Abstract:
An inexpensive surface acoustic wave apparatus uses a flip chip bonding technique and is capable of reducing an ohmic loss by reducing an electrode electrical resistance and also operates at a high-frequency range. A surface acoustic wave apparatus has a surface acoustic wave element bonded to a package by using a bonding flip chip technique. In the surface acoustic wave element, on a piezoelectric substrate, an IDT electrode, bus bar electrodes, reflector electrodes, relay electrodes, and electrode pads are disposed. Conductive films are disposed on the electrode pads and define a first metallic film, and the conductive films are also disposed on at least any of the bus bar electrodes and the relay electrodes to define a second metallic film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to surface acoustic wave elements, and more specifically, it relates to a surface acoustic wave element to be bonded to a package with metallic bumps and a manufacturing method thereof. 
     2. Description of the Related Art 
     Along with increases in the frequency of mobile radio communications, a surface acoustic wave apparatus used for the mobile radio communications is also required to be usable in a high-frequency range. The surface acoustic wave apparatus includes a surface acoustic wave element having a piezoelectric substrate and a package for accommodating the surface acoustic wave element. Because an acoustic velocity of the surface acoustic wave element on the surface of the piezoelectric substrate is several thousands m/second, the wavelength of an interdigital electrode of the surface acoustic wave element is as small as several μm, when forming the surface acoustic wave element operating at approximately 800 MHz, for example. Therefore, there has been a problem that the absolute value of the electrode film thickness for optimizing characteristics of the surface acoustic wave element is reduced so that a loss due to an electrode resistance, i.e., an ohmic loss, increases. 
     In order to solve such a problem, a surface acoustic wave apparatus as shown in FIGS. 5A and 5B is disclosed in Japanese Unexamined Patent Application Publication No. 7-212175. In addition, FIG. 5B is a schematic sectional view along the lines of FIG. 5A connecting the dash-dot lines P—P, Q—Q, and R—R, and the dash-dot lines S and T of FIG. 5B indicate boundaries of the connection. In addition, in the drawings attached to this specification, in the same way as FIGS. 5A and 5B, the sections shown along the lines P—P, Q—Q, and R—R in the plan view are shown in the sectional view corresponding to the plan view by connecting them along the dash-dot lines S and T. 
     In a surface acoustic wave apparatus  201  in the prior art, on a piezoelectric substrate  202 , an interdigital electrode  203  and reflector electrodes  204  and  205  disposed on both sides of the interdigital electrode  203  are arranged. Also, relay electrodes  206  and  207  are arranged to be electrically connected to the interdigital electrode  203 . Furthermore, electrode pads  208  and  209  are arranged so as to be electrically connected to the relay electrodes  206  and  207 , respectively. The electrode pads  208  and  209  correspond to parts to be electrically connected to electrodes of a package, and metallic bumps are arranged on the electrode pads  208  and  209 . The surface acoustic wave apparatus  201  includes the interdigital electrode  203  and the reflector electrodes  204  and  205  among various electrodes mentioned above and conductive films  212  shown in FIG.  5 B. As shown by the electrode pads  208  and  209  in FIG. 5B as representatives, on at least portions of the electrode pads  208  and  209  or the relay electrodes  206  and  207 , second conductive films  213  are deposited. That is, in comparison with the interdigital electrode  203 , portions of the relay electrodes  206  and  207  and the electrode pads  208  and  209  have an increased thickness, so that an ohmic loss due to an electrode resistance is reduced, thereby improving electrical characteristics. 
     By the way, because electronic components have recently been required to have a reduced size and height, a surface acoustic wave apparatus using a flip chip bonding technique is put into practical use. In the surface acoustic wave apparatus using the flip chip bonding technique, an electrode forming portion of the surface acoustic wave element opposes a mounting surface of a package, and electrodes of the surface acoustic wave element and electrodes of the package are bonded with metallic bumps. In this case, in order to increase the bonding strength between the metallic bump and the electrode pad, metallic films such as AU are disposed on the electrode pads of the surface acoustic wave element. On the metallic film made of AU, a metallic bump made of AU is disposed, which in turn is bonded to the package electrode surface. Alternatively, on the electrode pad, a metallic film made of Ag and having excellent solder wettability is disposed, so that a method in which solder bumps that are formed in the package in advance are bonded to the metallic films having excellent solder wettability is used. 
     When using the method described above, bonding wire is not required. Therefore, since the surface acoustic wave element is not required to have wire pads to be connected to the bonding wire, the planar area and height of the surface acoustic wave apparatus can be reduced. 
