Patent Publication Number: US-2023163702-A1

Title: Piezoelectric device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a piezoelectric device including a piezoelectric element and an electronic element. 
     TECHNICAL BACKGROUND 
     A piezoelectric device including a piezoelectric element and an electronic element is in widespread use. The piezoelectric element has a frequency-temperature characteristic in which its oscillation frequency varies with changes in temperature. The electronic element includes a temperature sensor therein, and controls operation of the piezoelectric element based on temperature information obtained through the temperature sensor so that the oscillation frequency is constant regardless of a change in temperature. Examples of such known piezoelectric devices include a temperature compensated crystal oscillator (hereinafter, referred to as a TCXO) (refer to, for example, Japanese Unexamined Patent Application Publication No. 2014-187641). The TCXO includes a crystal vibrator element, serving as a piezoelectric element, and an integrated circuit (IC), serving as an electronic element. 
     SUMMARY 
     The present disclosure provides a piezoelectric device including a piezoelectric element, a mounting plate, an electronic element including a temperature sensor therein, a base on which the piezoelectric element is mounted with the mounting plate therebetween and on which the electronic element is mounted, and a lid joined to the base and hermetically sealing at least the piezoelectric element and the mounting plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an exploded perspective view of a piezoelectric device according to Embodiment 1. 
         FIG.  2    is an exploded perspective view illustrating a piezoelectric element and a mounting plate in the piezoelectric device according to Embodiment 1. 
         FIG.  3    is a plan view of the piezoelectric device according to Embodiment 1. 
         FIG.  4    is a sectional view taken along line IV-IV in  FIG.  3   . 
         FIG.  5    is an exploded perspective view of a piezoelectric device according to Comparative Example. 
         FIG.  6    is a graph showing exemplary changes in temperature of an electronic element and exemplary changes in temperature of a piezoelectric element in the piezoelectric device according to Comparative Example while the ambient temperature of the piezoelectric device changed from T1 to T2. 
         FIG.  7    is a graph showing exemplary changes in frequency while the ambient temperature of the piezoelectric device according to Comparative Example is changed. 
         FIG.  8    is a graph showing exemplary changes in frequency while the ambient temperature of the piezoelectric device according to Embodiment 1 is changed. 
         FIG.  9    is a sectional view of a piezoelectric device according to Embodiment 2. 
         FIG.  10    is a sectional view of a piezoelectric device according to Embodiment 3. 
         FIG.  11    is a flowchart illustrating a first example of a method for making a typical piezoelectric device. 
         FIG.  12    is a flowchart illustrating a second example of a method for making a typical piezoelectric device. 
         FIG.  13    is a flowchart illustrating a method for making the piezoelectric device according to each of Embodiments 1 to 3. 
     
    
    
     DETAILED DESCRIPTION 
     A typical piezoelectric device includes a base and a lid, which define a hermetically enclosed space. The hermetically enclosed space contains a piezoelectric element and an electronic element that are mounted on the base. Mounting the piezoelectric element on the base requires high-temperature processing for, for example, curing a conductive adhesive and annealing for stabilization of properties. As illustrated in  FIG.  11   , if the electronic element is mounted on the base (S 11 ) before the piezoelectric element is mounted on the base (S 12 , S 13 ), the electronic element may be affected in its electrical characteristics by the high-temperature processing, which is unnecessary for the electronic element, performed on the electronic element in mounting the piezoelectric element. In contrast, as illustrated in  FIG.  12   , if the piezoelectric element is mounted on the base (S 21 , S 22 ) before the electronic element is mounted on the base (S 23 ), the piezoelectric element may be affected in its electrical characteristics by, for example, dust deposited on the piezoelectric element in mounting the electronic element. In other words, the electrical characteristics of the piezoelectric device may be impaired due to the order in which the piezoelectric element and the electronic element are mounted on the base. 
     A temperature change of an area surrounding the piezoelectric device propagates through the base and the like to the piezoelectric element and the electronic element. The electronic element detects a changing temperature at regular time intervals, and controls operation of the piezoelectric element in accordance with information on the detected temperature. However, the temperature of the piezoelectric element does not necessarily match the temperature of the electronic element. This may impair the frequency stability of the piezoelectric device. 
