JP2009049065A - Piezoelectric thin-film element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric thin-film element which can be greatly improved in piezoelectric characteristic by obtaining a piezoelectric thin film which contains (K, Na)NbO<SB>3</SB>as a main phase and has superior piezoelectric characteristics. <P>SOLUTION: The piezoelectric thin-film element constituted by disposing a lower electrode 2 and a piezoelectric thin film 3 in order on a substrate 1 is characterized in that: the piezoelectric thin film 3 is formed of a dielectric thin film having a perovskite structure containing (K, Na)NbO<SB>3</SB>as a main phase; and an intermediate layer 4 having a pseudo-cubic or cubic perovskite structure of ≥4.036 Å in lattice constant is inserted between the lower electrode 2 and piezoelectric thin film 3 in contact with the piezoelectric thin film 3. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a piezoelectric thin film element, and more particularly to a piezoelectric thin film element using a dielectric thin film having a perovskite structure having (K, Na) NbO ₃ as a main phase.

Piezoelectric materials are processed into various piezoelectric elements according to various purposes. In particular, they are widely used as functional electronic parts such as actuators that generate deformation by applying voltage and conversely sensors that generate voltage from deformation of the element. ing. As a piezoelectric material used for actuators and sensors, lead-based dielectrics having large piezoelectric characteristics, particularly lead zirconate titanate called PZT: Pb (Zr _1-x Ti _x ) O ₃ system (0 < Perovskite ferroelectrics with x <1) have been widely used so far. PZT is usually formed by sintering oxides composed of individual elements. In recent years, it has been desired to develop a piezoelectric material containing no lead in consideration of the environment, and lithium potassium sodium niobate (general formula: (Na _x K _y Li _z ) NbO ₃ (0 <x <1,0) <Y <1, 0 <z <1, x + y + z = 1) etc. Since this lithium potassium sodium niobate has piezoelectric properties comparable to PZT, it is a promising candidate for lead-free piezoelectric materials. As expected.

  On the other hand, as the various electronic components are now becoming smaller and higher in performance, there is a strong demand for smaller and higher performance piezoelectric elements. However, a piezoelectric material produced by a manufacturing method centering on a sintering method, which is a conventional manufacturing method, approaches the size of the crystal grains constituting the material, particularly when the thickness is 10 μm or less. Cannot be ignored. For this reason, problems such as significant variations in characteristics and deterioration occur, and in order to avoid such problems, methods for forming piezoelectric bodies using thin film technology instead of sintering have been recently studied. . Recently, PZT thin films formed by RF sputtering have been put into practical use as head actuators for high-definition high-speed inkjet printers. Needless to say, a lead-free material is required for the piezoelectric thin film as well as the piezoelectric sintered body.

For example, Patent Document 1 is a related technique.
JP 2002-151754 A

  Since it has become possible to produce a very thin piezoelectric film as described above, attempts have been made to use piezoelectric thin films as actuators and sensors. When a voltage is applied to the piezoelectric thin film, it deforms in proportion to the voltage. Conversely, when deformed, a voltage is generated by generating a charge on the surface in proportion to the displacement. Utilizing this phenomenon, studies have been made to apply it as a fine actuator, such as an ink discharge portion of an ink jet printer, and studies have been put into practical use for a small gyro sensor used for camera shake detection of a digital camera shake prevention mechanism.

When a piezoelectric thin film is applied to such a device, there is a piezoelectric constant d ₃₁ as a constant representing the piezoelectric characteristics of the piezoelectric thin film. This constant indicates mechanical displacement in a direction perpendicular to the direction of application when an electric field is applied to the piezoelectric body. When this value is large, it can be displaced greatly with the same voltage when used as an actuator, and when used as a sensor, a sensor with good sensitivity can be produced.
Therefore, in order to use the piezoelectric thin film for these applications, it is necessary to form a piezoelectric thin film having a large d ₃₁ . The actuator and sensor described above use a piezoelectric thin film having PZT as a main phase as the piezoelectric thin film. In order to make lead-free in the future, it is necessary to develop a lead-free piezoelectric thin film having a piezoelectric constant d ₃₁ comparable to this PZT thin film. Therefore, in recent years, a piezoelectric thin film having a main phase of potassium sodium niobate (K, Na) NbO ₃ as a lead-free piezoelectric material has attracted attention.
However, no lead-free piezoelectric thin film comparable to the PZT thin film has been reported so far.

An object of the present invention is to provide a piezoelectric thin film element capable of greatly improving piezoelectric characteristics by obtaining a piezoelectric thin film having excellent piezoelectric characteristics mainly composed of (K, Na) NbO _3. .

