Method for manufacturing electret film

Abstract

To provide a method for manufacturing an electret film capable of stably producing an electret film that is difficult to release charge and has a high charge.SOLUTION: A method for manufacturing an electret film includes: a supply process for supplying alkaline ion solution to one side of the silicon substrate to dry; an oxide film formation process for forming an alkali-doped oxide film containing the components of the alkali ion solution on one side of the silicon substrate by forming a silicon oxide film on one side of the silicon substrate by a vapor phase method and by heat treating the silicon substrate on which the silicon oxide film is formed; and a charging process for forming an electret film by applying a voltage to the silicon substrate and charging the alkali-doped oxide film.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for producing an electret film. [Background technology]

[0002] Patent Document 1 describes a method for manufacturing an electret film made of a silicon oxide film. This manufacturing method includes a step of forming irregularities, i.e., grooves and protrusions, on a silicon substrate, a thermal oxidation step of heating the silicon substrate to turn all of the protrusions into a silicon oxide film, a charging step of charging the silicon oxide film, and a step of removing the silicon oxide film in the thickness direction to partially expose the silicon substrate. In particular, in the thermal oxidation step, the silicon substrate is thermally oxidized in an atmosphere containing alkali ions to form a silicon oxide film containing potassium ions. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-152491 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the above-mentioned method for producing an electret film, a large amount of charge is contained near the surface of the silicon oxide film. Therefore, the charge is released from the surface of the silicon oxide film over time. Therefore, it is not possible to stably produce an electret film having a high charge. [Means for solving the problem]

[0005] The method for producing an electret film of the present invention includes a supplying step of supplying an alkali ion solution to one surface side of a silicon substrate and drying the solution; an oxide film forming step of forming a silicon oxide film on the one surface side of the silicon substrate by a vapor phase method, and heat-treating the silicon substrate on which the silicon oxide film has been formed, thereby forming an alkali-added oxide film containing a component of the alkali ion solution on the one surface side of the silicon substrate; and a charging step of applying a voltage to the silicon substrate to charge the alkali-added oxide film to form an electret film. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a perspective view showing a power generation device according to a first embodiment. [Diagram 2] 1 is a flowchart showing a method for manufacturing an electret film of a power generating device. [Diagram 3] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 4] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Diagram 5] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 6] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 7] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 8] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 9] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 10] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 11] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 12] FIG. 11 is a top view showing a power generation device according to a second embodiment. [Figure 13] FIG. 13 is an exploded perspective view of the power generation device of FIG. [Figure 14] 13 is a cross-sectional view taken along line AA in FIG. 12. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Hereinafter, the method for producing an electret film of the present invention will be described in detail based on the embodiments shown in the accompanying drawings.

[0008] First Embodiment Fig. 1 is a perspective view showing a power generating device according to a first embodiment. Fig. 2 is a flowchart showing a method for manufacturing an electret film of the power generating device. Figs. 3 to 11 are cross-sectional views for explaining the method for manufacturing an electret film. In the following, for convenience of explanation, the upper side in Fig. 1 is also referred to as the upper side, and the lower side is also referred to as the lower side.

[0009] 1 includes a first substrate 11, a plurality of electret films 12 arranged on the first substrate 11, a second substrate 13 arranged opposite the first substrate 11, a plurality of electrodes 14 arranged on the second substrate 13, and a rotating shaft 15 coaxially connecting the first substrate 11 and the second substrate 13. Note that in FIG. 1, for ease of explanation, the first substrate 11 and the second substrate 13 are shown with a large gap G therebetween, but in reality, they are arranged as close as possible to enhance power generation efficiency.

[0010] The first substrate 11 is made of a silicon substrate and has a disk shape. In addition, the first substrate 11 has a plurality of through holes 119 arranged in the circumferential direction. A plurality of electret films 12 are arranged on the surface of the first substrate 11 facing the second substrate 13. The electret films 12 are arranged between pairs of adjacent through holes 119, and the through holes 119 and the electret films 12 are arranged alternately. The electret films 12 are silicon oxide films and hold negative charges. A method for manufacturing the electret films 12 will be described in detail later.

