JP2009049355A - Piezoelectric thin film element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piozoelectric thin film element containing a potassium sodium niobate thin film having excellent piezoelectric characteristics. <P>SOLUTION: The piozoelectric thin film element includes: on a substrate 1; a lower electrode 2; a KNbO<SB>3</SB>thin film 3; a piezoelectric thin film 4 formed of potassium sodium niobate of perovskite structure (expressed by (K<SB>x</SB>Na<SB>1-x</SB>)NbO<SB>3</SB>(0<x<1)), which has a film thickness of 0.2 μm to 10 μm and a specific inductive capacity in the range of 50 or more and 200 or less; and an upper electrode 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a piezoelectric thin film element using potassium sodium niobate as a piezoelectric thin film.

  Piezoelectric bodies are processed into various piezoelectric elements according to various purposes, and are generated by actuators that perform various operations using deformation generated by applying voltage to the piezoelectric elements, and conversely, they are generated by deformation of piezoelectric elements. Widely used as functional electronic components such as sensors for detecting various physical quantities from voltage.

As the piezoelectric bodies are used in the actuator and sensor applications, excellent ferroelectric lead-based material having piezoelectric _{properties, Pb (Zr 1-x Ti} x) O 3 based perovskite ferroelectric particularly called PZT The body is widely used. These ferroelectrics are usually formed by sintering oxide powders composed of individual elements.

  Recently, various electronic components have been miniaturized and improved in performance, and piezoelectric elements such as actuators are required to be further reduced in size and performance. However, the piezoelectric material produced by the manufacturing method centering on the sintering method approaches the size of the crystal grains constituting the material as the thickness of the piezoelectric material is reduced, especially when it becomes a thin film of about 10 μm. The influence of the characteristic cannot be ignored, and there arises a problem that the dispersion and deterioration of the characteristic become remarkable.

  In order to avoid this, a method for forming a piezoelectric material using a thin film forming technique instead of the sintering method has been actively studied in recent years. Thin film formation techniques include, for example, sputtering, PLD (laser ablation), sol-gel, etc., and PZT thin films formed by RF sputtering have been put into practical use as head actuators for high-definition high-speed inkjet printers (for example, , See Patent Document 1).

On the other hand, a piezoelectric sintered body or a piezoelectric thin film made of PZT is not preferable from the viewpoint of ecological viewpoint and pollution prevention because it contains about 60 to 70% by weight of lead. Therefore, development of a piezoelectric material that does not contain lead is desired in consideration of the environment.
Currently, various lead-free piezoelectric materials have been studied, among which potassium sodium niobate (general formula: (K _x Na _1-x ) NbO ₃ (0 <x <1)). Potassium sodium niobate is a material having a perovskite structure and exhibits relatively good piezoelectric properties as a lead-free material, and is therefore expected as a promising candidate for lead-free piezoelectric materials. The potassium sodium niobate sintered body has excellent piezoelectric characteristics in the composition ratio x = 0.5 in (K _x Na _1-x ) NbO ₃ .

  In the prior art, there is one in which the relative dielectric constant of the PZT thin film is defined in order to improve the voltage resistance of the PZT piezoelectric thin film (see, for example, Patent Document 2). In addition, as for a niobic acid compound piezoelectric thin film element, in order to obtain sufficient piezoelectric characteristics, there has been studied the degree of orientation of a perovskite crystal structure (for example, see Patent Document 3).

Japanese Patent Laid-Open No. 11-135850 JP 2001-284670 A JP 2007-19302 A

  However, at present, excellent piezoelectric properties such as a sintered body such as PZT have not been realized with a potassium sodium niobate thin film. In addition, piezoelectric thin film elements fabricated on each substrate have variations in piezoelectric characteristics, but it has not been possible to easily inspect and detect the quality of piezoelectric characteristics.

  An object of the present invention is to solve the above-described problems and provide a piezoelectric thin film element using a potassium sodium niobate thin film having excellent piezoelectric characteristics.

A first aspect of the present invention is a substrate having a lower electrode and a film thickness of 0.2 μm or more and 10 μm or less, represented by a general formula (K _x Na _1-x ) NbO ₃ (0 <x <1). In the piezoelectric thin film element having a perovskite-structured piezoelectric thin film and an upper electrode, the piezoelectric thin film has a relative dielectric constant in the range of 50 to 200.

According to a second aspect of the present invention, in the invention according to the first aspect, a KNbO ₃ thin film is formed between the lower electrode and the piezoelectric thin film.

  According to a third aspect of the present invention, in the first or second aspect, the substrate is a Si substrate or a MgO substrate, and the lower electrode is a Pt electrode.

