Abstract:
A surface acoustic wave device includes a package having a cavity, a SAW chip housed in the cavity, a resin sealing the cavity, and a metal thin film provided on the resin.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a surface acoustic wave device, a package for the device, and a method of fabricating the device.  
         [0003]     2. Description of the Related Art  
         [0004]     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.  
         [0005]     The conventional SAW device has a SAW chip, which is hermetically sealed in a cavity. The SAW chip has a 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 2001-176995. 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.  
         [0006]     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 F-F shown in  FIG. 1 .  
         [0007]     Referring to these figures, the SAW device  100  has a package  101  having a cavity  102  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 a piezoelectric substrate  121 . The bottom surface of the cavity  102  (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 bonded on the die-attached surface in the facedown state by using metal bumps  112 . Thus, the package  101  and the SAW chip  120  can be electrically and mechanically connected to each other. The electrode pads on the die-attached surface are electrically connected to foot patterns  107  formed on the bottom surface of the package  101  (opposite to the surface defining the cavity  102 ) through interconnection lines and via interconnections  108 . The cavity  102  is sealed with a resin  103 .  
         [0008]     However, the cavity  102  cannot be sealed with only the resin  103  with high reliability. The above sealing structure does not show good results in a test of measuring moisture resistance.  
       SUMMARY OF THE INVENTION  
       [0009]     This is an object of the present invention to provide a surface acoustic wave device with improved moisture resistance and a method of fabricating such a device.  
         [0010]     This object of the present invention is achieved by a surface acoustic wave device comprising: a package having a cavity; a SAW chip housed in the cavity; a resin sealing the cavity; and a metal thin film provided on the resin.  
         [0011]     The above object of the present invention is also achieved by a method of fabricating a surface acoustic wave device comprising the steps of: (a) sealing a cavity of a package housing a surface acoustic wave chip with resin; and (b) providing a metal thin film on the resin.  
         [0012]     The above object of the present invention is also achieved by a method of fabricating a surface acoustic wave device comprising the steps of: (a) bonding surface acoustic wave chips in cavities arranged in rows and columns on a base substrate; (b) sealing the cavities with resin; (c) notching the resin on boundaries along which the base substrate can be split into individual surface acoustic wave devices, so that packages each having one of the cavities can be partially exposed, notches in the resin having a width greater than a cutoff width in splitting; (d) providing a metal thin film on the resin and exposed portions of the packages; and (e) splitting the base substrate into the individual surface acoustic wave devices. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Other objects, features and advantages of the present invention will become more apparent when read in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  is a perspective view of a conventional SAW device;  
         [0015]      FIG. 2  is a sectional view taken along a line F-F;  
         [0016]      FIG. 3  is a perspective view of a SAW device according to a first embodiment of the present invention;  
         [0017]      FIG. 4A  is a sectional view taken along a line A-A shown in  FIG. 3 ;  
         [0018]      FIG. 4B  is an enlarged view of a portion A shown in  FIG. 4A ;  
         [0019]      FIGS. 5A, 5B ,  5 C and  5 D show a method of fabricating the SAW device according to the first embodiment of the present invention;  
         [0020]      FIGS. 6A, 6B ,  6 C and  6 D show a method of fabricating the SAW device according to the first embodiment of the present invention following the steps of  FIGS. 5A through 5D ;  
         [0021]      FIG. 7  is a perspective view of a SAW device according to a second embodiment of the present invention;  
         [0022]      FIG. 8  is a cross-sectional view taken along a line B-B shown in  FIG. 7 ;  
         [0023]      FIG. 9  is a perspective view of a SAW device according to a third embodiment of the present invention; and  
         [0024]      FIG. 10  is a sectional view taken along a line C-C shown in  FIG. 9 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     A description will now be given of preferred embodiments of the present invention.  
         [heading-0026]     (First Embodiment)  
         [0027]      FIGS. 3, 4A  and  4 B show a SAW device  1 A according to a first embodiment of the present invention. More particularly,  FIG. 3  is a perspective view of the SAW device  1 A,  FIG. 4A  is a sectional view taken along a line A-A shown in  FIG. 3 , and  FIG. 4B  is an enlarged view of a portion A shown in  FIG. 4A .  
