Ultrasound probe and ultrasound diagnostic apparatus

An ultrasound probe including: a circuit substrate (23) having a recess in a first region on the lower surface side; a buffer layer (400) composed of an insulating material on a second region different from the first region of circuit substrate (23); and an element array layer (22) including a first piezoelectric element (100) for ultrasound transmission formed in the first region of the circuit substrate (23) without the buffer layer (400), and a second piezoelectric element (200) for ultrasound reception formed in the second region of the circuit substrate (23) on the buffer layer (400). The first piezoelectric element (100) vibrates in a flexural vibration mode on the circuit substrate (23), and the second piezoelectric element (200) vibrates in a thickness vibration mode on the circuit substrate (23).

CROSS REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2019-007259 filed on Jan. 18, 2019, is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure relates to an ultrasound probe and an ultrasound diagnostic apparatus.

Description of the Related Art

An ultrasound probe with an ultrasound emission surface on which multiple ultrasound transducers are arranged has been known. In recent years, an ultrasound probe using a piezoelectric element (also referred to as a piezoelectric micromachined ultrasound transducer (pMUT)) achieved by micro electro mechanical systems (MEMS) technology has been developed as an ultrasonic transducer (see PTL 1, for example).

An ultrasound probe using a pMUT can transmit and receive ultrasound by vibrating a diaphragm having a piezoelectric body like a drum (flexural vibration), for example. The pMUT is advantageous in that it can be made finer than a piezoelectric element obtained by dividing bulk lead zirconate titanate (PZT) by dicing or the like and can therefore be made higher in frequency and higher in resolution, and it is suitable for forming a two-dimensional array of piezoelectric elements for generating three-dimensional images, and it can be made compact and thin. However, when the same flexural vibration is used for transmission and reception as in the conventional pMUT, a problem of narrow band arises and the available frequency is limited to the vicinity of the resonance frequency.

Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2015-517752 (hereinafter, referred to as “PTL”) 1 discloses a pMUT array structure in which a wide band is achieved by arranging multiple pMUTs having different resonance frequencies (seeFIG. 7Bof PTL 1).

However, in the ultrasound probe having the pMUT array structure disclosed in PTL 1, a deep valley (that is, a frequency band that becomes a dead band) is generated between resonance peaks, and the image quality of the ultrasound image may therefore be deteriorated. Moreover, since multiple resonance frequencies are mixed, the transmission/reception sensitivity of the ultrasound probe as a whole may be lowered.

SUMMARY

An object of the present disclosure, which has been made in view of the above problems, is to provide an ultrasound probe and an ultrasound diagnostic apparatus that can achieve high transmission/reception sensitivity over a wide frequency band.

To achieve at least one of the abovementioned objects, according to an aspect of the present invention, an ultrasound probe reflecting one aspect of the present invention comprises:

a circuit substrate;

a buffer layer formed in a second region different from a first region of an upper surface of the circuit substrate and composed of an insulating material; and

an element array layer including a first piezoelectric element for ultrasound transmission formed in the first region of the circuit substrate without the buffer layer, and a second piezoelectric element for ultrasound reception formed in the second region of the circuit substrate on the buffer layer, wherein

the circuit substrate has a recess in a region of a lower surface, the region corresponding to the first region,

the first piezoelectric element vibrates in a flexural vibration mode on the circuit substrate, and

the second piezoelectric element vibrates in a thickness vibration mode on the circuit substrate.

To achieve at least one of the abovementioned objects, according to another aspect of the present invention, an ultrasound probe reflecting one aspect of the present invention comprises:

a circuit substrate;

a buffer layer that is formed of an insulating material, and has a first thickness in a first region of an upper surface of the circuit substrate and has a second thickness in a second region different from the first region, the second thickness being greater than the first thickness; and

an element array layer including a first piezoelectric element for ultrasound transmission formed in the first region of the circuit substrate on the buffer layer, and a second piezoelectric element for ultrasound reception formed in the second region of the circuit substrate on the buffer layer, wherein

the circuit substrate has a recess in a region of a lower surface corresponding to the first region,

the first piezoelectric element vibrates in a flexural vibration mode on the circuit substrate, and

the second piezoelectric element vibrates in a thickness vibration mode on the circuit substrate.

To achieve at least one of the abovementioned objects, according to another aspect of the present invention, an ultrasound probe reflecting one aspect of the present invention comprises: the ultrasound probe described above.

DETAILED DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same function are denoted by the same reference numerals, and redundant description is omitted.

Configuration of Ultrasound Diagnostic Apparatus

An example configuration of an ultrasound diagnostic apparatus according to this embodiment will now be described with reference toFIGS. 1 to 2.