According to the present invention, in a piezoelectric thin film element configured by sequentially arranging a lower electrode and a piezoelectric thin film on a substrate, the piezoelectric thin film is made of a dielectric thin film having a perovskite structure whose main phase is (K, Na) NbO _3. An intermediate layer having a pseudo-cubic or cubic perovskite structure having a lattice constant of 4.036 mm or more is inserted between the lower electrode and the piezoelectric thin film in contact with the piezoelectric thin film. A piezoelectric thin film element is provided.
The intermediate layer is a dielectric thin film having a perovskite structure in which BaZrO ₃ , BaSnO ₃ , BaMnO ₃ , KMnF ₃ , KFeF ₃ , a mixed crystal thereof, or a perovskite structure in which other elements are mixed in an amount of 10% or less. It is preferable that it is either. The intermediate layer is preferably preferentially oriented in the (001) plane in the normal direction of the substrate. Furthermore, the film thickness of the piezoelectric thin film is preferably 0.2 μm or more and 10 μm or less.

  According to the present invention, a piezoelectric thin film having excellent piezoelectric characteristics with KNN as a main phase can be obtained, and as a result, the piezoelectric characteristics can be greatly improved.

  Embodiments of the present invention will be described below.

In order to improve the piezoelectric characteristics of a piezoelectric thin film having (K, Na) NbO ₃ (hereinafter simply referred to as KNN) as a main phase, the present inventors paid attention to the orientation of the KNN piezoelectric thin film.
In the piezoelectric thin film as a piezoelectric body, a lower electrode is formed on a substrate, and a piezoelectric thin film is formed thereon. Furthermore, by forming an upper electrode on the upper part, a voltage can be applied to the piezoelectric thin film to drive as an actuator, or a voltage generated by receiving a mechanical displacement can be detected. By incorporating such a structure into a part of the element, the element functions as an actuator or a sensor. At this time, since the voltage is applied in the normal direction to the surface of the substrate or the generated voltage is detected, it is necessary to form the piezoelectric thin film so that the piezoelectric characteristics are maximized in this direction. .

  The piezoelectric body has a polarization axis, and the piezoelectric characteristics increase in that direction. The present inventors have found that this polarization axis is the <001> axis in the KNN piezoelectric thin film. Therefore, it is possible to greatly improve the piezoelectric constant by forming a KNN piezoelectric thin film oriented in the (001) plane in the normal direction of the substrate to which a voltage is applied or the voltage is detected.

However, when the KNN piezoelectric thin film is formed directly on the lower electrode made of platinum or the like, a film in which the (110) plane and the (001) plane are mixed is formed.
The present inventors formed a perovskite structure film having a lattice constant larger than that of the KNN piezoelectric thin film on the platinum lower electrode as a base dielectric thin film, and formed the KNN piezoelectric thin film on the upper part, thereby forming the (001) plane. It has been found that a more preferentially oriented KNN piezoelectric thin film can be formed, and that the base dielectric thin film is oriented in the (001) plane, so that the KNN film is more strongly oriented in the (001) plane.
Based on this knowledge, a piezoelectric thin film element according to a preferred embodiment of the present invention will be described below.

  As shown in FIG. 1, the piezoelectric thin film element according to the embodiment is integrally formed by sequentially arranging a conductive lower electrode 2, an intermediate layer 4 as a base dielectric thin film, and a piezoelectric thin film 3 on a substrate 1. Formed and configured. The piezoelectric thin film element according to the embodiment includes one in which a conductive upper electrode is disposed on the piezoelectric thin film 3.

The piezoelectric thin film 3 is a dielectric thin film having a perovskite structure whose main phase is KNN crystal. For example, it is made of a dielectric thin film of potassium sodium niobate represented by the general formula (K _x Na _y ) NbO ₃ (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y = 1).
Here, the unit cell of the KNN crystal defines the same unit cell as KNbO ₃ (ICDD number: 32-0822) described in ICDD, and sodium is partially substituted for the potassium site. The directions of the a-axis, b-axis, and c-axis are such that O <A <B when the unit cells are A, B, and O, respectively.

The intermediate layer 4 inserted between the lower electrode 2 and the piezoelectric thin film 3 is an intermediate layer in contact with the piezoelectric thin film 3 and having a pseudo cubic or cubic perovskite structure having a lattice constant of 4.036Å or more.
The intermediate layer satisfying such conditions is composed of, for example, any one of BaZrO ₃ , BaSnO ₃ , BaMnO ₃ , KMnF ₃ , KFeF ₃ , or a mixed crystal film thereof. Alternatively, the intermediate layer is composed of an intermediate layer of a layer having a perovskite structure in which a small amount of an additive, for example, another element having an atomic number concentration of 10% or less, such as Li, is mixed with these intermediate layer elements.
When the intermediate layer has a pseudo-cubic or cubic perovskite structure having a lattice constant of 4.036Å or more, a good piezoelectric thin film crystal having an excellent piezoelectric constant d ₃₁ can be formed on the lower electrode.

