Piezoelectric element, piezoelectric film substrate, and method for manufacturing piezoelectric film substrate

Abstract

A piezoelectric element (100) comprises: a substrate (10); a transmission-side unit (20) that is disposed on the substrate and transmits sound waves; and a reception-side unit (30) that is disposed on the substrate and receives sound waves. The transmission-side unit includes: a plurality of transmission-side piezoelectric layers (21); and a plurality of transmission-side electrodes (22) that are disposed so as to sandwich the respective transmission-side piezoelectric layers. The reception-side unit includes: one or more reception-side piezoelectric layers (31); and a plurality of reception-side electrodes (32) that are disposed so as to sandwich the respective one or more reception-side piezoelectric layers. The number of the transmission-side piezoelectric layers is greater than the number of the reception-side piezoelectric layers.

Description

Piezoelectric element, piezoelectric film substrate, and method for manufacturing piezoelectric film substrate

[0001] The present invention relates to a piezoelectric element, a piezoelectric film substrate, and a method for manufacturing a piezoelectric film substrate.

[0002] Piezoelectric elements that transmit and receive sound waves have been known in the past. Such piezoelectric elements are disclosed in, for example, Japanese Patent Application Laid-Open No. 2019-80249.

[0003] The above-mentioned Japanese Patent Application Laid-Open Publication No. 2019-80249 discloses an ultrasonic device including a piezoelectric element that transmits and receives ultrasonic waves. This ultrasonic device includes an element substrate, a support film laminated on the element substrate, and a transmitting piezoelectric element and a receiving piezoelectric element provided on the support film. The transmitting piezoelectric element includes a transmitting piezoelectric film for transmitting ultrasonic waves. The receiving piezoelectric element includes a receiving piezoelectric film for receiving ultrasonic waves. In the above-mentioned Japanese Patent Application Laid-Open Publication No. 2019-80249, the thickness of the transmitting piezoelectric film is greater than the thickness of the receiving piezoelectric film, thereby improving the transmission and reception characteristics of ultrasonic waves.

[0004] JP 2019-80249 A

[0005] In the ultrasonic device described in JP 2019-80249 A, the thickness of the transmitting piezoelectric film is greater than the thickness of the receiving piezoelectric film, which makes it possible to improve the ultrasonic transmission and reception characteristics. However, the following problem exists. That is, if the thickness of the transmitting piezoelectric film and the thickness of the receiving piezoelectric film are to be made different, each piezoelectric film must be formed (patterned) independently, which requires a complicated process to make the thicknesses of the piezoelectric films different. This poses the problem of a complicated manufacturing process.

[0006] The present invention has been made to solve the above-mentioned problems, and one object of the present invention is to provide a piezoelectric element that can improve the transmission and reception characteristics of sound waves and can simplify the manufacturing process. Another object of the present invention is to provide a piezoelectric film substrate and a method for manufacturing a piezoelectric film substrate that can improve the actuator characteristics.

[0007] In order to achieve the above object, a piezoelectric element according to a first aspect of the present invention comprises a substrate, a transmitting side portion arranged on the substrate and transmitting acoustic waves, and a receiving side portion arranged on the substrate and receiving acoustic waves, wherein the transmitting side portion includes a plurality of transmitting piezoelectric layers and a plurality of transmitting electrodes arranged to sandwich each of the plurality of transmitting piezoelectric layers, and the receiving side portion includes one or more receiving piezoelectric layers and a plurality of receiving electrodes arranged to sandwich each of the one or more receiving piezoelectric layers, and the number of layers of the plurality of transmitting piezoelectric layers is greater than the number of layers of the one or more receiving piezoelectric layers.

[0008] In the piezoelectric element according to the first aspect of the present invention, as described above, the number of the multiple transmitting piezoelectric layers is greater than the number of the one or more receiving piezoelectric layers. This allows the number of transmitting piezoelectric layers to be set to a number suitable for improving the transmission characteristics of sound waves, and the number of receiving piezoelectric layers to be set to a number suitable for improving the reception characteristics of sound waves. That is, the greater the number of transmitting piezoelectric layers, the greater the displacement of the transmitting piezoelectric layers in response to an applied voltage, allowing for the transmission of sound waves with higher pressure. As a result, the transmission characteristics of sound waves can be improved. Furthermore, the smaller the number of receiving piezoelectric layers, the higher the crystallinity and the lower the dielectric constant can be. This improves the sensor characteristics, thereby improving the signal-to-noise ratio. As a result, the reception characteristics of sound waves can be improved. Furthermore, unlike the piezoelectric element according to the first aspect, if the number of transmitting piezoelectric layers and the number of receiving piezoelectric layers are the same and the total thickness of the transmitting piezoelectric layers is made different from the total thickness of the receiving piezoelectric layers, each piezoelectric layer must be formed (patterned) independently, requiring a complicated process to make the thicknesses of each piezoelectric layer different. In contrast, the piezoelectric element according to the first aspect has a configuration in which the number of transmitting piezoelectric layers and the number of receiving piezoelectric layers are made different, so that for piezoelectric layers in the same layer counted from the substrate side, the transmitting piezoelectric layers and the receiving piezoelectric layers can be formed (patterned) in a common process. This simplifies the manufacturing process of the piezoelectric element. As a result, a piezoelectric element can be provided that can improve the sound wave transmission and reception characteristics and simplify the manufacturing process.

[0009] Here, among the multiple transmitting piezoelectric layers, the upper transmitting piezoelectric layers, with the substrate side at the bottom, tend to have lower crystallinity because they are less susceptible to the influence of the substrate, which has a higher crystallinity. Furthermore, the lower the crystallinity, the higher the piezoelectric constant and the improved actuator characteristics, thereby enabling the transmitting piezoelectric layers to be displaced more significantly with respect to an applied voltage. Therefore, in the piezoelectric element according to the above aspect, by having one or more transmitting piezoelectric layers than the number of receiving piezoelectric layers, the crystallinity of the transmitting piezoelectric layer located above the uppermost receiving piezoelectric layer is lower, thereby improving the sound wave transmission characteristics of the transmitting piezoelectric layer compared to the receiving piezoelectric layer. Note that, in this specification, the number of piezoelectric layers and electrodes is counted from the side closest to the substrate. In other words, the first piezoelectric layer and electrode refers to the piezoelectric layer and electrode closest to the substrate. Also, in this specification, "upper" refers to a direction away from the substrate, and "lower" refers to a direction toward the substrate.

[0010] In the piezoelectric element according to the first aspect, preferably, the number of layers of the plurality of transmitting piezoelectric layers is two or more, and the number of layers of one receiving piezoelectric layer is one. This allows the receiving piezoelectric layer to be configured with the minimum number of layers, thereby further improving the reception characteristics of sound waves. Here, the first receiving piezoelectric layer tends to have high crystallinity due to the influence of the highly crystalline substrate. The higher the crystallinity, the lower the dielectric constant and the improved sensor characteristics, thereby improving the signal-to-noise ratio. Therefore, by having only one receiving piezoelectric layer, the crystallinity of the receiving piezoelectric layer is high, and the reception characteristics of sound waves can be further improved.

[0011] In this case, the number of layers of the plurality of transmitting piezoelectric layers is preferably two. This allows the transmitting piezoelectric layers to be configured with the minimum number of layers, thereby simplifying the manufacturing process. In other words, by having two transmitting piezoelectric layers, the manufacturing process can be simplified while improving the transmission characteristics of sound waves.

[0012] In the above-described configuration in which the number of transmitter piezoelectric layers is two, the plurality of transmitter piezoelectric layers preferably includes a first transmitter piezoelectric layer and a second transmitter piezoelectric layer disposed farther from the substrate than the first transmitter piezoelectric layer, and the plurality of transmitter electrodes preferably include a transmitter lower electrode disposed between the substrate and the first transmitter piezoelectric layer, an intermediate electrode disposed between the first transmitter piezoelectric layer and the second transmitter piezoelectric layer, and a transmitter upper electrode disposed on the second transmitter piezoelectric layer, the transmitter lower electrode and the transmitter upper electrode being commonly connected to the first voltage application electrode, and the intermediate electrode being connected to the second voltage application electrode. This allows voltages to be applied to the first transmitter piezoelectric layer and the second transmitter piezoelectric layer so that the first transmitter piezoelectric layer and the second transmitter piezoelectric layer expand and contract in opposite directions. As a result, the transmitter section as a whole can be displaced significantly. This further improves the sound wave transmission characteristics in a configuration in which the number of transmitter piezoelectric layers is two.

[0013] In this case, the intermediate electrode is preferably located at the center of the entire transmitting section, including the vibrating section, in the thickness direction of the substrate. With this configuration, the direction of deformation can be reversed up and down from the center of the entire transmitting section, including the vibrating section, in the thickness direction of the substrate, so that the transmitting section, including the vibrating section, can be displaced more significantly as a whole.

[0014] In the piezoelectric element according to the first aspect, the first receiving-side piezoelectric layer, counting from the substrate side, of the one or more receiving-side piezoelectric layers is preferably an epitaxially grown layer. This allows the crystallinity of the first receiving-side piezoelectric layer to be high. As a result, the higher the crystallinity, the better the sensor characteristics, thereby improving the signal-to-noise ratio and further improving the reception characteristics of sound waves.

