Capacitive transducer and sample information acquisition apparatus

A capacitive transducer includes at least one cell that includes a first electrode and a vibrating membrane including a second electrode provided so as to be apart from the first electrode with a cavity sandwiched between the first electrode and the second electrode. An electrostatic shield is provided on the cell via a silicone rubber layer.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a capacitive electromechanical conversion device or transducer that performs transmission and reception of acoustic waves, such as ultrasonic waves, and a sample information acquisition apparatus using the capacitive electromechanical conversion device or transducer. The transmission and reception in this specification means at least one of transmission and reception. Although the acoustic waves are used as a term including sound waves, ultrasonic waves, and photoacoustic waves, the acoustic waves may be typified by the ultrasonic waves.

Description of the Related Art

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been proposed as transducers that performs transmission and reception of the ultrasonic waves (refer to A. S. Ergun, Y. Huang, X. Zhuang, O. Oralkan, G. G. Yarahoglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: fabrication technology,” Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on, vol. 52, no. 12, pp. 2242-2258, December 2005). The CMUTs are manufactured using a Micro Electro Mechanical Systems (MEMS) process that applies a semiconductor process.FIG. 13is a schematic cross-sectional view of an exemplary CMUT (transmission-reception element). Referring toFIG. 13, a set of a vibrating membrane101, and a first electrode102and a second electrode103, which are opposed to each other with a cavity105sandwiched therebetween, is referred to as a cell. The vibrating membrane101is supported by supporters104formed on a chip100. A direct-current voltage generating unit202is connected to the second electrode103. Certain direct-current voltage Va is applied from the direct-current voltage generating unit202to the second electrode103via a second conductive line302. The first electrode102is connected to a transmission-reception circuit201via a first conductive line301and has fixed potential near ground (GND) potential. This causes a potential difference of Vbias=Va−0V between the first electrode102and the second electrode103. Adjusting the value of the direct-current voltage Va causes the value of Vbias to coincide with a desired potential difference (around several ten volts to several hundred volts) determined on the basis of mechanical characteristics of the CMUT cells.

Application of alternating-current drive voltage to the first electrode102from the transmission-reception circuit201causes alternating-current electrostatic attractive force between the first electrode102and the second electrode103and causes the vibrating membrane101to vibrate at a certain frequency to transmit the ultrasonic waves. The vibration of the vibrating membrane101in response to the ultrasonic waves causes weak current in the first electrode102through electrostatic induction. Measurement of the value of the current with the transmission-reception circuit201allows a reception signal to be extracted. The potential difference between the CMUT electrodes causes the electrostatic attractive force between the electrodes to decrease the distance between the electrodes. Increase in electric field strength between the electrodes increases transmission sound pressure (transmission efficiency) when the same drive voltage is applied and increases an output signal (reception sensitivity) when the same ultrasonic waves are received.

SUMMARY OF THE INVENTION

It may be necessary to improve transmission and reception characteristics when a capacitive ultrasonic transducer (CMUT) is used in contact with a sample (charged sample), such as a living body. The present invention provides a capacitive transducer having excellent transmission and reception characteristics and a sample information acquisition apparatus using the capacitive transducer.

A capacitive transducer includes at least one cell that includes a first electrode and a vibrating membrane including a second electrode provided so as to be apart from the first electrode with a cavity sandwiched between the first electrode and the second electrode; a silicone rubber layer; and an electrostatic shield provided on the cell via the silicone rubber layer.

DESCRIPTION OF THE EMBODIMENTS

In embodiments of the present invention, in order to resolve the above problem, an electrostatic shield, such as a metal layer, having predetermined fixed potential is arranged between the surface of a transducer facing a sample and capacitive cells arranged on a chip or substrate. The metal layer is a typical example of the electrostatic shield. The electrostatic shield is not necessarily made of metal because it is sufficient for the electrostatic shield to have conductivity. However, since the electrostatic shield is desirably thin so as not to have undesired effects on the transmission characteristics of the ultrasonic waves when the electrostatic shield is actually used, it is preferred that the electrostatic shield be made of metal. While the embodiments of the invention will be described below, it will be recognized and understood that the present invention is not limited to the embodiments and that various modifications and changes may be made in the invention within the spirit and scope of the invention.

The embodiments of the present invention will herein be described with reference to the attached drawings.

