Abstract:
A surface acoustic wave device includes a SAW element that is mounted on a substrate. Grooves are provided in the substrate at the outer periphery of the SAW element, and a flexible resin layer is provided at the inner portion of the grooves so as to cover the SAW element. An outer resin layer that is harder than the flexible resin layer is provided at the exterior of the flexible resin layer. This configuration facilitates reduction in size and profile of the surface acoustic wave device, contributes to reduction in cost, and exhibits high environmental resistance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to surface acoustic wave devices for use in, for example, resonators, bandpass filters, and other devices, and a method for making the same. More particularly, the present invention relates to a surface acoustic wave device having a structure covered by an outer resin and a method for making the same.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 8 is a cross-sectional view of an exemplary conventional surface acoustic wave device. The surface acoustic wave device  101  has a packaging structure including a substrate  102  and an outer resin layer  103 . Electrodes  102   a  and  102   b  are provided on the upper surface of the substrate  102  for electrically connecting the surface acoustic wave element  104  to an outer package. A surface acoustic wave element  104  is fixed on the substrate  102  via an insulating adhesive material  105 . The surface acoustic wave element  104  is electrically connected to the electrodes  102   a  and  102   b  via bonding wires  106   a  and  106   b . A space A must be formed around the surface acoustic wave element  104  to facilitate vibration thereof, and the surface acoustic wave element  104  thereby exhibits satisfactory characteristics. In order to define the space A, the surface acoustic wave element is surrounded by a metal cap  107  having an opening at the bottom. The metal cap  107  is fixed on the substrate  102  via an adhesive  108 . Moreover, an outer resin layer  103  is disposed around the metal cap  107  to enhance moisture resistance.  
           [0005]    Reductions in size, profile and cost are required for surface acoustic wave devices, as with other electronic components.  
           [0006]    In the surface acoustic wave device  101 , the package is composed of the substrate  102 , the metal cap  107 , and the outer resin layer  103 . Thus, the package requires many components, which causes an inevitable increase in production cost. Furthermore, after the metal cap  107  is provided to define the space A, the protective layer  109  must be formed by resin molding. This process precludes reductions in size and profile.  
         SUMMARY OF THE INVENTION  
         [0007]    In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device and a method of making the same, which provide a surface acoustic wave device that has a greatly reduced size and profile and that is much less expensive to manufacture.  
           [0008]    According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a substrate, a surface acoustic wave element mounted on the substrate, the substrate being provided with at least one first groove provided in the substrate and located at the exterior of the surface acoustic wave element, a flexible resin layer provided on the substrate inside of the first groove so as to cover the surface acoustic wave element, the flexible resin layer being relatively soft, and an outer resin layer provided at the exterior of the flexible resin layer, the outer resin layer being harder than the flexible resin layer.  
           [0009]    In this surface acoustic wave device, the package structure includes the substrate, the flexible resin layer, and the outer resin layer. This SAW element packaging structure including these resinous materials facilitates reductions in size and profile of the surface acoustic wave device, whereas conventional surface acoustic wave devices using metal caps for providing spaces preclude reductions in size and profile.  
           [0010]    Since the package is completed by forming the resin layers without using a metal cap, the number of components and material cost are reduced and the manufacturing process is simplified. Accordingly, the surface acoustic wave device is produced at low cost.  
           [0011]    Since the flexible resin layer provided on the substrate does not extend outside of the first grooves, the flexible resin layer is reliably covered by the outer resin layer, thereby providing a surface acoustic wave device having superior environmental resistance.  
           [0012]    Preferably, at least one pair of first grooves opposing each other is provided on the substrate so as to sandwich the surface acoustic wave element.  
           [0013]    In such a case, these first grooves are preferably formed at both sides of the SAW element. Thus, the outer resin layer can be formed at both sides of the flexible resin layer, providing a surface acoustic wave device having superior moisture resistance.  
           [0014]    The surface acoustic wave device according to another preferred embodiment of the present invention may further include electrodes provided on the substrate for connection with an external component, and bonding wires for electrically connecting the electrodes and the surface acoustic wave element, wherein the surface acoustic wave device is fixed to the substrate.  
           [0015]    Alternatively, the surface acoustic wave device may further include electrodes disposed on the substrate for electrical connection with an external component, wherein the surface acoustic wave element is fixed on the substrate by a face down mounting process, and is electrically connected to the electrodes.  