[0015] In the piezoelectric element according to the first aspect, the area of ​​the receiving side, including the receiving vibration section provided on the substrate, is preferably smaller than the area of ​​the transmitting side, including the transmitting vibration section provided on the substrate, in a plan view. Here, if the number of transmitting piezoelectric layers is greater than the number of receiving piezoelectric layers, the transmitting side will be thicker than the receiving side, resulting in a higher resonant frequency proportional to the thickness in the transmitting side than in the receiving side. In this case, the resonant frequencies of the receiving side and transmitting side cannot be matched. Therefore, with the above configuration, the resonant frequency of the receiving side, which is inversely proportional to the area in a plan view, can be increased. Therefore, even if the transmitting side is thicker than the receiving side, the resonant frequencies of the receiving side and transmitting side can be matched. As a result, sound waves of a common resonant frequency can be transmitted from the transmitting side and the reflected sound waves of the common resonant frequency can be received by the receiving side.

[0016] In the piezoelectric element according to the first aspect, preferably, of the plurality of transmitter piezoelectric layers and the one or more receiver piezoelectric layers, the piezoelectric layers in the same layer, counting from the substrate side, are made of the same material and have the same thickness, and of the plurality of transmitter electrodes and the plurality of receiver electrodes, the electrodes in the same layer, counting from the substrate side, are made of the same material and have the same thickness. With this configuration, the piezoelectric layers and the electrodes can be formed (patterned) in a common process, thereby simplifying the manufacturing process of the piezoelectric element.

[0017] In the piezoelectric element according to the first aspect, the plurality of transmitter piezoelectric layers and the one or more receiver piezoelectric layers are preferably formed of lead zirconate titanate, thereby improving the transmission and reception characteristics of acoustic waves and simplifying the manufacturing process when the transmitter piezoelectric layer and the receiver piezoelectric layer are formed of lead zirconate titanate.

[0018] A piezoelectric film substrate according to a second aspect of the present invention comprises a substrate, a lower electrode disposed on the substrate, a first piezoelectric layer disposed on the lower electrode, an intermediate electrode which is not an epitaxially grown layer and disposed on the first piezoelectric layer, a second piezoelectric layer which is not an epitaxially grown layer and disposed on the intermediate electrode, and an upper electrode which is not an epitaxially grown layer and disposed on the second piezoelectric layer.

[0019] In the piezoelectric film substrate according to the second aspect of the present invention, the crystallinity of the second piezoelectric layer can be reduced by the above-described configuration, thereby improving actuator characteristics. Furthermore, if the second piezoelectric layer contains lead, undesirable lead diffusion from the second piezoelectric layer to the intermediate electrode can occur if the second piezoelectric layer is deposited at a relatively high temperature. However, since the second piezoelectric layer is not an epitaxially grown layer, it can be deposited at a relatively low temperature, thereby suppressing undesirable lead diffusion from the second piezoelectric layer to the intermediate electrode. Furthermore, depositing the second piezoelectric layer at a relatively low temperature can reduce film stress. Furthermore, since the intermediate electrode is not an epitaxially grown layer, it is possible to easily prevent the second piezoelectric layer from epitaxially growing. Furthermore, when the first piezoelectric layer contains lead, if the intermediate electrode is deposited at a relatively high temperature, undesirable diffusion of lead from the first piezoelectric layer to the intermediate electrode may occur, but since an intermediate electrode that is not an epitaxially grown layer can be deposited at a relatively low temperature, undesirable diffusion of lead from the first piezoelectric layer to the intermediate electrode can be suppressed. Furthermore, if the intermediate electrode is deposited at a relatively high temperature, the crystal structure of the first piezoelectric layer may change, but since an intermediate electrode that is not an epitaxially grown layer can be deposited at a relatively low temperature, it is possible to suppress change in the crystal structure of the first piezoelectric layer.

[0020] In the piezoelectric film substrate according to the second aspect, the first piezoelectric layer is preferably not an epitaxially grown layer. This configuration reduces the crystallinity of the first piezoelectric layer, thereby improving the actuator characteristics. Furthermore, the first piezoelectric layer that is not an epitaxially grown layer can be deposited at a relatively low temperature, thereby reducing film stress.

[0021] In the piezoelectric film substrate according to the second aspect, the intermediate electrode preferably includes a (111) preferentially oriented film. With this configuration, since the intermediate electrode includes a (111) preferentially oriented film, it is possible to easily prevent the second piezoelectric layer from growing epitaxially.

[0022] A method for manufacturing a piezoelectric film substrate according to a third aspect of the present invention includes the steps of preparing a substrate, forming a lower electrode on the substrate, forming a first piezoelectric layer on the lower electrode, forming an intermediate electrode on the first piezoelectric layer at a temperature that does not cause epitaxial growth, forming a second piezoelectric layer on the intermediate electrode at a temperature that does not cause epitaxial growth, and forming an upper electrode on the second piezoelectric layer at a temperature that does not cause epitaxial growth.

[0023] In the manufacturing method of a piezoelectric film substrate according to the third aspect of the present invention, the crystallinity of the second piezoelectric layer can be reduced by the above-described configuration, thereby improving actuator characteristics. Furthermore, if the second piezoelectric layer contains lead, undesirable lead diffusion from the second piezoelectric layer to the intermediate electrode can occur if the second piezoelectric layer is deposited at a relatively high temperature. However, since the second piezoelectric layer is not an epitaxially grown layer, it can be deposited at a relatively low temperature, thereby suppressing undesirable lead diffusion from the second piezoelectric layer to the intermediate electrode. Furthermore, depositing the second piezoelectric layer at a relatively low temperature can reduce film stress. Furthermore, since the intermediate electrode is not an epitaxially grown layer, it is possible to easily prevent the second piezoelectric layer from epitaxially growing. Furthermore, when the first piezoelectric layer contains lead, if the intermediate electrode is deposited at a relatively high temperature, undesirable diffusion of lead from the first piezoelectric layer to the intermediate electrode may occur, but since an intermediate electrode that is not an epitaxially grown layer can be deposited at a relatively low temperature, undesirable diffusion of lead from the first piezoelectric layer to the intermediate electrode can be suppressed. Furthermore, if the intermediate electrode is deposited at a relatively high temperature, the crystal structure of the first piezoelectric layer may change, but since an intermediate electrode that is not an epitaxially grown layer can be deposited at a relatively low temperature, it is possible to suppress change in the crystal structure of the first piezoelectric layer.

[0024] In the method for manufacturing a piezoelectric film substrate according to the third aspect, preferably, in the step of forming the first piezoelectric layer on the lower electrode, the first piezoelectric layer is not an epitaxially grown layer. This configuration reduces the crystallinity of the first piezoelectric layer, thereby improving the actuator characteristics. Furthermore, since the first piezoelectric layer is not an epitaxially grown layer, it can be formed at a relatively low temperature, thereby reducing film stress.

[0025] In the method for manufacturing a piezoelectric film substrate according to the third aspect, preferably, in the step of forming the intermediate electrode on the first piezoelectric layer at a temperature that does not cause epitaxial growth, the intermediate electrode includes a (111) preferentially oriented film. With this configuration, since the intermediate electrode includes a (111) preferentially oriented film, it is possible to easily prevent epitaxial growth of the second piezoelectric layer.

[0026] According to the present invention, as described above, it is possible to improve the transmission and reception characteristics of sound waves and to simplify the manufacturing process.

[0027] FIG. 1 is a schematic perspective view showing a piezoelectric element according to an embodiment; FIG. 2 is a schematic cross-sectional view showing a piezoelectric element according to an embodiment; FIG. 3 is a schematic diagram for explaining displacement of a transmitting side portion of a piezoelectric element according to an embodiment; FIG. 4 is a schematic diagram for explaining a cantilever model of a piezoelectric element; FIG. 5 is a schematic diagram for explaining a manufacturing process of a piezoelectric element according to an embodiment; FIG. 6 is a schematic diagram for explaining a manufacturing process of a piezoelectric element according to an embodiment; FIG. 7 is a schematic diagram for explaining a manufacturing process of a piezoelectric element according to an embodiment; FIG. 8 is a schematic cross-sectional view showing a piezoelectric element according to a first modified example of an embodiment; FIG. 9 is a schematic cross-sectional view showing a piezoelectric element according to a second modified example of an embodiment.

[0028] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0029] The structure of a piezoelectric element 100 according to one embodiment of the present invention will be described with reference to Figures 1 to 3. In the following description, the thickness direction of the substrate 10 is defined as the Z direction.

[0030] (Overall Configuration of Piezoelectric Element) The piezoelectric element 100 is an element for a piezoelectric transducer. The piezoelectric element 100 is used in ultrasonic transducers such as a PMUT (Piezoelectric Micromachined Ultrasonic Transducer). The piezoelectric element 100 is manufactured using MEMS (Micro Electro Mechanical Systems) technology.

