JP5531529B2 - Substrate with piezoelectric thin film, piezoelectric thin film element, actuator, and sensor - Google Patents

Description

  The present invention relates to a substrate with a piezoelectric thin film, a piezoelectric thin film element, an actuator, and a sensor. In particular, the present invention relates to a substrate with a piezoelectric thin film including a piezoelectric thin film having a perovskite structure, a piezoelectric thin film element, an actuator, and a sensor.

Piezoelectric elements having a piezoelectric body are used for actuators, sensors, and the like. As a piezoelectric body forming such a piezoelectric element, a Pb (Zr _1-x Ti _x ) O ₃ perovskite ferroelectric (PZT) is widely used. For example, a PZT thin film formed on a silicon substrate by a sputtering method has been put into practical use as a piezoelectric thin film for an actuator for a high-speed, high-definition inkjet printer head. However, since PZT contains about 60 wt% to 70 wt% of lead (Pb), a piezoelectric body that does not contain lead, that is, a lead-free piezoelectric body is used from the viewpoint of ecological viewpoint and pollution prevention. It is desirable to use it for a piezoelectric element. Among various lead-free piezoelectric materials being studied, potassium sodium niobate (general formula: (K _x Na _1-x ) NbO ₃ (0 <x <1)) is known. Since potassium sodium niobate is a material having a perovskite structure and exhibits relatively good piezoelectric characteristics as a lead-free material, it is expected as a promising candidate for a lead-free piezoelectric material.

  Conventionally, a potassium sodium niobate thin film has been attempted to be deposited on a silicon substrate by a film forming method such as a sputtering method, a sol-gel method, an aerosol deposition method, etc. (For example, refer nonpatent literature 1).

Applied Physics Epressl (2008) 011501

However, it is difficult to stably obtain a piezoelectric thin film having a piezoelectric constant d ₃₁ of −90 pm / V or more. For example, further improvement is required to apply the piezoelectric thin film to an inkjet printer head.

Accordingly, an object of the present invention is in conjunction with the piezoelectric constant d ₃₁ is good, a small piezoelectric thin film substrate with a deterioration of the characteristics caused by the temperature rise of the piezoelectric characteristics, the piezoelectric thin film element, an actuator, and to provide a sensor.

(1) In order to achieve the above object, the present invention is provided on the substrate, the lower electrode provided above the substrate, and the lower electrode, and is represented by the general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ x ≦ 0.7), and a piezoelectric thin film made of an alkali niobium oxide-based compound. The piezoelectric thin film corresponds to a temperature change ΔT when σ is the internal stress in the in-plane direction and T is the temperature. The ratio Δσ / ΔT of the change in internal stress Δσ has a negative slope, and the average value of Δσ / ΔT between 20 ° C. and 250 ° C. is −0.2495 × 10 ⁶ to −0.6866 × 10 ⁶ (Pa / range der of ° C.) is, the piezoelectric constant d ₃₁ at 20 ° C. is -100 pm / V or more (indicating that the absolute value is the "more" for the piezoelectric constant is large), and the rate of change with temperature of the piezoelectric constant _{d 31 (150 ℃) / d} 31 (20 ℃) 0.9 Ru on Der piezoelectric thin film with a substrate is provided.
(2) Moreover, it is provided on the substrate, the lower electrode provided above the substrate, and the lower electrode, and is represented by the general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ x ≦ 0.7). A piezoelectric thin film made of an alkali niobium oxide-based compound, and the piezoelectric thin film has a ratio Δσ / of the change Δσ of the internal stress to the temperature change ΔT when the internal stress in the in-plane direction is σ and the temperature is T ΔT has a negative slope, and the average value of Δσ / ΔT at 20 ° C. or more and 250 ° C. or less is in the range of −0.2495 × 10 ⁶ to −0.6866 × 10 ⁶ (Pa / ° C.), Piezoelectric constant d ₃₁ at −100 pm / V or more (“or more” for the piezoelectric constant indicates that the absolute value is large), and rate of change of piezoelectric constant with temperature d ₃₁ (150 ° C.) / D ₃₁ ( Substrate with piezoelectric thin film (20 ° C) is 0.9 or more (however, The piezoelectric thin film, the substrate with the piezoelectric thin film and the substrate temperature and is formed by setting the 550 ° C. or less of the substrate is excluded) is provided.

( 3 ) Moreover, as for the said board | substrate with a piezoelectric thin film, as for a piezoelectric thin film, it is preferable that the internal stress of the film surface direction in 20 degreeC exists in the range of -80 Mpa or more and 250 Mpa or less.

( 4 ) Further, in the above-mentioned substrate with piezoelectric thin film, the piezoelectric thin film has an inflection point in a temperature range of 250 ° C. or higher and 400 ° C. or lower in a graph showing the relationship between the internal stress in the in-plane direction of the piezoelectric thin film and the temperature. It is preferable that the average value of Δσ / ΔT in the range of 400 ° C. to 500 ° C. is smaller than the average value of Δσ / ΔT in the range of 20 ° C. to 250 ° C.

( 5 ) Further, in the substrate with the piezoelectric thin film, the piezoelectric thin film has an internal stress in the in-plane direction at a predetermined temperature when the piezoelectric thin film is formed on the lower electrode in the range of more than −500 MPa and not more than −200 MPa. der is, the predetermined temperature is a temperature der Rukoto corresponding to the maximum temperature at the time of forming the piezoelectric thin film is preferable.

( 6 ) Moreover, as for the said board | substrate with a piezoelectric thin film, it is preferable that a lower electrode is formed from platinum (Pt).

( 7 ) The piezoelectric thin film-attached substrate may further include a titanium (Ti) layer between the substrate and the lower electrode.

( 8 ) Moreover, the board | substrate with a said piezoelectric thin film can also have a thermal oxide film on the surface.

