Ultrasonic transducer

Abstract

The present invention is provided with a substrate (210) and a plurality of acoustic MEMS devices (100). The plurality of acoustic MEMS devices (100) are mounted on the substrate (210). At least one cavity (210eh) is formed in the substrate (210). Each cavity (210eh) of the at least one cavity (210eh) faces at least two acoustic MEMS devices (100) of the plurality of acoustic MEMS devices (100).

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] A prior art document disclosing the resistance that a fluid flowing through a pipe with a rectangular cross-section encounters from the wall of the pipe is "Flow in a Pipe of Rectangular Cross-Section," R. J. Cornish, October 1, 1928 (Non-Patent Document 1).

[0004] International Publication No. 2020 / 230484

[0005] "Flow in a Pipe of Rectangular Cross-Section," R. J. Cornish, October 1, 1928

[0006] In an ultrasonic transducer having multiple acoustic MEMS elements mounted on a substrate, variations in the resonant frequency of each acoustic MEMS element can cause the sound pressure to fluctuate up and down with changes in frequency in the frequency band in which the ultrasonic transducer is used, resulting in unstable sound pressure frequency characteristics.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide an ultrasonic transducer that can stabilize sound pressure frequency characteristics in the frequency band used.

[0008] An ultrasonic transducer according to the present invention includes a substrate and a plurality of acoustic MEMS elements. The plurality of acoustic MEMS elements are mounted on the substrate. At least one cavity is formed in the substrate. Each cavity in the at least one cavity faces at least two of the plurality of acoustic MEMS elements.

[0009] According to the present invention, it is possible to stabilize the sound pressure frequency characteristics in the frequency band in which the ultrasonic transducer is used.

[0010] 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 XI-XI. It is a cross-sectional view showing an ultrasonic transducer according to a comparative example. It is a graph showing the change in sound pressure depending on the frequency of ultrasonic waves emitted from the acoustic MEMS element in an ultrasonic transducer according to a comparative example. It is a graph showing the change in sound pressure depending on the frequency of ultrasonic waves emitted from the acoustic MEMS element in an ultrasonic transducer according to embodiment 1. It is a diagram showing the positional relationship between a cavity and an acoustic MEMS element. It is a graph showing the change in Q value at each coordinate calculated based on the equation for acoustic resistance R, assuming a = 1 cm and d = 0.5 mm. It is a cross-sectional view showing an ultrasonic transducer according to a first modified example of embodiment 1 of the present invention. It is a graph showing the change in sound pressure depending on the frequency of ultrasonic waves emitted from the acoustic MEMS element in an ultrasonic transducer according to embodiment 1 of the present invention. It is a cross-sectional view showing an ultrasonic transducer according to a second modified example of embodiment 1 of the present invention. It is a cross-sectional view showing an ultrasonic transducer according to embodiment 2 of the present invention. It is a cross-sectional view showing an ultrasonic transducer according to embodiment 3 of the present invention.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 1 and 2 , the acoustic MEMS element 100 includes a membrane portion 120. The membrane portion 120 extends from a base portion 110 located on the edge of the acoustic MEMS element 100 toward the center of the acoustic MEMS element 100. 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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).

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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).

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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 formed by epitaxial growth.

[0040] 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).

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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 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 be formed by epitaxial growth.

[0045] 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 .

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 seen from the direction of the arrows XI-XI. As shown in Figs. 10 and 11, an ultrasonic transducer 200 according to embodiment 1 of the present invention includes a plurality of acoustic MEMS elements 100 and a substrate 210.

[0050] Specifically, a plurality of acoustic MEMS elements 100 are 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 plurality of acoustic MEMS elements 100 are arranged adjacent to one another in a matrix. The frequencies of the ultrasonic waves emitted from each of the plurality of acoustic MEMS elements 100 may be different from one another. The acoustic MEMS elements 100 are fixed onto the main surface of the substrate 210 by a die bond agent 220. The die bond agent 220 is a thermosetting adhesive.

[0051] At least one cavity 210eh is formed in the substrate 210. Each cavity 210eh in the at least one cavity 210eh faces at least two of the multiple acoustic MEMS elements 100. In this embodiment, one cavity 210eh is formed in the substrate 210. One cavity 210eh faces all of the acoustic MEMS elements 100 mounted on the substrate 210. However, multiple cavities 210eh may be formed in the substrate 210. In this case, the multiple cavities 210eh face the corresponding multiple acoustic MEMS elements 100, respectively. In this embodiment, the support layer 15 located on the edge of the ultrasonic transducer 200 is located on the substrate 210, and the remaining support layer 15 is located in a position that overlaps with the cavity 210eh between the acoustic MEMS elements 100 when viewed from the thickness direction (Z-axis direction) of the substrate 210.

