G is a through hole penetrating the piezoelectric layer PL. According to the above configuration, each piezoelectric element 36 deforms individually for each pressure chamber CV, so that the propagation of vibration between the piezoelectric elements 36 is suppressed. This makes it possible to control the ink ejection characteristics of each nozzle N with high precision. The ejection characteristics are, for example, the ink ejection amount, the ink ejection speed, or the ink ejection direction.

[0036] The upper electrode TE is formed on the surface of the piezoelectric layer PL via an insulating layer IL1. Specifically, the upper electrode TE is a strip-shaped common electrode extending in the direction along the Y-axis so as to be continuous across the plurality of piezoelectric elements 36. A reference voltage is supplied to the upper electrode TE, which functions as a common electrode. In FIG. 4, in order to avoid complication of the drawing, the wiring connected to the lower electrode BE and supplying a drive signal to the lower electrode BE, and the wiring connected to the upper electrode TE and supplying a reference voltage to the upper electrode TE are omitted.

[0037] The piezoelectric layer PL deforms in response to the voltage difference between the drive signal supplied to the lower electrode BE and the reference voltage supplied to the upper electrode TE. For example, a portion of the piezoelectric layer PL sandwiched between the lower electrode BE and the upper electrode TE deforms. This deformation causes the piezoelectric element 36 to bend the vibration plate 35. That is, the piezoelectric element 36 vibrates the vibration plate 35 in response to the voltage difference between the drive signal applied to the lower electrode BE and the reference voltage supplied to the upper electrode TE. The piezoelectric element 36 is formed individually for each pressure chamber CV. Specifically, a plurality of piezoelectric elements 36 formed long in the direction along the X-axis are arranged in the direction along the Y-axis at intervals from each other.

[0038] Next, the insulating layer IL1 will be described.

[0039] In this embodiment, as described above, the insulating layer IL1 is provided between the upper electrode TE and the piezoelectric layer PL. That is, in this embodiment, the insulating layer IL1 is formed on the surface of the piezoelectric layer PL, and the upper electrode TE is formed on the surface of the insulating layer IL1. Note that, similar to the upper electrode TE, the insulating layer IL1 may be formed in a strip shape extending in the direction along the Y axis so as to be continuous across the multiple piezoelectric elements 36. Alternatively, similar to the lower electrode BE, the insulating layer IL1 may be formed individually for each piezoelectric element 36.

[0040] Details will be described with reference to FIG. 5, but the insulating layer IL1 provided between the upper electrode TE and the piezoelectric layer PL reduces the leakage current between the upper electrode TE and the lower electrode BE.

[0041] For example, in a conventional piezoelectric element not provided with an insulating layer IL1, when a drive signal including an AC waveform or a pulse waveform is supplied, a large leakage current flows instantaneously when the voltage between the lower electrode BE and the upper electrode TE changes across 0V. For this reason, in this embodiment, the insulating layer IL1 is provided mainly to suppress the leakage current that flows when the voltage between the lower electrode BE and the upper electrode TE changes across 0V. The thickness TH1 of the insulating layer IL1 is not particularly limited. However, when focusing on the voltage drop in the insulating layer IL1, it is preferable that the thickness TH1 of the insulating layer IL1 is thin.

[0042] For example, when the thickness TH1 of the insulating layer IL1 is thick, the insulation is improved compared to when the thickness TH1 of the insulating layer IL1 is thin, but the voltage drop in the insulating layer IL1 increases. When the voltage drop in the insulating layer IL1 is large, a sufficient voltage is not applied to the piezoelectric layer PL compared to when the voltage drop in the insulating layer IL1 is small. For this reason, when the voltage drop in the insulating layer IL1 is large, it may be more difficult to accurately control the ink ejection characteristics compared to when the voltage drop in the insulating layer IL1 is small.

[0043] When the thickness TH1 of the insulating layer IL1 is small, during a period in which a relatively large voltage is applied between the bottom electrode BE and the top electrode TE, the insulating layer IL1 can be substantially ignored due to the tunnel effect.

[0044] Here, for example, during a supply period in which a drive signal is supplied to the piezoelectric element 36, in a first period in which the voltage between the lower electrode BE and the upper electrode TE varies across 0V, the voltage between the lower electrode BE and the upper electrode TE is smaller than that in a second period excluding the first period. In other words, in the second period, a larger voltage is applied between the lower electrode BE and the upper electrode TE than that in the first period. Therefore, it is preferable that the thickness TH1 of the insulating layer IL1 is, for example, a thickness at which a tunnel effect occurs in the second period. Specifically, it is preferable that the thickness TH1 of the insulating layer IL1 is greater than 0 nm and equal to or less than 5 nm.

[0045] In this manner, in this embodiment, by reducing the thickness TH1 of the insulating layer IL1, the voltage drop in the insulating layer IL1 can be suppressed throughout the entire supply period in which the drive signal is supplied to the piezoelectric element 36. Note that, in the first period in which the voltage between the lower electrode BE and the upper electrode TE is small, even if the thickness TH1 of the insulating layer IL1 is thin, the tunnel effect is unlikely to occur. Therefore, in the first period in which the voltage between the lower electrode BE and the upper electrode TE is small, even if the thickness TH1 of the insulating layer IL1 is thin, the insulating properties of the insulating layer IL1 are effectively exhibited. Therefore, even if the thickness TH1 of the insulating layer IL1 is thin, the leakage current that flows when the voltage between the lower electrode BE and the upper electrode TE changes across 0V is suppressed by the insulating layer IL1.

[0046] Therefore, in this embodiment, by reducing the thickness TH1 of the insulating layer IL1, it is possible to suppress the voltage drop in the insulating layer IL1 while suppressing the leakage current that flows when the voltage between the lower electrode BE and the upper electrode TE changes across 0 V.

[0047] In addition, in this embodiment, since the leakage current is suppressed by the insulating layer IL1, the freedom of selection of materials for the lower electrode BE and the upper electrode TE is improved. For example, in this embodiment, as the electrode material for the lower electrode BE and the upper electrode TE, a material having a lower electrical resistivity than iridium (Ir) and platinum (Pt), which are mainly used as electrode materials in conventional piezoelectric elements, can be used.