FIG. 1is a diagram showing the appearance of ultrasound diagnostic apparatus1according to this embodiment.FIG. 2is a block diagram showing the main part of the control system of ultrasound diagnostic apparatus1according to this embodiment.

Ultrasound diagnostic apparatus1is used for visualizing the shape, properties, or dynamics in a subject in an ultrasound image and performing image diagnosis. Note that ultrasound diagnostic apparatus1may generate arbitrary ultrasound images such as B-mode images, color Doppler images, three-dimensional ultrasound images, or M-mode images. Similarly, an arbitrary probe such as a convex probe, a linear probe, a sector probe, or a three-dimensional probe may be used as ultrasound probe20.

Ultrasound probe20transmits ultrasound to the subject, receives an ultrasonic echo reflected by the subject, converts it to a reception signal, and transmits it to ultrasound diagnostic apparatus body10. The details of ultrasound probe20will be described later.

Ultrasound diagnostic apparatus body10visualizes the internal state of the subject in an ultrasound image, using the reception signal from ultrasound probe20. To be specific, ultrasound diagnostic apparatus body10includes operation input section11, transmission section12, reception section13, image processing section14, display section15, and control section16.

Operation input section11receives, for example, a command for instructing to start diagnosis or the like or an input of information on the subject. Operation input section11includes, for example, an operation panel having multiple input switches, a keyboard, and a mouse.

Transmission section12generates a transmission signal according to an instruction from control section16and transmits it to ultrasound probe20.

Reception section13acquires the reception signal generated by ultrasound probe20, performs reception processing (for example, phasing addition processing and filter processing) on the reception signal, and then outputs it to image processing section14.

Image processing section14performs predetermined signal processing (for example, logarithmic compression processing, detection processing, or FFT analysis processing) on the reception signal acquired from reception section13in accordance with an instruction from control section16, and generates ultrasound images showing the internal state (e.g., B-mode images, color Doppler images, or three-dimensional ultrasound images). Note that the content of the processing for generating ultrasound images is well known, and the description thereof will therefore be omitted here.

Display section15, for example, a liquid crystal display, displays an ultrasound image generated in image processing section14.

Control section16entirely controls ultrasound diagnostic apparatus1by controlling operation input section11, transmission section12, reception section13, image processing section14, and display section15according to their functions.

Schematic Configuration of Ultrasound Probe

FIG. 3is a diagram showing the configuration of ultrasound probe20according to this embodiment.

Ultrasound probe20includes protective layer21, element array layer22, circuit substrate23, and backing member24in this order from the Ultrasound emission surface side.

Protective layer21protects the surface of element array layer22(that is, the Ultrasound emission surface). Protective layer21is composed of a material (for example, silicone rubber) that does not give discomfort when being brought into contact with a human body and has an acoustic impedance relatively close to that of the human body. Note that protective layer21may include an acoustic lens or matching layer.

Element array layer22is composed of multiple piezoelectric elements that are two-dimensionally arrayed in the ultrasound emission surface of ultrasound probe20, transmits ultrasound toward the inside of the subject, and receives ultrasonic echoes reflected by the inside of the subject.

Circuit substrate23is a base on which element array layer22is formed. Circuit substrate23includes switching devices that drive and control the respective piezoelectric elements of element array layer22, and a transmission circuit that generates a transmission signal for ultrasound transmission through the switching devices, and a reception circuit that detects the reception signal (ultrasound signal). Circuit substrate23is connected to ultrasound diagnostic apparatus body10(transmission section12and reception section13) via cable30.

Note that circuit substrate23may include a transmission/reception switching circuit and a beam former (phased addition circuit). In addition, signals between circuit substrate23and ultrasound diagnostic apparatus body10(transmission section12and reception section13) may be transmitted and received by wireless communication instead of cable30.

Backing member24attenuates the unnecessary vibration generated in element array layer22.

FIGS. 4A and 4Bare plan views showing the configuration of element array layer22according to this embodiment, from the upper surface side of element array layer22, that is, the ultrasound emission surface side.FIG. 4Bis an enlarged view of region A enclosed by the thick line inFIG. 4A.

Element array layer22includes first piezoelectric element100and second piezoelectric element200that are two-dimensionally arrayed in the ultrasound emission surface of ultrasound probe20. First piezoelectric element100is a piezoelectric element for ultrasound transmission (hereinafter referred to as “transmission element100”), and second piezoelectric element200is a piezoelectric element for ultrasound reception (hereinafter referred to as “reception element200”).

The ultrasound emission surface of ultrasound probe20is divided into multiple channels CH along the scanning direction, and multiple transmission elements100and reception elements200are provided in each channel CH. Transmission elements100and reception elements200are driven and controlled for each channel CH.

