Ultrasonic transducer

Abstract

The present invention is provided with an acoustic MEMS element (100) and an acoustic path (P). The acoustic path (P) is connected to the acoustic MEMS element (100). Ultrasonic waves generated through vibration of the acoustic MEMS element (100) can acoustically resonate in the acoustic path (P). This ultrasonic transducer has, through a combination between sympathetic vibration of the acoustic MEMS element (100) and acoustic resonance in the acoustic path (P), sound pressure-frequency characteristics in which a plurality of sound pressure peaks appear. When f0 represents the sympathetic vibration frequency of the acoustic MEMS element (100) and f1 represents the acoustic resonance frequency in the acoustic path (P), the relationship -20≤(f1-f0) / f0×100≤80 is satisfied.

Description

Ultrasonic Transducer

[0001] The present invention relates to an ultrasonic transducer.

[0002] International Publication No. WO 2020 / 230484 (Patent Document 1) is a prior art document that discloses the configuration of an ultrasonic transducer. The ultrasonic transducer described in Patent Document 1 includes a mounting substrate and a piezoelectric device. The piezoelectric device is mounted on the mounting substrate. The piezoelectric device includes a substrate, a piezoelectric element, and a lid. A through hole is formed in the substrate. The piezoelectric element is located on a first main surface of the substrate. The mounting substrate faces a second main surface of the substrate. A through hole is formed in the mounting substrate. An end of the through hole in the mounting substrate on the piezoelectric device side is located in a position facing the through hole in the substrate.

[0003] International Publication No. 2020 / 230484

[0004] An ultrasonic transducer is sometimes required to be able to emit ultrasonic waves with a stable phase in a frequency band in which it is used by reducing the Q value in that frequency band.

[0005] An ultrasonic transducer according to the present invention includes an acoustic MEMS element and an acoustic path. The acoustic path is connected to the acoustic MEMS element. Ultrasonic waves generated by vibration of the acoustic MEMS element can resonate in the acoustic path. The ultrasonic transducer has sound pressure frequency characteristics in which multiple sound pressure peaks appear due to a combination of resonance of the acoustic MEMS element and resonance in the acoustic path. If the resonance frequency of the acoustic MEMS element is f0 and the resonance frequency of the acoustic path is fl, then the relationship -20≦(f1−f0) / f0×100≦80 is satisfied.

[0006] According to the present invention, it is possible to reduce the Q value in the frequency band to emit ultrasonic waves with a stable phase in that frequency band.

[0007] 1 is a perspective view showing the appearance of an acoustic MEMS element included in an ultrasonic transducer according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view of the acoustic MEMS element of FIG. 1 as seen from the direction of the arrows along line II-II. FIG. 2 is a cross-sectional view showing a state in which a second electrode layer is provided on a piezoelectric single crystal substrate, in a manufacturing method for an acoustic MEMS element according to a first embodiment of the present invention. FIG. 3 is a cross-sectional view showing a state in which a first support section is provided, in a manufacturing method for an acoustic MEMS element according to a first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a laminate is bonded to a first support section, in a manufacturing method for an acoustic MEMS element according to a first embodiment of the present invention. FIG. 5 is a cross-sectional view showing a state in which a piezoelectric layer is formed by grinding a piezoelectric single crystal substrate, in a manufacturing method for an acoustic MEMS element according to a first embodiment of the present invention. FIG. 6 is a cross-sectional view showing a state in which a first electrode layer is provided on a piezoelectric layer, in a manufacturing method for an acoustic MEMS element according to a first embodiment of the present invention. 11 is a cross-sectional view of the ultrasonic transducer of FIG. 10 as viewed from the direction of the arrows along line XI-XI. This is a graph showing the transitions in sound pressure and phase with frequency of ultrasonic waves emitted from one acoustic MEMS element. This is a graph showing the transitions in sound pressure and phase with frequency in the acoustic path of ultrasonic waves emitted from a virtual sound source arranged at the position of the membrane portion. This is a graph showing the transitions in phase with frequency of ultrasonic waves emitted from an ultrasonic transducer in which one acoustic MEMS element is combined with the acoustic paths of samples 1 to 4, respectively. This is a graph showing the transitions in sound pressure with frequency of ultrasonic waves emitted from an ultrasonic transducer in which one acoustic MEMS element is combined with the acoustic paths of samples 1 to 4, respectively. This is a graph showing the rate of change of phase θ with frequency f in the phase-frequency characteristics of ultrasonic waves emitted from an ultrasonic transducer in which one acoustic MEMS element is combined with the acoustic paths of samples 1 to 5, respectively. This is a graph showing the relationship between dθ / df and the resonant frequency of the acoustic path when the resonant frequency of the acoustic MEMS element is 40 kHz.Fig. 1 is a cross-sectional view showing an ultrasonic transducer according to a first modified example of embodiment 1, in which cavities forming acoustic paths are formed in a plurality of stacked substrates; Fig. 2 is a cross-sectional view showing an ultrasonic transducer according to a second modified example of embodiment 1, in which cavities forming acoustic paths do not penetrate the substrates; Fig. 3 is a cross-sectional view showing an ultrasonic transducer according to embodiment 2 of the present invention; Fig. 4 is a cross-sectional view showing an ultrasonic transducer according to a modified example of embodiment 2, in which cavities forming acoustic paths do not penetrate the substrates;

[0008] Hereinafter, ultrasonic transducers according to embodiments of the present invention will be described with reference to the drawings. In the following description of the embodiments, the same or corresponding parts in the drawings will be denoted by the same reference numerals, and the description thereof will not be repeated.

[0009] In this specification, "MEMS" is an abbreviation for Micro Electro Mechanical Systems. "Acoustic MEMS element" is a general term for MEMS microphones, pMUTs (piezoelectric micro-machined ultrasonic transducers), cMUTs (capacitive micro-machined ultrasonic transducers), MEMS speakers, etc.

