Patent Publication Number: US-8987978-B2

Title: Piezoelectric vibrating piece, piezoelectric vibrator, oscillator, electronic apparatus and radio timepiece

Description:
RELATED APPLICATIONS 
     This application claims benefit of priority under 35 U.S.C. §119 to Japanese Patent Application No. 2012-204611, filed Sep. 18, 2012, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piezoelectric vibrating piece, a piezoelectric vibrator, an oscillator, an electronic apparatus and a radio timepiece. 
     2. Description of the Related Art 
     In a cellular phone or portable information terminal devices, a piezoelectric vibrator using quartz crystal and so on is used as a time source, a timing source of a control signal, a reference-signal source and so on. Various types of piezoelectric vibrators have been provided, and a piezoelectric vibrator having a so-called tuning-fork type piezoelectric vibrating piece is known as one of the piezoelectric vibrators. 
       FIG. 16  is a cross-sectional view showing a related-art piezoelectric vibrating piece. 
     As shown in  FIG. 16 , a tuning-fork type piezoelectric vibrating piece  200  includes a pair of vibrating arm portions  210  and  211  arranged in parallel and a base portion (not shown) supporting base end portions of the pair of vibrating arm portions  210  and  211 . Additionally, an electrode film is formed on an outer surface of the piezoelectric vibrating piece  200 , and the pair of vibrating arm portions  210  and  211  can be vibrated in a direction moving close to or away from each other at a predetermined resonant frequency when a voltage is applied to the electrode film. 
     Incidentally, as apparatuses on which the piezoelectric vibrator is mounted are becoming small in size in recent years, the piezoelectric vibrating piece  200  is also desired to be small in size. However, for example, when the width of the vibrating arm portions  210  and  211  is narrowed, the forming width of the electrode film formed on the vibrating arm portions  210  and  211  is also narrowed, as a result, an equivalent series resistance value (crystal impedance (CI) value) is increased and accuracy of an output signal is deteriorated. 
     In response to the above, a structure in which groove portions  212  are formed on both main surfaces of the vibrating arm portions  210  and  211  by etching processing as shown in  FIG. 16  is known. According to the structure, excitation electrodes (not shown) to be paired face to each other on side surfaces of the groove portions  212 , therefore, an electric field can be acted in the facing direction efficiently. Accordingly, even when the width of the vibrating arm portions  210  and  211  is narrowed, the electric field efficiency can be increased and miniaturization can be realized while maintaining a resonant frequency F. 
     An example of the related art includes JP-A 2009-81520. 
     However, in the case where the groove portions  212  are formed in the vibrating arm portions  210  and  211  of the piezoelectric vibrating piece  200 , rigidity of the vibrating arm portions  210  and  211  is reduced. In particular, when the groove portions are formed in the vicinity of a connecting portion between the base end portions of the vibrating arm portions  210 ,  211  and the base portion, it is difficult to obtain sufficient strength of the vibrating arm portions  210  and  211 , and stress concentration may occur at the portion. Accordingly, when an external impact and the like are applied to the piezoelectric vibrating piece  200 , there is a danger that a fracture and so on occur from the vicinity of the connecting portion between the base end portions of the vibrating arm portions  210 ,  211  and the base portion. That is, there is a problem that the rigidity of the vibration arm portions is reduced when the groove portions  212  are provided in the vibrating arm portions  210  and  211 . 
     Moreover, in the case where the groove portions  212  are formed at the vibrating arm portions  210  and  211 , there is the following problem in addition to the above “problem that the rigidity of the vibrating arm portions is reduced”. That is, the groove portions  212  are formed by performing wet etching to a wafer made of quartz crystal or the like by using a mask pattern. The material of quartz crystal or the like has given crystal axes, having a property in which etching speed differs according to the crystal-axis direction. Such property is also called “etching anisotropy”. Specifically, it is known that the etching speed is reduced in the order of Z axis, +X axis, −X axis and Y axis in respective crystal axes (X axis, Y axis and Z axis) of quartz crystal. As the material has the “etching anisotropy”, it is known that the cross-sectional shape of the groove portion  212  obtained after the etching is not a simple rectangle but a shape having inclined surfaces as shown in  FIG. 16 . 
     Here, a portion of the inclined surfaces in the groove portion  212  as shown in  FIG. 16  is called an “etching residue  213 ”. Normally, when the tuning-fork type piezoelectric vibrating piece  200  is formed, the wafer is cut from a rude ore of quartz crystal so that the Z axis of the crystal axes approximately corresponds to a thickness direction of the piezoelectric vibrating piece  200 , the Y axis approximately corresponds to a length direction of the piezoelectric vibrating piece  200  and the X axis approximately corresponds to a width direction of the piezoelectric vibrating piece  200  for obtaining a desired outer shape by the etching processing. At the time of forming the groove portion  212 , the etching residue  213  is generated on the side surface of the groove portion  212  being affected by the etching anisotropy described above. Specifically, the delay occurs in the etching speed as coming from the +X axis direction side toward the −X axis direction side, therefore, a −X axis side surface  212   a  positioned on the −X axis side in side surfaces facing to each other in the X-axis direction in the groove portion  212  will be an inclined surface gradually inclining toward the +X-axis direction as coming toward a bottom portion of the groove portion  212 . Then, the inclined portion will be the etching residue  213  described above. Note that a +X axis side surface  212   b  positioned on the +X axis side will be a side surface parallel to the Z-axis direction (the side surface not having the etching residue  213 ). The etching residues are formed not only on the groove portions  212  but also on side surfaces of the vibrating arm portions  210  and  211 . In  FIG. 16 , the inclinations (etching residues) are formed on side surfaces on the +X axis side in both side surfaces of the vibrating arm portions  210  and  211 . 
     When the etching residue  213  is generated in the groove portion  212 , shapes are different on both sides with respect to a center line O′ dividing each of the vibrating arm portions  210  and  211  in half in the X-axis direction, and the weight balance is lost. As a result, there are problems that variation of drive-level characteristics of the vibrating arm portions  210  and  211  (behavior of the resonant frequency F with respect to a voltage to be applied on the piezoelectric vibrating piece  200 ), increase of the CI value due to vibration leakage and so on occur. That is, when the groove portions  212  are provided on the vibrating arm portions  210  and  211 , there is also “a problem that the vibration balance is lost” due to the etching residue  213  in addition to the “problem that the rigidity of the vibrating arm portions is reduced”. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to provide a piezoelectric vibrating piece, a piezoelectric vibrator, an oscillator, an electronic apparatus and a radio timepiece having the piezoelectric vibrating piece capable of correcting vibration imbalance while suppressing reduction of rigidity in the piezoelectric vibrating piece provided with the groove portions in the vibrating arms. 
