Patent Publication Number: US-2021194454-A1

Title: Resonance device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT/JP2019/040867 filed Oct. 17, 2019, which claims priority to JP Application No. 2018-200146, filed Oct. 24, 2018, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a resonance device. 
     BACKGROUND 
     Resonators manufactured by using micro electro mechanical systems (MEMS) technology (hereinafter, such resonators being also referred to as “MEMS vibrators”) have attracted attention. 
     The resonant frequency of such a MEMS vibrator sometimes varies due to manufacturing variations. Thus, during or after manufacturing such a MEMS vibrator, the frequency is adjusted by, for example, additional etching. 
     For example, Japanese Unexamined Patent Application Publication No. 2012-065293 (hereinafter “Patent Document 1”) discloses a known vibration piece that includes a base, a vibration arm extending from the base in the Y-axis direction and configured to bend and vibrate in the Z-axis direction, a piezoelectric element disposed on the vibration arm and configured to bend and vibrate the vibration arm, and a mass portion disposed closer to a tip of the vibration arm than the piezoelectric element. The piezoelectric element at least includes a first electrode layer, a second electrode layer, and a piezoelectric layer positioned between the first electrode layer and the second electrode layer. The mass portion includes at least one film. Moreover, the film is made of the same material as that for one of the layers forming the piezoelectric element. 
     Existing resonators have a configuration in which a rectangular vibration portion (vibration body) is connected to a frame via a holding arm and in which a surface of the frame is covered with an insulating film. In the resonators having such a configuration, an insulating film of the frame may be electrified due to, for example, an electric field generated during thin film formation. In addition, when such a frequency adjusting method using ion beams described in Patent Document 1 is used in existing resonators, an insulating film of the frame may be electrified due to the frame being irradiated with an ion beam. In addition, when a pyroelectric material is used for an insulating film of the frame, an electric charge is generated at the interface of the insulating film due to a pyroelectric effect caused by temperature changes. As described above, when an insulating film of the frame disposed around a vibration portion is electrified, a Coulomb force (e.g., an electrostatic force) is generated by the electric charge at the insulating film, and the spring constant of the vibration portion thus changes. As a result, the resonant frequency changes. 
     SUMMARY OF THE INVENTION 
     The exemplary embodiments of the present invention are made in view of such circumstances. Thus, it is an object of the present invention to provide a resonance device constructed to reduce the influence, on the resonant frequency of the resonance device, of the electric charge borne by an insulating film of a holding portion. 
     In one aspect, a resonance device is provided that includes a resonator including a vibration body and a frame disposed in at least a part of a vicinity of the vibration body. Moreover, the frame includes a holding body and an insulating film, with the holding body holding the vibration portion configured to vibrate and the insulating film being formed above the holding body. A lower cover is provided having a recess forming at least a part of a space in which the vibration portion vibrates. An inner side surface of the insulating film is disposed at a first distance from an inner surface of a side wall defining the recess. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view schematically illustrating the exterior of a resonance device according to a first exemplary embodiment. 
         FIG. 2  is an exploded perspective view schematically illustrating the structure of the resonance device illustrated in  FIG. 1 . 
         FIG. 3  is a plan view schematically illustrating the structure of a resonator illustrated in  FIG. 2 . 
         FIG. 4  is a sectional view schematically illustrating the configuration of a section of the resonance device along line IV-IV illustrated in  FIG. 2 . 
         FIG. 5  is a sectional view schematically illustrating the configuration of a section of a vibration arm along line V-V illustrated in  FIG. 3 . 
         FIG. 6  is a sectional view schematically illustrating the configuration of a section of the resonator along line VI-VI illustrated in  FIG. 3 . 
         FIG. 7  is a sectional view schematically illustrating the configuration of a resonance device according to a second exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below. In the following drawings, the same or similar components are represented by the same or similar reference signs. The drawings are illustrative and schematically illustrate the dimensions and the shapes of the components. Thus, the technical scope of the present invention should not be construed as being limited to the embodiments. 
     First Exemplary Embodiment 
     First, a schematic configuration of a resonance device according to a first exemplary embodiment (i.e., Embodiment 1) will be described with reference to  FIGS. 1 and 2 .  FIG. 1  is a perspective view schematically illustrating the exterior of a resonance device  1  according to Embodiment 1.  FIG. 2  is an exploded perspective view schematically illustrating the structure of the resonance device  1  illustrated in  FIG. 1 . 
     The resonance device  1  includes a lower cover  20 , a resonator  10 , and an upper cover  30 . That is, the resonance device  1  has a configuration in which the lower cover  20 , the resonator  10 , and the upper cover  30  are laminated in this order. The lower cover  20  and the upper cover  30  are disposed so as to face each other with the resonator  10  interposed therebetween. The lower cover  20  corresponds to an example of “first substrate” in an exemplary aspect of the present disclosure. The upper cover  30  corresponds to an example of “second substrate” in an exemplary aspect of the present disclosure. 
     Hereinafter, each configuration of the resonance device  1  will be described. In the following descriptions, the side of the resonance device  1  on which the upper cover  30  is disposed is an upper side (or a front side), and the side of the resonance device  1  on which the lower cover  20  is disposed is a lower side (or a back side). 
     According to an exemplary aspect, the resonator  10  is a MEMS vibrator manufactured by using MEMS technology. The resonator  10  and each of the lower cover  20  and the upper cover  30  are joined so as to seal the resonator  10  and to form a space in which the resonator  10  vibrates. The resonator  10  and the lower cover  20  are each made of a silicon (Si) substrate (hereinafter referred to as “Si substrate”), and the Si substrates are joined to each other. The resonator  10  and the lower cover  20  may be each made of an SOI substrate. 
     The upper cover  30  flatly extends along the XY-plane. A recess  31 , which has, for example, a flat cuboid shape, is formed at the back of the upper cover  30 . The recess  31  is surrounded by a side wall  33  and forms a part of the space for vibration that is the space in which the resonator  10  vibrates. The upper cover  30  may be configured to be flat and not to have the recess  31 . 
     The lower cover  20  has a bottom plate  22 , which has a flat rectangular shape and is disposed along the XY-plane, and a side wall  23 , which extends from the periphery of the bottom plate  22  in the Z-axis direction, that is, the direction in which the lower cover  20  and the resonator  10  are laminated. A recess  21 , which is defined by a surface of the bottom plate  22  and an inner surface of the side wall  23 , is formed on the surface facing the resonator  10  of the lower cover  20 . The recess  21  forms at least a part of the space in which the resonator  10  vibrates. 
