Patent Publication Number: US-2012023720-A1

Title: Piezoelectric devices and methods for manufacturing same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 12/237,301, filed Sep. 24, 2008, which claims priority to, and the benefit of, Japan Patent Application No. 2007-251821, filed on Sep. 27, 2007, in the Japan Patent Office, the disclosures of which are incorporated herein by reference in their entirety. 
     FIELD 
     This disclosure relates to, inter alia, piezoelectric devices of which the package is formed from a glass wafer with embedded metal wires. The disclosure also pertains to methods for manufacturing piezoelectric devices, including sealing of piezoelectric vibrating pieces in their respective packages. 
     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 oscillators used in this equipment must be progressively smaller and/or operate at higher frequency. Also required are piezoelectric oscillators that can be surface mounted on circuit boards. 
     Contemporary piezoelectric oscillators comprise piezoelectric vibrating pieces sealed in respective device packages, wherein the piezoelectric vibrating piece is connected to electrodes in the package. Generally, the package is made of ceramic, and defines a predetermined interior volume to contain the piezoelectric vibrating piece. At the bottom of the package, holes are formed through a bottom portion of the package, and the through-holes are sealed with solder. 
     Example package-sealing techniques using a ceramic package are discussed in Japan Unexamined Patent Publication No. 2003-158439, in which a first adhesive such as a brazing filler metal is applied on the top of the package. The brazing filler metal is melted in a heat chamber with application of pressure to bond the package. Then, a sealing material such as high-temperature solder is applied to the through-holes in a vacuum or inert-gas environment. The package is sealed by irradiating the high-temperature solder with a laser beam. 
     Further regarding the sealing method discussed in Japan Unexamined Patent Publication No. 2003-158439, the package-sealing material is inserted in the through-holes formed on the bottom of the package, and the through-holes are sealed by melting the sealing material using laser light. However, this sealing must be performed individually with each piezoelectric vibrating piece being packaged on an assembly line. Hence, mass-production (with consequent item cost reduction) is difficult. 
     The shortcomings of conventional methods summarized above are addressed by methods and devices as disclosed herein, in which a wafer of low-melting-point glass is formed by casting the glass in a mold in which metal wires having low electrical resistance are arranged at designated positions. After casting the glass is cooled, removed from the mold, and sliced to form the glass wafers with embedded lead lines formed from the metal wires. On the sliced wafers are formed multiple package pieces (notably package bases) each with respective embedded wires for use in packaging piezoelectric vibrating pieces. 
     SUMMARY 
     A piezoelectric device of a first aspect comprises a piezoelectric vibrating piece having first and second electrodes. The piezoelectric vibrating piece is sandwiched between and sealed to a glass base below and a lid above. The glass base has a first surface and an opposing second surface. The first surface is an interior surface of the piezoelectric device, and the second surface is an exterior (lower) surface. The glass base also has at least first and second metal wires extending through the glass base and extending from the first and second surfaces. The extending ends of the wires are denuded to the first and second surfaces. The ends denuded to the first surface are connected to the first and second electrodes when the glass base is attached to the piezoelectric vibrating piece. The first and second metal wires extend through the glass base and are inherently sealed to the glass base. (Sealing of the wires to the glass desirably is achieved by casting the glass, in a molten condition, around the wires.) The wires sealed to the glass base eliminate a need, with conventional bases, to perform a separate step to seal wires passing through the base, such as via through-holes. Thus, piezoelectric devices can be manufactured at lower cost than conventionally. 
     The glass base of the piezoelectric device desirably includes respective connection electrodes on its second surface that are connected to the first and second wires, respectively. Thus, each of the connection electrodes is connected surely to the first and second electrode patterns without any contact failure. 
     The metal wires can be made of any of various suitable metals such as, but not limited to, iron (Fe), copper (Cu), nickel (Ni), stainless (SUS), or kovar alloy. These metals are relatively inexpensive and have superior electrical conductivity. The wires desirably are coated with a metal such as gold (Au). The gold layer on the metal wires increases the affinity of the wires to glass and improves the electrical conductivity of the wires through the glass base. Gold also improves connection of the wires to the respective connection electrodes. The surface roughness of the wires can be increased to increase the affinity of the wires to the glass of the base. The gold layer can be formed after increasing the surface roughness of the wires. 
