Patent Publication Number: US-2013241362-A1

Title: Piezoelectric device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Japan application serial no. 2012-056677, filed on Mar. 14, 2012. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     TECHNICAL FIELD 
     This disclosure relates to a piezoelectric device where a plurality of lid portions and a plurality of base portions can be fabricated in a state of a wafer. 
     DESCRIPTION OF THE RELATED ART 
     As disclosed in Japanese Unexamined Patent Application Publication No. 11-136062 (hereinafter referred to as Patent Literature 1), a piezoelectric device that includes a pair of base castellations and a pair of side surface electrodes are proposed. The pair of base castellations are disposed at two sides that face each other on a base portion and depressed at the center side of the base portion. The pair of side surface electrodes are formed at the pair of base castellations and connect a first surface and a second surface. The castellation is disposed at a center of a short side of the base portion, and a connecting electrode is formed at a portion near the castellation only. 
     However, the base portion of the piezoelectric device disclosed in Patent Literature 1 includes a corner portion that may be chipped due to an impact or similar cause, which is applied during conveyance. Additionally, when the piezoelectric device is mounted to a printed circuit board with a solder, the corner portion of the base portion may be chipped by bending the printed circuit board. 
     A need thus exists for a piezoelectric device which is not susceptible to the drawbacks mentioned above. 
     SUMMARY 
     A piezoelectric device according to a first aspect includes a piezoelectric vibrating piece and a base portion in a square shape with four sides viewed from a first surface. The piezoelectric vibrating piece includes a pair of excitation electrodes on both principal surfaces, and a pair of extraction electrodes. The pair of extraction electrodes is extracted from the pair of excitation electrodes. The base portion includes a pair of connecting electrodes and two pairs of mounting terminals. The pair of connecting electrodes are disposed on the first surface at a side of the piezoelectric vibrating piece and connected to the pair of extraction electrodes. The two pairs of mounting terminals are disposed on a second surface. The second surface is an opposite surface of the first surface. The base portion has two sides that face one another. Two pairs of castellations and two pairs of side surface electrodes are formed at the two sides, the two pairs of castellations are depressed toward a center side of the base portion, and the two pairs of side surface electrodes are on the two pairs of castellations. The two pairs of side surface electrodes connect the first surface and the second surface. One pair among the two pairs of side surface electrodes connects to the pair of connecting electrodes and one pair of mounting terminals among the two pairs of mounting terminals. The mounting terminals are formed up to four corners of the base portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein: 
         FIG. 1  is an exploded perspective view of a first piezoelectric device  100  of a first Embodiment; 
         FIG. 2A  is a cross-sectional view taken along the line IIA-IIA of  FIG. 1 ; 
         FIG. 2B  is a bottom view of the first piezoelectric device  100 ; 
         FIG. 3  is a flowchart illustrating fabrication of the first piezoelectric device  100  of the first Embodiment; 
         FIG. 4  is a plan view of a quartz-crystal wafer  10 W; 
         FIG. 5  is a plan view of a lid wafer  11 W; 
         FIG. 6  is a plan view of a base wafer  12 W; 
         FIG. 7  is a bottom view of the base wafer  12 W; 
         FIG. 8A  is a cross-sectional view of a first piezoelectric device  100 ′ taken along a line VIIIA-VIIIA of  FIG. 8B  illustrating a modification of the first Embodiment; 
         FIG. 8B  is a bottom view of the first piezoelectric device  100 ′; 
         FIG. 9  is an exploded perspective view of a second piezoelectric device  200  of a second Embodiment; 
         FIG. 10A  is a cross-sectional view taken along the line XA-XA of  FIG. 9 ; 
         FIG. 10B  is a bottom view of the second piezoelectric device  200 ; 
         FIG. 11  is a plan view of a quartz-crystal wafer  20 W; 
         FIG. 12A  is a plan view of a quartz-crystal vibrating piece  20  of a modification of the second Embodiment viewed from the +Y′ side; 
         FIG. 12B  is a transparent view of the quartz-crystal vibrating piece  20 ′ of the modification of the second Embodiment viewed from the +Y′ side; 
         FIG. 12C  is a plan view of a base portion  22 ′ of the modification of the second Embodiment viewed from the +Y′ side; 
         FIG. 12D  is a transparent view of the base portion  22 ′ of the modification of the second Embodiment viewed from the +Y′ side; 
         FIG. 13  is a cross-sectional view taken along the line XIII-XIII of  FIG. 12B ; 
         FIG. 14  is a plan view of a quartz-crystal wafer  20 ′W; and 
         FIG. 15  is a plan view of a base wafer  22 W′. 
     
    
    
     DETAILED DESCRIPTION 
     In this disclosure, an AT-cut quartz-crystal vibrating piece as a piezoelectric vibrating piece is employed. The AT-cut quartz-crystal vibrating piece has a principal surface (in the Y-Z plane) that is tilted by 35° 15′ about the Y-axis of crystallographic axes (XYZ) in the direction from the Z-axis to the Y-axis direction around the X-axis. The new axes tilted with reference to the axis directions of the AT-cut quartz-crystal vibrating piece are denoted as the Y′-axis and the Z′-axis. This disclosure defines the longer side direction of a crystal unit as the X-axis direction, the height direction of the crystal unit as the Y′-axis direction, and the direction perpendicular to the X and Y′-axis directions as the Z′-axis direction. 
