Abstract:
In an exemplary method for making crystal vibrating devices, four wafers are provided: a crystal wafer, a base wafer, a first-lid wafer, and a second-lid wafer. The crystal wafer defines multiple crystal vibrating pieces including respective frames and respective electrodes formed on both main surfaces thereof. The base wafer defines multiple base plates bondable to one main surface of respective frames. The first-lid wafer defines multiple first lids bondable to the other main surface of the respective frames. Each first lid defines a void registrable with respective electrodes. The second-lid wafer is sized similarly to and bondable to the first-lid wafer so as to sealably close the voids. In a first bonding step the crystal wafer is bonded to the base wafer and first-lid wafer. In a subsequent adjustment step the thickness of at least one electrode per each crystal vibrating piece is adjusted to adjust the vibrational frequency of the respective vibrating portion. Thickness adjustment occurs through the respective voids. In a second bonding step, the second-lid wafer is bonded to the first-lid wafer. The resulting wafer sandwich is cut up into individual quartz crystal devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Japan Patent Application No. 2010-080195 filed on Mar. 31, 2010, in the Japan Patent Office, the disclosure of which is incorporated herein by reference in its entirety. 
       FIELD 
       [0002]    This disclosure relates to, inter alia, surface-mounted quartz-crystal devices of which the respective packages include lids and base plates made of crystal materials. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    At time of manufacture, crystal vibrating pieces that produce vibrations in the thickness-shear mode (e.g., AT-cut quartz-crystal vibrating pieces and devices) are made to produce resonant-vibration frequencies that are slightly higher than the nominal vibration frequencies in finished product. These vibrating pieces are generally “adjusted” to vibrate at their lower, nominal, vibration frequency by adding an “adjustment” metal film to the excitation electrodes while the resonance frequencies are being measured. This is done before the vibrating piece is enclosed within a “package” (at which time the vibrating piece becomes a vibrating “device”). After adjusting the vibration frequency, the vibrating piece is mounted in a package, which is sealed. Unfortunately, mounting a crystal vibrating piece in a package frequently causes an offset in the resonance vibration frequency produced by the vibrating piece. Relative to product specifications, this offset may be rather large due to electrical capacitance between the crystal vibrating piece and the package. 
         [0004]    Japan Patent Publication No. 1998-284971 discusses a way in which to solve this problem. Specifically, quartz-crystal vibrating pieces disclosed in this reference are mounted in their respective packages, and electrodes are added via openings defined in the lid to reduce resonance frequencies. Then, the openings in the lid are sealed. A disadvantage of the methods disclosed in JP &#39;971 is that each one of the openings formed in the lid must be individually covered and sealed, which increases production costs. 
         [0005]    In view of the difficulties with conventional practice as summarized above, a purpose of this invention is to provide methods for manufacturing quartz-crystal devices at lower cost without compromising the ability of the devices to vibrate at accurate and precise resonance frequencies. 
       SUMMARY 
       [0006]    In view of the foregoing, an object of this invention is to provide improved methods for manufacturing quartz-crystal vibrating devices comprising respective crystal vibrating pieces that exhibit stable vibrations. 
         [0007]    According to a first aspect of the invention, methods are provided for manufacturing a quartz-crystal vibrating device that includes a crystal vibrating piece. An embodiment of such a method comprises providing four wafers. The first wafer is a crystal wafer defining multiple crystal vibrating pieces each having a surrounding frame and respective electrodes formed on both main surfaces of the crystal vibrating piece. The frame is coupled to and circumferentially surrounds the crystal vibrating piece. The second wafer is a base wafer defining multiple base plates that are bondable to a first bonding surface of respective frames on the crystal wafer. The third wafer is a first-lid wafer defining multiple first lids that are bondable to a second bonding surface of respective frames on the crystal wafer. Each first lid defines a void situated to register with the electrode on the respective crystal vibrating piece whenever the first lids are bonded to the second bonding surface. The fourth wafer is a second-lid wafer sized similarly to the first-lid wafer. The second-lid wafer defines multiple second lids that are bondable to respective first lids of on the first-lid wafer so as to cover and seal the respective voids. In the method the crystal wafer is registered with, sandwiched between, and bonded to the base wafer and first-lid wafer. At this stage the vibrational frequencies of the crystal vibrating pieces are measured, followed by adjustment (as required on each vibrating piece) of the thickness of the electrode. The adjustment is performed through the void to the electrode facing the void. Adjustment results in the vibrational frequencies being brought to within specification for the finished product. Then, the second-lid wafer is registered with and bonded to the upper surface of the first-lid wafer to form a wafer assembly containing multiple “packaged” crystal vibrating pieces, now called “crystal vibrating devices.” The wafer assembly is cut along cut lines to separate the finished crystal vibrating devices into individual devices. 
         [0008]    During adjustment of the vibration resonance frequency, the resonance frequency (initially higher than required) is reduced by adding metal film (e.g., by sputtering) to the electrode visible through the void. This metal film is called an “adjustment metal film.” Alternatively, the resonance frequency (initially lower than required) is increased by removing metal from the electrode (e.g., by reverse-sputtering). 