[0031] 1, the piezoelectric element 100 includes a substrate 10, a transmitting side unit 20, a receiving side unit 30, voltage application electrodes 41 and 42, and charge detection electrodes 51 and 52. The piezoelectric element 100 transmits ultrasonic waves to a target using the transmitting side unit 20, and receives ultrasonic waves reflected from the target using the receiving side unit 30. The voltage application electrodes 41 and 42 are examples of the "first voltage application electrode" and the "second voltage application electrode" in the claims, respectively.

[0032] The substrate 10 is a semiconductor substrate and also serves as a support for the transmitting section 20, the receiving section 30, the voltage application electrodes 41 and 42, and the charge detection electrodes 51 and 52.

[0033] The transmitting side unit 20 is disposed on the substrate 10. The transmitting side unit 20 transmits ultrasonic waves.

[0034] The receiving side unit 30 is disposed on the substrate 10. The receiving side unit 30 receives ultrasonic waves. The transmitting side unit 20 and the receiving side unit 30 are disposed at positions spaced apart from each other in a direction perpendicular to the Z direction.

[0035] The voltage application electrodes 41 and 42 are electrically connected to the transmitting side unit 20 and apply a voltage to the transmitting side unit 20. The voltage applied to the transmitting side unit 20 is supplied from a power source (not shown) outside the piezoelectric element 100. The voltage application electrodes 41 and 42 are electrically connected to the power source. By applying an AC voltage to the transmitting side unit 20, the voltage application electrodes 41 and 42 vibrate the transmitting side unit 20 to generate (transmit) ultrasonic waves.

[0036] The charge detection electrodes 51 and 52 are electrically connected to the receiving side unit 30 and detect the charge generated in the receiving side unit 30 as a received signal. The received signal detected from the receiving side unit 30 is transmitted to a signal processing unit (not shown) outside the piezoelectric element 100. The charge detection electrodes 51 and 52 are electrically connected to the signal processing unit. When the receiving side unit 30 is vibrated by an ultrasonic wave, the charge detection electrodes 51 and 52 receive the ultrasonic wave by detecting the charge generated in the receiving side unit 30.

[0037] (Detailed Configuration of Piezoelectric Element) As shown in FIG. 2, the piezoelectric element 100 includes a substrate 10, a transmitting section 20, a receiving section 30, an insulating layer 60, and a wiring layer .

[0038] The substrate 10 is an SOI (Silicon on Insulator) substrate. The substrate 10 has a structure in which a first silicon layer 11, a silicon oxide layer 12 as an insulating layer, and a second silicon layer 13 as a single crystal are arranged in this order. The substrate 10 also has recesses 14 and 15 at positions overlapping the transmitting side section 20 and the receiving side section 30, respectively, in a plan view (as viewed from the Z direction). The recesses 14 and 15 are formed by removing at least the first silicon layer 11 and the silicon oxide layer 12, so as to recess from the first silicon layer 11 side toward the second silicon layer 13 side. The recesses 14 and 15 have vibrating portions 14a and 15a at their bottoms, respectively. The vibrating portions 14a and 15a are made of the second silicon layer 13. The transmitting side section 20 is arranged on the vibrating portion 14a, and the receiving side section 30 is arranged on the vibrating portion 15a. The vibration units 14a and 15a are examples of the "transmission side vibration unit" and the "reception side vibration unit" in the claims, respectively.

[0039] The transmitter section 20 includes a plurality of transmitter piezoelectric layers 21 and a plurality of transmitter electrodes 22 arranged so as to sandwich each of the plurality of transmitter piezoelectric layers 21. The transmitter section 20 has a structure in which the transmitter piezoelectric layers 21 and the transmitter electrodes 22 are alternately stacked. When an AC voltage is applied to the plurality of transmitter piezoelectric layers 21 from the voltage application electrodes 41 and 42 via the plurality of transmitter electrodes 22, the plurality of transmitter piezoelectric layers 21 vibrates. The vibration of the plurality of transmitter piezoelectric layers 21 then vibrates the vibrating section 14a, generating ultrasonic waves.

[0040] The receiving side section 30 includes one receiving piezoelectric layer 31 and multiple receiving electrodes 32 arranged to sandwich the single receiving piezoelectric layer 31. The receiving side section 30 has a structure in which the receiving piezoelectric layers 31 and the receiving electrodes 32 are alternately stacked. When the vibrating section 15a is vibrated by ultrasonic waves, the receiving piezoelectric layer 31 is vibrated. When the receiving piezoelectric layer 31 is vibrated, an electric charge is generated in accordance with the vibration, and the ultrasonic waves are detected by the charge detection electrodes 51 and 52 via the multiple receiving electrodes 32.

[0041] In this embodiment, the number of layers in the transmitting piezoelectric layer 21 is greater than the number of layers in the receiving piezoelectric layer 31. Specifically, the number of layers in the transmitting piezoelectric layer 21 is two or more, and the number of layers in the receiving piezoelectric layer 31 is one. More specifically, the number of layers in the transmitting piezoelectric layer 21 is two.

[0042] The plurality of transmitter piezoelectric layers 21 include a transmitter piezoelectric layer 21a and a transmitter piezoelectric layer 21b that is located farther from the substrate 10 than the transmitter piezoelectric layer 21a. The plurality of transmitter electrodes 22 include a transmitter lower electrode 22a that is located between the substrate 10 and the transmitter piezoelectric layer 21a, an intermediate electrode 22b that is located between the transmitter piezoelectric layer 21a and the transmitter piezoelectric layer 21b, and a transmitter upper electrode 22c that is located on the transmitter piezoelectric layer 21b. The transmitter lower electrode 22a, transmitter piezoelectric layer 21a, intermediate electrode 22b, transmitter piezoelectric layer 21b, and transmitter upper electrode 22c are arranged in this order from the substrate 10 side. The transmitter piezoelectric layers 21a and 21b are examples of a "first transmitter piezoelectric layer" and a "second transmitter piezoelectric layer," respectively.

[0043] The transmitter lower electrode 22a is disposed on the vibrating portion 14a of the substrate 10. The transmitter lower electrode 22a is an epitaxially grown layer formed by epitaxial growth on the single-crystal second silicon layer 13. The transmitter lower electrode 22a may be a single-layer or multi-layer electrode. If it is a multi-layer electrode, the transmitter lower electrode 22a includes an electrode layer and a buffer layer. In this case, the electrode layer is disposed on the substrate 10, the buffer layer is disposed on the electrode layer, and the transmitter piezoelectric layer 21a is disposed on the buffer layer. The electrode layer is formed of a material such as platinum (Pt) or iridium (Ir). The buffer layer is formed of strontium ruthenate (SrRuO ３ ), or lanthanum nickelate (LaNiO ３ As an example, the electrode layer and the buffer layer are made of platinum (Pt) and strontium ruthenate (SrRuO), respectively. ３ ) is formed.

[0044] The transmitter piezoelectric layer 21a is disposed on the transmitter lower electrode 22a. The transmitter piezoelectric layer 21a is an epitaxially grown layer formed on the transmitter lower electrode 22a by epitaxial growth. The transmitter piezoelectric layer 21a is made of lead zirconate titanate (PZT), which exhibits ferroelectricity.

[0045] The intermediate electrode 22b is disposed on the transmitting piezoelectric layer 21a. The intermediate electrode 22b may be a single-layer or multi-layer electrode. When the intermediate electrode 22b is a multi-layer electrode, it includes a first buffer layer, an electrode layer, and a second buffer layer. In this case, the first buffer layer is disposed on the transmitting piezoelectric layer 21a, the electrode layer is disposed on the first buffer layer, the second buffer layer is disposed on the electrode layer, and the transmitting piezoelectric layer 21b is disposed on the second buffer layer. The electrode layer may be made of titanium (Ti), gold (Au), platinum (Pt), or iridium dioxide (IrO ２ The first buffer layer and the second buffer layer are made of a material such as strontium ruthenate (SrRuO ３ ), or lanthanum nickelate (LaNiO ３ As an example, the first buffer layer and the second buffer layer are made of a material such as strontium ruthenate (SrRuO ３ The intermediate electrode 22b is located at the center of the entire transmitting section 20, including the vibrating section 14a provided on the substrate 10, in the thickness direction (Z direction) of the substrate 10.

[0046] The transmitter piezoelectric layer 21b is disposed on the intermediate electrode 22b and is made of lead zirconate titanate (PZT), which exhibits ferroelectricity.

[0047] The transmitter upper electrode 22c is disposed on the transmitter piezoelectric layer 21b. The transmitter upper electrode 22c is a single-layer or multi-layer electrode. When the transmitter upper electrode 22c is a multi-layer electrode, it includes a buffer layer and an electrode layer. In this case, the buffer layer is disposed on the transmitter piezoelectric layer 21b, and the electrode layer is disposed on the buffer layer. The electrode layer is made of titanium (Ti), gold (Au), platinum (Pt), or iridium dioxide (IrO ２ The buffer layer is made of a material such as strontium ruthenate (SrRuO ３ ), or lanthanum nickelate (LaNiO ３ As an example, the electrode layer and the buffer layer are made of platinum (Pt) and strontium ruthenate (SrRuO), respectively. ３ ) is formed.

