JP5980263B2 - Device including capacitance type electromechanical transducer - Google Patents

Device including capacitance type electromechanical transducer Download PDF

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JP5980263B2
JP5980263B2 JP2014094905A JP2014094905A JP5980263B2 JP 5980263 B2 JP5980263 B2 JP 5980263B2 JP 2014094905 A JP2014094905 A JP 2014094905A JP 2014094905 A JP2014094905 A JP 2014094905A JP 5980263 B2 JP5980263 B2 JP 5980263B2
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frequency
region
electromechanical transducer
electrode
current
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JP2014144376A (en
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貴弘 秋山
貴弘 秋山
高木 誠
誠 高木
一成 藤井
一成 藤井
水谷 英正
英正 水谷
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キヤノン株式会社
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Description

The present invention relates to an apparatus including a capacitive electromechanical transducer, such as an ultrasonic detection apparatus and an ultrasonic diagnostic apparatus.

Conventionally, a capacitive ultrasonic detector including a cell having electrodes arranged at intervals is known (see Patent Document 1). In particular, in recent years, a capacitive ultrasonic transducer (CMUT) using micromachining technology has been actively studied. This CMUT transmits or receives ultrasonic waves using a lightweight vibrating membrane, and can easily obtain excellent broadband characteristics even in liquid and gas. Using this CMUT, ultrasonic diagnosis with higher accuracy than conventional medical diagnostic modalities is attracting attention as a promising technology. The ultrasonic reception function of the CMUT is performed by a capacitance type electromechanical transducer and a subsequent electric circuit. The output of the capacitive electromechanical transducer in the previous stage is a current output because it is due to the time variation of the capacitance. Therefore, it is common to use a current-voltage conversion amplifier circuit in the subsequent stage.

On the other hand, piezoelectric materials (piezo) have been mainly used for practical ultrasonic transducers. Since the resolution of this piezoelectric material type device is proportional to the frequency, an ultrasonic transducer having a central sensitivity in the range of 3 MHz to 10 MHz is common. Compared to this, CMUT is characterized by having a wide frequency band. However, as an alternative to the piezoelectric material type, CMUT is intended to be used as a conventional sensor for general ultrasonic diagnosis. Also, 3 MHz to 10 MHz is normal. However, in order to effectively use a wide frequency band, a wide band is also required for the subsequent electrical circuit. The frequency characteristic of the ultrasonic reception function of the CMUT is usually configured as a bandpass type between the cutoff frequency of the capacitive electromechanical transducer and the cutoff frequency of the amplifier circuit. Therefore, an amplifier circuit whose cutoff frequency is sufficiently larger than the reception band is often used. In this regard, Non-Patent Document 1 shows an amplifier circuit having an explicit feedback resistor and a feedback capacitor that is a parasitic capacitance in the MOS transistor circuit. As a result, the frequency band of the CMUT described in Non-Patent Document 1 is 2 MHz to 7 MHz.

US Pat. No. 6,430,109

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol.55, No.2, Feb. 2008

In the above-described technical situation, development of an ultrasonic transducer that displays not only a morphological image but also a functional image in the examination of a specimen has been promoted in recent years. One such device is a device that uses photoacoustic spectroscopy. The frequency band of photoacoustic waves used in such photoacoustic spectroscopy is generally lower than the frequency band of ultrasonic waves used in ultrasonic echoes. For example, the frequency band of the photoacoustic wave is distributed in the range of 200 KHz to 2 MHz, which is lower than the ultrasonic center frequency of 3.5 MHz used in the ultrasonic echo. Therefore, it is necessary to develop an ultrasonic transducer that can detect a relatively low frequency band with high sensitivity.

