Patent Publication Number: US-2020298276-A1

Title: Ultrasonic Device And Ultrasonic Apparatus

Description:
The present application is based on, and claims priority from JP Application Ser. No. 2019-054529, filed Mar. 22, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an ultrasonic device and an ultrasonic apparatus. 
     2. Related Art 
     In the related art, there is an ultrasonic apparatus including a substrate provided with an opening, a vibration plate provided on the substrate to close the opening, and an ultrasonic device laminated on the vibration plate (for example, refer to JP-A-2002-271897). 
     In the ultrasonic apparatus disclosed in JP-A-2002-271897, the vibration plate is formed by laminating a membrane made of SiO 2  and a barrier layer made of ZrO 2 , and a piezoelectric layer made of PZT or the like is laminated on the barrier layer. In this configuration, a chemical interaction between an electrode layer formed in or on the membrane and the piezoelectric layer, that is, diffusion of Pb can be prevented by the barrier layer. In the ultrasonic apparatus disclosed in JP-A-2002-271897, the barrier layer has a bending rigidity less than that of the membrane. 
     However, in JP-A-2002-271897, a Young&#39;s modulus of SiO 2  forming the membrane is lower than a Young&#39;s modulus of ZrO 2  forming the barrier layer. Therefore, in order to make the bending rigidity of the barrier layer less than the bending rigidity of the membrane, it is necessary to make a thickness of the membrane considerably large. In this case, a thickness of the vibration plate is increased, and thus there is a problem in that drive characteristics of the ultrasonic device change. 
     For example, a resonance frequency of the ultrasonic device increases, and thus transmission and reception of ultrasonic waves with a desired frequency are difficult. When a width of an opening is increased to reduce a resonance frequency, displacement efficiency of the vibration plate deteriorates. In this case, when an ultrasonic wave is transmitted, power of the transmitted ultrasonic wave is reduced, and, when an ultrasonic wave is received, a reception sensitivity is reduced. 
     SUMMARY 
     An ultrasonic device according to a first application example includes a base material that has an opening; a vibration plate that is provided on the base material and closes the opening; and a piezoelectric element that is provided on the vibration plate, in which the vibration plate has a first layer provided on the base material and a second layer disposed between the first layer and the piezoelectric element, and a bending rigidity of the second layer is equal to or larger than a bending rigidity of the first layer. 
     In the ultrasonic device according to the application example, a width of the opening may be equal to or less than 100 μm. 
     In the ultrasonic device according to the application example, the second layer may have a thickness equal to or more than a predetermined defined value such that piezoelectric characteristics of the piezoelectric element is maintained. 
     The ultrasonic device according to the application example may further include an ultrasonic transducer that includes a vibration portion closing the opening in the vibration plate and the piezoelectric element, and, when a resonance frequency of the ultrasonic transducer is a first frequency when a thickness of the second layer is set to the defined value, and the bending rigidity of the first layer is the same as the bending rigidity of the second layer, the thickness of the second layer may be set to the defined value when the resonance frequency of the ultrasonic transducer is lower than the first frequency. 
     The ultrasonic device according to the application example may further include a vibration attenuation layer having a thickness corresponding to the resonance frequency is provided on the vibration plate when the resonance frequency of the ultrasonic transducer is lower than a second frequency lower than the first frequency. 
     In the ultrasonic device according to the application example, the first layer may be made of SiO 2 , and the second layer may be made of ZrO 2 . 
     An ultrasonic apparatus according to a second application example includes the ultrasonic device according to the first application example; and a controller that controls the ultrasonic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a schematic configuration of an ultrasonic apparatus of an embodiment. 
         FIG. 2  is a schematic plan view illustrating an ultrasonic device of the present embodiment. 
         FIG. 3  is a sectional view of the ultrasonic device taken along the line III-III in  FIG. 2 . 
         FIG. 4  is a graph illustrating a relationship between a width of an opening and a displacement amount of a vibration portion. 
         FIG. 5  is a graph illustrating a relationship between a thickness of a first layer and a bending rigidity ratio between a first layer and the second layer when a thickness of the second layer is set to a defined value. 
         FIG. 6  is a graph illustrating a relationship between a width of an opening and a bending rigidity ratio between the first layer and the second layer when a resonance frequency of an ultrasonic transducer is set to a predetermined frequency. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment will be described. 