     The space in which the resonator  10  vibrates is airtightly sealed with the upper cover  30  and the lower cover  20  described above and is maintained under vacuum. The space in which the resonator  10  vibrates may be filled with a gas such as an inert gas. 
     A projection  50 , which projects from the bottom plate  22  into the space for vibration, is formed on an inner surface of the lower cover  20 , that is, the surface of the bottom plate  22 . A specific configuration of the projection  50  will be described later. 
     Next, a schematic configuration of the resonator  10  according to an exemplary embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a plan view schematically illustrating the structure of the resonator  10  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 3 , the resonator  10  is a MEMS vibrator manufactured by using MEMS technology and generates out-of-plane vibration on the XY-plane of the orthogonal coordinate system in  FIG. 3 . It is noted that the resonator  10  is not limited to a resonator that uses an out-of-plane bending vibration mode. The resonator of the resonance device  1  may be a resonator that uses, for example, an expansion vibration mode, a thickness longitudinal vibration mode, a Lamb-wave vibration mode, an in-plane bending vibration mode, or a surface-acoustic-wave vibration mode. Such a vibrator is applied to, for example, timing devices, RF filters, duplexers, ultrasonic transducers, gyro sensors, and accelerometers. In addition, such a vibrator may be used for, for example, piezoelectric mirrors having an actuator function, piezoelectric gyroscopes, optical-scanner-type MEMS mirrors, piezoelectric microphones having a pressure sensor function, and ultrasonic vibration sensors. In addition, such a vibrator may be applied to electrostatic MEMS elements, electromagnetic-drive MEMS elements, and piezoresistive MEMS elements. In addition, such a vibrator is also usable for megahertz oscillators by being applied to megahertz vibrators. 
     The resonator  10  includes a vibration portion  120  (e.g., a vibration body), a holding portion  140  (e.g., a frame), a holding arm  110 , and vias V 1 , V 2 , V 3 , and V 4 . 
     The vibration portion  120  has a rectangular outline extending along the XY-plane of the orthogonal coordinate system in  FIG. 3 . The vibration portion  120  is disposed inside the holding portion  140 . A space is formed between the vibration portion  120  and the holding portion  140  with a predetermined distance therebetween. In the example in  FIG. 3 , the vibration portion  120  includes a base  130  and four vibration arms  135 A to  135 D (hereinafter also collectively referred to as “vibration arms  135 ”). It is noted that the number of vibration arms is not limited to four and is set to any number equal to or more than two, for example, according to alternative aspects. In the present embodiment, the vibration arms  135  and the base  130  are integrally formed. 
     When the main surface of the resonator  10  is viewed in plan view (hereinafter simply referred to as “plan view”), the base  130  has long sides  131   a  and  131   b  in the X-axis direction and short sides  131   c  and  131   d  in the Y-axis direction. The long side  131   a  is a side of a front end surface (hereinafter also referred to as “front end  131 A”) of the base  130 . The long side  131   b  is a side of a rear end surface (hereinafter also referred to as “rear end  131 B”) of the base  130 . In the base  130 , the front end  131 A and the rear end  131 B are disposed so as to face each other. 
     The base  130  is connected to the vibration arms  135  (described later) at the front end  131 A. The base  130  is connected to holding arms  111  and  112  (described later) at the rear end  131 B, which opposes the front end  131 A. In the example illustrated in  FIG. 3 , the base  130  has a substantially rectangular shape in plan view, but the shape of the base  130  is not limited thereto. For example, it is simply required that the base  130  be formed substantially symmetrically with respect to an imaginary plane P defined along a perpendicular bisector of the long side  131   a . The base  130  may have, for example, a trapezoidal shape in which the long side  131   b  is shorter than the long side  131   a  or a semicircular shape whose diameter is the long side  131   a . The long sides  131   a  and  131   b  and the short sides  131   c  and  131   d  are not limited to a straight line and may be a curved line. 
     In the base  130 , the base length (each length of the short sides  131   c  and  131   d  in  FIG. 3 ) that is the longest distance between the front end  131 A and the rear end  131 B in a direction from the front end  131 A toward the rear end  131 B is about 40 μm. The base width (each length of the long sides  131   a  and  131   b  in  FIG. 3 ) that is the longest distance between the side ends of the base  130  in the width direction orthogonal to the direction of the base length is about 285 μm. 
     The vibration arms  135  extend in the Y-axis direction and have the same size. The vibration arms  135  are each disposed parallel to the Y-axis direction between the base  130  and the holding portion  140 . As shown, one end of each of the vibration arms  135  is connected to the front end  131 A of the base  130  to be a fixed end, and the other end is an open end that is disposed opposite thereof. The vibration arms  135  are disposed side by side (e.g., in parallel) with a predetermined distance in the X-axis direction therebetween. The vibration arms  135  each have a width in the X-axis direction of about 50 μm and a length in the Y-axis direction of about 420 μm, for example. 
     Preferably, the vibration arms  135  each have a weight portion G at the corresponding open end. The weight portion G has a width in the X-axis direction larger than the other part of the vibration arm  135 . The weight portion G has a width in the X-axis direction of about 70 μm, for example. The weight portion G is integrally formed with the vibration arm  135  by the same process. The vibration arm  135  has the weight portion G, and a part closer to the open end of the vibration arm  135  is thus heavier than a part closer to the fixed end of the vibration arm  135  per unit length. Accordingly, the vibration arms  135  each having the weight portion G at the part closer to the corresponding open end enable an increase in the amplitude of vibration of each vibration arm in the up-down direction. 
     In the vibration portion  120  in the present embodiment, the two vibration arms  135 A and  135 D are disposed on the outside in the X-axis direction, and the two vibration arms  135 B and  135 C are disposed on the inside in the X-axis direction. A distance W 1  between the vibration arms  135 B and  135 C in the X-axis direction is set to be larger than a distance W 2  between the vibration arm  135 A ( 135 D), which is on the outside in the X-axis direction, and the vibration arm  135 B ( 135 C), which is on the inside in the X-axis direction and is adjacent to the vibration arm  135 A ( 135 D) on the outside. The distance W 1  is, for example, about 35 μm. The distance W 2  is, for example, about 25 μm. The vibration characteristics are improved by setting the distance W 2  to be smaller than the distance W 1 . To reduce the size of the resonance device  1 , the distance W 1  may be set to be smaller than the distance W 2  or may be set to be equal to the distance W 2 . 