FIGS. 1A and 1Bare schematic cross-sectional views of a capacitive transducer according to a first embodiment of the present invention. Referring toFIG. 1A, reference numeral100denotes a chip or substrate, reference numeral101denotes a vibrating membrane, reference numeral102denotes a first electrode, reference numeral103denotes a second electrode, reference numeral104denotes supporters, reference numeral105denotes a gap or cavity, and reference numeral106denotes an insulating film on the chip100. Reference numeral107denotes a conductive line connected to the first electrode102, reference numeral108denotes a conductive line connected to the second electrode103, reference numeral109denotes an external connection electrode connected to the conductive line107, and reference numeral110denotes an external connection electrode connected to the conductive line108. Reference numeral120denotes a flexible wire, reference numeral121denotes an external connection electrode, reference numeral122denotes a conductive layer, reference numeral123denotes a first insulating layer, reference numeral124denotes a second insulating layer, reference numeral131denotes a wire, reference numeral140denotes a supporting member, reference numeral401denotes an acoustic lens, reference numeral402denotes a silicone rubber layer, and reference numeral500denotes an electrostatic shield. The acoustic lens401is bonded to the electrostatic shield500via the silicone rubber layer402. In the first embodiment, the electrostatic shield is at least arranged at a position opposed to the cells. The electrostatic shield is composed of a single electrostatic shield layer that has no opening and that is uniformly extended.

The chip100and the flexible wire120are arranged on the supporting member140. In the first embodiment, a capacitive ultrasonic transducer (for example, a CMUT) is arranged on the chip100and is connected to a direct-current voltage generating unit202and a transmission-reception circuit201(refer toFIG. 1B) on the outside via the flexible wire120. The vibrating membrane101is supported by the supporters104on the chip100and vibrates in response to the ultrasonic waves. The first electrode102is arranged on the vibrating membrane101and the second electrode103is arranged at a position on the chip100, which is opposed to the first electrode102. A set of the vibrating membrane101, and the first electrode102and the second electrode103, which are opposed to each other with the cavity105sandwiched therebetween, composes a cell.

As illustrated inFIG. 1B, the first electrode102is extended to the outside of the chip100via a first conductive line301and is connected to the transmission-reception circuit201. The second electrode103is extended to the outside of the chip100via a second conductive line302and is connected to the direct-current voltage generating unit202. Potential difference from several ten volts to several hundred volts is generated between the first electrode102and the second electrode103by the direct-current voltage generating unit202. The vibration of the vibrating membrane101and the first electrode102varies the distance between the first electrode102and the second electrode103to vary the electrostatic capacitance between the electrodes. Since the potential difference exists between the electrodes, weak current occurs in response to the variation in capacitance. The weak current is converted into voltage in the transmission-reception circuit201connected to the first electrode102and the voltage is output from the transmission-reception circuit201. The transmission is performed by vibrating the vibrating membrane101with alternating-current electrostatic attractive force caused between the first electrode102and the second electrode103in response to application of alternating-current drive voltage to the first electrode102.

Multiple cells are arranged on the chip100. In the first embodiment, the second electrodes103in the respective cells on the chip100are electrically connected to each other and have the same potential on the chip100. In contrast, the first electrodes102on the chip100are electrically connected to each other in multiple groups and are electrically connected to the different transmission-reception circuits201for every group. Each group is referred to as an element in transmission and reception (for example, refer to an element20inFIG. 2). Typically, the transducer includes multiple elements each including at least one cell. The size (diameter) of each cell is several hundred micrometers to several millimeters and the number of the elements (the elements20) is from one hundred to several thousands. The CMUT on the chip is capable of easily being manufactured using the MEMS technology. The chip100may be made of, for example, silicon or glass. In the first embodiment, each first electrode102is connected to the transmission-reception circuit201and it is necessary to electrically separate the first electrodes102for every element (element20) in which the first electrodes102are connected to each other. However, since only changing the pattern of the uppermost electrode layer allows the first electrode and its conductive line to be formed, it is possible to manufacture the CMUT using a more simple method.

FIG. 2is a schematic view for describing the shapes of the electrodes in the capacitive transducer of the first embodiment. The first electrode102on the vibrating membrane101, the second electrode103on the surface of the chip100, an outer shape of each cell10, and an outer shape of the element20are illustrated inFIG. 2, which is a top view viewed from a sample800side (refer toFIG. 11andFIG. 12). The second electrode103on the chip100is connected to the direct-current voltage generating unit202and is arranged over the surface of the chip100.

Since noise in reception is increased if the first electrode102connected to the transmission-reception circuit201has a large parasitic capacitance, the area of the first electrode102is desirably small. In contrast, since the cell (the vibrating membrane101) is most deformed in a central portion in the vibration of the vibrating membrane101, only arranging each first electrode102only in the central portion of the cell allows reduction in the transmission efficiency and the reception sensitivity to be minimized. Accordingly, most of the area of the first electrode102is arranged in the central portion of the cell in the outer shape of the cell10and the first electrode102has a minimum width necessary for the connection between the first electrodes102in peripheral portions of the cell.