  (9) Moreover, it is preferable that predetermined | prescribed temperature is the range of 550 degreeC or more and 600 degrees C or less of the said board | substrate with a piezoelectric thin film.

(10) Further, the present invention is to achieve the above object, the above (1) Preparations ~ and piezoelectric thin film with board according to any one of (9), and an upper electrode provided on the piezoelectric thin film A piezoelectric thin film element is provided.

( 11 ) Moreover, in order to achieve the said objective, this invention provides an actuator provided with the board | substrate with a piezoelectric thin film as described in any one of said (1)-(9).

( 12 ) Moreover, in order to achieve the said objective, this invention provides a sensor provided with the board | substrate with a piezoelectric thin film as described in any one of said (1)-(9).

The piezoelectric thin film substrate with the present invention, a piezoelectric thin film element, an actuator, and according to the sensor, the piezoelectric constant d ₃₁ is good, a small piezoelectric thin film substrate with a deterioration of the characteristics caused by the temperature rise of the piezoelectric characteristics, the piezoelectric thin film Elements, actuators, and sensors can be provided.

It is sectional drawing of the board | substrate with a piezoelectric thin film which concerns on the 1st Embodiment of this invention. It is sectional drawing of the board | substrate with a piezoelectric thin film which concerns on the 2nd Embodiment of this invention. It is sectional drawing of the board | substrate with a piezoelectric thin film which concerns on the 3rd Embodiment of this invention. It is sectional drawing of the piezoelectric thin film element which concerns on the 4th Embodiment of this invention. It is a schematic diagram of the curvature radius measurement of the board | substrate with a thin film by an optical lever method. 3 is an X-ray diffraction pattern of a piezoelectric thin film according to Example 2. FIG. 3 is an X-ray diffraction pattern of a piezoelectric thin film according to Comparative Example 2. FIG. It is a figure which shows the temperature dependence of the internal stress of the piezoelectric thin film which concerns on Example 2. FIG. It is a figure which shows the temperature dependence of the internal stress of the piezoelectric thin film which concerns on Example 7. FIG. It is a figure which shows the temperature dependence of the internal stress of the piezoelectric thin film which concerns on the comparative example 1. It is a figure which shows the temperature dependence of the internal stress of the piezoelectric thin film which concerns on the comparative example 2. (A) and (b) is a schematic diagram of the evaluation method of the piezoelectric constant d ₃₁ of the piezoelectric element.

[Summary of embodiment]
In a substrate with a piezoelectric thin film including a piezoelectric thin film having a perovskite structure, a substrate, a lower electrode provided above the substrate, and a lower electrode are provided. The general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ and a piezoelectric thin film made of an alkali niobium oxide compound represented by x ≦ 0.7). The piezoelectric thin film has an internal stress with respect to a temperature change ΔT when σ is an internal stress in the in-plane direction and T is a temperature. The ratio Δσ / ΔT of the stress change Δσ has a negative slope, and the average value of Δσ / ΔT at 20 ° C. or more and 250 ° C. or less is −0.25 × 10 ⁶ to −0.68 × 10 ⁶ (Pa / ° C. The substrate with piezoelectric thin film is provided. Further, in a piezoelectric thin film element including a piezoelectric thin film having a perovskite structure, a substrate, a lower electrode provided above the substrate, and a lower electrode are provided. The general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ x ≦ 0.7), a piezoelectric thin film made of an alkali niobium oxide-based compound and an upper electrode provided on the piezoelectric thin film. The piezoelectric thin film has an internal stress in the in-plane direction of σ, a temperature The ratio Δσ / ΔT of the change in internal stress Δσ to the temperature change ΔT when T is T has a negative slope, and the average value of Δσ / ΔT at 20 ° C. to 250 ° C. is −0.25 × 10 ⁶ A piezoelectric thin film element in the range of −0.68 × 10 ⁶ (Pa / ° C.) is provided. Furthermore, an actuator and a sensor provided with the said board | substrate with a piezoelectric thin film are provided.

[First Embodiment]
(Outline of configuration of substrate 1 with piezoelectric thin film)
FIG. 1 shows an outline of a cross section of a substrate with a piezoelectric thin film according to a first embodiment of the present invention.

  The substrate 1 with a piezoelectric thin film according to the first embodiment includes a substrate 10, a lower electrode 20 provided in contact with the surface of the substrate 10, and a piezoelectric thin film 30 provided in contact with the surface of the lower electrode 20. The substrate with piezoelectric thin film according to the present embodiment is applied to a head of an ink jet printer, for example.

(Substrate 10)
As the substrate 10, for example, a silicon substrate having a (100) plane orientation can be used. As an example, the substrate 10 has a thickness of 200 μm or more and 1000 μm or less. Further, as the substrate 10, a silicon substrate having a plane orientation different from the (100) plane, a silicon on insulator (SOI) substrate, a quartz glass substrate, a compound semiconductor substrate made of GaAs, a sapphire substrate, a metal substrate made of stainless steel, An MgO substrate, a SrTiO ₃ substrate, or the like can also be used.

(Lower electrode 20)
For example, the lower electrode 20 is made of platinum (Pt). For example, the lower electrode 20 has a thickness of 0.02 μm to 0.5 μm. By forming the lower electrode 20 from Pt, the piezoelectric thin film 30 having high orientation can be formed on the lower electrode 20. Alternatively, the lower electrode 20 may be made of an alloy containing Pt, a metal material such as gold (Au), ruthenium (Ru), or iridium (Ir), or a metal oxide such as SrRuO ₃ or LaNiO ₃ .

(Piezoelectric thin film 30)
The piezoelectric thin film 30 has a perovskite structure and is formed of a piezoelectric material that does not contain lead (Pb). As an example, the piezoelectric thin film 30 is formed with a film thickness in the range of 0.2 μm to 5 μm. In the present embodiment, the piezoelectric thin film 30 includes an alkali niobium oxide compound represented by the general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ x ≦ 0.7) (for example, potassium niobate). Sodium, hereinafter referred to as “KNN”). Alternatively, the piezoelectric thin film 30 in which an additive element such as Li, Ta, Sb, Ca, Cu, Ba, Ti or the like of 5 wt% or less is added to KNN can be formed.

