Patent Publication Number: US-2011068659-A1

Title: Piezoelectric vibrating devices and methods for manufacturing same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japan Patent Application No. 2009-218703, filed on Sep. 24, 2009, and Japan Patent Application No. 2010-069444 filed on Mar. 25, 2010, in the Japan Patent Office, the disclosures of which are incorporated herein by reference in their respective entireties. 
     FIELD 
     The present invention relates to, inter alia, piezoelectric devices and to methods for manufacturing such devices at mass-production levels. 
     DESCRIPTION OF THE RELATED ART 
     With the progress of miniaturization and/or increases in the operating frequency of mobile communication apparatus and office automation (OA) equipment, piezoelectric devices used in this equipment must be made progressively smaller. For reducing manufacturing costs, the methods for manufacturing these devices must be optimized as much as possible. 
     According to the method for manufacturing piezoelectric device disclosed in Japan Unexamined Patent Application No. 2008-182468, individual lids are placed on and attached to respective “packages,” on a “package wafer” including multiple packages, wherein each package comprises a respective piezoelectric vibrating piece. The lids are fitted to the packages with the aid of “guide parts,” followed by hermetic bonding of the lids to the packages. Then the package wafer is cut device-by-device to separate the multiple individual piezoelectric devices from each other. This method is effective for preventing misalignments of lids with their respective packages and can be used for mass-production. However, the method disclosed in the &#39;468 reference must be performed device-by-device on the package wafer. Each lid is manufactured individually and individually attached to a respective package on the wafer. This device-by-device assembly is inefficient, perhaps too inefficient for modern mass-production. 
     An object of the invention is to provide piezoelectric vibrating devices exhibiting long-term stability and to provide efficient methods for their manufacture. 
     SUMMARY 
     According to a first aspect of the invention, methods are provided for manufacturing piezoelectric devices. An embodiment of such a method comprises preparing a base wafer defining multiple bases and preparing a lid wafer defining multiple lids. Each base has respective sides and a respective periphery, and includes a respective stripe of a first bonding film extending inboard of each edge around the periphery. Each base also includes at least one first indent formed adjacent each respective edge and contacting the respective stripe of the first bonding film. Each lid has respective sides and a respective periphery, and includes a respective stripe of a second bonding film extending inboard of each edge around the periphery. Each lid also includes at least one second indent formed adjacent each respective edge and contacting the respective stripe of the second bonding film. The stripes of bonding film and indents on the lid wafer are aligned with corresponding stripes and indents on the base wafer. A respective unit of bonding material is applied onto each of the first indents or each of the second indents. The lid wafer is aligned with the base wafer such that the wafers are separated from each other by the units of bonding material situated between respective opposing first and second indents. The units of bonding material are melted to produce flow of the molten bonding material from the indents along the stripes of the first and second bonding films. The bonding material is then solidified to bond the base wafer and lid wafer together to form a package wafer. 
     The package wafer is cut between adjacent stripes to release individual piezoelectric devices from the package wafer and to separate them from each other. This method provides mass-production of piezoelectric devices exhibiting long-term high stability. 
     Each of the first and second indents has a hemispherical shape, for example. The intents can be formed by etching using a mask having respective holes that define the shape and locations of the indents. Bonding together the lid wafer and base wafer can be performed under a vacuum state or in an inert gas environment. 
     The manufacturing method can further comprise, after forming the package wafer, cutting (“dicing”) the package wafer between adjacent stripes to separate the piezoelectric devices from the package wafer and from each other. The stripes of the first and second bonding films desirably are formed at respective regions in which the stripes thereof are not cut in the cutting step. 
     Preparing the base wafer desirably includes providing the base wafer with cutting grooves that are used in the cutting step. A respective cutting groove is located between flanking stripes of respective adjacent piezoelectric vibrating devices, so the cutting grooves collectively define the outline profiles of the piezoelectric vibrating devices in the package wafer. Similarly, preparing a lid wafer includes providing the lid wafer with cutting grooves that are used in the cutting step. A respective cutting groove is located between flanking stripes of respective adjacent piezoelectric vibrating devices, so the cutting grooves collectively define the outline profiles of the piezoelectric vibrating devices in the package wafer. The cutting grooves define the outline profiles of the piezoelectric vibrating devices. 
     The first and second indents are desirably formed on the stripes of the first and second bonding films of the base and lid, respectively. 
