A droplet-ejecting head including a substrate including a pressure chamber communicating with a nozzle hole and also a piezoelectric element that includes a lower electrode, a piezoelectric layer which is formed above the lower electrode, and an upper electrode formed above the piezoelectric element and that causes a change in pressure in a liquid contained in the pressure chamber. The piezoelectric layer includes a first piezoelectric sub-layer located on the lower electrode and a second piezoelectric sub-layer located between the first piezoelectric sub-layer and the upper electrode. The first piezoelectric sub-layer has a polarization axis predominantly directed in an in-plane direction of the first piezoelectric sub-layer. The second piezoelectric sub-layer is predominantly (100)-oriented in the pseudocubic coordinate system.

The entire disclosure of Japanese Patent Application No. 2009-227124, filed Sep. 30, 2009 is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a droplet-ejecting head, a droplet-ejecting apparatus, and a piezoelectric element. More particularly, the present invention relates to a droplet-ejecting head, apparatus, and piezoelectric element with improved ejection characteristics.

2. Related Art

Many ink jet processes and techniques for high-definition high-speed printing are currently used in the art. One commonly used technique using a piezoelectric actuator including electrodes and a piezoelectric layer sandwiched therebetween is useful in ejecting ink droplets. Typically, the material for forming the piezoelectric layer is lead zirconate titanate (Pb(Zr,Ti)O3, PZT), which is a perovskite-type oxide, as disclosed in, for example, JP-A-2001-223404.

Piezoelectric actuators for use in droplet-ejecting apparatuses such as ink jet printers need to have an increased displacement so as to eject larger droplets or such that they may be operated at lower voltage. This issue is not limited to the piezoelectric actuators but is common to piezoelectric elements for use in other apparatuses, including ultrasonic motors, pressure sensors, and ultrasonic devices such as ultrasonic oscillators.

BRIEF SUMMARY OF THE INVENTION

An advantage of some aspects of the invention is to provide a droplet-ejecting head having good displacement characteristics, resulting in superior ejection properties. Other aspects of the invention provide a droplet-ejecting apparatus including the droplet-ejecting head and an piezoelectric element for use in the droplet-ejecting head or in the droplet-ejecting apparatus.

A first aspect of the invention is a droplet-ejecting head which includes a substrate including a pressure chamber communicating with a nozzle hole and a piezoelectric element that includes a lower electrode, a piezoelectric layer which overlies the lower electrode and which is made of perovskite-type oxide, and an upper electrode overlying the piezoelectric element and that causes a change in pressure in a liquid contained in the pressure chamber. The piezoelectric layer includes a first piezoelectric sub-layer located on the lower electrode and a second piezoelectric sub-layer located between the first piezoelectric sub-layer and the upper electrode. The first piezoelectric sub-layer has a polarization axis predominantly directed in an in-plane direction of the first piezoelectric sub-layer. The second piezoelectric sub-layer is predominantly (100)-oriented in the pseudocubic coordinate system.

In the droplet-ejecting head, the polarization axis of the first piezoelectric sub-layer is predominantly directed in an in-plane direction of the first piezoelectric sub-layer and the second piezoelectric sub-layer is predominantly (100)-oriented in the pseudocubic coordinate system. This allows the droplet-ejecting head to have good displacement properties.

A second aspect of the invention is a piezoelectric element which includes a lower electrode, a piezoelectric layer which overlies the lower electrode and which is made of perovskite-type oxide, and an upper electrode overlying the piezoelectric element. The piezoelectric layer includes a first piezoelectric sub-layer located on the lower electrode and a second piezoelectric sub-layer located between the first piezoelectric sub-layer and the upper electrode. The first piezoelectric sub-layer has a polarization axis predominantly directed in an in-plane direction of the first piezoelectric sub-layer. The second piezoelectric sub-layer is predominantly (100)-oriented in the pseudocubic coordinate system.

In the piezoelectric element, the polarization axis of the first piezoelectric sub-layer is predominantly directed in an in-plane direction of the first piezoelectric sub-layer and the second piezoelectric sub-layer is predominantly (100)-oriented in the pseudocubic coordinate system. This allows the piezoelectric element to have good piezoelectric properties.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the description below, the term “an in-plane direction of the first piezoelectric sub-layer” refers to a direction in which crystal grains in the first piezoelectric sub-layer are grown, that is, a direction perpendicular to or substantially perpendicular to the thickness direction of the first piezoelectric sub-layer. The phrase “the polarization axis of the first piezoelectric sub-layer is predominantly directed in an in-plane direction of the first piezoelectric sub-layer” as used herein means that, in half or more (50% or more) of the area of the first piezoelectric sub-layer in an in-plane direction, the polarization axis is directed in the in-plane direction.

The term “pseudocubic” as used herein means that a crystal structure is almost cubic.

The term “predominantly (100)-oriented” as used herein covers the case where all of crystal grains are (100)-oriented and also covers the case where most of crystal grains (for example, 90% or more) are (100)-oriented and other crystal grains that are not (100)-oriented are (111)- or (110)-oriented.

In descriptions herein, the term “overlie” is used in, for example, the phrase “a specific thing (hereinafter referred to as “B”) overlying another specific thing (hereinafter referred to as “A”). This phrase covers the phrase “B lying on A” and the phrase “B lying above A with another thing present therebetween.”

The phrase “a crystal structure is a monoclinic structure” as used herein covers the case where all of crystal grains have a monoclinic structure in addition to the case where most of the crystal grains (for example, 90% or more) have a monoclinic structure and other crystal grains having no monoclinic structure have a tetragonal structure.

