Abstract:
A surface acoustic wave device includes a surface acoustic wave chip having a piezoelectric substrate on which comb-like electrodes and electrode pads are formed, a package housing the surface acoustic wave chip, and electrode patterns provided on a bottom surface of the package. The bottom surface of the package is wider than a top surface of the package. The electrode patterns are away from edges of the bottom surface of the package.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a surface acoustic wave device, a package for the device, and a method of fabricating the device. 
   2. Description of the Related Art 
   As electronic apparatuses with higher performances have become smaller in size, electronic devices to be mounted to such apparatuses are also expected to be smaller and have higher performances. Especially, surface acoustic wave (SAW) devices to be used as electronic parts such as filters, delay lines, and oscillators for electronic apparatuses that transmit or receive electric waves have been required to have downsized packages and high performance. 
   The conventional SAW device has a SAW chip, which is hermetically sealed in a cavity. The SAW chip has a piezoelectricity device substrate (hereinafter, referred to as piezoelectric substrate) on which interdigital transducers (hereinafter referred to as IDT) having comb-like electrodes are formed. This type of the SAW device is described in, for example, Japanese Patent Application Publication 2001-53577, particularly,  FIG. 3 ). The IDT on the input side converts an electric signal applied thereto into a SAW, which propagates on the piezoelectric substrate. The IDT on the output side converts the received SAW into an electric signal that has been subjected to a given modulation. 
   A description will now be given of a conventional SAW device  100  with reference to  FIGS. 1 and 2 .  FIG. 1  is a perspective view of the SAW device  100 , and  FIG. 2  is a sectional view taken along a line A–A′ shown in  FIG. 1 . 
   Referring to these figures, the SAW device  100  has a package  102  having a cavity  104  in which a SAW chip  120  is housed. Metal patterns, which include IDTs  122 , interconnection lines  124 , and electrode pads  123 , are formed on a circuit-formed surface (main surface) of the piezoelectric substrate  121 . The bottom surface of the cavity  104  (die-attached surface) is provided with metal patterns, which include electrode pads  109  aligned with the electrode pads  123  of the SAW chip  120 . The SAW chip  120  is flip-chip mounted on the die-attached surface in the facedown state by using metal bumps  112 . Thus, the package  102  and the SAW chip  120  are electrically and mechanically connected to each other. The electrode pads on the die-attached surface are electrically connected to foot patterns  114  formed on the bottom surface of the package  102  (opposite to the surface defining the cavity  104 ) through interconnection lines and via interconnections. 
   The cavity  104  is hermetically sealed with a metal cap  103  with high reliability. The cap  103  may be fixed to the package  102  by a bonding member  106  (called washer in the following) made of solder or metals such as gold and tin. A metal plating layer  105  is formed on a portion of the package  102  to which the cap  103  is attached. The metal plating layer  105  is electrically connected to a ground foot pattern  113  on the bottom surface of the package  102  via interconnection lines  110  and vias  111  formed in the package  102 . 
   A conventional fabrication method uses multiple-package substrate having multiple SAW devices arranged in rows and columns. The multiple-package substrate is divided into individual packages, each having the above-mentioned structure by dicing or scribing. 
   In the conventional fabrication process, the multiple-package substrate is divided into the individual packages so that the sidewalls of the packages stand vertically. Thus, this process may damage the metal patterns on the bottom surface of the package  102  on the dicing or scribing lines or in the vicinity thereof. For example, the ground foot patterns  113  and the foot patterns  114  are likely to have a chip by dicing or scribing. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to prevent the metal patterns on the bottom surface of the package from being broken or damaged. 
   This object of the present invention is achieved by a surface acoustic wave device comprising: a surface acoustic wave chip having a piezoelectric substrate on which comb-like electrodes and electrode pads are formed; a package housing the surface acoustic wave chip; and electrode patterns provided on a bottom surface of the package; the bottom surface of the package being wider than a top surface of the package, the electrode patterns being away from edges of the bottom surface of the package. 
   The above object of the present invention is also achieved by a package comprising: a cavity in which a surface acoustic wave chip is housed which has a piezoelectric substrate on which comb-like electrodes and electrode pads are formed; and electrode patterns formed on a bottom surface of the package, the bottom surface being wider than a top surface of the package, the electrode patterns being away from edges of the bottom surface of the package. 
