JP2007294593A - Element using piezoelectric thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an element using a lead-free piezoelectric thin film with an excellent piezoelectric characteristic, where relative permittivity or piezoelectric characteristic is not changed with circumferential temperature change. <P>SOLUTION: The element 10 using the piezoelectric thin film is obtained by successively forming a lower electrode layer 12, a piezoelectric layer composed of the piezoelectric thin film, and an upper electrode layer 12 on a substrate 11. The piezoelectric layer is an alkali niobic oxide film 1 expressed by (Na<SB>x</SB>K<SB>y</SB>Li<SB>z</SB>)NbO<SB>3</SB>(0<x<1, 0<y<1, 0<z≤0.05, x+y+z=1), and the crystalline structure of the alkali niobic oxide film 1 is tetragonal at normal temperature. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an element using a piezoelectric thin film in which a lower electrode layer, a piezoelectric layer composed of a piezoelectric thin film, and an upper electrode layer are sequentially formed on a substrate.

Piezoelectric materials are processed into various piezoelectric elements according to various purposes, and are widely used as functional electronic components such as actuators that generate deformation by applying voltage, and sensors that generate voltage from element deformation. ing. As a piezoelectric material used for actuators and sensors, a lead-based dielectric material having a large piezoelectric characteristic, in particular, a Pb (Zr _1-x Ti _x ) O ₃ perovskite type ferroelectric material called PZT has been used so far. It is widely used and is usually formed by sintering oxides composed of individual elements.

However, since the piezoelectric sintered body made of PZT contains about 60 to 70% by weight of lead oxide (PbO), it is not preferable from the viewpoint of ecological viewpoint and pollution prevention. Therefore, development of a piezoelectric body that does not contain lead is desired in consideration of the environment. Currently, various lead-free piezoelectric materials have been studied, among which 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) (see, for example, Patent Document 1) Since this lithium potassium sodium niobate thin film has piezoelectric characteristics comparable to PZT, it is a promising candidate for lead-free piezoelectric materials. Expected.

JP 2004-300012 A

  Generally, lithium potassium sodium niobate as a sintered body is orthorhombic at room temperature. When lithium potassium sodium niobate is heated, it changes from orthorhombic to tetragonal at a relatively low temperature of 100 to 200 ° C. (second phase transition point). For this reason, even when lithium potassium niobate is used near room temperature, there is a problem that the relative dielectric constant and piezoelectric characteristics change due to the increase in ambient temperature and the temperature of surrounding electrical components.

  In order to solve this problem, it was necessary to increase the lithium content of lithium potassium sodium niobate (0.06 ≦ z). By increasing the amount of lithium, it becomes tetragonal at room temperature, and the second phase transition point can be reduced to room temperature or lower.

  However, when the lithium content is increased, there are problems such as a decrease in piezoelectric constant and a decrease in mechanical quality factor Qm. When a piezoelectric body is used as an actuator, the piezoelectric constant must be large, and when it is used as a resonator, the mechanical quality factor Qm must be large.

  Accordingly, an object of the present invention is to provide an element using a piezoelectric thin film that does not change in relative permittivity and piezoelectric characteristics due to changes in ambient temperature, and is lead-free and excellent in piezoelectric characteristics.

The present invention has been devised to achieve the above object, and the invention of claim 1 comprises a piezoelectric thin film in which a lower electrode layer, a piezoelectric layer composed of a piezoelectric thin film, and an upper electrode layer are sequentially formed on a substrate. In the element used, the piezoelectric layer is represented by (Na _x K _y Li _z ) NbO ₃ (0 <x <1, 0 <y <1, 0 <z ≦ 0.05, x + y + z = 1). It is an element using a piezoelectric thin film which is an alkali niobium oxide film and whose crystal structure is tetragonal at room temperature.

  A second aspect of the present invention is an element using a piezoelectric thin film according to the first aspect, wherein the thickness of the alkali niobium oxide film is less than 20 μm.

  A third aspect of the invention is an element using the piezoelectric thin film according to the first or second aspect, wherein the substrate is any one of an MgO substrate, a Si substrate, a glass substrate, a stainless steel substrate, a Cu substrate, and an Al substrate.

  A fourth aspect of the present invention is an element using the piezoelectric thin film according to any one of the first to third aspects, wherein the lower electrode layer and the upper electrode layer are formed of a cubic material.