         [0009]    When the second-lid wafer is being bonded to the first-lid wafer, the bonding can be performed using an adjustment metal film previously formed on the upper surface of the first-lid wafer (i.e., on the surface of the first-lid wafer facing the second-lid wafer). 
         [0010]    In the second bonding step, an adjustment metal film is bonded to the first lid wafer using an adjustment metal film formed on the first lid wafer on the main surface of the second lid wafer. 
         [0011]    The first-lid wafer and second-lid wafer can be made of crystal material, in which event the second-lid wafer is bonded to the first-lid wafer by siloxane bonding. Performing this bonding is facilitated by the previously performed reverse-sputtering. 
         [0012]    The provided crystal wafer can include an array of through-holes formed at respective positions corresponding to points at which cut lines cross each other. Hence, the through-holes are located substantially outside the frames. Similarly placed through-holes are formed at respective locations on the base wafer that register with the locations of respective through-holes on the crystal wafer. If these features are included, the method can further comprise a step in which external electrodes are formed on the outer surface of the base wafer. The external electrodes are formed by forming a conductive metal film to the inner surface of each through-hole and to respective locations on the outer (lower) surface of the base wafer. This step is performed between the step of bonding the first-lid wafer and the step of adjusting the vibration frequency. 
         [0013]    In other embodiments, particularly directed to methods for manufacturing crystal vibrating devices that produce thickness-shear vibrations, respective electrodes are formed on both surfaces of the crystal wafer. In these devices the voids in the first lids have similar shape and size as the electrodes. 
         [0014]    In yet other embodiments each crystal vibrating piece is a tuning-fork type crystal vibrating piece including a pair of vibrating arms. On the distal end of each vibrating arm is a respective “weight.” In such pieces two voids are formed in the first lid, each having the same shape and area of the respective underlying weight. 
         [0015]    According to another aspect, quartz-crystal devices are provided. An embodiment of such a device comprises a crystal frame including a crystal vibrating piece with electrodes being formed on both main surfaces thereof. A frame circumferentially surrounds the crystal vibrating piece. A base is affixed to one surface of the frame, and a first lid is affixed to the other surface of the frame. The device includes a first lid having an area sufficient to cover the crystal vibrating piece. The first lid defines a void registered with the electrode on the upward-facing surface of the crystal vibrating piece. Extending over the entire upper surface of the first lid is an adjustment metal film, which also covers the facing surface of the facing electrode. The device includes a second lid bonded to the first lid. The second lid has an area sufficient to cover the first lid including the void. In this embodiment the surface of the second lid facing the first lid (i.e., lower surface of the second lid) is bonded to an adjustment metal film initially applied to the upper surface of the first lid and through the void to the upper surface of the crystal vibrating piece. 
         [0016]    The lower surface of second lid can also include a bonding-metal film, wherein the second lid is bonded to the first lid by bonding the bonding metal film on the second lid with the adjustment metal film on the first lid. 
         [0017]    The second lid can be made of a glass containing metal ions, wherein the adjustment metal film on the first lid is bonded to the second lid (e.g., by anodic bonding). 
         [0018]    Another embodiment of a quartz crystal device comprises a crystal frame including a crystal vibrating piece with respective electrodes formed on both main surfaces thereof and a frame circumferentially surrounding the crystal vibrating piece. A base is affixed to one main surface of the frame, and a first lid is affixed to the other main surface of the frame. The device includes a first lid having an area sufficient to cover the crystal vibrating piece. The first lid defines a void that is registered with the electrode on the crystal vibrating piece facing the void. The device also includes a second lid that is registered with and bonded to the first lid and that covers the void. Before the second lid is bonded, the upper surface of the electrode can be made thinner, e.g., by abrasion, followed by application of an adjustment metal film to the upper surface of the first lid, including through the void and to the upper surface of the electrode. The second lid is bonded to the adjustment metal film. If the first and second lids are made of crystal material, the first and second lids desirably are bonded together by siloxane bonding. 
         [0019]    In other embodiments the lower surface of the second lid includes a respective bonding metal film extending at least around the void. Applied over the upper surface of the first lid is a respective bonding metal film that is thicker than the electrode formed earlier. 
         [0020]    The electrodes are usually formed on respective surfaces of the crystal vibrating piece. If the crystal vibrating piece is configured to vibrate according to the thickness-shear mode, the voids  11  desirably have the same shape as of the electrodes on the vibrating piece. 
         [0021]    In other embodiments the crystal vibrating pieces are configured as tuning-fork type crystal vibrating pieces, each comprising a pair of vibrating arms having respective “weights” on the distal ends thereof. In such embodiments the first lid desirably defines two voids each placed to align with a respective weight and each being dimensioned in the length and width dimensions similarly to the respective weight. 
         [0022]    The various embodiments provide quartz crystal devices producing consistent, uniform vibrations at specification, while being manufactured at reduced cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is an oblique exploded view of a first embodiment of a crystal vibrating device. 
           [0024]      FIG. 2  is a cross-sectional view, along the line A-A in  FIG. 1 , of the first embodiment of a crystal vibrating device after completing adjustment of its vibration frequency. 
