Patent Publication Number: US-2023165153-A1

Title: Piezoelectric Device

Description:
The present application is based on, and claims priority from JP Application Serial Number 2021-188435, filed Nov. 19, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a piezoelectric device. 
     2. Related Art 
     In related art, piezoelectric devices having piezoelectric elements arranged in matrix forms are known. For example, JP-A-2021-106183 discloses a piezoelectric device including a sealing plate having an opening portion, a vibrating plate closing the opening portion, and a piezoelectric element placed on the vibrating plate and sandwiching a piezoelectric material by an upper electrode and a lower electrode. 
     According to the disclosure, the vibrating plate and the sealing plate are placed to face each other. The sealing plate restricts the range of the vibration of the vibrating plate. The frequency of the vibration of the vibrating plate is set according to the size of the vibrating plate surrounded by the opening portion of the sealing plate. 
     A pair of through electrodes are placed in the sealing plate. The upper electrode and the lower electrode are respectively electrically continuous to the through electrodes. The sealing plate is placed to face a wiring board. The wiring board includes pads. The through electrodes project toward the wiring board. The pads and the through electrodes electrically contact. The upper electrode and the lower electrode are respectively electrically continuous to the pads. 
     The through electrodes are formed using resin adhesives containing metal fillers. The sealing plate and the wiring board are bonded using an insulating adhesive. 
     In the piezoelectric device of JP-A-2021-106183, the range of the adhesive spreading between the sealing plate and the wiring board varies. In this case, the bonding range of the sealing plate and the wiring board affects rigidity of the sealing plate. Further, the rigidity of the sealing plate affects vibration characteristics of the vibrating plate. Accordingly, the variations of the range of the adhesive spreading between the sealing plate and the wiring board change the resonance frequency of the piezoelectric element and the vibrating plate. On this account, a piezoelectric device that may suppress changes in resonance frequency due to the adhesive is desired. 
     SUMMARY 
     A piezoelectric device includes a first substrate including a first surface on which a plurality of piezoelectric elements and a first electrode coupled to the piezoelectric elements are placed, a second substrate including a second surface on which a second electrode coupled to a control circuit is placed, a third substrate placed between the first substrate and the second substrate and including a third surface joined to the first surface and a fourth surface facing the second surface, and a bonding portion bonding the second substrate and the third substrate, wherein the third substrate includes a through hole penetrating from the third surface to the fourth surface and a third electrode provided in the through hole and coupled to the first electrode, the second electrode is coupled to the third electrode and electrically coupled to the first electrode via the third electrode, and the second substrate includes a wall suppressing an outflow of an adhesive on the second surface facing the third substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic exploded perspective view showing a configuration of a piezoelectric device in a first embodiment. 
         FIG.  2    is a schematic side sectional view showing the configuration of the piezoelectric device. 
         FIG.  3    is a schematic side sectional view showing the configuration of the piezoelectric device. 
         FIG.  4    is a schematic plan view showing a configuration of a first substrate. 
         FIG.  5    is a schematic side sectional view showing configurations of through electrodes. 
         FIG.  6    is a schematic side sectional view showing the configuration of the piezoelectric device. 
         FIG.  7    is a schematic side sectional view of a main part for explanation of adhesives and a wall. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
     In the embodiment, a characteristic example of a piezoelectric device is explained. 
     As shown in  FIG.  1   , in a piezoelectric device  1 , a second substrate  2 , a third substrate  3 , a first substrate  4 , and a fourth substrate  5  are sequentially stacked in Z directions. In the Z directions, the fourth substrate  5  side is in the Z positive direction and the second substrate  2  side is in the Z negative direction. The direction along the Z positive direction is referred to as “first direction  6 ”. The first direction  6  is the stacking direction of the first substrate  4 , the third substrate  3 , and the second substrate  2 . 
     The second substrate  2 , the third substrate  3 , the first substrate  4 , and the fourth substrate  5  have rectangular shapes as seen from the first direction  6 . The longitudinal directions of the second substrate  2 , the third substrate  3 , the first substrate  4 , and the fourth substrate  5  are the same. The third substrate  3 , the first substrate  4 , and the fourth substrate  5  have the same shape. The second substrate  2  is larger than the third substrate  3 , the first substrate  4 , and the fourth substrate  5 . 
