Patent Publication Number: US-2021184650-A1

Title: Electronic device and module including the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2017-104823 filed on May 26, 2017 and is a Continuation Application of PCT Application No. PCT/JP2018/018080 filed on May 10, 2018. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic device and a module including the same. 
     2. Description of the Related Art 
     In recent years, wafer level package (hereinafter, simply referred to as WLP) electronic devices have been developed, which are element size packages. 
     For example, Japanese Patent Laid-Open No. 2006-345075 discloses a surface acoustic wave resonator device which includes interdigitated electrodes on a piezoelectric substrate, a gap forming layer having a gap formed therein above the interdigitated electrodes, and a sealing layer sealing the interdigitated electrodes. In Japanese Patent Laid-Open No. 2006-345075, a conductive via and an external connection electrode are formed so that the conductive via passes through the sealing layer and the gap forming layer and is connected to the interdigitated electrodes; and the external connection electrode continues the conductive via. Then, the conductive via and the external connection electrode are covered with a solder resist, a portion of the solder resist is removed, and a solder bump is thereby formed. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide electronic devices and modules each including the same which can reduce degradation in characteristics of the electronic devices. 
     An electronic device according to a preferred embodiment of the present invention includes a piezoelectric substrate including a first main surface, a functional element provided on or above the first main surface, an insulation material layer provided on or above the first main surface and surrounding the functional element, and a protective layer provided on or above the insulation material layer. The piezoelectric substrate and the insulation material layer define a hollow portion that accommodates the functional element. In a cross-sectional view in a direction perpendicular to a thickness direction of the piezoelectric substrate, the protective layer includes a first portion, a second portion and a third portion. The first portion is above the hollow portion. The second portion is adjacent to the first portion at one end of the second portion. The third portion is adjacent to the second portion at another end of the second portion. A distance between the first main surface and a surface of the protective layer in the thickness direction is greatest at a location where the second portion is adjacent to or in a vicinity of the first portion, and the distance is shortest at a location where the second portion is adjacent to or in a vicinity of the third portion. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a surface acoustic wave filter according to a first preferred embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of a surface acoustic wave filter according to a Comparative Example. 
         FIG. 3  is a diagram showing how the surface acoustic wave filter according to the Comparative Example is flip-chip mounted on a circuit board. 
         FIG. 4  is a diagram showing how the surface acoustic wave filter according to the first preferred embodiment of the present invention is flip-chip mounted on a circuit board. 
         FIGS. 5A and 5B  are cross-sectional views of the surface acoustic wave filter according to the first preferred embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of a surface acoustic wave filter according to a variation of the first preferred embodiment of the present invention. 
         FIG. 7  is a cross-sectional view of a surface acoustic wave filter according to a second preferred embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of a surface acoustic wave filter according to a third preferred embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of a surface acoustic wave filter according to a fourth preferred embodiment of the present invention. 
         FIG. 10  is a cross-sectional view of a surface acoustic wave filter according to a variation of the fourth preferred embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of a surface acoustic wave filter according to a fifth preferred embodiment of the present invention. 
         FIG. 12  is a cross-sectional view of a surface acoustic wave filter according to the fifth preferred embodiment of the present invention. 
         FIG. 13  is a cross-sectional view of a surface acoustic wave filter according to a sixth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments according to the present invention will be described with reference to the accompanying drawings. Note that the same reference signs are used to refer to the same or corresponding portions in the drawings, and description thereof will not be repeated. 
     Note that when the preferred embodiments described below refer to a number, a quantity, etc., the scope of the present invention is not always limited to that number, that quantity, etc., unless otherwise specified. In the following preferred embodiments, each device is also not essential to the present invention unless otherwise specified. 
     First Preferred Embodiment 
     An electronic device  100  according to a first preferred embodiment is applied to, for example, an RF circuit included in a communication device, such as a mobile phone. Electronic device  100  operates through sound waves. Examples of electronic device  100  include surface acoustic wave (SAW) devices, bulk acoustic wave (BAW) devices, and MEMS devices. In the first preferred embodiment, a surface acoustic wave filter will be shown and described as one preferred embodiment of electronic device  100 . In the following, electronic device  100  will be also referred to as a “surface acoustic wave filter  100 ”. 
       FIG. 1  is a cross-sectional view of surface acoustic wave filter  100  according to the first preferred embodiment of the present invention. Surface acoustic wave filter  100  is preferably, for example, a ladder filter in which multiple surface acoustic wave resonators are connected in a ladder arrangement. The surface acoustic wave resonator corresponds to one preferred embodiment of “the functional element”. 
