Capacitance type sensor, acoustic sensor, and microphone

A chamber that penetrates vertically is formed in a silicon substrate. A diaphragm is arranged on the upper surface of the silicon substrate so as to cover the upper opening of the chamber. The diaphragm is divided by slits into a region located above the chamber (first diaphragm) and a region located above the upper surface of the silicon substrate (second diaphragm). A fixed electrode plate is arranged above the first diaphragm, and a low-volume first acoustic sensing portion is formed by the first diaphragm and the fixed electrode plate. Also, a high-volume second acoustic sensing portion is formed by the second diaphragm and the upper surface (electrically conducting layer) of the silicon substrate.

TECHNICAL FIELD

The present invention relates to a capacitive sensor, an acoustic sensor, and a microphone. Specifically, the present invention relates to a capacitive sensor constituted by a capacitor structure that is made up of a vibrating electrode plate (diaphragm) and a fixed electrode plate. The present invention also relates to an acoustic sensor (acoustic transducer) that converts acoustic vibration into an electrical signal and outputs it, and a microphone that uses the acoustic sensor. In particular, the present invention relates to a very small capacitive sensor and acoustic sensor that are produced using MEMS (Micro Electro Mechanical System) technology.

RELATED ART

Electret condenser microphones have conventionally been widely used as compact microphones installed in mobile phones and the like. However, electret condenser microphones are susceptible to heat, and are inferior to MEMS microphones in terms of adaptability to digitalization, size reduction, functionality advancement and multifunctionality, and power saving. For this reason, MEMS microphones have currently become prevalent.

A MEMS microphone includes an acoustic sensor (acoustic transducer) that detects acoustic vibration and converts it into an electrical signal (detection signal), a drive circuit that applies a voltage to the acoustic sensor, and a signal processing circuit that performs signal processing such as amplification on the detection signal from the acoustic sensor and outputs the resulting signal to the outside. The acoustic sensor used in MEMS microphones is a capacitive acoustic sensor manufactured using MEMS technology. Also, the drive circuit and the signal processing circuit are integrally manufactured as an ASIC (Application Specific Integrated Circuit) using semiconductor manufacturing technology.

In recent years, microphones have been required to detect sound from low sound pressures to high sound pressures with high sensitivity. In general, the maximum input sound pressure of a microphone is limited by the harmonic distortion ratio (Total Harmonic Distortion). This is because if a sound with a high sound pressure is to be detected by a microphone, harmonic distortion occurs in the output signal, thus impairing the sound quality and precision. Accordingly, if the harmonic distortion ratio can be reduced, it is possible to increase the maximum input sound pressure and widen the detectable sound pressure range (hereinafter, called the dynamic range) of the microphone.

However, with ordinary microphones, there is a tradeoff relationship between an improvement in acoustic vibration detection sensitivity and a reduction in the harmonic distortion ratio. For this reason, with highly sensitive microphones that can detect low-volume (low sound pressure) sounds, the harmonic distortion ratio of the output signal increases when a high-volume sound enters, and therefore the maximum detectable sound pressure is limited. This is because the output signal of the highly sensitive microphone increases, and harmonic distortion easily occurs. Conversely, when attempting to increase the maximum detectable sound pressure by reducing the harmonic distortion of the output signal, the sensitivity of the microphone degrades, and it is difficult to detect low-volume sounds with high quality. As a result, it has been difficult to give ordinary microphones a wide dynamic range from low-volume (low sound pressure) to high-volume (high sound pressure) sounds.

In light of this technical background, a microphone that employs multiple acoustic sensors having different detection sensitivities has been considered as a method for realizing a microphone that has a wide dynamic range. Microphones of this type are disclosed in Patent Documents 1 to 4, for example.

Patent Documents 1 and 2 disclose a microphone that is provided with multiple acoustic sensors, and switches or combines multiple signals from the acoustic sensors according to the sound pressure. This type of microphone switches between, for example, using a high-sensitivity acoustic sensor having a detectable sound pressure level (SPL) of approximately 30 dB to 115 dB and using a low-sensitivity acoustic sensor having a detectable sound pressure level of approximately 60 dB to 140 dB, thus making it possible to configure a microphone having a detectable sound pressure level of approximately 30 dB to 140 dB. Also, Patent Documents 3 and 4 disclose a microphone in which multiple independent acoustic sensors are formed on one chip.

FIG. 1(A)shows the relationship between the harmonic distortion ratio and sound pressure in the high-sensitivity acoustic sensor of Patent Document 1.FIG. 1(B)shows the relationship between the harmonic distortion ratio and sound pressure in the low-sensitivity acoustic sensor of Patent Document 1. Also,FIG. 2shows the relationship between the average amount of diaphragm distortion and the sound pressure in the high-sensitivity acoustic sensor and the low-sensitivity acoustic sensor of Patent Document 1. Now, assuming that the permissible harmonic distortion ratio is 20%, the maximum detectable sound pressure of the high-sensitivity acoustic sensor is approximately 115 dB. Also, in the high-sensitivity acoustic sensor, the S/N ratio degrades if the sound pressure is lower than approximately 30 dB, and therefore the minimum detectable sound pressure is approximately 30 dB. Accordingly, the dynamic range of the high-sensitivity acoustic sensor is approximately 30 dB to 115 dB as shown inFIG. 1(A). Similarly, assuming that the permissible harmonic distortion ratio is 20%, the maximum detectable sound pressure of the low-sensitivity acoustic sensor is approximately 140 dB. Also, the area of the diaphragm is smaller in the low-sensitivity acoustic sensor than in the high-sensitivity acoustic sensor, and the average amount of diaphragm distortion is also smaller than in the high-sensitivity acoustic sensor as shown inFIG. 2. Accordingly, the minimum detectable sound pressure of the low-sensitivity acoustic sensor is higher than in the high-sensitivity acoustic sensor, and is approximately 60 dB. As a result, the dynamic range of the low-sensitivity acoustic sensor is approximately 60 dB to 140 dB as shown inFIG. 1(B). If the high-sensitivity acoustic sensor and the low-sensitivity acoustic sensor are combined, the detectable sound pressure range is widened to approximately 30 dB to 140 dB as shown inFIG. 1(C).