     Also, in the surface acoustic wave apparatus obtained by using the flip chip bonding technique, when a surface acoustic wave is actually vibrated, while the film thickness of electrodes other than electrodes to be propagated is increased, the ohmic loss is reduced, and losses and the reduction in Q of a resonator can be suppressed. In this case, first, the electrodes, in which a surface acoustic wave is vibrated and propagated, i.e., the interdigital electrode, reflector electrodes, bus bar electrodes, relay electrodes, and electrode pads are formed of the same conductive film. Next, in electrodes disposed in portions other than where a surface acoustic wave is vibrated and propagated, the second conductive film is deposited or the first conductive film is increased in thickness. 
     Therefore, when performing the bonding with the metallic bumps, after depositing the metallic films such as Au on the thick conductive films on the electrode pads, the metallic bumps such as Au may be formed thereon. Also, when performing the bonding with the solder bumps, on the plurality of deposited conductive films or on the electrode pads thickly formed by the increase in the conductive film thickness, the metallic films made of Ag and having excellent solder wettability are provided. 
     FIGS. 6A and 6B are a plan view and schematic sectional view for illustrating a conventional method using the flip chip bonding technique for manufacturing a surface acoustic wave apparatus operating at low frequencies, respectively. 
     According to the method, by patterning a conductive film  222 , an IDT electrode  223 , bus bar electrodes  224  and  225 , reflector electrodes  226  and  227 , relay electrodes  228  and  229 , and electrode pads  230  and  231  are formed. Next, as shown in FIGS. 7A and 7B, on each of the electrode pads  230  and  231 , a conductive film  232  and a metallic film  233  are deposited. The conductive film  232  is arranged for improving adhesion to the conductive film  222 , and the metallic film  233  is arranged for improving the bonding strength between a metallic bump  234  shown in FIGS. 8A and 8B and the electrode pad. 
     Therefore, according to the method, after patterning the conductive film  222 , the conductive film  232  and metallic film  233  have to be deposited. 
     On the other hand, when obtaining the surface acoustic wave apparatus operating at a high frequency range, as described above, in order to reduce the ohmic loss, on the conductive film  232 , a conductive film  241  made of the same material as that of the conductive film  222  is deposited, and on the conductive film  241 , the conductive film  232  and metallic film  233  have to be deposited, as shown in FIGS. 9A and 9B. 
     When manufacturing the surface acoustic wave apparatus operating at a low frequency range by bonding between the surface acoustic wave element and the package with the solder bumps disposed on the package instead of the metallic bumps made of Au, after the patterned conductive film  222  is formed on the piezoelectric substrate  221  as shown in FIGS. 6A and 6B, the metallic film  232  for improving the bonding strength to the conductive film  222  and a metallic film  242  to be a solder barrier are deposited on the electrode pads, and finally, a metallic film  243  having excellent solder wettability has to be deposited, as shown in FIGS. 10A and 10B. That is, a deposited structure made of three metallic layers has to be formed. Also, in this case, in order to reduce the ohmic loss of the surface acoustic wave apparatus operating at high frequencies, as shown in FIGS. 11A and 11B, on the metallic film  232 , a conductive film  244  made of the same electrode material as that of the conductive film  222  is further deposited, and then, a multi-layered structure including a plurality of metallic films  242  and  243  has to be formed. 
     On the other hand, published application WO99/05788 discloses a surface acoustic wave apparatus, in which at least one of a bus bar and electrode pad includes a combination of a first conductive layer having Al as a principal ingredient, an intermediate layer, and a second conductive layer having Al as a principal ingredient. Therefore, when the electrode pad having the above-mentioned deposited structure with a large thickness, the mechanical strength is increased. In the WO99/05788 Publication, there is a description of a structure in that on the electrode pad, wire bonding or an Au bump is disposed. 
     As described above, when manufacturing a surface acoustic wave apparatus operating at a high frequency range, in any cases of the bonding between the surface acoustic wave element and the package with the metallic bumps on the surface acoustic wave element and of the bonding between the surface acoustic wave element and the package with the solder bumps disposed on the package, it has been necessary that one or more of the conductive films  241  and  244  are additionally deposited on a region other than where a surface acoustic wave is vibrated and propagated in comparison with the case of manufacturing the surface acoustic wave apparatus operating at a low frequency range. Therefore, there has been a problem that the manufacturing process is complicated and not cost effective. 
