Patent Publication Number: US-2005140252-A1

Title: Tuning fork type piezoelectric resonator element and method for producing a tuning fork type piezoelectric resonator

Description:
TECHNICAL FIELD  
      The present invention relates to a tuning fork type piezoelectric resonator element and a method for producing a tuning fork type piezoelectric resonator. More particularly, the present invention relates to a tuning fork type piezoelectric resonator element that is required to be highly reliable and a method for producing a tuning fork type piezoelectric resonator.  
     BACKGROUND ART  
      One type of tuning fork type piezoelectric resonator is a cylinder tuning fork type piezoelectric resonator having a tuning fork type piezoelectric resonator element disposed in a cylindrical container.  FIG. 5  is a sectional view of a related cylinder tuning fork type piezoelectric resonator. A tuning fork type piezoelectric resonator element  100  comprises a base  102  and a plurality of resonating arms  104  extending from the base  102 . An excitation electrode (not shown) is disposed at each vibratory arm  104 , and mount electrodes  106  for connection to the excitation electrodes are disposed at the base  102 . The tuning fork type piezoelectric resonator element  100  is disposed in a cylindrical container  108  having one open end so that the base  102  opposes the open end of the container  108 . The container  108  is hermetically sealed by joining a plug  116  to the open end of the container  108 . The plug  116  is formed by hermetically sealing a plurality of lead terminals  114  comprising inner leads  110  and outer leads  112 .  
      The tuning fork type piezoelectric resonator element  100  is small in size which permits a large number of tuning fork type piezoelectric resonator elements to be formed from one piece of wafer. Therefore, the width of the base  102  of the tuning fork type piezoelectric resonator element  100  is smaller than the distance between the lead terminals  114 .  
      Consequently, the ends of the leads  110  adjacent the mount electrodes  106  (“inner ends”) are bent with a jig so as to reduce the distance between the lead terminals  114  in accordance with the width of the base  102 . Thereafter, the inner leads  110  and the mount electrodes  106  are electrically and mechanically joined with solder  118 . An arrangement of a tuning fork cylinder piezoelectric resonator formed in this manner is disclosed in Japanese Unexamined Patent Application Publication No. 59-225605.  
      In recent years, vehicles are computerized and have various electronic devices installed which require time synchronization. A tuning fork type piezoelectric resonator is installed in a vehicle in order to generate a control clock for such various electronic devices. Since a tuning fork type piezoelectric resonator installed in a vehicle is constantly being vibrated, it uses a cylindrical metallic container  108 , such as that shown in  FIG. 5 , in order to prevent breakage by the vibration. In the tuning fork type piezoelectric resonator for vehicle use, mount electrodes  106  and lead terminals  114  at a piezoelectric resonator element  100  are joined with solder that provides excellent vibration resistance.  
      The tuning fork type piezoelectric resonator for vehicle use may be disposed in the engine compartment of a vehicle. In this location the tuning fork type piezoelectric resonator is exposed to varying temperatures depending upon the operating condition of the vehicle. More specifically, in midwinter, the temperature in the engine compartment may be less than 0° C. when the engine is stopped, whereas the temperature may rise to about 100° C. when the engine is operating. Therefore, the tuning fork type piezoelectric resonator for vehicle use is required to be highly reliable so that, for example, it will operate stably over a long period of time and in a wide temperature range which can vary from −40° C. to +125° C. Consequently, in the tuning fork type piezoelectric resonator for vehicle use, the mount electrodes  106  and the lead terminals  114  are joined with a high temperature solder preferably containing 90 wt % lead (Pb) and 10 wt % tin (Sn).  
      However, when the tuning fork type piezoelectric resonator is disposed in the engine compartment of an automobile where temperature variations fluctuate repeatedly between high and low temperatures and over a long period of time, the solder particles joining the mount electrodes and the lead terminals may diffuse due to temperature stress. This may cause the diffused solder to protrude from each mount electrode  106  resulting in the diffused solder from adjacent mount electrodes  106  making contact with one another which may cause shorting of the mount electrodes. In addition, the resonating arms may be chipped or bent when the container of the tuning fork type piezoelectric resonator and the resonating arms of the tuning fork type piezoelectric resonator element come into contact with each other due to intense vibration of the vehicle.  
