JP2008021786A - Electret, method of manufacturing the same, and acoustic sensing device having a capacitor equipped with the electret - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electret having high electric charge storing performance, and to provide a manufacturing method thereof. <P>SOLUTION: In the electret 12 consisting of an insulator film bearing electric charges, the electric charges are distributed so that an electric charge density can change in the insulator film. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electret, a method for manufacturing the same, and an acoustic device having a capacitor including the electret.

  A conventional electret (thin film capable of holding electric charges) will be described with reference to FIG. 5 by taking a condenser microphone having a condenser with a conventional electret as a specific example. FIG. 5 is a schematic cross-sectional view showing the structure of a condenser microphone having a condenser provided with a conventional electret.

  As shown in FIG. 5, the condenser microphone 5 includes, as main components, a vibrating electrode 50, a fixed electrode 51 made of a metal plate arranged to face the vibrating electrode 50, and the vibrating electrode 50 and the fixed electrode 51. And an external power supply 53 that is electrically connected to each of the vibrating electrode 50 and the fixed electrode 51. The condenser microphone 5 functions as a capacitor by storing static electricity between the electret fixed electrode 51 </ b> A in which the fixed electrode 51 and the electret 52 are laminated by the external power source 53 and the vibrating electrode 50. When the vibration of the air is detected, the condenser microphone 5 vibrates, and the capacitance between the vibration electrode 50 and the electret fixed electrode 51A changes. It can be detected as a signal.

  Here, a method for charging the electret 52 will be briefly described below.

After an insulator film made of resin is bonded to the fixed electrode 51 side, an electret 52 made of an insulator film charged with electric charge is formed by corona discharge or electron beam. Thereafter, the electric charge charged in the electret 52 is stabilized by a heat treatment such as annealing.
JP 2005-191467 A

  However, electrets that are charged with corona discharge or electron beam (that is, conventional electrets) have the following problems.

  Here, when corona discharge is used as means for charging the electret, ions are adsorbed on the surface of the electret by corona discharge.

  Further, when an electron beam is used as means for charging the electret, electrons are injected into the electret by the electron beam.

  Here, the electric charge is easily discharged when it comes into contact with a charge-eliminating substance floating in the air (mainly, moisture contained in the air or organic gas contained in tobacco smoke or the like).

  Therefore, in the case of electrets that have been charged by corona discharge, the ions are adsorbed on the surface of the electret, so there is a high probability of contact with the charge-removing material, and the charges are easily discharged by the ions coming into contact with the charge-removing material. There is a problem that.

  In addition, in the case of electrets charged with an electron beam, electrons are injected into the electret. However, since electrons are generally easy to thermally diffuse, the electrons injected into the electret are thermally diffused, and the electret There is a problem that the charge is easily removed by contacting the heat diffused electrons on the surface of the material with the charge removal material.

  As described above, in any case of an electret charged with a charge by corona discharge and an electret charged with an electron beam, the charged charge is easily removed by contact with a charge-removing substance. There is a problem that the charge retention characteristics of the electret are deteriorated without being able to retain the charged charge. For this reason, a capacitor provided with a conventional electret cannot function as a capacitor, and naturally the performance of a condenser microphone having a capacitor is lowered.

  In view of the above, an object of the present invention is to provide an electret having a high charge retention characteristic and a method for manufacturing the same.

  In order to achieve the above object, an electret according to the present invention is an electret made of an insulator film charged with charge, and the charge is distributed so that the charge density changes in the insulator film. It is characterized by that.

  The electret according to the present invention is an electret in which charges are distributed so that the charge density changes inside, and since the charges are charged inside the electret, the electret charged by corona discharge (that is, on the surface) Compared with an electret on which ions are adsorbed), it is possible to reduce the probability that the charge comes into contact with the charge removal material. For this reason, the electret which has a high electric charge retention characteristic can be provided.