[0011] The second substrate 13 is located above the first substrate 11. The second substrate 13 is made of a conductor such as a metal, and has approximately the same shape and size as the first substrate 11. The second substrate 13 also has through holes 139 aligned in the circumferential direction, the through holes 139 having the same number and shape as the through holes 119. The region between a pair of adjacent through holes 139 functions as an electrode 14 facing the electret film 12.

[0012] The rotating shaft 15 passes through the center of the first substrate 11 and the center of the second substrate 13, and supports them concentrically. The second substrate 13 is supported rotatably about the rotating shaft 15. On the other hand, although not shown, the first substrate 11 is fixed to a portable electrical device to which the power generation device 10 is attached. Although not shown, a weight for rotating the second substrate 13 about the rotating shaft 15 is connected to the second substrate 13.

[0013] In the power generation device 10 configured as described above, the second substrate 13 rotates in accordance with the movement of the portable electrical device itself. This rotation of the second substrate 13 causes each electrode 14 to move relative to the electret film 12 so that the overlapping area changes. When the overlapping area increases, the electrode 14 is charged with a charge of the opposite polarity to the electret film 12, i.e., a positive charge, due to electrostatic induction. Conversely, when the overlapping area decreases, the charge stored in the electrode 14 is discharged. Therefore, the rotation of the second substrate 13 causes each electrode 14 to repeatedly charge and discharge.

[0014] In this way, repeated charging and discharging of electric charge at each electrode 14 causes a current to flow from the power generation device 10 to the rectifier circuit side. This current is rectified by the rectifier circuit and input to the secondary battery. As a result, the power generated by the power generation device 10 is stored in the secondary battery. The power stored in the secondary battery is supplied to a load as needed. The secondary battery is a rechargeable battery such as a lithium secondary battery. The load is a circuit for realizing the functions of the portable electrical device, and operates by consuming power supplied from the secondary battery. For example, if the portable electrical device is a wristwatch, the load includes a timing circuit and the like, and performs various controls such as keeping track of the current time.

[0015] The power generation device 10 has been described above. However, the power generation device 10 is not particularly limited. For example, in the illustrated configuration, the second substrate 13 is disposed only on the upper side of the first substrate 11, but the present invention is not limited to this, and the second substrates 13 may be disposed on both the upper and lower sides of the first substrate 11. In this case, the electret film 12 may be formed on the lower surface of the first substrate 11 in the same manner.

[0016] Next, a method for manufacturing the electret film 12 will be described. As shown in Fig. 2, the method for manufacturing the electret film 12 includes a supplying step S1 of supplying an alkali ion solution L to the upper surface 110a side of the silicon substrate 110, which is the base material of the first substrate 11, and drying the supplying step S1, an oxide film forming step S2 of forming a silicon oxide film 121 by a gas phase method on the upper surface side of the silicon substrate 110, and forming an alkali-added oxide film 122 containing the components of the alkali ion solution L on the upper surface side of the silicon substrate 110 by heat-treating the silicon substrate 110 on which the silicon oxide film 121 is formed, a patterning step S3 of patterning the silicon substrate 110, and a charging step S4 of applying a voltage to the silicon substrate 110 to charge the alkali-added oxide film 122 to form the electret film 12.

[0017] ≪Supply process S1≫ 3, first, a silicon substrate 110 is prepared as a base material for the first substrate 11. The silicon substrate 110 has an upper surface 110a as one surface.

[0018] 4, an alkaline ion solution L is applied to the upper surface 110a of the silicon substrate 110. The alkaline ion solution L is not particularly limited, and may be, for example, a KOH (potassium hydroxide) aqueous solution, a NaOH (sodium hydroxide) aqueous solution, or the like. The method of applying the alkaline ion solution L to the upper surface 110a is not limited to application.

