JP6720973B2 - Piezoelectric element, inkjet head, inkjet printer, and method for manufacturing piezoelectric element - Google Patents

Description

The present invention relates to a piezoelectric element having a lower electrode, a piezoelectric thin film, and an upper electrode in this order from a supporting substrate side, an inkjet head including the piezoelectric element, an inkjet printer, and a method for manufacturing the piezoelectric element. is there.

In recent years, piezoelectric materials such as lead zirconate titanate (Pb(Zr,Ti)O ₃ ) have been used as electromechanical conversion elements for application to drive elements and sensors. Such a piezoelectric body is expected to be applied to a MEMS (Micro Electro Mechanical Systems) element by being formed as a thin film on a substrate such as silicon (Si).

In the manufacture of the MEMS device, high-precision processing using a semiconductor process technology such as photolithography can be applied, so that the device can be downsized and the density can be increased. In particular, by collectively fabricating the elements on a relatively large Si wafer having a diameter of 6 inches or 8 inches at a high density, the cost can be significantly reduced as compared with the single-wafer manufacturing in which the elements are individually manufactured. it can.

In addition, the thinning of the piezoelectric body and the use of MEMS in the device have improved the conversion efficiency of mechanical electricity, which has created new added values such as the sensitivity of the sensor and the characteristics of the driving element. For example, in a thermal sensor, it is possible to increase the measurement sensitivity by reducing the thermal conductance by using MEMS, and in an inkjet head for a printer, it is possible to perform high-definition patterning by increasing the density of nozzles.

As a material of the piezoelectric thin film (hereinafter also referred to as a piezoelectric thin film), a crystal called PZT made of lead (Pb), zirconium (Zr), titanium (Ti), and oxygen (O) is often used. Since PZT exhibits a good piezoelectric effect when it has the perovskite structure shown in FIG. 10, it is necessary to use a perovskite single phase. Here, the perovskite structure ideally has a cubic unit cell and is arranged at a metal (for example, Pb) arranged at each apex (A site) of the cubic crystal, and at a body center (B site). It is an ABO ₃ type crystal structure composed of a metal (for example, Zr or Ti) to be formed and oxygen O arranged in each face center of a cubic crystal. Crystals having a perovskite structure include tetragonal crystals in which cubic crystals are distorted, orthorhombic crystals, rhombohedral crystals, and the like.

On the other hand, if the crystallinity of the piezoelectric thin film is poor and the number of crystals or amorphous regions having a pyrochlore structure increases, the piezoelectric characteristics will deteriorate. Therefore, the crystallinity of the base of the piezoelectric thin film (for example, the electrode layer) and the film forming conditions of the piezoelectric thin film are very important for improving the piezoelectric characteristics.

Further, when a piezoelectric thin film is used as the MEMS driving element, the piezoelectric body must be formed in a thickness of 1 to 10 μm on a substrate such as Si in order to satisfy the necessary displacement generating force. As a method for forming a piezoelectric thin film on a substrate, a chemical film forming method such as a CVD method (Chemical Vapor Deposition), a physical method such as a sputtering method or an ion plating method, and a liquid phase growth such as a sol-gel method The method is known, and it is important to find the conditions for obtaining a perovskite single-phase film according to the film forming method.

A piezoelectric constant, which is one of the electromechanical conversion characteristics, is an important characteristic when the piezoelectric body is applied to a MEMS element. The higher the piezoelectric constant, the more efficiently it can be driven with respect to the input voltage. Therefore, increasing the piezoelectric constant of the piezoelectric body has been a development task. Depending on the specifications, for example, in the case of an inkjet head that uses high-viscosity ink or enables high-speed ejection, it is required that the value of the piezoelectric constant |d ₃₁ | be 150 pm/V or more, and further application A value of 300 pm/V or more is required for expansion. Further, in ultrasonic applications, a value such as 400 to 500 pm/V is required as the piezoelectric constant |d ₃₁ |.

As a piezoelectric material capable of obtaining a high piezoelectric constant, there is a relaxer-type single crystal material in bulk (thickness is several hundred μm or more). It has been confirmed that when the above single crystal material is used, a very high piezoelectric constant such as d ₃₃ =1500 to 2000 pm/V (d ₃₁ may be considered to be about half the value of d ₃₃ ) is obtained. Examples of the relaxor-based single crystal material include PMN-PT. PMN-PT is a solid solution of a perovskite compound containing Pb, magnesium (Mg) and niobium (Nb) and a perovskite compound containing Pb and Ti (lead titanate).

It is also possible to apply the relaxer-based single crystal material used in such a bulk to the thin film. For example, Non-Patent Document 1 reports an example in which PMN-PT is heteroepitaxially grown on a single crystal substrate (MgO substrate) made of magnesium oxide (MgO) to form a single crystal piezoelectric thin film (PMN-PT layer). Has been done. However, in Non-Patent Document 1, in order to form a device, an electrode layer (Pt layer) made of platinum is formed between an MgO substrate and a PMN-PT layer, and the crystal orientation of the PMN-PT layer is controlled. As a buffer layer (seed layer) for this purpose, a layer made of strontium ruthenate (SrRuO ₃ ; SRO) is formed between the Pt layer and the PMN-PT layer.

K. Wasa et al. "Electromechanical coupling factors of single-domain 0.67Pb(Mg1/3Nb2/3)O-3-0.33 PbTiO3 single-crystal thin films", APPLIED PHYSICS LETTERS 88, 122903 (2006)

However, even if the PMN-PT layer is formed by the method of Non-Patent Document 1, the piezoelectric constant d ₃₃ is 194 pC/N (≈194 pm/V) and the piezoelectric constant |d ₃₁ | is 104 pC/N (≈104 pm/ Only a value as low as V) is obtained. This is because, in the method of Non-Patent Document 1, PMN-PT grows epitaxially in a form in which the crystallinity (lattice constant, orientation, etc.) of MgO is inherited through the Pt layer and the SRO layer, and therefore grows in a perovskite single phase. This is considered to be because it is difficult to form the pyrochlore phase.