       FIG. 1  is a block diagram illustrating a schematic configuration of an ultrasonic apparatus  100  of the present embodiment. 
     As illustrated in  FIG. 1 , the ultrasonic apparatus  100  of the present embodiment includes an ultrasonic device  10 , and a controller  20  controlling the ultrasonic device  10 . In the ultrasonic apparatus  100  of the present embodiment, the controller  20  controls the ultrasonic device  10  via a drive circuit  30 , and transmits an ultrasonic wave to a target object from the ultrasonic device  10 . When the ultrasonic wave is reflected by the target object, and thus a reflected wave is received by the ultrasonic device  10 , the controller  20  calculates a distance from the ultrasonic device  10  to the target object based on a period of time from a transmission timing of the ultrasonic wave to a reception timing of the ultrasonic wave. 
     Hereinafter, a configuration of the ultrasonic apparatus  100  will be described in detail. 
     Configuration of Ultrasonic Device  10   
       FIG. 2  is a schematic plan view illustrating the ultrasonic device  10 .  FIG. 3  is a sectional view of the ultrasonic device  10  taken along the line III-III in  FIG. 2 . 
     As illustrated in  FIG. 3 , the ultrasonic device  10  is configured to include an element substrate  11  that is a base material, a vibration plate  12 , and a piezoelectric element  13 . 
     Configuration of Element Substrate  11   
     The element substrate  11  is a substrate that is made of Si and has a predetermined thickness for supporting the vibration plate  12 . The element substrate  11  has a first surface  11 A and a second surface  11 B on an opposite side to the first surface  11 A. Here, in the following description, a direction from the first surface  11 A toward the second surface  11 B is set to a Z direction, a direction orthogonal to the Z direction is set to an X direction, and a direction orthogonal to the X direction and the Z direction is set to a Y direction. The first surface  11 A and the second surface  11 B are surfaces parallel to an XY plane. In the present embodiment, as an example, the Y direction is orthogonal to the X direction, but the Y direction may be inclined at angles other than 90° with respect to the X direction. In the following description, regarding the X direction, the Y direction, and the Z direction, a case not including a direction may also be referred to as a case assumed to include a direction. 
     The element substrate  11  is provided with a plurality of openings  111  disposed in a two-dimensional array form along the X direction and the Y direction. The openings  111  are through-holes penetrating through the element substrate  11  from the first surface  11 A to the second surface  11 B in the Z direction. 
     The vibration plate  12  is provided on the first surface  11 A of the element substrate  11 , and an end of the opening  111  on the −Z side is closed by the vibration plate  12 . In other words, a portion of the element substrate  11  not provided with the openings  111  forms a wall portion  112 , and the vibration plate  12  is laminated on the wall portion  112 . 
     A vibration attenuation layer  14  may be provided in the opening  111  of the element substrate  11  as necessary. The vibration attenuation layer  14  is made of an elastomer such as a silicon rubber, and has a Young&#39;s modulus sufficiently lower than that of a first layer  121  or a second layer  122  (which will be described later) of the vibration plate  12 . The vibration attenuation layer  14  is provided to be in contact with the vibration plate  12  in the opening  111  and thus suppresses vibration of the vibration plate  12 . 
     Specifically, a thickness of the vibration attenuation layer  14  is from 10 μm to 30 μm, and is formed to satisfy the following Equation (1) when a resonance frequency of an ultrasonic transducer Tr is f, and a thickness of the vibration attenuation layer  14  is d. 
         f=α×d+β   (1)
 
     Here, α and β are coefficients generally determined depending on constituent materials of the first layer  121  and the second layer  122 . When the first layer is made of SiO 2 , and the second layer is made of ZrO 2 , α is −8.12, and β is 789. The coefficients α and β may be easily calculated according to a finite element method. 
     In  FIG. 3 , as an example, the vibration attenuation layer  14  is configured to be provided in the opening  111 , but is not limited thereto. The vibration attenuation layer  14  may be provided to cover the piezoelectric element  13  on the opposite side to the element substrate  11  on the vibration plate  12 . 
     Configuration of Vibration Plate  12   
     The vibration plate  12  is provided on the first surface  11 A of the element substrate  11  as described above. In other words, the vibration plate  12  is supported at the wall portion  112 , and closes the openings  111 . Here, the ultrasonic transducer Tr is formed by a vibration portion  12 A that is a portion closing the opening  111  in the vibration plate  12 , and the piezoelectric element  13  laminated on the vibration portion  12 A. 
