JP2015146972A - Capacitance type transducer and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a capacitance type transducer capable of reducing a side lobe.SOLUTION: In the capacitance type transducer including a component 14 that has a plurality of cells 12 in which a vibration membrane 9 containing one of a pair of electrodes formed with a gap kept between is supported to be capable of vibrating, the distance between a pair of electrodes in a cell at the end of the component is wider than the distance between a pair of electrodes in a cell in the central part of the component.

Description

  The present invention relates to a capacitive transducer and a method for manufacturing the same.

  Conventionally, a micromechanical member manufactured by a micromachining technique can be processed on the order of a micrometer, and various microfunctional elements are realized using these. A capacitive transducer using such a technique has been studied as an alternative to a piezoelectric element. According to such a capacitive transducer, ultrasonic waves can be transmitted and received using vibrations of the vibrating membrane, and excellent broadband characteristics can be easily obtained particularly in liquid.

  There is a capacitive transducer including elements in which cells are arranged in a square shape or a rectangular shape and the interval between adjacent cells is uniform (see Patent Document 1). In addition, there is a capacitive transducer in which the transmission efficiency or reception sensitivity of the cell at the element end is lower than the transmission efficiency or reception sensitivity of the cell at the center of the element (see Patent Document 2).

JP 2008-98697 A U.S. Pat. No. 8,456,958

  When ultrasonic waves are transmitted by a capacitive transducer that includes elements in which cells are arranged in a square or rectangular shape and the spacing between adjacent cells is uniform, uniform sound pressure is radiated at the end and center of the element. Therefore, side lobes are likely to occur in the ultrasonic beam. The image quality of an ultrasonic image using this ultrasonic beam may be deteriorated by side lobes. Similarly, in the case of reception, the image quality may deteriorate.

  In addition, in the capacitive transducer where the transmission efficiency or reception sensitivity of the cell at the element end of the capacitive transducer is lower than the cell at the center, the shape of the cell at the element end is different from the shape of the cell at the center. ing. While this configuration enables apodization to reduce side lobes, since the frequency characteristics of the transmission efficiency and reception sensitivity of each cell are different, it also acquires signals in unnecessary frequency bands especially when receiving ultrasonic waves, The signal-to-noise ratio may deteriorate.

  The present invention has been made on the basis of such problem recognition. An object of the present invention is to reduce side lobes in a capacitive transducer.

The present invention employs the following configuration. That is,
A capacitive transducer including an element having a plurality of cells in which a vibrating membrane including one electrode of a pair of electrodes formed with a gap is supported so as to be capable of vibrating,
The capacitance type transducer is characterized in that the distance between the pair of electrodes of the cell at the end of the element is wider than the distance between the pair of electrodes of the cell at the center of the element.

The present invention also employs the following configuration. That is,
A method of manufacturing a capacitive transducer including an element having a plurality of cells,
Forming a plurality of first electrodes;
A plurality of cells including a pair of the first electrode and the second electrode are formed by forming a vibrating membrane including a plurality of second electrodes respectively corresponding to the plurality of first electrodes so as to vibrate. And steps to
Have
In the forming step, the distance between the first electrode and the second electrode in the cell at the end of the element is the distance between the first electrode and the second electrode in the cell at the center of the element. It is a manufacturing method of a capacitive transducer characterized by being wider.

  According to the present invention, side lobes in a capacitive transducer can be reduced.

The top view and AB sectional drawing of the capacitive transducer of Example 1. FIG. The top view and AB sectional drawing of the capacitive transducer of Example 2. FIG. FIG. 6 is a cross-sectional view taken along a line A-B illustrating a method for manufacturing a capacitive transducer. The block diagram explaining the structure of the subject information acquisition apparatus.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described below should be changed as appropriate according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the following description.

  The present invention has been made with respect to an ultrasonic capacitive transducer, and can be applied to an apparatus and a method for transmitting or receiving ultrasonic waves using the transducer. Further, the subject of the present invention includes an apparatus using ultrasonic echo technology that transmits ultrasonic waves to a subject such as a living body and receives echo waves reflected and propagated inside the subject. By generating data based on echo waves, characteristic information reflecting the difference in acoustic impedance inside the subject can be acquired.