[0048] In this embodiment, the transmitter lower electrode 22a and the transmitter upper electrode 22c are commonly connected to the voltage application electrode 41, and the intermediate electrode 22b is connected to the voltage application electrode 42. Specifically, the transmitter lower electrode 22a and the transmitter upper electrode 22c are commonly connected to the voltage application electrode 41 via a wiring portion 71 (described later) of the wiring layer 70. The intermediate electrode 22b is connected to the voltage application electrode 42 via a wiring portion 72 (described later) of the wiring layer 70. In other words, a common potential is applied to the transmitter lower electrode 22a and the transmitter upper electrode 22c, and a potential different from the potentials applied to the transmitter lower electrode 22a and the transmitter upper electrode 22c is applied to the intermediate electrode 22b.

[0049] As shown in FIG. 3 , when a voltage is applied to the transmitter lower electrode 22 a, the intermediate electrode 22 b, and the transmitter upper electrode 22 c, the transmitter piezoelectric layers 21 a and 21 b are displaced. At this time, by adopting the electrode connection described above, the transmitter piezoelectric layers 21 a and 21 b are displaced so as to expand and contract in opposite directions. That is, when the transmitter piezoelectric layer 21 a expands in the longitudinal direction of the transmitter piezoelectric layer 21 a, the transmitter piezoelectric layer 21 b contracts in the longitudinal direction of the transmitter piezoelectric layer 21 b. Furthermore, when the transmitter piezoelectric layer 21 a contracts in the longitudinal direction of the transmitter piezoelectric layer 21 a, the transmitter piezoelectric layer 21 b expands in the longitudinal direction of the transmitter piezoelectric layer 21 b. This allows for greater displacement of the transmitter section 20 in the Z direction.

[0050] 2, the multiple receiver electrodes 32 include a receiver lower electrode 32a disposed between the substrate 10 and the receiver piezoelectric layer 31, and a receiver upper electrode 32b disposed on the receiver piezoelectric layer 31. The receiver lower electrode 32a, the receiver piezoelectric layer 31, and the receiver upper electrode 32b are disposed in this order from the substrate 10 side.

[0051] The receiver lower electrode 32a is disposed on the vibrating portion 15a of the substrate 10. The receiver lower electrode 32a is an epitaxially grown layer formed by epitaxial growth on the single-crystal second silicon layer 13. The receiver lower electrode 32a is a single-layer or multi-layer electrode. If it is a multi-layer electrode, the receiver lower electrode 32a includes an electrode layer and a buffer layer. In this case, the electrode layer is disposed on the substrate 10, the buffer layer is disposed on the electrode layer, and the receiver piezoelectric layer 31 is disposed on the buffer layer. The electrode layer is formed of a material such as platinum (Pt) or iridium (Ir). The buffer layer is formed of strontium ruthenate (SrRuO ３ ), or lanthanum nickelate (LaNiO ３ As an example, the electrode layer and the buffer layer are made of platinum (Pt) and strontium ruthenate (SrRuO), respectively. ３ ) is formed.

[0052] The receiver piezoelectric layer 31 is disposed on the receiver lower electrode 32 a. The receiver piezoelectric layer 31 is an epitaxially grown layer formed on the receiver lower electrode 32 a by epitaxial growth. The receiver piezoelectric layer 31 is made of lead zirconate titanate (PZT), which exhibits ferroelectricity.

[0053] The receiver upper electrode 32b is disposed on the receiver piezoelectric layer 31. The receiver upper electrode 32b is a single-layer or multi-layer electrode. When the receiver upper electrode 32b is a multi-layer electrode, it includes a first buffer layer, an electrode layer, and a second buffer layer. In this case, the first buffer layer is disposed on the receiver piezoelectric layer 31, the electrode layer is disposed on the first buffer layer, and the second buffer layer is disposed on the electrode layer. The electrode layer is made of titanium (Ti), gold (Au), platinum (Pt), or iridium dioxide (IrO ２ The first buffer layer and the second buffer layer are made of a material such as strontium ruthenate (SrRuO ３ ), or lanthanum nickelate (LaNiO ３ As an example, the first buffer layer and the second buffer layer are made of a material such as strontium ruthenate (SrRuO ３ ) and the electrode layer is formed of platinum (Pt).

[0054] The receiver lower electrode 32a is connected to the charge detection electrode 51, and the receiver upper electrode 32b is connected to the charge detection electrode 52. Specifically, the receiver lower electrode 32a is connected to the charge detection electrode 51 via a wiring portion 73 (described later) of the wiring layer 70. The receiver upper electrode 32b is connected to the charge detection electrode 52 via a wiring portion 74 (described later) of the wiring layer 70.

[0055] In this embodiment, as described below, the transmitter lower electrode 22a and the receiver lower electrode 32a are formed in a common process. Therefore, the transmitter lower electrode 22a and the receiver lower electrode 32a are formed of the same material and have the same thickness in the Z direction. Similarly, the transmitter piezoelectric layer 21a and the receiver piezoelectric layer 31 are formed in a common process. Therefore, the transmitter piezoelectric layer 21a and the receiver piezoelectric layer 31 are formed of the same material and have the same thickness in the thickness direction of the substrate 10. Similarly, the intermediate electrode 22b and the receiver upper electrode 32b are formed in a common process. Therefore, the intermediate electrode 22b and the receiver upper electrode 32b are formed of the same material and have the same thickness in the thickness direction of the substrate 10. In this embodiment, among the multiple transmitter piezoelectric layers 21 and receiver piezoelectric layers 31, piezoelectric layers in the same layer, counting from the substrate 10, are formed of the same material and have the same thickness. Furthermore, among the plurality of transmitter electrodes 22 and the plurality of receiver electrodes 32, electrodes in the same layer counted from the substrate 10 side are made of the same material and have the same thickness.

[0056] Note that, as long as electrodes formed in a common process are made of the same material and have the same thickness, electrodes formed in different processes may be made of different materials and have different thicknesses. Similarly, as long as piezoelectric layers formed in a common process are made of the same material and have the same thickness, piezoelectric layers formed in different processes may be made of different materials and have different thicknesses. In the example shown in FIG. 2 , the transmitter lower electrode 22 a, intermediate electrode 22 b, transmitter upper electrode 22 c, receiver lower electrode 32 a, and receiver upper electrode 32 b are made of the same material and have the same thickness. Also, in the example shown in FIG. 2 , the transmitter piezoelectric layer 21 a, transmitter piezoelectric layer 21 b, and receiver piezoelectric layer 31 are made of the same material and have the same thickness.

[0057] In this embodiment, as shown in FIG. 1 , the area of ​​the receiving unit 30, including the vibrating unit 15a provided on the substrate 10, is smaller than the area of ​​the transmitting unit 20, including the vibrating unit 14a provided on the substrate 10, in a plan view (viewed in the Z direction). Specifically, the transmitting unit 20 and the receiving unit 30 each have a circular shape when viewed in the Z direction, and the diameter 81 of the vibrating unit 15a of the receiving unit 30 is smaller than the diameter 82 of the vibrating unit 14a of the transmitting unit 20. The diameter 81 of the receiving unit 30 is adjusted so that the receiving unit 30 has the same resonant frequency as the resonant frequency of the transmitting unit 20. When an AC voltage of the resonant frequency is applied from the voltage application electrodes 41 and 42, the transmitting unit 20 can output ultrasonic waves of the resonant frequency. The receiving unit 30 can also detect reflected ultrasonic waves of the resonant frequency.

[0058] 2 , the insulating layer 60 electrically insulates the wiring layer 70 from the substrate 10, the electrodes (transmitter lower electrode 22 a, intermediate electrode 22 b, transmitter upper electrode 22 c, receiver lower electrode 32 a, and receiver upper electrode 32 b), and the piezoelectric layers (transmitter piezoelectric layer 21 a, transmitter piezoelectric layer 21 b, and receiver piezoelectric layer 31). The insulating layer 60 is formed to cover the substrate 10, the electrodes, and the piezoelectric layers. The insulating layer 60 is formed, for example, from a silicon oxide film.

[0059] The wiring layer 70 electrically connects the electrodes to each other. The wiring layer 70 includes wiring portions 71, 72, 73, and 74. The wiring portion 71 electrically connects the transmitter lower electrode 22a and the transmitter upper electrode 22c to the voltage application electrode 41. The wiring portion 72 electrically connects the intermediate electrode 22b to the voltage application electrode 42. The wiring portion 73 electrically connects the receiver lower electrode 32a to the charge detection electrode 51. The wiring portion 74 electrically connects the receiver upper electrode 32b to the charge detection electrode 52. The wiring layer 70 is formed of a material such as titanium (Ti) or gold (Au).

[0060] (Signal-to-Noise Ratio of Receiving Side) The S / N ratio of the receiving side 30 will be described with reference to Fig. 4. Here, a cantilever model shown in Fig. 4 was used to compare the S / N ratios when the number of piezoelectric layers is one and when the number is two. In Fig. 4, the cantilever 200 includes a substrate 210 and a piezoelectric layer 231 disposed on the substrate 210. The piezoelectric layer 231 corresponds to the receiving-side piezoelectric layer 31, and the substrate 210 corresponds to the vibration part 15a of the substrate 10. Note that the electrodes are ignored as they are sufficiently thin.

