Abstract:
A piezoelectric oscillator includes: a package including a floor surface and a step portion thereinside; a conductive adhesive applied on the step portion; and a piezoelectric vibrator including one end placed on the step portion via the conductive adhesive. The floor surface is provided with a recess filled with a resin material having a thermal deformation property. The package includes a sidewall having transparency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-244640, filed on Oct. 23, 2009, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a piezoelectric oscillator and a manufacturing method of the piezoelectric oscillator. 
       BACKGROUND 
       [0003]    In order to generate clock signals in information equipments, a piezoelectric oscillator is widely used, in which a piezoelectric material such as a crystal or barium titanate is used. The piezoelectric oscillator includes a substrate made of the piezoelectric material having a predetermined length and thickness, excitation electrode patterns formed on both surfaces of the substrate, and leading lines that extend from the electrode patterns. The substrate functions as a piezoelectric vibrator. One part of the substrate is mechanically fixed inside a package, and another part of the substrate is oscillated mechanically. 
         [0004]    One of known methods for mechanically supporting the piezoelectric vibrator is a cantilever method, in which one end of the piezoelectric vibrator is not mechanically fixed so as to be a free end. 
         [0005]      FIG. 4A  illustrates a plan view of a piezoelectric oscillator  10 , without a lid, including a piezoelectric vibrator  20  in a cantilever fashion.  FIG. 4B  illustrates a lateral view of the piezoelectric oscillator  10 , with a lid  40 , cut along a line A-A′ in  FIG. 4A . The piezoelectric vibrator  20  of the piezoelectric oscillator  10  is provided inside a package  30 . The piezoelectric vibrator  20  is a substrate made of a crystal or the like. An electrode pattern  21  and a leading line pattern  22  that extends from the electrode pattern  21  are formed on the upper surface of the piezoelectric vibrator  20  by a thin film formation technique such as a metallic vapor deposition technique. Similarly, a lower surface electrode pattern (not illustrated) and a leading line pattern  24  that extends from the lower surface electrode pattern are formed on the lower surface of the piezoelectric vibrator  20 . The package  30  is made of a material such as ceramic and has a step  31  formed therein. Metal pads  32  and  34  are formed on the step  31  by metalization. Conductive adhesives  33  and  35  are respectively applied to the metal pads  32  and  34 . Then, in this state, ends of the leading line patterns  22  and  24  formed on the piezoelectric vibrator  20  are respectively placed on the conductive adhesives  33  and  35 , and then the conductive adhesives  33  and  35  are cured. The curing of the conductive adhesives  33  and  35  allows the piezoelectric vibrator  20  to be supported on the step  31  substantially horizontally in a cantilever fashion. 
         [0006]    Each of the metal pads  32  and  34  formed on the step  31  is connected to an external electrode (not illustrated), which is formed on the bottom of the package  30  by metallization, through a conductor (not illustrated) provided inside the package  30 . The external electrode applies a voltage to the piezoelectric oscillator  10  provided on a circuit substrate. The piezoelectric vibrator  20  is deformed by a voltage effect and is therefore oscillated at a resonant frequency, thereby obtaining a voltage signal corresponding to the frequency. 
         [0007]    In this example, only the piezoelectric vibrator is accommodated in the package, but a semiconductor device of an oscillation circuit may be accommodated in the package along with the piezoelectric vibrator.  FIG. 5  illustrates a lateral view of a piezoelectric oscillator  11  accommodated with a semiconductor device  50  therein. This configuration allows a packaging density on the circuit substrate to be improved. Because the semiconductor device  50  in addition to the piezoelectric vibrator  20  is accommodated in the package  30 , the free space inside the package  30  becomes smaller. Therefore, control for keeping the piezoelectric vibrator  20  not to contact a lid  40  and the semiconductor device  50  becomes more difficult. 
         [0008]    A ceramic package made of alumina (Al 2 O 3 ) is typically used as the package of the piezoelectric oscillator. 