  In addition to the echo wave, the capacitive transducer of the present invention is a photoacoustic that is generated and propagated by a light absorber inside the subject due to a photoacoustic effect when the subject is irradiated with light from a light source. Can be used to receive waves. By analyzing the photoacoustic wave, functional information and optical characteristic information inside the subject can be acquired. Since these devices obtain characteristic information by processing the received echo waves and photoacoustic waves by the signal processing unit and then performing analysis by the information processing device, they can be called object information acquisition devices. . By making the characteristic information into image data and displaying it on the display unit, an internal inspection such as diagnosis can be performed.

  The present invention can also be understood as a method for controlling an object information acquiring apparatus, an object information acquiring method, or an acoustic wave measuring method. Furthermore, these methods can be understood as programs that are realized by an information processing unit such as a CPU or a circuit. It can also be understood as a method of manufacturing a capacitive transducer characteristic of the present invention and a method of manufacturing a probe using the same.

When a capacitive transducer is used for acquiring characteristic information, it is preferable to use a probe in which one or a plurality of elements are arranged. If the object is held and the probe is scanned, a wide range of measurement is possible. If the subject is a breast, for example, holding using a plate-like member or bowl-like member is suitable.
The ultrasonic wave referred to in the present invention is listed as a typical example of an acoustic wave also called a sound wave or an elastic wave, and does not limit the wavelength or the like.

  The embodiment of the present invention will be described below with reference to FIG. FIG. 1A is a top view of the capacitive transducer of the present invention, and FIG. 1B is a cross-sectional view taken along the line AB of FIG. The capacitance transducer element 14 of the present invention has a plurality of cells 12 formed therein. In FIG. 1, the number of elements included in the capacitive transducer is one, but any number may be used. Here, the element is one element of the capacitive transducer in which the signal extraction electrodes of all cells constituting the element are common. That is, an electrical signal is output for each element. In FIG. 1, the number of cells included in the element 14 is 15, but any number may be used.

  A vibrating membrane 9 is supported on the cell so as to vibrate. The vibration film 9 includes the second electrode 1, and the second electrode 1 is provided with the first electrode 2 and the gap 3 (cavity) interposed therebetween. In FIG. 1, the vibrating membrane has a configuration in which the second electrode 1 is sandwiched between the first membrane 7 and the second membrane 8, but the vibrating membrane is capable of vibrating and has the second electrode. What is necessary is just to have only the 2nd electrode or the structure of only a 1st membrane and a 2nd electrode. As will be described later, reference numeral 4 denotes an etching path, reference numeral 6 denotes a sealing portion, reference numeral 10 denotes a substrate, and reference numerals 11 and 15 denote first and second insulating films.

  The first electrode or the second electrode is used as an electrode for applying a bias voltage or an electrode for applying an electric signal or taking out an electric signal. In FIG. 1, the first electrode is used as the electrode to which the bias voltage is applied, and the second electrode is used as the signal extraction electrode. The electrode to which the bias voltage is applied is also common in the element. The bias voltage may be common between the elements. On the other hand, the signal extraction electrode must be electrically separated for each element.

  In the element of the capacitive transducer of FIG. 1, the distance between the pair of electrodes of the cell at the end of the element is wider than the distance between the pair of electrodes of the cell at the center of the element. Since the transmission efficiency or the reception sensitivity becomes lower as the distance between the pair of electrodes of the cell becomes wider, the transmission efficiency or the reception sensitivity of the cell at the element end can be lowered. Therefore, compared to a capacitive transducer having the same transmission efficiency or reception sensitivity from the central part to the end part of the element, side lobes generated again of the ultrasonic beam can be reduced. Therefore, it is possible to reduce the ultrasonic signal from the target that is not along the direction of the ultrasonic beam, and to form a high-quality ultrasonic image.

