Patent Publication Number: US-2020295732-A1

Title: Resonance device and method for producing resonance device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International application No. PCT/JP2018/034266, filed Sep. 14, 2018, which claims priority to Japanese Patent Application No. 2018-023946, filed Feb. 14, 2018, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a resonance device and a method for producing a resonance device. 
     BACKGROUND OF THE INVENTION 
     Hitherto, resonance devices produced by microelectromechanical systems (MEMS) technology have been widely used. Such a resonance device is produced, for example, by bonding an upper substrate to a lower substrate that includes a resonator. 
     Patent Document 1 discloses a bonding portion including a diffusion preventing layer that is stacked above a wafer and that is composed of a material having low wettability with AuSn, a bonding layer that is disposed on a surface of the diffusion preventing layer and that is spaced apart from edges of the diffusion preventing layer, and a functional layer that is disposed between the wafer and the diffusion preventing layer and that is easily degraded by the diffusion of AuSn. In the bonding portion, since the bonding layer is spaced apart from edges of the diffusion preventing layer, when AuSn eutectic bonding is performed with AuSn solder, molten AuSn solder does not easily spread over the surface of the diffusion preventing layer; thus, the flowing down of the solder to the functional layer due to the diffusion of AuSn is less likely to occur. 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2013-149599 
     SUMMARY OF THE INVENTION 
     A vibration space in which a resonator vibrates in a resonance device needs to be hermetically sealed to maintain a vacuum state in order to stabilize the resonance characteristics of the resonator. Outgassing from a material of the resonance device causes a decrease in the degree of vacuum in the vibration space. To prevent the occurrence of the outgassing, a method for performing degassing by heat treatment in the production of a resonance device has been used. 
     However, in a bonding portion bonded by eutectic bonding, when heat treatment for degassing is performed at a high temperature, thermal diffusion occurs easily to cause, for example, the deviation of a eutectic composition and the failure of a eutectic reaction during eutectic bonding. Thus, the heat treatment for degassing cannot be performed at a high temperature, and the degree of vacuum in the vibration space of the resonator may be decreased by outgassing. 
     The present invention has been accomplished in view of the foregoing circumstances. It is an object of the present invention to provide a resonance device that can maintain the vibration space of a resonator in a high vacuum and a method for producing a resonance device. 
     A resonance device according to an aspect of the present invention includes a first substrate including a resonator, a second substrate, and a bonding portion bonding the first substrate to the second substrate so as to seal a vibration space of the resonator. The bonding portion includes a eutectic layer composed of a eutectic alloy of germanium and a metal mainly containing aluminum, a first titanium layer, a first aluminum oxide film, and a first conductive layer consecutively arranged from the first substrate to the second substrate lies. 
     A method for producing a resonance device according to another aspect of the present invention includes forming a first layer including a metal layer mainly containing aluminum around a vibrating portion of a resonator of a first substrate, forming a second layer on a portion of a second substrate that faces the first layer when the first substrate faces the second substrate, the second layer including a first conductive layer, a first aluminum oxide film, a first titanium layer, and a germanium layer consecutively formed in this order from the second substrate, and bonding the metal layer of the first layer to the germanium layer of the second layer by eutectic bonding so as to seal a vibration space of the resonator. 
     According to the present invention, the vibration space of the resonator can be maintained in a high vacuum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of the appearance of a resonance device according to an embodiment of the present invention. 
         FIG. 2  is a schematic exploded perspective view of the structure of a resonance device according to an embodiment of the present invention. 
         FIG. 3  is a plan view of the structure of a resonator according to an embodiment of the present invention. 
         FIG. 4  is a schematic cross-sectional view of the resonance device taken along line IV-IV of  FIG. 1 . 
         FIG. 5  is a schematic enlarged fragmentary cross-sectional view of a bonding portion illustrated in  FIG. 4 . 
         FIG. 6A  is a schematic view illustrating a production process of a resonance device according to an embodiment. 
         FIG. 6B  is a schematic view illustrating the production process of a resonance device according to an embodiment. 
         FIG. 6C  is a schematic view illustrating the production process of a resonance device according to an embodiment. 
         FIG. 7  is an enlarged fragmentary cross-sectional view of a first modification of the bonding portion illustrated in  FIG. 5 . 
         FIG. 8  is an enlarged fragmentary cross-sectional view of a second modification of the bonding portion illustrated in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below. In the following description of the drawings, the same or similar components are denoted by the same or similar reference numerals. It is noted that the drawings are illustrative only and the dimensions and geometries are schematic only, and the technical scope of the present invention should not be understood as being limited to the embodiments. 
     EMBODIMENTS 
     The schematic structure of a resonance device  1  according to an embodiment of the present invention will be described with reference to  FIGS. 1 and 2 .  FIG. 1  is a schematic perspective view of the appearance of the resonance device  1  according to an embodiment of the present invention.  FIG. 2  is a schematic exploded perspective view of the structure of the resonance device  1  according to an embodiment of the present invention. 
     The resonance device  1  includes a lower cover  20 , a resonator  10  (hereinafter, the lower cover  20  and the resonator  10  are also collectively referred to as a “MEMS substrate  50 ”), an upper cover  30 , and a bonding portion  60 . That is, the resonance device  1  includes the MEMS substrate  50 , the bonding portion  60 , and the upper cover  30  stacked in this order. The MEMS substrate  50  corresponds to an example of a “first substrate” in the present description. The upper cover  30  corresponds to an example of a “second substrate” in the present description. 
     Each component of the resonance device  1  will be described below. In the following description, a side of the resonance device  1  on which the upper cover  30  lies will be described as an upper side (or front), and a side of the resonance device  1  on which the lower cover  20  lies will be described as a lower side (or back). 
     The resonator  10  is a MEMS vibrator produced by MEMS technology. The resonator  10  and the upper cover  30  are bonded with the bonding portion  60 , described below, provided therebetween. The resonator  10  and the lower cover  20  each include a silicon (Si) substrate (hereinafter, referred to as a “Si substrate”), and these Si substrates are bonded together. The MEMS substrate  50  (the resonator  10  and the lower cover  20 ) may include an SOI substrate. 