           [0025]      FIG. 3  is a flow-chart of an embodiment of a method for manufacturing crystal vibrating devices according to the first embodiment. 
           [0026]      FIG. 4A  is a plan view of a base wafer comprising multiple individual base plates formed thereon. 
           [0027]      FIG. 4B  is an enlarged view of a region of the base wafer, schematically depicting multiple base plates formed thereon. 
           [0028]      FIG. 5A  is a plan view of an AT-cut crystal wafer comprising multiple individual crystal frames formed thereon. 
           [0029]      FIG. 5B  is an enlarged view of a region of the AT-cut crystal wafer, schematically depicting multiple AT-cut crystal vibrating pieces formed thereon. 
           [0030]      FIG. 6  is a plan view of a first lid wafer on which multiple individual first lids are formed. 
           [0031]      FIG. 7  is a plan view of a second lid wafer on which multiple individual second lids are formed. 
           [0032]      FIG. 8  is a flow-chart of method for manufacturing crystal vibrating devices according to the second embodiment. 
           [0033]      FIG. 9  is a cross-sectional view of a crystal vibrating device according to the second embodiment, after completing adjustment of its vibration frequency. 
           [0034]      FIG. 10  is a cross-sectional view of a crystal vibrating device according to a third embodiment, after completing adjustment of its vibration frequency. 
           [0035]      FIG. 11  is an oblique exploded view of a crystal vibrating device according to a fourth embodiment. 
           [0036]      FIG. 12  is a cross-sectional view of a crystal vibrating device according to the fourth embodiment, after completing adjustment of its vibration frequency. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    The invention is described below in the context of representative embodiments, which are not intended to be limiting in any way. 
       First Embodiment of Crystal Vibrating Device 
       [0038]    This embodiment is an example of a crystal vibrating device  90   a  that vibrates in the thickness-shear mode.  FIG. 1  is an oblique exploded view of the device  90   a , which is configured as a surface-mountable device (SMD) and includes an AT-cut crystal frame  10 . The single crystal frame  10  shown in the figure is one of a few hundreds to a few thousands of such frames that are formed simultaneously on an AT-cut “frame wafer.”  FIG. 2  is a cross-sectional view of the device  90   a , along the line A-A in  FIG. 1 , after completing adjustment of its vibration frequency. 
         [0039]    The crystal vibrating device  90   a  comprises a base plate  20 , the AT-cut crystal frame  10 , a first lid  30   a , and a second lid  40   a . The base plate  20  is fabricated of piezoelectric material, e.g., a piezoelectric crystal material such as quartz crystal. The base plate  20  includes an upper (inner) main surface (visible in  FIG. 1 ) and a lower (outer) main surface, the latter also termed a “mounting surface.” The inner main surface faces the crystal frame  10  and defines a concavity  21  having length and width dimensions (in the XY plane) that are larger than corresponding dimensions of the vibrating portion  11  described below. Formed on the lower main surface of the base plate  20  are external electrodes  24 . Each of the four corners of the base plate  20  includes a respective quarter-circle cutout  22 , each being a respective portion of a respective through-hole BH formed in a base wafer, discussed later below with reference to  FIG. 4B . Conformably disposed in each cutout  22  is a respective connecting electrode  25  (see  FIG. 2 ) formed simultaneously with formation of the external electrodes  24 . Thus, the external electrodes  24  are electrically connected to the excitation electrodes  15  of the crystal frame  10 . 
         [0040]    The crystal frame  10 , fabricated of AT-cut crystal material, comprises a vibrating portion  11  and a frame portion  12  that circumferentially surrounds the vibrating portion  11 . Between the vibrating portion  11  and the frame portion  12  is a U-shaped through-channel (void)  13 , which extends on three sides of the vibrating portion. The remaining side is occupied by a connecting portion  14  coupling the vibrating portion  11  to the frame portion  12 . 
         [0041]    A respective excitation electrode  15  is disposed on the upper main surface and lower main surface of the vibrating portion  11 . Each excitation electrode  15  is connected to a respective extraction electrode  16  disposed on the respective main surface of the frame  12 . In this embodiment the through-channel  13  provides clearance for the vibrating portion  11  on three sides thereof; however, this clearance is not required. Also, whereas the vibrating portion  11  is shown having a rectangular plan profile, it alternatively can have either a trapezoidal or an inverted trapezoidal profile. 
         [0042]    The quarter-circle cutouts  17  on the corners of the crystal frame  10  are formed simultaneously with formation of the through-holes CH (discussed later below with reference to  FIG. 5B ). As noted, each quarter-circle cutout  17  includes a respective connecting electrode  25 . The external electrodes  24  and the extraction electrodes  16  are electrically connected together via the connecting electrodes  25  on the crystal frame  10  and via respective quarter-circle cutouts  22  on the base plate  20 . 
