WO2025173302A1 - Ultrasonic transducer - Google Patents

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

The present invention relates to an ultrasonic transducer.

Prior art document disclosing the configuration of an ultrasonic transducer is International Publication No. 2020/230484 (Patent Document 1). The ultrasonic transducer described in Patent Document 1 comprises 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. The end of the through hole in the mounting substrate facing the piezoelectric device is located in a position facing the through hole in the substrate.

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

International Publication No. 2020/230484

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

In ultrasonic transducers with 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 within the frequency band in which the ultrasonic transducer is used, resulting in unstable sound pressure frequency characteristics.

The present invention was made in consideration of the above problems, and aims to provide an ultrasonic transducer that can stabilize sound pressure frequency characteristics in the frequency band used.

An ultrasonic transducer according to the present invention comprises 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.

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.

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. 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. 4 is a cross-sectional view showing a state in which a second electrode layer is provided on a piezoelectric single crystal substrate in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a first support portion is provided in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a stacked body is bonded to a first support portion in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a piezoelectric layer is formed by cutting a piezoelectric single crystal substrate in a manufacturing method of an acoustic MEMS element according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a first electrode layer is provided on a piezoelectric layer in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a groove and a recess are provided in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. FIG. 4 is a cross-sectional view showing a state in which a first connection electrode layer and a second electrode connection layer are provided in the manufacturing method of the acoustic MEMS element according to the first embodiment of the present invention. FIG. 1 is a plan view showing an ultrasonic transducer 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. FIG. 10 is a cross-sectional view showing an ultrasonic transducer according to a comparative example. 10 is a graph showing the change in sound pressure depending on the frequency of ultrasonic waves emitted from an acoustic MEMS element in an ultrasonic transducer according to a comparative example. 4 is a graph showing the change in sound pressure depending on the frequency of ultrasonic waves emitted from the acoustic MEMS element in the ultrasonic transducer according to the first embodiment. FIG. 2 is a diagram showing the positional relationship between a cavity and an acoustic MEMS element. 10 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. FIG. 2 is a cross-sectional view showing an ultrasonic transducer according to a first modified example of the first embodiment of the present invention. 10 is a graph showing the change in sound pressure depending on the frequency of ultrasonic waves emitted from an acoustic MEMS element in an ultrasonic transducer according to a first modified example of the first embodiment. FIG. 10 is a cross-sectional view showing an ultrasonic transducer according to a second modified example of the first embodiment of the present invention. FIG. 10 is a cross-sectional view showing an ultrasonic transducer according to a second embodiment of the present invention. FIG. 10 is a cross-sectional view showing an ultrasonic transducer according to a third embodiment of the present invention.

The ultrasonic transducer according to each embodiment of the present invention will be described below with reference to the drawings. In the following description of the embodiments, the same or equivalent parts in the drawings will be given the same reference numerals, and their description will not be repeated.

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

(Embodiment 1)
Fig. 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.

As shown in Figures 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 generates and/or detects vibrations using a piezoelectric material. Note that the acoustic MEMS element 100 is not limited to being a piezoelectric element, and may be configured to generate and/or detect vibrations using an electrostatic method.

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 Figure 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.

As shown in FIG. 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 that portion.

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.

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.

The piezoelectric layer 11 is composed of a single crystal piezoelectric material. The cut orientation of the piezoelectric layer 11 is appropriately selected to achieve the 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.

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 ).

As shown in FIG. 2, the first electrode layer 12 is disposed above the piezoelectric layer 11. The second electrode layer 13 is disposed below the piezoelectric layer 11 so as to face at least a portion of the first electrode layer 12 across the piezoelectric layer 11. 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.

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 (LiNbO ₃ ), 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.

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 adhesion layer is, for example, 0.005 μm or more and 0.05 μm or less.

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.

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

As shown in FIG. 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).

The thickness of each of the first connection electrode layer 20 and the second connection electrode layer 30 is, for example, 0.1 μm or more and 1.0 μm or less. The thickness of each of the adhesion layer connected to the first connection electrode layer 20 and the adhesion layer connected to the second connection electrode layer 30 is, for example, 0.005 μm or more and 0.1 μm or less.

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 also 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.