The term “lattice constant” as used herein refers to the length of a side of a unit cell with a perovskite-type structure represented by the formula ABO3.

First Embodiment

A piezoelectric element100according to a first embodiment of the present invention will now be described.FIG. 1is a schematic sectional view of the piezoelectric element100.

With reference toFIG. 1, the piezoelectric element100includes a base200, a lower electrode10disposed on the base200, a piezoelectric layer12which is disposed on the lower electrode10and which contains perovskite-type oxides, and an upper electrode14disposed on the piezoelectric layer12. The base200may take various forms depending on applications of the piezoelectric element100.

The piezoelectric layer12includes a first piezoelectric sub-layer12adisposed on the lower electrode10and a second piezoelectric sub-layer12bdisposed between the first piezoelectric sub-layer12aand the upper electrode14. The first piezoelectric sub-layer12amay have a tetragonal structure. The second piezoelectric sub-layer12bmay have a monoclinic structure. The first and second piezoelectric sub-layers12aand12bare predominantly (100)-oriented in the pseudocubic coordinate system. For example, 90% or more of crystal grains in the second piezoelectric sub-layer12bare (100)-oriented in the pseudocubic coordinate system.

Examples of a perovskite-type oxide for forming the first piezoelectric sub-layer12ainclude lead titanate and a lead titanate solid solution. Lead titanate and the lead titanate solid solution may contain a small amount of an element for forming the second piezoelectric sub-layer12b(for example, 10% or less in a B site). Examples of such an element include zirconium and niobium. Lead titanate has a tetragonal structure.

The first piezoelectric sub-layer12ahas a thickness of, for example, 1 nm to 20 nm. When the thickness of the first piezoelectric sub-layer12ais less than 1 nm, the first piezoelectric sub-layer12ais poor in controlling the orientation of the second piezoelectric sub-layer12b. When the thickness of the first piezoelectric sub-layer12ais greater than 20 nm, the first piezoelectric sub-layer12ais likely to have poor piezoelectric properties.

Examples of a perovskite-type oxide for forming the second piezoelectric sub-layer12binclude lead zirconate titanate (Pb(Zr,Ti)O3, PZT) and a lead zirconate titanate solid solution. An example of the lead zirconate titanate solid solution is lead zirconate titanate niobate (Pb(Zr,Ti,Nb)O3, PZTN). Lead zirconate titanate or the lead zirconate titanate solid solution may contain a larger amount of lead as compared to the above formulas. The number of moles of lead contained in lead zirconate titanate or the lead zirconate titanate solid solution may be 1.0 to 130 when the number of moles of zirconium and titanium contained in lead zirconate titanate or the lead zirconate titanate solid solution is one. Half of lead atoms exceeding 1.0 in the composition of the second piezoelectric sub-layer12boccupy B sites of a perovskite-type structure represented by the formula ABO3. The term “B site” as used herein refers to a site coordinated by six oxygen atoms. This is described in JP-A-2008-258575 in detail. Similarly, half of lead atoms exceeding 1.0 in the composition of the first piezoelectric sub-layer12acan occupy B sites of a perovskite-type structure represented by the formula ABO3. The fact that an excessive amount of lead is present at a B site of a perovskite-type structure has been verified with an energy dispersive X-ray fluorescence spectrometer (EDX) by transmission electron microscopy (TEM).

When the second piezoelectric sub-layer12bis made of, for example, lead zirconate titanate (Pb(ZrxTi1-x)O3), x is preferably 0.4 to 0.6 and more preferably 0.45 to 0.55. When x is within the above range, the second piezoelectric sub-layer12bcan be readily controlled so as to have a monoclinic structure. However, the crystal structure of the second piezoelectric sub-layer12bdoes not solely depend on the value of x but depend on factors such as stress, lattice defect, and dislocation. In this embodiment, the second piezoelectric sub-layer12bhas a monoclinic structure as is clear from an example below.

The thickness of the second piezoelectric sub-layer12bis not particularly limited and may be, for example, 300 nm to 1,500 nm.

The lower electrode10is one for applying a voltage to the piezoelectric layer12. The lower electrode10may include, for example, a polycrystalline platinum (Pt) layer and a polycrystalline iridium (Ir) layer disposed thereon. The Ir layer may be converted into an iridium oxide layer through a step of firing a precursor of the piezoelectric layer12. The thickness of the lower electrode10is not particularly limited and may be 50 nm to 200 nm.

The upper electrode14applies a voltage to the piezoelectric layer12. The upper electrode14may include an iridium (Ir) layer. The thickness of the upper electrode14is not particularly limited and may be 50 nm to 200 nm.

The crystal structure of the piezoelectric layer12is described below with reference toFIGS. 1 to 3.FIG. 2is a schematic view illustrating the crystal structure of the first piezoelectric sub-layer12a.FIG. 3is a schematic view illustrating the crystal structure of the second piezoelectric sub-layer12b.