In view of the above problems, equipment of the present invention includes a capacitive electromechanical transducer and an electrical circuit, having the following characteristics. The capacitance-type electromechanical transducer has a cell including a first electrode and a second electrode disposed opposite to the first electrode and spaced from a cavity, and receives ultrasonic waves. The electric circuit converts a current output from the electromechanical converter into a voltage. The frequency characteristic of the output current of the electromechanical transducer with respect to the received ultrasonic wave has a first region where the value decreases as the frequency decreases, and the frequency characteristic of the output voltage of the electric circuit with respect to the input current is It has a 2nd field where a value becomes small when a frequency becomes high, and has a portion with which the 1st field and the 2nd field overlap. Another equipment of the present invention includes a capacitive electromechanical transducer and an electrical circuit, having the following characteristics. The capacitance-type electromechanical transducer has a cell including a first electrode and a second electrode disposed opposite to the first electrode and spaced from a cavity, and receives ultrasonic waves. The electric circuit converts a current output from the electromechanical converter into a voltage. The frequency characteristic of the output current of the electromechanical transducer with respect to the received ultrasonic wave has a characteristic that the value becomes smaller when the frequency becomes lower in the first region on the frequency side lower than the first frequency. The frequency characteristic of the output voltage of the electric circuit with respect to the current having a characteristic is such that the value decreases as the frequency increases in the second region on the frequency side higher than the second frequency, and the first region and the first It has a part which 2 area | regions overlap.

It by the present invention, conventional ultrasonic detection device for a band of low-frequency range than the ultrasonic probe can be achieved.

The principle of the present invention will be described by showing examples of the frequency characteristics of the capacitive electromechanical transducer of the previous stage of the ultrasonic detection apparatus of the present invention, the electrical circuit of the subsequent stage, and the entire apparatus, and the embodiment of the present invention. It is a figure which shows the structure of an ultrasonic detection apparatus. It is a block diagram of the ultrasonic detection apparatus of other embodiment of this invention. 1 is a configuration diagram of an ultrasonic diagnostic apparatus according to an embodiment of the present invention. It is a figure explaining the frequency characteristic of a prior art example.

Hereinafter, embodiments of the ultrasonic detection apparatus and the ultrasonic diagnostic apparatus of the present invention will be described. In the invention, to have a frequency band and overlapping portions of the frequency band as the electric circuit of the electromechanical converter. The meaning of the cut-off frequency, high-pass characteristics, and low-pass characteristics will be described. As will be described later, the frequency characteristic of the output of the capacitive electromechanical transducer is maximized at the resonance frequency of the vibrating membrane in vacuum. The frequency characteristic of the present invention refers to the frequency between the anti-resonance frequency of the diaphragm through the resonance frequency from the frequency (first cut-off frequency) lowering by about 3 dB on the lower frequency side than the maximum frequency. It is a frequency characteristic of a band pass characteristic. In a device that is actually manufactured, the first cut-off frequency can be defined by using an average value in the vicinity of the frequency that is the maximum output. The frequency characteristic of the capacitive electromechanical transducer at this time is a high-pass characteristic having a first cutoff frequency. In other words, the high-pass characteristic in this specification means that the gain increases with respect to the frequency with a substantially fixed slope in the frequency region lower than the cutoff frequency, and a flat distribution in the frequency region higher than the cutoff frequency. is there. On the other hand, the subsequent current-voltage conversion amplifier circuit has a low-pass frequency characteristic in which the second cut-off frequency is determined by the values of the feedback resistance and the feedback capacitance. The low-pass characteristic in this specification means that the gain decreases with respect to the frequency with a fixed slope in the frequency region higher than the cutoff frequency, and a flat distribution is obtained in the frequency region lower than the cutoff frequency. Here, the second cut-off frequency indicates a frequency indicating a gain that is reduced by about 3 dB from the gain in the low band. In the present specification , the “flat distribution” means a constant gain. Specifically, the low-pass characteristic becomes “flat” in a region lower than the cutoff frequency, and “gain decreases with a fixed slope” in a region higher than the cutoff frequency. Similarly, the high-pass characteristics “gain increases with a fixed slope” in a region lower than the cutoff frequency, and “flat” in a region higher than the cutoff frequency. The “flat distribution” in this specification includes not only a completely flat case but also a case where the relationship of the slope with respect to the frequency characteristic of the gain is so small that it can be ignored in the device design .

Based on the above concept, the basic form of the ultrasonic detection apparatus and ultrasonic diagnostic apparatus of the present invention has the configuration as described above. On the basis of this basic form, the following embodiments are possible. For example, a capacitance type electromechanical transducer is sandwiched between a first electrode disposed on a substrate, a second electrode disposed opposite to the first electrode, and the first electrode and the second electrode. And a vibrating membrane that supports the second electrode so as to vibrate up and down (see the first embodiment described later). Further, in the ultrasonic detection device, the capacitor configured by the first electrode and the second electrode includes a plurality of gaps and a plurality of second electrodes or vibration films. The characteristics of the output current of the capacitance type electromechanical transducer are determined by factors including the average of the mechanical characteristics of the plurality of second electrodes or diaphragms and the capacitance of the capacitor (second implementation described later). See form). In the ultrasonic detection apparatus, the capacitors are two-dimensionally arranged, and vibration information of the second electrode or the diaphragm can be detected two-dimensionally (see a third embodiment described later). .

