Abstract:
A crystal resonator includes: lower glass plates on which first electrodes are formed so as to extend from side surfaces to a bottom surface of the lower glass plates; a crystal plate which is provided over the lower glass plates and on which second electrodes to be coupled to the first electrodes are formed on a surface in contact with the lower glass plates; and an upper glass plate which is provided over the crystal plate; wherein the side surfaces of the lower glass plates on which the first electrodes are formed are provided with a protrusion that extends in parallel with a top surface and the bottom surface of the lower glass plates and that extends from one end to the other end of each of the side surfaces, and wherein the first electrodes are formed on the side surfaces that include surfaces of the protrusion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2012/001467 filed on Mar. 2, 2012 and designated the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to crystal resonators for use in electronic equipment and the like and production methods therefor. 
     BACKGROUND 
     As a structure of a conventional crystal resonator, a crystal resonator  100  has been known in which a pair of lid bodies  92 ,  93  are bonded through bonding films  94 ,  95 ,  96  made of metal material, for instance, onto top and bottom surfaces of a crystal resonator plate  91  having a crystal resonator piece and a frame integrally formed therein as illustrated in  FIG. 1A , for instance. External terminals  98  are formed on corner parts of the lower lid body  93  under the crystal resonator plate  91  so as to extend from side surfaces to a bottom surface of the lower lid body  93 . The external terminals  98  provide electrical connection between electrode pads on a circuit board on which the crystal resonator  100  is mounted and the bonding films  95 ,  96  on the bottom surface of the crystal resonator plate  91 . The bonding film  96  on the bottom surface of the crystal resonator plate  91  and the bonding film  94  on the top surface of the crystal resonator plate  91  are connected by through vias not illustrated. When a voltage is applied to the electrode pads on the circuit board, occurrence of a voltage difference between the bonding film  94  and the bonding film  95  causes resonance of the crystal resonator piece formed in the crystal resonator plate  91 , so that a predetermined frequency is generated. 
     For mounting of electronic components on a circuit board, commonly, solder paste is applied onto a plurality of electrode pads on the circuit board, the plurality of electronic components are placed on the solder paste so that electrodes of the electronic components come into contact with the solder paste, the solder paste is simultaneously melted by reflow of the entire circuit board, and the plurality of electronic components are collectively fixed to the circuit board. 
     On condition that the electronic components are mounted on only one surface of the circuit board, problems hardly occur because the electronic components are collectively mounted by the reflow at one time. On condition that the electronic components are mounted on both surfaces of the circuit board, however, problems may occur because the reflow is performed twice due to demands on production. 
       FIG. 1B  illustrates a problem that may occur when the crystal resonator  100  is mounted on a circuit board  80 . 
     For mounting of the crystal resonator  100  on the circuit board  80 , commonly, thin solder  82  is applied onto electrode pads  81  on the circuit board  80 , the crystal resonator  100  is placed so that the external terminals  98  of the crystal resonator  100  come into contact with the solder  82 , and the entire circuit board  80  is subjected to reflowing. Then the solder  82  is melted and soaks onto entire surfaces of the external terminals  98  of the crystal resonator  100 . Then an Au constituent of the external terminals  98  is diffused into the solder  82  and an Au alloy is thereby formed. A melting point on the order of 200° C. of the Au alloy is lower than a melting point of 240° C. of lead-free solder and thus so-called solder erosion may occur in which the Au alloy is absorbed into the solder  82 . When parts of the external terminals  98  undergo the solder erosion, the bonding films  95 ,  96  may be exposed on contact surfaces between the bonding films  95 ,  96  in the lower part of the crystal resonator  100  and the external terminals  98 . 
     Upon performance of the second reflow in this state, the melted solder  82  may come into contact with the bonding films  95 ,  96  in the lower part. On this occasion, an Al constituent of the bonding films  95 ,  96  in the lower part may be diffused into the solder  82  and an Al alloy is thereby formed. A melting point on the order of 200° C. of the Al alloy is also lower than the melting point of 240° C. of lead-free solder and thus solder erosion may occur in which the Al alloy is absorbed into the solder  82 . 
       FIG. 1B  illustrates a state in which the bonding film  95  on the bottom surface of the crystal resonator piece  91  is made to recede by the solder erosion at a site designated by “X” in  FIG. 1B . In such a state, the crystal resonator piece of the crystal resonator plate  91  fails to vibrate because the electrical connection between the bonding film  95  and the electrode pads  81  is lost. Then overall circuit malfunctions because the crystal resonator  100  generates a reference clock for operation of the circuit. 