     According to the present invention, multiple piezoelectric devices are manufactured from a package wafer. Each device includes a respective piezoelectric vibrating device exhibiting improved reliability and durability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view of the inner surface of a first embodiment of a piezoelectric vibrating device  100  in which a respective tuning-fork type crystal vibrating piece  30  is mounted. 
         FIG. 1B  is an elevational section along the line A-A of the first embodiment shown in  FIG. 1A . 
         FIG. 2  is a plan view of a first embodiment of a package wafer  80 W, as viewed from the lid wafer  10 W. 
         FIG. 3A  is an enlarged cross-sectional view along the line B-B in  FIG. 2 . The lid wafer  10 W and base wafer  40 W are shown vertically aligned with each other. 
         FIG. 3B  is an enlarged cross-sectional view of the region denoted “X” in  FIG. 3A . 
         FIGS. 4A-4F  constitute a flow-chart of a method for forming the indents  66 ,  67 . 
         FIG. 5  is a plan view of a second embodiment of a package wafer  80 WA, as viewed from the lid wafer  10 WA. 
         FIG. 5B  is an enlarged cross-sectional view along the line C-C in  FIG. 5A . 
         FIG. 6A  is a plan view of a third embodiment of a package wafer  80 WB, as viewed from the lid wafer  10 WB. 
         FIG. 6B  is an enlarged cross-sectional view along the line D-D in  FIG. 6A . 
         FIG. 7A  is a plan view of a fourth embodiment of a package wafer  80 WC, as viewed from the lid wafer  10 WC. 
         FIG. 7B  is an enlarged cross-sectional view along the line E-E in  FIG. 7A . 
         FIG. 8  is a flow-chart of an embodiment of a method for manufacturing piezoelectric devices. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment of Piezoelectric Device 
       FIGS. 1A and 1B  are schematic views of this first embodiment of a piezoelectric vibrating device  100  comprising a tuning-fork type crystal vibrating piece  30 .  FIG. 1A  is a plan view of the inner surface of the device, and  FIG. 1B  is an elevational section along the line A-A in  FIG. 1A . The piezoelectric device  100  comprises a lid  10  and a base  40 , which are made of a glass material, for example. The base  40  defines a concavity  47  facing the lid  10 . A mount  52  is formed in the concavity  47  and is made of the same glass material as the base  40 . The tuning-fork type crystal vibrating piece  30  is mounted to the mount  52 . 
     The base  40  includes a first bonding film  45  on the bonding surface of the base. The bonding surface extends just inboard of the peripheral edge of the base  40  and is the top edge of a frame portion  49  extending around the periphery of the base. The first bonding film  45  essentially comprises linear stripes extending on respective portions of the bonding surface, thereby forming a rectangular figure having four sides. As shown in  FIG. 1A , the first bonding film  45  also includes stripes extending diagonally from each corner of the rectangle to respective corners of the base  40 . 
     The lid  10  shown in  FIG. 1B  includes a second bonding film  15  on the bonding surface of the lid. The bonding surface extends just inboard of the peripheral edge of the lid  10  and is the lower edge of a frame portion extending around the periphery of the lid. The second bonding film  15  essentially comprises linear stripes extending on respective portions of the bonding surface, thereby forming a rectangular figure having four sides. Each of the first and second bonding films  45 ,  15  is a gold layer having a thickness of 400 Å to 2000 Å. 
     The base  40  defines a first through-hole  41  and a second through-hole  42  that extend from the inner surface to the outer (under) surface of the base. The concavity  47 , the mount  52 , the frame portion  49 , the first through-hole  41 , and the second through-hole  43  are all formed concurrently by etching. A first connecting electrode  42  and a second connecting electrode  44  are formed on the inner surface of the base  40 . A first external electrode  55  and a second external electrode  56  are metalized on the outer (under) surface of the base  40 . The first and second through-holes  41 ,  43  each include an interior metal film. The first and second through-holes  41 ,  43  are sealed by a sealing material  70 . 
     The lid  10  includes a concavity  17  facing the base  40 . Surrounding the concavity is a rim including a bonding surface. Applied to the bonding surface are stripes of a second bonding film  15 . The stripes form a rectangular pattern with four sides that extend just inboard of the extreme periphery of the lid. 