As shown inFIG. 2, a crystal120in the first piezoelectric sub-layer12ahas a lattice constant a(L1Y) in a first direction (Y-direction shown inFIGS. 1 and 2) that is one of in-plane directions of the first piezoelectric sub-layer12aand a lattice constant b(L1Z) in the thickness direction (Z-direction shown inFIGS. 1 and 2) of the first piezoelectric sub-layer12a, the lattice constant a(L1Y) being equal to or substantially equal to the lattice constant b(L1Z). The in-plane directions of the first piezoelectric sub-layer12aare parallel to the upper surface10cof the lower electrode10as shown inFIG. 1. The thickness direction of the first piezoelectric sub-layer12ais perpendicular to the upper surface10cof the lower electrode10as shown inFIG. 1. The crystal120also has a lattice constant c(L1X) in a second direction (X-direction) that is one of the in-plane directions of the first piezoelectric sub-layer12a, the lattice constant c(L1Y) being greater than the lattice constant b (L1Z) in the second direction (Y-direction) perpendicular to the first direction in the pseudocubic coordinate system. The relationship between these constants is represented by the following formula:
a(L1Y)=b(L1Z)<c(L1x).

This formula expresses that the first piezoelectric sub-layer12ahas a tetragonal structure with a c-axis parallel to an in-plane direction thereof.

As shown inFIG. 3, a crystal130in the second piezoelectric sub-layer12bhas a lattice constant a(L2Y) in an in-plane direction (X-direction shown inFIGS. 1 and 2) of the second piezoelectric sub-layer12b, a lattice constant b(L2Y) in an in-plane direction (Y-direction shown inFIGS. 1 and 2) of the second piezoelectric sub-layer12b, and a lattice constant c(L2Z) in the thickness direction (Z-direction shown inFIGS. 1 and 3) of the second piezoelectric sub-layer12b, the lattice constant a(L2X) and the lattice constant b(L2Y) being greater than the lattice constant c(L2Z). The in-plane directions of the second piezoelectric sub-layer12bare parallel to the upper surface10cof the lower electrode10as shown inFIG. 1. The lattice constant a(L2X) in a first direction (X-direction) that is one of the in-plane directions of the second piezoelectric sub-layer12bis equal to or substantially equal to the lattice constant b(L2Y) in a second direction (Y-direction) which is one of the in-plane directions of the second piezoelectric sub-layer12band which is perpendicular to the first direction in the pseudocubic coordinate system. The relationship between these constants is represented by the following formula:
a(L2X)=b(L2Y)>c(L2Z).

These prove that the inequality L1Z<L1Xholds for the first piezoelectric sub-layer12a, wherein L1Zis a lattice constant in the thickness direction of the first piezoelectric sub-layer12aand L1Xis the maximum of lattice constants in in-plane directions of the first piezoelectric sub-layer12a.

As is clear from the example below, it has been verified that lattice constants satisfy the above relationship when the first piezoelectric sub-layer12ais made of lead titanate or a lead titanate solid solution. The polarization axis P1of the first piezoelectric sub-layer12ais directed in a direction in which a lattice constant is large, that is, in the X-direction inFIG. 2.

The inequality L2Z<L2Xholds for the second piezoelectric sub-layer12b, wherein L2Zis a lattice constant in the thickness direction of the second piezoelectric sub-layer12band L2Xis the maximum of lattice constants in in-plane directions of the second piezoelectric sub-layer12b.

As is clear from the example below, it has been verified that lattice constants satisfy the above relationship when the second piezoelectric sub-layer12bis made of lead zirconate titanate. The examination of crystal symmetry by Raman scattering and X-ray diffractometry has verified that lead zirconate titanate has a monoclinic structure. Thus, the second piezoelectric sub-layer12bhas an engineered domain configuration in which the polarization axis P2is inclined to the thickness direction (the direction in which an electric field is applied) of the second piezoelectric sub-layer12bat a finite angle as shown inFIG. 4.

The second piezoelectric sub-layer12bhas the monoclinic structure and is predominantly (100)-oriented in the pseudocubic coordinate system as described above. The second piezoelectric sub-layer12bis formed on the first piezoelectric sub-layer12aby deposition. The second piezoelectric sub-layer12bis capable of having a large piezoelectric constant (d31), as described below.

The monoclinic structure, which is one of perovskite-type structures represented by a pseudocubic crystal, has a larger compliance constant (skj) for shear-mode deformation as compared to other structures. The piezoelectric constant (dij) is given by the following equation using the compliance (skj) and the piezoelectric stress constant (eik);
dij=eik·skj.

There is not much difference in piezoelectric stress constant (eik) between the monoclinic structure, a tetragonal structure, and a rhombohedral structure. The monoclinic structure has a significantly larger compliance constant (skj) as compared to other structures. Thus, the second piezoelectric sub-layer12b, which has the monoclinic structure, probably has a high piezoelectric constant (d31).

The direction of the polarization axis (polarization moment) of the piezoelectric layer12is described below with reference toFIGS. 1 to 4.FIG. 4is a schematic view illustrating the direction of the polarization axis P1of the first piezoelectric sub-layer12aand that of the polarization axis P2of the second piezoelectric sub-layer12b.

With reference toFIG. 4, the polarization axis P1of the first piezoelectric sub-layer12ais in a plane following an in-plane direction of the first piezoelectric sub-layer12a.FIG. 4shows that the in-plane direction of the first piezoelectric sub-layer12ais parallel to the upper surface10cof the lower electrode10. The term “plane following a direction parallel to the upper surface10cof the lower electrode10” as used herein covers not only planes following directions parallel to the upper surface10cof the lower electrode10but also planes slightly inclined to such directions. Crystal grains are deposited such that the first piezoelectric sub-layer12aforms an angle of, for example, zero to ten degrees with the lower electrode10of an underlayer (the lower electrode10) because the first piezoelectric sub-layer12ais affected by a material for forming the underlayer, the quality of the underlayer, and/or conditions for forming the underlayer as described below in detail. Therefore, a plane following a direction parallel to the upper surface10cof the lower electrode10refers to a plane that forms an angle of zero to ten degrees with the lower electrode10of the lower electrode10.