Transmission element100and reception element200each have, for example, a circular shape or a rectangular shape in a plan view (seeFIG. 4B). For example, transmission element100and reception element200are alternately arranged in a checkered pattern in the ultrasonic emission surface in a plan view. Here, a “plan view” refers to a state in which ultrasound probe20is viewed from the ultrasonic emission surface side (the same applies hereinafter).

To be specific, transmission element100and reception element200are adjacent to each other so that their element regions do not overlap in the ultrasonic emission surface. The “element region” is an effective region where ultrasound is transmitted or received and, in a laminated structure including a piezoelectric body and two electrodes disposed on the upper surface side and the lower surface side of the piezoelectric body, is a region where all of the piezoelectric body and the two electrodes overlap each other (seeFIG. 5).

In this embodiment, in order to increase the number of reception elements200, reception elements200are also disposed at the boundaries of adjacent channels. Reception elements200disposed at the boundaries of adjacent channels are alternately allocated to any one channel during operation.

In element array layer22, the element inter-center distance L between transmission element100and reception element200adjacent to each other is preferably L≤λc/2 where the wavelength in the living body (representative sound speed 1530 m/sec) with respect to the center frequency fcof the band characteristic of ultrasound probe 20 is λc. Hence, transmission element100and reception element200can be regarded as the same sound source, and the paths of the transmission ultrasonic beam and the reception ultrasonic beam can be matched.

An aspect of the arrangement of transmission elements100and reception elements200is not limited to that shown inFIGS. 4A and 4B: for example, they may be arranged in a triangular lattice or grid or in a random fashion. The arrangement and shape of transmission elements100or reception elements200may be different between on the outer side and the inner side with respect to the short axis direction of ultrasound probe20.

The number of transmission elements100and reception elements200in each channel CH may be the same or different. For example, in the case where the transmission sound pressure intensity can be secured with a small number of cells, it is desirable to reduce the number of transmission elements100and increase the number of reception elements200in each channel CH.

Detailed Configuration of Ultrasound Probe

FIG. 5is a sectional view showing a configuration of ultrasound probe20according to this embodiment. Note that illustration of backing member24is omitted inFIG. 5. The ultrasonic emission surface side of ultrasound probe20will be hereinafter referred to as “upper side”, and the opposite side to the ultrasonic emission surface will be referred to as “lower side”.

Ultrasound probe20has a structure in which insulating layer23A, element array layer22, and protective layer21are laminated on circuit substrate23in this order from the lower side. Buffer layer400is present between reception element200in element array layer22and insulating layer23A.

Circuit substrate23is, for example, a substrate for circuitry on which transmission transistor310, reception transistor320, and wiring layer330are formed on substrate300(for example, a Si substrate).

Transmission transistor310generates a transmission signal and controls the operation of transmission element100. Reception transistor320amplifies the reception signal generated by reception element200. On circuit substrate23, different CMOS circuits are made up of respective transmission transistors310corresponding to the respective transmission elements100, and different CMOS circuits are made up of reception transistors320corresponding to respective reception elements200.

Wiring layer330includes a wiring section that electrically connects transmission transistor310and transmission element100with each other, and a wiring section that electrically connects reception transistor320and reception element200with each other.

Substrate300has thin film section300a, in a plan view, in a portion corresponding to a region of the bottom surface of that substrate300(that is, the opposite side to the side on which transmission element100is formed; the same applies hereinafter) where each transmission element100is formed (that is, the element region). Thin film section300ais formed by etching a portion of the lower surface of substrate300corresponding to the region where transmission element100is formed, into a concave shape. In other words, thin film section300ais formed on the lower surface of substrate300in a position overlapping the region where each transmission element100is formed. Substrate300constitutes a diaphragm structure below the region where transmission element100is formed.

Meanwhile, a portion of the lower surface of substrate300corresponding to the region where each reception element200of substrate300is formed (that is, the element region) is not etched and is thicker than thin film section300a(hereinafter also referred to as “thick film section300b”). In other words, substrate300forms a non-diaphragm structure below the region where reception element200is formed (corresponding to the “second region” of the present invention).

Here, the upper surface of circuit substrate23is the surface on the upper side inFIG. 5and refers to the surface on the ultrasound transmission/reception surface side of ultrasound probe20(the same applies hereinafter). The lower surface of circuit substrate23is the surface on the lower side inFIG. 5and refers to the surface opposite to the ultrasound transmission/reception surface of ultrasound probe20(the same applies hereinafter).