  Furthermore, according to the electret according to the present invention, the electret is such that the charge is distributed so that the charge density changes inside, and since the charge is charged inside the electret, the charge is distributed three-dimensionally inside. Therefore, it is possible to provide an electret charged with a larger amount of charge compared to an electret charged by corona discharge (that is, an electret in which ions are two-dimensionally distributed on the surface). be able to. For this reason, for example, even if the miniaturization of the electret may progress with the progress of the miniaturization of the capacitor equipped with the electret, a sufficient amount of charge (that is, the amount of charge necessary to fulfill the function as the electret) ) Can be provided.

  In the electret according to the present invention, the charge is preferably a charge derived from ions.

  In this way, ions, not electrons, are charged inside the electret, and generally, ions are less likely to be thermally diffused than electrons. Therefore, electrets charged by an electron beam (that is, electrons are injected into the inside). Compared with the electret), it is possible to prevent the charged charge from being thermally diffused on the surface of the electret, so that the probability of the charge thermally diffused on the surface of the electret contacting the charge-removing substance can be reduced. it can. For this reason, the electret which has a high electric charge retention characteristic can be provided.

  In the electret according to the present invention, the charge is preferably a charge charged by ion implantation.

  In this way, at the time of ion implantation, for example, by selecting ions composed of atoms having a relatively large atomic weight based on the diffusion rate of ions in the electret, the diffusion rate in the electret is relatively slow. Ions can be selected. Therefore, compared with ions made of atoms having a relatively small atomic weight (in other words, atoms having a relatively high diffusion rate in electrets), it is further possible that the implanted ions are thermally diffused on the surface of the electret. Since it can prevent, it can prevent further that the ion thermally diffused on the surface of the electret contacts with a static elimination substance, Therefore The electret which has a much higher charge retention characteristic can be provided.

  Also, in this case, at the time of ion implantation, for example, by selecting ions composed of atoms having a relatively small atomic weight based on the projected range of ions onto the electret, the projected range onto the electret can be reduced. Relatively deep ions can be selected. As a result, ions can be easily implanted deeper into the electret compared to ions made up of atoms with a relatively large atomic weight (in other words, atoms with a relatively shallow projected range on the electret). Thus, it is possible to provide an electret charged with a larger amount of charge.

  As described above, by appropriately selecting the ion species to be injected into the electret, it is possible to provide an electret in which a charge is charged in a desired region and a desired charge density.

  In the electret according to the present invention, the ions are preferably at least one ion selected from the ion species of arsenic ions, phosphorus ions, and boron ions.

  In this case, for example, when a silicon oxide film is used as the insulator film constituting the electret, ions represented by arsenic, ions made of phosphorus, and ions made of boron are ions to the silicon oxide film. It is an ion generally used for implantation, and an ion implantation technique for a silicon oxide film can be applied. Therefore, based on the technical knowledge of the ion implantation technique to the silicon oxide film, the ion species to be implanted into the insulator film can be selected more easily, and the ion implantation conditions applied to the insulator film can be more easily performed. And it can set precisely.

  The electret according to the present invention preferably further includes a hydrophobic coating film that covers the surface of the insulator film.

  In this way, even if the charged electric charge is thermally diffused on the surface of the electret, it is possible to prevent the electric charge from coming into contact with the neutralizing material. Can be provided.

  The electret which concerns on this invention WHEREIN: It is preferable that the hydrophobic coating film is provided so that the side surface of an insulator film may be covered.

  In this way, even if the charged charge is thermally diffused on the surface of the electret, it is possible to further prevent the charge from coming into contact with the charge removal material.

  The electret according to the present invention preferably further includes an insulating coating film provided on the insulator film.

  In this way, since a laminated electret is formed by laminating an insulator film charged with an electric charge and an insulating coating film, the charged electric charge may be thermally diffused on the surface of the electret. Even if it exists, since it can prevent that an electric charge contacts a static elimination substance, the electret which has a much higher charge retention characteristic can be provided.

  In the electret according to the present invention, the insulating coating film is preferably charged.

  In this way, a laminated electret is formed by laminating a charge-charged insulator film and a charge-charged insulating coating film, so that the laminate-type electret is charged to the insulator film. The charge amount corresponding to the sum of the charge amount and the charge amount charged on the insulating coating film is configured to provide a stacked electret having a sufficient charge amount. it can.

