JP4101785B2 - Condenser microphone and manufacturing method thereof - Google Patents

Description

  The present invention relates to a micro condenser microphone made by silicon micromachining.

  Conventionally, a condenser-type microphone manufactured by micromachining using a silicon wafer is known. This condenser microphone is provided with a membrane that is a diaphragm for detecting sound pressure and a back plate opposite to it, so as to electrically detect a change in the gap between the membrane and the back plate. (For example, refer nonpatent literature 1, patent documents 2 and 3).

G.M.Sessler; "Silicon Microphones" J.Audio Eng.Soc.vol.44, No.1 / 2 p.16-22 (1996) Japanese Patent Laid-Open No. 2001-39796 JP 2003-163998 A

  When the condenser microphone is downsized, the membrane is naturally reduced, and the gap capacity is also reduced. For this reason, the sensitivity particularly in the low frequency region is lowered. In order to solve this problem, the membrane tension may be reduced. However, in the prior art, since the tension of the membrane is determined by the manufacturing conditions, adjustment by design does not work, and it is difficult to obtain a condenser microphone having a desired resonance frequency.

(1) The condenser microphone according to claim 1 is a condenser microphone manufactured by a photolithography method, and a diaphragm having a peripheral portion supported on the base plate, and a diaphragm provided at a predetermined interval from the back plate. And a beam-like member for suspending the diaphragm from the base plate, and a pair of electrodes for taking out a change in capacitance between the diaphragm and the back plate as an electric signal . The beam-like member has a cross shape. The beam-shaped member has a beam portion and a fixed end portion, and the beam portion is provided so as to be positioned in a cross-shaped opening formed in the back plate, and the fixed end portion supports the beam portion by both ends. It is fixed to the base plate as described above .
(2) In the above condenser microphone, a pair of electrodes and a fixed side electrode and the movable side electrode, the movable electrode is provided on the upper surface of the beam portion, the fixed-side electrode is provided on the upper surface of the back plate It is preferable.
(3) In the condenser microphone according to the first or second aspect , the base plate, the back plate, the vibration plate, and the beam-shaped member may be manufactured by photolithography on the SOI wafer.
(4) A method for producing a condenser microphone according to claim 4 includes a step of forming a back plate having a cross-shaped opening with a peripheral portion supported on a base plate, and a predetermined distance from the back plate. Forming a diaphragm, forming a beam-like member for suspending the diaphragm from the base plate, and forming a pair of electrodes for taking out a capacitance change between the diaphragm and the back plate as an electrical signal; In the step of forming the beam-shaped member , the beam-shaped member is configured to support the beam portion provided so as to be positioned in the cross-shaped opening formed in the back plate and the beam portion by both ends. It is formed as a cross shape having a fixing portion fixed to the base plate .
(5) A microphone system according to a fifth aspect of the present invention provides a diaphragm in the resonance frequency region of a condenser microphone from the condenser microphone according to any one of the first to third aspects and an electric signal extracted from the condenser microphone. And a detection circuit for extracting a signal corresponding to the sound pressure acting.
(6) A microphone system according to the invention of claim 6 has a microphone array in which a plurality of condenser microphones according to any one of claims 1 to 3 having different resonance frequencies are formed on the same wafer substrate, and a plurality of condenser microphones. A plurality of detection circuits each for extracting signals corresponding to sound pressure acting on the diaphragm in the resonance frequency region of each condenser microphone from the electrical signals taken out from each condenser microphone, and from each detection circuit And a combining circuit that combines the input signals and outputs a combined signal.
(7) The invention of claim 7 is the microphone system according to claim 6 , wherein each condenser microphone has a continuous frequency region so that a combination of the resonance frequency regions of the plurality of condenser microphones forms one continuous frequency region. characterized in that setting the resonant frequency.

  According to the present invention, since the diaphragm is supported by the beam-like member, the dimension and shape of the beam portion can be freely designed, so that an ultra-small condenser microphone having a desired resonance frequency can be provided. . Further, by providing a plurality of condenser microphones having different resonance frequencies, it is possible to detect sound with high sensitivity in one continuous frequency region.

Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings.
-First embodiment-
FIG. 1 is a perspective view schematically showing the overall configuration of the condenser microphone according to the present embodiment, in which directions are represented by three-dimensional orthogonal coordinates. 2 is an XZ cross-sectional view taken along the line II of FIG. FIG. 3 is a top view of FIG.

As shown in FIG. 1, the condenser microphone 1 will be described in detail later using an SOI (Silicon on Insulator) wafer 100 having a three-layer structure of a lower Si layer 30, an SiO ₂ layer 20, and an upper Si layer 10. As described above, it is manufactured by a micromachining technique or a photolithography technique. In particular, silicon micromachining is a technique for forming a fine structure or a movable system by performing film formation, etching, thermal diffusion, etc. on a silicon-based material.

