Abstract:
A surface acoustic wave element constituted by forming a comb electrode and a terminal electrode on a piezoelectric element and a base substrate including an element connection terminal and an external connection terminal are opposed to each other, the terminal electrode and the element connection terminal are connected to each other, and the peripheries of the base substrate and the surface acoustic wave element are sealed with a sealing member and bonded to each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a Continuation Application of PCT Application No. PCT/JP00/06374, filed Sep. 19, 2000, which was not published under PCT Article 21(2) in English. 
     
    
     
         [0002]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-266363, filed Sep. 20, 1999, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0003]    This invention relates to a miniaturized surface acoustic wave device and a method for manufacturing the same.  
           [0004]    In general, a surface acoustic wave device converts an electrical signal to a surface acoustic wave (SAW) and transmits/receives a signal using a thin-film metal comb electrode (IDT: Inter-Digital Transducer) formed on a piezoelectric element.  
           [0005]    Such a surface acoustic wave device is used in a surface acoustic wave filter, a surface acoustic wave resonator, a delay circuit, or the like.  
           [0006]    Recently, the surface acoustic wave device has been used widely in the field of a mobile communication system such as a cellular phone in particular because the device can be thinned and miniaturized.  
           [0007]    At present, the surface acoustic wave device is required to be miniaturized further as a cellular phone decreases in size.  
           [0008]    A surface acoustic wave device having a structure as disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 8-191181 is known as a prior art one.  
           [0009]    The surface acoustic wave device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-191181 will now be described with reference to FIGS.  10  to  15  that show a manufacturing process thereof.  
           [0010]    As shown in FIG. 10, comb electrodes  2  and terminal electrodes  3  are formed of a metal thin film such as aluminum (Al) on a piezoelectric substrate  1 , thereby forming a plurality of surface acoustic wave elements  4 ,  4 .  
           [0011]    Then, a metal bump  5  is formed on each of the terminal electrodes  3 , as illustrated in FIG. 11.  
           [0012]    As shown in FIG. 12, the piezoelectric substrate  1  is cut into individual surface acoustic wave elements  4  by dicing or the like.  
           [0013]    After that, the surface acoustic wave elements  4  are face-down-bonded by means of ultrasonic wave to their respective box-type ceramic packages  11  connected to each other, as shown in FIG. 13. Each of the packages  11  has an opening at the top and includes an external connection terminal and an element connection terminal in advance, neither of which is shown.  
           [0014]    In this state, the opening of each package  11  is blocked with a cap  12 , and the cap  12  is fixed to the package  11  by an adhesive  13 , as shown in FIG. 14. Thus, the surface acoustic wave element  4  is sealed in a space formed between the package  11  and the cap  12 .  
           [0015]    The respective packages  11  are separated from each other to form a surface acoustic wave device  15 , as illustrated in FIG. 15.  
           [0016]    However, the prior art surface acoustic wave device shown in FIGS.  10  to  15  is so constituted that the surface acoustic wave element  4  is mounted in the package  11 .  
           [0017]    The problem therefore occurs in which the size of the package  11  has to be about four times as large as that of the surface acoustic wave element  4  from the viewpoint of the accuracy of mounting the surface acoustic wave element  4  in the package  11 , the mechanism for holding the mounted element in the package, or the obtainment of space for mounting the cap  12  on the package  11 .  
           [0018]    Since, moreover, the cap  12  is mounted on the package  11  in order to seal the surface acoustic wave element  4 , the mounting step is added and the material costs of the cap  12  are required, which becomes uneconomical.  
           [0019]    Since, furthermore, the surface acoustic wave element  4  is face-down-bonded to the package  11  by means of ultrasonic wave or the like, there occurs an inconvenience of limiting the material of the package  11  or the piezoelectric substrate  1  to a hard material that does not absorb the ultrasonic wave.  
           [0020]    Accordingly, the present invention has been developed in consideration of the above situation and its object is to provide a small-sized surface acoustic wave device that increases in productivity and reliability and a method for manufacturing the device.  