     Overall Configuration of a Piezoelectric Device  100  According to a First Embodiment 
     A description will be given of the overall configuration of the piezoelectric device  100  with referring to  FIG. 1  to  FIG. 2B .  FIG. 1  is an exploded perspective view of the piezoelectric device  100 , and  FIG. 2A  is a cross-sectional view taken along the line IIA-IIA of  FIG. 1 . In  FIG. 1 , a low-melting point glass LG, which is a sealing material, is transparent such that the whole connecting electrodes  124   a  and  124   b  are viewable. 
     As illustrated in  FIG. 1  to  FIG. 2B , the piezoelectric device  100  includes a lid portion  11 , a base portion  12 , and a planar quartz-crystal vibrating piece  10 . The lid portion  11  includes a lid depressed portion  111 . The base portion  12  includes a base depressed portion  121 . The quartz-crystal vibrating piece  10  is placed on the base portion  12 . 
     The quartz-crystal vibrating piece  10  includes an AT-cut crystal wafer  101 . A pair of excitation electrodes  102   a  and  102   b  face each other and are disposed on both principal surfaces of the crystal wafer  101  close to the center of the surface. An extraction electrode  103   a , which extends to the −X side of the bottom surface of the crystal wafer  101  (+Z′ side), connects to an excitation electrode  102   a . An extraction electrode  103   b , which extends to the +X side of the bottom surface of the crystal wafer  101  (−Z′ side), connects to an excitation electrode  102   b . The quartz-crystal vibrating piece  10  may be a mesa type or an inverse mesa type. 
     Here, the excitation electrodes  102   a  and  102   b  and the extraction electrodes  103   a  and  103   b , for example, employ a chromium layer as a foundation layer and a gold layer over the top surface of the chromium layer. The chromium layer has a thickness of, for example, 0.05 μm to 0.1 μm, and the gold layer has a thickness of, for example, 0.2 μm to 2 μm. 
     The base portion  12  is made of a glass or a piezoelectric material. The base portion  12  includes a second end surface M 2 , which is formed at a peripheral area of a base depressed portion  121 , on its surface (+Y′ side surface). The base portion  12  also includes two base castellations  122   a  and  122   b  at the one side in the −X-axis direction. When a base through hole BH 1  (see  FIG. 6  and  FIG. 7 ) is formed, the base castellations  122   a  and  122   b  extend in the Z′-axis direction. Here, the base castellation  122   a  is formed at the +Z side, and the base castellation  122   b  is formed at the −Z side. Similarly, the base portion  12  includes two other base castellations  122   c  and  122   d  at the other side in the +X-axis direction. When the base through hole BH 1  (see  FIG. 6  and  FIG. 7 ) is formed, the base castellations  122   c  and  122   d  extend in the Z′-axis direction. Here, the base castellation  122   c  is formed at the −Z side, and the base castellation  122   d  is formed at the +Z side. That is, the base castellations  122   a  and  122   c  are diagonally disposed on the base portion  12 , and the base castellations  122   b  and  122   d  are diagonally disposed on the base portion  12 . 
     In the base portion  12 , tapered projecting portions  126  are formed on the respective base castellations  122   a  to  122   d . The projecting portion  126  protrudes outside at the approximately center portion in the Y′-axis direction. Additionally, the respective base castellations  122   a  to  122   d  include base side surface electrodes  123   a  to  123   d.    
     In this constitution, the base castellations  122   a  to  122   d  include an inclined region. This shortens time the taken for forming a film when forming the base side surface electrodes  123   a  to  123   d  by a method such as sputtering. 
     A pair of connecting electrodes  124   a  and  124   b  is formed on the second end surface M 2  of the base portion  12 . The connecting electrode  124   a  electrically connects to the base side surface electrode  123   a . The connecting electrode  124   b  electrically connects to a base side surface electrode  123   c , which is diagonally disposed on the base portion  12  relative to the base side surface electrode  123   a.    
     Further, the base portion  12  includes two pairs of mounting terminals  125   a  to  125   d  on a mounting surface M 3  that are electrically connected to the respective base side surface electrodes  123   a  to  123   d . The two pairs of mounting terminals  125   a  to  125   d  are formed on the four corners (the four corner portions) of the base portion  12 . The corner portions of the base portion  12  are easily chipped; therefore, the mounting terminals are formed up to the four corners to increase strength (see the round frame P in  FIG. 2B ). 
     Among the two pairs of mounting terminals  125   a  to  125   d , one pair of mounting terminals  125   a  and  125   c  are diagonally disposed on the base portion  12  and connects to the respective connecting electrodes  124   a  and  124   b  via the base side surface electrodes  123   a  and  123   c . The mounting terminals  125   a  and  125   c  are mounting terminals for an external electrode (hereafter referred to as external electrodes). In short, the external electrodes  125   a  and  125   c  are diagonally disposed on the base portion  12 . The external electrode  125   c  has a notch C (see  FIG. 2B ). The notch is formed to check the orientation of the piezoelectric device  100 . When an alternating voltage (a potential that alternates between positive and negative values) is applied across the external electrodes  125   a  and  125   c , the quartz-crystal vibrating piece  10  exhibits thickness-shear vibration. 