The piezoelectric thin film 30 according to the present embodiment has a ratio Δσ of the change Δσ of the internal stress to the temperature change ΔT when the internal stress in the in-plane direction is σ and the temperature (where the temperature of the film surface) is T. / ΔT (hereinafter sometimes referred to as temperature dependency of internal stress σ) has a negative slope, and the average value of Δσ / ΔT at 20 ° C. or higher and 250 ° C. or lower is −0.25 × 10 ⁶ to −0. It is formed so as to be in the range of 68 × 10 ⁶ (Pa / ° C.). The piezoelectric thin film 30 preferably has an internal stress in the in-plane direction at 20 ° C. in the range of −80 MPa (compression stress) to 250 MPa (tensile stress). And in the graph which shows the relationship between the internal stress of the film surface direction of the piezoelectric thin film 30, and temperature, it has an inflection point in the temperature range of 250 degreeC or more and 400 degrees C or less, and in the range of 20 degrees C or more and 250 degrees C or less. More preferably, the average value of Δσ / ΔT in the range of 400 ° C. or higher and 500 ° C. or lower is smaller than the average value of Δσ / ΔT. Further, the piezoelectric thin film 30 has an internal stress in the in-plane direction at a predetermined temperature when the piezoelectric thin film 30 is formed on the lower electrode 20 (for example, the highest temperature in the film forming process for forming the piezoelectric thin film 30). It is in the range of more than 500 MPa (compressive stress) and −200 MPa (compressive stress). In the present embodiment, the “maximum temperature in the film formation process” means the maximum temperature in the process (including the film formation process and the crystallization process) until the piezoelectric thin film 30 is formed. In addition, the formation of the piezoelectric thin film 30 includes only a step of forming the piezoelectric thin film 30 and a case of including a step of performing heat treatment for crystallization on the piezoelectric thin film 30 after the step of forming the piezoelectric thin film 30. is there. Therefore, in the present embodiment, the “maximum temperature of the film forming process” means the maximum temperature in the process of forming the piezoelectric thin film 30 in the case of only the process of forming the piezoelectric thin film 30, and the piezoelectric thin film 30 is formed. In the case where the step of applying the heat treatment to the piezoelectric thin film 30 is performed after the step of performing, the highest temperature in the step of forming the piezoelectric thin film 30 and the maximum temperature in the step of applying the heat treatment to the piezoelectric thin film 30 are high. Means the temperature.

  In the present embodiment, the stress generated in the piezoelectric thin film 30 (for example, a KNN film) means a stress in the in-film direction. The internal stress of the KNN film has a distribution in the film thickness direction, and the average internal stress is referred to as “internal stress” in the present embodiment. Internal stress is expressed as negative when compressive stress is generated in the KNN film and positive when tensile stress is generated in the KNN film.

  The reason for defining the change of internal stress with respect to the temperature change of the piezoelectric thin film 30 in this way is based on the following knowledge obtained by the present inventors.

(Knowledge obtained by the inventor)
First, as a film formation method of a piezoelectric thin film made of KNN, a method of sputtering film formation at a high temperature (high temperature sputtering method), a method of crystallizing at a high temperature after sputtering film formation at room temperature (room temperature sputtering method), a room temperature or a low temperature. A method of crystallizing at a high temperature after applying the raw material liquid on the substrate (MOD method or sol-gel method), a method of heat-treating at a high temperature after forming an aerosol deposition film by high-speed collision of the raw material fine particles with the substrate (aerosol deposition method), and Alternatively, a heat treatment method after film formation by a high temperature sputtering method, a room temperature sputtering method, a MOD method, or a sol-gel method can be employed. As a result of the study by the present inventor, the piezoelectric characteristics of the KNN thin film are largely influenced by the stress generated in the KNN thin film at the time of film formation and after film formation, although it slightly depends on the film forming method and the orientation of crystal grains. We obtained knowledge and found the following trends.

First, as a first tendency, when the internal stress of the KNN thin film at a temperature corresponding to the maximum temperature during the film forming process is in the range of compressive stress exceeding 500 MPa and not more than 200 MPa (that is, the internal stress is −500 MPa). It has been found that the piezoelectric constant d ₃₁ exhibits excellent piezoelectric characteristics of −100 pm / V or more when the value is in the range of more than −200 MPa. In this specification, the internal stress is measured using a 20 mm square sample in plan view.

Further, as a second tendency, the temperature dependence (Δσ / ΔT) of the internal stress σ of the piezoelectric thin film has a negative slope (that is, the compressive stress of the KNN thin film tends to increase as the temperature rises). When the average value of Δσ / ΔT from 20 ° C. to 250 ° C. is in the range of −0.25 × 10 ⁶ to −0.68 × 10 ⁶ (Pa / ° C.), the temperature rise of the piezoelectric characteristics of the KNN thin film It has been found that the deterioration of the characteristics accompanying the reduction becomes small. Specifically, it has been found that the ratio d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) between the piezoelectric constant at 150 ° C. and the piezoelectric constant at 20 ° C. is 0.9 or more.

  The inventor considers this tendency as follows. That is, first, the KNN crystal structure is formed at the highest temperature during the film formation process of the KNN thin film, and most of the quality of the KNN crystal is determined at that time. When the KNN crystal structure is determined, that is, when a large compressive stress of 500 MPa or more is generated in the KNN crystal at the highest temperature during the film forming process, the quality of the KNN crystal deteriorates (however, “the quality of the KNN crystal” "Deterioration of" means that crystal defects and strain are generated in the KNN crystal due to compressive stress), and as a result, the piezoelectric properties of the KNN thin film are considered to be degraded. When compressive stress of 200 MPa or less and / or tensile stress is generated in the KNN crystal at the maximum temperature during the film forming process, a good quality KNN crystal film is formed at the time when the KNN thin film is formed. Although it seems that when the temperature is cooled from the maximum temperature to room temperature, the KNN thin film has a thermal expansion coefficient larger than that of the silicon substrate, so that a very large tensile stress is generated in the KNN thin film at room temperature. It seems that the generation of a large tensile stress in the KNN thin film at room temperature deteriorates the piezoelectric properties of the KNN thin film.