  As shown in FIGS. 1 to 3, the condenser microphone 1 is provided with a base plate 31 formed by forming a space inside the lower Si layer 30 and an upper side of the base plate 31, and is formed in a cross shape above the space. Between the back plate 13, the cross-shaped region of the back plate 13, beam-like members (hereinafter referred to as bridges) 11 and 12, both ends of which are fixed to the base plate 31, and the back plate 13. And a diaphragm 32 suspended from the bridges 11 and 12 at a predetermined interval.

  The bridge 11 extending in the X direction has a beam portion 11a and two fixed end portions 11b, and the bridge 12 extending in the Y direction has a beam portion 12a and two fixed end portions 12b. The bridges 11 and 12 are an integral structure that intersects at the center of the beam portion. The back plate 13 has a plurality of acoustic holes 14 penetrating in the Z direction, and is separated from the bridges 11 and 12.

  As shown in FIGS. 2 and 3, a circular diaphragm 32 is integrally formed on the lower surfaces of the beam portions 11a and 12a. The diaphragm 32 forms a gap A between the diaphragm 32 and the back plate 13 while being suspended by the beam portions 11a and 12a. That is, the diaphragm 32 is separated from the central portion 13a of the back plate 13 by the thickness d of the spacer 21 in the Z direction. Further, the diaphragm 32 is spaced apart from the peripheral portion 13b of the back plate 13 by forming a ring-shaped space R having a width of distance r on the XY plane.

  The back plate 13 is formed integrally with the base plate 31 at the peripheral portion 13b. Although not shown, the bridges 11 and 12 are formed integrally with the base plate 31 at the fixed ends 11b and 12b. An electrode film 52 is provided on the surface of the back plate 13, and a part thereof is a fixed-side electrode outlet F. The fixed-side electrode outlet F is provided on the upper surface of the peripheral portion 13 b of the back plate 13. In addition, an electrode film 51 is provided on the surfaces of the bridges 11 and 12, and a part of the electrode film 51 is an extraction port M of the movable electrode. The take-out port M for the movable electrode is provided on the upper surface of the fixed end 12 b of the bridge 12. Thus, the electrode films 51 and 52 are provided on the same plane. The electrode films 51 and 52 are made of a metal layer 50 described later.

With reference to FIG. 2, each of the above components is compared with each layer of the SOI wafer 100.
The beam portions 11 a and 12 a have a structure in which the SiO ₂ layer 20, the upper Si layer 10, the base layer 40, and the metal layer 50 are sequentially provided. Similarly, the fixed end portions 11b and 12b also have a structure in which the SiO ₂ layer 20, the upper Si layer 10, the base layer 40, and the metal layer 50 are sequentially provided.

The central portion 13a of the back plate 13 has a structure in which the upper Si layer 10, the base layer 40, and the metal layer 50 are sequentially provided. On the other hand, the peripheral portion 13b of the back plate 13 has a structure in which the SiO ₂ layer 20, the upper Si layer 10, the base layer 40, and the metal layer 50 are sequentially provided.
The base plate 31 is composed only of the lower Si layer 30. The diaphragm 32 is mainly composed of the lower Si layer 30, and a base layer 40 and a metal layer 50 are sequentially provided at a joint portion between the beam portions 11 a and 12 a. The underlayer 40 formed at the coupling portion of the diaphragm 32 also covers the side surfaces of the beam portions 11a and 12a, but the metal layer 50 does not necessarily cover the side surfaces of the beam portions 11a and 12a. Therefore, the metal layer 50 formed on the coupling portion of the diaphragm 32 may not be electrically connected to the metal layer 50 formed on the beam portions 11a and 12a.

The sound collection by the condenser microphone 1 configured as described above will be described. When the diaphragm 32 receives sound pressure, it vibrates according to the amplitude and frequency of the sound. The state of vibration, that is, the temporal change in the capacity of the air gap A is transmitted to the beam portions 11a and 12a and is detected as a voltage change between the movable side electrode outlet M and the fixed side electrode outlet F. The frequency characteristics and sensitivity of the condenser microphone 1 depend on the thickness, width and length of the beam portions 11a and 12a and the capacity of the gap A (area of the diaphragm 31 × gap length d). Regarding the length and width of the beam portions 11a and 12a and the area of the diaphragm 31, there is a degree of freedom in design, and the beam portions 11a and 12a can be manufactured to arbitrary dimensions by a micromachining technique (photolithography technique). On the other hand, the thickness of the beam portions 11a and 12a is determined by the sum of the thicknesses of the upper Si layer 10 and the SiO ₂ layer 20, and the gap length d is determined by the thickness of the SiO ₂ layer 20, so Accordingly, the SOI wafer 100 may be selected as appropriate.

  Thus, in this embodiment, there is an effect that the frequency characteristics and sensitivity of the condenser microphone 1 can be arbitrarily set. In the conventional membrane structure, the frequency characteristics and sensitivity of the microphone depend not only on the capacity of the air gap A but also on the tension of the membrane, and this cannot be controlled.

  Next, the manufacturing process of the condenser microphone 1 of the present invention will be described in detail with reference to FIGS. The manufacturing process proceeds in order from (a) to (p). (A) to (h) are partially perspective views, and (i) to (p) are partially sectional views.