      The inner leads are fixed and joined to the mount electrodes in the tuning fork type piezoelectric resonator element by fusing solder that was previously applied to the inner leads. However, in bending the inner leads, the applied solder are peeled and raised at portions of the inner leads that are rubbed by a jig for bending the inner leads. That is, what are called solder burrs are produced. When the solder burrs are produced and the inner leads are joined to the mount electrodes, a short circuit may occur due to the raised solder at one of the mount electrodes or inner leads coming into contact with another of the mount electrodes or inner leads.  
      The tuning fork type piezoelectric resonator element of the present invention is not susceptible to temperature change even over a wide temperature range and has increased shock resistance.  
     SUMMARY OF THE INVENTION  
      The tuning fork type piezoelectric resonator element of the present invention comprises a base having opposite ends, a plurality of resonating arms protruding from one end of the base, a plurality of mount electrodes disposed at the other end of the base in substantial alignment with the resonating arms, a corresponding plurality of lead terminals extending from the mount electrodes and a solder conductive joining material for joining the lead terminals to the mount electrodes wherein the mount electrodes are spaced apart a minimum distance of at least about 60 μm so as to prevent shorting of the conductive joining material when subjected to repeated temperature changes. By virtue of this structure, the shorting of the mounting electrodes do not occur even if the conductive joining material is diffused by being subjected to temperature stress produced by a repetition of a temperature cycle of low temperature and high temperature.  
      It is desirable that the distance between the mount electrodes be equal to or greater than at least about 60 μm. Experiments were conducted on a tuning fork type piezoelectric resonator having lead terminals joined to mount electrodes using high temperature solder. The experiments show that, when a temperature cycle in the temperature range of from −40° C. to +125° C. is repeated 1000 times, the length of diffused solder protruding from each mount electrode (fillet length) is approximately 15 μm at the maximum. Therefore, if the distance between the mount electrodes is equal to or greater than 60 μm, it is possible to prevent shorting of the mount electrodes and, thus, to maintain the performance of the tuning fork type piezoelectric resonator when the temperature cycle in the aforementioned temperature range that is generally required for a tuning fork type piezoelectric resonator for vehicle use is repeated 1000 times.  
      When the temperature cycle in the temperature range of from −40° C. to +125° C. is repeated 2000 times (corresponding to approximately 10 years of vehicle use), the length of the solder protruding from each mount electrode is approximately 25 μm at the maximum. Therefore, if the distance between the mount electrodes is 60 μm, shorting of the mount electrodes can be prevented even if the temperature cycle is repeated 2000 times (corresponding to approximately 10 years of vehicle use). However, in order to more safely and reliably prevent shorting of the mount electrodes when the aforementioned temperature cycle is repeated 2000 times, it is desirable for the distance between the mount electrodes to be equal to or greater than 80 μm. Further, in order to reliably prevent shorting of the mount electrodes when a vehicle is used for an even longer period of time, it is desirable that the distance between the mount electrodes be equal to or greater than 120 μm.  
      In accordance with the present invention it was discovered that the width of the base in the direction transverse to the lead terminals may be extended to allow the lead terminals to be linearly joined without reducing the area of the mount electrodes even if the distance between the mount electrodes is increased, so that it is possible to provide mount electrodes that are large enough for joining the lead terminals thereto without bending the ends of the lead terminals. In addition, since it is no longer necessary to bend the lead terminals, the problem of the conductive joining material applied to the lead terminals being peeled and raised due to rubbing between the lead terminals and a jig for bending the lead terminals does not occur. Therefore, it is possible to eliminate the problem of a short circuit failure caused by the conductive joining material at one lead terminal coming into contact with another lead terminal or another conductive joining material.  