     First, the present disclosure aims to provide a piezoelectric device having improved electrical characteristics that can be attained by solving an issue arising from the order in which a piezoelectric element and an electronic element are mounted on a base. Second, the present disclosure aims to provide a piezoelectric device having improved frequency stability that can be attained by solving an issue arising from the difference in temperature between a piezoelectric element and an electronic element. 
     The present disclosure provides a piezoelectric device including a piezoelectric element mounted on a base with a mounting plate therebetween. Such a configuration allows only the piezoelectric element and the mounting plate to undergo high-temperature processing in mounting the piezoelectric element. This reduces or eliminates the likelihood that an electronic element may be affected in its electrical characteristics by the high-temperature processing, which is unnecessary for the electronic element, performed on the electronic element in mounting the piezoelectric element. If the mounting plate with the piezoelectric element is mounted after the electronic element is mounted, the electronic element will not undergo the high-temperature processing, which is unnecessary for the electronic element. Accordingly, the mounting plate with the piezoelectric element is mounted after the electronic element is mounted. This eliminates the likelihood of, for example, deposition of dust on the piezoelectric element in mounting the electronic element. Thus, the issue arising from the order in which the piezoelectric element and the electronic element are mounted on the base can be solved, leading to improved electrical characteristics of the piezoelectric device. 
     The piezoelectric device according to the present disclosure includes the piezoelectric element mounted on the base with the mounting plate therebetween. This configuration allows transfer of the ambient temperature of the piezoelectric device to the piezoelectric element to be delayed by an amount corresponding to the mounting plate. This can reduce the difference in temperature between the piezoelectric element and the electronic element if the temperature of the piezoelectric element changes earlier than the temperature of the electronic element. Thus, the issue arising from the temperature difference between the piezoelectric element and the electronic element can be solved, leading to improved frequency stability of the piezoelectric device. 
     Modes (hereinafter, referred to as “embodiments”) for embodying the present disclosure will be described below with reference to the accompanying drawings. In the specification and the drawings, the same reference signs are used for substantially the same components to avoid redundant description. The shapes of components illustrated in the drawings are not necessarily drawn to scale for clarity and ease of illustration. 
     Embodiment 1 
     A schematic configuration of a piezoelectric device  11  according to Embodiment 1 will be described with reference to  FIGS.  1  to  4   . In  FIG.  1   , a base  50  is illustrated with a portion cut away. 
     The piezoelectric device  11  includes a piezoelectric element  20 , a mounting plate  30 , an electronic element  40  including a temperature sensor  41  therein, the base  50  on which the piezoelectric element  20  is mounted with the mounting plate  30  therebetween and on which the electronic element  40  is mounted, and a lid  70  joined to the base  50  and hermetically sealing at least the piezoelectric element  20  and the mounting plate  30 . 
     Embodiment 1 provides the following configuration. The base  50  includes a substrate  60 , a first frame  61 , and a second frame  62 . The substrate  60  includes a first bottom surface  51 , on which the electronic element  40  is mounted. The first frame  61  is located on an outer peripheral edge of the first bottom surface  51  and includes a second bottom surface  52 , on which the piezoelectric element  20  is mounted with the mounting plate  30  therebetween. The second frame  62  is located on an outer peripheral edge of the second bottom surface  52 . The lid  70  is joined to the second frame  62  and hermetically seals the piezoelectric element  20 , the mounting plate  30 , and the electronic element  40 . 
     The configuration of the piezoelectric device  11  according to Embodiment 1 will now be described in further detail. 
     The piezoelectric element  20 , which is substantially quadrilateral in plan view, is a crystal vibrator element including a quartz crystal blank  27  and electrodes  23  and  24 . The quartz crystal blank  27  includes an upper surface  21  and a lower surface  22  opposite to the upper surface  21 . The electrodes  23  and  24  extend from the upper surface  21  to the lower surface  22  of the quartz crystal blank  27 . The quartz crystal blank  27  is, for example, an AT-cut quartz crystal plate. The electrodes  23  and  24  are isolated from each other. Each of the electrodes  23  and  24  divides into, for example, an excitation electrode, an extraction electrode, and a pad electrode, and extends from the upper surface  21  to the lower surface  22  over a side. The piezoelectric element  20  with such a configuration is a thickness shear mode vibrator element. Instead of such a vibrator element, a tuning-fork flexural vibrator element or a face shear mode vibrator element may be used. Instead of the crystal vibrator element, a piezoelectric element including ceramics etc. may be used. The shape in a plan view of the piezoelectric element  20  is not limited to a quadrilateral, but may be any other shape, such as a circle, an ellipse, or a polygon. 