[0019] Next, as shown in FIG. 5, the alkaline ion solution L supplied to the upper surface 110a of the silicon substrate 110 is dried. This causes the alkaline ions to settle on the upper surface 110a. In the figure, the alkaline ions are referred to as "K + The drying method is not particularly limited, and may be natural drying at room temperature or heated drying using a heater or the like.

[0020] ≪Oxide film formation step S2≫ In the oxide film forming step S2, first, as shown in Fig. 6, a silicon oxide film 121 is formed on the upper surface 110a side of the silicon substrate 110 by plasma CVD (chemical vapor deposition) using TEOS (tetra ethoxy silane) as a source gas. Alternatively, the silicon oxide film 121 may be formed by plasma CVD using SiH4 as a source gas. According to these methods, the silicon oxide film 121 can be easily formed.

[0021] Moreover, by forming the silicon oxide film 121 by using plasma CVD using TEOS or SiH4, the surface of the silicon oxide film 121 can be made to have good flatness. That is, the surface of the silicon oxide film 121 becomes flatter. Therefore, in the above-mentioned power generating device 10, the gap G between the first substrate 11 and the second substrate 13 can be reduced while suppressing contact between the first substrate 11 and the second substrate 13, that is, a short circuit between the electret film 12 and the electrode 14. Therefore, the power generation amount of the power generating device 10 can be increased. However, the method of forming the silicon oxide film 121 is not limited to this, and a gas phase method other than CVD, such as deposition or sputtering, may be used.

[0022] The thickness D of the silicon oxide film 121 is not particularly limited, but is preferably, for example, 3 μm or more and 10 μm or less. By setting the thickness D of the silicon oxide film 121 to 3 μm or more, the function of the silicon oxide film 121 can be fully exhibited. This function will be described later. Furthermore, by setting the thickness D of the silicon oxide film 121 to 10 μm or less, excessive thickening of the silicon oxide film 121 can be suppressed, and the time required for forming the silicon oxide film 121 can be shortened.

[0023] Next, the silicon substrate 110 is placed in a thermal oxidation furnace and heated in an oxygen atmosphere. As a result, a thermal oxide film integral with the silicon oxide film 121 is formed on the upper surface 110a side of the silicon substrate 110 as shown in Fig. 7. This thermal oxide film is an alkali-added oxide film 122 that confines the components of the alkali ion solution L, i.e., alkali ions, inside.

[0024] The alkali-added oxide film 122 is formed by thermal oxidation, and therefore is a denser film. The heating conditions are not particularly limited, but it is preferable to heat at a temperature of 900° C. or higher for 30 minutes or more. This allows the formation of a sufficiently thick alkali-added oxide film 122. The alkali ions in the alkali-added oxide film 122 are positively charged, but are paired with the electrons of the silicon atoms. Therefore, the positive charges of the alkali ions and the negative charges of the silicon atoms cancel each other out, making the film electrically neutral. Therefore, in this state, the alkali-added oxide film 122 is electrically neutral and has no surface potential.

[0025] <Patterning process S3> In the patterning step S3, as shown in Fig. 8, the alkali-added oxide film 122 and the silicon substrate 110 are sequentially etched to form the electret film 12 and the outer shape of the first substrate 11. The etching method is not particularly limited, but it is preferable to use dry etching, particularly RIE (reactive ion etching). By using dry etching, through holes with a high aspect ratio can be formed with high precision, enabling finer processing.

[0026] <Charging process S4> In the charging step S4, a voltage is applied to the silicon substrate 110 to charge the alkali-added oxide film 122 to form the electret film 12. For example, as shown in FIG. 9, the silicon substrate 110 is placed on the negative electrode plate E1, and the positive electrode terminal E2 is brought into contact with the surface of the alkali-added oxide film 122, and a voltage is applied between the negative electrode plate E1 and the positive electrode terminal E2. Then, as shown in FIG. 10, the positively charged alkali ions are attracted to the negative electrode plate E1, which becomes the negative electrode, and combine with electrons to lose the positive charge and become neutral. Then, as shown in FIG. 11, only the electrons that formed pairs with the alkali ions remain inside the alkali-added oxide film 122 as a result of losing the alkali ions. Since the electrons have a negative charge, the alkali-added oxide film 122 becomes the electret film 12 that is negatively charged and has a surface potential.