         [0009]    The package made of alumina is nontransparent, however, a transparent package made of a hard glass material is also known. Japanese Laid-open Patent Publication No. 2008-42512 discloses a related technique. 
         [0010]    A known method for forming a cantilever piezoelectric vibrator includes the following processes: placing a free end of the piezoelectric vibrator on a temporary supporting member (pillow member), which is positioned on a floor surface inside the package, made of a resin having a heat shrinkage property or a thermal deformation property; placing a supported end of the piezoelectric vibrator on the conductive adhesives applied to the step of the package; curing the conductive adhesives; and deforming the temporary supporting member by heating to form a gap between the piezoelectric vibrator and the temporary supporting member. The temporary supporting member is separated from the piezoelectric vibrator after the fixation of the supported end, thereby allowing the piezoelectric vibrator to be supported substantially horizontally in a cantilever fashion. Accordingly, contact of the free end with the package is suppressed. Japanese Laid-open Patent Publication No. 2006-80599 and Japanese Patent No. 4214753 disclose relates techniques. 
         [0011]    One end of the cantilever piezoelectric vibrator is free, and this allows the piezoelectric vibrator not to be mechanically restrained. Therefore, the cantilever piezoelectric vibrator is widely used because the supported portion is less affected by stress due to vibration. However, recently, the size of electronic devices has been decreasing, and electronic components have been required to have a low height. Accordingly, available space in the package to place the piezoelectric vibrator is reduced. In cases where the piezoelectric vibrator is fixed while being tilted, the piezoelectric vibrator may contact the package. Therefore, the piezoelectric vibrator is required to be more accurately supported horizontally in a cantilever fashion. Also in cases where the semiconductor device of the oscillation circuit is accommodated in the package along with the piezoelectric vibrator, the same requirement arises as in the case in which electronic components are required to have a low height. 
       SUMMARY 
       [0012]    According to an aspect of the present invention, provided is a piezoelectric oscillator and a manufacturing method thereof. The piezoelectric oscillator includes: a package including a floor surface and a step portion thereinside; a conductive adhesive applied on the step portion; and a piezoelectric vibrator including one end placed on the step portion via the conductive adhesive. The floor surface is provided with a recess filled with a resin material having a thermal deformation property. The package includes a sidewall having transparency. 
         [0013]    The manufacturing method includes: applying a conductive adhesive to a step formed inside a package made of a transparent material; positioning a temporary supporting member on a floor surface inside the package such that a height of a top of the temporary supporting member is higher than a height of a top of the conductive adhesive applied to the step, the temporary supporting member being made of a resin having a thermal deformation property; placing one end of a piezoelectric vibrator on the conductive adhesive applied to the step, the one end becoming a supported end of the piezoelectric vibrator; placing another end of the piezoelectric vibrator on the temporary supporting member, the another end becoming a free end of the piezoelectric vibrator; adjusting a height of the free end of the piezoelectric vibrator so as to be lower by a predetermined difference than a height of the supported end of the piezoelectric vibrator by radiating a laser to the temporary supporting member through a side wall of the package to thermally soften the temporary supporting member; curing the conductive adhesive by heating at a first temperature; and deforming the temporary supporting member by heating at a second temperature to form a gap between the piezoelectric vibrator and the temporary supporting member. 