The flexible wire120arranged on the supporting member140with the chip100has a structure in which the thin conductive layer122is sandwiched between the two insulating layers123and124. The conductive layer122is exposed from an end portion at the chip100side as the external connection electrode121. As illustrated inFIG. 1A, the external connection electrodes109and110on the chip100are electrically connected to the external connection electrode121on the flexible wire120via the wire131. The insulating layers of the flexible wire120are made of polyimide and the conductive layer of the flexible wire120is made of metal, such as copper or gold. The thickness of the entire flexible wire120is from several ten micrometers to one hundred micrometers.

The acoustic lens401is arranged on the chip100and the flexible wire120, which are arranged on the supporting member140, via the silicone rubber layer402in the first embodiment. One electrostatic shield500is arranged in the silicone rubber layer402. The silicone rubber layer402is mainly used to bond the acoustic lens401to the chip100. When common adhesive is used, for example, reflection may occur on the interface and/or vibration characteristics of the vibrating membrane may be affected by the hard adhesive because the acoustic impedance of the adhesive is different from the acoustic impedance of portions in contact with the silicone rubber layer. Accordingly, the bonding using the silicone rubber layer is essential or preferred. The silicone rubber layer may incidentally protect the surface of the transducer and ensure the insulation. In the embodiments of the present invention, the silicone rubber layer may be made of rubber containing polydimethylsiloxane (PDMS). Since the silicone rubber has a low Young's modulus (stiffness), the mechanical characteristics of the vibrating membrane are less affected by the silicone rubber.

The electrostatic shield500of the first embodiment is formed of a metal thin-film layer, is set so as to have fixed potential equal to reference voltage Vref of the transmission-reception circuit201, and has characteristics in which the ultrasonic waves are transmitted through the electrostatic shield500without deterioration. The electrostatic shield500may be made of, for example, aluminum, copper, nickel, or gold and is set so as to have a thickness sufficiently smaller than the wavelength of the ultrasonic waves that are used. It is sufficient for the electrostatic shield500to have the material and the thickness that ensure the transmission characteristics of the ultrasonic waves and a sufficiently low electrical resistance. In particular, the electrostatic shield having a thickness of several micrometers or less is desirably used. In contrast, the silicone rubber layer402that has no effect on the transmission and reception characteristics of the vibrating membrane101of the transducer and that enables the bonding between the acoustic lens401and the chip100is desirably used, as described above. In addition, the silicone rubber layer402is desirably made of a material consistent with the acoustic impedances of a living body, which is a sample, and the acoustic lens401. The silicone rubber layer402is desirably capable of minimizing the reflection of the ultrasonic waves from the interface between the acoustic lens401and the silicone rubber layer402.

When the transducer is used in contact with the sample information acquisition apparatus, the transducer may be used in a state in which a sample, such as a living body, is arranged near the surface of the acoustic lens401. Gel (ultrasound gel) is generally filled between the acoustic lens401and the sample so that the transmission characteristics of the ultrasonic waves are not deteriorated because of a bubble or the like. The surface of the sample is charged and may be greatly charged depending on the surface state. The transmission and reception characteristics of the transducer may be greatly affected by the charge of the surface of the sample. The inventor has found that there is a problem in that, when the capacitive transducer (the CMUT or the like) is used in contact with a sample, such as a living body, the transmission and reception characteristics of the transducer are affected by the electric charge existing on the surface of the sample to deteriorate the transmission and reception characteristics. In the configuration in the related art in which the electrostatic shield is not provided, when a sample with the charged surface comes close to the transducer, electrostatic coupling may occur between the sample and the first electrode102, the electric charge may be induced to the first electrode102, and noise may be generated in the reception. Simultaneously, the line of electric force between the first electrode102and the second electrode103may be varied to vary the strength of the electric field between the first electrode102and the second electrode103. As a result, the transmission efficiency of the output sound pressure in the transmission and the reception sensitivity of the sound pressure in the reception are varied. Such effects from the sample are liable to occur when the distance between the sample and the first electrode102is short.

In contrast, in the first embodiment, the electrostatic shield500is provided. Accordingly, even if the sample with the charged surface comes close to the transducer, the electric charge induced from the sample800occurs in the electrostatic shield500and almost no electric charge occurs in the first electrode102on the chip100. Since the first electrode102of the first embodiment is surrounded by the second electrode103having the fixed potential and the electrostatic shield500, the shape of the line of electric force between the first electrode102and the second electrode103is hardly varied due to the sample800outside the second electrode103and the electrostatic shield500. Accordingly, in the first embodiment, the transmission efficiency of the output sound pressure in the transmission and the reception sensitivity of the sound pressure in the reception are hardly varied by the sample. As described above, according to the first embodiment, since the transmission and reception characteristics are less affected by the electric charge of the sample on the surface of the transducer, it is possible to provide the capacitive transducer having excellent transmission and reception characteristics.