           [0016]    That is, the SAW element and the electrodes on the substrate may be bonded with bonding wires or electrically connected by a face down mounting process. Since the bonding wires and the conductive bond are covered with the flexible resin layer in any bonding process, reliability of electrical connection is greatly improved.  
           [0017]    Preferably, the surface acoustic wave device may also further include a moisture resistant material layer provided at the exterior of the outer resin layer, the moisture resistant material having higher moisture resistance than that of the outer resin layer.  
           [0018]    The moisture resistant material layer contributes to further improvements in environmental properties such as moisture resistance of the resulting surface acoustic wave device.  
           [0019]    The surface acoustic wave device may further include another electronic component mounted on the substrate and covered by the outer resin layer.  
           [0020]    In such a configuration, reductions in size, profile, and costs of a composite surface acoustic wave device including ICs and other elements are achieved.  
           [0021]    According to another preferred embodiment of the present invention, a method for making surface acoustic wave devices includes the steps of preparing a mother substrate having a plurality of first grooves on the upper surface thereof, mounting surface acoustic wave elements onto the mother substrate so that the first grooves are located at the exterior of the surface acoustic wave elements, forming a flexible resin layer of a relatively soft resin on the substrate closer to the surface acoustic wave element than the first grooves so as to cover each surface acoustic wave element, covering the flexible resin layer with an outer resin layer of a relatively hard resin, and cutting the outer resin layer and the mother substrate into individual surface acoustic wave devices.  
           [0022]    The method including these simplified steps provides inexpensive surface acoustic wave devices having a greatly reduced size and a greatly reduced profile.  
           [0023]    Preferably, the first grooves are at least one pair of first grooves formed at both sides of each surface acoustic wave element.  
           [0024]    In such a manner, the outer resin layer is reliably formed at both sides of the flexible resin layer.  
           [0025]    Preferably, the method further includes the step of forming second grooves, each extending from the outer resin layer to the mother substrate at outer portions of the first grooves after the formation of the outer resin layer, wherein the mother substrate is cut along the second grooves into the individual surface acoustic wave devices in the cutting step.  
           [0026]    The mother substrate is readily cut into surface acoustic wave devices at the second grooves.  
           [0027]    Preferably, the method further includes the step of covering the outer resin layer with a moisture resistant material having higher moisture resistance than that of the outer resin layer, this covering step being performed after the step of forming the second grooves and before the step of cutting the mother substrate into the individual surface acoustic wave devices.  
           [0028]    Since the outer surface of the outer resin layer is covered by the moisture resistant material layer, the resulting surface acoustic wave device exhibits higher moisture resistance.  
           [0029]    Preferably, the mother substrate has third grooves for facilitating cutting at the cutting positions into the individual surface acoustic wave devices on the lower surface thereof.  
           [0030]    According to another preferred embodiment of the present invention, a communication device includes the surface acoustic wave device according to the other preferred embodiments of the present invention.  
           [0031]    Other features, characteristics, elements and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a frontal cross-sectional view of a surface acoustic wave device according to a first preferred embodiment of the present invention;  
         [0033]    [0033]FIG. 2A is a partial frontal cross-sectional view illustrating a state of a surface acoustic wave element mounted on a mother substrate;  
         [0034]    [0034]FIG. 2B is a partial frontal cross-sectional view illustrating a state of the formation of a flexible resin layer;  
         [0035]    [0035]FIG. 2C is a partial frontal cross-sectional view illustrating a state of the formation of an outer resin layer;  
         [0036]    [0036]FIG. 2D is a partial frontal cross-sectional view illustrating a state of the formation of second grooves;  
         [0037]    [0037]FIG. 2E is a partial frontal cross-sectional view illustrating a state of the formation of a moisture resistant resin layer;  
         [0038]    [0038]FIG. 3A is a partial frontal cross-sectional view illustrating a method for making a surface acoustic wave device of a second preferred embodiment of the present invention;  
         [0039]    [0039]FIG. 3B is a partial frontal cross-sectional view illustrating the surface acoustic wave device in accordance with the second preferred embodiment;  
         [0040]    [0040]FIG. 4A is a partial frontal cross-sectional view illustrating a method for making a surface acoustic wave device of a third preferred embodiment of the present invention;  
         [0041]    [0041]FIG. 4B is a partial frontal cross-sectional view illustrating the surface acoustic wave device in accordance with the third preferred embodiment;  
         [0042]    [0042]FIGS. 5A and 5B are partial frontal cross-sectional views illustrating steps of producing surface acoustic wave devices of a fourth preferred embodiment and a modification thereof, respectively, of the present invention;  
         [0043]    [0043]FIG. 6 is an outline block diagram of a preferred embodiment of a communication device including a surface acoustic wave device according to other preferred embodiments of the present invention;  
         [0044]    [0044]FIG. 7 is an outline block diagram of another preferred embodiment of a communication device including a surface acoustic wave filter according to other preferred embodiments of the present invention; and  
         [0045]    [0045]FIG. 8 is a cross-sectional view of a conventional surface acoustic wave device. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0046]    The present invention is described in detail with reference to the following preferred embodiments.  