     A thickness dimension of the vibration plate  12  is sufficiently smaller than a thickness dimension of the element substrate  11 . 
     More specifically, the vibration plate  12  has the first layer  121  and the second layer  122  laminated on the first layer  121 . 
     The first layer  121  is made of SiO 2 . In the present embodiment, the element substrate  11  is made of Si, and the first layer  121  made of SiO 2  is formed by performing thermal oxidation treatment on the first surface side of the element substrate  11 . The element substrate  11  is subjected to etching treatment from the second surface side by using the first layer  121  made of SiO 2  as an etching stopper, and thus the element substrate  11  having the opening  111  and the wall portion  112  is formed. 
     The second layer  122  is made of ZrO 2 . The second layer  122  is formed by laminating a Zr layer on the first layer  121  and performing thermal oxidation treatment on the Zr layer. 
     The second layer  122  is a layer suppressing diffusion of Pb atoms of the piezoelectric element  13  made of PZT or the like. The second layer  122  having a sufficient thickness is provided, and thus piezoelectric characteristics of the piezoelectric element  13  can be maintained. 
     In the present embodiment, the second layer  122  has a Young&#39;s modulus higher than that of the first layer  121 , and a bending rigidity of the second layer  122  is larger than a bending rigidity of the first layer  121 . 
     A detailed description of a bending rigidity ratio between the first layer  121  and the second layer  122  will be described later. 
     Configuration of Piezoelectric Element  13   
     The piezoelectric element  13  is provided on a surface of the vibration portion  12 A of the vibration plate  12  on the opposite side to the element substrate  11 . 
     More specifically, as illustrated in  FIGS. 3 and 4 , the piezoelectric element  13  is formed by laminating a first electrode  131 , a piezoelectric membrane  132 , and a second electrode  133  in this order on the vibration plate  12 . 
     The piezoelectric membrane  132  in the present embodiment is made of a perovskite type transition metal oxide containing Pb, which is, for example, PZT consisting of Pb, Zr, and Ti in the present embodiment. 
     The piezoelectric element  13  extends and contracts when a voltage is applied between the first electrode  131  and the second electrode  133 . The piezoelectric element  13  extends and contracts such that the vibration portion  12 A of the vibration plate  12  on which the piezoelectric element  13  is provided vibrates, and thus an ultrasonic wave is transmitted from the ultrasonic transducer Tr. 
     When an ultrasonic wave is input to the vibration portion  12 A from the opening  111 , the vibration portion  12 A vibrates, and thus a potential difference occurs between the upper and lower sides of the piezoelectric membrane  132  of the piezoelectric element  13 . Therefore, the potential difference occurring between the first electrode  131  and the second electrode  133  is detected, and thus reception of the ultrasonic wave can be detected. 
     Disposition and Configuration of Ultrasonic Transducer Tr 
     In the present embodiment, as illustrated in  FIG. 2 , a plurality of ultrasonic transducers Tr are disposed in an array form along the X direction and the Y direction in the ultrasonic device  10 . 
     In the present embodiment, the first electrode  131  is linearly formed along the X direction, and is coupled to drive terminals  131 P provided at ±X ends. In other words, the first electrode  131  is used in common to the ultrasonic transducers Tr adjacent to each other in the X direction, and thus a single channel CH is formed. A plurality of channels CH are disposed along the Y direction. Thus, a separate drive signal can be input to the drive terminals  131 P corresponding to each channel CH, and thus each channel CH can be separately driven. 
     On the other hand, as illustrated in  FIG. 2 , the second electrode  133  is linearly formed along the Y direction, and ±Y side ends of the second electrodes  133  are coupled to each other and are coupled to a common terminal  133 P. The second electrodes  133  are electrically coupled to the drive circuit  30  via the common terminal  133 P, and thus an identical common potential is applied thereto. 
     Configuration of Controller  20   
     Referring to  FIG. 1  again, the controller  20  will be described. 
     The controller  20  is configured to include a drive circuit  30  driving the ultrasonic device  10 , and a calculation unit  40 . The controller  20  may include a storage unit storing various pieces of data or various programs for controlling the ultrasonic apparatus  100 . 
     The drive circuit  30  is a driver circuit controlling driving of the ultrasonic device  10 , and include, as illustrated in  FIG. 1 , for example, a reference potential circuit  31 , a switching circuit  32 , a transmission circuit  33 , and a reception circuit  34 . 