     An insulating film  235  is formed on a surface (e.g., the surface facing the upper cover  30 ) of the vibration portion  120  so as to cover the entire surface of the vibration portion  120 . In addition, a conductive film  236  is formed on a part of a surface of the insulating film  235  in each of the vibration arms  135 A to  135 D. The resonant frequency of the vibration portion  120  can be adjusted with the insulating film  235  and the conductive film  236 . Although the insulating film  235  does not necessarily have to cover the entire surface of the vibration portion  120 , the insulating film  235  preferably covers the entire surface of the vibration portion  120  to protect, from damage caused in frequency adjustment, an under electrode film such as a metal layer E 2  in  FIG. 4  and a piezoelectric film such as a piezoelectric thin film F 3  in  FIG. 4 . 
     The conductive film  236  is formed, such that a surface of the conductive film  236  is exposed, on the insulating film  235  in at least a part of a region of the vibration portion  120  whose average displacement due to vibration is larger than that of the other region. Specifically, the conductive film  236  is formed at the tip of the vibration arm  135 , that is, at the weight portion G. On the other hand, the surface of the insulating film  235  is exposed in the other region of the vibration arm  135 . In the embodiment, the conductive film  236  is formed so as to reach the tip of the vibration arm  135 , and the insulating film  235  is not exposed at the tip portion in the least. The conductive film  236  can also be configured not to be formed at the tip portion of the vibration arm  135  to expose a part of the insulating film  235 . Moreover, a second conductive film may be formed at a part closer to the base of the vibration arm  135  (e.g., a part closer to the position where the vibration arm  135  is connected to the base  130 ). In this case, a change in frequency temperature characteristics due to frequency adjustment can be reduced. 
     The holding portion  140  (or simply the “frame”) is formed into a rectangular frame shape along the XY-plane. The holding portion  140  has a frame shape in plan view and is disposed so as to surround the outer side portion of the vibration portion  120  along the XY-plane. As described above, the configuration in which the holding portion  140  has a frame shape in plan view enables the holding portion  140  surrounding the vibration portion  120  to be easily realized. 
     The holding portion  140  is simply required to be disposed in at least a part of the vicinity of the vibration portion  120 , and the shape of the holding portion  140  is not limited to a frame shape. The holding portion  140  is simply required to be disposed around the vibration portion  120  such that, for example, the holding portion  140  holds the vibration portion  120  and is joined to the upper cover  30  and the lower cover  20 . 
     In the present embodiment, the holding portion  140  includes frames  140   a  to  140   d , which have a rectangular columnar shape and are integrally formed. As illustrated in  FIG. 3 , the frame  140   a  is disposed so as to face the open ends of the vibration arms  135  with the long-side direction of the frame  140   a  parallel to the X-axis. As further shown, the frame  140   b  is disposed so as to face the rear end  131 B of the base  130  with the long-side direction of the frame  140   b  parallel to the X-axis. The frame  140   c  is disposed so as to face a side end (short side  131   c ) of the base  130  and the vibration arm  135 A with the long-side direction of the frame  140   c  parallel to the Y-axis. The frame  140   c  is connected to one end of the frame  140   a  and one end of the frame  140   b  at the corresponding ends of the frame  140   c . The frame  140   d  is disposed so as to face a side end (short side  131   d ) of the base  130  and the vibration arm  135 D with the long-side direction of the frame  140   d  parallel to the Y-axis. The frame  140   d  is connected to the other end of the frame  140   a  and the other end of the frame  140   b  at the corresponding ends of the frame  140   d.    
     In the exemplary aspect, the substantially entire surface of the holding portion  140  is covered with the insulating film  235 . 
     Moreover, the holding arms  111  and  112  are disposed inside the holding portion  140  and respectively connect the rear end  131 B of the base  130  to the frame  140   c  and the rear end  131 B of the base  130  to the frame  140   d . As illustrated in  FIG. 3 , the holding arms  111  and  112  are formed substantially symmetrically with respect to the imaginary plane P defined parallel to the YZ-plane along a center line of the base  130  in the X-axis direction. 
     As further shown, the holding arm  111  includes arms  111   a ,  111   b ,  111   c , and  111   d . One end of the holding arm  111  is connected to the rear end  131 B of the base  130 , and the holding arm  111  extends from the rear end  131 B toward the frame  140   b . The holding arm  111  then bends in a direction toward the frame  140   c  (that is, the X-axis direction). In addition, the holding arm  111  bends in a direction toward the frame  140   a  (that is, the Y-axis direction). The holding arm  111  bends in the direction toward the frame  140   c  (that is, the X-axis direction) again, and the other end of the holding arm  111  is connected to the frame  140   c.    
     The arm  111   a  is disposed between the base  130  and the frame  140   b  so as to face the frame  140   c  with the long-side direction of the arm  111   a  parallel to the Y-axis. One end of the arm  111   a  is connected to the base  130  at the rear end  131 B, and the arm  111   a  extends, from the rear end  131 B, substantially perpendicularly to the rear end  131 B, that is, in the Y-axis direction. An axis passing through the center of the arm  111   a  in the X-axis direction is preferably disposed more inside than a center line of the vibration arm  135 A. In the example in  FIG. 3 , the arm  111   a  is disposed between the vibration arms  135 A and  135 B. The other end of the arm  111   a  is connected to one end of the arm  111   b  at a side surface of the other end of the arm  111   a . The width of the arm  111   a  defined in the X-axis direction is about 20 μm. The length of the arm  111   a  defined in the Y-axis direction is 40 μm. 
     The arm  111   b  is disposed between the base  130  and the frame  140   b  so as to face the frame  140   b  with the long-side direction of the arm  111   b  parallel to the X-axis direction. The one end of the arm  111   b  is connected to the side surface facing the frame  140   c  of the other end of the arm  111   a , and the arm  111   b  extends, from the side surface, substantially perpendicularly to the arm  111   a , that is, in the X-axis direction. The other end of the arm  111   b  is connected to the side surface facing the vibration portion  120  of one end of the arm  111   c . The width of the arm  111   b  defined in the Y-axis direction is, for example, about 20 μm. The length of the arm  111   b  defined in the X-axis direction is, for example, about 75 μm. 
