Acoustic transducer and method for controlling sound waves

Abstract

In an acoustic transducer with multiple acoustic elements arranged in a grid, the focusing performance of the acoustic beam width is improved. [Solution] The acoustic transducer 1 comprises a first acoustic element 10 and a second acoustic element 20, each having diaphragms 11 and 21 driven by a piezoelectric film 35, and electrodes 12 and 22 provided on the piezoelectric film 35 for applying an electrical signal to the piezoelectric film 35. When a common electrical signal is applied to the electrodes 12 and 22 of the first acoustic element 10 and the second acoustic element 20, stresses in different directions are generated in the diaphragms 11 and 21 of the first acoustic element 10 and the second acoustic element 20, thereby generating sound waves with different phases.

Description

Technical Field

[0001] The present invention relates to an acoustic transducer and a method for controlling sound waves.

Background Art

[0002] In recent years, piezoelectric micromachined ultrasonic transducers (pMUTs) in which a plurality of piezoelectric micro ultrasonic elements are arrayed have been known. A pMUT is also called a piezoelectric MEMS ultrasonic transducer. Arrayed piezoelectric micro acoustic transducers such as pMUTs are expected to have a wide range of applications, such as medical ultrasonic probes, non-destructive inspections, ultrasonic fingerprint and blood vessel sensors. Since the horizontal resolution in acoustic imaging is limited by the width of the acoustic beam, it is important to adjust the width of the acoustic beam.

[0003] Techniques for adjusting the acoustic beam width by mounting an acoustic lens are known. However, in a piezoelectric micro acoustic transducer in which a plurality of piezoelectric micro ultrasonic elements are integrated, after forming the diaphragm structure for each element, it is technically difficult to align and mount the acoustic lens with high precision, which increases the labor and cost.

[0004] In addition, an ultrasonic FZP applying the principle of a Fresnel zone plate (FZP) has also been reported in recent years. An FZP (also called an FZP lens) is an acoustic beam focusing element that utilizes the refraction phenomenon of waves. For example, a technique of an acoustic transducer that integrates the functions of a sound source and an FZP has been proposed (Non-Patent Document 1) by patterning drive electrodes in an annular shape on a plate of a uniform bulk piezoelectric film so as to be arranged according to the FZP theory and vibrating the piezoelectric film selectively in regions.

Prior Art Documents

Non-Patent Documents

[0005] [Non-Patent Document 1] S. Hosseini, K. Laursen, A. Rashidi, T. Mondal, B. Corbett and F. Moradi, "S-MRUT: Sectored-Multiring Ultrasonic Transducer for Selective Powering of Brain Implants," in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 68, no. 1, pp. 191-200, Jan. 2021. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In acoustic transducers that use multiple piezoelectric microacoustic elements or other acoustic elements arranged as sound sources, aligning external acoustic lenses is not easy, and applying FZP is also not easy, resulting in a problem of low acoustic beam width focusing performance.

[0007] In one aspect, the present invention aims to improve the focusing performance of the acoustic beam width in an acoustic transducer in which multiple acoustic elements are arranged. [Means for solving the problem]

[0008] In one aspect, the acoustic transducer comprises a first acoustic element and a second acoustic element, each having a diaphragm driven by a piezoelectric film and an electrode provided on the piezoelectric film for applying an electrical signal to the piezoelectric film. When a common electrical signal is applied to the electrodes of the first and second acoustic elements, stresses in different directions are generated in the diaphragms of the first and second acoustic elements, thereby generating sound waves with different phases.

[0009] In other aspects, the acoustic transducer has a plurality of acoustic elements, each having a diaphragm driven by a piezoelectric film and an electrode provided on the piezoelectric film to which an electrical signal is applied. A first region where the plurality of acoustic elements are arranged and a second region where the plurality of acoustic elements are not arranged alternately, and the boundary between the first region and the second region is defined in a top view from a reference point corresponding to the focal point of the acoustic beam focused by the acoustic transducer by the following equation (1) r n It will be installed in a separate location.

number

[0010] Furthermore, when the present invention is considered as a method for controlling sound waves, this control method involves applying a common electrical signal to the electrodes of a first acoustic element and a second acoustic element, each having a diaphragm driven by a piezoelectric film and an electrode provided on the piezoelectric film for applying an electrical signal to the piezoelectric film, thereby generating sound waves with different phases in the diaphragms of the first and second acoustic elements, respectively. [Effects of the Invention]

[0011] In one aspect, the present invention can improve the focusing performance of the acoustic beam width in an acoustic transducer in which multiple acoustic elements are arranged. [Brief explanation of the drawing]

[0012] [Figure 1] This is a top view showing an example of an acoustic transducer in the first embodiment. [Figure 2] This is an enlarged cross-sectional view of the acoustic transducer portion shown in Figure 1. [Figure 3] This is a magnified top view of the acoustic transducer portion shown in Figure 1. [Figure 4]It is a top view showing a first modification example in which the shapes of the first upper electrode and the second upper electrode of the acoustic transducer are changed. [Figure 5] It is a top view showing a second modification example in which the shapes of the first upper electrode and the second upper electrode of the acoustic transducer are changed. [Figure 6] It is a diagram schematically showing the change in the thickness of the piezoelectric film depending on the polarization direction of the piezoelectric film and the direction of the applied voltage. [Figure 7] It is a diagram schematically showing the difference in stress between the first acoustic element and the second acoustic element. [Figure 8] It is a diagram showing the sound pressure simulation results of the acoustic transducer in the first embodiment and the acoustic transducer in the comparative example. [Figure 9] (a) to (f) are diagrams showing the manufacturing method of the acoustic transducer in the first embodiment. [Figure 10] It is an image showing a manufacturing example of the acoustic transducer. [Figure 11] It is an enlarged image of a part of FIG. 10. [Figure 12] It is a diagram showing a sound pressure distribution measurement system. [Figure 13] It is a diagram showing the measurement results of the sound pressure distribution by the acoustic transducer of FIG. 10. [Figure 14] It is a top view showing an example of an acoustic transducer in which the first acoustic element and the second acoustic element are arranged in an annular shape. [Figure 15] It is a top view showing a first example of the acoustic transducer in the second embodiment. [Figure 16] It is a top view showing a second example of the acoustic transducer in the second embodiment. [Figure 17] It is a top view showing a third example of the acoustic transducer in the second embodiment. [Figure 18] It is a top view showing an example of the acoustic transducer in the third embodiment.

