Electret

Abstract

To provide an electret capable of maintaining a high surface potential for a long time even in a high temperature environment and having excellent in thermal and temporal stability.SOLUTION: An electret 1 includes a substrate 10 and an electret layer 2 formed on its surface. The electret layer 2 is obtained by subjecting an inorganic dielectric film 20 including an outer layer film 3 and an inner layer film 4 stacked in a thickness direction X of the substrate 10 to a charge treatment. The outer layer film 3 is a composite metal compound containing two or more different metal elements, and is a film containing, as a main component, a first inorganic dielectric material containing at least a trivalent metal element and having a band gap energy of 3 eV or more. The inner layer film 4 is a film mainly containing a second inorganic dielectric material different from the first inorganic dielectric material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electret. [Background technology]

[0002] Electrets are electrically charged materials that generate an electrostatic field around them and have traditionally been used in applications such as electret condenser microphones and dust-collecting filters. Recently, they have been expected to be applied to vibration power generation, a type of energy harvesting technology. For example, there is a desire to commercialize small vibration power generation elements using electrets as integrated circuit-embedded elements for electrostatic vibration power generators that use environmental vibrations as a power source.

[0003] Organic polymer materials such as fluororesins are commonly used as materials for electrets. While organic polymer materials offer excellent control over the degree of freedom in shape and film thickness when forming thin films, there are concerns about the thermal stability of the surface potential and deterioration of performance over time. Therefore, inorganic compound materials with excellent thermal stability are being investigated. Known examples of inorganic compound materials include electrets using bulk sintered bodies of hydroxyapatite and electrets using complex oxides with a perovskite structure.

[0004] Furthermore, by incorporating a power generation element using an inorganic electret into an integrated circuit formed on a substrate, it becomes possible to miniaturize the power generation device and use it in a high-temperature environment, and it is expected to be applied to various uses. Therefore, the present inventors previously proposed an electret having a substrate and an electret layer formed on the surface thereof, in which the electret layer uses a thin film mainly composed of an inorganic dielectric material that is a composite metal compound containing two or more different metal elements and has a band gap energy of 4 eV or more (Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7390687 Summary of the Invention [Problem to be solved by the invention]

[0006] The electret described in Patent Document 1 is believed to have a high surface potential due to the high band gap energy of an inorganic dielectric material containing two or more metal elements, which controls defects and enables polarization treatment at high voltages at high temperatures. Therefore, a relatively stable surface potential can be obtained in a temperature environment above room temperature (e.g., about 100°C), but it is difficult to maintain the high surface potential immediately after electretization. It has been found that the surface potential is particularly prone to disappear over time in a high-temperature environment above 100°C (e.g., above 200°C), and there is a need to improve stability over time.

[0007] Electrets are known in which a thin film made of a conventional inorganic compound material or a fluorine-based organic polymer material, or an electret film made of a combination of these materials, is formed on a substrate. However, the inorganic compound materials generally used to form elements do not have a high surface potential, and fluorine-based organic polymer materials, although they exhibit a relatively high surface potential at room temperature, have low heat resistance and are difficult to apply to elements used in high-temperature environments.

[0008] The present invention has been made in view of the above problems, and aims to provide an electret that can maintain a high surface potential for a long time even in a high-temperature environment and has excellent thermal and temporal stability. [Means for solving the problem]

[0009] One aspect of the present invention is An electret (1) having a substrate (10) and an electret layer (2) formed on the surface (11) of the substrate, The electret layer is an inorganic dielectric film (20) including an outer layer film (3) and an inner layer film (4) laminated in the thickness direction (X) of the substrate, and is subjected to a charging treatment; the outer layer film is a composite metal compound containing two or more different metal elements, and is a film containing as a main component a first inorganic dielectric material containing at least a trivalent metal element and having a band gap energy of 3 eV or more; The inner layer film is an electret, which is a film mainly composed of a second inorganic dielectric material different from the first inorganic dielectric material. [Effects of the Invention]

[0010] In a configuration in which an electret layer is disposed on a substrate, it is thought that in a high-temperature environment, charges are more likely to migrate from the surface in contact with the substrate, leading to a decrease in surface potential. In contrast, the electret of this embodiment has a laminated structure in which the electret layer includes an outer layer film and an inner layer film, and therefore can stably maintain a high surface potential even in a high-temperature environment. The reason for this is not entirely clear, but it is thought that the outer layer film is a film mainly composed of a first inorganic dielectric material that is made of a specific composite metal oxide with a band gap energy of 3 eV or more and can exhibit a high surface potential by charging, and the inner layer film composed of a second inorganic dielectric material is disposed inside it, thereby achieving a high effect of accumulating charges at the laminate interface and suppressing the outflow of charges to the substrate.

[0011] As a result, the accumulated charge can be stably maintained, and changes in the surface potential can be suppressed regardless of the temperature environment. This allows the electret to maintain its initial high surface potential and exhibit stable performance even in harsh environments. Furthermore, in the manufacture of power generation devices using electrets, it becomes possible to apply high-temperature processes, which increases design freedom and contributes to reducing manufacturing costs.

[0012] As described above, according to the above-described embodiment, it is possible to provide an electret that can maintain a high surface potential for a long period of time even in a high-temperature environment and has excellent thermal and temporal stability. In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate the correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a schematic configuration of an electret in a first embodiment. [Figure 2] 1 is a schematic diagram showing a configuration example of an electret in the first embodiment. [Figure 3] FIG. 6 is a schematic diagram showing a configuration example of an electret in a second embodiment. [Figure 4] 1A and 1B are schematic diagrams showing the general configuration of an electret and a graph showing the relationship between heat treatment and surface potential in a comparative example. [Figure 5] FIG. 10 is a diagram showing the relationship between the film thickness of the inner layer film of the electret and the surface potential in the examples. [Figure 6] FIG. 10 is a diagram showing the relationship between the elapsed time after charging processing and the surface potential in an example. [Figure 7] FIG. 10 is a diagram showing the relationship between the film thickness of the outer layer of the electret and the surface potential in the examples. [Figure 8] FIG. 10 is a diagram showing the relationship between the elapsed time after charging processing and the surface potential in an example. [Figure 9] FIG. 10 is a diagram showing the relationship between the film thickness of the inner layer film of the electret and the surface potential in the examples. [Figure 10] FIG. 10 is a diagram showing the relationship between the elapsed time after charging processing and the surface potential in an example. [Figure 11] FIG. 10 is a diagram showing the relationship between the film thickness of the inner layer film of the electret and the surface potential in the examples. [Figure 12] FIG. 10 is a diagram showing the relationship between the film thickness of the inner layer film of the electret and the surface potential in the examples. [Figure 13]FIG. 10 is a diagram showing the relationship between the elapsed time after charging processing and the surface potential in an example. [Figure 14] FIG. 10 is a diagram showing the relationship between the elapsed time after charging processing and the surface potential in an example. [Figure 15] FIG. 4 is a schematic diagram showing a schematic configuration of an electret in a comparative example. [Figure 16] FIG. 10 is a graph showing the relationship between the time elapsed after charging processing and the surface potential in a comparative example. [Figure 17] FIG. 10 is a graph showing the relationship between the time elapsed after charging processing and the surface potential in a comparative example. [Figure 18] FIG. 10 is a graph showing the relationship between the time elapsed after charging and the surface potential in Examples and Comparative Examples. [Figure 19] FIG. 10 is a graph showing the relationship between the time elapsed after charging and the surface potential in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] (Embodiment 1) A first embodiment of the electret will be described with reference to the drawings. As shown in Fig. 1, the electret 1 of this embodiment has a substrate 10 and an electret layer 2 formed on the surface thereof. The electret layer 2 is an inorganic dielectric film 20 that has been electrified by being charged, and the inorganic dielectric film 20 includes an outer layer film 3 and an inner layer film 4 that are laminated in the thickness direction X of the substrate 10. The outer layer film 3 is a film whose main component is a first inorganic dielectric material, and the inner layer film 4 is a film whose main component is a second inorganic dielectric material.