Even when a relaxer-based single crystal material is used, only a low value as described above is obtained as the piezoelectric constant. Therefore, instead of the relaxer-based single crystal material, a single crystal PZT or a single crystal PZT is used. It is presumed that when an additive (for example, Nb) is used, only a piezoelectric constant lower than the above value can be obtained.

Therefore, it is desired to realize a piezoelectric element capable of sufficiently improving piezoelectric characteristics even with a structure using a single crystal piezoelectric thin film, but such a piezoelectric element has not been proposed at present.

The present invention has been made to solve the above problems, and an object thereof is a piezoelectric element having a configuration using a single crystal piezoelectric thin film and capable of sufficiently improving piezoelectric characteristics, and a piezoelectric element thereof. It is an object of the present invention to provide an inkjet head including the above, an inkjet printer, and a method for manufacturing the piezoelectric element.

A piezoelectric element according to one aspect of the present invention has a lower electrode, a single crystal film having a perovskite structure, and an upper electrode in this order from the supporting substrate side, and the single crystal film is a perovskite type from the supporting substrate side. It has a single crystal seed layer having a structure and a single crystal piezoelectric thin film having a perovskite structure, and the single crystal film has a crystallinity different from that of the supporting substrate.

Since the single crystal film including the single crystal seed layer and the single crystal piezoelectric thin film has crystallinity different from that of the supporting substrate, the single crystal seed layer can be reliably realized by the perovskite single phase, and the single crystal A single crystal piezoelectric thin film can be surely realized on the seed layer with a perovskite single phase. Therefore, the piezoelectric characteristics can be sufficiently improved with the configuration using the single crystal piezoelectric thin film.

FIG. 1 is an explanatory diagram showing a schematic configuration of an inkjet printer according to an embodiment of the present invention. FIG. 3 is a plan view showing a schematic configuration of an inkjet head included in the inkjet printer. FIG. 2B is a sectional view taken along the line A-A′ in the plan view of FIG. 2A. It is explanatory drawing which shows typically the typical crystal structure of a single crystal ferroelectric. It is explanatory drawing which shows the crystal structure of silicon typically. FIG. 3 is an explanatory diagram schematically showing the relationship between crystal orientation and the direction of application of an electric field in each crystal structure of the piezoelectric thin film of the inkjet head. FIG. 6 is a cross-sectional view showing a manufacturing process of the inkjet head. FIG. 6 is a cross-sectional view showing a manufacturing process of the inkjet head. FIG. 6 is a cross-sectional view showing a manufacturing process of the inkjet head. FIG. 6 is a cross-sectional view showing a manufacturing process of the inkjet head. It is explanatory drawing which shows the crystal structure of a piezoelectric material.

An embodiment of the present invention will be described below with reference to the drawings. In this specification, when the numerical range is described as A to B, the numerical range includes the lower limit A and the upper limit B.

[Configuration of inkjet printer]
FIG. 1 is an explanatory diagram showing a schematic configuration of the inkjet printer 1 of the present embodiment. The inkjet printer 1 is a so-called line head type inkjet recording device in which the inkjet heads 21 are provided in a line shape in the width direction of the recording medium in the inkjet head unit 2.

The inkjet printer 1 includes the inkjet head unit 2, the feeding roll 3, the winding roll 4, the two back rolls 5 and 5, the intermediate tank 6, the liquid feeding pump 7, the storage tank 8, and the fixing. The mechanism 9 is provided.

The inkjet head unit 2 discharges ink from the inkjet head 21 toward the recording medium P to perform image formation (drawing) based on image data, and is arranged in the vicinity of one back roll 5. The details of the inkjet head 21 will be described later.

The pay-out roll 3, the take-up roll 4, and the back rolls 5 are members each having a columnar shape and rotatable about an axis. The delivery roll 3 is a roll that delivers the long recording medium P, which is wound around the circumferential surface in multiple layers, toward a position facing the inkjet head unit 2. The feeding roll 3 feeds the recording medium P in the X direction of FIG. 1 by being rotated by a driving unit (not shown) such as a motor.

The take-up roll 4 is taken out from the take-up roll 3 and takes up the recording medium P on which the ink is ejected by the inkjet head unit 2 on the circumferential surface.

Each back roll 5 is arranged between the pay-out roll 3 and the winding roll 4. The one back roll 5 located on the upstream side in the transport direction of the recording medium P is a position facing the inkjet head portion 2 while winding and supporting the recording medium P fed by the feeding roll 3 around a part of the peripheral surface. Transport to. The other back roll 5 conveys the recording medium P while winding it around a part of the peripheral surface and supporting it from the position facing the inkjet head portion 2 toward the winding roll 4.

The intermediate tank 6 temporarily stores the ink supplied from the storage tank 8. Further, the intermediate tank 6 is connected to the plurality of ink tubes 10, adjusts the back pressure of the ink in each inkjet head 21, and supplies the ink to each inkjet head 21.

The liquid feed pump 7 supplies the ink stored in the storage tank 8 to the intermediate tank 6, and is arranged in the middle of the supply pipe 11. The ink stored in the storage tank 8 is pumped up by the liquid feed pump 7 and supplied to the intermediate tank 6 via the supply pipe 11.

The fixing mechanism 9 fixes the ink ejected onto the recording medium P by the inkjet head unit 2 onto the recording medium P. The fixing mechanism 9 is composed of a heater for heating and fixing the ejected ink on the recording medium P, a UV lamp for curing the ink by irradiating the ejected ink with UV (ultraviolet light), and the like. There is.

In the above configuration, the recording medium P fed from the feeding roll 3 is conveyed to a position facing the inkjet head unit 2 by the back roll 5, and ink is ejected from the inkjet head unit 2 onto the recording medium P. After that, the ink ejected onto the recording medium P is fixed by the fixing mechanism 9, and the recording medium P after the ink is fixed is taken up by the take-up roll 4. As described above, in the line head type inkjet printer 1, ink is ejected while the recording medium P is being conveyed while the inkjet head unit 2 is stationary, and an image is formed on the recording medium P.