Note that the harmonic distortion ratio is defined as follows. The waveform shown by the solid line inFIG. 3(A)is a sine wave of a frequency f1that serves as the reference. If this reference sine wave is subjected to Fourier transform, spectrum components only appear at frequency f1positions. Assume that the reference sine wave inFIG. 3(A)has become distorted to the waveform shown by the dashed line inFIG. 3(A)for some sort of reason. Assume that when this distorted waveform is subjected to Fourier transform, the frequency spectrum shown inFIG. 3(B)is obtained. In other words, assume that the distorted waveform has FFT intensities (Fast Fourier Transform intensities) V1, V2, . . . , and V5at frequencies f1, f2, . . . , and f5. At this time, the harmonic distortion ratio THD of the distorted waveform is defined by Expression 1 below.

RELATED ART DOCUMENTS

Patent Documents

Patent Document 1: US Patent Application No. 2009/0316916, Specification

Patent Document 2: US Patent Application No. 2010/0183167, Specification

Patent Document 4: US Patent Application No. 2007/0047746, Specification

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

However, in the microphones described in Patent Documents 1 to 4, regardless of whether multiple acoustic sensors are formed on separate chips, or multiple acoustic sensors are integrally formed on one chip (substrate), the acoustic sensors have capacitor structures that are independent from each other. For this reason, acoustic characteristic variation and mismatching occur among these microphones. Here, acoustic characteristic variation refers to deviation in acoustic characteristics between acoustic sensors among chips. Also, acoustic characteristic mismatching refers to deviation in acoustic characteristics between acoustic sensors in the same chip.

Specifically, in the case where acoustic sensors are formed on separate chips, inter-chip variation occurs with respect to detection sensitivity due to, for example, variation in the warping and thickness of the produced diaphragms. As a result, there is an increase in the amount of inter-chip variation with respect to the difference in detection sensitivity between acoustic sensors. Also, even in the case where independent acoustic sensors are integrally formed on a common chip, when the capacitor structures of the acoustic sensors are produced using MEMS technology, variation easily occurs in the gap distance between the diaphragm and the fixed electrode. Furthermore, the back chamber and the vent hole are formed separately, and therefore the intra-chip mismatching occurs with acoustic characteristics that are influenced by the back chamber and the vent hole, such as the frequency characteristic and phase.

The present invention was achieved in light of the aforementioned technical issues, and an object thereof is to provide a capacitive sensor and an acoustic sensor that have a wide dynamic range and little mismatching between sensing portions due to forming multiple sensing portions having different sensitivities in an integrated manner, and that enable size reduction and noise reduction.

Means for Solving the Problems

A capacitive sensor according to the present invention is a capacitive sensor that includes: a substrate having a cavity that is open at least at an upper surface; a vibrating electrode plate formed above the substrate so as to cover an upper portion of the cavity; a back plate formed above the substrate so as to cover the vibrating electrode plate; and a fixed electrode plate provided on the back plate, wherein the vibrating electrode plate is divided into a region located above the cavity and a region located above an upper surface of the substrate, a first sensing portion is formed by the fixed electrode plate and the region of the vibrating electrode plate located above the cavity, and a second sensing portion is formed by the upper surface of the substrate and the region of the vibrating electrode plate located above the upper surface of the substrate.

The region of the vibrating electrode plate that constitutes the first sensing portion (i.e., the region located above the cavity) and the region of the vibrating electrode plate that constitutes the second sensing portion (i.e., the region located above the upper surface of the substrate) are divided by a slit formed in the vibrating electrode plate, for example. Also, in order for the upper surface of the substrate to become an electrode of the second sensing portion, the upper surface of the substrate may be subjected to electrical conductivity processing by ion injection or the like, or a substrate electrode may be formed on the upper surface of the substrate in opposition to the region of the vibrating electrode plate that constitutes the second sensing portion.

According to the capacitive sensor of the present invention, the vibrating electrode plate is divided, and therefore multiple sensing portions (a variable capacitor structure) are formed between the vibrating electrode plate and the fixed electrode plate. Accordingly, electrical signals are output from the respective divided sensing portions, and change in pressure such as acoustic vibration can be converted into multiple electrical signals that are then output. According to this capacitive sensor, the sensing portions can be given different detection ranges and sensitivities by, for example, giving each vibrating electrode plate a different area or giving each vibrating electrode plate a different amount of displacement, and the detection range can be widened without reducing the sensitivity, by switching or combining the signals.

Also, the sensing portions are formed by dividing the vibrating electrode plate or the fixed electrode plate that is produced at a single time, and therefore the variation in characteristics between sensing portions is lower than in conventional technology in which the sensing portions are independent from each other and produced separately. As a result, it is possible to reduce variation in characteristics caused by differences in detection sensitivities between the sensing portions. Also, the vibrating electrode plate and the fixed electrode plate are used in common for the sensing portions, and therefore it is possible to reduce mismatching regarding characteristics such as the phase and the frequency characteristics.