[0048] Specifically, the lower electrode BE and the upper electrode TE may contain at least one of silver (Ag), copper (Cu), gold (Au), aluminum (Al), calcium (Ca), magnesium (Mg), rhodium (Rh), sodium (Na), and molybdenum (Mo). That is, the electrode material may contain, for example, any one of silver (Ag), copper (Cu), gold (Au), aluminum (Al), calcium (Ca), magnesium (Mg), rhodium (Rh), sodium (Na), and molybdenum (Mo) as a main component. Here, the main component is, for example, a component contained in the electrode material in an amount of 50% or more. Therefore, the electrode material contained in the upper electrode TE may contain a main component material and a material different from the main component material.

[0049] In this embodiment, the electrical resistance of the lower electrode BE and the upper electrode TE can be reduced by using a material having a lower electrical resistivity than iridium (Ir) as the electrode material for the lower electrode BE and the upper electrode TE. In this case, when the piezoelectric element 36 is driven at high speed, charges move quickly, improving the electric field response. That is, in this embodiment, the piezoelectric element 36 can be driven at high speed by using a material having a lower electrical resistivity than iridium (Ir) as the electrode material for the lower electrode BE and the upper electrode TE.

[0050] In addition, in this embodiment, a material that is less expensive than iridium (Ir) and platinum (Pt) can be used as the electrode material. This allows the manufacturing cost of the piezoelectric element 36 to be reduced in this embodiment. In addition, in this embodiment, a material that has a smaller Young's modulus than iridium (Ir) and platinum (Pt) can be used as the electrode material. This allows the shape of the piezoelectric element 36 to be easily displaced in this embodiment.

[0051] Next, with reference to FIG. 5, the relationship between the energy gaps of the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL will be described.

[0052] Fig. 5 is an explanatory diagram for explaining the relationship of the energy gap between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL. In Fig. 5, the energy band diagrams of the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL are shown in two parts, a first pattern and a second pattern. In Fig. 5, the vacuum level is set as a reference potential of 0 eV. In Fig. 5, the energy level E located in the lower direction of the paper is higher than the energy level E located in the upper direction of the paper in each of the first pattern and the second pattern.

[0053] In the following description, the electron affinity of the material contained in each layer may be referred to as the electron affinity of the layer. For example, the electron affinity EAp and ionization energy IEp of the piezoelectric material contained in the piezoelectric layer PL may be referred to as the electron affinity EAp and ionization energy IEp of the piezoelectric layer PL, respectively.

[0054] 5, it is assumed that the piezoelectric material contained in the piezoelectric layer PL is lead zirconate titanate (PZT). In this case, for example, the electron affinity EAp of the piezoelectric layer PL is 3.5 eV, and the ionization energy IEp of the piezoelectric layer PL is 7.0 eV.

[0055] Here, in this embodiment, the electron affinity indicates the magnitude of the energy from the vacuum level to the bottom of the conduction band, the ionization energy indicates the magnitude of the energy from the vacuum level to the top of the valence band, and the work function indicates the magnitude of the energy from the vacuum level to the Fermi level. For example, in the piezoelectric layer PL, the electron affinity EAp indicates the magnitude of the energy from the vacuum level to the bottom of the conduction band Ecp, and the ionization energy IEp indicates the magnitude of the energy from the vacuum level to the top of the valence band Evp. Similarly, in the insulating layer IL1, the electron affinity EAi indicates the magnitude of the energy from the vacuum level to the bottom of the conduction band Eci, and the ionization energy IEi indicates the magnitude of the energy from the vacuum level to the top of the valence band Evi. Also, for example, the work function WF of the upper electrode TE indicates the magnitude of the energy from the vacuum level to the Fermi level Ef of the upper electrode TE.

[0056] Although not shown in FIG. 5, the Fermi levels of the insulating layer IL1 and the piezoelectric layer PL cannot be measured directly, so the Fermi levels of the insulating layer IL1 and the piezoelectric layer PL are indirectly measured by measuring the Fermi levels of the metals that come into contact with the insulating layer IL1 and the piezoelectric layer PL. For example, the Fermi levels of the insulating layer IL1 and the piezoelectric layer PL move to positions where they are aligned with the Fermi levels of the metals by contacting the insulating layer IL1 and the piezoelectric layer PL. Therefore, the Fermi levels of the insulating layer IL1 and the piezoelectric layer PL can be indirectly measured by measuring the Fermi levels of the metals that come into contact with the insulating layer IL1 and the piezoelectric layer PL. For example, the Fermi level of the insulating layer IL1 is determined as the work function of the insulating layer IL1 by measuring the energy difference between the Fermi level indirectly measured based on the Fermi level of the metal that comes into contact with the insulating layer IL1 during measurement and the vacuum level. Similarly, in the piezoelectric layer PL, the energy difference between the Fermi level, which is indirectly measured based on the Fermi level of a metal in contact during measurement, and the vacuum level is specified as the work function of the piezoelectric layer PL.

[0057] Here, the work function WF of the upper electrode TE is an example of "the work function WF1 of the electrode material included in the first electrode." Also, the electron affinity EAi of the insulating layer IL1 is an example of "the electron affinity EAi1 of the insulating material included in the first insulating layer," and the ionization energy IEi of the insulating layer IL1 is an example of "the ionization energy IEi1 of the insulating material included in the first insulating layer."

[0058] In this embodiment, as described in FIG. 4, an insulating layer IL1 is provided between the upper electrode TE and the piezoelectric layer PL. Since the insulating layer IL1 has a large band gap, the charge injection of holes and the charge injection of electrons into the piezoelectric layer PL are suppressed by the insulating layer IL1. As a result, in this embodiment, the leakage current can be reduced. However, depending on the relationship between the energy gaps of the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL, the leakage current may not be suppressed. In FIG. 5, the appropriate relationship between the energy gaps of the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL is described for the first pattern and the second pattern.

[0059] The first pattern in Fig. 5 shows the relationship of the energy gap between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL when the work function WF of the upper electrode TE is greater than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL. The second pattern in Fig. 5 shows the relationship of the energy gap between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL when the work function WF of the upper electrode TE is less than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL. Note that (Ecp+Evp) / 2 in Fig. 5 indicates the energy level of half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL.