For convenience of explanation, wiring layer330is shown with a thickness equivalent to that of substrate300in the drawing. In reality, wiring layer330is extremely thinner than substrate300. Accordingly, the thickness of circuit substrate23substantially corresponds to the thickness of substrate300.

Insulating layer23A is a protective layer for circuit substrate23formed so as to cover the entire upper surface of circuit substrate23. Insulating layer23A insulates heat to prevent circuit substrate23from being deteriorated by being heated when the piezoelectric film of element array layer22is formed.

In insulating layer23A, through electrode331athat electrically connects the wiring section of wiring layer330connected to transmission transistor310with transmission element100, and through electrode332athat electrically connects the wiring section of wiring layer330connected to reception transistor320with reception element200are formed. On insulating layer23A, wiring section332bthat connects through electrode332aand through electrode400aprovided in buffer layer400is formed.

Insulating layer23A is composed of, for example, SiO2or porous silicon. Insulating layer23A may have a single layer structure or a multilayer structure. The thickness of insulating layer23A is, for example, 3 μm or more.

Transmission element100includes first lower electrode101, first piezoelectric body102, and first upper electrode103, which are laminated on insulating layer23A in this order from the lower side. Transmission element100has a unimorph structure in which first piezoelectric body102is sandwiched between first lower electrode101and first upper electrode103. Note that buffer layer400is not present between transmission element100and insulating layer23A.

First lower electrode101is electrically connected to transmission transistor310via through electrode331aprovided in insulating layer23A and wiring layer330of the circuit substrate. First upper electrode103is connected to GND via common electrode22G routed around the upper surface of element array layer22.

First lower electrode101and first upper electrode103are composed of, for example, a metal material such as Pt, Au, or Ti, or a conductive oxide. Note that first lower electrode101and first upper electrode103may be a laminate of multiple different metal materials or a laminate of a metal material and a conductive oxide.

First piezoelectric body102is typically composed of an inorganic piezoelectric material having excellent ultrasound transmission performance (that is, transmission sensitivity and available frequency band). First piezoelectric body102is preferably composed of a material having a large inverse piezoelectric constant, for example, lead zirconate titanate (PZT).

Transmission element100is formed on thin film section300aof substrate300constituting the diaphragm structure. Consequently, upon voltage application, transmission element100vibrates in the flexural vibration mode on circuit substrate23and emits ultrasound.

Here, a diaphragm structure refers to a structure that induces the bending mode resonance of the diaphragm held at an end in a frequency band (−40 dB bandwidth) in which ultrasound is transmitted. The bending of the diaphragm refers to the displacement of the diaphragm in the vertical direction (thickness direction) caused when the piezoelectric body (here, transmission element100) extends and contracts in the direction in which the plate surface of the diaphragm (here, thin film section300a) extends (that is, the direction orthogonal to the thickness direction of circuit substrate23).

Transmission element100preferably has one or more resonance points in the frequency band used for transmitting ultrasound. Hence, high transmission sensitivity can be obtained in the vicinity of the resonance point. The transmission sensitivity characteristic is the ultrasound intensity (sound pressure intensity) and is proportional to the product of the transducer displacement and the frequency, and thus does not abruptly attenuate even at frequencies higher than the resonance frequency, so that the bandwidth can be made wider even with a resonance point. Since transmission element100vibrates in the flexural vibration mode on circuit substrate23, the resonance point of that transmission element100also depends on the diaphragm structure of substrate300.

It is preferable that transmission element100be designed such that the effective acoustic impedance matches the acoustic impedance of the living body. Hence, ultrasound can be efficiently propagated in the living body. To be specific, the rigidity of the diaphragm structure of substrate300is preferably optimized. Depending on the required resonance frequency, transmission performance (transmission sensitivity or frequency band), and the like, the material of substrate300, the thickness of thin film section300a, the thickness of first piezoelectric body102, the element region of transmission element100, and the like are optimized as appropriate.

Reception element200is formed on insulating layer23A with buffer layer400is located therebetween. Reception element200includes second lower electrode201, second piezoelectric body202, and second upper electrode203, which are layered in this order on buffer layer400from the lower side. Reception element200has a unimorph structure in which second piezoelectric body202is sandwiched between second lower electrode201and second upper electrode203.

Second lower electrode201is electrically connected to reception transistor320through electrode400aprovided in buffer layer400, wiring section332bprovided on insulating layer23A, through electrode332aprovided in insulating layer23A, and wiring layer330of circuit substrate23.

Second upper electrode203is connected to GND via common electrode22G routed on the upper surface of element array layer22. In this embodiment, second upper electrode203is configured as a part of common electrode22G.