         [0047]    [0047]FIG. 1 is a cross-sectional view of a surface acoustic wave device in accordance with a first preferred embodiment of the present invention. FIGS. 2A to  2 E are cross-sectional views for illustrating a method for making the surface acoustic wave device of this preferred embodiment of the present invention.  
         [0048]    With reference to FIG. 2A, a mother substrate  1  is prepared. In this preferred embodiment, the mother substrate  1  is preferably made of alumina. In various preferred embodiments of the present invention, the mother substrate  1  may be made of any insulating material, such as, insulating ceramic, glass-epoxy, or synthetic resin, or other suitable materials.  
         [0049]    A plurality of first grooves  2  and  3  is preferably provided on the upper surface  1   a  of the mother substrate  1 , whereas a plurality of third grooves  15   a  is preferably provided on the lower surface thereof. The third grooves  15   a  are provided at positions for cutting the mother substrate  1  into individual surface acoustic wave devices. Electrodes  4  and  5  are provided in each region between a pair of first grooves  2  and  3 . The electrodes  4  and  5  are electrically connected to through-hole electrodes  6  and  7  that extend from the upper surface to the lower surface of the mother substrate  1 . Lower ends of the through hole electrodes  6  and  7  are electrically connected to electrodes  8  and  9 , respectively, disposed on the lower surface of the mother substrate  1 . Preferably, the electrodes  8  and  9  extend to the third grooves  15   a.    
         [0050]    The plurality of first grooves  2  and  3  and the third grooves  15   a  are also preferably provided in the mother substrate  1  in the direction that is substantially parallel to the drawings, respectively, so that the first grooves  2  and  3  define a plurality of substantially rectangular regions which become respective chips.  
         [0051]    Surface acoustic wave elements  10  and  10 A (hereinafter referred to as SAW elements) are fixed onto the mother substrate  1  preferably via a conductive bond or gold balls  11  and  12  by a face down mounting process in which the SAW element  10  is fixed onto the mother substrate  1  so that a surface provided with electrodes for connection to an external component is facing downwardly. The electrodes of the SAW element  10  are electrically connected to the electrodes  4  and  5  on the mother substrate  1  with conductive bonds  11  and  12 , respectively. In addition to fixation of the SAW element  10 , electrical connection between the SAW element  10  and the electrodes  4  and  5  is thereby achieved.  
         [0052]    The SAW element  10  is fixed in a region between the first grooves  2  and  3 . In other words, the SAW element  10  is fixed on the mother substrate  1  so that the first grooves  2  and  3  are located at the exterior of the SAW element.  
         [0053]    The electrode structure and arrangement of the other parts of the SAW element  10  are not limited, and well known SAW resonators and SAW filters may be used. The conductive bonds  11  and  12  are also not limited, and may be bonded balls, solder and conductive bonds.  
         [0054]    As shown in FIG. 2B, a flexible resin layer  13  is located in a region between the first grooves  2  and  3  on the mother substrate  1  so as to cover the SAW elements  10  and  10 A. The flexible resin layer  13  may be a resin having appropriate flexibility that does not adversely affect the properties of the SAW elements  10  and  10 A. The flexible resin layer  13  is preferably made of silicon rubber, silicone gel, epoxy gel or the like, and more preferably made of silicone gel or epoxy gel.  
         [0055]    In the formation of the flexible resin layer  13 , a flexible resin having fluidity is applied so as to cover the SAW element  10  between the first grooves  2  and  3  and is heated to modify the resin. When the first grooves  2  and  3  are narrow, the resin does not extend into the first grooves  2  and  3  due to the surface tension of the flexible resin layer in this preferred embodiment. Even if the flexible resin extends into the first grooves  2  and  3 , the resin will not extend to the exteriors of the first grooves  2  and  3 .  
