JP7064205B2 - Lead-free ferroelectric thin film - Google Patents

Description

The present invention relates to a lead-free ferroelectric thin film.

Conventionally, as a composition exhibiting ferroelectricity (ferroelectric ceramics), a ferroelectric composition containing lead such as lead zirconate titanate (PZT) is widely known. Such a ferroelectric composition is excellent in properties as a ferroelectric substance (ferroelectricity) such as dielectric constant and piezoelectricity, and is widely used in various industrial fields.

However, in recent years, from the viewpoint of environmental protection, the movement to use lead-free substances and products has become widespread, and this is also the case in the field of ferroelectric compositions. Therefore, a lead-free ferroelectric composition, that is, a lead-free ferroelectric composition is required.

As the lead-free ferroelectric composition, for example, a composition containing bismuth titanate (BT), bismuth bismuth titanate (BLT) and the like is being studied (see, for example, Patent Document 1). However, even these lead-free ferroelectric compositions do not have sufficient polarization characteristics and have not completely replaced lead-containing ferroelectric compositions such as lead zirconate titanate.

In recent years, as a base material for a magnetic thin film constituting a nanopillar type multiferroic, a lead-free ferroelectric thin film having crystal orientation in the c-axis direction and large spontaneous polarization along the a-axis direction, especially a bismuth layer. Development of structural ferroelectrics (BLSFs) thin films is required.

Japanese Patent Application Laid-Open No. 2003-82470

In view of the above situation, it is an object of the present invention to provide a novel lead-free ferroelectric thin film which does not contain lead and has excellent crystal orientation in the c-axis direction.

As a result of various studies by the present inventors in order to solve the above problems, a lead-free ferroelectric thin film composed of an oxide containing Bi, Ti and a lanthanoid has excellent orientation in the c-axis direction. We found that it shows, and came to the present invention.

That is, the present invention relates to a lead-free ferroelectric oxide thin film composed of an oxide containing Bi, Ti, and a lanthanide and having crystal orientation in the c-axis direction. Preferably, the lanthanoid is at least one selected from the group consisting of Sm, Eu, Gd, Tb, La, and Nd. Preferably, the content of the lanthanoid is 2 to 30 at%. Preferably, the lanthanoids are Nd and Eu. Preferably, in the oxide, the element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 15 at%, and 1 It is ~ 15 at%. Preferably, the orientation ratio of the oxide thin film in the c-axis direction is 97.0% or more. Further, the present invention may relate to a laminate including the lead-free ferroelectric oxide thin film and a substrate. The present invention may be a multiferroic element, a piezoelectric element, a ferroelectric memory, or a sensor or an actuator including the lead-free ferroelectric oxide thin film or the laminate.

According to the present invention, it is possible to provide a novel lead-free ferroelectric thin film that does not contain lead and has excellent crystal orientation in the c-axis direction.

A photograph of the surface of the oxide thin film of Example 1 taken with a field emission scanning electron microscope.

Hereinafter, embodiments of the present invention will be described in detail.

The lead-free ferroelectric thin film according to the present invention is made of an oxide containing bismuth (Bi), titanium (Ti) and a lanthanide without containing lead. The lanthanoid is preferably at least one selected from the group consisting of samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), lanthanum (La), and neodym (Nd). , Neodim (Nd) and Europium (Eu) are both more preferred. The oxide thin film may be a composite oxide, or may be a mixture of a simple oxide and / or a composite oxide.

When the oxide thin film contains bismuth (Bi), titanium (Ti), neodymium (Nd) and europium (Eu), each element of Bi, Ti, Nd and Eu with respect to the total amount of Bi, Ti, Nd and Eu. The ratio is preferably 40 to 65 at%, 35 to 55 at%, 1 to 15 at%, and 1 to 15 at%. When the ratio of the number of moles of each element is within the above range, the polarization performance exhibited by the lead-free ferroelectric thin film can be improved. The element ratios of Bi, Ti, Nd and Eu are more preferably 45 to 60 at%, 37 to 54 at%, 1 to 10 at%, and 1.1 to 10 at%, still more preferably 48 to 55 at. %, 37 to 52 at%, 1.1 to 9 at%, and 1.2 to 9 at%.