[0052] The cavity 210eh is a hole that penetrates the substrate 210. The cavity 210eh has a rectangular shape when viewed in the axial direction of the central axis C. However, the shape of the cavity 210eh may be a circle, an ellipse, or a polygon when viewed in the axial direction of the central axis C. The opening 101 of the acoustic MEMS element 100 and the cavity 210eh formed in the substrate 210 form an acoustic path P that communicates with the acoustic MEMS element 100. The acoustic path P is a region that faces the membrane portion 120.

[0053] In the thickness direction (Z-axis direction) of the substrate 210 , the dimension of the depth H 2 of the cavity 210 eh is greater than the dimension of the shortest distance H 1 from the substrate 210 to the membrane portion 120 .

[0054] The substrate 210 is preferably made of a material whose flow rate, as defined in JIS Z 8762, is half or less that of single-crystal silicon. This enhances the effect of reducing the Q value due to the viscous resistance of the wall surface of the cavity 210eh, which will be described later. Specifically, 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 such as alumina. The substrate 210 may also be a flexible substrate made of copper foil and polyimide, or a composite substrate with a reinforcing plate attached thereto.

[0055] 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.

[0056] Here, a mechanism by which the ultrasonic transducer 200 according to the first embodiment of the present invention can stabilize the sound pressure frequency characteristics in the frequency band used will be described.

[0057] Fig. 12 is a cross-sectional view showing an ultrasonic transducer according to a comparative example. As shown in Fig. 12, the ultrasonic transducer 900 according to the comparative example includes a plurality of MEMS elements 100 arranged in an array on a substrate 910. No cavities are formed in the substrate 910. In the ultrasonic transducer 900, the base 110 is provided only at the edge of the ultrasonic transducer 900. In other words, the support layer 15 located at the boundary between adjacent acoustic MEMS elements 100 is removed. In the ultrasonic transducer 900, since no cavities are formed in the substrate 910, the acoustic resistance, which is the viscous resistance in the acoustic path, is small, and the Q value of the ultrasonic waves radiated from each acoustic MEMS element 100 is maintained with almost no reduction.

[0058] 13 is a graph showing the change in sound pressure over time depending on the frequency of the ultrasonic waves emitted from the acoustic MEMS element in the ultrasonic transducer according to the comparative example. In Fig. 13, the vertical axis represents the ultrasonic sound pressure (Pa) and the horizontal axis represents the ultrasonic frequency (Hz). Furthermore, solid lines L1 to L5 represent data on the ultrasonic waves emitted from the acoustic MEMS elements 100 of samples 1 to 5 included in the ultrasonic transducer 900 according to the comparative example, and solid line LT represents data on the ultrasonic waves emitted from the ultrasonic transducer 900 as a combination of the ultrasonic waves emitted from the acoustic MEMS elements 100 of samples 1 to 5.

[0059] As shown in FIG. 13, in the ultrasonic transducer 900 according to the comparative example, when the frequencies of the ultrasonic waves radiated from the acoustic MEMS elements 100 of samples 1 to 5 vary, the Q value of the ultrasonic waves radiated from each of the acoustic MEMS elements 100 of samples 1 to 5 is maintained with almost no reduction, and therefore the ultrasonic waves L1 to L5 radiated from the acoustic MEMS elements 100 of samples 1 to 5 are combined to form the ultrasonic wave LT radiated from the ultrasonic transducer 900, and the sound pressure vibrates up and down as the frequency changes, making the sound pressure frequency characteristics unstable.

[0060] Fig. 14 is a graph showing the change in sound pressure depending on the frequency of the ultrasonic waves emitted from the acoustic MEMS element in the ultrasonic transducer according to embodiment 1. In Fig. 14, the vertical axis represents the ultrasonic sound pressure (Pa) and the horizontal axis represents the ultrasonic frequency (Hz). Furthermore, solid lines L1 to L5 respectively represent data on the ultrasonic waves emitted from the acoustic MEMS elements 100 of samples 1 to 5 included in the ultrasonic transducer 200 according to embodiment 1, and solid line LT represents data on the ultrasonic waves emitted from the ultrasonic transducer 200 as a combination of the ultrasonic waves emitted from the acoustic MEMS elements 100 of samples 1 to 5.

