Patent Publication Number: US-2018041187-A1

Title: Surface acoustic wave elements having improved resistance to cracking, and methods of manufacturing same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of co-pending U.S. Provisional Application No. 62/370,773 titled “SURFACE ACOUSTIC WAVE ELEMENTS AND METHODS OF MANUFACTURING SAME” and filed on Aug. 4, 2016, and of co-pending U.S. Provisional Application No. 62/512,813 titled “SURFACE ACOUSTIC WAVE ELEMENTS HAVING IMPROVED RESISTANCE TO CRACKING, AND METHODS OF MANUFACTURING SAME” filed on May 31, 2017, both of which are incorporated herein by reference in their entireties for all purposes. 
    
    
     BACKGROUND 
     Conventionally, an interdigital transducer (IDT) electrode forming a surface acoustic wave (SAW) resonator in a SAW element is configured as a double-layered structure that combines one layer made of a heavier metal such as molybdenum with another other layer made of a more conductive metal such as aluminum in order to provide both appropriate weight and conductivity to the electrode fingers. Further, certain devices include a protective film on the surface of such a double-layered IDT electrode to prevent issues such as void formation, fracturing, spurious emission and the like. Japanese Patent Application Publication No. 2006-109287, Japanese Patent Application Publication No. 2007-282294, and International Application No. WO2012/090698 disclose examples of such devices. 
       FIG. 1  is a cross-sectional view showing the structure of a conventional double-layered IDT electrode  110 . The IDT electrode  110  includes a first metal layer  111  made of molybdenum formed on a top surface of a piezoelectric substrate  120  and a second metal layer  112  made of aluminum formed on the first metal layer  111  and having a trapezoidal cross-section. An insulation layer  130  made of silicon dioxide (SiO 2 ) is formed over the top surface of the piezoelectric substrate  120  to cover the IDT electrode  110 . 
     SUMMARY OF INVENTION 
     Aspects and embodiments relate to a surface acoustic wave (SAW) element using a piezoelectric substrate and a method of manufacturing the same. 
     In the conventional IDT electrode  110  as shown in  FIG. 1 , tensile stress (represented by arrows  142 ) concentrates in regions  152  of the IDT electrode  110  where the second metal layer  112  has a cross-sectional shape with an acute angle, around a point where an interface between the first metal layer  111  of molybdenum and the second metal layer  112  of aluminum is in contact with the insulation layer  130  of silicon dioxide. Because this tensile stress can be repeatedly applied in a higher temperature environment in which the SAW element operates, a crack may develop in a cracking direction  162  from the acute-angled region  152  into the insulation layer  130 . Consequently, the insulation layer  130  may be fractured and the SAW element may be broken. Even using the technique of providing a protective layer over the double-layered electrode as disclosed in the above-mentioned applications, it has not been possible to sufficiently suppress the crack development along the cracking direction  162 . 
     Aspects and embodiments are directed to addressing the above-mentioned problem, and provide a SAW element having a multi-layered, for example double-layered, IDT electrode in which crack development can be sufficiently suppressed to prevent fractures, and a method of manufacturing the same. 
     According to certain embodiments, a surface acoustic wave device comprises a piezoelectric substrate, a comb-shaped electrode formed on a top surface of the piezoelectric substrate and including a plurality of electrode fingers, each electrode finger including a first metal layer made of a first metal and formed on the top surface of the piezoelectric substrate, a second metal layer made of a second metal and formed on the first metal layer, and a first protective film at least partially covering the second metal layer, and an insulation layer formed over the top surface of the comb-shaped electrode to cover the piezoelectric substrate. 
     In one example the first metal has a first conductivity and the second metal has a second conductivity greater than the first conductivity. In another example the first metal has a first density and the second metal has a second density less than the first density. In one example the first metal layer is made of molybdenum and the second metal layer is made of aluminum. In one example the first protective film is made of aluminum oxide. 
     In one example the second metal layer has a trapezoidal cross-section taken along a line perpendicular to an extending direction of the electrode finger. In one example the first protective film covers side surfaces and a top surface of the second metal layer. In another example the first protective film covers side surfaces of the second metal layer and does not extend over a top surface of the second metal layer. 
     In one example the insulation layer is made of a first material having a first fracture toughness value, and the first protective film is made of a second material having a second fracture toughness value greater than the first fracture toughness value. In one example the second material of the first protective film is oxidized from the second metal. In another example the second material of the first protective film is nitrided from the second metal. 
     In one example each electrode finger further includes a third metal layer made of the first metal and disposed on a top surface of the second metal layer. The second metal layer may be covered by the first protective film, the first metal layer, and the third metal layer. In another example each electrode finger further includes a fourth metal layer made of the second metal and disposed on a top surface of the third metal layer. In one example the fourth metal layer has a trapezoidal cross-section taken along a line perpendicular to an extending direction of the electrode finger. Each electrode finger may further include a second protective film covering side surfaces and a top surface of the fourth metal layer. 
     In one example each electrode finger further includes a second protective film at least partially covering the first metal layer, the first metal being covered by the second protective film and the top surface of the piezoelectric substrate. The second protective film may include material oxidized from the first metal. 
     According to certain embodiments a method of manufacturing a surface acoustic wave device comprises forming a comb-shaped electrode on a top surface of a piezoelectric substrate, and forming an insulation layer on the top surface of the piezoelectric substrate to cover the comb-shaped electrode. Forming the comb-shaped electrode may include forming a first metal layer made of first metal on the top surface of the piezoelectric substrate, forming a second metal layer made of second metal on the first metal layer, and oxidizing a surface of the second metal layer to form a protective film on the second metal layer, the second metal layer being covered by the protective film and the first metal layer. 