Second lower electrode201and second upper electrode203are composed of, for example, a metal material such as Pt, Au, or Ti, or a conductive oxide. Note that second lower electrode201and second upper electrode203may be a laminate of multiple different metal materials or a laminate of a metal material and a conductive oxide.

Second piezoelectric body202generates voltage by receiving ultrasound. Second piezoelectric body202is preferably composed of an organic piezoelectric material having excellent ultrasound reception performance (that is, reception sensitivity and available frequency band), for example, polyvinylidene fluoride (PVDF) resin (a copolymer based on PVDF resin). PVDF is inferior in ultrasound transmission performance to PZT, but has low dielectric constant and thus very high voltage reception performance. For example, a piezoelectric element with PVDF has a voltage reception sensitivity about10times that of a piezoelectric element with PZT.

Reception element200is formed on thick film section300bof substrate300of the non-diaphragm structure. Consequently, upon reception of ultrasound, reception element200deforms in the thickness direction and generates voltage. In other words, reception element200vibrates in the thickness vibration mode on circuit substrate23.

It is preferable that reception element200does not have a resonance point in the frequency band. Hence, a wide reception bandwidth can be obtained. This is because the reception sensitivity is equivalent to a voltage signal, is proportional to the displacement of the transducer, and sharply attenuates on the high frequency side from the resonance frequency and becomes a narrow band when a resonance point is in the frequency band. From this point of view, in order to drive reception element200in a frequency band lower than or equal to the resonance point, the total thickness of reception element200(second piezoelectric body202) and buffer layer400is ¼ or less of the wavelength of the ultrasound (the details will be described later).

Buffer layer400is formed on insulating layer23A and directly below the region where reception element200is formed so as to be present between insulating layer23A and reception element200. Buffer layer400reduces the degradation of the reception signal due to the parasitic capacitance of reception transistor32(hereinafter also referred to as “transistor parasitic capacitance”), and can function as a backing member that suppresses the reverberation of the ultrasound that travels from element array layer22toward circuit substrate23. The effect of buffer layer400in reducing the transistor parasitic capacitance will be described later with reference toFIGS. 6A and 6B.

However, buffer layer400may hinder the flexural vibration of transmission element100and degrade the transmission characteristics of transmission element100. For this reason, in ultrasound probe20according to this embodiment, buffer layer400is not provided in a region directly below transmission element100.

Buffer layer400is preferably an insulating material having a relative dielectric constant of 4 or less in order to reduce transistor parasitic capacitance, and is composed of, for example, silicon oxide, parylene, polyimide, polyethylene, or silicone rubber. In this embodiment, polyimide is used as buffer layer400.

However, in order for the function as the backing member to be effectively exhibited, it is preferable to use an organic insulating material such as parylene, polyimide, polyethylene, or silicone rubber for buffer layer400. In particular, buffer layer400is preferably composed of a material that satisfies the condition expressed by the following formula (1) or the following formula (2). When an anisotropic material is used for buffer layer400, the elastic modulus in the following formula (2) is a larger one of the tensile and compressive elastic modulus of the transmission piezoelectric material in the elongation direction (the in-plane direction vertical to the normal slope).
g2/g1<10   Formula (1)
where g1is the elastic modulus of second piezoelectric body202, and g2is the elastic modulus of buffer layer400.
Z2/Z1<1   Formula (2)
where Z1is the acoustic impedance of second piezoelectric body202, and Z2is the acoustic impedance of buffer layer400.

Buffer layer400is formed using, for example, a printing method.FIG. 5shows a mode in which buffer layer400is formed by a printing method. Buffer layer400surrounds, for example, the periphery of transmission element100, and is formed such that the position of the upper end of that buffer layer400is higher than the position of the upper end of transmission element100.

With the position of the upper end of buffer layer400made higher than the position of the upper end of transmission element100in this manner, when buffer layer400is formed on insulating layer23A, it can be formed in the entire region of insulating layer23A so as to cover transmission element100(which will be described later with reference toFIG. 7C). This makes the surface of buffer layer400smooth, and enables formation of a uniform film when second piezoelectric body202of reception element200is formed thereafter by application.

If a material having a low elastic modulus is used as buffer layer400, the phenomenon in which buffer layer400hinders operation of transmission element100can be eased even when buffer layer400is in contact with the side of transmission element100. Buffer layer400is preferably composed of a material having an elastic modulus of 1/10  or less of the elastic modulus of the piezoelectric material of transmission element100, and an organic material that does not contain a filler can be used as a material that satisfies this condition.

Next, the effect of buffer layer400in reducing the transistor parasitic capacitance will be described with reference toFIGS. 6A and 6B.

