Patent Application
20150016221
1. Field of the Invention
The present invention relates to a transducer and a subject information acquisition apparatus. More particularly, the present invention relates to a transducer for transmitting or receiving an acoustic wave and a subject information acquisition apparatus including the transducer.
2. Description of the Related Art
A capacitive micromachined ultrasonic transducer (CMUT) produced by using micro-machining techniques has been studied as a substitute of a piezoelectric element. A capacitance type transducer such as a CMUT can transmit or receive an acoustic wave (typically an ultrasonic wave) by using vibration of a diaphragm.
Each cell of the capacitance type transducer includes two electrodes disposed facing each other to sandwich a space called a cavity which is maintained in a substantially vacuum state. One of the two electrodes is fixed to a membrane, and functions as a diaphragm together with the membrane.
When the capacitance type transducer receives an acoustic wave in a state where a bias voltage is applied between the two electrodes of each cell, the diaphragm of each cell vibrates. Accordingly, the distance between the two electrodes changes, resulting in a change in the capacitance. An element composed of a plurality of cells outputs the change in the capacitance as a current signal. Further, when a voltage with amplitude varying with time (i.e., alternating-current (AC) voltage) is applied between the two electrodes, the diaphragm vibrates, allowing an acoustic wave to be transmitted on an element-by-element basis.
International Publication No. WO 09/008282 discusses a CMUT composed of a plurality of one-dimensionally arranged elements. In the CMUT discussed in International Publication No. WO 09/008282, a bias voltage is applied to lower electrodes (common electrodes) electrically connected to each other among the plurality of elements, and upper electrodes (signal electrodes) separated for each element are connected to the ground. Each electrode is connected to wiring of a flexible substrate with a wire.
It is desirable that each element of the capacitance type transducer has uniform reception characteristics or transmission characteristics (i.e., conversion characteristics such as conversion efficiency). However, in a state where a bias voltage is applied between the electrode (common electrodes) electrically connected to each other among the plurality of elements, and the other electrodes, when the transducer receives an acoustic wave and the diaphragm of each cell vibrates or when an AC voltage is applied between the electrodes, a current flows through the plurality of elements. In this case, a transient response may vary among the elements depending on the condition of wiring connecting the common electrodes to each other. Such a variation occurring in the transient response affects the reception characteristics or transmission characteristics of each element. Particularly in the case of a two-dimensional array composed of two-dimensionally arranged elements, the variation in the transient response may be larger among the elements.
The present invention is directed to a transducer which reduces an influence on the conversion characteristics due to a variation in the transient response among the elements.
According to an aspect of the present invention, a transducer includes a plurality of elements each including at least one cell structured in such a way that a diaphragm including one of a first electrode and a second electrode disposed facing each other via a space is vibratably supported, bias wiring for supplying a bias voltage to the first electrode to provide a potential difference between the first and the second electrodes, and for electrically connecting the first electrodes of the plurality of elements to each other, and a plurality of signal wiring lines each connected to a different one of the plurality of elements. The bias wiring includes a plurality of branch wiring lines to each of which the first electrodes of a part of the plurality of elements are connected, a plurality of first common wiring lines for connecting the plurality of branch wiring lines to each other, and a second common wiring line for connecting the plurality of first common wiring lines to each other.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings.
Elements and wiring of a capacitance type transducer according to an exemplary embodiment will be described below with reference to
The transducer according to the present exemplary embodiment includes a substrate 100, a plurality of elements 101 formed on the substrate 100, bias wiring 103, and a signal wiring line 104. The substrate 100 is provided with a terminal 105 to which the bias wiring 103 is connected. The terminal 105 and a wiring substrate (such as a flexible substrate) are connected to each other by a wire, an anisotropic conductive film (ACF), or through wiring to supply a bias voltage to the terminal 105 from a bias power source 102 provided outside the transducer.
The bias wiring 103 according to the present exemplary embodiment is arranged so as to reduce a time constant variation which affects the transient response. This point will be described in detail below.
The configuration of the elements 101 according to the present exemplary embodiment will be described below. Each of the elements 101 according to the present exemplary embodiment includes a plurality of cells 1 electrically connected to each other. Although, in
Each of the cells 1 is a minimum unit structure in which a diaphragm including one of two electrodes (a pair of electrodes) disposed facing each other via a cavity as a space is vibratably supported. More specifically, in
Each of the elements 101 includes at least one cell 1 to form an electrically independent component. Specifically, when one cell 1 is considered as one capacitance, capacitances of the plurality of cells 1 in the element 101 are electrically connected in parallel to each other. Signals are input and output in units of the element 101. Further, when a plurality of elements 101 is provided, the elements 101 are electrically independent of each other.
