JP2013229829A - Electromechanical conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromechanical conversion device capable of reducing an unnecessary signal, noise, and the like in a received signal due to reflection of an acoustic wave by wiring.SOLUTION: On a substrate 100 of the electromechanical conversion device, an electromechanical conversion element 101 including at least one cell being a capacitive micromachined ultrasonic transducer or the like and wiring 102 which is connected to the electromechanical conversion element 101 and transmits at least an electric signal of a received acoustic wave are provided. An attenuation layer 301 for attenuating an acoustic wave is provided on the wiring 102.

Description

The present invention relates to an electromechanical transducer such as a capacitive ultrasonic transducer and a device such as a subject diagnostic apparatus using the electromechanical transducer.

In recent years, a capacitive electromechanical transducer manufactured using a micromachining process has been actively researched, and there is a proposal regarding this (see Patent Document 1). An ordinary capacitive electromechanical transducer includes an electromechanical transducer element having a plurality of cells including a lower electrode, a vibrating membrane supported at a predetermined interval, and an upper electrode disposed on the surface of the vibrating membrane. Yes. This is used, for example, as a capacitive ultrasonic transducer (CMUT: Capacitive-Micromachined-Ultrasonic-Transducer). Such a CMUT transmits or receives an acoustic wave using a lightweight vibrating membrane, and can easily obtain a CMUT having excellent broadband characteristics in liquid and air. When this CMUT is used, diagnosis with higher accuracy than conventional medical diagnosis is possible, and therefore, it is attracting attention as a promising technology. In addition, in this specification, an acoustic wave includes what is called a sound wave, an ultrasonic wave, and a photoacoustic wave. For example, an acoustic wave generated inside the measurement target by irradiating the measurement target with light (electromagnetic wave) such as visible light or infrared, or a reflected acoustic wave transmitted inside the measurement target and reflected inside the measurement target including. In the following description, an acoustic wave may be referred to as an ultrasonic wave.

In addition, there is an example described in Non-Patent Document 1 as an example of an apparatus in which CMUT, IC, wiring, etc. are mounted for the purpose of acquiring an ultrasonic image. Furthermore, there is a proposal of an apparatus in which electromechanical conversion elements are arranged two-dimensionally in order to obtain a large amount of information at once (see Patent Document 2).

In the use of the measurement device as described above, if there is an interface between the sensor and the sound source in the direction in which the ultrasonic waves reach, the surface where the acoustic impedance changes discontinuously, the ultrasonic waves are reflected there. There is. In particular, when the ultrasonic wave is difficult to attenuate between the sensor surface and the interface, multiple reflections occur between the sensor surface and the interface, and this reflected wave enters the sensor as a false signal with a delay. May be.

In general, the electromechanical conversion element of CMUT is formed on a silicon wafer. A protective layer may be provided on the element. In addition, in the case of receiving a photoacoustic wave generated inside a measurement target by irradiating the measurement target with light such as visible light or infrared as an ultrasonic diagnosis, a light reflector is provided between the sound source and the CMUT element. There is a case. Such a protective layer or light reflector forms an interface between the sensor surface and the sound source. Therefore, an ultrasonic signal with a delay due to multiple reflection caused by it is generated, which lowers the resolution when image reconstruction is performed by the measuring apparatus, and causes a false image called tailing.

In addition, ultrasonic waves generated from the subject or the like may be reflected by portions other than the vibration film on the sensor surface. On the sensor surface, wiring connected to the element is installed, and there is actually about 30% of the sensor surface that is not a vibration film. For this reason, about 30% of the incident ultrasonic waves return to the sound source direction, and are reflected at the interface, and some of them can vibrate the vibrating membrane again. Such wiring is usually made of metal, and the reflection of ultrasonic waves by the wiring is higher than the surface of the silicon substrate.

JP 2006-319712 A JP 2008-193357 A

IEEE Ultrasonic Symposium, 1997, p.900-903.

As described above, it is required that the signal wave incident on the vibration film functioning as a sensor is not attenuated as much as possible and the reflected wave is made as small as possible. Therefore, it has been a problem to suppress reflection of acoustic waves from other than the vibrating membrane.

In view of the above-described problems, an electromechanical transducer according to the present invention is an electrical machine in which an electromechanical transducer element and a wiring that is connected to the electromechanical transducer element and transmits at least received electrical signals based on acoustic waves are provided on a substrate. In the mechanical conversion device, an attenuation layer that attenuates an acoustic wave is provided on the wiring.