         BRIEF SUMMARY OF THE INVENTION  
         [0021]    A surface acoustic wave device according to the present invention, comprises a base substrate having an external connection terminal and an element connection terminal electrically connected to the external connection terminal, a surface acoustic wave element arranged opposite to the base substrate and having a piezoelectric element, a comb electrode and a terminal electrode connected thereto being formed on an inner major surface of the piezoelectric element, a connection member for connecting the element connection terminal and the terminal electrode, and a frame-like resin member whose side continues with one side of the piezoelectric element and the base substrate, the frame-like resin member being sandwiched between an inner major surface of the base substrate and the inner major surface of the piezoelectric element, and holding and bonding the base substrate and the piezoelectric element with a predetermined gap therebetween.  
           [0022]    A method for manufacturing a surface acoustic wave device according to the present invention, comprises a first step of forming a comb electrode and a terminal electrode electrically connected thereto on each of a plurality of piezoelectric elements into which a piezoelectric wafer is cut, thereby forming a plurality of surface acoustic wave elements, a second step of forming a metal bump on the terminal electrode of each of the surface acoustic wave elements, a third step of forming an external connection terminal and an element connection terminal electrically connected thereto on each of a plurality of base substrates into which a base plate is cut, a fourth step of applying a resin member to a frame-like periphery of each of the plurality of base substrates, a fifth step of facing the surface acoustic wave element and the base substrate each other such that the terminal electrode and the element connection terminal are connected to each other through the metal bump, and a sixth step of curing the resin member to bond the surface acoustic wave element and the base substrate to each other.  
           [0023]    According to the above structure and method, the terminal electrode of the surface acoustic wave element and the element connection terminal of the base substrate are electrically connected through a metal bump, and the piezoelectric element and the base substrate are bonded by the frame-like resin member sandwiched therebetween. No clearance is therefore required between the surface acoustic wave element and the package wall surface. The device can be thinned and the package itself can be formed to have the same area as that of the surface acoustic wave element; consequently, the device can be miniaturized.  
           [0024]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0025]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
         [0026]    [0026]FIG. 1 is a sectional side view explaining one embodiment of a surface acoustic wave device according to the present invention;  
         [0027]    [0027]FIG. 2 is a sectional side view explaining one step of a manufacturing method of the surface acoustic wave device according to the embodiment;  
         [0028]    [0028]FIG. 3 is a sectional side view explaining a step subsequent to the step shown in FIG. 2 in the manufacturing method according to the embodiment;  
         [0029]    [0029]FIG. 4 is a sectional side view explaining a step subsequent to the step shown in FIG. 3 in the manufacturing method according to the embodiment;  
         [0030]    [0030]FIG. 5 is a sectional side view explaining a step subsequent to the step shown in FIG. 4 in the manufacturing method according to the embodiment;  
         [0031]    [0031]FIG. 6 is a sectional side view explaining a step subsequent to the step shown in FIG. 5 in the manufacturing method according to the embodiment  
         [0032]    [0032]FIG. 7 is a sectional side view explaining a step subsequent to the step shown in FIG. 6 in the manufacturing method according to the embodiment;  
         [0033]    [0033]FIG. 8 is a sectional side view explaining a step subsequent to the step shown in FIG. 3 in a modification to the manufacturing method according to the embodiment;  
         [0034]    [0034]FIG. 9 is a sectional side view explaining a step subsequent to the step shown in FIG. 5 in the manufacturing method according to the modification;  
         [0035]    [0035]FIG. 10 is a sectional side view explaining one step of a manufacturing method of a prior art surface acoustic wave device;  
         [0036]    [0036]FIG. 11 is a sectional side view explaining a step subsequent to the step shown in FIG. 10 in the prior art manufacturing method;  
         [0037]    [0037]FIG. 12 is a sectional side view explaining a step subsequent to the step shown in FIG. 11 in the prior art manufacturing method;  
         [0038]    [0038]FIG. 13 is a sectional side view explaining a step subsequent to the step shown in FIG. 12 in the prior art manufacturing method;  
         [0039]    [0039]FIG. 14 is a sectional side view explaining a step subsequent to the step shown in FIG. 13 in the prior art manufacturing method; and  
         [0040]    [0040]FIG. 15 is a sectional side view explaining a step subsequent to the step shown in FIG. 14 in the prior art manufacturing method. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    One embodiment of the present invention will be described in detail hereinafter with reference to the drawings. As FIG. 1 shows, a surface acoustic wave device  21  has a surface acoustic wave element  22 .  
         [0042]    In the surface acoustic wave element  22 , a comb electrode  24  serving as an inter-digital transducer (IDT) and terminal electrodes  25 , which are made of a metal thin film of aluminum (Al), are formed on one inner major surface of a plate-like piezoelectric element  23 .  