[0027] In the manufacturing method of the electret film 12, as described above, after alkali ions are fixed on the upper surface 110a of the silicon substrate 110, a silicon oxide film 121 is formed on the upper surface 110a of the silicon substrate 110, and then an alkali-added oxide film 122 is formed. Therefore, as shown in FIG. 11, electrons can be confined in a region sufficiently distant from the surface of the electret film 12. Therefore, it is possible to effectively prevent electrons from being released from the surface of the electret film 12 over time. As a result, it is possible to stably manufacture an electret film 12 that can maintain a high charge over time.

[0028] In particular, as described above, when the silicon oxide film 121 is formed by plasma CVD using TEOS as a raw material gas, organic components contained in TEOS are incorporated near the surface of the electret film 12 (on the surface side of the electrons). Then, this organic component can effectively prevent the emission of electrons from the surface of the silicon oxide film 121. Therefore, the above-mentioned effect becomes more prominent. Also, as described above, by setting the thickness D of the silicon oxide film 121 to 3 μm or more, the above-mentioned effect becomes more prominent.

[0029] The manufacturing method of the electret film 12 has been described above. As described above, the manufacturing method of the electret film 12 includes a supplying step S1 of supplying an alkali ion solution L to the upper surface 110a side, which is one surface of the silicon substrate 110, and drying the supplying step S1, an oxide film forming step S2 of forming an alkali-added oxide film 122 containing a component of the alkali ion solution on the upper surface 110a side of the silicon substrate 110 by forming a silicon oxide film 121 on the upper surface 110a side of the silicon substrate 110 by a gas phase method, and heating the silicon substrate 110 on which the silicon oxide film 121 is formed, and a charging step S4 of applying a voltage to the silicon substrate 110 to charge the alkali-added oxide film 122 to form the electret film 12. According to this manufacturing method, electrons can be confined in a region sufficiently away from the surface of the electret film 12. Therefore, electrons are less likely to be arranged near the surface of the electret film 12, and it is possible to effectively suppress the electrons from being released from the surface of the electret film 12 over time. As a result, it is possible to stably produce the electret film 12 capable of maintaining a high charge over time.

[0030] As described above, the vapor phase method is plasma CVD using TEOS as a raw material gas. By using this method, the silicon oxide film 121 can be easily formed.

[0031] As described above, the gas phase method is plasma CVD using SiH4 as a raw material gas. By using this method, the silicon oxide film 121 can be easily formed.

[0032] As described above, the thickness of the silicon oxide film 121 is 3 μm or more. This makes it more difficult for electrons to be arranged near the surface of the electret film 12. Therefore, it is possible to more effectively prevent electrons from being emitted from the surface of the electret film 12 over time.

[0033] As described above, the thickness of the silicon oxide film 121 is 10 μm or less, which can prevent the silicon oxide film 121 from becoming excessively thick, and can also shorten the time required to form the silicon oxide film 121.

[0034] As described above, in the heat treatment in the oxide film forming step S2, the silicon substrate 110 is heated to a temperature of 900° C. or higher for 30 minutes or longer. This allows the alkali-added oxide film 122 to be formed with a sufficient thickness.

[0035] <Second embodiment> Fig. 12 is a top view showing a power generation device according to a second embodiment, Fig. 13 is an exploded perspective view of the power generation device of Fig. 12, and Fig. 14 is a cross-sectional view taken along line AA in Fig. 12.