     On the other hand, among the two pairs of mounting terminals  125   a  to  125   d , the other one pair is mounting terminals for grounding electrodes  125   b  and  125   d  (hereafter referred to as grounding electrodes), which are connected to base side surface electrodes  123   b  and  123   d  for grounding. In short, the grounding electrodes  125   b  and  125   d  are diagonally disposed in a direction different from the external electrodes  125   a  and  125   c  on the base portion  12 . Here, the grounding electrodes  125   b  and  125   d  are employed for grounding; however, this disclosure includes the case where the grounding electrodes  125   b  and  125   d  are employed as terminals that are not electrically connected. The grounding electrodes  125   b  and  125   d  are employed to strongly bond the piezoelectric device  100  and a mounting printed circuit board (not shown) together. 
     The pair of external electrodes  125   a  and  125   c  and the pair of grounding electrodes  125   b  and  125   d  are disposed away from each other as illustrated in  FIG. 2B . The external electrode  125   a  and the grounding electrode  125   d  contact the corner portion of the base portion  12 . The external electrode  125   a  and the grounding electrode  125   d  are formed toward the center of the base portion  12  in the X-axis direction separated from one side in the +Z′ side by a distance SP 2 . The grounding electrode  125   b  and the external electrode  125   c  contact the corner portion of the base portion  12 . The grounding electrode  125   b  and the external electrode  125   c  are formed toward the center of the base portion  12  in the X-axis direction separated from another side in the −Z′ side by a distance SP 3 . 
     A distance SP 1  between the external electrode  125   a  and the grounding electrode  125   b , and between the external electrode  125   c  and the grounding electrode  125   d  in the Z′-axis direction is, for example, approximately 200 μm to 500 μm. Additionally, a distance SP 2  between the external electrode  125   a  or the grounding electrode  125   d  and one side at the +Z′ side of the base portion  12 ; and the distance SP 3  between the grounding electrode  125   b  or the external electrode  125   c  and the other side at the −Z′ side of the base portion  12  are, for example, approximately 100 μm to 150 μm. 
     In the piezoelectric device  100 , the length of the quartz-crystal vibrating piece  10  in the X-axis direction is longer than the length of the base depressed portion  121  in the X-axis direction. Accordingly, when the quartz-crystal vibrating piece  10  is placed on the base portion  12  with conductive adhesive  13 , both ends of the quartz-crystal vibrating piece  10  in the X-axis direction is placed on the second end surface M 2  of the base portion  12  as illustrated in  FIG. 2A . At this time, the extraction electrodes  103   a  and  103   b  of the quartz-crystal vibrating piece  10  electrically connect to the respective connecting electrodes  124   a  and  124   b  of the base portion  12 . 
     The lid portion  11  includes the lid depressed portion  111  and a first end surface M 1 . The lid depressed portion  111  has an area larger than the base depressed portion  121  in the X-Z′ plane. The first end surface M 1  is formed at the peripheral area of the lid depressed portion  111 . When the first end surface M 1  of the lid portion  11  and the second end surface M 2  of the base portion  12  are bonded together, the lid depressed portion  111  and the base depressed portion  121  form a cavity CT. The cavity CT houses the quartz-crystal vibrating piece  10 . The cavity CT is filled with an inert gas or is evacuated to a vacuum state. 
     Here, the first end surface M 1  of the lid portion  11  is bonded to the second end surface M 2  of the base portion  12 , for example, with a low-melting point glass LG, which is a sealing material (non-conductive adhesive). 
     In the lid portion  11 , the length of the lid depressed portion  111  in the X-axis direction is longer than the length of the quartz-crystal vibrating piece  10  in the X-axis direction and the length of the base depressed portion  121  in the X-axis direction. Further, the low-melting point glass LG bonds the lid portion  11  and the base portion  12  together outside of the second end surface M 2  (the width is approximately 300 μm) of the base portion  12  as illustrated in  FIG. 1  and  FIG. 2A . 
     While the quartz-crystal vibrating piece  10  is placed on the second end surface M 2  of the base portion  12 , the quartz-crystal vibrating piece  10  may be housed in the base depressed portion  121 . At this time, the connecting electrodes extend from the base castellations  122   a  and  122   c  to the bottom surface of the base depressed portion  121  via the second end surface M 2 . In this case, the lid portion may be planar where a lid depressed portion is not formed. 
     Fabrication Method of the Piezoelectric Device  100   
       FIG. 3  is a flowchart illustrating fabrication of the piezoelectric device  100 . In  FIG. 3 , the fabrication step of the quartz-crystal vibrating piece  10  (S 10 ), the fabrication step of the lid portion  11  (S 11 ), and the fabrication step of the base portion  12  (S 12 ) can be performed at the same time.  FIG. 4  is a plan view of a quartz-crystal wafer  10 W where a plurality of quartz-crystal vibrating pieces  10  can be fabricated at the same time.  FIG. 5  is a plan view of a lid wafer  11 W where a plurality of lid portions  11  can be fabricated at the same time.  FIG. 6  is a plan view of the base wafer  12 W where a plurality of base portions  12  can be fabricated at the same time.  FIG. 7  is a bottom view of the base wafer  12 W. 
     The quartz-crystal vibrating piece  10  is fabricated at step S 10 . Step S 10  includes steps S 101  to S 103 . In step S 101 , outlines of the plurality of quartz-crystal vibrating pieces  10  are formed on the quartz-crystal wafer  10 W of even thickness by etching as illustrated in  FIG. 4 . Here, each quartz-crystal vibrating piece  10  connects to the quartz-crystal wafer  10 W with a connecting portion  104 . 