         [0028]     Referring to  FIG. 3  and  4 A, the SAW device  1 A uses a package  1 , which may be a ceramics substrate. A cavity  2 , which is provided in the package  1 , houses a SAW chip  20 . In the present embodiment, the package  1  has a sidewall that is approximately 0.1 mm to 0.3 mm thick (0.2 mm for example), and a bottom plate having a thickness of 0.2 mm or less (for example, 0.15 mm). The cavity  2  has a depth capable of completely accommodating the SAW chip  20 . In other words, the depth of the cavity  2  is greater than the height of the SAW chip  20 . For example, when the SAW device  1 A is 0.6 mm high and the SAW chip  20  is 0.35 mm high, the cavity  2  has a depth of approximately 0.4 mm. This arrangement makes it possible to completely accommodate the SAW chip  20  and prevent the SAW chip  20  from being damaged at the time of mounting on the print-circuit board.  
         [0029]     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 lithium tantalate, 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.  
         [0030]     The metal patterns including the IDT  22 , the interconnection lines  24  and the electrode pads  23  may be 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.  
         [0031]     Metal patterns that include electrode pads  9  aligned with the electrode pads  23  of the SAW chip  20  and interconnection patterns integrally formed with the electrode pads  9  are formed on the bottom surface (die-attached surface) of the cavity  2 . These metal patterns may be 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.  
         [0032]     The SAW chip  20  may be flip-chip bonded on the die-attached surface in the facedown state by using metal bumps  12  of gold or solder, so that the SAW chip  20  can be electrically and mechanically connected to the package  1 .  
         [0033]     The electrode pads  9  on the die-attached surface are electrically connected, via interconnection patterns  6  formed thereon, to via interconnection lines (which may be referred to as inner via patterns) that penetrate the bottom plate of the package  1 . The via interconnection lines  8  are connected to foot patterns  7  formed on the back (bottom) surface of the package  1  opposite to the opening of the cavity  2 . Input and output terminals and ground terminals of the SAW device chip  20  are extended to the foot patterns  7  on the bottom surface of the package  1  via the interconnection patterns  6  and the via interconnection lines  8 .  
         [0034]     The cavity  2  that accommodates the SAW chip  20  is sealed with a resin  3 . The resin  3  does not reach the die-attached surface. In other words, it is preferable to define a space below the resin  3 . This avoids a problem that the resin  3  reaches some metal patterns on the SAW chip  20  and prevents the normal filtering operation of the SAW device  1 A. The resin  3  may be, for example, epoxy resin.  
         [0035]     The resin that seals the cavity  2  is covered with a thin film  4 , so that sealing can be improved. The resin coated with the metal thin film  4  does not directly receive external force at the time of mounting the SAW device  1 A on the print-circuit board, so that the reliability in mounting can be improved. The metal thin film  4  is made of a metal material such as copper (Cu). The metal thin film  4  may have single-layer structure or multilayer structure having layers of different metals.  
         [0036]     Exposed portions of the metal thin film  4  is plated or coated with an antioxidant film  5 . Preferably, the antioxidant film  5  may be a metal-based film that contains, for example, nickel (Ni), gold (Au), platinum (Pt). The antioxidant film  5  may also be a resin-based film that is, for example, epoxy resin, acrylic resin or fluorinated resin such as Teflon (registered trademark). Besides, the antioxidant film  5  may be made of other materials resistant to oxidizing and corrosion.  
         [0037]     As is shown in  FIG. 4B , the metal thin film  4  contacts a metal pattern  11  formed on the upper ends of the sidewalls of the package  1 . More specifically, the metal thin film  4  contacts the metal pattern  11  along outer edge portions of the package  1 . The metal pattern  11  is electrically connected to the interconnection patterns  6  via the via interconnection lines  10  formed within the sidewalls of the package  1 . Thus, the metal thin film  4  and the antioxidant film  5  are electrically connected to a corresponding foot pattern on the bottom surface of the package  1  (more specifically, a foot pattern connected to ground) via the metal pattern  11 , via interconnection lines  10 , the interconnection patterns  6  and the via interconnection lines  8 .  