[0061] The displacement Z of the tip of the cantilever 200 shown in FIG. ｘ－Ｌ can be calculated from the equation for the displacement of the tip of the cantilever 200 when a uniformly distributed load p is applied. Specifically, the displacement amount Z of the tip of the cantilever 200 ｘ－Ｌ is expressed by the following equation (1) using the uniformly distributed load p, the Young's modulus E of the cantilever 200, the second moment of area I of the cantilever 200, and the length L of the cantilever 200 in the longitudinal direction.

[0062] In addition, the total charge Q generated from position x = 0 to position x = L is the positive piezoelectric constant e ３１ｆ、 Poisson's ratio ν of the substrate 210 ｓ , width w, neutral axis e of the cantilever 200, and neutral axis z of the piezoelectric layer 231. ｐ is expressed by the following equation (2).

[0063] In addition, the primary resonance frequency f １ is expressed by the following equation (3) using the density ρ of the cantilever 200 and the cross-sectional area A of the cantilever 200.

[0064] Furthermore, when electrical noise is dominant in a piezoelectric element, the signal-to-noise ratio is 1 / ε ３３ where ε ３３ is the relative dielectric constant of the piezoelectric layer 231. Therefore, the noise N is ３３ When the piezoelectric layer 231 is two layers, they can be regarded as parallel connections with no correlation, so the noise N is the sum of squares and is expressed by the following equation (4).ｉ and ε ３３ｉ In this equation, i=1 and 2 represent the first and second piezoelectric layers 231, respectively.

[0065] Furthermore, from equation (4), the S / N ratio when the piezoelectric layer 231 is two layers is expressed by the following equation (5).

[0066] Furthermore, the S / N ratio when the piezoelectric layer 231 is a single layer is expressed by the following formula (6).

[0067] The inventors of the present application used equations (1), (2), (5), and (6) to calculate the S / N ratios for a single piezoelectric layer 231 and for a double piezoelectric layer 231 using representative values ​​of each parameter. In this calculation, the length L of the cantilever 200 was set to a representative value when the piezoelectric layer 231 was single-layered. On the other hand, in the calculation for a double piezoelectric layer 231, the length L of the cantilever 200 was derived from equation (3) under the condition that the resonant frequency is the same when the piezoelectric layer 231 is single-layered and when the piezoelectric layer 231 is double-layered. The density ρ and Young's modulus E of the cantilever 200 were calculated as weighted averages of the substrate 210 and the piezoelectric layer 231. The neutral axis e of the cantilever 200 was assumed to coincide with the thickness hs of the substrate 210.

[0068] As a result of calculation, the S / N ratio when there is one piezoelectric layer 231 is more than twice as high as the S / N ratio when there are two piezoelectric layers 231. In other words, when there is one piezoelectric layer 231, the amount of electric charge generated is more than twice as high as when there are two piezoelectric layers 231. This confirms that when there is one piezoelectric layer 231, the sound wave reception characteristics can be improved compared to when there are two piezoelectric layers 231.

[0069] (Manufacturing Method of Piezoelectric Element) A manufacturing method of the piezoelectric element 100 will be described with reference to Figures 5 to 12. The manufacturing method of the piezoelectric element 100 includes the following steps. a step of preparing the substrate 110; a step of forming an electrode 120 on the substrate 110; a step of forming a piezoelectric layer 130 on the electrode 120; a step of forming an electrode 140 on the piezoelectric layer 130; a step of forming a piezoelectric layer 150 on the electrode 140; a step of forming an electrode 160 on the piezoelectric layer 150; a step of forming a transmitter upper electrode 22c by processing the electrode 160; a step of forming a transmitter piezoelectric layer 21b by processing the piezoelectric layer 150; a step of forming an intermediate electrode 22b and a receiver upper electrode 32b by processing the electrode 140; a step of forming a transmitter piezoelectric layer 21a and a receiver piezoelectric layer 31 by processing the piezoelectric layer 130; a step of forming a transmitter lower electrode 22a and a receiver lower electrode 32a by processing the electrode 120; a step of forming an insulating layer 60; a step of forming a wiring layer 70; a step of forming a substrate 10 by processing the substrate 110.

[0070] 5 shows a process for preparing a substrate 10. In the process for preparing a substrate 10, a substrate 110 is prepared as an SOI substrate. The substrate 110 has a structure in which a first silicon layer 111, a silicon oxide layer 112 as an insulating layer, and a second silicon layer 113 as a single crystal are arranged in this order.

[0071] 6 shows the steps of forming an electrode 120 on a substrate 110, forming a piezoelectric layer 130 on the electrode 120, forming an electrode 140 on the piezoelectric layer 130, forming a piezoelectric layer 150 on the electrode 140, and forming an electrode 160 on the piezoelectric layer 150. The electrode 120 is an example of a "lower electrode" in the claims. The piezoelectric layer 130 is an example of a "first piezoelectric layer" in the claims. The electrode 140 is an example of an "intermediate electrode" in the claims. The piezoelectric layer 150 is an example of a "second piezoelectric layer" in the claims. The electrode 160 is an example of an "upper electrode" in the claims.

[0072] In the step of forming the electrode 120 on the substrate 110, the electrode 120, which is a multilayer film or a single layer film, is laminated on the second silicon layer 113 of the substrate 110 by sputtering. At this time, the electrode 120 is formed on the single-crystal second silicon layer 113 at a temperature at which epitaxial growth occurs. Furthermore, in the step of forming the piezoelectric layer 130 on the electrode 120, the piezoelectric layer 130 is laminated on the electrode 120 by sputtering. At this time, the piezoelectric layer 130 is formed on the electrode 120 at a temperature at which epitaxial growth occurs.

[0073] In the step of forming the electrode 140 on the piezoelectric layer 130, the electrode 140 of a multilayer film or a single layer film is laminated on the piezoelectric layer 130 by sputtering. At this time, the electrode 140 is formed on the piezoelectric layer 130 at a temperature that does not cause epitaxial growth. Furthermore, in the step of forming the piezoelectric layer 150 on the electrode 140, the piezoelectric layer 150 is laminated on the electrode 140 by sputtering. At this time, the piezoelectric layer 150 is formed on the electrode 140 at a temperature that does not cause epitaxial growth. Furthermore, in the step of forming the electrode 160 on the piezoelectric layer 150, the electrode 160 of a multilayer film or a single layer film is laminated on the piezoelectric layer 150 by sputtering. At this time, the electrode 160 is formed on the piezoelectric layer 150 at a temperature that does not cause epitaxial growth. In this way, a piezoelectric film substrate 300 is manufactured, which includes a substrate 110, an electrode 120 arranged on the substrate 110, a piezoelectric layer 130 arranged on the electrode 120, an electrode 140 that is not an epitaxially grown layer arranged on the piezoelectric layer 130, a piezoelectric layer 150 that is not an epitaxially grown layer arranged on the electrode 140, and an electrode 160 that is not an epitaxially grown layer arranged on the piezoelectric layer 150.

[0074] In the manufacturing method of the piezoelectric element 100 of this embodiment, the piezoelectric layer 130 constituting the receiving-side piezoelectric layer 31 is epitaxially grown and also constitutes the transmitting-side piezoelectric layer 21, while the piezoelectric layer 150 not constituting the receiving-side piezoelectric layer 31 is not epitaxially grown. That is, the piezoelectric layer 130 is formed to have high crystallinity, while the piezoelectric layer 150 is formed to have low crystallinity. Higher crystallinity results in a lower dielectric constant and improved sensor characteristics of the piezoelectric layer. Therefore, epitaxial growth of the piezoelectric layer 130 constituting the receiving-side piezoelectric layer 31 improves the sensor characteristics of the receiving-side piezoelectric layer 31. This makes it possible to improve the ultrasonic wave reception characteristics. Furthermore, lower crystallinity results in a higher piezoelectric constant and improved actuator characteristics of the piezoelectric layer. Therefore, while the piezoelectric layer 150 constituting the transmitting-side piezoelectric layer 21 and not epitaxial growth of the piezoelectric layer not constituting the receiving-side piezoelectric layer 31 improves the actuator characteristics of the transmitting-side piezoelectric layer 21. This makes it possible to improve the transmission characteristics of ultrasonic waves.In this way, it is possible to improve the transmission and reception characteristics of ultrasonic waves.

[0075] FIG. 7 shows a process of forming the transmitter upper electrode 22c by processing the electrode 160 and a process of forming the transmitter piezoelectric layer 21b by processing the piezoelectric layer 150.

[0076] In the step of forming the transmitter upper electrode 22c by processing the electrode 160, the shape of the electrode 160 is processed (patterned) by etching, thereby forming the transmitter upper electrode 22c. At this time, the part of the electrode 160 other than the part that will become the transmitter upper electrode 22c is removed. Also, in the step of forming the transmitter piezoelectric layer 21b by processing the piezoelectric layer 150, the shape of the piezoelectric layer 150 is processed (patterned) by etching, thereby forming the transmitter piezoelectric layer 21b. At this time, the part of the piezoelectric layer 150 other than the part that will become the transmitter piezoelectric layer 21b is removed.