The principle of the present invention will be described. It is not easy for the electrostatic capacity type electromechanical transducer to have a center frequency band around 1 MHz by design. In order to make the center frequency band around 1 MHz, it is necessary to soften the hardness of the second electrode as a membrane or the vibrating membrane (decrease the spring constant). This is because the degree of freedom of design is limited. That is, the deflection of the membrane increases, making it difficult to form a narrow gap structure for increasing sensitivity, and the voltage applied to the electrode must be reduced, resulting in a decrease in sensitivity. Therefore, in the present invention, the capacitance type electromechanical transducer is designed so that the center frequency is on the higher frequency side than about 1 MHz, and the cutoff frequency of the subsequent electric circuit is adjusted so that the total center is obtained. The system has a frequency band around 1 MHz. In such a target relatively low frequency band, it is relatively easy to adjust the cutoff frequency of the electric circuit while increasing the amplification gain to some extent without adversely affecting other characteristics. On the other hand, increasing the cut-off frequency while increasing the amplification gain of the electric circuit is equivalent to reducing the feedback resistance or reducing the feedback capacitance. Sensitivity will decrease. Alternatively, there is a limit in circuit performance.

The above will be further described with reference to FIGS. 1 (a), (b), and (c). When the capacitance change is approximated to a parallel plate, the frequency characteristic 1 (see FIG. 1A) of the output current I of the capacitive electromechanical transducer with respect to the input sound pressure (pressure of the input elastic wave) is as follows. Formulated by equation (1).
I = P / [(Zm + Zr) / (εA * V b / d 2 ) + jωC] (1)
Here, e is the dielectric constant of the vacuum, A is the area of the electrode of the electromechanical transducer (see upper electrode 7 described later), V b is the bias voltage applied between the electrodes, d is the vacuum equivalent distance between the electrodes, P Is the input sound pressure, Zm is the mechanical impedance of the vibrating membrane (see vibrating membrane 8 described later), Zr is the acoustic impedance of the medium around the electromechanical transducer, ω is the angular frequency of the input sound pressure, and C is the total capacitance. It is. In this equation, since the total capacitance is relatively small, it can be said that the function of the frequency is the mechanical impedance Zm of the diaphragm.

Zm is represented by the following equation (2).
Zm = j * km * {(ω / ω 0 2 ) -1 / ω} (2)
km is the spring constant of the diaphragm, and is proportional to the pressure P in the region where the frequency is lower than the resonance angular frequency ω 0 (which is near the first cutoff frequency 2 (see FIG. 1A)). As a result, the vibrating membrane is displaced. Zm approaches zero from the low frequency region to the resonance frequency in inverse proportion to the frequency. From this, the output current frequency characteristic 1 becomes a primary characteristic with respect to the frequency in a frequency region smaller than the resonance frequency of the vibrating membrane. In addition, about the curve of the frequency characteristic of Fig.1 (a)-(c), these are simplified and drawn easily for the principle explanation. Actually, for example, in the vicinity of the shoulder portion, the shape is slightly broken and gradually changed, and the cut-off frequency is not necessarily located at the corner of the shoulder portion as shown. In addition, the horizontal axis of FIG. 1A represents the logarithmically displayed frequency, and the primary characteristic means that it is a primary characteristic with respect to the logarithmically displayed frequency. Similarly, the inverse proportion also means inverse proportion with respect to the logarithmically displayed frequency.