   The above object of the present invention is also achieved by a method of fabricating surface acoustic wave devices comprising the steps of: mounting surface acoustic wave chips on a base substrate having a bottom surface on which electrode patterns are provided; and dividing the base substrate into individual surface acoustic wave devices by a dicing blade applied from a top surface of the base substrate, the dicing blade having a blade portion that becomes sharp towards a circumferential end thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a conventional SAW device; 
       FIG. 2  is a sectional view taken along a line A–A′ shown in  FIG. 1 ; 
       FIG. 3  is a perspective view of a surface acoustic wave device according to a first embodiment of the present invention; 
       FIG. 4A  is a sectional view taken along a line B–B′ shown in  FIG. 3 ; 
       FIG. 4B  is an enlarged view of an edge shown in  FIG. 4A ; 
       FIGS. 5A through 5E  show a process of fabricating the surface acoustic wave device according to the first embodiment of the present invention, in which  FIGS. 5A through 5E  are sectional views taken along a line C–C′ shown in  FIG. 3 ; 
       FIG. 6  is a perspective view of a surface acoustic wave device according to a second embodiment of the present invention; and 
       FIG. 7  is a sectional view taken along a line D–D′ shown in  FIG. 6 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given of preferred embodiments of the present invention with reference to the accompanying drawings. 
   (First Embodiment) 
     FIG. 3  is a perspective view of a SAW device  1  according to a first embodiment of the present invention.  FIG. 4A  is a sectional view taken along a line B–B′ shown in  FIG. 3 , and  FIG. 4B  is an enlarged view of an edge  2 A of a package  2  shown in  FIG. 4A . 
   Referring to  FIGS. 3 ,  4 A and  4 B, the SAW device has the package  2 , which may be a board made of ceramics or BT (Bismaleimide Triazine) resin. A cavity  4  is provided in the package  2 , and houses a SAW chip  20 . Preferably, the cavity  4  has a depth capable of completely accommodating the SAW chip  20 . In other words, the depth of the cavity  4  is greater than the height of the SAW chip  20 . However, the cavity  4  may be equal to or smaller than the height of the SAW chip  20  if a cap  3  has a cavity that accommodates the SAW chip  20 . 
   Metal patterns, which include IDTs  22 , interconnection lines  24  and electrode pads  23 , are formed on the circuit-formed surface (main surface) of a piezoelectric substrate  21 . The piezoelectric substrate  21  may be an LT substrate that is a piezoelectric single crystal of, or an LN substrate that is a piezoelectric single crystal of lithium niobate. Another piezoelectric material such as crystalline quartz may be used for the substrate. The insertion loss of the SAW device is comparatively small when the LT substrate is used. The metal patterns including the IDT  22 , the interconnection lines  24  and the electrode pads  23  may be a single layer structure or multilayer structure that contains, as the major component, a metal of aluminum (Al), copper (Cu), gold (Au), molybdenum (Mo), tungsten (W), tantalum (Ta), chromium (Cr), titanium (Ti), platinum (Pt), ruthenium (Ru) or rhodium (Rh). The metal patterns may be formed by photolithographic technology. 
   Metal patterns that include electrode pads  9  aligned with the electrode pads  23  of the SAW chip  20  are formed on the bottom surface (die-attached surface) of the cavity  4 . These metal patterns may be a single layer structure or multilayer structure that contains, as the major component, a metal of Al, Cu, Au, Mo, W, Ta, Cr, Ti, Pt, Ru or Rh, and may be formed by printing and plating technology or formed by photolithographic technology. 
   The SAW chip  20  may be flip-chip mounted on the die-attached surface in the facedown state by using metal bumps  12  of gold or solder, and is be electrically and mechanically connected to the package  2 . The electrode pads  9  on the die-attached surface are electrically connected to foot patterns  14  formed on the bottom or lower surface of the package (opposite to the bottom of the cavity  4 ) through interconnection lines and via interconnections formed in the package  2 . 
   The cavity  4  is hermetically sealed with a metal cap  3  made of, for example, KOVAR™ with high reliability. Preferably, the cap  3  has a size greater than the opening of the cavity  4  and is as large as 80% to 97% of the size of the top surface of the package  2 . Preferably, the cap  3  is 0.1 mm thick or less. The cap  3  may be fixed to the package  2  by a bonding member (washer)  6  using a metal of, for example, gold and tin. A fixed portion of the package  2 , to which the cap  3  is attached, that is, the upper surfaces of the sidewalls of the package  2  are plated with a metal  5 . The metal plating layer  5  has a width equal to or greater than 60% of the widths of the sidewalls of the package  2 . 