Also, in the capacitive sensor of the present invention, the second sensing portion is arranged so as to surround the first sensing portion, and therefore the size of the capacitive sensor can be reduced compared to the case where the first sensing portion and the second sensing portion are arranged side-by-side.

In an embodiment of the capacitive sensor according to the present invention, in a view from above the substrate, the fixed electrode plate is formed at a position that is not overlapped with a region of the vibrating electrode plate that constitutes the second sensing portion. According to this embodiment, it is possible to reduce the parasitic capacitance between the fixed electrode plate and the region of the vibrating electrode plate that constitutes the second sensing portion.

In another embodiment of the capacitive sensor according to the present invention, the vibrating electrode plate is divided into a region that constitutes the first sensing portion and a region that constitutes the second sensing portion, at a position shifted toward an interior of the cavity relative to an edge of an upper opening of the cavity. According to this embodiment, it is possible to reduce the parasitic capacitance between the region of the vibrating electrode plate that constitutes the first sensing portion and the upper surface of the substrate. Also, it is less likely to be influenced by the Brownian motion of air molecules between the region that constitutes the first sensing portion and the substrate upper surface, thus reducing noise in the signal from the first sensing portion.

In yet another embodiment of the capacitive sensor according to the present invention, a region of the vibrating electrode plate that constitutes the first sensing portion and a region of the vibrating electrode plate that constitutes the second sensing portion are partially continuous. According to this embodiment, the first sensing portion and the second sensing portion are electrically connected, and therefore the electrical wiring of the capacitive sensor is simplified. Also, if the vibrating electrode plate is supported by a fixing portion at the location where the first sensing portion and the second sensing portion are connected, the first sensing portion and the second sensing portion can be supported at the same time.

In still another embodiment of the capacitive sensor according to the present invention, a lower surface of an outer peripheral edge of a region of the vibrating electrode plate that constitutes the second sensing portion is supported by a fixing portion provided on the upper surface of the substrate. According to this embodiment, the second sensing portion can be reliably supported, and therefore it is possible to maintain independence between the vibration of the region of the vibrating electrode plate that constitutes the first sensing portion and the region of the vibrating electrode plate that constitutes the second sensing portion, and it is possible to prevent interference between signals from the first sensing portion and the second sensing portion.

In still another embodiment of the capacitive sensor according to the present invention, the area of a region of the vibrating electrode plate that constitutes the second sensing portion is smaller than the area of a region of the vibrating electrode plate that constitutes the first sensing portion. According to this embodiment, the first sensing portion is a high-sensitivity sensing portion, and the second sensing portion is a low-sensitivity sensing portion.

In still another embodiment of the capacitive sensor according to the present invention, a region of the vibrating electrode plate that constitutes the second sensing portion is further divided into a region that has a comparatively large area and a region that has a comparatively small area. According to this embodiment, it is possible to further widen the dynamic range of the capacitive sensor.

An acoustic sensor according to the present invention is an acoustic sensor employing the capacitive sensor according to the present invention, wherein a plurality of holes for allowing passage of acoustic vibration is formed in the back plate and the fixed electrode plate, and signals of different sensitivities are output from the first sensing portion and the second sensing.

With an acoustic sensor that has multiple sensing portions by using a common thin film and dividing an electrode, if acoustic vibration with a high sound pressure is applied, it is likely that the vibrating electrode plate in the high-sensitivity first sensing portion will collide with the back plate and distortion vibration will be generated. However, the second sensing portion of the acoustic sensor of the present invention has a structure that is not likely to be influenced by distortion vibration of the back plate, and therefore it is possible to prevent an increase in harmonic distortion in the second sensing portion on the low sensitivity side caused by distortion vibration generated on the high sensitivity side, and it is possible to prevent the dynamic range of the acoustic sensor from becoming narrow.

A microphone according to the present invention includes an acoustic sensor of the present invention and a circuit portion that amplifies a signal from the acoustic sensor and outputs the amplified signal to the outside. With the microphone of the present invention, it is possible to prevent an increase in harmonic distortion in the second sensing portion on the low sensitivity side caused by distortion vibration generated on the high sensitivity side, and it is possible to prevent the dynamic range of the microphone from becoming narrow.

In an embodiment of the microphone according to the present invention, the circuit portion includes a phase inversion circuit that inverts the phase of one output signal out of an output signal from the first sensing portion and an output signal from the second sensing portion. With an acoustic sensor (capacitive sensor) having the structure of the present invention, the signal phase is inverted between the signal output from the first sensing portion and the signal output from the second sensing portion. However, with this embodiment, it is possible to invert the phase of one output signal out of the output signal from the first sensing portion and the output signal from the second sensing portion with the phase inversion circuit, and therefore the output signal from the first sensing portion and the output signal from the second sensing portion can be handled with the same phase in the circuit portion.

Note that the solution to the problems in the present invention features an appropriate combination of the above-described constituent elements, and many variations of the present invention are possible according to the combination of the constituent elements.

INDEX TO THE REFERENCE NUMERALS

EMBODIMENTS OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments, and various design modifications can be made without departing from the gist of the present invention. Although the following description takes the example of an acoustic sensor and a microphone in particular, besides an acoustic sensor, the present invention is also applicable to a capacitive sensor such as a pressure sensor.