The inkjet printer 1 may be configured to form an image on a recording medium by a serial head method. The serial head method is a method in which an inkjet head is moved in a direction orthogonal to the transport direction while the recording medium is transported to eject ink to form an image. In this case, the inkjet head moves in the width direction of the recording medium while being supported by a structure such as a carriage. Further, as the recording medium, other than the long one, a sheet-like one which is cut into a predetermined size (shape) in advance may be used.

[Structure of inkjet head]
Next, the configuration of the above-described inkjet head 21 will be described. FIG. 2A is a plan view showing a schematic configuration of the inkjet head 21, and FIG. 2B is a sectional view taken along the line AA′ in the plan view.

The inkjet head 21 has a thermal oxide film 23, a lower electrode 24, a single crystal film 25, and an upper electrode 26 in this order on a support substrate 22 having a plurality of pressure chambers 22a (openings).

The support substrate 22 is composed of a semiconductor substrate made of a single crystal Si (silicon) simple substance having a thickness of, for example, about 100 to 300 μm. The supporting substrate 22 is a Si substrate having a thickness of about 750 μm adjusted to a thickness of about 100 to 300 μm by a polishing process, but the thickness of the supporting substrate 22 may be appropriately adjusted according to a device to be applied. .. The upper wall of the pressure chamber 22a in the support substrate 22 (the wall located on the single crystal film side of the pressure chamber 22a) constitutes a vibration plate 22b serving as a driven film, and the single crystal film 25 (especially a piezoelectric thin film described later) is formed. 28) is displaced (vibrated) by driving (expansion and contraction), and pressure is applied to the ink in the pressure chamber 22a. The support substrate 22 may be composed of an SOI (Silicon on Insulator) substrate in which two Si substrates are joined via an oxide film.

The thermal oxide film 23 is made of, for example, SiO ₂ (silicon oxide) having a thickness of about 0.1 μm, and is formed for the purpose of protecting and insulating the support substrate 22. The thermal oxide film 23 may be provided if necessary.

The lower electrode 24 is a common electrode provided commonly to the plurality of pressure chambers 22a, and is configured by stacking a Ti (titanium) layer and a Pt (platinum) layer. The Ti layer is formed to improve the adhesion between the thermal oxide film 23 and the Pt layer. The Ti layer has a thickness of, for example, about 0.02 μm, and the Pt layer has a thickness of, for example, about 0.1 μm.

The single crystal film 25 is composed of a single crystal film having a perovskite structure, and is provided corresponding to each pressure chamber 22a. The single crystal film 25 includes a single crystal seed layer 27 having a perovskite structure and a piezoelectric thin film 28 having a single crystal having a perovskite structure, which are arranged from the support substrate 22 side. The seed layer 27 is an orientation control layer (buffer layer) for controlling the crystal orientation of the piezoelectric thin film 28. The thickness of the seed layer 27 is set to, for example, 1 μm or less. This is because the seed layer 27 can be easily formed in the manufacturing process of the piezoelectric actuator 21a described later. On the other hand, the thickness of the piezoelectric thin film 28 is set to 1 μm or more and 10 μm or less. As a result, the displacement generation force required for ink ejection by the inkjet head 21 can be reliably ensured. The details of the single crystal film 25 will be described later.

The upper electrode 26 is an individual electrode provided corresponding to each pressure chamber 22a, and is configured by stacking a Ti layer and a Pt layer. The Ti layer is formed to improve the adhesion between the single crystal film 25 and the Pt layer. The Ti layer has a thickness of, for example, about 0.02 μm, and the Pt layer has a thickness of, for example, about 0.1 to 0.2 μm. The upper electrode 26 is provided so as to sandwich the single crystal film 25 with the lower electrode 24 in the film thickness direction. A layer made of gold (Au) may be formed instead of the Pt layer.

The lower electrode 24 and the upper electrode 26 are connected to a driving circuit 29, and the single crystal film 25 (particularly the piezoelectric thin film 28) is driven based on the voltage (driving signal) applied to the lower electrode 24 and the upper electrode 26. To be done.

A nozzle substrate 31 is joined to the pressure chamber 22a on the side opposite to the diaphragm 22b. The nozzle substrate 31 is formed with ejection holes (nozzle holes) 31a for ejecting the ink contained in the pressure chamber 22a as ink droplets to the outside. Ink supplied from the intermediate tank 6 is stored in the pressure chamber 22a.

In the above configuration, when a potential difference is applied between the lower electrode 24 and the upper electrode 26 by applying a voltage by the drive circuit 29, the piezoelectric thin film 28 of the single crystal film 25 causes the potential difference between the lower electrode 24 and the upper electrode 26. Accordingly, it expands and contracts in a direction perpendicular to the thickness direction (direction parallel to the surface of the support substrate 22). Then, due to the difference in length between the piezoelectric thin film 28 and the diaphragm 22b, a curvature is generated in the diaphragm 22b, and the diaphragm 22b is displaced (curved or vibrated) in the thickness direction.

Therefore, if the ink is stored in the pressure chamber 22a, the pressure wave is propagated to the ink in the pressure chamber 22a by the vibration of the vibrating plate 22b described above, and the ink in the pressure chamber 22a drops from the ejection hole 31a. Is discharged to the outside.

In the present embodiment, as will be described later, the configuration using the single crystal piezoelectric thin film 28 can sufficiently improve the piezoelectric characteristics. Therefore, a highly viscous ink is used or an ink jet capable of high-speed ejection of the ink is used. It is possible to realize the head 21 and the high-performance inkjet printer 1 capable of performing high-speed, high-quality drawing on the recording medium.

[Details of single crystal film]
Next, details of the single crystal film 25 described above will be described. As the single crystal material forming the seed layer 27 included in the single crystal film 25, a relaxor single crystal material or strontium titanate (SrTiO ₃ ; STO) can be used.