[0060] In the first pattern of FIG. 5, as described above, the work function WF of the upper electrode TE is greater than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL. That is, in the first pattern, the Fermi level Ef of the upper electrode TE is closer to the upper end Evp of the valence band than the lower end Ecp of the conduction band of the piezoelectric layer PL. In this case, as shown by the star in the first pattern of FIG. 5, a material in which the ionization energy IEi of the insulating layer IL1 is greater than the ionization energy IEp of the piezoelectric layer PL is used as the insulating material contained in the insulating layer IL1. That is, a material in which the upper end Evi of the valence band of the insulating layer IL1 is higher than the upper end Evp of the valence band of the piezoelectric layer PL is used as the insulating material contained in the insulating layer IL1. As a result, in this embodiment, the movement of holes between the upper electrode TE and the piezoelectric layer PL, particularly the injection of holes from the upper electrode TE to the piezoelectric layer PL, can be suppressed.

[0061] For example, in the first pattern, the insulating material contained in the insulating layer IL1 may be an oxide containing, for example, any one of molybdenum (Mo), chromium (Cr), vanadium (V), tungsten (W), titanium (Ti), and tantalum (Ta) as a main component. The main component is, for example, a component contained in the insulating material at an amount of 50% or more. Therefore, the insulating material contained in the insulating layer IL1 may contain an oxide of the main component and an oxide different from the oxide of the main component. That is, the insulating layer IL1 may be formed by combining a plurality of oxides. For example, it is considered that the ionization energy IEi of the insulating layer IL1 is likely to be large by forming the insulating layer IL1 by combining a plurality of oxides having large ionization energies IEi.

[0062] Thus, in the first pattern piezoelectric element 36, the work function WF of the upper electrode TE, the electron affinity EAp of the piezoelectric layer PL, the ionization energy IEp of the piezoelectric layer PL, and the ionization energy IEi of the insulating layer IL1 satisfy equation (2) when the following equation (1) is satisfied. WF>(EAp+IEp) / 2 (1) IEi>IEp ···(2)

[0063] Here, for example, if formula (1) is satisfied but formula (2) is not satisfied, the insulating layer IL1 does not act as a barrier, and holes may easily move from the upper electrode TE to the piezoelectric layer PL. Specifically, in the first pattern, if the ionization energy IEi of the insulating layer IL1 is equal to or greater than the work function WF of the upper electrode TE and equal to or less than the ionization energy IEp of the piezoelectric layer PL, holes may easily move from the upper electrode TE to the piezoelectric layer PL. In this case, the leakage current is not suppressed. Thus, even in a configuration in which the insulating layer IL1 is provided between the upper electrode TE and the piezoelectric layer PL, the leakage current may not be suppressed depending on the relationship of the energy gap between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL.

[0064] In this embodiment, as described above, when formula (1) is satisfied, a material that satisfies formula (2) is used as the insulating material contained in the insulating layer IL1, so that the insulating layer IL1 acts as a barrier and can suppress the leakage current. In other words, when both formulas (1) and (2) are satisfied, the leakage current can be suppressed more reliably than when formula (1) is satisfied but formula (2) is not satisfied.

[0065] In addition, in the second pattern of FIG. 5, as described above, the work function WF of the upper electrode TE is smaller than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL. That is, in the first pattern, the Fermi level Ef of the upper electrode TE is closer to the bottom end Ecp of the conduction band than the top end Evp of the valence band of the piezoelectric layer PL. In this case, as shown by the star in the second pattern of FIG. 5, a material that makes the electron affinity EAi of the insulating layer IL1 smaller than the electron affinity EAp of the piezoelectric layer PL is used as the insulating material contained in the insulating layer IL1. That is, a material that makes the bottom end Eci of the conduction band of the insulating layer IL1 lower than the bottom end Ecp of the conduction band of the piezoelectric layer PL is used as the insulating material contained in the insulating layer IL1. As a result, in this embodiment, the movement of electrons between the upper electrode TE and the piezoelectric layer PL, in particular, the injection of electrons from the upper electrode TE to the piezoelectric layer PL can be suppressed.

[0066] For example, in the second pattern, the insulating material contained in the insulating layer IL1 may be an oxide containing, for example, any one of copper (Cu), cobalt (Co), and zirconium (Zr) as a main component. In the second pattern, the insulating material contained in the insulating layer IL1 may also contain an oxide of the main component and an oxide different from the oxide of the main component. That is, in the second pattern, the insulating layer IL1 may also be formed by combining a plurality of oxides. For example, it is considered that the electron affinity EAi of the insulating layer IL1 is likely to be small by forming the insulating layer IL1 by combining a plurality of oxides having small electron affinities EAi.

[0067] Thus, in the second pattern piezoelectric element 36, the work function WF of the upper electrode TE, the electron affinity EAp of the piezoelectric layer PL, the ionization energy IEp of the piezoelectric layer PL, and the electron affinity EAi of the insulating layer IL1 satisfy equation (4) when the following equation (3) is satisfied. WF<(EAp+IEp) / 2 (3) E.A.i <EAp ···(4)

[0068] Here, for example, if formula (3) is satisfied but formula (4) is not satisfied, the insulating layer IL1 does not act as a barrier, and electrons may easily move from the upper electrode TE to the piezoelectric layer PL. Specifically, in the second pattern, if the electron affinity EAi of the insulating layer IL1 is equal to or greater than the electron affinity EAp of the piezoelectric layer PL and equal to or less than the work function WF of the upper electrode TE, electrons may easily move from the upper electrode TE to the piezoelectric layer PL. In this case, the leakage current is not suppressed. Thus, even in a configuration in which the insulating layer IL1 is provided between the upper electrode TE and the piezoelectric layer PL, the leakage current may not be suppressed depending on the relationship of the energy gap between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL.

[0069] In this embodiment, as described above, when formula (3) is satisfied, a material that satisfies formula (4) is used as the insulating material contained in the insulating layer IL1, so that the insulating layer IL1 acts as a barrier and can suppress the leakage current. That is, when formula (3) and formula (4) are both satisfied, the leakage current can be suppressed more reliably than when formula (3) is satisfied but formula (4) is not satisfied.

[0070] Here, with reference to the energy band diagram of the piezoelectric layer PL in FIG. 5, a brief description will be given of the relationship of the energy gap between the electrodes of a conventional piezoelectric element not provided with an insulating layer IL1 and the piezoelectric layer PL.