[0010] 1 is a perspective view showing the appearance of an acoustic MEMS element provided in an ultrasonic transducer according to embodiment 1 of the present invention. Fig. 2 is a cross-sectional view of the acoustic MEMS element of Fig. 1 as seen from the direction of the arrows along line II-II.

[0011] 1 and 2 , the acoustic MEMS element 100 includes a base portion 110 and a membrane portion 120. In this embodiment, the acoustic MEMS element 100 is a piezoelectric element. The membrane portion 120 performs at least one of vibration generation and vibration detection using a piezoelectric body. Note that the acoustic MEMS element 100 is not limited to a piezoelectric element, and may be configured to perform at least one of vibration generation and vibration detection using an electrostatic method.

[0012] In this embodiment, the base 110 has a square annular shape extending in the X-axis and Y-axis directions when viewed in the axial direction of the central axis C shown in FIG. 2 . The shape of the base 110 is not particularly limited as long as it is annular when viewed in the central axis direction (Z-axis direction). When viewed in the central axis direction (Z-axis direction), the outer peripheral side surface of the base 110 may be polygonal or circular, and the inner peripheral side surface of the base 110 may be polygonal or circular. For example, the length of one side of the inner peripheral side surface of the base 110 is 0.6 mm or more and 1.5 mm or less, and the thickness of the base 110 is 0.2 mm or more and 0.5 mm or less.

[0013] 2, the base 110 includes a support layer 15. An opening 101 is formed in the support layer 15. The vibration layer 10 is disposed above the support layer 15. The base 110 further includes a portion of the vibration layer 10 located above the support layer 15, and a first connection electrode layer 20 and a second connection electrode layer 30 disposed above the portion.

[0014] The support layer 15 includes an intermediate layer 15a and a substrate layer 15b. The intermediate layer 15a is formed on the substrate layer 15b. In this embodiment, the intermediate layer 15a is made of SiO2, and the substrate layer 15b is made of single crystal Si. Note that the material constituting the intermediate layer 15a and the substrate layer 15b is not limited to Si, and may be other semiconductor materials.

[0015] The vibration layer 10 has a piezoelectric layer 11, a first electrode layer 12, a second electrode layer 13, and an elastic layer 14. The thickness of the vibration layer 10 is, for example, not less than 0.5 μm and not more than 6.0 μm.

[0016] The piezoelectric layer 11 is made of a single crystal piezoelectric material. The cut orientation of the piezoelectric layer 11 is appropriately selected so as to achieve desired device characteristics. In this embodiment, the piezoelectric layer 11 is a thinned single crystal substrate, and the single crystal substrate is specifically a rotated Y-cut substrate. The cut orientation of the rotated Y-cut substrate is specifically 30°. The thickness of the piezoelectric layer 11 is, for example, 0.3 μm or more and 5.0 μm or less.

[0017] The material constituting the piezoelectric layer 11 is appropriately selected so that the acoustic MEMS element 100 exhibits desired characteristics. In this embodiment, the piezoelectric layer 11 is made of an inorganic material. Specifically, the piezoelectric layer 11 is made of an alkali niobate compound or an alkali tantalate compound. In this embodiment, the alkali metal contained in the alkali niobate compound or the alkali tantalate compound is at least one of lithium, sodium, and potassium. In this embodiment, the piezoelectric layer 11 is made of lithium niobate (LiNbO3) or lithium tantalate (LiTaO3).

[0018] 2 , the first electrode layer 12 is disposed on the upper side of the piezoelectric layer 11. The second electrode layer 13 is disposed on the lower side of the piezoelectric layer 11 so as to face at least a part of the first electrode layer 12 with the piezoelectric layer 11 in between. In this embodiment, adhesive layers (not shown) are disposed between the first electrode layer 12 and the piezoelectric layer 11 and between the second electrode layer 13 and the piezoelectric layer 11.

[0019] In this embodiment, the first electrode layer 12 and the second electrode layer 13 are each made of Pt. The first electrode layer 12 and the second electrode layer 13 may each be made of other materials such as Al. The adhesion layer is made of Ti. The adhesion layer may also be made of other materials such as a NiCr alloy. The first electrode layer 12, the second electrode layer 13, and the adhesion layer may each be an epitaxially grown film. When the piezoelectric layer 11 is made of lithium niobate (LiNbO3), it is preferable that the adhesion layer be made of a NiCr alloy in order to prevent the material constituting the adhesion layer from diffusing into the first electrode layer 12 or the second electrode layer 13. This improves the reliability of the acoustic MEMS element 100.

[0020] In this embodiment, the dimensions of each of the first electrode layer 12 and the second electrode layer 13 are, for example, 0.05 μm or more and 0.2 μm or less. The thickness of the adhesive layer is, for example, 0.005 μm or more and 0.05 μm or less.

[0021] The elastic layer 14 is disposed on the side of the piezoelectric layer 11 opposite the first electrode layer 12 and on the side of the second electrode layer 13 opposite the piezoelectric layer 11. The elastic layer 14 includes a first elastic layer 14a and a second elastic layer 14b laminated on the side of the first elastic layer 14a opposite the piezoelectric layer 11. In this embodiment, the first elastic layer 14a is made of SiO2, and the second elastic layer 14b is made of single crystal Si. In this embodiment, the thickness of the elastic layer 14 is preferably thicker than that of the piezoelectric layer 11, from the viewpoint of the flexural vibration of the membrane portion 120.

[0022] In addition, if the second elastic layer 14b is made of low-resistance Si, it is possible to have the second elastic layer 14b function as a lower electrode layer without providing the second electrode layer 13, and in this case, the first elastic layer 14a is not provided.

[0023] 2, the first connection electrode layer 20 is formed on the first electrode layer 12 via an adhesive layer (not shown). The second connection electrode layer 30 is formed on the second electrode layer 13 via an adhesive layer (not shown).