      Two sides of each vibratory arm may have the same length, and the resonating arms may extend symmetrically with respect to a centerline of the base. By virtue of this structure, it is possible to maintain vibrational balance when the resonating arms undergo bending vibration, and, thus, to achieve a predetermined oscillatory frequency.  
      Each vibratory arm may be disposed inwardly from the sides of the base, and the base may have an arc shape disposed between the resonating arms defined by a forked portion and with each side of the base having rounded shoulders of the same curvature as the forked portion disposed between the sides of the base and the resonating arms. By virtue of this structure, since the curvature of the inner side of the resonating arms and the curvature of the outer side of the resonating arms are the same, the resonating arms are all of the same length. Therefore, it is possible to maintain vibrational balance when the resonating arms undergo bending vibration, and, thus, to achieve a predetermined oscillatory frequency.  
      An end of each vibratory arm may have a convex surface. By virtue of this structure, since the problem of, for example, cracking or bending of the resonating arms caused by the resonating arms coming into contact with a container containing the tuning fork type piezoelectric resonator element is eliminated, it is possible to increase shock resistance.  
      Both sides of the base extending in the direction in which the resonating arms extend may have cut portions extending into the base. By virtue of this structure, it is possible to reduce vibration leakage caused by bending vibration of the resonating arms, so that the performance of the tuning fork type piezoelectric resonator is increased.  
      The width at the end of the base  12  ( FIG. 2 ) extending perpendicularly to the resonating arms may be greater than the width of the base extending at least over the portion where the mount electrodes are formed. By virtue of this structure, only the portion of the base where the mount electrodes are formed is enlarged.  
      The tuning fork type piezoelectric resonator of the present invention is formed by a method which comprises the steps of setting the distance between the mount electrodes to a value of at least 60 μm which prevents shorting caused by diffusion of the conductive joining material while; setting the width of the base to a value sufficient to allow the lead terminals to be linearly joined to the mount electrodes essentially without bending; and determining the location of one of the resonating arms with respect to the base so that curvatures of the base at the locations where the resonating arms are connected to the base are the same. By virtue of this arrangement, it is possible to eliminate the problem of shorting of the mount electrodes caused by dispersion of the conductive joining material. In addition, since the width of the base is increased at least at one end thereof, it is possible for the area of the mount electrodes to be large enough for joining the lead terminals thereto. Further, since conductive adhesive applied to the lead terminals does not rub against a jig and become raised, a short circuit failure does not occur. Still further, by forming the resonating arms with the same length, it is possible to maintain vibrational balance when the resonating arms undergo bending vibration, and, thus, to achieve a predetermined oscillatory frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view of a tuning fork type piezoelectric vibrator in accordance with a first embodiment of the present invention.  
       FIG. 2  is a plan view of a tuning fork type piezoelectric resonator element in accordance with the first embodiment.  
       FIG. 3  is a plan view of a tuning fork type piezoelectric resonator element in accordance with a second embodiment of the present invention.  
       FIG. 4  is a plan view of a tuning fork type piezoelectric resonator element in accordance with a third embodiment of the present invention.  
       FIG. 5  is a sectional view of a related cylinder tuning fork type piezoelectric vibrator.  
       FIG. 6  illustrates the relationship between a temperature cycle and solder fillet.  
       FIG. 7  is a plan view of a tuning fork type piezoelectric resonator element in accordance with a fourth embodiment of the present invention. and  
       FIG. 8  is a sectional view taken along line A-A of  FIG. 7 . 
    
    
      The preferred embodiments of the tuning fork type piezoelectric resonator element and method for producing a tuning fork type piezoelectric resonator in accordance with the present invention will hereafter be described in detail. The first embodiment will be described in connection with  FIG. 1  and  FIG.2 .  