[0077] FIG. 8 shows a process of forming the intermediate electrode 22b and the receiving upper electrode 32b by processing the electrode 140, and a process of forming the transmitting piezoelectric layer 21a and the receiving piezoelectric layer 31 by processing the piezoelectric layer 130.

[0078] In the step of forming the intermediate electrode 22b and the receiver upper electrode 32b by processing the electrode 140, the shape of the electrode 140 is processed (patterned) by etching, thereby forming the intermediate electrode 22b and the receiver upper electrode 32b. At this time, parts of the electrode 140 other than those that will become the intermediate electrode 22b and the receiver upper electrode 32b are removed. Also, in the step of forming the transmitter piezoelectric layer 21a and the receiver piezoelectric layer 31 by processing the piezoelectric layer 130, the shape of the piezoelectric layer 130 is processed (patterned) by etching, thereby forming the transmitter piezoelectric layer 21a and the receiver piezoelectric layer 31. At this time, parts of the piezoelectric layer 130 other than those that will become the transmitter piezoelectric layer 21a and the receiver piezoelectric layer 31 are removed.

[0079] 9 shows a process of forming the transmitter lower electrode 22a and the receiver lower electrode 32a by processing the electrode 120. In the process of forming the transmitter lower electrode 22a and the receiver lower electrode 32a by processing the electrode 120, the shape of the electrode 120 is processed (patterned) by etching to form the transmitter lower electrode 22a and the receiver lower electrode 32a. At this time, portions of the electrode 120 other than those to become the transmitter lower electrode 22a and the receiver lower electrode 32a are removed.

[0080] In the manufacturing method of the piezoelectric element 100 of this embodiment, the electrode 120, the piezoelectric layer 130, the electrode 140, the piezoelectric layer 150, and the electrode 160 are sequentially laminated, and then the electrode 160, the piezoelectric layer 150, the electrode 140, the piezoelectric layer 130, and the electrode 120 are sequentially processed to form the transmitting side section 20 and the receiving side section 30 so that the number of layers of the transmitting side piezoelectric layer 21 is greater than the number of layers of the receiving side piezoelectric layer 31. The greater the number of layers of the transmitting side piezoelectric layer 21, the greater the displacement of the transmitting side piezoelectric layer 21 with respect to the applied voltage, thereby improving the ultrasonic wave transmission characteristics. Furthermore, the smaller the number of layers of the receiving side piezoelectric layer, the more the signal-to-noise ratio can be improved, thereby improving the ultrasonic wave reception characteristics. As a result, the ultrasonic wave transmission and reception characteristics can be improved. Furthermore, after the layers are sequentially laminated, it is only necessary to sequentially process (pattern) each of the laminated layers. That is, the manufacturing process can be simplified because the transmitting piezoelectric layer 21 and the receiving piezoelectric layer 31 can be formed (patterned) in a common process for the piezoelectric layers in the same layer counted from the substrate 10. Unlike the present embodiment, if the number of transmitting piezoelectric layers and the number of receiving piezoelectric layers are the same but the thicknesses of the piezoelectric layers are made different, each piezoelectric layer must be formed (patterned) independently, which requires a complicated process.

[0081] 10 shows a process for forming the insulating layer 60. In the process for forming the insulating layer 60, an insulator is stacked on the substrate 10, the transmitting unit 20, and the receiving unit 30 by atomic layer deposition (ALD). Then, the shape of the insulator is processed (patterned) by etching, thereby forming the insulating layer 60.

[0082] 11 shows a process for forming the wiring layer 70. In the process for forming the wiring layer 70, a conductor is formed by sputtering on the insulating layer 60, the transmitting unit 20, and the receiving unit 30. Then, the shape of the conductor is processed (patterned) by etching, thereby forming the wiring layer 70 including wiring units 71 to 74.

[0083] 12 shows a process of forming the substrate 10 by processing the substrate 110. In the process of forming the substrate 10 by processing the substrate 110, the shape of the substrate 110 is processed by etching, thereby forming the substrate 10. That is, at least the first silicon layer 111 and the silicon oxide layer 112 are removed from the back surface side of the substrate 110 to form a recess 14 having the vibrating portion 14a and a recess 15 having the vibrating portion 15a. In this way, the substrate 10 is formed, including the first silicon layer 11, the silicon oxide layer 12, the second silicon layer 13, the recess 14, and the recess 15. In this manner, the piezoelectric element 100 is manufactured.

[0084] (Effects of this embodiment) In this embodiment, the following effects can be obtained.

[0085] In this embodiment, as described above, the number of layers of the multiple transmitting piezoelectric layers 21 is greater than the number of layers of the receiving piezoelectric layers 31. This allows the number of layers of the transmitting piezoelectric layers 21 to be set to a number suitable for improving the transmission characteristics of sound waves (ultrasound), and the number of layers of the receiving piezoelectric layers 31 to be set to a number suitable for improving the reception characteristics of sound waves (ultrasound). That is, the greater the number of layers of the transmitting piezoelectric layers 21, the greater the displacement of the transmitting piezoelectric layers 21 with respect to the applied voltage, allowing for the transmission of sound waves (ultrasound) at higher pressures. As a result, the transmission characteristics of sound waves (ultrasound) can be improved. Furthermore, the smaller the number of layers of the receiving piezoelectric layers 31, the higher the crystallinity and the lower the dielectric constant can be. This improves the sensor characteristics, thereby improving the signal-to-noise ratio. As a result, the reception characteristics of sound waves (ultrasound) can be improved. Furthermore, unlike the present embodiment, if the number of transmitting piezoelectric layers and receiving piezoelectric layers is the same but the total thickness of the transmitting piezoelectric layers is different from the total thickness of the receiving piezoelectric layers, each piezoelectric layer must be formed (patterned) independently, requiring a complicated process to differentiate the thicknesses of the piezoelectric layers. In contrast, the piezoelectric element 100 of the present embodiment is configured such that the number of transmitting piezoelectric layers 21 and the number of receiving piezoelectric layers 31 are different. Therefore, for piezoelectric layers in the same layer counted from the substrate 10, the transmitting piezoelectric layers 21 and the receiving piezoelectric layers 31 can be formed (patterned) in a common process. This simplifies the manufacturing process of the piezoelectric element. As described above, a piezoelectric element can be provided that can improve the transmission and reception characteristics of sound waves (ultrasound) and simplify the manufacturing process.

[0086] Here, among the multiple transmitting piezoelectric layers 21, the transmitting piezoelectric layers 21 located above the substrate 10 tend to have lower crystallinity because they are less susceptible to the influence of the substrate 10, which has a higher crystallinity. The lower the crystallinity, the higher the piezoelectric constant and the improved actuator characteristics, allowing the transmitting piezoelectric layers 21 to be displaced more greatly in response to an applied voltage. Therefore, in the piezoelectric element 100 of this embodiment, the number of layers of the multiple transmitting piezoelectric layers 21 is greater than the number of layers of the receiving piezoelectric layers 31, so that the crystallinity of the transmitting piezoelectric layers 21 located above the uppermost receiving piezoelectric layer 31 is lower, and the sound wave (ultrasound) transmission characteristics of the transmitting piezoelectric layers 21 can be improved compared to the receiving piezoelectric layers 31.

[0087] In this embodiment, as described above, the number of layers in the multiple transmitting piezoelectric layers 21 is two or more, and the number of layers in one receiving piezoelectric layer 31 is one. This allows the receiving piezoelectric layer 31 to be configured with the minimum number of layers, thereby further improving the reception characteristics of sound waves (ultrasound waves). Here, the first receiving piezoelectric layer 31 tends to have high crystallinity due to the influence of the highly crystalline substrate 10. The higher the crystallinity, the lower the dielectric constant and the improved sensor characteristics, thereby improving the signal-to-noise ratio. Therefore, by having only one layer in the receiving piezoelectric layer 31, the crystallinity of the receiving piezoelectric layer 31 is high, and the reception characteristics of sound waves (ultrasound waves) can be further improved.

[0088] In this embodiment, as described above, the number of layers in the plurality of transmitter piezoelectric layers 21 is two. This allows the transmitter piezoelectric layers 21 to be configured with the minimum number of layers, thereby simplifying the manufacturing process. In other words, by having two transmitter piezoelectric layers 21, the transmission characteristics of sound waves (ultrasound waves) can be improved while simplifying the manufacturing process.

[0089] In this embodiment, as described above, the plurality of transmitter piezoelectric layers 21 include a transmitter piezoelectric layer 21a and a transmitter piezoelectric layer 21b disposed farther from the substrate 10 than the transmitter piezoelectric layer 21a. The plurality of transmitter electrodes 22 include a transmitter lower electrode 22a disposed between the substrate 10 and the transmitter piezoelectric layer 21a, an intermediate electrode 22b disposed between the transmitter piezoelectric layer 21a and the transmitter piezoelectric layer 21b, and a transmitter upper electrode 22c disposed on the transmitter piezoelectric layer 21b. The transmitter lower electrode 22a and the transmitter upper electrode 22c are commonly connected to the voltage-application electrode 41, and the intermediate electrode 22b is connected to the voltage-application electrode 42. This allows voltages to be applied to the transmitter piezoelectric layer 21a and the transmitter piezoelectric layer 21b so that the transmitter piezoelectric layer 21a and the transmitter piezoelectric layer 21b expand and contract in opposite directions. As a result, the transmitter section 20 as a whole can be largely displaced. This makes it possible to further improve the transmission characteristics of sound waves (ultrasonic waves) in a configuration in which the number of layers of the transmitting piezoelectric layer 21 is two.