     The upper cover  30  extends in a flat plate shape along the XY plane and includes, for example, a recessed portion  31  having a flat rectangular parallelepiped shape on the back side thereof. The recessed portion  31  is surrounded by a side wall  33  and included as part of a vibration space in which the resonator  10  vibrates. A gettering layer  34  is disposed on a surface of the recessed portion  31  of the upper cover  30  adjacent to the resonator  10 . The upper cover  30  need not include the recessed portion  31  and may have a flat plate structure. 
     Two terminals T 4  are disposed on a surface of the upper cover  30 . Through vias V 3  filled with a conductive material are disposed below the respective terminals T 4 . The terminals T 4  are electrically connected to respective voltage application portions  141  on a holding portion  140  described below. 
     The lower cover  20  includes a rectangular flat base plate  22  disposed along the XY plane and a side wall  23  extending from the outer edge portion of the base plate  22  in the Z-axis direction, i.e., in the stacking direction of the lower cover  20  and the resonator  10 . The lower cover  20  includes a recessed portion  21  on a surface thereof facing the resonator  10 , the recessed portion  21  being defined by a surface of the base plate  22  and the inner surface of the side wall  23 . The recessed portion  21  is included as part of the vibration space of the resonator  10 . The lower cover  20  need not include the recessed portion  21  and may have a flat plate structure. A gettering layer may also be disposed on a surface of the recessed portion  21  of the lower cover  20  adjacent to the resonator  10 . 
     The schematic structure of the resonator  10  according to a first embodiment of the present invention will be described below with reference to  FIG. 3 .  FIG. 3  is a schematic plan view of the structure of the resonator  10  according to an embodiment of the present invention. 
     As illustrated in  FIG. 3 , the resonator  10  is a MEMS vibrator produced by MEMS technology and vibrates in an out-of-plane mode in the XY plane of a rectangular coordinate system in  FIG. 3 . The resonator  10  is not limited to a resonator used in an out-of-plane flexural vibration mode. For example, the resonator of the resonance device  1  may be used in an extensional vibration mode, a thickness longitudinal vibration mode, a Lamb wave vibration mode, an in-plane flexural vibration mode, or a surface acoustic wave vibration mode. These vibrators may be used for, for example, timing devices, RF filters, duplexers, ultrasonic transducers, gyro sensors, and acceleration sensors. Additionally, these vibrators may be used for, for example, piezoelectric mirrors having actuator functions, piezoelectric gyros, piezoelectric microphones having pressure sensor functions, or ultrasonic vibration sensors. Furthermore, these vibrators may be used for electrostatic MEMS devices, electromagnetically driven MEMS devices, or piezoresistive MEMS devices. 
     The resonator  10  includes a vibrating portion  120 , the holding portion  140 , and a holding arm  110 . 
     The vibrating portion  120  is disposed inside the holding portion  140 . A space is formed between the vibrating portion  120  and the holding portion  140  at a predetermined spacing. In the example illustrated in  FIG. 3 , the vibrating portion  120  includes a base portion  130  and four vibrating arms  135 A to  135 D (hereinafter, also collectively referred to as “vibrating arms  135 ”). The number of vibrating arms is not limited to four, and is set to, for example, any number equal to or more than one. In this embodiment, the vibrating arms  135 A to  135 D and the base portion  130  are integrally formed. 
     The base portion  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 when viewed in the plan view of  FIG. 3 . The long side  131   a  is a side of the front-end face (hereinafter, also referred to as a “front end  131 A”) of the base portion  130 . The long side  131   b  is a side of the back-end face (hereinafter, also referred to as a “back end  131 B”) of the base portion  130 . In the base portion  130 , the front end  131 A and the back end  131 B are disposed so as to be opposite each other. 
     The base portion  130  is connected to the vibrating arms  135  at the front end  131 A and is connected to the holding arm  110  described below at the back end  131 B. In the example illustrated in  FIG. 3 , the shape of the base portion  130  is, but not limited to, a substantially rectangular shape when viewed in plan. The base portion  130  may be disposed so as to be substantially symmetric with respect to a virtual plane P specified along the perpendicular bisector of the long side  131   a . For example, the base portion  130  may have a trapezoidal shape in which the long side  131   b  is shorter than the long side  131   a  or may have a semicircular shape having a diameter defined by the long side  131   a . Each of the faces of the base portion  130  is not limited to a flat face, and may be a curved face. The virtual plane P is a plane extending through the center of the vibrating portion  120  in the direction in which the vibrating arms  135  are arranged. 
     In the base portion  130 , the base-portion length, which is the longest distance between the front end  131 A and the back end  131 B, in a direction from the front end  131 A toward the back end  131 B is about 35 μm. The base-portion width, which is the longest distance between the side edges of the base portion  130 , in the width direction perpendicular to the direction of the base-portion length is about 265 μm. 
     The vibrating arms  135  extend in the Y-axis direction and have the same size. Each of the vibrating arms  135  is disposed between the base portion  130  and the holding portion  140  in parallel with the Y-axis direction. One end of each vibrating arm  135  is a fixed end connected to the front end  131 A of the base portion  130 , and the other end is an open end. The vibrating arms  135  are arranged in parallel at predetermined intervals in the X-axis direction. Each of the vibrating arms  135  has, for example, a width of about 50 μm in the X-axis direction and a length of about 465 μm in the Y-axis direction. 
     The holding portion  140  has a rectangular frame shape surrounding the outer side portion of the vibrating portion  120  along the XY plane. For example, the holding portion  140  is integrally formed from a prismatic frame. The holding portion  140  may be at least partially disposed around the vibrating portion  120  and is not limited to the frame shape. 
     The voltage application portions  141  are disposed in a region of the holding portion  140  facing the open ends of the vibrating arms  135  and in a region connected to the holding arm. The voltage application portions  141  are electrically connected to the terminals T 4  of the upper cover  30  and can apply an alternating electric field to the resonator  10 . 
     The holding arm  110  is disposed inside the holding portion  140  and connects the vibrating portion  120  to the holding portion  140 . 