     In step S 102 , first, a chromium layer and a gold layer are formed in this order on both of the surfaces and the side surfaces of the quartz-crystal wafer  10 W by sputtering or vacuum evaporation. Then, a photoresist is evenly applied over all surfaces of the metal layer. Then, the patterns of the excitation electrode and the extraction electrode described on a photomask is exposed onto the quartz-crystal wafer  10 W using all exposing device (not shown). Next, the metal layer exposed from the photoresist is etched. This forms excitation electrodes  102   a  and  102   b  and extraction electrodes  103   a  and  103   b  on both surfaces and the side surfaces of the quartz-crystal wafer  10 W as illustrated in  FIG. 4 . 
     In step S 103 , the quartz-crystal vibrating pieces  10  are diced into individual pieces. In the dicing process, the quartz-crystal vibrating pieces  10  are diced along a cut line CL indicated by the one dot chain line illustrated in  FIG. 4  using a dicing unit employing a laser beam, a dicing blade, or similar. 
     In step S 11 , the lid portion  11  is fabricated. Step S 11  includes steps S 111  and S 112 . In step S 111 , several hundred to several thousand of the lid depressed portions  111  are formed on the lid wafer  11 W of crystal planar with even thickness as illustrated in  FIG. 5 . The lid depressed portion  111  is formed on the lid wafer  11 W by etching or machining. The first end surface M 1  is formed at the peripheral area of the lid depressed portion  111 . 
     In step S 112 , the low-melting point glass LG is printed on the first end surface M 1  of the lid wafer  11 W by screen-printing. Then, by temporary hardening of the low-melting point glass LG, the low-melting point glass LG film is formed on the first end surface M 1  of the lid wafer  11 W. The low-melting point glass film is not foamed on a portion  112  corresponding to the base through hole BH 1  (the base castellations  122   a  to  122   d  in  FIG. 1 ). In this embodiment, the low-melting point glass LG is formed on the lid portion  11 ; however, the low-melting point glass LG may be formed on the second end surface M 2  of the base portion  12 . 
     In step S 12 , the base portion  12  is fabricated. Step S 12  includes steps S 121  and S 122 . In step S 121 , several hundred to several thousand of the base depressed portions  121  are formed on the base wafer  12 W of crystal planar with even thickness as illustrated in  FIG. 6 . The base depressed portion  121  is formed on the base wafer  12 W by etching. The second end surface M 2  is formed at the peripheral area of the base depressed portion  121 . At the same time, two base through holes BH 1  are formed on both sides of each base portion  12  in the X-axis direction. The base through hole BH 1  has a rounded rectangular shape and penetrates the base wafer  12 W. 
     In step S 121 , the base castellations  122   a  to  122   d  are formed by etching from the +Y′ side and the −Y′ side. When etching is performed from the +Y′ side, the base depressed portion  121  is formed at the same time. This forms a projecting region  127  at the base through hole BH 1  of the base wafer  12 W as illustrated in  FIG. 6 . Dividing the projecting region  127  into half forms the projecting portion  126  (see  FIG. 1  and  FIG. 6 ). Here, when the base through hole BH 1  of the rounded rectangular shape is divided into half, one of the base castellations  122   a  to  122   d  is formed (see  FIG. 1 ). 
     In step S 122 , sputtering from the +Y′ side and the −Y′ side forms the base side surface electrodes  123   a  to  123   d  at the base castellations  122   a  to  122   d.    
     In step S 122 , gold (Au) layers are formed on the surfaces of chromium (Cr) layers, which are foundation layers, at both surfaces of the base wafer  12 W by sputtering. Then, etching forms the connecting electrodes  124   a  and  124   b  on the second end surface M 2  as illustrated in  FIG. 6 . 
     At the same time, a pair of external electrodes  125   a  and  125   c  and a pair of grounding electrodes  125   b  and  125   d  are formed on the bottom surface of the base wafer  12 W as illustrated in  FIG. 7 . Here, an external electrode and a grounding electrode formed adjacent each other in the X-axis direction are integrally formed at the base portion  12 . Four base portions ( 12 A to  12 D) enclosed by the dotted line in  FIG. 7  will be described as one example. The external electrode  125   a  of a base portion  12 B, the grounding electrode  125   d  of a base portion  12 C, and the base side surface electrodes  123   a  and  123   d  of the base through hole BH 1  are integrally formed. Further, the external electrode  125   c  of the base portion  12 B, the grounding electrode  125   b  of the base portion  12 A, and the base side surface electrodes  123   b  and  123   c  of the base through hole BH 1  are integrally formed. These electrodes employ a chromium (Cr) layer as a foundation layer and nickel tungsten (Ni/W) alloy is sputtered. Then a gold (Au) layer is formed on the sputtered surface. 