     The concavity  17  in the lid  10  and the concavity  47  in the base collectively form a cavity  22 . The piezoelectric vibrating device  100  includes the tuning-fork type crystal vibrating piece  30  mounted within the cavity  22  using an electrically conductive adhesive  71 . 
     The tuning-fork type crystal vibrating piece  30  comprises a pair of vibrating arms  21  and a basal portion  23 . A first base electrode  31  and a second base electrode  32  are formed on the basal portion  23 . Each vibrating arm  21  includes a respective excitation electrode, namely a first excitation electrode  33  and a second excitation electrode  34 , respectively. The excitation electrodes are formed on the upper, lower, and side surfaces of the respective vibrating arms  21 . The first excitation electrode  33  is connected to a first base electrode  31 , and the second excitation electrode  34  is connected to a second base electrode  32 . 
     Each of the first base electrode  31 , the second base electrode  32 , the first excitation electrode  33 , and the second excitation electrode  34  comprises respective metal layers. Example metal layers are 400-2000 Ångstroms (thickness) of gold (Au) layered on 150-700 Ångstroms (thickness) of chromium (Cr). A titanium (Ti) layer can be used instead of the chromium (Cr) layer, and a silver (Ag) layer can be used instead of the gold (Au) layer. 
     The first base electrode  31  and the second base electrode  32  are connected to a first bonding electrode  42  and a second bonding electrode  44 , respectively, using the electrically conductive adhesive  71 . The first connecting electrode  42  is connected to the first external electrode  55 , on the under-surface of the base  40 , via the through-hole  41 . Similarly, the second connecting electrode  44  is connected to the second external electrode  56 , on the under-surface of the base  40 , via the through-hole  43 . Thus, the first base electrode  31  is electrically connected to the first external electrode  55 , and the second base electrode  32  is electrically connected to the second external electrode  56 . 
     One piezoelectric vibrating device  100  is depicted in  FIG. 1  for ease of description. However, during actual manufacture, hundreds or thousands of devices  100  are manufactured simultaneously for higher productivity. That is, multiple bases  40  are formed on a base wafer  40 W (see  FIGS. 2 and 3 ), and a respective tuning-fork type crystal vibrating piece  30  is mounted on each base. Similarly, multiple lids  10  are formed on a lid wafer  10 W (see  FIGS. 2 and 3 ). On the lid wafer  10 W, the multiple lids  10  are located so as to be alignable with respective bases  40  on the base wafer  40 W. 
     After aligning the base wafer  40 W and lid wafer  10 W in this way, the wafers are bonded together by bonding together all the lids  10  with their respective bases  40 , thereby forming a package wafer  80 W having all the attached piezoelectric devices  100 . Finally, the package wafer  80 W is diced to separate the individual piezoelectric devices  100  from one another. 
       FIG. 2  is a plan view of the package wafer  80 W, as viewed from the lid wafer  10 W. For comprehension, the lid wafer  10 W is depicted as if it were transparent, and the figure mainly shows the tuning-fork type crystal vibrating piece  30  mounted on the base  40 . An area (X-Y plane) corresponding to one piezoelectric vibrating device  100  is delineated with a virtual line (two-dotted chain line) on the package wafer  80 W. Also, the cavities  22  are depicted as meshed zones to distinguish the tuning-fork type crystal vibrating piece  30  from other structure. 
     As shown in  FIG. 2 , cutting grooves  60  are formed on the lid wafer  10 W. Corresponding cutting grooves  60 , formed on the under-surface of the base wafer  40 W (see  FIG. 3A ), are aligned (in the X-Y plane) with the cutting grooves  60  on the lid wafer  10 W. The cutting grooves  60  are situated between adjacent virtual lines (two-dotted chain lines). The package wafer  80 W is affixed to a dicing film (not shown) and is cut along the cutting grooves  60  using a dicing saw. The cutting grooves  60  prevent cracks from forming on the piezoelectric devices  100  whenever the package wafer  80 W is being cut by the dicing saw. During cutting the dicing saw moves linearly between the walls of the cutting grooves  60  of the lid wafer  10 W and base wafer  40 W. The depth of each cutting groove  60  is in the range of 20 to 70 μm. By providing the lid wafer  10 W and base wafer  40 W with cutting grooves  60 , the cutting load for the dicing saw is reduced, which improves work efficiency. The cutting grooves also prevent chipping or cracking of the package wafer  80 W during dicing. 