  In the electret according to the present invention, the insulator film is preferably made of a perfluoro amorphous fluoropolymer resin.

  In this way, according to the present inventors' experimental investigation, an electret made of perfluoro amorphous fluoropolymer resin is compared with a conventional electret made of, for example, PTFE (polytetrafluoroethylene) or BCB, It has been found that a high charge retention property is exhibited, and an electret having a high charge retention property can be provided by using an electret made of a perfluoro amorphous fluoric acid polymer resin.

  In the electret according to the present invention, the insulator film is preferably made of a silicon oxide film formed on a semiconductor substrate.

  In this way, the electret according to the present invention can be easily incorporated into a semiconductor device including, for example, a transistor formed over a semiconductor substrate.

  A capacitor provided with an electret according to the present invention is a capacitor that functions as an acoustic sensing device, and is provided between a vibrating electrode, a fixed electrode disposed so as to face the vibrating electrode, and the vibrating electrode and the fixed electrode. And an electret made of an insulator film charged with electric charge. The electret is characterized in that the electric charge is distributed so that the charge density changes in the insulator film.

  According to the capacitor provided with the electret according to the present invention, since the electret having high charge retention characteristics can be used, a long-life acoustic sensing device can be realized.

  According to the electret and the manufacturing method thereof according to the present invention, an electret having high charge retention characteristics can be provided. For this reason, when the capacitor including the electret according to the present invention is applied to an acoustic device such as a condenser microphone, a long-life acoustic device can be realized.

  Furthermore, according to the electret and the manufacturing method thereof according to the present invention, it is possible to provide an electret charged with a larger amount of charge. Therefore, even if the electret is further miniaturized as the acoustic sensitive device is further miniaturized, it is possible to provide an electret having a sufficient charge amount.

  Embodiments of the present invention will be described below with reference to the drawings.

(First embodiment)
Hereinafter, the electret according to the first embodiment of the present invention will be described with reference to FIG. 1, taking a capacitor having the electret as a specific example. FIG. 1 is a cross-sectional view showing the structure of an electret according to the first embodiment of the present invention.

  As shown in FIG. 1, a capacitor including an electret according to the present embodiment includes, as main components, a vibrating electrode (not shown), a fixed electrode 11 made of a metal plate, and an insulator film charged with a charge. As shown in FIG. 1, the electret according to this embodiment is configured so that the charge density changes inside, that is, the charge density changes from the surface toward the inside. It is an electret in which charges derived from ions are distributed three-dimensionally.

  Below, the manufacturing method of the electret which concerns on the 1st Embodiment of this invention is demonstrated easily, referring a capacitor | condenser provided with the electret as a specific example with reference to FIG.

  As shown in FIG. 1, after fixing a fixed electrode 11 made of a metal plate and an insulator film made of a perfluoro non-crystalline hydrofluoric acid polymer resin or a silicon oxide film, ions are implanted into the insulator film by ion implantation. Is injected to form an electret 12 made of an insulator film charged with electric charge.

  As described above, in the present embodiment, ion implantation is employed as means for charging the electret 12 instead of the conventional corona discharge and electron beam. Therefore, ion species to be implanted into the insulator film constituting the electret 12 are appropriately selected, and ion implantation conditions (specifically, implantation energy, etc.) applied to the insulator film constituting the electret 12 are appropriately adjusted. Thus, the electret 12 can be charged so that the charge density changes from the surface toward the inside, for example, so that the maximum charge density is located not inside the surface but inside.

  Here, in general, when ions composed of a certain atom are implanted into a certain film, the diffusion rate of ions in the film is determined according to the atomic weight of the atom. In general, when ions made of a certain atom are implanted into a certain film, the projected range of ions in that film is determined according to the atomic weight of the atom. For example, when ions made of arsenic are implanted into perfluoro amorphous fluoropolymer resin under an implantation energy of about 1000 keV, the projected range of ions is about 1 mm, whereas ions made of boron are When implanted, the projected range of ions is about 2.5 mm.

