JP5328007B2 - Piezoelectric element and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric element whose polarization fatigue is suppressed, and a manufacturing method thereof. <P>SOLUTION: The piezoelectric element of the present invention includes a substrate having a pair of opposed main surfaces, a lower electrode layer disposed on one main surface of the substrate, a piezoelectric layer disposed on the lower electrode layer, and an upper electrode layer disposed on the piezoelectric layer, the lower electrode layer being made of nickel acid lanthanum-based ceramics. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a piezoelectric element having an electromechanical conversion function and a manufacturing method thereof.

An oxide dielectric thin film having a perovskite structure is represented by the general formula ABO ₃ and exhibits excellent ferroelectricity, piezoelectricity, pyroelectricity, and electro-optical properties, and is an effective material for a wide range of devices such as various sensors and actuators. The range of use is expected to expand rapidly in the future.

Since a thin film of lead zirconate titanate (PZT: general formula Pb (Zr _x Ti _1-x ) O ₃ (0 <x <1)) ceramic, which is a perovskite oxide, has high piezoelectricity, It is used as a piezoelectric element such as a sensor or a piezoelectric actuator. The piezoelectric sensor uses a ferroelectric piezoelectric effect. A ferroelectric has spontaneous polarization inside and generates positive and negative charges on its surface. In the steady state in the atmosphere, it is in a neutral state combined with the charge of the molecules in the atmosphere. When an external pressure is applied to the piezoelectric body, an electrical signal corresponding to the amount of pressure can be extracted from the piezoelectric body. The piezoelectric actuator also uses the same principle. When a voltage is applied to the piezoelectric body, the piezoelectric body expands and contracts in accordance with the voltage, and displacement can be generated in the expansion / contraction direction or a direction perpendicular to the direction.

  A thin film of PZT-based ceramics can be formed by vapor deposition methods represented by vapor deposition, sputtering (sputtering), chemical vapor deposition (CVD), or the like, or chemical solution (CSD): hydrothermal. Fabrication has been attempted using a liquid phase growth method typified by a synthesis method, an oxidation thermal decomposition method, or the like. Among them, the CSD method is characterized by easy composition control, easy production of a thin film with good reproducibility, and low cost required for manufacturing equipment and mass production.

FIG. 7 is a schematic cross-sectional view showing the structure of a conventional piezoelectric element. A conventional piezoelectric element includes a substrate 11, a lower electrode layer 12 disposed on the substrate 11, a piezoelectric layer 13 disposed on the lower electrode layer 12, and an upper electrode disposed on the piezoelectric layer 13. Layer 14. An adhesion layer 15 is disposed between the substrate 11 and the lower electrode layer 12 to adhere each of them. The substrate 11 includes a (100) -oriented single crystal silicon (Si) layer 16 and a silicon oxide (SiO ₂ ) layer 17 formed on the surface by thermally oxidizing the layer. The lower electrode layer 12 is made of platinum (Pt), the piezoelectric layer 13 is made of lead zirconate titanate (PZT), the upper electrode layer 14 is made of gold (Au), and the adhesion layer 15 is made of titanium (Ti). .

As a method for manufacturing this conventional piezoelectric element using the CSD method, for example, the method of Patent Document 1 has been proposed. That is, after a PZT precursor solution made of a composite alkoxide solution containing Pb, Ti, and Zr is applied on a substrate on which a lower electrode layer made of Pt is formed and dried, the organic matter is thermally decomposed at 400 to 450 ° C. Crystallization annealing is performed at a rate of 30 ° C./sec at 700 ° C. for 1 minute in an O ₂ atmosphere. By repeating the steps from application of the precursor solution on the substrate to crystallization annealing, piezoelectrics composed of a plurality of PZT layers are obtained. A body layer is produced. Here, since the second and subsequent PZT layers use the first PZT layer as a nucleation site, the crystal orientation of the piezoelectric layer composed of the plurality of PZT layers is the crystal orientation of the first PZT layer. Depends on gender.

JP-A-11-220185

In the above-described conventional piezoelectric element, Pt is used as the lower electrode layer 12, but there is a problem that Ti as the adhesion layer 15 diffuses due to repeated polarization inversion, and the piezoelectric characteristics are fatigued.

  An object of the present invention is to solve the above-mentioned problems and to provide a piezoelectric element that suppresses fatigue of piezoelectric characteristics even when polarization inversion is repeated and a method for manufacturing the same.