     The arm  111   c  is disposed between the base  130  and the frame  140   c  so as to face the frame  140   c  with the long-side direction of the arm  111   c  parallel to the Y-axis direction. The one end of the arm  111   c  is connected to the other end of the arm  111   b  at the side surface of the one end of the arm  111   c . The other end of the arm  111   c  is connected to one end of the arm  111   d . The width of the arm  111   c  defined in the X-axis direction is, for example, about 20 μm. The length of the arm  111   c  defined in the Y-axis direction is, for example, about 140 μm. 
     The arm  111   d  is disposed between the base  130  and the frame  140   c  so as to face the frame  140   a  with the long-side direction of the arm  111   d  parallel to the Y-axis direction. The one end of the arm  111   d  is connected to the side surface facing the frame  140   c  of the other end of the arm  111   c . The other end of the arm  111   d  is connected to the frame  140   c  at a position facing the vicinity of the part where the vibration arm  135 A and the base  130  are connected. The arm  111   d  extends, from the position, substantially perpendicularly to the frame  140   c , that is, in the X-axis direction. The width of the arm  111   d  defined in the Y-axis direction is, for example, about 20 μm. The length of the arm  111   d  defined in the X-axis direction is, for example, about 10 μm. 
     As described above, the holding arm  111  is configured to be connected to the base  130  at the arm  111   a , to bend at the part where the arm  111   a  and the arm  111   b  are connected, the part where the arm  111   b  and the arm  111   c  are connected, and the part where the arm  111   c  and the arm  111   d  are connected, and to be then connected to the holding portion  140 . 
     Similarly, the holding arm  112  includes arms  112   a ,  112   b ,  112   c , and  112   d . One end of the holding arm  112  is connected to the rear end  131 B of the base  130 , and the holding arm  112  extends from the rear end  131 B toward the frame  140   b . The holding arm  112  then bends in a direction toward the frame  140   d  (that is, the X-axis direction). In addition, the holding arm  112  bends in the direction toward the frame  140   a  (that is, the Y-axis direction). The holding arm  112  bends in the direction toward the frame  140   d  (that is, the X-axis direction) again, and the other end of the holding arm  112  is connected to the frame  140   d.    
     Otherwise, it is noted that the configuration of the arms  112   a ,  112   b ,  112   c , and  112   d  is symmetrical to the configuration of the arms  111   a ,  111   b ,  111   c , and  111   d  and is thus not specifically described. 
     The vias V 1 , V 2 , V 3 , and V 4  are holes that are filled with a metal and that are formed close to the respective tip portions of the vibration arms  135 . In the exemplary aspect, the vias V 1 , V 2 , V 3 , and V 4  electrically couple the conductive film  236 , which is formed on each of the vibration arms  135 A to  135 D, to the metal layer E 2  (described later with regard to  FIG. 6 , for example). In  FIG. 6 , dashed lines represent electrical coupling, and, in particular, dotted lines represent electrical coupling through the vias V 1 , V 2 , V 3 , and V 4 . 
     The vias V 1 , V 2 , V 3 , and V 4  are each formed close to the boundary between the region in which the conductive film  236  is exposed and the region in which the insulating film  235  is exposed in a corresponding one of the tip portions of the vibration arms  135 A,  135 B,  135 C, and  135 D. This will be specifically described later. In the present embodiment, the vias V 1 , V 2 , V 3 , and V 4  are formed in the corresponding end portions closer to the fixed ends of the weight portions G. 
     Vias V 6  and V 7  are preferably formed close to the part where the holding portion  140  and the holding arm  111  or  112  are connected. In the example in  FIG. 3 , the via V 6  is formed close to the part where the frame  140   c  and the holding arm  111  (arm  111   d ) are connected, and the via V 7  is formed close to the part where the frame  140   d  and the holding arm  112  (arm  112   d ) are connected. It is noted that the positions where the vias V 6  and V 7  are formed are not limited to this example, and the vias V 6  and V 7  may be formed at any position in the holding portion  140  in alternative aspects. 
     A terminal for connecting the metal layer E 2  (e.g., the upper electrode) described later to an external drive power supply is formed in each of the vias V 6  and V 7 . This configuration enables supply of drive power to the vibration portion  120 . 
     The projection  50  is formed on the lower cover  20  so as to project between the vibration arms  135 B and  135 C. In the present embodiment, the projection  50  is formed into a rectangular columnar shape extending parallel to the vibration arms  135 B and  135 C. The length of the projection  50  in a direction along the vibration arm  135  is about 240 μm. The length (width) of the projection  50  orthogonal to the direction along the vibration arm  135  is about 15 μm. The number of the projections  50  is not limited to one and may be two or more. 
     Next, the laminated structure of the resonance device  1  according to an embodiment of the present invention will be described with reference to  FIGS. 4 to 6 .  FIG. 4  is a sectional view schematically illustrating the configuration of a section of the resonance device  1  along line IV-IV illustrated in  FIG. 2 .  FIG. 5  is a sectional view schematically illustrating the configuration of a section of the vibration arm  135 D along line V-V illustrated in  FIG. 3 .  FIG. 6  is a sectional view schematically illustrating the configuration of a section of the resonator  10  along line VI-VI illustrated in  FIG. 3 . The line IV-IV illustrated in  FIG. 2  is a line parallel to the frames  140   a  and  140   b.    
     As illustrated in  FIG. 4 , the upper cover  30  is made of a silicon (Si) wafer (hereinafter referred to as “Si wafer”) S 3 , which has a predetermined thickness. The upper cover  30  and the holding portion  140  of the resonator  10  are joined, at the periphery (side wall  33 ) of the upper cover  30 , by a joining layer  40  (described later). The surface facing the resonator  10  and the back surface of the upper cover  30  are preferably covered with a silicon oxide film (not illustrated). The silicon oxide film is formed on surfaces of the Si wafer S 3  by, for example, oxidation of the surfaces of the Si wafer S 3  or chemical vapor deposition (CVD). 
     Moreover, a terminal (not illustrated) is formed in the surface opposite to the surface facing the resonator  10  of the upper cover  30 . The terminal is formed by filling a through hole formed in the upper cover  30  with a conductive material such as impurity-doped polycrystalline silicon (Poly-Si), copper (Cu), gold (Au), or impurity-doped monocrystalline silicon. The terminal is connected to the vias V 6  and V 7  and functions as wiring that electrically couples an external power supply to the resonator  10 . 
     The bottom plate  22  and the side wall  23  of the lower cover  20  are integrally formed and made of a silicon (Si) wafer S 1 . The lower cover  20  and the holding portion  140  of the resonator  10  are joined by an upper surface of the side wall  23 . The thickness of the lower cover  20  defined in the Z-axis direction is, for example, 150 μm. The depth of the recess  21  defined in the Z-axis direction is, for example, 50 μm. The Si wafer S 1  is made of non-degenerate silicon having a resistivity of, for example, 16 mΩ·cm or more. 