Further, the oxide thin film has excellent crystal orientation in the c-axis direction. The orientation ratio in the c-axis direction is preferably 97.0% or more, more preferably 98.0% or more, still more preferably 99.0% or more. When the orientation ratio is 97.0% or more, the oxide thin film can be nanoprocessed by reactive ion etching or the like and used as a base material for the magnetic thin film constituting the nanopillar type multiferroic device. When used for such applications, the a / b axis direction is located on the side surface of the pillar, so that the polarization axis can be stimulated efficiently, and a high electromagnetic effect can be expected to be exhibited. The a-axis direction means the [h00] direction when the plane growth direction of the crystal is expressed as [hkl], and the c-axis direction means the [00l] direction.

Next, the method for producing the thin film according to the present invention will be specifically described, but the present invention is not limited to the following production method.

(Raw material preparation process)
First, bismuth oxide or its precursor, titanium oxide or its precursor, and an oxide or precursor of lanthanoid are prepared as raw materials. As the precursor, a substance that can be converted into an oxide by a firing step or the like described later can be used, and specifically, a hydroxide, a carbonate, a nitrate or the like of each element can be used.

(Grinding and mixing process)
Next, each prepared raw material is crushed and mixed. Grinding and mixing can be performed using known methods. The mixing method may be dry mixing or wet mixing, but in the case of wet mixing, it is preferable to perform wet mixing in an alcohol solvent such as methanol or ethanol. As the device used for crushing and mixing, for example, various mixing devices such as a ball mill, a mortar, and a stirrer can be used. After performing the wet mixing, it may be appropriately dried.

(Baking process)
Next, the mixed raw material is fired to produce a lead-free ferroelectric composition constituting a lead-free ferroelectric thin film. The firing temperature is preferably in the range of 500 ° C. to 1000 ° C., but the optimum temperature can be selected depending on the composition of the raw material.

The firing time is preferably in the range of 1 hour to 10 hours, but the optimum time can be selected depending on the composition of the raw material and the size of the raw material. The calcination can be performed under various conditions such as a normal atmosphere atmosphere, an atmosphere in which an inert gas and oxygen are mixed, or an oxygen atmosphere.

(Molding process)
Next, the powder of the lead-free ferroelectric composition obtained by the firing step is molded. Molding can be performed using various known molding equipment such as a die and a press machine. The pressure applied during molding is preferably in the range of, for example, 10 MPa to 400 MPa, but the optimum pressure can be selected depending on the composition of the composition and the size of the powder.

By selecting the pressure and time of this molding step, it is preferable that the relative density of the obtained molded product is 40 to 60%. The relative density of the molded product is more preferably 42 to 55%, still more preferably 44 to 50%.

(Sintering process)
When producing a sintered body, the above-mentioned raw materials can be mixed and the sintered body can be produced by hot pressing. In that case, each powdered raw material can be filled in a carbon die, and hot pressing can be performed under the conditions of a temperature of 500 to 1000 ° C. and a pressure of 10 to 200 MPa. Further, the sintered body can also be produced by producing the molded body as described above and then firing the molded body in an air atmosphere. In producing the sintered body, water and / or a binder (for example, polyvinyl alcohol, etc.) may be added to the raw material as a molding auxiliary material.

By selecting the temperature, pressure, and time of the sintering step, it is preferable that the relative density of the obtained sintered body is 80% or more and 100% or less. The relative density of the sintered body is more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more.

(Manufacturing process of oxide thin film)
The lead-free ferroelectric thin film of the present invention can be produced, for example, by a sputtering method. Specifically, the oxide thin film can be formed on a substrate by preparing a target made of a molded body or a sintered body of a lead-free ferroelectric composition and using a high-frequency magnetron sputtering device or the like. ..

The material constituting the substrate may be any material as long as the lead-free strong dielectric oxide thin film of the present invention grows epitaxially in the c-axis direction and exhibits excellent crystal orientation. For example, magnesium oxide single crystal and strontium titanate. A single crystal, a cobalt oxide single crystal, and a copper oxide single crystal can be used. Further, a platinum film may be formed on the substrate, and the lead-free ferroelectric oxide thin film of the present invention may be formed on the platinum film.