         [0043]    Hence, whenever an alternating voltage signal is applied to the external electrodes  24 , the signal is conducted to the excitation electrodes  15  to cause the vibrating portion  11  to vibrate at its designated fundamental frequency. The fundamental frequency of the crystal frame  10  is a function of several variables including the thickness of the AT-cut vibrating portion  11 . The fundamental frequency also varies with corresponding changes in the thickness of the excitation electrodes  15 . Hence, the vibration frequency of the crystal frame  10  can be minutely adjusted to a desired frequency by appropriately changing the thickness of the excitation electrodes  15 . For example, the vibration frequency of the crystal vibrating device  90   a  is increased by making the excitation electrodes  15  thinner, and the vibration frequency is decreased by making the excitation electrodes thicker. 
         [0044]    With respect to the crystal vibrating piece  90   a  of this first embodiment, the excitation electrodes  15  are initially formed thicker than ultimately desired (which causes the vibrating portion  11  to vibrate at a frequency slightly lower than desired). To increase the vibration frequency to within specification, the excitation electrodes  15  are thinned in situ by execution of a reverse sputtering step performed during the frequency-adjustment stage of the manufacturing process. 
         [0045]    The first lid  30   a  is a rectangular piece of the crystal material. The first lid  30   a  defines a void  31  having the same size and shape as the adjacent excitation electrode  15  on the vibrating portion  11 . Defined on the lower main surface of the first lid  30   a  is a concavity  32  that is disposed adjacent the upper main surface of the crystal frame  10 . The concavity  32  is larger than the vibrating portion. On the lower main surface of the first lid  30   a , the area that is peripheral to the concavity  32  is a lower bonding zone  33  for bonding the first lid  30   a  to the frame portion  12 . On the upper main surface of the first lid  30   a  is a peripheral upper bonding zone  33  for bonding the second lid  40   a  to the first lid  30   a.    
         [0046]    The second lid  40   a  in this embodiment desirably is made of a rectangular unit of crystal material. The second lid  40   a , when bonded to the upper bonding zone  33 , seals the void  31 . 
       Embodiment of Method for Manufacturing Crystal Vibrating Devices 
       [0047]    This method is appropriate for manufacturing crystal vibrating devices according to the first embodiment described above. An embodiment of this method is depicted in the flow-chart of  FIG. 3 . In step S 11  a base wafer  200  is formed of a thin planar unit of crystal material. 
         [0048]    In step S 111  through-holes BH (see  FIG. 4B ) are formed in the base wafer  200  using conventional techniques including photolithography and etching. These through-holes BH are destined to become the quarter-circle cutouts  22 . In step S 112 , concavities  21  are formed on the base wafer  200  in a similar manner using photolithography and etching. If the AT-cut crystal vibrating portions  11  are to have inverted trapezoidal shapes (in which the vibrating portions  11  are thinner than the respective frame portions  12 ), step S 112  may not be necessary. 
         [0049]      FIGS. 4A and 4B  are plan views of the upper main surface of the base wafer  200  formed in step S 11 .  FIG. 4B  is an enlargement of a region of base plates  20  formed on the base wafer  200 .  FIG. 4A  shows an exemplary base wafer  200  made of a circular wafer of crystal material of which both major surfaces are planar to very tight tolerances. The base wafer  200  includes an orientation flat OF on a part of the peripheral edge thereof, wherein the flat OF provides a reference crystal orientation of the wafer. On a typical 3-4 inch diameter base wafer  200 , a few hundred to a few thousand base plates  20  are formed simultaneously. Upon completing fabrication of the base plates  20 , the base wafer  200  is cut up, as described later, along the cut lines CL to release all the individual base plates  20  from the base wafer  200 . 
         [0050]    As shown in  FIG. 4B , circular through-holes BH are formed on the base wafer  200  at the corners of the cut lines CL. Since each through-hole BH circumscribes the four corners at the respective location on the base wafer  200 , after cutting the corners of each base plate  20  have respective quarter-circle cutouts  22 . Thus, each through-hole BH forms four quarter-circle cutouts  22 . The base through-holes BH can be formed by photolithography and etching, or other suitable technique. 
         [0051]    Returning to  FIG. 3 , step S 12  involves forming the AT-cut quartz-crystal wafer  100 . The resonance frequencies of the AT-cut crystal frame  10  to be formed on the AT-cut crystal wafer  100  are initially configured to be slightly lower than the desired frequencies. In step S 121  the U-shaped through-channels  13  are formed in the crystal wafer  100 . In step S 122  circular through-holes CH ( FIG. 5B ) are formed on the crystal wafer  100  at the corners of the cut lines CL for cutting out crystal frames  10  from the crystal wafer  100 . The through-channels  13  and the through-holes CH can be formed by photolithography and etching, or other suitable technique. In step S 123 , the excitation electrodes  15  and extraction electrodes  16  are formed simultaneously in each of the crystal frames  10  on the wafer  100  by the technique such as sputtering. The excitation electrodes  15 , at least on the main surface facing the first lid  30   a , are formed slightly thicker than ultimately required, to allow sufficient thickness for later adjustment by thinning. Thickness reduction (electrode thinning) is performed later by reverse sputtering at the frequency-adjustment stage. 