According to a fourth aspect of the present invention, in the invention described in the second or third aspect, the lower electrode, the piezoelectric thin film, and the KNbO ₃ thin film are formed by an RF magnetron sputtering method.

  According to the present invention, a piezoelectric thin film element using a potassium sodium niobate thin film having excellent piezoelectric characteristics can be obtained.

The present inventors evaluated the piezoelectric characteristics and the like by forming a potassium sodium niobate thin film under various sputter deposition conditions in order to find out why the thin film of potassium sodium niobate does not provide excellent piezoelectric characteristics. It has been found that there is a correlation between the relative dielectric constant of the sodium potassium niobate thin film and the piezoelectric constant, and when the relative dielectric constant is set to 200 or less, the piezoelectric constant increases abruptly about three times (see FIG. 5). When the relative dielectric constant was in the range of 50 to 200, the piezoelectric constant was substantially constant, and a potassium sodium niobate thin film having excellent piezoelectric characteristics was obtained.
Based on this result, the present invention realizes a potassium sodium niobate thin film having excellent piezoelectric characteristics by forming so that the relative dielectric constant of the potassium sodium niobate thin film is in the range of 50 to 200. The main purpose.
According to the examination results of the present inventors, when a potassium sodium niobate thin film is formed by a general sputtering method, a potassium sodium niobate thin film having a relative dielectric constant of 50 to 200 is, for example, ( It can be obtained by increasing the orientation ratio of at least one of the (001) plane orientation, (100) plane orientation, and (010) plane orientation.

FIG. 1 shows a cross-sectional view of a piezoelectric thin film element according to an embodiment of the present invention. As shown in FIG. 1, the piezoelectric thin film element of this embodiment includes a lower electrode 2, a potassium niobate (KNbO ₃ ) thin film 3, and a potassium sodium niobate having a perovskite structure ((K _x Na _1). in _{-x) NbO 3 (0 <x} <1)), thickness 0.2Myuemu～10myuemu, and the ratio between the piezoelectric thin film 4 having a dielectric constant in the range of 50 to 200, and the upper electrode 5 are sequentially formed ing.

As a method for forming the KNbO ₃ thin film 3 and the piezoelectric thin film 4, a sputtering method, a CVD method, a PLD method, a sol-gel method, and the like are conceivable. In the present embodiment, the lower electrode 2 is also formed by the RF magnetron sputtering method.
A small amount of additive (for example, Li having an atomic number concentration of 8% or less) may be mixed in the piezoelectric thin film 4 of potassium sodium niobate. Also in this case, improvement in piezoelectric characteristics can be expected.

  The substrate 1 is preferably a Si substrate or a MgO substrate, and the lower electrode 2 is preferably a Pt electrode. For the growth of the piezoelectric film, an electrode material as an underlayer is important. The Pt (platinum) formed on the Si substrate becomes a single crystal Pt film oriented in the (111) plane orientation due to self-orientation, and the Pt formed on the MgO (100) substrate is a (100) plane. Although it is a polycrystalline film oriented in the orientation, each crystal grain becomes a Pt film regularly aligned in the in-plane direction of the substrate 1. Therefore, the piezoelectric films of potassium niobate and potassium sodium niobate formed on the lower electrode 2 of the Pt film are also strongly oriented in the (100) plane orientation, and each crystal grain is in-plane even if it is a polycrystalline film. The film has regularity in the direction, and is excellent in piezoelectric characteristics and uniform.

The KNbO ₃ thin film 3 on the lower electrode 2 is easily oriented in the (001) plane orientation. For this reason, the piezoelectric thin film 4 of potassium sodium niobate formed on the KNbO ₃ thin film 3 that is the base film has at least one of (001) plane orientation, (100) plane orientation, and (010) plane orientation. The ratio of the orientation of the plane orientation becomes high. The piezoelectric thin film 4 of potassium sodium niobate having a high proportion of the orientation of one or more of the (001) plane orientation, the (100) plane orientation, and the (010) plane orientation has a low dielectric constant.

Various kinds of sputtering film formation conditions were changed to form potassium sodium niobate thin films, and the relationship between the relative permittivity of these thin films and the piezoelectric constant was investigated. The results are shown in FIG. As shown in FIG. 5, the piezoelectric constant d ₃₁ greatly changes with the relative dielectric constant 200 as a boundary. In the range where the relative dielectric constant exceeds 200 (the relative dielectric constant is about 400 for the potassium sodium niobate piezoelectric thin film manufactured by the conventional method), the piezoelectric constant d ₃₁ is approximately 20 to 40 [−pm / V]. the piezoelectric constant _{d 31} becomes remarkably and 80 [-pm / V] or more large dielectric constant of 200 or less.
In addition, although the present inventors changed the sputtering film-forming conditions and formed various potassium sodium niobate thin films using the RF magnetron sputtering method, forming a potassium sodium niobate thin film having a relative dielectric constant of 50 or less. I could not.