In the present exemplary embodiment, the first electrodes 13 function as common electrodes electrically connected to each other among the elements 101. A bias voltage is applied to the first electrodes 13. The second electrodes 4 function as signal electrodes from which an output signal for each element 101 is taken out. Specifically, the first electrodes 13 are electrically connected to each other among the elements 101 via the bias wiring 103 illustrated in
Although, in the present exemplary embodiment, the first electrodes 13 are electrically connected to each other among the plurality of elements 101 included in the transducer, the first electrodes 13 do not necessarily require electrical connection among all of the elements 101 included in the transducer. Specifically, it is possible that all of the elements 101 are divided into several groups, the first electrodes 13 are electrically connected to each other among a plurality of elements 101 within each of the groups, and the first electrodes 13 are not electrically connected across the groups. For example, it is assumed here that the transducer includes a total of “m” elements 101 (m is an integer equal to or larger than 4), out of which “n” elements 101 (n is an integer equal to or larger than 2, and smaller than m) are included in a first group, and “1” elements 101 (l is an integer equal to or larger than 2, and smaller than m) are included in a second group. In this case, the bias wiring 103 of the first and the second groups may be connected to the same single terminal 105, allowing the first electrodes 13 of the first and the second groups to be connected to each other, or the bias wiring 103 of the first and the second groups may be connected to different terminals 105.
Although, in the present exemplary embodiment, the electrode on the substrate 100 side serves as the first electrode 13 (common electrode) and the electrode on the diaphragm side serves as the second electrode 4 (signal electrode), these electrodes may be configured in reverse way. Specifically, the electrode on the substrate 100 side may serve as a signal electrode separated for each element 101, and the electrode on the diaphragm side may serve as a common electrode conducting among the elements 101.
Although, in
In the present exemplary embodiment, the first electrode 13 is disposed above the substrate 100 via the first insulating film 12, and the second insulating film 14 is disposed on the first electrode 13. However, the first electrode 13 may be disposed directly on the substrate 100 without using the first insulating film 12. Further, the first electrode 13 may be exposed without disposing the second insulating film 14 thereon.
A silicon substrate or a glass substrate can be used as the substrate 100. A metal such as titanium and aluminum, or an aluminum silicon alloy can be used as the first electrode 13, the second electrode 4, the bias wiring line 103, and the signal wiring line 104. A silicon nitride film or a silicon oxide film can be used as the first insulating film 12, the second insulating film 14, and the membrane 15. A transducer can be produced by using well-known methods such as a sacrifice layer type in which a cavity is formed by etching a sacrifice layer, and a junction type in which an active layer (surface silicon layer) of a silicon-on-insulator (SOI) substrate is used as a membrane.
The driving principle of the capacitance type transducer will be described below. When the capacitance type transducer receives an ultrasonic wave, a potential difference is first produced between the first electrode 13 and the second electrode 4. More specifically, a bias voltage (direct current (DC) voltage) is applied from the bias power source 102 to the first electrode 13, and the second electrode 4 is connected to the ground potential via a receiving circuit (not illustrated). When the transducer receives an ultrasonic wave in this state, the diaphragm having the second electrode 4 vibrates, the distance between the second electrode 4 and the first electrode 13 changes, and accordingly the capacitance changes. This capacitance change causes a signal (current) to be output from the second electrode 4, allowing a current to flow through the signal wiring line 104. This current is converted into a voltage by a current-voltage conversion unit (not illustrated) such as a transimpedance circuit using an operational amplifier included in the receiving circuit (not illustrated), and then transmitted to an external signal processing unit (not illustrated) as a received signal.
When transmitting an ultrasonic wave, an AC voltage is applied from a transmitting circuit (not illustrated) to the second electrode 4 in a state where a potential difference is present between the first electrode 13 and the second electrode 4. A plurality of pulses having a certain number of waves or a sine wave is applied to the second electrode 4 as the AC voltage. The electrostatic force produced by the application of the AC voltage vibrates the diaphragm, enabling ultrasonic wave transmission. The capacitance type transducer according to the present exemplary embodiment is capable of performing at least one of transmission and reception of an ultrasonic wave (acoustic wave).
The configuration of the bias wiring 103 according to the present exemplary embodiment will be described below. First of all, the influence of a variation in the transient response among the elements 101 will be described below.