     Additionally, mounting terminals of the base portion  12 B (the external electrode and the grounding electrode) are formed away from mounting terminals of the base portion  12 D, which is adjacent to the base portion  12 B in the Z′-axis direction, by a distance SP 4 . Here, the distance SP 4  is approximately 240 μm to 280 μm. Assuming that, for example, the distance SP 4  is 240 μm and the width for dicing in step S 17 , which will be described below, is 40 μm. The distance SP 3  indicated in  FIG. 2B  becomes 100 μm. That is, an external electrode and a grounding electrode formed adjacent to each other on the base portion  12  in the X-axis direction are connected, while an external electrode and a grounding electrode formed adjacent to each other on the base portion  12  in the Z′-axis direction are not connected. On the other hand, mounting terminals corresponding to the four corners (the corner portions) of the base portion  12 B and the base portion  12 D are formed in contact with each other in the Z′-axis direction. This is because even if a dicing width changes, the mounting terminals  125   a  to  125   d  are formed up to the four corners (the four corner portions) of the base portion  12 . 
     In step S 13 , the individual quartz-crystal vibrating piece  10 , which is fabricated in step S 10 , is placed on the second end surface M 2  of the base portion  12  formed on the base wafer  12 W with the conductive adhesive  13 . At this time, the quartz-crystal vibrating piece  10  is placed on the second end surface M 2  of the base portion  12  so as to align the extraction electrodes  103   a  and  103   b  of the quartz-crystal vibrating piece  10  with the respective connecting electrodes  124   a  and  124   b  of the second end surface M 2  of the base portion  12 . Thus, several hundred to several thousand of the quartz-crystal vibrating pieces  10  are placed on the base wafer  12 W. 
     In step S 14 , a pair of probes for frequency measurement (not shown) contact the pair of respective external electrodes  125   a  and  125   c  on the same base portion  12 , and thus the frequency of each quartz-crystal vibrating piece  10  is measured. 
     In step S 15 , the thickness of the excitation electrode  102   a  of the quartz-crystal vibrating piece  10  is adjusted. The thickness can be adjusted by sputtering a metal onto the excitation electrode  102   a  to increase its mass (and to decrease its frequency), or by evaporating metal from the excitation electrode  102   a  to decrease its mass (and to increase its frequency) by reverse sputtering. The details of the frequency adjustment are disclosed in Japanese Unexamined Patent Application Publication No. 2009-141825 by the applicants of this application. If the measured frequency result is within its predetermined range, adjustment of the frequency is not required. 
     In step S 14 , after a frequency of one quartz-crystal vibrating piece  10  is measured, the frequency of one quartz-crystal vibrating piece  10  may be adjusted in step S 15 . The sequence of this step is repeated for all the quartz-crystal vibrating pieces  10  on the base wafer  12 W. Alternatively, after a frequency of all the quartz-crystal vibrating pieces  10  on the base wafer  12 W is measured in step S 14 , the frequency of the quartz-crystal vibrating pieces  10  may be adjusted one by one in step S 15 . 
     In step S 16 , the low-melting point glass LG is heated, and the lid wafer  11 W and the base wafer  12 W are pressurized. Thus, the lid wafer  11 W and base wafer  12 W are bonded together by the low-melting point glass LG. 
     In step S 17 , the bonded-together lid wafer  11 W and the base wafer  12 W are individually diced. In the dicing process, using a dicing unit employing a laser beam, a dicing blade, or similar, separates the wafer into individual piezoelectric devices  100  by dicing along the scribe lines SL, denoted by the one dot chain line illustrated in  FIGS. 5 to 7 . This fabricates several hundred to several thousand of the piezoelectric devices  100 . As illustrated in  FIG. 7 , a part of the mounting terminals corresponding to the four corners (the corner portions) of the base portion  12  contacts in the Z′-axis direction while other parts are formed providing the distance SP 4 . In view of this, at usage of a dicing blade, clogging the blade with a metal (so-called clogging) is reduced to a minimum. 
     Overall Configuration of a First Piezoelectric Device  100 ′ According to a Modification of the First Embodiment 
     A description will be given of the overall configuration of the first piezoelectric device  100 ′ with referring to  FIG. 8A  and  FIG. 8B .  FIG. 8A  is a cross-sectional view of the first piezoelectric device  100 ′ taken along a line VIIIA-VIIIA of  FIG. 8B  illustrating a modification of the first Embodiment.  FIG. 8B  is a bottom view of the first piezoelectric device  100 ′. 
     As illustrated in  FIG. 8B , the first piezoelectric device  100 ′ includes an external electrode and a grounding electrode of a different shape with those of the first piezoelectric device  100 . The embodiment will now be described wherein like reference numerals designate corresponding or identical elements with the first piezoelectric device  100  throughout the embodiments. 
     A base portion  12 ′ includes two pairs of mounting terminals  125   a ′ to  125   d ′ on a mounting surface M 3 . The two pairs of mounting terminals  125   a ′ to  125   d ′ electrically connect to the respective base side surface electrodes  123   a  to  123   d . Each of the two pairs of mounting terminals  125   a ′ to  125   d ′ extends to the corner portion, one side at the +Z′ side, and one side at the −Z′ side of the base portion  12 ′ to enhance a strength of the four corners (see  FIG. 8B ). 
     Among the two pairs of mounting terminals  125   a ′ to  125   d ′, one pair are external electrodes  125   a ′ and  125   c ′ that are diagonally disposed on the base portion  12 ′ and connect to the respective connecting electrodes  124   a  and  124   b  via the base side surface electrodes  123   a  and  123   c . The external electrode  125   c ′ includes a notch (see  FIG. 8B ) to check the orientation of the piezoelectric device  100 ′. 