     In order to achieve the above object, the present invention provides a tuning-fork type piezoelectric vibrating piece including a pair of vibrating arm portions arranged in parallel to each other, a base portion integrally fixing base end portions of the pair of vibrating arm portions in a length direction, groove portions formed on main surfaces of the pair of vibrating arm portions and extending along the length direction, in which a thickness direction of the vibrating arm portions is a Z axis direction of crystal axes, the length direction of the vibrating arm portions is a Y axis direction of crystal axes and a width direction orthogonal to the length direction and the thickness direction of the vibrating arm portions is an X axis direction of crystal axes, and each of the groove portions includes a first groove portion formed on a tip portion side of the vibrating arm portions and a second groove portion formed closer to the base-end portion side of the vibrating arm portions with respect to the first groove portion, and the second groove portion is arranged so as to be offset in a −X axis direction with respect to the first groove portion in the X axis direction. 
     According to the structure, the groove portion is divided into the first groove portion and the second groove portion in the base end side of the vibrating arm portions, therefore, it is possible to suppress stress concentration to the base end portions of the vibrating arm portions and to increase the rigidity of the vibrating arms as compared with a case where one communicating groove portion is formed in the vibrating arm portion. Furthermore, as the second groove portion is arranged so as to be offset in the −X axis direction with respect to the first groove portion, it is possible to suppress variation in weight on both side portions of each vibrating arm portion in the X axis direction, which is caused by the etching residue generated at the time of forming the first groove portion. That is, the etching residue is generated in the −X axis direction side of the first groove portion when forming the first groove portion, and thus, weight on the −X axis direction side is increased with respect to the center line dividing the first groove portion in the X axis direction. However, when the second groove portion is formed to be offset to the −X axis direction side, the weight in −X axis direction is reduced as compared with the weight in +X axis direction in the second groove portion, therefore, variation in weight balance generated in the first groove portion can be reduced. As a result, it is possible to reduce variation in weight balance of right and left with respect to the center line along a longitudinal direction of the vibrating arm as the vibration center as the whole view of the vibrating arm portions. 
     In the above structure, a groove width of the second groove portion is narrower than a groove width of the first groove portion. 
     According to the structure, as the groove widths of the second groove portions can be narrowed on the base end side of the vibrating arm portions, stress concentration to the base end portions of the vibrating arm portions can be efficiently suppressed, which can further increase the rigidity of the vibrating arm portions. 
     A piezoelectric vibrator according to an embodiment of the invention includes the above piezoelectric vibrating piece and a package having a base substrate and a lid substrate bonded to each other, housing the piezoelectric vibrating piece in a cavity formed between both substrates. Accordingly, it is possible to provide the piezoelectric vibrator having excellent impact resistance and further, having excellent drive level characteristics. 
     In an oscillator according to an embodiment of the present invention, the piezoelectric vibrator is electrically connected to an integrated circuit as a resonator. Moreover, in an electronic apparatus according to an embodiment of the present invention, the piezoelectric vibrator is electrically connected to a timer unit. Furthermore, in a radio timepiece according to an embodiment of the present invention, the piezoelectric vibrator is electrically connected to a filter unit. 
     As the oscillator, the electronic apparatus and the ratio timepiece includes the above-described piezoelectric vibrator, high-quality products having excellent reliability and durability can be provided. 
     According to the embodiment of the present invention, as the piezoelectric vibrating piece provided with the groove portions in the vibrating arms, the piezoelectric vibrating piece capable of correcting the vibration imbalance while suppressing the reduction of rigidity can be provided, and also the piezoelectric vibrator, the oscillator, the electronic apparatus and the radio timepiece, each having the piezoelectric vibrating piece, can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a piezoelectric vibrating piece according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view taken along A-A line of  FIG. 1 ; 
         FIG. 3  is a flowchart showing a manufacturing method of the piezoelectric vibrating piece; 
         FIG. 4  is a process view showing the manufacturing method of the piezoelectric vibrating piece, which is a plan view of a crystal wafer; 
         FIG. 5  is a process view showing the manufacturing method of the piezoelectric vibrating piece, which is a cross-sectional view taken along B-B line of  FIG. 4 ; 
         FIG. 6  is a process view showing the manufacturing method of the piezoelectric vibrating piece, which is a cross-sectional view taken along B-B line of  FIG. 4 ; 
         FIG. 7  is a process view showing the manufacturing method of the piezoelectric vibrating piece, which is a cross-sectional view taken along B-B line of  FIG. 4 ; 
         FIG. 8  is a process view showing the manufacturing method of the piezoelectric vibrating piece, which is a cross-sectional view taken along B-B line of  FIG. 4 ; 
         FIG. 9  is an external perspective view showing a piezoelectric vibrator; 
         FIG. 10  is an inside structure view of the piezoelectric vibrator shown in  FIG. 9 , which is a plan view in a state of removing a lid substrate; 
         FIG. 11  is a cross-sectional view taken along C-C line of  FIG. 10 ; 
         FIG. 12  is an exploded perspective view of the piezoelectric vibrator shown in  FIG. 9 ; 
         FIG. 13  is a view showing an embodiment of the present invention, which is a structure view of an oscillator; 
         FIG. 14  is a view showing an embodiment of the present invention, which is a structure view of an electronic apparatus; 
         FIG. 15  is a view showing an embodiment of the present invention, which is a structure view of a radio timepiece; and 
         FIG. 16  is a cross-sectional view showing a related-art piezoelectric vibrating piece. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, an embodiment of the present invention will be explained based on the drawings. 
     (Piezoelectric Vibrating Piece) 
       FIG. 1  is a plan view of a piezoelectric vibrating piece according to the embodiment of the present invention and  FIG. 2  is a cross-sectional view taken along A-A line of  FIG. 1 . 