[0071] In conventional piezoelectric elements, an electrode material is used in which the Fermi level of the electrode is located in the center between the valence band and the conduction band of the piezoelectric layer PL. For example, when the electron affinity EAp of the piezoelectric layer PL is 3.5 eV and the ionization energy IEp is 7.0 eV, iridium (Ir) and platinum (Pt) having a work function WF of about 5.6 eV are used as the electrode material. In iridium (Ir) or platinum (Pt), the work function WF is half the sum of 3.5 eV and 7.0 eV, that is, 5.6 eV, which is close to the center between the valence band and the conduction band of the piezoelectric layer PL, so that the charge injection of holes and the charge injection of electrons into the piezoelectric layer PL are suppressed. However, the electrical resistivity of iridium (Ir) and platinum (Pt) is not so small, so there is a problem with the responsiveness when driving the piezoelectric element at high speed.

[0072] In contrast, in this embodiment, as described above, an insulating layer IL1 that suppresses the movement of holes and electrons between the upper electrode TE and the piezoelectric layer PL is provided between the upper electrode TE and the piezoelectric layer PL. Therefore, in this embodiment, as described in Fig. 4, a material having a lower electrical resistivity than iridium (Ir) can be used as the electrode material for the lower electrode BE and the upper electrode TE. As a result, in this embodiment, the piezoelectric element 36 can be driven at high speed.

[0073] Next, a preferred work function of the insulating layer IL1 will be briefly described with reference to the energy band diagram of the piezoelectric layer PL in FIG.

[0074] Since the Fermi level of the piezoelectric layer PL changes due to the transfer of charges between the piezoelectric layer PL and a material in contact with the piezoelectric layer PL, the work function of the piezoelectric layer PL changes within a range from the lower end Ecp of the conduction band to the upper end Evp of the valence band of the piezoelectric layer PL. For this reason, it is preferable to use a material whose work function of the insulating layer IL1 falls within a range from the lower end Ecp of the conduction band to the upper end Evp of the valence band of the piezoelectric layer PL as the insulating material contained in the insulating layer IL1.

[0075] For example, when the work function of the insulating layer IL1 does not fall within the range from the lower end Ecp of the conduction band of the piezoelectric layer PL to the upper end Evp of the valence band, a very large band bending occurs at the interface between the insulating layer IL1 and the piezoelectric layer PL and at the interface between the insulating layer IL1 and the upper electrode TE. In this case, charge is easily transferred from the upper electrode TE to the piezoelectric layer PL via the insulating layer IL1. Therefore, the fact that the work function of the insulating layer IL1 does not fall within the range from the lower end Ecp of the conduction band of the piezoelectric layer PL to the upper end Evp of the valence band is one of the factors that increases the leakage current. In other words, in order to suppress the increase in the leakage current, it is preferable to set the work function of the insulating layer IL1 within the range from the lower end Ecp of the conduction band of the piezoelectric layer PL to the upper end Evp of the valence band. For example, in a FET (Field Effect Transistor), when there is an energy gap of about 1 eV, the current tends to flow very little. Therefore, for example, when the lower end Ecp of the conduction band of the piezoelectric layer PL is 3.5 eV and the upper end Evp of the valence band is 7.0 eV, it is preferable to use a material having a work function WF of 3.5 eV to 7.0 eV as the insulating material, and it is further preferable to use a material having a work function WF of 4.5 eV to 6.0 eV as the insulating material.

[0076] The relationship of the energy gap between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL is not limited to the example shown in FIG. 5. For example, regardless of the first pattern and the second pattern, a material that satisfies both of the above formulas (2) and (4) may be used as the insulating material contained in the insulating layer IL1. That is, a material in which the ionization energy IEi of the insulating layer IL1 is greater than the ionization energy IEp of the piezoelectric layer PL and the electron affinity EAi of the insulating layer IL1 is smaller than the electron affinity EAp of the piezoelectric layer PL may be used as the insulating material contained in the insulating layer IL1. In this case, the movement of both holes and electrons between the upper electrode TE and the piezoelectric layer PL can be suppressed. Therefore, it is preferable that a material that satisfies both of the above formulas (2) and (4) is used as the insulating material contained in the insulating layer IL1. In addition, the insulating material contained in the insulating layer IL1 may be an organic material. When the insulating material is an organic material, the insulating layer IL1 having a thin thickness TH1 can be easily formed.

[0077] In addition, the piezoelectric material contained in the piezoelectric layer PL is not limited to lead zirconate titanate (PZT). For example, the piezoelectric material contained in the piezoelectric layer PL may be potassium sodium niobate (KNN), barium titanate, or bismuth sodium titanate (BNT).

[0078] As described above, in this embodiment, the liquid ejection head 3 includes a piezoelectric element 36, a pressure chamber CV filled with ink, the pressure of which changes when the piezoelectric element 36 is driven, and a nozzle N that ejects the ink filled in the pressure chamber CV when the piezoelectric element 36 is driven. The piezoelectric element 36 includes an upper electrode TE, a lower electrode BE, a piezoelectric layer PL provided between the upper electrode TE and the lower electrode BE, and an insulating layer IL1 provided between the upper electrode TE and the piezoelectric layer PL and having insulating properties. When the electron affinity of the piezoelectric material included in the piezoelectric layer PL is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material included in the upper electrode TE is WF, and the ionization energy of the insulating material included in the insulating layer IL1 is IEi, both WF>(EAp+IEp) / 2 and IEi>IEp are satisfied.

[0079] That is, in this embodiment, when the work function WF of the electrode material is greater than half the sum of the electron affinity EAp and ionization energy IEp of the piezoelectric material, the insulating layer IL1 contains an insulating material whose ionization energy IEi is greater than the ionization energy IEp of the piezoelectric material. This makes it possible to suppress the movement of holes between the upper electrode TE and the piezoelectric layer PL in this embodiment. As a result, in this embodiment, it is possible to suppress the leakage current flowing between the upper electrode TE and the lower electrode BE. For example, in this embodiment, it is possible to suppress the occurrence of leakage current between the upper electrode TE and the piezoelectric layer PL.