  The invention according to claim 5 is the element using the piezoelectric thin film according to claim 4, wherein the lower electrode layer and the upper electrode layer are made of Pt.

  According to the present invention, the relative permittivity and the piezoelectric characteristics are not changed by the ambient temperature change, and the lead-free and excellent piezoelectric characteristics are exhibited.

  As a result of diligent research for obtaining a piezoelectric thin film capable of obtaining a desired piezoelectric characteristic by setting the second phase transition point to room temperature or lower and reducing the lithium content as much as possible, the present inventors have found an example of an alkali niobium oxide film. A device using the piezoelectric thin film of the present invention including a lithium potassium sodium niobate thin film was invented.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a cross-sectional view of an element using a piezoelectric thin film (piezoelectric thin film element) showing a preferred embodiment of the present invention.

As shown in FIG. 1, the piezoelectric thin film element 10 according to the present embodiment has a lower electrode layer 12 and a piezoelectric layer made of a piezoelectric thin film on a substrate 11 as (Na _x K _y Li _z ) NbO ₃ (0 < An alkali niobium oxide film 1 represented by x <1, 0 <y <1, 0 <z ≦ 0.05, x + y + z = 1) and an upper electrode layer 13 are sequentially formed.

The alkali niobium oxide film 1 has a tetragonal crystal structure at normal temperature, preferably 100 to 200 ° C. or less, more preferably −40 to 80 ° C. in the actual use temperature range. As the alkali niobium oxide film 1, a lithium potassium sodium niobate (for example, (Na _{0.5-a / 2} K _{0.5-a / 2} Li _a ) NbO ₃ , 0 <a ≦ 0.05) film is used.

Lithium potassium sodium niobate (Na _x K _y Li _z ) NbO ₃ which is an example of the alkali niobium oxide film 1 has a large piezoelectric constant d ₃₁ when the values of x and y are in the range of 0.4 to 0.6. Is particularly effective.

  The thickness of the alkali niobium oxide film 1 is preferably less than 20 μm. As will be described later, in order to obtain a tetragonal alkali niobium oxide film 1 at room temperature, it is necessary to constrain the crystal structure of the alkali niobium oxide film 1 by the lower electrode layer 12 made of a cubic material as a base. There is. For this reason, when the film thickness is 20 μm or more, the restraining force of the lower electrode layer 12 on the alkali niobium oxide film 1 becomes weak, and a sufficient restraining force cannot be obtained.

  As the substrate 11, any one of an MgO substrate, a Si substrate, a glass substrate, a stainless steel substrate, a Cu substrate, and an Al substrate is used.

  The lower electrode layer 12 and the upper electrode layer 13 are formed of a cubic material such as Pt (platinum), Lu (ruthenium), Ir (iridium), Cu, Au, or Ag. In the present embodiment, the lower electrode layer 12 and the upper electrode layer 13 are made of Pt, which is a general electrode material.

  As a method for producing the alkali niobium oxide film 1, it is desirable to use a sputtering method, a CVD method, a PLD (Pulsed Laser Deposition) method, a coating method or the like that can produce a high-quality and high-density crystalline thin film. Further, even if a small amount of additive is mixed in the alkali niobium oxide film 1, the same effect as that obtained when no additive is present can be obtained.

  The operation of this embodiment will be described.

  Conventionally, lithium potassium sodium niobate is generally sintered and formed as a bulk sintered body. In the piezoelectric thin film element 10, a lower electrode layer 12 is formed on a substrate 11, and the lower electrode layer 12 is formed. Further, as an example of the alkali niobium oxide film 1, lithium potassium sodium niobate is formed in a thin film state. When lithium potassium sodium niobate is formed in a thin film state on a substrate with an electrode, it is possible to use a binding force due to the crystal structure of the substrate and the electrode material.

  Thereby, in the piezoelectric thin film element 10, even when the lithium composition ratio is in the range of 0 <z ≦ 0.05, a tetragonal lithium potassium sodium niobate thin film can be formed at room temperature.

  More specifically, a lithium potassium sodium niobate thin film can be generally formed using a sputtering method or the like. In order to form lithium potassium sodium niobate by sputtering or the like, it is usually formed at a temperature of 500 ° C. or higher. Since this temperature is higher than the Curie point (first phase transition point) of lithium potassium sodium niobate, cubic crystals are formed when the film is formed.