Further, as can be seen from the above formula (1), the output current frequency characteristic 1 depends not only on the mechanical impedance Zm of the diaphragm but also on the constant acoustic impedance Zr of the usage environment. Capacitance type electromechanical transducers are usually used in a liquid. The acoustic impedance of the liquid is larger than the mechanical impedance of the vibrating membrane. In this case, the acoustic impedance of the liquid is dominant for the frequency characteristic 1. As described above, the frequency at which the mechanical impedance Zm of the diaphragm is 0 is the resonance frequency of the diaphragm, and the output current frequency characteristic 1 takes the maximum value here. The mechanical impedance of the diaphragm is essentially infinite at the anti-resonance frequency of the diaphragm, but the anti-resonance frequency is irrelevant when used in a region lower than the resonance frequency. In the output current frequency characteristic 1, the region near the anti-resonance frequency is omitted. Taking these points into consideration, the output current frequency characteristic 1 represented by the above equation (1) is shown in FIG.

On the other hand, the frequency characteristic 3 (see FIG. 1B) of the current-voltage conversion amplifier circuit is formulated by the following equation (3), and the second cutoff frequency 4 is represented by the following equation (4). .
G = Rf / (1 + jωRf * Cf) (3)
f = 1 / (2πRf * Cf) (4)
G is the gain of the electric circuit, Rf is the feedback resistor, Cf is the feedback capacitance, and f and ω are the frequency and angular frequency of the input current. As the electric circuit used in the configuration of the present invention, it is desirable to use a circuit having a primary characteristic with respect to the frequency as shown in the expression (3) (similar to the frequency characteristic 1 is a characteristic relating to the logarithmically displayed frequency). It is not preferable to use a circuit having higher-order characteristics.

In the present invention, by combining the frequency characteristic 1 and the frequency characteristic 3 of the output of the electrical circuits of the output current of the capacitive electromechanical transducer, than conventional ultrasound probe to a band of low-frequency range ultrasonic detection Try to realize the device. In this combination, in order to realize an ultrasonic detection device having the target characteristic 5 (see FIG. 1C), the first cutoff frequency of the frequency characteristic 1 of the output current of the capacitive electromechanical transducer than 2 to reduce the second cut-off frequency 4 of the frequency characteristics third output of the electric circuitry. The reason is as described above.

In this way, the output current frequency characteristic 1 of the capacitive electromechanical transducer and the frequency characteristic 3 of the current-voltage conversion amplifier circuit are combined to form the output frequency characteristic 5 of the ultrasonic detector. As shown in FIG. 1C, the effective frequency band is between the low-frequency cutoff frequency 101 and the high-frequency cutoff frequency 102. At this time, the low-frequency cutoff frequency 101 and the high-frequency cutoff frequency 102 do not necessarily match the second cutoff frequency 4 and the first cutoff frequency 2, respectively. This is because when the first cut-off frequency 3 and the second cut-off frequency 4 are close to each other, the output frequency characteristic 5 of the ultrasonic detection device has a low-frequency cut-off frequency 101 and a high-frequency cut-off frequency. This is because it is difficult to obtain a substantially flat distribution between the regions 102. The frequency characteristic 1 and the frequency characteristic 3 are preferably designed to have a substantially flat distribution while maintaining a certain level between the low-frequency cutoff frequency 101 and the high-frequency cutoff frequency 102. For this purpose, for example, the slope of the slope part of the frequency characteristic 1 and the slope of the slope part of the frequency characteristic 3 are preferably the same as possible but having the opposite sign. In addition, it is preferable to increase the gain of the frequency characteristic 3.

From the above, in the ultrasonic detection apparatus with a wide band and high sensitivity, for example, the frequency that is the geometric mean of the cutoff frequency 2 and the cutoff frequency 4 is in the range of 0.4 MHz to 1.0 MHz. It is preferable to have a frequency characteristic 5 as shown in FIG. When the value obtained by dividing the flat frequency band of the frequency characteristic 5 by its center value is 130%, if the geometric mean of the cutoff frequency 2 and the cutoff frequency 4 is set to 0.4 MHz, an ultrasonic wave of 0.2 MHz It can be detected. Similarly, if the geometric mean of the cutoff frequency 2 and the cutoff frequency 4 is set to 1.0 MHz, an ultrasonic wave of 2.0 MHz can be detected.