First, referring to FIG.
In the step (a), an SOI wafer 100 is prepared in which the thicknesses of the lower Si layer 30, the SiO ₂ layer 20, and the upper Si layer 10 are 500 μm, 1 μm, and 25 μm, respectively.
In the step (b), a resist 41 is applied to the surface of the upper Si layer 10 by a spin coater under the conditions of 3000 rpm and 30 sec, and baked at 90 ° C. for 5 minutes.
In the step (c), the resist 41 is exposed to ultraviolet rays for 4.0 sec using a mask having a pattern corresponding to the peripheral portion 15 of the beam portions 11a and 12a, and developed for 1.5 min. The resist 41 is removed.

In the step (d), regions corresponding to the peripheral portion 15 of the upper Si layer 10 are etched by ICP-RIE (inductively coupled plasma-reactive ion etching) to form beam portions 11a and 12a. FIG. 4E is a cross-sectional view taken along the line II-II in FIG. 4D, and etching is performed until the surface of the SiO <SUB> 2 </ SUB> layer 20 is exposed as illustrated. ICP-RIE etches a sample under a relatively low pressure of 0.05 to 1 Pa using a chemical reaction between ions of a process gas in a high-density plasma and the sample surface. High etching processing is possible. As the process gas, an oxidizing gas such as CCl ₂ F ₂ or CF ₄ is used.

The next step will be described with reference to FIG.
In step (e), the resist 41 is removed by washing at 90 ° C. for 5 minutes with sulfuric acid / hydrogen peroxide (H ₂ SO ₄ + H ₂ O ₂ ), and the exposed SiO ₂ layer 20 is removed by etching with strong hydrofluoric acid. The polycrystalline silicon film 40 is deposited by 600 nm by LPCVD (low pressure chemical vapor deposition). FIG. 5B is a cross-sectional view taken along the line III-III in FIG. 5A, and the exposed portion of the beam portion 12 a is covered with the polycrystalline silicon film 40 as shown in the drawing.

LPCVD is a film forming method in which a sample is heated under a reduced pressure of 10 to 10 ³ Pa, and a film is generated on the sample surface by a gas phase chemical reaction by thermal energy. This method has an advantage that a uniform film thickness can be obtained with excellent film adhesion. In film formation of polycrystalline silicon, SiCl ₄ + H ₂ or SiH ₄ is used as a process gas. The reason why the polycrystalline silicon film 40 is formed is to strengthen the coupling between the beam portions 11a and 12a and the diaphragm 32. If a contact hole is provided at the bonding position and a polycrystalline silicon film is formed, an anchor effect can be expected.

  After the polycrystalline silicon film 40 is formed, an OCD resist is applied with a spin coater under conditions of 4000 rpm and 30 seconds, baked under conditions of 150 ° C. and 30 minutes, and then thermal diffusion of phosphorus (P) under conditions of 1000 ° C. and 30 minutes. Process. Due to the thermal diffusion of P into the polycrystalline silicon film 40, the electrical resistance of the polycrystalline silicon film 40 is reduced. After the thermal diffusion treatment, the OCD resist is removed by washing with BHF solution for 5 minutes.

  In the step (f), a thick film resist 42 is applied on the thin polycrystalline silicon film 40 by a spin coater under the conditions of 2000 rpm and 25 sec, and baked at 110 ° C. for 10 minutes. Thereafter, using the mask 200 shown in FIG. 8, ultraviolet exposure is performed for 60 seconds, development is performed for 2 minutes, and the thick film resist 42 is patterned as shown in FIG. 5C. At the time of ultraviolet exposure, the mask 200 is accurately positioned with the beam portions 11a and 12a formed in the step (e).

  In FIG. 8, the ultraviolet transmissive region of the mask 200 is a portion not hatched and corresponds to a portion where the thick film resist 42 in FIG. 5C is removed. Reference numerals 110 and 120 correspond to the bridges 11 and 12, and reference numerals 130, 140, and 150 correspond to the back plate 13, the acoustic hole 14, and the peripheral portion 15 of the beam portion, respectively.

In the step (g), the upper Si layer 10 and the polycrystalline silicon film 40 are etched by ICP-RIE to form the outer shape of the bridges 11 and 12, the outer shape of the back plate 13, and the acoustic hole 14. Etching is performed at 25.6 μm, which is the sum of the thicknesses of the upper Si layer 10 and the polycrystalline silicon film 40.
In the step (h), the thick film resist 42 is removed by washing at 90 ° C. for 5 minutes with sulfuric acid / hydrogen peroxide. A polycrystalline silicon film 40 exists on the outermost surface. Thereafter, in order to protect the etched surface by the etching in the step (g), the thick film resist is applied again and baked.

Through the above series of steps, the upper structure of the condenser microphone 1 is completed, and then the lower structure is manufactured. The upper structure is as illustrated in step (h) of FIG.
This will be described with reference to FIG.
In step (i), an aluminum (Al) layer 31 is formed to a thickness of 0.1 μm on the lower Si layer 30 by vacuum deposition.