         [0043]    In FIG. 1, reference numeral  26  indicates a base substrate. The base substrate  26  is formed like a plate, the shape of which is substantially the same as that of the surface acoustic wave element  22  and the area of which is the same as that of the element  22 .  
         [0044]    An element connection terminal  27  is formed on one inner major surface of the base substrate  26 . An external connection terminal  28  is formed on the other surface of the base substrate  26 .  
         [0045]    The base substrate  26  includes a contact hole  29  formed in a portion sandwiched between the element connection terminal  27  and the external connection terminal  28 .  
         [0046]    In the base substrate  26 , the element connection terminal  27  and the external connection terminal  28  are electrically connected to each other via a connecting section  30  provided in the contact hole  29 .  
         [0047]    The surface acoustic wave element  22  is face-down-bonded to the base substrate  26  such that they face each other with a predetermined interval therebetween.  
         [0048]    In this case, the element connection terminal  27  of the base substrate  26  and the terminal electrode  25  of the surface acoustic wave element  22  are arranged so as to face each other.  
         [0049]    The element connection terminal  27  and the terminal electrode  25  are mechanically bonded and electrically connected to each other through a connection member  33 . The connection member  33  includes a convex conductive metal bump  31  and a conductive adhesive  32 .  
         [0050]    A sealing member  34  is formed of a conductive adhesive and interposed between opposing peripheral portions of the base substrate  26  and the surface acoustic wave element  22 .  
         [0051]    The sealing member  34  is a frame-like resin member, and the resin member is sandwiched between the piezoelectric element  23  and the base substrate  26  such that its side surface continues with that of the piezoelectric element  23  or the base substrate  26 . In this embodiment, the piezoelectric element  23 , the side surface of the base substrate  26 , and the resin member constitute a package wall surface.  
         [0052]    The sealing member  34  allows the base substrate  26  and the surface acoustic wave element  22  to be mechanically bonded to each other with a predetermined interval therebetween.  
         [0053]    The sealing member  34  has a function of sealing the periphery of the comb electrode  24  between the piezoelectric element  23  and the base substrate  26  and serves as a conductive path used for establishing a ground.  
         [0054]    A method for manufacturing the surface acoustic wave device  21  described above will now be described with reference to FIGS.  2  to  7 .  
         [0055]    Referring first to FIG. 2, a comb electrode  24  and a terminal electrode  25  are formed of a metal thin film of aluminum (Al) or the like on a piezoelectric wafer  41  to form a plurality of surface acoustic wave elements  22 ,  22 .  
         [0056]    As FIG. 3 illustrates, a metal bump  31  is formed on the terminal electrode  25  of each surface acoustic wave element  22  by bonding, screen printing or the like.  
         [0057]    As FIG. 4 shows, a contact hole  29  is formed in a given position of a base plate  42  in which a plurality of base substrates  26  are formed.  
         [0058]    An element connection terminal  27  is formed on one surface of the base plate  42 . An external connection terminal  28  is formed on the other surface of the base plate  42 .  
         [0059]    After that, a connecting section  30  is formed in the contact hole  29  to electrically connect the element connection terminal  27  and the external connection terminal  28  with each other.  
         [0060]    As shown in FIG. 5, a conductive adhesive  32  is applied to the element connection terminal  27  of each of the base substrates  26  of the base plate  42  in the applying step.  
         [0061]    At the same time, a sealing member  34 , which is formed of a conductive adhesive having the same material as that of the conductive adhesive  32 , is applied to the periphery portion of each of the base substrates  26  of the base plate  42  by screen printing.  
         [0062]    After that, as illustrated in FIG. 6, the surface acoustic wave element  22  of the piezoelectric wafer  41  is provided above and opposite to its corresponding one of the base substrates  26  of the base plate  42  in a mounting step.  
         [0063]    In this case, the element connection terminal  27  of the base substrate  26  and the terminal electrode  25  of the surface acoustic wave element  22  are arranged so as to face each other.  
         [0064]    As FIG. 7 shows, in a fixing step, the conductive adhesive  32  is cured by thermosetting to form a connection member  33  together with the metal bump  31 . Thus, the element connection terminal  27  and the terminal electrode  25  are electrically connected to each other.  