Moreover, it is considered that the deterioration of the piezoelectric characteristics of the KNN thin film due to the temperature rise is mainly caused by the change in the internal stress of the KNN thin film accompanying the temperature change. As a result of investigation by the inventors, the average value of the temperature dependence (Δσ / ΔT) of the internal stress σ of the piezoelectric thin film from 20 ° C. to 250 ° C. is −0.25 × 10 ⁶ to −0.68 × 10 ⁶ (Pa / ° C), it was found that the deterioration of the characteristics due to the temperature rise of the piezoelectric characteristics becomes small. The value of Δσ / ΔT varies depending on the thermal expansion coefficient, Young's modulus, and Poisson's ratio of the KNN thin film. However, the thermal expansion coefficient, Young's modulus, and Poisson's ratio of the KNN thin film disposed on the substrate 10 affect each other in a complicated manner, and it is practically difficult to obtain individual accurate values. However, the inventor has found that the value of Δσ / ΔT is changed by changing the (001) plane orientation state of the KNN thin film, the Na composition ([Na] / [K] + [Na]), etc. I found out that it can be controlled.

(Effects of the first embodiment)
In the substrate with piezoelectric thin film 1 according to the first embodiment, the ratio Δσ / ΔT of the change Δσ of the internal stress to the temperature change ΔT when the internal stress in the in-plane direction is σ and the temperature is T is negative slope And an average value of Δσ / ΔT at 20 ° C. or more and 250 ° C. or less is within the range of −0.25 × 10 ⁶ to −0.68 × 10 ⁶ (Pa / ° C.), For example, it is possible to provide a substrate 1 with a piezoelectric thin film having piezoelectric characteristics that can be substituted for a PZT thin film for an inkjet printer head. That is, in order for the piezoelectric thin film 30 made of KNN to be widely applied to an ink jet printer head, at least the piezoelectric constant d ₃₁ is −90 pm / V or more (in this embodiment, “above” relating to the piezoelectric constant is −100. , −110, etc., indicating that the absolute value is large), the piezoelectric constant d ₃₁ according to the first embodiment has a piezoelectric constant d ₃₁ of −90 pm / V. The piezoelectric thin film-equipped substrate 1 including the above KNN piezoelectric thin film can be provided stably.

In addition, it is a conventional recognition that the piezoelectric characteristics of the piezoelectric thin film 30 made of KNN have a large decrease in characteristics due to a temperature rise (specifically, the piezoelectric constant at 20 ° C. is d ₃₁ (20 ° C.), The conventional recognition is that when the piezoelectric constant at 150 ° C. is d ₃₁ (150 ° C.), the value of d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) is less than 0.75. ), Which differs somewhat depending on the applied product of the piezoelectric film, but the value of d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) is desired to be 0.8 or more. According to the substrate 1 with a piezoelectric thin film, it is possible to improve the problem that the characteristic of the KNN thin film is greatly deteriorated due to a temperature rise. For example, d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) With a KNN thin film having a value of 0.8 or more A substrate with a thin film 1 can be provided.

[Second Embodiment]
FIG. 2 shows an outline of a cross section of a substrate with a piezoelectric thin film according to the second embodiment of the present invention.

  The substrate with a piezoelectric thin film 1a according to the second embodiment is different from the substrate with a piezoelectric thin film 1 according to the first embodiment except that an adhesion layer 40 is further provided on the substrate 10. It has substantially the same configuration and function as the piezoelectric thin film-coated substrate 1 according to the embodiment. Therefore, a detailed description is omitted except for differences.

  The substrate with piezoelectric thin film 1 a is provided in contact with the substrate 10, the surface of the substrate 10, an adhesion layer 40 for improving adhesion between the lower electrode 20 and the substrate 10, and a lower electrode provided in contact with the adhesion layer 40. 20 and a piezoelectric thin film 30 provided on the lower electrode 20. The adhesion layer 40 is formed from a metal material such as titanium (Ti), for example. For example, the adhesion layer 40 has a thickness of 0.001 μm or more and 0.05 μm or less. The adhesion layer 40 can also be formed from tantalum (Ta).

[Third Embodiment]
FIG. 3 shows an outline of a cross section of a substrate with a piezoelectric thin film according to the third embodiment of the present invention.

  The substrate 1b with a piezoelectric thin film according to the third embodiment is substantially the same as the substrate 1a with a piezoelectric thin film according to the second embodiment except that a thermal oxide film 12 is formed on the surface of the substrate 10. It has configuration and functions. Therefore, a detailed description is omitted except for differences.

  The substrate with piezoelectric thin film 1b according to the third embodiment has, for example, a substrate 10 formed of silicon or the like, and a predetermined depth from the surface of the substrate 10 by thermally oxidizing the surface of the substrate 10. A thermal oxide film 12 formed in contact with the thermal oxide film 12, an adhesive layer 40 provided in contact with the thermal oxide film 12, a lower electrode 20 provided in contact with the adhesive layer 40, and a piezoelectric thin film 30 provided on the lower electrode 20. Is provided. The thermal oxide film 12 is formed from silicon dioxide when the substrate 10 is formed from silicon. As an example, the thermal oxide film 12 has a thickness of 0.1 μm or more and 0.3 μm or less.