In step (j), a resist 43 is applied to the surface of the Al layer 31 by a spin coater under the conditions of 3000 rpm and 30 seconds, baked at 90 ° C. for 5 minutes, and then exposed to ultraviolet light for 4.0 seconds and developed for 1. After 5 minutes, the resist 43 is patterned as shown in FIG. The pattern of the resist 43 is a ring pattern.
In the step (k), the Al layer 31 is etched for pattern formation by immersing in a mixed acid P solution (H ₃ PO ₄ + HNO ₃ + CH ₃ COOH + H ₂ O ₂ ) for 2 min, and RIE, that is, oxygen gas The resist 43 is removed by ashing using.

  In the step (l), a thick film resist 44 is applied to the surface of the lower Si layer 30 from which the Al layer 31 has been removed using a spin coater under the conditions of 2000 rpm and 25 seconds, and baked under the conditions of 110 ° C. and 5 minutes, followed by ultraviolet exposure. For 60 seconds and development for 2 minutes, and the resist 43 is patterned as shown in FIG. The Al layer 31 and the thick film resist 44 are separated from each other by a distance r. That is, the Al layer 31 and the thick film resist 44 exist with the ring-shaped region R therebetween.

The description will be continued with reference to FIG.
In step (m), the ring-shaped region R of the lower Si layer 30 is etched by about 55 μm by ICP-RIE. This etching amount determines the thickness of the diaphragm 32.
In the step (n), the thick film resist 44 is removed by a remover (resist stripping solution). At this time, since the protective thick film resist applied to the upper surface of the condenser microphone 1 in step (h) is also peeled off, the protective thick film resist is again applied to the upper surface with a spin coater under the conditions of 2000 rpm and 25 sec. Apply and bake at 110 ° C. for 10 min.

In step (o), the lower Si layer 30 is etched by about 445 μm by ICP-RIE using the Al layer 31 as a mask. Thereby, the lower Si layer 30 is entirely removed in the ring-shaped region R, and the outer shape of the diaphragm 32 having a thickness of 55 μm is formed inside the ring-shaped region R.
In the step (p), the SiO ₂ layer 20 is removed with strong hydrofluoric acid after washing at 90 ° C. for 5 minutes with sulfuric acid / hydrogen peroxide. Thereby, the diaphragm 32 is completely separated from the back plate 32.

  Finally, an Al metal layer 50 having a thickness of 0.05 μm is formed on the upper surface of the condenser microphone 1 to complete the condenser microphone 1 shown in the cross-sectional view of FIG. Of the Al metal layer 50, the Al film 51 provided on the bridges 11 and 12 and the Al film 52 provided on the back plate 13 exist on the same plane, so that the movable side electrode outlet M and the fixed side electrode take out. The mouth F is also on the same plane, can be easily connected to an electric circuit, and a simple configuration as a whole can be realized.

  In the embodiment described above, the cross-shaped bridges 11 and 12 in which the two beam portions 11a and 12a are crossed are used, but one bridge may be used. Since the sound wavelength is sufficiently longer than that of the diaphragm 32, the sound pressure is uniformly applied to the diaphragm 32. Therefore, even in the configuration in which the diaphragm 32 is suspended by a single bridge, the vibration of the diaphragm 32 is accurately transmitted to the beam portion.

As in the manufacturing method described above, the condenser microphone of this embodiment is manufactured using a silicon micromachining technique in all steps using an SOI wafer as a material. Accordingly, a large number of condenser microphones of the same size can be manufactured in an array on a single wafer, and an assembly process is not required, so that the manufacturing cost can be greatly reduced. Further, this manufacturing method has very high dimensional accuracy. For example, when the gap length d determined by the thickness of the SiO ₂ layer 20 is 1 μm, it can be manufactured with an accuracy of 5 nm or less.

<< Explanation of electric circuit >>
Next, an electric circuit of a microphone system using the above-described condenser microphone will be described. As an electric circuit for extracting an electric signal corresponding to sound pressure from a microphone element, there are generally an FM method and a voltage detection method.

[Voltage detection method]
FIG. 9 shows an electric circuit in the voltage detection system, and shows an equivalent circuit for alternating current when the capacitance of the condenser microphone is Cm and the electrode gradient between the electrodes is G. U is the displacement of the diaphragm and R0 is the load resistance. The electrostatic capacitance Cd (F) of the microphone element can be obtained by the following equation (1).
Cm = ε ₀ · ε _r · (S / d) (1)
However, (epsilon) ₀ is a dielectric constant of a vacuum and is (epsilon) ₀ = 8.854 * 10 < ^-12 > (F / m). Further, ε _r is a relative dielectric constant, and in the case of air, ε _r ≈1. S (m ² ) is the area of the electrode, and d (m) is the distance between the electrodes.