     The mounting plate  30 , which is substantially quadrilateral in a plan view, includes a first main surface  31  and a second main surface  32  opposite to the first main surface  31 . The mounting plate  30  includes a multilayered ceramic plate made by firing a stack of multiple green sheets. As illustrated in  FIG.  2   , piezoelectric-element pads  33  and  34  are located on the first main surface  31 , and mounting-plate electrodes  35 ,  36 ,  37 , and  38  are located on the second main surface  32 . The piezoelectric-element pads  33  and  34  are arranged so as to face the electrodes  23  and  24  of the piezoelectric element  20 , and are electrically connected to the electrodes  23  and  24  by piezoelectric-element joining members  25  and  26 , respectively. The piezoelectric-element joining members  25  and  26  are, for example, a conductive adhesive, such as epoxy resin containing silver, and have fluidity before cured. As illustrated in  FIG.  2   , the mounting-plate electrodes  35 ,  36 ,  37 , and  38  are located at four respective corners of the second main surface  32 . The piezoelectric-element pads  33  and  34  on the first main surface  31  are respectively electrically connected to the mounting-plate electrodes  35  and  36  on the second main surface  32  by interconnection (not illustrated). The interconnection includes a conductive pattern printed on the green sheets or via-hole conductors. The mounting-plate electrodes  37  and  38  are not electrically connected to anywhere, and are simply mechanically connected to the base  50 . The piezoelectric-element pads  33  and  34  and the mounting-plate electrodes  35 ,  36 ,  37 , and  38  each include a gold (Au) layer, serving as a surface layer, and a nickel (Ni) layer underlying the Au layer. For the mounting plate  30 , for example, a quartz crystal plate may be used instead of the ceramic plate. The shape in a plan view of the mounting plate  30  is not limited to a quadrilateral, but may be any other shape, such as a circle, an ellipse, a triangle, a pentagon, or a polygon including more than five corners. 
     The electronic element  40  is an IC functioning as the temperature sensor  41  and functioning as, for example, an oscillation circuit for the piezoelectric element  20 , and also a flip chip (FC) with bumps, serving as connection terminals  42 . The bumps are made of, for example, gold or solder, and are electrically connected to electronic-element pads  53 . The connection terminals  42  are equal in number to the electronic-element pads  53 . In other words, a circuit formation surface, which includes the connection terminals  42 , of the electronic element  40  is directed to the electronic-element pads  53  on the first bottom surface  51 , or faced down, and the electronic element  40  is mounted on the base  50  with the connection terminals  42  therebetween. The temperature sensor  41  uses, for example, a forward voltage across a pn junction in the IC. Such a forward voltage across the pn junction decreases with increasing temperature. Measuring a forward voltage across the pn junction at a constant current through the pn junction yields voltage information. Conversion of the voltage information yields temperature information on the electronic element  40 , or temperature information on the piezoelectric element  20 . The electronic element  40  may consist of a temperature sensor, or may be a thermistor or a diode, for example. For the connection terminals  42 , wires made of, for example, aluminum or gold, may be used instead of the bumps. 
     The substrate  60 , the first frame  61 , and the second frame  62 , which constitute the base  50 , each include a multilayered ceramic plate made by firing a stack of multiple green sheets. The first frame  61  is provided in a continuous manner on an outer peripheral edge of the substrate  60 . The second frame  62  is provided in a continuous manner on an outer peripheral edge of the first frame  61 . Interconnection (not illustrated) includes a conductive pattern printed on the green sheets or via-hole conductors. The electronic-element pads  53  are arranged on the first bottom surface  51  in a recessed space  63 , and mounting-plate pads  55 ,  56 ,  57 , and  58  are arranged on the second bottom surface  52 . The electronic-element pads  53  and the mounting-plate pads  55 ,  56 ,  57 , and  58  each include a gold (Au) layer, serving as a surface layer, and a nickel (Ni) layer underlying the Au layer. 