[0036] For ease of explanation, the three mutually perpendicular axes are referred to as the X-axis, Y-axis, and Z-axis below. The direction along the X-axis is also referred to as the X-axis direction, the direction along the Y-axis is also referred to as the Y-axis direction, and the direction along the Z-axis is also referred to as the Z-axis direction. The arrow side of each axis is also referred to as the plus side, and the opposite side is also referred to as the minus side. The plus side of the Z-axis direction is also referred to as the upper side, and the minus side is also referred to as the lower side.

[0037] The power generating device 100 shown in Figures 12 and 13 is manufactured by bonding a first silicon substrate 2 patterned by a semiconductor process to an SOI (Silicon on Insulator) substrate 3 patterned by a semiconductor process. As shown in Figure 14, the SOI substrate 3 is a substrate in which a silicon oxide layer 3B is inserted between a first silicon layer 3A and a second silicon layer 3C.

[0038] Such a power generation device 100 has a support portion 4, a spring portion 5 connected to the support portion 4, a movable portion 6 connected to the support portion 4 via the spring portion 5, a movable electrode 7 connected to the movable portion 6, and a first fixed electrode 8 and a second fixed electrode 9 connected to the support portion 4.

[0039] 13, the support 4 is formed of a laminate of a first support 41 formed from the first silicon substrate 2 and a second support 42 formed from the first silicon layer 3A of the SOI substrate 3. The first support 41 is divided into a first fixed electrode support 411 to which the first fixed electrode 8 is connected and a second fixed electrode support 412 to which the second fixed electrode 9 is connected, and these are insulated from each other. An electrode pad PAD1 for extracting charges from the first fixed electrode 8 is formed on the first fixed electrode support 411, and an electrode pad PAD2 for extracting charges from the second fixed electrode 9 is formed on the second fixed electrode support 412.

[0040] As shown in FIG. 14, the movable portion 6 and the spring portion 5 are each formed from a first silicon layer 3A of an SOI substrate 3. The movable electrode 7 is formed from a second silicon layer 3C of the SOI substrate 3. The movable electrode 7 has a comb-like shape and includes an electrode finger support portion 71 connected to the movable portion 6 via a silicon oxide layer 3B, and a plurality of movable electrode fingers 72 extending from the electrode finger support portion 71. The plurality of movable electrode fingers 72 include a plurality of first movable electrode fingers 721 extending from the electrode finger support portion 71 to the positive side in the X-axis direction, and a plurality of second movable electrode fingers 722 extending from the electrode finger support portion 71 to the negative side in the X-axis direction. An electret film EL is formed on the first and second movable electrode fingers 721 and 722.

[0041] 12 and 13, first fixed electrode 8 and second fixed electrode 9 are each formed from first silicon substrate 2. First fixed electrode 8 is comb-shaped and has an electrode finger support portion 81 connected to first fixed electrode support portion 411, and a plurality of first fixed electrode fingers 82 as fixed electrode fingers extending from electrode finger support portion 81. In a stationary state, the plurality of first fixed electrode fingers 82 and the plurality of first movable electrode fingers 721 face each other with a predetermined meshing length in the X-axis direction and with a gap in the Y-axis direction.

[0042] The second fixed electrode 9 is formed symmetrically to the first fixed electrode 8. The second fixed electrode 9 is comb-shaped and has an electrode finger support portion 91 connected to the second fixed electrode support portion 412, and a plurality of second fixed electrode fingers 92 as fixed electrode fingers extending from the electrode finger support portion 91. The plurality of second fixed electrode fingers 92 and the plurality of second movable electrode fingers 722 face each other with a predetermined meshing length in the X-axis direction and with a gap in the Y-axis direction in a stationary state.