         [0052]      FIGS. 5A and 5B  are plan views of the AT-cut crystal wafer  100  formed in step S 12 .  FIG. 5B  is an enlarged view of a region of the surface of the wafer  100 , showing fewer crystal frames  10 . In  FIG. 5A  the AT-cut crystal wafer  100  is configured as a precisely planar plate. An orientation flat OF is provided to denote a specified crystal orientation of the wafer. On each crystal wafer  100  a few hundreds to a few thousands of crystal frames  10  are formed simultaneously. The outline profile of each crystal frame  10 , when cut from the wafer  100  is established by the cut lines CL. 
         [0053]    In  FIG. 5B , on the crystal wafer  100 , a few hundreds to a few thousands of U-shaped through-channels  13  (one for each crystal frame  10 ) are formed simultaneously with formation of the through-holes CH. On the crystal wafer the positions of the through-holes CH correspond to respective positions of corresponding through-holes BH of the base wafer  200 . 
         [0054]    During fabrication of the AT-cut crystal wafer  100 , first a layer of chromium is formed on the surface of the wafer for use as a substrate for the excitation electrodes  15  and the extraction electrodes  16 . A layer of gold is formed over the chromium layer. Formation of the gold layer is prefaced by formation of the chromium layer because gold adheres better to chromium than to piezoelectric crystal. Thus, the excitation electrodes  15  and extraction electrodes  16  can be formed with good conductivity and good adhesion to the crystal. The gold layer desirably is thicker than the chromium layer to make the initial resonance frequencies lower than the desired frequencies. 
         [0055]    Returning to  FIG. 3 , Step S 13  involves formation of the first-lid wafer  300 . In step S 131  the voids  31  are formed in the first-lid wafer  300  using photolithography and etching or other suitable technique. Each void  31  has the same size as the respective excitation electrodes  15  on the crystal frames  10 . In step S 132  the concavities  32  are formed on the first-lid wafer  300  by photolithography and etching. Step S 132  is not always required if the vibrating portion  11  is shaped as an inverted trapezoid, wherein the vibrating portion  11  is thinner than the frame portion  12 . 
         [0056]      FIG. 6  is a plan view of the first-lid wafer  300  formed in step S 13 . The first-lid wafer  300  is fabricated of the crystal material and configured as a precisely planar wafer. The outline profile of each first lid  30   a  is defined by cut lines CL, as described later. The crystal orientation of the first-lid wafer is referenced to an orientation flat OF. 
         [0057]    Returning to  FIG. 3 , in step S 14  the second-lid wafer  400   a  is fabricated of the crystal material, and configured as a precisely planar wafer. Steps S 11  to S 14  can be performed simultaneously.  FIG. 7  is a plan view of the second-lid wafer  400   a  formed in step S 14 . The outline profile of each second lid  40   a  is defined by cut lines CL, as described later. An orientation flat OF defines a reference crystal orientation. 
         [0058]    Returning to  FIG. 3 , a first bonding step is performed in step S 15 , in which the base wafer  200  formed in step S 11 , the crystal wafer  100  formed in step S 12 , and the first-lid wafer  300  formed in step S 13  (in this order) are lap-bonded together. In this step the orientation flats OF are used as respective lattice references for precise positioning of the wafers. The precision with which this first bonding step is performed enables the through-holes BH on the base wafer and the through-holes CH on the crystal wafer  100  to be accurately registered with each other. 
         [0059]    Since all three wafers are made of crystal material, siloxane bonds are used for bonding them together. Siloxane bonding requires that two opposing (and clean) surfaces of crystal material be brought together and heated to approximately 200° C. while applying compression. 
         [0060]    In step S 16  the connecting electrodes  25  and external electrodes  24  are formed simultaneously by masking and sputtering. The external electrodes  24  are formed on the lower main surface of the base wafer  200  (after being bonded in step S 15 ). The connecting electrodes  25  are formed on the walls of the aligned crystal through-holes CH and base through-holes BH previously formed on the crystal wafer  100  and base wafer  200 . Electrode-related failures, including contact failures, open circuits, etc., between the external electrodes  24  formed on the base wafer  200  and the excitation electrodes formed on the crystal wafer  100  can be eliminated by forming the connecting electrodes  25  in the contiguously aligned through-holes CH and BH. 
         [0061]    In step S 17  vibration frequency is adjusted. This step is performed on all the crystal frames  10  on the crystal wafer  100  using reversed sputtering via the voids  31 . Frequency adjustments are performed, after finishing the first bonding step and after forming all the electrodes, to every one of the AT-cut crystal frames  10  by applying respective voltages. For example, a train of alternating voltages is applied to the external electrodes  24  of the base wafer  200  under a vacuum. Then, each crystal frame  10  is vibrated to measure its vibration frequencies. The thickness of the excitation respective electrodes  15  is reduced, by reverse-sputtering, according to the measurements. 
         [0062]    In this first embodiment, since each one of the AT-cut crystal frames  10  is initially configured to vibrate at lower frequencies, the desired frequencies are attained by increasing them. This is done by reverse-sputtering performed on the excitation electrodes  15  on the upper surface of the crystal frames  10 . The frequency adjustment is performed on each and every one of the crystal frames  10  on the wafer. The first-lid wafer  300  is used as a mask for performing reverse-sputtering. The bonding zones  33  (except the voids  31 ) are cleaned to provide suitable conditions for the siloxane bonding involving the second-lid wafer  400   a.    