[0024] The thickness of each of the first connection electrode layer 20 and the second connection electrode layer 30 is, for example, not less than 0.1 μm and not more than 1.0 μm. The thickness of each of the adhesion layers connected to the first connection electrode layer 20 and the adhesion layers connected to the second connection electrode layer 30 is, for example, not less than 0.005 μm and not more than 0.1 μm.

[0025] In this embodiment, the first connection electrode layer 20 and the second connection electrode layer 30 are each made of Au. The first connection electrode layer 20 and the second connection electrode layer 30 may be made of other conductive materials such as Al. The adhesion layer connected to the first connection electrode layer 20 and the adhesion layer connected to the second connection electrode layer 30 are each made of, for example, Ti. These adhesion layers may also be made of a NiCr alloy.

[0026] 1 and 2, when viewed from the central axis direction (Z-axis direction), a slit SL is formed in the vibration layer 10 in a portion located inside the base 110. From the viewpoint of suppressing sound leakage, the width of the slit SL is preferably 10 μm or less. Furthermore, from the viewpoint of lowering the Q value at the resonant frequency of the acoustic MEMS element 100, it may be preferable to set the width of the slit SL to 3 μm or more.

[0027] 2, the membrane part 120 is positioned so as to cover the opening 101. The membrane part 120 extends along an imaginary plane when the acoustic MEMS element 100 is not driven. The membrane part 120 is a vibration part that includes the piezoelectric layer 11.

[0028] The membrane portion 120 is configured to be able to vibrate when a voltage is applied to the piezoelectric layer 11. Furthermore, the vibration acting on the membrane portion 120 is converted into a voltage by the piezoelectric layer 11, thereby enabling the vibration to be detected. Note that the membrane portion 120 is not limited to the configuration in which vibration is generated and detected by a piezoelectric method as described above, and may be configured to generate and detect vibration by an electrostatic method.

[0029] From the viewpoint of facilitating bending vibration, it is preferable that the length dimension in the extension direction of membrane portion 120 is at least five times the thickness dimension in the central axis direction (Z-axis direction) of membrane portion 120. Note that in Figure 2, the extension length and thickness of membrane portion 120 are shown schematically and not to the actual ratio.

[0030] When generating ultrasonic waves using the acoustic MEMS element 100, a voltage is applied between the first connection electrode layer 20 and the second connection electrode layer 30 as shown in FIG. 2 . Then, a voltage is applied between the first electrode layer 12 connected to the first connection electrode layer 20 and the second electrode layer 13 connected to the second connection electrode layer 30. Furthermore, in the membrane portion 120, a voltage is also applied between the first electrode layer 12 and the second electrode layer 13 that face each other via the piezoelectric layer 11. As a result, the piezoelectric layer 11 expands and contracts along an in-plane direction perpendicular to the thickness direction (Z-axis direction), causing the membrane portion 120 to flexurally vibrate along the thickness direction (Z-axis direction). As a result, a force is applied to the medium surrounding the membrane portion 120 of the acoustic MEMS element 100, which further vibrates the medium, generating ultrasonic waves.

[0031] Furthermore, in the acoustic MEMS element 100 according to this embodiment, the membrane portion 120 has a specific mechanical resonance frequency. Therefore, if the applied voltage is a sine wave voltage and the frequency of the sine wave voltage is close to the value of the resonance frequency, the amount of displacement when the membrane portion 120 is bent becomes large.

[0032] When ultrasonic waves are detected by the acoustic MEMS element 100, the ultrasonic waves vibrate the medium surrounding the membrane portion 120, and a force is applied from the surrounding medium to the membrane portion 120, causing the membrane portion 120 to undergo bending vibration. When the membrane portion 120 undergoes bending vibration, stress is applied to the piezoelectric layer 11. The application of stress to the piezoelectric layer 11 induces charges in the piezoelectric layer 11. The charges induced in the piezoelectric layer 11 generate a potential difference between the first electrode layer 12 and the second electrode layer 13, which face each other via the piezoelectric layer 11. This potential difference is detected by the first connection electrode layer 20 connected to the first electrode layer 12 and the second connection electrode layer 30 connected to the second electrode layer 13. This allows the acoustic MEMS element 100 to detect ultrasonic waves.

[0033] Furthermore, if the ultrasonic waves to be detected contain a large amount of specific frequency components and these frequency components are close to the value of the resonant frequency, the amount of displacement when the membrane part 120 flexurally vibrates increases. As the amount of displacement increases, the potential difference increases.

[0034] Thus, when the acoustic MEMS element 100 according to this embodiment is used as an ultrasonic transducer for non-contact haptics, etc., the resonant frequency of the membrane portion 120 is 20 kHz or more and 60 kHz or less, for example, 40 kHz. When the acoustic MEMS element 100 is used as an audio device such as a speaker or microphone, the resonant frequency of the membrane portion 120 is set to less than 20 kHz, which is in the audible range.

[0035] A method for manufacturing the acoustic MEMS element 100 according to the first embodiment of the present invention will be described below. Fig. 3 is a cross-sectional view showing a state in which a second electrode layer is provided on a piezoelectric single crystal substrate in the method for manufacturing the acoustic MEMS element according to the first embodiment of the present invention. Fig. 3 and Figs. 4 to 9 shown below are illustrated in the same cross-sectional view as Fig. 2.

[0036] As shown in Figure 3, first, an adhesive layer (not shown) is provided on the lower surface of the piezoelectric single crystal substrate 11a, and then a second electrode layer 13 is provided on the side of the adhesive layer opposite the piezoelectric single crystal substrate 11a. The second electrode layer 13 is formed to have a desired pattern by a vapor deposition lift-off method. The second electrode layer 13 may also be formed by laminating it over the entire lower surface of the piezoelectric single crystal substrate 11a by sputtering, and then forming the desired pattern by an etching method. The second electrode layer 13 and the adhesive layer may also be grown epitaxially.