According to the present invention, since the attenuation layer as described above is provided on the wiring, it is possible to reduce unnecessary signals and noise of the signal received by the electromechanical transducer used in the capacitive ultrasonic probe and the like. As a result, when the electromechanical transducer of the present invention is used for a probe such as an image diagnostic apparatus, effects such as an improvement in image contrast and resolution and elimination of false images can be expected.

1 is a schematic perspective view of a substrate and a partially broken protective film of an electromechanical transducer such as a capacitive ultrasonic transducer according to the present invention. A-A 'sectional view in Drawing 2 of Embodiments 1-3 of the present invention. The schematic perspective view of the probe using the electromechanical transducer of this invention. The figure which shows the upper surface and cross section of the structural example of an electrostatic capacitance type electromechanical transducer. The figure which shows Embodiment 4 which concerns on the subject diagnostic apparatus of this invention.

The present invention is characterized in that an attenuation layer for attenuating an acoustic wave is provided on a wiring that is connected to the electromechanical conversion element and transmits at least the electrical signal of the received acoustic wave. Since the attenuation layer only needs to attenuate the acoustic wave that is going to return to the sound source side, the acoustic wave may be absorbed, scattered, or diffused regardless of the attenuation process. The larger the acoustic wave attenuation rate, the better. However, in the case where a protective layer is provided, the acoustic wave attenuation rate may be larger than that of the protective layer. For example, when the attenuation layer is an acoustic wave absorption layer, the absorption layer has, for example, a difference in acoustic impedance with a substance in contact thereof within about 20% and an acoustic wave attenuation rate of about 3 [dB / cm] or more. . Further, the present invention is not limited to the capacitance type electromechanical transducer described in the embodiments described later, and can also be applied to a piezoelectric electromechanical transducer. Usually, it is necessary to provide wiring on the substrate in order to arrange a plurality of elements, but the reflected wave of the acoustic wave there is likely to be incident on the sensor surface with a delay and become noise or the like. However, by arranging an attenuation layer for attenuating the acoustic wave on the wiring as in the present invention to reduce the return of the acoustic wave, the incidence on the sensor surface again can be reduced.

Embodiments of the present invention will be described below. FIG. 1A shows a semiconductor substrate of an embodiment of an electromechanical transducer such as a capacitive ultrasonic transducer. In the present embodiment, a plurality of elements 101 configured by collecting at least one cell such as a capacitive ultrasonic transducer corresponding to a sensor on a substrate 100 are arranged in a two-dimensional array, and wiring 102 from each element is arranged. Is connected to an electrode 103 arranged around the substrate. The wiring 102 is located between the adjacent electromechanical conversion elements 101.

As shown in FIG. 1B, a protective layer 201 for protecting the element 101 is usually provided. Since it is necessary for the protective layer 201 to pass incident ultrasonic waves, it is preferable to use a material that is less attenuated with respect to the ultrasonic waves. Further, when an ultrasonic propagation medium (not shown) from the sound source is used, the acoustic impedance of the protective film 201 is protected in order to reduce reflection of ultrasonic waves caused by a difference in acoustic impedance between the propagation medium and the protective film 201. It is desirable to be close to the propagation medium with which the film 201 is in contact. For example, when the subject is a living tissue, its acoustic impedance is close to water (acoustic impedance is 1.5 MRayls). Therefore, water, silicon gel, or the like is used as the propagation medium assuming that the acoustic impedance is close to water. Therefore, the protective film 201 in contact with the propagation medium preferably has acoustic characteristics close to that of the propagation medium, and preferably has an acoustic impedance in the range of about 1 to 3 MRayls. For example, silicone, fluorosilicone, chloroprene, urethane, or the like can be used.

FIG. 2A shows a cross-sectional structure of the first embodiment having the above configuration as a basic configuration. A protective film 201 is formed on the upper surface of the element 101 including a plurality of cells that are capacitive ultrasonic transducers, and a reflected wave absorbing layer or the like is formed on the upper surface of the semiconductor substrate 100 where the wiring 102 is present but the element 101 is not present. The attenuation layer 301 is formed. Here, the protective film 201 is entirely formed on the element 101 and the attenuation layer 301. In addition, it is preferable that the attenuation layer 301 hardly reflects ultrasonic waves at the interface between the attenuation layer 301 and the protective film 201. That is, the attenuation layer 301 preferably has an acoustic impedance close to that of the protective film 201 and easily absorbs ultrasonic waves. Such a material can be realized by dispersing fine particles such as metal in the resin used for the protective film 201. As the fine particles, tungsten, aluminum, iron, copper, platinum or a compound thereof is suitable. The suppression and absorption of acoustic wave reflection can be adjusted by changing the concentration of the fine particles. In the present embodiment, the surface of the attenuation layer 301 can be a rough surface or the like to scatter or diffuse incident acoustic waves.