Configuration of Embodiment 1

Hereinafter, the structure of an acoustic sensor according to Embodiment 1 of the present invention will be described with reference toFIGS. 4 to 6.FIG. 4is an exploded perspective view of an acoustic sensor11according to Embodiment 1 of the present invention.FIG. 5is a cross-sectional diagram of the acoustic sensor11, and also shows an enlargement of one portion.FIG. 6(A)is a plan view of the acoustic sensor11from which a back plate18has been removed, and shows how a diaphragm13(vibrating electrode plate) and a fixed electrode plate19are overlapped above a silicon substrate12(substrate).FIG. 6(B)is a plan view of the acoustic sensor11from which the back plate18and the fixed electrode plate19have been removed, and shows the arrangement of the diaphragm13on the upper surface of the silicon substrate12.

This acoustic sensor11is a capacitive element manufactured using MEMS technology. As shown inFIGS. 4 and 5, this acoustic sensor11includes the diaphragm13provided on the upper surface of the silicon substrate12(substrate) via anchors16(fixing portions), and a canopy portion14that is arranged above the diaphragm13via a very small air gap20(air gap) and is fixed to the upper surface of the silicon substrate12.

A chamber15(cavity) that penetrates from the upper surface to the lower surface is formed in the silicon substrate12, which is made of single crystal silicon. The chamber15shown in the figures is constituted by wall surfaces that are inclined surfaces formed by (100) surfaces, (111) surfaces of the silicon substrate, and surfaces equivalent to the (111) surfaces, but the wall surfaces of the chamber15may be vertical surfaces. Also, the upper surface of the silicon substrate12has been given electrical conductivity by ion injection, and serves as an electrically conducting layer21. The electrically conducting layer21is connected to an electrode pad33provided on the upper surface of the back plate18.

In this way, if the electrically conducting layer21is formed on the upper surface of the silicon substrate12by ion injection and used as a substrate-side electrode for a later-described second acoustic sensing portion23b, there is no need to lay wiring as in the case of forming a wiring pattern by metal films on the upper surface of the silicon substrate12, and the process for producing the acoustic sensor11can be simplified.

The diaphragm13is arranged on the upper surface of the silicon substrate12so as to cover the upper opening of the chamber15. As shown inFIGS. 4 and 6(B), the diaphragm13is formed with an approximately rectangular shape. The diaphragm13is formed by an electrically conductive polysilicon thin film, and the diaphragm13itself is a vibrating electrode plate. The diaphragm13is produced at one time and as a single body, and is then divided into two regions by slits17that extend approximately parallel with the outer peripheral sides. Note that the diaphragm13is not completely divided into two by the slits17, and is mechanically and electrically connected in the vicinity of the end portions of the slits17(in the corner portions of the diaphragm13). In the following, out of the two regions divided by the slits17, the approximately rectangular region that is located in the central portion and has a large area is called a first diaphragm13a(the region that constitutes the first sensing portion of the vibrating electrode plate), and the region formed so as to surround the first diaphragm13ais called the second diaphragm13b(the region that constitutes the second sensing portion of the vibrating electrode plate).

It is possible to completely mechanically and electrically separate the first diaphragm13aand the second diaphragm13bfrom each other, or the second diaphragms13bof respective sides from each other, but in this case, the portions each need to be supported by an anchor, and the portions each need to be connected a wiring pattern. For this reason, in the present embodiment, the first diaphragm13aand the second diaphragm13bare separated by the slits17while also being connected in the corner portions, thus simplifying the support structure and eliminating the need for connections by a wiring pattern.

Leg pieces26provided in the corner portions of the diaphragm13, that is to say the first diaphragm13aand the second diaphragm13b, are supported by anchors16on the upper surface of the silicon substrate12, and the diaphragm13is supported so as to float above the upper opening of the chamber15and the upper surface of the silicon substrate12. Also, a lead-out interconnect27is drawn from the diaphragm13, and the lead-out interconnect27is connected to an electrode pad31provided on the upper surface of the back plate18.

As shown inFIG. 5, the canopy portion14is obtained by providing a fixed electrode plate19made of polysilicon on the lower surface of the back plate18made of SiN. The canopy portion14is formed with a dome shape and has a cavity portion underneath the dome, and the diaphragm13is covered by the cavity portion. A very small air gap20(air gap) is formed between the lower surface of the canopy portion14(i.e., the lower surface of the fixed electrode plate19) and the upper surface of the diaphragm13. Also, a lead-out interconnect28is drawn from the fixed electrode plate19, and the lead-out interconnect28is connected to an electrode pad32provided on the upper surface of the back plate18.

Many acoustic holes24(acoustic holes) for allowing the passage of acoustic vibration are formed in the canopy portion14(i.e., the back plate18and the fixed electrode plate19) so as to penetrate from the upper surface to the lower surface. As shown inFIGS. 4 and 6(A), the acoustic holes24are arranged regularly. In the illustrated example, the acoustic holes24are arranged in a triangular manner along three directions that form 120° angles with each other, but they may be arranged in a rectangular manner or a concentric circle manner.

In this acoustic sensor11, the fixed electrode plate19and the first diaphragm13aoppose each other via the air gap20so as to constitute a capacitor structure, and form a first acoustic sensing portion23a(first sensing portion). Similarly, the second diaphragm13band the surface of the silicon substrate12(the electrically conducting layer21) oppose each other via the air gap22so as to constitute a capacitor structure, and form a second acoustic sensing portion23b(second sensing portion).