      In  FIGS. 1 and 2 , The resonator element  10  includes conventional excitation electrodes (not shown) which are disposed in the resonating arms. The tuning fork type piezoelectric resonator element  10  comprises a base  12  of substantially rectangular configuration having opposite sides  13   a  and  13   b  and opposite ends, a plurality of resonating arms  18  extend from one end of the base  12 which has a curved portion  15  disposed between the pair of resonating arms  18  and has curved shoulders  17  disposed between the sides  13   a  and  13   b  of the base  12  and the corresponding resonating arms  18  respectively. The opposite end  14  of the base  12  is flat and lies perpendicular to the resonating arms  18 . Mount electrodes  16  are formed adjacent the flat end  14  of the base  12  at each opposite corner of the sides  13   a  and  13   b  respectively and are connected to the excitation electrodes (not shown) of the resonating arms  18 .  
      The width of the base  12  at the end  14  which lies in a direction perpendicular to the direction of the resonating arms  18  is greater than the lateral distance between lead terminals  32  as is shown in  FIG. 1 . The lead terminals  32  are electrically and mechanically joined to the mount electrodes  16  located on both corners of the end  14  of the base  12  using a conductive joining material  22  having increased heat resistance, such as a solder  22  composition of lead and tin in a mixing ratio adjusted for high heat resistance.  
      The distance between the mount electrodes  16  is greater than that in the related tuning fork type piezoelectric resonator element  10 , and does not allow shorting of the mount electrodes  16  even if solder diffusion occurs when temperature stress produced by repetition of a temperature cycle of low temperature and high temperature is exerted upon the solder  22 . Since the diffusion distance of the solder changes with the difference between low temperature and high temperature, the diffusion distance of the solder in a specification temperature of the tuning fork type piezoelectric resonator  20  is previously checked, and the distance between the mount electrodes  16  is set greater than this previously checked distance. In this embodiment, the preferred minimum distance between the pair of mount electrodes  16  is 60 μm.  
      The lead terminals  32  are joined to the mount electrodes  16  in the tuning fork type piezoelectric resonator element  10  using a solder  22  for high-temperature application containing 90 wt % lead and 10 wt % tin. The inventor et al. conducted a temperature cycling test in a range of from −40° C. to +125° C. on the joined lead terminals  32  and the mount electrodes  16  in order to observe the diffusion state of the solder with a microscope. The results are shown in  FIG. 6 . In  FIG. 6 , the horizontal axis represents the number of temperature cycles in the range of from −40° C. to +125° C., and the vertical axis represents the maximum length of protrusion of the solder from the mount electrodes in each tuning fork type piezoelectric resonator element (that is, fillet length) in μm. The temperature cycling test was carried out in a cycle of one hour such that the temperature is kept at −40° C. for 30 minutes, then, at +125° C. for 30 minutes, and then at −40° C. for 30 minutes.  
      As shown in  FIG. 6 , when 1000 cycles of the temperature cycling required with regard to a heat resistance specification for vehicle use are to be performed, the fillet length is a little less than 15 μm at the maximum. Therefore, if the distance between the mount electrodes  16  is 60 μm, the solder fillets at the respective mount electrodes  16  will not contact each other, thereby making it possible to prevent shorting of the mount electrodes  16 . When 2000 cycles of the temperature cycling corresponding to of the order of 10 years of use of a vehicle are to be performed, the maximum fillet length of the solder is approximately 25 μm. Therefore, if the distance between the mount electrodes is equal to or greater than 60 μm, it is possible to prevent shorting of the mount electrodes. However, in order to more reliably prevent the shorting of the mount electrodes, it is desirable for the distance between the mount electrodes to be equal to or greater than 80 μm. If the distance between the mount electrodes is equal to greater than 80 μm, the distance between the fillets is at least approximately equal to or greater than 30 μm, so that shorting does not occur. In order to safely and reliably prevent shorting of the mount electrodes when more than 2000 cycles of the temperature cycling which may produce larger fillets are to be performed, the distance between the mount electrodes is equal to or greater than 120 μm.  
      From the aforementioned results, the distance between the mount electrodes  16  can be set at 60 μm or greater, at 80 μm or greater, and at 120 μm or greater in accordance with the required number of cycles of temperature cycling.  