     The stacked structure of the resonance device  1  according to the first embodiment of the present invention will be described below with reference to  FIG. 4 , which is a schematic cross-sectional view of the resonance device  1  taken along line IV-IV of  FIG. 1 . 
     As illustrated in  FIG. 4 , in the resonance device  1 , the holding portion  140  of the resonator  10  is bonded to the side wall  23  of the lower cover  20 , and the holding portion  140  of the resonator  10  is bonded to the side wall  33  of the upper cover  30 . The resonator  10  is held between the lower cover  20  and the upper cover  30 . The lower cover  20 , the upper cover  30 , and the holding portion  140  of the resonator  10  define the vibration space in which the vibrating arms  135  vibrate. 
     The upper cover  30  is formed of a silicon (Si) wafer (hereinafter, referred to as a “Si wafer”) L 3  having a predetermined thickness. The peripheral portion (side wall  33 ) of the upper cover  30  is bonded to the holding portion  140  of the resonator  10  by the bonding portion  60  described below. The front surface of the upper cover  30  facing the resonator  10 , the back surface thereof, and side faces of the through vias V 3  are preferably covered with a silicon oxide film L 31 . The silicon oxide film L 31  is formed on surfaces of the Si wafer L 3  by, for example, oxidation of the Si wafer L 3  or chemical vapor deposition (CVD). 
     The gettering layer  34  is disposed on a surface of the recessed portion  31  of the upper cover  30  facing the resonator  10 . The gettering layer  34  is composed of, for example, titanium (Ti) and adsorbs a gas generated in the vibration space. In the upper cover  30  according to this embodiment, the gettering layer  34  is disposed on almost the entire surface of the recessed portion  31  facing the resonator  10 ; thus, a decrease in the degree of vacuum in the vibration space can be suppressed. 
     The through vias V 3  in the upper cover  30  are formed by filling through holes in the upper cover  30  with a metal such as polycrystalline silicon (poly-Si). The through vias V 3  function as lines that electrically connect the terminals T 4  to the voltage application portions  141 . Connecting lines W 1  are disposed between the through vias V 3  and the voltage application portions  141 . The connecting lines W 1  are formed by, for example, eutectic bonding between an aluminum (Al) film and a germanium (Ge) film. 
     The base plate  22  and the side wall  23  of the lower cover  20  are integrally formed of a Si wafer L 1 . The upper surface of the side wall  23  of the lower cover  20  is bonded to the holding portion  140  of the resonator  10 . The thickness of the lower cover  20  defined in the Z-axis direction is, for example, 150 μm. The depth of the recessed portion  21  is, for example, 50 μm. The Si wafer L 1  is composed of non-degenerate silicon and has a resistivity of, for example, 16 mΩ·cm or more. 
     The holding portion  140 , the base portion  130 , the vibrating arms  135 , and the holding arm  110  of the resonator  10  are integrally formed by the same process. In the resonator  10 , a piezoelectric thin film F 3  is disposed on a Si substrate F 2 , which is an example of a substrate, so as to cover the Si substrate F 2 . A metal layer E 1  is stacked on the piezoelectric thin film F 3 . A piezoelectric thin film F 3  is stacked on the metal layer E 1  so as to cover the metal layer E 1 . A metal layer E 2  is stacked on the piezoelectric thin film F 3 . A protective film  235  is stacked on the metal layer E 2  so as to cover the metal layer E 2 . 
     The Si substrate F 2  is composed of, for example, a degenerate n-type 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 degenerate Si used for the Si substrate F 2  has a resistance of, for example, less than 16 mΩ·cm, preferably 1.2 mΩ·cm or less. A silicon oxide (for example, SiO 2 ) layer F 21  serving as an example of a temperature characteristic correction layer is disposed on the lower surface of the Si substrate F 2 . This enables an improvement in temperature characteristics. The silicon oxide layer F 21  may be disposed on the upper surface of the Si substrate F 2  or may be disposed on each of the upper surface and the lower surface of the Si substrate F 2 . 
     Each of the metal layers E 1  and E 2  is formed by using, for example, molybdenum (Mo) or aluminum (Al) having a thickness of, for example, about 0.1 μm to about 0.2 μm. 
     Each of the metal layers E 1  and E 2  is formed into a desired shape by, for example, etching. For example, the metal layer E 1  is formed so as to function as a lower electrode on the vibrating portion  120 . Additionally, the metal layer E 1  is formed so as to, on the holding arm  110  and the holding portion  140 , function as a line that connects the lower electrode to an alternating-current power supply disposed outside the resonator  10 . 
     The metal layer E 2  is formed so as to function as an upper electrode on the vibrating portion  120 . 
     Additionally, the metal layer E 2  is formed so as to, on the holding arm  110  and the holding portion  140 , function as a line that connects the upper electrode to a circuit disposed outside the resonator  10 . 
     The protective film  235  is formed of a nitride film composed of, for example, aluminum nitride (AlN) or silicon nitride (SiN) or an oxide film composed of, for example, tantalum pentoxide (Ta 2 O 5 ) or silicon dioxide (SiO 2 ). A portion of the protective film  235  on the holding portion  140  is removed so as to expose the metal layer E 2 . A portion where the protective film  235  is removed is filled with a metal, such as aluminum (Al), to form the voltage application portion  141 . 
     The piezoelectric thin film F 3  is a piezoelectric thin film that converts an applied voltage into vibration and can be mainly composed of, for example, aluminum nitride (AlN) or an oxide. Specifically, the piezoelectric thin film F 3  can be composed of scandium aluminum nitride (ScAlN). Scandium aluminum nitride is a compound in which aluminum in aluminum nitride is partially replaced with scandium. The piezoelectric thin film F 3  has a thickness of, for example, 1 μm and may have a thickness of about 0.2 μm to about 2 μm. 
     The piezoelectric thin film F 3  expands and contracts in the in-plane direction of the XY plane, i.e., in the Y-axis direction, in accordance with an electric field applied from the metal layers E 1  and E 2  to the piezoelectric thin film F 3 . Due to the expansion and contraction of the piezoelectric thin film F 3 , the vibrating arms  135  displace their free ends toward the inner surfaces of the lower cover  20  and the upper cover  30  and vibrate in an out-of-plane flexural vibration mode. 