  As a modification of the third embodiment, a form in which the adhesion layer 40 is removed from the piezoelectric thin film-coated substrate 1b, that is, the substrate 10, the thermal oxide film 12 formed on the surface of the substrate 10, and the thermal oxidation. A configuration including a lower electrode 20 provided on the film 12 and a piezoelectric thin film 30 provided on the lower electrode 20 may be employed.

[Fourth Embodiment]
FIG. 4 shows an outline of a cross section of a piezoelectric thin film element according to the fourth embodiment of the present invention.

  The piezoelectric thin film element 2 according to the fourth embodiment further includes an upper electrode 50 as compared with the piezoelectric thin film-coated substrate 1a according to the second embodiment, and a part of the surface of the lower electrode 20 is exposed to the outside. Except for this point, it has substantially the same configuration and function as the piezoelectric thin film-coated substrate 1a. Therefore, a detailed description is omitted except for differences.

  As shown in FIG. 4, the piezoelectric thin film element 2 according to the fourth embodiment includes a substrate 10, an adhesion layer 40 provided in contact with the substrate 10, and a lower electrode 20 provided in contact with the adhesion layer 40. A piezoelectric thin film 30 provided in contact with a part of the surface of the lower electrode 20, and an upper electrode 50 provided in contact with the piezoelectric thin film 30. That is, the piezoelectric thin film element 2 has a configuration in which the upper electrode 50 is further formed on the piezoelectric thin film 30 provided in the substrate 1 with the piezoelectric thin film, and a part of the surface of the lower electrode 20 is exposed. The piezoelectric thin film element 2 can be driven by applying a voltage between the upper electrode surface 50a and the lower electrode surface 20a.

  The upper electrode 50 according to the fourth embodiment is made of a metal material, and is made of Pt as an example. For example, the upper electrode 50 has a thickness of 0.005 μm or more and 0.2 μm or less. The upper electrode 50 can also be formed from other metal materials except Pt, or alloy materials containing metal materials such as Pt. The piezoelectric element 2 is formed in a strip shape in plan view.

(Calculation method of internal stress)
The internal stress generated in the piezoelectric thin film 30 can be calculated as follows.

  FIG. 5 shows an outline of the measurement of the radius of curvature of the substrate with a thin film by the optical lever method.

  The internal stress of the piezoelectric thin film 30 disposed directly or indirectly on the substrate 10 can be calculated from the curvature radius of the warp of the substrate 1 with the piezoelectric thin film. The curvature radius of the warp of the substrate with piezoelectric thin film 1 can be measured by an optical lever method. As can be seen from FIG. 2, the laser light emitted from the laser 100 is branched by the beam splitter 110 into laser light propagating in the direction toward the substrate 15 and laser light propagating in the direction toward the mirror 112. The laser light applied to the substrate 15 from the beam splitter 110 is reflected by the thin film 35 on the substrate 15, and the reflected laser light is detected by the detector 120. On the other hand, the laser light applied to the substrate 15 from the mirror 112 is reflected by the thin film 35 on the substrate 15, and the reflected laser light is detected by the detector 122. And based on the detection result in the detector 120 and the detector 122, the curvature radius of the curvature of the board | substrate 1 with a piezoelectric thin film is calculated.

As an example, the structure to be measured in the first to fourth embodiments is “the lower electrode 20 made of KNN piezoelectric thin film 30 / Pt (thickness is 200 nm) / adhesive layer 40 made of Ti ( However, the thickness is 2 nm) / thermal oxide film 12 (thickness is 200 nm) / substrate 10 made of Si (thickness is 0.5 mm). Here, the “substrate 15” is “the lower electrode 20 made of Pt / the adhesion layer 40 made of Ti / the substrate 10 made of the thermal oxide film 12 / Si”. Because the substrate 15 itself has a slight warp upon the initial state and a high temperature, and the radius of curvature R _1. Moreover, the radius of curvature of warping of the piezoelectric thin film 30 / substrate 10 consisting of KNN was _{R 2.} At this time, the curvature radius R of the warp caused by the piezoelectric thin film 30 made of KNN can be calculated from the following equation (1).

R can be calculated from R ₁ and R ₂ at each temperature measured by the optical lever method. Further, the internal stress σ of the piezoelectric thin film 30 and the curvature radius R of the warp caused by the piezoelectric thin film 30 have the relationship of the following formula (2) or formula (3).

  Here, E is the Young's modulus (Pa) of the substrate, ν is the Poisson's ratio of the substrate, E / (1-ν) is the biaxial elastic modulus of the substrate, h is the thickness (m) of the substrate, and t is the film thickness of the thin film. (M), R is the curvature radius (m) of the warp of the substrate, and σ is the average value (Pa) of the internal stress of the thin film.

  In each embodiment, 180.5 GPa is used as the biaxial elastic modulus E / (1-ν) of the substrate 10 made of silicon. The internal stress is negative when compressive stress is generated in the thin film and positive when tensile stress is generated in the thin film. Basically, the internal stress generated in the piezoelectric thin film 30 is substantially the same both when the temperature is raised and when the temperature is lowered. In each embodiment, the temperature characteristics at the time of temperature rise are used. The average value of Δσ / ΔT from 20 ° C. to 250 ° C. (that is, the slope of the internal stress characteristic graph of the piezoelectric thin film consisting of temperature-KNN) is σ (stress) −T (temperature) in the range of 20 ° C. to 250 ° C. The slope of a straight line that can be obtained by linearly approximating the graph by the least square method is used. The internal stress of the piezoelectric thin film at the maximum temperature of the film forming process in each embodiment is that the substrate with the piezoelectric thin film made of KNN after film formation is raised again to the same temperature as the maximum temperature of the film forming process. It can be considered to be the same as the internal stress of the piezoelectric thin film 30 made of KNN when measured by the method.