      Moreover, if the width of the base  12  is increased due to an increase in the distance between the mount electrodes  16 , the area of the mount electrodes  16  will still be large enough for joining the lead terminals  32 . see comment Therefore, a reduction in the area of the mount electrodes caused by simply increasing only the distance between the mount electrodes compared to the prior art related tuning fork type piezoelectric resonator element does not occur.  
      The lengths of linear portions of the left and right sides of the resonating arms  18  extending from the base  12  are the same. In the embodiment, shown in  FIG. 2 , the pair of the resonating arms  18  are disposed at symmetrical positions situated inwardly of the opposite sides  13   a  and  13   b  of the base  12 . The end of the base  12  from which the resonating arms  18  extend is formed with an arc shape disposed between the pair of resonating arms  18  to define a forked portion  15  and has shoulders  17  disposed between the sides  13   a  and  13   b  and the respective resonating arms  18  with each shoulder  17  having a curvature conforming to the curvature of the forked portion  15  on the side opposite the respective shoulder. Therefore, the shapes of the resonating arms  18  are symmetrical with respect to a centerline extending in the lengthwise direction of the resonating arms  18 . Since the plurality of such resonating arms  18  extend from the base  12 , it is necessary for the shapes of all of the resonating arms  18  to be the same. Therefore, in the tuning fork type piezoelectric resonator element  10 , the resonating arms  18  extend from the base  12  symmetrically with respect to the centerline of the base  12  extending in the direction of extension of the resonating arms  18 . Thus, the curvature of the portion disposed inwardly of the resonating arms  18  and the curvature of the portion disposed outwardly of the resonating arms  18  are the same(mirror image) Since the natural frequency of the tuning fork type piezoelectric resonator element is determined by the length and the width of the resonating arms, the length and the width of the resonating arms  18  in the embodiment are the same as those of the resonating arms in the related art.  
      The above-described tuning fork type piezoelectric resonator element  10  is disposed in a cylinder  24  for forming the cylinder tuning fork type piezoelectric resonator  20 . of the present invention shown in  FIG. 1 . More specifically, the tuning fork type piezoelectric resonator element  10  is disposed in a cylindrical metallic container  26  having an open end adjacent the base  12 . The container  26  is hermetically sealed at the open end by inserting a plug  34  into the open end of the container  26 . The plug  34  is formed by hermetically sealing the lead terminals  32  to form linear inner leads  28  internal of the container  26  and outer leads  30  which lie external of the container  26 . The inner leads  28  and the mount electrodes  16  are electrically and mechanically joined with a solder  22  having increased heat resistance.  
      A preferred method for producing the tuning fork type piezoelectric resonator element  10  of the present invention for use in the tuning fork type piezoelectric resonator  20  will be hereafter described. First, the distance of solder diffusion at a specification temperature of the tuning fork type piezoelectric resonator  20  is checked in order to determine whether the distance between the mount electrodes  16  is greater than the checked distance. Since the area of the mount electrodes  16  is reduced when the distance between the mount electrodes  16  is increased, the joining strength of the mount electrodes  16  and the lead terminals  32  is reduced. Therefore, the width of the base  12  at which the mount electrodes  16  are disposed is determined so that the lead terminals  32  can be linearly joined, and the mount electrodes  16  is widened in the direction in which the width of the base  12  is increased in order to ensure joining strength of the mount electrodes  16  with the lead terminals  32 .  
      Next, in order to form the left and right sides of one of the resonating arms  18  with the same length, the curvature at the location where the vibratory arm  18  and the base  12  are connected is determined. This is because, if the left and right sides of the vibratory arm  18  have different lengths, a predetermined oscillatory frequency cannot be achieved due to a loss in bending vibrational balance. The location of the vibratory arm  18  extending from the base  12  is determined so that the curvatures at the resonating arms  18  are the same. Accordingly, the shape of the tuning fork type piezoelectric resonator element  10  is determined.  