     As shown in  FIG. 1  and  FIG. 2 , a piezoelectric vibrating piece  1  according to the embodiment is a tuning-fork type vibrating piece made of a piezoelectric material such as quartz crystal, lithium tantalate and lithium niobate, including a piezoelectric plate  13  having a base portion  12  integrally fixing a pair of vibrating arm portions  10 ,  11  and base end portions of the pair of vibrating arm portions  10 ,  11 , and an electrode film  14  formed on the piezoelectric plate  13 . The piezoelectric plate  13  according to the embodiment is formed by slicing a Lambert ore of quartz crystal at a given angle with respect to an X axis, a Y axis and a Z axis orthogonal to one another as crystal axes of quartz crystal to make a crystal wafer  40  (refer to  FIG. 4 ) and by performing wet etching to the crystal wafer  40  as described later. In the piezoelectric plate  13  according to the embodiment, the Z axis of the crystal axes of quartz crystal approximately corresponds to a thickness direction of the piezoelectric plate  13 , the Y axis approximately corresponds to a length direction of the piezoelectric plate  13  (the vibrating arm portions  10  and  11 ) and the X-axis corresponds to a width direction of the piezoelectric plate  13  (an arrangement direction of the vibrating arm portions  10  and  11 ). 
     The base portion  12  includes a first base portion  21  to which the vibrating arm portions  10  and  11  are connected and a second base portion  22  connected to a base end portion of the first base portion  21  and having a wider width than the first base portion  21  (a so-called two-step base portion type). Additionally, a connecting portion between the first base portion  21  and the base end portion of each of the vibrating arm portions  10 ,  11  and a connecting portion between respective base portions  21  and  22  are inclined surfaces  21   a  and  22   a  in which the width is gradually widened from a tip side toward the base end side along a Y axis direction. Though the shape of the base portion  12  is the two-step base portion type in the present embodiment, the shape of the base portion  12  is not limited to this, and a straight type having no step on a side surface of the base portion  12  or a notch type having a notch in an X axis direction of the base portion  12  may be applied. In the notch type, it is possible to obtain an effect of reducing “vibration leakage” in which vibration of the vibrating arm portions  10  and  11  is leaked to the package side through the base portion  12 . 
     A pair of vibrating arm portions  10  and  11  extends along the Y axis direction as well as formed side by side in parallel to the X axis direction. −X axis side surfaces  10   a  and  11   a  positioned in a −X axis direction in both side surfaces facing in the X axis direction of respective vibrating arm portions  10  and  11  are formed approximately vertical to both main surfaces  10   b  and  11   b . +X axis side surfaces  10   c  and  11   c  positioned in a +X axis direction are formed to be inclined surfaces inclining toward the +X axis direction as coming from the outside along a Z-axis direction (the main surfaces  10   b ,  11   b  side) to a central portion. That is, a portion swelling in the +X axis direction with respect to a first virtual line L 1  extending from an end edge on the +X axis side of each of the both main surfaces  10   b  and  11   b  in each of the vibration arm portions  10  and  11  toward the Z-axis direction correspond to an etching residue  23  generated due to the etching anisotropy in a later-described outline forming process (S 20 ). 
     Groove portions  25  which are concave toward the inside in the Z-axis direction and extending along the Y-axis direction are formed on the both main surfaces  10   b  and  11   b  of respective vibrating arm portions  10  and  11 . Each groove portion  25  further includes a first groove portion  26  formed on a tip portion side along the Y-axis direction of each of the vibrating arm portions  10 ,  11  and a second groove portion  27  formed on a base-end portion side of the vibrating arm portions  10  and  11  with respect to the first groove portion  26 . That is, the first groove portion  26  and the second groove portion  27  are formed side by side in the Y-axis direction on the both main surfaces  10   b  and  11   b  of respective vibrating arm portions  10  and  11  in the present embodiment. It is sufficient that the groove portions  25  are formed in one of the front surface and the rear surface of the vibration arm portions  10  and  11 . 
     The first groove portion  26  extends from the base end portion of each of the vibrating arm portions  10  and  11  to the vicinity of an intermediate portion, which is formed over the entire portion in the Y-axis direction. Additionally, when the vibrating arm portions  10  and  11  are seen from the Z-axis direction, each first groove portion  26  is formed so as to be line symmetry with respect to a center line O passing through the center in the X-axis direction (a central position in the width direction) of each of the main surfaces  10   b  and  11   b  in respective vibrating arm portions  10  and  11 . That is, the first groove portion  26  is formed in each the vibrating arm portions  10  and  11  so that a center line dividing the opening width of the first groove portion  26  in half in the X-axis direction approximately corresponds to the center line O of the vibrating arm. 
     The second groove portion  27  extends from the connecting portion between the vibrating arm portions  10 ,  11  and the base portion  12  to the vicinity of the base end portion of the first groove portion  26 , which is arranged with a gap with respect to the first groove portion  26  in the Y-axis direction. Moreover, in the second groove portion  27 , a length TL2 along the Y-axis direction is shorter than a length TL1 of the first groove portion  26  as well as a width TW2 along the X-axis direction is narrower than a width TW1 of the first groove  26  (refer to  FIG. 1 ). 
     Additionally, as shown in  FIG. 2 , a +X axis side surface  27   a  positioned in the +X axis direction in both side surfaces facing to each other in the second groove portion  27  in the X-axis direction is formed approximately vertical to both main surfaces  10   b  and  11   b . An −X axis side surface  27   b  positioned in the −X axis direction is an inclined surface inclining toward the +X axis direction as coming from the outer side to the inner side along the Z-axis direction. That is, the −X axis side surface  27   b  in the second groove portion  27  inclines so that the width TW2 in the second groove portion  27  in the X-axis direction gradually narrows toward the inner side of the Z-axis direction. 
     In this case, a portion swelling in the +X axis direction with respect to a second virtual line L 2  extending from an end edge on the −X axis side of in an opening edge of the second groove portion  27  to the Z axis direction corresponds to an etching residue  24  generated due to the etching anisotropy in a later-described groove-portion forming process (S 30 ). A bottom surface  27   c  of the second groove portion  27  is formed so as to be parallel to the both main surfaces  10   b  and  11   b.    