[0090] In this embodiment, as described above, the intermediate electrode 22b is located at the center, in the thickness direction of the substrate 10, of the entire transmitting unit 20, including the vibrating unit 14a provided on the substrate 10. With this configuration, the direction of deformation can be reversed up and down from the center, in the thickness direction of the substrate 10, of the entire transmitting unit 20, including the vibrating unit 14a, so that the transmitting unit 20, including the vibrating unit 14a, can be displaced more greatly as a whole.

[0091] In this embodiment, as described above, the first receiving piezoelectric layer 31 counting from the substrate 10 side is an epitaxially grown layer. This makes it possible to increase the crystallinity of the first receiving piezoelectric layer 31. As a result, the higher the crystallinity, the better the sensor characteristics, and therefore the signal-to-noise ratio can be improved, thereby further improving the reception characteristics of sound waves (ultrasound waves).

[0092] In this embodiment, as described above, the area of ​​the receiving section 30, including the vibrating section 15a provided on the substrate 10, is smaller in plan view than the area of ​​the transmitting section 20, including the vibrating section 14a provided on the substrate 10. Here, if the number of layers of the transmitting-side piezoelectric layer 21 is greater than the number of layers of the receiving-side piezoelectric layer 31, the thickness of the transmitting section 20 is greater than that of the receiving section 30, and therefore the resonant frequency, which is proportional to the thickness, is higher in the transmitting section 20 than in the receiving section 30. In this case, it is not possible to match the resonant frequencies of the receiving section 30 and the transmitting section 20. Therefore, with the above-described configuration, it is possible to increase the resonant frequency, which is inversely proportional to the area in plan view, in the receiving section 30. Therefore, even if the thickness of the transmitting section 20 is greater than that of the receiving section 30, it is possible to match the resonant frequencies of the receiving section 30 and the transmitting section 20. As a result, sound waves (ultrasound waves) of the common resonant frequency can be transmitted from the transmitting side unit 20 and the reflected sound waves (ultrasound waves) of the common resonant frequency can be received by the receiving side unit 30 .

[0093] In this embodiment, as described above, among the plurality of transmitter piezoelectric layers 21 and one or more receiver piezoelectric layers 31, the piezoelectric layers in the same layer, counting from the substrate 10 side, are made of the same material and have the same thickness, and among the plurality of transmitter electrodes 22 and the plurality of receiver electrodes 32, the electrodes in the same layer, counting from the substrate 10 side, are made of the same material and have the same thickness. This allows the piezoelectric layers and electrodes to be formed (patterned) in a common process, thereby simplifying the manufacturing process of the piezoelectric element 100.

[0094] In this embodiment, as described above, the plurality of transmitter piezoelectric layers 21 and receiver piezoelectric layers 31 are formed of lead zirconate titanate, which makes it possible to improve the transmission and reception characteristics of sound waves (ultrasound waves) and simplify the manufacturing process when the transmitter piezoelectric layers 21 and receiver piezoelectric layers 31 are formed of lead zirconate titanate.

[0095] In this embodiment, as described above, the piezoelectric film substrate 300 comprises a substrate 110, an electrode 120 arranged on the substrate 110, a piezoelectric layer 130 arranged on the electrode 120, an electrode 140 arranged on the piezoelectric layer 130 and not an epitaxially grown layer, a piezoelectric layer 150 arranged on the electrode 140 and not an epitaxially grown layer, and an electrode 160 arranged on the piezoelectric layer 150 and not an epitaxially grown layer. The method for manufacturing the piezoelectric film substrate 300 includes the steps of preparing a substrate 110, forming an electrode 120 on the substrate 110, forming a piezoelectric layer 130 on the electrode 120, forming an electrode 140 on the piezoelectric layer 130 at a temperature at which epitaxial growth does not occur, forming a piezoelectric layer 150 on the electrode 140 at a temperature at which epitaxial growth does not occur, and forming an electrode 160 on the piezoelectric layer 150 at a temperature at which epitaxial growth does not occur. This reduces the crystallinity of the piezoelectric layer 150, thereby improving the actuator characteristics. Furthermore, if the piezoelectric layer 150 contains lead and is deposited at a relatively high temperature, undesirable diffusion of lead from the piezoelectric layer 150 to the electrode 140 may occur. However, since the piezoelectric layer 150 is not an epitaxially grown layer, it is possible to deposit the piezoelectric layer 150 at a relatively low temperature, thereby suppressing undesirable diffusion of lead from the piezoelectric layer 150 to the electrode 140. Furthermore, depositing the piezoelectric layer 150 at a relatively low temperature reduces film stress. Furthermore, since the electrode 140 is not an epitaxially grown layer, it is possible to easily prevent the piezoelectric layer 150 from epitaxially growing. Furthermore, if the piezoelectric layer 130 contains lead and the electrode 140 is deposited at a relatively high temperature, undesirable diffusion of lead from the piezoelectric layer 130 to the electrode 140 may occur. However, since the electrode 140 is not an epitaxially grown layer, it is possible to suppress undesirable diffusion of lead from the piezoelectric layer 130 to the electrode 140. Furthermore, if the electrode 140 is deposited at a relatively high temperature, the crystal structure of the piezoelectric layer 130 may change. However, since the electrode 140 is not an epitaxially grown layer, it is possible to deposit it at a relatively low temperature, and therefore it is possible to prevent the crystal structure of the piezoelectric layer 130 from changing.

[0096] (Modifications) The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the description of the above-mentioned embodiments, and includes all modifications (modifications) within the meaning and scope of the claims.

[0097] For example, in the above embodiment, the vibrating portion 14a on the transmitting side 20 side and the vibrating portion 15a on the receiving side 30 side have the same thickness, but the present invention is not limited to this. In the present invention, the vibrating portion on the transmitting side and the vibrating portion on the receiving side may have different thicknesses. For example, in a first modified example shown in FIG. 13 , the thickness of the vibrating portion 315a on the receiving side 30 side is smaller than the thickness of the vibrating portion 14a on the transmitting side 20 side. With this configuration, the vibrating portion 315a can be vibrated more strongly in response to sound waves (ultrasound waves), and therefore the receiving-side piezoelectric layer 31 can be vibrated more strongly in response to the vibration of the vibrating portion 315a.

[0098] Although the above embodiment illustrates an example in which the multiple transmitter piezoelectric layers 21 have the same thickness, the present invention is not limited to this. In the present invention, the multiple transmitter piezoelectric layers may have different thicknesses. For example, in a second modified example shown in FIG. 14 , the thickness of the transmitter piezoelectric layer 421b is greater than the thickness of the transmitter piezoelectric layer 21a. With this configuration, the greater the thickness, the better the voltage resistance of the piezoelectric layer, allowing a larger voltage to be applied to the transmitter piezoelectric layer 421b. As a result, the transmitter piezoelectric layer 421b can be displaced more significantly, thereby improving the transmission characteristics of sound waves (ultrasound waves). Note that, unlike the above embodiment, if the thickness of the transmitter piezoelectric layer 421b is greater than the thickness of the transmitter piezoelectric layer 21a, it is preferable to connect the transmitter lower electrode 22a and the transmitter upper electrode 22c to separate voltage application electrodes and apply a voltage greater than the voltage applied to the transmitter piezoelectric layer 421b. Furthermore, regardless of the thickness of each transmitter piezoelectric layer, the transmitter lower electrode and the receiver upper electrode may be connected to separate voltage application electrodes.

[0099] In the above embodiment, the piezoelectric element 100 is configured to transmit and receive ultrasonic waves, but the present invention is not limited to this. In the present invention, the piezoelectric element may be configured to transmit and receive sound waves in the audible range.

[0100] In the above embodiment, the number of layers of the transmitter piezoelectric layer 21 is two, but the present invention is not limited to this. In the present invention, the number of layers of the transmitter piezoelectric layer may be three or more.

[0101] Furthermore, in the above embodiment, an example was shown in which the number of layers in the receiving-side piezoelectric layer 31 was one, but the present invention is not limited to this. In the present invention, the number of layers in the receiving-side piezoelectric layer may be two or more, as long as it is smaller than the number of layers in the transmitting-side piezoelectric layer. When the number of layers in the receiving-side piezoelectric layer is two or more, the second and subsequent receiving-side piezoelectric layers may also be epitaxially grown layers. Furthermore, when there are multiple receiving-side piezoelectric layers, the multiple receiving-side piezoelectric layers may have the same thickness or different thicknesses. Furthermore, when there are multiple receiving-side piezoelectric layers, the receiving-side upper electrode and the receiving-side lower electrode may be connected to the same charge detection electrode or to separate charge detection electrodes.