Therefore, the ratio of the orientation of one or more of the (001) plane orientation, the (100) plane orientation, or the (010) plane orientation is high, and the relative permittivity of the sodium potassium niobate is as low as 50 to 200. by forming a piezoelectric thin film 4, so that the piezoelectric thin film element piezoelectric constant d ₃₁ was excellent in large piezoelectric characteristics can be obtained.
Further, in order to obtain the piezoelectric constant of a piezoelectric thin film element in which a piezoelectric thin film is formed on a substrate, conventionally, as described later, it is necessary to measure the amount of piezoelectric displacement when a voltage is applied to the piezoelectric thin film element. It was. However, according to the present invention, the relative dielectric constant can be easily calculated by simply measuring the capacitance of a piezoelectric thin film element using a potassium sodium niobate thin film using an LCR meter or the like. It is also useful as a selection method for selecting piezoelectric thin film elements having excellent piezoelectric characteristics with a rate of 50 to 200.

The film thickness of the piezoelectric thin film 4 is preferably 0.2 μm to 10 μm. If the film thickness is less than 0.2 μm, sufficient piezoelectric performance cannot be obtained. On the other hand, if the film thickness exceeds 10 μm, the piezoelectric thin film element cannot be miniaturized.
The film thickness of the KNbO ₃ thin film 3 is preferably about 0.1 μm or less. However, since the KNbO ₃ thin film 3 is a film for increasing the orientation ratio of the piezoelectric thin film 4 in the [001] direction, it is necessary to have a film thickness sufficient to exhibit its function.
Further, the piezoelectric thin film 4 of (K _x Na _1-x ) NbO ₃ should have a composition x = 0.5 in order to obtain excellent piezoelectric characteristics, and the composition x of the piezoelectric thin film 4 is It is preferable to make the composition x uniform such that the difference between the maximum value and the minimum value of the composition x in the film thickness direction is 0.05 or less.

In the above embodiment, the lower electrode 2 and the piezoelectric thin film 4 are used in order to increase the orientation ratio of at least one of (001) plane orientation, (100) plane orientation, and (010) plane orientation. The KNbO ₃ thin film 3 is provided between the piezoelectric thin film 4 and the (001) plane orientation, (100) plane orientation, or (010) plane of the piezoelectric thin film 4 by adjusting the sputtering film forming conditions. It is possible to increase the orientation ratio of any one or more plane orientations of the orientation. Specifically, the substrate temperature, RF discharge power, introduced gas composition / pressure, target composition, distance between the target and the substrate, and the like are adjusted. For example, when the distance between the target and the substrate is increased, the incident of source atoms sputtered from the target and flying straight to the substrate is aligned in the direction perpendicular to the substrate, and the (001) plane orientation, (100) plane orientation, Alternatively, the orientation ratio of any one or more of (010) plane orientations is increased.

An actuator can be obtained by connecting at least a voltage applying means to the lower electrode 2 and the upper electrode 5 of the piezoelectric thin film element according to the embodiment shown in FIG. Various members can be driven by applying a voltage to the piezoelectric thin film element of the actuator to deform the piezoelectric thin film element. Moreover, a sensor is obtained by connecting at least a voltage detection means to the lower electrode 2 and the upper electrode 5 of the piezoelectric thin film element according to the present embodiment. When the piezoelectric thin film element of this sensor is deformed along with any change in physical quantity, a voltage is generated along with the deformation, so that various physical quantities can be detected by detecting this voltage.
The actuator is used in an inkjet printer, a scanner, an ultrasonic generator, and the like. Sensors are used for gyros, ultrasonic sensors, pressure sensors, speed / acceleration sensors, and the like.
Moreover, although the said embodiment demonstrated the case where a potassium sodium niobate thin film was applied as a piezoelectric thin film, the application to various uses, for example, a pyroelectric element and a surface acoustic wave device, can be considered. .

  Next, examples of the present invention will be described.