Among the elements 101 in which the first electrodes 13 (common electrodes) are electrically connected to each other by the bias wiring 103, when the diaphragm of a certain element 101 vibrates, the diaphragm is also influenced by the bias wiring line and the other elements 101 connected to the bias wiring line. For example, when a certain element 101 receives an acoustic wave, the diaphragm of the element 101 vibrates resulting in change in distance between the first electrode 13 and the second electrode 4. To keep the potential difference between the electrodes at the bias voltage even after the distance changes, it is necessary that charges are supplied between the electrodes from the bias power source 102, or absorbed to the bias power source 102. A response time (transient response) from when the charges are supplied (absorbed) until when the potential difference between the electrodes returns to a predetermined bias voltage is influenced by the resistance of the wiring of a charge supply path and the capacitances of the elements 101 connected to the wiring of the charge supply path. Regularly, since a plurality of elements 101 receives an acoustic wave with almost no time difference, some of the elements 101 have a short response time (during which charges vary) and the other elements 101 have a long response time.
When the diaphragms of the plurality of elements 101 vibrate, the elements 101 having a long response time (during which charges vary) are influenced by charge supply to elements 101 having a short response time, resulting in further longer response time. This means that the current transient response of a certain element 101 (temporarily referred to as “first element”) is influenced by the current transient responses of the other elements 101 to which the certain element 101 is connected via the bias wiring 103. The above descriptions of the case of acoustic wave reception also apply to the case of acoustic wave transmission. Such a transient response variation among the elements 101 affects the acoustic wave transmission and reception characteristics of the transducer. More specifically, such a transient response variation affects the waveform of a transmitting acoustic wave and the waveform of a signal output from the transducer upon reception of an acoustic wave.
The transient response of the first element depends on the time constant between the bias power source 102 and the first element. Influences on the transient response increase with a larger time constant, and decrease with a lower time constant. A time constant τ is represented by the product of an electrical resistance R and a capacitance C (τ=R×C). Specifically, to reduce the transient response of the first element, it is necessary to decrease the electrical resistance and the capacitance existing between the bias power source 102 and the first element. However, the time constant of the path leading from the bias power source 102 to the terminal 105 on the substrate 100 is considered to be constant for all the elements 101. The above-described potential variation generated by vibration is generated in the elements 101 on the substrate 100. Therefore, when considering the transient response variation among the elements 101, the time constant of a path having the terminal 105 as an input and each of the plurality of elements 101 as an output, has to be considered.
Specifically, the time constant which affects the transient response of a certain element 101 can be approximated by the product of the wiring resistance of the bias supply path from the terminal 105 to the certain element 101 and the capacitances connected to the above-described bias supply path along the above-described bias supply path (total capacitance of the elements 101 other than the certain element 101).
The present exemplary embodiment is characterized by reducing the time constant variation. More specifically, the time constant variation can be reduced by reducing the time constant of the element 101 having a large time constant. To reduce the time constant of a certain element 101 having a large time constant, the following methods can be considered:
1. Reducing the total capacitances of the elements 101 connected to the bias supply path connected to the certain element 101
2. Reducing the wiring resistance of the bias supply path connected to the certain element 101
An aspect of the present exemplary embodiment for reducing the time constant variation will be described below with reference to
Accordingly, another bias wiring 300 is provided to bypass bias wiring 200 ranging from the transmitting circuit serving as a pulse generation source to the plurality of elements 101. When the bias wiring 300 is connected to an element 101A which is distant from the pulse generation source, the phase of the current output from this element 101A is almost the same as the phase of the current output from an element 101B which is closest to the pulse generation source. By providing the bias wiring 300 having a small capacitive component, the variation in the transient characteristics among the elements 101 can be suppressed, which is equivalent to decreasing the time constant.
Referring to
As described above, the present exemplary embodiment is provided with a path having a small time constant to reduce the time constant variation among the elements 101. An aspect of the present exemplary embodiment is characterized in that a plurality of bias wiring lines which functions as first common wiring lines is provided. Referring to
Further, the present exemplary embodiment is provided with second common wiring lines for connecting the first common wiring lines to each other. Referring to
Thus, by providing the plurality of first common wiring lines for connecting the branch wiring lines 103A to each other, and the second common wiring lines for connecting the first common wiring lines to each other can provide an element 101 distant from the terminal 105 with a bias supply path having a small number of connected elements. Thus, the time constant variation among the elements 101 can be reduced.