     The stripes of the second bonding film  15  formed on the lid wafer  10 W and the stripes of the first bonding film  45  formed on the base wafer  40 W are situated so as to be in registration with each other in the package wafer  80 W. Additional stripes of the first and second bonding films  45 ,  15  extend from respective corners of each rectangle toward the respective cutting grooves  60 . The additional stripes extend from respective corners of the rectangles toward the X-axis at angles of + or −45°. The additional stripes cross each other at loci identical to loci (on the X-Y plane) at which respective cutting grooves  60  cross each other. First and second indents  66 ,  67  are situated at loci at which stripes of the bonding films  45 ,  15  cross each other. 
       FIG. 3A  is an enlarged elevational view of a portion of the package wafer  80 W along the line B-B in  FIG. 2 . The lid wafer  10 W and base wafer  40 W are shown aligned with each other.  FIG. 3B  is an enlarged cross-sectional view of the region denoted “X” in  FIG. 3A . As shown in  FIG. 3A , the first indents  66  are formed on a major surface of the base  40  (i.e., the inner major surface) that is opposite the major surface on which the cutting grooves  60  are formed. Similarly, the second indents  67  are formed on a major surface of the lid (i.e., the inner major surface) that is opposite the major surface on which the cutting grooves  60  are formed. 
     As shown in  FIG. 3B , each of the first and second indents  66 ,  67  is formed as a hemispherical concavity. Respective stripes of the first bonding film  45  extend onto each first indent  66 , and respective stripes of the second bonding film  15  extend onto each second indent  67 . A respective bonding ball  75  is placed on each of the first indents  66  on the base wafer  40 W. Thus, the bonding ball  75  becomes sandwiched between the respective first indent  66  and second indent  67 . The bonding balls  75  serve as placement guides for aligning and placing the lid wafer  10 W on the base wafer  40 W, thereby avoiding misalignment of the base wafer  40 W and lid wafer  10 W. The bonding ball  75  is a eutectic metal ball comprising, for example, a gold-silicon alloy (Au 3.15 Si) or a gold-germanium (Au 12 Ge) alloy. 
     The base wafer  40 W and lid wafer  10 W are bonded together by material of the bonding balls  75 . To such end, there desirably is a space (see  FIG. 3B ) between the first bonding film  45  and the second bonding film  15  before the bonding balls  75  are melted. This space allows the interior cavities  22  of the package wafer  80 W to be evacuated to a desired vacuum level whenever the package wafer  80 W is kept/heated in a reflow furnace under vacuum conditions. This space also allows the cavities  22  to be filled with an inert gas if the reflow furnace operates under an inert gas environment. 
     As the bonding balls  75  melt, the resulting eutectic melt flows along the respective stripes of the first and second bonding films  45 ,  15 , thereby “wetting” the surfaces of the stripes by capillary action. Upon completion of wetting, the melt is allowed to cool sufficiently to complete bonding. As noted above, the bonding films  45 ,  15  can be bonded together while the inside of the cavities  22  is evacuated or filled with an inert gas. 
     Returning to  FIG. 2 , the first indents  66  are formed just outboard of areas delineated by the two-dotted chain line (these lines delineate the extreme peripheries of adjacent piezoelectric vibrating devices  100 ). As a result, stripes of the first bonding film  45  and second bonding film  15  forming the rectangular patterns of such stripes are formed just inboard of the extreme peripheries. In these inboard areas there are no indents. The absence of indents in these stripes facilitates capillary flow of the eutectic melts from the molten bonding balls  75 . As a result, bonding of the lid wafer  10 W to the base wafer  40 W can be done securely. 
     The first indents  66  and second indents  67  are aligned with the cutting grooves  60  in the Z-direction, which may allow chips of wafer material generated by contact with a dicing blade to attach to the blade. To minimize possible adverse effects of this, regions of the stripes of first bonding film  45  and second bonding film  15  near the first indent  66  and second indent  67  are preferably as thin as possible. 
     Forming the First and Second Indents 
       FIGS. 4A-4F  constitute a flow-chart of an embodiment of a method for forming the indents  66 ,  67 . Cross-sections of the wafer (made of a glass material), showing respective results of each step are provided along on the right side of the flow-chart. Although the indents  66 ,  67  are formed concurrently with formation of the concavities  17 ,  47  on the lid wafer  10 W and base wafer  40 W, respectively, only formation of an indent is shown and described. 