     The reference potential circuit  31  is coupled to the common terminal  133 P of the second electrodes  133  of the ultrasonic device  10 , and applies a reference potential to the second electrodes  133 . 
     The switching circuit  32  is coupled to the drive terminal  131 P, the transmission circuit  33 , and the reception circuit  34 . The switching circuit  32  is formed of a circuit using switching elements, and performs switching between transmission coupling of coupling each drive terminal  131 P to the transmission circuit  33  and reception coupling of coupling each drive terminal  131 P to the reception circuit  34 . 
     The transmission circuit  33  is coupled to the switching circuit  32  and the calculation unit  40 . When the switching circuit  32  switches to the transmission coupling, the transmission circuit  33  outputs a pulsed drive signal to each ultrasonic transducer Tr under the control of the calculation unit  40 , and thus transmits an ultrasonic wave from the ultrasonic device  10 . 
     The calculation unit  40  is configured with, for example, a central processing unit (CPU), and controls the ultrasonic device  10  via the drive circuit  30 , and thus the ultrasonic device  10  performs ultrasonic wave transmission and reception processes. 
     In other words, the calculation unit  40  causes the switching circuit  32  to switch to the transmission coupling, and thus drives the ultrasonic device  10  from the transmission circuit  33  to perform an ultrasonic wave transmission process. The calculation unit  40  causes the switching circuit  32  to switch to the reception coupling immediately after the ultrasonic wave is transmitted, and thus the ultrasonic device  10  receives a reflected wave that is reflected from a target object. The calculation unit  40  calculates a distance from the ultrasonic device  10  to the target object according to a time of flight (ToF) method by using a period of time from a transmission timing at which the ultrasonic wave is transmitted from the ultrasonic device  10  to a reception timing at which a received signal is received, and a sonic speed in the air. 
     Relationship between Bending Rigidity Ratio between first Layer  121  and Second Layer  122  and Resonance Frequency 
     Next, a description will be made of a relationship between a bending rigidity ratio between the first layer  121  and the second layer  122  of the vibration plate  12  and a resonance frequency of the ultrasonic transducer Tr. 
     A frequency of an ultrasonic wave transmitted and received in the ultrasonic transducer Tr substantially matches a resonance frequency of the ultrasonic transducer Tr. In order to adjust the resonance frequency of the ultrasonic transducer Tr to a desired frequency, it is necessary to appropriately set a width of the opening  111  of the element substrate  11  and the rigidity of the vibration plate  12 . 
       FIG. 4  is a graph illustrating a relationship between a width of the opening  111  and a displacement amount of the vibration portion  12 A. 
     In the ultrasonic transducer Tr, when a drive voltage is applied to the piezoelectric element  13 , the vibration portion  12 A vibrates. As illustrated in  FIG. 4 , regarding the vibration in the vibration portion  12 A, when a width of the opening  111  is equal to or less than 100 μm, as the width is increased, a displacement amount of the vibration portion  12 A is increased, that is, displacement efficiency is not reduced. 
     On the other hand, when the width of the opening  111  exceeds 100 μm, the displacement efficiency is gradually reduced, and, when the width thereof exceeds 200 μm, the displacement efficiency is considerably reduced. This is because an unnecessary vibration mode occurs in the vibration portion  12 A when the width of the opening  111  exceeds 100 μm. In other words, in order to output an ultrasonic wave with a high sound pressure from the ultrasonic transducer Tr, the vibration portion  12 A is preferably caused to vibrate with an end of the opening  111  as a node and the center of the opening  111  at which the piezoelectric element  13  is disposed as an antinode. However, when the unnecessary vibration mode occurs, a plurality of nodes and antinodes are generated in the vibration plate  12  closing the opening  111 , and thus a sound pressure of an ultrasonic wave is reduced. Thus, the width of the opening  111  is preferably equal to or less than 100 μm. 
     On the other hand, when the width of the opening  111  is equal to or less than 100 μm such that the displacement efficiency of the vibration portion  12 A is improved, it is necessary to control a resonance frequency of the ultrasonic transducer Tr by using bending rigidities of the first layer  121  and the second layer  122  forming the vibration plate  12 . 
     For example, when the width of the opening  111  is set to 100 μm, and thus the resonance frequency is more than a desired value, it is necessary to reduce the rigidity of the vibration portion  12 A by thinning the vibration plate  12 . 