     In this embodiment, the phase of an electric field applied to the outer vibrating arms  135 A and  135 D and the phase of an electric field applied to the inner vibrating arms  135 B and  135 C are set to be opposite to each other. Accordingly, the outer vibrating arms  135 A and  135 D and the inner vibrating arms  135 B and  135 C are displaced in opposite directions. For example, when the outer vibrating arms  135 A and  135 D displace their free ends toward the inner surface of the upper cover  30 , the inner vibrating arms  135 B and  135 C displace their free ends toward the inner surface of the lower cover  20 . 
     The bonding portion  60  is disposed in a rectangular ring shape along the XY plane between the MEMS substrate  50  (the resonator  10  and the lower cover  20 ) and the upper cover  30  around the vibrating portion  120  of the resonator  10 , for example, on the holding portion  140 . The bonding portion  60  bonds the MEMS substrate  50  and the upper cover  30  so as to seal the vibration space of the resonator  10 . This hermetically seals the vibration space to maintain a vacuum state. 
     In this embodiment, the bonding portion  60  includes a first layer  70  disposed on the MEMS substrate  50  and a second layer  80  disposed on the upper cover  30 . The MEMS substrate  50  and the upper cover  30  are bonded by eutectic bonding between the first layer  70  and the second layer  80 . 
     The stacked structure of the bonding portion  60  according to the first embodiment of the present invention will be described below with reference to  FIG. 5 .  FIG. 5  is a schematic enlarged fragmentary cross-sectional view of the structure of the bonding portion  60  illustrated in  FIG. 4 . 
     As illustrated in  FIG. 5 , the bonding portion  60  includes a eutectic layer  65 , a first titanium (Ti) layer  63 , a first aluminum oxide film  62 , and a first conductive layer  61  consecutively arranged from the MEMS substrate  50  (the resonator  10  and the lower cover  20 ) to the upper cover  30 . 
     The eutectic layer  65  includes a germanium (Ge) layer  65   a  and a metal layer  65   b  mainly containing aluminum. In an example illustrated in  FIG. 5 , each of the germanium (Ge) layer  65   a  and the metal layer  65   b  is illustrated as an independent layer. In fact, however, the interface therebetween is formed by eutectic bonding. In other words, the eutectic layer  65  is composed of a eutectic alloy of germanium (Ge) and a metal mainly containing aluminum (Al). As used herein, the term “mainly containing” means the most abundant component of that article, and not necessarily more than 50% of the article. For example, “a metal mainly containing aluminum” could mean that more than 50% of that metal is aluminum, or aluminum is less than 50% or that metal but is the most abundant component of that metal. 
     The material of the metal layer  65   b  is preferably aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). Aluminum or an aluminum alloy is a metal that is often used for, for example, lines in resonance devices and so forth. In the case where aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy) is used for the metal layer  65   b , the germanium (Ge) layer  65   a  and the metal layer  65   b  can be easily bonded together by eutectic bonding, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     The metal layer  65   b  of the bonding portion  60  is included in the first layer  70 . The first conductive layer  61 , the first aluminum oxide film  62 , the first titanium (Ti) layer  63 , and the germanium (Ge) layer  65   a  are included in the second layer  80 . 
     The first conductive layer  61  is disposed on a surface of the silicon oxide film L 31  on the back surface side of the upper cover  30 . The material of the first conductive layer  61  is preferably aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). When the first conductive layer  61  is composed of an aluminum-copper alloy (AlCu alloy), copper (Cu) is preferably contained in an amount of about 0.5% by weight. In this case, the first conductive layer  61  is electrically conductive, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     The first aluminum oxide film  62  is disposed on the first conductive layer  61  (below the first conductive layer  61  in  FIG. 5 ). The first aluminum oxide film  62  is composed of aluminum oxide. The first aluminum oxide film  62  is formed on the first conductive layer  61  by exposing a surface of the first conductive layer  61  to an oxygen plasma or air. When the surface of the first conductive layer  61  is exposed to air, the first aluminum oxide film  62  having a thickness of about 5 nm is formed. The first aluminum oxide film  62  preferably has a thickness of 3 nm to 10 nm. In this case, it is possible to suppress an increase in resistance to conduction due to the first aluminum oxide film  62 . 
     The first titanium (Ti) layer  63  is disposed on the first aluminum oxide film  62  (below the first aluminum oxide film  62  in  FIG. 5 ). The first titanium (Ti) layer  63  is composed of titanium (Ti). The first titanium (Ti) layer  63  functions as a close-contact layer for providing close contact between the eutectic layer  65  and the first titanium (Ti) layer  63 . Titanium (Ti) has excellent wettability with a eutectic alloy of germanium (Ge) and a metal mainly containing aluminum (Al) obtained by eutectic bonding. Since the bonding portion  60  includes the eutectic layer  65  and the first titanium (Ti) layer  63  disposed consecutively, the eutectic layer  65  can spread over the first titanium (Ti) layer  63  to suppress the possible formation of voids between the eutectic layer  65  and the first titanium (Ti) layer  63 . Accordingly, the airtightness of the vibration space of the resonator  10  can be enhanced. 
     Titanium (Ti) has the feature that the material cost is low, compared with, for example, tantalum (Ta) and tantalum nitride (TaN). Since the bonding portion  60  includes the first titanium (Ti) layer  63 , it is possible to reduce the production cost of the bonding portion  60 . 
     The first aluminum oxide film  62  and the first titanium (Ti) layer  63  function as diffusion-preventing layers for preventing thermal diffusion. Here, thermal diffusion is less likely to occur between the aluminum oxide film and the titanium (Ti) than between, for example, aluminum (Al) and titanium (Ti). 
     To verify the function as the diffusion-preventing layers, the upper cover  30  on which the second layer  80  including the first conductive layer  61 , the first aluminum oxide film  62 , the first titanium (Ti) layer  63 , and the germanium (Ge) layer  65   a  was disposed was subjected to heat treatment for degassing at 435° C. The results indicated that in the second layer  80  after the heat treatment, no migration of aluminum or an aluminum alloy of the first conductive layer  61  due to thermal diffusion was observed. 