As a result of intensive studies by the inventors of the present invention in order to achieve the above object, it has been found that the substrate and the piezoelectric layer can be prevented from diffusing into the lower electrode layer by using lanthanum nickelate ceramics for the lower electrode layer. The present invention has been completed.
That is, the piezoelectric element of the present invention includes a substrate having a pair of opposing main surfaces, a lower electrode layer disposed on one main surface of the substrate, a piezoelectric layer disposed on the lower electrode layer, An upper electrode layer disposed on the piezoelectric layer, and the lower electrode layer is a lanthanum nickelate ceramic and is made of a perovskite oxide preferentially oriented in the (100) plane of the pseudo cubic system, The piezoelectric layer is made of a lead zirconate titanate ceramic and is made of a perovskite oxide preferentially oriented in a rhombohedral or tetragonal (001) plane, the substrate is made of Si , At least one concave portion is provided in at least a part of the other main surface, and the concave portion has an opposing thickness (t) defined by a distance between the deepest portion and one main surface of the substrate. And the opposing thickness ( A ratio to the thickness of the substrate (T) in) (t / T) is equal to or less than 0.4.

  In the present invention, the lower electrode layer may be a lanthanum nickelate ceramic and a perovskite oxide preferentially oriented in a pseudo cubic (100) plane.

  The piezoelectric layer can be made of lead zirconate titanate ceramics.

  The piezoelectric layer may also be a perovskite oxide made of lead zirconate titanate ceramic and preferentially oriented in the rhombohedral or tetragonal (001) plane.

  Also, a piezoelectric layer formed by a liquid phase growth method can be used.

  The lower electrode layer is made of lanthanum nickelate ceramics and a perovskite oxide preferentially oriented in the pseudo cubic (100) plane, and the piezoelectric layer is made of lead zirconate titanate ceramics. A perovskite oxide preferentially oriented in the rhombohedral or tetragonal (001) plane can also be used.

The method for manufacturing a piezoelectric element according to the present invention includes a step of providing at least one concave portion in at least a part of one main surface of a substrate having a pair of opposed main surfaces, and the other main surface of the substrate. Forming a lower electrode layer made of lanthanum nickelate-based ceramics; forming a piezoelectric layer on the lower electrode layer; and forming an upper electrode layer on the piezoelectric layer. It is characterized by that.

  In the production method of the present invention, the step of forming the lower electrode layer is a step of forming an electrode layer made of a perovskite oxide that is a lanthanum nickelate ceramic and is preferentially oriented in the (100) plane of the pseudo cubic system. The step of forming the piezoelectric layer is to form a piezoelectric layer made of a perovskite oxide that is preferentially oriented in the rhombohedral or tetragonal (001) plane of a lead zirconate titanate ceramic. It can be a process.

  The piezoelectric layer can also be formed by a liquid phase growth method.

  The step of forming the lower electrode layer includes at least a coating step of applying a precursor solution containing an organometallic compound or an inorganic metal compound of lanthanum and nickel on a substrate, and a firing step of firing the obtained coating film. Can also be included.

  Further, the baking step includes a drying step, a preliminary baking step, and a crystallization step, and the temperature of the drying step is set lower than the temperature at which the organic matter in the precursor solution starts thermal decomposition, and the temperature of the temporary baking step is set as the precursor. The temperature of the crystallization process is higher than the temperature at which the organic matter in the body solution is higher than the temperature at which thermal decomposition starts and lower than the temperature at which the organic matter is thermally decomposed and crystallization of the precursor proceeds. The temperature can be higher than the temperature and lower than the temperature at which the crystallized precursor is decomposed.

  Moreover, the temperature of a drying process can be over 100 degreeC and less than 200 degreeC, the temperature of a temporary baking process can be 200 to 500 degreeC, and the temperature of a crystallization process can be 500 to 750 degreeC.

  According to the present invention, by using nickel lanthanum ceramics for the lower electrode layer, it is possible to prevent the substrate and the piezoelectric layer from diffusing into the lower electrode layer, so that the piezoelectric characteristics are fatigued even by repeated polarization inversion. It is possible to provide a piezoelectric element without the above.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments.
Embodiment 1 FIG.
FIG. 1 is a cross-sectional view of the piezoelectric element according to Embodiment 1 of the present invention. The piezoelectric element includes a substrate 1 made of Si, a lower electrode layer 2 made of lanthanum nickelate formed on the substrate 1, a piezoelectric layer 3 formed on the lower electrode layer 2, and a piezoelectric layer 3 The upper electrode layer 4 is formed on the upper electrode layer 4.