Here, the relaxor is a general term for ferroelectric materials that exhibit the property (Dielectric relaxation) in which the temperature at which the maximum value of the dielectric constant moves to high temperature with frequency and the maximum value of the dielectric constant further decreases. Generally, it refers to a composite perovskite compound containing lead, that is, a material having a composition formula represented by Pb(B ₁ , B ₂ )O ₃ . B ₁ is an element that becomes a divalent cation such as magnesium (Mg) or zinc (Zn), or a trivalent cation, and B ₂ is a pentavalent cation such as niobium (Nb). , Or an element that becomes a hexavalent cation. The relaxor system collectively refers to a relaxor simple substance, a solid solution of the relaxor and the relaxor, and a solid solution of the relaxor and another perovskite compound.

Specific examples of the relaxor-based single crystal material include, for example, PMN-PT described above, PINO-PMNO, PIMNT, PZNT, PMNT, and PSNT. PINO-PMNO is a perovskite compound containing Pb, indium (In) and Nb (Pb(In _1/2 Nb _1/2 )O ₃ ; PIN) and a perovskite compound containing Pb, Mg and Nb (Pb(Mg ₁ It is a solid solution with _/3 Nb _2/3 )O ₃ ; PMN). PIMNT is a solid solution of PIN, PMN, and a perovskite compound containing Pb and Ti (lead titanate; PbTiO ₃ , PT).

PZNT is a solid solution of a perovskite compound containing Pb, Zn, and Nb (Pb(Zn _1/3 Nb _2/3 )O ₃ ; PZN) and PT, and in particular, PZN is 85 to 95 mol%. , PT is contained in a proportion of 15 to 5 mol %. PMNT is a solid solution of PMN and PT, and in particular, contains PT in an amount of 30 mol% with respect to 70 mol% of PMN. Psnt is a solid solution of perovskite compound containing Pb, scandium (Sc), and Nb (Pb(Sc _1/2 Nb _1/2 )O ₃ ; PSN) and PT, and particularly, PSN is 58 to 67 mol %. On the other hand, PT was contained in a ratio of 42 to 33 mol %.

The piezoelectric thin film 28 included in the single crystal film 25 is composed of the same relaxor single crystal material as the seed layer 27 or a film containing Pb, Zr, and Ti. The film containing Pb, Zr, and Ti includes not only lead zirconate titanate (PZT) but also PZT to which an additive such as La or Nb is added.

The single crystal film 25 including the seed layer 27 and the piezoelectric thin film 28 has a crystallinity different from that of the support substrate 22. Here, the support substrate 22 is composed of the Si substrate or the SOI substrate including the Si substrate as described above. Si has a crystal structure called a diamond structure (see FIG. 4), and its lattice constant is about 0.54 nm. On the other hand, the crystal structure of the single crystal film 25 (seed layer 27, piezoelectric thin film 28) is a perovskite type structure as described above, and for example, in PZT which is a constituent material of the piezoelectric thin film 28, its lattice constant is Is about 0.39 nm for a tetragonal crystal and about 0.41 nm for a rhombohedral crystal, and is the same value as PZT for a Pb-containing relaxor single crystal material and STO that form the seed layer 27.

Both the single crystal film 25 and the supporting substrate 22 are single crystals, but since the crystallinity (lattice constant) is different from each other, the single crystal film 25 is not formed on the supporting substrate 22 by epitaxial growth. Can be said. Actually, the single crystal film 25 is formed by bonding a part of the single crystal piezoelectric substrate on the support substrate 22 (details will be described later), and thus the single crystal film 25 and the support substrate 22 are formed. The crystallinity different from that of No. 22 was realized.

As described above, the single crystal film 25 has crystallinity different from that of the support substrate 22 and is formed without being affected by the crystallinity of the support substrate 22, so that both the seed layer 27 and the piezoelectric thin film 28 are formed. It can be composed of a single perovskite phase containing almost no pyrochlore phase. Since the single-crystal piezoelectric thin film 28 is a perovskite single phase, it is possible to realize a piezoelectric constant d ₃₁ that exceeds, for example, 200 pm/V in absolute value, and it is possible to sufficiently improve piezoelectric characteristics. Further, since the single-crystal piezoelectric thin film 28 is formed on the single-crystal seed layer 27 having the perovskite structure, the single-crystal piezoelectric thin film 28 follows the crystallinity (perovskite structure) of the single-crystal seed layer 27. The piezoelectric thin film 28 can be formed and the perovskite single-phase piezoelectric thin film 28 can be easily and surely realized.

Since the single crystal seed layer 27 is made of a relaxor material or STO, the perovskite single phase single crystal piezoelectric thin film 28 is formed on the seed layer 27 to ensure the piezoelectric characteristics. Can be improved.

Further, since the single crystal piezoelectric thin film 28 is composed of the relaxor material, PZT, or PZT to which an additive is added, high piezoelectric characteristics can be easily realized. In particular, by forming the single crystal piezoelectric thin film 28 using a relaxor material, high piezoelectric characteristics can be surely realized.

Hereinafter, the description will be supplemented with respect to the difference in crystallinity between the single crystal film 25 and the support substrate 22. FIG. 3 schematically shows a typical crystal structure (crystal system) of a single crystal ferroelectric (single crystal piezoelectric material). The single crystal film 25 (seed layer 27, piezoelectric thin film 28) has any one of the three crystal structures shown in FIG. Structure), which forms a single crystal film.

Here, the rhombohedral crystal refers to a crystal system (α=β=γ≠90°) in which all the crystal axes have the same length (a=b=c) and the axial angles are not all 90°. The orthorhombic crystal refers to a crystal system (α=β=γ=90°) in which all crystal axes have different lengths (a≠b≠c) and all axial angles are 90°. A tetragonal crystal refers to a crystal system in which two crystal axes have the same length (a=b≠c) and all axial angles are 90° (α=β=γ=90°).

