JP2010245797A - Capacitor microphone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a capacitor microphone excelling in high-frequency characteristics. <P>SOLUTION: A graphene 101 is used as a diaphragm of the capacitor microphone. In order to obtain high-frequency characteristics more superior than conventional capacitor microphones, a conductive material having higher Young's modulus and lower cotton density and being suited for thinning is required and the graphene, that has ideal characters as the diaphragm of the capacitor microphone is optimal. The capacitor microphone employing the graphene is available for recording music and measuring ultrasonic waves. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an audible sound and ultrasonic measurement microphone.

  In order to record music with higher sound quality, there is an increasing demand for microphones that can measure higher frequencies than ever. For example, in terms of format, a CD can only record sound up to a frequency of 22 kHz, whereas a DVD-Audio format can record sound up to a frequency of 96 kHz in principle. In order to record the actual sound, a microphone capable of handling from an audible frequency to a high frequency is necessary.

  On the other hand, there is an increasing demand for microphones that can measure ultrasonic waves for preventing robot collisions and medical diagnosis. Here, the performance of the audible sound is not necessarily required, but it is required to cope with a higher frequency than that for music.

  As such a microphone for measuring a wide frequency and a high frequency, a condenser microphone is most widely used. As shown in FIG. 2, the condenser microphone is a device having a structure in which a fixed electrode 202 and a conductive diaphragm 201 are arranged in parallel by a support 204 so as to form a condenser. The vibration plate is vibrated by sound waves or ultrasonic waves to change the distance between the electrodes. This change in the distance between the electrodes changes the capacitance of the capacitor. By detecting this capacitance change electrically by the circuit 203, it is possible to measure sound waves or ultrasonic waves. Condenser microphones are described in detail in Non-Patent Document 1.

Journal of the Acoustical Society of Japan, Vol. 64, No. 11

  In the case of a condenser microphone, the upper limit of the frequency that can be measured is limited by the resonance frequency of the diaphragm. In order to measure higher frequency ultrasonic waves, it is necessary to use a diaphragm having a higher resonance frequency.

  The resonance frequency of the diaphragm is proportional to 1/2 of the stiffness of the diaphragm and inversely proportional to 1/2 of the mass of the diaphragm. Therefore, in order to increase the resonance frequency, it is necessary to use a film having a higher stiffness and a smaller mass. The stiffness depends on the shape of the film, but in the case of the same shape, the higher the Young's modulus of the material, the higher the stiffness. On the other hand, the mass is the product of the density and volume of the material. At first glance, a material with lower density may seem advantageous. However, in order to function as a condenser microphone, it is necessary to always maintain the shape as a film. For example, if a crack occurs during vibration, it will no longer function as a microphone. A film with a simply low density often requires more thickness to maintain its shape as a film, which may result in an increased mass. Therefore, what is important is not a low volume density, but a low surface density at a thickness that functions as a film.

  For example, single crystal diamond has a Young's modulus of 1 TPa. This is the largest value among known substances. In order to use this material as a diaphragm for a condenser microphone, the shape needs to be a thin film. However, there is currently no technique for growing a single crystal into a thin film. The only thing that can be produced by the technique of growing a crystal in a thin film is a polycrystalline material composed of fine crystal grains or an amorphous material called diamond-like carbon. The Young's modulus of these films is about 150 GPa even if they are good quality, which is far from that of single crystals. It is possible to produce a thin plate shape by cutting single crystal diamond. However, diamond is very poor in machinability and thin films cannot be produced. Also, the low electrical conductivity of diamond is a problem. Since the diaphragm of the condenser microphone needs to function as one electrode of the condenser, it needs electrical conductivity. However, diamond is an insulator. For these reasons, it is difficult to make a good quality condenser microphone with diamond.

  An example of a material used as a diaphragm for a condenser microphone is a silicon nitride film. The Young's modulus of the silicon nitride film is usually about 300 GPa although it depends on the manufacturing method. Further, since the silicon nitride film is an insulating material, the silicon nitride film alone does not function as a diaphragm for a condenser microphone, and needs to be laminated with a metal material such as aluminum. Since the silicon nitride film is an amorphous material, a thickness of at least about 100 nm is necessary to maintain the properties as a film. Also, the aluminum film for providing conductivity needs to have the same thickness. Therefore, the surface density is at least about 6g per square meter.

  In order to obtain better high-frequency characteristics than conventional condenser microphones, a conductive material suitable for thinning is required, which has a higher Young's modulus and a lower areal density. The inventor of the present application has searched for a material satisfying these properties, and has found an optimal material as a diaphragm for a condenser microphone.

  In the present invention, graphene is used as the diaphragm of the condenser microphone.

  If the method of this invention is used, the microphone excellent in high frequency performance can be comprised.

The figure which shows the condenser microphone of this invention Diagram showing the components of a condenser microphone Diagram showing the molecular structure of graphene

(Embodiment 1)
FIG. 1 shows an example of the structure of the condenser microphone of the present invention. The feature of the present invention is that graphene is used as the diaphragm 101 of the condenser microphone. In the case of the example illustrated in FIG. 1, the counter electrode 102 uses a conductive substrate such as highly doped silicon, and an insulating film deposited on the substrate and removing a part under the graphene is used as the support 103. Used. Reference numerals 104 and 105 denote electrodes for connecting a capacitance detection circuit (not shown) to the vibrating membrane 101 and the counter electrode 102.

(Graphene structure)
Graphene is a two-dimensional sheet-like substance in which carbon atoms are arranged on a hexagonal lattice as shown in FIG. Although only a part of the molecule is shown in FIG. 3, the hexagonal lattice is continuous to the end of the molecule in the actual molecule. As the size of the molecule, one having a diameter exceeding 100 micrometers can be obtained. In other words, molecules large enough to be used as a diaphragm for a condenser microphone can be obtained.

  Graphene has a two-dimensional shape with a thickness of one atomic layer, whereas most other crystals have a three-dimensional structure. Therefore, the thickness necessary for maintaining the shape as a film is only one atomic layer, and is about 0.3 nanometer.

(Young's modulus)
The Young's modulus of graphene is 1 TPa, which is the highest value among known substances. This value is the same as that of diamond, but in the case of diamond, this value cannot be obtained as a thin film, whereas graphene can achieve this value with a thin film having a thickness of one atomic layer. .

(Area density and strength)
The surface density of graphene is 0.25mg per square meter. It is not only light but also has sufficient strength to be used as a diaphragm alone. Therefore, if graphene is used, a surface density of 1 / 10,000 or less can be achieved as compared with a conventional silicon nitride film.

(Electrical conductivity)
Graphene has good electrical conductivity. Therefore, even when used as a vibration film for a condenser microphone, it is not necessary to stack a metal film or the like.

  Thus, graphene has an ideal property as a diaphragm for a condenser microphone, and a condenser microphone using the graphene exhibits higher high-frequency characteristics than before.

  The microphone of the present invention can be used for music recording and ultrasonic measurement that require high frequency characteristics.

DESCRIPTION OF SYMBOLS 101 Graphene 102 Counter electrode 103 Support body 104 Electrode 105 Electrode 201 Diaphragm 202 Counter electrode 203 Capacitance detection circuit 204 Support body

Claims (1)

A conductive substrate;
A diaphragm made of graphene;
A condenser microphone comprising a support disposed between the substrate and the vibration plate, the support having a space formed by the substrate, the support, and the vibration plate.

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