Various materials can be selected for the substrate 1 according to the device to be manufactured, such as Si, SiO ₂ , metals, alloys, and the like. Preferably, a single crystal substrate made of Si oriented in the (100) plane can be used.

For the lower electrode layer 2, lanthanum nickelate (chemical formula LaNiO ₃ ) (hereinafter referred to as LNO) based ceramics can be used. The LNO-based ceramic used in the present invention is a ceramic mainly composed of this LNO, and includes not only LNO but also a ceramic in which a part of nickel is replaced with another metal. The other metal includes at least one metal selected from the group consisting of iron, aluminum, manganese, and cobalt. Examples thereof include LaNiO ₃ —LaFeO ₃ , LaNiO ₃ —LaAlO ₃ , LaNiO ₃ —LaMnO ₃ , and LaNiO ₃ —LaCoO ₃ . Moreover, what was substituted with 2 or more types of metals can also be used as needed. A preferred LNO ceramic is LNO.

Here, LNO has a space group of Rc and is a rhombohedral-distorted perovskite structure (rhombohedral crystal system: a0 = 5.461Å (a0 = ap), α = 60 °, pseudocubic system: a0 = 3 .84 cm), a resistivity of 1 × 10 ⁻³ (Ω · cm, 300 K), and an oxide having metallic electrical conductivity, and the metal-insulator transition occurs even when the temperature is changed. Absent.

For the piezoelectric layer 3, PZT ceramics can be used. The PZT ceramics used in the present invention include not only PZT represented by the general formula Pb (Zr _x Ti _1-x ) O ₃ (0 <x <1)) but also PZT as a main component, and Sr, Nb, And what added the trace amount of at least 1 sort (s) of metal selected from the group which consists of Al is also contained. Also included are those containing lead magnesium niobate (PMN) and lead zinc niobate (PZN). A preferred PZT ceramic is PZT.

  Here, the Zr / Ti composition of PZT is preferably Zr / Ti = 30/70 to 70/30, and more preferably Zr / Ti = 40/60 to 60/40. Preferably, a composition (Zr / Ti = 53/47) near the boundary between the tetragonal system and the rhombohedral system (morphotropic phase boundary) can be used. The film thickness of the piezoelectric layer 3 is not particularly limited as long as it is in the range of 0.5 to 5.0 μm.

  For the upper electrode layer 4, a conductive material such as a metal, an alloy of the metal, or a conductive metal oxide containing the metal can be used. The metal can be one metal selected from the group consisting of gold, silver, platinum, iridium, and ruthenium, with gold being preferred. Moreover, the film thickness of the upper electrode layer 4 will not be specifically limited if it is the range of 0.1-0.4 micrometer.

Next, a method for manufacturing the piezoelectric element will be described.
1. Preparation of substrate First, a substrate of Si (100) is prepared.

2. Fabrication of the base electrode The base electrode made of LNO ceramics can be formed using a vapor phase growth method such as a sputtering method, or a liquid phase growth method such as a CSD method, a hydrothermal synthesis method, or an oxidation thermal decomposition method. It is preferable to use a liquid phase growth method capable of reducing the manufacturing cost as compared with the vapor phase growth method. The CSD method is more preferable. The CSD method is not particularly limited, and any method can be used in the present invention as long as the LNO precursor solution is applied to a substrate, the coating is thermally decomposed, and LNO is crystallized.

Hereinafter, an example of a method for manufacturing a base electrode using the CSD method will be described.
(Preparation of LNO precursor solution)
As starting materials, lanthanum nitrate hexahydrate and nickel acetate tetrahydrate can be used, and as the solvent, 2-methoxyethanol and 2-aminoethanol can be used. Since 2-methoxyethanol contains a slight amount of water, it is preferable to remove the water beforehand using a molecular sieve of 0.3 nm.

Take lanthanum nitrate hexahydrate _{(La (NO 3) 3 ·} 6H 2 O) in a beaker, and dried over 1 hour at 0.99 ° C. for the removal of hydrates. Next, 2-methoxyethanol is added after cooling to room temperature, and lanthanum nitrate is dissolved by stirring at room temperature for 3 hours (solution A).