         [0038]     As will be described in detail later, the metal thin film  4  and the metal pattern  11  can be electrically made by notching the resin  3  on the metal pattern  11  and growing the metal film  4  on exposed surfaces of the metal pattern  11  by electroplating. At that time, as shown in  FIG. 4B , preferably, a ratio W/T is set equal to or greater than 0.05 where T is the notched depth of the resin  3  (depth up to the metal pattern  11 ) and W is half the width of the notch. It will be noted that the slope of the resin  3  shown in  FIG. 3B  is half of the notch on the resin  3  formed by notching in the fabrication process that will be described later. The ratio W/T indicates the degree of inclination of the slope on the surface of the resin  3 . The forming of the slope having the degree of inclination equal to or less than 5% makes it possible to an underlying layer (which is another metal film  3   a  described later) below the metal thin film  4  from breaking away at the time of electroplating. For example, the depth T is equal to 0.02 mm, and the width W is equal to 0.2 mm. The slope cut deeply into the resin  3  may be flat or curved as shown in  FIG. 4B .  
         [0039]     The metal thin film  4  is joined directly to the metal pattern  11  or the sidewalls of the package, as shown in  FIG. 4B . This results in perfect hermetic sealing of the cavity  2  with the package material and metal thin film, and thus improves the reliability of hermetic sealing. The antioxidant film  5  may be formed by growing nickel or the like by electroplating after the metal thin film  4  is formed by electroplating. This will be described in detail later.  
         [0040]     A description will now be given of a method of fabricating the SAW device  1 A with reference to  FIGS. 5A through 5D  and  6 A through  6 D. As shown in  FIG. 5A , a multi-package base substrate  10 A is formed in which groups are arranged in rows and columns, each of which groups has the cavity  2 , metal patterns including the metal pattern  11 , the via interconnection lines  10 , the electrode pads  9  and the interconnection patterns  6 , the via interconnection lines  8 , and the foot patterns  7 . Next, the SAW chips  20  are flip-chip bonded in the cavities  2  in the facedown state, as shown in  FIG. 5B . In the mounting on the print-circuit board, the electrode pads of the SAW chips  20  are aligned with the electrode pads  9  in the cavities  2 , and are bonded thereto by metal bumps  12 . Thus, the SAW chips  20  are electrically connected to the packages  1  and are mechanically fixed thereto.  
         [0041]     Then, as shown in  FIG. 5C , the cavities  2  in which the SAW chips  20  have been bonded are sealed with the resin  3 . Thereafter, a dicing blade  91  is used to form notches  15  on boundaries along which the base substrate should be split into individual packages, as shown in  FIG. 5D . The resin  3  is cut in or notched so that the metal patterns  11  and the sidewalls of the packages  1  can be exposed. The dicing blade  91  has a blade capable of realizing the ratio W/T equal to or greater than 0.05 in which each of the notches  15  has a width equal to twice the width W that defines the ratio W/T. In other words, the relationship between the width W equal to half the width of each notch  15  and the depth T of the resin  3  up to the metal pattern  11  satisfies a ratio of 0.05 or greater. It can also be said that the step of  FIG. 4D  uses the blade  91  capable of realizing the degree of inclination of the slope equal to or less than 5%.  
         [0042]     Next, the metal film  3   a  is deposited on the resin  3  and the notches  15  as a preprocess for electroplating, as shown in  FIG. 6A . The presence of the notches  15  having the degree of inclination equal to or less than 5% avoids breaking away in deposition. After that, a resist  3   b  is provided on the back surface of the base substrate  10 A (which corresponds to the back surfaces of the packages  1 ) in order to protect the foot patterns  7 . Then, copper is grown on the upper surface of the base substrate  10 A by electroplating, so that the metal thin film  4  can be formed on the metal film  3   a , as shown in  FIG. 6B . Further, nickel is provided on the metal thin film  4  by electroplating so that the antioxidant metal film  5  can be formed thereon, as shown in  FIG. 6B . In the above-mentioned method, the reliability in hermetic sealing of the cavities  2  can be improved and the metal thin film  4  can be grounded by forming the notches  15  through which the metal pattern  11  or the package  1  are exposed and then providing the metal thin film  4  on the entire surface including the notches  15 .  