     According to another aspect, methods for manufacturing piezoelectric devices are provided. An embodiment of such a method comprises preparing a lid wafer on which is formed an array of multiple lids, preparing a piezoelectric wafer on which is formed an array of multiple piezoelectric vibrating pieces each having a first electrode and a second electrode, and preparing a glass base wafer on which is defined an array of multiple bases. The glass base wafer has a first surface and an opposing second surface. Each base includes at least a first metal wire and a second metal wire extending through the base and having respective ends denuded to the first surface. The method includes the step of sandwiching the piezoelectric wafer between the lid wafer and the glass base wafer in a manner resulting in registration of each piezoelectric vibrating piece with a respective base and a respective lid. The wafers are bonded together in a manner resulting in bonding each of the piezoelectric vibrating pieces to a respective lid and respective base, and automatic connection of the denuded metal wire-ends on the first surface of the base to the first and second electrode patterns of the piezoelectric vibrating piece. The resulting multiple piezoelectric devices are then cut or otherwise detached from the bonded three-layered wafer. According to this method embodiment, assembly and sealing of individual piezoelectric devices as performed in conventional methods are avoided because multiple piezoelectric devices are made simultaneously by performing all process steps at the wafer level. This allows piezoelectric devices to be manufactured at lower cost than conventionally. 
     In the method, the step of preparing the glass base wafer can include the steps of arranging the first and second metal wires substantially parallel to each other at a designated spacing in a mold; fusing glass fragments at a fusing temperature of 900 C or less; forming a glass block containing the first and second metal wires by pouring the fused glass into the mold around the wires and allowing the glass to harden; and slicing the glass block in a direction perpendicular to the metal wires to form glass wafers. By forming glass wafers in this manner, each having embedded wires, mass production of bases can be achieved. Bubbles in the glass of the glass block can be reduced by pouring the fused glass into the mold after fusing the glass fragments. 
     In an alternative embodiment, the step of preparing the glass base wafer can further comprise the steps of placing the glass fragments along with arranged wires (sufficient to provide at least first and second metal wires for each base), arranged substantially parallel to each other in a designated space in a mold; fusing the glass fragments in the mold at 900 C. or less to form a glass block containing the metal wires; allowing the fused glass to harden to form a glass block; and slicing the glass block in a direction perpendicular to the metal wires to form glass wafers. Thus, this embodiment differs from the first method embodiment by forming a glass block, containing the metal wires, in the mold by pouring the glass fragments (rather than a glass melt) into the mold and heating the mold to a fusing temperature. The glass wafers are manufactured by slicing them from the block so that mass production of glass wafers can be achieved. 
     If desired, a glass film can be formed on the surfaces of the metal wires to confer high affinity of the wires to the fused glass. After forming the glass film on the wires, the wires are arranged in the mold at designated positions. Thus, the first and second metal wires for each piezoelectric device formed from the glass wafer are strongly bonded to their respective glass base. This allows bonding of the wafers to be performed in a vacuum so as to seal a vacuum environment inside each piezoelectric device. Maintenance of the vacuum is assured due to the absence of any space between the first and second wires and the glass base of each piezoelectric device. 
     As noted above, the surface roughness of the metal wires can be increased to increase the affinity of the wires to the fused glass. The roughened wires are arranged at their designated positions in the mold. The consequent strong bonding of the first and second metal wires to the respective bases allows processing in a vacuum to produce a vacuum environment inside each piezoelectric device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of the exterior of the crystal unit of the first embodiment and the crystal unit of the second embodiment. 
         FIG. 2A  is a schematic plan view of an embodiment of a base glass wafer. 
         FIG. 2B  is a cross-sectional view along the B-B line in  FIG. 2A . 
         FIGS. 2C ,  2 D, and  2 E are schematic plan views showing respective possible positions of metal wires in the base. 
         FIG. 2F  is a plan view of a portion of a base glass wafer on which multiple bases are formed. 
         FIG. 3A  is a schematic perspective view of a mold used for forming a glass block from which base glass wafers are obtained. 
         FIG. 3B  is a cross-sectional view along the A-A line in  FIG. 3A . 
         FIG. 3C  is a cross-sectional view along the B-B line in  FIG. 3A . 
         FIG. 3D  is a top view of a portion of a glass block, showing wires and lines along which base glass wafers are cut from the block. 
         FIG. 3E  is a top edge view of a base glass wafer. 
         FIG. 4A  is a schematic elevational section of a crystal unit according to a first embodiment, the section being along the line X-X in  FIG. 4C . 
         FIG. 4B  is a plan view of the under-surface of a lid used for packaging, the lid including a concave region. 
         FIG. 4C  is a top view of the crystal unit without the lid to show interior detail. 
         FIG. 4D  is a plan view of the upper surface of a glass base used for packaging, the lid including a concave region and connection electrodes. 
         FIG. 4E  is a schematic view showing a setup for performing anodic bonding. 
         FIG. 5A  is a schematic elevational section of a crystal unit according to a second embodiment. 
         FIG. 5B  is a plan view of the under-surface of a lid used for packaging, the lid including a concave region. 
         FIG. 5C  is a top view of the crystal unit without the lid to show interior detail. 
         FIG. 5D  is a plan view of the upper surface of a glass base used for packaging, the lid including a concave region and connection electrodes. 