  Furthermore, the spring constant of the vibration film of the cell at the element end can be made smaller than the spring constant of the vibration film of the cell at the center of the element. The frequency characteristic of transmission efficiency or reception sensitivity is determined by the effective spring constant of the cell. This effective spring constant depends on the force obtained by subtracting the electrostatic force from the restoring force of the vibrating membrane spring. In the capacitive transducer of this configuration, the distance between the pair of electrodes of the cell at the element end is wider than the distance between the pair of electrodes at the cell at the center of the element. It is smaller than the electrostatic force of the cell.

As in this configuration, by making the spring constant of the vibration film of the cell at the element end smaller than the spring constant of the vibration film of the cell at the center of the element, the effective spring constant of all the cells in the element can be reduced. Can be the same. Then, the frequency characteristics of transmission efficiency and reception sensitivity of all the cells constituting the element are almost the same. Therefore, since an unnecessary frequency band signal is not acquired when receiving an ultrasonic wave, the SN ratio is not deteriorated, and deterioration of image quality can be prevented. Therefore, the capacitive transducer of this configuration enables apodization to reduce the side lobes and can reduce the deterioration of the SN ratio, so that a high-quality ultrasonic image can be formed.

  Furthermore, the sum of the restoring force and electrostatic attraction force due to the vibration film of the cell at the end of the element may be equal to the sum of the restoring force and electrostatic attraction force of the vibration film of the cell at the center of the element. In the capacitive transducer of this configuration, the effective spring constant of all the cells in the element can be made the same, and the frequency characteristics of the transmission efficiency and the reception sensitivity of all the cells constituting the element are the same. . Therefore, since an unnecessary frequency band signal is not acquired when receiving an ultrasonic wave, the SN ratio is not deteriorated, and deterioration of image quality can be prevented. Therefore, the capacitive transducer of this configuration enables apodization to reduce the side lobes and can reduce the deterioration of the SN ratio, so that a high-quality ultrasonic image can be formed.

ここで、ε_０は真空の誘電率、Ｓは振動膜面積、Ｖはバイアス電圧、ｄはバイアス電圧を印加する前の実効距離である。 For example, if the vibration film shape is a circle, the restoring force F _M of the spring can be described as (1).
F _M = k _M · x (1)
Here, K _M is the spring constant of the vibrating film, x is a displacement of the vibrating membrane.
Moreover, the electrostatic force _{F E} can be described as (2).

Here, ε ₀ is the dielectric constant of vacuum, S is the diaphragm area, V is the bias voltage, and d is the effective distance before the bias voltage is applied.

従って、セルの実効的なばね定数は、（４）式のように記述できる。

The total sum of the restoring force and electrostatic attractive force of the vibrating membrane of the cell can be described as in equation (3).

Therefore, the effective spring constant of the cell can be described as in equation (4).

When the sum of the restoring force and electrostatic attraction force due to the vibrating membrane of the cell at the end of the element and the sum of the restoring force and electrostatic attraction force of the vibrating membrane of the cell at the center of the element are exactly the same As well as a range where each effective spring constant is equal. If the effective spring constants of the cell at the center and the end of the element are described by adding c and e to the subscript at the lower right, respectively, it can be described as in equation (5).

Here, since d _e > d _c , k _Me <k _Mc is satisfied. If the spring constant of the end part of the element and the spring constant of the center part are determined so as to satisfy the expression (5), the frequency characteristics of the transmission efficiency and the reception sensitivity of all the cells constituting the element can be made the same. Therefore, since an unnecessary frequency band signal is not acquired when receiving an ultrasonic wave, the SN ratio is not deteriorated, and deterioration of image quality can be prevented. Therefore, the capacitive transducer of this configuration enables apodization to reduce the side lobes and can reduce the deterioration of the SN ratio, so that a high-quality ultrasonic image can be formed.

  The driving principle of the present invention will be described. The capacitive transducer can extract an electric signal from the second electrode by using a signal extraction wiring. In the present embodiment, an electrical signal is drawn out by a lead wiring, but a through wiring or the like may be used. In this embodiment, an electrical signal is drawn from the second electrode, but it may be drawn from the first electrode.