         [0056]    When the resin constituting the flexible resin layer  13  has low viscosity and low surface tension, the flexible resin is applied so that the flexible resin does not extend to the exteriors of the first grooves  2  and  3  even if the flexible resin reaches the first grooves  2  and  3 . In order to define the flexible resin layer  13  having a predetermined shape, it is preferable that the first grooves  2  and  3  are preferably arranged to surround each SAW element. Alternatively, it is preferable that other structures to prevent the flexible resin layer from flowing out of the predetermined area cooperate with the first grooves  2  and  3 .  
         [0057]    [0057]FIGS. 1, 2B and so on show that the flexible resin layer  13  penetrates into a space provided between the mother substrate  1  and the SAW element  10 . However, it is not necessary that the flexible resin layer  13  be provided between the mother substrate  1  and the SAW element  10 . The space between the mother substrate  1  and the SAW element  10  may be left vacant.  
         [0058]    With reference to FIG. 2C, then, the upper surface of the mother substrate  1  is covered by an outer resin layer  14 . The material for constituting the outer resin layer  14  is preferably an insulating resin that has higher hardness than that of the flexible resin layer  13 . Examples of such resins are epoxy resin and glass-epoxy resin.  
         [0059]    In this preferred embodiment, the mother substrate  1  provided with the SAW elements  10  and  10 A covered by the flexible resin layer  13  is set into a mold. Then, a resin is injected into the mold and is cured therein to form an outer resin layer  14  by resin molding.  
         [0060]    As shown in FIG. 2D, a plurality of second grooves  15  is provided above the third grooves  15   a  from the upper surface of the outer resin layer  14 . The second grooves  15  are located outside of the first grooves  2  and  3 . These second grooves  15  are used for finally cutting the mother substrate  1  into individual surface acoustic wave devices. Thus, the second grooves  15  are provided at positions for cutting the mother substrate into the individual surface acoustic wave devices. Accordingly, each pair of the second grooves  15  is disposed between a first groove  3  at a SAW element  10  and another first groove  2  at the adjoining SAW element  10 A in FIG. 3A. The second grooves  15  pass through the outer resin layer  14  and partly pass through the mother substrate  1 .  
         [0061]    Next, as shown in FIG. 2E, a moisture resistant material layer  16  is provided on the outer resin layer  14 . The moisture resistant material layer  16  may be made of a material that exhibits higher moisture resistance than that of the resin constituting the outer resin layer  14 . Examples of such materials are moisture resistant resins, e.g., polyimide resin, and inorganic materials, e.g., SiO 2  and metal.  
         [0062]    When the moisture resistant material is applied to form the moisture resistant material layer  16 , it is preferable that the moisture resistant material extends into the second grooves  15 . That is, the material is applied so that the moisture resistant material layer  16  extends to a level that is lower than the interface between the outer resin layer  14  and the mother substrate  1  so as to cover the entire outer surfaces of the outer resin layer  14 .  
         [0063]    Next, the mother substrate  1  is cut along dotted lines B shown in FIG. 2E. In other words, the mother substrate  1  is cut along the center of the second grooves  15  to prepare a surface acoustic wave device  17  in this preferred embodiment. It is to be noted that the cutting is also performed along the approximate center of the second grooves  15  arranged substantially parallel to the FIG. 2E. The cutting method is not limited and may use a dicer, a laser, or a slicer, or other suitable cutting device or method. The third grooves  15   a  that are provided below the second grooves  15  facilitate separation of the SAW elements  10  and  10 A. However, there is no difference in electrical properties if the third grooves  15   a  are not provided.  
         [0064]    As shown in FIG. 1, in the surface acoustic wave device  17 , the SAW element  10  is fixed on the substrate  1 A, which is prepared by cutting the mother substrate  1 . The SAW element  10  is preferably surrounded by the flexible resin layer  13 . Moreover, the outer surfaces of the flexible resin layer  13  are preferably surrounded by the outer resin layer  14 , and the moisture resistant material layer  16  covers the outer surfaces of the outer resin layer  14 . Thus, the resulting surface acoustic wave device  17  exhibits superior environmental resistance, such as moisture resistance.  
         [0065]    Moreover, the packaging structure is provided on the substrate  1 A by forming the flexible resin layer  13 , the outer resin layer  14 , and the moisture resistant material layer  16  by the application and curing processes. Thus, the number of the components and the material cost are greatly reduced compared with conventional methods using metal caps.  