     For comparison with the second layer  80  according to this embodiment, a second layer that does not contain the first aluminum oxide film  62 , specifically, an upper cover on which a virtual second layer including a conductive layer composed of aluminum (Al), a titanium (Ti) layer, and a germanium (Ge) layer were consecutively arranged in this order, was subjected to heat treatment for degassing at 360° C. In the virtual second layer after the heat treatment, aluminum (Al) in the conductive layer diffused to the germanium (Ge) layer through the titanium (Ti) layer. This thermal diffusion causes a deviation of a eutectic composition and the failure of a eutectic reaction during eutectic bonding. 
     As described above, since the bonding portion  60  includes the first titanium (Ti) layer  63  and the first aluminum oxide film  62  consecutively arranged, the thermal diffusion between the aluminum oxide film and titanium (Ti) is less likely to occur; thus, it is possible to raise the temperature of the heat treatment for degassing. 
     Accordingly, a gas contained in the resonance device  1  can be released (evaporated) by high-temperature heat treatment to suppress the occurrence of outgassing, thereby providing a high vacuum in the vibration space of the resonator  10 . 
     A production process of the resonance device  1  according to an embodiment will be described below with reference to  FIGS. 6A to 6C .  FIGS. 6A to 6C  are schematic views illustrating the production process of the resonance device  1  according to an embodiment. In the production process of the resonance device  1 , a procedure for bonding the MEMS substrate  50  to the upper cover  30  will be selected and described. In  FIGS. 6A to 6C , for the sake of convenience, one resonance device  1  of the multiple resonance devices  1  formed on or in a wafer is illustrated and described. As with normal MEMS processes, the resonance device  1  is obtained by forming multiple resonance devices on or in one wafer and then separating the wafer. 
     In a step illustrated in  FIG. 6A , the MEMS substrate  50  including the resonator  10  and the upper cover  30  described above are provided. 
     In a step illustrated in  FIG. 6B , the first layer  70  including the metal layer  65   b  mainly containing aluminum (Al) is formed around the vibrating portion  120  of the resonator  10  of the provided MEMS substrate  50 . 
     Specifically, for example, aluminum (Al) is stacked on the piezoelectric thin film F 3  of the resonator  10 . Then the stacked aluminum (Al) is formed into a desired shape by, for example, etching, thereby forming the metal layer  65   b  outside the vibrating portion  120  of the MEMS substrate  50 . The metal layer  65   b  is formed around the resonant space of the resonator  10  when the MEMS substrate  50  is viewed in a plan view thereof. 
     After the formation of the first layer  70 , the MEMS substrate  50  is subjected to heat treatment for degassing at a high temperature, for example, about 435° C. Since the first layer  70  includes the metal layer  65   b  alone, the effect of thermal diffusion is small. 
     The second layer  80  including the first conductive layer  61 , the first aluminum oxide film  62 , the first titanium (Ti) layer  63 , and the germanium (Ge) layer  65   a  consecutively arranged from the upper cover  30  is formed on the provided upper cover  30 . 
     Specifically, for example, aluminum (Al) is stacked on a surface of the silicon oxide film L 31  on the back surface of the upper cover  30 . Then the stacked aluminum (Al) is formed into a desired shape by, for example, etching, thereby forming the first conductive layer  61  at a predetermined portion of the upper cover  30 . The predetermined portion where the first conductive layer  61  is formed is, for example, a portion of the back surface of the upper cover  30  that faces or substantially faces the first layer  70  formed on the surface of the MEMS substrate  50  when the surface of the MEMS substrate  50  faces the back surface of the upper cover  30 . Then the first aluminum oxide film  62  is formed on the first conductive layer  61  (below the first conductive layer  61  in  FIG. 6B ). Titanium (Ti) is stacked on the first aluminum oxide film  62  (below the first aluminum oxide film  62  in  FIG. 6B ) to provide the first titanium (Ti) layer  63 . Furthermore, germanium (Ge) is stacked on the first titanium (Ti) layer  63  (below the first titanium (Ti) layer  63  in  FIG. 6B ) to provide the germanium (Ge) layer  65   a.    
     After the formation of the second layer  80 , the upper cover  30  is subjected to heat treatment for degassing at a high temperature, for example, about 435° C. This enables a sufficient release (evaporation) of a gas contained in the upper cover  30  and the second layer  80 , thereby reducing the occurrence of outgassing. 
     In a step illustrated in  FIG. 6C , the metal layer  65   b  of the first layer  70  is bonded to the germanium (Ge) layer  65   a  of the second layer  80  by eutectic bonding. 
     Specifically, the MEMS substrate  50  and the upper cover  30  are aligned in such a manner that the first layer  70  is matched to the second layer  80 . After the alignment, the MEMS substrate  50  and the upper cover  30  are sandwiched by, for example, a heater and subjected to heat treatment for eutectic bonding. At this time, the upper cover  30  is moved toward the MEMS substrate  50 . Thus, as illustrated in  FIG. 6C , the germanium (Ge) layer  65   a  of the second layer  80  comes into contact with the metal layer  65   b  of the first layer  70 . 
     The temperature in the heat treatment for eutectic bonding is preferably equal to or higher than a eutectic temperature and lower than the melting point of aluminum (Al) alone, i.e., about 424° C. or higher and about lower than 620° C. The heating time is preferably about 10 minutes to 20 minutes. In this embodiment, the heat treatment is performed at a temperature of 430° C. to 500° C. for about 15 minutes. 
     The resonance device  1  is pressed from the upper cover  30  to the MEMS substrate  50  at a pressure of, for example, about 15 MPa during the heating. The pressure applied is preferably about 5 MPa to about 25 MPa. 
     After the heat treatment for eutectic bonding, cooling treatment is performed, for example, by natural cooling. The cooling treatment is not limited to natural cooling. It is sufficient that the eutectic layer  65  can be formed at the bonding portion  60 . Various cooling temperatures and various cooling rates can be selected. 
     Performing the step illustrated in  FIG. 6C  results in the formation of the bonding portion  60  including the eutectic layer  65  obtained by eutectic bonding between the germanium (Ge) layer  65   a  and the metal layer  65   b  mainly containing aluminum (Al) as illustrated in  FIG. 5 . 