[0037] 4 is a cross-sectional view showing a state in which a first support portion has been provided in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. As shown in FIG. 4, a first elastic layer 14a is provided on the lower surface of each of the piezoelectric single crystal substrate 11a and the second electrode layer 13 by a method such as CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition). Immediately after providing the first elastic layer 14a, a portion of the lower surface of the first elastic layer 14a located on the opposite side of the first elastic layer 14a from the second electrode layer 13 is raised. Therefore, the lower surface of the first elastic layer 14a is polished and flattened by a method such as chemical mechanical polishing (CMP).

[0038] 5 is a cross-sectional view showing the state in which the stacked body is bonded to the first support in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. As shown in FIG. 5 , a stacked body 16 consisting of a second elastic layer 14 b and a support layer 15 is bonded to the lower surface of the first elastic layer 14 a by surface activated bonding or atomic diffusion bonding. In this embodiment, the stacked body 16 is an SOI (Silicon on Insulator) substrate. Note that the yield of the acoustic MEMS element 100 is improved by planarizing the upper surface of the second elastic layer 14 b in advance by CMP or the like. Furthermore, if the second elastic layer 14 b is made of low-resistivity Si, the second elastic layer 14 b can function as a lower electrode layer, which eliminates the need to form the second electrode layer 13 and the first elastic layer 14 a.

[0039] 6 is a cross-sectional view showing a state in which a piezoelectric layer is formed by grinding a piezoelectric single crystal substrate in a manufacturing method of an acoustic MEMS element according to embodiment 1 of the present invention. As shown in Fig. 6, the upper surface of the piezoelectric single crystal substrate 11a is thinned by grinding with a grinder. The upper surface of the thinned piezoelectric single crystal substrate 11a is further polished by CMP or the like to form the piezoelectric single crystal substrate 11a into the piezoelectric layer 11.

[0040] It is also possible to form a release layer by implanting ions into the upper surface of the piezoelectric single crystal substrate 11a in advance, and then peel off the release layer to form the piezoelectric single crystal substrate 11a into the piezoelectric layer 11. It is also possible to form the piezoelectric single crystal substrate 11a into the piezoelectric layer 11 by further polishing the upper surface of the piezoelectric single crystal substrate 11a after peeling off the release layer by CMP or the like.

[0041] 7 is a cross-sectional view showing a state in which a first electrode layer is provided on a piezoelectric layer in a manufacturing method of an acoustic MEMS element according to the first embodiment of the present invention. As shown in FIG. 7 , an adhesive layer (not shown) is provided on the upper surface of the piezoelectric layer 11, and then the first electrode layer 12 is provided on the side of the adhesive layer opposite the piezoelectric layer 11. The first electrode layer 12 is formed to have a desired pattern by a vapor deposition lift-off method. The first electrode layer 12 may also be formed by laminating it over the entire upper surface of the piezoelectric layer 11 by sputtering, and then forming the desired pattern by an etching method. The first electrode layer 12 and the adhesive layer may also be grown epitaxially.

[0042] 8 is a cross-sectional view showing a state in which grooves and recesses are formed in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. As shown in FIG. 8 , in a region corresponding to a region inside the base 110 of the acoustic MEMS element 100 as viewed from the stacking direction, a slit is formed in the piezoelectric layer 11 and the first elastic layer 14a by dry etching using reactive ion etching (RIE) or the like. The slit may also be formed by wet etching using fluoronitric acid or the like. Furthermore, the second elastic layer 14b exposed in the slit is etched by deep reactive ion etching (DRIE) so that the slit reaches the top surface of the support layer 15. This forms a groove 17 shown in FIG. 8 , which corresponds to the slit SL shown in FIGS. 1 and 2 .

[0043] 8, in the portion corresponding to the base portion 110 of the acoustic MEMS element 100, the piezoelectric layer 11 is etched by the dry etching or wet etching so as to expose a portion of the second electrode layer 13. As a result, a recess 18 is formed.

[0044] 9 is a cross-sectional view showing the state in which the first connection electrode layer and the second electrode connection layer have been provided in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. As shown in FIG. 9, in the portion corresponding to the base portion 110, an adhesive layer (not shown) is provided on each of the first electrode layer 12 and the second electrode layer 13, and then the first connection electrode layer 20 and the second connection electrode layer 30 are provided on the upper surface of each adhesive layer by a vapor deposition lift-off method. The first connection electrode layer 20 and the second connection electrode layer 30 may be formed by laminating them over the entire surfaces of the piezoelectric layer 11, the first electrode layer 12, and the exposed second electrode layer 13 by sputtering, and then forming the desired pattern by an etching method.

[0045] Finally, a portion of the substrate layer 15b of the support layer 15 is removed by DRIE, and then a portion of the intermediate layer 15a is removed by RIE. As a result, an opening 101 is provided and a membrane portion 120 is formed, as shown in Fig. 2. Through the above steps, the acoustic MEMS element 100 according to the first embodiment of the present invention, as shown in Figs. 1 and 2, is manufactured.

[0046] Fig. 10 is a plan view showing an ultrasonic transducer according to embodiment 1 of the present invention. Fig. 11 is a cross-sectional view of the ultrasonic transducer of Fig. 10 as viewed from the direction of the arrows along line XI-XI. As shown in Figs. 10 and 11 , an ultrasonic transducer 200 according to embodiment 1 of the present invention includes an acoustic MEMS element 100 and an acoustic path P. The acoustic path P communicates with the acoustic MEMS element 100. In this embodiment, the acoustic path P is configured by the opening 101 of the acoustic MEMS element 100 and a cavity 210h formed in the substrate 210.

