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 concavity formation process for forming a concave area on one side of a silicon substrate; a supply process for supplying alkaline ion solution in the concave area; 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 heating the silicon substrate; and a charging process for applying a voltage to the silicon substrate to charge the alkali-doped oxide film to form an electret 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 recess forming step of forming a recess on one surface of a silicon substrate, a supplying step of supplying an alkali ion solution into the recess; an oxide film forming step of heating the silicon substrate to form 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] FIG. 2 is a plan view for explaining a manufacturing method of an electret film. [Figure 6] FIG. 2 is a plan view for explaining a manufacturing method of 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] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 13] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 14] 1A to 1C are cross-sectional views for explaining a method for manufacturing an electret film. [Figure 15]FIG. 11 is a top view showing a power generation device according to a second embodiment. [Figure 16] FIG. 16 is an exploded perspective view of the power generation device of FIG. [Figure 17] 16 is a cross-sectional view taken along line AA in FIG. 15. 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 and 4 are cross-sectional views for explaining the method for manufacturing an electret film. FIGS. 5 and 6 are plan views for explaining the method for manufacturing an electret film. FIGS. 7 to 14 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 recess forming step S1 for forming a recess 111 on the upper surface 110a of the silicon substrate 110, which is the base material of the first substrate 11, a supplying step S2 for supplying an alkali ion solution L into the recess 111, a removing step S3 for removing a part of the alkali ion solution L in the recess 111, an oxide film forming step S4 for heating the silicon substrate 110 to form an alkali-added oxide film 122 containing a component of the alkali ion solution L on the upper surface 110a, and a charging step S5 for applying a voltage to the silicon substrate 110 to charge the alkali-added oxide film 122 to form the electret film 12.

[0017] <Recess formation process S1> In the recess forming step S1, first, as shown in FIG. 3, a silicon substrate 110 that is to be the base material of the first substrate 11 is prepared. The silicon substrate 110 has an upper surface 110a as one surface. Next, the silicon substrate 110 is etched from the upper surface 110a side to form a recess 111 on the upper surface 110a of the silicon substrate 110 as shown in FIG. 4. The etching method is not particularly limited, but it is preferable to use dry etching, particularly RIE (reactive ion etching). By using dry etching, holes with a high aspect ratio can be formed with high precision, so that a finer recess 111 can be formed. However, the method of forming the recess 111 is not particularly limited.

[0018] In this embodiment, four formation regions Q12 for the electret film 12 are set on the silicon substrate 110, and recesses 111 are formed in each formation region Q12. In addition, in each formation region Q12, a plurality of recesses 111 are formed adjacent to each other at a narrow pitch. This allows a sufficient amount of alkaline ion solution L to be uniformly supplied into each formation region Q12 in the subsequent supply step S2. In this embodiment, as shown in FIG. 5, a plurality of arc-shaped recesses 111 are formed concentrically. However, the configuration of the recesses 111 is not particularly limited, and may be, for example, a configuration extending linearly from the center to the outer edge as shown in FIG. 6.

[0019] In this manner, by providing a plurality of formation regions Q12 on the silicon substrate 110, a plurality of electret films 12 can be selectively formed on the silicon substrate 110.

[0020] 4, the width W of each recess 111 is 0.5 to 1.0 μm, and the interval D between a pair of adjacent recesses 111 is also approximately 0.5 to 1.0 μm. This allows a sufficient amount of alkaline ion solution L to be uniformly supplied to each formation region Q12 in the subsequent supplying step S2. However, the width W and the interval D are not particularly limited.

[0021] Moreover, the depth H of each recess 111 is preferably about 70% of the target thickness of the alkali-added oxide film 122 to be formed in the subsequent oxide film forming step S4. Since silicon expands in volume 2.2 to 2.3 times by thermal oxidation, it becomes easier to form the alkali-added oxide film 122 of a desired thickness by setting the depth H of each recess 111 to about 70% of the target thickness of the alkali-added oxide film 122. However, the depth H of each recess 111 is not particularly limited.

[0022] In the recess forming step S1, the silicon substrate 110 is patterned by etching to form the first substrate 11, as shown in Fig. 7. The etching method is not particularly limited, but it is preferable to use dry etching, particularly RIE (reactive ion etching). By using dry etching, holes with a high aspect ratio can be formed with high precision, enabling finer processing.