     The joining layer  40  is formed between the periphery of the upper cover  30  and the holding portion  140 . The upper cover  30  and the holding portion  140  are joined by the joining layer  40 . The joining layer  40  is made of, for example, a gold (Au) film and a tin (Sn) film. 
     The projection  50  is integrally formed with the Si wafer S 1  of the lower cover  20 . The projection  50  projects between the vibration arms  135 B and  135 C from the bottom plate  22  of the lower cover  20 . As described above, the projection  50  is disposed between the vibration arms  135 B and  135 C and projects from the bottom plate  22  of the lower cover  20 . As a result, it is possible to increase the rigidity of the lower cover  20  and to reduce the occurrence of deflection of the resonator  10  formed on the lower cover  20  and the occurrence of a warp in the lower cover  20 . 
     The holding portion  140 , the base  130 , the vibration arms  135 , and the holding arms  111  and  112  of the resonator  10  are integrally formed by the same process. In the resonator  10 , a metal layer E 1  is laminated on a silicon (Si) substrate, which is an example of a substrate, (hereinafter referred to as “Si substrate F 2 ”). The piezoelectric thin film F 3  is then laminated on the metal layer E 1  so as to cover the metal layer E 1 . In addition, the metal layer E 2  is laminated on a surface of the piezoelectric thin film F 3 . The insulating film  235  is laminated on the metal layer E 2  so as to cover the metal layer E 2 . In addition, on the vibration portion  120 , the conductive film  236  is laminated on the insulating film  235 . In the present embodiment, the metal layer E 2  is configured not to extend to the tips of the vibration arms  135 . This configuration enables characteristic changes due to shorting between the metal layer E 2  and the metal layer E 1  or the conductive film  236  to be reduced. As described above, although patterning is preferably performed such that the metal layer E 2  does not extend to the tips of the vibration arms  135 , the metal layer E 2  may extend to the tips of the vibration arms  135 . When a degenerate silicon substrate having a low resistance is used for the Si substrate F 2 , the Si substrate F 2  can also serve as the metal layer E 1 , and it is also possible to omit the metal layer E 1 . The metal layer E 1  corresponds to an example of “first electrode” in an exemplary aspect of the present disclosure. The metal layer E 2  corresponds to an example of “second electrode” in an exemplary aspect of the present disclosure. 
     The Si substrate F 2  is made of, for example, a degenerate n-type silicon (Si) semiconductor having a thickness of about 6 μm and can contain, for example, phosphorus (P), arsenic (As), or antimony (Sb) as an n-type dopant. The resistivity of degenerate Si used for the Si substrate F 2  is, for example, less than 1.6 mΩ·cm, more preferably 1.2 mΩ·cm or less. In addition, a silicon oxide (for example, SiO 2 ) layer F 21  is formed, as an example of a temperature-characteristic correction layer, on a lower surface of the Si substrate F 2 . This configuration enables an improvement in temperature characteristics. 
     In the present embodiment, the silicon oxide layer F 21  is a layer constructed for having a function of reducing, at at least a temperature close to room temperature, the temperature coefficient of the frequency, that is, the rate of change in the frequency, per unit temperature change, of the vibration portion  120  in which a temperature-characteristic correction layer is formed on the Si substrate F 2  compared with the case in which the silicon oxide layer F 21  is not formed on the Si substrate F 2 . The vibration portion  120  includes the silicon oxide layer F 21 , and as a result it is possible to reduce changes, due to temperature changes, in the resonant frequency of the laminated structure composed of the Si substrate F 2 , the metal layers E 1  and E 2 , the piezoelectric thin film F 3 , and the silicon oxide layer F 21 , for example. The silicon oxide layer F 21  and the Si substrate F 2  in the holding portion  140  correspond to an example of “holding body” in an exemplary aspect of the present disclosure. 
     In the resonator  10 , the silicon oxide layer F 21  is preferably formed so as to have a uniform thickness. For purposes of this disclosure, it is noted that the uniform thickness denotes the thickness of the silicon oxide layer F 21  whose variations are within a range of ±20% of the average thickness thereof. 
     The silicon oxide layer F 21  may be formed on an upper surface of the Si substrate F 2  and formed on both the upper surface and the lower surface of the Si substrate F 2 . In the holding portion  140 , the silicon oxide layer F 21  does not have to be formed on the lower surface of the Si substrate F 2 . 
     The metal layers E 1  and E 2  each have a thickness of, for example, about 0.1 μm or more and about 0.2 μm or less. The metal layers E 1  and E 2  are each patterned into a desired shape by, for example, etching after a film-forming process. The metal layers E 1  and E 2  are made of, for example, molybdenum (Mo) or tungsten (W). As described above, the metal layers E 1  and E 2  each preferably contain, as a main component, a metal whose crystal structure is a body-centered cubic structure. This configuration enables the metal layers E 1  and E 2  respectively suitable for a lower electrode and an upper electrode of the resonator  10  to be easily realized. 
     The metal layer E 1  is formed so as to function as, for example, a lower electrode, a float electrode, or a ground electrode in the vibration portion  120 . In the present embodiment, the metal layer E 1  is configured as a lower electrode. In addition, the metal layer E 1  is formed so as to function as wiring, on the holding arms  111  and  112  or the holding portion  140 , for connecting a lower electrode or a ground electrode to an alternating-current power supply disposed outside the resonator  10 . 
     On the other hand, the metal layer E 2  is formed so as to function as an upper electrode in the vibration portion  120 . In addition, the metal layer E 2  is formed so as to function as wiring, on the holding arms  111  and  112  or the holding portion  140 , for connecting an upper electrode to a circuit disposed outside the resonator  10 . 
     A configuration in which an electrode is formed on an outer surface of the upper cover  30  and connects the circuit to lower wiring or upper wiring or a configuration in which a via formed in the upper cover  30  is filled with a conductive material to prepare wiring that connects the alternating-current power supply to the lower wiring or the upper wiring may be used to connect the alternating-current power supply or a ground to the lower wiring or the upper wiring. 
     Moreover, in an exemplary aspect, the piezoelectric thin film F 3  is a thin film made of a piezoelectric material that converts an applied voltage into vibration. 