FIG. 2B shows a cross-sectional structure of the second embodiment that is different from the first embodiment in the layer configuration on the substrate. In the present embodiment, the light reflector 401 is disposed on the attenuation layer 301 and the protective film 201 on the wiring 102. The upper surface of the attenuation layer 301 and the upper surface of the protective film 201 are flat at the same level, and the light reflector 401 is formed on the flat surface. The light reflector 401 reflects scattered light from the measurement target or light from the light source that excites the measurement target, and preferably has a high reflectance at the wavelength of such light. Usually, light in the visible to infrared region is often used, and as the reflective film of the light reflector 401, an Au film, an Au alloy film, an Ag film, or the like is suitable. In such a light reflector, the reflective film is typically formed on a resin film or the like. As this resin film, films such as polymethylpentene, polyethylene terephthalate, polyethylene, polycarbonate, ethyl vinyl acetate, and acrylic resin can be used. The light reflector can also be disposed on the protective layer having the configuration shown in FIG. Thus, a light reflector can be provided on at least one of the attenuation layer and the protective layer.

The attenuation layer 301 such as a reflected wave absorption layer is preferably a material that easily absorbs ultrasonic waves. The attenuation layer 301 preferably does not easily reflect ultrasonic waves at the interface between the attenuation layer 301 and the protective film 201 and at the interface between the attenuation layer 301 and the propagation medium. That is, it is preferable that the acoustic impedance is close to that of the propagation medium and the protective film 201 and it is easy to absorb ultrasonic waves, and examples of such materials include those mentioned in the first embodiment.

The attenuation layer 301 may be covered with the protective film 201 as in the first embodiment, or may be arranged without being covered with the protective film 201 as shown in FIG. The thickness of the protective film 201 on the element 101 is preferably thin in order to reduce attenuation of ultrasonic waves, and the attenuation layer 301 is preferably thick in order to attenuate ultrasonic waves. As described above, it is preferable that the attenuation layer 301 is thick because the attenuation of acoustic wave reflection is large. However, if the attenuation layer 301 is too thick, the signal incident on the element 101 itself may be blocked or the signal during transmission may be blocked. It is necessary to consider the aspect ratio because it will be overwhelmed.

Such a situation will be described with reference to FIG. However, Embodiment 3 in FIG. 2C is a form in which the protective layer 201 and the reflector 401 are not provided, and the attenuation layer 301 is provided on the wiring 102. Such a form is also included in the present invention.

As shown in FIG. 2C, the directivity angle of the acoustic wave beam of the capacitive ultrasonic transducer (cell) of the electromechanical transducer element 101 is β (rad). The directivity angle means half of the angle surrounding the main lobe of the acoustic wave generated from the element cell. The directivity angle can be obtained from the frequency of the acoustic wave, the cell size, and the sound speed of the propagation medium. Further, the invalid area ratio of the area of the invalid area having no cell on the substrate to the area of the substrate is α. The invalid area is an area including an area with wiring. If the shape of the element area (the area including the element and the surrounding wiring part) is a square with one side L (that is, the length (pitch) between the electromechanical transducer elements is L), the length of the invalid area width Assuming that a is 2a = √α * L. At this time, the distance b from the center to the end of the element 101 is b = (L−2a) / 2, and d <b * tan (π / 2−β) where d is the thickness of the attenuation layer 301. Thus, it is preferable to limit the film thickness.

Summarizing the above, the ratio of the invalid area to the substrate area of the invalid area without cells on the substrate is α, the length (pitch) between the electromechanical conversion elements is L (m), and the cells of the electromechanical conversion element When the directivity angle of the acoustic wave beam is β (rad), the thickness d (m) of the attenuation layer should satisfy the following condition.
d <(L / 2) * (1-√α) * tan (π / 2-β)

The shape of the element is not necessarily square. In the case of a rectangle, the same can be said for each side, so it is preferable to select a small film thickness. In the case of a circle, the upper limit film thickness can be similarly determined from the diameter, and in the case of a polygon, the upper limit film thickness can be similarly determined from the diameter of a circle corresponding to the area.