[0080] Further, in the present embodiment, when the electron affinity of the insulating material is E Ai, it may further satisfy E Ai < E Ap. That is, in the present embodiment, the insulating layer IL1 may include an insulating material having an ionization energy I Ei greater than the ionization energy I Ep of the piezoelectric material and an electron affinity E Ai smaller than the electron affinity E Ap of the piezoelectric material. In this case, compared with the case where a material having an electron affinity E Ai greater than the electron affinity E Ap of the piezoelectric material is used as the insulating material included in the insulating layer IL1, the electron affinity E Ai of the insulating material is separated from the work function WF of the electrode material. Therefore, the movement of electrons between the upper electrode TE and the piezoelectric layer PL is suppressed. That is, when further satisfying E Ai < E Ap, the movement of both holes and electrons between the upper electrode TE and the piezoelectric layer PL can be suppressed.

[0081] Further, in the present embodiment, the insulating material may be an oxide containing any one of molybdenum (Mo), chromium (Cr), vanadium (V), tungsten (W), titanium (Ti), and tantalum (Ta). Even in this case, since the movement of holes between the upper electrode TE and the piezoelectric layer PL can be suppressed, the leakage current flowing between the upper electrode TE and the lower electrode BE can be suppressed.

[0082] Further, in the present embodiment, when the work function WF of the electrode material is smaller than one-half of the sum of the electron affinity E Ap and the ionization energy I Ep of the piezoelectric material, the following conditions are satisfied. When the electron affinity of the piezoelectric material contained in the piezoelectric layer PL is E Ap, the ionization energy of the piezoelectric material is I Ep, the work function of the electrode material contained in the upper electrode TE is WF, and the electron affinity of the insulating material contained in the insulating layer IL1 is E Ai, both WF < (E Ap + I Ep) / 2 and E Ai < E Ap are satisfied.

[0083] That is, in this embodiment, when the work function WF of the electrode material is smaller than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric material, the insulating layer IL1 includes an insulating material whose electron affinity EAi is smaller than the electron affinity EAp of the piezoelectric material. This makes it possible to suppress the movement of electrons between the upper electrode TE and the piezoelectric layer PL in this embodiment. As a result, it is possible to suppress the leakage current flowing between the upper electrode TE and the lower electrode BE in this embodiment.

[0084] In addition, in this embodiment, when the work function WF of the electrode material is smaller than half the sum of the electron affinity EAp and ionization energy IEp of the piezoelectric material, IEi>IEp may be further satisfied when the ionization energy of the insulating material is IEi. That is, in this embodiment, the insulating layer IL1 may include an insulating material whose ionization energy IEi is larger than the ionization energy IEp of the piezoelectric material and whose electron affinity EAi is smaller than the electron affinity EAp of the piezoelectric material. In this case, the ionization energy IEi of the insulating material is more distant from the work function WF of the electrode material than when a material whose ionization energy IEi is smaller than the ionization energy IEp of the piezoelectric material is used as the insulating material contained in the insulating layer IL1. Therefore, the movement of electrons between the upper electrode TE and the piezoelectric layer PL is suppressed. That is, when IEi>IEp is further satisfied, the movement of both holes and electrons between the upper electrode TE and the piezoelectric layer PL can be suppressed.

[0085] In this embodiment, when the work function WF of the electrode material is smaller than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric material, the insulating material may be an oxide containing any one of copper (Cu), cobalt (Co) and zirconium (Zr). Even in this case, the movement of electrons between the upper electrode TE and the piezoelectric layer PL can be suppressed, so that the leakage current flowing between the upper electrode TE and the lower electrode BE can be suppressed.

[0086] In this embodiment, the thickness TH1 of the insulating layer IL1 may be 5 nm or less. In this case, during the entire supply period in which the drive signal is supplied to the piezoelectric element 36, it is possible to suppress the voltage drop in the insulating layer IL1 and also suppress the leakage current that flows when the voltage between the lower electrode BE and the upper electrode TE changes across 0 V.

[0087] In this embodiment, the piezoelectric material is lead zirconate titanate (PZT). In this case, the movement of electrons between the upper electrode TE and the piezoelectric layer PL can be suppressed, and therefore the leakage current flowing between the upper electrode TE and the lower electrode BE can be suppressed.

[0088] In this embodiment, the electrode material includes any one of silver (Ag), copper (Cu), gold (Au), aluminum (Al), calcium (Ca), magnesium (Mg), rhodium (Rh), sodium (Na), and molybdenum (Mo). Even in this case, the movement of electrons between the upper electrode TE and the piezoelectric layer PL can be suppressed, so that the leakage current flowing between the upper electrode TE and the lower electrode BE can be suppressed. In this embodiment, the upper electrode TE and the lower electrode BE can be formed without using iridium (Ir) and platinum (Pt), so that the manufacturing cost of the piezoelectric element 36 can be reduced. In addition, silver (Ag), copper (Cu), gold (Au), aluminum (Al), calcium (Ca), magnesium (Mg), rhodium (Rh), sodium (Na), and molybdenum (Mo) have a smaller electrical resistivity than iridium (Ir). Therefore, in this embodiment, the electrical resistance of the upper electrode TE and the lower electrode BE can be reduced. As a result, in this embodiment, the piezoelectric element 36 can be driven at high speed.

[0089] [2. Modifications] Each of the above embodiments may be modified in various ways. Specific modified embodiments are exemplified below. Two or more embodiments arbitrarily selected from the following examples may be appropriately combined within a range that does not contradict each other. In the modified examples exemplified below, the elements whose actions and functions are equivalent to those of the embodiment will be appropriately omitted by using the symbols referenced in the above explanation.

[0090] [First Modification] In the above-described embodiment, the insulating layer IL1 is provided between the upper electrode TE and the piezoelectric layer PL, but the present invention is not limited to this embodiment. For example, as shown in Fig. 6, instead of the insulating layer IL1 provided between the upper electrode TE and the piezoelectric layer PL, an insulating layer IL2 may be provided between the lower electrode BE and the piezoelectric layer PL.

[0091] Fig. 6 is an explanatory diagram for explaining a piezoelectric element 36A according to a first modified example. Fig. 6, like Fig. 4, also shows a schematic cross section of the piezoelectric element 36A when the liquid ejection head 3 is cut along the line aa shown in Fig. 2. The liquid ejection head 3 according to this modified example is similar to the liquid ejection head 3 of the above-mentioned embodiment, except that it has a piezoelectric element 36A instead of the piezoelectric element 36 shown in Fig. 2 etc.