         FIG. 6A  is a schematic perspective view of a crystal wafer  10  on which are formed multiple crystal frames and respective tuning-fork type crystal vibrating pieces. 
         FIG. 6B  is an enlarged plan view of three crystal frames on the crystal wafer of  FIG. 6A . 
         FIG. 7  is a schematic perspective view showing a lid glass wafer (on which multiple first lids are formed), a crystal wafer (on which multiple tuning-fork type crystal vibrating pieces and their crystal frames are formed), and a base glass wafer (on which multiple first bases are formed). For simultaneous manufacture of multiple crystal units, these wafers are stacked in registration to form a wafer sandwich. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Crystal Units 
       FIG. 1  is a schematic view of a first embodiment of a crystal unit  100  as an exemplary piezoelectric device. The crystal unit  100  is a surface-mountable piezoelectric device suitable for mounting to a printed base. The crystal unit  100  mainly comprises an insulating glass base  40 , a crystal frame  50 , and a glass lid  65 . The crystal frame  50 , the glass base  40 , and the glass lid  65  are manufactured in wafer units to increase productivity. First, a base glass wafer  20  for the glass base  40  is described below. 
     Configuration of Base Glass Wafer 
     The base glass wafer  20  is described with reference to  FIGS. 2A-2E .  FIG. 2A  is a plan view of the base glass wafer  20 .  FIG. 2B  is a cross-sectional view along the line B-B in  FIG. 2A .  FIGS. 2C ,  2 D, and  2 E are plan views of exemplary respective patterns of metal wires  23  formed in bases  40  cut from the wafer  20 . 
     In  FIG. 2A , metal wires  23  are arranged at designated positions and embedded in the base glass wafer  20 . The metal wires  23  are comprised of nickel (Ni), stainless (SUS), iron (Fe), or kovar alloy (Fe, Ni, and Co). The base glass wafer  20  is sliced from a glass block  25 , as explained later with reference to  FIG. 3B . I.e., a method for manufacturing the base glass wafer  20  is described later with reference to  FIGS. 3A and 3B . 
     In  FIG. 2B  the metal wires  23  pass through the thickness of the base glass wafer  20  from the front surface to the rear surface thereof. In the figure, the metal wires  23  extend in directions that are normal to the front and back surfaces. However, this is not intended to be limiting; the wires can extend through the wafer  20  at non-normal angles relative to the front and back surfaces. The thickness D of the base glass wafer  20  is established when the block is sliced using a saw, and usually ranges from 0.3 mm to 1.0 mm. 
       FIGS. 2C ,  2 D, and  2 E show individual glass base units  40 - 1 ,  40 - 2 ,  40 - 3 , respectively, with respective patterns of metal wires  23 . On the type-A base  40 - 1  of  FIG. 2C , two metal wires  23  are arranged near the left edge, with space between the wires and the adjacent edges. On the type-B base  40 - 2  of  FIG. 2D , two metal wires  23  are arranged substantially along a diagonal (one wire in the upper left corner, and the other wire in the lower right corner), with space between the wires and the adjacent edges. On the type-C base  40 - 3  of  FIG. 2E , a respective metal wire  23  is situated in each of the four corners, with space between the wires and the adjacent edges. These positions of the metal wires  23  on the bases depend upon the respective positions of electrodes, of the crystal vibrating pieces to be formed on the crystal frames  50 , intended to be connected to the wires. 
       FIG. 2F  is a plan view of a portion of the base glass wafer  20  on which multiple type-A bases  40 - 1  have been formed. In  FIG. 2F  the multiple type-A bases  40 - 1  have been formed, by etching, on the rectangular base glass wafer  20 . The regions that are hatched are actually voids  22  formed in the base glass wafer  20  by etching, resulting in the formation of the bases  40 - 1  nearly surrounded by respective voids. In the central area of each of the type-A bases  40 - 1 , a concave region  47  is formed by etching a selected depth into the glass of the base. The bases  40 - 1  are arranged in blocks of three in the figure, wherein each group is substantially surrounded by voids  22 . Alternatively to forming surrounding voids  22 , the bases  40  can be cut from the wafer  20  piece by piece using a dicing saw. 
     Further shown in  FIG. 2F  is an orientation flat  20   c  on one corner of the rectangular glass wafer  20 . The orientation flat  20   c  provides a directional reference for orienting the bases  40 . The bases  40  are connected to the glass wafer by narrow connection portions  11 . Thus, multiple bases  40  can be handled and processed simultaneously as a single unit by processing the base glass wafer  20 . 
     Manufacturing the Base Glass Wafer 
     Aspects of an embodiment of a process for manufacturing the base glass wafer  20 , described above in connection with  FIGS. 2A-2F , are now described with reference to  FIGS. 3A-3C , which depict different views of an exemplary mold  70  used for forming a glass block  25  from which base glass wafers  20  are cut. 