     In one example of the method forming the second metal layer includes forming the second metal layer to have a trapezoidal cross-section taken along a line perpendicular to an extending direction of an electrode finger of the comb-shaped electrode. 
     According to certain aspects and embodiments, crack development is suppressed or prevented from occurring in a SAW element including a multi-layered, for example double-layered, IDT electrode, such that the SAW element is prevented from fracturing. 
     Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments, are discussed in detail below. Any embodiment disclosed herein may be combined with any other embodiment in any manner consistent with at least one of the objects, aims, and needs disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. The figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures: 
         FIG. 1  is a cross-sectional view showing the structure of a conventional electrode; 
         FIG. 2  is a cross-sectional view showing one example of an arrangement of electrodes according to aspects of the present invention; 
         FIG. 3  is a cross-sectional view showing a structure of an electrode corresponding to the example shown in  FIG. 2 ; 
         FIG. 4A  is a cross-sectional view showing stress distribution in an electrode according to the example of  FIG. 2 ; 
         FIG. 4B  is a cross-sectional view showing stress distribution in a comparative example of an electrode; 
         FIG. 5  is a graph showing a relationship between a lifetime and a frequency of an electrode according to the example of  FIG. 2 ; 
         FIGS. 6A, 6B, and 6C  are cross-sectional views showing structures of electrodes corresponding to lines A, B and C, respectively, indicated in  FIG. 5 ; 
         FIG. 7  is a graph showing a relationship between a thickness of the protective film and a lifetime of the electrode according to the example of  FIG. 2 ; 
         FIGS. 8A to 8D  are cross-sectional views showing electrode structures corresponding to a series of method steps of manufacturing an electrode according to the example of  FIG. 2 ; 
         FIG. 9  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 10  is a graph showing a relationship between a thickness of the protective film and a lifetime of the electrode according to the example of  FIG. 9 ; 
         FIG. 11  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 12  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 13  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 14  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 15  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 16  is a cross-sectional view showing the structure of another example of an electrode according to aspects of the present invention; 
         FIG. 17  is a block diagram of one example of a filter module including a surface acoustic wave device having an electrode structure according to aspects of the present invention; and 
         FIG. 18  is a block diagram of an example of a wireless device including the filter module of  FIG. 17  according to aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of surface acoustic wave (SAW) elements and methods of manufacturing the same are described below in detail with reference to the drawings. 
       FIG. 2  is a cross-sectional view of a SAW resonator comb-shaped IDT electrode included in a SAW element  200  according to a first embodiment, the cross-section being taken along a line perpendicular to an extending direction of electrode fingers of the IDT electrode.  FIG. 2  shows two electrodes  210  representing a plurality of electrode fingers of the IDT electrode. Each of the electrodes  210  has a certain width. The electrodes  210  are disposed on a top surface  222  of a piezoelectric substrate  220  with a certain pitch relative to each other. 
       FIG. 3  is an enlarged cross-sectional view showing an example of the structure of one of the electrodes  210 . For simplicity,  FIG. 3  shows a cross-section of a single electrode  210  representing one of the electrode fingers of the IDT electrode included in the SAW element  200  to illustrate the structure of the electrode  210 . Similarly in the following cross-sectional views, a cross-section of a single electrode  210  is shown representing the IDT electrode. 
     Referring to  FIGS. 2 and 3 , according to the first embodiment, the electrode  210  forming the IDT electrode of the SAW resonator is formed on a flat top surface  222  of the piezoelectric substrate  220 , which is made of piezoelectric material (also called a piezoelectric body) such as lithium niobate (LiNbO 3 ), for example. An insulation layer  230  made of silicon dioxide (SiO 2 ) is formed on the piezoelectric substrate  220 , covering the electrode  210 . 
     In the electrode  210 , a first metal layer  211  made of molybdenum is formed on the top surface  222  of the piezoelectric substrate  220 , the first metal layer  211  having a certain width and height. A second metal layer  212  made of aluminum is formed on the first metal layer  211 . The second metal layer  212  has a trapezoidal cross-section, such that its width becomes less as its height becomes more elevated from the top surface  222  of the piezoelectric substrate  220 . 
     The density of molybdenum included in the first metal layer  211  is greater than the density of aluminum included in the second metal layer  212 , and therefore the first metal layer  211  of molybdenum can bear weight for providing an adequate mass for the electrode  210 . In other words, the density of the second metal layer  212  is less than that of the first metal layer  211 . The conductivity of aluminum included in the second metal layer  212  is greater than the conductivity of the first metal layer  211 , and therefore the second metal layer  212  of aluminum provides adequate conductivity for the electrode  210 . 
     The second metal layer  212  has side and top surfaces, the aluminum of which is converted into aluminum oxide (Al 2 O 3 ) to a certain depth to form a protective film  213 . A side surface of the first metal layer  211  of molybdenum is smoothly connected at the upper edge thereof to a corresponding side surface of the protective film  213  of aluminum oxide at the lower edge thereof. The first metal layer  211  of molybdenum or the protective film  213  of aluminum oxide is disposed between the second metal layer  212  of aluminum and the insulation layer  230  of silicon dioxide, such that the aluminum of the second metal layer  212  can be covered and protected by the molybdenum of the first metal layer  211  and/or by the aluminum oxide of the protective film  213 . 
     As listed in TABLE 1 below, each of molybdenum (Mo) and aluminum oxide (Al 2 O 3 ) has a coefficient of thermal expansion (such as linear expansion coefficient) lower than that of aluminum (Al). Further, each of molybdenum and aluminum oxide has a Young&#39;s modulus greater than that of aluminum. Accordingly, the first metal layer  211  of molybdenum and the protective film  213  of aluminum oxide that cover the second metal layer  212  may mitigate thermal expansion of the second metal layer  212  of aluminum to suppress any effect thereof on the insulation layer  230  of silicon dioxide. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Linear expansion coefficient  
                   