  When ultrasonic waves are received by the capacitive transducer, a direct current voltage is applied to the first electrode 2 by a voltage application unit (not shown) to generate a potential difference between the electrodes. In this case, the second electrode 1 is preferably fixed to the ground voltage. The ground voltage indicates a DC reference potential included in a current-voltage conversion circuit (reception circuit) (not shown). When the ultrasonic wave is incident, the vibration film 9 having the second electrode 1 is deformed, so that the distance of the gap 3 between the second electrode 1 and the first electrode 2 is changed, resulting in the capacitance. Changes. Due to this change in capacitance, a current is output from the second electrode 1 and a current flows through the lead-out wiring. This current can be converted into a voltage by a current-voltage conversion circuit (not shown) and ultrasonic waves can be received. As described above, by changing the configuration of the lead-out wiring, a DC voltage may be applied to the second electrode and an electric signal may be drawn from the first electrode. The current-voltage conversion circuit is preferably provided in the probe 402 of FIG.

  When transmitting an ultrasonic wave, an AC voltage (including a pulse voltage) is transmitted as a transmission signal to the second electrode 1 in a state where a potential difference is generated between the first electrode 2 and the second electrode 1. And the vibrating membrane 9 can be vibrated by electrostatic force. Thereby, an ultrasonic wave can be transmitted. Also in the case of transmission, the vibration film may be vibrated by applying an AC voltage to the first electrode by changing the configuration of the lead wiring.

  A manufacturing method of the present invention will be described with reference to FIGS. FIG. 3 is a cross-sectional view of the capacitive transducer of the present invention, and has substantially the same configuration as FIG. 3 is a cross-sectional view taken along the line AB of FIG.

  As shown in FIG. 3A, a first insulating film 61 is formed on the substrate 60. The substrate 60 is a silicon substrate, and the first insulating film 61 is provided to form insulation with the first electrode. When the substrate 60 is an insulating substrate such as a glass substrate, the first insulating film 61 need not be formed. The substrate 60 is preferably a substrate having a small surface roughness. When the surface roughness is large, the surface roughness is transferred even in the film-forming process in the subsequent step of this process, and the distance between the first electrode and the second electrode due to the surface roughness is between each cell. , It will vary between each element. This variation is a variation in transmission and reception sensitivity. Therefore, the substrate 60 is preferably a substrate having a small surface roughness.

  Next, the first electrode 51 is formed. The first electrode 51 is preferably a conductive material having a small surface roughness, such as titanium or aluminum. As with the substrate, when the surface roughness of the first electrode is large, the distance between the first electrode and the second electrode due to the surface roughness varies between cells and between elements. A small conductive material is desirable.

  Next, a second insulating film 65 is formed. The second insulating film 65 is preferably made of an insulating material having a small surface roughness, and the electric current between the first electrode and the second electrode when a voltage is applied between the first electrode and the second electrode. It is formed to prevent mechanical short circuit or dielectric breakdown. In the case of driving at a low voltage, since the first membrane layer described later is an insulator, the second insulating film 65 need not be formed. As with the substrate, when the surface roughness of the second insulating film is large, the distance between the first electrode and the second electrode due to the surface roughness varies between cells and between elements. A second insulating film with low roughness is desirable. For example, a silicon nitride film, a silicon oxide film, or the like.

  Next, as shown in FIG. 3B, a sacrificial layer 53 is formed. The sacrificial layer 53 is preferably made of a material having a small surface roughness. Similar to the substrate, when the surface roughness of the sacrificial layer is large, the distance between the first electrode and the second electrode due to the surface roughness varies between cells and between elements. A small sacrificial layer is desirable. In addition, in order to shorten the etching time for etching to remove the sacrificial layer, a material having a high etching rate is desirable.

  Further, there is a demand for a sacrificial layer material that does not substantially etch the second insulating film, the first membrane layer, and the second electrode against an etching solution or an etching gas that removes the sacrificial layer. When the second insulating film, the first membrane layer, and the second electrode are substantially etched with respect to the etching solution or etching gas for removing the sacrificial layer, the thickness variation of the vibration film, the first electrode and the first electrode Variation in the distance between the two electrodes occurs. Variations in the thickness of the vibrating membrane and variations in the distance between the first electrode and the second electrode result in sensitivity variations between cells and between elements. When the second insulating film and the first membrane layer are a silicon nitride film or a silicon oxide film, the surface roughness is small, and the second insulating film, the first membrane layer, and the second electrode are not etched. Chrome is preferred.