     The mounting-plate pads  55 ,  56 ,  57 , and  58  are arranged so as to face the mounting-plate electrodes  35 ,  36 ,  37 , and  38  ( FIG.  2   ) of the mounting plate  30 , and are electrically connected to the mounting-plate electrodes  35 ,  36 ,  37 , and  38  by mounting-plate joining members  65 ,  66 ,  67 , and  68 , respectively. The mounting-plate joining members  65 ,  66 ,  67 , and  68  are, for example, a conductive adhesive, such as epoxy resin containing silver, and have fluidity before cured. External terminals  54  for surface mounting are arranged on angled portions at four corners of the substrate  60 . Examples of the external terminals  54  include a frequency control terminal, a ground terminal, an output terminal, and a power supply voltage terminal. The mounting-plate pads  55  and  56 , the electronic-element pads  53 , and the external terminals  54  are electrically connected to each other by interconnection (not illustrated). The mounting-plate pads  57  and  58  are not electrically connected to anywhere. For the mounting-plate joining members  65 ,  66 ,  67 , and  68 , for example, solder may be used instead of the conductive adhesive. 
     The lid  70  is made of, for example, ceramic or metal, such as Kovar, and is a rectangular flat plate. The lid  70  is joined to the base  50  by, for example, electric welding or glass sealing, thus hermetically sealing the recessed space  63 . 
     The recessed space  63  is a space surrounded by the substrate  60 , the first frame  61 , the second frame  62 , and the lid  70 . In other words, the recessed space  63  is defined in the base  50  and contains the piezoelectric element  20 , the mounting plate  30 , and the electronic element  40 . 
     The piezoelectric device  11  is configured such that the piezoelectric element  20 , the mounting plate  30 , and the electronic element  40  are hermetically enclosed in the recessed space  63  by joining the lid  70  to the base  50 , on which the piezoelectric element  20 , the mounting plate  30 , and the electronic element  40  are mounted, through seam welding or glass sealing. As described above, the piezoelectric device  11  is a surface-mount crystal oscillator including the piezoelectric element  20 . The crystal oscillator, which serves as a reference clock signal source of an apparatus, is required to have higher reliability than other electronic components. 
     A method for assembling the piezoelectric device  11  will now be described. 
     (First Step: S 1 , S 2  in  FIG.  13   ) As illustrated in  FIG.  2   , the piezoelectric-element joining members  25  and  26 , which are made of a conductive adhesive, are applied to the piezoelectric-element pads  33  and  34  on the first main surface  31  of the mounting plate  30 . The electrodes  23  and  24  of the piezoelectric element  20  are respectively placed on the piezoelectric-element joining members  25  and  26 , and are subjected to heat treatment for 10 to 30 minutes at a high temperature of, for example, from 300° C. to 350° C., thereby curing the piezoelectric-element joining members  25  and  26 . At this time, the piezoelectric element  20  is fixed as a cantilever. The piezoelectric-element joining members  25  and  26  are immediately cured at a high temperature so that the piezoelectric element  20  is unlikely to tilt, come in contact with an upper or lower member, and be fixed to the upper or lower member. The piezoelectric element  20  is mounted on the mounting plate  30  in this manner. Then, the mounting plate  30  with the piezoelectric element  20  mounted thereon is subjected to annealing for five to 15 minutes at a high temperature of, for example, from 300° C. to 350° C. in order to stabilize the properties of the piezoelectric element  20 . 
     (Second Step: S 3  in  FIG.  13   ) Separately from the first step, the formation surface, which includes the connection terminals  42 , of the electronic element  40  is directed to the first bottom surface  51  in the recessed space  63 , the connection terminals  42  are aligned with the electronic-element pads  53 , and the connection terminals  42  are pressed against the electronic-element pads  53  under application of heat or ultrasound. Thus, the connection terminals  42  are joined to the electronic-element pads  53 . 