Here, as shown inFIGS. 5 and 6(A), the fixed electrode plate19is provided in a region that opposes the first diaphragm13a, and the fixed electrode plate19is arranged so as to not be overlapped with the second diaphragm13bin a view from a direction perpendicular to the upper surface of the silicon substrate12. If the fixed electrode plate19is provided at a position opposing the second diaphragm13bas well as in the comparison example shown inFIG. 7, a parasitic capacitance Cs is generated between the fixed electrode plate19and the second diaphragm13b, and the signal from the first acoustic sensing portion23aand the signal from the second acoustic sensing portion23binterfere with each other. In contrast, if the fixed electrode plate19is provided at only the position opposing the first diaphragm13aas in the present embodiment, it is possible to reduce the parasitic capacitance between the fixed electrode plate19and the second diaphragm13band prevent the signals from the first acoustic sensing portion23aand the second acoustic sensing portion23bfrom interfering with each other.

Also, as shown inFIGS. 5 and 6(B), the slits17are shifted toward the interior of the chamber15relative to the edge of the upper opening of the chamber15, with the exception of the two end portions.

Operation of Embodiment 1

In the acoustic sensor11, when acoustic vibration enters the chamber15(front chamber), the diaphragms13aand13b, which are thin-films, vibrate with the same phase due to the acoustic vibration. When the diaphragms13aand13bvibrate, the capacitances of the acoustic sensing portions23aand23bchange. As a result, in the first acoustic sensing portion23a, the acoustic vibration (change in sound pressure) detected by the first diaphragm13ais change in the capacitance between the first diaphragm13aand the fixed electrode plate19, and this is output as an electrical signal. Also, in the second acoustic sensing portion23b, the acoustic vibration (change in sound pressure) detected by the second diaphragm13bis change in the capacitance between the second diaphragm13band the electrically conducting layer21of the silicon substrate12, and this is output as an electrical signal. Also, in a different use mode, such as a use mode in which the chamber15is the back chamber, acoustic vibration enters the air gap20in the canopy portion14through the acoustic holes24and causes the diaphragms13aand13b, which are thin films, to vibrate.

Also, since the area of the second diaphragm13bis smaller than the area of the first diaphragm13a, the second acoustic sensing portion23bis a low-sensitivity acoustic sensor for the sound pressure range from mid-volume to high-volume, and the first acoustic sensing portion23ais a high-sensitivity acoustic sensor for the sound pressure range from low-volume to mid-volume. Accordingly, the dynamic range of the acoustic sensor11can be widened by obtaining a hybrid form for the two acoustic sensing portions23aand23band outputting signals from a later-described processing circuit. For example, assuming that the dynamic range of the first acoustic sensing portion23ais approximately 30 to 120 dB, and that the dynamic range of the second sensing portion23bis approximately 50 to 140 dB, combining these two acoustic sensing portions23aand23bmakes it possible to widen the dynamic range to approximately 30 to 140 dB. Also, if the output of the acoustic sensor11is switched between the first acoustic sensing portion23afor low-volume to mid-volume and the second acoustic sensing portion23bfor mid-volume to high-volume, it is possible to not use output from the first acoustic sensing portion23awhen the volume is high. As a result, even if harmonic distortion increases in the high sound pressure range in the first acoustic sensing portion23a, the signal from it is not output from the acoustic sensor11, and thus does not influence the performance of the acoustic sensor11. Consequently, it is possible to raise the sensitivity of the first acoustic sensing portion23awith respect to low-volume.

Furthermore, in this acoustic sensor11, the first acoustic sensing portion23aand the second acoustic sensing portion23bare formed on the same substrate. Moreover, the first acoustic sensing portion23aand the second acoustic sensing portion23buse the first diaphragm13aand the second diaphragm13bthat are obtained by using the slits17to divide the diaphragm13produced at a single time and as an integrated body. In other words, a sensing portion that is originally one portion is divided into two portions, and the first acoustic sensing portion23aand the second acoustic sensing portion23bhave a hybrid form. For this reason, compared to a conventional example in which two independent sensing portions are provided on one substrate, and a conventional example in which sensing portions are respectively provided on separate substrates, the first acoustic sensing portion23aand the second acoustic sensing portion23bhave similar variation regarding detection sensitivity. As a result, variation in detection sensitivity between the two acoustic sensing portions23aand23bcan be reduced. Also, since one diaphragm is used in common for the two acoustic sensing portions23aand23b, it is possible to suppress mismatching regarding acoustic characteristics such as phase and frequency characteristics.

Application in Microphone

FIG. 8(A)is a plan view showing a partial breakaway view of a microphone41having the acoustic sensor11according to Embodiment 1 built therein, and shows the interior thereof with the upper surface of the cover43removed.FIG. 8(B)is a longitudinal sectional view of the microphone41.

This microphone41has the acoustic sensor11and a signal processing circuit44(ASIC), which is a circuit portion, built into a package made up of a circuit substrate42and a cover43. The acoustic sensor11and the signal processing circuit44are mounted on the upper surface of the circuit substrate42. A sound introduction hole45for the introduction of acoustic vibration into the acoustic sensor11is formed in the circuit substrate42. The acoustic sensor11is mounted on the upper surface of the circuit substrate42such that the lower opening of the chamber15is aligned with the sound introduction hole45and covers the sound introduction hole45. Accordingly, the chamber15of the acoustic sensor11is the front chamber, and the space inside the package is the back chamber.