As shown in Figures 1 and 2, a slit SL is formed in the vibration layer 10 in a portion located inside the base 110 when viewed from the central axis direction (Z-axis direction). From the perspective of suppressing sound leakage, the width of the slit SL is preferably 10 μm or less. Furthermore, from the perspective 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.

As shown in FIG. 2, the membrane portion 120 is positioned so as to cover the opening 101. When the acoustic MEMS element 100 is not driven, the membrane portion 120 extends along an imaginary plane. The membrane portion 120 is a vibration portion that includes the piezoelectric layer 11.

The membrane portion 120 is configured to be able to vibrate when a voltage is applied to the piezoelectric layer 11. The piezoelectric layer 11 converts the vibrations acting on the membrane portion 120 into voltage, allowing the vibrations to be detected. Note that the membrane portion 120 is not limited to being configured to generate and detect vibrations using a piezoelectric method as described above, and may also be configured to generate and detect vibrations using an electrostatic method.

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

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 applied between the first electrode layer 12 and the second electrode layer 13, which 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.

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 sinusoidal voltage and the frequency of the sinusoidal voltage is close to the value of the resonance frequency, the amount of displacement when the membrane portion 120 is bent will be large.

When ultrasonic waves are detected using the acoustic MEMS element 100, the ultrasonic waves cause the medium surrounding the membrane portion 120 to vibrate, 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 charge in the piezoelectric layer 11. The charge induced in the piezoelectric layer 11 generates 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.

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 portion 120 flexurally vibrates increases. As this amount of displacement increases, the potential difference increases.

In this way, when the acoustic MEMS element 100 according to this embodiment is used as an ultrasonic transducer such as a non-contact haptic, 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.

The following describes a method for manufacturing an acoustic MEMS element 100 according to embodiment 1 of the present invention. Figure 3 is a cross-sectional view showing the state in which a second electrode layer has been provided on a piezoelectric single crystal substrate in the method for manufacturing an acoustic MEMS element according to embodiment 1 of the present invention. Figure 3 and Figures 4 to 9 shown below are illustrated using the same cross-sectional view as Figure 2.

As shown in FIG. 3, first, an adhesive layer (not shown) is provided on the underside 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 side. The second electrode layer 13 is formed to have a desired pattern using a vapor deposition lift-off method. The second electrode layer 13 may also be formed by laminating it over the entire underside of the piezoelectric single crystal substrate 11a by sputtering, and then forming the desired pattern using an etching method. The second electrode layer 13 and adhesive layer may also be formed by epitaxial growth.

Figure 4 is a cross-sectional view showing the state after the first support portion has been provided in the manufacturing method of the acoustic MEMS element according to embodiment 1 of the present invention. As shown in Figure 4, a first elastic layer 14a is provided on the underside 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, the portion of the underside 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. For this reason, the underside of the first elastic layer 14a is polished and flattened by chemical mechanical polishing (CMP) or the like.

FIG. 5 is a cross-sectional view showing the state in which a laminate has been bonded to a first support member in a manufacturing method for an acoustic MEMS element according to embodiment 1 of the present invention. As shown in FIG. 5, a laminate 16 consisting of a second elastic layer 14b and a support layer 15 is bonded to the lower surface of the first elastic layer 14a by surface activated bonding or atomic diffusion bonding. In this embodiment, the laminate 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 14b in advance by CMP or the like. Furthermore, if the second elastic layer 14b is made of low-resistivity Si, the second elastic layer 14b can function as a lower electrode layer, eliminating the need to form the second electrode layer 13 and first elastic layer 14a.

FIG. 6 is a cross-sectional view showing the state after a piezoelectric single crystal substrate has been ground to form a piezoelectric layer in the manufacturing method for an acoustic MEMS element according to embodiment 1 of the present invention. As shown in FIG. 6, the top surface of the piezoelectric single crystal substrate 11a is thinned by grinding it with a grinder. The top 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.

It is also possible to form a release layer on the upper surface of the piezoelectric single crystal substrate 11a by implanting ions in advance, and then peeling off the release layer to form the piezoelectric single crystal substrate 11a into the piezoelectric layer 11. Furthermore, 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.