[0015] In the inorganic dielectric film 20, the outer layer film 3 is located on the outer side in the thickness direction X (i.e., the stacking direction) of the substrate 10, and the inner layer film 4 is located on the inner side. The first inorganic dielectric material that is the main component of the outer layer film 3 can be selected from inorganic dielectric materials that are complex metal compounds containing two or more different metal elements, contain at least a trivalent metal element, and have a band gap energy of 3 eV or more. The second inorganic dielectric material that is the main component of the inner layer film 4 can be selected from inorganic dielectric materials different from the first inorganic dielectric material.

[0016] The inorganic dielectric film 20 can exhibit desired electret properties by appropriately setting the combination of inorganic dielectric materials and film thicknesses that form the outer layer film 3 and inner layer film 4, the conditions for electret formation, etc. Note that in the film compositions that form the outer layer film 3 and inner layer film 4, the term "main component" means that the first and second inorganic dielectric materials may be the only constituent materials, or that impurities resulting from the raw materials of the first and second inorganic dielectric materials may be included, or that some other components may be added in the process of forming the first and second inorganic dielectric materials.

[0017] Electret 1 is a charged substance that holds a positive or negative charge on its surface and provides an electrostatic field to its surroundings. When an inorganic dielectric film 20 formed on a substrate 10 is charged, it exhibits electret performance and becomes an electret layer 2. "Electretization" means, in other words, developing a surface potential by performing a charging process to turn the substance into a charged substance. Such electret 1 can be used as a power generation element with an integrated circuit built in various devices that convert mechanical energy into electrical energy and vice versa, such as a small electrostatic vibration power generation device that uses ambient vibration as a power source.

[0018] The composition of the first inorganic dielectric material constituting the outer layer 3 is not particularly limited, as long as it is a composite metal compound material in which two or more different metal elements, including a trivalent metal element, have a bandgap energy of 3 eV or greater. In this case, the desired physical properties can be obtained depending on the combination of the two or more metal elements and the structure of the composition containing them. Having a relatively large bandgap energy of 3 eV or greater, preferably 4 eV or greater, increases the breakdown voltage. This allows for the application of a high voltage during charging, enabling the development of a desired high surface potential. Preferably, the first inorganic dielectric material has a bandgap energy of 4.5 eV or greater, more preferably 5.5 eV or greater.

[0019] In this embodiment, the electret layer 2 has a structure in which an inner layer film 4 and an outer layer film 3 are laminated in this order on a substrate 10. In this case, the outer layer film 3 is made of a first inorganic dielectric material having a relatively large band gap energy, which enables the development of a high surface potential. Furthermore, the inner layer film 4 made of a second inorganic dielectric material is disposed inside the outer layer film 3, thereby achieving the effect of charge accumulation at the layer interface. The charge accumulation effect at the interface of such a layered structure is generally known as the Maxwell-Wagner effect, and the combination of the outer layer film 3 made of the first inorganic dielectric material and the inner layer film 4 made of the second inorganic dielectric material exhibits an unprecedentedly high effect on the retention and stabilization of the amount of charge developed on the surface of the outer layer film 3.

[0020] With this configuration, it has been found that the electret 1 retains almost the same initial surface potential as before exposure to the high-temperature environment, even in a high-temperature environment of, for example, over 200°C, and can exhibit stable electret performance even in applications with harsh temperature environments.

[0021] Specific configuration examples of the electret 1 will be described in detail below. The electret 1 has any outer shape corresponding to the shape of the substrate 10. The shape of the substrate 10 is, for example, a rectangular plate or a disk. The substrate 10 has a plurality of thin films of an inorganic dielectric film 20 that become the electret layer 2 sequentially laminated on one side in the thickness direction X (here, the up-and-down direction in the drawing). That is, the outer shape of the outer layer film 3 and the inner layer film 4, which are a plurality of thin films, has the same shape as the substrate 10, and the lamination direction thereof coincides with the thickness direction X of the substrate 10. Hereinafter, of the two surfaces of the substrate 10, the surface on which the electret layer 2 is laminated will be referred to as the upper surface 11, and the opposite surface will be referred to as the lower surface 12.

[0022] The first inorganic dielectric material that becomes the outer layer film 3 can have a basic composition of, for example, a composite oxide containing two different metal elements A and B. In this case, the metal elements A and B are selected so that the band gap energy is 3 eV or more and at least one of them contains a trivalent metal element. It is preferable to use a material in which the metal element A is a divalent or trivalent metal element and the metal element B is a trivalent metal element.

[0023] An example of such a first inorganic dielectric material is a complex oxide having a perovskite-type composition. That is, a first complex oxide containing two different trivalent metal elements A and B and represented by the composition formula ABO3 can be used as the basic composition. A complex oxide having a perovskite-type composition is typically a complex oxide having a perovskite-type crystal structure with a cubic unit lattice, in which metal element A (A site) is located at each vertex of the cubic crystal, metal element B (B site) is located at the center of the cubic crystal, and oxygen atoms O are coordinated in a regular octahedron with respect to each metal element A and B. In the perovskite structure, a non-stoichiometric composition is often obtained due to a deficiency of oxygen atoms, and in such cases, the composition formula ABO x It can be expressed as (x≦3).

[0024] Here, the first composite oxide only needs to have a basic composition represented by the composition formula ABO3, and may be crystalline with a perovskite-type crystal structure or amorphous. In either case, it is desirable that the ratio of the basic constituent metal elements A, B, and O in the first composite oxide as a whole be A:B:O=1:1:3 or a relationship close to this. In this case, if the amount of oxygen in the first composite oxide is less than the stoichiometric ratio of the basic composition, defects are likely to be introduced, and the surface potential is likely to increase.

[0025] In the electret layer 2, the form of the outer layer film 3 containing the first inorganic dielectric material as the main component is not particularly limited, and may be, for example, a film of a first complex oxide having an amorphous structure (hereinafter referred to as an amorphous film) or a film containing a first complex oxide having a crystalline structure (hereinafter referred to as an oxide crystal film). In this case, the entire outer layer film 3 does not need to be an oxide of uniform composition, and may partially have a composition different from the basic composition.

[0026] In the present embodiment, the electret layer 2 using an amorphous film as the outer layer 3 will be mainly described below. In an amorphous film, defects due to unbonded dangling bonds are more likely to form than in an oxide crystalline film with a perovskite structure of the same composition. In the electret layer 2, the presence of defects is considered to be important for the generation of the surface potential in the outer layer 3, and a high surface potential can be obtained by using an amorphous film. In addition, an amorphous film can be formed at a lower temperature than an oxide crystalline film, so thermal damage to wiring, etc. can be suppressed during device fabrication.

[0027] The first composite oxide that serves as the first inorganic dielectric material preferably has a composition in which the metal element A in the composition formula ABO3 is at least one element selected from rare earth elements R, and the metal element B is Al. Composite oxides (RAlO3; rare earth aluminates) with a perovskite composition that combines a trivalent rare earth element R and trivalent Al have a relatively large band gap energy (e.g., 4 eV or more) and a relatively small relative dielectric constant (e.g., 100 or less), which allows for a high surface potential to be achieved. Furthermore, they can be produced using relatively inexpensive materials, which is advantageous in terms of production costs.

[0028] The trivalent rare earth element R may be, for example, at least one element selected from Y, Sc, and lanthanoids. Examples of lanthanoids include La, Pr, Nd, Sm, and Gd. Preferably, LaAlO3 (lanthanum aluminate) containing La and Al as the trivalent rare earth element R may be used.