Also, nickel acetate tetrahydrate ((CH ₃ COO) ₂ Ni · 4H ₂ O) was taken in another separable flask, dried at 150 ° C. for 1 hour to remove the hydrate, and then at 200 ° C. for 1 hour. Allow to dry for a total of 2 hours. Next, 2-methoxyethanol and 2-aminoethanol are added and stirred at 110 ° C. for 30 minutes (solution B).

  After cooling the solution B to room temperature, the solution A is put into a separable flask containing the solution B. By stirring these mixed liquids at room temperature for 3 hours, an LNO precursor solution is prepared.

(Baking of LNO coating)
The firing of the LNO coating film can be composed of a coating film drying process, a temporary firing process for thermally decomposing the coating film, and a LNO crystallization process.

  First, the LNO precursor solution is applied onto the substrate using an application means such as a spin coat method. The conditions for performing the spin coating can be, for example, 30 seconds at a rotational speed of 3500 rpm.

(1) Drying process Next, the coating film is dried for the purpose of removing physically adsorbed moisture in the precursor solution. The temperature is desirably greater than 100 ° C. and less than 200 ° C. This is because decomposition of residual organic components in the precursor solution starts at 200 ° C. or higher, and prevents moisture from remaining in the produced film. For example, drying is performed at 150 ° C. for 10 minutes.

(2) Temporary firing step Thereafter, the coating film is thermally decomposed at a temperature of 200 ° C or higher and lower than 500 ° C. When the temperature is 500 ° C. or higher, crystallization of the dried precursor solution proceeds greatly, which is not preferable. When the temperature is lower than 200 ° C., an organic component tends to remain in the manufactured film. For example, heat treatment is performed at 350 ° C. for 10 minutes.

(3) LNO crystallization step The crystallization temperature is preferably 500 ° C or higher and 750 ° C or lower. Crystallization is performed by rapid heating using a rapid thermal annealing (RTA). For example, the heating rate can be 200 ° C./min at 700 ° C. for 5 minutes.

  In addition, it is preferable that the coating of the LNO precursor solution on the substrate, drying, and pre-baking be one cycle, the cycle is repeated a plurality of times to obtain a desired film thickness, and then crystallization is performed.

3. Production of Piezoelectric Layer A piezoelectric layer made of PZT-based ceramics can be produced using a vapor phase growth method such as a sputtering method, or a liquid phase growth method such as a CSD method, a hydrothermal synthesis method, or an oxidation thermal decomposition method. However, it is preferable to use a liquid phase growth method capable of reducing the manufacturing cost as compared with the vapor phase growth method. The CSD method is more preferable. There is no particular limitation on the CSD method, and any method can be used in the present invention as long as it is a method in which a PZT precursor solution is applied to a substrate and pyrolyzed to crystallize PZT.

An example of a method for manufacturing a piezoelectric layer using the CSD method will be described below.
(Preparation of PZT precursor solution)
In this preparation method, ethanol is used as a solvent. In order to prevent hydrolysis of the metal alkoxide by water in ethanol, anhydrous ethanol that has been subjected to dehydration treatment in advance is used. First, lead acetate (II) trihydrate (Pb (OCOCH ₃ ) ₂ .3H ₂ O) is used as a starting material for the Pb precursor solution. This is taken into a separable flask and dried at 150 ° C. for 2 hours or more to remove the hydrate. Next, absolute ethanol is added and dissolved, and refluxed at 78 ° C. for 4 hours to prepare a Pb precursor solution. As starting materials for preparing the Ti-Zr precursor solution, titanium isopropoxide (Ti (OCH (CH ₃ ) ₂ ) ₄ ) and zircon normal propoxide (Zr (OCH ₂ CH ₂ CH ₃ ) ₄ ) are used. This is also taken in another separable flask, dissolved by adding absolute ethanol, and refluxed at 78 ° C. for 4 hours to prepare a Ti—Zr precursor solution. The Ti / Zr ratio is weighed so that, for example, the molar ratio is Ti / Zr = 47/53. This Ti—Zr precursor solution is mixed with the Pb precursor solution. At this time, the stoichiometric composition of Pb component _{(Pb (Zr 0.53, Ti 0.47} ) O 3) is preferably set to 20 mol% excess with respect to. This is to compensate for the shortage due to volatilization of the lead component during annealing. The mixed solution is refluxed at 78 ° C. for 4 hours, 0.5 mol equivalent of acetylacetone as a stabilizer is added to the total amount of metal cations, and further refluxed at 78 ° C. for 1 hour to prepare a PZT precursor solution. be able to.