               
               
                 Material 
                 [10 −6 /° C.] 
                 Young&#39;s modulus 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Al 
                 23 
                 70 
               
               
                 Al 2 O 3   
                 7 
                 ~400 
               
               
                 Mo 
                 5 
                 324 
               
               
                   
               
            
           
         
       
     
     Further, as listed in TABLE 2, each of molybdenum (Mo) and aluminum oxide (Al 2 O 3 ) has a fracture toughness greater than that of silicon dioxide (SiO 2 ). Accordingly, the first metal layer  211  of molybdenum and the protective film  213  of aluminum oxide may not be fractured prior to the fracturing of the insulation layer  230  of silicon dioxide. Therefore, due to the presence and arrangement of the first metal layer  211  of molybdenum and/or the protective film  213  of aluminum oxide, cracking can be prevented from developing into the insulation layer  230  of silicon dioxide. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Material 
                 Fracture toughness value Kc 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 SiO 2   
                 1.5 
               
               
                   
                 AlN 
                 3 
               
               
                   
                 Al 2 O 3   
                 4 
               
               
                   
                 Si 3 N 4   
                 7 
               
               
                   
                 Mo 
                 50 
               
               
                   
                   
               
            
           
         
       
     
     As listed in TABLE 2, each of aluminum nitride (AlN) and silicon nitride (Si 3 N 4 ) has fracture toughness greater than that of silicon dioxide. Accordingly, even if the protective film  213  is made of aluminum nitride or silicon nitride instead of aluminum oxide, the protective film  213  may not be fractured prior to the fracturing of the insulation layer  230  of silicon dioxide. Therefore, due to presence and arrangement of the first metal layer  211  of molybdenum and/or the protective film  213  of aluminum nitride or silicon nitride, cracking can be prevented from developing into the insulation layer  230  of silicon dioxide. 
     Further, the protective film  213  of aluminum oxide has higher fracture toughness by itself. Accordingly, even if cracking occurs in the protective film  213  along an interface between the second metal layer  212  of aluminum and the protective film  213  of aluminum oxide, the crack development into the insulation layer  230  can be suppressed. 
     Each of aluminum nitride and silicon nitride also has higher fracture toughness by itself. Accordingly, when the protective film  213  is made of aluminum nitride or silicon nitride instead of aluminum oxide, crack development can be suppressed even if cracking occurs in the protective film  213  along an interface between the second metal layer  212  of aluminum and the protective film  213  of aluminum nitride or silicon nitride. 
       FIGS. 4A and 4B  show how stresses (represented by arrows  310 ) act on the insulation layer  230  of silicon dioxide.  FIG. 4A  shows the electrode  210  of the first embodiment. According to the first embodiment, the protective film  213  of aluminum oxide is formed on side and top surfaces of the second metal layer  212  of aluminum, as discussed above. Therefore, the stresses  310  are unlikely to be concentrated within the protective film  213 . 
       FIG. 4B  shows another structure as a comparative example in which a protective film  240  of aluminum oxide having a certain film thickness is formed covering the side and top surfaces of the electrode  210 . In the comparative example, like reference numerals refer to like components corresponding to first embodiment. The difference in expansion coefficient between the first metal layer  211  of molybdenum and the second metal layer  212  of aluminum may allow the stresses  310  to be concentrated in the protective film  240  at a portion in contact with an acute-angled region  320  of the second metal layer  212 , such that cracking may tend to occur in a direction  330  from the interface with the insulation layer  230 . 
       FIG. 5  is a graph showing a relationship between a lifetime of a SAW element and an electrode structure of the SAW element, as a function of frequency. In  FIG. 5 , the horizontal axis represents normalized frequency relative to the resonance frequency of the SAW element, and the vertical axis represents the device lifetime in arbitrary units.  FIGS. 6A-6C  illustrate cross-sectional views showing structures of the electrodes corresponding to lines A, B and C, respectively, as shown in the graph of  FIG. 5 .  FIG. 6A  corresponds to line A, and shows the structure of electrode  210  according to the first embodiment similar to that of  FIG. 3 .  FIG. 6B  corresponds to line B, and shows the structure of the electrode  210  as a comparative example, similar to that of  FIG. 4B , in which the side and top surfaces of the electrode  210  are entirely covered with the protective film  240  of aluminum oxide having a certain thickness.  FIG. 6C  corresponds to line C, and shows the structure of the electrode  210  as another comparative example in which none of the side and top surfaces are covered with a protective film. In  FIGS. 6B and 6C , like numerals refer to like components corresponding to the first embodiment of  FIG. 6A . 
     The lifetime of the SAW element shown in  FIG. 5  was measured as follows: a power of 1 W was applied across the SAW element maintaining the temperature at 100° C. and then the time was measured until the insulation layer  230  was fractured. The film thicknesses of the protective film  213  and the protective film  240  were each 20 nm. The lifetime decreased in each of the comparative examples (lines B and C) at the resonance frequency or greater as compared to the SAW element of the first embodiment (line A). 
     Referring to  FIGS. 4A and 4B  showing the distribution of stresses  310 , and to  FIG. 5 , it can be seen that the structure according to the first embodiment of  FIG. 6A  suppressed cracking and extended the lifetime of the SAW element over the entire frequency range. Further, the structure of  FIG. 6B , corresponding to  FIG. 4B  and having the protective film  240  formed over the entire surfaces of the electrode  210 , allowed the stresses to be concentrated on the interface with the acute-angled region  320  and cracking occurred such that the lifetime was shorter at the resonance frequency or greater as compared to the lifetime of the SAW device of  FIG. 6C  having no protective film. 
