JP5103790B2 - Piezoelectric thin film, element using piezoelectric thin film, and method for manufacturing piezoelectric thin film element - Google Patents

Description

The present invention relates to a piezoelectric thin film formed on an electrode layer, an element using the piezoelectric thin film , and a method for manufacturing the piezoelectric thin film element .

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 ferroelectric material called PZT has been used so far. It is widely used and is usually formed by sintering oxides composed of individual elements. At present, as various electronic components have been reduced in size and performance, there has been a strong demand for miniaturization and high performance in piezoelectric elements.

  However, a piezoelectric material manufactured 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 when the thickness becomes 10 μm or less. The influence on the material properties by can not be ignored. For this reason, problems such as significant variation in characteristics and deterioration have occurred, and in order to avoid such problems, methods for forming piezoelectric bodies using thin film technology instead of sintering have recently been studied. . Recently, PZT thin films formed by RF (high frequency) sputtering have been put into practical use as head actuators for high-definition high-speed inkjet printers.

  The prior art document information related to the invention of this application includes the following.

JP 2004-300012 A

On the other hand, the piezoelectric sintered body or piezoelectric thin film made of PZT described above contains lead oxide (PbO) in an amount of about 60 to 70% by weight, which is not preferable from the viewpoint of ecology 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) Since this lithium potassium sodium niobate thin film has piezoelectric properties comparable to PZT, it is expected as a promising candidate for a lead-free piezoelectric material.

In order to realize high piezoelectric characteristics in the lithium potassium sodium niobate thin film, it is necessary to form a crystal thin film having a perovskite structure. Currently, some research institutions have realized the formation of highly oriented lithium potassium sodium niobate crystal thin films on SrTiO ₃ single crystal substrates, but perovskite structures on general electrodes (currently platinum is the mainstream) The crystal thin film cannot be obtained. Therefore, a piezoelectric thin film element having excellent piezoelectric characteristics in this material has not been realized.

  That is, when a lithium potassium sodium niobate thin film is to be formed on a general electrode (for example, platinum), there is a problem that a lithium potassium sodium niobate thin film having a perovskite structure cannot be successfully formed if formed directly on the electrode.

  Accordingly, an object of the present invention is to provide a lead-free piezoelectric thin film having excellent piezoelectric characteristics.

The present invention has been made in order to achieve the above object, the invention of claim 1, a piezoelectric thin film formed on the electrode _{layer, (Na x K y Li z} ) NbO 3 (0 <x <1 , 0 <y <1, 0 <z <1, x + y + z = 1), the value of z in the film being different in the thickness direction, and the electrode layer side a piezoelectric thin film increases phased in accordance with distance from the.

According to another aspect of the invention, the lower electrode layer on a substrate, a piezoelectric layer, in a device using a piezoelectric thin film formed of the upper electrode layer, the piezoelectric layer is made of an alkali niobate oxide film according to claim 1, wherein This is an element using a piezoelectric thin film.

The invention according to claim 3 is the element using the piezoelectric thin film according to claim 2 , 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.

The invention according to claim 4 is the element using the piezoelectric thin film according to claim 2 or 3 , wherein the lower electrode layer and the upper electrode layer are made of Pt.
According to a fifth aspect of the present invention, there is provided a substrate, a lower electrode layer formed on the substrate, and a (Na _x K _y ) NbO ₃ (0 <x <1, 0 <y < 1, x + y = 1) formed on the first alkali niobium oxide film and (Na _x K _y Li _z ) NbO ₃ (0 <x <1,0 <Y <1, 0 <z <1, x + y + z = 1), the second alkali niobium oxide film, and the first alkali niobium oxide film and the second alkali niobium oxide film, (Na _x K _y Li _z ) NbO ₃ (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1) reduced in lithium composition as compared with the two-alkali niobium oxide film A third alkali niobium oxide film and an upper electrode layer formed on the second alkali niobium oxide film. A method of forming the lower electrode layer on the substrate, a step of forming the first alkali niobium oxide film on the lower electrode layer, and the first alkali niobium element. Forming the third alkali niobium oxide film on the oxide film; forming the second alkali niobium oxide film on the third alkali niobium oxide film; and the second alkali niobium oxide. And forming the upper electrode layer on the film. A method for producing a piezoelectric thin film element, comprising:

  According to the present invention, the excellent effect of lead-free and excellent piezoelectricity is exhibited.