  The silicon substrate with a piezoelectric thin film according to Examples 1 to 9 and the silicon substrate with a piezoelectric thin film according to Comparative Examples 1 to 6 were respectively produced. The outline of the cross section of the silicon substrate with piezoelectric thin film according to Examples 1 to 9 and Comparative Examples 1 to 6 is substantially the same as that of the substrate with piezoelectric thin film 1a according to the second embodiment.

(Examples 1-9, Comparative Examples 1-3)
Specifically, a silicon substrate with a piezoelectric thin film provided with a piezoelectric thin film 30 made of KNN having a thickness of 3 μm was produced. As the substrate 10, a silicon substrate with a thermal oxide film (however, a (100) plane orientation, a thickness of 0.525 mm, a thermal oxide film thickness of 200 nm, a size of 20 mm × 20 mm) was used. First, an adhesion layer 40 made of Ti (with a film thickness of 2 nm) and a lower electrode 20 made of Pt (with a (111) plane preferred orientation, film thickness of 200 nm) were formed on the substrate 10 by RF magnetron sputtering. . The adhesion layer 40 made of Ti and the lower electrode 20 made of Pt are respectively formed under the conditions of a substrate temperature of 300 ° C., a discharge power of 200 W, an introduced gas Ar atmosphere, a pressure of 2.5 Pa, a film formation time of 1 to 3 minutes, and 10 minutes. Filmed. A (K _1-x Na _x ) NbO ₃ thin film having a thickness of 3 μm was formed thereon by RF magnetron sputtering. The (K _1-x N _x ) NbO ₃ piezoelectric thin film has a composition ratio (K + Na) /Nb=1.0 and Na / (K + Na) = 0.4 to 0.7 (K _1-x Na _x ) NbO ₃ Using the bonded body as a target, a film was formed under conditions of a substrate temperature of 550 ° C. to 575 ° C., a discharge power of 75 W to 100 W, an introduced gas Ar atmosphere, and a pressure of 1.3 Pa. The film formation time was finely adjusted so that the film thickness was about 3 μm in each film formation. In the RF magnetron sputtering method, film formation was performed using an RF magnetron sputtering apparatus in which the substrate revolves on the target, and the distance between the target and the substrate was in the range of 100 mm to 150 mm. And in Example 9 and Comparative Examples 1-6, the heat processing of 600 to 750 degreeC was implemented for 2 hours in air | atmosphere after film-forming.

(Comparative Example 4)
In the silicon substrate with a piezoelectric thin film according to Comparative Example 4, the piezoelectric thin film was produced by a room temperature sputtering method. The substrate 10, the adhesive layer 40 provided on the substrate 10, and the lower electrode 20 provided on the adhesive layer 40 were formed in the same manner as in Examples 1 to 9 and Comparative Examples 1 to 3. On the other _{hand, (K 1-x N x} ) NbO 3 piezoelectric thin film, the composition ratio (K + Na) /Nb=1.0,Na/ (K + Na) = 0.5 of _{(K 0.5} Na 0.5) NbO ₃ The sintered body was used as a target, and the film was formed under the conditions of no substrate heating, discharge power of 75 W, introduced gas Ar atmosphere, and pressure of 0.4 Pa. The film formation time was adjusted so that the film thickness was about 3 μm. After the film formation, heat treatment was performed at 650 ° C. for 2 hours in an air atmosphere using an electric furnace to produce a silicon substrate with a piezoelectric thin film provided with a piezoelectric thin film made of polycrystalline KNN according to Comparative Example 4. The Na composition of the piezoelectric thin film made of KNN was 0.47.

(Comparative Example 5)
In the silicon substrate with a piezoelectric thin film according to Comparative Example 5, the piezoelectric thin film was produced by the MOD method (coating pyrolysis method). The substrate 10, the adhesive layer 40 provided on the substrate 10, and the lower electrode 20 provided on the adhesive layer 40 were formed in the same manner as in Examples 1 to 9 and Comparative Examples 1 to 3. Thereafter, a MOD raw material solution (however, Na composition 0.5) for forming a (K, Na) NbO ₃ thin film was applied onto the lower electrode 20 made of Pt by a spin coater, and was heated on a hot plate set at 400 ° C. A drying process for 5 minutes was performed. Then, the silicon substrate with a piezoelectric thin film provided with the piezoelectric thin film which consists of polycrystalline KNN was produced by implementing the crystallization process for 30 minutes at 700 degreeC in an atmospheric condition in an electric furnace. The Na composition of the piezoelectric thin film made of KNN was 0.45.

(Comparative Example 6)
In the silicon substrate with a piezoelectric thin film according to Comparative Example 6, the piezoelectric thin film was produced by an aerosol deposition method. The substrate 10, the adhesive layer 40 provided on the substrate 10, and the lower electrode 20 provided on the adhesive layer 40 were formed in the same manner as in Examples 1 to 9 and Comparative Examples 1 to 3. Thereafter, a piezoelectric thin film made of KNN having a thickness of 3 μm was formed on the lower electrode 20 made of Pt by an aerosol deposition method. The raw material powder is (K _0.5 Na _0.5 ) NbO ₃ fine particles having an average particle size of 0.5 μm, and is heated to the substrate (more specifically, the surface of the lower electrode 20) without heating the substrate. The film was formed by setting the collision speed of the KNN fine particles at a condition of about 100 m / sec. After the film formation, a silicon substrate with a piezoelectric thin film provided with a piezoelectric thin film made of polycrystalline KNN was manufactured by performing a heat treatment at 750 ° C. for 2 hours in an electric furnace in an air atmosphere. The Na composition of the piezoelectric thin film made of KNN was 0.49.

(X-ray diffraction measurement of piezoelectric thin film made of KNN)
In the silicon substrate with piezoelectric thin film provided with the piezoelectric thin film made of KNN according to Examples 1 to 9 and Comparative Examples 1 to 6, X-ray diffraction measurement (2θ / θ scan) was performed to investigate the crystal structure and the orientation state. .