         [0063]    In step S 18 , a second bonding step is performed to seal the crystal vibrating devices  90   a . For sealing, the second-lid wafer  400   a  (formed under a vacuum atmosphere) in step S 14  is bonded to the upper main surface of the first-lid wafer to preserve the quality of the frequency-adjusted crystal frames  10 . In this first embodiment, since the second-lid wafer  400   a  is made of the crystal material, the first-lid wafer  300  and the second-lid wafer  400   a  are bonded together by siloxane bonding after cleaning the bonding surfaces. Again, the orientation flats OF are used as references for positioning the second-lid wafer  400   a  precisely for siloxane bonding. 
         [0064]    In step S 19  the crystal vibrating devices  90   a  are separated from one another by a process called “dicing.” Dicing can be performed using a laser, cutting blade, or other implement, and results in singulation of the crystal vibrating devices from one another. Thus, a few hundreds to a few thousands of individual crystal vibrating devices  90   a  are produced, each having a precisely adjusted vibration frequency. 
         [0065]    After the base wafer  200 , the crystal wafer  100  and the first-lid wafer  300  are bonded together, frequency adjustment is performed as in step S 17 . Therefore, the frequencies of the crystal vibrating pieces  90   a  will be substantially unchanged after steps S 18  and S 19 . 
       Second Embodiment of Crystal Vibrating Device 
       [0066]    A crystal vibrating device  90   b  according to this embodiment comprises a second lid made from a precisely planar second-lid wafer  400   b  having the same shape as the second-lid wafer  400   a  of the first embodiment. However, the material from which the second-lid wafer  400   b  is fabricated is different from the material of the second-lid wafer  400   a  of the first embodiment. Specifically, the second-lid wafer  400   b  is made of a glass material, such as Pyrex glass, borosilicate glass, or soda glass. These are all glasses that contain metal ions such as sodium ions. The description below covers the features of this embodiment that are different from the corresponding features in the first embodiment. In this second embodiment, components that are the same as corresponding components of the first embodiment have the same respective reference numerals and are not described further. 
         [0067]      FIG. 8  is a flow-chart of an embodiment of a method for fabricating the crystal vibrating devices  90   b  of this embodiment. The method depicted in  FIG. 8  differs from the method shown in  FIG. 3  in steps S 123 , S 14 , S 17 , and S 18 . Hence, the following description covers only these steps and includes steps T 11  to T 19  and corresponding steps T 123 , T 14 , T 17 , and T 18  of  FIG. 3 . 
         [0068]    In step T 123 , the AT-cut crystal frames  10  formed on the AT-cut crystal wafer  100  are configured to produce resonance oscillations that are slightly higher in frequency to the desired oscillation frequencies. In other words, assuming the excitation electrodes  15  are made thicker by sputtering at the frequency-adjustment stage, these electrodes are deliberately made thinner when initially formed. During deposition of the chromium and gold layers, the gold layer is initially made thinner than desired, which produces vibration frequencies that are a little higher than ultimately desired. 
         [0069]    In step T 14  the second-lid wafer  400   b  is formed of glass having a desired thickness. 
         [0070]    In step T 17  vibration frequency is adjusted. This step is performed simultaneously on all the crystal frames  10  on the frame wafer  10 , and is performed by sputtering, using the voids  31  on the first-lid wafer  300  as a mask. Thus, an adjustment metal film  35  is formed on the upper surface of the excitation electrode  15  of each crystal frame  10 , similar to the metal film formed on the bonding areas  33  of the first-lid wafer  300 . 
         [0071]      FIG. 9  is a cross-sectional view of the crystal vibrating device  90   b  after performing frequency adjustment. Since each crystal frame  10  is initially configured to vibrate at a frequency slightly higher than desired, frequency adjustment is performed by sputtering additional material onto the target regions, especially onto the upper surface of the vibrating portion  11 . Adding this additional material (as an “adjustment metal film”) to the upper excitation electrode  15  reduces the vibrational resonance frequency of the vibrating portion  11  to the desired level, as shown in  FIG. 9 . 
         [0072]    The adjustment metal film  35  is either the chromium layer overlaid by the gold layer, the gold layer alone, or an aluminum layer alone. If frequency adjustment is unnecessary, or if only a small amount of the adjustment metal film  35  is required because the required adjustment is very small, sufficient metal film cannot be formed on the first-lid wafer  300 . In such a situation, the adjustment metal film may be formed beforehand on the first-lid wafer  300  to be used for the second bonding step, described later. The chromium layer may be formed on the first-lid wafer  300  as a substrate for a subsequently applied gold layer formed by sputtering in the frequency-adjustment step. 
