Abstract:
A surface acoustic wave device includes a piezoelectric substrate, an interdigital transducer (IDT) formed on the piezoelectric substrate, an interconnection electrode that is provided on the piezoelectric substrate and is connected to the IDT, the IDT being made of a metal identical to that of the IDT, an inorganic insulation layer that is provided on the piezoelectric substrate so that at least the interconnection electrode is exposed, an insulative resin layer that is located on an interface between the inorganic insulation layer and a portion of the interconnection electrode exposed from the inorganic insulation layer and is formed so as to cover a side surface of the interconnection electrode, and a metal layer that is provided on the interconnection electrode and the insulative resin layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-074885, filed on Mar. 24, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present invention generally relates to surface acoustic wave devices, and more particularly, to a surface acoustic wave device having an interdigital transducer (IDT) composed of comb electrodes. 
       BACKGROUND 
       [0003]    Recently, mobile communications devices such as cellular phones have been progressively spread due to the development of information-oriented society. Acoustic wave devices are used in the mobile communications devices as duplexers, transmission bandpass filters or reception bandpass filters, and are required to be downsized, weight-lightened and more reliable. 
         [0004]    A surface acoustic wave device composed of an IDT and reflectors formed on a piezoelectric substrate is a typical example of the acoustic wave devices. An acoustic wave is excited by an electric signal applied to the IDT and a resultant electric signal of a frequency in the pass band is output. 
         [0005]    Now, a SAW device described in Japanese Patent No. 3405329 (Document 1) is described below. Document 1 describes a technique of providing a reaction restraining film and a metal layer on an interconnection electrode. 
         [0006]      FIG. 1A  is a plan view of a SAW device  100 . Reflectors  4  and IDTs  6  made of a metal such as aluminum are formed on a piezoelectric substrate  2  made of lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ). Further, there are provided interconnections  10  connected to the IDTs  6  and external connection terminals  8  connected to the interconnections  10 . 
         [0007]      FIG. 1B  is a cross-sectional view taken along a line A-A depicted in  FIG. 1A . Referring to  FIG. 1B , each of the interconnections  10  is composed of an interconnection electrode  12 , a first metal layer  14 , a second metal layer  16 , a third metal layer  18 , and an inorganic insulation layer  20 . The interconnection electrode  12  is formed on the piezoelectric substrate  2  and may be made of aluminum. The first metal layer  14  is provided on the interconnection electrode  12  and may be made of titanium. The second metal layer  16  is provided on the first metal layer  14  and may be made of palladium. The inorganic insulation layer  20  is interposed between the interconnection electrode  12  and the first metal layer  14 . 
         [0008]      FIG. 2A  illustrates a variation of the SAW device illustrated in  FIG. 1B . The inorganic insulation layer  20  is provided so as to cover the side surfaces of the interconnection electrode  12 . Document 1 describes that reaction of the interconnection electrode  12  with the metal layers can be restrained, so that the junctions between the interconnection electrode  12  and the metal layers and the junctions between the interconnections  10  and the external connection terminals  8  can be strengthened. 
         [0009]    However, in the structure illustrated in  FIG. 1B , since side surfaces  12   a  of the interconnection electrode  12  are exposed, the interconnections  10  do not have good moisture resistance. Moisture that enters into the interconnection electrode  12  corrodes it and degrades the reliability of the SAW device. 
         [0010]      FIG. 2B  is an enlarged view of one of the side surfaces  12   a  of the interconnection electrode  12  and its vicinity. Referring to  FIG. 2B , there is a great step between the piezoelectric substrate  2  and the interconnection electrode  12 , and the thickness of the inorganic insulation layer  20  may be reduced unwillingly (backward tapered). In this case, the interconnection electrode does not have good moisture resistance, and therefore, the reliability of the SAW device may be degraded. 
       SUMMARY 
       [0011]    According to an aspect of the present invention, there is provided a surface acoustic wave device includes a piezoelectric substrate, an interdigital transducer (IDT) formed on the piezoelectric substrate, an interconnection electrode that is provided on the piezoelectric substrate and is connected to the IDT, the IDT being made of a metal identical to that of the IDT, an inorganic insulation layer that is provided on the piezoelectric substrate so that at least the interconnection electrode is exposed, an insulative resin layer that is located on an interface between the inorganic insulation layer and a portion of the interconnection electrode exposed from the inorganic insulation layer and is formed so as to cover a side surface of the interconnection electrode, and a metal layer that is provided on the interconnection electrode and the insulative resin layer. 