   The sidewalls of the package  2  are 0.1 mm to 0.3 mm wide. Castellations  7  are formed on the sidewalls of the package  2 . The castellations  7  are located at the corners of the package  2 . The castellations  7  start from the top surface of the package  2  and do not reach the bottom surface thereof. The castellations  7  have hollows, which have surfaces plated with a metal. Castellations  16  are formed on the inner walls of the package  2  that define the cavity  4 . The lengths of the castellations  16  are shorter than the height of the cavity  4 . The castellations  16  have hollows, which have surfaces plated with a metal. The metal plating on the castellations  7  and the metal plating on the castellations  16  are electrically connected by interconnection lines  15  provided in the sidewalls of the package  2 . The interconnection lines  15  are inner layers sandwiched between layers of the package  2 . The term “castellations” also has the meaning that includes the plated metal. The castellations  7  are electrically connected to the plating metal layer  5 . The castellations  16  are connected to the interconnection lines  10  formed on the bottom of the cavity on the sides opposite to the sides on which the castellations  16  are connected to the interconnection lines  15 . The interconnection lines  10  are electrically connected to ground foot patterns  13  formed on the bottom surface of the package  2  through via interconnections  11  that penetrate the bottom portion of the package  2 . Thus, the cap  3  is grounded via the plating metal layer  5 , the castellations  7 , the interconnection lines  15 , the castellations  16 , the interconnection lines  10 , the via interconnections  11  and the ground foot pattern  13 . Preferably, the bottom portion of the package is 0.2 mm thick or less. 
   As shown in  FIG. 4B , the sidewalls of the package  2  have lower portions (the edges  2 A shown in  FIG. 4A ) that extend outwards or splay out. The side surfaces of the package  2  are defined by dividing the multiple-package substrate into the individual packages by dicing or scribing. The multiple-package substrate is cut so that the lower edges  2 A in the vicinity of the ground foot patterns  13  and the foot patterns  14  splay out. In other words, the bottom surface of the package  2  is wider than the top surface thereof including the opening of the cavity  4 . The ground foot patterns  13  and the foot patterns  14  are away from the edges of the bottom surface of the package  2 . The splaying-out edges  2 A of the package prevent the ground foot patterns  13  and the foot patterns  14  from being exposed to cutting, and prevent these metal patterns from being broken at the time of cutting or mounting. 
   Preferably, a ratio W/H ranges from 0.02 to 0.10 (0.02≦W/H≦0.10) where W is half the difference between the length of at least one side of the top surface of the package  2  in the longitudinal or lateral direction and the length of the side on the bottom surface of the package  2  parallel to the above-mentioned one side of the top surface, and H is the height of the package  2 . The upper limit of the ratio W/H equal to 0.10 prevents the package  2  (and the SAW device  1 ) from having an exclusively large size. The lower limit of the ratio W/H equal to 0.02 makes it possible to secure a sufficient distance from the ground foot patterns  13  and the foot patterns  14  to the lower edges  2 A and to thus prevent these patterns from being broken with high reliability. For example, W/H is equal to 0.02 when the top surface of the package has a size of 2.0 mm and 1.6 mm in the longitudinal and lateral directions, the bottom surface has a size of 2.02 mm and 1.62 mm in the longitudinal and lateral directions, and the height H of the package  2  is 0.5 mm. The ratio W/H of 0.02 makes it possible to prevent the package  2  from becoming large and prevent the ground foot patterns  13  and the foot patterns  14  from being broken. The height H of the package  2  may include the thickness of the cap  3  and that of the washer  6 . That is, the package  2  may be defined so as to include the cap  3  and the washer  6 . 
   A description will now be given of a method of fabricating the SAW device  1  with reference to  FIGS. 5A through 5E . 
   As shown in  FIG. 5A , a base substrate  2 B, on which multiple packages  2  can be integrally arranged in rows and columns, is produced. Next, as shown in  FIG. 5B , the SAW chips  20  are flip-chip mounted in the cavities  4  formed in the base substrate  2 B through metal bumps  12 . 