[0023] ≪Supply process S2≫ 8, in the supplying step S2, an alkaline ion solution L is supplied into each recess 111 of each formation region Q12. The alkaline ion solution L is not particularly limited, and for example, a KOH (potassium hydroxide) aqueous solution, a NaOH (sodium hydroxide) aqueous solution, etc. can be used. The method of supplying the alkaline ion solution L to the upper surface 110a is not particularly limited, and for example, spin coating, spray coating, immersion, etc. can be used.

[0024] ≪Removal step S3≫ In the removal step S3, as shown in FIG. 9, a part of the alkaline ion solution L in the recess 111 is removed. In the figure, about half of the alkaline ion solution L is removed from the recess 111, but the amount to be removed is not particularly limited. In addition, the method for removing the alkaline ion solution L from the recess 111 is not particularly limited. In the present embodiment, for example, an ashing process, particularly a plasma ashing process, is used to remove the resist, which is an organic matter attached to the upper surface 110a of the silicon substrate 110 in the recess formation step S1. When the ashing process is performed on the upper surface 110a of the silicon substrate 110, the organic matter attached to the upper surface 110a is removed, and a part of the alkaline ion solution L in the recess 111 is vaporized and removed. According to such a method, a part of the alkaline ion solution L can be easily removed from the recess 111 without substantially adding a step for the removal step S3.

[0025] In addition, in the removal step S3, before proceeding to the next oxide film formation step S4, the alkali ion solution L in the recesses 111 may be dried as necessary to fix the alkali ions in the recesses 111. The drying method is not particularly limited, and may be natural drying at room temperature or heated drying using a heater or the like. Also, the removal step S3 may be omitted.

[0026] ≪Oxide film formation step S4≫ In the oxide film forming step S4, the silicon substrate 110 is placed in a thermal oxidation furnace and heated in an oxygen atmosphere. As a result, as shown in FIG. 10, an alkali-added oxide film 122 made of a thermal oxide film is formed on the upper surface 110a side of the silicon substrate 110. In this way, by forming the alkali-added oxide film 122 by thermal oxidation, a dense alkali-added oxide film 122 can be easily formed. In addition, the surface area of ​​the upper surface 110a is increased by the recesses 111, and a thick alkali-added oxide film 122 can be formed in a short time. Since the oxide film grows isotropically from the surface of the silicon substrate 110, it closes the recesses 111 during the growth process and finally has a flat upper surface. In addition, as shown in FIG. 12, the alkali-added oxide film 122 takes in the alkali ions settled in the recesses 111. In the figure, the alkali ions are referred to as "K + " is expressed as follows.

[0027] Also, the thickness H1 of the alkali-added oxide film 122 is larger than the depth H of the recess 111. That is, H1>H. Also, the upper end of the alkali-added oxide film 122 is located above the upper end of the recess 111, and the lower end of the alkali-added oxide film 122 is located below the lower end of the recess 111. This makes it possible to more reliably form the alkali-added oxide film 122 that has alkali ions incorporated therein. Also, the heating conditions for the silicon substrate 110 are not particularly limited, but are preferably, for example, 900° C. or higher and 30 minutes or longer. This makes it possible to more reliably form the alkali-added oxide film 122 that is thicker than the depth H of the recess 111.

[0028] The alkali ions in the alkali-added oxide film 122 are positively charged, but they form pairs 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.

[0029] In the oxide film forming step S4, next, as shown in Fig. 11, unnecessary portions of the alkali-added oxide film 122 are removed by etching. In this manner, the outer shapes of the first substrate 11 and the electret film 12 are formed. In addition, in the oxide film forming step S4, a process for planarizing the surface of the alkali-added oxide film 122 may be performed as necessary. This process can be performed, for example, by using CPM (chemical mechanical polishing) or the like.

[0030] <Charging process S5> In the charging step S5, 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. 13, 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. In this state, a voltage is applied between the negative electrode plate E1 and the positive electrode terminal E2. Then, 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. 14, 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.

[0031] In such a method for producing the electret film 12, as described above, after supplying the alkali ion solution L to the recess 111, the alkali-added oxide film 122 is formed. Therefore, as shown in FIG. 14, electrons can be confined in a region sufficiently distant from the surface of the electret film 12. In other words, the electret film 12 has a higher charge inside the film than on the surface. 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 produce the electret film 12 that can maintain a high charge over time.

[0032] In particular, as described above, in the removal step S3, a part of the alkaline ion solution L is removed from inside the recesses 111. Therefore, electrons are less likely to exist near the surface of the electret film 12, and the electrons can be more reliably confined in a region sufficiently away from the surface of the electret film 12.