     In  FIG. 4A  (step S 202 ) a corrosion-resistant film  20 , such as gold (Au) or silver (Ag), is formed on the major surfaces of the lid wafer  10 W and base wafer  40 W by sputtering or vacuum-deposition. Since the lid wafer  10 W and base wafer  40 W are made of glass, gold (Au) or silver (Ag) can be formed directly on the respective surfaces of the wafers.  FIG. 4A  includes a cross-section of the lid wafer  10 W or base wafer  40 W upon completion of this step. 
     In  FIG. 4B  (step S 204 ) a photoresist film  36  is evenly applied by spin-coating on the major surfaces of the lid wafer  10 W and base wafer  40 W on which the corrosion-resistant film  20  was formed. An exemplary photoresist film  36  is a positive photoresist made of novolak.  FIG. 4B  includes a cross-section of the lid wafer  10 W or base wafer  40 W upon completion of this step. 
     Next, in  FIG. 4C  (step S 206 ), using a lithographic exposure device (not shown), an indent pattern is exposed onto the photoresist  36  on the lid wafer  10 W and base wafer  40 W. Each indent  66 ,  67  is a hemisphere having a diameter in the range of 150 μm to 200 μm, while the diameter of each hole in the indent pattern is 50 μm.  FIG. 4C  includes a cross-section of the lid wafer  10 W or base wafer  40 W upon completion of this step. 
     In  FIG. 4D  (step S 208 ) the photoresist films  36  on the lid wafer  10 W and base wafer  40 W are developed, followed by removal of the exposed photoresist film  38 . Respective regions of the gold layer revealed by removal of exposed resist  36  are etched using an aqueous solution of iodine and potassium iodide. The concentrations of etching solutes, temperature, and etching time are controlled to avoid over-etch. Thus, revealed portions of the corrosion-resistant film  20  are removed.  FIG. 4D  includes a cross-section of the lid wafer  10 W or base wafer  40 W in which holes for the indents have been formed. 
     In  FIG. 4E  (step S 210 ) portions of the lid wafer  10 W and base wafer  40 W revealed by removal of regions of corrosion-resistant film  20  are wet-etched using a hydrofluoric acid solution to form the profile outlines of the indents  66  and  67 . The duration of wet-etching is a function of concentration, type, and temperature of the hydrofluoric acid solution. The glass wafer is etched radially from the small hole so that the indent assumes a hemispherical shape.  FIG. 4E  includes a cross-section of the lid wafer  10 W or base wafer  40 W after etching, depicting the hemispherical indents  66 ,  67 . The hemispherical shapes are covered by the corrosion-resistant film  20  and the photoresist film  36 .  FIG. 4E  includes a cross-section of an indent  66 ,  67  having a greater diameter than the diameter of the hole in the indent pattern. 
     In  FIG. 4F  (step S 212 ) the hemispherical indents  66  and  67  are formed by removing the remaining photoresist film  36  and the corrosion-resistant film  20 . For comprehension, in the figure the indent  66 ,  67  is depicted enlarged. At least one stripe of bonding film  45 ,  15  extends onto the indent  66 ,  67  in the course of forming the stripe.  FIG. 4F  includes a cross-section of an indent  66 ,  67  thus formed. 
     Second Embodiment of Piezoelectric Device 
       FIG. 5A  is a plan view of a package wafer  80 WA, as viewed from above the lid wafer  10 WA, used for producing multiple piezoelectric devices  110  according to the second embodiment. The lid wafer  10 WA is depicted as if it were transparent, and the figure mainly shows tuning-fork type crystal vibrating pieced  30  mounted on respective bases  40 A. For comprehension, respective areas (in the X-Y plane) corresponding to single respective piezoelectric vibrating devices  110  are delineated using a virtual line (two-dotted chain line) on the package wafer  80 WA. Voids  22  are depicted as meshed zones to distinguish the tuning-fork type crystal vibrating piece  30  in each device  110 .  FIG. 5B  is an enlarged elevational section along the line C-C in  FIG. 5A . For comprehension,  FIG. 5B  shows the constituent wafers  10 WA and  40  WA separated from each other in the Z-direction. 