FIG. 7 is a cross-sectional view showing the state in which a first electrode layer is provided on a piezoelectric layer in a manufacturing method for an acoustic MEMS element according to embodiment 1 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 adhesive layer may also be formed by epitaxial growth.

Figure 8 is a cross-sectional view showing the state in which grooves and recesses have been formed in the manufacturing method of the acoustic MEMS element according to embodiment 1 of the present invention. As shown in Figure 8, in a region corresponding to the region inside the base 110 of the acoustic MEMS element 100 when viewed from the stacking direction, a slit is formed in the piezoelectric layer 11 and the first elastic layer 14a by dry etching using RIE (Reactive Ion Etching) 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 DRIE (Deep Reactive Ion Etching) so that the slit reaches the top surface of the support layer 15. This forms the groove 17 shown in Figure 8, which corresponds to the slit SL shown in Figures 1 and 2.

Furthermore, as shown in FIG. 8, in the portion corresponding to the base 110 of the acoustic MEMS element 100, the piezoelectric layer 11 is etched by the above-mentioned dry etching or wet etching so that a portion of the second electrode layer 13 is exposed. This forms a recess 18.

FIG. 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 embodiment 1 of the present invention. As shown in FIG. 9, in the portion corresponding to the base 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.

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 formed and a membrane portion 120 is formed, as shown in FIG. 2. Through the above process, an acoustic MEMS element 100 according to embodiment 1 of the present invention, as shown in FIGS. 1 and 2, is manufactured.

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, the ultrasonic transducer 200 according to embodiment 1 of the present invention comprises multiple acoustic MEMS elements 100 and a substrate 210.

Specifically, multiple acoustic MEMS elements 100 are mounted on a substrate 210. In this embodiment, multiple acoustic MEMS elements 100 are arranged in an array on the substrate 210. The multiple acoustic MEMS elements 100 are arranged adjacent to one another in a matrix. The frequencies of the ultrasonic waves emitted from each of the multiple 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.

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 a corresponding multiple acoustic MEMS elements 100. 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 in the thickness direction (Z-axis direction) of the substrate 210.

The cavity 210eh is a hole that penetrates the substrate 210. The cavity 210eh is rectangular when viewed in the axial direction of the central axis C. However, the shape of the cavity 210eh may be circular, elliptical, or polygonal 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 leads to the acoustic MEMS element 100. The acoustic path P is an area that faces the membrane portion 120.

In the thickness direction (Z-axis direction) of the substrate 210, the depth H2 of the cavity 210eh is greater than the shortest distance H1 from the substrate 210 to the membrane portion 120.

Substrate 210 is preferably made of a material whose flow rate, as specified 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 cavity 210eh, which will be described later. Specifically, substrate 210 is made of a material that combines glass fiber with a resin such as glass epoxy, low-temperature co-fired ceramics (LTCC), or ceramics made from alumina or the like. Substrate 210 may also be a flexible substrate made of copper foil and polyimide, or a composite substrate with a reinforcing plate attached.

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. On the other hand, as described above, the acoustic MEMS element 100 has a natural resonant frequency of the membrane portion 120.

Here, we will explain the mechanism by which the ultrasonic transducer 200 according to embodiment 1 of the present invention can stabilize the sound pressure frequency characteristics in the frequency band used.

Figure 12 is a cross-sectional view showing an ultrasonic transducer according to a comparative example. As shown in Figure 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 has been removed. Because no cavities are formed in the substrate 910 in the ultrasonic transducer 900, the acoustic resistance, which is the viscous resistance in the acoustic path, is small, and the Q value of the ultrasonic waves emitted from each acoustic MEMS element 100 is maintained with almost no reduction.

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

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. 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 oscillates up and down as the frequency changes, resulting in an unstable sound pressure frequency characteristic.

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

As shown in FIG. 14, in the ultrasonic transducer 200 according to embodiment 1, the Q value of the ultrasonic waves emitted 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 emitted 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.

As shown in Figure 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. 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. Instead of the sound pressure oscillating up and down with changes in frequency, the ultrasonic wave LT exhibits stable sound pressure frequency characteristics where the sound pressure monotonically increases to a sound pressure peak as the frequency changes, and then monotonically decreases from the sound pressure peak.