[0029] The first composite oxide may have a composition in which part of the metal element A, part of the metal element B, or both in the composition formula ABO3 are substituted with a dopant element D made of a different metal element. In this case, if the dopant element D has a lower valence than the metal elements A and B, defects due to oxygen vacancies are likely to occur in the structure. For example, when the metal element A is a trivalent rare earth element R, a divalent alkaline earth metal element is preferably used, and when the metal element B is trivalent Al, one or more elements selected from a divalent alkaline earth metal element and Zn are preferably used. Examples of alkaline earth metal elements include Mg, Ca, Sr, and Ba.

[0030] The combination of metal elements A and B with dopant element D is not particularly limited, and substitution with a dopant element D of a lower valence generates defects due to oxygen deficiency in the perovskite composition to maintain electrical neutrality, contributing to an improvement in the surface potential. Since there is a correlation between the amount of substitution with dopant element D and the number of defects, controlling the amount of dopant element D introduced makes it possible to control the number of defects that affect the surface potential, resulting in stable surface potential characteristics.

[0031] Specifically, a typical example of a rare earth aluminate is lanthanum aluminate (LaAlO), in which a portion of La is replaced with an alkaline earth metal element (e.g., Ca). In this case, the composition formula (La, Ca)AlO is determined taking into consideration the amount of substitution by the dopant element D and the amount of oxygen that varies depending on the atmosphere, etc. x (x<3). Alternatively, for convenience, it is expressed by the basic composition formula before substitution. For example, when only the dopant element D is considered, if the substitution ratio is Y (atm%), the composition formula is La (1-Y) Ca Y AlO 3-Y / 2 It can be expressed as:

[0032] The substitution ratio of the dopant element D substituting the metal element A can be appropriately set, for example, to 20 atm% or less, preferably in the range of 0.5 atm% to 20 atm%. Similarly, the substitution ratio of the dopant element D substituting the metal element B can be, for example, 20 atm% or less, preferably in the range of 0.5 atm% to 20 atm%. When the substitution ratio is 0.5 atm% or more, the effect of improving the surface potential is obtained compared to when the dopant element D is not introduced. However, as the substitution ratio approaches 20 atm%, the effect of introducing the dopant element D tends to decrease. The reason for this is not necessarily clear, but it is presumed that an increase in the relative dielectric constant acts to suppress the increase in the surface potential. Therefore, it is best to appropriately set the substitution ratio so that the desired characteristics are obtained within a range not exceeding 20 atm%.

[0033] In the electret layer 2, it is desirable that the relative permittivity of the first inorganic dielectric material, which is the main component of the outer layer film 3, is greater than the relative permittivity of the second inorganic dielectric material, which is the main component of the inner layer film 4. The relative permittivity of the first inorganic dielectric material can be adjusted, for example, by the combination of metal elements A and B, the dopant element D introduced into the metal elements A and B, and the substitution ratio thereof. Note that the relative permittivity is an intrinsic value expressed as the ratio of the dielectric constant of each material to the dielectric constant of a vacuum (i.e., relative permittivity = dielectric constant ε / dielectric constant of a vacuum ε0).

[0034] In the electret layer 2, the outer layer film 3 forming its outer surface exhibits a high surface potential, and the presence of the inner layer film 4 between the outer layer film 3 and the substrate 10 provides a high charge accumulation effect at the laminated interface, which is thought to stabilize the surface potential. In this case, the outer layer film 3 is made of a material with a relatively large dielectric constant, which contributes to an increase in the amount of charge due to the charging process, and the inner layer film 4 is made of a material with a relatively small dielectric constant, which suppresses the movement of charges and contributes to stabilizing the charges accumulated at the interface.

[0035] This configuration prevents the charge accumulated in the electret layer 2 from leaking out through the substrate 10, making it possible to maintain the surface potential developed by the charging process even in a high-temperature environment. The relative dielectric constant of the first inorganic dielectric material is not particularly limited, but is preferably 10 or more, which allows for a high surface potential (for example, an absolute value of 1000 V or more). The upper limit of the relative dielectric constant is not particularly limited, but is preferably, for example, about 100 or less, and the first inorganic dielectric material can be selected so that a desired surface potential can be obtained in combination with the band gap energy.

[0036] The second inorganic dielectric material is not particularly limited and can be appropriately selected from inorganic dielectric materials having a lower dielectric constant than the first inorganic dielectric material. Preferably, the second inorganic dielectric material is an inorganic compound having a dielectric constant of about 10 or less. Examples of such inorganic compounds include oxides, nitrides, oxynitrides, and mixtures thereof containing metal elements such as Si or Al. Preferred examples include Si compounds such as SiO and SiN, Al compounds such as AlO (e.g., AlO), and mixtures of two or more of these compounds. The second inorganic dielectric material can be appropriately selected taking into consideration the materials of the first inorganic dielectric material and substrate 10, the method of film formation on the substrate 10, and the like.

[0037] The material of the substrate 10 is not particularly limited, but for example, conductive Si can be used. Alternatively, a conductive substrate using a conductive material such as (Nb,Sr)TiO3 or a metal, or an insulating substrate using an insulating material such as Al2O3 or a glass material can be used.

[0038] As shown in FIG. 2 (upper diagram), the electret layer 2 is made of, for example, a first inorganic dielectric material constituting the outer layer 3, which has the composition formula (La, Ca)AlO xThe first composite oxide represented by (x<3) can be used, and the second inorganic dielectric material constituting the inner layer film 4 can be SiO2. An inorganic dielectric film 20 combining such an outer layer film 3 and inner layer film 4 is formed, for example, directly on the upper surface 11 of a substrate 10 made of conductive Si, and is electretized to form an electret 1 having a three-layer structure as a whole.

[0039] As shown in a modified example in FIG. 2 (lower diagram), the electret layer 2 can have, for example, an inner layer film 4 with a multi-layer structure of two or more layers. It can also be configured as a film with such a multi-layer structure (hereinafter referred to as a multi-layer film). In this case, two or more of the metal compounds exemplified as the second inorganic dielectric material can be arbitrarily combined. Here, the inner layer film 4 is a multi-layer film having a first layer 4a (e.g., SiO2) in contact with the upper surface 11 of the substrate 10 and a second layer 4b (e.g., SiN).

[0040] There are no particular limitations on the method for forming the thin film that will become the inorganic dielectric film 20, and any method can be used. Specific examples include thermal oxidation, physical vapor deposition (PVD) methods such as sputtering, chemical vapor deposition (CVD), deposition, and sol-gel methods, and the method can be selected taking into consideration the desired film quality and film thickness for each of the outer layer film 3 and inner layer film 4 that make up the inorganic dielectric film 20.

[0041] For example, when sputtering is used to form the outer layer 3 and the inner layer 4, crystals of first and second inorganic dielectric materials having the same composition as the films to be formed are used as targets, and a high voltage is applied in an inert gas to cause accelerated ions to collide with the targets, thereby forming a thin film of the desired composition on the upper surface of the substrate 10. Also, as shown in Fig. 2, when the second inorganic dielectric material constituting the inner layer 4 contains SiO2, a thermal oxidation film of SiO2 can be formed on the surface of the Si substrate, which is the substrate 10, by thermal oxidation.

[0042] The film formation temperature is usually within the range of room temperature to 1000°C and may be set to a temperature appropriate for the material. By using such a method to form the film under a temperature condition of 1000°C or less, it is possible to form thin films of the first and second inorganic dielectric materials that will become the inorganic dielectric film 20 while suppressing damage caused by high temperatures to the substrate 10 and the wiring on the substrate 10. The first and second inorganic dielectric materials that will become the target raw materials can be those manufactured in a high-temperature process exceeding 1000°C.

[0043] The thin film formed on the substrate 10 can be made to have any thickness, for example, 0.01 μm or more, by adjusting the film formation conditions, etc. Preferably, by making the inorganic dielectric film 20 a thin film in the range of 0.1 μm to 10 μm, an electret 1 suitable for use in small devices such as vibration power generation elements and memory circuits can be obtained. In this case, the surface potential of the electret layer 2 has a positive correlation with the film thickness of the inner layer film 4, and the thicker the film thickness of the inner layer film 4, the higher the surface potential. Preferably, the film thickness of the inner layer film 4 is adjusted to a range of 0.1 μm or more, more preferably 0.5 μm or more, so as to obtain desired characteristics.