     The stripes of the first bonding film  45  and second bonding film  15  in this device  110  have a different pattern than in the first embodiment. Also the positions of the first indents  66 A and the second indents  67 A of this device  110  are different from corresponding positions of the indents in the first embodiment  100 . Only the differences from the first embodiment  100  are described below. 
     The package wafer  80 WA comprises a lid wafer  10 WA defining multiple lids  10 A and a base wafer  40 WA defining multiple corresponding bases  40 A. The tuning-fork type crystal vibrating piece  30  is mounted on a mount  52 , which is part of the base  40 A. 
     As shown in  FIGS. 5A and 5B , the stripes of the second bonding film  15  on the lid wafer  10 WA face the base wafer  40 WA. The base wafer  40 WA includes corresponding stripes of the first bonding film  45 . The stripes of both bonding films  45 ,  15  not only form a rectangular pattern in each device  110 , but also have short extensions extending from the mid-point of each stripe outward toward the respective cutting groove  60 . Thus, with respect to each stripe of the first bonding film  45  on adjacent bases  40 A and each stripe of the second bonding film  15  on adjacent lids, a respective short perpendicular stripe crosses the adjacent cutting groove  60 . 
     As shown in  FIG. 5B , first indents  66 A are formed on the inner major surface of the base wafer  40 WA, which is opposite the outer major surface on which the cutting grooves  60  are formed. Second indents  67 A are formed on the inner major surface of the second lid  10 A opposite the outer major surface on which the cutting grooves  60  are formed. In this embodiment the first and second indents  66 A and  67 A are not formed on the intersections of cutting grooves  60  extending in the X-axis direction and cutting grooves  60  extending in the Y-axis direction (compare to first embodiment). Nevertheless, individual first and second indents  66 A and  67 A have a hemispherical shape. A bonding ball  75  is placed in each first indent  66 A. As shown in  FIG. 5A , the first indents  66 A are formed just outboard of the area corresponding to the piezoelectric vibrating device  110  (i.e., outside the two-dotted chain line). As a result, the stripes of the first bonding film  45  and second bonding film  15  located just inboard of the periphery of each piezoelectric vibrating device  110  (two-dotted chain line) are planar and lack any indents. 
     Third Embodiment of Piezoelectric Vibrating Device 
       FIG. 6A  is a plan view of a package wafer  80 WB of this embodiment, as viewed from above the lid wafer  10 WB.  FIG. 6B  is an enlarged elevational view along the line D-D of  FIG. 6A . The lid wafer  10 WB is depicted as if it were transparent, and the figure mainly shows the tuning-fork type crystal vibrating piece  30  mounted on the base  40 B. For comprehension, respective areas (in the X-Y plane) of individual piezoelectric vibrating devices  120  are delineated using a virtual line (two-dotted chain line) on the package wafer  80 WB. Voids are shown as meshed zones to distinguish the tuning-fork type crystal vibrating piece  30 . 
     The piezoelectric vibrating device  120  of this embodiment differs from the piezoelectric vibrating device  100  of the first embodiment in that each first indent  66 B and second indent  67 B of the third embodiment is located at substantially the mid-length of the respective stripe of bonding film. Further description below will focus only on the differences of this embodiment  120  from the first embodiment  100  of a piezoelectric vibrating device. Similar components in each embodiment have the same reference numerals. 
     In  FIG. 6A  the stripes of second bonding film  15  are formed on the lid wafer  10 WB so as to be aligned (in the X-Y plane) with respective stripes of the first bonding film  45  on the base wafer  40 WB. The stripes of the second bonding film  15  and first bonding film  45  are located inboard of the outline profile of each piezoelectric vibrating device  120  so as not to touch the cutting grooves  60 . 
     The first indents  66 B are located at mid-length of the respective stripes of the first bonding film  45 . Similarly, the second indents  67 B are located at mid-length of the respective stripes of the second bonding film  15 . Each of the first indents  66 A and second indents  67 A has a hemispherical shape. Since the distance between adjacent (in the Z-direction) stripes of the first and second indents  66 B,  67 B is substantially constant, as the bonding ball  75  melts, the melt flows along and between the adjacent surfaces of the bonding films  45 ,  15  by capillary action, which “wets” the surfaces of the bonding films  45 ,  15  with the melt. 