     When the first layer  70  and the second layer  80  are formed, an aluminum (Al) film and a germanium (Ge) film may be formed and bonded together by eutectic bonding to form the connecting lines W 1  configured to connect the through vias V 3  to the respective voltage application portions  141  as illustrated in  FIG. 4 . 
     In this embodiment, the bonding portion  60  including the eutectic layer  65 , the first titanium (Ti) layer  63 , the first aluminum oxide film  62 , and the first conductive layer  61  has been exemplified from  FIG. 5  to  FIG. 6C , but is not limited thereto. 
     (First Modification) 
       FIG. 7  is an enlarged fragmentary view of a first modification of the bonding portion  60  illustrated in  FIG. 5 . In a second modification, the same configuration as that of the bonding portion  60  illustrated in  FIG. 5  are designated using the same reference numerals, and the description is omitted as appropriate. No mention will be made of the same operation and effect of the same configuration. 
     As illustrated in  FIG. 7 , the bonding portion  60  further includes a second conductive layer  66  and a second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 . The second conductive layer  66  and the second titanium (Ti) layer  67  are included in the first layer  70 . 
     The second conductive layer  66  is disposed on the piezoelectric thin film F 3  of the resonator  10 . The material of the second conductive layer  66  is preferably aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). When the second conductive layer  66  is composed of an aluminum-copper alloy (AlCu alloy), copper (Cu) is preferably contained in an amount of about 0.5% by weight. In this case, the second conductive layer  66  is electrically conductive, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     The second titanium (Ti) layer  67  is disposed on the second conductive layer  66 . The second titanium (Ti) layer  67  is composed of titanium (Ti). The second titanium (Ti) layer  67  functions as a close-contact layer for providing close contact between the eutectic layer  65  and the second titanium (Ti) layer  67 . Since the bonding portion  60  includes the second titanium (Ti) layer  67  and the eutectic layer  65  arranged consecutively, the eutectic layer  65  can spread over the second titanium (Ti) layer  67  to suppress the possible formation of voids between the eutectic layer  65  and the second titanium (Ti) layer  67 . Accordingly, the bonding portion  60  can further enhance the airtightness of the vibration space of the resonator  10 . 
     As described above, since the bonding portion  60  includes the second conductive layer  66  and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 , wiring can be routed from the second conductive layer  66  at the MEMS substrate  50 . 
     In the production process according to the first modification, the second conductive layer  66  and the second titanium (Ti) layer  67  are continuously formed from the MEMS substrate  50  side to the metal layer  65   b  in the step of forming the first layer  70 . 
     Specifically, for example, aluminum (Al) is stacked on the piezoelectric thin film F 3  of the resonator  10 . Then the stacked aluminum (Al) is formed into a desired shape by, for example, etching, thereby forming the second conductive layer  66 . The second conductive layer  66  is formed around the resonant space of the resonator  10  when the MEMS substrate  50  is viewed in the plan view thereof. Titanium (Ti) is stacked on the second conductive layer  66  to form the second titanium (Ti) layer  67 . Furthermore, for example, aluminum (Al) is stacked on the second titanium (Ti) layer  67  to form the metal layer  65   b . Similar to the step illustrated in  FIG. 6C , the metal layer  65   b  and the germanium (Ge) layer  65   a  are bonded together by eutectic bonding; thus, the bonding portion  60  includes the second conductive layer  66 , the second titanium (Ti) layer  67 , and the eutectic layer  65  arranged consecutively. 
     (Second Modification) 
       FIG. 8  is an enlarged fragmentary view of a second modification of the bonding portion  60  illustrated in  FIG. 5 . In the second modification, the same configuration as that of the bonding portion  60  illustrated in  FIG. 5  are designated using the same reference numerals, and the description is omitted as appropriate. No mention will be made of the same operation and effect of the same configuration. 
     As illustrated in  FIG. 8 , the bonding portion  60  further includes the second conductive layer  66 , a second aluminum oxide film  68 , and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 . The second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  are included in the first layer  70 . 
     The second conductive layer  66  is disposed on the piezoelectric thin film F 3  of the resonator  10 . As with the first modification, the material of the second conductive layer  66  is preferably aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). 
     The second aluminum oxide film  68  is disposed on the second conductive layer  66 . The second aluminum oxide film  68  is composed of aluminum oxide. The second aluminum oxide film  68  is formed on the second conductive layer  66  by exposing a surface of the second conductive layer  66  to an oxygen plasma or air. When the surface of the second conductive layer  66  is exposed to air, the second aluminum oxide film  68  has a thickness of about 5 nm. The second aluminum oxide film  68  preferably has a thickness of 3 nm to 10 nm. In this case, it is possible to suppress an increase in resistance to conduction due to the second aluminum oxide film  68 . 
     The second titanium (Ti) layer  67  is disposed on the second aluminum oxide film  68 . The second titanium (Ti) layer  67  is composed of titanium (Ti). As with the first modification, the second titanium (Ti) layer  67  functions as a close-contact layer for providing close contact between the eutectic layer  65  and the second titanium (Ti) layer  67 . 
     Furthermore, the second aluminum oxide film  68  and the second titanium (Ti) layer  67  function as diffusion-preventing layers for preventing thermal diffusion. 
     To verify the function as the diffusion-preventing layers, the MEMS substrate  50  on which the first layer  70  including the second conductive layer  66 , the second aluminum oxide film  68 , the second titanium (Ti) layer  67 , and the metal layer  65   b  was disposed was subjected to heat treatment for degassing at 435° C. The results indicated that in the first layer  70  after the heat treatment, no migration of aluminum or an aluminum alloy of the second conductive layer  66  due to thermal diffusion was observed. 
     For comparison with the first layer  70  according to the second modification, a first layer that does not contain the second aluminum oxide film  68 , specifically, a MEMS substrate on which a virtual first layer including a conductive layer composed of aluminum (Al), a titanium (Ti) layer, and a metal layer composed of aluminum (Al) were consecutively arranged in this order, was subjected to heat treatment for degassing at 360° C. In the virtual first layer after the heat treatment, aluminum (Al) in the conductive layer diffused to the metal layer through the titanium (Ti) layer. This thermal diffusion causes a deviation of a eutectic composition and the failure of a eutectic reaction during eutectic bonding. 