Embodiments for Carrying Out the Invention

[0013] Embodiments of the present invention will now be described with reference to the drawings. However, the embodiments described below are merely illustrative and are not intended to exclude various modifications or applications of techniques not explicitly stated below. For example, these embodiments can be implemented with various modifications without departing from their spirit. In the drawings used in the following description, parts denoted by the same reference numerals represent the same or similar parts unless otherwise specified.

[0014] [First Embodiment] The following describes a method for improving the focusing performance of the acoustic beam width, using the acoustic transducer in the first embodiment as an example.

[0015] Figure 1 is a top view showing an example of an acoustic transducer 1 in the first embodiment. The acoustic transducer 1 comprises a plurality of first acoustic elements 10 and a plurality of second acoustic elements 20.

[0016] Acoustic transducer 1 may be a pMUT (piezoelectric MEMS ultrasonic transducer). Acoustic transducer 1 is a device that combines ultrasonic transmission (speaker) and reception (microphone). Acoustic transducer 1 has multiple acoustic elements arranged in an array to increase the sound wave output. Furthermore, to increase the sound wave output, the elements of acoustic transducer 1 may be operated at the resonant frequency.

[0017] The first acoustic element 10 is an example of a first acoustic element, and the second acoustic element 20 is an example of a second acoustic element. The first acoustic element 10 and the second acoustic element 20 may be piezoelectric microacoustic elements. The wavelength range of the acoustic waves emitted by the first acoustic element 10 and the second acoustic element 20 may be ultrasonic.

[0018] Figure 2 is a cross-sectional view along the PP' line, showing an enlarged portion of the acoustic transducer 1 in Figure 1. Figure 3 is an enlarged view of portion S of the acoustic transducer 1 in Figure 1.

[0019] As shown in Figure 2, of the main surfaces of the piezoelectric film 35 (also called the piezoelectric transducer thin film), the surface on which the first upper electrode 12 and the second upper electrode 22 are formed is defined as the upper surface 4, and the other main surface of the piezoelectric film 35 is defined as the lower surface 5. The direction perpendicular to the upper surface 4 of the piezoelectric film 35 is defined as the Z-axis direction. In the in-plane direction of the upper surface 4 of the piezoelectric film 35, the mutually orthogonal directions are defined as the X-axis direction and the Y-axis direction. "Top view" means viewing the upper surface 4 along the Z-axis direction.

[0020] In Figure 1, multiple first acoustic elements 10 and multiple second acoustic elements 20 are arranged in an array that forms a rectangular region in rows (X direction) and columns (Y direction). In Figure 1, elements shown as circles are first acoustic elements 10, and elements shown as double circles are second acoustic elements 20. In Figure 1, the Y direction may be the short axis direction of the array of acoustic transducers 1, and the X direction may be the long axis direction.

[0021] In Figure 1, the first acoustic elements 10 and second acoustic elements 20, arranged in the Y direction, are connected to each other via wiring 2, with their electrodes (i.e., the first upper electrode 12 and second upper electrode 22 in Figure 2) being connected to electrode pads 3-N (where N is an integer; hereafter, these may be collectively referred to as electrode pads 3). Electrode pads 3-1, 3-2, ..., 3-N are provided in each row (Y direction) of the array of acoustic transducer 1. An electrical signal (i.e., voltage) common to the first upper electrode 12 and second upper electrode 22 of the elements in each row is applied from the electrode pad 3 in each row. Additionally, the lower electrodes 30 of each first acoustic element 10 and second acoustic element 20 are provided on the upper surface 4.

[0022] Multiple acoustic elements of the same type may be arranged in a row (X direction). That is, multiple first acoustic elements 10 may be arranged in one row direction, and multiple second acoustic elements 20 may be arranged in other row directions.

[0023] The upper surface 4 is divided into multiple regions Z1, Z2, Z3, Z4, and Z5 depending on their position in the Y direction. The number of regions is not limited to this case. Regions Z1, Z3, and Z5 are examples of first regions where the first acoustic element 10 is placed, and regions Z2 and Z4 are examples of second regions where the second acoustic element 20 is placed. Each region Z1 to Z5 may contain multiple first acoustic elements 10 or multiple second acoustic elements 20. Furthermore, in Figure 1, each region Z1 to Z5 is arranged symmetrically in the Y direction with respect to a reference point O corresponding to the focal point of the acoustic beam in a top view (i.e., vertically symmetrical in the plane of the paper).

[0024] In the Y-axis direction, the first and second regions are arranged alternately. The boundary between the first and second regions is located at positions r1, r2, r3, and r4 away from the reference point O corresponding to the focal point of the acoustic beam, as specified by equation (1) in a top view. As a result, each Fresnel band is composed of an array of multiple piezoelectric microacoustic elements, and the whole system functions as an FZP. That is, by distributing multiple microacoustic element sound sources according to FZP theory, the FZP and sound sources can be used interchangeably.

[0025]

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[0026] In the Y-axis, which is the short axis direction, the FZP focuses the acoustic beam. In the X-axis direction, which is the long axis direction, the acoustic beam may be focused by phased array technology, which changes the focal length by adjusting the phase of the voltage applied to each row (i.e., the electrodes that provide a potential signal to the linear array). In this case, the phase of the voltage applied to electrode pads 3-1, 3-2, ... 3-N corresponding to each row is adjusted. The phased array technology is the same as conventional methods, so a detailed explanation is omitted.

[0027] Furthermore, if the acoustic transducer 1 is a device that focuses the acoustic beam uniaxially rather than focusing it to a single point, then the phase of the voltage applied to each column does not need to be adjusted.