[0092] The piezoelectric element 36A is similar to the piezoelectric element 36 shown in Fig. 4, except that instead of the insulating layer IL1 shown in Fig. 4 being provided between the upper electrode TE and the piezoelectric layer PL, an insulating layer IL2 is provided between the lower electrode BE and the piezoelectric layer PL. That is, in this modification, the lower electrode BE is formed on the second surface SF2 of the vibration plate 35, and an insulating layer IL2 having insulating properties is formed on the surface of the lower electrode BE. Then, the piezoelectric layer PL is formed on the surface of the insulating layer IL2, and the upper electrode TE is formed on the surface of the piezoelectric layer PL. Note that the insulating layer IL2 may be formed so as to cover the lower electrode BE.

[0093] The thickness TH2 of the insulating layer IL2 is preferably greater than 0 nm and equal to or less than 5 nm, similar to the insulating layer IL1 shown in Fig. 4. The relationship of the energy gaps among the lower electrode BE, the insulating layer IL2, and the piezoelectric layer PL can be explained by replacing the "upper electrode TE" and the "insulating layer IL1" in Fig. 5 with the "lower electrode BE" and the "insulating layer IL2," respectively.

[0094] For example, in this modification, when the work function WF of the electrode material contained in the lower electrode BE and the ionization energy IEi of the insulating material contained in the insulating layer IL2 are satisfied and the formula (1) described in Fig. 4 is satisfied, the formula (2) is satisfied. That is, in this modification, when the work function WF of the lower electrode BE is greater than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL, a material having an ionization energy IEi greater than the ionization energy IEp of the piezoelectric layer PL is used as the insulating material contained in the insulating layer IL2. In this case, the insulating material contained in the insulating layer IL2 may be, for example, an oxide containing any one of molybdenum (Mo), chromium (Cr), vanadium (V), tungsten (W), titanium (Ti), and tantalum (Ta) as a main component.

[0095] Alternatively, for example, in this modification, when the electron affinity of the insulating material contained in the insulating layer IL2 is EAi and the formula (3) described in Fig. 4 is satisfied, the formula (4) is satisfied. That is, in this modification, when the work function WF of the lower electrode BE is smaller than half the sum of the electron affinity EAp and the ionization energy IEp of the piezoelectric layer PL, a material whose electron affinity EAi is smaller than the electron affinity EAp of the piezoelectric layer PL is used as the insulating material contained in the insulating layer IL2. In this case, the insulating material contained in the insulating layer IL2 may be, for example, an oxide containing any one of copper (Cu), cobalt (Co), and zirconium (Zr) as a main component.

[0096] In this modification, similarly to the insulating layer IL1, the insulating material contained in the insulating layer IL2 may contain a main component oxide and an oxide different from the main component oxide. That is, in this modification, the insulating layer IL2 may be formed by combining a plurality of oxides. Also, in this modification, similarly to the insulating layer IL1, it is preferable that a material whose work function of the insulating layer IL2 falls within the range from the lower end Ecp of the conduction band of the piezoelectric layer PL to the upper end Evp of the valence band is used as the insulating material contained in the insulating layer IL2.

[0097] In this modification, the lower electrode BE is an example of a "first electrode", the upper electrode TE is an example of a "second electrode", and the insulating layer IL2 is an example of a "first insulating layer". Therefore, the work function WF of the lower electrode BE is an example of a "work function WF1 of the electrode material contained in the first electrode". In addition, the electron affinity EAi of the insulating layer IL2 is an example of an "electron affinity EAi1 of the insulating material contained in the first insulating layer", and the ionization energy IEi of the insulating layer IL2 is an example of an "ionization energy IEi1 of the insulating material contained in the first insulating layer".

[0098] As described above, the present modification can also provide the same effects as the above-mentioned embodiment. For example, the present modification can also suppress the leakage current flowing between the upper electrode TE and the lower electrode BE. Specifically, for example, the present modification can suppress the occurrence of the leakage current between the lower electrode BE and the piezoelectric layer PL.

[0099] [Second modified example] In the above-described embodiment, the insulating layer IL1 is provided between the upper electrode TE and the piezoelectric layer PL, but the present invention is not limited to this. For example, as shown in FIG. 7, an insulating layer IL2 may be further provided between the lower electrode BE and the piezoelectric layer PL.

[0100] Fig. 7 is an explanatory diagram for explaining a piezoelectric element 36B according to a second modified example. Fig. 7, like Fig. 4, also shows a schematic cross section of the piezoelectric element 36B when the liquid ejection head 3 is cut along the line aa shown in Fig. 2. The liquid ejection head 3 according to this modified example is similar to the liquid ejection head 3 of the above-mentioned embodiment, except that it has a piezoelectric element 36B instead of the piezoelectric element 36 shown in Fig. 2 etc.

[0101] The piezoelectric element 36B is similar to the piezoelectric element 36 shown in FIG. 4, except that it has an insulating layer IL2 provided between the lower electrode BE and the piezoelectric layer PL. That is, in this modification, the lower electrode BE is formed on the second surface SF2 of the vibration plate 35, and the insulating layer IL2 is formed on the surface of the lower electrode BE. Then, the piezoelectric layer PL is formed on the surface of the insulating layer IL2, the insulating layer IL1 is formed on the surface of the piezoelectric layer PL, and the upper electrode TE is formed on the surface of the insulating layer IL1. Note that the insulating layer IL2 may be formed so as to cover the lower electrode BE. In this modification, the insulating layer IL2 is an example of a "second insulating layer."

[0102] The insulating layer IL1 is similar to the insulating layer IL1 in the above-described embodiment, and the insulating layer IL2 is similar to the insulating layer IL2 in the above-described first modified example. For example, each of the thickness TH1 of the insulating layer IL1 and the thickness TH2 of the insulating layer IL2 is preferably greater than 0 nm and equal to or less than 5 nm. The relationship of the energy gaps between the upper electrode TE, the insulating layer IL1, and the piezoelectric layer PL is explained in FIG. 5. The relationship of the energy gaps between the lower electrode BE, the insulating layer IL2, and the piezoelectric layer PL is explained by replacing the "upper electrode TE" and "insulating layer IL1" in FIG. 5 with the "lower electrode BE" and "insulating layer IL2," respectively.