[0044] On the other hand, the surface potential of the electret layer 2 does not depend on the thickness of the outer layer film 3, and exhibits a substantially constant surface potential due to the presence of the inner layer film 4 between the electret layer 2 and the substrate 10. Therefore, the thickness of the outer layer film 3 may be 0.01 μm or more, and preferably 0.1 μm or more, so that a surface potential according to the composition, charging conditions, etc. can be stably expressed. More preferably, the thickness of the outer layer film 3 is adjusted to a sufficient thickness within the range of 0.5 μm or more so that dielectric breakdown does not occur due to the applied voltage during charging processing.

[0045] The electret 1 is obtained by subjecting an inorganic dielectric film 20 formed on the upper surface 11 of a substrate 10 to a charging treatment in the stacking direction (i.e., thickness direction X). The charging method is not particularly limited, and for example, a method of applying a voltage under heating conditions between a ground electrode and a counter electrode connected to the inorganic dielectric film 20 using corona discharge or the like can be employed. Alternatively, charging can be performed by a thermal electretization method in which a high voltage is applied at high temperature.

[0046] Since the surface potential is proportional to the voltage applied to the inorganic dielectric film 20 formed on the substrate 10, a required voltage may be applied to achieve the surface potential required for the application. Alternatively, the film thickness may be increased accordingly so that dielectric breakdown does not occur for the required voltage.

[0047] (Embodiment 2) A second embodiment of the electret will now be described. The basic configuration of the electret 1 of this embodiment is the same as that of the first embodiment, and includes a substrate 10 shown in FIG. 1 and an electret layer 2 formed on the surface thereof. The electret layer 2 has a two-layer structure including an outer layer film 3 and an inner layer film 4, and in this embodiment, the first inorganic dielectric material that is the main component of the outer layer film 3 is changed. The following mainly describes the differences. Note that, among the symbols used in the second and subsequent embodiments, the same symbols as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0048] The form of the outer layer film 3 constituting the electret layer 2 is the same as that of the above-mentioned embodiment 1, and can be an amorphous film or an oxide crystal film mainly composed of the first inorganic dielectric material. The material of the substrate 10, the second inorganic dielectric material constituting the inner layer film 4, the film formation method, film thickness, and the like can also be the same as those of the above-mentioned embodiment 1.

[0049] In this embodiment as well, the first inorganic dielectric material has a basic composition of a complex oxide having a band gap energy of 3 eV or more and at least one of the metal elements A and B being a trivalent metal element. Preferably, a material combining a divalent or trivalent metal element A with a trivalent metal element B is used, and as such a complex oxide, a second complex oxide having a garnet-type composition or a third complex oxide having a spinel-type composition can be used instead of the complex oxide having a perovskite-type composition of embodiment 1.

[0050] The first inorganic dielectric material contains two different trivalent metal elements A and B and has a composition formula of A3B5O 12 The second composite oxide may have a basic composition represented by the formula: In this case, the second composite oxide may be crystalline having a cubic garnet-type crystal structure, or may be amorphous. In either case, it is desirable that the ratio of the basic constituent elements, metal elements A, B, and O, of the second composite oxide as a whole is A:B:O=3:5:12 or a relationship close to this.

[0051] In the second composite oxide, the combination of the trivalent metal elements A and B is not particularly limited. Preferably, the second composite oxide has the composition formula A3B5O 12 The metal element A in the above formula (1) can be at least one element selected from rare earth elements R, and the metal element B can be Al. As the trivalent rare earth element R, for example, at least one element selected from Y, Sc, and lanthanoids can be used. Examples of lanthanoids include La, Pr, Nd, Sm, and Gd. Specific examples of the second composite oxide having such a composition include Y3Al5O, which contains Y as the trivalent rare earth element R and Al. 12 etc.

[0052] Composition formula A3B5O 12In the above, two or more rare earth elements can be used in combination as the metal element A. Also, Al and at least one element selected from trivalent typical elements (e.g., Ga) can be used in combination as the metal element B. Specific examples of the second composite oxide having such a composition include Gd(Al,Ga)O, which contains Gd as the trivalent rare earth element R, and Al and Ga. 12 etc.

[0053] Also, the composition formula is A3B5O 12 In the above, a part of the metal element A, a part of the metal element B, or both may be substituted with a dopant element D made of a different metal element. In this case, it is preferable that the dopant element D is a metal element having a lower valence than the metal elements A and B. For example, when the metal element A is a trivalent rare earth element R, a divalent alkaline earth metal element is preferably used, and when the metal element B is trivalent Al, one or more elements selected from a divalent alkaline earth metal element and Zn are preferably used. Examples of alkaline earth metal elements include Mg, Ca, Sr, and Ba.

[0054] The substitution ratio of the dopant element D substituting for the metal elements A and B can be set appropriately, for example, to 20 atm% or less, preferably in the range of 0.1 atm% to 20 atm%. More preferably, a substitution ratio of 0.5 atm% facilitates the introduction of defects, contributing to an improvement in the surface potential. The same applies to the metal element B; it is preferable to set the substitution ratio appropriately so that the desired characteristics are obtained within a range not exceeding 20 atm%.

[0055] The first inorganic dielectric material may also have a third complex oxide containing a divalent metal element A and a trivalent metal element B and having a composition formula of AB2O4 as its basic composition. The third complex oxide may be crystalline with a cubic spinel-type crystal structure, or may be amorphous. In either case, it is desirable that the ratio of the basic constituent elements, metal elements A and B, to O, throughout the third complex oxide as a whole be A:B:O=1:2:4 or a relationship close to this.

[0056] In the third composite oxide, the combination of the divalent metal element A and the trivalent metal element B is not particularly limited. Preferably, the composition has the composition formula AB2O4, in which the divalent metal element A is at least one element selected from alkaline earth metal elements and transition metal elements, and the trivalent metal element B is Al. Examples of alkaline earth metal elements include Mg, Ca, Sr, and Ba. Examples of transition metal elements include Fe, Zn, and Mn.

[0057] Specific examples of the third composite oxide having such a composition include MgAl2O4 (spinel), as well as SrAl2O4, FeAl2O4, ZnAl2O4, and MnAl2O4, all of which have a relatively high band gap energy (for example, 4 eV or more).

[0058] In this embodiment, the inorganic dielectric film 20 also includes an outer layer film 3 and an inner layer film 4, each of which uses such second and third complex oxides as the first inorganic dielectric material. The outer layer film 3 does not need to be an oxide of uniform composition as a whole, and may, for example, have a composition that is partially different from the second and third complex oxides that form the basic composition. The electret layer 2 is obtained by charging such an inorganic dielectric film 20. The charging method can be the same as in the first embodiment.

[0059] As with the first composite oxide, the second and third composite oxides may have defects in their structure due to oxygen vacancies or the like, resulting in a lower oxygen content than in the basic composition formula. x (x≦12), composition formula AB2O x (x≦4). Alternatively, for convenience, it is expressed by the basic composition formula before substitution.

[0060] Specifically, for example, the second composite oxide YAlO 12 In this case, a part of Y can be replaced with an alkaline earth metal element (e.g., Mg) which is a dopant element D. In this case, the composition formula (Y,Mg)3Al5O is determined taking into consideration the amount of substitution by the dopant element D and the amount of oxygen which varies depending on the atmosphere, etc. x (x<12). Alternatively, for convenience, it is expressed by the basic composition formula before substitution.