  As a result of diligent research for a lead-free piezoelectric thin film having excellent piezoelectric properties, which has been required for recent piezoelectric materials, the present inventors have found a lithium potassium sodium niobate thin film as an example of an alkali niobium oxide film. Invented the piezoelectric thin film of the present invention.

  As described in the background art, there is currently no product obtained by forming a lithium potassium sodium niobate thin film having a perovskite structure on a general electrode such as platinum.

In order to solve the above problems, the present inventors have
(1) Method of forming a layer containing no lithium on the electrode and forming a layer containing lithium on the upper part (2) Forming a layer having a small lithium composition ratio on the electrode, and forming lithium on the upper part Of forming a layer having a high composition ratio (3) To obtain a lithium potassium sodium niobate thin film having a perovskite structure by using a method in which the composition ratio of lithium gradually increases from the electrode layer side (substrate side) Found that it would be possible.

  Hereinafter, a preferred first embodiment of the present invention will be described with reference to the accompanying drawings.

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

As shown in FIG. 1, the piezoelectric thin film 1 according to the first embodiment is formed on an electrode layer, and (Na _x K _y ) NbO ₃ (0 <x <1, 0 <y <1, x + y = 1) formed on the first alkali niobium oxide film (orientation control layer) 2 and the first alkali niobium oxide film 2, and (Na _x K _y Li _z ) NbO ₃ (0 < x <1, 0 <y <1, 0 <z <1, x + y + z = 1) and the second alkali niobium oxide film 3.

As the first alkali niobium oxide film 2, a potassium sodium niobate (for example, Na _0.5 K _0.5 ) NbO ₃ ) film is used. As the second alkali niobium oxide film 3, a lithium potassium sodium niobate (for example, (Na _0.47 K _0.47 Li _0.06 ) NbO ₃ ) film is used.

  The piezoelectric thin film 1 is manufactured by the above-described method (1), that is, a method in which a layer not containing lithium is formed on an electrode and a layer containing lithium is formed on top of the layer.

  An element (piezoelectric thin film element) 10 using the piezoelectric thin film 1 is formed by sequentially forming a lower electrode layer 12 and a piezoelectric thin film 1 and an upper electrode layer 13 as a piezoelectric layer on a substrate 11.

  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 made of Pt (platinum), Lu (ruthenium), Ir (iridium), or an oxide thereof. 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 first alkali niobium oxide film 2, 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 first alkali niobium oxide film 2, the same effect as that obtained when no additive is present can be obtained.

  The operation of the first embodiment will be described.

  When an attempt is made to form a lithium potassium sodium niobate thin film directly on a general electrode such as Pt, lithium niobate is formed on a part of the electrode. Since lithium niobate is not a perovskite structure having piezoelectricity but an illuminite structure having no piezoelectricity, a film having a perovskite structure is not formed thereon. However, if the film formation is continued, niobium, potassium, sodium, and oxygen, which try to take a perovskite structure, fly as reactive species, so these substances lose their place, and an amorphous layer or the like is formed.

  On the other hand, when potassium sodium niobate containing no lithium is formed on the electrode, both potassium niobate and sodium niobate formed as crystal nuclei on the electrode have a perovskite structure. Crystals are formed. From this, it is easy to form potassium sodium niobate on the electrode.

  In addition, since potassium sodium niobate has almost the same lattice constant and the like as lithium potassium sodium niobate having a perovskite structure, lithium potassium sodium niobate is easily formed on potassium sodium niobate.

  The piezoelectric thin film 1 uses these characteristics to form a potassium sodium niobate thin film as an orientation control layer on the lower electrode 12 as an example of the first alkali niobium oxide film 2, and a second alkali on the upper part. As an example of the niobium oxide film 3, a lithium potassium sodium niobate thin film is formed.