       FIG. 7  is a graph showing a relationship between the lifetime of the SAW element and a film thickness of the protective film  213  according to the first embodiment. In  FIG. 7 , the horizontal axis represents normalized frequency relative to the resonance frequency of the SAW element, and the vertical axis represents the lifetime of the device in arbitrary units. In  FIG. 7 , line A (♦) corresponds to the SAW element having no protective film  213 , and lines B (□), C (▴), D (x), E (*), F (∘) and G (|) correspond to film thicknesses of 5 nm, 10 nm, 20 nm, 50 nm, 80 nm and 100 nm, respectively, for the protective film  213 . 
     The lifetime of the SAW element shown in  FIG. 7  was measured as follows: a power of 1 W was applied across the SAW element maintaining the temperature at 100° C. and then the time was measured until the insulation layer  230  was fractured, similar to the case of  FIG. 5 . The results presented in  FIG. 7  show that the protective film  213  having film thickness of 5 nm (line B) indicates a longer lifetime at the resonance frequency or higher frequencies as compared to the lifetime of a device having no protective film (line A). As shown by lines C to G, the protective film  213  having a film thickness of 10 nm or greater may extend the lifetime of the device at the resonance frequency or higher frequencies as compared to either the case with no protective film (corresponding to line A) or the case where the protective film  213  has a film thickness of 5 nm (corresponding to line B). However, a convergence of lines E to G can be seen corresponding to the protective film  213  having a film thickness of 50 nm or greater. 
     In view of the results shown in  FIG. 7  and discussed above, the film thickness of the protective film  213  may advantageously range between 5 nm and 50 nm, preferably between 10 nm and 20 nm, to extend the lifetime of the SAW element according to the first embodiment.  FIGS. 8A to 8D  show structures corresponding to a series of method steps of manufacturing a SAW element according to the first embodiment. As shown in  FIG. 8A , a first metal layer  211   a  is formed by depositing molybdenum on a flat top surface  222  of a piezoelectric substrate  220  made of tantalum niobate to a certain height, and then a second metal layer  212   a  is formed by depositing aluminum on the first metal layer  211   a  to a certain height. 
     The first metal layer  211   a  and the second metal layer  212   a  stacked on the piezoelectric substrate  220  as shown in  FIG. 8A  are processed by photolithography to remove certain portions such that the first metal layer  211  and the second metal layer  212  forming the electrode  210  are created as shown in  FIG. 8B . As discussed above, the second metal layer  212  of aluminum is formed to have a trapezoidal cross-section. 
     As shown in  FIG. 8C , the second metal layer  212  of aluminum is oxidized from the surface and the aluminum is converted into aluminum oxide to a certain depth, such that the protective film  213  of aluminum oxide is formed. As shown in  FIG. 8D , the insulation layer  230  of silicon dioxide is formed to cover the electrode  210  including the protective film  213  formed as shown in  FIG. 8C . 
     Thus, by way of the steps corresponding to the structures shown in  FIGS. 8A to 8D , the SAW element  200  of the first embodiment is formed. The SAW element of the first embodiment can be manufactured using known techniques such as chemical vapor deposition (CVD) and sputtering. 
     It is to be appreciated that although the metal of the first metal layer  211  is described above as being molybdenum, it is not limited thereto. The metal of the first metal layer  211  may be any metal that can support an adequate weight and has a coefficient of thermal expansion lower than that of the second metal layer  212 , a Young&#39;s modulus higher than that of the second metal layer  212 , and fracture toughness higher than that of the insulation layer  230 . For example, the metal may be elemental tungsten, an alloy including molybdenum, or an alloy including tungsten. 
     Further, although the metal of the second metal layer  212  is described above as being aluminum, it is not limited thereto. The metal of the second metal layer  212  may be any metal that can provide an adequate conductivity to the second metal layer  212 . For example, the metal may be elemental copper, an alloy including aluminum, or an alloy including copper. 
     The material of the protective film  213  is also not limited to aluminum oxide, and may be a material that has a linear expansion coefficient lower than that of the second metal layer  212 , a Young&#39;s modulus higher than that of the second metal layer  212 , and fracture toughness higher than that of the insulation layer  230 . For example, the material may be aluminum nitride (AlN) or may be a coating formed by depositing molybdenum, tungsten, or an appropriate alloy. 
     In certain examples the piezoelectric substrate  220  is a substrate made of lithium niobate, but it is not limited thereto. For example, another piezoelectric body, such as lithium tantalate (LiTaO 3 ), can be used for the piezoelectric substrate  220 . 
       FIG. 9  is a cross-sectional view showing the structure of an electrode according to a second embodiment. The second embodiment is similar to the first embodiment, and like reference numerals indicate like elements, but is different from the first embodiment in that the top surface of the second metal layer  212  of aluminum is not covered with the protective film  213 . 
     According to the second embodiment, a first metal layer  211  of molybdenum is formed on the top surface  222  of the piezoelectric substrate  220 , and a second metal layer  212  of aluminum is formed on the first metal layer  211 . In one example the piezoelectric substrate  220  is made of tantalum niobate. The second metal layer  212  of aluminum is formed to have a trapezoidal cross-section and a side surface of the second metal layer  212  is converted from aluminum into aluminum oxide to a certain depth to form a protective film  213 , as discussed above. An insulation layer  230  of silicon dioxide is formed on the top surface  222  of the piezoelectric substrate  220  to cover the electrode  210 . 
     According to the second embodiment, the protective film  213  of aluminum oxide is formed on the side surfaces of the second metal layer  212  of aluminum to cover the second metal layer  212  together with the first metal layer  211 . Therefore, the second metal layer  212  of aluminum has an acute-angled region  320  covered by the molybdenum of the first metal layer  211  and the aluminum oxide of the protective film  213 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does aluminum. Further, the aluminum oxide of the protective film  213  has higher fracture toughness by itself. Still further, the protective film  213  of aluminum oxide only covers the side surfaces of the second metal layer  212  of aluminum. Accordingly, stresses applied within the protective film  213  may be prevented from concentrating on a region in contact with the acute-angled region  320  of the second metal layer  212  of aluminum such that cracking initiated from the protective film  213  toward the insulation layer  230  can be suppressed. 