     The pair of external electrodes  125   a ′ and  125   c ′ and the pair of grounding electrodes  125   b ′ and  125   d ′ are disposed away from each other as illustrated in  FIG. 8B . The external electrode  125   a ′ and the grounding electrode  125   d ′ are formed in contact with the corner portion and one side at the +Z′ side of the base portion  12 ′. The grounding electrode  125   b ′ and the external electrode  125   c ′ are formed in contact with the corner portion and the other side at the −Z′ side of the base portion  12 ′. Here, the distance SP 2  between the external electrode  125   a ′ and the grounding electrode  125   b ′ or between the external electrode  125   c ′ and the grounding electrode  125   d ′ in the Z′-axis direction is, for example, approximately 100 μm to 150 μm. 
       FIG. 8A  and  FIG. 8B  illustrate the two pairs of mounting terminals  125   a ′ to  125   d ′ according to the first Embodiment, this applies to a modification of the second Embodiment, which will be described below. 
     Overall Configuration of a Second Piezoelectric Device  200  According to a Second Embodiment 
     A description will be given of the overall configuration of the second piezoelectric device  200  with referring to  FIG. 9 ,  FIG. 10A , and  FIG. 10B .  FIG. 9  is an exploded perspective view of the second piezoelectric device  200 .  FIG. 10A  is a cross-sectional view taken along the line XA-XA of  FIG. 9   
     As illustrated in  FIG. 9  and  FIG. 10A , the second piezoelectric device  200  includes a lid portion  21  that includes a lid depressed portion  211 , a base portion  22  that includes a base depressed portion  221 , and a rectangular quartz-crystal vibrating piece  20 . The quartz-crystal vibrating piece  20  is sandwiched between the lid portion  21  and the base portion  22 . 
     The quartz-crystal vibrating piece  20  includes a crystal vibrator  201  and a framing body  208  that surrounds the crystal vibrator  201 . The crystal vibrator  201  includes excitation electrodes  202   a  and  202   b  on both of the surfaces. A pair of supporting portions  204   a  and  204   b  is formed between the crystal vibrator  201  and the framing body  208 . The pair of respective supporting portions  204   a  and  204   b  extends from the crystal vibrator  201  along both of the sides in the X-axis direction and connects to the framing body  208 . Accordingly, a pair of L-shaped through openings  205   a  and  205   b  is formed between the crystal vibrator  201  and the framing body  208 . Two by two crystal castellations  206   a  to  206   d  are disposed on both sides in the X-axis direction of the quartz-crystal vibrating piece  20  when forming a crystal through hole CH of a rounded rectangular shape (see  FIG. 11 ). Both of the sides extend in the Z′-axis direction. The crystal castellations  206   a  to  206   d  include crystal side surface electrodes  207   a  to  207   d , respectively. 
     A supporting portion  204   a  includes an extraction electrode  203   a  on its surface Me. The extraction electrode  203   a  connects an excitation electrode  202   a  and a crystal side surface electrode  207   a , which is formed at the +Z side of one side in the −X-axis direction of the quartz-crystal vibrating piece  20 . Here, the crystal side surface electrode  207   a  extends to a back surface Mi of the quartz-crystal vibrating piece  20  to form a connection pad  207 M. The connection pad  207 M securely and electrically connects to a connection pad  223 M of a base side surface electrode  223   a , which will be described below. Similarly, a supporting portion  204   b  includes an extraction electrode  203   b  on its back surface Mi. The extraction electrode  203   b  connects an excitation electrode  202   b  and a crystal side surface electrode  207   c , which is formed at the −Z side of the other side in the +X-axis direction of the quartz-crystal vibrating piece  20 . Here, the extraction electrode  203   b  is connected to a connection pad  223 M of a base side surface electrode  223   c  that will be described below. 
     The base portion  22  is made of a glass or a quartz-crystal material, and includes a second end surface M 2  formed at a peripheral area of the base depressed portion  221  on its surface (+Y′ side surface). Additionally, the base portion  22  includes two by two base castellations  222   a  to  222   d  when the base through holes BH 1  (see  FIG. 6  and  FIG. 7 ) are formed on both of the sides in the X-axis direction. Furthermore, the base castellations  222   a  to  222   d  form respective base side surface electrodes  223   a  to  223   d . Here, the base side surface electrode  223   a  formed at the +Z′ side of one side in the −X-axis direction of the base portion  22  is connected to the connection pad  207 M via the connection pad  223 M formed on the second end surface M 2 . The connection pad  207 M is formed on a crystal side surface electrode  207   a  formed on the quartz-crystal vibrating piece  20 . This connects the base side surface electrode  223   a  and the extraction electrode  203   a  together via the connection pad  207 M and the crystal side surface electrode  207   a . Further, the base side surface electrode  223   c  formed at the −Z side of another side in the +X-axis direction of the base portion  22  is connected to an extraction electrode  203   b  formed at the quartz-crystal vibrating piece  20 . 
     On the other hand, the base portion  22  includes a pair of diagonally disposed external electrodes  225   a  and  225   c  and a pair of diagonally disposed grounding electrodes  225   b  and  225   d  on the mounting surface M 3  (see  FIG. 10A  and  FIG. 10B ). The pair of external electrodes  225   a  and  225   c  and the pair of grounding electrodes  225   b  and  225   d , as illustrated in  FIG. 10B , are formed to be the same shape as the external electrode and the grounding electrode of the base portion  12  of the first piezoelectric device  100 . 