Further, the single crystal film 25 can realize an ABO ₃ type perovskite structure in any crystal system. For example, by arranging Pb at the apex (A site) of each crystal lattice shown in FIG. 3, arranging Zr or Ti at the body center (B site), and arranging oxygen O at the face center, a perovskite structure is obtained. PZT can be realized by rhombohedral, orthorhombic and tetragonal crystal systems.

On the other hand, FIG. 4 schematically shows the crystal structure of Si. The crystal structure of Si is a so-called diamond structure in which two sets of face-centered cubic lattices composed of the same atoms are displaced from each other by 1/4 L in the diagonal direction with the diagonal length being L. It is clear from FIGS. 3 and 4 that the support substrate 22 and the single crystal film 25 have different crystal structures.

Therefore, when a substrate having Si, that is, a Si substrate or an SOI substrate is used as the support substrate 22, the crystallinity (diamond structure) of the support substrate 22 and the crystallinity of the single crystal film 25 (ABO ₃ type perovskite type) are used. Structure) can be definitely different. Further, since the Si substrate or the SOI substrate is easily processed, it can be easily applied to the MEMS element, and the piezoelectric actuator 21a as the piezoelectric element can be obtained at low cost.

[About engineered domain structure]
The piezoelectric thin film 28 of the single crystal film 25 preferably has an engineered domain structure in each of the above crystal structures (rhombohedron, orthorhombic, tetragonal). The engineered domain structure will be described below.

FIG. 5 schematically shows the relationship between the crystal orientation and the direction in which the electric field is applied in each of the above-described crystal structures of the piezoelectric thin film 28 (rhombohedral, orthorhombic, tetragonal). In the figure, for the sake of convenience, the crystal lattices of each crystal structure are shown in the same shape, but the shape of each crystal lattice is exactly as shown in FIG.

A crystal having a perovskite structure such as PZT has a spontaneous polarization Ps when no voltage is applied. A region in which the directions of the spontaneous polarization Ps are aligned is called a polarization domain here. The possible directions of the spontaneous polarization Ps vary depending on the shape of the unit cell of the crystal. The rhombohedral crystal has the <111> direction, the orthorhombic crystal has the <110> direction, and the tetragonal crystal has the tetragonal crystal. If there is, it is the <100> direction. Note that the <111> direction refers to the [111] direction and a direction equivalent thereto (for example, the [-111] direction) collectively. Similarly, the <110> direction collectively refers to the [110] direction and its equivalent direction, and the <100> direction collectively refers to the [100] direction and its equivalent direction. In FIG. 5, the direction of the spontaneous polarization Ps is shown by a broken line.

The engineered domain structure means that the electric field E is applied from a direction different from the polarization direction Ps of the polarization domain to rotate the polarization domain other than 180°, thereby increasing the piezoelectric strain and improving the piezoelectric characteristics. The obtained domain structure is also called a non-180° domain structure. For example, in the case of a rhombohedral crystal, the polarization direction is the <111> direction. Therefore, by setting the electric field application direction to the [100] direction or the [110] direction different from the polarization direction, non-180° rotation of the polarization domain is caused. It can be used to increase the piezoelectric strain, which can improve the piezoelectric properties. Similarly, in the case of the orthorhombic crystal, the polarization direction is the <110> direction. Therefore, the piezoelectric characteristics can be improved by setting the electric field application direction to the [100] direction or the [111] direction. Further, in the case of a tetragonal crystal, the polarization direction is the <100> direction. Therefore, the piezoelectric characteristics can be improved by setting the electric field application direction to the [110] direction or the [111] direction.

Therefore, in the piezoelectric thin film 28, a piezoelectric domain is formed so that a polarization domain whose polarization direction is different from the electric field application direction, that is, the direction in which the lower electrode 24 and the upper electrode 26 face each other (the film thickness direction of the piezoelectric thin film 28 ). By controlling the crystal orientation of the thin film 28, the piezoelectric characteristics can be improved by the engineered domain structure. The crystal orientation of the piezoelectric thin film 28 can be controlled by controlling the crystal orientation of the seed layer 27 that is the underlying layer. That is, the crystal orientation of the piezoelectric thin film 28 can be controlled by forming the seed layer 27 oriented in a predetermined plane direction on the lower electrode 24 and forming the piezoelectric thin film 28 thereon.

As described above, the piezoelectric thin film 28 has the polarization domain polarized in a direction different from the facing direction (electric field applying direction) of the lower electrode 24 and the upper electrode 26, and thus, when the electric field is applied, Engineered domain structures can be realized that utilize non-180° rotation to improve piezoelectric properties.

[Method for manufacturing inkjet head]
Next, a method of manufacturing the inkjet head 21 of the present embodiment, including the method of manufacturing the single crystal film 25 described above, will be described below. 6 to 9 are cross-sectional views showing the manufacturing process of the inkjet head 21.

First, as shown in FIG. 6, a single crystal piezoelectric substrate 41 is prepared. Since part of the piezoelectric substrate 41 later becomes the seed layer 27, the piezoelectric substrate 41 is made of the same material as the seed layer 27 (for example, PZNT). The piezoelectric substrate 41 has a thickness of, for example, 500 to 600 μm.

Subsequently, by ion implantation into the piezoelectric substrate 41, an altered layer for separating the piezoelectric substrate 41 inside the piezoelectric substrate 41 and at a depth position of, for example, 500 nm from the surface of the piezoelectric substrate 41. 41b (separation layer, ion implantation layer) is formed. Ion implantation is performed by ionizing these gases using a gas such as hydrogen, argon, or nitrogen. The ion implantation conditions at this time are, for example, an ion acceleration energy of the implanting device: 100 to 170 keV, a dose amount: 1×10 ¹⁶ to 1×10 ¹⁸ atom/cm ³ . As a result, the piezoelectric substrate 41 has a structure in which the support layer 41a, the altered layer 41b, and the outermost layer 41c are sequentially stacked. The altered layer 41b has a thickness of, for example, several tens of nm, and the outermost layer 41c has a thickness of, for example, 500 nm. The thickness of the outermost layer 41c is preferably 1 μm or less from the viewpoint of easily forming the altered layer 41b by ion implantation and controlling ion implantation.