[0047] Specifically, the acoustic MEMS element 100 is mounted on a substrate 210. In this embodiment, a plurality of acoustic MEMS elements 100 are arranged in an array on the substrate 210. The frequencies of the ultrasonic waves emitted from each of the plurality of acoustic MEMS elements 100 may be different from one another. However, only one acoustic MEMS element 100 may be arranged on the substrate 210. The acoustic MEMS element 100 is fixed onto the main surface of the substrate 210 by a die bond agent 220. The die bond agent 220 is a thermosetting adhesive.

[0048] A cavity 210h that constitutes an acoustic path P is formed in a position of the substrate 210 facing the acoustic MEMS element 100. In this embodiment, a plurality of cavities 210h are formed in the substrate 210 in one-to-one correspondence with each of the plurality of acoustic MEMS elements 100. The cavity 210h is a hole that penetrates the substrate 210. The cavity 210h has a cylindrical shape. However, the shape of the cavity 210h is not limited to a cylindrical shape and may be a rectangular parallelepiped shape, for example.

[0049] The thickness H2 of the substrate 210 is greater than the shortest distance H1 from the substrate 210 to the membrane portion 120. The diameter W2 of the cavity 210h is smaller than the width W1 of the opening 101.

[0050] The material of the substrate 210 is a combination of a resin such as glass epoxy and glass fiber, low-temperature co-fired ceramics (LTCC), or ceramics made of alumina, etc. The substrate 210 may also be a flexible substrate made of copper foil and polyimide, or a composite substrate with a reinforcing plate attached.

[0051] The acoustic path P has a frequency response to ultrasonic waves and resonates at a specific natural frequency. In other words, the acoustic path P has a resonant frequency. Ultrasonic waves generated by vibration of the acoustic MEMS element 100 can resonate in the acoustic path P. Meanwhile, the acoustic MEMS element 100 has a natural resonant frequency of the membrane portion 120, as described above.

[0052] Here, a mechanism by which the ultrasonic transducer 200 according to the first embodiment of the present invention can reduce the Q value in the frequency band to be used and emit ultrasonic waves with a stable phase in that frequency band will be described.

[0053] Fig. 12 is a graph showing the transitions in sound pressure and phase depending on the frequency of ultrasound emitted from a single acoustic MEMS element. In Fig. 12, the left vertical axis represents the ultrasound sound pressure (au), the right vertical axis represents the ultrasound phase (deg), and the horizontal axis represents the ultrasound frequency (kHz). The solid line L0 represents the sound pressure frequency characteristic, which is the transition of sound pressure depending on the ultrasound frequency, and the dotted line L0 represents the phase frequency characteristic, which is the transition of phase depending on the ultrasound frequency.

[0054] 12, the sound pressure of the ultrasonic waves emitted from one acoustic MEMS element 100 reaches a peak at 40 kHz, which is the resonant frequency of the acoustic MEMS element 100. The phase of the ultrasonic waves emitted from the acoustic MEMS element 100 is inverted at the boundary where the frequency reaches 40 kHz. In other words, near the resonant frequency of 40 kHz, the phase of the ultrasonic waves emitted from the acoustic MEMS element 100 is suddenly displaced.

[0055] 13 is a graph showing the transitions of sound pressure and phase with frequency in the acoustic path of ultrasound emitted from a virtual sound source located at the position of the membrane part. Specifically, without placing the acoustic MEMS element 100, a virtual sound source without resonance was placed at the position of the membrane part 120, and simulation analysis was performed on the transitions of sound pressure and phase with frequency for five samples of acoustic paths with different resonance frequencies.

[0056] In Figure 13, the left vertical axis represents the ultrasonic sound pressure (au), the right vertical axis represents the ultrasonic phase (deg), and the horizontal axis represents the ultrasonic frequency (kHz). The solid line represents the sound pressure frequency characteristic, which is the transition of sound pressure depending on the ultrasonic frequency, and the dotted line represents the phase frequency characteristic, which is the transition of phase depending on the ultrasonic frequency. L1 to L5 represent the data for samples 1 to 5, respectively.

[0057] As shown in FIG. 13 , the sound pressure of the ultrasonic waves radiated from Sample 1 peaks at 32 kHz, which is the resonant frequency of the acoustic path P. The phase of the ultrasonic waves radiated from Sample 1 is inverted at the frequency of 32 kHz. The sound pressure of the ultrasonic waves radiated from Sample 2 peaks at 35 kHz, which is the resonant frequency of the acoustic path P. The phase of the ultrasonic waves radiated from Sample 2 is inverted at the frequency of 35 kHz. The sound pressure of the ultrasonic waves radiated from Sample 3 peaks at 40 kHz, which is the resonant frequency of the acoustic path P. The phase of the ultrasonic waves radiated from Sample 3 is inverted at the frequency of 40 kHz. The sound pressure of the ultrasonic waves radiated from Sample 4 peaks at 42 kHz, which is the resonant frequency of the acoustic path P. The phase of the ultrasonic waves radiated from Sample 4 is inverted at the frequency of 42 kHz. The sound pressure of the ultrasonic waves radiated from Sample 5 peaks at 73 kHz, which is the resonant frequency of the acoustic path P. The phase of the ultrasonic waves emitted from the sample 5 is inverted at the boundary where the frequency is 73 kHz.

[0058] Fig. 14 is a graph showing the phase transition with frequency of ultrasonic waves emitted from an ultrasonic transducer in which one acoustic MEMS element is combined with each of the acoustic paths of samples 1 to 4. In Fig. 14, the vertical axis represents the phase (deg) of the ultrasonic waves, and the horizontal axis represents the frequency (kHz) of the ultrasonic waves. L0 represents the phase-frequency characteristic of only acoustic MEMS element 100, and L11 to L14 represent the phase-frequency characteristics of ultrasonic transducers in which acoustic MEMS element 100 is combined with each of the acoustic paths P of samples 1 to 4.