         [0073]    In step T 18  a second bonding step is performed to seal the crystal vibrating devices  90   b . For sealing, the second-lid wafer  400   b  formed in step S 14  is bonded in a vacuum atmosphere to the upper surface of the first-lid wafer  300  after the frequency-adjustment step has been completed (step T 17 ). Bonding involves the adjustment metal film  35  that was formed in the bonding zones  33  on the upper surfaces of the first lids  30   a  and on the upper-surface excitation electrodes  15 . The particular bonding method is anodic bonding that occurs between the adjustment metal film  35  in the bonding zones  33  and the lower surfaces of the second lids  40   b.    
         [0074]    Anodic bonding is an effective way to bond a metal to a glass containing metal ions, as in this embodiment. The metal (adjustment metal film  35 ) and glass (second lid) are placed in contact with each other in a heated vacuum environment or inert-gas atmosphere. Using the metal as an anode, a cathode is placed on the plane facing the bonding plane of the glass material. An electric field is impressed between the metal and the glass, which urges migration of the metal ions (e.g., sodium ions) from the glass to the cathode. As a result, the metal contacting the glass at the bond interface is oxidized and becomes bonded to the glass. 
       Third Embodiment of Crystal Vibrating Device 
       [0075]    To produce crystal vibrating devices  90   c  according to this embodiment, a second-lid wafer  400   c  is used having the same shape as the second-lid wafer  400   a  used in the first embodiment. The second-lid wafer  400   c  is precisely planar, and can be made of any of various materials, including crystal, glass, or ceramic, on which a metal film can be formed. 
         [0076]    Crystal vibrating devices according to this third embodiment can be manufactured using a method having many similarities to the method ( FIG. 9 ) used for manufacturing crystal vibrating devices  90   b  according to the second embodiment. The only differences are in the bonding steps T 14  and T 18 . Referring to  FIG. 9  in the following description of the method according to the third embodiment, only the differences in these steps are described. 
         [0077]    In step T 14 , the second-lid wafer  400   c , made of crystal material, is formed having the desired thickness. As shown in  FIG. 10 , a first bonding metal film  45  is formed on the lower surface of the second-lid wafer  400   c  destined to be bonded to the upper surface of the first-lid wafer  300 .  FIG. 10  is a cross-sectional view of a crystal vibrating piece  90   c  after completion of frequency adjustment. 
         [0078]    The first bonding metal film  45  is applied to the entire lower surface of the second-lid wafer  400   c  by sputtering or other suitable technique. (Alternatively, the first bonding metal film  45  can be applied only to those regions of the lower surface that correspond to the bonding zone  33 , on the upper surface of the first-lid wafer  300 , surrounding the void  31 .) The first bonding metal film  45  is, for example, a gold layer or copper layer applied over a chromium layer. 
         [0079]    If frequency adjustment is not required or if the amount of required adjustment is very small such that only a small amount of the metal film  45  is required, sufficient metal film  45  for bonding cannot be formed on the first-lid wafer  300 . In such a case, sufficient metal film  45  for bonding purposes can be formed beforehand on the upper surface of the first-lid wafer  300 . If the metal film  45  is gold or copper, a chromium layer may be formed beforehand on the upper surface of the first-lid wafer  300  to improve adhesion of the metal layer  45 , especially since it is being used for bonding. 
         [0080]    In step T 18  the crystal vibrating pieces  90   c  are sealed inside their respective packages. For sealing, the second-lid wafer  400   c  formed in step S 14  is bonded to the first-lid wafer in a manner that preserves the quality of the frequency-adjusted crystal frames  10 . The first bonding metal film  45  on the second-lid wafer  400   c  and the adjustment metal film  35  (both either copper or gold) in the bonding zones  33  on the first-lid wafer  300  are bonded together using a conventional pressure-heating technique. The pressure-heating bonding technique is performed using a device that bond wafers together by melting the metal layers formed on the wafer surfaces in regions to be bonded together. First, the wafers are aligned with each other and then are subjected to pressure heating using the pressure-heating device. The pressure-heating device heats the wafers to a specified bonding temperature as the wafers are being compressed at a specified pressure for a specified length of time. Thus, the metal layers in contact with each other bond together, which in turn bonds the wafers together. 
         [0081]    In the second and third embodiments, in step T 123  the vibration frequencies of the AT-cut crystal frames  10  are initially a little higher than ultimately desired. In step T 17  the frequencies are reduced by sputtering, as described earlier. 
         [0082]    The frequencies of the AT-cut crystal frames  10  can be configured so that, during adjustment, the frequencies are first reduced slightly to below the ultimately desired frequencies in the same way as in the first embodiment, followed by reverse-sputtering to increase the frequencies. In such a case, in step T 123  the resonance frequencies of the AT-cut crystal frames  10  formed on the crystal wafer  100  are configured to become a little lower than desired. For the frequency adjustment of these crystal frames  10 , assuming the thickness of the excitation electrodes  15  is adjusted to be made thinner by reverse-sputtering at the frequency-adjustment stage, the excitation electrodes  15  are formed with sufficient thickness to allow performance of the required frequency adjustment. Therefore, the excitation electrodes  15  formed on the upper surface of the first-lid wafer  300  are configured to be initially thicker than their ultimate thickness. 