The electrode pads31,32, and33of the acoustic sensor11are connected to pads47of the signal processing circuit44by respective bonding wires46. Multiple terminals48for electrically connecting the microphone41to the outside are provided on the lower surface of the circuit substrate42, and electrode portions49in conduction with the terminals48are provided on the upper surface of the circuit substrate42. Pads50of the signal processing circuit44mounted on the circuit substrate42are connected to the electrode portions49by respective bonding wires51. Note that the pads50of the signal processing circuit44have a function of supplying electrical power to the acoustic sensor11and a function of outputting capacitance change signals from the acoustic sensor11to the outside.

The cover43is attached to the upper surface of the circuit substrate42so as to cover the acoustic sensor11and the signal processing circuit44. The package has an electromagnetic shielding function, and protects the acoustic sensor11and the signal processing circuit44from mechanical shock and electrical disturbances from the outside.

In this way, acoustic vibration that has entered the chamber15through the sound introduction hole45is detected by the acoustic sensor11, and then output after being subjected to amplification and signal processing by the signal processing circuit44. Since the space inside the package is the back chamber in this microphone41, the volume of the back chamber can be increased, and the sensitivity of the microphone41can be increased.

Note that in this microphone41, the sound introduction hole45for introducing acoustic vibration into the package may be formed in the upper surface of the cover43. In this case, the chamber15of the acoustic sensor11is the back chamber, and the space inside the package is the front chamber.

FIG. 9is a circuit diagram of the MEMS microphone41shown inFIG. 8. As shown inFIG. 9, the acoustic sensor11includes the first acoustic sensing portion23aon the high sensitivity side and the second acoustic sensing portion23bon the low sensitivity side, whose capacitances change according to acoustic vibration.

Also, the signal processing circuit44includes a charge pump52, a low-sensitivity amplifier53, a high-sensitivity amplifier54, ΣΔ (ΔΣ) ADCs (Analog-to-Digital Converters)55and56, a reference voltage generator57, a buffer58, and a phase inversion circuit59.

The charge pump52applies a high voltage HV to the first acoustic sensing portion23aand the second acoustic sensing portion23b, the electrical signal output from the second acoustic sensing portion23bis amplified by the low-sensitivity amplifier53, and the electrical signal output from the first acoustic sensing portion23ais amplified by the high-sensitivity amplifier54. However, the first acoustic sensing portion23aoutputs the capacitance between the upper surface of the first diaphragm13aand the fixed electrode plate19, and the second acoustic sensing portion23boutputs the capacitance between the lower surface of the second diaphragm13band the electrically conducting layer21. For this reason, if the air gap20of the first acoustic sensing portion23ahas become narrow (wide), the air gap22of the second acoustic sensing portion23bbecomes wide (narrow), and the phases of the output of the first acoustic sensing portion23aand the output of the second acoustic sensing portion23bbecome inverted (the phases deviate by) 180°. For this reason, the phase of the output of the second acoustic sensing portion23bis inverted by the phase inversion circuit59such that this output is input to the low-sensitivity amplifier53in a state of having eliminated the phase difference with the output of the first acoustic sensing portion23a. Of course, the phase inversion circuit59may be inserted between the first acoustic sensing portion23aand the high-sensitivity amplifier54.

The signal amplified by the low-sensitivity amplifier53is converted into a digital signal by the ΣΔ ADC55. Similarly, the signal amplified by the high-sensitivity amplifier54is converted into a digital signal by the ΣΔ ADC56. The digital signals obtained by the ΣΔ ADCs55and56are output to the outside on one data line as a PDM (Pulse Density Modulation) signal via the buffer58. Although not shown, the digital signals consolidated on the one data line are selected according to the signal strength, and thus the first acoustic sensing portion23aand the second acoustic sensing portion23bare automatically switched according to the sound pressure.

Note that although two digital signals obtained by the ΣΔ ADCs55and56are consolidated and output on one data line in the example ofFIG. 9, these two digital signals may be output on separate data lines.

Furthermore, according to this structure of the acoustic sensor11, the size of the acoustic sensor11can be reduced. The Applicant of the present invention has proposed the structure shown inFIG. 10(e.g., Japanese Patent Application No. 2012-125526) as an acoustic sensor in which two acoustic sensing portions are achieved in a hybrid manner to widen the dynamic range, and mismatching between the acoustic sensing portions and the like are reduced. In an acoustic sensor61(reference example) inFIG. 10, a diaphragm64provided on the upper surface of a silicon substrate62is divided into left and right portions by a slit63, thus forming a first diaphragm64ahaving a large area and a second diaphragm64bhaving a small area. A fixed electrode plate65ahaving a large area is provided above the first diaphragm64aand in opposition to the first diaphragm64a, and a high-sensitivity first acoustic sensing portion66ais constituted by the first diaphragm64aand the fixed electrode plate65a. Similarly, a fixed electrode plate65bhaving a small area is provided above the second diaphragm64band in opposition to the second diaphragm64b, and a low-sensitivity second acoustic sensing portion66bis constituted by the second diaphragm64band the fixed electrode plate65b. With this acoustic sensor61, the first acoustic sensing portion66aand the second acoustic sensing portion66bare aligned side-by-side, and therefore size in a view from above is large, and occupied area in the case of mounting on a interconnect substrate or the like is large.

In contrast, with the acoustic sensor11of the present embodiment, the first acoustic sensing portion23aand the second acoustic sensing portion23bare constituted in the central portion and the outer peripheral portion, and therefore there is almost no change in size from a conventional acoustic sensor that has a signal acoustic sensing portion. Therefore, according to the acoustic sensor11of the present embodiment, it is possible to reduce the sensor size in comparison with the acoustic sensor61of the reference example.