       FIG. 3A  is a perspective view of the mold  70  to form the glass block  25 . The mold  70  is formed of machined plates of graphite assembled into a box. The mold  70  has a glass inlet  72  at one end, the inlet extending down the side of the mold  70 . In the lateral plates of graphite are holes  73 , in designated positions, in which metal wires  23  are inserted. The wires  23  extend across the inside of the mold to corresponding holes  73  in the opposing lateral plate. In the depicted mold, the holes  73  formed on the side surface are formed at the same positions on both X and Y surfaces. The interior of the mold  70  has a depth of 100 mm to 150 mm, a width of 20 mm to 50 mm, and a length of 100 mm to 150 mm. The metal wires  23  are pre-processed to have an affinity to glass to ensure good and close bonding of the wires to glass added to the mold. 
     If the wires  23  are made of copper wire, they are preprocessed by heating to 800 C and annealed to remove stresses, cooled, and soaked in a saturated aqueous solution of borax (Na 2 B 4 O 7 .10H 2 O). The copper wire is heated again, to 900 C., resulting in formation of a borax glass (Na 2 B 4 O 7 ) coating on the surface of the copper wire. 
     If the metal wires  23  are made of nickel, stainless, iron, or kovar alloy, the surface of the wire is scabbled (roughened) by sandblasting to increase its roughness, and then washed. A layer of gold is formed on the wire by electroplating. The gold layer not only enhances the affinity of the wire for glass but also improves conductivity of the wire. 
       FIG. 3B  is a cross-sectional view of the mold  70  along the line B-B of  FIG. 3A . The wires  23  that have been preprocessed have been threaded through the holes  73 , followed by application of graphite paste on the outer surface  74  of the mold. The graphite paste is cured by placing the mold  70  into a first electrical furnace, not shown. 
       FIG. 3C  is a cross-sectional view along the line C-C in  FIG. 3A .  FIG. 3C  shows an inlet guide  75  attached to the glass inlet  72  of the mold  70 . The inlet guide  75  is not fixed to the inlet  72  so that it is easy to remove from the mold. In this condition the mold  70  is placed in a second electrical furnace in which the mold is pre-heated to 650 C. in preparation for receiving molten glass. 
     The molten glass desirably is prepared from 100% cullet of sodium borosilicate glass. The cullet is glass waste obtained by comminuting glass after fusion and cooling. In other processes, cullet is used to facilitate fusion of glass, wherein cullet having the same composition as the glass to be fused is mixed 20% to 50% to a glass formulation to cause fusion. For the present purposes, the glass for the block is made entirely of cullet of sodium borosilicate glass. To make molten glass, the cullet is added to a crucible, and the crucible is heated in the second electrical furnace. The crucible can be made of, for example, porcelain, graphite, or platinum. 
     The softening temperature of the cullet is about 650 C., which is less than the fusion temperature. The grain size of the cullet is 1-2 mm. Desirably, the cullet does not include significant amounts of fine glass powder because glass powder in the cullet causes, during fusion, persistent entrainment of bubbles that are very difficult to remove, and a glass block  25  containing entrained bubbles may be unsatisfactory for making glass wafers. In the second electrical furnace the crucible containing the cullet increases in temperature to 800 C., at which fusion begins, and then to 900 C. at which the cullet is completely fused. It is important to control fusion temperature; if the cullet is fused at significantly greater than 900 C., then the metal of the wires (copper, nickel, stainless, iron, or kovar alloy) may become fused or the metal wires  23  may be broken when the molten glass is introduced to the mold. 
     During fusion of the cullet, it is desirable to allow bubbles to rise and dissipate. At the appropriate time, the crucible containing the fused glass is moved to the pre-heated mold  70 , and the fused glass is poured gently into the mold  70  through the inlet guide  75  and the inlet  72 . The fused glass moves in the directions indicated by the arrows in the inlet  72  ( FIG. 3C ) and fills the mold  70  from bottom to top with molten glass. 
       FIGS. 3D and 3E  schematically depict formation of the base glass wafer  20  from a glass block  25 .  FIG. 3D  is a partial side elevational view of the glass block  25 , and  FIG. 3E  is an edge view of a wafer that has been sliced from the block. The glass block  25  shown in  FIG. 3D  is cooled down in the first electrical furnace after the block is cast in the mold  70 . Then, after cooling to approximately room temperature, the block is removed from the mold  70 . 