  That is, an ion composed of atoms having a relatively small atomic weight has a relatively high diffusion rate in the electret compared to an ion composed of atoms having a relatively large atomic weight. While there is a demerit that the charge may be removed by contact with the charge-removing substance, the maximum charge density is located deeper inside the electret compared to ions consisting of atoms with relatively large atomic weight. There is a merit that ions can be implanted into the substrate.

  In contrast, ions with relatively large atomic weights have a relatively slow diffusion rate at the electret compared to ions with relatively small atomic weights, so that ions are thermally diffused on the surface of the electret. While it has the merit of effectively preventing this, it is possible to implant ions so that the maximum charge density is located deeper inside the electret compared to ions consisting of atoms with relatively small atomic weights. There is a demerit that it is difficult.

  Therefore, for example, when ions made of atoms having a relatively small atomic weight are selected as ions to be injected into the electret, the ion implantation conditions (specifically, the implantation is performed so that the maximum charge density is located deeper inside the electret. By adjusting energy etc., it is possible to effectively prevent ions from being thermally diffused on the surface of the electret.

  In addition, for example, when ions made of atoms having a relatively large atomic weight are selected as ions to be injected into the electret, by adjusting the ion implantation conditions so that ions are injected into the electret, ions can be formed on the surface of the electret. Can be effectively prevented from being thermally diffused.

  Specifically, when, for example, boron is selected as the ion to be injected into the electret, the atomic weight is smaller than when arsenic or phosphorus is selected, so that ions can be easily implanted deeper into the electret. On the other hand, when, for example, arsenic or phosphorus is selected as an ion to be injected into the electret, the atomic weight is larger than when boron is selected, so the arsenic diffusion rate and the phosphorus diffusion rate are slower than the boron diffusion rate. Therefore, it is possible to effectively prevent ions from being thermally diffused on the surface of the electret.

  As described above, in this embodiment, by appropriately adjusting the ion implantation conditions applied to the electret according to the ion species to be implanted into the electret, charges are charged in a desired region and a desired charge density is set. The electret which has can be implement | achieved.

  As described above, according to the electret according to the present embodiment, since the charges derived from ions are charged inside the electret, the electret charged by corona discharge (that is, the electret having ions adsorbed on the surface) and In comparison, the probability that the charge comes into contact with the charge removal material can be reduced. In addition, ions, not electrons, are injected into the inside of the electret. In general, ions are less likely to be thermally diffused than electrons, and therefore, electrets charged by an electron beam (that is, electrets into which electrons are injected). Compared with the above, since it is possible to prevent the charged charges from being thermally diffused on the surface of the electret, it is possible to reduce the probability that the charges thermally diffused on the surface of the electret come into contact with the charge removal material. For this reason, the electret which has a high electric charge retention characteristic can be provided.

  Furthermore, according to the electret according to the present embodiment, since the ions are injected into the electret so that the charge density changes from the surface toward the inside, the electret in which the ions are distributed three-dimensionally is provided. Since it can be realized, it is possible to provide an electret charged with a larger amount of charge compared to an electret charged by corona discharge (that is, an electret in which ions are distributed two-dimensionally on the surface). . For this reason, for example, even if the miniaturization of the electret proceeds with the progress of the miniaturization of the acoustic device having a capacitor equipped with the electret, a sufficient amount of charge (that is, the function as the electret can be achieved). It is possible to provide an electret that is charged with a necessary amount of charge).

  In this embodiment, a specific example is the case where arsenic, phosphorus, or boron, which are ions generally used in ion implantation into a silicon oxide film, is used as ions to be implanted into the insulator film constituting the electret. However, the present invention is not limited to this, and in consideration of each of the above-mentioned merits and demerits, an ion species is appropriately selected and an ion implantation condition is appropriately adjusted to obtain a desired one. It is possible to realize an electret having a charge density in a region and a desired charge density.