Conventionally, when ordinary semiconductors and micromachining-related materials are used, the frequency characteristics of the capacitive electromechanical transducer are stable and stable at about 3 MHz or more in a liquid that easily transmits ultrasonic waves from a living body. is there. However, as described above, it is difficult to obtain a CMUT having a center near 1 MHz and high sensitivity. According to the present invention based on the above principle, such difficulties can be solved. For comparison, FIGS. 4A, 4B, and 4C show the frequency characteristics of a conventional capacitance type electromechanical transducer, the frequency characteristics of an electric circuit, and the frequency characteristics of an ultrasonic detector. The frequency characteristic of FIG. 4A is not much different from the frequency characteristic of FIG. 1A, but the frequency characteristic of FIG. 4B is higher in cutoff frequency 4 than the frequency characteristic of FIG. The overall gain is low. As a result, in the frequency characteristic of FIG. 4C, the range of the low-frequency cutoff frequency 101 and the high-frequency cutoff frequency 102 is on the high frequency side, for example, 3 MHz to 10 MHz.

Hereinafter, an embodiment having a configuration of a capacitance type electromechanical conversion device and a current-voltage conversion amplifier circuit embodied based on the above principle will be described with reference to the drawings.
(First embodiment)
The ultrasonic detection apparatus according to the first embodiment will be described. FIG. 1D shows the configuration of the capacitance type electromechanical transducer 6 (hereinafter also referred to as a cell) and the electric circuit 14 of the present embodiment. A capacitance type electromechanical transducer 6 shown as one cell includes an upper electrode 7, a vibration film 8, a cavity 9, an insulating layer 10, a support part 11 that supports the vibration film 8, a lower electrode 12, and a substrate that supports these. 13. The electric circuit 14 includes an operational amplifier including a resistor R1, a feedback resistor Rf, and a feedback capacitor Cf connected to the upper electrode 7 and the lower electrode 12. The conversion device 6 and the electric circuit 14 are configured to have the above-described frequency characteristics.

FIG. 1D is an example of the configuration. If the vibration film 8 is an insulator, the insulating layer 10 may or may not be present. In this case, the vibration film 8 and the support portion 11 may be the same material. The insulating layer 10 and the support portion 11 may be the same material. In terms of configuration, the upper electrode 7 and the vibration film 8 are bonded, and vibrate integrally. From the viewpoint of improving sensitivity, the cavity 9 is preferably maintained at a pressure lower than that of the atmosphere. When the substrate 13 is a conductive substrate such as a semiconductor substrate such as silicon, the substrate 13 and the lower electrode 12 may be integrated. The output current frequency characteristic 1 depends on the mechanical impedance of the diaphragm 8 and the acoustic impedance of the usage environment. The electrostatic capacity type electromechanical transducer is usually immersed in the liquid 18 and used in many cases. The acoustic impedance of the liquid 18 is larger than the mechanical impedance of the vibrating membrane 8, and specifically, the liquid is water, oil for ultrasonic diagnosis, oil such as castor oil, or the like.

In general, the upper electrode 7 and the lower electrode 12 may be metal, but may be a low-resistance semiconductor or the like. For example, the upper electrode 7 as the second electrode is made of a conductor selected from Al, Cr, Ti, Au, Pt, Cu, Ag, W, Mo, Ta, Ni, a semiconductor such as Si, AlSi, AlCu, and the like. , AlTi, MoW, AlCr, TiN, AlSiCu, etc., and can be formed of at least one material. Further, the upper electrode 7 is provided in at least one of the upper surface, the back surface, and the inside of the vibration film 8, or when the vibration film 8 is formed of a conductor or a semiconductor, the vibration film serves as the upper electrode 7. You can also The lower electrode 12 that is the first electrode can also be formed of the same conductor or semiconductor as the upper electrode 7. Moreover, the electrode materials of the lower electrode 12 and the upper electrode 7 may be different.

The dimensions of each part in this embodiment are exemplified as follows. For example, the height of the cavity 9 is about 100 nm, but may be in the range of 10 nm to 500 nm. The length of one piece of the cavity 9 is, for example, in the range of 10 μm to 200 μm. The vibration film 8 is made of SiN, for example, but may be other insulating materials. The cavity 9 is kept at a reduced pressure with respect to the atmospheric pressure, and the vibrating membrane 8 has a slightly concave shape. The vibrating membrane and the electrode are, for example, rectangular, but may be circular, polygonal, or the like. The shape of the cell cavity 9 is also, for example, a square, but may be other shapes.