  When the film formation is completed, the substrate 11 starts to be cooled, and the film temperature is cooled below the Curie point. Below the Curie point, the film changes from cubic to tetragonal. At this time, it is considered that the lattice constants of the a-axis and b-axis of the crystal structure of the film do not change greatly.

  However, when it is further cooled to around 100 to 200 ° C., when it is not formed on the substrate as in the prior art (bulk sintered body), there is a second phase transition point and the phase transition from tetragonal to orthorhombic. At this time, the lattice constant changes greatly.

  In contrast, the present inventors formed a lower electrode layer 12 made of a cubic electrode material on the substrate 11 as in this embodiment, and formed lithium potassium niobate on the lower electrode layer 12. It was found that sodium did not have a second phase transition point between the Curie point and room temperature, and was tetragonal even at room temperature.

  This is because the crystal structure of the lower electrode layer 12 made of a cubic electrode material as a base constrains the crystal structure of the alkali niobium oxide film 1, that is, the restraint force due to the crystal structure of the lower electrode layer 12. This is probably because the lithium potassium sodium niobate thin film maintains a tetragonal structure even at room temperature.

  With the above-described structure, the piezoelectric thin film element 10 is obtained by reducing a lithium potassium sodium niobate thin film, which is an example of the alkali niobium oxide film 1, to a square at room temperature even in the range of 0 <z ≦ 0.05 by reducing the lithium content as much as possible. It becomes possible to make a crystal.

  Accordingly, the piezoelectric thin film element 10 does not change in relative permittivity or piezoelectric characteristics due to a change in ambient temperature, is lead-free, has a large piezoelectric constant, residual polarization value, and mechanical quality factor Qm, and has excellent piezoelectric characteristics. ing.

  By using this piezoelectric thin film element 10, it becomes possible to manufacture devices such as actuators and resonators that operate stably with respect to temperature changes such as changes in ambient temperature and temperature changes due to heat generation of surrounding electrical components. .

  The piezoelectric thin film element 10 can be used as an electromechanical energy conversion component of an inkjet printer, a scanner, a gyroscope, and an ultrasonic generator, and as an ultrasonic sensor, a pressure sensor, a speed sensor, an acceleration sensor, and a surface wave filter.

  Moreover, although the piezoelectric thin film element 10 according to the above-described embodiment has been described for the piezoelectric application, the application is not limited to the piezoelectric application. For example, the piezoelectric thin film element 10 is applicable to various applications such as a ferroelectric component. Application is conceivable.

Example 1
Hereinafter, as an example of the alkali niobium oxide film 1 having a thickness of 2 μm using the present invention, a lithium potassium sodium niobate film (in the example 1, (Na _{0.5-a / 2} K _{0.5-a / 2} Li _a ) NbO _{3 is used.} , 0 <a ≦ 0.05) An example in which the piezoelectric thin film element 10 including 1 is formed will be described. Here, x = 0.5−a / 2, y = 0.5−a / 2, and z = a.

  The substrate 11 was an MgO substrate ((001), 20 mm × 20 mm, thickness 0.5 mm). A Pt ((001) plane orientation, film thickness 0.2 μm) lower electrode layer 12 is formed on the MgO substrate 11 by RF (radio frequency) magnetron sputtering, and the alkali niobium oxide film 1 is formed thereon by RF magnetron sputtering. 1 was formed, and a Pt upper electrode layer 13 was formed thereon to produce the piezoelectric thin film element 10 of FIG.

  The formation conditions of the Pt lower electrode 12 are a substrate temperature of 700 ° C., a discharge power of 200 W, an introduced gas Ar atmosphere, a pressure of 2.5 Pa, a film formation time of 10 minutes, and the formation conditions of the alkali niobium oxide film 1 are a substrate temperature of 600 ° C. Discharge power 75 W, introduction gas Ar atmosphere, pressure 0.4 Pa, film formation time 3 hours and 30 minutes, Pt upper electrode 13 is formed at a substrate temperature of 200 ° C., discharge power 200 W, introduction gas Ar atmosphere, pressure 2.5 Pa, The film formation time was 1 minute.

(Conventional example)
For comparison with Example 1, a bulk sintered body of lithium potassium sodium niobate was formed by a conventional technique, and a piezoelectric element similar to that of Example 1 was formed using this bulk sintered body.