Since the polarization axis P1of the first piezoelectric sub-layer12ais in the plane that forms an angle of zero to ten degrees with the lower electrode10of the lower electrode10, the inequality ∈x<∈z holds for the first piezoelectric sub-layer12a, wherein ∈x is a dielectric constant in the direction of the polarization axis P1and ∈z is a dielectric constant in the direction perpendicular to the polarization axis P1in the pseudocubic coordinate system, that is, the direction (Z-direction) perpendicular to the upper surface10cof the lower electrode10. According to the Landolt-Bornstein database, it is known that the dielectric constant ∈ of lead titanate with a tetragonal structure is least (∈=about 100) in the direction of the polarization axis (c-axis) thereof and is greatest (∈=about 200) in the direction perpendicular to the polarization axis thereof. Thus, when a polarization axis is parallel to an in-plane direction, an effective electric field applied to the first piezoelectric sub-layer12acan be reduced and an effective electric field applied to the second piezoelectric sub-layer12bcan be increased.

The relationship between the dielectric constant of the first piezoelectric sub-layer12aand the voltage applied to the second piezoelectric sub-layer12bis described below with reference toFIG. 5. InFIG. 5, the horizontal axis represents the dielectric constant ∈1of the first piezoelectric sub-layer12aand the vertical axis represents the ratio (V2/V) of the partial voltage V2applied to the second piezoelectric sub-layer12bto the voltage V applied to the whole of the piezoelectric layer12.

The graph shown inFIG. 5is determined by calculation as described below. In this example, the piezoelectric layer12is composed of a capacitor C1that is the first piezoelectric sub-layer12aand a capacitor C2that is the second piezoelectric sub-layer12b, the capacitor C1and the capacitor C2being connected to each other in series. The relationship between the dielectric constant ∈1of the first piezoelectric sub-layer12aand the partial voltage V2applied to the second piezoelectric sub-layer12bis determined by a known calculation method, when the thickness of the first piezoelectric sub-layer12ais 10 nm, the dielectric constant ∈2of the second piezoelectric sub-layer12bis 2,000, and the thickness of the second piezoelectric sub-layer12bis 1,000 nm.

FIG. 5shows that an increase in the dielectric constant ∈1of the first piezoelectric sub-layer12aincreases the partial voltage V2applied to second piezoelectric sub-layer12b. That is, when the dielectric constant ∈1is large, a large partial voltage is applied to the thickest sub-layer (the second piezoelectric sub-layer12b) in the piezoelectric layer12and therefore a large piezoelectric displacement can be expected.

The first piezoelectric sub-layer12ais made of lead titanium (PbTiO3) with a hexagonal structure. A larger dielectric constant ∈2can be obtained by setting the c-axis (polarization axis) of PbTiO3in parallel to an in-plane direction as compared to the case where the c-axis (polarization axis) thereof is directed in the thickness direction perpendicular to the in-plane direction. Thus, a larger piezoelectric displacement can be obtained by setting the c-axis (polarization axis) of PbTiO3in parallel to the in-plane direction.

FIG. 6is a schematic sectional view of a modification of the piezoelectric element100. In the modification, the polarization axis P1of the first piezoelectric sub-layer12amay include a polarization axis P3that does not follow a direction parallel to the upper surface10cof the lower electrode10. In this embodiment, in all regions of the first piezoelectric sub-layer12a, the polarization axis P1preferably follows a direction parallel to the upper surface10cof the lower electrode10. The polarization axis P3may be partly present depending on deposition conditions and/or the state of the underlayer (the lower electrode10).

According to this embodiment, the polarization axis P1of the first piezoelectric sub-layer12ais predominantly directed along a plane parallel to an in-plane direction (which is parallel to the upper surface10cof the lower electrode10); hence, the effective voltage applied to the first piezoelectric sub-layer12acan be substantially minimized and therefore the effective voltage applied to the second piezoelectric sub-layer12bcan be substantially maximized. This allows the second piezoelectric sub-layer12bto have a large piezoelectric displacement.

If the polarization axis of the first piezoelectric sub-layer12ais directed in an in-plane direction in a finite area of the plane area of the piezoelectric layer12, a region thereof can be increased in piezoelectric displacement. If the polarization axis of the first piezoelectric sub-layer12ais directed in an in-plane direction in 50% or more of the plane area of the piezoelectric layer12, substantially the whole of the piezoelectric element100can be can be increased in piezoelectric displacement.

Since the second piezoelectric sub-layer12bis deposited on the first piezoelectric sub-layer12a, the second piezoelectric sub-layer12bcan be predominantly (100)-oriented in the pseudocubic coordinate system and is allowed to have the monoclinic structure. This allows the piezoelectric element100to have high piezoelectric properties.

An exemplary method for manufacturing the piezoelectric element100will now be described with reference toFIG. 1. In descriptions below, the following case is used as an example: the first piezoelectric sub-layer12ais made of lead titanate and the second piezoelectric sub-layer12bis made of lead zirconate titanate.

(1) A conductive layer used to form the lower electrode10is formed over the base200. The base200is varied in structure depending on applications of the piezoelectric element100and therefore an example of the manufacture thereof is described below.