  In the piezoelectric thin film 1, since the first alkali niobium oxide film 2 containing no lithium functions as a buffer layer when forming the second alkali niobium oxide film 3 containing lithium, a perovskite having piezoelectricity in the film A thin film having a structure can be formed uniformly.

  Thus, the piezoelectric thin film 1 has a two-layer structure of a potassium sodium niobate film as an example of the first alkali niobium oxide film 2 and a lithium potassium sodium niobate film as an example of the second alkali niobium oxide film 3. By doing so, the above-described problems can be solved, lead-free and excellent in piezoelectricity. The piezoelectric thin film element 10 using the piezoelectric thin film 1 is also lead-free and excellent in piezoelectricity for the same reason.

  Next, a second embodiment will be described.

As shown in FIG. 5, the piezoelectric thin film 51 is formed on the electrode layer, and (Na _x K _y Li _z ) NbO ₃ (0 <x <1, 0 <y <1, 0 <z <1). , X + y + z = 1), the value of z in the film is different in the thickness direction, and continuously increases as the distance from the surface on the electrode layer side increases. is there.

  The piezoelectric thin film 51 is manufactured by the method (3) described above, that is, a method in which the composition ratio of lithium gradually increases from the electrode layer side.

  FIG. 5 shows an example in which the value of z in the film continuously increases as the piezoelectric thin film 51 moves away from the surface on the electrode layer side, but the piezoelectric thin film according to the second embodiment includes a plurality of layers. Thus, the value of z in the film (each layer) may increase stepwise as the distance from the surface on the electrode layer side increases.

  In the piezoelectric thin film according to the second embodiment, the value of z in the film increases stepwise as the distance from the surface on the electrode layer side increases. A layer having a small composition ratio is formed, and a layer having a large composition ratio of lithium is formed thereon.

  That is, in the piezoelectric thin film according to the second embodiment, a potassium sodium niobate film containing no lithium (or slightly containing lithium) is formed on the electrode, and the concentration of lithium is gradually increased on the upper part. The structure is to form a lithium potassium sodium niobate film.

  As a result, the piezoelectric thin film 51 is a more crystalline lithium potassium sodium niobate film because the difference in lattice constant between the films can be alleviated as compared with the case of simply having a two-layer structure.

  In addition, the piezoelectric thin film element 50 according to the second embodiment uses a piezoelectric thin film 51 in place of the piezoelectric thin film 1 in the piezoelectric thin film element 10 of FIG.

Further, like the piezoelectric thin film 61 according to the third embodiment shown in FIG. 6, the second alkali niobium oxide film 3 is interposed between the first alkali niobium oxide film 2 and the second alkali niobium oxide film 3. A third alkali niobium oxide represented by (Na _x K _y Li _z ) NbO ₃ (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1) For example, a Na _0.48 K _0.48 Li _0.04 ) NbO ₃ ) film (second alignment control layer) 62 may be formed.

  The piezoelectric thin film 61 was manufactured by the method (2) described above, that is, a method in which a layer having a small lithium composition ratio was formed on the electrode and a layer having a large lithium composition ratio was formed thereon. It is.

  The piezoelectric thin film 61 can provide the same effects as the piezoelectric thin film 51 of FIG. In addition, the piezoelectric thin film element 60 according to the third embodiment uses a piezoelectric thin film 61 in place of the piezoelectric thin film 1 in the piezoelectric thin film element 10 of FIG.

In the above embodiment, lithium potassium sodium niobate (Na _x K _y Li _z ) NbO ₃ is particularly effective when the piezoelectric constant d ₃₁ is increased in the range of 4 <x, y <6, z <10.

  The piezoelectric thin film element according to the above embodiment can be used as an electromechanical energy conversion component for 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 and the piezoelectric thin film element according to the above-described embodiment are described for the piezoelectric application, the application is not limited to the piezoelectric application, and various applications such as a ferroelectric component, for example. Application to is considered.