[0061] At this time, as shown in FIG. 3, the electret layer 2 is made of, for example, a first inorganic dielectric material constituting the outer layer 3, which has the composition formula (Y, Mg)3Al5O x The second composite oxide represented by (x<12) can be used, and the second inorganic dielectric material constituting the inner layer film 4 can be SiO2. An inorganic dielectric film 20 combining such an outer layer film 3 and inner layer film 4 is formed, for example, directly on the upper surface 11 of a substrate 10 made of conductive Si, and is made into an electret, thereby constituting an electret 1 having a three-layer structure as a whole.

[0062] In the second and third complex oxides, which serve as the first inorganic dielectric material in this embodiment, the molar fraction of metal element B relative to the total of metal elements A and B is larger than that of the first complex oxide. That is, the first complex oxide having a perovskite composition used in the first embodiment 1 above has metal element A and metal element B in an equal ratio (A:B=1:1), whereas the second complex oxide having a garnet composition has an A:B ratio of 3:5, and the third complex oxide having a spinel composition has an A:B ratio of 1:2, meaning that the proportion of metal element B is higher.

[0063] The first inorganic dielectric material has a basic composition of first to third composite oxides containing metal elements A, B, and O, which are basic constituent elements, with the ratio A:B:O being a predetermined relationship. These first to third composite oxides exhibit desired properties, such as a band gap energy of 3 eV or more and a relative dielectric constant of 100 or less, which is higher than that of the second inorganic dielectric material, by combining different metal elements A and B or by introducing a dopant element D. An inorganic dielectric film 20 combining the first and second inorganic dielectric materials exhibits a high surface potential by becoming an electret, and is able to stably maintain the surface potential that is developed. [Example]

[0064] Example 1 An electret 1 having the configuration of embodiment 1 shown in Fig. 2 (upper diagram) was produced by the following method: A first composite oxide having a perovskite composition was used as the first inorganic dielectric material. <Formation of inorganic dielectric film 20> First, the upper surface 11 of the substrate 10 made of conductive Si was thermally oxidized to form a thermal oxide film (SiO2), which served as the inner layer 4 made of a thin film of the second inorganic dielectric material. The thickness of the substrate 10 was 625 μm, and the thickness of the inner layer 4 was 1.2 μm. Next, an outer layer 3 made of the first inorganic dielectric material was formed on the upper surface of the inner layer 4 by a sputtering method. The sputtering conditions were 360°C in an Ar atmosphere, and a crystal target having the same composition as the first composite oxide used as the first inorganic dielectric material was used to deposit an amorphous film that would become the outer layer 3 to a thickness of 1.3 μm.

[0065] The first composite oxide was a lanthanum aluminate (LaAlO3; hereinafter referred to as LAO)-based composite oxide containing La and Al as metal elements A and B in the composition formula ABO3, with part of the La being substituted with a dopant element D. Here, Ca was used as the dopant element D, and the substitution ratio was 1 atm% (hereinafter referred to as 1% Ca-added LAO). In addition, a polycrystalline material [(La 0.99Ca 0.01 )AlO x In this way, an amorphous film containing Ca, La, Al, and O in a La:Al:O ratio of approximately 1:1:3 was obtained.

[0066] The band gap energy of lanthanum aluminate (LaAlO3), a typical composition of LAO-based composite oxides, is 5.6 eV, and a composition in which part of the La is replaced with Ca, which is the dopant element D, also has approximately the same band gap energy. Furthermore, the relative dielectric constant of the LAO-based composite oxide (first inorganic dielectric material) is approximately 22, which is larger than that of SiO2 (second inorganic dielectric material), which has a relative dielectric constant of 3.8.

[0067] <Electret> In this way, a sample was prepared in which an inorganic dielectric film 20 was formed on a substrate 10, and then a charging process was performed to obtain an electret 1 having an electret layer 2 (Example 1). A corona discharge method was used for the charging process, and the substrate 10 in contact with the inner layer 4 of the inorganic dielectric film 20 was used as a ground electrode, and a discharge needle serving as a corona discharge electrode was placed opposite the outer layer 3 side, and a negative voltage was applied to generate corona discharge. At this time, the sample was placed on a heater plate, and the lower surface 12 of the substrate 10 was placed in contact with the grounded heater plate. The heating temperature by the heater plate and the conditions for the charging process by corona discharge were as follows: Heating temperature: 200℃ Discharge needle voltage: -10kV Discharge time: 15 seconds Distance between discharge needle and sample: 20 mm

[0068] <Surface potential measurement>

[0069] As a result, ions charged by corona discharge collide with the exposed surface of the inorganic dielectric film 20 formed on the substrate 10 (i.e., the surface of the outer layer film 3), which becomes negatively charged and turns into an electret. Next, the surface potential of the obtained electret 1 (Example 1) was measured. For the measurement, a surface potential meter (MODEL 341-B: manufactured by Trek) was used, and the measurement probe was placed opposite the electret 1 placed on a stage to measure the surface potential in a non-contact manner. At this time, multiple samples (number of samples n = 2) with the same configuration were prepared, each was charged, and then heated at 200 ° C for 30 minutes. The surface potential after the heat treatment was measured, and all of them obtained a high surface potential of over 1100 V (absolute value).

[0070] (Comparative Example 1) For comparison, an electret 1 was produced using a single-layer inorganic dielectric film 21, as shown in FIG. 4 (upper diagram). The inorganic dielectric film 21 that becomes the electret layer 2 has a film configuration mainly composed of the same inorganic dielectric material as the outer layer film 3, that is, a single-layer film that does not have the inner layer film 4 in the configuration of Example 1. The rest was the same as Example 1. Specifically, an amorphous film of an LAO-based composite oxide containing 1 atm % of Ca was formed directly on the upper surface 11 of a substrate 10 made of conductive Si by a sputtering method to form the inorganic dielectric film 21, and then a charging process was performed in the same manner to form the electret 1. The thickness of the substrate 10 was 625 μm, and the thickness of the inorganic dielectric film 21 was 1.3 μm.

[0071] The surface potential of the electret 1 of Comparative Example 1 was measured immediately after the charging process using the same method as in Example 1. At this time, the surface potential was measured immediately after the electret was formed by the charging process, after heating at 100°C for 30 minutes, and after heating at 200°C for 30 minutes, and the effects of the thermal environment and the passage of time were evaluated. The results are shown in Figure 4 (lower diagram) for each of the following: immediately after charging, after heating at 100°C (30 minutes), and after heating at 200°C (30 minutes).

[0072] In FIG. 4 (lower diagram), the surface potential immediately after charging was approximately 400 V (absolute value), less than half of the surface potential in Example 1. After heating at 100°C, the surface potential was more than 200 V (absolute value), half the surface potential immediately after charging, and after heating at 200°C, the surface potential further decreased and almost disappeared. Thus, it was confirmed that it is difficult to maintain the surface potential in a high-temperature environment when the electret layer 2 formed on the substrate 10 is configured to be made of a single layer of inorganic dielectric film 21.

[0073] Examples 2 to 5 Next, electrets 1 were produced in which the film structure of the electret layer 2 was the same as in Example 1, but the film thickness of the inner layer film 4 was changed. Specifically, a thermal oxide film (SiO2) that would become the inner layer film 4 was formed on the upper surface 11 of a substrate 10 (conductive Si) with a film thickness of 0.1 μm, 0.3 μm, 0.5 μm, or 1.0 μm, respectively. An amorphous film (LAO with 1% Ca added) that would become the outer layer film 3 was then formed by sputtering to form an inorganic dielectric film 20. Thereafter, each was subjected to a charging process in the same manner as in Example 1, to produce electrets 1 of Examples 2 to 5. The thickness of the substrate 10 was 625 μm, and the film thickness of the outer layer film 3 was 1.3 μm.

[0074] The surface potential of the electrets 1 of Examples 2 to 5 was measured in the same manner as in Example 1. At this time, a plurality of samples (number of samples n=2) were prepared for each Example, and after a charging treatment, they were heated at 200°C for 30 minutes, and the surface potential after the heat treatment was measured. In addition, the surface potential of the electret 1 of Example 1 was also measured in the same manner after heating at 200°C for 30 minutes, and these results are shown in comparison in Figure 5.