      In this way, since the distance between the mount electrodes  16  disposed at the tuning fork type piezoelectric resonator element  10  is increased, shorting of the mount electrodes  16  caused by the solder  22  of each mount electrodes  16  coming into contact with each other due to solder diffusion does not occur even if temperature stress is applied. If the mount electrodes  16  are not shorted even if solder diffusion occurs, various characteristics of the tuning fork type piezoelectric resonator  20  are not affected. In addition, the width of the base  12  where the mount electrodes  16  are disposed is greater than the distance between the lead terminals  32 , and the mount electrodes  16  is larger in the direction in which the width of the base  12  is increased. Therefore, the area of the mount electrodes  16  is not reduced, so that the joining strength of the mount electrodes  16  and the lead terminals  32  can be ensured. Further, since the width of the base  12  is large enough with respect to the plug  34 , the supporting capability is greater than that in a related tuning fork type piezoelectric resonator element. Still further, since the lead terminals  32  can be linearly joined to the mount electrodes  16 , solder burrs produced when the inner leads are bent with a jig are not produced, thereby making it possible to eliminate short circuit failure. Consequently, it is possible to provide a tuning fork type piezoelectric resonator  20  which is required to be highly reliable.  
      Since the inner leads  28  are not bent, it is possible to increase productivity of the tuning fork type piezoelectric resonator  20 . Since a large investment in plant and equipment is not required, it is possible to minimize an increase in cost of producing a tuning fork type piezoelectric resonator element  10  having a new shape.  
      A second embodiment will now be described. Since the second embodiment is a modification of the tuning fork type piezoelectric resonator element  10  of the first embodiment, corresponding parts to those of the first embodiment will be given the same reference numerals, and will not be described below.  FIG. 3  is a plan view of a tuning fork type piezoelectric resonator element in accordance with the second embodiment. In  FIG. 3 , excitation electrodes that are disposed at resonating arms are not shown.  
      When a tuning fork type piezoelectric resonator  20  is used in an environment in which an intense vibration is applied thereto, and is installed and used in, for example, a vehicle, resonating arms  18  may become, for example, chipped or bent as a result of a tuning fork type piezoelectric resonator element  10  being shaken and coming into contact with a container  26 . In addition, even when producing the tuning fork type piezoelectric resonator  20 , the resonating arms  18  may become, for example, cracked or chipped when corners of the resonating arms  18  get caught by, for example, a manufacturing jig. Therefore, ends of the resonating arms  18  have convex curved portions  36  (see  FIG. 3 ( a )). The curvatures of the curved portions  36  of the left and right resonating arms  18  are the same. The curved portions  36  are formed by etching. When the ends of the resonating arms  18  are curved, they will not chip or bend compared to the case in which the ends of the resonating arms  18  are angular. Therefore, shock resistance is increased, thereby making it possible to provide a highly reliable tuning fork type piezoelectric resonator element  10 . In addition, since, for example, cracking or chipping does not occur when producing the tuning fork type piezoelectric resonator  20 , yield is increased, so that the cost of manufacturing the tuning fork type piezoelectric resonator  20  can be reduced.  
      When the tuning fork type piezoelectric resonator element  10  vibrates, the resonating arms  18  undergo bending vibration. Here, what is called vibration leakage may occur in which the vibration is transmitted to portions at the base  12  where mount electrodes  16  and lead terminals  32  are joined. Therefore, cut portions  38  are formed in both sides  13   a  and  13   b  of the base  12  extending in the direction in which the resonating arms  18  extend (see  FIG. 3 ( b )). The cut portions  38  are formed at locations where the area of the mount electrodes  16  is not reduced. The cut portions  38  formed in both sides have the same shape, are formed by etching, and can reduce the vibration leakage.  
      The above-described structure in which the ends of the resonating arms  18  have the curved portions  36  and the structure in which the cut portions  38  are formed in the base  12  may both be used at the same time (see  FIG. 3 ( c )).  
      Next, a third embodiment of the present invention will be described in connection with  FIG. 4 . In  FIG. 4 , excitation electrodes disposed at resonating arms are not illustrated. The form of the tuning fork type piezoelectric resonator element in accordance with the third embodiment is different from that of the tuning fork type piezoelectric resonator element in accordance with the first embodiment, but the method for designing the resonator element and the advantages provided by the resonator element are the same. Therefore, parts corresponding to those in the first embodiment are not described below.  