[0028] As shown in Figure 2, the first acoustic element 10 and the second acoustic element 20 may be provided adjacent to each other. In this embodiment, a silicon on insulator (SOI) substrate is used, in which a silicon oxide layer 32 is provided on a silicon substrate 31, and a silicon layer 33 is provided on the silicon oxide layer 32. A lower electrode layer 34 may be provided on the silicon layer 33. The lower electrode layer 34 may be formed of a conductive material such as metal. However, the silicon layer 33 may be used as the lower electrode layer 34. In this case, a separate lower electrode layer 34 is omitted. A piezoelectric film 35 is provided on the lower electrode layer 34. The lower electrode 30 is electrically connected to the lower electrode layer 34.

[0029] The piezoelectric film 35 is a piezoelectric material that converts stress into electricity and vice versa. Piezoelectric materials include PZT(Pb(Zr,Ti)O3), PbTiO3, PbNb2O6, BaTiO3, and BiTiO3. 12 ,PMN-PT(Pb(Mg 1 / 3 Nb 2 / 3 ))O3-PbTiO3,PZN-PT(Pb(Zn 1 / 3 Nb 2 / 3 The piezoelectric film 35 may contain at least one material selected from the group consisting of O3-PbTiO3, quartz, LiNbO3, LiTiO3, (Bi,Na)TiO3, KNN(K,Na)NbO3, ZnO, AlN, ScAlN, PVDF (polyvinylidene fluoride), and PLA (polylactic acid). Specifically, the piezoelectric film 35 may be one of these materials, a mixed material of two or more of these materials, or a material in which other additives are added to one of these materials or a mixed material. In this embodiment, the piezoelectric film 35 is formed of AlN.

[0030] The thickness of the piezoelectric film 35 may be between 100 nm and 10 μm. When the piezoelectric film 35 is formed by deposition, the thickness of the piezoelectric film 35 may be between 1 μm and 3 μm from the viewpoint of suppressing the deposition time from being long. However, a method of thinning the film by attaching the bulk material to the substrate and polishing it can be employed, in which case a piezoelectric film 35 of 3 μm or more can be formed. The silicon oxide layer 32 and the silicon layer 33 function as elastic plates. The material of the elastic plate is not limited to the silicon oxide layer and silicon, but may be other inorganic or organic materials. The thickness of the lower electrode layer 34 (or the silicon layer 33 may also serve as the lower electrode layer 34) may be between 10 nm and 1 μm.

[0031] The first acoustic element 10 has a first upper electrode 12. In the region where the first acoustic element 10 is provided, the silicon substrate 31 has a first recess 13 below the first upper electrode 12. The silicon oxide layer 32, silicon layer 33, lower electrode layer 34, and piezoelectric film 35 in the region corresponding to the first recess 13 constitute the first diaphragm 11.

[0032] The second acoustic element 20 has a second upper electrode 22. In the region where the second acoustic element 20 is provided, the silicon substrate 31 has a second recess 23 below the second upper electrode 22. The silicon oxide layer 32, silicon layer 33, lower electrode layer 34, and piezoelectric film 35 in the region corresponding to the second recess 23 constitute the second diaphragm 21.

[0033] The thickness of the elastic plate (i.e., the sum of the thicknesses of the silicon oxide layer 32 and the silicon layer 33) may be between 100 nm and 10 μm. The thickness of the elastic plate may be determined such that the stress neutral axis when the first diaphragm 11 and the second diaphragm 21 are bent is on the elastic plate side with respect to the boundary between the elastic plate and the piezoelectric film 35 (the lower electrode layer 34, or the silicon layer 33 if there is no lower electrode layer 34) (conversely, the stress neutral axis is not on the piezoelectric film 35 side). The stress neutral axis is determined by the balance between the thickness and hardness of the piezoelectric film 35 (piezoelectric body) and the thickness and hardness of the elastic plate, ignoring the thin lower electrode layer 34.

[0034] Note that the first upper electrode 12 and the second upper electrode 22 are electrically connected to each other by wiring 2, as shown in Figure 1, but wiring 2 is omitted from the illustration in Figure 2. A common electrical signal is applied to the first upper electrode 12 and the second upper electrode 22.

[0035] The thickness of the first upper electrode 12 and the second upper electrode 22 may be between 10 nm and 1 μm.

[0036] As shown in Figures 1 to 3, the first upper electrode 12 and the second upper electrode 22 have different shapes. Due to these different shapes, the first diaphragm 11 and the second diaphragm 21 experience stress, or in other words, deformation force, in opposite directions.

[0037] Specifically, in a top view, the second upper electrode 22 has an opening 24, while the first upper electrode 12 does not have an opening 24. The second upper electrode 22 is an example of a first electrode having an opening 24, and the first upper electrode 12 is an example of a second electrode not having an opening 24.

[0038] For example, the diameter of the first diaphragm 11 corresponds to the diameter d1 of the first recess 13 and may be φ10 μm or more and 2 mm. For example, the diameter of the second diaphragm 21 corresponds to the diameter d2 of the second recess 23 and may be φ10 μm or more and 2 mm. Basically, the acoustic transducer 1 is driven resonantly to increase the amplitude of sound waves, and the diameters of the first diaphragm 11 and the second diaphragm 21 are related to the resonant frequency. Therefore, once the resonant frequency of the sound wave used (e.g., ultrasound) is determined, the sizes of the first diaphragm 11 and the second diaphragm 21 are determined to match the determined resonant frequency. If the first diaphragm 11 and the second diaphragm 21 are made larger, they become softer, so the resonant frequency is lowered and the amplitude can be increased. However, if the first diaphragm 11 and the second diaphragm 21 are made larger, they will interfere with adjacent acoustic elements when adjacent acoustic elements are arranged in an array, so there is a practical limit to the size of the first diaphragm 11 and the second diaphragm 21.

[0039] The diameter d3 of the first upper electrode 12 is smaller than the diameter of the first diaphragm 11. The diameter d4 of the second upper electrode 22 may be 0.9·d2 or greater and less than d2. The diameter d5 of the opening 24 of the second upper electrode 22 is smaller than the diameter d4 of the second upper electrode 22.