     The second groove portion  27  according to the embodiment is formed to be offset in the −X axis direction with respect to the first groove portion  26 . Specifically, the second groove portion  27  is provided so that a center line extending in the Y axis direction of the second groove portion  27  is offset in the −X axis direction from the center line O of the vibrating arm portions  10  and  11 . In further other words, a width WB from an opening edge on the +X axis side of each second groove portion  27  to an end edge on the +X axis side of the both main surfaces  10   b  and  11   b  of each of vibrating arm portions  10  and  11  is wider than a width WA from an opening edge on the −X axis side of each second groove portion  27  to an end edge on the −X axis side of the both main surfaces  10   b  and  11   b  of each of the vibrating arm portions  10  and  11  (WB&gt;WA). 
     Accordingly, a weight on the −X axis direction side with respect to the center line O of each of the vibrating arm portions  10  and  11  is increased in the first groove portion  26  due to the etching residue, and the weight balance between both sides of the center line O of each of the vibrating arm portions  10  and  11  in the X axis direction varies, however, the weight balance between both sides of the center line O in the X axis direction of each of the vibrating arm portions  10  and  11  is approximately equal or the weight in the +X axis direction is increased in the region where the second groove portion  27  is formed as the second groove portion  27  is formed to be offset in the in the −X axis direction. Accordingly, as the whole view of the vibrating arm portions  10  and  11 , variation in the weight balance generated in the first groove portion  26  (variation caused by the increase of weight on the −X axis direction side) is reduced by the weight balance in the second groove portion  27 , therefore, the weight balance between both sides of the center line O in the X axis direction can be approximately equivalent. The weight balance in the root (base-end side) largely affects the vibration of the vibrating arm portions  10  and  11  as compared with the weight balance of the tip side of the vibrating arm portions  10  and  11 . Accordingly, in the case where the weight balance in the whole vibrating arm portions  10  and  11  is not achieved in both sides in the X-axis direction even when the second groove portion  27  is formed to be offset (in the case where the weight on the −X axis direction side is still higher than the weight on the +X axis direction side), drive level characteristics of the vibrating arm portions  10  and  11  can be largely increased and thus an accurate and stable resonant frequency can be obtained as long as the weight balance between both sides of the center line O in the X axis direction is achieved in the second groove portion  27 . 
     Additionally, the electrode film  14  for allowing these vibrating arms to perform bending vibration is formed in the vibrating arm portions  10  and  11 . The electrode film  14  includes a first excitation electrode  31  and a second excitation electrode  32  formed in the pair of vibrating arm portions  10  and  11  for vibrating the pair of vibrating arm portions  10  and  11 , a first mount electrode  33  and a second mount electrode  34  electrically connected to the first excitation electrode  31  and the second excitation electrode  32  respectively, and lead-out electrodes  35 ,  36  electrically connecting the excitation electrodes  31 ,  32  to the mount electrodes  33 ,  34  respectively. The electrode film  14  is formed by, for example, a stacked film of chrome (Cr) and gold. 
     The excitation electrodes  31  and  32  are formed on an outer surface of the pair of vibrating arm portions  10  and  11 . The excitation electrodes  31  and  32  vibrate the pair of vibrating arm portions  10  and  11  so as to move close to or away from each other in the X axis direction at a given resonant frequency F when a voltage is applied. 
     The pair of excitation electrodes  31  and  32  is formed by being patterned respectively in an electrically separated state on the outer surfaces of the pair of vibrating arm portions  10  and  11 . 
     Specifically, the first excitation electrode  31  is mainly formed on an inner surface of the groove portion  25  of the vibrating arm  10 , the +X axis side surface  11   c  and the −X axis side surface  11   a  of the vibrating arm  11 . The second excitation electrode  32  is mainly formed on an inner surface of the groove portion  25  of the vibrating arm  11 , +X axis side surface  10   c  and the −X axis side surface  10   a  of the vibrating arm  10 . 
     The mount electrodes  33  and  34  are formed side by side in the X axis direction at base end portions on the main surface of the base portion  12 . 
     The first lead-out electrode  35  of the lead-out electrodes  35  and  36  connects the first excitation electrode  31  to the first mount electrode  33 , and the second lead-out electrode  36  connects the second excitation electrode  32  to the second mount electrode  34 . It is sufficient that the mount electrodes  33 ,  34  and the lead-out electrodes  35 ,  36  are formed at least one main surface of both main surfaces of the piezoelectric plate  13 . 
     At tip portions of the vibrating arm portions  10  and  11 , a weighted metal film  37  including a coarse adjustment film  37   a  and a fine adjustment film  37   b  for performing adjustment (frequency adjustment) so that the arm portions vibrate within a given frequency. As the frequency adjustment is performed by using the weighted metal film  37 , the frequency of the pair of vibrating arm portions  10  and  11  can fall within the range of a nominal frequency of the device. 
     (Manufacturing Method of Piezoelectric Vibrator) 
     Next, a manufacturing method of the piezoelectric vibrating piece  1  will be explained. 
       FIG. 3  is a flowchart showing a manufacturing method of the piezoelectric vibrating piece.  FIG. 4  to  FIG. 8  are process views for explaining the manufacturing method of the piezoelectric vibrating piece, and  FIG. 5  to  FIG. 8  are cross-sectional views corresponding to B-B line of  FIG. 4 . 
     First, as shown in  FIG. 3  and  FIG. 4 , a Lambert ore of quartz crystal is sliced at a given angle to make the crystal wafer  40  having a predetermined thickness (S 10 ). At this time, the crystal wafer is sliced so that the Z axis approximately corresponds to the thickness direction of the piezoelectric vibrating piece  1 , the Y axis approximately corresponds to the length direction of the piezoelectric vibrating piece  1  (the vibrating arm portions  10  and  11 ) and the X-axis corresponds to the width direction of the piezoelectric vibrating piece  1  (an arrangement direction of the vibrating arm portions  10  and  11 ). Next, after the crystal wafer  40  is wrapped and coarse processing is performed, an affected layer is removed by etching, then, mirror processing such as polishing is performed to obtain a given thickness. 
     Subsequently, the outline forming process (S 20 ) in which outer shapes of plural piezoelectric plates  13  are formed on the crystal wafer  40  is performed as shown in  FIG. 3  to  FIG. 5 . The outline forming process (S 20 ) has a mask-pattern forming process (S 21 ) in which an etching protection film  41  is formed on both main surfaces of the crystal wafer  40  and a mask pattern  42  corresponding to the outer shapes of the piezoelectric vibrating pieces  1  from the etching protection film  41  by a photolithography technique, and an etching process (S 22 ) in which etching processing is performed from the sides of both main surfaces of the crystal wafer  40  by wet etching using the mask pattern  42  as a mask. 