Here, the acoustic resistance of the ultrasonic transducer 200 according to embodiment 1 was calculated with reference to the viscous resistance calculation formula described in Non-Patent Document 1. Figure 15 is a diagram showing the positional relationship between the cavity and the acoustic MEMS element. In Figure 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).

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

The "..." part in the above equation for acoustic resistance R is a higher-order term that has almost no effect on the value of acoustic resistance R and does not need to be taken into consideration.

A in the above acoustic resistance R formula is expressed by the following formula:

B in the above acoustic resistance R formula is expressed by the following formula:

Figure 16 is a graph showing the progression of the Q value at each coordinate calculated based on the acoustic resistance R formula, where a = 1 cm and d = 0.5 mm. In Figure 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.

As shown in Figure 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 emitted from the acoustic MEMS element 100. Therefore, by arranging multiple acoustic MEMS elements 100 so that the acoustic paths of the acoustic MEMS elements 100 that emit ultrasonic waves at frequencies deviated from the desired frequency are located near the wall surface of the cavity 210eh, it is possible to achieve a stable sound pressure frequency characteristic 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.

The arrangement of multiple acoustic MEMS elements 100 is not limited to the above-described arrangement in which the acoustic path of the acoustic MEMS elements 100 that emit ultrasonic waves at a frequency deviated from the desired frequency is located near the wall surface of the cavity 210eh. Even if multiple acoustic MEMS elements 100 are arranged without regard to variations in the frequency of the ultrasonic waves emitted by each element, the Q value of the ultrasonic waves emitted from each acoustic MEMS element 100 is reduced, and the ultrasonic waves emitted 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.

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.

Furthermore, by making the substrate 210 from a material whose flow rate, as specified in JIS Z 8762, is half or less that of the single crystal Si that makes up 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. Furthermore, from the perspective of preventing the sound pressure of the emitted ultrasound from becoming too low, it is preferable that the substrate 210 be made from a material whose flow rate, as specified in JIS Z 8762, is between 1/50 and 1/2 that of single crystal Si.

The following describes an ultrasonic transducer according to a first modified example of embodiment 1 of the present invention. Figure 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 Figure 17, in the 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. In other words, the support layer 15 located at the boundary between adjacent acoustic MEMS elements 100 has been removed. In other words, the acoustic MEMS element 100 located at the edge of the ultrasonic transducer 200a, which is one of the multiple acoustic MEMS elements 100, has a base 110 that includes the support layer 15.

Figure 18 is a graph showing the change in sound pressure over time depending on the frequency of the ultrasound waves emitted from the acoustic MEMS element in the ultrasonic transducer according to the first modified example of embodiment 1. In Figure 18, the vertical axis represents the ultrasound sound pressure (Pa) and the horizontal axis represents the ultrasound frequency (Hz). Furthermore, solid lines L1 to L5 respectively represent data on the ultrasound 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 on the ultrasound waves emitted from the ultrasonic transducer 200a as a combination of the ultrasound waves emitted from the acoustic MEMS elements 100 of samples 1 to 5.

As shown in FIG. 18, in the ultrasonic transducer 200a according to the first modified example of embodiment 1, 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 becomes. 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.

As shown in FIG. 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. As a result, 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 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.

The following describes an ultrasonic transducer according to a second modified example of embodiment 1 of the present invention. Figure 19 is a cross-sectional view showing an ultrasonic transducer according to a second modified example of embodiment 1 of the present invention. As shown in Figure 19, in ultrasonic transducer 200b according to the second modified example of embodiment 1 of the present invention, multiple acoustic MEMS elements 100 are mounted on a laminated substrate. Specifically, substrate 210 is joined to substrate 230 by adhesive 240 on the main surface opposite to the main surface on which acoustic MEMS elements 100 are mounted. Cavity 230eh is formed in substrate 230 at a position corresponding to cavity 210eh. Cavity 230eh is a hole that penetrates substrate 230. Cavity 230eh is rectangular when viewed in the axial direction of central axis C. However, the shape of cavity 230eh may be circular, elliptical, or polygonal when viewed in the axial direction of 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 between the acoustic MEMS elements 100 in a position that overlaps with the cavity 210eh and the cavity 230eh when viewed in the thickness direction (Z-axis direction) of the substrate 210.