FIG. 6Ais a diagram showing parasitic capacitance existing between reception element200and substrate300, andFIG. 6Bis a diagram showing a circuit equivalent to a reception circuit that detects a reception signal generated by reception element200.

InFIGS. 6A and 6B, Vsigis the signal source (that is, reception element200), Cpdis the capacitance of reception element200, Cbaris the capacitance of buffer layer400, Cparais the parasitic capacitance of reception transistor320, Cgis the input capacitance (that is, the gate capacitance) of reception transistor320, and Vgis the gate input voltage to reception transistor320.

In general, in order to improve the reception performance of ultrasound probe20, it is important to maximize the input signal (that is, the gate input voltage) to reception transistor320when reception element200generates a reception signal.

In addition, from the viewpoint of impedance matching of the signal path from reception element200to reception transistor320, reception transistor320and reception element200are preferably set so that input capacitance Cgof reception transistor320and electrostatic capacitance Cpdof reception element200match.

From this point of view, when reception transistor320side is viewed from signal source Vsigside, the parasitic capacitance Cparaof that reception transistor320is required to be equivalently close to zero.

Note that parasitic capacitance Cparaof reception transistor320is, for example, gate-source capacitance, gate-drain capacitance, and the like that appear in parallel with input capacitance Cgof reception transistor320formed between the gate (here, the input electrode) and substrate300. Parasitic capacitance Cparaof reception transistor320is a factor that causes signal degradation of the reception signal generated by reception element200.

In this regard, as shown inFIG. 6B, the capacitance provided by buffer layer400is equivalent to electrostatic capacitance Cbuffconnected in series with parasitic capacitance Cparaof reception transistor320when reception transistor320side is viewed from signal source Vsig. In other words, combined capacitance Callcan be expressed by Formula (3) below when reception transistor320side is viewed from signal source Vsig.
Call=Cg+(Cpara×Cbuff)/(Cpara+Cbuff)   Formula (3)

Here, reducing electrostatic capacitance Cbuffof buffer layer400can bring the series-combined capacitance of electrostatic capacitance Cbuffof buffer layer400and parasitic capacitance Cparaof reception transistor320(the second term of Formula (3)) close to zero. At this time, combined capacity Callobtained when reception transistor320side is viewed from signal source Vsigis equivalent to only input capacitance Cgof reception transistor320formed between the gate of reception transistor320and substrate300.

As described above, buffer layer400provided between reception element200and insulating layer23A can increase the sensitivity of reception transistor320. In other words, this makes it possible to increase the gate input signal in reception transistor320.

How the thickness of buffer layer400is to be set will now be described.

Since buffer layer400is composed of a material having a low elastic modulus (for example, polyimide) like second piezoelectric body (for example, PVDF)202, that buffer layer400vibrates together and integrally with reception element200. Therefore, the frequency at which reception element200resonates is determined by the total thickness of second piezoelectric body202and buffer layer400. At this time, insulating layer23A can be regarded as a fixed end during vibration along the thickness of reception element200and buffer layer400.

Accordingly, in order to drive reception element200in the frequency band lower than or equal to the resonance point, the sum of the thickness of second piezoelectric body202and the thickness of buffer layer400needs to be ¼ or less of the wavelength of the ultrasound used for transmission and reception, as shown in Formula (4) below.
t≤v/4 fmaxFormula (4)

Here, in Formula (4), t is the sum of the thickness of reception element200and the thickness of buffer layer400; v is the average of the sound speed in reception element200(that is, second piezoelectric body202) and the sound speed in buffer layer400; and fmaxis the maximum frequency in the frequency band used for transmission/reception of ultrasound, and represents, for example, the frequency twice the center frequency (transmission frequency) of the frequency band characteristic of ultrasound probe20.

Hence, in reception element200, resonance in the thickness direction can be prevented and a wide bandwidth can be ensured.

Here, it is preferable that the thickness of buffer layer400be increased in order to reduce the capacitance of that buffer layer400. On the other hand, increasing the thickness of buffer layer400decreases the maximum frequency that can be used for transmission and reception of ultrasound. On the other hand, increasing the thickness of buffer layer400under the condition expressed by Formula (4) reduces the thickness of second piezoelectric body202of reception element200, and reduces the reception sensitivity of reception element200. For this reason, the thickness of buffer layer400is preferably set as appropriate in consideration of, in addition to the capacitance of that buffer layer400, the maximum frequency used for transmission/reception of ultrasound and the reception sensitivity of reception element200.

Table 1 below shows an example of gate input voltage Vg, the amplification sensitivity by buffer layer400, and maximum frequency fmaxobtained when the material and thickness of buffer layer400are changed. Maximum frequency fmaxis calculated by fmax=v/4t as in Formula (4) above. Here, PVDF is used as second piezoelectric body202of reception element200.