[0061] 14, in the ultrasonic transducer 200 according to the first embodiment, the Q value of the ultrasonic waves radiated from each of the acoustic MEMS elements 100 of samples 1 to 5 decreases due to viscous resistance from the wall surface of the cavity 210eh in the acoustic path P. As will be described later, the closer the acoustic path is located to the wall surface of the cavity 210eh, the lower the Q value of the ultrasonic waves radiated from the acoustic MEMS element 100. In the example shown in FIG. 14, the acoustic paths P of the acoustic MEMS elements 100 of samples 1 and 5 are located closest to the wall surface of the cavity 210eh, and the acoustic path P of the acoustic MEMS element 100 of sample 3 is located farthest from the wall surface of the cavity 210eh.

[0062] As shown in FIG. 14 , when the frequencies of the ultrasonic waves radiated from the acoustic MEMS elements 100 of samples 1 to 5 vary, the Q value of the ultrasonic waves radiated from each of the acoustic MEMS elements 100 of samples 1 to 5 decreases, and therefore the ultrasonic waves L1 to L5 radiated from the acoustic MEMS elements 100 of samples 1 to 5 are combined to form the ultrasonic wave LT radiated from the ultrasonic transducer 200, and the sound pressure does not oscillate up and down as the frequency changes, but rather monotonically increases to a sound pressure peak as the frequency changes and then monotonically decreases from the sound pressure peak, resulting in a stable sound pressure frequency characteristic.

[0063] Here, the acoustic resistance of the ultrasonic transducer 200 according to embodiment 1 was calculated with reference to the equation for calculating viscous resistance described in Non-Patent Document 1. Fig. 15 is a diagram showing the positional relationship between the cavity and the acoustic MEMS element. In Fig. 15, when viewed from the central axis direction (Z-axis direction), the shape of the cavity 210eh is a square with a side length of a, the shape of the acoustic path P of the acoustic MEMS element 100 is a square with a side length of d, the center position of the cavity 210eh is O, and the position of the acoustic path P is shown as (x, y).

[0064] From the calculation formula for viscous resistance described in Non-Patent Document 1, when the length of the pipe is L and the viscosity coefficient of air is μ, the acoustic resistance R, which is the viscous resistance within the cavity 210eh of the substrate 210, is expressed by the following formula.

[0065]

[0066] The part "..." in the above formula for acoustic resistance R is a higher-order term, which has almost no effect on the value of acoustic resistance R and therefore need not be taken into consideration.

[0067] A in the above formula for acoustic resistance R is expressed by the following formula.

[0068]

[0069] B in the above formula for acoustic resistance R is expressed by the following formula.

[0070]

[0071] 16 is a graph showing the transition of the Q value at each coordinate calculated based on the equation for acoustic resistance R, where a = 1 cm and d = 0.5 mm. In Fig. 16, the Q value is a value normalized based on the Q value of an acoustic MEMS element having an acoustic path at the center position of the cavity. The Q value is proportional to the reciprocal of the acoustic resistance R.

[0072] 16, the closer the acoustic path is located to the wall surface of the cavity 210eh, the lower the Q value of the ultrasonic waves radiated from the acoustic MEMS element 100. Therefore, by arranging a plurality of acoustic MEMS elements 100 so that the acoustic paths of the acoustic MEMS elements 100 that radiate ultrasonic waves at frequencies deviated from the desired frequency are located near the wall surface of the cavity 210eh, it is possible to achieve stable sound pressure frequency characteristics in which a sound pressure peak appears at the desired frequency, and then monotonically increases to the sound pressure peak as the frequency changes, and then monotonically decreases from the sound pressure peak.

[0073] The arrangement of the multiple acoustic MEMS elements 100 is not limited to the above-described case in which the acoustic paths of the acoustic MEMS elements 100 that radiate ultrasonic waves at frequencies deviated from the desired frequency are positioned near the wall surfaces of the cavity 210eh. Even if multiple acoustic MEMS elements 100 are arranged without regard to variations in the frequencies of the ultrasonic waves radiated by each of them, the Q value of the ultrasonic waves radiated from each acoustic MEMS element 100 is reduced, and therefore the ultrasonic waves radiated from the ultrasonic transducer 200 are prevented from vibrating up and down in sound pressure as the frequency changes, thereby stabilizing the sound pressure frequency characteristics in the frequency band in which the ultrasonic transducer 200 is used.