The conductive layer, which is used to form the lower electrode10, is formed by, for example, sputtering. The conductive layer is not particularly limited and is preferably a laminate including a platinum sub-layer with a thickness of, for example, 20 nm to 150 nm and an iridium sub-layer which is disposed on the platinum sub-layer and which has a thickness of, for example, 10 nm to 60 nm. The iridium sub-layer may be converted into an oxide sub-layer by subsequent heat treatment.

In this step, the iridium sub-layer may be subjected to reverse sputtering for a short time, for example, 30 seconds at 100 W. This step may be shifted to subsequent Step (2) in a short time, for example, 30 seconds. Conditions for orienting the polarization of the first piezoelectric sub-layer12aare sensitive to conditions for forming the lower electrode10, which is an underlayer; hence, the iridium sub-layer is preferably processed as described above.

(2) A titanium layer is formed over the conductive layer by sputtering. The titanium layer may have a thickness of, for example, 0.5 nm to 12 nm in consideration of the thickness of the first piezoelectric sub-layer12a. Titanium is converted into an oxide by heat treatment performed to crystallize the piezoelectric layer12and the oxide reacts with lead contained in the second piezoelectric sub-layer12bto produce lead titanate. The layer of lead titanate may contain zirconium, which is contained in the second piezoelectric sub-layer12b. Since lead and zirconium react with titanium by diffusion during heat treatment, a transition region in which the composition ratio of zirconium to titanium varies is present at the boundary between the first piezoelectric sub-layer12aand the second piezoelectric sub-layer12b. The first piezoelectric sub-layer12ahas B sites which are located on the lower electrode10side and which is rich in titanium. The composition ratio of zirconium to titanium in the first piezoelectric sub-layer12aincreases toward the second piezoelectric sub-layer12band approaches the composition ratio of zirconium to titanium in the second piezoelectric sub-layer12b. Therefore, the first piezoelectric sub-layer12acan be referred to as a solid solution prepared by doping lead titanate with zirconium.

The titanium layer preferably has a thickness of 1 nm to 20 nm in consideration of the transition region. When the thickness of the titanium layer is less than 1 nm, the thickness of the first piezoelectric sub-layer12ais too small to control the orientation of the second piezoelectric sub-layer12b. When the thickness of the titanium layer is greater than 20 nm, the thickness of the first piezoelectric sub-layer12ais excessively large and therefore piezoelectric properties of the piezoelectric layer12are likely to be insufficient.

(3) A precursor of the second piezoelectric sub-layer12bis then formed by, for example, a sol-gel process (solution process).

Metal compounds containing metals forming lead zirconate titanate (PZT) are mixed together such that the metals satisfy a desired molar ratio. The mixed metal compounds are dissolved in an organic solvent such as an alcohol, whereby a source solution is prepared. The source solution is applied over the titanium layer by a spin coating process or another process. The composition ratio (Zr:Ti) of Zr to Ti can be controlled by varying the mixing ratio of source solutions each containing a corresponding one of Zr and Ti in this solution. The source solutions may be mixed together such that Zr composition (Zr/(Zr+Ti)) is 0.5. Zr can be used at a ratio ranging from, for example, 0.45:1 to 0.55:1 with respect to the sum of Ti and Zr. Pb composition can be controlled by varying the mixing ratio of source solutions. In consideration of the volatilization of Pb by heat treatment, Pb can be used in an amount greater than a stoichiometric composition ratio.

Known compounds containing Pb, Zr, and Ti may be used. Usable metal compounds may be metal alkoxides, organic acid salts, and other compounds. Examples of carboxylates or acetylacetonato complexes containing metals forming PZT are those described below. An organometal containing lead (Pb) is, for example, lead acetate. An organometal containing zirconium (Zr) is, for example, zirconium butoxide. An organometal containing titanium (Ti) is, for example, titanium isopropoxide. Organometals containing metals forming PZT are not limited to those described above.

(4) The piezoelectric layer12precursor can be formed by performing heat treatment in a drying step and a degreasing step. The temperature of the drying step is preferably, for example, 150° C. to 200° C. The time of the drying step is preferably, for example, five minutes or more. In the degreasing step, organic components remaining in the dried piezoelectric layer12precursor can be removed by thermally decomposing the organic components into NO2, CO2, H2O, and the like. The temperature of the degreasing step is, for example, about 300° C.

The precursor need not be formed in one batch and may be formed in several batches. In particular, the application, drying, and degreasing of a piezoelectric material may be repeated several times.

The precursor is then fired. In the firing step, the precursor can be crystallized by heating. The temperature of the firing step is preferably, for example, 650° C. to 800° C. The time of the firing step is preferably, for example, five to 30 minutes. Examples of an apparatus used in the firing step include, but are not limited to, diffusion furnaces and rapid thermal annealing (RTA) furnaces. Firing may be performed in each of the application, drying, and degreasing of the piezoelectric material.

The titanium layer is converted into a zirconium-containing lead titanate layer (the first piezoelectric sub-layer12a) through the heat-treating step.

The first piezoelectric sub-layer12a, which is made of lead titanate or the lead titanate solid solution, and the second piezoelectric sub-layer12b, which is made of lead zirconate titanate or the lead zirconate titanate solid solution, can be formed through the above steps. Lead zirconate titanate used may be doped with an element such as Ca, La, or Nb.

(6) A columnar portion with a desired shape can be formed by patterning, for example, the upper electrode14and the piezoelectric layer12. For example, the lower electrode10may be then patterned. For example, a lithographic technique and an etching technique can be used to pattern each layer. The lower electrode10, the piezoelectric layer12, and the upper electrode14may be patterned in the formation of each layer or patterned in the formation of a plurality of layers in one batch.