  Next, as shown in FIG. 3C, a sacrificial layer 83 and a sacrificial layer 93 are formed by repeating the process of FIG. The sacrificial layer 53 is for forming a cell gap at the center of the device, and the sacrificial layer 83 and the sacrificial layer 93 are for forming a cell gap toward the end of the device. Therefore, the sacrificial layer 53 is thinner than the sacrificial layer 83, and the sacrificial layer 83 is thinner than the sacrificial layer 93.

  From FIG. 3D to FIG. 3F, the process of forming the gap by forming the vibration film including the second electrode and removing the sacrificial layer will be described. In FIG. 3, the vibrating membrane is composed of the first membrane, the second electrode, and the second membrane, but may be composed of any number of layers including the second electrode.

  As shown in FIG. 3D, a first membrane layer 57 including a first membrane is formed. The first membrane layer 57 desirably has a low tensile stress. For example, a tensile stress of 300 MPa or less is good. The silicon nitride film can be stress-controlled and can have a low tensile stress of 300 MPa or less. When the first membrane has a compressive stress, the first membrane causes sticking or buckling and deforms greatly. In addition, in the case of a large tensile stress, the first membrane may be broken. Therefore, the first membrane layer 57 desirably has a low tensile stress.

  Next, as shown in FIG. 3E, a second electrode 52 is formed, and further an etching hole (not shown) is formed. Thereafter, the sacrificial layer 53, the sacrificial layer 83, and the sacrificial layer 93 are removed from the etching hole via an etching path (not shown). The second electrode 52 is preferably made of a material having a small residual stress and heat resistance. When the residual stress of the second electrode is large, it causes a large deformation of the vibration film. Therefore, the second electrode having a small residual stress is desirable. Further, a material that does not cause alteration or increase in stress depending on the temperature at the time of forming the second membrane layer or the sealing layer for forming the sealing portion is desirable.

  Further, when removing the sacrificial layer with the second electrode exposed, it is necessary to perform sacrificial layer etching while applying a photoresist or the like that protects the second electrode. Since the first membrane is likely to stick due to the stress of the photoresist or the like, it is desirable to have a second electrode having etching resistance so that the sacrificial layer can be etched without the photoresist and with the second electrode exposed. The sticking means that a vibration film as a structure adheres after the sacrificial layer is removed. For example, titanium, aluminum silicon alloy, etc. are desirable.

  Next, as shown in FIG. 3F, a second membrane layer 58 including a second membrane is formed. In this step, a second membrane is formed and a sealing portion for sealing an etching hole (not shown) is formed. By forming the second membrane layer 58, the second membrane can be formed to form a vibration film having a desired spring constant, and the etching hole can be sealed.

When the process of sealing the etching hole and the process of forming the second membrane are the same as in this process, the vibration film can be formed only by the film forming process. Therefore, it is easy to control the thickness of the diaphragm, and it is possible to suppress variations in the spring constant or deflection of the diaphragm due to thickness variations, reducing variations in reception or transmission sensitivity between cells or elements. can do.
The step of sealing the etching hole and the step of forming the second membrane may be separate steps. The sealing part can be formed after forming the second membrane, or the second membrane can be formed after forming the sealing part.

  The second membrane layer is preferably made of a material having a low tensile stress. Similar to the first membrane, when the second membrane has a compressive stress, the first membrane causes sticking or buckling and is greatly deformed. In addition, in the case of a large tensile stress, the second membrane may be broken. Therefore, a low tensile stress is desirable for the second membrane layer. The silicon nitride film can be stress-controlled and can have a low tensile stress of 300 MPa or less.