     In this case, as described above, the second layer  122  is a layer suppressing diffusion of Pb atoms contained in the piezoelectric element  13 , and thus there is concern that piezoelectric characteristics of the piezoelectric element  13  may deteriorate when a thickness thereof is reduced. Thus, in order to maintain the piezoelectric characteristics of the piezoelectric element  13 , a thickness of the second layer  122  is required to be equal to or more than a defined value such that diffusion of Pb atoms contained in the piezoelectric membrane  132  is suppressed. However, a film thickness of the second layer  122  is set to a value more than the defined value, a probability that peeling between the second layer  122  and the first layer  121  or cracks may occur increases, and thus the piezoelectric element  13  deteriorates. For example, in the present embodiment, the defined value is 400 nm. Therefore, even when a thickness of the vibration plate  12  is made small, the second layer  122  is preferably maintained to have a thickness of about the defined value. 
       FIG. 5  is a graph illustrating a relationship between a thickness of the first layer  121  and a bending rigidity ratio between the first layer  121  and the second layer  122  when a thickness of the second layer  122  is set to the defined value. The bending rigidity ratio in the present disclosure is a value obtained by dividing a bending rigidity of the first layer  121  by a bending rigidity of the second layer  122  (that is, the bending rigidity of the first layer/the bending rigidity of the second layer). 
       FIG. 6  is a graph illustrating a relationship between a width of the opening  111  and a bending rigidity ratio (that is, the bending rigidity of the first layer/the bending rigidity of the second layer) between the first layer  121  and the second layer  122  when a resonance frequency of the ultrasonic transducer Tr is set to a predetermined frequency when a thickness of the second layer  122  is fixed to the defined value. 
     In  FIG. 6 , a first frequency f 1  is a resonance frequency of the ultrasonic transducer Tr when a width of the opening  111  is set to 100 μm, a thickness of the second layer  122  is set to the defined value, and a thickness of the first layer  121  is set such that the bending rigidities of the first layer  121  and the second layer  122  are the same as each other. In other words, the first frequency f 1  is a resonance frequency when the bending rigidity ratio is 1. The first frequency f 1  changes depending on materials forming the first layer  121  and the second layer  122 . 
     In the present embodiment, when a width of the opening  111  is set to 100 μm, and a resonance frequency of the ultrasonic transducer Tr is set to be lower than the first frequency f 1 , a thickness of the second layer  122  is set to the defined value. A thickness of the first layer  121  is set such that the bending rigidity ratio is less than 1, and the bending rigidity ratio has a value corresponding to a resonance frequency of the ultrasonic transducer Tr. 
     The vibration attenuation layer  14  is provided when a resonance frequency of the ultrasonic transducer Tr is set to be lower than a second frequency f 2  lower than the first frequency f 1 . 
     The second frequency f 2  is a resonance frequency of the ultrasonic transducer Tr when a bending rigidity ratio corresponding to the resonance frequency is equal to or less than a predetermined threshold value. For example, in the example illustrated in  FIG. 6 , when a bending rigidity ratio is 0 when a width of the opening  111  is 100 μm, that is, the first layer  121  is not provided, a resonance frequency of the ultrasonic transducer Tr is set to the second frequency f 2 . 
     When a width of the opening  111  is 100 μm, and a resonance frequency of the ultrasonic transducer Tr is set to be lower than the second frequency f 2 , a thickness of the second layer  122  is required to be small in order to control a resonance frequency by using only a thickness of the vibration plate  12 . In this case, piezoelectric characteristics of the piezoelectric element  13  deteriorate. Therefore, in the present embodiment, a thickness of the second layer  122  is maintained to have the defined value, and the vibration attenuation layer  14  is provided, so that a resonance frequency is set. 
     In  FIG. 6 , a resonance frequency when an opening width is 100 μm and a bending rigidity ratio is 0 is the second frequency f 2 , but is not limited thereto. For example, there is a lower limit value in a thickness of the first layer  121  that is formable in the ultrasonic device  10 . Therefore, a bending rigidity ratio when a thickness of the first layer  121  is set to the lower limit value and a thickness of the second layer  122  is set to the defined value maybe used as a threshold value. In this case, the second frequency f 2  is a resonance frequency of the ultrasonic transducer Tr when a thickness of the first layer  121  is set to the lower limit value and a thickness of the second layer  122  is set to the defined value. 