     In the mask-pattern forming process (S 21 ), first, the etching protection film  41  is formed over the whole of both main surfaces of the crystal wafer  40 . As the etching protection film  41 , for example, a metal film formed by stacking a base film made of chrome and a finishing film made of gold can be cited, which is formed by deposition using a sputtering method, a deposition method and so on. 
     Then, the mask pattern  42  corresponding to the outer shapes of the piezoelectric vibrating pieces  1  is formed by patterning the etching protection film  41 . Specifically, after a not-shown photoresist film is formed on the etching protection film  41 , patterning is performed, for example, to be the outer shapes of the piezoelectric vibrating pieces  1  by a normal photoresist technique. Then, etching processing is performed by using the photoresist film as a mask to selectively remove portions of the etching protection film  41  which are not masked. Then, after etching processing, the photoresist film used as the mask is removed. 
     Next, etching processing (wet etching) is performed by using the patterned mask pattern  42  as a mask (S 22 ). Specifically, the crystal wafer  40  on which the mask pattern  42  is formed is dipped in a not-shown etching etchant (for example, a fluorine-based etchant) for a predetermined period of time to selectively remove portions in the crystal wafer  40  not masked by the mask pattern  42 . Accordingly, the crystal wafer  40  can be etched in accordance with the shape of the mask pattern  42 , thereby forming the outer shapes of the piezoelectric plates  13 . Note that plural piezoelectric plates  13  are connected to the crystal wafer  40  through not-shown connecting portions until a subsequent cutting process (S 70 ) is performed. 
     Next, as shown in  FIG. 7 , the groove-portion forming process (S 30 ) of forming the groove portions  25  (refer to  FIG. 1 ) on the both main surfaces  10   a ,  11   b  of the vibrating arm portions  10  and  11  in the piezoelectric plates  13 . Specifically, the mask pattern  42  is patterned again so that forming regions of the groove portions  25  open in the above mask pattern  42 . In this case, forming regions of the first groove portions  26  are formed so as to be line symmetry with respect to the center line O, and forming regions  42   a  of the second groove portions  27  are formed so as to be offset in the −X axis direction with respect to center line O in opening portions of the mask pattern  42 . Then, etching processing (wet etching) is performed by using the patterned mask pattern  42  as the mask in the same manner as the above outline forming process (S 20 ). Accordingly, regions not masked by the mask pattern  42  in the crystal wafer  40  are selectively removed as shown in  FIG. 8 , thereby forming the groove portions  25  respectively on the both main surfaces  10   b  and  11   b  of pairs of vibrating arm portions  10  and  11 . After that, the mask pattern  42  is removed. 
     In the outline forming process (S 20 ) and the groove-portion forming process (S 30 ), the above-described etching residues  23 ,  24  are generated by being affected by the etching anisotropy. In this case, as etching time of the groove-portion forming process (S 30 ) is set to be shorter than the outline forming process (S 20 ), the etching residues  24  generated in the groove-portion forming process (S 30 ) will be larger than the etching residues  23  generated in the outline-forming process (S 20 ). 
     Next, an electrode forming process (S 40 ) of respectively forming the excitation electrodes  31 ,  32 , the mount electrodes  33 ,  34  and the lead-out electrodes  35 ,  36  on plural piezoelectric plates  13  is performed by patterning the electrode film  14  using, for example, the sputtering method and the like. Moreover, a weighted-metal film forming process (S 50 ) of forming the weighted metal films  37  at tips of the pairs of vibrating arm portions  10  and  11  is performed. 
     The electrode forming process (S 40 ) and the weighted-metal film forming process (S 50 ) may be performed in separate processes or may be performed at the same time in the same process. 
     Next, a coarse adjustment process (S 60 ) of coarsely adjusting frequencies is performed to all vibrating arm portions  10  and  11  formed on the crystal wafer  40 . At this time, frequencies of all vibrating arm portions  10  and  11  formed on the crystal wafer  40  are simultaneously measured, and trimming amounts are calculated based on the difference between measured frequencies and a predetermined target frequency. Then, the coarse adjustment films  37   a  of the weighted metal films  37  are irradiated with laser light to evaporate part of the films, thereby removing the coarse adjustment films  37   a  in accordance with the trimming amounts. The fine adjustment adjusting the resonant frequency F more accurately is performed after the piezoelectric vibrating piece  1  is mounted. 
     Next, a cutting process (S 70 ) of cutting connecting portions connecting between the crystal wafer  40  and the piezoelectric plates  13  and separating plural piezoelectric plates  13  from the crystal wafer  40  into individual pieces is performed. 
     According to the above processes, plural tuning-fork type piezoelectric vibrating pieces  1  are manufactured from the single crystal wafer  40  at a time. 
     As described above, the present embodiment has a structure in which the first groove portions  26  formed on the tip portion side of the vibrating arm portions  10  and  11  extending along the Y axis and the second groove portions  27  formed on the base-end portion side of the vibrating arm portions  10  and  11  extending along the Y axis with respect to the first groove portions  26 , in which the width along the X axis is narrower than the first groove portions  26 . 
     When applying the structure, the width TW2 of the second groove portion  27  is narrower than the width TW1 of the first groove portion  26 , thereby increasing the rigidity of the base end portions in the vibrating arm portions  10  and  11 , therefore, it is possible to suppress the stress to be concentrated on the base end portions of the vibrating arm portions  10  and  11 . As a result, it is possible to prevent occurrence of fracture and so on of the piezoelectric vibrating piece  1  even when an external impact and the like are applied to the piezoelectric vibrating piece  1 . 
     Particularly, the second groove portion  27  is formed to be offset in the −X axis direction with respect to the center line O of each of the vibrating arm portions  10  and  11 . 
     According to the structure, it is possible to suppress variation in weight balance of the vibrating arm portions  10  and  11  due to the etching residues  24  generated at the time of forming a groove portion. Accordingly, the weight balance of the vibrating arm portions  10  and  11  can be kept, which can improve drive level characteristics and can reduce the CI value by suppressing vibration leakage. As a result, it is possible to provide the piezoelectric vibrating piece  1  capable of reducing the size while suppressing reduction of rigidity and having excellent vibration characteristics. 