At this time, the output voltage E is expressed by the following equation (2).
E = GU / {1+ (1 / jωCmR0)} (2)
In order to obtain a voltage change according to the sound pressure by the voltage detection method, a DC voltage is applied to one of the electrodes. Alternatively, a charged film (electret) may be attached to one of the electrodes like an electret condenser microphone.

[FM method]
FIG. 10 shows an electric circuit in the FM system. Reference numeral 80 denotes a microphone element composed of the above-described condenser microphone, 81 is an oscillation circuit, 82 is an FM detection circuit, and 83 is a speaker. The oscillation circuit 81 is provided with capacitors 81b and 81c, a coil 81d, and an inverting amplifier 81a. The capacitance Cm of the microphone element 80 is calculated by the above-described equation (1). However, since the distance between the diaphragm and the electrode changes depending on the displacement amount of the diaphragm 32, the capacitance Cm changes depending on the sound pressure. It will be.

When the microphone element 80 is not connected, the oscillation frequency Fosc of the oscillation circuit 81 including the inverting amplifier 81a, the capacitors 81b and 81c, and the coil 81d is calculated by the following equation (3).
Fosc = 1 / 2π {L × C1 × C2 / (C1 + C2)} ^1/2 (3)

  Equation (3) means that when the inductance (L) of the coil 81d is not changed, the oscillation frequency Fosc changes when any of the capacitances (C1, C2) of the capacitors 81b, 81c is changed. ing. Therefore, the frequency change according to the sound pressure applied to the microphone element 80 can be obtained by connecting the microphone element 80 in parallel to the capacitor C1 as shown in FIG. A voltage that changes in accordance with the sound pressure can be taken out through the FM detection circuit 82, and can be output as a sound from the speaker 83.

  FIG. 11 is a cross-sectional view schematically showing a schematic configuration of a microphone head 90 using the above-described condenser microphone. Reference numeral 91 denotes a chip on which a condenser microphone is formed. The chip 91 is mounted on a substrate 92. A circuit element 93 and the like are also mounted on the substrate 92. The substrate 92 is housed in the casing 94 of the microphone head 90. When sound pressure acts on the microphone head 90, the condenser microphone diaphragm 32 (see FIG. 2) vibrates and a signal is output from the circuit element 93.

-Second Embodiment-
In the first embodiment described above, one condenser microphone is formed on the substrate, but a plurality of condenser microphones having different frequency characteristics may be formed in an array on one substrate. The plurality of condenser microphones are used as one microphone system. Specific application examples include a sound detection unit of a wearable auscultation system, which is one of wearable physical condition monitoring systems, and a voice input terminal of a portable communication device.

  FIG. 12 is a diagram showing an example of a microphone array in which a plurality of condenser microphones having different resonance frequencies are formed, and shows a part of a wafer on which a plurality of the same microphone arrays 300 are formed. In the microphone array 300, five condenser microphones 301, 302, 303, 304, and 305 having different resonance frequencies are formed. Hereinafter, these condenser microphones 301 to 305 will be referred to as microphone elements.

  Each microphone array 300 shown in FIG. 12 is cut out from the wafer as a chip, and is incorporated into the microphone head 90 of FIG. 11 as in the first embodiment. FIG. 13 is a diagram showing an electric circuit in the case of the FM system, and corresponds to FIG. 10 of the first embodiment. For each of the microphone elements 301 to 305, a transmission circuit 81 and an FM detection circuit 82 are provided.

  The FM detection circuit 82 performs FM detection and adjusts the gain so that the output peak values of the FM detection circuits 82 become substantially the same value. The output of each FM detection circuit 82 is input to a voltage synthesis circuit 84 where voltage synthesis is performed. The synthesized voltage is output to an output device such as a speaker as shown in FIG.

  In the example shown in FIG. 12 described above, the five microphone elements 301 to 305 are collectively formed as one microphone array 300. However, the microphone elements 301 to 305 may be separately formed on the wafer 100 as shown in FIG. good. In the example illustrated in FIG. 14, microphone elements 301, 302, 303, and 305 having the same size are combined into a microphone array 300 </ b> A, and the microphone element 304 having a large area is formed in another area.

  In this case, the cut out microphone array 300A and the microphone element 304 are mounted on the same substrate and used as a microphone system. By arranging the microphone elements 301 to 305 as shown in FIG. 14, the wafer 100 can be used effectively. The microphone elements 301 to 305 formed in the microphone array 300 have the same basic structure. However, in order to make the resonance frequency different as described later, the dimensions of the diaphragm and the number and dimensions of the beams are different from each other. It is set every 305.

FIG. 15 is a plan view of a microphone element 305 having two beams, where (a) shows the electrode surface side and (b) shows the diaphragm surface side. FIG. 16 is a perspective view showing a IV-IV cross section of FIG. As described above, the SOI wafer 100 has a three-layer structure including the upper Si layer 10, the lower Si layer 30, and the SiO ₂ layer 20 provided as an insulating layer therebetween (see FIG. 16). ).