  In order to know the crystal structure of the prepared lithium potassium sodium niobate thin film of Example 1, X-ray diffraction measurement (ω-2θ scan) was performed. As a result, it was found that a lithium potassium sodium niobate thin film oriented in the (001) direction was formed.

From this XRD (X-ray diffraction) peak map of this film, the length (lattice constant) of the a-axis (■ in FIG. 2) and c-axis (♦ in FIG. 2) of the crystal structure with respect to the lithium composition ratio (mol ratio). FIG. 2 shows the result of calculating (ii) (10 ⁻¹⁰ m)).

  As shown in FIG. 2, the alternate long and short dash line indicates the a-axis lattice constant of the crystal structure with respect to the lithium composition ratio in the conventional bulk sintered body. In the case of a bulk sintered body, since the lithium composition ratio is orthorhombic when the lithium composition ratio is 0.05 or less and tetragonal when 0.05 or more, the a-axis lattice constant changes discontinuously (the dotted line portion in FIG. 2). ).

  In contrast, the lithium potassium sodium niobate thin film of Example 1 has no discontinuity in the data obtained for both the a-axis and the c-axis. From this, Example 1 in which the lower electrode layer 12 was formed on the substrate 11 and the lithium potassium sodium niobate was formed as a thin film on the lower electrode layer 12 had a tetragonal crystal structure even when the lithium composition ratio was 0.05 or less. You can see that it is maintained. That is, in Example 1, since there is no change in length in the vicinity of 0.05 for both the a-axis and the c-axis, it indicates that there is no phase change in the vicinity of 0.05.

  Therefore, unlike the case of the conventional bulk sintered body, it was found that in Example 1, tetragonal crystals were formed even when x was 0.05 or less.

  Next, the piezoelectric characteristics of the piezoelectric thin film element 10 of Example 1 were examined.

  First, a simple unimorph cantilever was formed by cutting out a rectangular shape having a length of 20 mm and a width of 3 mm from the piezoelectric thin film element 10 and fixing the end in the longitudinal direction with a clamp. In this state, by applying a voltage to the lithium potassium sodium niobate thin film between both electrodes, the thin film was expanded and contracted to operate the lever tip. The amount of tip displacement was measured with a laser Doppler displacement meter.

As a result, it was found that the piezoelectric thin film element 10 having the piezoelectric constant d ₃₁ = −57 pm / V and good piezoelectric characteristics was formed at x = 0.02.

(Example 2)
Moreover, when the same examination was performed using a Si substrate, a glass substrate, a stainless steel substrate, a Cu substrate, and an Al substrate as the substrate 11 in addition to the MgO substrate, the same effects as in Example 1 were obtained.

It is sectional drawing of the element using the piezoelectric thin film which shows suitable embodiment of this invention. The figure which shows the lithium composition ratio dependence of the a-axis and c-axis of the crystal structure in the piezoelectric thin film produced in the example, and the lithium composition ratio dependence of the a-axis of the crystal structure in the piezoelectric bulk sintered body produced in the conventional example. It is.

Explanation of symbols

1 Alkaline niobium oxide film (lithium potassium sodium niobate thin film)
DESCRIPTION OF SYMBOLS 10 Piezoelectric thin film element 11 Substrate 12 Lower electrode layer 13 Upper electrode layer

Claims (5)

In an element using a piezoelectric thin film in which a lower electrode layer, a piezoelectric layer composed of a piezoelectric thin film, and an upper electrode layer are sequentially formed on a substrate, the piezoelectric layer has (Na _x K _y Li _z ) NbO ₃ (0 < x <1, 0 <y <1, 0 <z ≦ 0.05, x + y + z = 1), and the crystal structure of the alkali niobium oxide film is tetragonal at room temperature. An element using a piezoelectric thin film characterized by being.   2. A device using a piezoelectric thin film according to claim 1, wherein the thickness of the alkali niobium oxide film is less than 20 [mu] m.   The element using the piezoelectric thin film according to claim 1 or 2, wherein the substrate is an MgO substrate, a Si substrate, a glass substrate, a stainless steel substrate, a Cu substrate, or an Al substrate.   The element using the piezoelectric thin film according to claim 1, wherein the lower electrode layer and the upper electrode layer are formed of a cubic material. 5. The element using a piezoelectric thin film according to claim 4, wherein the lower electrode layer and the upper electrode layer are made of Pt.

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