     Also in the present embodiment, the first groove portion  26  formed without being offset with respect to the vibrating arm portions  10  and  11 , that is, the first groove portion  26  is formed so that the center line of the first groove portion  26  approximately corresponds to the center line O of each of the vibrating arm portions  10  and  11 . However, the shape of the first groove portion  26  is not limited to this, and it is also preferable that the first groove portion  26  may be offset in the −X axis direction side. According to the structure, variation in weight balance due to the etching residue of the first groove portion  26  can be reduced by alignment of the first groove portion  26  in the X axis direction. When the second groove portion  27  is also formed to be offset with respect to the first groove portion  26  in addition to the above, it is possible to reduce variation in weight balance of the vibrating arm portions  10  and  11  more effectively. 
     Although the groove width of the first groove portion  26  is formed to be larger than the groove width of the second groove portion  27 , the size relation of the groove width is not limited to the above. That is, the groove width may be the same in the groove portion  26  and the groove portion  27  as long as the rigidity of vibrating arms can be secured even when the groove width of the second groove portion  27  is not narrowed. 
     Next, a piezoelectric vibrator  50  including the above piezoelectric vibrating piece  1  will be explained.  FIG. 9  is an external perspective view showing the piezoelectric vibrator and  FIG. 10  is an inside structure view of the piezoelectric vibrator shown in  FIG. 9 , which is a plan view in a state of removing a lid substrate.  FIG. 11  is a cross-sectional view taken along C-C line of  FIG. 10 , and  FIG. 12  is an exploded perspective view of the piezoelectric vibrator shown in  FIG. 9 . In the following description, structures common to the above structures are denoted by the same reference numerals and explanation thereof is omitted. Also in the embodiment, the etching residues  23 ,  24 , the electrode film  14 , the weighted metal film  37  and the like are not shown. 
     As shown in  FIG. 9  to  FIG. 12 , the piezoelectric vibrator  50  according to the embodiment is a surface mounted type including a package  53  in which a base substrate  51  and a lid substrate  52  are bonded, for example, by anodic bonding, or through a not-shown bonding film, solder, a brazing material and so on, and the piezoelectric vibrating piece  1  housed in a cavity C formed inside the package  53 . The explanation will be made in a case of using a glass material as a material for the package, however, the package capable of housing the piezoelectric vibrating piece according to the invention is not limited to this, and a ceramic package using a ceramic material as the base substrate and a ceramic or metal lid as the lid substrate may be applied. 
     The base substrate  51  and the lid substrate  52  are transparent insulating substrates made of a glass material such as soda-lime glass, which is formed to have an approximately plate shape. In the lid substrate  52 , a concave portion  52   a  for housing the piezoelectric vibrating piece  1  on a bonding surface side to which the base substrate  51  is bonded. The concave portion  52   a  forms the cavity C housing the piezoelectric vibrating piece  1  when the base substrate  51  and the lid substrate  52  overlap each other. 
     In the base substrate  51 , a pair of through holes  54  and  55  penetrating the base substrate  51  in the thickness direction is formed. The through holes  54  and  55  are formed at positions to be housed inside the cavity. In more detail, the through holes  54  and  55  according to the embodiment are provided so that one through hole  54  is formed at a position corresponding to the base portion  12  side of the mounted piezoelectric vibrating piece  1  and the other through hole  55  is formed at a position corresponding to the tip portion side of the vibrating arm portion  11 . 
     In the pair of through holes  54  and  55 , a pair of through electrodes  56  and  57  formed so as to fill in these through holes  54  and  55  is formed. These through electrodes  56  and  57  are, for example, conductive core materials integrally fixed to the through holes  54  and  55 , which are formed so that both ends are flat as well as to be approximately the same thickness as the thickness of the base substrate  51 . Accordingly, electrical conductivity is secured on both surfaces of the base substrate  51  while maintaining airtightness inside the cavity C. 
     The through electrodes  56  and  57  are not limited to the above structure, and may be formed by, for example, inserting not-shown metallic pins into the through holes  54  and  55 , filling between the through holes  54 ,  55  and the metallic pins with glass frit and calcining them. It is further preferable that through electrodes  56  and  57  may be conductive adhesive buried in the through holes  54 ,  55 . 
     On an upper surface of the base substrate  51  (a bonding surface to which the lid substrate  52  is bonded), a pair of layout electrodes  61  and  62  are patterned. Additionally, bumps B respectively made of gold and the like are formed on the pair of layout electrodes  61  and  62 , and the pair of mount electrodes  33  and  34  are mounted by using the bumps B. Accordingly, one mount electrode  33  of the piezoelectric vibrating piece  1  is electrically connected to one through electrode  56  through one layout electrode  61 , and the other mount electrode  34  is electrically connected to the other through electrode  57  through the other layout electrode  62 . 
     On a lower surface of the base substrate  51 , a pair of external electrodes  64  and  65  is formed. The pair of external electrodes  64  and  65  is formed on both end portions of the base substrate  51  in a longitudinal direction and is electrically connected to the pair of through electrodes  56  and  57 . 
     When the piezoelectric vibrator  50  having the above structure is activated, a given drive voltage is applied to the external electrodes  64  and  65  formed on the base substrate  51 . Accordingly, it is possible to allow electric current to flow in the excitation electrodes of the piezoelectric vibrating piece  1 , which can vibrate the pair of vibrating arm portions  10  and  11  in a direction moving close to or away from each other at a predetermined frequency. Additionally, the piezoelectric vibrator  50  can be used as a time source, a timing source of a control signal, a reference-signal source and so on by utilizing the vibration of the pair of vibrating arm portions  10  and  11 . 
     As the piezoelectric vibrator  50  according to the embodiment includes the high-quality and small piezoelectric vibrating piece  1  having stable vibration characteristics, in which breakage due to the external impact hardly occurs in the vibrating arm portions  10  and  11 , therefore, the high-quality piezoelectric vibrator  50  having improved reliability and durability in activation can be realized. 
     (Oscillator) 
     Next, an oscillator according to an embodiment of the present invention will be explained with reference to  FIG. 13 . 