         [0012]    According to another aspect of the present invention, there is provided a method of fabricating a surface acoustic wave device comprising: forming an interdigital transducer (IDT) and an interconnection electrode connected thereto on a piezoelectric substrate, the IDT and the interconnection electrode being made of an identical metal; forming an inorganic insulation layer on the piezoelectric substrate so that at least the interconnection electrode is exposed; forming an insulative resin layer on an interface between the inorganic insulation layer and a part of the interconnection electrode exposed from the inorganic insulation layer; and forming a metal layer on the interconnection electrode and the insulative resin layer. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1A  is a plan view of a surface acoustic wave device related to the present invention, and  FIG. 1B  is a cross-sectional view taken along a line A-A depicted in  FIG. 1A ; 
           [0014]      FIG. 2A  is a cross-sectional view of a variation of the surface acoustic wave device depicted in  FIG. 1B , and  FIG. 2B  is an enlarged view of one of the side surfaces  12   a  of the interconnection electrode  12  and its vicinity; 
           [0015]      FIG. 3A  is a plan view of a surface acoustic wave device in accordance with a first embodiment of the present invention; and  FIG. 3B  is a cross-sectional view taken along a line A-A depicted in  FIG. 3A ; 
           [0016]      FIG. 4  is a flowchart of a method of fabricating the surface acoustic wave device illustrated in  FIGS. 3A and 3B ; 
           [0017]      FIGS. 5A through 5D  are cross-sectional views illustrating the method of fabricating the surface acoustic wave device illustrated in  FIGS. 3A and 3B ; 
           [0018]      FIG. 6  is a cross-sectional view illustrating the method of fabricating the surface acoustic wave device illustrated in  FIGS. 3A and 3B ; and 
           [0019]      FIG. 7  is a cross-sectional view illustrating a method of a surface acoustic wave device in accordance with a second embodiment. 
       
    
    
       [0020]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0021]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the inventions as claimed. 
       DESCRIPTION OF EMBODIMENTS 
       [0022]    A description is now given of embodiments of the present invention with reference to the accompanying drawings. 
       First Embodiment 
       [0023]      FIG. 3A  is a plan view of a SAW device  200  in accordance with a first embodiment. As illustrated in  FIG. 3A , the SAW device  200 , which may be a filter, has the piezoelectric substrate  2  of, for example, LiNbO 3  or LiTaO 3 , on which the reflectors  4 , the IDTs  6 , the interconnections  10  and the external connection terminals  8  are provided. The conductive patterns on the piezoelectric substrate  2  may be made of Al or an alloy of Al—Cu. The inorganic insulation layer  20  is provided on the piezoelectric substrate  2  so that at least the external connection terminals  8  and the interconnection electrodes  12  are exposed, as will be described later. In  FIG. 3 , the reflectors  4  and the IDTs  6  are seen through the inorganic insulation layer  20 . 
         [0024]      FIG. 3B  is a cross-sectional view taken along a line A-A in  FIG. 3A . As illustrated in  FIG. 3B , the interconnection electrode  12 , which may, for example, be 350 nm thick, is provided on the piezoelectric substrate  2 . The inorganic insulation layer  20 , which may, for example, be 50 nm thick, is provided so that the interconnection electrode  12  can be exposed. An insulative resin layer  22  is provided on an interface between the interconnection electrode  12  exposed from the inorganic insulation layer  20  and the inorganic insulation layer  20  so as to cover the interface. The insulative resin layer  22  may, for example, be 1 μm thick and may be made of, for example, photosensitive resin such as photosensitive polyimide. The first metal layer  14 , which may, for example, be 200 nm, is provided on the interconnection electrode  12  and the insulative resin layer  22 . The third metal layer  18 , which may, for example, be 150 nm, is provided on the first metal layer  14 . An end of the insulative resin layer  22  is a forward tapered portion  22   a.    
         [0025]    The interconnection electrode  12  may be made of the same metal as the reflectors  4  and the IDTs  6  and may be made of, for example Al or an alloy of Al—Cu. The third metal layer  18  is made of a metal having a relatively low resistance and may be gold. The first metal layer  14  may be made of a metal having a good adhesiveness to Al for the interconnection electrode  12  and Au for the third metal layer  18 , and may be titanium. 
         [0026]    A description is now given of a method of fabricating the SAW device  200  in accordance with the first embodiment.  FIG. 4  is a flowchart of the fabrication method, and  FIGS. 5A through 6  are respectively cross-sectional views of the device in the fabrication method. 
         [0027]      FIG. 5A  is a cross-sectional view at step S 10  depicted in  FIG. 4  at which the piezoelectric substrate  2  is prepared. 
         [0028]      FIG. 5B  is a cross-sectional view at step S 11  at which an Al film or an Al—Cu alloy film is grown on the piezoelectric substrate  2  by sputtering. Then, the film is patterned into the reflectors  4 , the IDTs  6 , the external connection terminals  8  and the interconnection electrodes  12  by the photolithographic process. That is, the identical metal is used to form the reflectors  4 , the IDTs  6 , the external connection terminals  8  and the interconnection electrodes  12 . 