In addition, in the first embodiment, the direct-current high voltage is applied to the second electrode103and the second electrode103is covered with the first electrode102the voltage of which is generally fixed to a value near the reference voltage of the transmission-reception circuit201. Accordingly, since the electrode to which the direct-current high voltage is applied is arranged at a position more apart from the sample, it is possible to increase the insulation from the sample to provide the transducer with high safety.

A second embodiment differs from the first embodiment in the configuration of electrodes in a capacitive transducer. The second embodiment is the same as the first embodiment in the other points.FIG. 3is a schematic cross-sectional view for describing a capacitive transducer according to the second embodiment. As illustrated inFIG. 3, the second embodiment is characterized in that the direct-current voltage generating unit202is connected to the first electrode102and the transmission-reception circuit201is connected to the second electrode103.

The line of electric force between the first electrode102and the second electrode103when the electrostatic shield500is not provided will now be considered. As described above in the first embodiment, the first electrode102has a pattern and has a surface area smaller than that of the second electrode103(refer toFIG. 2). Accordingly, the line of electric force between the first electrode102and the second electrode103has a shape extended toward the second electrode103. Accordingly, when the charged sample exists on the first electrode102, the shape of the line of electric force is affected and is liable to be deformed. The variation in the shape of the line of electric force varies the transmission efficiency of the output sound pressure in the transmission and the reception sensitivity of the sound pressure in the reception. Accordingly, the transmission and reception characteristics of the capacitive transducer, such as the CMUT, are varied depending on the surface state of the sample to lead deterioration of the performance as the transducer. The effect from the sample is liable to occur when the distance between the second electrode103connected to the transmission-reception circuit201and the sample is short.

However, since the electrostatic shield500is provided between the sample and the transducer in the second embodiment, the line of electric force between the first electrode102and the second electrode103is less affected by the surface state of the sample and the variation in the transmission and reception characteristics is suppressed. In addition, since the second electrode103connected to the transmission-reception circuit201is relatively apart from the sample, the second electrode103is less affected by the sample.

A third embodiment differs from the above embodiments in the components arranged on the surface of the transducer. The third embodiment is the same as the first and second embodiments in the other points.FIG. 4is a schematic cross-sectional view for describing a capacitive transducer according to the third embodiment.

The capacitive transducer of the third embodiment has a configuration in which the acoustic lens401is not provided. The transducer having no acoustic lens is preferably used as a transmission-reception transducer that performs electronic focusing or a photoacoustic transducer that receives ultrasonic waves (photoacoustic waves) caused by a photoacoustic effect.

In the configuration of the third embodiment, the acoustic lens401generally having a thickness of several hundred micrometers to several millimeters is not provided and the sample800is in contact with the surface of the transducer via the silicone rubber layer402having a thickness of several ten micrometers to one hundred micrometers. Accordingly, the distance between the sample800and the electrodes in the transducer is greatly decreased, compared with the case in which the acoustic lens401is provided, and the transducer is liable to be greatly affected from the surface of the sample800. However, since the electrostatic shield500is provided between the sample800and the electrodes in the transducer also in the third embodiment, the deterioration in the transmission and reception characteristics hardly occurs also in the configuration in which the distance between the sample800and the electrodes in the transducer is very short. Accordingly, it is possible to provide the capacitive transducer the transmission and reception characteristics of which are less affected by the surface state of the sample also in the configuration in which the acoustic lens401is not provided.

Modifications of the third embodiment will now be described with reference toFIG. 5andFIG. 6.FIG. 5illustrates an exemplary capacitive transducer that differs from the above embodiments in the connection state between the chip100and the flexible wire120, which connects the electrodes on the chip100to the direct-current voltage generating unit202and the transmission-reception circuit201outside the transducer. Referring toFIG. 5, the flexible wire120is arranged so as to be opposed to the external connection electrodes109and110on the chip100for electrical connection. Specifically, the flexible wire electrically connected to the external connection electrodes is provided so as to be opposed to the face of the chip on which the cells are provided. The external connection electrodes109and110on the chip100are capable of easily being connected to the external connection electrode121in the flexible wire120using, for example, anisotropic conductive film (ACF). With the connection method illustrated inFIG. 5, the height of the protrusions on the surface of the chip100is decreased, compared with the case illustrated inFIG. 4in which the wire131is used. Accordingly, the thickness of the silicone rubber layer402on the chip100may be decreased. Since attenuation of the ultrasonic waves occurs in the silicone rubber layer402, the transmission and reception characteristics are improved with the decreasing thickness of the silicone rubber layer402. In contrast, the transmission and reception characteristics are more liable to be affected by the surface state of the sample with the decreasing thickness of the silicone rubber layer402. However, since the use of the configuration of the third embodiment including the electrostatic shield500causes the transducer to be less affected by the sample, the excellent transmission and reception characteristics are kept.