     (Third Step: S 4  in  FIG.  13   ) After the first and second steps, the mounting-plate joining members  65 ,  66 ,  67 , and  68 , which are the conductive adhesive, are applied to the mounting-plate pads  55 ,  56 ,  57 , and  58  on the second bottom surface  52 , respectively. Then, the mounting-plate electrodes  35 ,  36 ,  37 , and  38  ( FIG.  2   ) of the mounting plate  30  are placed on the mounting-plate joining members  65 ,  66 ,  67 , and  68 , respectively. The mounting-plate joining members  65 ,  66 ,  67 , and  68  are cured at room temperature or high temperature. At this time, a lower curing temperature than that in the first step can be used because it is unnecessary to take into account a tilt and the like of the mounting plate  30 . 
     (Fourth Step) After the third step, the recessed space  63  of the base  50  is closed by the lid  70 . Thus, the piezoelectric device  11  is completed. 
     The advantageous effects of the piezoelectric device  11  will now be described. 
     Comparative Example illustrated in  FIG.  5    will be described first. A piezoelectric device  10  according to Comparative Example includes no mounting plate  30 , which has been described in Embodiment 1. In other words, the piezoelectric device  10  includes the same configuration as that of the piezoelectric device  11  according to Embodiment 1, except that the piezoelectric element  20  is mounted directly on the base  50  without the mounting plate  30  therebetween. 
     (1) The piezoelectric device  10  according to Comparative Example is configured such that the electronic element  40  is located deeper than the piezoelectric element  20  in the recessed space  63 . It is therefore necessary to mount the electronic element  40  before mounting the piezoelectric element  20  ( FIG.  11   ). Accordingly, after the electronic element  40  is mounted on the base  50  (S 11  in  FIG.  11   ), the piezoelectric element  20  is mounted on the base  50  (S 12  in  FIG.  11   ). The piezoelectric element  20  is then subjected to high-temperature processing (S 13  in  FIG.  11   ). At this time, the electronic element  40  may be affected in its electrical characteristics by the high-temperature processing, which is unnecessary for the electronic element  40 , performed on the electronic element  40  in mounting the piezoelectric element  20 . 
     In contrast, the piezoelectric device  11  according to Embodiment 1 includes the piezoelectric element  20  mounted on the base with the mounting plate  30  therebetween. Such a configuration allows only the piezoelectric element  20  and the mounting plate  30  to undergo the high-temperature processing in mounting the piezoelectric element  20  ( 51 , S 2  in  FIG.  13   ). This reduces or eliminates the likelihood that the electronic element  40  may be affected in its electrical characteristics by the high-temperature processing, which is unnecessary for the electronic element  40 , performed on the electronic element  40  in mounting the piezoelectric element  20 . Thus, the issue arising from the order in which the piezoelectric element  20  and the electronic element  40  are mounted on the base  50  can be solved, leading to improved electrical characteristics of the piezoelectric device  11 . 
     (2) The following expression may hold. 
       |τ s−τx 1|&lt;|τ s−τx 2|
 
     where τs ( FIG.  6   ) denotes the thermal time constant of heat transferred from the base  50  to the electronic element  40 , τx1 denotes the thermal time constant of heat transferred from the base  50  to the piezoelectric element  20  through the mounting plate  30 , and τx2 ( FIG.  6   ) denotes the thermal time constant of heat transferred from the base  50  mounted directly to the piezoelectric element  20  without the mounting plate  30  therebetween (refer to Comparative Example of  FIG.  5   ). 
     Each of the thermal time constants may be the time that it takes for a respective one of the electronic element  40  and the piezoelectric element  20  to change from T1 to {T1+(T2−T1)×0.632} while the ambient temperature of the piezoelectric device  11  changes from T1 to T2. 
     In this case, as illustrated in  FIG.  6   , the thermal time constant τx2 for the piezoelectric element  20  in the piezoelectric device  10  according to Comparative Example is considerably smaller than the thermal time constant τs for the electronic element  40 . Accordingly, as illustrated in  FIG.  7   , a frequency change (df/f0) in increasing the ambient temperature of the piezoelectric device  10  (−40° C.→85° C.) differs from that in reducing the ambient temperature of the piezoelectric device  10  (85° C.→−40° C.). 