The sealing unit may prevent liquid or outside air from entering the gap. In particular, when sealing is performed under reduced pressure, the vibration film is deformed by atmospheric pressure, and the distance between the first electrode and the second electrode is shortened. Since the sensitivity of transmission or reception is proportional to the effective distance between the first electrode and the second electrode to the 1.5th power, sealing under reduced pressure and keeping the gap lower than atmospheric pressure Transmission or reception sensitivity can be improved. The effective distance is the sum of the value obtained by dividing the insulating film between the first electrode and the second electrode by the relative dielectric constant and the gap.
After this step, wirings connected to the first electrode and the second electrode are formed by a step (not shown). The wiring material may be aluminum or the like.

According to such a manufacturing method, a capacitive transducer having a configuration necessary to achieve the object of the present invention can be manufactured.
Hereinafter, the present invention will be described in detail with reference to more specific examples.

<Example 1>
The embodiment of the present invention will be described below with reference to FIG. FIG. 1A is a top view of the capacitive transducer of the present invention, and FIG. 1B is a cross-sectional view taken along the line AB of FIG. The element 14 of the capacitive transducer of the present invention is composed of 15 cells 12. In FIG. 1, the number of elements included in the capacitive transducer is one, but any number may be used.

  In the cell 12, a vibrating membrane 9 including the second electrode 1 provided with the first electrode 2 and the gap 3 interposed therebetween is supported so as to be able to vibrate. The vibrating membrane 9 has a configuration in which the second electrode 1 is sandwiched between the first membrane 7 and the second membrane 8. The first electrode 2 is an electrode to which a bias voltage is applied, and the second electrode 1 is a signal extraction electrode. The diaphragm shape of the present embodiment is a circle, but the shape may be a rectangle, a hexagon, or the like. In the case of a circular shape, the vibration mode is axisymmetric, and hence vibration of the vibration film due to unnecessary vibration modes can be suppressed.

The first insulating film 11 on the silicon substrate 10 is a silicon oxide film having a thickness of 1 μm formed by thermal oxidation. The second insulating film 15 is a silicon oxide film with a thickness of 0.1 μm formed by Plasma Enhanced Chemical Vapor Deposition (PE-CVD). The first electrode is aluminum having a thickness of 50 nm, and the second electrode 4 is aluminum having a thickness of 100 nm. The first membrane 7 and the second membrane 8 are silicon nitride films produced by PE-CVD and are formed with a tensile stress of 200 MPa or less. The diameters of the first membrane 7 and the second membrane 8 are 25 μm, and the thicknesses are 0.4 μm and 0.7 μm, respectively.

  In the element of the capacitive transducer of FIG. 1, the distance between the pair of electrodes of the cell at the end of the element is wider than the distance between the pair of electrodes of the cell at the center of the element. The gap depth of the cell at the element end is 0.25 μm, and the gap depth toward the cell at the center of the element is 0.2 μm and 0.15 μm, respectively. With this configuration, the transmission efficiency or the reception sensitivity becomes lower as the distance between the pair of electrodes of the cell becomes wider, so the transmission efficiency or the reception sensitivity of the cell at the element end can be lowered. Therefore, compared to a capacitive transducer having the same transmission efficiency or reception sensitivity from the central part to the end part of the element, side lobes generated again of the ultrasonic beam can be reduced. Therefore, it is possible to reduce the ultrasonic signal from the target that is not along the direction of the ultrasonic beam, and to form a high-quality ultrasonic image.

  The gap between the pair of electrodes in the cell at the element end is only required to be larger than the gap between the pair of electrodes in the cell at the center of the element. Is larger than the cell gap depth, the first insulating film thickness, and the vibration film thickness. When the vibration film thickness of the cell at the element end is larger than the vibration film thickness of the cell at the element center, the vibration film size of the cell at the element end is larger than the vibration film size of the cell at the element center. Thereby, the spring constant of the vibration film of the cell at the element end and the spring constant of the vibration film of the cell at the center of the element can be made the same, and the frequency characteristics of the transmission efficiency or reception sensitivity of all the cells in the element are Can be the same.