  In the present embodiment, the case where a perfluoro non-crystalline hydrofluoric acid polymer resin is used as the insulator film constituting the electret has been described as a specific example, but the present invention is not limited to this. For example, when a silicon oxide film is used as the insulator film constituting the electret, the ion species (for example, arsenic, phosphorus, boron, etc.) can be selected more easily based on the technical knowledge of the ion implantation technique to the silicon oxide film. In addition, the ion implantation conditions can be set more easily and precisely.

<First Modification>
Hereinafter, as an electret according to the first modification of the present invention, a capacitor having an electret will be described as a specific example, and description will be made with reference to FIG. FIG. 2 is a cross-sectional view showing the structure of the electret according to the first modification of the present invention. In FIG. 2, the same components as those of the capacitor shown in FIG. Therefore, in the present modification, the same description as in the first embodiment is not repeated.

  As shown in FIG. 2, a capacitor including an electret according to this modification includes a vibrating electrode (not shown), a fixed electrode 11 made of a metal plate, and an insulator film charged with electric charge, as shown in FIG. The electret according to this modification is an electret in which charges derived from ions are three-dimensionally distributed so that the charge density changes from the surface toward the inside. The difference between the first embodiment and this modification is that a hydrophobic coating film 23 is provided so as to cover the surface and side surfaces of the electret 12 as shown in FIG.

  According to this modification, even if ions injected into the electret 12 may be thermally diffused on the surface, it is possible to reliably prevent the ions from coming into contact with the charge-removing substance. An electret having characteristics can be provided.

<Second Modification>
Hereinafter, as an electret according to the second modification of the present invention, a capacitor having an electret will be described as a specific example, and description will be made with reference to FIG. FIG. 3 is a cross-sectional view showing the structure of the electret according to the second modification of the present invention. In FIG. 3, the same components as those of the capacitor shown in FIG. Therefore, in the present modification, the same description as in the first embodiment is not repeated.

  As shown in FIG. 3, a capacitor including an electret according to this modification includes, as main components, a vibrating electrode (not shown), a fixed electrode 11 made of a metal plate, and an insulator film charged with electric charge. The electret according to this modification is an electret in which charges derived from ions are three-dimensionally distributed so that the charge density changes from the surface toward the inside. The difference between the first embodiment and this modification is that an insulating coating film 33 is provided so as to cover the surface of the electret 12 as shown in FIG. Thereby, it is possible to realize a laminated electret 34 in which the electret 12 made of an insulator film and the insulating coating film 33 are laminated.

  Here, each of the case where the insulating coating film 33 is used without being charged and the case where the insulating coating film 33 is used after being charged is described below.

  First, when the insulating coating film 33 is used without being charged, ions injected into the electret 12 may be thermally diffused on the surface as in the first modification. However, since it is possible to prevent ions from coming into contact with the charge removal material, an electret having even higher charge retention characteristics can be provided.

  Second, when the insulating coating film 33 is used by being charged, a further effect can be obtained in addition to the same effect as in the first modified example. Specifically, the multilayer electret 34 having a charge amount corresponding to the sum of the charge amount charged on the electret 12 and the charge amount charged on the insulating coating film 33 can be configured. A stacked electret 34 having a charge amount charged can be realized.

  In particular, for example, when the electret of the present invention charged by ion implantation is used instead of the conventional electret charged by corona discharge or electron beam as the electret formed by charging the insulating coating film 33, The electret of the present invention (that is, an electret in which ions are implanted in three dimensions) is compared with a conventional electret (that is, an electret in which ions are adsorbed in two dimensions or an electret in which electrons are implanted in three dimensions). Since an electret having a higher charge amount and a higher charge retention characteristic can be realized, it is possible to effectively increase the amount of charge that the stacked electret 34 has.

  As described above, according to the electret and the manufacturing method thereof according to the first embodiment, the first modification, and the second modification of the present invention, an electret having high charge retention characteristics can be provided. Therefore, a capacitor provided with an electret having high charge retention characteristics can be realized.

(Second Embodiment)
FIG. 4 shows an acoustic sensing device according to the second embodiment of the present invention, specifically, a condenser microphone 4 having the capacitor 1 provided with the electret 12 according to the first embodiment of the present invention. The description will be given with reference. FIG. 4 shows an acoustic device according to the second embodiment of the present invention, specifically, the structure of the capacitor 1 including the electret 12 according to the first embodiment of the present invention, and the capacitor 1 electrically It is a figure shown about the circuit to connect.