      In accordance with the third embodiment shown in  FIG. 4 ( a ) the mount electrodes  42  are spaced from each other so that the mount electrodes  42  are not shorted by solder diffusion. The side of the base  44  of the tuning fork type piezoelectric resonator element  40  is formed so that only a base portion  44   a  where the mount electrodes  42  are formed is wider than the distance between lead terminals. Accordingly, this leaves a base portion  44   b  where the mount electrodes  42  are not formed which is thinner i.e. smaller in width than the base portion  44   a  where the mount electrodes  42  are formed. In order for the lengths of both sides of resonating arms  46  to be the same, the curvatures of the resonating arms  46  at locations where both sides of the resonating arms  46  and the base  44  are connected should be the same. The resonating arms  46  extend symmetrically from the base  44  with respect to a centerline of the base  44  extending in the direction of extension of the resonating arms  46 . The width of the base  44  in the direction transverse to the arms  46  is smaller than the corresponding width of the base  12  in the first embodiment. Therefore, the distance between the resonating arms  46  of the tuning fork type piezoelectric resonator element  40  in accordance with the third embodiment is smaller than the distance between the resonating arms  18  in accordance with the first embodiment. Since the length and width of the resonating arms  46  correspond to the counter part length and width of the resonating arms  18  in accordance with the first embodiment, they have the same natural frequency.  
      Even the tuning fork type piezoelectric resonator element  40  in accordance with the third embodiment may have the forms illustrated in the second embodiment. In other words, ends of the resonating arms  46  may have convex curved portions  48  in order to prevent the resonating arms  46  from becoming, for example, chipped or bent (see  FIG. 4 ( b )). In addition, cut portions  50  may be formed at locations of both sides of the base  44  where the area of the mount electrodes  42  is not reduced in order to reduce vibration leakage (see  FIG. 4 ( c )). Further, the structure in which the ends of the resonating arms  46  have the curved portions  48  and the structure in which the cut portions  50  are formed in the base  44  may both be used at the same time (see  FIG. 4 ( d )).  
      Although, in each of the above-described embodiments, the cylinder tuning fork type piezoelectric resonator  20  having the tuning fork type piezoelectric resonator element  10  or  40  inserted in the container  26  is described, a surface-mount tuning fork type piezoelectric resonator having one side of the above-described tuning fork type piezoelectric resonator element  10  or  40  mounted to a ceramic or metallic package may be used. In this case, the tuning fork type piezoelectric resonator element is mounted to the mount electrodes formed at the package.  
       FIG. 7  is a plan view of a tuning fork type piezoelectric resonator element in accordance with a fourth embodiment of the present invention. In  FIG. 7 , a tuning fork type piezoelectric resonator element  60  comprises a base  62  and a pair of resonating arms  64  ( 64   a  and  64   b ) protruding from one end of the base  62 . Each vibratory arm  64  has grooves  66  at its base end side so as to extend in the longitudinal direction of the resonating arms  64 . The grooves  66  are formed at locations corresponding to the upper and lower surfaces of each vibratory arm  64 . Therefore, as shown in  FIG. 8 , each vibratory arm  64  has an H shape in cross section. In the tuning fork type piezoelectric resonator element  60 , excitation electrodes  68  and  70  are formed at the respective resonating arms  64 . The excitation electrodes  68  and  70  comprise side electrode portions  68   a  and side electrode portions  70   a,  respectively, formed at both side surfaces of the resonating arms  64 , and groove electrode portions  68   b  and groove electrode portions  70   b,  respectively, formed at inner surfaces defining the respective grooves  66 . The side electrode portions  68   a  and the side electrode portions  70   a  formed at both side surfaces of the respective resonating arms  64  are connected to each other through an end electrode portion  68   c  and an end electrode portion  70   c,  respectively, formed at an end of its corresponding vibratory arm  64 . The end electrode portions  68   c  and  70   c  are used to adjust the oscillatory frequency of the tuning fork type piezoelectric resonator element  60 .  