As described above, in the modification illustrated inFIG. 5, it is possible to realize the capacitive transducer that has the excellent transmission and reception characteristics and that are less affected by the surface state of the sample also in the configuration in which the acoustic lens401is not provided.

FIG. 6illustrates another exemplary capacitive transducer that differs from the above embodiments in the connection state between the chip100and the flexible wire120, which connects the electrodes on the chip100to the direct-current voltage generating unit202and the transmission-reception circuit201outside the transducer. The configuration inFIG. 6differs from the configuration inFIG. 5in that the external connection electrodes109and110on the chip100, which are electrically connected to the flexible wire120, are arranged on a face (rear face) opposite to the face of the chip100on which the cells are formed. Specifically, the flexible wire electrically connected to the external connection electrodes is provided so as to be opposed to the face opposite to the face of the chip on which the cells are provided. In the configuration inFIG. 6, the chip100includes a thorough line111for the electrical connection to the flexible wire120on the rear face of the chip100. With the connection method illustrated inFIG. 6, the protrusions are not provided on the surface of the chip100. Accordingly, it is possible to decrease the thickness of the silicone rubber layer402on the chip100to a thickness that achieves the most excellent transmission and reception characteristics. In the modification illustrated inFIG. 6, it is possible to provide the capacitive transducer that has the especially excellent transmission and reception characteristics and that are less affected by the surface state of the sample also in the configuration in which the acoustic lens401is not provided.

A fourth embodiment differs from the above embodiments in an area where the electrostatic shield is arranged. The fourth embodiment is the same as any of the first to third embodiments in the other points.FIG. 7is a schematic cross-sectional view for describing a capacitive transducer according to the fourth embodiment.

The fourth embodiment is characterized in that an electrostatic shield501is arranged in an area opposed to the area on the chip100where the first electrodes102and the second electrodes103are arranged. In the fourth embodiment, the arrangement of the electrostatic shield501only in the area opposed to the area where the cells are arranged allows the configuration to be simplified, compared with the case in which the electrostatic shield is entirely arranged. In addition, the transducer is manufactured by arranging the cells on the chip100, arranging the electrostatic shield501, and electrically connecting the chip to the flexible wire120. Accordingly, since the restriction on the manufacturing method is small, it is possible to manufacture the transducer using the easier manufacturing method.

A fifth embodiment differs from the above embodiments in the shape of an electrostatic shield502. The fifth embodiment is the same as any of the first to fourth embodiments in the other points.FIGS. 8A to 8Eare diagrams for describing a capacitive transducer according to the fifth embodiment.FIG. 8Ais a schematic cross-sectional view of the capacitive transducer.FIG. 8Bis a schematic view when the electrostatic shield is viewed from above.

The electrostatic shield502of the fifth embodiment is characterized in that multiple openings503are two-dimensionally arranged periodically, as illustrated inFIG. 8B. In other words, the electrostatic shield has the multiple openings when the electrostatic shield is viewed from above the cells. Since the size of the openings503is sufficiently smaller than the surface area of the sample800, the shielding effect is hardly reduced even when the electrostatic shield502has the multiple openings503. The size and the arrangement cycle of the openings503may be set to arbitrary values as long as the transmission and reception characteristics are not affected by the sample via the electrodes on the chip100. Although the multiple openings503are opposed to the multiple cells in an irregular pattern in the configuration inFIG. 8A, the arrangement is not limited to this.

Small parasitic capacitance occurs between the electrostatic shield502and the first electrode102(or the second electrode103) on the chip100. The parasitic capacitance is increased in size with the decreasing distance between the electrostatic shield502and the first electrode102(or the second electrode103) on the chip100. The parasitic capacitance at the electrodes connected to the transmission-reception circuit201causes a reduction in the reception sensitivity and an increase in the output noise in the reception. Against such a situation, in the electrostatic shield502including the openings503in the fifth embodiment, the surface area of the electrostatic shield502is capable of being decreased in response to an increase in the total area of the openings503. Accordingly, the magnitude of the parasitic capacitance occurring between the electrostatic shield502and the first electrode102(or the second electrode103) on the chip100is capable of being suppressed while the effect of the electrostatic shield is being kept. In particular, when the effects on the reception characteristics, such as a reduction in the reception sensitivity and an increase in the output noise, are undesirably caused, the use of the electrostatic shield of the fifth embodiment reduces the effects on the reception characteristics.