  The thickness of the KNN piezoelectric thin film is preferably 0.2 μm or more and 10 μm or less. If the film thickness is 0.2 μm or more, effective piezoelectric characteristics as a piezoelectric thin film element can be obtained. If a piezoelectric thin film having a film thickness of 10 μm or more is desired, a sintered body is used as in the past. That's fine.

  In addition, the thickness of the intermediate layer is basically not limited, but is approximately equal to or less than one fifth, preferably less than one fifth, in order to apply a sufficient electric field to the piezoelectric film. However, if the thickness of the intermediate layer is too thin, it becomes difficult to control the orientation of the KNN piezoelectric thin film, so the lower limit is set to several nm.

The intermediate layer is preferably preferentially oriented in the (001) plane in the normal direction of the substrate. A film having a perovskite structure having a lattice constant larger than that of the KNN piezoelectric thin film is formed on the lower electrode, and a KNN piezoelectric thin film is formed thereon, whereby the KNN piezoelectric thin film is more preferentially oriented on the (001) plane. Can be formed. Further, since the intermediate layer is oriented in the (001) plane, the KNN film can be more strongly oriented in the (001) plane. This makes it possible to increase the piezoelectric properties d ₃₁ of KNN piezoelectric thin film.

  As the substrate, an inexpensive Si substrate, a silicon substrate with a thermal oxide film, or a glass substrate is used. In addition, a magnesium oxide substrate (MgO substrate), a stainless steel substrate, a copper substrate, or an aluminum substrate can also be used. When the substrate is conductive, an insulating film may be formed on the surface.

  As the lower electrode and the upper electrode, any of those conventionally used as electrode materials can be applied, and although not particularly limited, for example, a platinum (Pt) electrode is preferable.

  The method for forming the KNN piezoelectric thin film and the intermediate layer is not particularly limited as long as it is a method capable of forming a high-quality and high-density crystalline thin film. For example, sputtering (RF magnetron sputtering), CVD, PLD It is desirable to use a method or a coating method.

According to the piezoelectric thin film element of the present embodiment, the piezoelectric thin film is formed between the lower electrode and the piezoelectric thin film while using a dielectric thin film having a perovskite structure having (K, Na) NbO ₃ as a main phase. In contact therewith, an intermediate layer having a pseudo-cubic or cubic perovskite structure having a lattice constant of 4.036 or more was inserted, so that the piezoelectric characteristic d ₃₁ of the KNN piezoelectric thin film was larger than that without the intermediate layer inserted. And excellent piezoelectric characteristics can be exhibited. Moreover, since the piezoelectric thin film element uses a dielectric thin film of KNN that does not contain lead, it is very useful from the viewpoint of ecology and pollution prevention.

  The same effect can be expected when a small amount of additive (for example, Li having an atomic number concentration of 8% or less) is mixed in the KNN piezoelectric thin film. In this embodiment, the KNN piezoelectric thin film is described as being applied as a piezoelectric thin film. However, the application is not limited to the piezoelectric application, and the application to various other applications is also possible. Conceivable. For example, it can be used for an inkjet printer, a scanner, a gyroscope, an ultrasonic generator, an ultrasonic sensor, a pressure sensor, a speed sensor, and an acceleration sensor.

  As described above, the present invention is not limited to the above-described embodiment, and it goes without saying that various other things are assumed.

Next, although this invention is demonstrated based on an Example, this invention is not limited to these Examples.
(Example 1)

  Examples in which a piezoelectric thin film element including a 3 μm thick KNN piezoelectric thin film is formed using the present invention will be described below. A sectional view of the formed piezoelectric thin film element structure is shown in FIG.

  A Si substrate with a thermal oxide film ((100) plane orientation, thickness 05 mm, size 20 mm × 20 mm) was used as the substrate.

  First, a Ti adhesion layer (film thickness: 2 nm) and a platinum lower electrode ((111) plane single orientation, film thickness: 0.2 μm) were formed on a Si substrate by RF magnetron sputtering. The Ti adhesion layer and the platinum lower electrode were formed under conditions of a substrate temperature of 350 ° C., a discharge power of 200 W, an introduced gas Ar atmosphere, a pressure of 2.5 Pa, and a film formation time of 1 minute and 10 minutes, respectively.

Next, a 0.2 μm BaSnO ₃ layer was formed on the platinum lower electrode by RF magnetron sputtering. A BaSnO ₃ sintered body was used as a target, and the film was formed under conditions of a substrate temperature of 750 ° C., a discharge power of 150 W, an Ar atmosphere, a pressure of 0.1 Pa, and a film formation time of 10 mm.