     A thickness of the vibration attenuation layer  14  provided on the vibration plate  12  is a thickness corresponding to a resonance frequency of the ultrasonic transducer Tr, a width of the opening  111 , and a bending rigidity ratio. 
     When a resonance frequency of the ultrasonic transducer Tr is set to a frequency higher than the first frequency f 1 , a thickness of the second layer  122  is set to a thickness corresponding to the resonance frequency such that a width of the opening  111  is set to a predetermined value of 100 μm or less, and a bending rigidity ratio between the first layer  121  and the second layer  122  is equal to or less than 1. 
     When the ultrasonic transducer Tr with a resonance frequency higher than the first frequency f 1  is to be obtained, a width of the opening  111  is preferably small up to a width of the formable opening  111 . In this case, as illustrated in  FIG. 4 , a displacement amount of the vibration portion  12 A linearly increases at an opening width from 0 to 100 μm, and thus the displacement efficiency does not deteriorate at the opening width from 1 to 100 μm. Peeling between the second layer  122  and the first layer  121  or cracks can be suppressed from occurring by increasing a thickness of the vibration plate  12 , and thus it is possible to suppress deterioration in the piezoelectric element  13 . 
     A width of the opening  111 , a thickness of the first layer  121 , and a thickness of the second layer  122  are set as mentioned above, and thus it is possible to suppress deterioration in the piezoelectric element  13  and also to provide the high performance ultrasonic transducer Tr in which displacement efficiency of the vibration portion  12 A is high and an increase of a total thickness of the ultrasonic transducer Tr is suppressed. 
     Advantageous Effects of Present Embodiment 
     The ultrasonic apparatus  100  of the present embodiment includes the ultrasonic device  10  and the controller controlling the ultrasonic device  10 . The ultrasonic device  10  includes the element substrate  11  having the openings  111 , the vibration plate  12  closing the openings  111 , and the piezoelectric element  13  disposed on the vibration plate  12 . The vibration plate  12  has the first layer  121  laminated on the element substrate  11  and the second layer  122  that is provided between the first layer  121  and the piezoelectric element  13  and suppresses diffusion of a Pb atom that is a component contained in the piezoelectric element  13 . A bending rigidity of the second layer  122  is larger than a bending rigidity of the first layer  121 . In other words, in the present embodiment, a bending rigidity ratio obtained by dividing the bending rigidity of the first layer  121  by the bending rigidity of the second layer  122  is equal to or less than 1. 
     In this configuration, since the bending rigidity ratio is equal to or less than 1, it is possible to reduce a total thickness of the vibration plate  12  for setting a resonance frequency to a predetermined value. In other words, when the bending rigidity ratio is equal to or more than 1, a thickness of the first layer  121  is required to be large when a resonance frequency of the ultrasonic transducer Tr is made higher than the first frequency f 1 , but, in this case, a Young&#39;s modulus of the first layer  121  is smaller than a Young&#39;s modulus of the second layer  122 , and thus it is necessary to excessively increase a thickness of the first layer  121 . In contrast, in the present embodiment, a thickness of the second layer  122  with the large Young&#39;s modulus may be increased, and thus it is possible to suppress an excessive increase of a total thickness of the vibration plate  12 . 
     In a case where a resonance frequency of the ultrasonic transducer Tr is made lower than the first frequency f 1 , it is necessary to reduce a thickness of the second layer  122  or to increase a width of the opening  111  when the bending rigidity ratio is equal to or more than 1. In contrast, in the present embodiment, in a state in which the second layer  122  is fixed to the defined value, a thickness of the first layer  121  may be reduced, deterioration in piezoelectric characteristics of the piezoelectric element  13  can be suppressed, and a width of the opening  111  is not required to be changed. 
     As described above, in the present embodiment, it is possible to provide the ultrasonic device  10  having desired drive characteristics. 
     In the present embodiment, a width of the opening  111  is equal to or less than 100 μm. 
     Thus, in the vibration portion  12 A, it is possible to suppress a problem that an unnecessary vibration mode occurs and to improve displacement efficiency of the vibration portion  12 A. 
     In the present embodiment, the second layer  122  has a thickness of the defined value or greater such that diffusion of Pb atoms is suppressed and piezoelectric characteristics are maintained. 
     In other words, in the present embodiment, even when a resonance frequency of the ultrasonic transducer Tr is set to the first frequency f 1  or higher or is set to below the first frequency f 1 , a thickness of the second layer  122  is equal to or more than the defined value. Consequently, deterioration in the performance of the piezoelectric element  13  can be suppressed, and thus it is possible to maintain the performance of the ultrasonic device  10 . 