     The following are reference documents: 
     [Document 1] Japanese Laid-open Patent Publication No. 2011-176787 and 
     [Document 2] Japanese Laid-open Patent Publication No. 2000-286671. 
     SUMMARY 
     According to an aspect of the invention, a crystal resonator includes: lower glass plates on which first electrodes are formed so as to extend from side surfaces to a bottom surface of the lower glass plates; a crystal plate which is provided over the lower glass plates and on which second electrodes to be coupled to the first electrodes are formed on a surface in contact with the lower glass plates; and an upper glass plate which is provided over the crystal plate; wherein the side surfaces of the lower glass plates on which the first electrodes are formed are provided with a protrusion that extends in parallel with a top surface and the bottom surface of the lower glass plates and that extends from one end to the other end of each of the side surfaces, and wherein the first electrodes are formed on the side surfaces that include surfaces of the protrusion. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  illustrate a conventional problem; 
         FIG. 2  illustrates a structure of a crystal resonator according to a first embodiment; 
         FIGS. 3A and 3B  illustrate the structure of the crystal resonator according to the first embodiment; 
         FIGS. 4A and 4B  illustrate effects of the crystal resonator according to the first embodiment; 
         FIGS. 5A and 5B  illustrate a production method for the crystal resonator according to the first embodiment; 
         FIGS. 6A to 6C  illustrate the production method for the crystal resonator according to the first embodiment; 
         FIG. 7  illustrates the production method for the crystal resonator according to the first embodiment; and 
         FIGS. 8A and 8B  illustrate a structure of a crystal resonator according to a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Herein below, preferred embodiments of techniques disclosed herein will be described in detail with reference to the accompanying drawings. 
       FIG. 2  is an exploded perspective view of a crystal resonator  10  according to a first embodiment. 
     The crystal resonator  10  of the first embodiment includes a crystal resonator plate  20  having a crystal resonator piece  26  that is made of crystal (SiO 2 ), for instance, and that is shaped like a tuning fork, a lid glass  30  bonded to a first surface on the front side of the crystal resonator plate  20  and configured to hermetically seal the crystal resonator piece  26  in a vibratable state and to form a lid body of the crystal resonator plate  20 , and a base glass  32  bonded to a second surface on the back side of the crystal resonator plate  20  and configured to hermetically seal the crystal resonator piece  26  in a vibratable state and to form a lid body of the crystal resonator plate  20 . 
     In a center part of the crystal resonator plate  20 , the crystal resonator piece  26  surrounded by an opening part and shaped like the tuning fork is formed and excitation electrode films  28  configured to vibrate the crystal resonator piece  26  are formed on surfaces of the crystal resonator piece  26 . An upper bonding film  22  that is made of the same material as the material of the excitation electrode films  28  and that becomes an actual bonding part for the lid glass  30  is provided on a surface of a frame-like part that surrounds the crystal resonator piece  26 . 
     Though not illustrated, the excitation electrode film  28  configured to vibrate the crystal resonator piece  26  is formed on the surface of the crystal resonator piece  26  on the back side of the crystal resonator plate  20  as on the front side. Lower bonding films  24 ,  25  that are made of the same material as the material of the excitation electrode films  28  and that become actual bonding parts for the base glass  32  are provided on a surface of the frame-like part that surrounds the crystal resonator piece  26 . 
     The excitation electrode films  28 , the upper bonding film  22 , and the lower bonding films  24 ,  25  are 1100 Å Al layers formed by sputtering or vacuum deposition, for instance. 
     The base glass  32  is made of a three-layer structure having three glass plates formed of soda-lime glass, silicon, or the like. In the three glass plates, having generally even thicknesses, a length in a longitudinal direction of a second glass plate  37  at the middle position is greater than a length in the longitudinal direction of a first glass plate  36  on the top of the second glass plate  37 . The length in the longitudinal direction of the second glass plate  37  at the middle position is greater than a length in the longitudinal direction of a third glass plate  38  on the bottom of the second glass plate  37 . 
       FIG. 3A  is a perspective view of the crystal resonator  10  according to the embodiment in which components in the exploded perspective view illustrated in  FIG. 2  are assembled without modification. In the three glass plates under the crystal resonator plate  20 , as seen looking on a short side surface of the crystal resonator  10 , the second glass plate  37  at the middle position protrudes relative to the first glass plate  36  and the third glass plate  38 . 
       FIG. 3B  illustrates a section of the crystal resonator  10  illustrated in  FIG. 3A  and cut in a direction of arrows from a line IIIB-IIIB. External terminals  40  are formed so as to extend from side surfaces of the three glass plates to the bottom surface of the third glass plate  38  at the lowest position. 