  As shown in FIG. 4, the condenser microphone 4 includes a capacitor 1 having a structure similar to that of the capacitor shown in FIG. 1, and the capacitor 1 includes, as shown in FIG. The vibration electrode 10, the fixed electrode 11 made of a metal plate disposed so as to face the vibration electrode 10, and the electret 12 bonded to the fixed electrode 11 side so as to be disposed between the vibration electrode 10 and the fixed electrode 11. It has.

  As shown in FIG. 4, in the condenser microphone 4, the vibration electrode 10 and the FET 43 are electrically connected, and the fixed electrode 11 and the GND 49 are electrically connected. A first resistor 44 is provided between the drain side of the FET 43 and the power supply 47 electrically connected to the drain side of the FET 43, and the source side of the FET 43 and the source side of the FET 43 are electrically connected. The capacitor 46 is electrically connected to the output terminal 48 connected to the second resistor, while the second resistor is connected between the source side of the FET 43 and the GND 49 electrically connected to the source side of the FET 43. 45 is provided.

  According to the acoustic device according to the present embodiment, since the electret having high charge retention characteristics is used as the electret constituting the capacitor, a long-life acoustic device can be realized.

  In the present embodiment, the condenser microphone has been described as a specific example of the acoustic sensing device. However, the present invention is not limited to this, and for example, a pressure sensor or the like may be used.

  INDUSTRIAL APPLICABILITY The present invention can improve the charge retention characteristics of electrets, and is therefore useful for electrets and methods for producing the same.

It is sectional drawing shown about the structure of the electret which concerns on the 1st Embodiment of this invention. It is sectional drawing shown about the structure of the electret which concerns on the 1st modification of this invention. It is sectional drawing shown about the structure of the electret which concerns on the 2nd modification of this invention. It is a figure shown about the acoustic response apparatus which concerns on the 2nd Embodiment of this invention. It is a schematic sectional drawing shown about the structure of the capacitor | condenser provided with the conventional electret.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Capacitor 10 Vibrating electrode 11 Fixed electrode 12 Electret 23 Hydrophobic coating film 33 Insulating coating film 34 Laminated type electret 4 Condenser type microphone 43 FET
44 1st resistor 45 2nd resistor 46 Capacitor 47 Power supply 48 Output terminal 49 GND
5 Condenser microphone 50 Vibrating electrode 51 Fixed electrode 51A Electret fixed electrode 52 Electret 53 External power supply

Claims (11)

An electret comprising an insulator film charged with electric charge,
The electret characterized in that the charge is distributed so that a charge density changes in the insulator film. The electret according to claim 1,
The electret characterized in that the charge is a charge derived from ions. The electret according to claim 2,
The electret characterized in that the charge is a charge charged by ion implantation. The electret according to claim 2,
The electret is characterized in that the ion is at least one ion selected from an arsenic ion, a phosphorus ion and a boron ion species. The electret according to claim 1,
An electret, further comprising a hydrophobic coating film that covers a surface of the insulator film. The electret according to claim 5,
The electret characterized in that the hydrophobic coating film is provided so as to cover a side surface of the insulator film. The electret according to claim 1,
An electret, further comprising an insulating coating film provided on the insulator film. The electret according to claim 7,
The electret wherein the insulating coating film is charged. The electret according to claim 1,
The electret is characterized in that the insulator film is made of a perfluoro amorphous fluoropolymer resin. The electret according to claim 1,
The electret according to claim 1, wherein the insulator film is made of a silicon oxide film formed on a semiconductor substrate. A capacitor that functions as an acoustic device,
A vibrating electrode;
A fixed electrode arranged to face the vibrating electrode;
An electret formed of an insulator film provided between the vibrating electrode and the fixed electrode and charged with electricity;
The electret is a capacitor characterized in that the charge is distributed so that a charge density changes in the insulator film.

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