(Baking of PZT precursor coating)
The firing of the PZT coating film can be composed of a coating film drying process, a preliminary firing process for thermally decomposing the coating film, and a PZT crystallization process.

  First, the PZT precursor solution is coated on the base electrode using a known coating means such as a spin coating method. As conditions for performing the spin coating, for example, 30 seconds can be used at a rotational speed of 2500 rpm.

(1) Drying process The drying is intended to remove physically adsorbed moisture in the precursor solution, and the temperature is desirably higher than 100 ° C and lower than 200 ° C. This is because decomposition of residual organic components in the precursor solution starts at 200 ° C. or higher, and prevents moisture from remaining in the produced film. For example, it is dried at 115 ° C. for 10 minutes.

(2) Temporary firing step Thereafter, the coating film is thermally decomposed at a temperature of 200 ° C or higher and lower than 500 ° C. When the temperature is 500 ° C. or higher, crystallization of the dried precursor solution proceeds greatly, which is not preferable. When the temperature is lower than 200 ° C., an organic component tends to remain in the manufactured film. For example, heat treatment is performed at 350 ° C. for 10 minutes.

(3) Crystallization step of PZT The crystallization temperature is desirably 500 ° C. or higher and 750 ° C. or lower. When the temperature is higher than 750 ° C., Pb contained in the film evaporates at the time of film formation, resulting in insufficient crystallinity. When the temperature is lower than 500 ° C., crystallization is insufficient. Crystallization is performed by rapid heating using a rapid heating furnace. For example, the temperature can be raised at 650 ° C. for 5 minutes and the temperature rising rate can be 200 ° C./min.

  In addition, it is preferable to perform application | coating on the lower electrode layer of a PZT precursor solution, drying, and pre-baking as 1 cycle, and repeating the cycle in multiple times to obtain a desired film thickness, rapid heating, and performing crystallization. .

  Alternatively, a method in which a PZT precursor solution is applied on the lower electrode layer, dried, pre-baked, and crystallized as one cycle and the cycle is repeated a plurality of times to obtain a desired film thickness can be used.

4). Production of upper electrode Next, the upper electrode layer 4 made of, for example, gold is formed on the piezoelectric layer 3. As a method for forming the upper electrode layer 4, an ion beam evaporation method, a resistance heating evaporation method, a sputtering method, or the like can be used.

  According to the present embodiment, by using LNO for the lower electrode layer, it is possible to suppress the substrate and the piezoelectric layer from diffusing into the lower electrode layer as compared with the case of using platinum. Thereby, a very high value of about −80 pC / N can be obtained as the piezoelectric constant d31 of the piezoelectric element.

  In addition, since the piezoelectric layer made of PZT is formed on the base electrode layer made of LNO, compared with the case where the piezoelectric layer made of PZT is formed on the Pt electrode as in the conventional piezoelectric element, A remarkably high crystal orientation can be obtained. An X-ray diffraction diagram of this piezoelectric layer is shown in FIG. It can be seen that a film with very high crystal orientation is obtained. The piezoelectric layer has a (001) plane orientation degree α (001) of about 98%.

  This is because LNO not only has a role as an electrode but also has good lattice matching with PZT. Lattice matching refers to the lattice matching between the unit lattice of PZT and the unit lattice on the surface of LNO. In general, when a certain crystal plane is exposed on the surface, a force for matching the crystal lattice with the crystal lattice of a film to be deposited thereon works, so that an epitaxial layer is formed at the substrate-film interface. It has been reported that it is easy to form crystal nuclei. Therefore, the PZT (001) plane of PZT (lattice constants: a = 4.036, c = 4.146 の) having good lattice matching with the (100) -oriented LNO surface (lattice constant: 3.84Å) and ( 100) The surface is selectively generated.