     In the structure disclosed in the WO99/05788 Publication, the electrode pad includes a combination of a first conductive layer having Al as a principal ingredient, an intermediate layer, and a second conductive layer having Al as a principal ingredient. However, in this structure, when forming the solder bump, since the top layer of the electrode pad is formed of the second conductive layer having Al as a principal ingredient, there has been a problem that the solder diffuses so that sufficient bonding strength cannot be obtained. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems describe above, preferred embodiments of the present invention provide a surface acoustic wave element and a manufacturing method thereof, the surface acoustic wave element being accommodated in a package by a flip chip bonding technique for use at a high frequency range, and is capable of simplifying the manufacturing process and reliably obtaining excellent electrical characteristics due to the reduction in an ohmic loss, and which is excellent in the bump bonding strength when using solder bumps. 
     In accordance with a first preferred embodiment of the present invention, a surface acoustic wave element is accommodated within a package and is to be bonded to the package with solder bumps provided on the package, the surface acoustic wave element includes a piezoelectric substrate, at least one interdigital electrode provided on the piezoelectric substrate, a pair of bus bar electrodes connected to the interdigital electrode, relay electrodes connected to the bus bar electrodes, electrode pads connected to the relay electrodes, and electrically connected to the package, first metallic films provided on the electrode pads for improving bonding strength to the solder bumps, and second metallic films made of the same material as that of the first metallic film, and provided on at least one of the bus bar electrodes and the relay electrodes. 
     Preferably, the first and second metallic films have a multi-layered structure formed by depositing a plurality of metallic layers. In this case, the top metallic layer of the first and second metallic films is made of a metallic material having excellent bonding properties to solder bumps while the other metallic layers are made of a metallic material with low electric resistance, for example, so that both the improvement in the bonding strength to bumps and improvement in the reduction of the ohmic loss are achieved. 
     Preferably, the first and second metallic films have a multi-layered structure formed by depositing a plurality of metallic layers, and the top metallic layer is preferably made of one of Ag and Au. In this case, since the top metallic layer in the first and second metallic films has excellent solderability, the first metallic film can be rigidly and readily bonded to an electrode land of the package with solder bumps therebetween. 
     Preferably, the interdigital electrode has a multi-layered structure formed by depositing a plurality of metallic layers, and wherein at least one metallic layer in the first and second metallic films is preferably made of a metal having a smaller specific electric resistance in comparison with the bottom metallic layer of the metallic layers constituting the interdigital electrode. Thereby, the specific electric resistance of at least one metallic layer in the first and second metallic films is relatively small, so that the ohmic loss is even further reduced in the metallic film while the thickness of the metallic layer made of a metal with relatively small specific electrical resistance is greatly reduced. 
     In accordance with a second preferred embodiment of the present invention, a manufacturing method of a surface acoustic wave element configured according to other preferred embodiments of the present invention includes the steps of forming the interdigital electrode, the bus bar electrodes, the relay electrodes, and the electrode pads on the piezoelectric substrate, forming the first metallic films on the electrode pads, and forming the second metallic films on at least one of the bus bar electrodes and the relay electrodes. 
     In accordance with a third preferred embodiment of the present invention, a communication apparatus includes a surface acoustic wave element configured according other preferred embodiments of the present invention as a band-pass filter. 
     In the surface acoustic wave element and the manufacturing method thereof according to preferred embodiments of the present invention, the second metallic film, preferably made of the same material as that of the first metallic film disposed on the electrode pad for increasing the bonding strength to the solder bump in the package side, is disposed on at least one of the bus bar electrode and relay electrode. That is, in the process for forming the first metallic film on the electrode pad for increasing the bonding strength to the solder bump in the package, also on at least one of the bus bar electrode and relay electrode, the second metallic film made of the same material is formed. Therefore, without complicating the manufacturing process, the electrical resistance loss of the electrode is greatly reduced. Thereby, an additional process is not required for increasing the film thickness of the bus bar electrode and relay electrode, so that the cost reduction for manufacturing the surface acoustic wave apparatus using the flip chip bonding technique can be achieved. 
     Additionally, since the increasing of the film-thickness of the bus bar electrode and relay electrode is performed by the metallic material having bonding strength to the conductive film constituting the IDT electrode, at least the same characteristics as those in the case of the film-thickness increasing with the same electrode material as those of the IDT electrode and relay electrode can be obtained. 