[0103] Also, the insulating material included in the insulating layer IL1 is the same as the insulating layer IL1 in the above-described embodiment, and the insulating material included in the insulating layer IL2 is the same as the insulating layer IL2 in the first modified example described above. Note that the insulating material included in the insulating layer IL2 may be the same as the insulating material included in the insulating layer IL1 or may be different from the insulating material included in the insulating layer IL1. Also, the electrode material included in the lower electrode BE may be the same as the electrode material included in the upper electrode TE or may be different from the electrode material included in the upper electrode TE.

[0104] Note that in this modified example, the lower electrode BE is an example of the "second electrode", and the insulating layer IL2 is an example of the "second insulating layer". Therefore, the work function WF of the lower electrode BE is an example of the "work function WF2 of the electrode material included in the second electrode". Also, the electron affinity EAi of the insulating layer IL2 is an example of the "electron affinity EAi2 of the insulating material included in the second insulating layer", and the ionization energy IEi of the insulating layer IL2 is an example of the "ionization energy IEi2 of the insulating material included in the second insulating layer".

[0105] As described above, in this modified example, for example, the piezoelectric element 36B is provided between the lower electrode BE and the piezoelectric layer PL, and further includes an insulating layer IL2 having insulating properties. When the work function of the electrode material included in the lower electrode BE is WF and the ionization energy of the insulating material included in the insulating layer IL2 is IEi, both WF > (EAp + IEp) / 2 and IEi > IEp are satisfied. Alternatively, when the work function of the electrode material included in the lower electrode BE is WF and the electron affinity of the insulating material included in the insulating layer IL2 is EAi, both WF < (EAp + IEp) / 2 and EAi < EAp are satisfied. As described above, also in this modified example, the same effects as those of the above-described embodiment can be obtained. For example, also in this modified example, the leakage current flowing between the upper electrode TE and the lower electrode BE can be suppressed. Specifically, for example, in this modified example, the generation of leakage current can be suppressed between the upper electrode TE and the piezoelectric layer PL, and further, the generation of leakage current can be suppressed between the lower electrode BE and the piezoelectric layer PL.

[0106] [Third Modification] In the above-described embodiment and modified example, the lower electrode BE is an individual electrode and the upper electrode TE is a common electrode, but the present invention is not limited to such an embodiment. For example, the lower electrode BE may be a common electrode that is continuous across multiple piezoelectric elements 36, and the upper electrode TE may be an individual electrode for each piezoelectric element 36. As described above, the same effects as those of the above-described embodiment can be obtained in this modified example as well.

[0107] [Fourth Variation] In the above-mentioned embodiment and modification, the method of forming the lower electrode BE and the upper electrode TE is not particularly limited, but the lower electrode BE and the upper electrode TE may be formed by a vapor phase method. As described above, the present modification can also provide the same effects as those of the above-mentioned embodiment and modification.

[0108] [Fifth Modification] In the above-described embodiment and modified example, the liquid ejection device 1 is a serial printer, but the present invention is not limited to this embodiment. For example, the liquid ejection device 1 may be a so-called line printer in which the multiple nozzles N are provided in the liquid ejection head 3 so as to extend wider than the width of the medium PP. As described above, the modified example can also achieve the same effects as the above-described embodiment and modified example.

[0109] [Sixth Modification] The liquid ejection device 1 exemplified in the above-mentioned embodiment and modified example can be adopted in various devices such as facsimile machines and copy machines, in addition to devices dedicated to printing. However, the use of the liquid ejection device of the present invention is not limited to printing. For example, a liquid ejection device that ejects a solution of a color material is used as a manufacturing device that forms a color filter for a liquid crystal display device. Moreover, a liquid ejection device that ejects a solution of a conductive material is used as a manufacturing device that forms wiring and electrodes of a wiring board. As described above, the same effects as those of the above-mentioned embodiment and modified example can be obtained in this modified example as well.

[0110] [3. Notes] From the forms exemplified above, for example, the following configurations can be grasped.

[0111] The piezoelectric element according to Mode 1, which is a preferred mode, includes a first electrode, a second electrode, a piezoelectric layer provided between the first electrode and the second electrode, and a first insulating layer provided between the first electrode and the piezoelectric layer and having insulating properties. When the electron affinity of the piezoelectric material contained in the piezoelectric layer is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material contained in the first electrode is WF1, and the ionization energy of the insulating material contained in the first insulating layer is IEi1, both WF1 > (EAp + IEp) / 2 and IEi1 > IEp are satisfied. According to Mode 1, since the movement of holes between the first electrode and the piezoelectric layer can be suppressed, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0112] In the piezoelectric element according to Mode 2, which is a specific example of Mode 1, when the electron affinity of the insulating material is EAi1, EAi1 < EAp is satisfied. According to Mode 2, since the movement of both holes and electrons between the first electrode and the piezoelectric layer can be suppressed, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0113] In the piezoelectric element according to Mode 3, which is a specific example of Mode 1 or 2, the insulating material is an oxide containing any one of molybdenum (Mo), chromium (Cr), vanadium (V), tungsten (W), titanium (Ti), and tantalum (Ta). Also in Mode 3, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0114] The piezoelectric element according to Embodiment 4, which is another preferred embodiment, includes a first electrode, a second electrode, a piezoelectric layer provided between the first electrode and the second electrode, and a first insulating layer provided between the first electrode and the piezoelectric layer and having insulation properties. When the electron affinity of the piezoelectric material contained in the piezoelectric layer is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material contained in the first electrode is WF1, and the electron affinity of the insulating material contained in the first insulating layer is EAi1, both WF1 < (EAp + IEp) / 2 and EAi1 < EAp are satisfied. According to Embodiment 4, since the movement of electrons between the first electrode and the piezoelectric layer can be suppressed, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0115] In the piezoelectric element according to Embodiment 5, which is a specific example of Embodiment 4, when the ionization energy of the insulating material is IEi1, IEi1 > IEp is satisfied. According to Embodiment 5, since the movement of both holes and electrons between the first electrode and the piezoelectric layer can be suppressed, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0116] In the piezoelectric element according to Embodiment 6, which is a specific example of Embodiment 4 or 5, the insulating material is an oxide containing any one of copper (Cu), cobalt (Co), and zirconium (Zr). Also in Embodiment 6, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0117] In the piezoelectric element according to Embodiment 7, which is a specific example of any one of Embodiments 1 to 6, the thickness of the first insulating layer is 5 nm or less. According to Embodiment 7, during the entire supply period when a drive signal is supplied to the piezoelectric element, while suppressing the voltage drop in the insulating layer, the leakage current flowing when the voltage between the first electrode and the second electrode changes across 0 V can be suppressed.