[0040] The distance between adjacent acoustic elements (i.e., the first acoustic element 10 or the second acoustic element 20) (i.e., the pitch, which is the distance between the centers of adjacent diaphragms) is preferably narrower than the wavelength of the sound wave λ / 2. This suppresses the generation of unwanted sound wave beams (e.g., ultrasonic beams) such as grating lobes or side lobes. However, the distance is not limited to less than λ / 2, depending on the application and other factors.

[0041] The shapes of the first upper electrode 12 and the second upper electrode 22 are not limited to those shown in Figures 1 to 3. Figure 4 is a top view showing a first modified example in which the shapes of the first upper electrode 12b and the second upper electrode 22b of the acoustic transducer 1b are changed. Figure 5 is a top view showing a second modified example in which the shapes of the first upper electrode 12c and the second upper electrode 22c of the acoustic transducer 1c are changed.

[0042] As shown in Figure 3, in a top view, the first upper electrode 12 may be disc-shaped. The second upper electrode 22 may be annular (donut-shaped). As shown in Figure 4, the first upper electrode 12b of the first acoustic element 10b of the acoustic transducer 1b may be rectangular in shape in a top view. The second upper electrode 22b of the second acoustic element 20b may have a rectangular opening 24b formed therein, giving it a rectangular ring shape. As shown in Figure 5, the first upper electrode 12c of the first acoustic element 10c of the acoustic transducer 1c may have a polygonal shape in a top view, for example, a hexagonal shape. The second upper electrode 22c of the second acoustic element 20c may have a polygonal shape in a top view, for example, a hexagonal opening 24c formed therein, giving it a polygonal ring shape. In Figure 4, reference numeral 11b indicates the first diaphragm in the first acoustic element 10b, and reference numeral 21b indicates the second diaphragm in the second acoustic element 20b. In Figure 5, reference numeral 11c indicates the first diaphragm in the first acoustic element 10c, and reference numeral 21c indicates the second diaphragm in the second acoustic element 20c.

[0043] However, the shapes of the first upper electrode 12 and the second upper electrode 22 are not limited to those shown in Figures 3 to 5. For example, the shape of the second upper electrode 22 may be a ring shape with a notch in part and divided in the circumferential direction (a C-shape).

[0044] Figure 6 schematically shows the change in the thickness of the piezoelectric film 35 depending on the polarization direction of the piezoelectric film 35 and the direction of the applied voltage. The piezoelectric film 35 has a polarization direction. In Figure 6, the direction is from negative charge to positive charge (i.e., the positive side is the tip of the arrow). Positive polarization is dielectric polarization that occurs in the piezoelectric film 35 without the action of an external electric field. The polarization direction is often the direction of the thickness of the piezoelectric film 35 (Z axis direction). Depending on the film deposition state, the polarization direction may be from the lower electrode layer 34 to the first upper electrode 12 (second upper electrode 22) (+Z direction), or it may be the opposite (-Z direction). Figure 6 illustrates the case where the polarization direction is the +Z direction. Due to the polarization, a positive charge is generated on one of the upper surface 4 and lower surface 5 of the piezoelectric film 35, and a negative charge is generated on the other.

[0045] In this state, for example, a voltage (i.e., an electrical signal) is applied between the first upper electrode 12 and the lower electrode 30 (i.e., the lower electrode layer 34). When a voltage is applied so that an electric field is generated in the same direction as the polarization direction, the piezoelectric film 35 becomes thinner in the Z direction and expands in the X and Y directions. When a voltage is applied so that an electric field is generated in a direction different from the polarization direction, the piezoelectric film 35 becomes thicker in the Z direction and contracts in the X and Y directions.

[0046] Figure 7 schematically shows the difference in stress between the first acoustic element 10 and the second acoustic element 20. As shown in Figure 7, consider the case where a voltage is applied so that an electric field is generated between the first upper electrode 12 and the second upper electrode 22 and the lower electrode layer 34 (lower electrode 30) in the same direction as the polarization direction. As a result, the piezoelectric film 35 becomes thinner in the Z direction and expands in the X and Y directions in the parts that are in contact with the first upper electrode 12 and the second upper electrode 22. Consequently, stress (i.e., deformation force) is generated in the first diaphragm 11 and the second diaphragm 21 in the parts that are in contact with the piezoelectric film 35, causing the piezoelectric film 35 side (upper surface 4 side in Figure 2) to become convex.

[0047] In the first acoustic element 10, the first upper electrode 12 is positioned at the center of the first diaphragm 11, so the first diaphragm 11 is driven so that the piezoelectric film 35 side is convex at the center of the first diaphragm 11. On the other hand, in the second acoustic element 20, the second upper electrode 22 is positioned at the outer edge of the second diaphragm 21, so the second diaphragm 21 is driven so that the piezoelectric film 35 side is convex at the outer edge of the second diaphragm 21, in other words, so that the silicon oxide layer 32 side is convex near the center of the second diaphragm 21.

[0048] Conversely, consider the case where a voltage is applied such that an electric field is generated between the first upper electrode 12 and the second upper electrode 22 and the lower electrode layer 34 (lower electrode 30) in a direction different from the polarization direction. As a result, the piezoelectric film 35 in the portion in contact with the first upper electrode 12 and the second upper electrode 22 becomes thicker in the Z direction and contracts in the X and Y directions. Consequently, stress (i.e., deformation force) is generated in the portion of the first diaphragm 11 and the second diaphragm 21 in contact with the piezoelectric film 35, causing the side opposite to the piezoelectric film 35 (the silicon oxide layer 32 side, or the bottom surface 5 side in Figure 7) to become convex.

[0049] In the first acoustic element 10, the first diaphragm 11 is driven so that the silicon oxide layer 32 side becomes convex at the center of the first diaphragm 11. On the other hand, in the second acoustic element 20, since the second upper electrode 22 is positioned on the outer edge of the second diaphragm 21, the second diaphragm 21 is driven so that the silicon oxide layer 32 side becomes convex at the outer edge of the second diaphragm 21, in other words, so that the piezoelectric film 35 side becomes convex near the center of the second diaphragm 21.