  In the conventional piezoelectric element, as shown in FIG. 7, the piezoelectric layer 14 made of PZT is formed on the lower electrode layer 13 made of Pt by the CSD method. It needs to be very hot. Therefore, Pb and Ti in the film diffuse during the heat treatment, so that a desired composition cannot be obtained due to formation of a Pb—Pt compound, diffusion of Ti in the adhesion layer 13, and the like. Therefore, the crystal orientation of the PZT layer forming the piezoelectric layer 13 is low. FIG. 8 shows an X-ray diffraction pattern of the crystal structure of the PZT layer in Patent Document 1. The PZT layer is oriented not only in the (100) plane direction but also in the (110) and (111) plane directions. In contrast, in the piezoelectric element according to the present embodiment, LNO of the lower electrode layer suppresses the diffusion of the substrate and the piezoelectric layer into the lower electrode layer and good lattice matching with PZT. , It has the effect of providing a PZT film with very high crystal orientation.

Here, the degree of orientation of the (100) plane (α (100)) is
α (100) = I (100) / ΣI (hkl)
In other words, α (100) = 88%, which is a low value. Therefore, the piezoelectric element has low piezoelectric characteristics. For example, when used as an actuator, the amount of displacement is insufficient, and the change in the amount of displacement with respect to the applied voltage becomes nonlinear. Here, ΣI (hkl) is the sum of diffraction peak intensities from each crystal plane in PZT of the perovskite crystal structure with 2θ of 10 to 70 ° when using Cu-Kα rays in the X-ray diffraction method. is there. Note that the (002) plane and the (200) plane are equivalent to the (001) plane and the (100) plane, and thus are not included in ΣI (hkl).

LNO is easily oriented in the (100) plane direction regardless of the substrate material. Therefore, the degree of freedom of selection of the substrate is large, and various materials can be selected according to the device to be manufactured such as Si, SiO ₂ , metal, alloy, and the like.

Embodiment 2. FIG.
The piezoelectric element according to the present embodiment has the same structure as that of the first embodiment except that a concave portion is provided on the main surface facing the main surface on the lower electrode layer side of the substrate.
FIG. 3 is a schematic cross-sectional view showing the structure of the piezoelectric element in the present embodiment. The same components as those in the first embodiment are given the same reference numerals. The piezoelectric element includes a substrate 5 having a pair of opposed main surfaces, a lower electrode layer 2 formed on one main surface of the substrate 5, and a piezoelectric layer 3 formed on the lower electrode layer 2. And an upper electrode layer 4 formed on the piezoelectric layer 3. The substrate 5 is a single crystal substrate made of Si oriented in the (100) plane, and has at least one recess 6 on the other main surface.

  The shape of the recess is not particularly limited as long as it extends from one main surface of the substrate to the other main surface on the lower electrode layer side, and is circular, elliptical, polygonal such as a triangle or square in plan view, Grooves or irregular shapes are included. Further, the concave portion may be formed on at least a part of the main surface of the substrate, but is preferably formed on the entire surface. In addition, the number of recesses is not particularly limited as long as it is 1 or more, and includes those in which a large number of recesses are regularly arranged in various patterns. The concave portion can be formed by etching using a mask having a predetermined pattern, for example. The diameter of the recess is not particularly limited as long as it can be formed by etching.

Hereinafter, a method for manufacturing the piezoelectric element according to the present embodiment will be described with reference to the drawings. The piezoelectric element manufacturing method according to the present embodiment is different from the piezoelectric element manufacturing method according to the first embodiment only in that it includes a step of forming a recess. Therefore, the description of the overlapping part is omitted, and only the method for forming the recess will be described.

FIG. 4 is a schematic cross-sectional view showing an example of the manufacturing process of the recess 6. It shows a method of forming by etching using a mask having a predetermined pattern. First, an oxide film 7 made of SiO ₂ is subjected to heat treatment at 1100 ° C. for 5 hours in a thermal oxidation furnace on a substrate 100 made of (100) -oriented Si as a mask material during wet etching by a thermal oxidation method. It forms on a pair of main surface which the board | substrate 5 opposes. Thereby, an oxide film 7 made of 1.3 μm of SiO ₂ is formed (step (a)).

  Next, a resist 8 is applied on one oxide film 7 by a spin coating method. For the resist 8, for example, OFPR800 manufactured by Tokyo Ohka Kogyo can be used (step (b)).

  Next, the resist 8 is patterned in accordance with the diameter of the recess 6 to be formed (step (c)).