[0102] In the above embodiment, the transmitter piezoelectric layer 21 and the receiver piezoelectric layer 31 are formed from lead zirconate titanate, but the present invention is not limited to this. In the present invention, the transmitter piezoelectric layer and the receiver piezoelectric layer may be formed from a piezoelectric material other than lead zirconate titanate. Furthermore, multiple transmitter piezoelectric layers may be formed from the same piezoelectric material or different piezoelectric materials. Furthermore, when there are multiple receiver piezoelectric layers, the multiple receiver piezoelectric layers may be formed from the same piezoelectric material or different piezoelectric materials. According to the manufacturing method of the above embodiment, the transmitter piezoelectric layer and the receiver piezoelectric layer of the same layer, counting from the substrate side, are formed from the same material and to the same thickness.

[0103] Furthermore, in the above embodiment, an example was shown in which the first receiving-side piezoelectric layer 31 (piezoelectric layer 130) was an epitaxially grown layer, but the present invention is not limited to this. In the present invention, as long as the receiving side section has reception characteristics that are practically acceptable, the first receiving-side piezoelectric layer does not have to be an epitaxially grown layer. In other words, the first piezoelectric layer of the piezoelectric film substrate does not have to be an epitaxially grown layer. In this case, the crystallinity of the first piezoelectric layer can be reduced, thereby improving the actuator characteristics. Furthermore, a first piezoelectric layer that is not an epitaxially grown layer can be formed at a relatively low temperature, thereby reducing film stress.

[0104] In the above embodiment, the area of ​​the receiving section 30 is smaller than the area of ​​the transmitting section 20 in order to match the resonant frequencies of the transmitting section 20 and the receiving section 30, but the present invention is not limited to this. In the present invention, if it is not necessary to match the resonant frequencies of the transmitting section and the receiving section, the area of ​​the receiving section may be larger than the area of ​​the transmitting section. Also, the areas of the receiving section and the transmitting section may be the same.

[0105] Furthermore, in the above embodiment, an example was shown in which the piezoelectric element 100 is provided with one transmitting side unit 20 and one receiving side unit 30, but the present invention is not limited to this. In the present invention, the piezoelectric element may be provided with multiple transmitting side units or multiple receiving side units. Furthermore, the number of transmitting side units and the number of receiving side units may or may not be the same. That is, one or more transmitting side units and a larger number of receiving side units than the transmitting side units may be provided, or one or more receiving side units and a larger number of transmitting side units than the receiving side units may be provided.

[0106] In the above embodiment, the substrate 10 (110) is an SOI substrate, but the present invention is not limited to this. In the present invention, the substrate may be a semiconductor substrate other than an SOI substrate.

[0107] In the above embodiment, the electrode 120 (transmitter lower electrode 22a, receiver lower electrode 32a), piezoelectric layer 130 (transmitter piezoelectric layer 21a, receiver piezoelectric layer 31), electrode 140 (intermediate electrode 22b, receiver upper electrode 32b), piezoelectric layer 150 (transmitter piezoelectric layer 21b), and electrode 160 (transmitter upper electrode 22c) are formed by sputtering. However, the present invention is not limited to this. In the present invention, each electrode and each piezoelectric layer may be formed by a method other than sputtering. For example, each electrode may be formed by vacuum deposition. Furthermore, each piezoelectric layer may be formed by PLD (pulsed laser deposition), CVD (chemical vapor deposition), a sol-gel method, or the like.

[0108] In the above embodiment, the electrode 140 (intermediate electrode) of the piezoelectric film substrate 300 may include a (111) preferentially oriented film. In this case, since the intermediate electrode includes a (111) preferentially oriented film, it is possible to easily prevent the piezoelectric layer 150 (second piezoelectric layer) from growing epitaxially.

[0109] REFERENCE SIGNS LIST 10 Substrate 14a Vibration section (transmission side vibration section) 15a Vibration section (reception side vibration section) 20 Transmission side section 21 Transmission side piezoelectric layer 21a Transmission side piezoelectric layer (first transmission side piezoelectric layer) 21b Transmission side piezoelectric layer (second transmission side piezoelectric layer) 22 Transmission side electrode 22a Transmission side lower electrode 22b Intermediate electrode 22c Transmission side upper electrode 30 Reception side section 31 Reception side piezoelectric layer 32 Reception side electrode 32a Reception side lower electrode 32b Reception side upper electrode 41 Voltage application electrode (first voltage application electrode) 42 Voltage application electrode (second voltage application electrode) 100 Piezoelectric element 110 Substrate 120 Electrode (lower electrode) 130 Piezoelectric layer (first piezoelectric layer) 140 Electrode (intermediate electrode) 150 Piezoelectric layer (second piezoelectric layer) 160 Electrode (upper electrode) 300 Piezoelectric film substrate

Claims

1. A piezoelectric element comprising: a substrate; a transmitting side portion arranged on the substrate for transmitting acoustic waves; and a receiving side portion arranged on the substrate for receiving acoustic waves, wherein the transmitting side portion includes a plurality of transmitting piezoelectric layers and a plurality of transmitting electrodes arranged to sandwich each of the plurality of transmitting piezoelectric layers; the receiving side portion includes one or more receiving piezoelectric layers and a plurality of receiving electrodes arranged to sandwich each of the one or more receiving piezoelectric layers, and the number of layers of the plurality of transmitting piezoelectric layers is greater than the number of layers of the one or more receiving piezoelectric layers.

2. The piezoelectric element according to claim 1, wherein the number of layers in the plurality of transmitting piezoelectric layers is two or more, and the number of layers in the single receiving piezoelectric layer is one.

3. The piezoelectric element according to claim 2, wherein the number of layers of the plurality of transmitting piezoelectric layers is two.

4. The piezoelectric element described in claim 3, wherein the multiple transmitting side piezoelectric layers include a first transmitting side piezoelectric layer and a second transmitting side piezoelectric layer located farther from the substrate than the first transmitting side piezoelectric layer, and the multiple transmitting side electrodes include a transmitting side lower electrode located between the substrate and the first transmitting side piezoelectric layer, an intermediate electrode located between the first transmitting side piezoelectric layer and the second transmitting side piezoelectric layer, and a transmitting side upper electrode located on the second transmitting side piezoelectric layer, wherein the transmitting side lower electrode and the transmitting side upper electrode are commonly connected to a first voltage application electrode, and the intermediate electrode is connected to a second voltage application electrode.

5. A piezoelectric element as described in claim 4, wherein the intermediate electrode is located at the center of the entire transmitting side section including the transmitting side vibration section provided on the substrate in the thickness direction of the substrate.

6. The piezoelectric element according to claim 1, wherein the first receiving piezoelectric layer, counting from the substrate side, of the one or more receiving piezoelectric layers is an epitaxially grown layer.

7. A piezoelectric element as described in claim 1, wherein, in a plan view, the area of ​​the receiving side portion, including the receiving vibration portion provided on the substrate, is smaller than the area of ​​the transmitting side portion, including the transmitting vibration portion provided on the substrate.

8. A piezoelectric element as described in claim 1, wherein, among the plurality of transmitting piezoelectric layers and the one or more receiving piezoelectric layers, piezoelectric layers in the same layer as counted from the substrate side are formed of the same material and have the same thickness, and, among the plurality of transmitting electrodes and the plurality of receiving electrodes, electrodes in the same layer as counted from the substrate side are formed of the same material and have the same thickness.

9. The piezoelectric element according to claim 1, wherein said plurality of transmitter piezoelectric layers and said one or more receiver piezoelectric layers are formed from lead zirconate titanate.

10. A piezoelectric film substrate comprising: a substrate; a lower electrode disposed on the substrate; a first piezoelectric layer disposed on the lower electrode; an intermediate electrode which is not an epitaxially grown layer and disposed on the first piezoelectric layer; a second piezoelectric layer which is not an epitaxially grown layer and disposed on the intermediate electrode; and an upper electrode which is not an epitaxially grown layer and disposed on the second piezoelectric layer.

11. The piezoelectric film substrate according to claim 10, wherein the first piezoelectric layer is not an epitaxially grown layer.

12. The piezoelectric film substrate according to claim 10, wherein the intermediate electrode includes a (111) preferentially oriented film.

13. A method for manufacturing a piezoelectric film substrate, comprising the steps of: preparing a substrate; forming a lower electrode on the substrate; forming a first piezoelectric layer on the lower electrode; forming an intermediate electrode on the first piezoelectric layer at a temperature that does not cause epitaxial growth; forming a second piezoelectric layer on the intermediate electrode at a temperature that does not cause epitaxial growth; and forming an upper electrode on the second piezoelectric layer at a temperature that does not cause epitaxial growth.

14. The method for manufacturing a piezoelectric film substrate as described in claim 13, wherein in the step of forming the first piezoelectric layer on the lower electrode, the first piezoelectric layer is not an epitaxially grown layer.

15. The method for manufacturing a piezoelectric film substrate as described in claim 13, wherein in the step of forming the intermediate electrode on the first piezoelectric layer at a temperature that does not cause epitaxial growth, the intermediate electrode includes a (111) preferentially oriented film.

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