     The glass block  25  is sliced using a slicing saw along the dotted lines denoted “CUT.”  FIG. 3E  shows the edge of a sliced base glass wafer  20 . On the front and back major surfaces of the base glass wafer  20  (as cut by the slicing saw), unevenness or flexure is usually evident. To remove unevenness or flexure, and to reveal the ends of the metal wires  23  on the surfaces of the base glass wafer  20 , etching is performed on the “first and second surfaces” (front and rear major surfaces) of the wafer to reduce surface irregularities to 100-300 Ångstroms. Etching also exposes and denudes the ends of the metal wires  23  so that they extend slightly from the first and second surfaces of the base glass wafer  20  (see  FIG. 2B ). 
     As described, the glass block  25  is formed by pouring the fused glass, produced separately by fusing the cullet in the second electric furnace, into the mold  70 . Alternatively, the cullet can be added to and fused in the mold  70  placed in the first electric furnace heated to 900 C. The alternative procedure eliminates having to transfer the fused glass into the mold. In the alternative procedure, bubbles must not be left in the glass block  25 . Whenever anodic bonding is performed as shown in  FIG. 4E , metal ions need to be added to the cullet. If the glass melt can be formed at a temperature that does not damage or melt the metal wires  23 , any of various types of glass can be used. 
     First Embodiment of Crystal Unit 
     A crystal unit  100  (as an exemplary piezoelectric device) according to this embodiment is shown in  FIGS. 4A-4D , which are schematic views.  FIG. 4A  is an elevational section along the line X-X in  FIG. 4C ,  FIG. 4B  is a plan view of the under (inner) surface of the lid  65  of the package,  FIG. 4C  is a top (plan) view of the crystal frame  50 , and  FIG. 4D  is a top (plan) view of the first base  40 . 
     In  FIG. 4A  the crystal unit  100  includes a crystal frame  50  in which a tuning-fork type crystal vibrating piece  30  is situated centrally. A first base  40  is attached to the bottom surface of the crystal frame  50 , and a first lid  65  is attached to the top surface of the crystal frame  50 . In the crystal frame  50  the tuning-fork type crystal vibrating piece  30  was formed from a crystal wafer  10  by etching. 
     As  FIG. 4B  shows, the lid  65  has a lid concave portion  67  formed by etching of the surface of the first lid on the crystal-frame side thereof. The first lid  65  can be formed of Pyrex® glass, borosilicate glass, or sodium glass (hereinafter called the “lid glass wafer”  60 ). 
     As shown in  FIG. 4C , the crystal frame  50  includes the so-called tuning-fork type crystal vibrating piece  30  located substantially centrally, and an outer frame  51  surrounding the piece  30 , with a void  52  being defined between the tuning-fork crystal vibrating piece  30  and the outer frame  51 . The void  52 , which defines the outline of the tuning-fork type crystal vibrating piece  30 , is formed by etching of the crystal material. The tuning-fork type crystal vibrating piece  30  comprises a base portion  32  and a pair of vibrating aims  31  extending horizontally from the base portion  32 . The base portion  32  and the outer frame  51  are formed as a single unit. A first base electrode  33  and a second base electrode  34  are formed on a “first surface” (upper surface) of the base portion  32 , on a “second surface” (lower surface) of the base portion, and on the outer frame  51 . 
     On the first and second surfaces of the crystal vibrating piece  30 , first and second excitation electrodes  35 ,  36 , respectively, are formed. The first excitation electrode  35  is connected to the first base electrode  33  on the base portion  32  and the outer frame  51 , and the second excitation electrode  36  is connected to the second base electrode  34  on the base portion  32  and the outer frame  51 . Also, near the distal ends of the vibrating arms of the tuning-fork type crystal vibrating piece  30 , respective “weights”  37 ,  38  are formed. The first and second base electrodes  33 ,  34 , the first and second excitation electrodes  35 ,  36 , and the weights  37 ,  38  are formed earlier using a process involving photolithography and etching. Whenever a voltage is applied to these electrodes, the arms of the tuning-fork type crystal vibrating piece  30  vibrate with a designated frequency. The weights  37 ,  38  facilitate ease of vibration of the vibrating arms  31  and adjustment of the vibration frequency. 
     Each of the upper and lower surfaces of the outer frame  51  comprises a metal film  53 . The metal films  53  are formed by a sputtering or vacuum-deposition technique. Each metal film  53  desirably is a layer of aluminum (Al) having a thickness of approximately 1000-1500 Ångstroms. 
     In  FIG. 4D  the first base  40  is formed by etching of the surface of a base glass wafer  20  to form a base concave portion  47  and step portion  49 . As described above, the ends of the two metal wires  23  are denuded and caused to protrude slightly from the first and second surfaces of the base glass wafer  20  by etching. At the areas where the metal wires  23  protrude, a first connection electrode  42  and a second connection electrode  44  are formed, respectively. The protruding wires  32  are easily bonded to the first and second connection electrodes  42 ,  44 . 