It is desirable that the cell gap depth and the first insulating film thickness at the element end are larger than the cell gap depth and the first insulating film thickness at the element center. In this configuration, the size of the vibration film of all the cells in the element can be made the same, so that the design can be facilitated. Further, the radiation impedance of all the cells in the element can be made the same, and the etching time of the sacrificial layer of all the cells in the element can be made the same.
The distance between the pair of electrodes of the cell at the end of the element and the distance between the pair of electrodes of the cell at the center of the element may be designed depending on the shape of the ultrasonic beam to be formed. For example, if a Gaussian beam is used, it may be designed according to its distribution.

  Further, since the vibration film shape of the cell is the same in the element, the frequency characteristics of the transmission efficiency and the reception sensitivity of all the cells constituting the element are substantially the same. Therefore, since an unnecessary frequency band signal is not acquired when receiving an ultrasonic wave, the SN ratio is not deteriorated, and deterioration of image quality can be prevented. Therefore, the capacitive transducer of this configuration enables apodization to reduce the side lobes and can reduce the deterioration of the SN ratio, so that a high-quality ultrasonic image can be formed. The “same cell shape” includes not only the case where the shape is strictly the same, but also an error that can be regarded as the same frequency characteristics of the cell conversion efficiency, such as an error in the manufacturing process.

<Example 2>
The configuration of the capacitive transducer according to the second embodiment will be described with reference to FIG. FIG. 2A is a top view of the capacitive transducer of the present invention, and the configuration of the capacitive transducer of the second embodiment is substantially the same as that of the first embodiment. Therefore, it demonstrates centering on a different point.

The capacitive transducer of FIG. 2 includes a second electrode 31, a first electrode 32, a gap 33,
The etching path 34, the sealing portion 36, the first membrane 36, the second membrane 37, the vibration film 39, the substrate 40, the first insulating film 41, the cell 42, the element 44, and the second insulating film 45 are included.

  In the element of the capacitive transducer of FIG. 2, the spring constant of the diaphragm of the cell at the end of the element is smaller than the spring constant of the diaphragm of the cell at the center of the element. The frequency characteristic of transmission efficiency or reception sensitivity is determined by the effective spring constant of the cell. This effective spring constant depends on the force obtained by subtracting the electrostatic force from the restoring force of the vibrating membrane spring. In the capacitive transducer of this configuration, the distance between the pair of electrodes of the cell at the end of the element is wider than the distance between the pair of electrodes of the cell at the center of the element. It is smaller than the electrostatic force of the cell at the center of the element.

  As in this configuration, the spring constant of the diaphragm of the cell at the end of the element is smaller than the spring constant of the diaphragm of the cell at the center of the element. Constants can be the same. Therefore, the frequency characteristics of the transmission efficiency and the reception sensitivity of all cells constituting the element are almost the same. Therefore, since an unnecessary frequency band signal is not acquired when receiving an ultrasonic wave, the SN ratio is not deteriorated, and deterioration of image quality can be prevented. Therefore, the capacitive transducer of this configuration enables apodization to reduce the side lobes and can reduce the deterioration of the SN ratio, so that a high-quality ultrasonic image can be formed.

  As described above, in the capacitive transducer according to the present invention, the distance between the pair of electrodes in the cell at the element end is wider than the distance between the pair of electrodes in the cell at the center of the element. Since the transmission efficiency or the reception sensitivity becomes lower as the distance between the pair of electrodes of the cell becomes wider, the transmission efficiency or the reception sensitivity of the cell at the element end can be lowered, and the transmission efficiency or the reception sensitivity of the cell at the center of the element can be lowered. Therefore, compared to a capacitive transducer having the same transmission efficiency or reception sensitivity from the central part to the end part of the element, side lobes generated again of the ultrasonic beam can be reduced. Therefore, it is possible to reduce the ultrasonic signal from the target that is not along the direction of the ultrasonic beam, and to form a high-quality ultrasonic image.