         [0066]    [0066]FIGS. 3A and 3B are cross-sectional views illustrating another preferred embodiment of the method for making the surface acoustic wave device of the present invention. In this preferred embodiment, a wide groove  2 A is formed between the two adjoining SAW elements  10  and  10 A, instead of the narrow first grooves  2  and  3  in the preferred embodiment described above. When a flexible resin layer is formed in this case, a resin constituting the flexible resin layer is also applied so that the SAW element  10  is covered by the resin at the inner side of the wide grooves  2 A. After an outer resin layer, second grooves, and a moisture resistant resin layer are formed as in the first preferred embodiment, the substrate is cut along the second grooves. In this case, the second grooves are formed in the approximate center of the wide groove  2 A in the width direction, as shown by a broken line in FIG. 3A.  
         [0067]    A surface acoustic wave device  21  shown in FIG. 4B is thereby prepared. In the second preferred embodiment, the flexible resin layer  13  is formed in an inner region between the wide grooves  2 A. Thus, the resulting surface acoustic wave device  21  is compact and inexpensive and exhibits superior environmental resistance, as in the first preferred embodiment.  
         [0068]    [0068]FIGS. 4A and 4B are cross-sectional views illustrating a method for making the surface acoustic wave device in accordance with yet another preferred embodiment of the present invention.  
         [0069]    In the third preferred embodiment, SAW elements  30  and  30 A are electrically connected to electrodes  4  and  5 , respectively, provided on a mother substrate  1  with bonding wires  31  and  32 , respectively. That is, the SAW elements  30  and  30 A are fixed on the mother substrate  1  preferably via an insulating bond  33 . The other steps are substantially the same as those in the first preferred embodiment. Thus, a surface acoustic wave device  33  shown in FIG. 4B is prepared by the steps shown in the first preferred embodiment.  
         [0070]    As described above, in various preferred embodiments of the present invention, fixation of the SAW element to the substrate and electrical connection of the SAW element to the electrodes  4  and  5  on the substrate may be performed by a process using the bonding wires  31  and  32 , instead of the face down mounting process.  
         [0071]    [0071]FIGS. 5A and 5B are fragmental front cross-sectional views illustrating a method for making the surface acoustic wave device in accordance with a further preferred embodiment of the present invention.  
         [0072]    In the present preferred embodiment, an IC  41  defining another electronic component, in addition to the SAW element  10 , is fixed on the mother substrate  1  with an insulating bond  42 . Chain lines D in FIG. 5A represent cutting positions when the mother substrate is cut into individual surface acoustic wave devices are finally prepared. Thus, in this preferred embodiment, the resulting surface acoustic wave device is a composite-type electronic component including the SAW element  10  and the IC  41 . In this case, the IC  41  is arranged at the outer region of the first groove  3  and in an inner region between the second grooves  15  which are cutting positions. Thus, the IC  41  is not covered by the flexible resin layer  13  but covered by the outer resin layer  14 .  
         [0073]    As shown in FIG. 5B, the IC  41  may be arranged at an inner region than the first groove  3  so that the IC  41  is also covered by the flexible resin layer  13 . In this case, the IC  41  is covered by the outer resin layer  14  with the flexible resin layer  13  provided therebetween.  
         [0074]    The IC  41  is exemplified as another electronic component in the present preferred embodiment. Alternatively, the IC may be replaced with any other electronic component, such as a capacitor or a resistor.  
         [0075]    Moreover, electrical connection between the IC  41  and the SAW element  10  and electrical connection between the IC  41  and the external component may be performed by providing through hole electrodes in the mother substrate  1  or providing lead electrodes on both surfaces or the lower surface of the mother substrate  1 .  
         [0076]    [0076]FIGS. 6 and 7 are outline block diagrams of communication devices  60  using surface acoustic wave filters in accordance with various preferred embodiments of the present invention.  
         [0077]    In FIG. 6, a duplexer  62  is connected to an antenna  61 . A surface acoustic wave filter  64  and an amplifier  65  are connected between the duplexer  62  and a receiving mixer  63 . An amplifier  67  and a surface acoustic wave filter  68  are connected to the duplexer  62  and a transmitter mixer  66 .  
         [0078]    As shown in FIG. 7, when an amplifier  65 A used in the transmitter side corresponds to unbalanced signals, the surface acoustic wave filter in accordance with various preferred embodiments of the present invention can be preferably used as a surface acoustic wave filter  64 A.  
         [0079]    While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made without departing from the spirit and scope of the invention.