     An oscillator  100  according to the embodiment uses the piezoelectric vibrator  50  as a resonator electrically connected to an integrated circuit  101  as shown in  FIG. 13 . The oscillator  100  includes a substrate  103  on which an electronic component  102  such as a capacitor is mounted. The integrated circuit  101  for the oscillator is mounted on the substrate  103  and the piezoelectric vibrator  50  is mounted in the vicinity of the integrated circuit  101 . The electronic component  102 , the integrated circuit  101  and the piezoelectric vibrator  50  are electrically connected to one another by a not-shown wiring pattern. Note that respective components are molded by a not-shown resin. 
     In the oscillator  100  having the above structure, when a voltage is applied to the piezoelectric vibrator  50 , the piezoelectric vibrating piece  1  inside the piezoelectric vibrator  50  vibrates. The vibration is converted into an electric signal by piezoelectric characteristics possessed by the piezoelectric vibrating piece  1  and inputted into the integrated circuit  101  as the electric signal. Various processing is performed to the inputted electric signal by the integrated circuit  101  and outputted as a frequency signal. 
     Accordingly, the piezoelectric vibrator  50  functions as the resonator. 
     In the configuration of the integrated circuit  101 , for example, an RTC (real time clock) module is selectively set according to a request, thereby adding functions of controlling a single-function oscillator for a timepiece, operation dates, time of the device or external devices, as well as providing time, a calendar and so on. 
     As described above, the oscillator  100  according to the embodiment includes the high-quality piezoelectric vibrator  50  having improved reliability and durability in activation, therefore, it is possible to provide the oscillator  100  having high quality with excellent reliability and durability as well as capable of obtaining a stable and highly-accurate frequency signal for a long period of time. 
     (Electronic Apparatus) 
     Next, an electronic apparatus according to an embodiment of the present invention will be explained with reference to  FIG. 14 . The explanation will be made by citing a portable information device  110  including the above piezoelectric vibrator  50  as an example of the electronic apparatus. First, the portable information device  110  according to the embodiment is typified by a cellular phone, which is obtained by developing and improving a wrist watch in related art. An appearance of the device is analogous to a wrist watch, and a liquid crystal display is arranged at a portion corresponding to an hour plate to thereby display present time and the like on a screen thereof. When using the device as a communication tool, communication can be performed similarly as a cellular phone in related art, by removing the device from a wrist and using a speaker and a microphone included in an inside portion of a band. However, size and weight of the device has been drastically reduced as compared with the related-art cellular phone. 
     Subsequently, a configuration of the portable information device  110  according to the embodiment will be explained. The portable information device  110  includes the piezoelectric vibrator  50  and a power supply unit  111  for supplying electric power as shown in  FIG. 14 . The power supply unit  111  is formed by, for example, a lithium secondary battery. A control unit  112  performing various control, a timer unit  113  performing counting of time and the like, a communication unit  114  performing communication with the outside, a display unit  115  displaying various information and a voltage detection unit  116  detecting voltages of respective function units are connected in parallel to the power supply unit  111 . Then, electric power is supplied to respective function units by the power supply unit  111 . 
     The control unit  112  controls respective function units to control operations in the entire system such as transmission/reception of audio data and measurement/display of present time. The control unit  112  also includes a ROM in which programs are previously written, a CPU reading and executing programs written in the ROM, a RAM used as a work area of the CPU and so on. 
     The timer unit  113  includes an integrated circuit having an oscillating circuit, a register circuit, a counter circuit, an interface circuit and so on, and the piezoelectric vibrator  50 . When the voltage is applied to the piezoelectric vibrator  50 , the piezoelectric vibrating piece  1  vibrates, and the vibration is converted into an electric signal by piezoelectric characteristics possessed by quartz crystal to be inputted into the oscillating circuit as the electric signal. An output of the oscillating circuit is binarized and counted by the register circuit and the counter circuit. Then, signal transmission/reception is performed with respect to the control unit  112  through the interface circuit, and present time/present date or calendar information and so on are displayed on the display unit  115 . 
     The communication unit  114  has similar functions as the related-art cellular phone, including a radio unit  117 , an audio processing unit  118 , a switching unit  119 , an amplification unit  120 , an audio input/output unit  121 , a telephone-number input unit  122 , a ring-tone generation unit  123  and a call-control memory unit  124 . 
     The radio unit  117  performs transmission/reception of various data such as audio data with respect to base stations through an antenna  125 . The audio processing unit  118  encodes and decodes an audio signal inputted from the ratio unit  117  or the amplification unit  120 . The amplification unit  120  amplifies a signal inputted from the audio processing unit  118  or the audio input/output unit  121  to a given level. The audio input/output unit  121  is formed by a speaker, a microphone and the like, amplifying a ring tone or receiver audio as well as collecting audio. 
     The ring-tone generation unit  123  generates the ring tone in accordance with calling from the base station. Only when receiving a call, the switching unit  119  switches the amplification unit  120  connected to the audio processing unit  118  to the ring-tone generation unit  123 , the ring tone generated in the ring-tone generation unit  123  is outputted to the audio input/output unit  121  through the amplification unit  120 . 
     The call-control memory unit  124  stores programs concerning incoming/outgoing call control of communication. The telephone-number input unit  122  has, for example, number keys from “0” to “9” and other keys, and a telephone number of a called party and so on is inputted by pressing these number keys. 
     When the voltage applied to respective function units such as the control unit  112  by the power supply unit  111  becomes lower than a given value, the voltage detection unit  116  detects the voltage decrease and notifies the control unit  112  of the decrease. The given voltage value set at this time is a value previously set as the minimum voltage necessary for stably operating the communication unit  114 , which is for example, approximately 3V. The control unit  112  which has received notification of voltage decrease from the voltage detection unit  116  prohibits operations of the radio unit  117 , the audio processing unit  118 , the switching unit  119  and the ring-tone generation unit  123 . Particularly, stop of the operation of the radio unit  117  having large power consumption is fundamental. Moreover, information indicating that the communication unit  114  is unavailable due to the insufficient remaining amount of a battery is displayed on the display unit  115 . 
     That is, it is possible to prohibit the operation of the communication unit  114  and display the prohibition on the display unit  115  by the voltage detection unit  116  and the control unit  112 . The display may be made as a message of characters, or it is also preferable that a cross mark is put as a more intuitive display on a telephone icon displayed on an upper part of a display surface of the display unit  115 . 
     It is possible to stop the function of the communication unit  114  more positively by providing a power-off unit  126  which can selectively power off portions concerning the function of the communication unit  114 . 