In general, a frequency band of about 10 MHz is required as the maximum frequency of ultrasound used in medical ultrasound diagnostic apparatus1. In this regard, Table 1 shows that, regardless of whether an inorganic material or an organic material is used as buffer layer400, the maximum frequency of ultrasound of about 10 MHz can be ensured when the thickness of buffer layer400is 30 μm or less.

Meanwhile, Table 1 shows that, regardless of whether an inorganic material or an organic material is used as buffer layer400, the amplification effect of the gate input voltage can be obtained when the thickness of buffer layer400is 2 μm or more.

From such a viewpoint, it is preferable that the thickness of buffer layer400be 2 μm to 30 μm. However, the thickness of that buffer layer400may be changed as appropriate from the above range according to the material of second piezoelectric body202of reception element200, the maximum frequency of the required ultrasound, and the like. When the thickness of buffer layer400is less than 2 μm, the effect of reducing the parasitic capacitance disappears, and when the thickness of buffer layer400is greater than 30 μm, the band is narrowed because the resonance frequency is lowered.

Process of Manufacturing Ultrasound Probe

Next, a process of manufacturing ultrasound probe20according to this embodiment will be described with reference toFIGS. 7A to 7F.

FIGS. 7A to 7Fare diagrams showing the process of manufacturing ultrasound probe20according to this embodiment in time series.

FIG. 7Ashows the step of preparing circuit substrate23. In this step, a reception circuit, such as reception transistor320, and a transmission circuit, such as transmission transistor310, are formed on substrate300, and wiring layer330and insulating layer23A are then formed on the reception circuit and the transmission circuit. After insulating layer23A is formed, through electrodes331aand332aare formed in insulating layer23A, and wiring section332bis formed on insulating layer23A. In this step, recess300ais formed in the portion corresponding to the region where transmission element100is to be formed on the lower surface of substrate300.

FIG. 7Bshows the step of forming transmission element100. In this step, materials for forming first lower electrode101, first piezoelectric body102, and first upper electrode103are sequentially formed on insulating layer23A. Subsequently, the resist patterns provided on the upper part of the region where transmission element100is to be formed are etched to pattern first lower electrode101, first piezoelectric body102, and first upper electrode103, thereby forming transmission element100.

FIG. 7Cshows the step of forming buffer layer400. In this step, for example, buffer layer400is formed on insulating layer23A and up to a position higher than the upper end of transmission element100by a printing method. Subsequently, through electrode400ais formed in buffer layer400.

FIG. 7Dshows the step of forming reception element200. In this step, a pattern of second lower electrode201is first formed on buffer layer400. Afterwards, a material (here, PVDF) for forming second piezoelectric body202is formed on the entire surface of buffer layer400by, for example, a printing method.

FIG. 7Eshows the step of forming common electrode22G. In this step, an opening communicating with first upper electrode103of transmission element100is formed in the piezoelectric material for second piezoelectric body202and buffer layer400by etching (for example, dry etching using O2plasma). Subsequently, common electrode22G is formed entirely on second piezoelectric body202and first upper electrode103of transmission element100. Consequently, a part of common electrode22G becomes second upper electrode203and reception element200is formed.

FIG. 7Fshows the step of forming protective layer21. In this step, protective layer21is formed so as to cover the entire transmission element100and reception element200.

As described above, with the configuration of ultrasound probe20according to this embodiment, ultrasound probe20having high transmission/reception sensitivity over a wide frequency band can be manufactured with a simple manufacturing process.

Advantageous Effects

As described above, in ultrasound probe20according to this embodiment, transmission element100is configured to vibrate in the flexural vibration mode, and reception element200is configured to vibrate in the thickness vibration mode. Hence, high transmission/reception sensitivity can be attained over a wide frequency band. With this configuration, since it is not necessary to form a recess (thin film section300a) on substrate300in the region where reception element200is formed, there is the advantage that transmission element100can be formed in high density.

In particular, with ultrasound probe20according to this embodiment, buffer layer400provided in the region immediately below reception element200can substantially increase the reception sensitivity of reception element200without causing the transmission characteristics of transmission element100to deteriorate.

In addition, with ultrasound probe20according to this embodiment, buffer layer400also functions as a backing member, so that deterioration in the reception performance of reception element200due to reflection by circuit substrate23can be suppressed. This can also reduce crosstalk between the two transmission elements100adjacent to each other.

The configuration of ultrasound probe20according to Embodiment 2 will be described with reference toFIG. 8.FIG. 8is a sectional view showing the configuration of ultrasound probe20according to Embodiment 2.