     The longitudinal directions of the second substrate  2  are X directions. The lateral directions of the second substrate  2  are Y directions. The X directions, the Y directions, and the Z directions are orthogonal to one another. 
     The first substrate  4  includes a first surface  4   a  at the side facing the third substrate  3 . A plurality of piezoelectric elements  7  are arranged in a matrix form on the first surface  4   a . Alternating-current voltages are applied to the piezoelectric elements  7 , and thereby, the piezoelectric device  1  may vibrate the first substrate  4  and output ultrasonic wave. The first substrate  4  is also referred to as “vibrating plate”. The plurality of piezoelectric elements  7  form an element array  8  in which the piezoelectric elements are arranged and placed. 
     The number of the piezoelectric elements  7  is not particularly limited. In the embodiment, for example, the piezoelectric elements  7  form an arrangement of four rows and four columns and the number of the piezoelectric elements  7  is 16. 
     The fourth substrate  5  includes four fourth holes  9  elongated in the Y directions. The shape of the fourth hole  9  is a parallelogram as seen from the first direction  6 . The fourth substrate  5  is formed using a silicon single-crystal substrate. The fourth holes  9  are formed by wet-etching. The side surface of the fourth hole  9  is a crystal face with a lower etching rate. In the silicon single-crystal substrate, the crystal face with the lower etching rate has a parallelogram shape and the shape of the fourth hole  9  is the parallelogram. The fourth holes  9  penetrate the fourth substrate  5 . The fourth holes  9  are placed in locations facing the arrangement of the piezoelectric elements  7 . Note that the number of fourth holes  9  is not particularly limited. 
     The third substrate  3  is placed between the first substrate  4  and the second substrate  2 . The third substrate  3  has a third surface  3   a  and a fourth surface  3   b . The third surface  3   a  faces in the Z positive direction. The fourth surface  3   b  faces in the Z negative direction. The third surface  3   a  is joined to the first surface  4   a  of the first substrate  4 . The third substrate  3  includes four second grooves  11  as sealing spaces elongated in the X directions in the third surface  3   a . The shape of the second groove  11  as seen from the first direction  6  is a parallelogram. The third substrate  3  is formed using a silicon single-crystal substrate. The second grooves  11  are formed by wet-etching. Accordingly, the shape of the second groove  11  is the parallelogram. The second grooves  11  are placed in locations facing the arrangement of the piezoelectric elements  7 . 
     The piezoelectric elements  7  are placed in locations where the fourth holes  9  and the second grooves  11  cross as seen from the first direction  6 . Accordingly, the first substrate  4  may vibrate in the Z positive direction and the Z negative direction in the locations where the piezoelectric elements  7  are placed. 
     The first substrate  4  and the fourth substrate  5  are integrally provided. The material of the first substrate  4  is silicon oxide and the first substrate  4  is formed by oxidation of the fourth substrate  5 . 
     On the first surface  4   a  of the first substrate  4 , a common terminal  12  as a first electrode and a drive terminal  13  as a first electrode are placed. The common terminal  12  and the drive terminal  13  are electrically coupled to the piezoelectric elements  7 . 
     The third substrate  3  has a first through hole  14  as a through hole penetrating from the third surface  3   a  to the fourth surface  3   b  in a position corresponding to the common terminal  12 . The third substrate  3  has a second through hole  15  as a through hole penetrating from the third surface  3   a  to the fourth surface  3   b  in a position corresponding to the drive terminal  13 . The thickness of the third substrate  3  is about 400 μm. 
     A first through electrode  16  as a third electrode is provided at the Z negative direction side of the common terminal  12  in the first through hole  14  of the third substrate  3 . The first through electrode  16  is electrically continuous to the common terminal  12 . A second through electrode  17  as a third electrode is provided at the Z negative direction side of the drive terminal  13  in the second through hole  15  of the third substrate  3 . The second through electrode  17  is electrically continuous to the drive terminal  13 . 
     The third substrate  3  includes an open hole  18  at the X positive direction side of the second grooves  11 . The open hole  18  penetrates from the third surface  3   a  to the fourth surface  3   b . The open hole  18  and the second grooves  11  are connected by a first communication groove  19 . The four second grooves  11  are connected by second communication grooves  21  to one another. 