  The microphone element 305 includes an electrode part 310, a diaphragm 311, a bridge 312 that holds the diaphragm 311 with respect to the electrode part 310 at a predetermined interval, and a base part 313. The vibration plate 311 is formed of a Si layer on a disk separated from the base portion 313 by forming a circular groove 320 in the lower Si layer 30. On the other hand, the cross-shaped bridge 312 is obtained by etching the upper Si layer 10 into a cross shape, and includes a fixed end portion 312a, a beam portion 312b, and a diaphragm fixing portion 312c. Two beams 330 are formed on the beam portion 312b.

The groove 321 shown in FIG. 15A reaches the lower Si layer 30, and the Si layer 10 constituting the electrode part 310 and the bridge 312 is separated from the Si layer 10 of the base part 313 by this groove 321. On the other hand, the circular groove 320 shown in FIG. 15B reaches the upper Si layer 10 through the lower Si layer 30 and the SiO ₂ layer 20. In the region inside the circular groove 320, the SiO ₂ layer 20 remains only in the cross-hatched region of FIG. 15B, and in the region outside the circular groove 320, the entire region excluding the groove 321 is left. The SiO ₂ layer 20 remains.

That is, the fixed end 312 a of the bridge 312 is fixed to the lower Si layer 30 of the base 313 via the SiO ₂ layer 20. The diaphragm 311 is fixed to the fixing portion 312c of the bridge 312 via the SiO ₂ layer 20 in the shaded area. Since the gap between the vibration plate 311 and the electrode portion 310 is a gap from which the SiO ₂ layer 20 is removed, the beam portion 312b of the bridge 312 is elastically deformed by the sound pressure acting on the vibration plate 311, and the vibration plate 311 is Vibrate.

  Similar to the first embodiment described above, a metal layer is formed on the electrode portion side surfaces of the electrode portion 310, the bridge 312 and the base portion 313. That is, the electrode part 310 constitutes one electrode plate of the condenser microphone, and the diaphragm 311 constitutes the other electrode plate. Lead terminal portions 310 d and 312 d are formed on the electrode portion 310 and the bridge 312, respectively. Note that a plurality of through holes 310 a are formed in the electrode portion 310, and these through holes 310 a function as holes for releasing air between the electrode portion 310 and the vibration plate 311 when the vibration plate 311 vibrates. .

  As described above, the five microphone elements 301 to 305 formed in the microphone array 300 have different resonance frequencies. The microphone elements 301 to 305 have a structure in which the diaphragm 311 is supported by four beam portions 312b, and the diaphragm 311 vibrates when the beam portion 312b is deformed by the action of sound pressure. Therefore, in the present embodiment, the resonance frequencies of the microphone elements 301 to 305 are set to different values by changing the dimensions of the diaphragm 311, the dimensions of the beam portion 312 b, and the number of beams.

FIG. 17 is an enlarged view of the beam portion 312 b of the microphone element 305. The dimensions of the beam 330 are represented by a width W, a length L, and a height t. In the case of the microphone element 305, the number N of beams is two. FIG. 18 is a diagram showing dimensions and characteristics of each of the microphone elements 301 to 305. The dimensions W, L, t, the number N of the beams 330, the area and mass m of the diaphragm 311, and the sectional secondary moment I of the beams 330 are shown. It shows the amplitude of a _V spring constant k, the resonance frequency f of the microphone element ₀ and the diaphragm 311 of microphone elements.

<Description of resonance system model>
Then, moment of inertia of area I in FIG. 18, the spring constant k, the resonance frequency _{f 0} and an amplitude weight _{A V} is described. In the present embodiment, as an example, a configuration as shown in FIG. 19 is adopted as a resonance system model. In this resonance system model, the fixed portion 312c of the bridge 312 and the diaphragm 311 are arranged at the free ends (point C in FIG. 19) of the plurality of single support beams 330 having the fixed end 312a as the fixed end (point B in FIG. 19). Is fixed.

It is assumed that the external force acting on the diaphragm 311 by the sound pressure is F = PScos ωt. For example, when the number N of the beams 330 is N = 1 as in the microphone element 301 (see FIG. 18), the force applied to the point _C is 1 / F of the force F applied to the four beams 330 as a whole. 4, that is, F _C = F / 4. Therefore, the deflection angle theta _C by the force F _C at point C is expressed by the following equation (4). Further, the deflection angle θ _MC due to the moment _{MC at the} point _C is expressed as the following equation (5).
^{_{θ C = F C L 2 /}} 2EI = FL 2 / 8EI ... (4)
θ _MC = M _C L / EI (5)

On the other hand, since it can be assumed that dy / dl = 0 at the point C, the above θ _C and θ _MC satisfy θ _C = θ _MC . As a result, the moment MC at the unknown point _C can be expressed as shown in Equation (6).
M _C = FL / 8 (6)

Therefore, the deflection X at the point C is calculated by the following equation (7) using a superposition method of the deflection X1 due to the force and the deflection X2 due to the moment.
X = X1 + X2
^{_{= F C L 3 / 3EI +}} (- M C L 2 / 2EI)
^{= FL 3 / 12EI-FL 3} / 16EI
= FL ³ / 48EI (7)