During the reception operation, a direct current voltage V is applied by the voltage source 15 in order to generate a potential difference between the upper electrode 7 and the lower electrode 12 of the cell 6 of the ultrasonic detector. When receiving an ultrasonic wave, the vibrating membrane 8 vibrates, and a current flows by an amount corresponding to a change in capacitance accompanying the vibration. The current is amplified by the current-voltage conversion amplifier circuit 14.

(Second Embodiment)
An ultrasonic detection apparatus according to the second embodiment will be described. The configuration of this embodiment is shown in FIGS. 2 (a) and 2 (b). The broken line portion in the figure indicates that the drawing of the structure is omitted except for the perspective portion of the cell 6. In the present embodiment, a plurality of cells 6 are arranged on the substrate 13. The structure of each cell 6 and the electric circuit 14 is as described in the first embodiment. The upper electrode 7 and the lower electrode 12 of the plurality of cells 6 are electrically connected by the electrode coupling wiring portions 16 and 17, respectively, and are electrically connected between the plurality of cells 6. As shown in FIG. 2 (b), the cells 6 are two-dimensionally arranged at equal intervals to form one element 20. The apparatus is used in a state in which the upper electrode 7 of the element 20 is in contact with, for example, the liquid 18 having good propagation of ultrasonic waves. From the viewpoint of detection sensitivity, ease of signal processing, etc., it is desirable that the mechanical characteristics of the vibrating membrane 8 and the depth of the cavity 9 are uniform between the plurality of cells 6. In the element 20, the arrangement of the cells 6 is a square lattice shape in the illustrated example, but may be a staggered shape, a hexagonal close-packed shape, or the like. What is necessary is just to determine suitably the arrangement | sequence form and number of the cells 6 in the element 20 according to the case. In the illustrated example, the shape of the vibrating membrane 8 is depicted as a circle, but it may be a polygon. As described above, in this embodiment, the capacitor including the lower electrode 12 (first electrode) and the upper electrode 7 (second electrode) includes a plurality of gaps 9 and a plurality of second electrodes or vibration films 8. Consists of. The frequency characteristics of the output current of the element 20 are determined by factors including the average of the mechanical characteristics of the plurality of second electrodes or diaphragms 8 and the capacitance of the capacitor.

In the present embodiment, an area where the plurality of upper electrodes 7 are conductive becomes an ultrasonic detection area, and sensitivity is increased as compared with the first embodiment including one cell. In this embodiment, it can be said that the element 20 including a plurality of cells constitutes one capacitance type electromechanical transducer. The frequency characteristic 1 (see FIG. 1A) of the capacitive electromechanical transducer at this time is determined by the average value of the mechanical characteristics of the plurality of vibrating membranes 8 as described above. The magnitude of the current output of the element 20 is substantially proportional to the total area of the upper electrodes 7 on the plurality of vibrating membranes 8. The other points are the same as in the first embodiment.

(Third embodiment)
An ultrasonic detection apparatus according to a third embodiment will be described. The configuration of this embodiment is shown in FIGS. FIG. 2D, which is a top view, shows the overall configuration of the ultrasonic detection device 32. 2C and 2D, the broken line portion indicates that the drawing of the structure is omitted. The ultrasonic detection device 30 of the present embodiment has a configuration in which the elements 20 of the second embodiment are two-dimensionally arranged. One of the upper electrode 7 connected by the wiring portion 16 and the lower electrode 12 connected by the wiring portion 17 is electrically separated for each element 20. Also in this embodiment, the upper electrode 16 is in contact with the liquid 18 with good ultrasonic propagation. By sending the output of each element 20 to the current-voltage conversion amplification circuit 14 via the wiring 31 and converting the voltage, it is possible to detect the ultrasonic signal as a two-dimensional distribution. Again, the frequency characteristic 1 of each element 20 is determined by the average value of the mechanical characteristics of the plurality of vibrating membranes 8 or the like. The magnitude of the current output of each element 20 is substantially proportional to the total area of the plurality of upper electrodes 7. In the ultrasonic detection apparatus of the present embodiment, the capacitors are two-dimensionally arranged, and vibration information of the second electrode or the vibrating membrane 8 can be detected two-dimensionally. The other points are the same as in the first embodiment.