         [0014]    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 
         [0015]      FIG. 1  is a diagram illustrating an exploded perspective view of a piezoelectric oscillator according to an embodiment of the present invention; 
           [0016]      FIGS. 2A to 2H  are diagrams illustrating a process for manufacturing a piezoelectric oscillator according to an embodiment of the present invention, each figure including a plan view and a lateral view of the piezoelectric oscillator; 
           [0017]      FIG. 3  is a diagram illustrating a perspective view of a piezoelectric oscillator before attachment of a lid according to an embodiment of the present invention; 
           [0018]      FIGS. 4A and 4B  are diagrams illustrating a plan view and a lateral view, respectively, of a piezoelectric oscillator, without a lid, including a piezoelectric vibrator in a cantilever fashion; and 
           [0019]      FIG. 5  is a diagram illustrating a lateral view of a piezoelectric oscillator accommodated with a semiconductor device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    In a conventional method for supporting the cantilever piezoelectric vibrator, one end of the piezoelectric vibrator is placed on the conductive adhesive applied to the step of the package, and another end thereof is placed on the temporary supporting member. Then, the piezoelectric vibrator is heated to cure the conductive adhesive in this state. The conductive adhesive is shrunk by the heating and then is cured while the end of the piezoelectric vibrator on the temporary supporting member is raised. Accordingly, in order to prevent the piezoelectric vibrator from contacting the lid or the like of the package, strict control of the height of the applied conductive adhesive and the height of the temporary supporting member may be required. For example, it is considered that control of the viscosity and amount of conductive adhesive applied may be required and that temperature control during heating may be required, and such a requirement generates a problem that a lot of time is spent for such control. 
         [0021]    After the conductive adhesive is cured by heating, the temporary supporting member is thermally deformed by heating at a higher temperature to form the gap between the piezoelectric vibrator and the temporary supporting member. The temporary supporting member deformed to form the gap is left in the package, and then the lid is sealed. When the temporary supporting member is fixed to the floor surface inside the package, no problem may arise. If the temporary supporting member is removed from the floor surface during transportation or handling in a mounting process, the temporary supporting member may move about inside the package, and may adhere to the piezoelectric vibrator. In this case, the piezoelectric oscillator may be unsteadily oscillated, thereby causing a problem. 
         [0022]      FIG. 1  illustrates components of a piezoelectric oscillator  100  according to the present embodiment. A package  200  made of transparent material, such as transparent ceramic, has a step  210  therein. Metal pads  230  and  240  are formed on the step  210  by metallization. In addition, the package  200  has a floor surface  215  and has an opening  220  formed on the floor surface  215 . The opening  220  has a predetermined shape and size, for example, a 0.5 mm square. The opening  220  leads into a hollow portion (not illustrated) having a size larger than the predetermined size. 
         [0023]    For example, the ceramic used for the package  200  may be produced from yttrium oxide (Y 2 O 3 ) by adding an additive and sintering, and the produced ceramic is transparent. Such transparent ceramic is disclosed in the following web site: 
         [0000]    http//www.covalent.co.jp/eng/rd/developments/transparent.html, for example. An external electrode (not illustrated) used for the piezoelectric oscillator  100  is provided on the bottom surface of the package  200 , and the external electrode is electrically connected to the metal pads  230  and  240  through internal wiring. 
         [0024]    A temporary supporting member  300  temporarily supports a piezoelectric vibrator  400 . For example, the temporary supporting member  300  is prepared by forming a thermoplastic fluorine resin into a spherical shape having a size of 0.8 mm and is thermally deformed at a temperature in the range from 200° C. to 400° C. 
         [0025]    In the embodiment, a quartz plate having a length of 4.5 mm and a width of 2 mm is used for the piezoelectric vibrator  400 . An evaporated metal film is deposited as an electrode pattern  410  on an upper surface of the quartz plate, and a leading line pattern  420  is formed so as to extend from the electrode pattern  410  to the left end of the quartz plate. A lower surface electrode pattern (not illustrated) is formed on a lower surface of the quartz plate as in the case of the electrode pattern  410 , and a leading line pattern  430  is formed so as to extend from the lower surface electrode pattern to the left end of the quartz plate. When the piezoelectric vibrator  400  is attached to the package  200 , the left end of the piezoelectric vibrator  400  becomes the supported end which is supported by a conductive adhesive, and the right end thereof becomes the free end. The conductive adhesives are applied to the metal pads  230  and  240  and connect the leading line patterns  420  and  430  of the piezoelectric vibrator  400  to the metal pads  230  and  240 , respectively. In addition, the conductive adhesives mechanically fix the piezoelectric vibrator  400  to the step  210 . 