Ultrasound probe20according to this embodiment is different from Embodiment 1 in that buffer layer400is not in contact with transmission element100. Description of the configuration shared with Embodiment 1 will be omitted (the same applies to the other embodiments).

As described above, a material having a low elastic modulus (typically, an organic insulating material) is used as buffer layer400. Consequently, even if that buffer layer400is in contact with transmission element100, the degree of inhibition of operation of transmission element100by that buffer layer400is small. However, when an inorganic material (for example, silicon oxide) having an elastic modulus larger than 1/10 of the piezoelectric material is used as buffer layer400, that buffer layer400may hinder operation of transmission element100. In addition, buffer layer400may hinder the travel of the ultrasound beam emitted from transmission element100, and may narrow the opening of the ultrasound beam.

For this reason, in ultrasound probe20according to this embodiment, buffer layer400is formed such that this buffer layer400is not in contact with transmission element100. To be specific, buffer layer400according to this embodiment is provided apart from transmission element100, in a region surrounding transmission element100. Such a configuration is achieved, for example, by etching buffer layer400into a desired pattern after formation of buffer layer400in the step shown inFIG. 7Cand before formation of the piezoelectric material for second piezoelectric body202in the step ofFIG. 7D.

At this time, the position of buffer layer400is such that the line connecting the center position on the lower surface side of transmission element100and a position facing transmission element100at the upper end of buffer layer400forms 60 degrees or more with respect to the direction of transmission of the ultrasound beam from transmission element100(that is, the normal direction) (see angle θ inFIG. 8). As a result, a sufficient opening for the ultrasound beam emitted from transmission element100can be ensured.

As described above, with ultrasound probe20according to this embodiment, the phenomenon in which the vibration of transmission element100is hindered by buffer layer400can be suppressed. This leads to the improved transmission characteristic of transmission element100.

The configuration of ultrasound probe20according to Embodiment 3 will now be described with reference toFIG. 9.FIG. 9is a sectional view showing the configuration of ultrasound probe20according to Embodiment 3.

Ultrasound probe20according to this embodiment is different from Embodiment 1 in that thin buffer layer400is formed immediately below transmission element100.

As described above, buffer layer400may hinder the flexural vibration of transmission element100and degrade the transmission characteristic of transmission element100. However, since buffer layer400also functions as a backing member, considering suppression of reflection by circuit substrate23, thin buffer layer400is also provided directly below transmission element100as necessary.

Such a configuration can be achieved, for example, by forming thin buffer layer400in the step shown inFIG. 7B, forming transmission element100, and then forming buffer layer400again.

Other Embodiments

The present invention is not limited to the aforementioned embodiments and there are various potential modifications.

The aforementioned embodiments show the aspect in which a recess is provided only in the region immediately below transmission element100on the lower surface side of substrate300. However, a recess may be provided in a region directly below reception element200on the lower surface side of substrate300such that the resonance frequency in the flexural vibration mode does not exist in the frequency band used for transmission/reception of ultrasound.

In the aforementioned embodiments, buffer layer400is formed by a printing method. However, depending on the material used for buffer layer400, buffer layer400may be formed by a technique (for example, CVD) other than the printing method.

The aforementioned embodiments show the aspect in which buffer layer400is formed up to a position higher than the position of the upper end of transmission element100. However, depending on how buffer layer400is formed, buffer layer400may be formed such that the position of the upper end of buffer layer400is lower than the position of the upper end of transmission element100.

In the aforementioned embodiments, first piezoelectric body102of transmission element100is composed of PZT. However, an inorganic piezoelectric material other than PZT (for example, lead magnesium niobate titanate (PMN-PT) or lead zirconate magnesium niobate titanate (PMN-PZT)) may be used as first piezoelectric body102.

In the above embodiment, second piezoelectric body202of reception element200is composed of PVDF. However, second piezoelectric body202may be composed of an organic piezoelectric material other than PVDF (for example, a urea resin). Second piezoelectric body202may have a single layer structure composed of only a piezoelectric material, or may have a laminated structure in which a metal or non-metal thin film layer is sandwiched between piezoelectric materials.

Specific examples of the present invention which have been described in detail above are merely illustrative and do not limit the scope of the claims The techniques described in the claims include various modifications and changes of the specific examples illustrated above.

INDUSTRIAL APPLICABILITY

The ultrasound probe according to the present disclosure can achieve high transmission/reception sensitivity over a wide frequency band.

REFERENCE SIGNS LIST

10Ultrasound diagnostic apparatus body

11Operation input section

14Image processing section

22Element array layer

300aThin film section

300bThick film section