The thickness of the lead-free ferroelectric oxide thin film of the present invention is not particularly limited, but is preferably in the range of 500 nm to 5000 nm.

If there is a possibility that bismuth may re-evaporate from the sample surface due to sputtering conditions or sintered body manufacturing conditions, the purpose is to maintain the originally designed chemical composition by compensating for the re-evaporated bismuth. As a result, bismuth oxide or a precursor thereof can be added in excess of the desired amount in the thin film to the molded body or sintered body.

The crystal orientation ratio in the c-axis direction of the lead-free ferroelectric oxide thin film can be controlled by the temperature of the substrate during sputtering. The optimum temperature range may differ depending on the sputtering device and the composition of the oxide thin film. For example, when the element ratios of Bi, Ti, Nd and Eu are 54 at%, 36 at%, 8 at% and 2 at%, respectively. When the substrate temperature is set to 600 to 700 ° C., an oxide thin film having an orientation ratio in the c-axis direction exceeding 98.0% can be produced.

As other sputtering conditions, for example, a mixed gas having an arbitrary mixing ratio of argon and oxygen is used as the atmosphere gas, the flow rate is 10 to 100 sccm, the degree of vacuum is 1.0 × 10 ^-3 Pa or less, and the film forming time is 30. The active power can be set to 100 to 200 W for about 300 min.

The lead-free ferroelectric oxide thin film of the present invention is not limited to that produced by the above method. It suffices as long as the finally obtained oxide thin film has a predetermined composition and the orientation ratio in the c-axis direction is within a predetermined range. As a method for producing a lead-free strong dielectric oxide thin film other than the above-mentioned method, for example, a sol-gel method, a coating method, etc. Organic Metal Chemical Vapor Deposition (MOCVD), Physical Vapor Deposition (PVD), or Chemical Vapor Deposition (CVD) to form a thin film, or after firing a molded body, a high frequency magnetron sputtering device. A method of forming a thin film using the above can be mentioned.

Specific applications to which the lead-free ferroelectric oxide thin film of the present invention is applied are not particularly limited, and examples thereof include multiferroic elements, piezoelectric elements, ferroelectric memories, sensors, actuators, and the like. ..

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

(Examples 1 to 4 and Comparative Example 1)
Raw material preparation process As raw materials, powdered bismuth oxide (purity 99.9% or more, particle size 2.6 μm), powdered titanium oxide (purity 99.9% or more, particle size 3.1 μm), powdered neodymium oxide (Purity 99.9% or more, particle size 1.6 μm) and powdered europium oxide (purity 99.9% or more, particle size 1.8 μm) were prepared. Each raw material was adjusted and weighed so that the element ratio finally obtained was the element ratio shown in Table 1.

Grinding and mixing steps Next, the prepared powders of each raw material were crushed and mixed. The pulverization and mixing were carried out by wet mixing with ethanol using a mortar. Then, it was dried at 90 ° C. for 2 hours using a dryer (Labostar GRAVITY OVEN LG-122 type).

Firing Step Next, the mixed raw materials were calcined to produce a lead-free ferroelectric composition. The firing was carried out in an air atmosphere using an electric furnace (MPH-1N type manufactured by AS ONE) at 750 ° C. for 3 hours. The temperature rising rate was 5 ° C./min and the cooling rate was 4 ° C./min. After firing, it was cooled to room temperature to obtain a powdery lead-free ferroelectric composition.

Molding Step Next, a lead-free ferroelectric composition was molded. Molding was carried out by using a mold having a diameter of 100 mm and maintaining a pressure of 40 MPa for 2 minutes. As a result, a cylindrical molded body having a diameter of 100 mm and a thickness of about 5 mm and having a relative density of 48% composed of a lead-free ferroelectric composition was obtained.

Film formation process An MgO single crystal substrate was used as the substrate for film formation. The size of the substrate is 7.0 × 7.0 mm and the thickness is 0.4 mm.