Then, RF magnetron spa ivy ring method _{(K 0. 5 Na 0.} 5) to NbO ₃ thin film was 3μm formed. The KNN piezoelectric thin film uses a KNN sintered body having a composition ratio (K + Na) /Nb=1.0 and K / (K + Na) = 0.5 as a target, a substrate temperature of 650 ° C., a discharge power of 100 W, an Ar atmosphere, and a pressure of 0. The film was formed under the conditions of 4 Pa and film formation time of 4 hours.

(Comparative Example 1)
In embodiments, the platinum lower electrode _{(K 0. 5 Na 0.} 5) NbO 3 except that did not form a BaSnO ₃ layer formed between the piezoelectric thin film, the piezoelectric thin-film element under the same conditions as in Example Formed. A cross-sectional view of the formed structure is shown in FIG.

(Evaluation of Example 1 and Comparative Example 1)
In both examples and comparative examples, to evaluate the piezoelectric constant d _31, a piezoelectric characteristic evaluation method shown in FIG.
A platinum upper electrode 5 (thickness: 0.02 μm) is formed on these piezoelectric thin film elements by RF magnetron sputtering, and a rectangular shape having a length of 20 mm and a width of 2.5 mm is cut out to include the KNN piezoelectric thin film 3. Element 6 was produced. A simple unimorph cantilever 7 was constructed by fixing the longitudinal end of the piezoelectric thin film element 6 with a clamp 9 (FIG. 3A).

  Next, in this state, a voltage (sin wave) is applied to the KNN piezoelectric thin film 3 between the upper and lower electrodes 2 and 5, and the KNN piezoelectric thin film 3 is expanded and contracted to bend the cantilever 7 and to operate the lever tip 7a. I let you. The tip displacement was measured with a laser Doppler displacement meter 10 (FIG. 3B).

The piezoelectric constant d ₃₁ (displacement in the direction in which the piezoelectric thin film element contracts) is calculated from the displacement amount of the cantilever tip 7a, the length of the cantilever 7, the thickness and Young's modulus of the substrate 1 and the KNN piezoelectric thin film 3, and the applied voltage. The value of d ₃₁ of Example and Comparative Example shown in FIG. From this result, it is apparent that d ₃₁ is significantly larger in the example than in the comparative example.

(Example 2)
A piezoelectric thin film element was fabricated in the same process as in the above example. However, a KNN piezoelectric thin film was formed on an intermediate layer having a pseudo cubic or cubic perovskite structure having several different lattice constants. Then, measurement of the piezoelectric constant d ₃₁ in the same manner as the piezoelectric characteristic evaluation method described above. The value of d ₃₁ of Example and Comparative Example shown in FIG. The applied voltage is 18V.

As can be seen from FIG. 5, when the lattice constant of the intermediate layer is 4.05 or more, the piezoelectric constant d ₃₁ is 100-pm / V, which is good and has an improvement effect, but below that value, 70-pm. / V or less. Therefore, it was found that there is an improvement effect when the lattice constant is 4.05 or more. In addition to BaSnO ₃ described above, BaZrO ₃ , BaMnO ₃ , KMnF ₃ , and KFeF ₃ were suitable as the effective intermediate layer material.

It is a schematic diagram which shows the cross-section of the piezoelectric thin film element based on the Example of this invention. It is a schematic diagram which shows the cross-sectional structure of the piezoelectric thin film element which concerns on a comparative example. It is a schematic diagram which shows the shape and measurement state of a unimorph cantilever. It is a characteristic view which shows the relationship between the applied voltage of a piezoelectric thin film element, and a piezoelectric constant. It is a characteristic view which shows the relationship between the lattice constant of an intermediate | middle layer, and a piezoelectric constant.

Explanation of symbols

1 Substrate 2 Lower electrode 3 Piezoelectric thin film 4 Intermediate layer

Claims (4)

In a piezoelectric thin film element configured by sequentially arranging a lower electrode and a piezoelectric thin film on a substrate,
The piezoelectric thin film comprises a dielectric thin film having a perovskite structure having (K, Na) NbO ₃ as a main phase,
An intermediate layer having a pseudo-cubic or cubic perovskite structure having a lattice constant of 4.036 Å or more is inserted between the lower electrode and the piezoelectric thin film, in contact with the piezoelectric thin film. Piezoelectric thin film element. The intermediate layer is a dielectric thin film having a perovskite structure in which BaZrO ₃ , BaSnO ₃ , BaMnO ₃ , KMnF ₃ , KFeF ₃ , a mixed crystal thereof, or a perovskite structure in which other elements are mixed in an amount of 10% or less. The piezoelectric thin film element according to claim 1, wherein the piezoelectric thin film element is any one of the following.   3. The piezoelectric thin film element according to claim 1, wherein the intermediate layer is preferentially oriented in a (001) plane in a normal direction of the substrate.   4. The piezoelectric thin film element according to claim 1, wherein a film thickness of the piezoelectric thin film is 0.2 μm or more and 10 μm or less.

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