  6 shows an X-ray diffraction pattern of the piezoelectric thin film according to Example 2, and FIG. 7 shows an X-ray diffraction pattern of the piezoelectric thin film according to Comparative Example 2.

  6 and 7, it can be seen that the piezoelectric thin film made of KNN has a complete perovskite structure, is a pseudo-cubic crystal, and is preferentially oriented in the (001) plane orientation of KNN. As an indication of the orientation state of the (001) plane orientation, the intensity of the (001) diffraction peak (2θ = 21-23 ° peak) of KNN in the piezoelectric thin film-attached silicon substrate provided with the piezoelectric thin film composed of each KNN was read. .

(Measurement of internal stress of piezoelectric thin film made of KNN and temperature dependence of internal stress)
The internal stress of the piezoelectric thin film composed of KNN at each temperature of the piezoelectric thin film-attached substrates according to Examples 1 to 9 and Comparative Examples 1 to 6 was measured by an optical lever method (details are as described above).

  8 shows the temperature dependence of the internal stress of the piezoelectric thin film according to Example 2, FIG. 9 shows the temperature dependence of the internal stress of the piezoelectric thin film according to Example 7, and FIG. FIG. 11 shows the temperature dependence of the internal stress of the piezoelectric thin film according to Comparative Example 2. FIG.

  Based on the measurement results, the internal stress of the piezoelectric thin film made of KNN at 20 ° C., the internal stress in the in-plane direction of the piezoelectric thin film at the highest temperature in the film forming process, and the average value of Δσ / ΔT from 20 ° C. to 250 ° C. Asked. For other examples and comparative examples, the internal stress of the KNN piezoelectric thin film at 20 ° C. in the same manner, the internal stress in the in-plane direction of the piezoelectric thin film at the highest temperature in the film forming process, and 20 ° C. to 250 ° C. The average value of Δσ / ΔT was determined.

(Piezoelectric property evaluation, actuator / sensor prototype)
Thereafter, in order to evaluate the piezoelectric constant d ₃₁ of the piezoelectric thin film made of KNN, an upper electrode made of Pt (provided that the film is formed on the piezoelectric thin film made of KNN of each of Examples 1 to 9 and Comparative Examples 1 to 6) 20 nm thick) was formed by RF magnetron sputtering, and a rectangular shape having a length of 20 mm and a width of 2.5 mm was cut out to produce a piezoelectric element including a KNN piezoelectric thin film.

12A and 12B show an outline of a method for evaluating the piezoelectric constant d ₃₁ of the piezoelectric element.

  The piezoelectric constant d31 was evaluated as follows. That is, first, as shown in FIG. 12A, the end of the piezoelectric element in the longitudinal direction was fixed with the clamp 200 to constitute a simple unimorph cantilever. Next, in this state, a predetermined voltage was applied to the KNN thin film as the piezoelectric thin film 30 between the upper electrode 50 and the lower electrode 20. As a result, the KNN thin film was expanded and contracted, and the entire unimol flavor was bent, and the tip of the cantilever was moved as shown in FIG. At this time, a displacement amount 300, which is a displacement between the surface position 300a of the upper electrode surface 50a before voltage application and the surface position 300b after voltage application, was measured with a laser Doppler displacement meter 250.

The piezoelectric constant d ₃₁ is calculated from the displacement amount 300 at the tip of the cantilever, the cantilever length, the thickness of the substrate and the piezoelectric thin film, the Young's modulus of the substrate and the piezoelectric thin film, and the applied voltage. The value of the piezoelectric constant d _{31 when} the applied electric field was 30 kV / cm was measured. The method for calculating the piezoelectric constant d ₃₁ is described in the literature (T. Mino, S. Kuwajima, T. Suzuki, I. Kanno, H. Kotera, and K. Wasa: Jpn. J. Appl. Phys. 46 (2007) 6960). According to the method described.

The Young's modulus of the KNN thin film was 104 GPa. By controlling the ambient atmosphere temperature, the temperature of the KNN thin film was controlled at 20 ° C., and the piezoelectric constant d ₃₁ (20 ° C.) at 20 ° C. was calculated. In addition, a hot plate 270 was installed in a non-contact manner directly below the cantilever provided with the KNN thin film, and the KNN thin film cantilever was heated with radiant heat. The temperature of the KNN thin film was measured from above using a handy type radiation thermometer 260, and the same piezoelectric constant measurement was performed when the temperature of the KNN thin film was 150 ° C. to calculate d ₃₁ (150 ° C.).

FIG. 13 shows an outline of a method for measuring d ₃₁ (150 ° C.).

Film forming method, film forming temperature, film forming process maximum temperature, Na composition, KNN (001) plane diffraction peak intensity in X-ray diffraction measurement, KNN at 20 ° C. The internal stress of the thin film, the internal stress of the KNN thin film at the maximum temperature of the film forming process, the temperature dependence of the internal stress of the KNN thin film from 20 ° C. to 250 ° C., the piezoelectric constant d ₃₁ at 20 ° C., and the change of the piezoelectric constant with temperature The rate d31 (150 ° C.) / D31 (20 ° C.) is shown in Table 1.

Referring to Table 1, when the internal stress of the KNN thin film at the maximum temperature of the film forming process is in the range of −500 to −200 MPa, the piezoelectric constant d ₃₁ at 20 ° C. has a high value of −100 pm / V or more. It was shown that the value of the piezoelectric constant d ₃₁ suddenly decreases outside this range. When the temperature dependence (Δσ / ΔT) of the internal stress of the KNN thin film at 20 ° C. to 250 ° C. is in the range of −0.25 × 10 ⁶ to −0.68 × 10 ⁶ (Pa / ° C.), the piezoelectric constant The rate of change d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) with a temperature of 0.9 has a high value of 0.9 or more, and it is shown that it becomes a small value of 0.71 or less outside this range. It was. The simple unimorph cantilever can also be used as a sensor that outputs a displacement amount at the tip of the cantilever as a voltage by connecting a voltage detector between the upper and lower electrodes.