     In contrast, since the piezoelectric device  11  according to Embodiment 1 includes the piezoelectric element  20  mounted on the base with the mounting plate  30  therebetween, the thermal time constant τx1 for the piezoelectric element  20  can be increased and can be close to the thermal time constant τs for the electronic element  40 . Thus, as illustrated in  FIG.  8   , the difference between the frequency change (df/f0) in increasing the ambient temperature of the piezoelectric device  11  (−40° C.→85° C.) and that in reducing the ambient temperature of the piezoelectric device  11  (85° C.→−40° C.) is smaller than that in  FIG.  7   . 
     As described above, the piezoelectric device  11  according to Embodiment 1 includes the piezoelectric element  20  mounted on the base with the mounting plate  30  therebetween. Such a configuration allows transfer of the ambient temperature of the piezoelectric device  11  to the piezoelectric element  20  to be delayed by an amount corresponding to the mounting plate  30 . This can reduce the difference in temperature between the piezoelectric element  20  and the electronic element  40  if the temperature of the piezoelectric element  20  changes earlier than the temperature of the electronic element  40 . Thus, the issue arising from the temperature difference between the piezoelectric element  20  and the electronic element  40  can be solved, leading to improved frequency stability of the piezoelectric device  11 . Examples of methods for obtaining an actual temperature of the piezoelectric element  20  include measurement with a thermocouple, measurement with a very small hole formed in the base  50  or the lid  70  and a radiation thermometer, and calculation based on computer simulation. 
     (3) The piezoelectric element  20 , the electronic element  40 , and the mounting plate  30  interposed therebetween may be superposed on each other in a plan view. Such a configuration allows the piezoelectric device  11  to occupy less area, resulting in a reduction in size of the piezoelectric device  11 . In addition, the electrodes  23  and  24  of the piezoelectric element  20  sealing the electronic element  40  can reduce noise to be added to a signal of the electronic element  40 . 
     (4) In a plan view, the mounting plate  30  may be equal in size to the piezoelectric element  20  or may be greater in size to the piezoelectric element  20  to encompass the piezoelectric element  20 . Such a configuration allows the mounting plate  30  to have a larger heat capacity and further allows the mounting plate  30  to function as a shield against heat radiated from the base  50 . Thus, the thermal time constant τx1 for the piezoelectric element  20  can be increased. 
     (5) The mounting plate  30  may be made of the same material (e.g., ceramic) as that of the base  50 . Such a configuration causes the mounting plate  30  and the base  50  to have the same coefficient of thermal expansion. This can reduce distortion of the mounting plate  30  and the base  50  that is caused by a change in temperature. 
     Embodiment 2 
     As illustrated in  FIG.  9   , the structure of a base  150  of a piezoelectric device  12  according to Embodiment 2 differs from that in Embodiment 1. The base  150  includes a substrate  160  and a frame  161 . The substrate  160  includes an inner surface  151  and an outer surface  152  opposite to the inner surface  151 . The piezoelectric element  20  is mounted on the inner surface  151  with the mounting plate  30  therebetween. The electronic element  40  is mounted on the inner surface  151 . The frame  161  is located on an outer peripheral edge of the inner surface  151 . A lid  170  is joined to the frame  161  and hermetically seals the piezoelectric element  20 , the mounting plate  30 , and the electronic element  40 . 
     The substrate  160 , the frame  161 , and the lid  170  define a space, which is a recessed space  163 . In Embodiment 2, the base  150  is recessed, and the lid  170  is flat. Conversely, the base  150  may be flat, and the lid  170  may be recessed. 
     In the piezoelectric device  12  according to Embodiment 2, the piezoelectric element  20  and the electronic element  40  are located on the inner surface  151 , or the same plane, in the recessed space  163 . Unlike the configuration in Embodiment 1, such a configuration enables the piezoelectric element  20  to be mounted before the electronic element  40  is mounted ( FIG.  12   ) and also enables the electronic element  40  to be mounted before the piezoelectric element  20  is mounted ( FIG.  11   ). If the mounting plate  30  with the piezoelectric element  20  is mounted (S 4  in  FIG.  13   ) after the electronic element  40  is mounted (S 3  in  FIG.  13   ), the electronic element  40  will not undergo the high-temperature processing, which is unnecessary for the electronic element. Accordingly, after the electronic element  40  is mounted (S 3  in  FIG.  13   ), the mounting plate  30  with the piezoelectric element  20  is mounted (S 4  in  FIG.  13   ). This eliminates the likelihood of, for example, deposition of dust on the piezoelectric element  20  in mounting the electronic element  40  (S 3  in  FIG.  13   ). 