Example 1
Hereinafter, a piezoelectric thin film element 10 including a second alkali niobium oxide film 3 ((Na _0.47 K _0.47 Li _0.06 ) NbO _{3 in} Example 1), which is a lithium potassium sodium niobate film having a thickness of 2 μm, using the present invention. An example in which is formed will be described.

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 magnetron sputtering, and a potassium sodium niobate film (orientation control layer) is formed thereon. 1 Alkali niobium oxide film 2 ((Na _0.5 K _0.5 ) NbO ₃ , film thickness 160 nm in Example 1) was formed. A second alkali niobium oxide film 3 ((Na _0.47 K _0.47 Li _0.06 ) NbO _{3 in} Example 1), which is a lithium potassium sodium niobate film, is formed thereon, and an upper electrode layer 13 is formed thereon. The piezoelectric thin film element 10 of FIG. 1 was produced.

  The thickness of each layer is 0.5 mm for the substrate 11, 0.2 μm for the lower electrode layer 12, 160 nm for the first alkali niobium oxide film 2, 2 μm for the second alkali niobium oxide film 3, and 0.2 mm for the upper electrode 13. 2 μm.

(Comparative example)
For comparison with Example 1, a sample similar to Example 1 was prepared by forming a lithium potassium sodium niobate film by a conventional technique without forming a potassium sodium niobate film.

  For these two samples of Example 1 and Comparative Example, the Pt upper electrode 13 was formed on the upper part thereof. 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 first and second alkali niobium oxide films 2 and 3. The substrate temperature is 600 ° C., the discharge power is 75 W, the introduced gas Ar atmosphere, the pressure is 0.4 Pa, the film formation time is 3 hours and 30 minutes, and the formation conditions of the Pt upper electrode 13 are the substrate temperature 700 ° C., the discharge power 200 W, and the introduced gas Ar. It was performed in an atmosphere, a pressure of 2.5 Pa, and a film formation time of 1 minute.

  In order to know the crystal structure of the prepared lithium potassium sodium niobate thin film of Example 1 and Comparative Example, X-ray diffraction measurement (ω-2θ scan) was performed. FIG. 2 shows the X-ray diffraction pattern of the piezoelectric thin film 1 of Example 1, and FIG. 3 shows the X-ray diffraction pattern of the piezoelectric thin film of the comparative example.

  As shown in FIG. 2, in the lithium potassium sodium niobate thin film of Example 1, the (001) plane and (002) plane diffraction peaks of the perovskite structure are prominent except for the diffraction peak of the MgO substrate and the diffraction peak of Pt. It can be confirmed that a crystal thin film having a perovskite structure can be formed.

  In contrast, as shown in FIG. 3, in the lithium potassium sodium niobate thin film of the comparative example, the diffraction peak on the crystal plane can be confirmed, but the value of the diffraction peak itself is very small.

  Next, the piezoelectric characteristics of the piezoelectric thin film element 10 of Example 1 and the piezoelectric thin film element of the comparative example were compared. First, a strip shape having a length of 20 mm and a width of 3 mm was cut out from these piezoelectric thin film elements, and a simple unimorph cantilever was formed by fixing a longitudinal end 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. FIG. 4 shows the relationship between the applied voltage (V) and the amount of piezoelectric displacement (maximum tip displacement) (nm) (Example 1 is shown by ● and Comparative Example is shown by ■).

  In the piezoelectric thin film element 10 of Example 1, the tip of the cantilever is greatly displaced, and it can be seen that the piezoelectric operation is performed. On the other hand, in the piezoelectric thin film element of the comparative example, the tip of the cantilever is not displaced, and actually the piezoelectric operation is hardly performed.

Further, when the piezoelectric constant d ₃₁ of both samples was measured from the relationship between the applied voltage and the amount of piezoelectric displacement, the piezoelectric thin film element of Example 1 was −35 pm / V.

  From these facts, it was confirmed that by using the present invention, a lithium potassium sodium niobate thin film having better piezoelectric characteristics than the prior art can be produced.