For example, if the size of each element 101 including the cell is about 1 mm, the frequency of the ultrasonic wave to be handled is about 1 MHz, and the propagation speed is about 1000 m / s, the directivity of the cell is determined, and β is the vertical direction of the cell. ± 30 ° with respect to the direction. If the effective area is 70% (invalid area is 30%) and the element 101 has a side of 1 mm, the thickness of the attenuation layer 301 needs to be 720 μm or less in accordance with the above conditional expression. The ultrasonic attenuation material forming the attenuation layer 301 is suitable to have substantially the same acoustic impedance as the propagation medium. Since the thickness of the ultrasonic attenuating material is limited to be thicker than those described above, polyurethane (14 to 36 dB / cm / MHz) or silicon rubber (15 to 5 MHz at 5 MHz) having a large attenuation rate per unit thickness is large. It is preferable to use one having an attenuation factor of 15 dB / cm or higher, such as 27 dB / cm). However, it is not limited to this. In addition, the attenuation factor can be increased by dispersing fine particles such as tungsten as the fine metal particles.

A configuration example of an actual probe is shown in FIG. In this configuration, as an electromechanical transducer in which a reflection film 601 for laser light, which is a photoacoustic wave excitation source, is installed on the protective film 201 of the element on the substrate, and generation of false signals due to laser light is suppressed. Yes. This is installed at the end of the holding portion 602 that is mechanically coupled to the housing that is the first-stage amplification substrate storage portion 603. Through the wiring inside the holding unit 602, the electromechanical converter is electrically connected to the first stage amplification unit in the storage unit 603, and a signal is transmitted to the first stage amplification unit.

Hereinafter, more specific examples will be described with reference to the drawings.
Example 1
As shown in FIG. 4, in the element 705 of the capacitive ultrasonic transducer of Example 1, the first electrode 702 on the substrate 701 and the second electrode 703 on the membrane 707 are provided facing each other with a cavity 704 therebetween. It has been. The substrate 701 is a silicon wafer. The first electrode 702 and the second electrode 703 are Ti films, but other electrode materials may be used. A silicon oxide film which is an insulating film 706 is disposed between the first and second electrodes. The second electrode 703 is disposed on the membrane 707 and connected to the electrode pad 709 of the second electrode 703 via the wiring 102. The first electrode 702 is connected to the electrode pad 708 of the first electrode. Four cells, which are the minimum unit of the capacitive electromechanical transducer, are connected by the second electrode 703 to form one element 705. Of course, the configuration of the element 705 is not limited to this, and can be appropriately configured. Here, the membrane 707 uses a silicon nitride film.

On the wiring 102 between the elements 705 on the substrate of such a capacitive electromechanical transducer, a commercially available thermosetting urethane was applied by screen printing and cured to form an attenuation layer. In the urethane resin, tungsten fine particles were dispersed in advance. Fine particles were added in a concentration range of 10 to 30 wt%. Next, a urethane resin in which tungsten is not dispersed is applied by screen printing to form a protective layer, and the attenuation layer of the tungsten-dispersed urethane resin is covered with the protective layer, as shown in FIG. A capacitive electromechanical transducer having a cross-sectional configuration was obtained. In this way, when a capacitive electromechanical transducer in which the attenuation layer 301 is provided in the portion of the wiring 102 was used to receive ultrasonic waves in water, a good received signal with little noise was obtained.

(Example 2)
In Example 2, a capacitive electromechanical transducer was formed as in Example 1. Next, a commercially available thick film resist was applied onto the substrate 701, and patterning was performed to create an opening on the element portion. Then, polydimethylsiloxane (PDMS) was filled in the opening and thermally cured to form a protective layer.

Next, the resist was peeled off, and the remaining portion was filled with polydimethylsiloxane in which silica particles were dispersed, followed by thermosetting to form an attenuation layer. Subsequently, the PET film on which the Au film was vapor-deposited was adhered to the silicone surface cured with the polydimethylsiloxane using an adhesive to form a light reflector. In this way, a capacitive electromechanical transducer having a cross-sectional configuration as shown in FIG. 2B was obtained. In this way, in the capacitive electromechanical transducer having the attenuation layer 301 provided in the portion of the wiring 102, when the ultrasonic phantom is irradiated with light in water to generate a photoacoustic wave and received, there is little noise. A good received signal was obtained.

(Example 3)
In Example 3, a capacitive electromechanical transducer was formed in the same manner as in Example 1. Next, a thick film resist (for example, Tokyo Ohka Kogyo TMMR S2000, JSR THB-430H) is applied on the substrate, and a layer corresponding to the protective film is applied, patterned, and opened on the element portion. Part was formed. A protective film was prepared by filling the opening with polydimethylsiloxane and thermosetting. A resist was further applied thereon and patterned to leave a resist on the portion of the wiring 102, which was used as an attenuation layer. The thickness of the attenuation layer can be controlled by the coating thickness of the resist. In this way, an electromechanical conversion device having a cross-sectional configuration as shown in FIG. 2C (however, in this embodiment, a protective layer is provided on the element 101) was formed.