     As described above, since the bonding portion  60  includes the second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 , wiring can be routed from the second conductive layer  66  at the MEMS substrate  50 . Furthermore, the thermal diffusion between the aluminum oxide film and titanium (Ti) is less likely to occur; thus, the MEMS substrate  50  can be subjected to heat treatment for degassing at a high temperature. 
     In the production process according to the second modification, the second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  are consecutively arranged from the MEMS substrate  50  to the metal layer  65   b  in the step of forming the first layer  70 . 
     Specifically, for example, aluminum (Al) is stacked on the piezoelectric thin film F 3  of the resonator  10 . Then the stacked aluminum (Al) is formed into a desired shape by, for example, etching, thereby forming the second conductive layer  66 . The second conductive layer  66  is formed around the resonant space of the resonator  10  when the MEMS substrate  50  is viewed in plan. The second aluminum oxide film  68  is formed on the second conductive layer  66 . Titanium (Ti) is stacked on the second aluminum oxide film  68  to form the second titanium (Ti) layer  67 . Furthermore, for example, aluminum (Al) is stacked on the second titanium (Ti) layer  67  to form the metal layer  65   b . Similar to the step illustrated in  FIG. 6C , the metal layer  65   b , and the germanium (Ge) layer  65   a  are bonded together by eutectic bonding; thus, the bonding portion  60  includes the second conductive layer  66 , the second aluminum oxide film  68 , the second titanium (Ti) layer  67 , and the eutectic layer  65  arranged consecutively. 
     The exemplary embodiments of the present invention have been described above. The resonance device  1  according to an embodiment of the present invention includes the MEMS substrate  50  including the resonator  10 , the upper cover  30 , and the bonding portion  60  that bonds the MEMS substrate  50  to the upper cover  30  so as to seal the vibration space of the resonator  10 . The bonding portion  60  includes the eutectic layer  65  composed of a eutectic alloy of germanium (Ge) and a metal mainly containing aluminum (Al), the first titanium (Ti) layer  63 , the first aluminum oxide film  62 , and the first conductive layer  61  consecutively arranged from the MEMS substrate  50  to the upper cover  30 . Titanium (Ti) has excellent wettability with the eutectic alloy of germanium (Ge) and the metal mainly containing aluminum (Al) obtained by eutectic bonding. Since the bonding portion  60  includes the eutectic layer  65  and the first titanium (Ti) layer  63  arranged consecutively, the eutectic layer  65  can spread over the first titanium (Ti) layer  63  to suppress the possible formation of voids between the eutectic layer  65  and the first titanium (Ti) layer  63 . Accordingly, the airtightness of the vibration space of the resonator  10  can be enhanced. Additionally, titanium (Ti) has the feature that the material cost is low, compared with, for example, tantalum (Ta) and tantalum nitride (TaN). Since the bonding portion  60  includes the first titanium (Ti) layer  63 , it is possible to reduce the production cost of the bonding portion  60 . Furthermore, since the bonding portion  60  includes the first titanium (Ti) layer  63  and the first aluminum oxide film  62  arranged consecutively, the thermal diffusion between the aluminum oxide film and titanium (Ti) is less likely to occur; thus, it is possible to raise the temperature of the heat treatment for degassing. 
     Accordingly, a gas contained in the resonance device  1  can be released (evaporated) by high-temperature heat treatment to suppress the occurrence of outgassing, thereby providing a high vacuum in the vibration space of the resonator  10 . 
     In the resonance device  1  described above, the first aluminum oxide film  62  may have a thickness of 3 nm to 10 nm. It is thus possible to suppress an increase in resistance to conduction due to the first aluminum oxide film  62 . 
     In the resonance device  1  described above, the material of the first conductive layer  61  may be aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). Aluminum or an aluminum alloy is a metal that is often used for, for example, lines in resonance devices and so forth. In the case where aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy) is used for the first conductive layer  61 , the first conductive layer  61  is electrically conductive, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     In the resonance device  1  described above, the metal mainly containing aluminum may be aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). In this case, the germanium (Ge) layer  65   a  and the metal layer  65   b  can be easily bonded together by eutectic bonding, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     In the resonance device  1  described above, the bonding portion  60  may further include the second conductive layer  66  and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 . In the case where the bonding portion  60  includes the second titanium (Ti) layer  67  and the eutectic layer  65  arranged consecutively, the eutectic layer  65  can spread over the second titanium (Ti) layer  67  to suppress the possible formation of voids between the eutectic layer  65  and the second titanium (Ti) layer  67 . Accordingly, the bonding portion  60  can further enhance the airtightness of the vibration space of the resonator  10 . Additionally, in the case where the bonding portion  60  includes the second conductive layer  66  and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 , wiring can be routed from the second conductive layer  66  at the upper cover  30 . 
     In the resonance device  1  described above, the bonding portion  60  may further include the second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 . In the case where the bonding portion  60  includes the second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  consecutively arranged from the MEMS substrate  50  to the eutectic layer  65 , wiring can be routed from the second conductive layer  66  at the MEMS substrate  50 . Furthermore, the thermal diffusion between the aluminum oxide film and titanium (Ti) is less likely to occur; thus, the MEMS substrate  50  can be subjected to heat treatment for degassing at a high temperature. 
     In the resonance device  1  described above, the second aluminum oxide film  68  may have a thickness of 3 nm to 10 nm. In this case, it is possible to suppress an increase in resistance to conduction due to the second aluminum oxide film  68 . 