[0075] In Figure 5, the horizontal axis represents the thickness of the inner layer film 4 (SiO2), and the results for Comparative Example 1, which does not have an inner layer film 4, are also shown. That is, when the thickness of the inner layer film 4 is 0 μm, the surface potential is 0 V. In contrast, in Examples 1 to 5, which have an inner layer film 4, a surface potential (absolute value) of approximately 140 V was obtained at 0.1 μm (Example 2), and the surface potential increased in proportion to the film thickness. Specifically, a high surface potential of approximately 400 V was obtained at 0.3 μm (Example 3), approximately 600 V at 0.5 μm (Example 4), and approximately 1000 V at 1.0 μm (Example 5).

[0076] FIG. 6 shows the change in surface potential of each sample of electret 1 of Examples 1 to 5 when it was left at room temperature after the heat treatment. At this time, the surface potential of the sample after the heat treatment was taken as the surface potential at the start of the test (elapsed time: 0), and the change was examined until a predetermined time had elapsed (for example, about 5 to 10 minutes). As shown in FIG. 6, almost no change in surface potential was observed over time in any of Examples 1 to 5.

[0077] 5 and 6, when the electret layer 2 has a film configuration including an outer layer film 3 and an inner layer film 4 (Examples 1 to 5), the surface potential is maintained even after heat treatment at 200°C, and no disappearance of the surface potential was observed as in Comparative Example 1. In this case, when the film thickness of the inner layer film 4 was in the range of 0.1 μm (Example 2) to 1.2 μm (Example 1), the thicker the film thickness of the inner layer film 4, the higher the surface potential, and no change in the surface potential was observed over time.

[0078] (Examples 6 to 7) Next, an electret 1 was produced in which the film thickness of the outer layer film 3 was changed in the configuration of the electret layer 2 of Example 2. Specifically, a thermal oxide film (SiO2) that becomes the inner layer film 4 was formed on the upper surface 11 of the substrate 10 (conductive Si) with a film thickness of 0.1 μm, and then an amorphous film (LAO with 1% Ca added) that becomes the outer layer film 3 was formed with a film thickness of about 0.4 μm by sputtering to form an inorganic dielectric film 20. Thereafter, a charging process was performed to obtain the electret 1 of Example 6. In addition, an inorganic dielectric film 20 was formed in the same manner except that the film thickness of the outer layer film 3 was formed with a film thickness of about 0.9 μm, and a charging process was performed to obtain the electret 1 of Example 7.

[0079] For the electrets 1 of Examples 6 and 7, after the charging treatment, they were heated at 200°C for 30 minutes, and then the surface potential was measured in the same manner as in Example 1.These results are shown in Figure 7 for comparison with the results of Example 2.

[0080] 7, the surface potential (absolute value) of Examples 6 and 7 was around 140 V, which was equivalent to the result of Example 2. As such, the magnitude of the surface potential does not depend on the film thickness of the outer layer film 3, and similar results are obtained if the same first inorganic dielectric material is used to form the outer layer film 3. Furthermore, as shown in Examples 1 to 5 in FIGS. 5 and 6, the magnitude of the surface potential has a positive correlation with the film thickness of the inner layer film 4, and stable results are obtained thermally and over time at any film thickness.

[0081] 8 shows the results of examining the change in surface potential over time for the electret 1 of Example 5 after further storage for a longer period of time. Specifically, multiple samples (number of samples n = 3) with the same configuration were prepared, and for each, the surface potential was measured after charging treatment and heating at 200 ° C for 30 minutes, and furthermore, the surface potential was measured after storing in the atmosphere for a predetermined time. The surface potential of each sample was measured at the same timing for up to about 2700 minutes.

[0082] 8, for all samples of Example 5, the surface potential (absolute value) immediately after the heat treatment was around 1000 V. Thereafter, the surface potential gradually decreased with time (for example, up to more than 500 minutes), but the decrease was slight, and thereafter almost no decrease in the surface potential was observed.

[0083] These results show that the electret 1 exhibits a surface potential that corresponds to the combination of the outer layer film 3 and the inner layer film 4 that constitute the electret layer 2 and the film thickness of the inner layer film 4, suppresses the effects of the temperature environment and long-term storage, and exhibits stable properties thermally and over time.

[0084] Examples 9 to 10 Next, an electret 1 was produced in which the second inorganic dielectric material constituting the inner layer film 4 was changed in the configuration of the electret layer 2 of Example 1, and its thermal and temporal stability was examined. Specifically, a thin film made of SiN was formed as the inner layer film 4 on the upper surface 11 of the substrate 10 (conductive Si) by plasma CVD. The film formation by plasma CVD method used SiH4 and NH3 as raw material gases, and the substrate temperature was set at 300°C, forming a CVD thin film made of SiN with a film thickness of 0.3 μm.

[0085] On the upper surface of the inner layer film 4, an amorphous film (1% Ca added LAO) which becomes the outer layer film 3 was further formed by a sputtering method to a film thickness of 1.3 μm to form an inorganic dielectric film 20. Thereafter, a charging treatment was carried out in the same manner as in Example 1 to obtain the electret 1 of Example 9. Moreover, the electret 1 of Example 10 was produced in the same manner except that a CVD thin film made of SiN with a thickness of 0.5 μm was formed as the inner layer film 4.

[0086] The surface potential of the electrets 1 of Examples 9 and 10 was measured in the same manner as in Example 1. At this time, a plurality of samples (number of samples n=2) having the same configuration were prepared, and the surface potential of each was measured after being charged and heated at 200°C for 30 minutes. These results are shown in Figure 9.

[0087] 9, the relationship between the thickness of the inner layer film 4 and the surface potential is similar to that shown in Fig. 5, and as the thickness of the inner layer film 4 increases from 0.3 µm (Example 9) to 0.5 µm (Example 10), the surface potential (absolute value) tends to increase from approximately 125 V to over 210 V. Thus, it was confirmed that, regardless of the type of second inorganic dielectric material that forms the inner layer film 4, thermal stability is improved and a stable surface potential is expressed that has a positive correlation with the thickness of the inner layer film 4.

[0088] 10 shows the change in surface potential of each sample of electret 1 of Examples 9 to 10 when left at room temperature after heat treatment. At this time, the surface potential of the sample after heat treatment was set as the surface potential at the start of the test (elapsed time: 0), and the change was examined until a predetermined time had passed (for example, about 5 to 10 minutes). In FIG. 10, almost no change in surface potential over time was observed in any of Examples 9 to 10.

[0089] Example 11 Next, the electret 1 having the configuration of the second embodiment shown in Fig. 3 was produced by the following method: As the first inorganic dielectric material, a second composite oxide having a garnet-type composition was used.

[0090] First, in the same manner as in Example 1, the upper surface 11 of the substrate 10 (conductive Si) was thermally oxidized to form a thermally oxidized film (SiO2) of the second inorganic dielectric material, which served as the inner layer film 4. The thickness of the substrate 10 was 625 μm, and the film thickness of the inner layer film 4 was 1.2 μm. Next, an outer layer film 3 made of the first inorganic dielectric material was formed on the upper surface of the inner layer film 4 by a sputtering method. The sputtering conditions were 360°C in an Ar atmosphere, and a crystal having the same composition as the second composite oxide serving as the first inorganic dielectric material was used as a target to form an amorphous film serving as the outer layer film 3 with a film thickness of 0.6 μm, which served as the inorganic dielectric film 20. Thereafter, a charging process was performed to obtain the electret 1 of Example 11.

[0091] The second complex oxide is a garnet-type oxide with the formula A3B5O 12YAlO contains Y and Al as metal elements A and B, and a part of Y is replaced with dopant element D. 12 Here, Mg was used as the dopant element D, and the substitution ratio was 4 atm% (hereinafter referred to as Mg4%-doped YAG). In addition, a polycrystalline YAG-based composite oxide containing 4 atm% Mg [(Y 2.88 Mg 0.12 AlO x In this way, an amorphous film containing Mg, Y, Al, and O in a ratio of Y:Al:O of approximately 3:5:12 was obtained.