     Other features, elements, characteristics, steps and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are a schematic plan view and schematic sectional view along the lines P—P, Q—Q, and R—R of FIG. 1A connected with the dash-dot lines S and T therebetween for illustrating a surface acoustic wave element according to a preferred embodiment of the present invention; 
     FIGS. 2A to  2 D are schematic sectional views for illustrating an electrode structure of the surface acoustic wave element according to a preferred embodiment of the present invention; 
     FIG. 3 is a drawing showing impedance/frequency characteristics of the surface acoustic wave element according to a preferred embodiment, a surface acoustic wave apparatus, and a surface acoustic wave element formed according to a conventional method, which are prepared for comparison purposes; 
     FIG. 4 is a schematic sectional view for illustrating a surface acoustic wave apparatus according to a preferred embodiment of the present invention; 
     FIGS. 5A and 5B are a schematic plan view and schematic sectional view along the lines P—P, Q—Q, and R—R of FIG. 5A connected with the dash-dot lines S and T therebetween for illustrating an example of a conventional surface acoustic wave apparatus; 
     FIGS. 6A and 6B are a schematic plan view and sectional view along the lines P—P, Q—Q, and R—R of FIG. 6A connected with the dash-dot lines S and T therebetween for illustrating a manufacturing process of the conventional surface acoustic wave apparatus shown in FIGS. 5A and 5B; 
     FIGS. 7A and 7B are a schematic plan view and sectional view at the lines P—P, Q—Q, and R—R of FIG. 7A connected with the dash-dot lines S and T therebetween for illustrating an example of a conventional surface acoustic wave apparatus used at a high-frequency range; 
     FIGS. 8A and 8B are a schematic plan view and sectional view along the lines P—P, Q—Q, and R—R of FIG. 8A connected with the dash-dot lines S and T therebetween for illustrating another example of a conventional surface acoustic wave apparatus; 
     FIGS. 9A and 9B are a schematic plan view and sectional view at the lines P—P, Q—Q, and R—R of FIG. 9A connected along the dash-dot lines S and T therebetween for illustrating still another example of a conventional surface acoustic wave apparatus used at a high-frequency range; 
     FIGS. 10A and 10B are a schematic plan view and sectional view along the lines P—P, Q—Q, and R—R of FIG. 10A connected with the dash-dot lines S and T therebetween for illustrating still another example of a conventional surface acoustic wave apparatus; and 
     FIGS. 11A and 11B are a schematic plan view and sectional view along the lines P—P, Q—Q, and R—R of FIG. 11A connected with the dash-dot lines S and T therebetween for illustrating still another example of a conventional surface acoustic wave apparatus. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described by illustrating specific preferred embodiments according to the present invention below. 
     FIGS. 1A and 1B are a plan view and schematic sectional view, respectively, for illustrating a surface acoustic wave element according to a preferred embodiment of the present invention. FIGS. 2A to  2 D are schematic sectional views for illustrating a manufacturing method of the surface acoustic wave element according to the present preferred embodiment. 
     According to the present preferred embodiment, a surface acoustic wave element is bonded to a package with solder bumps arranged in the package by using a flip chip bonding technique. 
     According to the present preferred embodiment, a surface acoustic wave element  54  shown in FIGS. 1A and 1B is prepared. In the surface acoustic wave element  54 , on a substantially rectangular plate-type piezoelectric substrate  42 , an IDT electrode  43 , a pair of bus bar electrodes  44  and  45 , reflector electrodes  46  and  47 , relay electrodes  48  and  49 , and electrode pads  50  and  51  are formed with conductive films Xa. 
     As the piezoelectric substrate  42 , a piezoelectric single crystal such as LiTaO 3 , LiNbO 3 , or crystal, and piezoelectric ceramics such as lead zirconate titanate ceramics, or other suitable material may be used. 
     The conductive film Xa is made of an appropriate conductive material such as Al. The method for forming the conductive film Xa on the piezoelectric substrate  42  is not specifically limited. An appropriate method such as vapor deposition, sputtering, or plating may be used. 
     On the bus bar electrodes  44  and  45 , relay electrodes  48  and  49 , and electrode pads  50  and  51 , metallic films Xb, metallic films Xd to be solder barrier layers, and metallic films Xe with excellent bonding properties to solder bumps are deposited except on a portion of the vicinities of the electrode pads. The metallic film Xb is preferably made of NiCr or Ti, for example, for improving the bonding strength to the metallic film Xd to be the solder barrier layer and the conductive film Xa. The metallic films Xe is preferably made of a metal having excellent bonding properties to solder bumps such as Ag. The metallic film Xd to be the solder barrier layer is preferably made of an appropriate metal, such as Ni, that is difficult to produce a solder leach. 
     In addition, the IDT electrode  43  is formed according to the present preferred embodiment. Alternatively, a plurality of IDT electrodes may be formed, and the reflector may not be necessarily provided. 
     When obtaining the surface acoustic wave element according to the present preferred embodiment, on the entire surface of the piezoelectric substrate  42 , the conductive film Xa is formed. Then, the patterned conductive films Xa are formed on the piezoelectric substrate  42  by patterning, or other suitable process. 