     The pair of external electrodes  225   a  and  225   c  are respectively connected to the base side surface electrodes  223   a  and  223   c , which are connected to the extraction electrodes  203   a  and  203   b  of the quartz-crystal vibrating piece  20 . Additionally, the pair of grounding electrodes  225   b  and  225   d  are respectively connected to the other base side surface electrodes  223   b  and  223   d . The mounting terminals  225   a  to  225   d  extend up to the four corners (four corner portions) of the base portion  22  to enhance the strength of the four corners (see  FIG. 10B ). 
     The external electrode  225   c  includes a notch (see  FIG. 10B ) to check the orientation of the piezoelectric device  200 . As illustrated in  FIG. 10A , the cavity CT that houses the crystal vibrator  201  of the quartz-crystal vibrating piece  20  is formed by the lid portion  21 , the framing body  208  of the quartz-crystal vibrating piece  20 , and the base portion  22 . Here, between the lid portion  21  and the quartz-crystal vibrating piece  20 , and between the quartz-crystal vibrating piece  20  and the base portion  22  are bonded together with a low-melting point glass LG, which is a sealing material. 
     Fabrication Method of the Second Piezoelectric Device  200   
     The method for fabricating the second piezoelectric device  200  is approximately the same as the fabrication method illustrated in  FIG. 3 . The difference is that a dicing process, which individually dices the quartz-crystal vibrating pieces  10  illustrated in step S 103 , is eliminated in this method. Additionally, the low-melting point glass LG is disposed between both of the principal surfaces of the framing body  208  and the lid portion  21  and the base portion  22 . Therefore, a detailed flowchart is omitted. 
       FIG. 11  is a plan view of a quartz-crystal wafer  20 W where a plurality of quartz-crystal vibrating pieces  20  can be fabricated at the same time. As illustrated in  FIG. 11 , the quartz-crystal wafer  20 W includes excitation electrodes  202   a  and  202   b  and the extraction electrodes  203   a  and  203   b  that are formed on both surfaces and side surfaces. 
     Overall Configuration of a Second Piezoelectric Device  200 ′ According to a Modification of the Second Embodiment 
     A description will be given of the overall configuration of the second piezoelectric device  200 ′ of a modification of the second Embodiment with referring to  FIG. 12A  to  FIG. 12D  and  FIG. 13 .  FIG. 12A  is a plan view of a quartz-crystal vibrating piece  20 ′ of a modification of the second Embodiment viewed from the +Y′ side.  FIG. 12B  is a transparent view of the quartz-crystal vibrating piece  20 ′ of the modification of the second Embodiment viewed from the +Y′ side.  FIG. 12C  is a plan view of a base portion  22 ′ of the modification of the second Embodiment viewed from the +Y′ side.  FIG. 12D  is a transparent view of the base portion  22 ′ of the modification of the second Embodiment viewed from the +Y′ side.  FIG. 13  is a cross-sectional view taken along the line XIII-XIII of  FIG. 12B . 
     As illustrated in  FIG. 12A  and  FIG. 12B , a quartz-crystal vibrating piece  20 ′ of a second piezoelectric device  200 ′ includes the crystal vibrator  201  and the framing body  208 . The crystal vibrator  201  includes excitation electrodes  202   a  and  202   b  on both surfaces. The framing body  208  surrounds the crystal vibrator  201 . A pair of supporting portions  204   a ′ and  204   b ′, which each extends from the crystal vibrator  201  to the −X side, are formed between the crystal vibrator  201  and the framing body  208 . Therefore, a rectangular through opening  205   a ′, which is opened at one side (−X side), is formed between the crystal vibrator  201  and the framing body  208 . A rectangular through opening  205   b ′ is formed between the pair of supporting portions  204   a ′ and  204   b′.    
     As illustrated in  FIG. 13 , an extraction electrode  203   a ′ is connected to an excitation electrode  202   a  formed on the surface Me of the quartz-crystal vibrating piece  20 ′. The extraction electrode  203   a ′ is extended from the front surface Me to the back surface Mi of the quartz-crystal vibrating piece  20 ′ via a side surface M 4  of the through opening  205   a′.    
     Referring to  FIG. 12A , the extraction electrode  203   a ′ extending to the back surface Mi of the quartz-crystal vibrating piece  20 ′ is formed at one corner at the −X side and the +Z′ side of the quartz-crystal vibrating piece  20 ′. Since the quartz-crystal vibrating piece  20 ′ is fabricated in a state of a wafer, the extraction electrode  203   a ′ is formed providing a distance SP 5  from one side at the +Z′ side of the quartz-crystal vibrating piece  20 ′ such that the extraction electrode  203   a ′ is not affected by the adjacent quartz-crystal vibrating piece  20 ′ at measurement of frequency. 
     The extraction electrode  203   b ′ formed at the back surface Mi of the quartz-crystal vibrating piece  20 ′ extends from the −X side of the crystal vibrator  201 , goes along the framing body  208 , and is formed at another corner at the +X side and the −Z′ side of the quartz-crystal vibrating piece  20 ′. Here, as described in the second Embodiment, since the quartz-crystal vibrating piece  20 ′ is fabricated in a state of a wafer, the extraction electrode  203   b ′ is formed providing the distance SP 5  from the other side at the −Z′ side of the quartz-crystal vibrating piece  20 ′ such that the extraction electrode  203   b ′ is not affected by the adjacent quartz-crystal vibrating piece  20 ′ (see  FIG. 12B  and  FIG. 14 ). 