It is desirable to provide the bias wiring serving as the first common wiring lines so as to connect the ends of the plurality of branch wiring lines 103A to each other. Referring to
It is desirable to provide a plurality of second common wiring lines. In particular, as illustrated in
It is also desirable that the first and second common wiring lines provide a smaller electrical resistance (wiring resistance) than the branch wiring lines 103A, and more specifically the first and second common wiring lines are thicker than the branch wiring lines 103A. According to the present exemplary embodiment, referring to the top view as illustrated in
Naturally, the signal wiring lines 104 and the bias wiring 103 (branch wiring lines 103A) can be arranged so that they vertically overlap each other by providing an insulating film between the signal wiring lines 104 and the branch wiring lines 103A. For example, referring to the sectional view as illustrated in
However, even with the same element interval of the two-dimensional array (when the elements 101 are two-dimensionally arranged equally in the vertical and horizontal directions), it is possible to increase the wiring width of the first common wiring lines. This is because no signal wiring line 104 is provided between the elements 101 in the vertical direction. It is also desirable that the second common wiring lines are provided on the outer side of the branch wiring lines 103A, and are thicker than the branch wiring lines 103A. A state where the branch wiring lines 103A are “in parallel with the direction in which the signal wiring lines 104 extend” includes not only a case where the branch wiring lines 103A are completely in parallel with the signal wiring lines 104 but also a state where the branch wiring lines 103A can be considered to be substantially in parallel with the signal wiring lines 104 to the extent that does not intersect with the signal wiring lines 104.
In the present exemplary embodiment, it is further desirable to form the first common wiring line which intersects with each branch wiring line 103A to halve each branch wiring line 103A. Hereinafter, the first common wiring line that halves each branch wiring line 103A is particularly referred to as a central common wiring line. More specifically, referring to
It is desirable to provide the central common wiring line in a position to intersect with the center of each branch wiring line 103A and halve each branch wiring line 103A. To provide the central common wiring line, it is desirable to form a plurality of signal wiring lines 104 so as to extend toward one of the end common wiring lines closer to the respective elements 101 to which the signal wiring lines 104 are connected.
More specifically, referring to
It is desirable that the signal wiring terminals (not illustrated), to which the signal wiring line 104 pulled out toward the side of the end common wiring line is connected, are provided on the inner side of the end common wiring line (inside the closed circuit). The signal wiring terminals are connected to a wiring substrate such as a flexible wiring substrate by using wires. Alternatively, the signal wiring terminals may be connected to a circuit substrate provided on the rear surface of the substrate 100 via through wiring which penetrates the substrate 100. Further, instead of being formed to be pulled out from the first electrode 13 onto the substrate 100, the signal wiring line 104 itself may be through wiring and pulled out from the bottom portion of the first electrode 13 onto the rear surface of the substrate 100 as through wiring.
On the other hand, according to another aspect of the present exemplary embodiment for reducing the time constant variation among the elements 101, the present exemplary embodiment is characterized in that a plurality of bias voltage supply paths is provided for each of the elements 101. Although, according to the above-described aspect of the present exemplary embodiment, the time constant variation is reduced by providing the plurality of first common wiring lines and the plurality of second common wiring lines, a plurality of bias voltage supply paths may be provided for each of the elements 101 by using a method other than the method with the common wiring lines.
According to yet another aspect of the present exemplary embodiment, as a result of intensive research, the inventor of the present invention has found it desirable that the time constant of each of the elements 101 is 1/10 times or less the reciprocal of the center frequency of the transducer. The reciprocal of the center frequency of the element 101 indicates the period of a signal output from the element 101. Specifically, with a large time constant, the transient response increases, thereby making it impossible to sufficiently reproduce the waveform of the signal output from the element 101 at predetermined intervals.
Therefore, it is desirable that the time constant of each of the elements 101 to which a common electrode is electrically connected is 1/10 times or less the reciprocal of the center frequency f of the transducer (i.e., τ≦ 1/10f). It is further desirable that the time constant of each of the elements 101 is 1/100 times or less the reciprocal of the center frequency f of the transducer. The center frequency of the transducer is typically 1 MHz or more and 20 MHz or less. However, for a transducer for receiving an acoustic wave generated by a photoacoustic effect (described below), it is desirable that the center frequency is 1 MHz or more and 10 MHz or less.
The transducer described in the first exemplary embodiment is applicable to a subject information acquisition apparatus using an acoustic wave including an ultrasonic wave. Such a subject information acquisition apparatus is capable of receiving an acoustic wave from a subject by using a transducer, and acquiring, by using an electrical signal output from the transducer, subject information reflecting the optical characteristic values of the subject such as the light absorption coefficient, and subject information reflecting an acoustic impedance difference.
The probes 2022 and 2122 may be a mechanically scanning type or a handheld type which is held and moved on a subject by a user such as a doctor or a technician. In the case of an apparatus using a reflection wave, as illustrated in
Further, it is also possible to combine the functions of the subject information acquisition apparatuses illustrated in
According to the exemplary embodiments of the present invention, it is possible to reduce the influence of a variation in the transient response among the elements on the conversion characteristics.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2013-144753 filed Jul. 10, 2013, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-144753 | Jul 2013 | JP | national |