(Example)
In this example, a piezoelectric thin film element having the same structure as that of the above embodiment shown in FIG. 1 was produced.
The substrate 1 is a Si substrate ((001) plane orientation, thickness 0.5 mm), and the platinum lower electrode 2 ((111) plane single orientation, film thickness 0.2 μm is formed on the Si substrate by RF magnetron sputtering. ) Was formed. The platinum lower electrode 2 was formed under conditions of a substrate temperature of 350 ° C., a discharge power of 200 W, an introduction gas Ar, a pressure of 2.5 Pa, and a film formation time of 10 minutes.
Next, a KNbO ₃ thin film 3 (film thickness: 0.1 μm) was formed on the lower electrode 2 by an RF magnetron sputtering method. The KNbO ₃ thin film 3 was formed using a KNbO ₃ sintered body as a target, a substrate temperature of 650 ° C., a discharge power of 100 W, an introduction gas Ar, a pressure of 0.4 Pa, and a film formation time of 10 minutes.
Furthermore, a piezoelectric thin film 4 (thickness: 2.9 μm) of (K _0.5 Na _0.5 ) NbO ₃ was formed on the KNbO ₃ thin film 3 by an RF magnetron sputtering method. The piezoelectric thin film 4 of (K _0.5 Na _0.5 ) NbO ₃ has a composition ratio of (K + Na) /Nb=1.0 and K / (K + Na) = 0.5 (K _0.5 Na _{0 .5} ) Using a NbO ₃ sintered body, a film was formed at a substrate temperature of 650 ° C., a discharge power of 100 W, an introduction gas Ar, a pressure of 0.4 Pa, and a film formation time of 3 hours and 50 minutes.
Finally, a platinum upper electrode (film thickness 0.02 μm) 5 was formed on the (K _0.5 Na _0.5 ) NbO ₃ piezoelectric thin film 4 by RF magnetron sputtering. The platinum upper electrode 5 was formed at a substrate temperature of 350 ° C., a discharge power of 200 W, an introduction gas Ar, a pressure of 2.5 Pa, and a film formation time of 1 minute.

(Comparative example)
As a comparative example, a piezoelectric thin film element having a cross-sectional structure shown in FIG.
In the comparative example, the KNbO ₃ thin film 3 in the example is not formed, and the film thickness of the piezoelectric thin film 6 of (K _0.5 Na _0.5 ) NbO ₃ is set to 3.0 μm (film formation time 4 hours). Except for this point, the piezoelectric thin film element shown in FIG.

The dielectric constant measurement and the piezoelectric property evaluation measurement were performed on the piezoelectric thin film elements of the example and the comparative example.
Strip samples 10 each having a length of 20 mm and a width of 2.5 mm were cut out from the piezoelectric thin film elements (substrates) shown in the example of FIG. 1 and the comparative example of FIG. The capacitance of the produced sample 10 was measured using an LCR meter (frequency: 1 kHz), and the dielectric constant of the piezoelectric thin film element of the example and the comparative example was taken into consideration in consideration of the electrode area of the sample 10 and the thickness of the piezoelectric thin film. The rate was calculated. The relative dielectric constant of the piezoelectric thin film 4 in the example was 180, and the relative dielectric constant of the piezoelectric thin film 6 in the comparative example was 400.

Next, piezoelectric characteristic evaluation measurement was performed. FIG. 3A shows a schematic configuration of the measurement method.
As shown in FIG. 3 (a), one end of the sample 10 was fixed to the clamp 20 to constitute a simple unimorph cantilever. The clamp 20 was installed on the anti-seismic table 21 to remove the vibration. In this state, a voltage is applied between the upper electrode 1 and the lower electrode 5, and the piezoelectric thin films 3 and 4 and the piezoelectric thin film 6 are expanded and contracted, thereby bending the entire sample 10 and moving the tip of the sample 10 up and down. It was. FIG. 3B shows a state in which the entire sample 10 is bent and the tip of the sample 10 is displaced upward. The tip displacement amount Δ of the sample 10 moving up and down was measured with a laser Doppler displacement meter 22.
FIG. 4 shows the relationship between the applied voltage and the maximum tip displacement in the examples and comparative examples. FIG. 4 shows that the maximum displacement of the tip of the sample 10 due to the piezoelectric is about three times that of the piezoelectric thin film element of the example compared to the piezoelectric thin film element of the comparative example.

The piezoelectric constant d ₃₁ was calculated from the relationship between the size of the sample 10 and the applied voltage and the maximum tip displacement. As a result, the piezoelectric constant d ₃₁ of the piezoelectric thin film element manufactured according to the comparative example was 30 [−pm / V], whereas the piezoelectric constant d ₃₁ of the piezoelectric thin film element manufactured according to the present example was 90 [−pm. / V]. From this, it was confirmed that by using the present invention, a piezoelectric thin film of (K _x Na _1-x ) NbO _{3 having} excellent piezoelectric characteristics can be produced.