     According to the second embodiment, the protective film  213  is formed only on the side surfaces of the second metal layer  212  of aluminum. Therefore, as compared to first embodiment in which the protective film  213  is formed on both of the side surfaces and the top surface of the second metal layer  212 , a step of removing the protective film  213  formed on the top surface of the second metal layer  212  may be performed. 
       FIG. 10  is a graph showing a relationship between a lifetime of the SAW element and a film thickness of the protective film  213  according to the second embodiment. In  FIG. 10 , the horizontal axis represents a normalized frequency relative to the resonance frequency of the SAW element  200 , and the vertical axis represents the lifetime of the SAW element in arbitrary units. In  FIG. 10 , similar to  FIG. 7 , line A (+) corresponds to an example in which no protective film is included, and lines B (□), C (▴), D (x), E (*), F (∘) and G(|) correspond to film thicknesses of 5 nm, 10 nm, 20 nm, 50 nm, 80 nm, and 100 nm respectively. 
     Similar to the examples shown in  FIGS. 5 and 7 , the lifetime of the SAW element was measured as follows: a power of 1 W was applied across the SAW element maintaining the temperature at 100° C. and then the time was measured until the insulation layer  230  of the SAW element was fractured. In the results presented in  FIG. 10 , each of lines B to G corresponding to the film thicknesses of 5 nm to 100 nm of the protective film  213  shows a lifetime longer than line A corresponding to the example with no protective film. Accordingly, it is demonstrated that forming the protective film  213  having a film thickness of 5 nm or greater, for example, can extend the lifetime of the SAW element of the second embodiment. 
       FIG. 11  is a cross-sectional view showing the structure of an electrode according to a third embodiment. The third embodiment is similar to the first embodiment, but is different therefrom in that a third metal layer  214  of molybdenum is formed on the top surface of the second metal layer  212  of aluminum, rather than the top surface of the second metal layer  212  being covered with the protective film  213 . 
     According to the third embodiment, a first metal layer  211  of molybdenum is formed on the top surface  222  of the piezoelectric substrate  220 , a second metal layer  212  of aluminum is formed on the first metal layer  211 , and a third metal layer  214  of molybdenum is formed on the second metal layer  212 . In one example the piezoelectric substrate  220  is made of tantalum niobate. The second metal layer  212  of aluminum is formed to have a trapezoidal cross-section and a side surface of the second metal layer  212  is converted from aluminum into aluminum oxide to a certain depth to form a protective film  213 . An insulation layer  230  of silicon dioxide is formed on the top surface  222  of the piezoelectric substrate  220  to cover the electrode  210 . 
     According to the third embodiment, the protective film  213  of aluminum oxide is formed on the side surfaces of the second metal layer  212  of aluminum, as shown in  FIG. 11 . Therefore, the second metal layer  212  of aluminum has an acute-angled region  320  covered by the molybdenum of the first metal layer  211  and the aluminum oxide of the protective film  213 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does aluminum. Further, the aluminum oxide of the protective film  213  has higher fracture toughness by itself. Still further, the protective film  213  of aluminum oxide only covering the side surfaces of the second metal layer  212  of aluminum can prevent the concentration of stresses applied within the protective film  213 . Accordingly, cracking initiated from the interface between the acute-angled region  320  of the second metal layer  212  of aluminum and the protective film  213  can be suppressed. 
     According to the third embodiment, the first metal layer  211  and the third metal layer  214  are made of molybdenum having sufficient density to support the weight of other layers, and the second metal layer  212  is made of aluminum that provides adequate conductivity for the electrode  210 . Thus, sandwiching the second metal layer  212  between the first metal layer  211  and the third metal layer  214 , both of which can adequately bear weight, can vertically distribute the weight of the electrode  210  to ensure a stable oscillation of the electrode  210 . 
       FIG. 12  is a cross-sectional view of the structure of an electrode according to a fourth embodiment. The fourth embodiment is similar to the first embodiment, but different in that the second metal layer  212  of aluminum has a top surface provided with a third metal layer  214  of molybdenum and a fourth metal layer  215  of aluminum, rather than being covered with the protective film  213 . 
     According to the fourth embodiment, a first metal layer  211  of molybdenum is formed on the top surface  222  of the piezoelectric substrate  220 , which may be made of tantalum niobate, for example. A second metal layer  212  of aluminum is formed on the first metal layer  211 , and a third metal layer  214  of molybdenum is formed on the second metal layer  212 . Further, a fourth metal layer  215  of aluminum is formed on the third metal layer  214  of molybdenum. Each of the second metal layer  212  and the fourth metal layer  215  of aluminum is formed to have a trapezoidal cross-section. A side surface of the second metal layer  212  and a side surface and a top surface of the fourth metal layer  215  are converted from aluminum into aluminum oxide to a certain depth to form a first protective film  213  and a second protective film  216 . An insulation layer  230  of silicon dioxide is formed on the top surface  222  of the piezoelectric substrate  220  to cover the electrode  210 . 
     According to the fourth embodiment, the first protective film  213  of aluminum oxide is formed on the side surfaces of the second metal layer  212  of aluminum to cover the second metal layer  212  together with the first metal layer  211  and the third metal layer  214 . Therefore, the second metal layer  212  of aluminum has an acute-angled region  320  covered by the molybdenum of the first metal layer  211  and the aluminum oxide of the first protective film  213 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does aluminum. Further, the aluminum oxide of the first protective film  213  has higher fracture toughness by itself. Still further, the first protective film  213  of aluminum oxide only covering the side surfaces of the second metal layer  212  of aluminum can prevent the concentration of stresses applied within the first protective film  213 . Accordingly, cracking initiated from the interface between the acute-angled region  320  of the second metal layer  212  of aluminum and the first protective film  213  can be suppressed. 