     The piezoelectric thin film F 3  can contain, as a main component, for example, a nitride or an oxide such as aluminum nitride (AlN), scandium aluminum nitride (ScAlN), zinc oxide (ZnO), gallium nitride (GaN), or indium nitride (InN). In scandium aluminum nitride, a part of aluminum in aluminum nitride is substituted with scandium and may be substituted with, in place of scandium, two chemical elements such as magnesium (Mg) and niobium (Nb) or magnesium (Mg) and zirconium (Zr). As described above, the piezoelectric thin film F 3  preferably contains, as a main component, a piezoelectric material whose crystal structure is a hexagonal wurtzite structure. This configuration enables the piezoelectric thin film F 3  suitable for the resonator  10  to be easily realized. 
     The piezoelectric thin film F 3  in the vibration portion  120  and the piezoelectric thin film F 3  in the holding portion  140  may be made of different materials in one exemplary aspect. However, the piezoelectric thin film F 3  in the vibration portion  120  and the piezoelectric thin film F 3  in the holding portion  140  are preferably made of the same material. This configuration enables the vibration portion  120  and the holding portion  140  to be formed by the same process and the manufacturing process of the resonator  10  to be simplified. 
     The piezoelectric thin film F 3  has a thickness of, for example, 1 μm or can also have a thickness of about 0.2 to 2 μm. 
     The piezoelectric thin film F 3  expands and contracts in the in-plane direction of the XY-plane, that is, the Y-axis direction, in response to an electric field applied to the piezoelectric thin film F 3  by the metal layers E 1  and E 2 . By expanding and contracting the piezoelectric thin film F 3 , the open ends of the vibration arms  135  are displaced toward each inner surface of the lower cover  20  and the upper cover  30 , and the vibration arms  135  are vibrated in an out-of-plane bending vibration mode. 
     In the present embodiment, in the out-of-plane bending vibration mode using four arms, two inner arms and two outer arms are configured to bend and vibrate in respective directions opposite to each other by dividing the upper electrode and connecting each divided upper electrode to the alternating-current power supply. However, it is noted that the exemplary embodiment of the present invention is not limited thereto. For example, a configuration in which the number of vibration arms is one or a configuration in which vibration is performed in an in-plane bending vibration mode may be used. 
     The insulating film  235  is a layer made of an insulating material. In one aspect, the insulating film  235  is made of a material whose mass reduction rate in etching is slower than that of the conductive film  236 . The mass reduction rate is expressed by an etching rate, that is, the product of a density and a thickness by which thickness reduction is performed per unit time. 
     The insulating film  235  is formed by a piezoelectric film made of, for example, aluminum nitride (AlN), scandium aluminum nitride (ScAlN), zinc oxide (ZnO), gallium nitride (GaN), or indium nitride (InN), or an insulating film made of, for example, silicon nitride (SiN), silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), or tantalum pentoxide (Ta 2 O 5 ). The insulating film  235  is formed so as to have a thickness half or less of the thickness of the piezoelectric thin film F 3 . In the present embodiment, the insulating film  235  has a thickness of, for example, about 0.2 μm. More preferably, the thickness of the insulating film  235  is about a quarter of the thickness of the piezoelectric thin film F 3 . In addition, when the insulating film  235  is made of a piezoelectric material such as aluminum nitride (AlN), a piezoelectric material whose orientation is the same as that of the piezoelectric thin film F 3  is preferably used for the insulating film  235 . 
     The conductive film  236  is a layer made of a conductive material. The conductive film  236  is formed on the substantially entire surface of the vibration portion  120  and is then formed only in a predetermined region by processing such as etching. The conductive film  236  is made of a material whose mass reduction rate in etching is faster than that of the insulating film  235 . The conductive film  236  is made of a metal such as molybdenum (Mo), tungsten (W), gold (Au), platinum (Pt), nickel (Ni), aluminum (Al), or titanium (Ti). 
     The etching rate of the insulating film  235  and the etching rate of the conductive film  236  may have any relationship as long as the above relationship between the mass reduction rate of the insulating film  235  and the mass reduction rate of the conductive film  236  is satisfied. 
     In the resonance device  1  according to the exemplary embodiment, a trimming process in which the film thickness of the conductive film  236  is adjusted is performed after the formation of the resonator  10  described above. 
     In the trimming process, first, the resonant frequency of the resonator  10  is measured, and a deviation from a target frequency is calculated. Subsequently, the film thickness of the conductive film  236  is adjusted on the basis of the calculated frequency deviation. The film thickness of the conductive film  236  can be adjusted by, for example, irradiating the entire surface of the resonance device  1  with an argon (Ar) ion beam to etch the conductive film  236 . It is possible to irradiate an area wider than that of the resonator  10  with an ion beam. The exemplary embodiment illustrates an example in which etching is performed with an ion beam, but it is noted that the etching method is not limited thereto. In addition, it is preferable to clean the resonator  10  and to remove scattered films after the adjustment of the film thickness of the conductive film  236 . 
     As illustrated in  FIG. 5 , the metal layer E 2  is formed by adjusting the area thereof such that the region in which the metal layer E 2  and the conductive film  236  overlap each other is as small as possible. 
     The via V 4  is formed by filling, with a conductive material  237 , a hole that is formed by removing a part of the insulating film  235 , a part of the metal layer E 2 , and a part of the piezoelectric thin film F 3  so as to expose a surface of the metal layer E 1 . The conductive material  237 , with which the via V 4  is filled, is molybdenum (Mo) or aluminum (Al), for example. The conductive film  236  and the metal layer E 1  are electrically coupled via the conductive material  237 , with which the via V 4  is filled. This configuration enables the electric charge borne by the insulating film  235  to move to the metal layer E 1 . The electric charge that has moved to the metal layer E 1  can be then discharged to the outside of the resonance device  1  via a connection terminal for connection with the outside, the connection terminal being connected to the metal layer E 1 . As described above, the resonator  10  according to the present embodiment can inhibit an electric charge from being borne by the insulating film  235  formed on the vibration portion  120  and can thus prevent the resonant frequency of the resonator  10  from changing due to the electric charge borne by the vibration portion  120 . 