As described above, according to the fifth embodiment, it is possible to realize the capacitive transducer in which the undesirable effects on the reception characteristics, such as a reduction in the reception sensitivity and an increase in the output noise, are reduced and which is less affected by the surface state of the sample.

A modification of the fifth embodiment will now be described with reference toFIG. 8C.FIG. 8Cis a schematic view when the electrostatic shield is viewed from the top face of the chip100(the face on which the cells are arranged). Only the relationship between the first electrode102on the vibrating membrane101, the second electrode103on the chip100, and the outer shapes of the cells10is illustrated inFIG. 8C. The configuration inFIG. 8Cis characterized in that each of the openings503of the electrostatic shield502is arranged at a position corresponding to a central portion of the cell. In other words, the multiple openings of the electrostatic shield are regularly arranged at the positions corresponding to the cells. In the configuration inFIG. 8C, the first electrode102is connected to the transmission-reception circuit201, as in the configuration in the first embodiment. Since the area of the first electrode102is largest around the central portion of the cell, the arrangement of each of the openings503of the electrostatic shield502at the position opposed to the central portion of the cell allows the magnitude of the parasitic capacitance between the first electrode102and the electrostatic shield502to be further decreased.

With the configuration illustrated inFIG. 8C, it is possible to realize the capacitive transducer in which the undesirable effects on the reception characteristics, such as a reduction in the reception sensitivity and an increase in the output noise, are further reduced and which is less affected by the surface state of the sample.

Another modification of the fifth embodiment will now be described with reference toFIG. 8D.FIG. 8Dis a schematic view when the electrostatic shield is viewed from the top face of the chip100(the face on which the cells are arranged). Only the relationship between the first electrode102, the second electrode103, and the outer shapes of the cells10is illustrated also inFIG. 8D. The configuration inFIG. 8Dis characterized in that each of the openings503of the electrostatic shield502is shifted from the position corresponding to the central portion of the cell10as much as possible. In other words, the multiple openings of the electrostatic shield are arranged at the positions corresponding to areas shifted from the cells. In the configuration inFIG. 8D, the second electrode103is connected to the transmission-reception circuit201, as in the configuration in the second embodiment (refer toFIG. 3). The second electrode103is covered with the first electrode102having the pattern near the central portion of the cell. In contrast, the second electrode103is not almost covered with the first electrode102in an area apart from the central portion of the cell. Accordingly, the shift of each of the openings503of the electrostatic shield502from the position corresponding to the central portion of the cell as much as possible allows the magnitude of the parasitic capacitance between the second electrode103connected to the transmission-reception circuit201and the electrostatic shield502to be further decreased. Since the effect of the electrostatic shield502is little varied depending on the positions of the openings503, the capacitive transducer is hardly affected by the surface state of the sample.

As illustrated inFIG. 8E, the second electrode103may be configured so as to have a pattern in which no electrode is provided in areas opposed to the openings503of the electrostatic shield502. This configuration allows an occurrence of the parasitic capacitance to be further suppressed. Accordingly, it is possible to provide the capacitive transducer having more excellent reception characteristics. With the configurations illustrated inFIG. 8DandFIG. 8E, it is possible to realize the capacitive transducers in which the undesirable effects on the reception characteristics, such as a reduction in the reception sensitivity and an increase in the output noise, are further reduced and which are less affected by the surface state of the sample.

A sixth embodiment differs from the above embodiments in the shape of the electrostatic shield500. The sixth embodiment is the same as any of the first to fifth embodiments in the other points.FIGS. 9A to 9Care diagrams for describing a capacitive transducer according to the sixth embodiment. The sixth embodiment is characterized in that the electrostatic shield500is composed of multiple electrostatic shield layers. The multiple layers are set so as to have to the same fixed potential.

In the sixth embodiment, two electrostatic shield layers504and505are used, as illustrated inFIG. 9A, which is a schematic cross-sectional view. The electrostatic shield layers504and505are arranged at different heights.FIG. 9Bis a top view for describing the first electrostatic shield layer504and the second electrostatic shield layer505. In the first electrostatic shield layer504, the electrodes are arranged so as to draw vertical stripes on the plane of paper. In contrast, in the second electrostatic shield layer505, the electrodes are arranged so as to draw vertical stripes slightly shifted from the vertical stripes of the first electrostatic shield layer504on the plane of paper. No gap exists when the first electrostatic shield layer504is deposited on the second electrostatic shield layer505and the first electrostatic shield layer504and the second electrostatic shield layer505are viewed from above.

Since the electrostatic shield is divided into the multiple layers in the sixth embodiment, the effective distance between the first electrode102(or the second electrode103) on the chip100and the electrostatic shield500is increased, compared with the case in which the electrostatic shield is composed of one layer. Accordingly, the magnitude of the parasitic capacitance occurring between the first electrode102(or the second electrode103) and the electrostatic shield500is further reduced.