     In the present embodiment, when a resonance frequency of the ultrasonic transducer Tr is the first frequency f 1  at a bending rigidity ratio of 1, when the resonance frequency of the ultrasonic transducer Tr is made lower than the first frequency f 1 , the second layer  122  has a thickness of the defined value such that diffusion of Pb atoms is suppressed. 
     In other words, when a resonance frequency of the ultrasonic transducer Tr is made equal to or lower than the first frequency f 1 , a thickness of the second layer  122  with a large Young&#39;s modulus is set to the defined value such that diffusion of Pb atoms can be suppressed, and a thickness of the first layer  121  with a small Young&#39;s modulus is set such that a bending rigidity ratio is equal to or less than 1. Consequently, it is possible to provide the ultrasonic transducer Tr having a desired resonance frequency in which deterioration in piezoelectric characteristics of the piezoelectric element  13  is suppressed by the second layer  122 . 
     In the present embodiment, when a resonance frequency of the ultrasonic transducer Tr is equal to or lower than the predetermined second frequency f 2  lower than the first frequency f 1 , the vibration attenuation layer  14  having a thickness corresponding to the resonance frequency is provided on the vibration plate  12 . 
     When a resonance frequency of the ultrasonic transducer Tr is made lower than the second frequency f 2 , the bending rigidity ratio is required to be lower. However, there is a lower limit value in a thickness of the first layer  121  that is formable on the element substrate  11 , and thus it is difficult to form the first layer  121  having a thickness less than the lower limit value. In a case where the vibration plate  12  is formed of only the second layer  122  having a thickness of the defined value, a thickness of the vibration plate  12  cannot be reduced any longer. In contrast, in the present embodiment, in this case, the vibration attenuation layer  14  having a thickness corresponding to a resonance frequency is provided. Consequently, it is possible to provide the ultrasonic transducer Tr having a resonance frequency lower than the second frequency f 2 . 
     In the present embodiment, the first layer  121  is made of SiO 2 , and the second layer  122  is made of ZrO 2 . When the element substrate  11  is made of Si, thermal oxidation treatment is performed on one surface thereof, and thus the first layer  121  can be easily formed. When PZT is used for the piezoelectric membrane  132  of the piezoelectric element  13 , it is possible to suppress diffusion of Pb atoms by using ZrO 2  for the second layer  122 . Consequently, it is possible to provide the high performance ultrasonic device  10  at low cost. 
     Modification Examples 
     The present disclosure is not limited to the embodiments and modification examples, and configurations obtained through modifications, alterations, and combinations of the embodiments within the scope of being capable of achieving the object of the present disclosure are also included in the present disclosure. 
     In the embodiment, SiO 2  is used for the first layer  121  and ZrO 2  is used for the second layer  122 , but are not limited thereto. In other words, as long as a Young&#39; s modulus of the first layer  121  is smaller than a Young&#39;s modulus of the second layer  122 , materials of the first layer  121  and the second layer  122  are not limited. For example, Al 2 O 3  or TiO 2  may be used for the second layer  122 . 
     As an example, PZT is used for the piezoelectric membrane  132 , but various piezoelectric materials such as a perovskite type oxide containing Pb may be used. 
     In the embodiment, a width of the opening  111  is equal to or less than 100 μm, but may be equal to or less than 200 μm. As illustrated in  FIG. 4 , displacement efficiency of the vibration portion  12 A is reduced when a width of the opening  111  is from 100 μm to 200 μm, but a reduction ratio is low, and the displacement efficiency is considerably reduced when the opening width exceeds 200 μm. Therefore, a width of the opening  111  may be equal to or less than 200 μm. 
     In the embodiment, a single channel CH is formed of one row of the ultrasonic transducers Tr arranged in the X direction, but the channel CH may be formed of a plurality of ultrasonic transducers Tr arranged in the X direction and the Y direction. 
     A plurality of channels CH are disposed along the Y direction, but a plurality of channels CH may be disposed along the X direction, and a plurality of channels CH may be disposed in the X direction and the Y direction. 
     A description has been made of an example in which a single channel CH is formed of a plurality of ultrasonic transducers Tr, but there may be a configuration in which each of the plurality of ultrasonic transducers Tr can be separately driven.