         [0026]    A lid  500  is made of alumina ceramic and is glass-sealed with the upper surface of the package  200 . 
         [0027]    Examples of the conductive adhesive include an epoxy adhesive, a urethane adhesive, and a silicon adhesive based on a type of binder. An epoxy adhesive that exhibits high hardness is employed in the present embodiment. 
         [0028]      FIGS. 2A to 2H  illustrate a process for manufacturing the piezoelectric oscillator  100  according to the present embodiment. A method of manufacturing the piezoelectric oscillator  100 , which has been discussed with reference to  FIG. 1 , will be discussed in order of the process steps with reference  FIGS. 2A to 2H . Each of  FIGS. 2A to 2H  includes a plan view and a lateral view thereof cut along a line B-B′ in the plan view. 
         [0029]      FIG. 2A  illustrates a plan view and a lateral view of the package  200  made of the transparent ceramic. In the package  200 , the above discussed metal pads  230  and  240  are formed on the step  210 , and the opening  220  leading into the hollow portion  221  is formed on the floor surface  215 . 
         [0030]      FIG. 2B  illustrates a process in which a dispenser  250  is used to eject and apply the conductive adhesives to the metal pads  230  and  240 . The conductive adhesives  251  and  252  are ejected onto the metal pads  230  and  240  so as to be raised on the metal pads  230  and  240 , respectively. 
         [0031]      FIG. 2C  illustrates a process in which the temporary supporting member  300  is positioned on the opening  220  on the floor surface  215  of the transparent package  200 . The temporary supporting member  300  is sucked by a vacuum chuck of a handler  310 , and a vacuum state is canceled above the opening  220 . As a result, the bottom of the spherical temporary supporting member  300  engages with the opening  220  to determine the position of the temporary supporting member  300 . The height of the top of the temporary supporting member  300  placed on the opening  220  is equal to or slightly higher than that of each of the conductive adhesives  251  and  252 . In order to provide such a state, the size of the spherical temporary supporting member  300  is preliminarily determined. 
         [0032]      FIG. 2D  illustrates a state in which the piezoelectric vibrator  400  is placed over the conductive adhesives  251  and  252  respectively applied to the metal pads  230  and  240 , as well as the temporary supporting member  300 . The placed piezoelectric vibrator  400  has the free end at a position higher than that of the supported end (inclined leftward). Even when the height of each top of the conductive adhesives  251  and  252  is positioned equal to that of the temporary supporting member  300 , because each of the conductive adhesives  251  and  252  is not cured in this state, the supported end of the piezoelectric vibrator  400  is positioned at a slightly low position owing to the weight of the piezoelectric vibrator  400 , thereby providing the state of inclined leftward. The piezoelectric vibrator  400  is placed by a handler (not illustrated) similar to the handler  310  such that leading line patterns  420  and  430  formed on the piezoelectric vibrator  400  are respectively aligned with the conductive adhesives  251  and  252  provided on the metal pads  230  and  240 . For example, the piezoelectric vibrator  400  is positioned such that the end or end corner of the piezoelectric vibrator  400  overlaps a half to a quarter of each area of the metal pads  230  and  240 . 
         [0033]      FIG. 2E  illustrates a process to adjust the height of the free end of the piezoelectric vibrator  400  inclined leftward. The temporary supporting member  300  is irradiated with a laser, to be thermally softened, from the outside of a side surface of the package  200  through a side wall thereof. Then, the temporary supporting member  300  is deformed owing to the weight of the piezoelectric vibrator  400 , thereby decreasing the height of the temporary supporting member  300 . The laser irradiation is stopped at the time that the free end is positioned at a height lower than that of the supported end by a predetermined difference (namely, a state is provided, in which the piezoelectric vibrator  400  is inclined rightward). An yttrium aluminum garnet (YAG) laser apparatus  600  is used to radiate the laser to an upper portion (a portion that contacts the piezoelectric vibrator  400 ) of the temporary supporting member  300 . The positions of the two ends of the piezoelectric vibrator  400  are measured with a laser length measuring machine  610  using a Neon-Helium (Ne—He) laser. With the measuring result, the irradiation time or the like by the YAG laser apparatus  600  is controlled, thereby performing adjustment to provide a predetermined height of the piezoelectric vibrator  400 . Because the package  200  is transparent, the laser may be radiated through the side surface of the package  200 . 