[0050] As described above, even though a common voltage is applied to the first acoustic element 10 and the second acoustic element 20, the difference in the shapes of the first upper electrode 12 and the second upper electrode 22 causes stress in the first diaphragm 11 and the second diaphragm 21 in opposite directions. As a result, the sound waves generated by the first diaphragm 11 and the sound waves generated by the second diaphragm 21 are in opposite phase. Specifically, the first acoustic element 10 and the second acoustic element 20 generate sound waves that are out of phase by π.

[0051] Thus, the first acoustic element 10 and the second acoustic element 20, which generate sound waves with opposite phases to each other, are distributed in a plurality of specific regions Z1, Z2, Z3, Z4, and Z5 as shown in Figure 1. The first regions where the plurality of first acoustic elements 10 are each located (Z1, Z3, and Z5 in Figure 1) correspond to the transmission regions of the FZP with respect to light, and the second regions where the plurality of second acoustic elements 20 are each located (Z2 and Z4 in Figure 1) correspond to the phase inversion regions of the FZP with respect to light.

[0052] Sound waves generated in the second region (i.e., the phase inversion region) are out of phase by π compared to sound waves generated in the first region (i.e., the transmission region). These sound waves interfere with each other, resulting in an acoustic beam focused at a set focal point. Furthermore, since each of the first and second regions is equipped with multiple piezoelectric microacoustic elements instead of a single bulk annular acoustic element, the sound pressure level can be improved through the superposition effect of sound waves.

[0053] The acoustic transducer 1 of this embodiment can realize a phase-type FZP that can utilize both sound waves from a plurality of first acoustic elements 10 arranged in a first region (e.g., Z1, Z3, and Z5) and sound waves from a plurality of second acoustic elements 20 arranged in a second region (e.g., Z2 and Z4). The acoustic transducer 1 of this embodiment may be named an FZP-pMUT array.

[0054] The reverse-phase drive due to the difference in shape between the first upper electrode 12 and the second upper electrode 22, as explained in Figure 7, was confirmed by finite element simulation. Commercial software (Femtet®, Murata Software Co., Ltd.) was used. The following material constants were used for the AlN piezoelectric film 35 used in the simulation: C is the stiffness constant, e is the dielectric constant, and ε is the piezoelectric constant.

[0055]

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[0056] The simulation model reproduced the structure of the upper part of the device's diaphragm. The drive voltage applied between the upper electrodes (first upper electrode 12 and second upper electrode 22) and the lower electrode 30 was set to 10V. The simulation results confirmed that the reverse-phase drive described in Figure 7 was achieved.

[0057] Next, we will describe the sound wave analysis of the ultrasonic beam emitted from the acoustic transducer 1. Figure 8 shows the sound pressure simulation results for the acoustic transducer 1 in the first embodiment and the acoustic transducer in the comparative example.

[0058] In the simulation shown in Figure 8, for simplification, only one row of the linear array in which the first acoustic element 10 and the second acoustic element 20 are connected in a row was analyzed, and the analysis was performed in two dimensions. The diameters of the first diaphragm 11 and the second diaphragm 21 were set to 60 μm, and the pitch between adjacent elements (i.e., the distance between the centers of the diaphragms) was set to 84 μm, so that the resonant frequency was 15 MHz. In Example (a-1), the total number of first acoustic elements 10 and second acoustic elements 20 was 29. Eight second acoustic elements 20 were provided as phase inversion driving elements at positions determined based on the principle of phase-type FZP (see equation (1) above). In Comparative Example (b-1), all 29 elements were first acoustic elements 10. Based on the specifications of the propagation medium used, the sound velocity of the ultrasonic beam propagation medium was set to 680 m / s. The focal length was set to 4.0 mm. The element driving frequency was set to 12 MHz.

[0059] In Figure 8, the horizontal axis represents the array direction (Y-axis direction), and the vertical axis represents sound pressure (arbitrary unit). The maximum sound pressure of acoustic beam 6 was between 3 and 4 times the maximum sound pressure of the acoustic beam in the comparative example. Furthermore, the full width at half maximum of acoustic beam 6 was reduced to about one-tenth, confirming an improvement in focusing performance.

[0060] Next, an example of a method for manufacturing the acoustic transducer 1 in the first embodiment will be described. Figure 9 is a diagram showing a method for manufacturing the acoustic transducer 1 in the first embodiment.

[0061] (1) A conductive film to form the lower electrode layer 34 was deposited on the prepared SOI substrate (silicon substrate 31, silicon oxide layer 32, silicon layer 33) as needed. Furthermore, AlN was deposited on the lower electrode layer 34 as a piezoelectric film 35 (piezoelectric transducer thin film). Photoresist 40 was applied and an opening 41 was formed by patterning [see Figure 9(a)].

[0062] (2) The lower electrode layer 34 was exposed by etching AlN. If the silicon layer 33 is low-resistance Si, the silicon layer 33 may be used as the lower electrode layer. In that case, the silicon layer 33 is exposed [see Figure 9(b)].

[0063] (2) Lift-off resists 42 and photoresist 43 were deposited to form the first upper electrode 12, the second upper electrode 22, and the lower electrode 30. Patterning formed an opening 44a for the lower electrode 30, an opening 44b for the first upper electrode 12, and an opening 44c for the second upper electrode 22 [see Figure 9(c)].

[0064] (4) Al was sputter-deposited as the conductive material. The first upper electrode 12, the second upper electrode 22, and the lower electrode 30 were formed by patterning using lift-off [see Figure 9(d)].

[0065] (5) After the back surface of the silicon substrate 31 was polished, photoresist 45 was applied to the back surface. Openings 46a and 46b were formed by patterning [see Figure 9(e)].

[0066] (6) The back surface of the silicon substrate 31 is deep-etched to form the first recess 13 and the second recess 23 in the silicon substrate 31 [see Figure 9(f)]. However, the method for manufacturing the acoustic transducer 1 in the first embodiment is not limited to that shown in Figure 9.