     The external terminal  40  at the left in  FIG. 3B  is in contact with the lower bonding film  25  on the bottom surface of the crystal resonator plate  20 . The lower bonding film  25  is connected by through vias not illustrated to the upper bonding film  22  on the top surface of the crystal resonator plate  20 . There is continuity between the upper bonding film  22  and the excitation electrode film  28  on the top surface of the crystal resonator piece  26 . 
     The external terminal  40  at the right in  FIG. 3B  is in contact with the lower bonding film  24  on the bottom surface of the crystal resonator plate  20 . There is continuity between the lower bonding film  24  and the excitation electrode film  28  on the bottom surface of the crystal resonator piece  26 . 
     With the crystal resonator  10  mounted on a circuit board, application of a voltage to electrode pads on the circuit board provides a voltage difference between the excitation electrode films  28  on the top and bottom surfaces of the crystal resonator piece  26  and thereby causes resonance of the crystal resonator piece  26 , so that a predetermined frequency is generated. 
     Subsequently, effects of the crystal resonator  10  of the embodiment will be described with reference to  FIG. 4 .  FIG. 4A  and  FIG. 4B  are enlarged views of a bonding part between the electrode pad  81  on the circuit board  80  and the external terminal  40  of the crystal resonator  10 , as seen looking from a side surface, in a state in which the crystal resonator  10  is mounted on the circuit board  80 . 
     For mounting of the crystal resonator  10  of the embodiment on the circuit board  80 , thin solder  82  is applied onto the electrode pads  81  on the circuit board  80 , and the crystal resonator  10  is placed so that bottom surfaces of the external terminals  40  of the crystal resonator  10  come into contact with the solder. 
     Upon subsequent performance of reflow, the solder  82  soaks and spreads under protrusions  33 . Then a decrease in temperature solidifies the solder  82  under the protrusions  33 , as illustrated in  FIG. 4A . Even if the external terminals  40  undergo solder erosion in some degree, borders of the lower bonding film  24  on the crystal resonator plate  20  undergo no change. Upon subsequent performance of the second reflow in this state, the melted solder  82  does not go above the protrusions  33 , and thus the lower bonding film  24  undergoes no solder erosion. 
     Even if too large amount of the solder  82  is applied onto the electrode pads  81  on the circuit board  80  as illustrated in  FIG. 4B , the melted solder  82  does not come into contact with the lower electrode  24  because the melted solder  82  flows into recesses  34  above the protrusions  33 . Even if the external terminals  40  undergo the solder erosion in some degree, the borders of the lower bonding film  24  on the crystal resonator plate  20  undergo no change. Upon subsequent performance of the second reflow in this state, the melted solder  82  does not reach the lower bonding film  24  because the melted solder  82  remains in the recesses  34  above the protrusions  33 . Therefore, the lower bonding film  24  undergoes no solder erosion. 
     In accordance with the crystal resonator  10  of the embodiment, in soldering of the crystal resonator  10  on the circuit board  80 , the protrusions  33  on the side surfaces of the base glass  32  to be mounted make it difficult for the melted solder to come into contact with the lower electrodes  24 ,  25 . As a result, the crystal resonator  10  may be provided that resists the solder erosion of the electrodes and that is highly reliable. 
     Herein below, production processes for the crystal resonator  10  will be described with reference to  FIGS. 5, 6 , and a flow chart of  FIG. 7 . The crystal resonator  10  is formed of a five-layer structure as described with reference to the exploded perspective view of  FIG. 2 . 
     As illustrated in  FIG. 5A , initially, five wafers, that is, a glass wafer  30 W for lid that is material of the lid glass  30 , a crystal wafer  20 W that is material of the crystal resonator plate  20 , a first glass wafer  36 W that is material of the first glass plate  36 , a second glass wafer  37 W that is material of the second glass plate  37 , and a third glass wafer  38 W that is material of the third glass plate  38  are prepared. An appellation of “wafer” refers to plate-like material having a size equivalent to sizes of semiconductor wafers. The wafers are used because the wafers are convenient for production using production apparatus for semiconductor devices. 
     In the embodiment, for instance, a glass plate with a thickness of 0.4 mm is used as the glass wafer  30 W for lid. A crystal plate with a thickness of 0.13 mm is used as the crystal wafer  20 W, for instance. For instance, 0.13 mm glass plates are used as the first glass wafer  36 W, the second glass wafer  37 W, and the third glass wafer  38 W. 