     Further, the second protective film  216  of aluminum oxide is formed on the side surfaces and the top surface of the fourth metal layer  215  of aluminum to cover the fourth metal layer  215  together with the third metal layer  214 . Accordingly, an acute-angled region  325  of the fourth metal layer  215  of aluminum is covered by the molybdenum of the third metal layer  214  and the aluminum oxide of the second protective film  216 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does of aluminum. Still further, the aluminum oxide of the second protective film  216  has higher fracture toughness by itself. Yet still further, the second protective film  216  of aluminum oxide only covering the side surfaces and the top surface of the fourth metal layer  215  of aluminum can prevent the concentration of stresses applied within the second protective film  216 . Accordingly, cracking initiated from the interface between the acute-angled region  325  of the fourth metal layer  215  of aluminum and the second protective film  216  can be suppressed. 
     In the SAW element of the fourth embodiment, the first metal layer  211  and the third metal layer  214  are made of molybdenum, which as discussed above has sufficient density to support the weight of other layers, and the second metal layer  212  and the fourth metal layer  215  are made of aluminum which provides adequate conductivity for the electrode  210 . Thus, arranging the first metal layer  211  and the third metal layer  214  of molybdenum to alternate with the second metal layer  212  and the fourth metal layer  215  can vertically distribute the weight of the electrode  210  to ensure a stable oscillation of the electrode  210 . Further, both electric current and generated heat are advantageously distributed between the second metal layer  212  and the fourth metal layer  215  due to the aluminum having relatively high conductivity. 
       FIG. 13  is a cross-sectional view showing the structure of an electrode according to a fifth embodiment. The fifth embodiment similar to the first embodiment, but is different in that instead of having the top surface of the second metal layer  212  of aluminum being covered with the protective film  213 , a third metal layer  214  of molybdenum, a fourth metal layer  215  of aluminum, and a fifth metal layer  217  of molybdenum are formed over the top surface of the second metal layer  212 . 
     According to the fifth embodiment, a first metal layer  211  of molybdenum is formed on the top surface  222  of the piezoelectric substrate  220 , which may be made of tantalum niobate, for example. A second metal layer  212  of aluminum is formed on the first metal layer  211 , and a third metal layer  214  of molybdenum is formed on the second metal layer  212 . Further, a fourth metal layer  215  of aluminum is formed on the third metal layer  214  of molybdenum and a fifth metal layer  217  of molybdenum is formed on the fourth metal layer  215 . Each of the second metal layer  212  and the fourth metal layer  215  of aluminum is formed to have a trapezoidal cross-section, and a side surface thereof is converted from aluminum into aluminum oxide to a certain depth to form a first protective film  213  and a second protective film  216 . An insulation layer  230  of silicon dioxide is formed on the top surface  222  of the piezoelectric substrate  220  to cover the electrode  210 . 
     According to the fifth embodiment, the first protective film  213  of aluminum oxide is formed on the side surfaces of the second metal layer  212  of aluminum to cover the second metal layer  212  together with the first metal layer  211  and the third metal layer  214 . Therefore, the second metal layer  212  of aluminum has an acute-angled region  320  covered by the molybdenum of the first metal layer  211  and the aluminum oxide of the first protective film  213 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does aluminum. Further, the aluminum oxide of the first protective film  213  has higher fracture toughness by itself. Still further, the first protective film  213  of aluminum oxide only covering the side surfaces of the second metal layer  212  of aluminum can prevent the concentration of stresses applied within the first protective film  213 . Accordingly, cracking initiated from the interface between the acute-angled region  320  of the second metal layer  212  of aluminum and the first protective film  213  can be suppressed. 
     Further, the second protective film  216  of aluminum oxide is formed on the side surfaces of the fourth metal layer  215  of aluminum to cover the fourth metal layer  215  together with the third metal layer  214  and the fifth metal layer  217 . Accordingly, an acute-angled region  325  of the fourth metal layer  215  of aluminum is covered by the molybdenum of the third metal layer  214  and the aluminum oxide of the second protective film  216 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does of aluminum. Still further, the aluminum oxide of the second protective film  216  has higher fracture toughness by itself. Yet still further, the second protective film  216  of aluminum oxide only covering the side surfaces of the fourth metal layer  215  of aluminum can distribute the stresses. Accordingly, cracking initiated from the interface between the acute-angled region  325  of fourth metal layer  215  of aluminum and the second protective film  216  can be suppressed. 
     In the SAW element of fifth embodiment, the first metal layer  211 , the third metal layer  214  and the fifth metal layer  217  are made of molybdenum, providing weight-bearing capability, and the second metal layer  212  and the fourth metal layer  215  are made of aluminum having higher conductivity. Thus, sandwiching the second metal layer  212  and the fourth metal layer  215  with the first metal layer  211 , the third metal layer  214  and the fifth metal layer  217  of molybdenum can vertically distribute the weight of the electrode  210  to ensure a stable oscillation of the electrode  210 . Further, both electric current and generated heat are advantageously distributed between the second metal layer  212  and the fourth metal layer  215  due to the conductivity of the aluminum. 
       FIG. 14  is a cross-sectional view showing the structure of an electrode according to a sixth embodiment. The sixth embodiment is similar to the first embodiment, but is different therefrom in that the side surfaces of the first metal layer  211  of molybdenum are covered with a first protective film  218 . 
     According to the sixth embodiment, a first metal layer  211  of molybdenum is formed on the top surface  222  of the piezoelectric substrate  220 , which may be made of tantalum niobate, for example, and a second metal layer  212  of aluminum is formed on the first metal layer  211 . A first protective film  218  is formed by converting the molybdenum of the first metal layer  211  into molybdenum oxide to a certain depth to cover the first metal layer  211  together with the top surface  222  of the piezoelectric substrate  220  and the second metal layer  212 . The second metal layer  212  of aluminum is formed to have a trapezoidal cross-section and a side surface of the second metal layer  212  is converted from aluminum into aluminum oxide to a certain depth to form a second protective film  213 . The second metal layer  212  of aluminum is protected by the first metal layer  211  of molybdenum between the first protective film  218  of molybdenum oxide and the second metal layer  212  of aluminum. An insulation layer  230  of silicon dioxide is formed on the top surface  222  of piezoelectric substrate  220  to cover the electrode  210 . 