   Then, as is shown in  FIG. 5C , the base substrate  2 B is divided into separate packages  2  by a dicing blade  90 . This dicing blade  90  has a circumferential end that becomes gradually narrow towards the tip end. In other words, the dicing blade  90  has an end having a tapered cross section or having a thickness gradation area in which the thickness of the dicing blade  90  becomes gradually or continuously small toward the tip end. The side surfaces of the package are slightly curved. The circumferential end of the dicing blade  90  is used to divide the base substrate  2 B into the individual packages  20  from the upper side of the base substrate  2 B. The blade portion having gradation in thickness is used to divide the base substrate  2 B into the individual packages. Thus, the packages  2  having the splay-out lower edges  2 A can be produced. The dicing blade  90  used in the process has a first portion involved in cutting the bottom surface of the package  2 , and a second portion involved in cutting the top surface of the package  2 . The first and second portions have a ratio W/H that ranges from 0.02 to 0.10 (0.02≦W/H≦0.10) where W is half the difference between the thickness of the top portion and that of the bottom portion, and H is the height of the package  2 . 
   Thereafter, as shown in  FIG. 5D , the washer  6  is provided on the top surface of the package  2 , more particularly, the upper surfaces of the sidewalls of the package  2 . The cap  3  is stacked on the washer  6 . Then, the package  2  with the cap  3  being stacked through the washer  6  is pressurized and heated by a pressurizing machine  91  and a heating mechanism  92 . The washer  6  is fused so that the cap  3  can be attached to the package  2 . Preferably, the washer  6  has a metal such as solder having a thickness of 0.1 mm, which is coated with gold/tin. In this manner, the SAW device  1  can be produced, as shown in  FIG. 5E . The washer  6  that flows out by fusing is confined in the castellations  6 , so that it does not reach the bottom surface of the package  2 . The ground foot patterns  13  and the foot patterns  14  can be prevented from being short-circuited each other. 
   According to one aspect of the present invention, the bottom surface of the package  2  is wider than the top surface thereof including the opening of the cavity  4 . This is a very simple means for preventing the metal patterns on the bottom surface of the package from being broken, and easily improves the production yield. 
   (Second Embodiment) 
   A description will now be given of a second embodiment of the present invention with reference to  FIGS. 6 and 7 .  FIG. 6  is a perspective view of a SAW device  1 B according to the second embodiment, and  FIG. 7  is a sectional view taken along a line D–D′ shown in  FIG. 6 . In  FIGS. 6 and 7 , parts that are the same as those shown in the previously described figures are given the same reference numerals. 
   In the above-mentioned first embodiment, the plating metal layer  5  on the upper surfaces of the sidewalls of the package  2  are electrically connected to the interconnection lines  15  embedded in the sidewalls via the castellations  7  formed on the sidewalls of the package  2 . In contrast, according to the second embodiment, the plating metal layer  5  is electrically connected to the interconnection lines  15  via holes  7 B extending from the upper surfaces of the sidewalls of the package  2  to an intermediate position in the sidewalls, as shown in  FIG. 7 . The via holes  7 B are formed in the sidewalls of the package  2 . The inner surfaces of the via holes  7 B are plated with a metal as in the case of the castellations  7 . The plating metal layer  5  and the interconnections  15  can be electrically connected via the plating metal layers on the inner walls of the via holes  7 B. 
   For example, the via holes  7 B are positioned at the four corners and have a cylindrical shape of 0.35 mm. The depths of the via holes  7 B, namely, the intermediate positions of the via holes  7 B do not reach the die-attached surface that is the bottom surface of the cavity  4 . For example, the via holes  7 B have a depth approximately equal to half the depth of the cavity. When the cavity  4  is 0.3 mm deep, the via holes  7 B are 0.15 mm in depth. In this case, the package  2  can be formed by stacking layers having an identical thickness. 
   The second embodiment has the same advantages as those of the first embodiment. In addition, the second embodiment has another advantage in that the washer  6  that flows out by fusing can be confined in the via holes  7 B so that the ground foot patterns  13  and the foot patterns  14  can be prevented from being short-circuited. The other parts of the second embodiment are the same as those of the first embodiment. 
   The present invention is not limited to the specifically disclosed embodiments, and other embodiments, variations and modifications may be made without departing from the scope of the present invention. 
   The present invention is based on Japanese Patent Application No. 2003-169896 filed on Jun. 13, 2003, and the entire disclosure of which is hereby incorporated by reference.