[0059] 14, in the ultrasonic transducers in which the acoustic MEMS element 100 is combined with the acoustic paths P of samples 1 to 4, the frequency at which the phase of the radiated ultrasonic waves is inverted is shifted higher compared to the case of only the acoustic MEMS element 100. In other words, by combining the acoustic path P with the acoustic MEMS element 100, it is possible to stabilize the phase of the ultrasonic waves when the frequency of the ultrasonic waves radiated from the ultrasonic transducer is 40 kHz, which is the resonant frequency of the acoustic MEMS element 100. Furthermore, by changing the resonant frequency of the acoustic path P combined with the acoustic MEMS element 100, it is possible to adjust the frequency at which the phase of the ultrasonic waves radiated from the ultrasonic transducer is inverted.

[0060] Fig. 15 is a graph showing the transition of sound pressure with frequency of ultrasonic waves emitted from an ultrasonic transducer in which one acoustic MEMS element is combined with each of the acoustic paths P of samples 1 to 4. In Fig. 15, the vertical axis represents the ultrasonic sound pressure (au) and the horizontal axis represents the ultrasonic frequency (kHz). L0 represents the sound pressure frequency characteristic of only acoustic MEMS element 100, and L11 to L14 represent the sound pressure frequency characteristics of ultrasonic transducers in which acoustic MEMS element 100 is combined with each of the acoustic paths P of samples 1 to 4.

[0061] As shown in FIG. 15 , an ultrasonic transducer in which the acoustic MEMS element 100 is combined with each of the acoustic paths P of Samples 1 to 4 has two sound pressure peaks. Note that two or more sound pressure peaks may also appear. In this way, by combining the resonance of the acoustic MEMS element 100 with the resonance in the acoustic paths P of Samples 1 to 4, an ultrasonic transducer is constructed that has sound pressure frequency characteristics in which multiple sound pressure peaks appear. An ultrasonic transducer in which the acoustic MEMS element 100 is combined with each of the acoustic paths P of Samples 1 to 4 can radiate ultrasonic waves with high sound pressure over a wider frequency band than when only the acoustic MEMS element 100 is used. Furthermore, by changing the resonant frequency of the acoustic path P combined with the acoustic MEMS element 100, the frequency at which the sound pressure peak occurs in the ultrasonic waves radiated from the ultrasonic transducer can be adjusted.

[0062] Fig. 16 is a graph showing the rate of change of phase θ with respect to frequency f in the phase-frequency characteristics of ultrasonic waves emitted from an ultrasonic transducer in which one acoustic MEMS element is combined with each of the acoustic paths P of samples 1 to 5. In Fig. 16, the vertical axis shows dθ / df (deg / kHz) and the horizontal axis shows the frequency (kHz) of the ultrasonic waves. L0 shows data in the phase-frequency characteristics of only acoustic MEMS element 100, and L11 to L15 show data in the phase-frequency characteristics of ultrasonic transducers in which acoustic MEMS element 100 is combined with each of the acoustic paths P of samples 1 to 5.

[0063] In the phase-frequency characteristics of ultrasonic waves emitted from an ultrasonic transducer configured by combining acoustic MEMS element 100 with acoustic path P, the allowable range of dθ / df (deg / kHz), which is the rate of change of phase θ with respect to frequency f, is 15 or less. When dθ / df≦15 is satisfied, it is possible to reduce the Q value in the frequency band used by the ultrasonic transducer and emit ultrasonic waves with a stable phase in that frequency band. Therefore, when the resonant frequency of acoustic MEMS element 100 is 40 kHz, the resonant frequency of acoustic path P must be equal to or less than the threshold value of dθ / df=15, as indicated by dotted line Ls at the intersection with dotted line Lt in FIG. 16 .

[0064] Fig. 17 is a graph showing the relationship between dθ / df and the resonant frequency of the acoustic path when the resonant frequency of the acoustic MEMS element is 40 kHz. In Fig. 17, the vertical axis shows dθ / df (deg / kHz) and the horizontal axis shows the resonant frequency (kHz) of the acoustic path P. In Fig. 17, the threshold value of dθ / df = 15 is shown by a dotted line.

[0065] As shown in FIG. 17, when the resonant frequency of the acoustic MEMS element 100 is 40 kHz, the relationship dθ / df≦15 is satisfied when the resonant frequency of the acoustic path P is within the range of 32 kHz to 73 kHz.

[0066] Therefore, if the resonant frequency of the acoustic MEMS element 100 is f0 and the resonant frequency of the acoustic path P is fl, by satisfying the relationship -20≦(f1−f0) / f0×100≦80, it is possible to reduce the Q value in the frequency band used by the ultrasonic transducer 200 and emit ultrasonic waves with a stable phase in that frequency band.

[0067] In the ultrasonic transducer 200 according to this embodiment, the acoustic path P is composed of the opening 101 of the acoustic MEMS element 100 and a cavity 210h formed in the substrate 210 on which the acoustic MEMS element 100 is mounted. This allows the acoustic path P to be configured with a simple structure.

[0068] The cavity 210h penetrates the substrate 210. This allows ultrasonic waves to be emitted at high sound pressure through the cavity 210h.

[0069] The dimension of the thickness H2 of the substrate 210 is greater than the dimension of the shortest distance H1 from the substrate 210 to the membrane portion 120. Since the surface roughness of the substrate 210 at the cavity 210h is greater than the surface roughness of the substrate layer 15b at the opening 101, by satisfying the relationship H2>H1, the resistance when the ultrasonic waves pass through the cavity 210h can be increased, and the Q value of the ultrasonic waves can be efficiently reduced.