On the other hand, as shown in FIG. 7, a Si substrate, which is often used for MEMS, is prepared as the support substrate 22. As the support substrate 22, an SOI substrate may be used instead of the Si substrate. Then, each layer of Ti and Pt is sequentially formed by the sputtering method to form the lower electrode 24 on the supporting substrate 22. Before forming the lower electrode 24, the supporting substrate 22 is put in a heating furnace and kept at about 1500° C. for a predetermined time, so that the thermal oxide film 23 made of silicon oxide (SiO ₂ ) is formed on the supporting substrate 22 (see FIG. 2B). (See) and the lower electrode 24 may be formed thereon. The lower electrode 24 may be formed on the piezoelectric substrate 41 side, that is, on the surface of the outermost layer 41c.

Next, as shown in FIG. 8, the piezoelectric substrate 41 on which the altered layer 41b is formed is bonded to the support substrate 22 via the lower electrode 24. At this time, the lower electrode 24 of the support substrate 22 and the outermost layer 41c of the piezoelectric substrate 41 are opposed to each other, and the piezoelectric substrate 41 and the support substrate 22 are bonded to each other by thermocompression bonding or the like.

After the bonding, the piezoelectric substrate 41 is cleaved by the altered layer 41b, and the outermost layer 41c which is a part of the piezoelectric substrate 41 is left on the support substrate 22 side. Is formed on the lower electrode 24. The above cleavage is performed under heating conditions of 400 to 600° C. (2 to 5 hours). As a result, the ions injected into the altered layer 41b are gasified, and the support layer 41a is peeled off with the outermost layer 41c left on the support substrate 22 side. In order to flatten the roughness of the cleavage plane, it is desirable to perform etching treatment using acid or the like or CMP (Chemical Mechanical Polishing) treatment.

Then, after that, the single crystal piezoelectric thin film 28 is formed on the single crystal seed layer 27. For example, a relaxor single crystal material such as PMN-PT is deposited on the seed layer 27 to form a single crystal piezoelectric thin film 28 having a thickness of 1 to 10 μm. At this time, as a method of forming the piezoelectric thin film 28, a chemical film forming method (chemical vapor deposition) such as a CVD method, a physical method (physical vapor deposition) such as a sputtering method or an ion plating method, and a sol-gel A liquid phase growth method such as a method can be used.

Next, as shown in FIG. 9, a photosensitive resin 51 is applied on the piezoelectric thin film 28 by a spin coating method, and an unnecessary portion of the photosensitive resin 51 is removed by exposing and etching through a mask to form the photosensitive resin 51. The shape of the single crystal film 25 to be transferred is transferred. Then, the shapes of the piezoelectric thin film 28 and the seed layer 27 are processed by the reactive ion etching method using the photosensitive resin 51 as a mask to form the single crystal film 25.

Subsequently, each layer of Ti and Pt is sequentially formed on the lower electrode 24 by a sputtering method so as to cover the single crystal film 25, thereby forming a layer 26a which is a base of the upper electrode 26. Subsequently, a photosensitive resin 52 is applied on the layer 26a by a spin coating method, and an unnecessary portion of the photosensitive resin 52 is removed by exposing and etching through a mask to transfer the shape of the upper electrode 26 to be formed. To do. Then, using the photosensitive resin 52 as a mask, the shape of the layer 26a is processed by the reactive ion etching method to form the upper electrode 26.

Next, a photosensitive resin 53 is applied to the back surface of the support substrate 22 (the surface opposite to the side where the single crystal film 25 is formed) by a spin coat method, and the photosensitive resin 53 is exposed and etched through a mask to form a photosensitive resin. The unnecessary portion of 53 is removed, and the shape of the pressure chamber 22a to be formed is transferred. Then, using the photosensitive resin 53 as a mask, the supporting substrate 22 is removed by a reactive ion etching method to form a pressure chamber 22a to form the piezoelectric actuator 21a.

After that, the support substrate 22 of the piezoelectric actuator 21a and the nozzle substrate 31 having the ejection holes 31a are bonded together with an adhesive or the like. As a result, the inkjet head 21 is completed.

As described above, by adhering a part of the layer (outermost layer 41c) separated from the single crystal piezoelectric substrate 41 to the support substrate 22 via the lower electrode 24, the outermost layer 41c is formed on the lower electrode 24. A single crystal seed layer 27 made of is formed (seed layer forming step). That is, the single-crystal seed layer 27 is composed of a part of layers separated from the single-crystal piezoelectric substrate 41, and is not formed by film formation. Therefore, by bonding the outermost layer 41c onto the lower electrode 24, the seed layer 27 having a single crystal structure is surely formed on the lower electrode 24 without being affected by the crystallinity of the underlying lower electrode 24 and the supporting substrate 22. Can be formed.

Further, by performing ion implantation on the single crystal piezoelectric substrate 41 to form the altered layer 41b inside the piezoelectric substrate 41, it is easy to separate (cleavage) the piezoelectric substrate 41 at the altered layer 41b. Become. This facilitates formation of the single crystal seed layer 27 while leaving a part of the layer (outermost layer 41c) of the piezoelectric substrate 41 on the lower electrode 24.