The piezoelectric element100, which includes the base200, the lower electrode10, the piezoelectric layer12, and the upper electrode14, is formed through the above steps.

Second Embodiment

FIG. 9is a schematic sectional view of a droplet-ejecting head1000according to a second embodiment of the present invention.FIGS. 10 and 11are schematic sectional views of modifications of the droplet-ejecting head1000.FIG. 12is an exploded perspective view of the droplet-ejecting head1000, which is shown in an upside down manner. InFIG. 12, a driving portion30is simply shown.

The droplet-ejecting head1000includes the piezoelectric element100according to the first embodiment. In this embodiment, the base200, which is included in the piezoelectric element100, includes a pressure chamber-including substrate (hereinafter referred to as “pressure chamber substrate”)20, an elastic plate26, a nozzle plate28, and the driving portion30. The driving portion30is disposed on the elastic plate26.

The pressure chamber substrate20may be made of (110) single-crystalline silicon (<110> orientation). The pressure chamber substrate20includes pressure chambers20ahaving openings.

The elastic plate26is disposed on the pressure chamber substrate20. The elastic plate26may include, for example, an etching stopper layer22and an elastic layer24disposed on the etching stopper layer22. The etching stopper layer22is made of, for example, silicon oxide (SiO2). The etching stopper layer22has a thickness of, for example, 1 μm. The elastic layer24is made of, for example, zirconium oxide (ZrO2). The elastic layer24has a thickness of, for example, 1 μm. The elastic plate26need not necessarily include the etching stopper layer22, which is not shown.

The driving portion30is disposed on the elastic plate26. The driving portion30can bend the elastic plate26. The driving portion30includes the lower electrode10, which is disposed on the elastic plate26(particularly on the elastic layer24), the piezoelectric layer12, which is disposed on the lower electrode10, and the upper electrode14, which is disposed on the piezoelectric layer12. The piezoelectric layer12includes the first piezoelectric sub-layer12aand the second piezoelectric sub-layer12b. The lower electrode10, the piezoelectric layer12, and the upper electrode14, which form the driving portion30, have been described above with respect to the first embodiment and therefore will not be described again in detail.

In this embodiment, the piezoelectric layer12and the upper electrode14, which are included in the driving portion30, are arranged above the pressure chambers20a. The lower electrode10, which is included in the lower electrode10, may extend on the pressure chamber substrate20to functions as a common electrode.

The nozzle plate28has nozzle holes28acommunicating with the pressure chambers20a. Droplets of ink or the like are ejected through the nozzle holes28a. In the nozzle plate28, a large number of the nozzle holes28aare arranged in a line. Examples of the nozzle plate28include rolled sheets made of stainless steel (SUS) and silicon substrates. The nozzle plate28is fixed under the pressure chamber substrate20(on the pressure chamber substrate20inFIG. 12) in a normal use mode. The droplet-ejecting head1000can be enclosed in a housing56as shown inFIG. 12. For example, various resin materials and metal materials can be used to form the housing56.

With reference toFIG. 12, a reservoir (liquid storage portion)523, supply ports524, and a plurality of cavities (pressure chambers)20aare arranged in a spaced which is located between the nozzle plate28and the elastic plate26and which is partitioned with the pressure chamber substrate20. The elastic plate26has a through-hole531extending therethrough in the thickness direction thereof. The reservoir523temporarily stores a liquid or dispersion (hereinafter referred to as “ink”), such as ink, supplied from an external unit such as an ink cartridge through the through-hole531. The ink is supplied to the cavities20athrough the supply ports524.

Each of the cavities20ais connected to a corresponding one of the nozzle holes28a. The cavities20acan be varied in volume by the distortion of the elastic plate26. The variation in volume of the cavities20aallows the ink to be ejected from the cavities20a.

The driving portion30is electrically connected to a piezoelectric element-driving circuit (not shown) and can be operated (vibrated or distorted) in response to a signal transmitted from the piezoelectric element-driving circuit. The elastic plate26is distorted by the distortion of the driving portion30, whereby the pressure in each cavity20acan be rapidly increased.

Since the droplet-ejecting head1000includes the piezoelectric element100, the droplet-ejecting head1000can increase the piezoelectric displacement of the piezoelectric layer12and has a good droplet-ejecting function. This feature applies to the modifications below.

FIGS. 10 and 11are schematic sectional views of the modifications of the droplet-ejecting head1000. Substantially the same members as those shown inFIG. 9are denoted by the same reference numerals as those used inFIG. 9and will not be described. Portions different from those of the droplet-ejecting head1000shown inFIG. 9are mainly described below.

In the modification shown inFIG. 10, electrodes included in a driving portion30are different in configuration from those shown inFIG. 9. In particular, a lower electrode10and a second piezoelectric sub-layer12bare located only above cavities (pressure chambers)20a. A first piezoelectric sub-layer12aand an upper electrode14extend out of these pressure chambers20ain plan view. In the modification, this upper electrode14functions as a common electrode.

In the modification shown inFIG. 10, this first piezoelectric sub-layer12aextends over an elastic layer24. This allows this first piezoelectric sub-layer12a, which serves as an orientation control layer, to be located in a region which is disposed under this second piezoelectric sub-layer12band which is wider than this second piezoelectric sub-layer12band also allows this second piezoelectric sub-layer12bto have high crystallinity.