     During assembly, the first connection electrode  42  overlaps and thus contacts the first base electrode  33 , and the second connection electrode  44  overlaps and thus contacts the second base electrode  34  of the crystal vibrating piece  30 . Since the first and second connection electrodes  42 ,  44 , respectively, and the first and second base electrodes  33 ,  34  are layered, a space having a thickness substantially equal to the thickness of the respective stacked electrodes exists between the outer frame  51  and the crystal portion of the glass base  40 . To reduce this space, the first and second connection electrodes  42 ,  44  are formed at the step portion  49 , which is several hundred Ångstroms lower than the surface of the glass base  40 . 
     The first and second connection electrodes  42 ,  44  of the first base  40  are formed using a method involving photolithography and etching. The first base  40  includes first and second external electrodes  45 ,  46  which are respective metalized regions on the lower surface of the first base  40 . The first connection electrode  42  is connected to the first external electrode  45  (situated outside the first base  40 ) via a metal wire  23 . Similarly, the second connection electrode  44  is connected to the second external electrode  46  (situated outside the first base  40 ) via a metal wire  23 . 
       FIG. 4E  is an elevational section schematically depicting the results of anodic bonding used for forming and sealing the package. The first lid  65  and the first base  40  are made of Pyrex® glass, borosilicate glass, or sodium glass, which are all glasses containing metal ions such as sodium ions. The outer frame  51  of the crystal frame  50  includes the metal film  53 , which can be made of aluminum, on its upper and lower surfaces. The crystal frame  50 , including the centrally located tuning-fork type crystal vibrating piece  30 , is sandwiched between the first lid  65  having the lid concave portion  67  and the first base  40  having the base concave portion  47 . Alternatively to aluminum, the metal film  53  can comprise a layer of gold formed on a base layer of chromium. 
     To form the layered package structure, the crystal frame  50 , the first lid  65 , and the first base  40  are heated up to a temperature of 200-400 C. in a vacuum or inert-gas environment. The upper surface of the first lid  65  and the lower surface of the first base are set to a negative dc potential, and the metal films  53  of upper and lower surfaces of the outer frame  51  are set to a positive dc potential. Then, the crystal frame  50 , first lid  65 , and first base  40  are bonded together by anodic bonding including application, from a dc power source  90 , of 400 Vdc for 10 minutes while the package is in a vacuum or inert-gas atmosphere. Anodic bonding causes the first and second base electrodes  33 ,  34  and first and second connection electrodes  42 ,  43  of the crystal unit  100  to be connected together tightly. 
     Anodic bonding involves chemical reactions that oxidize the metal on joint interfaces. To achieve anodic bonding, the metal film is temporarily used as a positive electrode while the surface facing the connecting surface of glass is temporarily used as a negative electrode, and an electrical field is impressed between these electrodes. The field causes metal ions such as sodium in the glass to move toward the negative electrode, which oxidizes the metal film contacting the glass material at the joint interface and causes bonding of the surfaces. In this embodiment, by way of example, the metal was Al and the contacting dielectrics were heated (200-400 C) while applying a dc voltage of 500 V to 1 kV, yielding chemical bonding (and thus sealing together) of the metal and glass at the joint interfaces. 
     Second Embodiment of Crystal Unit 
     A crystal unit  110  according to this embodiment is described with reference to the schematic views shown in  FIGS. 5A-5D .  FIG. 5A  is an elevational section along the line X-X in  FIG. 5C ,  FIG. 5B  is a plan view of the lower (inner) surface of the second lid  65 ′,  FIG. 5C  is a top plan view of the crystal frame  50 , and  FIG. 5D  is a top view of the second base  40 ′. 
     In  FIG. 5A  the crystal unit  110  comprises a crystal frame  50  of which a tuning-fork type crystal vibrating piece  30  is a central portion. A second base  40 ′ is attached to the lower surface of the crystal frame  50  and a second lid  65 ′ is attached to the upper surface of the crystal frame  50 . 
     Turning to  FIG. 5B , the second lid  65 ′ includes a lid concave portion  67 ′ formed on the under-surface thereof. This under-surface will be situated inside the completed package. The lid concave portion  67 ′ is formed by etching of the lid material to a defined depth. The under-surface of the second lid  65 ′ also includes a metal film  68  having substantially the same shape and composition as of the metal film  53  formed on the outer frame  51  of the crystal frame  50  (made of, e.g., a gold layer on a base chrome layer). The second lid  65 ′ can be made from a lid glass wafer made of for example, Pyrex® glass, borosilicate glass, or sodium glass. 
     As shown in  FIG. 5C , the crystal frame  50  includes a so-called tuning-fork type crystal vibrating piece  30  centrally located and surrounded by an outer frame  51 . Between the tuning-fork crystal vibrating piece  30  and the outer frame  51  is a void  52 . The crystal vibrating piece  30  has substantially the same configuration and electrodes as in the first embodiment ( FIG. 4C ); consequentially, components of the second embodiment that are similar to corresponding components in the first embodiment have the same respective reference numerals and are not described further. A metal film  53  is formed on the upper and lower surfaces of the outer frame  51 . 