     In the resonance device  1  described above, the material of the second conductive layer  66  may be aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). In this case, the second conductive layer  66  is electrically conductive, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     A method for producing a resonance device according to an embodiment of the present invention includes forming the first layer  70  including the metal layer  65   b  mainly containing aluminum (Al) around the vibrating portion  120  of the resonator  10  of the MEMS substrate  50 , forming the second layer  80  on a portion of the upper cover  30  that faces the first layer  70  when the MEMS substrate  50  faces the upper cover  30 , the second layer  80  including the first conductive layer  61 , the first aluminum oxide film  62 , the first titanium (Ti) layer  63 , and the germanium (Ge) layer  65   a  consecutively formed in this order from the upper cover  30 , and bonding the metal layer  65   b  of the first layer  70  to the germanium (Ge) layer  65   a  of the second layer  80  by eutectic bonding so as to seal the vibration space of the resonator  10 . Thereby, the bonding portion  60  including the eutectic layer  65  obtained by eutectic bonding between the metal layer  65   b  mainly containing aluminum (Al) and the germanium (Ge) layer  65   a  is formed. Since the bonding portion  60  includes the first titanium (Ti) layer  63  and eutectic layer  65  formed consecutively, the eutectic layer  65  can spread over the first titanium (Ti) layer  63  to suppress the possible formation of voids between the eutectic layer  65  and the first titanium (Ti) layer  63 . Accordingly, the airtightness of the vibration space of the resonator  10  can be enhanced. Since the bonding portion  60  includes the first titanium (Ti) layer  63 , it is possible to reduce the production cost of the bonding portion  60 . Furthermore, since the bonding portion  60  includes the first aluminum oxide film  62  and the first titanium (Ti) layer  63  consecutively formed, the thermal diffusion between the aluminum oxide film and titanium (Ti) is less likely to occur; thus, it is possible to raise the temperature of the heat treatment for degassing. Accordingly, a gas contained in the resonance device  1  can be released (evaporated) by high-temperature heat treatment to suppress the occurrence of outgassing, thereby providing a high vacuum in the vibration space of the resonator  10 . 
     In the foregoing method for producing a resonance device, the first aluminum oxide film  62  may have a thickness of 3 nm to 10 nm. In this case, it is possible to suppress an increase in resistance to conduction due to the first aluminum oxide film  62 . 
     In the foregoing method for producing a resonance device, the material of the first conductive layer  61  may be aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). In this case, the first conductive layer  61  is electrically conductive, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     In the foregoing method for producing a resonance device, the material of the metal layer  65   b  may be aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). In this case, the germanium (Ge) layer  65   a  can be easily bonded to the metal layer  65   b  by eutectic bonding, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     In the foregoing method for producing a resonance device, the forming of the first layer  70  may further include consecutively forming the second conductive layer  66  and the second titanium (Ti) layer  67  from the MEMS substrate  50  to the metal layer  65   b . Since the bonding portion  60  includes the second titanium (Ti) layer  67  and the eutectic layer  65  formed consecutively, the eutectic layer  65  can spread over the second titanium (Ti) layer  67  to suppress the possible formation of voids between the eutectic layer  65  and the second titanium (Ti) layer  67 . Accordingly, the bonding portion  60  can further enhance the airtightness of the vibration space of the resonator  10 . Furthermore, since the bonding portion  60  includes the second conductive layer  66  and the second titanium (Ti) layer  67  consecutively formed from the MEMS substrate  50  to the eutectic layer  65 , wiring can be routed from the second conductive layer  66  at the MEMS substrate  50 . 
     In the foregoing method for producing a resonance device, the forming of the first layer  70  may further include consecutively forming the second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  from the MEMS substrate  50  to the metal layer  65   b . Since the bonding portion  60  includes the second conductive layer  66 , the second aluminum oxide film  68 , and the second titanium (Ti) layer  67  consecutively formed from the MEMS substrate  50  to the eutectic layer  65 , wiring can be routed from the second conductive layer  66  at the MEMS substrate  50 . Furthermore, the thermal diffusion between the aluminum oxide film and titanium (Ti) is less likely to occur; thus, the MEMS substrate  50  can be subjected to heat treatment for degassing at a high temperature. 
     In the foregoing method for producing a resonance device, the second aluminum oxide film  68  may have a thickness of 3 nm to 10 nm. In this case, it is possible to suppress an increase in resistance to conduction due to the second aluminum oxide film  68 . 
     In the foregoing method for producing a resonance device, the material of the second conductive layer  66  may be aluminum (Al), an aluminum-copper alloy (AlCu alloy), or an aluminum-silicon-copper alloy (AlSiCu alloy). In this case, the second conductive layer  66  is electrically conductive, the production process can be simplified, and the bonding portion  60  that seals the vibration space of the resonator  10  can be easily formed. 
     It is noted that the embodiments have been described above for ease of understanding of the present invention and are not intended to limit the scope of the present invention. Changes and improvements may be made to the present invention within the scope of the invention, and the present invention includes equivalents thereof. In other words, design changes may be suitably made to the embodiments by those skilled in the art, and such embodiments are also within the scope of the present invention as long as they have the features of the present invention. For example, the elements included in the embodiments and the arrangements, materials, conditions, shapes, sizes, and the like of the elements are not limited to those described above as examples, and they may be suitably changed. The embodiments are exemplary embodiments, and configurations described in the different embodiments can be partially replaced or combined and are also included in the scope of the present invention as long as the configurations include the features of the present invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  resonance device 
               10  resonator 
               20  lower cover 
               21  recessed portion 
               22  base plate 
               23  side wall 
               30  upper cover 
               31  recessed portion 
               33  side wall 
               34  gettering layer 
               50  MEMS substrate 
               60  bonding portion 
               61  first conductive layer 
               62  first aluminum oxide film 
               63  first titanium (Ti) layer 
               65  eutectic layer 
               65   a  germanium (Ge) layer 
               65   b  metal layer 
               66  second conductive layer 
               67  second titanium (Ti) layer 
               68  second aluminum oxide film 
               70  first layer 
               80  second layer 
               110  holding arm 
               120  vibrating portion 
               130  base portion 
               135 ,  135 A,  135 B,  135 C,  135 D vibrating arm 
               140  holding portion 
               141  voltage application portions 
               235  protective film 
             E 1 , E 2  metal layer 
             F 2  Si substrate 
             F 3  piezoelectric thin film 
             F 21  silicon oxide layer 
             L 1  Si wafer 
             L 3  Si wafer 
             L 31  silicon oxide film 
             T 4  terminal 
             V 3  through via 
             W 1  connecting line