         [0034]    The reason for employing the predetermined state in which the piezoelectric vibrator  400  is inclined rightward is that the piezoelectric vibrator  400  will be horizontal in a subsequent process step of curing the conductive adhesives  251  and  252 . Namely, with the application of heat during the curing of the conductive adhesives  251  and  252 , the viscosity of the conductive adhesives  251  and  252  is temporarily decreased, and the conductive adhesives  251  and  252  becomes slightly squashed owing to the weight of the piezoelectric vibrator  400 . Then solvents contained in the conductive adhesives  251  and  252  are volatilized to cause the shrinkage of the conductive adhesives  251  and  252 . Because these phenomena enable the free end to be raised, the free end of the piezoelectric vibrator  400  is configured so as to be lowered by the presumed amount of the rise. 
         [0035]      FIG. 2F  illustrates a state in which the piezoelectric vibrator  400  is heated to cure the conductive adhesives  251  and  252  after the height adjustment. The heating is performed at a temperature in the range from 150° C. to 180° C. for a predetermined period of time in an electric furnace. As illustrated in  FIG. 2F , the piezoelectric vibrator  400  inclined rightward is released from the temporary supporting member  300  owing to the curing and shrinkage of the conductive adhesives  251  and  252 , thereby becoming horizontal. The viscosity of the conductive adhesives  251  and  252  is decreased in the initial period of the heating with the result that the conductive adhesives  251  and  252  extend to the upper surface of the piezoelectric vibrator  400 . Therefore the conductive adhesive  251  contacts the leading line pattern  420  formed on the upper surface of the piezoelectric vibrator  400 . The temporary supporting member  300  is not deformed at such a temperature that the conductive adhesives  251  and  252  are cured (a temperature in the range from 150° C. to 180° C.). 
         [0036]      FIG. 2G  illustrates a state in which the temporary supporting member  300  is heated to be deformed at a temperature of 300° C. in the electric furnace after the curing of the conductive adhesives  251  and  252 . The temporary supporting member  300  enters the hollow portion  221  to be cured. 
         [0037]      FIG. 2H  illustrates a state in which the lid  500  is placed on the package  200  and the piezoelectric oscillator  100  is glass-sealed. The temporary supporting member  300  partially enters the hollow portion  221  to be fixed, and therefore the temporary supporting member  300  does not move about inside the package  200 . 
         [0038]      FIG. 3  illustrates the piezoelectric oscillator  100  before the attachment of the lid  500 . One end of the piezoelectric vibrator  400  is supported by the conductive adhesives  251  and  252  applied respectively to the metal pads  230  and  240  provided on the step  210 . The temporary supporting member  300 , which has been thermally deformed, is found on the floor surface  215  of the package  200  below the free end of the piezoelectric vibrator  400 . As discussed above, the temporary supporting member  300  partially enters the hollow portion  221  formed under the floor surface  215 , to be fixed, and therefore the temporary supporting member  300  does not move about inside the package  200 . 
         [0039]    Although transparent ceramic is used for the package  200  in the present embodiment, the transparent material for the package  200  may be transparent hard glass or any other material that is transparent. 
         [0040]    In the present embodiment, the height of the piezoelectric vibrator  400  is adjusted before the curing of the conductive adhesives  251  and  252 , and therefore one end of the piezoelectric vibrator  400  may be accurately held horizontally after the curing, thereby providing a method for manufacturing a piezoelectric oscillator using a package having a low height. 
         [0041]    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 discussed 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.