[0033] Furthermore, the recesses 111 absorb or reduce the expansion caused by thermal oxidation, so that warping of the first substrate 11 can be effectively suppressed. This makes it possible to reduce the gap G between the first substrate 11 and the second substrate 13 while preventing them from coming into contact with each other. This makes it possible to further increase the power generation efficiency of the power generation device 10.

[0034] 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 recess forming step S1 for forming a recess 111 on the upper surface 110a, which is one surface of the silicon substrate 110, a supplying step S2 for supplying an alkali ion solution L into the recess 111, an oxide film forming step S4 for heating the silicon substrate 110 to form an alkali-added oxide film 122 containing the components of the alkali ion solution L on the upper surface 110a side of the silicon substrate 110, and a charging step S5 for 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, the alkali ion solution L is supplied to the recess 111, and then the alkali-added oxide film 122 is formed, so that electrons can be confined in a region sufficiently distant 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.

[0035] As described above, the method for producing the electret film 12 further includes a removal step S3, which is performed between the supply step S2 and the oxide film forming step S4, and removes a part of the alkaline ion solution L in the recesses 111. By performing such a removal step S3, it becomes even more difficult for electrons to exist near the surface of the electret film 12, and the electrons can be more reliably confined in a region sufficiently separated from the surface of the electret film 12.

[0036] As described above, the removal step S3 is performed by ashing the upper surface 110a of the silicon substrate 110. According to such a method, part of the alkaline ion solution L can be easily removed from inside the recess 111.

[0037] As described above, in the oxide film forming step S4, the alkali-added oxide film 122 is formed to be thicker than the depth H of the recess 111. This makes it possible to more reliably form the alkali-added oxide film 122 that has alkali ions incorporated therein.

[0038] As described above, in the recess forming step S1, a plurality of recesses 111 are formed in the formation region Q12 for forming the electret film 12. This allows a sufficient amount of the alkali ion solution L to be uniformly supplied into the formation region Q12 in the supplying step S2.

[0039] As described above, the silicon substrate 110 has a plurality of formation regions Q12. This allows a plurality of electret films 12 to be selectively formed on the silicon substrate 110.

[0040] <Second embodiment> Fig. 15 is a top view showing a power generation device according to a second embodiment, Fig. 16 is an exploded perspective view of the power generation device of Fig. 15, and Fig. 17 is a cross-sectional view taken along line AA in Fig. 15.

[0041] 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.

[0042] The power generating device 100 shown in Figures 15 and 16 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 17, 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.

[0043] 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.

[0044] 16, 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.

[0045] As shown in FIG. 17, 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.

[0046] 15 and 16, 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.

[0047] 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.

[0048] 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.

[0049] 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 forming a recess on the upper surface of the second silicon layer 3C, supplying an alkali ion solution into the recess, heating the second silicon layer 3C to form an alkali-added oxide film, and applying a voltage to the second silicon layer 3C to charge the alkali-added oxide film.

[0050] The second embodiment as described above can also achieve the same effects as the first embodiment.

[0051] 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]

[0052] 10...power generation device, 100...power generation device, 11...first substrate, 110...silicon substrate, 110a...upper surface, 111...recess, 119...through hole, 12...electret 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...spring portion, 6...movable portion, 7...movable electrode, 7 1...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...spacing, E1...negative electrode plate, E2...positive electrode terminal, EL...electret film, G...spacing, H...depth, H1...thickness, L...alkaline ion solution, PAD1...electrode pad, PAD2...electrode pad, Q12...forming area, S1...recess forming step, S2...supply step, S3...removal step, S4...oxide film forming step, S5...charging step, W...width

Claims

1. a recess forming step of forming a recess on one surface of a silicon substrate; a supplying step of supplying an alkali ion solution into the recess; an oxide film forming step of heating the silicon substrate to form 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. The method for producing an electret film according to claim 1 , further comprising a removing step, which is carried out between the supplying step and the oxide film forming step, and which removes a part of the alkali ion solution in the recess.

3. The method for producing an electret film according to claim 2 , wherein the removing step is performed by an ashing process performed on the one surface of the silicon substrate.

4. The method for producing an electret film according to claim 1 , wherein in the oxide film forming step, the alkali-added oxide film is formed to have a thickness greater than the depth of the recess.

5. The method for producing an electret film according to claim 1 , wherein in the recess forming step, a plurality of recesses are formed in a region where the electret film is to be formed.

6. The method for producing an electret film according to claim 5 , wherein the silicon substrate has a plurality of regions in which the electret film is formed.

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