     The third substrate  3  and the first substrate  4  are bonded and fixed. The second grooves  11  are connected to the open hole  18 , the first communication groove  19 , and the second communication grooves  21  and not tightly closed. When the first substrate  4  vibrates, the air within the second grooves  11  communicates with the outside air and the air pressure is hard to fluctuate. Accordingly, the first substrate  4  easily vibrates. 
     The second substrate  2  includes a second surface  2   a  facing the fourth surface  3   b  of the third substrate  3 . On the second surface  2   a  of the second substrate  2 , a common connecting terminal  22  as a second electrode is placed in a location corresponding to the first through electrode  16 . The common connecting terminal  22  is coupled to the first through electrode  16  and electrically coupled to the common terminal  12  via the first through electrode  16 . On the second surface  2   a  of the second substrate  2 , a drive connecting terminal  23  as a second electrode is placed in a location corresponding to the second through electrode  17 . The drive connecting terminal  23  is coupled to the second through electrode  17  and electrically coupled to the drive terminal  13  via the second through electrode  17 . Electric power is supplied to the common connecting terminal  22  and the drive connecting terminal  23 , and thereby, the electric power may be supplied to the piezoelectric elements  7 . The third substrate  3  and the second substrate  2  are flip-chip packaged. 
     The second substrate  2  includes an external common terminal  24  at the X negative direction side of the common connecting terminal  22 . The external common terminal  24  and the common connecting terminal  22  are electrically coupled by a common connecting wire  25 . The second substrate  2  includes an external drive terminal  26  at the X negative direction side of the drive connecting terminal  23 . The external drive terminal  26  and the drive connecting terminal  23  are electrically coupled by a drive connecting wire  27 . 
     The external common terminal  24  and the external drive terminal  26  are electrically coupled to a control circuit  29  by external wires  28 . Therefore, the common connecting terminal  22  and the drive connecting terminal  23  are electrically coupled to the control circuit  29 . 
     The common connecting wire  25  and the drive connecting wire  27  are covered by resists  31 . The common connecting terminal  22 , the drive connecting terminal  23 , the external common terminal  24 , and the external drive terminal  26  are exposed, not covered by the resists  31 . The common connecting terminal  22  is electrically continuous to the first through electrode  16 . The drive connecting terminal  23  is electrically continuous to the second through electrode  17 . 
     The piezoelectric device  1  includes bonding portions  32  bonding the second substrate  2  and the third substrate  3  on the second surface  2   a  of the second substrate  2 . The bonding portions  32  include a first bonding portion  32   a  and second bonding portions  32   b . The second bonding portions  32   b  are placed on the second substrate  2  in locations overlapping with a corner at the Y positive direction side and a corner at the Y negative direction side at an end at the X positive direction side of the third substrate  3  as seen from the first direction  6 . On the second substrate  2  corresponding to an end at the X negative direction side of the third substrate  3 , the first bonding portion  32   a  is placed around the first through electrode  16  and the second through electrode  17 . The common connecting terminal  22  and the drive connecting terminal  23  face the first through electrode  16  and the second through electrode  17 , respectively. Therefore, the first bonding portion  32   a  is placed around the common connecting terminal  22  and the drive connecting terminal  23 . The first bonding portion  32   a  and the second bonding portions  32   b  are apart, and thereby, bending of the third substrate  3  due to bonding is suppressed. 
     In an assembly process of the second substrate  2  and the third substrate  3 , adhesives are applied to the locations where the bonding portions  32  are formed. The adhesives are applied to one of the second substrate  2  and the third substrate  3 . The adhesives are sandwiched by the second substrate  2  and the third substrate  3 . The first through electrode  16  and the common connecting terminal  22  are pressed against each other. The second through electrode  17  and the drive connecting terminal  23  are pressed against each other. The adhesives spread by pressure by the first through electrode  16 , the second through electrode  17 , the common connecting terminal  22 , and the drive connecting terminal  23 . The second substrate  2  includes a wall  33  suppressing an outflow of the adhesives on the second surface  2   a  facing the third substrate  3 . The spread of the adhesives is suppressed by the wall  33 . The adhesives are non-conductive. The non-conductive adhesive is called as NCP (Non-Conductive Paste). The adhesives are solidified to form the bonding portions  32 . 