By the way, the structure of the said embodiment can also be used as an apparatus which generates a sound wave. By applying a voltage obtained by superimposing a minute alternating voltage on a DC voltage with the voltage source 15 between the upper electrode 7 (or the upper electrode coupling wiring portion 16) and the lower electrode 12 (or the lower electrode coupling wiring portion 17), The vibrating membrane 8 is forcibly vibrated to generate sound waves. The frequency characteristics at this time mainly have transmission characteristics similar to the output current frequency characteristics 1 of the capacitive electromechanical transducer. As in the second embodiment and the third embodiment described above, this sound wave generator can generate a larger sound wave by arranging the vibrating membranes 8 two-dimensionally. Further, by increasing the generation area, the directivity of the sound wave can be increased and the diffraction can be reduced.

(Fourth embodiment)
An ultrasonic diagnostic apparatus according to a fourth embodiment will be described. The configuration of the present embodiment is shown in FIG. When light 41 emitted from the light source 40 propagates and strikes the living tissue 42, an ultrasonic wave 43 called a photoacoustic wave is emitted. That is, light is absorbed at a location where the light absorption coefficient exists in the living tissue and the location is heated. And the heated part expand | swells and an elastic wave generate | occur | produces with expansion | swelling. The frequency of the ultrasonic wave 43 varies depending on the substances and individuals constituting the living tissue, but as described above, it is, for example, about 200 kHz to 2 MHz. The ultrasonic wave (photoacoustic wave) 43 passes through the liquid 18 having good propagation and is detected by the ultrasonic detector 32. The current-voltage converted / amplified signal is sent to the signal processing system 45 via the signal bundle 44. The detection result signal is signal-processed by a signal processing system 45 to extract biological information. If the ultrasonic detector 32 is configured as in the above-described third embodiment, a two-dimensional ultrasonic distribution can be detected, and a wide range of distribution can be captured by scanning the detector 32. it can. Since ultrasonic waves are at the speed of sound, it is possible to obtain time information by analyzing the time difference of the arrival wave (time waveform) and acquire information in the depth direction. In this case, the signal processing system 45 is provided with a reconstruction function. In other words, three-dimensional biological information can be extracted. In addition, an image or the like can be acquired by obtaining a frequency characteristic by performing a Fourier transform on the received signal.

A technique for obtaining a tomographic image, a three-dimensional image, etc. of a sample (inspection object) using the photoacoustic effect is generally known as photoacoustic tomography, and is referred to as a PAT technique after taking its initials. ing.

6 ... Capacitance type electromechanical transducer (cell), 7 ... Upper electrode (second electrode), 8 ... Vibration membrane, 9 ... Cavity (gap), 12 ... Lower electrode (first electrode), 14 ... Current- Voltage conversion amplifier circuit, 15 ... Voltage source, 32 ... Ultrasonic detection device, 40 ... Light source, 42 ... Living tissue (test object), 43 ... Photoacoustic wave (ultrasound), 45 ... Signal processing system

Claims (26)