     As illustrated in  FIG. 12C  and  FIG. 12D , the base portion  22 ′ of the modification of the second Embodiment includes two pairs of mounting terminals  225   a ′ to  225   d ′ that are electrically connected to the respective base side surface electrodes  223   a  to  223   d  on the mounting surface M 3 . The two pairs of mounting terminals  225   a ′ to  225   d ′ are formed to the four corners (the four corner portions) of the base portion  22 ′ to enhance strength of the four corners. 
     Among the two pairs of mounting terminals  225   a ′ to  225   d ′, one pair are external electrodes  225   a ′ and  225   c ′ that are diagonally disposed on the base portion  22 ′ and connect to the respective connection pad  223 M via the base side surface electrodes  223   a  and  223   c . The external electrode  225   c ′ includes a notch (see  FIG. 12D ) to check the orientation of the piezoelectric device  200 ′. 
     The method for fabricating the second piezoelectric device  200 ′ is approximately the same as the fabrication method of the second Embodiment. However, when the quartz-crystal vibrating piece  20 ′ is formed in a state of the quartz-crystal wafer  20 W, as illustrated in  FIG. 14 , a distance between the adjacent quartz-crystal vibrating pieces  20 ′ differs. Additionally, when the base portion  22 ′ is formed in a state of the base wafer  22 ′W, as illustrated in  FIG. 15 , a distance from the adjacent base portions  22 ′ differs. 
     In step S 102  of  FIG. 3 , the extraction electrodes  203   a ′ and  203   b ′ are formed from adjacent extraction electrodes  203   a ′ and  203   b ′ by a distance SP 6  (see  FIG. 14 ). The distance SP 6  is approximately 40 μm to 100 μm. For example, assume that the distance SP 6  is 40 μm and dicing width diced in step S 17  is also 40 μm, the distance SP 5  illustrated in  FIG. 12A  and  FIG. 12B  becomes 0 μm. 
     In step S 122  of  FIG. 3 , the mounting terminal of the base portion  12 B illustrated in  FIG. 15  is formed from the mounting terminal formed at the base portion  12 A adjacent in the Z′-axis direction by the distance SP 6 . Here, the distance SP 6  is approximately 40 μm to 100 μm. Similarly, for example, assume that the distance SP 6  is 40 μm, dicing width diced in step S 17  also becomes 40 μm, the distance SP 5  illustrated in  FIG. 15  becomes 0 μm. 
     Representative embodiments are described in detail above; however, as will be evident to those skilled in the relevant art, this disclosure may be changed or modified in various ways within its technical scope. 
     While in this disclosure, for example, a base wafer, a quartz-crystal wafer, and a lid wafer are bonded together using low-melting point glass, a polyimide resin may be employed instead of the low-melting point glass. When using polyimide resin, the fabrication process may employ screen-printing, and an exposure step may be performed after applying photolithographic polyimide resin on the entire surface. 
     While in this application, a quartz-crystal vibrating piece is used, piezoelectric materials such as lithium tantalate and lithium niobate may be used in addition to quartz-crystal. Further, this disclosure may be directed to a piezoelectric oscillator in which an IC accommodating an oscillator circuit is mounted inside the package as a piezoelectric device. 
     In the first aspect, the piezoelectric device according to a second aspect is configured as follows. The two pairs of mounting terminals include a pair of external electrodes energized outside and a pair of grounding electrodes employed for grounding. The pair of external electrodes and the pair of grounding electrodes are diagonally formed on the second surface. In the first or second aspect, the piezoelectric device according to a third aspect is configured as follows. The base portion includes a depressed portion depressed from the first surface. The piezoelectric vibrating piece is disposed at the base portion with a conductive adhesive such that the pair of extraction electrodes and the pair of connecting electrodes are connected together. 
     In the third aspect, the piezoelectric device according to a fourth aspect is configured as follows. The piezoelectric device further includes a rectangular lid portion bonded to the first surface of the base portion. The lid portion and the base portion are bonded together with a sealing material. In the first or second aspect, the piezoelectric device according to a fifth aspect is configured as follows. The piezoelectric vibrating piece includes a vibrator and a rectangular framing body. The vibrator includes the pair of excitation electrodes. The rectangular framing body includes the extraction electrodes. The framing body surrounds a peripheral area of the vibrator. The piezoelectric vibrating piece is disposed such that the pair of extraction electrodes and the pair of connecting electrodes are connected together. 
     In the fifth aspect, the piezoelectric device according to a sixth aspect is configured as follows. The piezoelectric device further includes a lid portion that is bonded to one principal surface of the framing body. The lid portion is bonded to the one principal surface of the framing body with sealing material. The base portion is bonded to another principal surface of the framing body with sealing material. In any one of the first to sixth aspect, the piezoelectric device according to a seventh aspect is configured as follows. The first surface and the second surface are connected at a side surface of the castellation. The castellation has a cross section that includes a projecting portion. The projecting portion is protruded outside at the center portion from the first surface to the second surface. 
     With the fabrication method according to the embodiments, a piezoelectric device where a corner portion of a base portion is less damaged is obtained. 
     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.