Using a high-frequency magnetron sputtering device (SPF-210B manufactured by Anerva Co., Ltd.) as a target for a molded body composed of the lead-free ferroelectric composition obtained in the molding step, a film thickness is applied on the MgO single crystal substrate. An oxide thin film composed of a 1.8 μm lead-free ferroelectric composition and having excellent crystal orientation in the c-axis direction was formed. Sputtering was performed under the following conditions.
・ Atmosphere: Ar / O ₂ = 9: 1 (50sccm)
-Substrate temperature: 580 to 650 ° C (shown in Table 1)
・ Vacuum degree: 5.0 × 10 ^-4 Pa
・ Film formation time: 200 min
・ Active power: 120W

A Ti electrode is formed on the back surface of the MgO single crystal substrate using a high-frequency multilayer sputtering device (manufactured by Anerva Co., Ltd.), and a Pt electrode is further formed on the upper surface of an oxide thin film composed of the lead-free ferroelectric composition. Was prepared, and then the characteristics of the oxide thin film composed of the lead-free ferroelectric composition were evaluated.

(Evaluation methods)
(FE-SEM observation)
The surface of the oxide thin film was observed using a field emission scanning electron microscope (JSM-7001F manufactured by JEOL) under the conditions of an acceleration voltage of 5 kV and a WD of 10 mm. FIG. 1 shows a micrograph of the surface of the oxide thin film of Example 1.

(Measuring method of element ratio of oxide thin film)
Using the energy dispersive X-ray spectroscope attached to the electric field radiation scanning electron microscope (JEOL JSM-7001F), adjust the irradiation current so that the count count is 8000 cps or more under the conditions of acceleration voltage of 15 kV and magnification of 500 times. And the measurement was performed. Table 1 shows the average value of the five values obtained by five-field photography.

(Measurement method of film thickness)
A stylus type surface shape measuring instrument (Dektak-3030 manufactured by Bruker) was used to scan from the platinum lower electrode to the oxide thin film, and the measured step was taken as the film thickness.

(Measuring method of crystal orientation)
The crystal orientation was measured using X-ray diffraction (D8μ-HR manufactured by Bruker) equipped with a two-dimensional detector. The measurement conditions were a tube voltage of 40 kV, a tube current of 40 mV, a step time of 200 s / step, and 2θ: 10-55 °. From the obtained X-ray diffraction peak, the orientation rate in the c-axis direction was calculated by the following equation.
Orientation rate = (total of integrated intensities of X-ray diffraction peaks of [00l] ÷ total of integrated intensities of all X-ray diffraction peaks) × 100)
However, X-ray diffraction peaks derived from the substrate and electrodes are excluded.

表１より、実施例１～４は、ｃ軸方向に結晶配向性が優れていることが分かる。一方、比較例１は、実施例１～４と比較して低いｃ軸方向の配向率を示した。

From Table 1, it can be seen that Examples 1 to 4 have excellent crystal orientation in the c-axis direction. On the other hand, Comparative Example 1 showed a lower orientation rate in the c-axis direction as compared with Examples 1 to 4.

Claims (6)

A lead-free ferroelectric oxide thin film composed of an oxide containing Bi, Ti, and a lanthanide and having crystal orientation in the c-axis direction .
The lanthanoids are Nd and Eu,
In the oxide, the element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 15 at% and 1 to 15 at%. And
A lead-free ferroelectric oxide thin film having an orientation ratio of the oxide thin film in the c-axis direction of 97.0% or more . A laminate including the lead-free ferroelectric oxide thin film according to claim 1 and a substrate. A multiferroic device comprising the lead-free ferroelectric oxide thin film according to claim 1 or the laminate according to claim 2 . A piezoelectric element including the lead-free ferroelectric oxide thin film according to claim 1 or the laminate according to claim 2 . A ferroelectric memory comprising the lead-free ferroelectric oxide thin film according to claim 1 or the laminate according to claim 2 . A sensor or actuator comprising the lead-free ferroelectric oxide thin film according to claim 1 or the laminate according to claim 2 .

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