     As shown in  FIG. 6B , the first indents  66 B and second indents  67 B of this embodiment are not situated on the cutting grooves  60 . As a result, when the package wafer  80 WB is being cut using a dicing blade, chipped metal does not clog the blade. 
     Fourth Embodiment of a Piezoelectric Vibrating Device 
       FIG. 7A  is a plan view of the package wafer  80 WC of this embodiment, as viewed from above the lid wafer  10 WC.  FIG. 7B  is an enlarged elevational section along the line E-E in  FIG. 7A . The lid wafer  10 WC is shown as if it were transparent, and the figure mainly shows the tuning-fork type crystal vibrating piece  30  mounted on the base  40 C. For comprehension, areas (in the X-Y plane) corresponding to respective piezoelectric vibrating devices  130  are delineated with a virtual line (two-dotted chain line) on the package wafer  80 WC. Voids are depicted as meshed zones to distinguish the tuning-fork type crystal vibrating piece  30 . 
     In this embodiment stripes of the bonding films  45 ,  15  form rectangular patterns. A first indent  66 C is formed at each corner of the rectangle formed by stripes of the first bonding film  45 , and a second indent  67 C is formed at each corner of the rectangle formed by stripes of the second bonding film  15 . This arrangement of stripes and indents distinguishes this embodiment from the third embodiment. Below, only differences from the third embodiment are described, wherein similar components have similar respective reference numerals. 
     As shown in  FIGS. 7A and 7B , the stripes of the second bonding film  15  on the lid wafer  10 WC face corresponding stripes of the first bonding film  45  on the base wafer  40 WC, with a second indent  67 C at each corner of the rectangular stripe pattern. The base wafer  40 WC includes stripes of the first bonding film  45 , with a first indent  66 C at each corner of the rectangular stripe pattern. The first indents  66 C and second indents  67 C are located on the respective “bonding surfaces” of the lid wafer  10 WC and base wafer  40 WC. The first indents  66 C and second indents  67 C each have a hemispherical shape, and a respective bonding ball  75  is placed on each pair of opposing indents. 
     Exemplary Method for Manufacturing Piezoelectric Devices 
     An embodiment of a method for manufacturing a piezoelectric device  100  according to the first embodiment is described below. A flow-chart of the method is shown in  FIG. 8 . 
     Steps S 102  and S 104  are applied to the lid wafer  10 W, steps S 112  and S 114  are applied to the crystal wafer used for forming vibrating pieces, and steps S 122  and S 124  are applied to the base wafer  40 W. Step S 152  and subsequent steps are applied to package wafers. 
     In step S 102  multiple lids  10  (including the concavity  17 , cutting grooves  60 , and second indents  67 ) are formed on the lid wafer  10 W, made of glass. Hundreds or thousands of such lids  10  are formed on the lid wafer  10 W, depending upon the size of the lid wafer and the size of each lid. 
     In step S 112  multiple tuning-fork type crystal vibrating pieces  30  are formed on a crystal wafer by wet-etching. Hundreds or thousands of tuning-fork type crystal vibrating pieces  30  are formed on the crystal wafer, depending upon the size of the crystal wafer and the size of each vibrating piece. 
     In step S 114  respective excitation electrodes  33 ,  34  and base electrodes  31 ,  32  are formed on the each crystal vibrating piece  30  formed on the crystal wafer. Each thus-formed tuning-type crystal vibrating piece  30  is cut and separated from the crystal wafer. 
     In step S 122 , multiple bases  40  (having the concavity  47 , cutting grooves  60 , first indents  66 , and first and second through-holes  41 ,  43 ) are formed on the base wafer  40 W, made of glass. Hundreds or thousands of bases  40  are formed on the base wafer  40 W, depending upon the size of the base wafer and the size of each base. 
     In step S 124  respective first and second connecting electrodes  42 ,  44  and first and second external electrodes  55 ,  56  are formed on each base wafer  40 W. 
     The first and second through-holes  41 ,  42 , previously formed on the base wafer  40 W, are sealed by melting a sealing material  70 . The sealing material  70  is a ball of eutectic metal such as gold-silicon (Au 3.15 Si) alloy or gold-germanium (Au 12 Ge) alloy. 