     According to the sixth embodiment, the second protective film  213  of aluminum oxide is formed on the side surfaces and the top surface of the second metal layer  212  of aluminum to cover the second metal layer  212  together with the first metal layer  211 . Further, the first metal layer  211  of molybdenum is disposed between the first protective film  218  of molybdenum oxide and the second metal layer  212  of aluminum. Therefore, the second metal layer  212  of aluminum has an acute-angled region  320  covered by the molybdenum of the first metal layer  211  and the aluminum oxide of the second protective film  213 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than does aluminum. Still further, the aluminum oxide of the second protective film  213  has higher fracture toughness by itself. Yet still further, the second protective film  213  of aluminum oxide only covering the side surfaces and the top surface of the second metal layer  212  of aluminum can prevent the concentration of stresses applied within the second protective film  213 . Accordingly, cracking initiated from the interface between the acute-angled region  320  of the second metal layer  212  of aluminum and the second protective film  213  can be suppressed. 
     According to the sixth embodiment, the first protective film  218  of molybdenum oxide is formed on the side surfaces of the first metal layer  211  of molybdenum to protect the first metal layer  211  of molybdenum. The second metal layer  212  of aluminum is protected by the first metal layer  211  of molybdenum and the second protective film  213  of aluminum oxide. 
       FIG. 15  is a cross-sectional view showing the structure of an electrode according to a seventh embodiment. According to the seventh embodiment, a first metal layer  211  of aluminum is formed on the top surface  222  of the piezoelectric substrate  220  and a second metal layer  212  of molybdenum is formed on the first metal layer  211 . The piezoelectric substrate may be made of tantalum niobate, for example. The first metal layer  211  of aluminum is formed to have a trapezoidal cross-section and a side surface thereof is converted from aluminum into aluminum oxide to a certain depth to form a first protective film  218 . 
     According to the seventh embodiment, the first protective film  218  of aluminum oxide is formed on the side surfaces of the first metal layer  211  of aluminum to cover the first metal layer  211  together with the top surface  222  of the piezoelectric substrate  220  and the second metal layer  212 . A cross-sectional shape of the first metal layer  211  has an obtuse angle around a point where an interface between the first metal layer  211  and the second metal layer  212  is in contact with the first protective film  218 . The acute-angled region  320  according to any of the first to sixth embodiments does not exist such that stresses are not concentrated within the first protective film  218 . 
     According to the seventh embodiment again, the first metal layer  211  of aluminum is covered by the molybdenum of the second metal layer  212  and the aluminum oxide of the first protective film  218 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than those of aluminum. Further, the aluminum oxide of the first protective film  218  has higher fracture toughness by itself. Accordingly, cracking initiated from the interface between the first metal layer  211  of aluminum and the first protective film  218  can be suppressed. 
     According to the seventh embodiment, the second metal layer  212  of molybdenum is formed on the top surface of the first metal layer  211  of aluminum. Therefore, the top surface of the first metal layer  211  of aluminum is protected by the second metal layer  212  of molybdenum. 
       FIG. 16  is a cross-sectional view showing the structure of an electrode according to an eighth embodiment. The eighth embodiment is similar to the seventh embodiment, but is different therefrom in that a second protective film  213  of molybdenum oxide is formed on the side surfaces of the second metal layer  212  of molybdenum. 
     According to the eighth embodiment, a first metal layer  211  of aluminum is formed on the top surface  222  of the piezoelectric substrate  220  of tantalum niobate to cover the first metal layer  211  together with the top surface  222  of the piezoelectric substrate  220  and the second metal layer  212 . A second metal layer  212  of molybdenum is formed on the first metal layer  211 . The first metal layer  211  of aluminum is formed to have a trapezoidal cross-section and a side surface thereof is converted from aluminum into aluminum oxide to a certain depth to form a first protective film  218 . A second protective film  213  is formed by converting the molybdenum on a side surface of the second metal layer  212  into molybdenum oxide to a certain depth to cover the second metal layer  212  together with the first metal layer  211 . 
     According to the eighth embodiment, the second metal layer  212  of molybdenum is disposed between the second protective film  213  of molybdenum oxide and the first metal layer  211  of aluminum. The first metal layer  211  of aluminum is covered by the molybdenum of second metal layer  212  and the aluminum oxide of the first protective film  218 , each of which has a lower coefficient of thermal expansion, a higher Young&#39;s modulus, and higher fracture toughness than those of aluminum. Further, the aluminum oxide of the first protective film  218  has higher fracture toughness by itself. Accordingly, cracking initiated from an interface of the first metal layer  211  of aluminum can be suppressed. 
     According to the eighth embodiment, the top surface of the first metal layer  211  of aluminum is protected by the second metal layer  212  of molybdenum. The molybdenum of the second metal layer  212  is disposed between the first metal layer  211  of aluminum and the second protective film  213  of molybdenum oxide and protects the first metal layer  211  of aluminum. 
     It is to be appreciated that although electrodes  210  have been described and illustrated as including two to five metal layers, any of the metal layers may be configured as multi-layered structures having two or more layers. In certain embodiments, when a plurality of metal layers are stacked on the top surface of the piezoelectric substrate, the odd-numbered layers counted from the lowermost layer (closest to the piezoelectric substrate  220 ) may be made of a metal configured to bear the weight of other layers, whereas the even-numbered layers may be made of a metal having sufficiently high conductivity for operation of the electrode  210 . In other embodiments, the even-numbered layers counted from the lowermost layer may be made of a metal configured to bear the weight of other layers, whereas the odd-numbered layers may be made of a metal having sufficiently high conductivity for operation of the electrode  210 . In other embodiments, the weight-bearing and high-conductivity (or conductivity-supplying) metal layers need not be strictly alternating; instead some of the metal layers in a multi-layered structure may be made of a metal configured to bear the weight of other layers, whereas other layers may be made of a metal having sufficiently high conductivity for operation of the electrode  210 . 