[0067] Figure 10 is an image showing a manufacturing example of the acoustic transducer 1, and Figure 11 is an enlarged image of section C in Figure 10. In the acoustic transducer 1 shown in Figures 10 and 11, a silicon layer 33, which is low-resistance Si, is used as the lower electrode layer, and a separate lower electrode layer 34 is omitted. Also, in Figure 10, the wiring 2 connecting the upper electrodes (first upper electrode 12, second upper electrode 22) in each row are electrically connected across multiple rows, so that the same voltage is applied to the upper electrodes in each row. In this way, the voltage applied to the upper electrodes was standardized in the manufacturing example created for the demonstration experiment.

[0068] The manufactured acoustic transducer 1 was subjected to operational testing in air using a laser Doppler vibrometer (MAS-500 Polyltec).

[0069] A 16MHz sine wave (3Vpp: peak-to-peak [V]) was applied to the first upper electrode 12 and the second upper electrode 22, and a two-dimensional map of the vibrations was obtained. The results confirmed that the first acoustic element 10 and the second acoustic element 20 were driven in opposite phases to each other. Therefore, the opposite-phase driving due to the difference in shape between the first upper electrode 12 and the second upper electrode 22, as explained in Figure 7, was confirmed in the actually manufactured acoustic transducer 1.

[0070] Next, the sound pressure distribution was measured using the acoustic transducer 1 shown in Figure 10. Figure 12 shows the sound pressure distribution measurement system 50. The acoustic transducer 1 shown in Figure 10 was fixed to a printed circuit board. The acoustic transducer 1 fixed to the printed circuit board was placed in a container 52. A liquid propagation medium 53 was placed in the container 52. The acoustic transducer 1 was immersed in the propagation medium 53. FLUORINERT® FC-70 was used as the propagation medium 53.

[0071] Next, the function generator 51 applied a 40Vpp, 12MHz square wave drive voltage to the acoustic transducer 1, causing the acoustic transducer 1 to generate an acoustic beam 6, specifically an ultrasonic beam. The tip of the hydrophone scope 54 (NH0200, Precision Acoustics) was positioned approximately 4mm above the top surface 4 of the acoustic transducer 1, and the hydrophone scope 54 scanned along the short axis direction (Y-axis direction) of the acoustic transducer 1. The signal received by the hydrophone scope 54 was amplified by the preamplifier 55 and the booster amplifier 56 and recorded by the oscilloscope 57. At this time, the peak-to-peak of the received voltage waveform measured by the hydrophone scope 54 was recorded and plotted on a graph.

[0072] Figure 13 shows the measurement results of the sound pressure distribution by the acoustic transducer 1 shown in Figure 10. The horizontal axis of Figure 13 represents the scanning position of the tip of the hydrophone scope 54. Position 0 corresponds to the focal position. The vertical axis represents the received voltage (in arbitrary units) received by the hydrophone scope 54. The dotted line represents the noise level, and the solid line represents the measurement result.

[0073] In Figure 13, the noise level was 0.7 mVpp. Measurements revealed that in an acoustic transducer 1, which includes a first acoustic element 10 and a second acoustic element 20, and where the second acoustic element 20 is positioned as a phase-inverting drive element based on the principle of phase-type FZP, a peak was observed in the center of the acoustic transducer 1.

[0074] Furthermore, in a transducer in which only the first acoustic element 10 was placed in all multiple regions Z1, Z2, Z3, Z4, and Z5, without including the second acoustic element 20, no peaks were observed. This is thought to be because the sound pressure of the acoustic beam 6 was weak and was buried in noise. In other words, achieving beam focusing with a linear array including the first acoustic element 10 and the second acoustic element 20, whose phases are inverted relative to each other, means that the sound pressure was increased to an observable level in this measurement system.

[0075] According to the acoustic transducer 1 of the first embodiment, even in an arrayed acoustic transducer in which a plurality of piezoelectric microacoustic elements, namely the first acoustic element 10 and the second acoustic element 20, are arranged as sound sources, the focusing performance of the acoustic beam width can be improved.

[0076] Figure 1 illustrates an acoustic transducer 1 in which the first acoustic element 10 and the second acoustic element 20 are arranged in a rectangular shape, but this embodiment is not limited to this case.

[0077] Figure 14 is a top view showing an example of an acoustic transducer 1d in which a first acoustic element 10 and a second acoustic element 20 are arranged concentrically (in other words, in a concentric ring). In Figure 14, the region of the top surface 4 is divided into multiple concentric regions Z1, Z2, Z3, Z4, and Z5 according to the radial distance from the reference point corresponding to the focal point of the acoustic beam 6. The number of regions is not limited to this case. Regions Z1, Z3, and Z5 are examples of first regions in which the first acoustic element 10 is arranged, and regions Z2 and Z4 are examples of second regions in which the second acoustic element 20 is arranged. Each region Z1 to Z5 may contain multiple first acoustic elements 10 or multiple second acoustic elements 20.

[0078] In the radial direction, the first and second regions are arranged alternately. The boundary between the first and second regions is located at positions r1, r2, r3, and r4 away from the reference point O corresponding to the focal point of the acoustic beam, as specified by equation (1) above, when viewed from above. As a result, each Fresnel band is composed of an array of multiple piezoelectric microacoustic elements, and the whole system functions as an FZP. That is, by distributing multiple microacoustic element sound sources according to FZP theory, the FZP and sound sources can be used interchangeably.

number

[0079] As described above, as shown in Figures 1 to 14, in the acoustic transducer 1 of the first embodiment, instead of a single bulk annular acoustic element, multiple first acoustic elements 10 and second acoustic elements 20 are provided as piezoelectric microacoustic elements in the first and second regions of the FZP, respectively, so that the generated sound pressure level can be improved.

[0080] In particular, by distributing the first acoustic element 10 and the second acoustic element 20 in multiple regions Z1, Z2, Z3, Z4, and Z5, the increase in the region where elements are not placed (shielded region) can be suppressed, thereby improving the generated sound pressure level.

[0081] Furthermore, even when the first acoustic element 10 and the second acoustic element 20 are arranged in a rectangular shape with the X-axis as the long axis and the Y-axis as the short axis, it is not necessary to apply phased array technology to the Y-axis direction, for example. In other words, it is not necessary to use an active matrix device instead of a linear array. As a result, the complexity of the device structure and wiring can be suppressed, and the bloat of the control system can be prevented.