A BHF solution (46 wt% HF: 40 wt% NH ₄ F = 1: 6) is used for etching the oxide film 7. Since the etching rate of the oxide film 7 by this BHF solution is about 0.1 μm / min, the etching time is set to 15 min and patterning is performed (step (d)).

  Next, the resist 8 is removed (step (e)).

  Finally, KOH and TMAH (tetrahydroxyammonium hydroxide) are used for etching the substrate 5 and the oxide film 7, and the concentration of the solution, the etching temperature, and the etching time are adjusted, so that a large number of desired depths can be obtained. The recess 6 is formed (step (f)).

  After forming a large number of recesses 6 on one main surface of the substrate 5 in this way, the lower electrode layer 2, the piezoelectric layer 3 and the upper electrode layer 4 are formed on the other main surface of the substrate 5. A piezoelectric element is produced. The manufacturing method of the lower electrode layer 2, the piezoelectric layer 3, and the upper electrode layer 4 is the same as the method shown in the first embodiment.

Hereinafter, the effect of the recess will be described.
Table 1 shows thermal expansion coefficients of Si, which is the material of the substrate 5, LNO, which is the material of the lower electrode layer 2, PZT, which is the material of the piezoelectric layer 3, and Pt, which is the conventional electrode material.

  According to Table 1, when the piezoelectric layer 3 made of PZT is laminated on one main surface of the substrate 5 made of Si, stress in the tensile direction is applied to the piezoelectric layer 3 in the cooling step. On the other hand, when the piezoelectric layer 3 made of PZT is laminated on the lower electrode layer 2 made of Pt and LNO, stress in the compressive direction is applied to the piezoelectric layer 3 (LNO having a larger thermal expansion coefficient in Pt and LNO) Is more stressed in the compression direction).

  Here, when the recess 6 is provided on the other main surface of the substrate 5 made of Si, the tensile stress applied to the lower electrode layer 2 made of LNO is relieved. Conversely, the stress in the compression direction applied to the piezoelectric layer 3 made of PZT increases. The stress in the compression direction applied to the piezoelectric layer 3 made of PZT further increases as the depth of the recess 6 increases and the residual Si decreases.

Here, the distance between the bottom of the recess 6, that is, the deepest part, and the main surface of the substrate on the lower electrode layer 2 side is defined as an opposing thickness (t), and the thickness of the substrate 5 with this opposing thickness (t) ( The relationship between the ratio (t / T) to T) and the residual stress in the compression direction applied to the piezoelectric layer 3 is shown in FIG. The relationship with the piezoelectric constant d ₃₃ is shown in FIG. In FIG. 6, an example using platinum as the lower electrode layer is shown for comparison. Here, since it is difficult to measure stress in the vicinity of the MPB composition of PZT, the stress value shown in FIG. 5 is measured using the piezoelectric layer 3 made of PZT having a Zr / Ti ratio of 30/70. .

  As apparent from FIGS. 5 and 6, the stress in the compression direction applied to the piezoelectric layer 3 can be increased by setting (t / T) to 0.4 or less. Specifically, when (t / T) is 0.4 or less, a stress in the compression direction of 330 MPa or more can be obtained. That is, when (t / T) is 0.4 or less, it is possible to apply a stress in the compression direction of 330 MPa or more, and it is possible to obtain a very large stress-induced piezoelectric characteristic.

  According to the present embodiment, not only the effects of the first embodiment but also the following effects are obtained. That is, by providing a recess in the main surface opposite to the main surface on the lower electrode layer side of the substrate, the tensile stress applied to the lower electrode layer made of LNO is relaxed, and the stress in the compression direction applied to the piezoelectric layer made of PZT is reduced. It is possible to increase the piezoelectric characteristics. Furthermore, by setting the ratio (t / T) of the opposing thickness (t) to the thickness (T) of the substrate to 0.4 or less, it becomes possible to apply a stress in the compressing direction of 330 MPa or more, and the piezoelectric characteristics are greatly improved. Can be improved.

  As described above, according to the present invention, it is possible to provide a piezoelectric element having a piezoelectric layer with high crystal orientation that can suppress polarization fatigue, and various types such as an angular velocity sensor and an infrared sensor used in various electronic devices. It is useful for applications such as sensors, various actuators such as piezoelectric actuators and ultrasonic motors, optical devices such as optical waveguides and optical switches, micropumps, ceramic capacitors and ferroelectric memories.