     That is, as shown in FIG. 2A, on the piezoelectric substrate  42 , the patterned conductive films Xa are disposed. Thereby, the IDT electrode  43 , bus bar electrodes  44  and  45 , reflector electrodes  46  and  47 , relay electrodes  48  and  49 , and electrode pads  50  and  51  are formed with the conductive films Xa. 
     Thereafter, as shown in FIG. 2A, a resist  61  is deposited on the entire surface. 
     Then, by the development using exposure and a photo-mask, the resist portion that is not required is removed, so that the resist  61  is patterned. 
     In such a manner, as shown in FIG. 2B, the patterned resist  61 A is formed. In this state, the IDT electrode  43 , reflector electrodes  46  and  47 , and portions of the electrode pads  50  and  51  are covered with the resist  61 A. 
     Thereafter, as shown in FIG. 2C, the metallic films Xb preferably made of NiCr or Ti are formed. Then, on the entire surfaces of the metallic films Xb, the metallic films Xd made of Ni for functioning as a solder barrier and the metallic films Xe preferably made of Ag and having excellent solder wettability are sequentially formed. The forming of these metallic films Xb, Xd, and Xe may be performed by an appropriate method such as vapor deposition and sputtering. 
     Thereafter, the metallic films Xb, Xd, and Xe on the resist  61 A are lifted off together with the resist  61 A. In such a manner, as shown in FIG. 2D, on the bus bar electrodes  44  and  45 , relay electrodes  48  and  49 , and electrode pads  50  and  51 , the deposited metallic films constituting the metallic films Xb, Xd, and Xe are deposited so as to obtain the surface acoustic wave element  54  shown in FIGS. 6A and 6B. 
     When bonding the surface acoustic wave element  54  to a package, as shown in FIG. 4, the surface acoustic wave element  54  is placed on electrodes  14  and  15  on a package  11  from the side of the electrode forming surface so that the electrode pads  50  and  51  are brought into contact with solder bumps  12  and  13  disposed on the electrodes  14  and  15  on the package  11 , respectively. Then, by heating, the surface acoustic wave element  54  is bonded to the electrodes  14  and  15  on the package  11  with the respective solder bumps  12  and  13  therebetween so as to obtain the surface acoustic wave apparatus according to the present preferred embodiment. 
     According to the present preferred embodiment, on each of the bus bar electrode, relay electrode, and electrode pad, the metallic films Xb, Xd, and Xe are deposited. The metallic films Xb, Xd, and Xe on the electrode pad constitute the first metallic film according to preferred embodiments of the present invention, whereas the metallic films Xb, Xd, and Xe on each of the bus bar electrode and relay electrode constitute the second metallic film. Therefore, just like the structure that the conductive film made of the same electrode material as that of the conductive film Xa is deposited on the conductive film Xa, the electrical resistance of the electrode elements is greatly reduced, resulting in the reduction in the ohmic loss. Therefore, without using the process for depositing the conductive film made of the same electrode material as that of the conductive film Xa for reducing the ohmic loss, that is, without using the complicated electrode forming process, the loss of the surface acoustic wave apparatus and the reduction in Q are minimized. 
     In addition, according to the present preferred embodiment, the film thickness of the metallic film Xe for bonding to the solder bumps is not necessarily larger than that of the conductive film Xa defining the IDT electrode  43 . That is, when the specific electrical resistances of the metallic films Xb to Xe are smaller than that of the conductive film Xa, it is not necessary that the thickness of the metallic films Xb, Xd, and Xe be larger than that of the conductive film Xa. 
     FIG. 3 shows impedance/frequency characteristics of the surface acoustic wave apparatus according to the present preferred embodiment with the dotted lines. For comparison purposes, characteristics of a comparative example, in which the film thickness of the bus bar electrode and relay electrode is not increased, are shown with the solid lines while electrical characteristics of a conventional example, in which a conductive film element made of the same electrode material as that of the IDT electrode is increased according to a conventional method, are shown with the dash-dot lines. As is apparent from FIG. 3, it is understood that according to the present preferred embodiment, resonance characteristics with large Q are obtained in comparison with the comparative example, in which the film thickness of the bus bar electrode is not increased, while at least the same resonance characteristics as the conventional example are obtained. 
     In addition, according to preferred embodiments of the present invention, the second metallic films are disposed on the bus bar electrode and relay electrode, the second metallic film is provided for reducing the ohmic loss, and it may be disposed on one of the bus bar electrode and relay electrode or it may be partially disposed in portions of the bus bar electrode and relay electrode. However, it is preferable that as described in the present preferred embodiment, the second metallic films be formed on both the bus bar electrode and relay electrode. 
     While preferred embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.