(Example 2)
In Example 1, a potassium sodium niobate ((Na _0.5 K _0.5 ) NbO ₃ ) thin film is used for the orientation control layer, and a lithium potassium sodium niobate ((Na _0.47 K _0.47 Li _0.06 ) having a desired composition ratio is formed thereon. NbO ₃ ) thin film was formed.

In Example 2, a third alkali niobium oxide film 62 (second alignment control layer) ((Na _0.48 K _0.48 ) in which the lithium composition is reduced as compared with the second alkali niobium oxide film 3 between these two layers. Li _0.04 ) NbO ₃ ) was formed to produce a piezoelectric thin film 61, and a piezoelectric thin film element 60 was produced using this. This makes it possible to form a lithium potassium sodium niobate thin film with better crystallinity.

For the sample thus prepared, a simple unimorph cantilever was prepared in the same manner as in Example 1. As a result, d ₃₁ was -41 pm / V, and a lithium potassium sodium niobate thin film having superior piezoelectric characteristics compared to the prior art. It turned out that it can produce.

  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 other than the MgO substrate, the same effects as in Examples 1 and 2 were obtained. .

It is sectional drawing of the element using the piezoelectric thin film containing the lithium potassium sodium niobate thin film which shows the suitable 1st Embodiment of this invention. 2 is an XRD spectrum of a film with a potassium sodium niobate orientation control layer prepared in Example 1. FIG. It is an XRD spectrum of the film | membrane without the potassium sodium niobate orientation control layer produced in the comparative example. It is a graph which shows the relationship between the applied voltage and tip maximum displacement of the simple unimorph cantilever produced in the Example and the comparative example. It is sectional drawing of the element using the piezoelectric thin film which shows the 2nd Embodiment of this invention. It is sectional drawing of the element using the piezoelectric thin film which shows the 3rd Embodiment of this invention.

Explanation of symbols

1 Piezoelectric thin film 2 First alkali niobium oxide film (potassium sodium niobate film)
3 Second alkali niobium oxide film (lithium potassium sodium niobate film)
10 Element using piezoelectric thin film (piezoelectric thin film element)
11 Substrate 12 Lower electrode layer 13 Upper electrode layer

Claims (5)

In the piezoelectric thin film formed on the electrode layer, alkali niobium oxidation represented by (Na _x K _y Li _z ) NbO ₃ (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1) made from the object film, the value of z in the film are different in the thickness direction, and the piezoelectric thin film characterized by increases in phases in accordance with distance from the electrode layer side. An element using a piezoelectric thin film in which a lower electrode layer, a piezoelectric layer, and an upper electrode layer are formed on a substrate, wherein the piezoelectric layer is made of the alkali niobium oxide film according to claim 1. Device using. The element using a piezoelectric thin film according to claim 2 , 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. 4. The element using a piezoelectric thin film according to claim 2, wherein the lower electrode layer and the upper electrode layer are made of Pt. A substrate,
A lower electrode layer formed on the substrate;
Formed on the lower electrode layer, (Na _x  K _y  ) NbO _Three  A first alkali niobium oxide film represented by (0 <x <1, 0 <y <1, x + y = 1);
Formed on the first alkali niobium oxide film, (Na _x  K _y  Li _z  ) NbO _Three  A second alkali niobium oxide film represented by (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1),
The lithium composition was reduced between the first alkali niobium oxide film and the second alkali niobium oxide film as compared with the second alkali niobium oxide film (Na _x  K _y  Li _z  ) NbO _Three  A third alkali niobium oxide film represented by (0 <x <1, 0 <y <1, 0 <z <1, x + y + z = 1);
An upper electrode layer formed on the second alkali niobium oxide film;
A method of manufacturing a piezoelectric thin film element having
Forming the lower electrode layer on the substrate;
Forming the first alkali niobium oxide film on the lower electrode layer;
Forming the third alkali niobium oxide film on the first alkali niobium oxide film;
Forming the second alkali niobium oxide film on the third alkali niobium oxide film;
Forming the upper electrode layer on the second alkali niobium oxide film;
A method for manufacturing a piezoelectric thin film element, comprising:

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