     As described above, as the portable information device  110  according to the embodiment includes the high-quality piezoelectric vibrator  50  having improved reliability and durability in activation, it is possible to provide the high-quality portable information device  110  having excellent reliability and durability as well as capable of displaying stable and highly-accurate timepiece information for a long period of time. 
     (Radio Timepiece) 
     Next, a radio timepiece according to an embodiment of the present invention will be explained with reference to  FIG. 15 . 
     A radio timepiece  130  according to the embodiment includes the piezoelectric vibrator  50  electrically connected to a filter unit  131  as shown in  FIG. 15 , which is a timepiece having a function of receiving standard radio waves including timepiece information and displaying accurate time after performing automatic correction. 
     In Japan, there are transmitting stations (transmitter stations), which transmit standard radio waves, in Fukushima prefecture (40 kHz) and Saga prefecture (60 kHz), and respectively transmit standard radio waves. Since long waves such as 40 kHz or 60 kHz have both a property to propagate the ground surface and a property to propagate while being reflected between an ionized layer and the ground surface, therefore, a wide range of propagation is achieved, so that the above-described two transmitting stations cover the entire part of Japan. 
     Hereinafter, a functional configuration of the radio timepiece  130  will be explained in detail. 
     An antenna  132  receives a long-wave standard radio wave of 40 kHz or 60 kHz. The long standard radio wave is time information referred to as a time code and subjected to an AM modulation to a carrier wave of 40 kHz or 60 kHz. The received long standard wave is amplified by an amplifier  133  and is filtered and synchronized by the filter unit  131  having plural piezoelectric vibrators  50 . 
     The piezoelectric vibrators  50  according to the embodiment respectively include quartz vibrator units  138  and  139  having resonant frequencies of 40 kHz and 60 kHz which are the same as the above-described carrier frequencies, respectively. 
     Moreover, the filtered signal having a given frequency is detected and demodulated by a detection/rectification circuit  134 . 
     Subsequently, the time code is acquired through a waveform shaping circuit  135 , and counted by a CPU  136 . The CPU  136  reads information such as the current year, day of year, day of the week, time of day and the like. The read information is reflected on an RTC  137 , and correct time of day information is displayed. 
     Since the carrier wave has 40 kHz or 60 kHz, vibrators having the above-described turning-fork type structure are suitable for the quarts vibrator units  138  and  139 . 
     The above description is based on an example in Japan, and frequencies of the long standard radio waves are different in foreign countries. For example, in Germany, a standard radio wave of 77.5 kHz is used. Therefore, when the radio timepiece  130  which can be used in the foreign countries is incorporated in a portable apparatus, another piezoelectric vibrator  50  having a frequency different from that in Japan is required. 
     As described above, the radio timepiece  130  according to the embodiment has the high-quality piezoelectric vibrator  50  having improved reliability and durability in activation, therefore, it is possible to provide the high-quality radio timepiece  130  having excellent reliability and durability as well as capable of counting time stably and highly accurately for a long period of time. 
     Although the embodiments of the present invention have been described as the above with reference to the drawings, the specific structure is not limited to the above embodiments and includes design modification and so on not departing from the gist of the invention. 
     For example, the surface-mounted type piezoelectric vibrator  50  has been explained by being cited as an example of the piezoelectric vibrator, however, the present invention is not limited to the piezoelectric vibrator  50 . For example, a cylinder-package type piezoelectric vibrator and a surface-mounted type piezoelectric vibrator formed by fixing the cylinder-package type piezoelectric vibrator by being molded by a mold resin portion can be applied. 
     Also in the above embodiment, the case where the piezoelectric vibrating piece  1  is mounted by bumps made of gold and so on has been explained, however, it is not limited to this, and for example, the piezoelectric vibrating piece  1  may be mounted on a ceramic substrate by using conductive adhesive. 
     Furthermore, the offset amount of the second groove portion  27  can be appropriately changed in design depending on the etching residue  24 . Additionally, the width, length, shape and the like of the first groove portion  26  and the second groove portion  27  can be appropriately changed in design. 
     Additionally, it is appropriately possible to replace the components according to the above embodiment with well-known components in a scope not departing from the gist of the invention, or above modification examples may be appropriately combined. 
     (Reference Example) 
     In the above description, the type in which the first groove portion  26  and the second groove portion  27  are provided has been explained, however, it is not always necessary to provide the second groove portion  27  when the device gives weight to a point of suppressing variation in weight balance in the X axis direction of the vibrating arm portions  10  and  11 . In such case, only the first groove portions  26  are provided at the vibrating arm portions  10  and  11 , and further, each first groove portion  26  is offset in the −X axis direction with respect to the center line O in each of the vibrating arm portions  10  and  11 . According to the above structure, it is possible to reduce variation in weight balance of the vibrating arms  10  and  11  caused by the etching residues of the first groove portions  26 . 
     That is, a tuning-fork type piezoelectric vibrating piece including a pair of vibrating arm portions arranged in parallel to each other, a base portion integrally fixing base end portions of the pair of vibrating arm portions in a length direction, groove portions formed on main surfaces of the pair of vibrating arm portions and extending along the length direction, in which a thickness direction of the vibrating arm portions is a Z axis direction of crystal axes, the length direction of the vibrating arm portions is a Y axis direction of crystal axes and a width direction orthogonal to the length direction and the thickness direction of the vibrating arm portions is an X axis direction of crystal axes, and each of the groove portions is arranged so as to be offset in a −X direction with respect to a center line extending in the length direction of each of the vibrating arm portions. 
     Similarly, the piezoelectric vibrator, the oscillator, the electric apparatus, the radio timepiece can obtain an accurate output signal having excellent drive level characteristics by applying the piezoelectric vibrating piece. 
     Additionally, the length of the groove portions is not particularly limited in this case, and it is sufficient that the groove portions extend from the base end portions of the vibrating arms to the vicinity of middle portions of the vibrating arms. The configurations of the piezoelectric vibrator, the oscillator, the electric apparatus, the radio timepiece are as described above. 
     The offset amount may be determined in view of the amount of etching residues, however, when the offset amount is large, it is difficult to form the excitation electrodes on the main surface on the −X axis direction side on the main surface of the vibrating arm portions, therefore, it is necessary to consider the easiness in forming the excitation electrodes as the upper limit of the offset amount.