     Further, layers made of the conductivity-supplying metal may have a side surface provided with a protective film, or layers made of the weight-bearing metal may have a side surface provided with a protective film. Still further, a protective film may be formed on a top surface of the uppermost layer (furthest from the piezoelectric substrate) among the plurality of metal layers. 
     Various examples and embodiments of the SAW element  200  may be incorporated into and packaged as a module that may ultimately be used in an electronic device, such as a wireless communications device, for example. In certain examples the SAW element  200  can be configured as a SAW filter.  FIG. 17  is a block diagram illustrating one example of a filter module  400  including a SAW filter  410 . The SAW filter  410  may be implemented on one or more die(s)  420  including one or more connection pads  422 . The packaged module  400  includes a packaging substrate  430  that is configured to receive a plurality of components, including the die  420 . A plurality of connection pads  432  can be disposed on the packaging substrate  430 , and the various connection pads  422  of the SAW filter die  410  can be connected to the connection pads  432  on the packaging substrate  430  via electrical connectors  434 , which can be solder bumps or wirebonds, for example, to allow for passing of various signals to and from the SAW filter  410 . The module  400  may optionally further include other circuitry die  440 , such as, for example one or more additional filter(s), amplifiers, pre-filters, modulators, demodulators, down converters, and the like, as would be known to one of skill in the art of semiconductor fabrication in view of the disclosure herein. In some embodiments, the module  400  can also include one or more packaging structures to, for example, provide protection and facilitate easier handling of the module  400 . Such a packaging structure can include an overmold formed over the packaging substrate  430  and dimensioned to substantially encapsulate the various circuits and components thereon. 
     As discussed above, various examples and embodiments of the SAW filter  410  can be used in a wide variety of electronic devices. For example, the SAW filter  410  can be used in an antenna duplexer, which itself can be incorporated into a variety of electronic devices, such as RF front-end modules and communication devices. 
       FIG. 18  is a block diagram of one example of a wireless device  500  including a filter module  400 . The wireless device  500  can be a cellular phone, smart phone, tablet, modem, communication network or any other portable or non-portable device configured for voice or data communication. The wireless device  500  can receive and transmit signals from an antenna  510 . The antenna  510  is coupled to an antenna switch module  520  that can enable switching between a transmit mode and a receive mode, for example, or between different frequency bands within the transmit mode or receive mode, for example. The wireless device  500  further includes a transceiver  530  that is configured to generate signals for transmission and/or to process received signals. Signals generated for transmission are received by a power amplifier (PA)  540 , which amplifies the generated signals from the transceiver  530 . Received signals are amplified by a low noise amplifier (LNA)  545  and then provided to the transceiver  530 . As is also shown in  FIG. 18 , the antenna  510  both receives signals that are provided to the transceiver  530  via the antenna switch module  520  and the LNA  545  and also transmits signals from the wireless device  500  via the transceiver  530 , the PA  540 , and the antenna switch module  520 . However, in other examples multiple antennas can be used. 
     The power amplifier  540  can be used to amplify a wide variety of RF or other frequency-band transmission signals. For example, the power amplifier  540  can receive an enable signal that can be used to pulse the output of the power amplifier to aid in transmitting a wireless local area network (WLAN) signal or any other suitable pulsed signal. The power amplifier  540  can be configured to amplify any of a variety of types of signal, including, for example, a Global System for Mobile (GSM) signal, a code division multiple access (CDMA) signal, a W-CDMA signal, a Long Term Evolution (LTE) signal, or an EDGE signal. In certain embodiments, the power amplifier  540  and associated components including switches and the like can be fabricated on GaAs substrates using, for example, pHEMT or BiFET transistors, or on a Silicon substrate using CMOS transistors. 
     In the example shown in  FIG. 18 , the filter module  400  is shown in the transmit path positioned between the power amplifier  540  and the antenna switch module  520 . However, a variety of other configurations can be implemented. For example, the wireless device  500  can include one or more filter modules  400  in the transmit path or the receive path. Further, the filter module(s)  400  can be positioned before or after amplifiers or switches in either path. 
     The wireless device  500  of  FIG. 18  further includes a power management sub-system  550  that is connected to the transceiver  530  and manages the power for the operation of the wireless device  500 . The power management system  550  can also control the operation of a baseband sub-system  560  and various other components of the wireless device  500 . The power management system  550  can include, or can be connected to, a battery (not shown) that supplies power for the various components of the wireless device  500 . 
     In certain embodiments, the baseband sub-system  560  is connected to a user interface  570  to facilitate various input and output of voice and/or data provided to and received from the user. 
     The baseband sub-system  560  can also be connected to memory  580  that is configured to store data and/or instructions to facilitate the operation of the wireless device, and/or to provide storage of information for the user. 
     Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. It is to be appreciated that embodiments discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the foregoing description. The concepts and technology disclosed herein are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. Accordingly, the foregoing description is by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.