     When the metal layer E 1  and the conductive film  236  are connected to each other, respective electric fields whose directions are opposite to each other are applied to the insulating film  235  and the piezoelectric thin film F 3 . Thus, when the region in which the metal layer E 2  and the conductive film  236  overlap each other is large, the vibration of the resonator  10  is hindered. The resonator  10  according to the present embodiment is set such that the region in which the metal layer E 2  and the conductive film  236  overlap each other is as small as possible. This configuration enables hindrance to the vibration of the piezoelectric thin film F 3  due to the electric field applied to the insulating film  235  to be reduced. For example, the connection configuration and the effect of the vias V 1 , V 2 , and V 3 , and the material for the vias V 1 , V 2 , and V 3  are similar to those of the via V 4  and are thus not illustrated and described. 
     As illustrated in  FIG. 6 , in the present embodiment, the phase of the electric field applied to the vibration arms  135 A and  135 D, which are positioned on the outside, and the phase of the electric field applied to the vibration arms  135 B and  135 C, which are positioned on the inside, are set to be opposite to each other. Thus, the vibration arms  135 A and  135 D on the outside and the vibration arms  135 B and  135 C on the inside are displaced in the respective directions opposite to each other. For example, when the open ends of the vibration arms  135 A and  135 D on the outside are displaced toward the inner surface of the upper cover  30 , the open ends of the vibration arms  135 B and  135 C on the inside are displaced toward the inner surface of the lower cover  20 . 
     Thus, in the resonator  10  according to the present embodiment, when the vibration arms  135 A and  135 B vibrate in the respective phases opposite to each other, the vibration arms  135 A and  135 B vibrate in the respective up-down directions opposite to each other around a central axis r 1  extending parallel to the Y-axis. In addition, when the vibration arms  135 C and  135 D vibrate in the respective phases opposite to each other, the vibration arms  135 C and  135 D vibrate in the respective up-down directions opposite to each other around a central axis r 2  extending parallel to the Y-axis. This causes torsional moments in the directions opposite to each other to be generated at the respective central axes r 1  and r 2  and the base  130  to thus bend and vibrate. 
     Hereinafter, the positional relationship between the holding portion  140  and the side wall  23  of the lower cover  20  will be described. 
     In the related art, the inside side surface (hereinafter referred to as “inner side surface”) of the holding portion  140  is disposed so as to coincide or substantially coincide with an inner surface  23   a  of the side wall  23 . Thus, the holding portion  140  including the insulating film  235  is disposed around the vibration portion  120  including the vibration arms  135 . 
     The insulating film  235  of the holding portion  140  may be electrified due to various factors. The insulating film  235  of the holding portion  140  is electrified due to, for example, an electric field generated during thin film formation in the resonator  10 . In addition, when a film for frequency adjustment is formed on the vibration portion  120 , and the resonant frequency is adjusted by using, for example, an ion beam, the holding portion  140  is also irradiated with an ion beam, and as a result the insulating film  235  of the holding portion  140  is electrified. In addition, when a pyroelectric material is used for the insulating film  235 , an electric charge is generated at the interface of the insulating film  235  due to a pyroelectric effect caused by temperature changes. 
     When the insulating film  235  of the holding portion  140  disposed around the vibration portion  120  is electrified due to these factors, a Coulomb force (i.e., an electrostatic force) is generated by the electric charge at the insulating film  235 , and the spring constant of the vibration portion  120  thus changes. As a result, the resonant frequency changes. 
     On the other hand, in the present embodiment, as illustrated in  FIG. 4 , the inner side surface of the frame  140   c  is disposed at a first distance D 1  from the inner surface  23   a  of the side wall  23 . Similarly, the inner side surface of the frame  140   d  is disposed at the first distance D 1  from the inner surface  23   a  of the side wall  23 . 
     In addition, similarly, each inner side surface of the frames  140   a  and  140   b  (not illustrated) is also disposed at the first distance D 1  from the inner surface  23   a  of the side wall  23 . The first distance D 1  is not limited to the same first distance D 1  at which each inner side surface of the frames  140   a  to  140   d  is disposed and may be substantially the same first distance D 1 . 
     As described above, the inner side surface of the insulating film  235  included in the holding portion  140  is disposed at the first distance D 1  from the inner surface  23   a  of the side wall  23 . Thus, it is possible to increase the distance between the holding portion  140  and the vibration portion  120  including the vibration arms  135  compared with the case in which the inner side surface of the insulating film  235  is disposed at no distance from the inner surface  23   a  of the side wall  23 . Accordingly, it is possible to reduce the generation, around the vibration portion  120 , of the electric charge borne by the insulating film  235  of the holding portion  140 . As a result, it is possible to reduce the influence, on the resonant frequency, of the electric charge borne by the insulating film  235  of the holding portion  140 . 
     Preferably, the first distance D 1  is, for example, 5 μm or more and 50 μm or less. This configuration enables the influence, on the resonant frequency, of the electric charge at the holding portion  140  to be effectively reduced with the resonance device  1  reduced in thickness and size. 
     In an exemplary aspect, to dispose the inner side surface of the holding portion  140  at a distance from the inner surface  23   a  of the side wall  23 , the etching width of the holding portion  140  is required to be larger than that in the related art when a part between the vibration arms  135  and the holding portion  140  is released by, for example, etching. As described above, the inner side surface of the holding portion  140  can be disposed at the first distance D 1  from the inner surface  23   a  of the side wall  23  without increasing the number of masks or forming processes. Thus, it is possible to reduce the influence, on the resonant frequency, of the electric charge at the holding portion  140  at a low cost. 
     Second Exemplary Embodiment 
     Next, a second exemplary embodiment (hereinafter referred to as “Embodiment 2”) will be described. For Embodiment 2, points in common with Embodiment 1 are not described, and only different points are described. In particular, similar operational effects resulting from similar configurations are not mentioned in each embodiment. 
       FIG. 7  is a sectional view schematically illustrating the configuration of a resonance device  2  according to Embodiment 2 of the present invention.  FIG. 7  is a figure corresponding to  FIG. 4  in Embodiment 1. 
     As illustrated in  FIG. 7 , in the resonance device  2 , a step is formed in a part of the holding portion  140 . That is, each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2  in the holding portion  140  is disposed so as to substantially coincide with the inner surface  23   a  of the side wall  23 . Each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2  in the frame  140   c  may project in the positive X-axis direction. Each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2  in the frame  140   d  may project in the negative X-axis direction. 
     On the other hand, in the holding portion  140 , each inner side surface of the metal layer E 1 , the piezoelectric thin film F 3 , the metal layer E 2 , the insulating film  235 , and the conductive film  236  is disposed at the first distance D 1  from the inner surface  23   a  of the side wall  23 . 