  In addition, X-ray diffraction pattern measurement (2θ-ω scan) was performed on the piezoelectric thin film elements of the example and the comparative example in the state before forming the upper electrode 5 in order to investigate the orientation state of the piezoelectric thin film. It was. FIG. 6 shows the result of the X-ray diffraction pattern measurement of the example, and FIG. 7 shows the result of the X-ray diffraction pattern measurement of the comparative example.

As shown in FIG. 6, in the piezoelectric thin film element of the example, the crystal plane in which the diffraction peak of (K _x Na _1-x ) NbO ₃ (KNN) was observed was the (001) plane orientation of KNN, (100 ) Any one or more of the plane orientations or (010) plane orientations and any one or more of the (002), (200) or (020) plane orientations of KNN Was almost. From this, the (K _x Na _1-x ) NbO ₃ piezoelectric thin film of the example is high in any one or more of (001) plane orientation, (100) plane orientation, or (010) plane orientation. It was found that they were oriented at a ratio.
On the other hand, in the piezoelectric thin film element of the comparative example shown in FIG. 7, the crystal plane where the NKK diffraction peak was observed is the (001) plane orientation, (100) plane orientation, or (010) plane orientation of KNN. Any one or more plane orientations, any one or more plane orientations of (002) plane orientation, (200) plane orientation, or (020) plane orientation of KNN, and (110) plane orientation of NKK, There are (101) plane orientation and one or more plane orientations of (011) plane orientation, and the (K _x Na _1-x ) NbO ₃ piezoelectric thin film of the comparative example has a (001) plane orientation, (100 It has been found that the degree of orientation of at least one of the plane orientation and the (010) plane orientation is lower than that of the example.
The plane orientation of the KNN crystal (piezoelectric thin film) here is the diffraction pattern in the X-ray diffraction measurement (2θ-ω).
The diffraction peak at 2θ = 22.011 ° to 22.890 ° is a (001) plane, a (100) plane, or a (010) plane,
The diffraction peak of 2θ = 31.260 ° to 32.484 ° is a (110) plane, a (101) plane, or a (011) plane,
The diffraction peak at 2θ = 44.879 ° to 46.788 ° is taken as the (002) plane, (200) plane, or (020) plane.
The diffraction peak angle has a range as described above for the following two reasons.
One reason is that the KNN crystal lattice is distorted by the influence of internal stress caused by the thermal expansion difference between the substrate 1 and the KNN crystal (piezoelectric thin film) and the influence of the base (lower electrode 2).
Another reason is that the crystal lattice size changes depending on the Na / (K + Na) composition ratio.

It is sectional drawing of the piezoelectric thin film element in the embodiment and Example of this invention. It is sectional drawing of the piezoelectric thin film element in a comparative example. It is a schematic block diagram explaining the measuring method of piezoelectric property evaluation. It is a figure which shows the relationship between the applied voltage with respect to the piezoelectric thin film element of an Example and a comparative example, and a tip maximum displacement amount. A piezoelectric constant d ₃₁ of the piezoelectric thin film is a diagram showing the relationship between the relative dielectric constant. It is a figure which shows the X-ray-diffraction pattern of the piezoelectric thin film element of an Example. It is a figure which shows the X-ray-diffraction pattern of the piezoelectric thin film element of a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Lower electrode 3 KNbO ₃ Thin film 4 Piezoelectric thin film 5 Upper electrode 6 Piezoelectric thin film 10 Sample 20 Clamp 21 Isolation table 22 Laser Doppler displacement meter

Claims (4)

On the substrate, a lower electrode, a piezoelectric thin film having a perovskite structure having a thickness of 0.2 μm or more and 10 μm or less and represented by a general formula (K _x Na _1-x ) NbO ₃ (0 <x <1), In a piezoelectric thin film element having an upper electrode,
A piezoelectric thin film element, wherein a relative dielectric constant of the piezoelectric thin film is in a range of 50 to 200. 2. The piezoelectric thin film element according to claim 1, wherein a KNbO ₃ thin film is formed between the lower electrode and the piezoelectric thin film.   3. The piezoelectric thin film element according to claim 1, wherein the substrate is a Si substrate or a MgO substrate, and the lower electrode is a Pt electrode. 4. The piezoelectric thin film element according to claim 2, wherein the lower electrode, the piezoelectric thin film, and the KNbO ₃ thin film are formed by an RF magnetron sputtering method.

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