     The components of the crystal resonator  10  are formed at the same position in the glass wafer  30 W for lid, the crystal wafer  20 W, the first glass wafer  36 W, the second glass wafer  37 W, and the third glass wafer  38 W. 
     As illustrated in  FIG. 5B , initially, a plurality of crystal resonator plate regions  20 S are formed on the crystal wafer  20 W. Subsequently, the opening part  29  shaped like a letter “E” is formed at a generally center part in each of the crystal resonator plate regions  20 S. A part surrounded by the opening part  29  forms the crystal resonator piece  26 , and a part surrounding the opening part  29  forms the frame-like part (step S 1  in  FIG. 7 ). 
     Subsequently, the top and bottom surfaces of the crystal resonator piece  26  are etched so that the thickness of the crystal resonator piece  26  becomes smaller than the thickness of the frame-like part (step S 2 ). 
     Subsequently, metal films are deposited and formed by sputtering or the like on the surfaces of the crystal resonator piece  26  and the surfaces of the frame-like part (step S 3 ). Though material of the metal films is not especially limited, aluminum (Al), chromium (Cr), or the like is preferably used therefor, for instance, and aluminum is used in the embodiment. 
     Subsequently, the metal film is patterned so that the excitation electrode film  28  is formed on the crystal resonator piece  26  and so that the upper bonding film  22  is formed along the overall periphery on a part corresponding to the frame-like part there around (step S 4 ). 
     Similarly, on the back side of each crystal resonator plate region  20 S, the excitation electrode film  28  is formed on the crystal resonator piece  26  and the lower bonding films  24 ,  25  are formed along the overall periphery on a part corresponding to the frame-like part there around. 
     Referring to  FIG. 5A , opening parts  36 H are formed on both sides of each region to be formed into the first glass plate  36  on the first glass wafer  36 W (step S 6 ). 
     Similarly, opening parts  38 H are formed on both sides of each region to be formed into the third glass plate  38  on the third glass wafer  38 W (step S 7 ). 
     Subsequently, as illustrated in  FIG. 6A , the third glass wafer  38 W, the second glass wafer  37 W, the first glass wafer  36 W, the crystal wafer  20 W, and the glass wafer  30 W for lid are laminated in alignment with one another (step S 8 ). 
     Subsequently, as illustrated in  FIG. 6B , the laminated wafer group is anodically bonded in inert gas with application of a voltage to the wafer group (step S 9 ). It is then preferable to heat the wafer group to a temperature between 100° C. and 150° C., for instance, which is lower than the softening point of glass and to apply a DC voltage between 3 kV and 5 kV to the wafer group. In the embodiment, for instance, the wafers are heated to about 120° C. and the DC voltage of about 3.5 kV is applied to the wafers. 
     Subsequently, with use of a device to dice semiconductor wafers, the anodically bonded wafer group is diced at positions illustrated by arrows in  FIG. 6B , and singulation is attained as illustrated in  FIG. 6C  (step S 10 ). 
     Finally, sputtering with 600 Å Cr and 1500 Å Au is repeated three times to form the external terminals  40  with a thickness of 0.63μ on parts extending on the side surfaces of the first glass plate  36 , the second glass plate  37 , and the third glass plate  38  and the bottom surface of the third glass plate  38  so that the external terminals  40  are connected to the lower bonding film  24  on the bottom surface of the crystal resonator plate  20  (step S 11 ). 
     Finally, a structure of a crystal resonator  10 B of a second embodiment will be described with reference to  FIG. 8 . 
       FIG. 8A  is a perspective view of the crystal resonator  10 B according to the second embodiment.  FIG. 8B  illustrates a section of the crystal resonator  10 B illustrated in  FIG. 8A  and cut in a direction of arrows from a line VIIIB-VIIIB. 
     In the crystal resonator  10 B according to the second embodiment, glass plates under the crystal resonator plate  20  are composed of two plates. In the two glass plates, referring to  FIG. 8B , left and right end surfaces of a lower glass plate  38 B are recessed relative to end surfaces of an upper glass plate  37 B. The external terminals  40  are formed so as to extend from left and right side walls of the two glass plates in  FIG. 8B  to the bottom surface of the lower glass  38 B. 
     Even if portions of the external terminals  40  undergo solder erosion in the first soldering, solder melted in the second reflow does not come into direct contact with the lower electrodes  24 ,  25  under the crystal resonator plate  20  because the external terminals  40  are formed so as to cover the side surfaces of the lower electrodes  24 ,  25  under the crystal resonator plate  20 . Thus the crystal resonator  10 B that resists solder erosion of the electrodes and that is highly reliable may be provided. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.