[0070] The cavity 210h has a cylindrical shape. If the thickness H2 of the substrate 210 is x (mm), the diameter W2 of the cavity 210h is y (mm), the density of air is ρ, the speed of sound is c, the fluid viscosity of air is μ, and ω=2πf0, the radiation impedance when an ultrasonic wave is radiated to the outside from the cavity 210h is {ρc(ω / c)} 2 / 4π}, and the resistance when air passes through the cavity 210h, which is calculated from Poiseuille's law, is 128 μx / πy 4 is.

[0071] In order to reduce the Q value of the acoustic path P to 80% or less, the resistance when air passes through the cavity 210h needs to be 0.25 times or more the radiation impedance. 2 / 4π}×0.25≦128μx / πy 4 For example, when the resonant frequency of the acoustic MEMS element 100 is 40 kHz and the diameter W2 of the cavity 210h is 0.6 mm, the thickness H2 of the substrate 210 is 0.8 mm or more. This also reduces the Q value in the frequency band used by the ultrasonic transducer 200, making it possible to emit ultrasonic waves with a stable phase in that frequency band.

[0072] On the other hand, in order to make the Q value of the acoustic path P 10% or more so that the output of the ultrasonic waves does not become too small, the resistance when the air passes through the cavity 210h needs to be 9 times or less the radiation impedance. 2 / 4π}×0.25≦128μx / πy 4 ≦{ρc(ω / c) 2 / 4π}×9.

[0073] In this embodiment, a plurality of acoustic MEMS elements 100 are arranged in an array on a substrate 210, and a plurality of cavities 210h are formed in the substrate 210 in one-to-one correspondence with each of the plurality of acoustic MEMS elements 100. This reduces the Q value of the ultrasonic waves radiated from each of the plurality of acoustic MEMS elements 100 in the frequency band used by the ultrasonic transducer 200, thereby stabilizing the phase in that frequency band, and therefore it is possible to stably superimpose the ultrasonic waves radiated from each of the plurality of acoustic MEMS elements 100 and radiate ultrasonic waves with a high sound pressure.

[0074] 18 is a cross-sectional view showing an ultrasonic transducer according to a first modification of the first embodiment, in which cavities that form acoustic paths are formed in multiple stacked substrates. As shown in FIG. 18 , in the ultrasonic transducer according to the first modification of the first embodiment, the substrate 210 is bonded to the substrate 230 with an adhesive 240 on the main surface opposite the main surface on which the acoustic MEMS element 100 is mounted. A cavity 230h is formed in the substrate 230 at a position corresponding to the cavity 210h. The cavity 230h is a hole that penetrates the substrate 230. The cavity 230h has a cylindrical shape. However, the shape of the cavity 230h is not limited to a cylindrical shape and may be a rectangular parallelepiped shape, for example. The diameter W3 of the cavity 230h is larger than the diameter W2 of the cavity 210h. However, the diameter W3 of the cavity 230h may be smaller than the diameter W2 of the cavity 210h.

[0075] The dimension of the shortest distance H3 from the main surface of the substrate 210 on which the acoustic MEMS element 100 is mounted to the main surface of the substrate 230 opposite the substrate 210 side is greater than the dimension of the shortest distance H1 from the substrate 210 to the membrane portion 120. In other words, when a cavity is formed across multiple stacked substrates, the thickness x of the substrate is the sum of the thicknesses of the multiple substrates and the thickness of the adhesive.

[0076] The ultrasonic transducer according to the first modification of the first embodiment can also reduce the Q value in the frequency band in which the ultrasonic transducer is used, and emit ultrasonic waves with a stable phase in that frequency band.

[0077] Fig. 19 is a cross-sectional view showing an ultrasonic transducer according to a second modified example of embodiment 1, in which a cavity that forms an acoustic path does not penetrate the substrate. As shown in Fig. 19, in the ultrasonic transducer according to the second modified example of embodiment 1, a cavity 210c is formed in the substrate 210. The cavity 210c does not penetrate the substrate 210. Since the cavity 210c does not penetrate the substrate 210, it is possible to prevent foreign matter from entering the acoustic path P.

[0078] The ultrasonic transducer according to the second modification of the first embodiment can also reduce the Q value in the frequency band in which the ultrasonic transducer is used, and emit ultrasonic waves with a stable phase in that frequency band.

[0079] (Embodiment 2) An ultrasonic transducer according to embodiment 2 of the present invention will be described below with reference to the drawings. Note that the ultrasonic transducer according to embodiment 2 differs from the ultrasonic transducer according to embodiment 1 in that one cavity is formed corresponding to a plurality of acoustic MEMS elements, and therefore, description of the configuration that is the same as that of the ultrasonic transducer according to embodiment 1 will not be repeated.

[0080] Fig. 20 is a cross-sectional view showing an ultrasonic transducer according to embodiment 2 of the present invention. As shown in Fig. 20, in the ultrasonic transducer according to embodiment 2 of the present invention, a plurality of acoustic MEMS elements 100 are arranged in an array on a substrate 210. At least one cavity 210eh is formed in the substrate 210 so that one cavity 210eh faces at least two acoustic MEMS elements 100 out of the plurality of acoustic MEMS elements 100.

[0081] In this embodiment, one cavity 210eh is formed in the substrate 210. One cavity 210eh faces four acoustic MEMS elements 100. The cavity 210eh is a hole that penetrates the substrate 210. The cavity 210eh has a cylindrical shape with a diameter W4. However, the shape of the cavity 210eh is not limited to a cylindrical shape and may be a rectangular parallelepiped shape, etc. The dimension of the thickness H2 of the substrate 210 is greater than the dimension of the shortest distance H1 from the substrate 210 to the membrane portion 120.

[0082] In the ultrasonic transducer according to the second embodiment, by combining the acoustic MEMS element 100 with the acoustic path P, it is possible to reduce the Q value in the frequency band in which the ultrasonic transducer is used and to emit ultrasonic waves with a stable phase in that frequency band.