     Furthermore, this configuration, in which the piezoelectric element  20  and the electronic element  40  undergo heat conduction through the same substrate  160 , enables the thermal time constant for the piezoelectric element  20  to be closer to that for the electronic element  40 . The rest of the configuration and the advantageous effects in Embodiment 2 are the same as and/or similar to those in Embodiment 1. 
     Embodiment 3 
     As illustrated in  FIG.  10   , the structure of a base  250  of a piezoelectric device  13  according to Embodiment 3 differs from that in Embodiment 1. The base  250  includes a substrate  260 , a first frame  261 , and a second frame  262 . The substrate  260  includes an inner surface  251  and an outer surface  252  opposite to the inner surface  251 . The piezoelectric element  20  is mounted on the inner surface  251  with the mounting plate  30  therebetween. The electronic element  40  is mounted on the outer surface  252 . The first frame  261  is located on an outer peripheral edge of the outer surface  252 . The second frame  262  is located on an outer peripheral edge of the inner surface  251 . A lid  270  is joined to the second frame  262  and hermetically seals the piezoelectric element  20  and the mounting plate  30 . 
     The substrate  260 , the second frame  262 , and the lid  270  define a space, which is a recessed space  263 . Although the second frame  262  is located in the base  250  in Embodiment 3, the second frame  262  may be located in the lid  270 . In other words, the lid  270 , which is flat, may be recessed. 
     In the piezoelectric device  13  according to Embodiment 3, the piezoelectric element  20  is located on the inner surface  251  of the substrate  260 , and the electronic element  40  is located on the outer surface  252 . Unlike the configuration in Embodiment 1, such a configuration enables the piezoelectric element  20  to be mounted before the electronic element  40  is mounted ( FIG.  12   ) and also enables the electronic element  40  to be mounted before the piezoelectric element  20  is mounted ( FIG.  11   ). If the mounting plate  30  with the piezoelectric element  20  is mounted (S 4  in  FIG.  13   ) after the electronic element  40  is mounted (S 3  in  FIG.  13   ), the electronic element  40  will not undergo the high-temperature processing, which is unnecessary for the electronic element. Accordingly, after the electronic element  40  is mounted (S 3  in  FIG.  13   ), the mounting plate  30  with the piezoelectric element  20  is mounted (S 4  in  FIG.  13   ). This eliminates the likelihood of, for example, deposition of dust on the piezoelectric element  20  in mounting the electronic element  40  (S 3  in  FIG.  13   ). The rest of the configuration and the advantageous effects in Embodiment 3 are the same as and/or similar to those in Embodiment 1. 
     &lt;Other Supplements&gt; 
     The piezoelectric device with such a configuration is mounted on the surface of a printed circuit board included in an electronic device by fixing bottom surfaces of the external terminals to the printed circuit board through, for example, soldering, Au bumps, or a conductive adhesive. The piezoelectric device can be used as an oscillating source in various electronic devices including a personal computer, a clock, a game machine, a communications device, and an in-vehicle device, such as a car navigation system. In this piezoelectric device, the difference between a temperature obtained by conversion of a voltage outputted from the temperature sensor and an actual ambient temperature of the piezoelectric device can be reduced. Thus, the piezoelectric device can be readily corrected by an IC of an electronic device and thus output a stable oscillation frequency. This enables an electronic device including the piezoelectric device according to any of the above-described embodiments to operate reliably and precisely. 
     Although the present disclosure has been described with reference to the above-described embodiments, the embodiments are not intended to be construed as limiting the present disclosure. Various changes that can be understood by those skilled in the art can be made to the configurations and details of the present disclosure. An appropriate combination of some or all of the features of the above-described embodiments is intended to be embraced within the scope of the present disclosure. 
     This application claims the benefit of priority from Japanese Patent Application No. 2020-128278, filed Jul. 29, 2020, which is hereby incorporated by reference herein in its entirety.