[0082] This eliminates the need to separately manufacture external acoustic lenses and external FZPs, and also eliminates the need for alignment.

[0083] Multiple first acoustic elements 10 and multiple second acoustic elements 20 constituting a linear array corresponding to one row can be connected by applying a common voltage, i.e., a common electrical signal. The difference in shape between the first upper electrode 12 and the second upper electrode 22 makes it possible to invert their phases. Therefore, the burden of separate electrical control required to invert the phases can be reduced.

[0084] The difference in shape between the first upper electrode 12 and the second upper electrode 22 is a simple difference of whether or not there is an opening, so it can be easily manufactured by patterning.

[0085] [Second Embodiment] In the first embodiment, a configuration is described in which multiple first acoustic elements 10 and multiple second acoustic elements 20 are alternately arranged in multiple regions Z1, Z2, Z3, Z4, and Z5, thereby realizing the functions of a phase-type FZP and a sound source. A phase-type FZP is desirable to increase the sound pressure of the output acoustic beam, but depending on the requirements, it is also possible to realize the functions of an amplitude-type FZP instead of a phase-type FZP.

[0086] Figure 15 is a top view showing a first example of an acoustic transducer 1e in a second embodiment. In the acoustic transducer 1e of Figure 15, the upper surface 4 of the piezoelectric film 35 is divided into multiple regions Z1, Z2, Z3, Z4, and Z5. In regions Z1, Z3, and Z5, multiple first acoustic elements 10 are arranged in each region, as in the case of Figure 1. On the other hand, unlike in the case of Figure 1, neither the first acoustic elements 10 nor the second acoustic elements 20 are arranged in regions Z2 and Z4.

[0087] Regions Z1, Z3, and Z5 are examples of first regions where multiple acoustic elements are arranged, while regions Z2 and Z4 are examples of second regions where neither the first acoustic element 10 nor the second acoustic element 20 is arranged. In other words, the second regions correspond to the shielding regions in the FZP for light. In the second regions, dummy electrodes (not shown) that are not connected to wiring 2 may be arranged. In Figure 15, each region Z1 to Z5 is arranged symmetrically in the Y direction with respect to the reference point O corresponding to the focal point of the acoustic beam in a top view (i.e., vertically symmetrical in the plane of the paper).

[0088] In amplitude-type FZP, a first region (a transmission region, so to speak) and a second region (a shielding region, so to speak) are provided concentrically and alternately. The width between these regions is designed so that sound waves generated from the first region interfere with each other and converge at a set focal point.

[0089] The first and second regions are arranged alternately. The boundary between the first and second regions is located at positions r1, r2, r3, and r4 away from the reference point O corresponding to the focal point of the acoustic beam, as specified by equation (1) above, when viewed from above. As a result, the acoustic transducer 1 functions as an amplitude-type FZP. That is, by distributing multiple minute acoustic element sound sources according to FZP theory, the FZP and sound sources can be used interchangeably.

[0090] Figure 16 is a top view showing a second example of the acoustic transducer 1f in the second embodiment. In the acoustic transducer 1f of Figure 16, the upper surface 4 of the piezoelectric film 35 is divided into multiple regions Z1, Z2, Z3, Z4, and Z5. In regions Z2 and Z4, multiple second acoustic elements 20 are arranged in each region, as in the case of Figure 1. On the other hand, unlike in the case of Figure 1, neither the first acoustic element 10 nor the second acoustic element 20 are arranged in regions Z1, Z3, and Z5.

[0091] In this case, regions Z2 and Z4 are examples of first regions where multiple acoustic elements are arranged, while regions Z1, Z3, and Z5 are examples of second regions where neither the first acoustic element 10 nor the second acoustic element 20 is arranged. In other words, the second regions correspond to the shielding regions in the FZP for light. Furthermore, in Figure 16, each region Z1 to Z5 is arranged symmetrically in the Y direction with respect to the reference point O, which corresponds to the focal point of the acoustic beam, when viewed from above (i.e., vertically symmetrical in the plane of the paper).

[0092] Figure 17 is a top view showing a third example of the acoustic transducer 1g in the second embodiment.

[0093] In Figure 17, the region of the upper surface 4 is divided into multiple concentric regions Z1, Z2, Z3, Z4, and Z5 according to the radial distance from the reference point (marked with an "x") corresponding to the focal point of the acoustic beam 6. The number of regions is not limited to this case. Multiple first acoustic elements 10 are arranged in each of regions Z1, Z3, and Z5. In this case, regions Z1, Z3, and Z5 are examples of first regions where multiple acoustic elements are arranged. Regions Z2 and Z4 do not have either the first acoustic element 10 or the second acoustic element 20 arranged. In this case, regions Z2 and Z4 are examples of second regions where neither the first acoustic element 10 nor the second acoustic element 20 is arranged.

[0094] Unlike Figure 17, regions Z2 and Z4 may each have multiple second acoustic elements 20, while regions Z1, Z3, and Z5 may each have neither the first acoustic element 10 nor the second acoustic element 20. In this case, regions Z2 and Z4 are examples of first regions where multiple acoustic elements are arranged. Regions Z1, Z3, and Z5 are examples of second regions where neither the first acoustic element 10 nor the second acoustic element 20 is arranged.

[0095] According to the acoustic transducers 1e, 1f, and 1g of the second embodiment, the focusing performance of the acoustic beam width can be improved in acoustic transducers 1e, 1f, and 1g in which a plurality of acoustic elements are arranged.

[0096] [Third Embodiment] Figure 18 is a top view showing an example of an acoustic transducer 1h in the third embodiment. The acoustic transducer 1h is provided with a first additional electrode 61 and a second additional electrode 62 in addition to the acoustic transducer 1 in the first embodiment. The first additional electrode 61 is provided in the opening 24 of the second upper electrode 22d of the second acoustic element 20d. The second additional electrode 62 is provided on the outer circumference of the first upper electrode 12d of the first acoustic element 10d.