In the modification shown inFIG. 10, the presence of this first piezoelectric sub-layer12aallows the natural frequency of a elastic plate26to be controllable.

In the modification shown inFIG. 11, electrodes included in a driving portion30are different in configuration from those shown inFIG. 9. In particular, a lower electrode10is located only above pressure chambers20a. A first piezoelectric sub-layer12a, a second piezoelectric sub-layer12b, and an upper electrode14extend out of these pressure chambers20ain plan view. In this modification, this upper electrode14can functions as a common electrode.

In the modification shown inFIG. 11, as well as the modification shown inFIG. 10, this first piezoelectric sub-layer12aextends over an elastic layer24. This allows this first piezoelectric sub-layer12a, which serves as an orientation control layer, to be located in a region which is disposed under this second piezoelectric sub-layer12band which is wider than this second piezoelectric sub-layer12band also allows this second piezoelectric sub-layer12bto have high crystallinity.

In the modification shown inFIG. 11, as well as the modification shown inFIG. 10, the presence of this first piezoelectric sub-layer12aand this second piezoelectric sub-layer12ballows the natural frequency of a elastic plate26to be controllable.

A method for manufacturing the droplet-ejecting head1000according to this embodiment will now be described with reference toFIG. 9.

(1) The elastic plate26is formed on, for example, a (110) single-crystalline silicon substrate. In particular, the etching stopper layer22and the elastic layer24are formed on the (110) single-crystalline silicon substrate in that order. This allows the elastic plate26, which includes the etching stopper layer22and the elastic plate26, to be formed. The etching stopper layer22can be formed by, for example, a thermal oxidation process. The elastic layer24can be formed by, for example, sputtering.

(2) The driving portion30is formed on the elastic plate26. In particular, the lower electrode10, the piezoelectric layer12, and the upper electrode14are formed over the elastic plate26in that order. A procedure for forming the lower electrode10, the piezoelectric layer12, and the upper electrode14, which are included in the driving portion30, is the same as that described in the Formula and therefore is not described in detail.

(3) The pressure chambers20aare formed by patterning the (100) single-crystalline silicon substrate, whereby the pressure chamber substrate20is obtained. For example, a lithographic technique and an etching technique can be used to pattern the (100) single-crystalline silicon substrate. The pressure chambers20aare formed in such a manner that the (100) single-crystalline silicon substrate is partly etched such that, for example, the etching stopper layer22is exposed. In the etching step, the etching stopper layer22can function as an etching stopper. In the etching of the pressure chamber substrate20, the etching rate of the etching stopper layer22is less than the etching rate of the pressure chamber substrate20.

(4) The nozzle plate28is bonded to the lower end of the pressure chamber substrate20. In this step, the nozzle holes28a, which are arranged in the nozzle plate28, are aligned with the pressure chambers20a, which are arranged in the pressure chamber substrate20, such that each of the nozzle holes28ais connected to a corresponding one of the pressure chambers20a.

The droplet-ejecting head1000is formed through the above steps.

In the modifications shown inFIGS. 10 and 11, the driving portions30, the first piezoelectric sub-layers12a, the second piezoelectric sub-layers12b, and the upper electrodes14, which are included in the driving portions30, can be formed by known lithographic processes and etching processes as described above.

The droplet-ejecting head1000can be used as, for example, a ink jet recording head; a colorant-ejecting head used to manufacture a color filter for liquid crystal displays; an electrode material-ejecting head used to form an electrode for organic electroluminescent displays, field emission displays (FEDs), and other displays; and a bio-organic substance-ejecting head used to manufacture a bio-chip.

Third Embodiment

A droplet-ejecting apparatus600according to a third embodiment of the present invention will now be described. The droplet-ejecting apparatus600includes the droplet-ejecting head1000according to the second embodiment and corresponds to an ink jet droplet-ejecting apparatus.FIG. 13is a schematic perspective view of the droplet-ejecting apparatus600.

The droplet-ejecting apparatus600includes a head unit630, a head unit-driving section610, and a control section660. The droplet-ejecting apparatus600may further include an apparatus body620, a sheet-feeding section650, a tray621for carrying a recording sheet P, a discharge port622for discharging the recording sheet P, and an operating panel670disposed on the upper surface of the apparatus body620.

The head unit630includes the droplet-ejecting head1000. The head unit630further includes an ink cartridge631supplying ink to the droplet-ejecting head1000and a carrying section (carriage)632carrying the droplet-ejecting head1000and the ink cartridge631.

The head unit-driving section610can reciprocate the head unit630. The head unit-driving section610includes a carriage motor641serving as a driving source of the head unit630and a reciprocating mechanism642that receives torque from the carriage motor641to reciprocate the head unit630.

The reciprocating mechanism642includes a carriage guide shaft644of which both ends are supported with a frame (not shown) and a timing belt643extending in parallel to the carriage guide shaft644. The carriage guide shaft644supports the carriage632such that the carriage632can freely reciprocate. The carriage632is fixed to a portion of the timing belt643. When the timing belt643is run by the operation of the carriage motor641, the head unit630is guided with the carriage guide shaft644to reciprocate. Ink is appropriately ejected from the droplet-ejecting head1000during the reciprocation, whereby the recording sheet P is subjected to printing.

The control section660can control the head unit630, the head unit-driving section610, and the sheet-feeding section650.