[0083] Fig. 21 is a cross-sectional view showing an ultrasonic transducer according to a modification of embodiment 2, in which a cavity that forms an acoustic path does not penetrate the substrate. As shown in Fig. 21, in the ultrasonic transducer according to the modification of embodiment 2, a cavity 210ec is formed in a substrate 210. The cavity 210ec does not penetrate the substrate 210. Since the cavity 210ec does not penetrate the substrate 210, it is possible to prevent foreign matter from entering the acoustic path P.

[0084] The ultrasonic transducer according to the modified example of the second embodiment can also reduce the Q value in the frequency band in which the ultrasonic transducer is used, and emit ultrasonic waves with a stable phase in that frequency band.

[0085] (Note) It will be understood by those skilled in the art that the exemplary embodiments described above are specific examples of the following aspects.

[0086] <1> An ultrasonic transducer comprising: an acoustic MEMS element; and an acoustic path that communicates with the acoustic MEMS element and that allows ultrasonic waves generated by vibration of the acoustic MEMS element to resonate; wherein the ultrasonic transducer has sound pressure frequency characteristics in which multiple sound pressure peaks appear due to a combination of resonance of the acoustic MEMS element and resonance in the acoustic path; and wherein the ultrasonic transducer satisfies the relationship -20≦(f1−f0) / f0×100≦80, where f0 is the resonance frequency of the acoustic MEMS element and fl is the resonance frequency of the acoustic path.

[0087] <2> The ultrasonic transducer according to <1>, further comprising a substrate on which the acoustic MEMS element is mounted, wherein a cavity that forms the acoustic path is formed in a position of the substrate facing the acoustic MEMS element.

[0088] <3> The cavity has a cylindrical shape, and when the thickness of the substrate is x, the diameter of the cavity is y, the density of air is ρ, the speed of sound is c, the fluid viscosity of air is μ, and ω=2πf0, the following equation is satisfied: {ρc(ω / c) 2 / 4π}×0.25≦128μx / πy 4 ≦{ρc(ω / c) 2 / 4π}×9。 The ultrasonic transducer according to <2>, wherein the relationship is satisfied.

[0089] <4> The ultrasonic transducer according to <2> or <3>, wherein the acoustic MEMS element includes a base and a membrane portion, and the thickness of the substrate is greater than the shortest distance from the substrate to the membrane portion.

[0090] <5> The ultrasonic transducer according to any one of <2> to <4>, wherein the cavity penetrates the substrate.

[0091] <6> The ultrasonic transducer according to any one of <2> to <5>, wherein a plurality of the acoustic MEMS elements are arranged in an array on the substrate, and a plurality of the cavities are formed in the substrate corresponding to each of the plurality of acoustic MEMS elements.

[0092] <7> The ultrasonic transducer according to <2>, wherein a plurality of the acoustic MEMS elements are arranged in an array on the substrate, and at least one of the cavities is formed in the substrate so that one of the plurality of acoustic MEMS elements faces at least two of the acoustic MEMS elements.

[0093] <8> The ultrasonic transducer according to <7>, wherein the cavity penetrates the substrate.

[0094] In the above-described embodiments, configurations that can be combined may be combined with each other.

[0095] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0096] 10 vibration layer, 11 piezoelectric layer, 11a single crystal substrate, 12 first electrode layer, 13 second electrode layer, 14 elastic layer, 14a first elastic layer, 14b second elastic layer, 15 support layer, 15a intermediate layer, 15b substrate layer, 16 laminate, 17 groove portion, 18 recess, 20 first connection electrode layer, 30 second connection electrode layer, 100 acoustic MEMS element, 101 opening, 110 base portion, 120 membrane portion, 200 ultrasonic transducer, 210, 230 substrate, 210c, 210ec, 210eh, 210h, 230h cavity, 220 die bond agent, 240 adhesive, P acoustic path, SL slit.

Claims

1. An ultrasonic transducer comprising: an acoustic MEMS element; and an acoustic path communicating with said acoustic MEMS element and capable of resonating with ultrasonic waves generated by vibration of said acoustic MEMS element; wherein the ultrasonic transducer has sound pressure frequency characteristics in which multiple sound pressure peaks appear due to a combination of resonance of said acoustic MEMS element and resonance in said acoustic path; and wherein, where the resonance frequency of said acoustic MEMS element is f0 and the resonance frequency of said acoustic path is fl, the ultrasonic transducer satisfies the relationship -20≦(f1-f0) / f0×100≦80.

2. An ultrasonic transducer as described in claim 1, further comprising a substrate on which the acoustic MEMS element is mounted, wherein a cavity that forms the acoustic path is formed in a position of the substrate facing the acoustic MEMS element.

3. The cavity has a cylindrical shape, and when the thickness of the substrate is x, the diameter of the cavity is y, the density of air is ρ, the speed of sound is c, the fluid viscosity of air is μ, and ω=2πf0, then {ρc(ω / c) 2 / 4π}×0.25≦128μx / πy 4 ≦{ρc(ω / c) 2 3. The ultrasonic transducer according to claim 2, wherein the relationship satisfies: ∇ / 4π}×9.

4. An ultrasonic transducer according to claim 2 or 3, wherein the acoustic MEMS element includes a base and a membrane portion, and the thickness of the substrate is greater than the shortest distance from the substrate to the membrane portion.

5. An ultrasonic transducer according to any one of claims 2 to 4, wherein the cavity penetrates through the substrate.

6. An ultrasonic transducer according to any one of claims 2 to 5, wherein a plurality of the acoustic MEMS elements are arranged in an array on the substrate, and a plurality of the cavities are formed in the substrate corresponding to each of the plurality of acoustic MEMS elements.

7. An ultrasonic transducer as described in claim 2 or claim 5, wherein a plurality of the acoustic MEMS elements are arranged in an array on the substrate, and at least one of the cavities is formed in the substrate so that one of the plurality of acoustic MEMS elements faces at least two of the acoustic MEMS elements.

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