[0097] The first additional electrode 61 and the second additional electrode 62 may be electrically connected to the additional electrode pad 64 via wiring 63. The first additional electrode 61 and the second additional electrode 62 may be subjected to a second electrical signal that is in a different phase from the first electrical signal applied to the first upper electrode 12b and the second upper electrode 22b. It is desirable that the second electrical signal is out of phase (opposite phase) with the first electrical signal by π. In the third embodiment, in addition to the power supply 7 that supplies the first electrical signal, a power supply 8 that supplies the second electrical signal is provided.

[0098] The second upper electrode 22b, like the second upper electrode 22 shown in Figure 4, is annular (donut-shaped), but has a cut-out shape in the circumferential direction (i.e., the letter C). The wiring 63 connected to the first additional electrode 61 may be arranged to pass through this cut-out portion. The second additional electrode 62 is annular (donut-shaped), but has a cut-out shape in the circumferential direction. The wiring 2 connected to the first upper electrode 12b may be arranged to pass through this cut-out portion.

[0099] In the acoustic transducer 1h of the third embodiment, in the first acoustic element 10d, the sound waves generated by the first upper electrode 12b and the sound waves generated by the second additional electrode 62 overlap and are amplified. Specifically, these sound waves are shifted in phase by π due to the difference in electrode shape, and further shifted in phase by π due to the difference in the phase of the applied signal, so they end up in a state of being in phase or nearly in phase.

[0100] Similarly, in the second acoustic element 20d, the sound waves generated by the second upper electrode 22b and the sound waves generated by the first additional electrode 61 overlap and are amplified. That is, these sound waves are shifted in phase by π due to the difference in electrode shape, and further shifted in phase by π due to the difference in the phase of the applied signal, so they end up in phase or nearly in phase.

[0101] Therefore, according to the acoustic transducer 1h of the third embodiment, the first acoustic element 10d and the second acoustic element 20d generate sound waves with opposite phases to each other, and the sound pressure of each sound wave can be increased compared to the first embodiment, thereby increasing the sound pressure of the acoustic beam 6. Accordingly, when it is permissible to add wiring 63 and power supply 8, the sound pressure of the acoustic beam 6 can be increased by the configuration of the third embodiment.

[0102] Although Figure 18 illustrates the case where both the first additional electrode 61 and the second additional electrode 62 are provided, this embodiment is not limited to this case. Only one of the first additional electrode 61 or the second additional electrode 62 may be provided.

[0103] The above embodiments are merely illustrative examples, and there is no intention to exclude various modifications or applications of techniques not explicitly stated in these embodiments. Each configuration of these embodiments can be modified in various ways without departing from their intended purpose. Furthermore, each configuration of these embodiments can be selected or combined as needed. [Explanation of Symbols]

[0104] 1, 1b, 1c, 1d, 1e, 1f, 1g, 1h Acoustic transducer 2 Wiring 3. 3-1~3-N electrode pads 4 Top side 5 Bottom side 6 Acoustic beam 10, 10b, 10c, 10d 1st acoustic element 11, 11b, 11c First diaphragm 12, 12b, 12c, 12d 1st upper electrode 13. First recess 20, 20b, 20c, 20d 2nd acoustic element 21, 21b, 21c Second diaphragm 22, 22b, 22c, 22d 2nd upper electrode 23 Second recess 24 aperture 30 Lower electrode 31 Silicon substrate 32 Silicon oxide layer 33 Silicon layer 34 Lower electrode layer 34 35 Piezoelectric film 40, 43, 45 Photoresist 41, 44a, 44b, 44c, 46a, 46b opening 42 Resist 43 Photoresist 51 Function Generators 52 Container 53 Propagation medium 54 Hydrophone Scope 55 Preamplifier 56 Booster Amplifier 57 Oscilloscope 61 1st additional electrode 62 2nd additional electrode

Claims

1. The device comprises a first acoustic element and a second acoustic element, each having a diaphragm driven by a piezoelectric film and an electrode provided on the piezoelectric film for applying an electrical signal to the piezoelectric film. When a common electrical signal is applied to the electrodes of the first and second acoustic elements, stresses in different directions are generated in the diaphragms of the first and second acoustic elements, thereby generating sound waves with different phases. Acoustic transducer.

2. As for the electrodes, the first acoustic element has a first electrode, and the second acoustic element has a second electrode. The first electrode and the second electrode have different shapes, and due to these different shapes, stress is generated in each of the diaphragms in different directions. The acoustic transducer according to claim 1.

3. In a top view, the first electrode has an opening, and the second electrode does not have an opening. The acoustic transducer according to claim 2.

4. In a top view, a first additional electrode is positioned within the opening of the first electrode, which applies an electrical signal having a different phase from the aforementioned electrical signal. The acoustic transducer according to claim 3.

5. In a top view, a second additional electrode is arranged on the outer circumference of the second electrode, which applies an electrical signal with a different phase from the aforementioned electrical signal. The acoustic transducer according to claim 3 or claim 4.

6. The first region where the first acoustic element is located and the second region where the second acoustic element is located are arranged alternately, and the boundary between the first region and the second region is defined in a top view from a reference point corresponding to the focal point of the acoustic beam focused by the acoustic transducer by the following equation (1) ｎ It is located at a distance. The acoustic transducer according to claim 1. [Math 1]

7. An acoustic transducer comprising a plurality of acoustic elements, each having a diaphragm driven by a piezoelectric film and an electrode provided on the piezoelectric film to which an electrical signal is applied, Each region consists of alternating first regions where the plurality of acoustic elements are arranged and second regions where the plurality of acoustic elements are not arranged, and the boundary between the first region and the second region is defined in a top view from a reference point corresponding to the focal point of the acoustic beam focused by the acoustic transducer by the following equation (1) ｎ It is located at a distance. Acoustic transducer. [Math 2]

8. A first acoustic element and a second acoustic element each have a diaphragm driven by a piezoelectric film and an electrode provided on the piezoelectric film for applying an electrical signal to the piezoelectric film, wherein a common electrical signal is applied to each of the electrodes, By generating stresses in different directions on the diaphragms of the first acoustic element and the second acoustic element, sound waves with different phases are generated. Methods for controlling sound waves.

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