[0074] As shown in Figure 11, in this embodiment, the dimension of the depth H2 of the cavity 210eh in the thickness direction (Z-axis direction) of the substrate 210 is greater than the dimension of the shortest distance H1 from the substrate 210 to the membrane portion 120, thereby enhancing the effect of reducing the Q value due to the viscous resistance of the wall surface of the cavity 210eh.

[0075] Furthermore, by making the substrate 210 from a material whose flow rate, as defined in JIS Z 8762, is half or less that of the single crystal Si constituting the substrate layer 15b, the effect of reducing the Q value due to the viscous resistance of the wall surface of the cavity 210eh can be further enhanced. Also, from the viewpoint of preventing the sound pressure of the radiated ultrasonic waves from becoming too low, it is preferable that the substrate 210 be made from a material whose flow rate, as defined in JIS Z 8762, is between 1 / 50 and 1 / 2 that of the single crystal Si.

[0076] An ultrasonic transducer according to a first modified example of embodiment 1 of the present invention will be described below. Fig. 17 is a cross-sectional view showing an ultrasonic transducer according to a first modified example of embodiment 1 of the present invention. As shown in Fig. 17, in an ultrasonic transducer 200a according to the first modified example of embodiment 1 of the present invention, the base 110 is provided only at the edge of the ultrasonic transducer 200a. That is, the support layer 15 located at the boundary between adjacent acoustic MEMS elements 100 has been removed. That is, the acoustic MEMS elements 100 located at the edge of the ultrasonic transducer 200a, which are part of the multiple acoustic MEMS elements 100, have bases 110 that include the support layer 15.

[0077] 18 is a graph showing the transition of sound pressure with the frequency of ultrasonic waves emitted from the acoustic MEMS element in the ultrasonic transducer according to the first modified example of embodiment 1. In Fig. 18, the vertical axis represents ultrasonic sound pressure (Pa) and the horizontal axis represents ultrasonic frequency (Hz). Furthermore, solid lines L1 to L5 respectively represent data for ultrasonic waves emitted from the acoustic MEMS elements 100 of samples 1 to 5 included in the ultrasonic transducer 200a according to the first modified example of embodiment 1, and solid line LT represents data for ultrasonic waves emitted from the ultrasonic transducer 200a as a combination of the ultrasonic waves emitted from the acoustic MEMS elements 100 of samples 1 to 5.

[0078] 18 , in the ultrasonic transducer 200a according to the first modified example of the first embodiment, the Q value of the ultrasonic waves radiated from each of the acoustic MEMS elements 100 of samples 1 to 5 decreases due to viscous resistance from the wall surface of the cavity 210eh in the acoustic path P. The closer the acoustic path is located to the wall surface of the cavity 210eh, the lower the Q value of the ultrasonic waves radiated from the acoustic MEMS element 100. In the example shown in FIG. 18 , the acoustic paths P of the acoustic MEMS elements 100 of samples 1 and 5 are located closest to the wall surface of the cavity 210eh, and the acoustic path P of the acoustic MEMS element 100 of sample 3 is located farthest from the wall surface of the cavity 210eh.

[0079] 18, even in the ultrasonic transducer 200a according to the first modified example of embodiment 1, when the frequencies of the ultrasonic waves radiated from the acoustic MEMS elements 100 of samples 1 to 5 vary, the Q value of the ultrasonic waves radiated from each of the acoustic MEMS elements 100 of samples 1 to 5 is reduced, and therefore the ultrasonic waves L1 to L5 radiated from the acoustic MEMS elements 100 of samples 1 to 5 are combined to radiate the ultrasonic wave LT from the ultrasonic transducer 200a, and the sound pressure does not fluctuate up and down with changes in frequency, but rather has a stable sound pressure-frequency characteristic that monotonically increases to a sound pressure peak with changes in frequency and then monotonically decreases from the sound pressure peak. However, as shown in FIG. 14, the ultrasonic transducer 200 according to embodiment 1 is more preferable because it can effectively reduce the Q value and has a sound pressure-frequency characteristic that is a gentle curve with one peak.