Since F = kX is established between the external force F and the displacement (deflection) X, the spring constant k of the microphone element is expressed as in Expression (8). This equation (8) is a relationship that holds regardless of the number N of beams 330.
k = F / X
= 48EI / L ³ (8)
At this time, the resonance frequency f ₀ of the microphone element is expressed as in Expression (9) if the mass of the diaphragm 311 is m.
f ₀ = (1 / 2π) √ (k / m)
= (1 / 2π) √ (48EI / mL ³ ) (9)

Since the beam 330 has a width W and a height t, the cross-sectional secondary moment I of the beam portion 312b in the equation (9) is calculated by the equation (10).
^{I = (Wt 3/12)} × N ... (10)
N is the number of beams 330 formed in the beam portion 312b. That is, by adjusting the dimension W, t, L and the number N of beams 330, the resonance frequency f ₀ of the microphone element can be set to a desired value.

When considering a resonance system of a microphone element, an equation of motion such as equation (11) is considered in consideration of mechanical resistance r that contributes to attenuation of vibration amplitude.
m (dx ² / d ² t) + r (dx / dt) + kx = PScos ωt (11)
The right side is the external force due to sound pressure. In equation (11), if γ = r / 2m and ω ₀ = 2πf ₀ = √ (k / m), then equation (11) is transformed into equation (12) below. However, it is assumed that ω ₀ > γ.
dx ² / d ² t + 2γ (dx / dt) + ω ₀ ² x = (PS / m) cos ωt (12)

By solving the equation shown in equation (12), the vibration amplitude _AV is given by equation (13).
A _V = (PS / m) × 1 / {(ω ₀ ² −ω ² ) ² + 4γ ² ω ² } ^1/2 (13)
Here, gamma = because it is r / 2m, the amplitude A _V is contains mechanical resistance r as unknowns, the mechanical resistance r is measured resonant sharpness Q (= f 0 _/ Δf of the formula (13) ) Using r = ω ₀ m / Q. 20, with respect to the microphone element 304 of FIG. 18 shows the actual measurement value and the theoretical value of the amplitude of A _V. From Figure 20, between the measured value and the amplitude of A _V calculated from the model described above, it can be seen that sufficiently consistent is taken.

  Conventionally, when a condenser microphone formed on a Si substrate is used, a region R1 in which the amplitude is almost constant over a wide frequency range is used. The microphone system of the present embodiment is characterized in that the resonance frequency of the diaphragm 311 is used. As can be seen from FIG. 20, in the resonance region R2, the amplitude amount is larger than that in the region R1, and the sensitivity of the microphone can be improved. In the example of FIG. 20, the peak value in the region R2 is about three times that in the region R1.

However, since one microphone element cannot cope with a wide frequency range, the resonance frequency f ₀ of the microphone elements 301 to 305 formed in the microphone array 300 is set to be shifted little by little, and a predetermined frequency as shown in FIG. Made it possible to detect frequency domain audio.

  FIG. 21 shows output characteristics of signals obtained from the respective microphone elements 301 to 305. The horizontal axis represents frequency and the vertical axis represents output voltage. A curved line L1 represents the output characteristics of the microphone element 301, and similarly, curved lines L2 to L5 represent the output characteristics of the microphone elements 302 to 305, respectively. Since the peak heights of the characteristics L1 to L5 vary, as described above, gain adjustment is performed by the FM detection circuit 82 of FIG. 13 to equalize the output peak values so as to be substantially the same value.

  Then, the equalized outputs are synthesized by the voltage synthesis circuit 84 of FIG. 13 to obtain a synthesis characteristic L10 having a constant level in a wide frequency region R3. Since the resonance frequency of the characteristic L1 is 0.3 kHz and the resonance frequency of the characteristic L5 is 3 kHz (see FIG. 18), the frequency region R3 covers a range of at least 0.3 kHz to 3 kHz. Furthermore, since the output voltage value of the composite characteristic L10 is substantially the same as the peak values of the characteristics L1 to L5, the sensitivity is improved as compared with the conventional case.

  As described above, the microphone system according to the second embodiment includes a plurality of microphone elements that perform sound detection in the resonance frequency region, and can detect sound in a wide frequency region by slightly shifting the resonance frequency of each microphone element. It was configured as follows. As a result, the microphone can be miniaturized and voice detection can be performed with high sensitivity in a wide frequency range.

  In addition, the present invention is not limited to the above embodiment as long as the characteristics of the present invention are not impaired. For example, the bridge 312 may not have a cross shape, and the diaphragm 311 may have a rectangular shape.