With the configuration illustrated inFIG. 9AandFIG. 9B, it is possible to provide the capacitive transducer in which the undesirable effects on the reception characteristics, such as a reduction in the reception sensitivity and an increase in the output noise, are further reduced and which is less affected by the surface state of the sample.

A modification of the sixth embodiment will now be described with reference toFIG. 9C. In the modification inFIG. 9C, the first electrostatic shield layer504has the electrodes arranged so as to draw vertical stripes, as inFIG. 9B. The second electrostatic shield layer505has the electrodes arranged so as to draw horizontal stripes. The configuration inFIG. 9Cdiffers from the configuration inFIG. 9Bin that, when the first electrostatic shield layer504is deposited on the second electrostatic shield layer505, openings are periodically formed. The arrangement of the openings may be varied by appropriately designing the vertical and horizontal strip patterns and the manner in which the shield layers are deposited.

With the configuration illustrated inFIG. 9C, the effective distance between the first electrode102(or the second electrode103) on the chip100and the electrostatic shield500is increased and the magnitude of the parasitic capacitance is greatly reduced due to the presence of the openings. With the configuration illustrated inFIG. 9C, it is possible to provide the capacitive transducer in which the undesirable effects on the reception characteristics, such as a reduction in the reception sensitivity and an increase in the output noise, are further reduced and which is less affected by the surface state of the sample.

Although the electrostatic shield is composed of the two layers in the sixth embodiment described above, the sixth embodiment is not limited to the above configurations and may have a configuration in which three or more shield layers are used. In addition, the shield layers may have no pattern (that is, the electrostatic shield layers having no opening). The shield layers of such configurations have the advantages in that a problem involved in the stress or the like, which may occur in the formation of the thick shield layers, is avoided and in that the multiple thin shield layers having slightly high resistance are arranged to reduce the entire resistance of the shield layers.

A seventh embodiment differs from the above embodiments in that a layer that supports the electrostatic shield is included. The seventh embodiment is the same as any of the first to sixth embodiments in the other points.FIGS. 10A and 10Bare schematic cross-sectional views for describing a capacitive transducer according to the seventh embodiment.

The seventh embodiment is characterized in that an insulating film provided with the electrostatic shield is used. Specifically, the electrostatic shield is arranged on the insulating film. An insulating film403may be formed of a thin insulating film and may be made of a material that can be formed into a thin film, such as polyethylene terephthalate (PET), polyimide (PI), polyethylene (PE), or methylpentene (TPX). The insulating film403is set so as to have a thickness that is sufficiently small for the wavelength of the ultrasonic waves that are used and desirably has a thickness of several micrometers to a dozen or so micrometers.

FIG. 10Ais a schematic view in a configuration in which the seventh embodiment is applied to the configuration of the first embodiment (refer toFIGS. 1A and 1B).FIG. 10Bis a schematic view in a configuration in which the seventh embodiment is applied to the configuration of the third embodiment (refer toFIG. 4). The seventh embodiment is not limited to these configurations and may be applied to the configurations of the other embodiments in the same manner.

Since the electrostatic shield layer is formed on the flat insulating film403for usage in the seventh embodiment, the uniform and excellent film is provided even when the thickness of the electrostatic shield layer is decreased. Accordingly, the resistance of the electrostatic shield is sufficiently suppressed. Since the thickness of the electrostatic shield layer is decreased, the transmission characteristics of the ultrasonic waves through the electrostatic shield are greatly reduced.

When the seventh embodiment is used in the fifth embodiment or the sixth embodiment, the formation of the electrostatic shield layer on the insulating film provides the electrostatic shield having a shape more close to the desired shape because the electrostatic shield layer has a pattern. When the seventh embodiment is used in the fourth embodiment in which the electrostatic shield is limitedly arranged in a certain area, the manufacturing method is further simplified because the insulating film403is easily aligned with the chip100. When the seventh embodiment is used in the sixth embodiment, the openings of the electrostatic shield are easily aligned with the cells on the chip100with high accuracy because the insulating film403is easily and accurately aligned with the chip100. Accordingly, the parasitic capacitance is more effectively reduced. Furthermore, when the seventh embodiment is used in the sixth embodiment, the parasitic capacitance is more effectively reduced because the alignment of the patterns of the multiple electrostatic shield layers is performed with high accuracy.

The capacitive transducer according to any of the first to seventh embodiments is capable of being used for the reception of the photoacoustic waves (the ultrasonic waves) using the photoacoustic effect and is applicable to a sample information acquisition apparatus using the capacitive transducer.