         [0043]     The above-mentioned process of forming the metal thin film  4  shown in  FIG. 6B  employs electroplating, which may be replaced by sputtering, vacuum deposition or a combination thereof. For example, in the combination of sputtering and electroplating, the metal thin film  3   a  is formed first by sputtering, and the metal thin film  4  is then formed thereon by electroplating. In the combination of vacuum deposition and electroplating, the metal thin film  3   a  is formed first by vacuum deposition, and the metal thin film  4  is then formed thereon by electroplating.  
         [0044]     Thereafter, a dicing tape  92   a  is adhered to the back surface of the base substrate  10 A, which is then split into the individual packages  1 A by a dicing blade  92  applied to the base substrate  10 A from the upper side thereof, as shown in  FIG. 6C . Preferably, the dicing blade  92  has a width W 2  narrower than a width W 1  of the exposed portions of the packages  1 . In other words, the notches  15  are 0.1 mm wider than the cutoff portions. This condition secures the joining of the metal thin film  4  and the packages  1 . For example, the dicing blade  92  has the width W 2  that is 0.1 mm narrower than the width W 1  or more, so that the sufficient exposed portions of the packages  1  and the metal patterns  11  can be secured.  
         [0045]     According to the first embodiment of the present invention, the reliability in hermetically sealing of the cavities  2  can be improved by forming the notches on the resin  3  provided to the openings of the cavities  2  of the packages  1  so as to expose the sidewalls of the packages  1  and forming the metal thin film  4  so as to be joined to the exposed sidewall portions. The reliability in mounting the SAW device  1 A on the print-circuit board can be improved because the resin  3  is coated with the metal thin film  4  and is thus prevented from directly receiving external force. The metal thin film  4  is grounded by making contacts with the exposed portions of the metal patterns  11 , so that the electrical characteristics of the SAW device  1 A can be improved. The metal thin film  4  can be formed with high reliability by forming the notches  15  that are provided to expose the metal patterns  11  and the packages  1  and have the degree of inclination equal to or less than 5% with respect to the upper surface of the resin  3 .  
         [heading-0046]     (Second Embodiment)  
         [0047]     A description will now be given of a SAW device according to a second embodiment of the present invention. The second embodiment has a different way of making a connection between the metal pattern  11  formed on the upper surface of the package  1  and the interconnection lines  6  provided on the die-attached surface.  
         [0048]      FIGS. 7 and 8  show a SAW device  1 B according to the second embodiment of the present invention. More particularly,  FIG. 7  is a perspective view of the SAW device  1 B, and  FIG. 8  is a sectional view taken along a line B-B shown in  FIG. 7 . In these figures, parts that are the same as those shown in the previously described figures are given the same reference numerals. In the aforementioned first embodiment, the metal pattern  11  on the upper surfaces of the sidewalls of the package  1  are connected to the interconnection patterns  6  on the die-attached surface. According to the second embodiment, as shown in  FIGS. 7 and 8 , the metal pattern  11  is connected to the interconnection patterns  6  via castellations  10   b  plated with metal and formed on the outer surfaces of the package  1 . The metal-plated castellations  10   b  shown in  FIG. 7  are provided at the corners of the package  1 . The metal-plated castellations  10   b  do not reach the bottom surface of the package  1 . The metal-plated castellations  10   b  are used to make electrical connections between the metal thin film  4  and the foot patterns  7  (ground foot patterns) together with the interconnection lines  6  and the via interconnections  8 . The other structures of the second embodiment are the same as those of the first embodiment. The SAW device  1 B may be fabricated by the same method as that for the first embodiment.  