Further, by forming the piezoelectric thin film 28 on the single crystal seed layer 27 by vapor phase growth or liquid phase growth, the piezoelectric thin film 28 can be reliably formed as a single crystal in accordance with the crystallinity of the seed layer 27. You can

[Other materials for piezoelectric thin film]
In the present embodiment, as the Pb-containing perovskite compound that constitutes the single crystal piezoelectric thin film 28, PZT or PZT with La or Nb added has been described as an example. However, the additive is La or Nb described above. It is not limited to. The piezoelectric thin film 28 can also be made of a perovskite compound containing the following elements at the A site or B site of the ABO ₃ type perovskite structure. That is, the A site element of the ABO ₃ type perovskite structure contains lead (Pb), and further barium (Ba), lanthanum (La), strontium (Sr), bismuth (Bi), lithium (Li), It may contain at least one of sodium (Na), calcium (Ca), cadmium (Cd), magnesium (Mg), and potassium (K). The B site element includes zirconium (Zr) and titanium (Ti), and further includes vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W). ), manganese (Mn), scandium (Sc), cobalt (Co), copper (Cu), indium (In), tin (Sn), gallium (Ga), cadmium (Cd), iron (Fe), nickel (Ni). ) May be included.

[Other]
The piezoelectric actuator as the piezoelectric element described in the present embodiment is applicable to various devices (MEMS, such as an ultrasonic sensor, an infrared sensor (pyroelectric sensor), a frequency filter, a non-volatile memory, and a projector, in addition to an inkjet head and an inkjet printer. Actuator, MEMS sensor).

[Supplement]
The piezoelectric element, the inkjet head, the inkjet printer, and the method for manufacturing the piezoelectric element according to the present embodiment described above can also be expressed as follows, and the following operational effects are achieved thereby.

The piezoelectric element of the present embodiment has a lower electrode, a single crystal film having a perovskite structure, and an upper electrode in this order from the supporting substrate side, and the single crystal film is a single film having a perovskite structure from the supporting substrate side. It has a crystal seed layer and a single crystal piezoelectric thin film having a perovskite structure, and the single crystal film has crystallinity different from that of the supporting substrate.

According to the above configuration, the lower electrode, the single crystal seed layer, the single crystal piezoelectric thin film, and the upper electrode are formed in this order from the supporting substrate side. Since the single crystal film including the single crystal seed layer and the single crystal piezoelectric thin film has crystallinity different from that of the supporting substrate, it can be said that the single crystal film was formed without being affected by the crystallinity of the supporting substrate. Therefore, both the single crystal seed layer and the single crystal piezoelectric thin film can have a perovskite type structure (perovskite single phase) regardless of the crystallinity of the supporting substrate. As described above, since the single crystal piezoelectric thin film has the perovskite single phase, the piezoelectric characteristics can be sufficiently improved. Further, since the single crystal piezoelectric thin film is formed on the single crystal seed layer having the perovskite structure, such a perovskite single phase single crystal piezoelectric thin film can be reliably realized.

The thickness of the single crystal seed layer is preferably 1 μm or less. The single crystal seed layer is formed, for example, by implanting ions into a single crystal piezoelectric substrate to form an altered layer inside the piezoelectric substrate, and cleaving and dividing the piezoelectric substrate with the altered layer. .. If the thickness of the single crystal seed layer is 1 μm or less, it is not necessary to perform ion implantation to a deep portion of the piezoelectric substrate to form an altered layer in the deep portion. Easy to control.

The thickness of the single crystal piezoelectric thin film is preferably 1 μm or more and 10 μm or less. In this case, the displacement generation force required for the device can be reliably ensured.

The single crystal seed layer may be made of a relaxor single crystal material. In this case, the perovskite single-phase single-crystal piezoelectric thin film can be formed on the single-crystal seed layer made of the relaxor-based single-crystal material to reliably improve the piezoelectric characteristics.

The single crystal seed layer may be made of strontium titanate. In this case, the perovskite single-phase single-crystal piezoelectric thin film can be formed on the single-crystal seed layer made of STO (SrTiO ₃ ) to surely improve the piezoelectric characteristics.

The single crystal piezoelectric thin film may be made of a relaxor single crystal material. When the single crystal piezoelectric thin film is made of a relaxor single crystal material, it becomes easy to obtain high piezoelectric characteristics.

The single crystal piezoelectric thin film may be a film containing lead, zirconium, and titanium. The above-mentioned film containing Pb, Zr, and Ti includes, in addition to PZT, a perovskite compound obtained by adding an additive such as lanthanum (La) or Nb to PZT. Thus, by forming a single crystal piezoelectric thin film with a film containing Pb, Zr, and Ti, high piezoelectric characteristics can be obtained.

The single crystal seed layer is preferably composed of a part of layer separated from the single crystal piezoelectric substrate. The single crystal seed layer is not a layer (thin film) formed by film formation but is a partial layer of a single crystal piezoelectric substrate, that is, a substrate having a single crystal structure. By adhering on the lower electrode, a seed layer having a single crystal structure can be obtained. That is, the seed layer having a single crystal structure can be reliably realized without being affected by the crystallinity of the lower electrode of the base.

The single crystal film may have a rhombohedral, orthorhombic, or tetragonal crystal structure. Thereby, a single crystal film can be surely realized.

The single crystal piezoelectric thin film included in the single crystal film preferably has a polarization domain polarized in a direction different from a direction in which the lower electrode and the upper electrode face each other. In this case, since the electric field application direction, which is the opposing direction of the lower electrode and the upper electrode, and the polarization direction of the polarization domain are different, the piezoelectric characteristics are improved by utilizing the non-180° rotation of the polarization domain when the electric field is applied, An engineered domain structure can be realized.

The support substrate may be composed of a silicon substrate or an SOI substrate. Since the crystal structure of silicon is a diamond structure, the crystallinity of the single crystal film having the ABO ₃ type perovskite structure and the crystallinity of the supporting substrate can be surely made different.

The method for manufacturing a piezoelectric element of the present embodiment is a method for manufacturing a piezoelectric element having the above-described configuration, wherein a part of the layer separated from the single crystal piezoelectric substrate is provided on the support substrate via the lower electrode. There is a seed layer forming step of forming the single crystal seed layer composed of the partial layer on the lower electrode by adhering.

A part of the layer (single crystal structure layer) separated from the single crystal piezoelectric substrate is bonded to the lower electrode to form a single crystal seed layer, so that it is affected by the crystallinity of the underlying lower electrode. The single crystal seed layer can be formed without fail.