[0080] An ultrasonic transducer according to a second modified example of the first embodiment of the present invention will be described below. FIG. 19 is a cross-sectional view showing an ultrasonic transducer according to the second modified example of the first embodiment of the present invention. As shown in FIG. 19 , in an ultrasonic transducer 200b according to the second modified example of the first embodiment of the present invention, a plurality of acoustic MEMS elements 100 are mounted on a laminated substrate. Specifically, the substrate 210 is bonded to a substrate 230 with an adhesive 240 on the main surface opposite to the main surface on which the acoustic MEMS elements 100 are mounted. A cavity 230eh is formed in the substrate 230 at a position corresponding to the cavity 210eh. The cavity 230eh is a hole penetrating the substrate 230. The cavity 230eh has a rectangular shape when viewed in the axial direction of the central axis C. However, the shape of the cavity 230eh may be circular, elliptical, or polygonal when viewed in the axial direction of the central axis C. In this modified example, the support layer 15 located on the edge of the ultrasonic transducer 200b is located on the substrate 210, and the remaining support layer 15 is located in a position overlapping with the cavity 210eh and the cavity 230eh between the acoustic MEMS elements 100 when viewed from the thickness direction (Z-axis direction) of the substrate 210.

[0081] 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 of the substrate is the sum of the thicknesses of the multiple substrates and the thickness of the adhesive.

[0082] The ultrasonic transducer 200b according to the second modification of the first embodiment can also stabilize the sound pressure frequency characteristics in the frequency band used.

[0083] (Embodiment 2) An ultrasonic transducer according to embodiment 2 of the present invention will be described below with reference to the drawings. Ultrasonic transducer 300 according to embodiment 2 of the present invention differs from ultrasonic transducer 200 according to embodiment 1 in that the cavity is a recess that does not penetrate the substrate, and therefore, description of the configuration that is similar to ultrasonic transducer 200 according to embodiment 1 will not be repeated.

[0084] 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 an ultrasonic transducer 300 according to embodiment 2 of the present invention, a cavity 210ec is a recess that does not penetrate through the substrate 210. In this embodiment, the support layer 15 located on the edge of the ultrasonic transducer 300 is located on the substrate 210, and the remaining support layer 15 is located in a position that overlaps with the cavity 210ech between the acoustic MEMS elements 100 when viewed in the thickness direction (Z-axis direction) of the substrate 210.

[0085] The ultrasonic transducer 300 according to the second embodiment of the present invention can also stabilize the sound pressure frequency characteristics in the frequency band being used. Because the cavity 210ec does not penetrate the substrate 210, it is possible to prevent foreign matter from entering the acoustic path P. Furthermore, because the cavity 210ec is closed by a recess, it is possible to prevent unnecessary leakage of the ultrasonic waves emitted from the acoustic MEMS element 100.

[0086] (Embodiment 3) An ultrasonic transducer according to embodiment 3 of the present invention will be described below with reference to the drawings. Ultrasonic transducer 400 according to embodiment 3 of the present invention differs from ultrasonic transducer 200 according to embodiment 1 in that multiple cavities are formed therein, and therefore, description of the configuration that is the same as ultrasonic transducer 200 according to embodiment 1 will not be repeated.

[0087] 21 is a cross-sectional view showing an ultrasonic transducer according to embodiment 3 of the present invention. As shown in FIG. 21, in an ultrasonic transducer 400 according to embodiment 3 of the present invention, a plurality of cavities 210eh are formed in a substrate 210.

[0088] The ultrasonic transducer 400 according to the third embodiment of the present invention can also stabilize the sound pressure frequency characteristics in the frequency band used. The multiple cavities 210eh allow the ultrasonic transducer 400 to be divided into multiple regions. At least two acoustic MEMS elements 100 out of the multiple MEMS elements 100 are located in each of the multiple regions. In the example shown in Fig. 21 , the ultrasonic transducer 400 is divided into a first region R1 and a second region R2. For example, if the frequency of the ultrasonic waves emitted from the acoustic MEMS element 100 having an acoustic path in the first region R1 is around 100 kHz and the frequency of the ultrasonic waves emitted from the acoustic MEMS element 100 having an acoustic path in the second region R2 is around 150 kHz, then in the first region R1, the Q value of the ultrasonic waves emitted from the acoustic MEMS element 100 having an acoustic path in the first region R1 can be reduced to stabilize the sound pressure frequency characteristics in the frequency band around 100 kHz, and in the second region R2, the Q value of the ultrasonic waves emitted from the acoustic MEMS element 100 having an acoustic path in the second region R2 can be reduced to stabilize the sound pressure frequency characteristics in the frequency band around 150 kHz. In this embodiment, the support layers 15 located on the edges of each of the multiple regions of the ultrasonic transducer 400 are located on the substrate 210, and the remaining support layers 15 are located in positions that overlap with the corresponding cavities 210eh between the acoustic MEMS elements 100 when viewed from the thickness direction (Z-axis direction) of the substrate 210.