The sheet-feeding section650can transport the recording sheet P from the tray621to the head unit630. The sheet-feeding section650includes a sheet-feeding motor651serving as a driving source thereof and a sheet-feeding roller652rotated by the operation of the sheet-feeding motor651. The sheet-feeding roller652includes a driven sub-roller652aand driving sub-roller652bwhich are vertically arranged on opposite sides of the passage of the recording sheet P. The driving sub-roller652bis connected to the sheet-feeding motor651.

The head unit630, the head unit-driving section610, the control section660, and the sheet-feeding section650are arranged in the apparatus body620.

In this embodiment, the droplet-ejecting apparatus600has been described to be an ink jet droplet-ejecting apparatus. The droplet-ejecting apparatus600can be used as an industrial droplet-ejecting apparatus. Examples of a liquid (liquid material) ejected from the droplet-ejecting apparatus600include various functional materials and materials each having a viscosity appropriately adjusted with a dispersion medium.

EXAMPLE

A sample was prepared as described below. A silicon oxide layer with a thickness of 1000 nm and a zirconium oxide layer with a thickness of 500 nm were formed on a (110) single-crystalline silicon substrate in that order. The silicon oxide layer was formed by thermally oxidizing the silicon substrate. The zirconium oxide layer was formed in such a manner that a zirconium layer was formed by a sputtering process and was then thermally oxidized. A platinum layer was formed on the zirconium oxide layer by a sputtering process so as to have a thickness of 100 nm. An iridium layer was formed on the platinum layer by a sputtering process so as to have a thickness of 100 nm. A titanium layer was formed on the iridium layer by a sputtering process so as to have a thickness of 5 nm. A sol-gel source material for PZT was applied to the titanium layer by spin coating. The ratio of Pb to Zr to Ti contained in the sol-gel source material was 1.15:1:1.

A piezoelectric layer was formed as described below. A coating of the sol-gel source material was subjected to rapid thermal annealing (RTA) at 780° C. for 15 seconds in an oxygen atmosphere, whereby a PZT coating with a thickness of 200 nm was obtained. This operation was repeated five times, whereby a PZT layer with a thickness of about 1.0 μm was obtained. In this procedure, a lead titanate layer was formed under the PZT layer.

An iridium layer was formed on the PZT layer by a sputtering process so as to have a thickness of 200 nm. The sample was prepared as described above.

The sample was evaluated as described below. The PZT layer of the sample was subjected to X-ray diffraction, whereby the rocking curve of a PZT (200) peak was determined at θ-2θ. The rocking curve had a full width at half maximum of 21 degrees. This verified that the (100) orientation ratio of PZT was 90%.

The piezoelectric layer of the sample was observed with a transmission electron microscope (TEM).FIG. 7shows a TEM image of the piezoelectric layer. FromFIG. 7, it was confirmed that the lead titanate layer (first piezoelectric sub-layer12a) was disposed on a lower electrode, had a thickness of about 16 nm, and was inclined to the upper surface of the lower electrode (IrO2) at an angle of about 3.2 degrees. The lead titanate layer had a lattice constant (L1Z) of 0.393 nm in the thickness direction thereof. From the TEM image, the lattice constant of the lead titanate layer in an in-plane direction was determined to be 0.410 nm.

These show that the lead titanate layer has the largest lattice constant (L1X) in an in-plane direction and also has a tetragonal structure having a polarization axis parallel to the lower electrode.

A lead zirconate titanate layer disposed on the lead titanate layer was measured for lattice constant by X-ray diffractometry. The lattice constants (L2Xand L2Y) thereof in the X-direction and the Y-direction were both 0.418 nm and the lattice constant (L2Z) thereof in the Z-direction was 0.411 nm.

FIG. 8shows results obtained by analyzing the sample by Raman scattering. Analysis conditions were as follows: an excitation laser beam wavelength of 514.5 nm, an analysis temperature of 4.2 K, an objective lens magnification of 50×, and an analysis temperature of 20 minutes. An analysis system used was a backscattering arrangement type.

Degeneracy or fragmentation occurs in natural resonance peaks located at a wavenumber (Raman shift) of 250-300 cm−1because of a reduction in crystal symmetry. This can be used to evaluate crystal symmetry. In particular, when lead zirconate titanate has a structure, having high crystal symmetry, such as a tetragonal or rhombohedral structure among perovskite-type structures, the peaks degenerate into one. When lead zirconate titanate has a structure, such as a monoclinic structure, having low crystal symmetry, each of the peaks fragments into two. Therefore, evaluation is performed by checking whether there is one peak or two peaks.

As shown inFIG. 8, the fragmentation of the natural resonance peaks is observed when the composition ratio (Zr/Ti) of Zr to Ti is 40/60 or more (c in the figure) and 50/50 or less (k in the figure).

This shows that the lead zirconate titanate layer has a monoclinic structure and an engineered domain configuration in which a polarization axis is inclined to the thickness direction thereof at a certain angle.

In the Raman spectrum shown inFIG. 8, no monoclinic structure cannot be confirmed when Zr exceeds 50. However, when the Zr/Ti composition ratio is 60/40, the fact that the inequality L2X>L2Zholds is confirmed from analysis by X-ray diffraction. This proves that, in the composition, a second piezoelectric sub-layer12bhas monoclinic symmetry.

The present invention provides a technique commonly applicable to piezoelectric elements, such as pyroelectric sensors and ultrasonic sensors, having a capacitor structure.

While the embodiments of the present invention have been described above in detail, those skilled in the art can readily appreciate that various modifications can be made without departing from the spirit and scope of the present invention. Therefore, such modifications are within the scope of the present invention.