         [0083]    In  FIG. 10 , when the first-lid wafer  300  is being formed in step T 13 , a second bonding metal film  65  is formed on the upper surface of the first-lid wafer  300 . Since the second bonding metal film  65  is destined to be made thinner by reverse-sputtering in the frequency-adjustment step T 123 , the second bonding metal film  55  is initially formed at a thickness sufficient for performing frequency adjustment. 
         [0084]    In the frequency-adjustment step T 17 , the vibration frequencies are adjusted in the same way as described in the first embodiment. Hence, frequency adjustment can be performed by reverse-sputtering in step T 18 , and the second bonding step S 18  can be performed using the second bonding metal films  55  of sufficient thickness on the excitation electrodes of the crystal frames  10  and on the first-lid wafer  300 . The second bonding step can be performed either by anodic bonding or by pressure heating, depending upon the materials of the bonding plane of the second-lid wafer  400 . 
       Fourth Embodiment of Crystal Vibrating Device 
       [0085]    Crystal vibrating devices  90   d  according to this fourth embodiment comprise tuning-fork type crystal vibrating frames  50  instead of the AT-cut crystal frames  10  used in the first, second, and third embodiments.  FIG. 11  is an exploded oblique view of one of a few hundreds to a few thousands of crystal vibrating devices  90   d  normally formed on a crystal wafer.  FIG. 12  is a cross-sectional view of a vibrating device along the line B-B in  FIG. 11 . A crystal vibrating device  90   d  in this embodiment comprises a tuning-fork type vibrating piece  50 , a first lid  30   b , and a second lid  40   a.    
         [0086]    The tuning-fork type vibrating devices  50  are made of a piezoelectric material. Each tuning-fork type vibrating frame  50  comprises a tuning-fork type vibrating portion  51  and a frame portion  52  circumferentially surrounding the tuning-fork type vibrating portion  51 . 
         [0087]    Each tuning-fork type vibrating portion  51  comprises a pair of vibrating arms  53 , each having a respective groove  54  formed on each main surface of the vibrating arm  53 . The tuning-fork type vibrating portion  51  is connected to the frame portion  52  by supporting arms  55 . 
         [0088]    The distal end of each vibrating arm  53  widens to form a respective “hammer-head.” On these distal ends are “weights”  56  that can be trimmed to achieve adjustment of vibration frequency. The vibrating arms  53  are urged to vibrate whenever a voltage is applied to them. The vibrations are normally very stable. The weights  56  are metal films that can function either whenever an alternating voltage is applied or whenever voltage is not being applied. 
         [0089]    After forming the outline profile of the tuning fork type vibrating portion  51 , the grooves  54  are formed. The weights  56 , excitation electrodes  57 , and extraction electrodes  58  are formed simultaneously. The excitation electrodes  57  are formed on the vibrating arms  53 , and the grooves  54  and extraction electrodes  58  are formed on both main surfaces of the frame portion  52 . 
         [0090]    On the first lid  30   b , voids  34  are formed at respective positions corresponding to the positions of the weights  56  on the vibrating arms  53 . 
         [0091]    Adjusting the vibration frequency of this embodiment can be performed using a technique similar to that described in the first embodiment or the third embodiment. In general, frequency adjustment is performed by reducing the resonance oscillations of the crystal vibrating devices  90   d . I.e., the thickness of each weight  56  is increased by sputtering occurring through the voids  34  on the first lid  30   b . Frequency also can be adjusted by reducing the thicknesses of the weights  56  by reverse-sputtering occurring through the voids  34 . If reverse-sputtering is used, the weights  56  need to be initially formed with sufficient thickness for use of this technique. 
         [0092]    In the embodiments described above, the base wafer  200  or the first-lid wafer  300  may be formed using a material other than a crystal material, in which event the first bonding step must comprise a different bonding method. 
         [0093]    In the fourth embodiment a circular mono-crystalline wafer is used. Alternatively, for example, a rectangular mono-crystalline wafer may be used. 
         [0094]    It will be understood by persons of ordinary skill in the relevant art that the disclosed embodiments are exemplary but not limiting. The invention encompasses various alternatives, modifications, and equivalents that are within the spirit and scope of the invention, including alternatives, modifications, and equivalents of parametric ranges. Although this disclosure describes crystal vibrating devices, it will be understood that the disclosure also applies to, for example, quartz crystal oscillators comprising the subject crystal vibrating devices as well as at least one integrated circuit defining an appropriate oscillation-driving circuit. 
         [0095]    Whereas the foregoing description is made with reference to AT-cut crystal vibrating devices, similar effects can be realized if BT-cut crystal vibrating devices are used. In addition, the shape of an excitation electrode is not limited to the rectangles with rounded corners discussed and shown herein. The shape can be modified variously. For example, the shape can be any of various polygons, such as a hexagon with rounded corners can be used. 
         [0096]    Furthermore, the materials of which the excitation electrodes and extraction electrodes are made are not limited to the metals described above. Alternatively, aluminum (Al) can be used or an alloy of gold (Au) and silver (Ag), wherein the content of gold is 1 to 40% by weight.