In the graphs (FIGS. 8 and 9) showing the relationship between the internal stress σ and the temperature of the piezoelectric thin films of Examples 2 and 7, the graphs have clear inflection points in the temperature range of 250 ° C. or more and 400 ° C. or less. The average value in the range of 400 ° C. or more and 500 ° C. or less is higher than the average value in the range of 20 ° C. or more and 250 ° C. or less of the temperature dependence (Δσ / ΔT) of the internal stress σ of the piezoelectric thin film. It was shown to be a small value (ie an absolute value was shown to be large). Further, referring to Table 1, the internal stress in the in-plane direction of the piezoelectric thin film at 20 ° C. is in the range of −80 MPa (compression stress) to 250 MPa (tensile stress), and the Na composition is 0.4 or more and 0.7. It was shown that the piezoelectric constant d ₃₁ at 20 ° C. was good and the rate of change of the piezoelectric constant d ₃₁ with temperature was good in the following range.

  While the embodiments and examples of the present invention have been described above, the embodiments and examples described above do not limit the invention according to the claims. It should be noted that not all combinations of features described in the embodiments and examples are necessarily essential to the means for solving the problems of the invention.

DESCRIPTION OF SYMBOLS 1, 1a, 1b Substrate with piezoelectric thin film 2, 2a Piezoelectric thin film element 10 Substrate 12 Thermal oxide film 15 Substrate 20 Lower electrode 20a Lower electrode surface 30 Piezoelectric thin film 35 Thin film 40 Adhesion layer 50 Upper electrode 50a Upper electrode surface 100 Laser 110 Beam splitter 112 Mirror 120, 122 Detector 200 Clamp 250 Laser Doppler Displacement Meter 260 Radiation Thermometer 270 Hot Plate 300 Displacement 300a Surface Position 300b Surface Position

Claims (12)

A substrate,
A lower electrode provided above the substrate;
A piezoelectric thin film provided on the lower electrode and made of an alkali niobium oxide compound represented by the general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ x ≦ 0.7);
In the piezoelectric thin film, the ratio Δσ / ΔT of the change Δσ of the internal stress to the temperature change ΔT when the internal stress in the in-plane direction is σ and the temperature is T has a negative slope, and is 20 ° C. or higher and 250 ° C. or lower. in the average value of .DELTA..sigma / [Delta] T is - 0.2495 from × 10 ⁶ - 0.6866 Ri range der of ^{× 10 6 (Pa / ℃)} ,
Piezoelectric constant d ₃₁ at 20 ° C. is −100 pm / V or more (“above” relating to the piezoelectric constant indicates that the absolute value is large), and rate of change of piezoelectric constant with temperature d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) are piezoelectric thin film substrate with a Ru der least 0.9. A substrate,
A lower electrode provided above the substrate;
A piezoelectric thin film provided on the lower electrode and made of an alkali niobium oxide compound represented by the general formula (K _1-x Na _x ) NbO ₃ (0.4 ≦ x ≦ 0.7);
In the piezoelectric thin film, the ratio Δσ / ΔT of the change Δσ of the internal stress to the temperature change ΔT when the internal stress in the in-plane direction is σ and the temperature is T has a negative slope, and is 20 ° C. or higher and 250 ° C. or lower. in the average value of .DELTA..sigma / [Delta] T is - 0.2495 from × 10 ⁶ - 0.6866 Ri range der of ^{× 10 6 (Pa / ℃)} ,
Piezoelectric constant d ₃₁ at 20 ° C. is −100 pm / V or more (“above” relating to the piezoelectric constant indicates that the absolute value is large), and rate of change of piezoelectric constant with temperature d ₃₁ (150 ° C.) / D ₃₁ (20 ° C.) a substrate with a piezoelectric thin film Ru der 0.9 or more (but before Symbol piezoelectric thin film, der Ru pressure conductive thin film which was formed by setting the substrate temperature before Kimoto plate 550 ° C. or less Excluding attached board).   3. The substrate with a piezoelectric thin film according to claim 1, wherein the piezoelectric thin film has an internal stress in a film plane direction at 20 ° C. within a range of −80 MPa to 250 MPa.   The piezoelectric thin film has an inflection point in a temperature range of 250 ° C. or higher and 400 ° C. or lower in a graph showing the relationship between the internal stress in the in-plane direction of the piezoelectric thin film and temperature, and is 20 ° C. or higher and 250 ° C. or lower. The substrate with a piezoelectric thin film according to claim 3, wherein the average value of Δσ / ΔT in the range of 400 ° C. or more and 500 ° C. or less is smaller than the average value of Δσ / ΔT in the range of 5. The piezoelectric thin film state, and are within the scope internal stress of the film plane direction is less -200MPa exceed -500MPa at a given temperature at the time of forming the piezoelectric thin film on the lower electrode,
Wherein the predetermined temperature is, the piezoelectric thin film with substrate according to the temperature der Ru claim 4, which corresponds to the maximum temperature at the time of forming the piezoelectric thin film.   The substrate with a piezoelectric thin film according to claim 5, wherein the lower electrode is made of platinum (Pt).   The substrate with a piezoelectric thin film according to claim 6, further comprising a titanium (Ti) layer between the substrate and the lower electrode.   The substrate with a piezoelectric thin film according to claim 7, wherein the substrate has a thermal oxide film on a surface thereof. The substrate with a piezoelectric thin film according to claim 5 , wherein the predetermined temperature is in a range of 550 ° C to 600 ° C. The piezoelectric thin film element comprising: a piezoelectric thin film Tsukemoto plate according, and an upper electrode provided on the piezoelectric thin film in any of claims 1-9. Actuator comprising a piezoelectric thin film with substrate according to any one of claims 1-9. Sensor comprising a piezoelectric thin film with substrate according to any one of claims 1-9.

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