         [0065]    Simultaneously, the sealing member  34  is cured to bond the piezoelectric element  23  and the base substrate  26  to each other and seal the periphery of the comb electrode  24  therebetween.  
         [0066]    Since the base substrate  26  and the surface acoustic wave element  22  have substantially the same shape as shown in FIG. 1, the piezoelectric wafer  41  and the base plate  42  are diced together and cut into individual base substrates  26  and surface acoustic wave elements  22 . The surface acoustic wave device  21  is formed accordingly.  
         [0067]    A modification to the above-described embodiment will now be described. The metal bump  31  is formed on the terminal electrode  25  of each of the surface acoustic wave elements  22  as shown in FIG. 3. Then, the surface acoustic wave elements  22  are separated from each other as illustrated in FIG. 8.  
         [0068]    After the step shown in FIG. 5, each of the surface acoustic wave elements  22  can independently be connected to the base plate  42 , as illustrated in FIG. 9. In this modification, the separation of the piezoelectric wafer  41  and that of the base plate  42  are performed in different steps.  
         [0069]    In the foregoing embodiment, the sealing member  34  is made of the same material as that of the conductive adhesive  32  for connecting the element connection terminal  27  and the terminal electrode  25  and can be formed concurrently with the conductive adhesive  32 . This reduces the number of steps and improves the operability.  
         [0070]    However, the sealing member  34  need not always be made of the same material as that of the conductive adhesive  32 . For example, an insulative adhesive can be used. In particular, the insulative adhesive has the advantage that it contains no conductive particles and its mechanical strength is greater than that of the conductive adhesive.  
         [0071]    For example, a silver-epoxy material in which silver particles are scattered in epoxy resin can be used as the conductive adhesive.  
         [0072]    Epoxy resin is favorable for insulative resin when a user places importance on resistance to moisture. Silicon resin and polyimide resin are also favorable for the insulative resin when a user places importance on elasticity to mitigate distortion due to the stress of thermal expansion of a substrate.  
         [0073]    In the surface acoustic wave device  21  formed in the above-described embodiment, a protective film can cover the surface of the piezoelectric element  23  on which neither the comb electrode  24  nor the terminal electrode  25  is formed.  
         [0074]    As the protective film, for example, copper, nickel, aluminum, silicon dioxide, epoxy resin, silicon resin can be employed.  
         [0075]    If the protective film is formed in the stage before the piezoelectric element  23  is separated or in the stage where the element  23  has not been separated from one piezoelectric wafer  41 , the operability is improved.  
         [0076]    Since, moreover, the element connection terminal  27  of the base substrate  26  and the terminal electrode  25  of the surface acoustic wave element  22  are electrically connected to each other, the conductive adhesive  32  is not necessarily required.  
         [0077]    The reason for the above is as follows: Adequate electrical connection can be achieved by sandwiching the metal bump  31  between the element connection terminal  27  and the terminal electrode  25  and crushing the metal bump  31  by pressure applied thereto.  
         [0078]    In short, the mechanical coupling between the base substrate  26  and the surface acoustic wave element  22  is held firmly by the sealing member  34 . The connection between the element connection terminal  27  and the terminal electrode  25  needs no great mechanical strength.  
         [0079]    The above-described embodiment is directed to an example using a so-called single-layer wiring substrate in which the element connection terminal  27  is formed on one surface of the base substrate  26  and the external connection terminal  28  is formed on the other surface thereof.  
         [0080]    However, the present invention is not limited to the single-layer wiring substrate. Using a multi-layer wiring substrate in which an insulation substrate and an electrode pattern are stacked in layers, the degree of freedom of wiring can be heightened further.  
         [0081]    The present invention is not limited to the above embodiment. Various changes and modifications can be made without departing from the scope of the subject matter of the invention.  
         [0082]    As described above in detail, according to the present invention, the terminal electrode of the surface acoustic wave element and the element connection terminal of the base substrate are electrically connected to each other by the connection member, and the piezoelectric element and the base substrate are bonded to each other by the frame-like resin member sandwiched therebetween. No clearance is therefore required between the surface acoustic wave element and the package wall surface. The surface acoustic wave device can be thinned and the package size can be set to the same as that of the surface acoustic wave element. The device can be miniaturized accordingly. Consequently, there is a high possibility that the device is used widely in the field of a mobile communication system such as a cellular phone.  
         [0083]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.