     According to the configuration, the second substrate  2  includes the wall  33 . When the second substrate  2  and the third substrate  3  are bonded, the adhesives are applied to the second substrate  2 . The outflow of the adhesives is suppressed by the wall  33 . In the bonding portions  32 , the second substrate  2  and the third substrate  3  are bonded and fixed by the adhesives. When the adhesives flow out to an area overlapping with the element array  8  in a plan view of the piezoelectric device  1 , internal stress is generated in the third substrate  3  by bonding and affects the resonance frequency of the piezoelectric device  1 . The wall  33  is placed, and thereby, the adhesives do not reach the area overlapping with the element array  8  in the plan view of the piezoelectric device  1 . Therefore, the changes in resonance frequency due to the adhesives may be suppressed in the piezoelectric device  1 . 
     The wall  33  is the resist  31 . According to the configuration, the common connecting terminal  22  and the drive connecting terminal  23  are placed on the second substrate  2 . The common connecting wire  25  connecting from the common connecting terminal  22  to the external common terminal  24  is coupled. The drive connecting wire  27  connecting from the drive connecting terminal  23  to the external drive terminal  26  is coupled. The common connecting wire  25  and the drive connecting wire  27  are coated with the resists  31 . The wall  33  is the resist  31 , and thereby, the wall  33  and the resists  31  as coating on the common connecting wire  25  and the drive connecting wire  27  may be formed at the same time. Therefore, the piezoelectric device  1  may be manufactured with higher productivity compared to a case where the wall  33  and the resists  31  for the common connecting wire  25  and the drive connecting wire  27  are formed in separate processes. 
       FIG.  2    shows a section along line AA in  FIG.  1   .  FIG.  3    shows a section along line BB in  FIG.  1   . As shown in  FIGS.  2  and  3   , the piezoelectric elements  7  are placed in the locations where the fourth holes  9  and the second grooves  11  cross as seen from the first direction  6 . The piezoelectric elements  7  are placed on the first surface  4   a  of the first substrate  4 . The piezoelectric element  7  includes a drive electrode  7   a , a piezoelectric film  7   b , and a common electrode  7   c  placed to be stacked in the Z negative direction from the first surface  4   a.    
     The piezoelectric film  7   b  is formed using e.g. transition metal oxide having a perovskite structure. Specifically, the piezoelectric film  7   b  is formed using lead zirconate titanate containing Pb, Ti, and Zr. 
     A plurality of the drive electrodes  7   a  are electrically continuous to drive wires  34  extending in the X directions. The drive electrodes  7   a  and the drive wires  34  are formed using the same material. A plurality of the common electrodes  7   c  are electrically continuous to common wires  35  extending in the Y directions. The common electrodes  7   c  and the common wires  35  are formed using the same material. 
     Ultrasonic transducers  36  are formed by the first substrate  4  and the piezoelectric elements  7 . The drive electrodes  7   a  and the common electrodes  7   c  are electrically coupled to the control circuit  29 . The common electrodes  7   c  are maintained at a predetermined reference potential. Drive pulse signals are input to the drive electrodes  7   a , and the piezoelectric elements  7  are deformed and the first substrate  4  vibrates. Thereby, the ultrasonic transducers  36  transmit ultrasonic waves toward the Z positive direction side. When there is an object at the Z positive direction side of the piezoelectric device  1 , the ultrasonic waves are reflected by the object. The reflected ultrasonic waves pass through the fourth holes  9  of the fourth substrate  5  and reach the ultrasonic transducers  36 , and the first substrate  4  vibrates according to the sound pressure of the ultrasonic waves. The piezoelectric films  7   b  are deformed by the vibration of the first substrate  4  and potential differences are generated between the drive electrodes  7   a  and the common electrodes  7   c . Thereby, reception signals according to the sound pressure of the received ultrasonic waves are output from the drive electrodes  7   a  of the ultrasonic transducer  36 . That is, the ultrasonic waves are detected. 