Here, an attenuation layer having a film thickness of 700 μm was formed with an element pitch of 1 mm. With this capacitive electromechanical transducer, when an ultrasonic wave was received in water, a good signal with little noise was obtained. Further, even when ultrasonic waves were transmitted, it was confirmed that the output did not decrease and the attenuation layer did not block the transmission beam.

Example 4
FIG. 5 shows a subject diagnostic apparatus or photoacoustic measurement apparatus that is Embodiment 4 of the present invention. The light source 50 is, for example, a light source that generates laser light, and the light 24 is, for example, pulsed laser light.

In this apparatus, the irradiation light 24 emitted from the light source 50 toward the subject 17 strikes the light absorber 51 inside the subject, whereby an acoustic wave 52 called a photoacoustic wave is emitted due to the photoacoustic effect. The frequency of the acoustic wave 52 is about 300 kHz to 10 MHz, although it varies depending on the substance constituting the light absorber 51 and the size of the individual. The acoustic wave 52 passes through the acoustic impedance matching material 25 with good propagation, and is detected by the electromechanical transducer 53 of the present invention. The signal subjected to the current voltage amplification is sent to the signal processing unit 55 via the signal line 54. The detected signal is signal-processed by the signal processing unit 55, and the physical information of the subject is extracted. The signal processing unit 55 is mainly a computer, but part of the signal processing unit 55 may be an integrated circuit and can reconstruct a two-dimensional or three-dimensional image. By using the electromechanical transducer 53 of the present invention, a good signal with less noise was obtained, and the contrast and resolution of the image were improved, and the false image was eliminated. Of course, the electromechanical transducer of the present invention can also be used in a subject diagnostic apparatus that detects acoustic waves from a subject to which acoustic waves are applied. In this case as well, the acoustic wave from the subject is detected by the electromechanical transducer, and the converted signal is processed by the signal processing unit to acquire information inside the subject.

The electromechanical transducer of the present invention can be applied to an optical imaging apparatus that obtains information in a measurement target such as a living body, a conventional ultrasonic diagnostic apparatus, and the like. Furthermore, it can be applied to other applications such as an ultrasonic flaw detector.

100 ... Board, 101 ... Element (capacitive ultrasonic transducer), 102 ... Wiring 201 ... Protective film 301 ... Attenuation layer 401 ... Light reflector

Claims (10)

An electromechanical conversion device provided on a substrate with an electromechanical conversion element including a cell, and a wiring connected to the electromechanical conversion element and transmitting an electric signal by at least received acoustic waves,
An electromechanical conversion device, wherein an attenuation layer for attenuating acoustic waves is provided on the wiring. A plurality of the electromechanical transducer elements are provided on the substrate;
2. The electromechanical transducer according to claim 1, wherein the wiring is located between the adjacent electromechanical transducer elements, and the attenuation layer is provided on at least the wiring. The electromechanical transducer according to claim 1, wherein the attenuation layer is an absorption layer that absorbs and attenuates an acoustic wave. The thickness d (m) of the attenuation layer is such that the invalid area ratio of the area of the invalid area without cells on the substrate to the area of the substrate is α, the length between the electromechanical conversion elements is L (m), and When the directivity angle of the acoustic beam of the electromechanical transducer element cell is β (rad),
d <(L / 2) * (1-√α) * tan (π / 2-β)
The electromechanical transducer according to any one of claims 1 to 3, wherein The electromechanical transducer according to claim 1, wherein a protective layer is provided at least on the electromechanical transducer. The electromechanical transducer according to claim 5, wherein an attenuation rate at which the attenuation layer attenuates an acoustic wave is greater than an attenuation rate at which the protective layer attenuates an acoustic wave. The electromechanical transducer according to claim 1, wherein a light reflector is provided on at least one of the attenuation layer and the protective layer. 8. The electromechanical conversion element according to claim 1, wherein the electromechanical conversion element is an element configured by collecting at least one capacitive ultrasonic transducer that is a cell. 9. Electromechanical converter. The electromechanical transducer according to any one of claims 1 to 8,
A signal processing unit for processing a signal output from the electromechanical transducer;
Have
An object diagnostic apparatus characterized in that an acoustic wave from a subject is detected by the electromechanical conversion device, and information inside the subject is acquired by processing the converted signal by the signal processing unit. The electromechanical transducer according to any one of claims 1 to 8,
A light source that generates light in pulses;
A signal processing unit for processing a signal output from the electromechanical transducer;
Have
A photoacoustic wave generated by the light emitted from the light source and applied to the subject is detected by the electromechanical conversion device, and the converted signal is processed by the signal processing unit to acquire information inside the subject. An object diagnostic apparatus characterized by:

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