         [0029]      FIG. 5C  is a cross-sectional view at step S 12 . At step S 12 , an SiO 2  film is grown on the piezoelectric substrate  2 . Portions of the SiO 2  film located on the external connection terminals  8  and the interconnection electrodes  12  are removed by the photolithographic process. Thus, the inorganic insulation layer  20  is formed so that the external connection terminals  8  and the interconnection electrodes  12  can be exposed. 
         [0030]      FIG. 5D  is a cross-sectional view at steps S 13  and S 14 . Referring to  FIG. 5D , photosensitive polyimide resin is provided on the piezoelectric substrate  2  by spin coating. Then, the photosensitive polyimide resin is shaped by the photolithographic process, so that the insulative resin layer  22  is formed so as to cover the side surfaces  12   a  of the interconnection electrode  12  at the interfaces between the exposed interconnection electrode  12  and the inorganic insulation layer  20 . 
         [0031]    At step S 14 , the insulative resin layer  22  is annealed and cured. At this time, the piezoelectric substrate  2  is preferably heated from both the opposite sides, one of which has the IDTs  6  and the other has no IDTs  6 . This process may use an oven. The end of the insulative resin layer  22  after beating is shaped into the forward tapered portion  22   a.    
         [0032]      FIG. 6  is a cross-sectional view at step S 15 . The first metal layer  14  is formed on the interconnection electrode  12  and the insulative resin layer  22  by evaporation and liftoff, and the third metal layer  18  is formed on the first metal layer  14  by evaporation and liftoff. Through the above-described steps, the SAW device  200  is completed. 
         [0033]    According to the first embodiment, the insulative resin layer  22  covers the side surfaces  12   a  of the interconnection electrodes  12 , so that the interconnection electrodes  12  can be protected. Further, the first metal layer  14  and the third metal layer  18  completely cover the interconnection electrodes  12  and the insulative resin layer  22 . It is thus possible to increase the moisture resistance of the interconnections  10  and improve the reliability of the SAW device  200 . Since the ends of the insulative resin layer  22  are the forward tapered portions  22   a,  the steps between the inorganic insulation layer  20  and the insulative resin layer  22  can be gentled. It is thus possible to prevent the insulative resin layer  22  from being thinned and to prevent a crack from occurring in the first metal layer  14  and/or the third metal layer  18 . This improves the moisture resistance of the interconnections  10 . 
         [0034]    As has been described previously, the first metal layer  14  is made of a metal having good adhesiveness to the metal of the interconnection electrodes  12  and the metal of the third metal layer  18 . The first metal layer  14  is sandwiched between the interconnection electrode  12  and the third metal layer  18 , whereby the interconnections  10  can be strengthened and the reliability of the SAW device  200  can be improved. Further, the resistance of the interconnections  10  can be reduced because the third metal layer  18  can be formed of a metal having a relatively low resistance such as gold. 
         [0035]    The inorganic insulation layer  20  may be made of a silicon compound such as silicon nitride (SiN) or silicon oxycarbide (SiOC) instead of SiO 2 . The inorganic insulation layer  20  is required to be provided so that the external connection terminals  8  and the interconnection electrodes  12  can be exposed. 
         [0036]    The insulative resin layer  22  is not limited to the photosensitive resin, especially, photosensitive polyimide. However, in practice, the insulative resin layer  22  is preferably photosensitive resin because the insulative resin layer  22  can be precisely formed by the photolithographic process at step S 13 . Further, it is preferable to use photosensitive resin because the forward tapered portions  22   a  can easily be formed by annealing at step S 14 . 
         [0037]    The annealing process at step S 14  may be carried out by heating one of the opposite surfaces of the piezoelectric substrate  2 , one of which has the IDTs  6  and the other has no IDTs. In practice, it is preferable to heat the piezoelectric substrate  2  from both the opposite sides in order to precisely and efficiently form the ends of the insulative resin layer  22  into the forward tapered portions  22   a.    
       Second Embodiment 
       [0038]      FIG. 7  is a cross-sectional view of a SAW device  300  in accordance with a second embodiment. 
         [0039]    Referring to  FIG. 7 , the inorganic insulation layer  20  is formed so as to overlap the interconnection electrode  12 . The insulative resin layer  22  is provided so as to be located on the interfaces between the interconnection electrode  12  and the inorganic insulation layer  20  so as to cover the side surfaces  12   a  of the interconnection electrode  12  across the inorganic insulation layer  20 . 
         [0040]    According to the second embodiment, the side surfaces  12   a  of the interconnection electrode  12  are covered with the inorganic insulation layer  20 , so that the moisture resistance can further be improved and the reliability of the SAW device  300  can be improved. 
         [0041]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.