<Application example>
The above capacitive transducer can be applied to a probe that receives or transmits an acoustic wave using the capacitive transducer. For example, in FIG. 4, the probe 402 includes a plurality of elements 403. When the transmission unit 405 controls the transmission acoustic wave according to a command from the information processing unit 406, an acoustic wave is generated from each element. On the other hand, at the time of reception, the electric signal output from each element is subjected to processing (for example, amplification or AD conversion) by the signal processing unit 404.

FIG. 4 shows a state in which the above probe is used as a constituent element of the subject information acquiring apparatus.
First, as the characteristic information, a case where a light absorber inside the subject 401 absorbs light from a light source (not shown) to generate a photoacoustic wave will be described. At this time, the photoacoustic wave propagates inside the subject and is received by the element. The electric signal output from the element is input to the signal processing unit and subjected to signal processing. The information processing unit generates an initial sound pressure distribution, an absorption coefficient distribution, and the like in the subject by a known image reconstruction process based on the signal input from the signal processing unit. In diagnosis, these pieces of information may be converted into image data and displayed on the display unit 407 as necessary. In the present specification, a configuration including a signal processing unit and an information processing unit may be referred to as a processing unit.

Next, a case where echo information inside the subject is acquired will be described. At this time, an acoustic wave is transmitted from each element by a control signal sent by the transmitter. The acoustic wave reflected at the acoustic impedance boundary inside the subject is received by the element again. The received signal output from the element is subjected to known signal processing, reconstruction processing, and image data conversion as in the case of the photoacoustic wave. In the case of an apparatus using this reflected wave, the probe that transmits the acoustic wave may be provided separately from the probe that receives the acoustic wave.
Furthermore, the capacitive transducer of the present invention can also be applied to a device having the functions of a device using photoacoustic waves and a device using echo waves.

  Note that the probe may be one that scans mechanically, or one that a user such as a doctor or an engineer holds the probe and moves relative to the subject (handheld type). In particular, when a subject that is a living body is mechanically scanned, stable measurement can be performed by holding the subject by a holding unit. If the subject is a breast, a plate-shaped or bowl-shaped holding means is suitable.

  1: second electrode, 2: first electrode, 3: gap, 9: vibrating membrane, 12: cell, 14: element

Claims (8)

A capacitive transducer including an element having a plurality of cells in which a vibrating membrane including one electrode of a pair of electrodes formed with a gap is supported so as to be capable of vibrating,
The capacitive transducer characterized in that the distance between the pair of electrodes of the cell at the end of the element is wider than the distance between the pair of electrodes of the cell at the center of the element. 2. The capacitive transducer according to claim 1, wherein the spring constant of the vibration film of the cell at the end of the element is smaller than the spring constant of the vibration film of the cell at the center of the element. The sum of the restoring force and electrostatic attraction force due to the vibration film of the cell at the end of the element is equal to the sum of the restoring force and electrostatic attraction force due to the vibration film of the cell at the center of the element. 3. The capacitive transducer according to 1 or 2.   A probe comprising the capacitive transducer according to any one of claims 1 to 3. The capacitive transducer according to any one of claims 1 to 3,
A processing unit that acquires characteristic information in the subject using an electrical signal output from the capacitive transducer when an acoustic wave propagating from the subject is incident on the vibrating membrane. A subject information acquisition apparatus. A transmission unit that transmits an acoustic wave by vibrating the vibrating membrane by applying a voltage to the pair of electrodes;
The object information acquiring apparatus according to claim 5, wherein the acoustic wave is obtained by reflecting the transmitted acoustic wave inside the object. A light source,
The object information acquiring apparatus according to claim 5, wherein the acoustic wave is a photoacoustic wave generated from the object irradiated with light from the light source. A method of manufacturing a capacitive transducer including an element having a plurality of cells,
Forming a plurality of first electrodes;
A plurality of the cells including the pair of the first electrodes and the second electrode are formed by oscillating a vibrating membrane including the plurality of first electrodes and a plurality of second electrodes each paired with the plurality of first electrodes. Forming step;
Have
In the forming step, the distance between the first electrode and the second electrode in the end cell of the element is set such that the distance between the first electrode and the second electrode in the center cell of the element is The manufacturing method of the capacitive transducer characterized by being wider than this distance.

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