         [0049]     According to the second embodiment of the present invention, the reliability in hermetically sealing of the cavities  2  can be improved by forming the notches  15  on the resin  3  provided to the openings of the cavities  2  of the packages  1  so as to expose the sidewalls of the packages  1  and forming the metal thin film  4  so as to be joined to the exposed sidewall portions. The reliability in mounting the SAW device  1 B on the print-circuit board can be improved because the resin  3  is coated with the metal thin film  4  and is thus prevented from directly receiving external force. The metal thin film  4  is grounded by making contacts with the exposed portions of the metal patterns  11 , so that the electrical characteristics of the SAW device  1 A can be improved. The metal thin film  4  can be formed with high reliability by forming the notches  15  that are provided to expose the metal patterns  11  and the packages  1  and have the degree of inclination equal to or less than 5% with respect to the upper surface of the resin  3 . The castellations  10   b  that do not reach the bottom surface of the package  1  can prevent the resin  3  that seals the cavity  2  from flowing out to the back surface of the package  1 , particularly, the foot patterns  7 .  
         [heading-0050]     (Third Embodiment)  
         [0051]     A third embodiment of the present invention employs a different arrangement of the castellations  10   c . More particularly, the castellations  10   c  used in the third embodiment are arranged on the inner walls of the package  1 , and face the cavity  2 . This arrangement will now be described with reference to  FIGS. 9 and 10 .  
         [0052]      FIGS. 9 and 10  show a SAW device  1 C according to the third embodiment. More particularly,  FIG. 9  is a perspective view of the SAW device  1 C, and  FIG. 10  is a sectional view taken along a line C-C shown in  FIG. 9 . In these figures, parts that are the same as those shown in the previously described figures are given the same reference numerals.  
         [0053]     Castellations  10   c - 1  and  10   c - 2  are formed on the inner walls of the package  1  that defines the cavity  2 . The surfaces of the castellations  10   c - 1  and  10   c - 2  are plated with a metal. The castellations  10   c - 1  and  10   c - 2  are arranged at different levels in the vertical direction. The metal-plated castellations  10   c - 1  and  10   c - 2  are electrically connected to each other via interconnection patterns  13 . The castellation  10   c - 1  at the upper level of the package  1  is electrically connected to the metal pattern  11  formed on the upper surfaces of the sidewalls of the package  1 . The castellation  10   c - 2  at the lower level of the package  1  is electrically connected to the metal patterns  6  formed on the die-attached surface of the cavity  2 . Thus, the metal pattern  11  is electrically connected to the foot patterns (particularly, the ground foot patterns)  7  formed on the backside of the package  1  via the castellation  10   c - 1 , the interconnection patterns  13 , the castellation  10   c - 2 , the interconnection patterns  6  and the via interconnection lines  8 . The other structures of the third embodiment are the same as those of the first embodiment, and a description thereof will be omitted here.  
         [0054]     According to the third embodiment of the present invention, the reliability in sealing of the cavities  2  can be improved by forming the notches  15  on the resin  3  provided to the openings of the cavities  2  of the packages  1  so as to expose the sidewalls of the packages  1  and forming the metal thin film  4  so as to be joined to the exposed sidewall portions. The reliability in mounting the SAW device  1 C on the print-circuit board can be improved because the resin  3  is coated with the metal thin film  4  and is thus prevented from directly receiving external force. The metal thin film  4  is grounded by making contacts with the exposed portions of the metal patterns  11 , so that the electrical characteristics of the SAW device  1 A can be improved. The metal thin film  4  can be formed with high reliability by forming the notches  15  that are provided to expose the metal patterns  11  and the packages  1  and have the degree of inclination equal to or less than 5% with respect to the upper surface of the resin  3 . The castellations  10   c - 1  and  10   c - 2  provided on the inner walls of the package  1  and arranged at different levels in the vertical direction prevent the resin  3  that seals the cavity  2  from flowing out to the die-attached surface of the cavity  2  and the metal patterns of the SAW device  20 .  
         [0055]     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 claimed invention.  
         [0056]     The present invention is based on Japanese Patent Application No. 2003-193152 filed on Jul. 7, 2003, and the entire disclosure of which is hereby incorporated by reference.