Also, with the acoustic sensor61shown inFIG. 10, the harmonic distortion of the acoustic sensor on the low sensitivity side increases due to interference between the first acoustic sensing portion66aon the high sensitivity side (low volume side) and the second acoustic sensing portion66bon the low sensitivity side (high volume side), and as a result, there is a risk of a decrease in the maximum detectable sound pressure of the acoustic sensor and a narrower dynamic range. According to the acoustic sensor11of Embodiment 1 of the present invention, it is possible to prevent this increase in harmonic distortion. The reason for this is as follows.

First, the case of the acoustic sensor61will be described. The first diaphragm64aon the high sensitivity side has a larger area and is more flexible than the second diaphragm64bon the low sensitivity side. For this reason, when acoustic vibration with a high sound pressure is applied to the acoustic sensor61, there are cases where the first diaphragm64acollides with the back plate67aas shown inFIG. 11.FIG. 11shows how the first diaphragm64acollides with the back plate67adue to high sound pressure in the acoustic sensor61.

If the first diaphragm64acollides with the back plate67aas shown inFIG. 11, the vibration of the back plate67abecomes distorted due to this collision, and distortion vibration occurs as shown inFIG. 12(A). Note that although the back plate also vibrates due to acoustic vibration similarly to the diaphragm, the amplitude of the back plate is around 1/100 the amplitude of the diaphragm, and therefore acoustic vibration is not shown inFIG. 12. The distortion vibration occurring in the back plate67ais transmitted to the back plate67b, and therefore the distortion vibration shown inFIG. 12(B)occurs in the back plate67bas well due to the collision with the first diaphragm64a. On the other hand, the second diaphragm64bundergoes less distortion than the first diaphragm64a, and therefore undergoes sine wave vibration as shown inFIG. 12(C), for example, and does not collide with the back plate67b. If the distortion vibration of the back plate67bis added to the sine wave vibration of the second diaphragm64b, the gap distance between the back plate67band the second diaphragm64bin the second acoustic sensing portion66bchanges as shown inFIG. 12(D). As a result, the output signal from the second acoustic sensing portion66bbecomes distorted, and the harmonic distortion ratio of the second acoustic sensing portion66bdegrades. For this reason, the acoustic sensor61needs to have a configuration for preventing the distortion vibration occurring on the first acoustic sensing portion side from being transmitted to the second acoustic sensing portion side.

In contrast, in the case of the acoustic sensor11of Embodiment 1, as shown inFIG. 13, even if distortion vibration occurs due to the first diaphragm13acolliding with the back plate18due to high sound pressure, the distortion vibration is not likely to influence the second acoustic sensing portion23b, and the harmonic distortion ratio of the second acoustic sensing portion23bis not likely to degrade. In other words, the second acoustic sensing portion23bdoes not have the back plate18or the fixed electrode plate19as constituent elements, and therefore the output of the second acoustic sensing portion23bis not influenced by distortion vibration of the back plate18. As a result, it is possible to prevent the dynamic range of the acoustic sensor11from becoming narrower due to distortion vibration in the first acoustic sensing portion23a.

As shown inFIG. 5, with the acoustic sensor11of Embodiment 1, the slits17are shifted toward the interior of the chamber15relative to the edges of the upper opening of the chamber15, with the exception of the two end portions. As a result, in a view from a direction perpendicular to the upper surface of the silicon substrate12, the first diaphragm13ais not overlapped with the upper surface (the electrically conducting layer21) of the silicon substrate12, it is possible to reduce the parasitic capacitance between the first diaphragm13aand the electrically conducting layer21, and it is possible to reduce signal interference of the first acoustic sensing portion23a.

Also, since air is trapped between the diaphragm13and the upper surface of the silicon substrate12, there are cases where Brownian motion of air molecules trapped there is a cause of acoustic noise. However, with the acoustic sensor11of Embodiment 1, the slits17are provided between the first diaphragm13aand the second diaphragm13b, and moreover the slits17are shifted toward the interior of the chamber15relative to the edges of the upper opening of the chamber15. For this reason, it is possible to eliminate the air trapped between the first diaphragm13aand the upper surface of the silicon substrate12, and since the influence of acoustic noise caused by air molecules trapped there is eliminated, it is possible to reduce acoustic noise in the first acoustic sensing portion23a.

Examples of Different Anchor Arrangements

In Embodiment 1, the leg pieces26provided in the corner portions of the diaphragm13are supported by the anchors16, but various modes are conceivable for the diaphragm13support structure as shown inFIGS. 14(A), 14(B), and15.

InFIG. 14(A), an anchor16is added at the outer peripheral edge of each side of the second diaphragm13b. InFIG. 14(B), an anchor16is provided for the entire outer peripheral edge of the second diaphragm13b. InFIG. 15, anchors16are provided at intervals along the outer peripheral edge of the second diaphragm13b. According to these modified examples, the diaphragm13and the second diaphragm13bin particular can be reliably supported by anchors16, thus making it possible to maintain independence between the vibration of the first acoustic sensing portion23aand the second acoustic sensing portion23b, and making it possible to prevent signals from interfering with each other.

Different Structures of Electrically Conducting Layer

Also, the electrically conducting layer21on the upper surface of the silicon substrate12may be a substrate electrode formed by subjecting a metal thin film to patterning on the upper surface of the silicon substrate12as shown inFIG. 16. In this modified example, the area of the electrically conducting layer21is determined by the patterning area of the metal thin film, and therefore variation in the area of the electrically conducting layer21decreases.