[0089] As described above, in the ultrasonic transducer 400 according to the third embodiment of the present invention, it is possible to stabilize the sound pressure frequency characteristics in a plurality of frequency bands. Note that at least one cavity among the plurality of cavities 210eh may be a recess that does not penetrate the substrate 210 as in the second embodiment.

[0090] (Note) It will be understood by those skilled in the art that the exemplary embodiments described above are specific examples of the following aspects.

[0091] <1> An ultrasonic transducer comprising: a substrate; and a plurality of acoustic MEMS elements mounted on the substrate, wherein at least one cavity is formed in the substrate, and each cavity in the at least one cavity faces at least two acoustic MEMS elements of the plurality of acoustic MEMS elements.

[0092] <2> The ultrasonic transducer according to <1>, wherein the at least one cavity is a recess that does not penetrate the substrate.

[0093] <3> The ultrasonic transducer according to <1> or <2>, wherein a plurality of cavities are formed in the substrate, the substrate is divided into a plurality of regions by the plurality of cavities, and at least two acoustic MEMS elements of the plurality of acoustic MEMS elements are located in each of the plurality of regions.

[0094] <4> The ultrasonic transducer according to any one of <1> to <3>, wherein each of the plurality of acoustic MEMS elements includes a membrane portion, and a depth dimension of the at least one cavity in a thickness direction of the substrate is greater than a dimension of a shortest distance from the substrate to the membrane portion.

[0095] <5> The ultrasonic transducer according to any one of <1> to <4>, wherein the substrate is a laminated substrate.

[0096] <6> The ultrasonic transducer according to any one of <1> to <5>, wherein the substrate is made of a material whose flow rate, as defined in JIS Z 8762, is half or less that of single crystal silicon.

[0097] <7> The ultrasonic transducer according to any one of <1> to <6>, wherein at least some of the acoustic MEMS elements among the plurality of acoustic MEMS elements have a base including a support layer, the support layer includes a substrate layer, and at least a portion of the support layer is located on the substrate.

[0098] <8> The ultrasonic transducer according to <7>, wherein a portion of the support layer is located at a position overlapping the at least one cavity when viewed in the thickness direction of the substrate.

[0099] <9> The ultrasonic transducer according to <8>, wherein the portion of the support layer is located between the plurality of acoustic MEMS elements when viewed in the thickness direction of the substrate.

[0100] In the above-described embodiments, configurations that can be combined may be combined with each other.

[0101] 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.

[0102] 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, 200a, 200b, 300, 400, 900 ultrasonic transducer, 210, 230, 910 substrate, 210ec, 210eh, 230eh cavity, 220 die bond agent, 240 adhesive, P acoustic path, R1 first region, R2 second region, SL Slit.

Claims

1. An ultrasonic transducer comprising: a substrate; and a plurality of acoustic MEMS elements mounted on the substrate, wherein at least one cavity is formed in the substrate, and each cavity in the at least one cavity faces at least two acoustic MEMS elements of the plurality of acoustic MEMS elements.

2. The ultrasonic transducer of claim 1, wherein said at least one cavity is a recess that does not extend all the way through said substrate.

3. An ultrasonic transducer as described in claim 1 or claim 2, wherein a plurality of cavities are formed in the substrate, the substrate is divided into a plurality of regions by the plurality of cavities, and at least two acoustic MEMS elements of the plurality of acoustic MEMS elements are located in each of the plurality of regions.

4. An ultrasonic transducer according to any one of claims 1 to 3, wherein each of the plurality of acoustic MEMS elements includes a membrane portion, and the depth dimension of the at least one cavity in the thickness direction of the substrate is greater than the dimension of the shortest distance from the substrate to the membrane portion.

5. An ultrasonic transducer according to any one of claims 1 to 4, wherein the substrate is a laminated substrate.

6. An ultrasonic transducer according to any one of claims 1 to 5, wherein the substrate is made of a material whose flow rate as defined in JIS Z 8762 is half or less that of single crystal silicon.

7. An ultrasonic transducer as described in any one of claims 1 to 6, wherein at least some of the acoustic MEMS elements of the plurality of acoustic MEMS elements have a base including a support layer, the support layer includes a substrate layer, and at least a portion of the support layer is located on the substrate.

8. The ultrasonic transducer according to claim 7, wherein a portion of the support layer is located at a position overlapping the at least one cavity when viewed in the thickness direction of the substrate.

9. The ultrasonic transducer according to claim 8, wherein the portion of the support layer is located between the plurality of acoustic MEMS elements when viewed in the thickness direction of the substrate.

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