In the seed layer forming step, a step of performing ion implantation into the single crystal piezoelectric substrate to form an altered layer for separating the piezoelectric substrate inside the piezoelectric substrate, and the piezoelectric substrate. A step of adhering to the supporting substrate via the lower electrode, and after the adhering, cleaving the piezoelectric substrate with the altered layer and leaving the part of the layer of the piezoelectric substrate on the lower electrode. And forming the single crystal seed layer.

By forming the altered layer inside the single crystal piezoelectric substrate by ion implantation, it becomes easy to separate (cleavage) the piezoelectric substrate by the altered layer. This facilitates formation of the single crystal seed layer while leaving a part of the layer of the piezoelectric substrate on the lower electrode.

The method for manufacturing a piezoelectric element may further include a step of forming the single crystal piezoelectric thin film on the single crystal seed layer by vapor phase growth or liquid phase growth. In this case, the single crystal piezoelectric thin film can be reliably formed on the single crystal seed layer, following the crystallinity of the single crystal seed layer.

The inkjet head of the present embodiment includes the piezoelectric element having the above-described configuration, and a nozzle substrate having a nozzle hole for ejecting ink contained in an opening formed in the support substrate of the piezoelectric element to the outside. ing. According to the above-described configuration of the piezoelectric element, since the piezoelectric characteristics can be sufficiently improved with the configuration using the single crystal piezoelectric thin film, high-viscosity ink can be used and high-speed ejection of ink can be realized. It will be possible.

The inkjet printer of the present embodiment includes the inkjet head having the above-described configuration, and ejects ink from the inkjet head toward the recording medium. As a result, it is possible to realize a high-performance inkjet printer capable of high-speed, high-quality drawing on a recording medium.

The piezoelectric element of the present invention can be used for inkjet heads, inkjet printers, and the like.

1 Inkjet Printer 21 Inkjet Head 21a Piezoelectric Actuator (Piezoelectric Element)
22 Support substrate 22a Pressure chamber (opening)
24 Lower Electrode 25 Single Crystal Film 26 Upper Electrode 27 Seed Layer (Single Crystal Seed Layer)
28 Piezoelectric thin film (single crystal piezoelectric thin film)
31 Nozzle Substrate 31a Ink Ejection Hole 41 Piezoelectric Substrate 41b Altered Layer

Claims (16)

From the supporting substrate side, a lower electrode, a single crystal film having a perovskite structure, and an upper electrode are provided in this order,
The single crystal film, from the support substrate side,
A single crystal seed layer having a perovskite structure,
And a single crystal piezoelectric thin film having a perovskite structure,
The single crystal film has a crystallinity different from that of the supporting substrate ,
The single crystal seed layer is a piezoelectric element made of a relaxer single crystal material . From the supporting substrate side, a lower electrode, a single crystal film having a perovskite structure, and an upper electrode are provided in this order,
The single crystal film, from the support substrate side,
A single crystal seed layer having a perovskite structure,
A single crystal piezoelectric thin film having a perovskite structure,
The single crystal film has a crystallinity different from that of the supporting substrate,
The piezoelectric element, wherein the single crystal seed layer is composed of a part of layer separated from the single crystal piezoelectric substrate. The piezoelectric element according to claim 2, wherein the single crystal seed layer is made of a relaxor single crystal material. The piezoelectric element according to claim 2, wherein the single crystal seed layer is made of strontium titanate. The piezoelectric element according to claim 1, 3 or 4, wherein the single crystal piezoelectric thin film is made of a relaxor single crystal material. The piezoelectric element according to claim 1, 3 or 4, wherein the single crystal piezoelectric thin film is a film containing lead, zirconium, and titanium. The piezoelectric element according to claim 1, wherein the single crystal seed layer has a thickness of 1 μm or less. The piezoelectric element according to claim 1, wherein the single crystal piezoelectric thin film has a thickness of 1 μm or more and 10 μm or less. 9. The piezoelectric element according to claim 1, wherein the single crystal film has a rhombohedral, orthorhombic, or tetragonal crystal structure. 10. The piezoelectric element according to claim 9, wherein the single crystal piezoelectric thin film included in the single crystal film has a polarization domain polarized in a direction different from a facing direction of the lower electrode and the upper electrode. .. The piezoelectric element according to claim 1, wherein the support substrate is a silicon substrate or an SOI substrate. From the supporting substrate side, a lower electrode, a single crystal film having a perovskite structure, and an upper electrode are provided in this order,
The single crystal film, from the support substrate side,
A single crystal seed layer having a perovskite structure,
And a single crystal piezoelectric thin film having a perovskite structure,
The single crystal film is a method of manufacturing a piezoelectric element having crystallinity different from that of the supporting substrate ,
A part of the layer separated from the single crystal piezoelectric substrate is bonded to the support substrate via the lower electrode to form the single crystal seed layer composed of the part of the layer on the lower electrode. A method of manufacturing a piezoelectric element, comprising the step of forming a seed layer. In the seed layer forming step,
Ion implantation is performed on the single crystal piezoelectric substrate to form an altered layer for separating the piezoelectric substrate inside the piezoelectric substrate,
Bonding the piezoelectric substrate to the support substrate via the lower electrode,
Forming a single crystal seed layer by cleaving the piezoelectric substrate with the altered layer after bonding and leaving the part of the layer of the piezoelectric substrate on the lower electrode. 13. The method for manufacturing a piezoelectric element according to item 12. 14. The method of manufacturing a piezoelectric element according to claim 12, further comprising a piezoelectric thin film forming step of forming the single crystal piezoelectric thin film on the single crystal seed layer by vapor phase growth or liquid phase growth. A piezoelectric element according to any one of claims 1 to 11,
An inkjet head, comprising: a nozzle substrate having nozzle holes for ejecting ink contained in an opening formed in the support substrate of the piezoelectric element to the outside. An inkjet printer comprising the inkjet head according to claim 15, and ejecting ink from the inkjet head toward a recording medium.

Images