[0118] In the piezoelectric element according to Aspect 8, which is a specific example of any one of Aspects 1 to 7, the piezoelectric material is lead zirconate titanate. Also in Aspect 8, the leakage current flowing between the first electrode and the second electrode can be suppressed.

[0119] In the piezoelectric element according to Aspect 9, which is a specific example of any one of Aspects 1 to 8, the electrode material contains any one of silver (Ag), copper (Cu), gold (Au), aluminum (Al), calcium (Ca), magnesium (Mg), rhodium (Rh), sodium (Na), and molybdenum (Mo). Also in Aspect 9, the leakage current flowing between the first electrode and the second electrode can be suppressed. Further, according to Aspect 9, since the electrical resistance of the first electrode and the second electrode can be reduced, the piezoelectric element can be driven at high speed.

[0120] In the piezoelectric element according to Aspect 10, which is a specific example of any one of Aspects 1 to 9, a second insulating layer having insulating properties is further provided between the second electrode and the piezoelectric layer. When the work function of the electrode material contained in the second electrode is WF2 and the ionization energy of the insulating material contained in the second insulating layer is IEi2, both WF2 > (EAp + IEp) / 2 and IEi2 > IEp are satisfied. Also in Aspect 10, the leakage current flowing between the first electrode and the second electrode can be suppressed. For example, according to Aspect 10, the generation of leakage current can be suppressed between the first electrode and the piezoelectric layer, and further, the generation of leakage current can be suppressed between the second electrode and the piezoelectric layer.

[0121] In the piezoelectric element according to Aspect 11, which is a specific example of any one of Aspects 1 to 9, a second insulating layer having insulating properties is further provided between the second electrode and the piezoelectric layer. When the work function of the electrode material contained in the second electrode is WF2 and the electron affinity of the insulating material contained in the second insulating layer is EAi2, both WF2 < (EAp + IEp) / 2 and EAi2 < EAp are satisfied. Also in Example 11, the leakage current flowing between the first electrode and the second electrode can be suppressed. For example, according to Example 11, the leakage current can be suppressed between the first electrode and the piezoelectric layer, and further, the leakage current can be suppressed between the second electrode and the piezoelectric layer.

[0122] A liquid ejection head according to a preferred embodiment includes a piezoelectric element according to any one of embodiments 1 to 11, a pressure chamber filled with liquid, the pressure of the liquid changing when the piezoelectric element is driven, and a nozzle that ejects the liquid filled in the pressure chamber when the piezoelectric element is driven. In this embodiment as well, the leakage current flowing between the first electrode and the second electrode can be suppressed. [Explanation of symbols]

[0123] 1...liquid ejection device, 14...liquid container, 2...control unit, 20...wiring board, 3...liquid ejection head, 31...nozzle substrate, 32...vibration absorber, 33...flow path substrate, 34...pressure chamber substrate, 35...diaphragm, 36...piezoelectric element, 37...sealing plate, 38...case, 71...medium transport mechanism, 72...carriage transport mechanism, BE...lower electrode, CV...pressure chamber, N...nozzle, PL...piezoelectric layer, PP...medium, RS...liquid storage chamber, TE...upper electrode.

Claims

1. A first electrode; A second electrode; a piezoelectric layer provided between the first electrode and the second electrode; a first insulating layer provided between the first electrode and the piezoelectric layer and having insulating properties; Equipped with When the electron affinity of the piezoelectric material contained in the piezoelectric layer is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material contained in the first electrode is WF1, and the ionization energy of the insulating material contained in the first insulating layer is IEi1, Both WF1>(EAp+IEp) / 2 and IEi1>IEp are satisfied. A piezoelectric element characterized by:

2. When the electron affinity of the insulating material is EAi1, EAi1<EAp is satisfied. The piezoelectric element according to claim 1 .

3. The insulating material is an oxide containing any one of molybdenum, chromium, vanadium, tungsten, titanium, and tantalum. The piezoelectric element according to claim 1 .

4. A first electrode; A second electrode; a piezoelectric layer provided between the first electrode and the second electrode; a first insulating layer provided between the first electrode and the piezoelectric layer and having insulating properties; Equipped with When the electron affinity of the piezoelectric material contained in the piezoelectric layer is EAp, the ionization energy of the piezoelectric material is IEp, the work function of the electrode material contained in the first electrode is WF1, and the electron affinity of the insulating material contained in the first insulating layer is EAi1, Both of WF1<(EAp+IEp) / 2 and EAi1<EAp are satisfied. A piezoelectric element characterized by:

5. When the ionization energy of the insulating material is IEi1, IEi1>IEp is satisfied; The piezoelectric element according to claim 4.

6. The insulating material is an oxide containing any one of copper, cobalt, and zirconium. The piezoelectric element according to claim 4.

7. The thickness of the first insulating layer is 5 nm or less. The piezoelectric element according to claim 1 .

8. The piezoelectric material is lead zirconate titanate. The piezoelectric element according to claim 1 .

9. The electrode material includes any one of silver, copper, gold, aluminum, calcium, magnesium, rhodium, sodium, and molybdenum; The piezoelectric element according to claim 1 .

10. a second insulating layer provided between the second electrode and the piezoelectric layer and having insulating properties; When the work function of the electrode material contained in the second electrode is WF2 and the ionization energy of the insulating material contained in the second insulating layer is IEi2, Both WF2>(EAp+IEp) / 2 and IEi2>IEp are satisfied. The piezoelectric element according to claim 1 .

11. a second insulating layer provided between the second electrode and the piezoelectric layer and having insulating properties; When the work function of the electrode material contained in the second electrode is WF2 and the electron affinity of the insulating material contained in the second insulating layer is EAi2, Both of WF2<(EAp+IEp) / 2 and EAi2<EAp are satisfied. The piezoelectric element according to claim 1 .

12. A piezoelectric element according to any one of claims 1 to 6, a pressure chamber filled with liquid, the pressure of the liquid being changed by driving the piezoelectric element; a nozzle that ejects the liquid filled in the pressure chamber by driving the piezoelectric element; Equipped with A liquid ejection head comprising:

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