An exemplary operation of a sample information acquisition apparatus of an eighth embodiment will now be specifically described with reference toFIG. 11. First, a light source901is caused to generate light702(pulsed light) on the basis of a light emitting instruction signal701to irradiate the sample (object to be measured)800with the light702. Photoacoustic waves (ultrasonic waves)703are generated in the object800to be measured in response to the irradiation with the light702and the ultrasonic waves703are received by multiple capacitive transducers802in an ultrasound probe. Information about the size, the shape, and the time of the reception signals are supplied to an image information generating unit803, which is a processing unit, as reception signals704of the photoacoustic waves. Information (light emission information) about the size, the shape, and the time of the light702generated in the light source901is stored in the image information generating unit803for the photoacoustic signals. In the image information generating unit803for the photoacoustic signals, an image signal of the object800to be measured is generated on the basis of the reception signals704of the photoacoustic waves and the light emission information and the generated image signal is supplied to an image display unit804as reproduced image information705generated from the photoacoustic signals. In the image display unit804, an image of the object800to be measured is displayed on the basis of the reproduced image information705generated from the photoacoustic signals. As described above, in the eighth embodiment, the capacitive transducers receive the photoacoustic waves generated by the irradiation of the sample with the light generated by the light source and the processing unit (a sample image information generating unit here) acquires information about the sample using photoacoustic reception signals.

Since the reception characteristics of the capacitive transducers according to the eighth embodiment are less affected by the electric charge of the sample, the capacitive transducers are capable of acquiring the accurate information from the photoacoustic waves. Accordingly, the capacitive transducers are capable of generating a high-quality image.

In a ninth embodiment, the capacitive transducer according to any of the first to seventh embodiments is used in a sample information acquisition apparatus in a mode different from that of the eighth embodiment.FIG. 12is a schematic view of a sample information acquisition apparatus according to the ninth embodiment. Referring toFIG. 12, reference numeral706denotes ultrasonic transmission-reception signals, reference numeral707denotes transmitted ultrasonic waves, reference numeral708denotes reflected ultrasonic waves, and reference numeral709denotes reproduced image information generated through transmission and reception of the ultrasonic waves. The same reference numerals are used inFIG. 12to identify the same components illustrated inFIG. 11. The image information generating unit, which is the processing unit, uses a pulse echo method (transmission and reception of the ultrasonic waves), in addition to the reception of the photoacoustic waves, to form an image. Since the reception of the photoacoustic waves is performed in the same manner as that in the eighth embodiment, the pulse echo method (transmission and reception of the ultrasonic waves) will be described here.

The ultrasonic waves707are output (transmitted) from the multiple capacitive transducers802to the object800to be measured on the basis of the ultrasonic transmission signals706. The ultrasonic waves are reflected in the object800to be measured due to the difference in acoustic impedance specific to the substance existing in the object800to be measured. The reflected ultrasonic waves708are received by the multiple capacitive transducers802and information about the size, the shape, and time of the received signals are supplied to the image information generating unit803as the ultrasonic reception signals706. Information about the size, the shape, and the time of the transmitted ultrasonic waves is stored in the image information generating unit803as ultrasonic transmission information. An image signal of the object800to be measured is generated in the image information generating unit803on the basis of the ultrasonic reception signals706and the ultrasonic transmission information and the image signal is output as the reproduced image information709generated through transmission and reception of the ultrasonic waves.

An image of the object800to be measured is displayed in the image display unit804on the basis of the reproduced image information705generated from the photoacoustic signal and the reproduced image information709generated through transmission and reception of the ultrasonic waves. Since the transmission and reception characteristics of the ultrasonic waves in the capacitive transducers in the ninth embodiment are less affected by the electric charge of the sample, reception information from a different measurement method, that is, the transmission and reception of the ultrasonic waves is also capable of being accurately acquired, in addition to the photoacoustic waves, to form an image. Accordingly, it is possible to accurately acquire an image having a greater amount of information and display the image.

In the ninth embodiment, the capacitive transducers at least receive the ultrasonic waves from the sample and the processing unit acquires information about the sample using the ultrasonic reception signals from the capacitive transducers. Although the capacitive transducers also transmit the ultrasonic waves to the sample, the transmission of the ultrasonic waves may be performed by another transducer. Although the capacitive transducers also receive the photoacoustic waves generated in response to the irradiation of the sample with the light generated by the light source and the processing unit acquires information about the sample also using the photoacoustic reception signals, the capacitive transducers may receive only the ultrasonic waves without receiving the photoacoustic waves.

According to the present invention, the provision of the electrostatic shield allows the capacitive transducer having excellent transmission and reception characteristics to be provided.

This application claims the benefit of Japanese Patent Application No. 2014-236048, filed in Nov. 20, 2014, which is hereby incorporated by reference herein in its entirety.