     The metal film  53  can be formed by a sputtering or vacuum-deposition technique. In this embodiment the metal film  53  comprises a gold layer on a chromium base layer. Alternatively, the metal film  53  can comprise a layer of gold or silver on a base layer of nickel (Ni) or titanium (Ti). With respect to a chromium-gold metal film, by way of example, the thickness of the chromium layer is in the range of approximately 500-1000 Ångstroms, and the thickness of the gold layer is approximately 500-1500 Ångstroms. 
     In  FIG. 5D  the second base  40 ′ is formed from a base glass wafer  20  and has base concave portion  47 ′ formed by etching of the first surface (upper surface) facing the crystal frame  50 . As a result of etching of the base glass wafer, the denuded ends of the at least two metal wires  23 ′ protrude slightly from the surface of the second base  40 . Where the metal wires  23 ′ protrude, a first connection electrode  42 ′ and second connection electrode  44 ′ are formed. The protrusion of the metal wires  23 ′ simplifies connection thereto of the first and second connection electrodes  42 ′,  44 ′. 
     Also formed on the upper surface of the second base  40 ′ is a metal film  48  having substantially the same shape as of the metal film  53  formed on the lower surface of the outer frame  51  of the crystal frame  50 . 
     As they overlap each other, the first connection electrode  42 ′ contacts the first base electrode  33 , and the second connection electrode  44 ′ contacts the second base electrode  34  of the crystal vibrating piece  30 . As a result of the layered configurations of the first and second connection electrodes  42 ′,  44 ′ and of the first and second base electrodes  33 ,  34 , a space having the thickness of an electrode exists between the outer frame  51  and the crystal portion of the glass base  40 . To narrow this space, the first and second connection electrodes  42 ′,  44 ′ are formed on a step portion  49  that is several hundred Ångstroms lower than the first surface of the glass base  40 . 
     The gold layer of the second lid  65 ′ and the gold layer of the metal film  53  formed on the upper surface of the outer frame  51  of the crystal frame  50  are multi-layered films, as are the gold layer of the second base  40 ′ and the gold layer of the metal layer  53  formed on the lower surface of the outer frame  51  of the crystal frame  50 . When these multi-layered structures are impressed at high temperature (e.g., 300 C.) in a chamber in which the environment is a vacuum or inert gas, the metals become bonded. As a result of this bonding method, the interior of the package  80  of the crystal unit  110  is either a vacuum or filled with an inert gas. 
     At the time at which the second base  40 ′ and the crystal frame  50  are bonded together, the first and second base electrodes  33 ,  34 , formed on the second main surface of the outer frame  51 , are connected to the first and second connection electrodes  42 ′,  44 , respectively, on the second base  40 ′. 
     Process for Manufacturing Crystal Frames and Tuning-Fork Type Crystal Vibrating Pieces 
       FIG. 6A  is a schematic perspective view of an upper surface of a crystal wafer  10  on which multiple tuning-fork type crystal vibrating pieces  30  have been formed. As understood from  FIG. 6A , the tuning-fork type crystal vibrating pieces  30  and crystal frames  50  are formed at the same time by etching a square crystal wafer  10 . More specifically, the shaded regions in the figure are etched away, forming corresponding voids  12 ,  52 . Thus, the tuning-fork type crystal vibrating pieces  30  and their crystal frames  50  are defined on the wafer  10 .  FIG. 6A  shows, by way of example, fifteen blocks arranged on the wafer, wherein each block includes three respective tuning-fork type crystal vibrating pieces  30  each with its respective crystal frame  50 . To define an axial direction of the square crystal wafer  10 , an orientation flat  10   c  defining the crystal-lattice direction is formed at the appropriate region of the periphery  10   e  of the crystal wafer  10 . In  FIG. 6A  forty-five tuning-fork type crystal vibrating pieces  30  are shown on the crystal wafer  10  for ease of illustration; on an actual crystal wafer  10 , hundreds or thousands of tuning-fork crystal vibrating pieces  30  are typically formed. 
     In  FIG. 6A , the tuning-fork type crystal vibrating pieces  30  are still attached to their respective crystal frames  50 , and the crystal frames  50  are still attached to the crystal wafer  10 . The crystal wafer  10  with all these attached structures can be processed as a unit rather than having to process individual crystal vibrating pieces in their respective frames. 