     In step S 126  a respective tuning-fork type crystal vibrating piece  30  is mounted, using electrically conductive adhesive  71 , on a respective mount  52  in the cavity  22  of each base on the base wafer  40 W. First, a unit of the electrically conductive adhesive  71  is applied from a dispenser to a site on the mount  52 , followed by placement of the respective tuning-fork type crystal vibrating piece  30 , held by a holding device (not shown), on the unit of electrically conductive adhesive. The tuning-fork type crystal vibrating pieces  30  are thus mounted one at a time to the respective mounts  52  in the cavities  22  in the base wafer  40 W. After mounting all the tuning-fork type crystal vibrating pieces  30  on the mounts  52 , the electrically conductive adhesive  71  is cured to solidify it. Then, each tuning-fork type crystal vibrating piece  30  is connected to the respective first connecting electrode  42  and second connecting electrode  44  on the base wafer  40 W, both mechanically and electrically. For example, each unit of adhesive  71  is obtained from a paste of silicon-based electrically conductive adhesive or epoxy-based electrically conductive adhesive. 
     In step S 152  a respective bonding ball  75  is placed on each first indent  66  on the stripes of the first bonding film  45  of the base wafer  40 W. The first indents  66  each have a hemispherical shape to accommodate a respective bonding ball  75 . As the lid wafer  10 W is lowered onto the base wafer  40 W, the second indents  67  of the lid wafer  10 W are placed atop respective bonding balls  75 . Thus, the bonding balls  75  serve as alignment guides for achieving alignment of the lid wafer  10 W with the base wafer  40 W. The second indents  67  also have a hemispherical shape to facilitate fitting to the respective bonding balls  75 , thereby ensuring that the lid wafer  10 W does not move relative to the base wafer  40 W. This stability of the lid wafer  10 W relative to the base wafer  40 W is maintained through placement of the stacked wafers in a reflow furnace (not shown). 
     In the reflow furnace, the bonding balls are melted (step S 154 ). A portion of the melt flows over the bonding surface of the first base wafer  40 W, as facilitated by capillary action of the respective stripes. Melting of the bonding balls  75  can be conducted, in the furnace, in either a vacuum or inert-gas environment. Thus, the void  22  is evacuated or filled with an inert gas. As the bonding balls  75  melt, the resulting melt flows and spreads to the stripes of the first and second bonding films  45 ,  15 . After completion of melt flow, the temperature of the reflow furnace is reduced to a predetermined temperature. This bonding together the first and second bonding films  45 ,  15  forms the package wafer  80 W. 
     In step S 156 , the package wafer  80 W is affixed to a dicing film (not shown) and cut along the cutting grooves  60  using a dicing saw. By providing appropriate space between the dicing saw and the cutting grooves  60 , the dicing saw can cut the package wafer without touching the dicing sheet or the adhesive. As a result, burrs or chips are not produced during dicing. 
     Upon completing the foregoing steps, fabrication of the piezoelectric devices  100  is completed. Since the interior of each piezoelectric device  100  is in a vacuum state or filled with an inert gas, each device produces stable oscillations. 
     By forming each package using the respective bonding films and indents formed on the lid and base, hermetic sealing of each piezoelectric device is ensured. 
     In the foregoing method embodiment, the lid and base are made of glass. In other embodiments other materials may be used such as a crystal material (e.g., quartz crystal). The reason for allowing this substitution is as follows. One of the indicators of hardness of an industrial material is the “Knoop hardness.” A higher Knoop hardness number indicates greater hardness, and a lower number indicates greater softness. The Knoop hardness number of borosilicate glass (commonly used for making lids and bases) is 590 kg/mm 2 , and the Knoop hardness number of quartz crystal is 710 to 790 kg/mm 2 . Thus, making the lids and bases of crystal instead of glass produces vibrating devices having a higher degree of hardness. If the lids and bases are made of glass, the thickness of glass would have to be correspondingly thicker to meet a designated degree of hardness and strength. But, when fabricated of crystal, these components can be made with a thinner profile while achieving the same strength and hardness. I.e., in fabricating piezoelectric devices in which the lids and bases are made of crystal instead of glass, devices having the same strength and hardness as obtained when they are made of glass can be made that are more miniaturized and thinner than if they were made of glass. 
     In the embodiments described above, the vibrating devices included respective tuning-fork type crystal vibrating pieces. In alternative embodiments, AT-cut crystal panels can be used instead that exhibit thickness shear vibrations. In addition, in various alternative embodiments, other combinations of bonding surfaces and/or shapes of bonding materials can be used.