[0092] The YAG composite oxide has a typical composition of Y3Al5O 12 The band gap energy of YAG-based composite oxide is approximately 5 eV, and even in a configuration in which part of the Y is replaced with Mg, which is the dopant element D, the band gap energy is approximately the same. Furthermore, the relative dielectric constant of the YAG-based composite oxide (first inorganic dielectric material) is approximately 12, which is larger than that of SiO2 (second inorganic dielectric material), which has a relative dielectric constant of 3.8.

[0093] The surface potential of the electret 1 of Example 11 was measured in the same manner as in Example 1. At this time, a plurality of samples (number of samples n=3) having the same configuration were prepared, and after each was charged, they were heated at 200°C for 30 minutes, and the surface potential after the heat treatment was measured. As a result, a high surface potential (absolute value) of about 900V to 1200V was obtained.

[0094] (Examples 12 to 14) Furthermore, for the electret 1 of Example 11, the film thickness of the inner layer film 4 was changed to 0.3 μm, 0.5 μm, or 1.0 μm, and otherwise the inorganic dielectric film 20 was formed in the same manner as above, and a charging treatment was performed to produce the electrets 1 of Examples 12 to 14. For the samples of the electrets 1 of Examples 12 to 14 (sample number n = 1 to 3), after the charging treatment, the samples were heated at 200 ° C. for 30 minutes in the same manner as in Example 1, and then the surface potential was measured. These results, along with the results of Example 11 and Comparative Example 1, are shown in FIG. 11.

[0095] 11, the surface potentials (absolute values) of Examples 12 to 14 are approximately 400 V, 600 V, and 800 V, respectively, and the thermal stability is improved compared to Comparative Example 1. Furthermore, the results of Examples 11 to 14 show that the magnitude of the surface potential has a positive correlation with the film thickness of the inner layer film 4, and that the same tendency as in FIG. 5 is observed when the second composite oxide is used as the first inorganic dielectric material.

[0096] (Examples 15 to 16) Next, an electret 1 was produced in which the second inorganic dielectric material constituting the inner layer film 4 was changed in the configuration of the electret layer 2 of Example 11, and its thermal and temporal stability was examined. Specifically, a thin film made of SiN was formed as the inner layer film 4 on the upper surface 11 of the substrate 10 (conductive Si) by plasma CVD. The film formation by plasma CVD method used SiH4 and NH3 as raw material gases, and the substrate temperature was set at 300°C, forming a CVD thin film made of SiN with a film thickness of 0.3 μm.

[0097] On the upper surface of the inner layer film 4, an amorphous film (YAG with 4% Mg added) that would become the outer layer film 3 was further formed to a thickness of 0.6 μm by a sputtering method to form an inorganic dielectric film 20. Thereafter, a charging treatment was carried out in the same manner as in Example 11 to obtain the electret 1 of Example 15. Moreover, the electret 1 of Example 16 was produced in the same manner except that a CVD thin film made of SiN with a thickness of 0.5 μm was formed as the inner layer film 4.

[0098] The surface potential of the electrets 1 of Examples 15 and 16 was measured in the same manner as in Example 1. At this time, a plurality of samples (number of samples n=2) having the same configuration were prepared, and the surface potential of each was measured after being charged and heated at 200°C for 30 minutes. These results are shown in Figure 12.

[0099] 12, the relationship between the thickness of the inner layer film 4 and the surface potential is similar to that shown in Fig. 9, and as the thickness of the inner layer film 4 increases from 0.3 µm (Example 15) to 0.5 µm (Example 16), the surface potential (absolute value) tends to increase from approximately 120 V to over 200 V. Thus, it was confirmed that, regardless of the type of second inorganic dielectric material that forms the inner layer film 4, thermal stability is improved and a stable surface potential is expressed that is positively correlated with the thickness of the inner layer film 4.

[0100] 13 shows the change in surface potential when the samples of electret 1 of Examples 15 and 16 were left at room temperature after the heat treatment. At this time, the surface potential of the sample after the heat treatment was set as the surface potential at the start of the test (elapsed time: 0), and the change was examined until a predetermined time had passed (for example, about 5 to 10 minutes). In FIG. 13, almost no change in surface potential over time was observed in any of Examples 15 and 16.

[0101] (Examples 17 to 18) Next, an electret 1 was produced in which the configuration of the inner layer film 4 was changed in the electret layer 2 of Example 1, and its thermal and temporal stability was examined. The electret layer 2 has the configuration of embodiment 1 shown in Figure 2 (lower diagram), and the inner layer film 4 is configured as a multi-layer film having a first layer 4a and a second layer 4b.

[0102] Specifically, a thermal oxide film (SiO2) with a thickness of 1.2 μm was formed on the upper surface 11 of the substrate 10 (conductive Si) by thermal oxidation, to form the first layer 4a of the inner layer film 4. Next, a CVD thin film made of SiN with a thickness of 0.3 μm was formed by plasma CVD to form the second layer 4b. On the upper surface thereof, an amorphous film (LAO with 1% Ca added) with a thickness of 1.3 μm, which would become the outer layer film 3, was further formed by sputtering to form the inorganic dielectric film 20. Thereafter, a charging process was performed to obtain the electret 1 of Example 17. Furthermore, the electret 1 of Example 18 was produced in the same manner, except that a CVD thin film made of SiN with a thickness of 0.5 μm, was used as the second layer 4b of the inner layer film 4.

[0103] The surface potential of each of the electrets 1 of Examples 17 and 18 was measured in the same manner as in Example 1. Specifically, after the charging treatment, the electrets were heated at 200°C for 30 minutes, and then stored in the atmosphere. The surface potential was measured periodically, and the changes over a period of more than 6 minutes were investigated. The results are shown in Figure 14. Figure 14 also shows the results for the electret 1 of Example 1 in Figure 6.

[0104] 14, the electret 1 of Example 17 had a surface potential of about 800 V (absolute value), and the electret 1 of Example 18 had a surface potential of about 900 V (absolute value). In addition, none of the electrets 1 of Examples 17 to 18 showed a decrease in surface potential over time, and the surface potential remained almost constant even after more than 6 minutes had passed, showing a similar tendency to the electret 1 of Example 1.

[0105] In this way, even when the inner layer film 4 is a multi-layer film combining a plurality of second inorganic dielectric materials, by further laminating an outer layer film 3 made of a first inorganic dielectric material to form the electret layer 2, it is possible to improve the thermal and temporal stability of the electret 1. In addition, the surface potential of the electret 1 is determined depending on the combination of the first and second inorganic dielectric materials that make up the inner layer film 4 and the outer layer film 3, and the film thickness of the inner layer film 4, and it can be seen that if the layer structure of the electret layer 2 is the same, the thicker the film thickness of the inner layer film 4, the higher the surface potential.

[0106] (Comparative Examples 2 to 3) For comparison, as shown in FIG. 15, an electret 1 was fabricated in which the inorganic dielectric film 22 serving as the electret layer 2 was a multilayer film made only of the second inorganic dielectric material. That is, the inorganic dielectric film 22 did not have an outer layer 3 made of the first inorganic dielectric material, and had a first layer 22a in contact with the substrate 10 and a second layer 22b on the outside thereof. Specifically, a thermally oxidized film (SiO2) with a thickness of 1.2 μm was formed on the upper surface 11 of the substrate 10 (conductive Si) to form the first layer 22a, and a CVD thin film made of SiN with a thickness of 0.3 μm was formed on the upper surface of the first layer 22a by a plasma CVD method to form the second layer 22b, thereby forming the inorganic dielectric film 22. Thereafter, a charging process was performed to obtain the electret 1 of Comparative Example 2. In addition, an inorganic dielectric film 22 was formed in the same manner except that the second layer 22b was formed to a thickness of 0.5 μm, and a charging process was performed to obtain the electret 1 of Comparative Example 3.