It is a schematic cross section which shows an example of the structure of the piezoelectric element which concerns on Embodiment 1 of this invention. FIG. 2 is an X-ray diffraction diagram of a piezoelectric layer of the piezoelectric element in FIG. 1. It is a schematic cross section which shows an example of the structure of the piezoelectric element which concerns on Embodiment 2 of this invention. FIG. 4 is a schematic cross-sectional view showing a manufacturing process of a recess of the piezoelectric element of FIG. 4 is a graph showing the relationship between the ratio of the opposing thickness of the recesses to the substrate thickness in the piezoelectric element of FIG. 3 and the residual stress in the compression direction of the piezoelectric layer. FIG. 4 is a graph showing a relationship between a ratio of a facing thickness of a recess to a substrate thickness and a piezoelectric constant in the piezoelectric element of FIG. 3. It is a schematic cross section showing the structure of a conventional piezoelectric element. FIG. 8 is an X-ray diffraction diagram of a piezoelectric layer of the piezoelectric element of FIG. 7.

Explanation of symbols

1 Substrate 2 Lower electrode layer 3 Piezoelectric layer 4 Upper electrode layer 5 Substrate 6 Recess 7 Oxide film 8 Resist

Claims (8)

A substrate having a pair of opposing main surfaces, a lower electrode layer disposed on one main surface of the substrate, a piezoelectric layer disposed on the lower electrode layer, and an upper electrode disposed on the piezoelectric layer And having a layer
The lower electrode layer is a lanthanum nickelate ceramic and is made of a perovskite oxide preferentially oriented in the pseudo cubic (100) plane, and the piezoelectric layer is a lead zirconate titanate ceramic. It consists of a perovskite oxide preferentially oriented in the rhombohedral or tetragonal (001) plane,
The substrate is made of Si, and at least part of the other main surface of the substrate is provided with at least one concave portion, and the concave portion is a distance between the deepest portion and one main surface of the substrate. A piezoelectric element having a defined opposing thickness (t) and a ratio (t / T) of the opposing thickness (t) to the thickness (T) of the substrate being 0.4 or less.   The piezoelectric element according to claim 1, wherein the piezoelectric layer is formed by a liquid phase growth method.   The method for manufacturing a piezoelectric element according to claim 1, wherein at least one recess is provided in at least a part of one main surface of a substrate having a pair of opposed main surfaces, and the other of the substrates. Forming a lower electrode layer made of a lanthanum nickelate ceramic on the main surface, forming a piezoelectric layer on the lower electrode layer, forming an upper electrode layer on the piezoelectric layer, A method for manufacturing a piezoelectric element having The step of forming the lower electrode layer is a step of forming an electrode layer made of a perovskite-type oxide preferentially oriented on a pseudo cubic (100) plane made of lanthanum nickelate ceramics, and the piezoelectric layer And forming a piezoelectric layer made of a perovskite oxide preferentially oriented in a rhombohedral or tetragonal (001) plane. 3. A method for manufacturing a piezoelectric element according to 3.   The method for manufacturing a piezoelectric element according to claim 3, wherein the piezoelectric layer is formed by a liquid phase growth method.   The step of forming the lower electrode layer includes at least a coating step of applying a precursor solution containing an organometallic compound or an inorganic metal compound of lanthanum and nickel on the substrate, and a firing step of firing the obtained coating film. The method for manufacturing a piezoelectric element according to claim 3, wherein:   The firing step includes a drying step, a preliminary firing step, and a crystallization step, and the temperature of the drying step is lower than the temperature at which the organic matter in the precursor solution starts thermal decomposition, and the temperature of the temporary firing step Is higher than the temperature at which the organic matter in the precursor solution begins to thermally decompose and is lower than the temperature at which the organic matter is thermally decomposed and the crystallization of the precursor proceeds, and the temperature of the crystallization step is the temperature of the precursor The method for manufacturing a piezoelectric element according to claim 6, wherein the temperature is higher than a temperature at which crystallization proceeds and lower than a temperature at which the crystallized precursor is decomposed.   The temperature of the drying step is higher than 100 ° C and lower than 200 ° C, the temperature of the preliminary baking step is 200 ° C or higher and lower than 500 ° C, and the temperature of the crystallization step is 500 ° C or higher and 750 ° C or lower. 6. A method for manufacturing a piezoelectric element according to 6.

Images