       FIG.  4    shows the first substrate  4  as seen from the third substrate  3  side. As shown in  FIG.  4   , the four drive wires  34  extending in the X directions are placed on the first surface  4   a . The respective drive wires  34  are integrated at the X negative direction side and electrically coupled to the drive terminal  13 . The four common wires  35  extending in the Y directions are placed on the first surface  4   a . The respective common wires  35  are integrated at the Y negative direction side and electrically coupled to the common terminal  12 . 
     An area where the element array  8  is placed is an array area  37 . An area surrounding the array area  37  is a non-array area  38 . The common terminal  12  and the drive terminal  13  are placed in the non-array area  38 . 
     A signal output from the control circuit  29  to the external common terminal  24  passes through the common connecting wire  25 , the common connecting terminal  22 , the first through electrode  16 , the common terminal  12 , and the common wires  35  and is supplied to the common electrodes  7   c . A signal output from the control circuit  29  to the external drive terminal  26  passes through the drive connecting wire  27 , the drive connecting terminal  23 , the second through electrode  17 , the drive terminal  13 , and the drive wires  34  and is supplied to the drive electrodes  7   a . The control circuit  29  is electrically coupled to the piezoelectric elements  7 . The control circuit  29  outputs drive signals to the piezoelectric elements  7 . 
       FIG.  5    shows a section along line CC in  FIG.  1   . As shown in  FIG.  5   , the first through electrode  16  electrically couples the common terminal  12  and the common connecting terminal  22 . The second through electrode  17  electrically couples the drive terminal  13  and the drive connecting terminal  23 . 
     The second substrate  2  is placed to face the fourth surface  3   b  of the third substrate  3 . The second substrate  2  has the common connecting terminal  22  electrically continuous to the first through electrode  16 . The second substrate  2  has the drive connecting terminal  23  electrically continuous to the second through electrode  17 . The electric power is supplied to the common connecting terminal  22  and the drive connecting terminal  23 , and thereby, the electric power is supplied to the piezoelectric elements  7 . 
     The first through electrode  16  and the second through electrode  17  are formed using conductive resin adhesives solidified by heating and drying. Specifically, the first through electrode  16  and the second through electrode  17  are resins containing silver fillers. For the resin adhesives, e.g. epoxy resin, urethane resin, and silicone resin adhesives are used. Note that silver bumps may be formed on the common connecting terminal  22  and the drive connecting terminal  23 . The adhesion between the first through electrode  16  and the common connecting terminal  22  may be increased. The adhesion between the second through electrode  17  and the drive connecting terminal  23  may be increased. 
       FIG.  6    shows a section along line DD in  FIG.  1   . As shown in  FIG.  6   , the second grooves  11  that enable the vibration of the first substrate  4  are placed between the first substrate  4  and the third substrate  3 . The third substrate  3  includes the open hole  18  for communication between the second grooves  11  and the fourth surface  3   b . The wall  33  is placed between the first through hole  14  and second through hole  15  and the open hole  18 . 
     According to the configuration, the wall  33  suppresses the flow of the adhesives. Therefore, the flow of the adhesives does not reach the open hole  18  in the plan view of the piezoelectric device  1 . When the open hole  18  is closed, the vibration of the first substrate  4  fluctuates the air pressure within the second grooves  11 . Accordingly, the vibration of the first substrate  4  is suppressed and the resonance frequency changes. The piezoelectric device  1  of the embodiment may suppress the changes in resonance frequency due to entry of the adhesives into the open hole  18 . 
     In the plan view in the first direction  6 , the wall  33  is placed between the first through hole  14  and second through hole  15  and the element array  8 . When the adhesives spread to the array area  37 , the rigidity of the third substrate  3  becomes higher. Accordingly, the resonance frequency of the piezoelectric device  1  changes. As shown in  FIG.  7   , when the amounts of adhesives in the bonding portions  32  are larger, the adhesives reach the wall  33 . Then, the wall  33  stops the flow of the adhesives. Then, the wall  33  suppresses the spread of the adhesives. 
     According to the configuration, the wall  33  suppresses the flow of the adhesives. Therefore, in the plan view in the first direction  6  in which the first substrate  4 , the second substrate  2 , and the third substrate  3  are stacked, the adhesives do not reach the array area  37  overlapping with the element array  8 . Therefore, the changes in resonance frequency due to the adhesives may be suppressed in the piezoelectric device  1 .