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.

The ultrasonic transducer 200b according to the second modification of embodiment 1 can also stabilize the sound pressure frequency characteristics in the frequency band used.

(Embodiment 2)
An ultrasonic transducer according to a second embodiment of the present invention will be described below with reference to the drawings. An ultrasonic transducer 300 according to the second embodiment of the present invention differs from the ultrasonic transducer 200 according to the first embodiment in that the cavity is a recess that does not penetrate the substrate. Therefore, the description of the same configuration as the ultrasonic transducer 200 according to the first embodiment will not be repeated.

Figure 20 is a cross-sectional view showing an ultrasonic transducer according to embodiment 2 of the present invention. As shown in Figure 20, in the ultrasonic transducer 300 according to embodiment 2 of the present invention, the cavity 210ec is a recess that does not penetrate 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.

The ultrasonic transducer 300 according to embodiment 2 of the present invention also stabilizes 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 ultrasonic waves emitted from the acoustic MEMS element 100.

(Embodiment 3)
An ultrasonic transducer according to a third embodiment of the present invention will be described below with reference to the drawings. The ultrasonic transducer 400 according to the third embodiment of the present invention differs from the ultrasonic transducer 200 according to the first embodiment in that a plurality of cavities are formed therein. Therefore, the description of the same configuration as the ultrasonic transducer 200 according to the first embodiment will not be repeated.

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

The ultrasonic transducer 400 according to embodiment 3 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 Figure 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 the corresponding cavities 210eh between the acoustic MEMS elements 100 when viewed in the thickness direction (Z-axis direction) of the substrate 210.

As described above, the ultrasonic transducer 400 according to embodiment 3 of the present invention can stabilize sound pressure frequency characteristics across multiple frequency bands. Note that at least one of the multiple cavities 210eh may be a recess that does not penetrate the substrate 210, as in embodiment 2.

(Addendum)
It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

<1>
A substrate;
a plurality of acoustic MEMS elements mounted on the substrate;
at least one cavity formed in the substrate;
An ultrasonic transducer, wherein 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 according to <1>, wherein the at least one cavity is a recess that does not penetrate the substrate.

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

＜4＞
each of the plurality of acoustic MEMS elements includes a membrane portion;
An ultrasonic transducer according to any one of <1> to <3>, wherein 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>
The ultrasonic transducer according to any one of <1> to <4>, wherein the substrate is a laminated substrate.

<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.

＜７＞
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;
The ultrasonic transducer according to any one of <1> to <6>, wherein at least a portion of the support layer is located on the substrate.

＜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.

＜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.

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

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

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 portion, 20 first connection electrode layer, 30 second connection electrode layer, 100 acoustic MEMS element, 1 01 Opening, 110 Base, 120 Membrane, 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 (9)

A substrate;
a plurality of acoustic MEMS elements mounted on the substrate;
at least one cavity formed in the substrate;
An ultrasonic transducer, wherein each cavity in the at least one cavity faces at least two acoustic MEMS elements of the plurality of acoustic MEMS elements. The ultrasonic transducer of claim 1, wherein the at least one cavity is a recess that does not extend all the way through the substrate. a plurality of cavities formed in the substrate;
The cavity is divided into a plurality of regions,
3. The ultrasonic transducer according to claim 1, wherein at least two acoustic MEMS elements of the plurality of acoustic MEMS elements are located in each of the plurality of regions. each of the plurality of acoustic MEMS elements includes a membrane portion;
4. The ultrasonic transducer according to claim 1, wherein 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. An ultrasonic transducer as described in any one of claims 1 to 4, wherein the substrate is a laminated substrate. An ultrasonic transducer as described in any one of claims 1 to 5, wherein the substrate is made of a material whose flow rate as specified in JIS Z 8762 is half or less that of single crystal silicon. 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;
The ultrasonic transducer according to claim 1 , wherein at least a portion of the support layer is located on the substrate. An ultrasonic transducer as described in claim 7, wherein a portion of the support layer is located in a position overlapping with at least one cavity when viewed in the thickness direction of the substrate. An ultrasonic transducer as described in claim 8, wherein the portion of the support layer is located between the multiple acoustic MEMS elements when viewed in the thickness direction of the substrate.