       FIG. 6B  is an enlarged plan view of a block comprising three tuning-fork type crystal vibrating pieces  30  and their respective crystal frames  50 . The hatched regions  12 ,  52  are actually etched away, forming respective through-voids in the crystal wafer  10 . The remaining structures in the block are the tuning-fork type crystal vibrating pieces  30  and their respective crystal frames  50 . Respective connecting portions  11  are situated on a portion of the periphery of the crystal frame  50 , connecting the frames  50  and their respective pieces  30  to the crystal wafer  10 . By thus connecting the various structures together on one wafer, multiple crystal frames  50  and crystal vibrating pieces  30  can be manipulated and processed at one time by manipulating and processing the entire crystal wafer  10 . 
     The respective first and second base electrodes  33 ,  34 , first and second excitation electrodes  35 ,  36 , and weights  37 ,  38  are formed on the crystal vibrating pieces  30  while the pieces are still attached to the crystal wafer  10 . Also formed are groove regions  39  on the pairs of vibrating arms  31  extending from the base  32 . The first and second excitation electrodes  35 ,  36  are formed in the groove regions  39  to lower the CI (crystal impedance) value of the pieces  30 . The first and second base electrodes  33 ,  34 , the first and second excitation electrodes  35 ,  36 , and the weights  37 ,  38  on the tuning-fork type crystal vibrating piece  30  are formed by photolithography and etching steps performed after forming respective metal films by sputtering or vacuum-deposition. Particularly, metal films comprising a layer of gold or silver on a base layer of chrome, nickel, or titanium are formed by sputtering for use as base electrodes. In such an embodiment, the thickness of the chrome layer is 500-1000 Ångstroms, and the thickness of the gold layer is 500-1000 Ångstroms. Hence, the total thickness of each of the first and second base electrodes  33 ,  34  and of the first and second excitation electrodes  35 ,  36  is 1500-2000 Ångstroms. 
     Method for Manufacturing Crystal Unit 
       FIG. 7  depicts the results of three different steps in an embodiment of a method for manufacturing crystal units (as exemplary piezoelectric devices). The upper frame of  FIG. 7  shows a lid glass wafer  60  on which multiple first lids  65  have been formed. The middle frame of  FIG. 7  shows a crystal wafer  10  on which multiple tuning-fork type crystal vibrating pieces  30  and their respective crystal frames  50  have been formed. The lower frame of  FIG. 7  shows a base glass wafer  20  on which multiple first bases  40  have been formed. Similar to the crystal vibrating pieces  30  connected to the crystal wafer  10 , the first lids  65  and first bases  40  are connected to the lid glass wafer  60  and base glass wafer  20 , respectively, by connecting portions  11 . For simultaneous assembly of multiple crystal units, the wafers  60 ,  10 ,  20  are registered with each other and layered superposedly with respect to each other so that all the bases  40  are registered with respective crystal vibrating pieces  30 , and all the crystal vibrating pieces  30  are registered with respective lids  65 . 
     In preparation for simultaneous assembly, the first lids  65  include downward-facing concave portions  67  that have been formed by etching. The first bases  40  include upward-facing lid concave portions  47  that have been formed by etching and also include the first and second connection electrodes  42 ,  44 . As described in connection with  FIGS. 6A and 6B , the tuning-fork type crystal vibrating pieces  30  include the first and second base electrodes  33 ,  34  and first and second excitation electrodes  35 ,  36 . 
     By way of example, the lid glass wafer  60 , the base glass wafer  20 , and the crystal wafer  10  are each four inches on a side. Since each wafer includes a respective orientation flat  60   c ,  20   c ,  10   c , the three wafers can be accurately positioned for layering atop one another to form a three-wafer sandwich in which all the components  65 ,  30 ,  40  are registered with each other in a manner that will simultaneously form multiple crystal units. After forming the sandwich, the wafers are anodically bonded together in a vacuum chamber or in an inert gas environment. During the anodic bonding, in each crystal unit thus formed, the first and second base electrodes  33 ,  34  are bonded tightly to the first and second connection electrodes  42 ,  44 . 
     After bonding together the layered lid glass wafer  60 , base glass wafer  20 , and crystal wafer  10 , the connecting portions  11  are broken in common to release each crystal unit from the wafer sandwich. This step completes the simultaneous formation of the multiple crystal units  100 . This process achieves electrode connections and the bonding together of piezoelectric vibrating pieces and their package components all at the same time for the multiple crystal units of the wafer sandwich. Also, the process advantageously can be performed all in a wafer-manufacturing unit so that productivity is improved. Note that the crystal units  110  of the second embodiment also can be manufactured simultaneously in a wafer-manufacturing unit. 
     Representative embodiments are described above. It will be understood that these embodiments can be modified or changed while not departing from the spirit and scope of them and/or of the appended claims. 
     Also, although the embodiments have been described in the context of tuning-fork type crystal vibrating pieces  30 , it will be understood that the subject methods, for example, can be applied to AT vibrating devices and SAW elements.