     In addition, in the holding portion  140 , each inner side surface of the metal layer E 1 , the piezoelectric thin film F 3 , the metal layer E 2 , the insulating film  235 , and the conductive film  236  is disposed at a second distance D 2  from each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2 . The second distance D 2  from each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2  is set to be equal to or more than the first distance D 1  from the inner surface  23   a  of the side wall  23  (i.e., second distance D 2  first distance D 1 ). 
     As described above, in the holding portion  140 , the inner side surface of the insulating film  235  is disposed at the second distance D 2  equal to or more than the first distance D 1  from each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2 . Thus, the silicon oxide layer F 21  and the Si substrate F 2  of the holding portion  140  are disposed close to the vibration portion  120 . Accordingly, for example, when the vibration arms  135  of the vibration portion  120  are displaced due to an impact applied to the resonator  10 , the silicon oxide layer F 21  and the Si substrate F 2  of the holding portion  140  function as a stopper and can reduce the displacement of the vibration arms  135 . As a result, it is possible to prevent the vibration arms  135  from being broken. 
     To dispose each inner side surface of the metal layer E 1 , the piezoelectric thin film F 3 , the metal layer E 2 , the insulating film  235 , and the conductive film  236  in the holding portion  140  at a distance from each inner side surface of the silicon oxide layer F 21  and the Si substrate F 2 , first, the metal layer E 1 , the piezoelectric thin film F 3 , the metal layer E 2 , the insulating film  235 , and the conductive film  236  are removed by, for example, etching, and the silicon oxide layer F 21  and the Si substrate F 2  are left. As a result, a recess is formed in a part to be the holding portion  140  in the resonator  10 . Subsequently, a part between the vibration arms  135  and the holding portion  140  is released by, for example, etching. 
     It is noted that exemplary embodiments of the present invention are described above. In a resonance device according to an embodiment of the present invention, an inner side surface of an insulating film included in a holding portion is disposed at a first distance from an inner surface of a side wall defining a recess. Thus, it is possible to increase the distance between the holding portion and a vibration portion including vibration arms compared with the case in which the inner side surface of the insulating film is disposed at no distance from the inner surface of the side wall. Accordingly, the generation, around the vibration portion, of the electric charge borne by the insulating film of the holding portion can be reduced. As a result, the influence, on the resonant frequency, of the electric charge borne by the insulating film of the holding portion can also be reduced. 
     In addition, in the resonance device, the inner side surface of the insulating film in the holding portion is disposed at a second distance equal to or more than the first distance from each inner side surface of a silicon oxide layer and a Si substrate. Thus, the silicon oxide layer and the Si substrate of the holding portion are disposed close to the vibration portion. Accordingly, for example, when the vibration arms of the vibration portion are displaced due to an impact applied to a resonator, the silicon oxide layer and the Si substrate of the holding portion function as a stopper and can reduce the displacement of the vibration arms. As a result, the vibration arms can be prevented from being broken. 
     In addition, in the resonance device, the first distance is 5 μm or more and 50 μm or less. This configuration enables the influence, on the resonant frequency, of the electric charge at the holding portion to be effectively reduced with the resonance device reduced in thickness and size. 
     In addition, in the resonance device, the holding portion has a frame shape when a main surface of the resonator is viewed in plan view. This configuration enables the holding portion surrounding the vibration portion to be easily realized. 
     In addition, in the resonance device, a projection is disposed between adjacent two of the vibration arms and projects from the recess. As a result, it is possible to increase the rigidity of a lower cover and to reduce the occurrence of deflection of the resonator formed on the lower cover and the occurrence of a warp in the lower cover. 
     In addition, in the resonance device, a piezoelectric thin film contains, as a main component, a piezoelectric material having a hexagonal wurtzite structure. This configuration enables a piezoelectric thin film suitable for the resonator to be easily realized. 
     In addition, in the resonance device, the piezoelectric thin film in the holding portion and the piezoelectric thin film in the vibration portion are made of the same material. This configuration enables the vibration portion and the holding portion to be formed by the same process and the manufacturing process of the resonator to be simplified. 
     In addition, in the resonance device, at least one of metal layers contains, as a main component, a metal having a body-centered cubic structure. This configuration enables metal layers suitable for a lower electrode and an upper electrode of the resonator to be easily realized. 
     In addition, the resonance device includes an upper cover disposed so as to face the lower cover with the resonator interposed between the upper cover and the lower cover. This configuration enables the space in which the resonator vibrates to be airtightly sealed and to be maintained under a high vacuum. 
     The exemplary embodiments described above are intended to facilitate understanding of the present invention and are not intended to construe the present invention in any limiting manner. It should be appreciated that the present invention can be modified and improved without departing from the spirit of the present invention and includes equivalents thereof. That is, design changes appropriately made to each embodiment by those skilled in the art are also included in the scope of the present invention as long as the design changes have the features of the present invention. For example, the components in each embodiment, the dispositions, the materials, the conditions, the shapes, and the sizes of the components are not limited to those exemplified above and can be appropriately modified. Naturally, each embodiment is an exemplary embodiment, and configurations described in different embodiments can be partially substituted or combined. Such substitutions and combinations are also included in the scope of the present invention as long as the substitutions and combinations have the features of the present invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  resonance device 
               2  resonance device 
               10  resonator 
               20  lower cover 
               21  recess 
               22  bottom plate 
               23  side wall 
               23   a  inner surface 
               30  upper cover 
               31  recess 
               33  side wall 
               40  joining layer 
               50  projection 
               110  holding arm 
               111  holding arm 
               111   a ,  111   b ,  111   c ,  111   d  arm 
               112  holding arm 
               112   a ,  112   b ,  112   c ,  112   d  arm 
               120  vibration portion 
               130  base 
               131   a  long side 
               131 A front end 
               131   b  long side 
               131 B rear end 
               131   c  short side 
               131   d  short side 
               135 ,  135 A,  135 B,  135 C,  135 D vibration arm 
               140  holding portion 
               140   a ,  140   b ,  140   c ,  140   d  frame 
               235  insulating film 
               236  conductive film 
               237  conductive material 
             D 1  first distance 
             D 2  second distance 
             E 1 , E 2  metal layer 
             F 2  Si substrate 
             F 3  piezoelectric thin film 
             F 21  silicon oxide layer 
             G weight portion 
             P imaginary plane 
             r 1 , r 2  central axis 
             S 1  Si wafer 
             S 3  Si wafer 
             V 1 , V 2 , V 3 , V 4 , V 6 , V 7  via 
             W 1 , W 2  distance