FIG. 17is a schematic plan view of an acoustic sensor71according to Embodiment 2 of the present invention, in a state in which a back plate has been removed. In this embodiment, the first diaphragm13aand four second diaphragms13bon the sides thereof are completely separated from each other by the slits17. Also, the first diaphragm13ais connected to the electrode pad31aon the back plate by a lead-out interconnect27adrawn from the first diaphragm13a. Moreover, a lead-out interconnect27bis drawn from each of the four second diaphragms13b, and these lead-out interconnects27bare connected to the electrode pad31bon the back plate by wiring72.

According to this embodiment, the electrical wiring of the first diaphragm13aof the first acoustic sensing portion23aand the electrical wiring of the second diaphragm13bof the second acoustic sensing portion23bcan be provided separately and independently, thus making it possible to reduce the parasitic capacitance between the first acoustic sensing portion23aand the second acoustic sensing portion23band to make it unlikely for signals to interfere with each other.

FIG. 18is a schematic plan view of an acoustic sensor74according to Embodiment 3 of the present invention, in a state in which a back plate has been removed. In this embodiment, the first diaphragm13aand the second diaphragm13bare completely separated from each other by the slit17. Meanwhile, the portions of the second diaphragm13bon the respective sides are mechanically and electrically connected to each other in the corner portions of the diaphragm13. Also, the first diaphragm13ais connected to the electrode pad31aon the back plate by a lead-out interconnect27adrawn from the first diaphragm13a. Moreover, the second diaphragm13bis connected to the electrode pad31bon the back plate by the lead-out interconnect27bdrawn from the second diaphragms13b.

According to this embodiment, the electrical wiring of the first diaphragm13aof the first acoustic sensing portion23aand the electrical wiring of the second diaphragm13bof the second acoustic sensing portion23bcan be provided separately and independently, thus making it possible to reduce the parasitic capacitance between the first acoustic sensing portion23aand the second acoustic sensing portion23band to make it unlikely for signals to interfere with each other. Moreover, there is no need to connect the portions of the second diaphragm13bon the respective sides to each other, thus making it possible to simplify the wiring.

FIG. 19is a schematic plan view of an acoustic sensor76according to Embodiment 4 of the present invention, in a state in which a back plate has been removed. In this embodiment, the first diaphragm13aand four second diaphragms13bon the sides thereof are completely separated from each other by the slits17. Out of two second diaphragms13baand13bbamong the four second diaphragms13b, the area of the one second diaphragm13bbis larger than the area of the other second diaphragm13ba. Also, a low-volume (high-sensitivity) first acoustic sensing portion23a(first sensing portion) is constituted by the capacitance between the first diaphragm13aand the fixed electrode plate19. Moreover, a mid-volume (mid-sensitivity) second acoustic sensing portion23c(second sensing portion) is constituted by the capacitance between the second diaphragm13bband the upper surface of the silicon substrate12, and a high-volume (low-sensitivity) second acoustic sensing portion23b(second sensing portion) is constituted by the capacitance between the second diaphragm13baand the upper surface of the silicon substrate12. Also, the first diaphragm13ais connected to the electrode pad31aon the back plate by a lead-out interconnect27adrawn from the first diaphragm13a. The second diaphragm13bbis connected to an electrode pad31bbon the back plate by a lead-out interconnect27bb. The second diaphragm13bais connected to an electrode pad31baon the back plate by a lead-out interconnect27ba. Also, the fixed electrode plate19is connected to an electrode pad32on the back plate by a lead-out interconnect28, and the upper surface (the electrically conducting layer21) of the silicon substrate12is connected to the electrode pad33.

According to this embodiment, the low-volume first acoustic sensing portion23a, the mid-volume second acoustic sensing portion23c, and the high-volume second acoustic sensing portion23bare provided, thus making it possible to further widen the sound pressure range (dynamic range) of the acoustic sensor76.

FIG. 20is a schematic plan view of an acoustic sensor81according to Embodiment 5 of the present invention, in a state in which a back plate has been removed. In this embodiment, the circular diaphragm13is divided by the slit17into the arc-shaped second diaphragm13blocated on the outer peripheral side and the circular first diaphragm13alocated inward thereof. Also, the fixed electrode plate19is formed on the lower surface of the back chamber so as to oppose the first diaphragm13a.

The first diaphragm13ais supported in a cantilever manner above the chamber15due to the lead-out interconnect27abeing fixed to the silicon substrate12or the like. The low-volume first acoustic sensing portion23ais constituted by the first diaphragm13aand the fixed electrode plate19.

The outer peripheral portion of the lower surface of the second diaphragm13bis supported by an approximately arc-shaped anchor16. The high-volume second acoustic sensing portion23bis constituted by the second diaphragm13band the electrically conducting layer21of the silicon substrate12.

Note that anchors16may be provided at intervals along the outer peripheral portion of the lower surface of the second diaphragm13bas shown inFIG. 21(A).

Also, as shown inFIG. 21(B), a configuration is possible in which a portion of the upper surface of the silicon substrate12protrudes toward the interior of the circular chamber15, a half-moon shaped anchor82is provided on this portion, and an end of the first diaphragm13ais supported in a cantilever manner by the anchor82. Using this anchor82makes it possible to improve the strength of the first diaphragm13a.

Although acoustic sensors and microphones using the acoustic sensors have been described above, the present invention is also applicable to a capacitance sensor other than an acoustic sensor, such as pressure sensor.