  1. A capacitance-type electromechanical transducer having a cell comprising a first electrode and a second electrode disposed opposite to the first electrode and spaced from a cavity, and receiving ultrasonic waves;
    An electric circuit for converting a current output from the electromechanical converter into a voltage;
    The A including equipment,
    The frequency characteristic of the output current of the electromechanical transducer with respect to the received ultrasonic wave has a first region in which the value decreases as the frequency decreases,
    Frequency characteristics of the output voltage of the electric circuit for the current input, when the frequency becomes higher the value has a second region smaller,
    Equipment you characterized by having a portion where the first region and the second region overlap.
  2. The cells of the electro-mechanical conversion apparatus, equipment according to claim 1, characterized in that it comprises a vibration film to vibrate supporting the second electrode.
  3. The first region is equipment according to claim 2, characterized in that in the region lower than the resonance frequency of the vibrating membrane.
  4. 4. The first region according to claim 1, wherein the first region is in a region lower than a frequency at which an output current of the electromechanical transducer with respect to received ultrasonic waves shows a maximum value . 5. equipment.
  5. The first region is in a region lower than a frequency at which an output current of the electromechanical transducer with respect to the received ultrasonic wave is lower than a frequency at which the output current is 3 dB lower than a frequency at which the output current is lower. equipment according to any one of claim 1 to 3.
  6. Said second region, equipment according to any one of claims 1 to 5 output voltage of the electric circuit for the current input is characterized in that it is in the region higher than the frequency showing the maximum value .
  7. 6. The method according to claim 1, wherein the second region is in a region higher than a frequency at which an output voltage of the electric circuit with respect to an input current shows a value that is 3 dB lower than a value in a low region. equipment according to item 1 or.
  8. A capacitance-type electromechanical transducer having a cell comprising a first electrode and a second electrode disposed opposite to the first electrode and spaced from a cavity, and receiving ultrasonic waves;
    An electric circuit for converting a current output from the electromechanical converter into a voltage;
    The A including equipment,
    The frequency characteristic of the output current of the electromechanical transducer with respect to the received ultrasonic wave has a characteristic that the value decreases as the frequency decreases in the first region on the frequency side lower than the first frequency,
    The frequency characteristic of the output voltage of the electric circuit with respect to the input current has a characteristic that the value decreases as the frequency increases in the second region on the frequency side higher than the second frequency,
    Equipment you characterized by having a portion where the first region and the second region overlap.
  9. The electric cell of the transducer device, equipment according to claim 8, characterized in that it comprises a vibration film to vibrate supporting the second electrode.
  10. Said first frequency, equipment of claim 9, wherein the the resonance frequency of the vibrating film.
  11. Said first frequency, equipment according to any one of claims 8 to 10, characterized in that the frequency at which the maximum value of the output current of the electro-mechanical conversion apparatus for ultrasonic wave reception.
  12. 11. The first frequency is a frequency indicating a value that is 3 dB lower on a lower frequency side than a frequency indicating a maximum value of an output current of the electromechanical transducer with respect to received ultrasonic waves. equipment according to any one of.
  13. Said second frequency, equipment according to any one of claims 8 to 12, characterized in that the frequency showing the maximum value of the output voltage of the electric circuit for the current input.
  14. The frequency according to any one of claims 8 to 12, wherein the second frequency is a frequency indicating a value that is 3 dB lower than a value in a low frequency range of the output voltage of the electric circuit with respect to an input current. equipment described.
  15. The electromechanical transducer receives ultrasonic waves in a state where a potential difference is generated between the first electrode and the second electrode . equipment.
  16. The electromechanical transducer has an element including a plurality of the cells,
    The frequency characteristic of the output current of the electromechanical transducer with respect to the received ultrasonic wave includes an average of the mechanical characteristics of the plurality of second electrodes arranged in the plurality of cells and the capacitance of the element. equipment according to any one of claims 1 to 15, characterized in that it is determined by.
  17. The electromechanical transducer has an element including a plurality of the cells,
    The frequency characteristics of the output current of the electromechanical transducer with respect to the received ultrasonic waves are the average of the mechanical characteristics of the plurality of second electrodes or diaphragms disposed in the plurality of cells, the capacitance of the element, equipment according to claim 2, 3, 9 or 10, characterized in that it is determined by factors including.
  18. The frequency characteristic of the output current of the electromechanical transducer with respect to the received ultrasonic wave is the frequency characteristic of the output current of the electromechanical transducer with respect to the unit sound pressure of the received ultrasonic wave. equipment according to any one of.
  19. The frequency characteristic of the output voltage of the electric circuit with respect to the input current is a frequency characteristic of a conversion gain of the output voltage of the electric circuit with respect to the input current. equipment described.
  20. 20. The electromechanical transducer according to claim 1, wherein the electromechanical transducer outputs an electric current associated with a change in capacitance between the first electrode and the second electrode by receiving an ultrasonic wave. equipment according to any one.
  21. The electromechanical conversion device, equipment according to any one of claims 1 to 20, characterized in that it comprises a plurality of elements having a plurality of said cells.
  22. Wherein the plurality of elements, equipment according to claim 21, characterized in that it is disposed upon a substrate.
  23. The electromechanical conversion device, equipment according to any one of claims 1 to 22, characterized in that the light receiving ultrasonic waves generated in the test object by being absorbed.
  24. The electromechanical conversion device, equipment according to any one of claims 1 to 23, characterized in that for transmitting ultrasonic waves.
  25. Further comprising a light source,
    The electromechanical conversion device, according to any one of claims 1 to 24 light emitted from said light source, wherein the benzalkonium to receive the ultrasonic wave caused by being devoted to the inspection target.
  26. 26. The apparatus according to claim 1, further comprising a signal processing system that processes a signal output from the electric circuit.
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