1 is a perspective view schematically showing an overall configuration of a condenser microphone according to a first embodiment of the present invention. It is XZ sectional drawing cut | disconnected along II of FIG. It is a fragmentary perspective view for demonstrating the manufacturing process (ad) of the condenser microphone which concerns on 1st Embodiment. It is a fragmentary perspective view for demonstrating the manufacturing process (ad) of the condenser microphone which concerns on 1st Embodiment. It is a fragmentary perspective view for demonstrating the manufacturing process (eh) of the condenser microphone which concerns on 1st Embodiment. It is a fragmentary sectional view for demonstrating the manufacturing process (i-1) of the capacitor | condenser microphone which concerns on 1st Embodiment. It is a fragmentary sectional view for demonstrating the manufacturing process (mp) of the condenser microphone which concerns on 1st Embodiment. It is a top view of the mask used for the manufacturing process (f) of the condenser microphone which concerns on 1st Embodiment. It is a figure which shows the electric circuit in a voltage detection system. It is a figure which shows the electric circuit in FM system. 2 is a cross-sectional view showing a schematic configuration of a microphone head 90. FIG. It is a figure which shows the 2nd Embodiment of this invention, and shows an example of the microphone array used for a microphone system. It is a figure which shows the electric circuit of the FM system in 2nd Embodiment. It is a figure which shows the other example of arrangement | positioning of the microphone element on a wafer. 4A and 4B are plan views of the condenser microphone 305, where FIG. 5A shows the electrode surface side, and FIG. 5B shows the diaphragm surface side. It is a perspective view which shows the IV-IV cross section of FIG. It is the perspective view which expanded and showed the beam part 312b. It is a figure which shows the various dimensions and various characteristics of the microphone elements 301-305. It is a figure which shows the resonance system model of a microphone element. It illustrates and compares the measured value with the theoretical value of the amplitude of A _V. It is a figure which shows the output characteristics L1-L5 of the microphone elements 301-305, and the synthetic | combination characteristic L10.

Explanation of symbols

1,301 to 305 Condenser microphone 10 Upper Si layer 11, 12, 312 Bridge 11a, 12a Beam portion 13 Back plate 20 SiO ₂ layer 21 Spacer 30 Lower Si layer 31 Base plate 32, 311 Vibration plate 40 Underlayer (polycrystalline silicon film) )
50 Metal layer 51, 52 Electrode film (Al film)
DESCRIPTION OF SYMBOLS 81 Oscillator 82 FM detector 90 Microphone head 100 SOI wafer 200 Mask 300 Microphone array 310 Electrode part 312b Beam part 313 Base part 330 Beam A Cavity d Gap length F Fixed side electrode outlet M Movable side electrode outlet R Ring-shaped region (Ring space)

Claims (7)

A condenser microphone manufactured by photolithography,
A back plate whose periphery is supported on the base plate;
A diaphragm provided at a predetermined interval from the back plate;
A beam-like member for suspending the diaphragm from the base plate;
A pair of electrodes for taking out capacitance change between the diaphragm and the back plate as an electrical signal ;
The beam-like member has a cross shape,
The cross-shaped beam-shaped member has a beam portion and a fixed end portion,
The beam portion is provided so as to be positioned in a cross-shaped opening formed in the back plate,
The condenser microphone is characterized in that the fixed end portion is fixed to the base plate so as to support the beam portion with both ends . The condenser microphone according to claim 1 ,
The pair of electrodes includes a movable side electrode and a fixed side electrode,
The condenser microphone is characterized in that the movable side electrode is provided on an upper surface of the beam portion, and the fixed side electrode is provided on an upper surface of the back plate. The condenser microphone according to claim 1 or 2 ,
The condenser microphone, wherein the base plate, the back plate, the vibration plate, and the beam-like member are manufactured by photolithography on an SOI wafer. A method for producing a condenser microphone by photolithography,
Forming a back plate having a cross-shaped opening with a peripheral portion supported on the base plate;
Forming a diaphragm with a predetermined distance from the back plate;
Forming a beam-like member for suspending the diaphragm from the base plate;
Forming a pair of electrodes for taking out a change in capacitance between the diaphragm and the back plate as an electrical signal,
In the step of forming the beam member,
The beam-shaped member includes a beam portion provided so as to be positioned in a cross-shaped opening formed in the back plate, and a fixed portion fixed to the base plate so as to support the beam portion with both ends. A method of manufacturing a condenser microphone, characterized in that the condenser microphone is formed as a cross shape . A condenser microphone according to any one of claims 1 to 3 ,
A microphone system comprising: a detection circuit that extracts a signal corresponding to a sound pressure acting on the diaphragm in a resonance frequency region of the condenser microphone from an electrical signal extracted from the condenser microphone. A microphone array in which a plurality of condenser microphones according to any one of claims 1 to 3 having different resonance frequencies are formed on the same wafer substrate;
A plurality of signals that are provided in each of the plurality of condenser microphones and that extract signals corresponding to sound pressure acting on the diaphragm in a resonance frequency region of the condenser microphones from electrical signals extracted from the condenser microphones. A detection circuit;
A microphone system comprising: a signal input from each detection circuit; and a combining circuit that combines the input signals and outputs a combined signal. The microphone system according to claim 6 , wherein
A microphone system, wherein a resonance frequency of each condenser microphone is set so that a combination of the resonance frequency areas of the plurality of condenser microphones forms one continuous frequency area.

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