      The groove electrode portions  68   b  and  70   b  are electrically connected to the side electrode portions of the respective other resonating arms  64 . In other words, the groove electrode portions  68   b  of the vibratory arm  64   a  are electrically connected to the side electrode portions  70   a  of the other vibratory arm  64   b.  The groove electrode portions  70   b  of the vibratory arm  64   b  are electrically connected to the side electrode portions  68   a  of the vibratory arm  64   a.  In addition, the tuning fork type piezoelectric resonator element  60  has a pair of mount electrodes  72  ( 72   a  and  72   b ) formed on the base  62 . The mount electrode  72   a  is connected to the excitation electrode  68 , and the mount electrode  72   b  is connected to the excitation electrode  70 . Cut portions  38  are formed in both sides of the base  62 .  
      In the tuning fork type piezoelectric resonator element, the oscillatory frequency is basically determined by the width and length of the resonating arms. In the tuning fork type piezoelectric resonator element  60  for vehicle use in accordance with the embodiment, in order to increase temperature resistance cycle, a width c of the base  62  is greater than that in a related tuning fork type piezoelectric resonator element and the curvature of a forked portion  15  and that of shoulders  17  are small (that is, the radius of curvature is large). Therefore, in the tuning fork type piezoelectric resonator element  60  in accordance with the embodiment, when a width W of the resonating arms  64  is the same as the width of the resonating arms of a related tuning fork type piezoelectric resonator element, the width of the base end of the resonating arms  64  is large, so that the same advantage as that provided when the resonating arms  64  is shortened is essentially provided. Detailed investigations and experiments confirmed that, when the width W and a length b of the resonating arms  64  are the same as those of the related resonating arms, the oscillatory frequency of the tuning fork type piezoelectric resonator element  60  of the embodiment is less than the oscillatory frequency of a related tuning fork type piezoelectric resonator element comprising a base having a smaller width. Therefore, when the tuning fork type piezoelectric resonator element  60  of the embodiment having the same frequency as a related tuning fork type piezoelectric resonator element is to be formed, the length of the resonating arms  64  is made slightly longer than that of the resonating arms in the related tuning fork type piezoelectric resonator element in order to adjust the oscillatory frequency.  
      For example, when a tuning fork type piezoelectric vibrator having an oscillatory frequency of 32.768 kHz is to be formed, a related tuning fork type piezoelectric resonator element is formed so that, with reference to  FIG. 7 , a base length a=1102 μm, a base width c=640 μm, a vibratory arm length b=2358 μm, a vibratory arm width W=236 μm, an overall length L=3460 μm, and a curvature radius R of shoulders and a forked portion is equal to 66 μm, whereas the tuning fork type piezoelectric resonator element  60  of the embodiment is formed so that a length a of the base  62  is 1102 μm, a width c of the base  62  is 1000 μm, the length b of the resonating arms  64  is 2478 μm, the width W of the resonating arms  64  is 236 μm, an overall length L=3580 μm, and a curvature radius R of the shoulders  17  and the forked portion  15  is equal to 132 μm. In other words, the length b of the resonating arms  64  of the tuning fork type piezoelectric resonator element  60  of the embodiment is larger than that of the resonating arms of the related tuning fork type piezoelectric resonator element by 120 μm. In the tuning fork type piezoelectric resonator element  60  of the embodiment, the locations of formation of the cut portions  38 , that is, a distance e from the other end of the base  62  is 570 to 680 μm, and a depth g of the cut portions  38  is 100 to 220 μm.  
      The oscillatory frequency of the tuning fork type piezoelectric resonator element  60  of the embodiment formed in this way is slightly less than 32.768 kHz. Therefore, the oscillatory frequency of the tuning fork type piezoelectric vibrator using the tuning fork type piezoelectric resonator element  60  of the embodiment is easily adjusted to a value of 32.768 kHz by removing the end electrode portions  68   c  and  70   c  by laser.