[0043] The configuration of the power generation device 100 has been briefly described above. In the power generation device 100 configured as above, when a force in the X-axis direction is applied, the movable part 6 vibrates in the X-axis direction while elastically deforming the spring part 5. Then, the vibration of the movable part 6 changes the meshing length between the first and second movable electrode fingers 721, 722 and the first and second fixed electrode fingers 82, 92 in opposite phases, thereby generating power. Specifically, when the movable part 6 is displaced to the positive side in the X-axis direction, the meshing length between the first fixed electrode finger 82 and the first movable electrode finger 721 increases, and a positive charge is charged, and the meshing length between the second fixed electrode finger 92 and the second movable electrode finger 722 decreases, and the charge is discharged. Conversely, when movable part 6 is displaced to the negative side in the X-axis direction, the meshing length of first fixed electrode finger 82 with first movable electrode finger 721 decreases and the stored charge is discharged, and the meshing length of second fixed electrode finger 92 with second movable electrode finger 722 increases and a positive charge is stored in second fixed electrode finger 92. In this way, charging and discharging of charges are repeated in opposite phases between first and second fixed electrode fingers 82, 92.

[0044] The electret film EL of the power generating device 100 can be formed in the same manner as in the first embodiment described above. Specifically, the electret film EL can be formed by supplying an alkali ion solution to the upper surface of the second silicon layer 3C and drying it, forming a silicon oxide film on the upper surface side of the second silicon layer 3C by a gas phase method, heating the second silicon layer 3C to form an alkali-added oxide film containing the components of the alkali ion solution on the upper surface side of the second silicon layer 3C, patterning the second silicon layer 3C into the shape of the movable electrode 7, applying a voltage to the second silicon layer 3C, and charging the alkali-added oxide film.

[0045] The second embodiment as described above can also achieve the same effects as the first embodiment.

[0046] Although the method for producing an electret film of the present invention has been described above based on the illustrated embodiment, the present invention is not limited thereto, and the configuration of each part can be replaced with any configuration or any step having a similar function. In addition, any other configuration or any step may be added to the present invention. [Explanation of symbols]

[0047] 10...power generation device, 100...power generation device, 119...through hole, 11...first substrate, 110...silicon substrate, 110a...upper surface, 12...electret film, 121...silicon oxide film, 122...alkali-added oxide film, 13...second substrate, 139...through hole, 14...electrode, 15...rotating shaft, 2...first silicon substrate, 3...SOI substrate, 3A...first silicon layer, 3B...silicon oxide layer, 3C...second silicon layer, 4...support, 41...first support, 411...support for first fixed electrode, 412...support for second fixed electrode, 42...second support, 5...battery Neck portion, 6...movable portion, 7...movable electrode, 71...electrode finger support portion, 72...movable electrode finger, 721...first movable electrode finger, 722...second movable electrode finger, 8...first fixed electrode, 81...electrode finger support portion, 82...first fixed electrode finger, 9...second fixed electrode, 91...electrode finger support portion, 92...second fixed electrode finger, D...thickness, E1...negative electrode plate, E2...positive electrode terminal, EL...electret film, G...gap, L...alkaline ion solution, PAD1...electrode pad, PAD2...electrode pad, S1...supply step, S2...oxide film formation step, S3...patterning step, S4...charging step

Claims

1. A supplying step of supplying an alkali ion solution to one surface side of the silicon substrate and drying the solution; an oxide film forming step of forming a silicon oxide film on the one surface side of the silicon substrate by a vapor phase method, and heat-treating the silicon substrate on which the silicon oxide film has been formed, thereby forming an alkali-added oxide film containing a component of the alkali ion solution on the one surface side of the silicon substrate; and a charging step of applying a voltage to the silicon substrate to charge the alkali-added oxide film to form an electret film.

2. 2. The method for producing an electret film according to claim 1, wherein the vapor phase method is a plasma CVD method using TEOS as a raw material gas.

3. The gas phase method is SiH ４ 2. The method for producing an electret film according to claim 1, wherein the method is a plasma CVD method using the above as a raw material gas.

4. 4. The method for producing an electret film according to claim 1, wherein the silicon oxide film has a thickness of 3 [mu]m or more.

5. The method for producing an electret film according to claim 4, wherein the silicon oxide film has a thickness of 10 μm or less.

6. The method for producing an electret film according to claim 1 , wherein the heat treatment comprises heating the silicon substrate at a temperature of 900° C. or higher for 30 minutes or longer.

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