[0107] The electrets 1 of Comparative Examples 2 and 3 were charged and heated at 200°C for 30 minutes using the same method as in Example 1, and then the surface potential was measured. Furthermore, the change in surface potential when left at room temperature (for example, for more than 5 to 10 minutes) was examined. The results are shown in FIG. 16. In FIG. 16, the electrets 1 of Comparative Examples 2 and 3 exhibit a high surface potential of approximately 800V (Comparative Example 2) to 900V (Comparative Example 3) in absolute value until approximately 1 to 2 minutes have elapsed since the charging process, but a gradual decrease is observed, and the surface potential continues to decrease gradually thereafter. Therefore, in Comparative Example 2, the surface potential decreases to approximately 700V (absolute value) after approximately 6 minutes have elapsed, and in Comparative Example 3, the surface potential reaches approximately 600V (absolute value) after approximately 10 minutes have elapsed.

[0108] 17, when the measurement of the surface potential of the electret 1 of Comparative Example 3 was continued, the surface potential decreased to about 200 V (absolute value) after about 20 minutes. After that, although the rate of decrease became slower, it was found that the surface potential continued to decrease until it reached almost 0 after about 80 minutes.

[0109] Thus, even if the electret 1 has a structure in which multiple different inorganic dielectric materials are laminated, it is not possible to suppress the decrease in surface potential in the electret 1 that does not have an outer layer film 3 made of the first inorganic dielectric material, and it is difficult to ensure the stability of the surface potential over time.

[0110] 18 and 19 show the results for the electrets 1 of Examples 17 and 18, along with the results for the electrets 1 of Comparative Examples 2 and 3, respectively. FIG. 18 compares the results of Example 17 with those of Comparative Example 2 in FIG. 15, and shows that the electret 1 of Example 17 has a surface potential that is almost constant at about 800 V (absolute value), while the electret 1 of Comparative Example 2 decreases in surface potential over time, resulting in a larger difference from the surface potential of Example 17. The same is true for the comparison results in FIG. 19, where the electret 1 of Example 18 has a surface potential that is almost constant at about 900 V (absolute value), while the electret 1 of Comparative Example 3 decreases in surface potential over time, resulting in a larger difference from the surface potential of Example 18.

[0111] Thus, the electret 1 has an electret layer 2 on a substrate 10 that includes an outer layer 3 made of a first inorganic dielectric material and an inner layer 4 made of a second inorganic dielectric material, thereby improving thermal stability and exhibiting a high surface potential in high-temperature environments of 200°C or higher. It also has improved stability over time, making it possible to maintain a stable surface potential for a long period of time. Such an electret 1 can also be applied to processes that include high-temperature processes such as solder reflow, such as the manufacturing process of a power generation device, and this increases the flexibility of process design, contributing to reduced manufacturing costs. Furthermore, not only is performance degradation during the manufacturing process suppressed, but stable performance can also be maintained even in harsh temperature environments during subsequent use.

[0112] In the above embodiments, the outer layer film 3 of the electret layer 2 is mainly an amorphous film, but when an oxide crystal film containing a complex oxide with a crystalline structure is used, it may be a polycrystalline film or a mixed film containing complex oxide particles in a heat-resistant base film. Also, the electret layer 2 may have a multi-layer structure in which the outer layer film 3 has two or more layers, for example.

[0113] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments within the scope of the present invention. The features of the present invention are as follows. [1] An electret (1) having a substrate (10) and an electret layer (2) formed on its surface (11), The electret layer is an inorganic dielectric film (20) including an outer layer film (3) and an inner layer film (4) laminated in the thickness direction (X) of the substrate, and is subjected to a charging treatment; the outer layer film is a composite metal compound containing two or more different metal elements, and is a film containing as a main component a first inorganic dielectric material containing at least a trivalent metal element and having a band gap energy of 3 eV or more; The inner layer film is an electret film mainly composed of a second inorganic dielectric material different from the first inorganic dielectric material. [2] The first inorganic dielectric material is a material having a basic composition of a complex oxide containing two different metal elements A and B, wherein the metal element A is a divalent or trivalent metal element, and the metal element B is a trivalent metal element. [1] The electret. [3] The composite oxide is a first composite oxide represented by the composition formula ABO3, a second composite oxide represented by the composition formula ABO 12 or a third composite oxide represented by a composition formula AB2O4. [4] In the first composite oxide and the second composite oxide, the metal element A is at least one element selected from rare earth elements, and in the third composite oxide, the metal element A is at least one element selected from alkaline earth metal elements and transition metal elements. [3] The electret according to. [5] The electret according to [4], wherein in the first composite oxide, the second composite oxide, and the third composite oxide, the metal element B is Al. [6] The electret according to any one of [1] to [5], wherein the first inorganic dielectric material has a higher relative dielectric constant than the second inorganic dielectric material. [7] The electret according to [6], wherein the second inorganic dielectric material is one or a mixture of two or more compounds selected from a Si compound and an Al compound. [8] The electret according to any one of [1] to [7], wherein the magnitude of the surface potential of the electret layer has a positive correlation with the film thickness of the inner layer film. [9] The electret according to [8], wherein the thickness of the inner layer film is 0.1 μm or more.

[10] The electret according to any one of [1] to [9], wherein the inner layer film is a film of a single layer or a multi-layer structure, and the outer layer film is a film of the composite metal compound having an amorphous structure or a film of the composite metal compound having a crystalline structure. [Explanation of symbols]

[0114] 1 electret 10 Substrate 2 electret layer 20 Inorganic dielectric film 3 Outer membrane 4. Inner membrane

Claims

1. An electret (1) having a substrate (10) and an electret layer (2) formed on the surface (11) of the substrate, The electret layer is an inorganic dielectric film (20) including an outer layer film (3) and an inner layer film (4) laminated in the thickness direction (X) of the substrate, and is subjected to a charging treatment; the outer layer film is a composite metal compound containing two or more different metal elements, and is a film containing as a main component a first inorganic dielectric material containing at least a trivalent metal element and having a band gap energy of 3 eV or more; An electret, wherein the inner layer film is a film mainly composed of a second inorganic dielectric material different from the first inorganic dielectric material.

2. The first inorganic dielectric material has a basic composition of a complex oxide containing two different metal elements A and B, wherein the metal element A is a divalent or trivalent metal element, and the metal element B is a trivalent metal element. The electret according to claim 1.

3. The composite oxide has the composition formula ABO 3 The first composite oxide represented by composition formula A 3 B 5 O 12 A second composite oxide represented by the composition formula AB 2 O 4 The electret according to claim 2, wherein the third composite oxide is represented by the formula:

4. In the first composite oxide and the second composite oxide, the metal element A is at least one element selected from rare earth elements, and in the third composite oxide, the metal element A is at least one element selected from alkaline earth metal elements and transition metal elements. The electret according to claim 3.

5. 5. The electret according to claim 4, wherein in the first composite oxide, the second composite oxide, and the third composite oxide, the metal element B is Al.

6. The electret according to claim 1 , wherein the relative dielectric constant of the first inorganic dielectric material is greater than the relative dielectric constant of the second inorganic dielectric material.

7. 7. The electret according to claim 6, wherein the second inorganic dielectric material is one or a mixture of two or more compounds selected from a Si compound and an Al compound.

8. 2. The electret according to claim 1, wherein the magnitude of the surface potential of the electret layer has a positive correlation with the film thickness of the inner layer film.

9. The electret according to claim 8, wherein the inner layer film has a thickness of 0.1 μm or more.

10. 2. The electret according to claim 1, wherein the inner layer film is a film having a single layer or a multi-layer structure, and the outer layer film is a film of the composite metal compound having an amorphous structure or a film of the composite metal compound having a crystalline structure.

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