     Referring to  FIG. 1 , surface acoustic wave filter  100  according to the first preferred embodiment includes a piezoelectric substrate  1 , a surface acoustic wave resonator  2  (the functional element), a line  3 , a support layer  4 , a cover layer  5 , a connection electrode  6 , a line electrode  7 , a pillar  8 , an external connection terminal  9 , and a protective layer  10 . 
     Piezoelectric substrate  1  is preferably made of piezoelectric crystal, for example, lithium tantalate (LiTaO 3 ) or lithium niobate (LiNbO 3 ). Piezoelectric substrate  1  may be made of piezo ceramic or may be a substrate including a piezoelectric thin film on the primary surface. Piezoelectric substrate  1  has a cuboid shape, for example, and preferably has a rectangular or substantially rectangular shape in plan view in the thickness direction of piezoelectric substrate  1 . Piezoelectric substrate  1  includes a primary surface  1   a.    
     Surface acoustic wave resonator  2  and line  3  are provided on primary surface  1   a  of piezoelectric substrate  1 . Surface acoustic wave resonator  2  includes a conductor layer provided on primary surface  1   a , and includes a pair of interdigitated electrodes (hereinafter, also referred to as an “inter digital transducer (IDT) electrode”), the conductor layer preferably including aluminum (Al), copper (Cu), nickel (Ni), gold (Au), or platinum (Pt), for example. Surface acoustic wave resonator  2  may further include two reflectors provided on both sides of the IDT electrodes in a direction of propagation of a surface acoustic wave. 
     Multiple surface acoustic wave resonators  2  define a ladder filter. The size of each surface acoustic wave resonator is determined according to predetermined filtering characteristics of the ladder filter. Features of surface acoustic wave resonator  2  are the same as or similar to those of general surface acoustic wave resonators, and thus description thereof will be omitted. 
     Line  3  includes a conductor layer preferably including, for example, Al, Cu, Ni, Au, or Pt, which is provided on primary surface  1   a . At least one end of line  3  is connected to one of the interdigitated electrodes of surface acoustic wave resonator  2 . Line  3  may be defined by multiple conductor layers. 
     Cover layer  5  is provided on support layer  4  described below, and faces primary surface  1   a  of piezoelectric substrate  1 . Cover layer  5  preferably has a rectangular or substantially rectangular shape that is the same or substantially the same as primary surface  1   a . Cover layer  5  includes an insulating material. For example, a resin such as epoxy or polyimide, or an insulating ceramic such as silicon oxide (SiO 2 ) or Al 2 O 3  may preferably be used. Cover layer  5  may include multiple layers. 
     Support layer  4  is provided between piezoelectric substrate  1  and cover layer  5 , and defines a hollow portion  11  that accommodates surface acoustic wave resonator  2 , between piezoelectric substrate  1  and cover layer  5 . Specifically, in order to provide hollow portion  11 , support layer  4  that surrounds a region in which surface acoustic wave resonator  2  is provided on primary surface  1   a.    
     As shown in  FIG. 1 , a portion of support layer  4  is located on the outer periphery of piezoelectric substrate  1 . Due to the portion of support layer  4 , hollow portion  11  is provided within the outer periphery of piezoelectric substrate  1 . In the following description, the portion of support layer  4  that is located on the outer periphery of piezoelectric substrate  1  will be also referred to as “support layer  4 ”. 
     Support layer  4  is preferably made of an insulating material, and, for example, a resin or an insulating ceramic can be used. Support layer  4  is preferably made of an insulating material (e.g., polyimide) that has excellent water resistance to prevent ingress of water into surface acoustic wave filter  100 . Support layer  4  and cover layer  5  form an insulation material layer. 
     Line electrode  7  is provided on a second surface of cover layer  5  which is a surface opposite a first surface facing piezoelectric substrate  1 . Line electrode  7  is connected to surface acoustic wave resonator  2  by a penetrating electrode not shown. Line electrode  7  defines a line pattern, for example, an inductor. 
     Connection electrode  6  is provided on the second surface of cover layer  5 , extends to piezoelectric substrate  1  along side surfaces of cover layer  5  and side surfaces of support layer  4 , and is connected to line  3  provided on piezoelectric substrate  1 . 
     Protective layer  10 , sealing support layer  4 , cover layer  5 , connection electrode  6 , and line electrode  7  are provided on primary surface  1   a  of piezoelectric substrate  1 . Protective layer  10  preferably has a rectangular or substantially rectangular shape that is the same or substantially the same as primary surface  1   a . Protective layer  10  is preferably made of an insulating material, and, for example, a resin such as epoxy or polyimide, or an insulating ceramic such as silicon oxide (SiO 2 ) or Al 2 O 3  can be used. Protective layer  10  may be made of the same or similar material as cover layer  5 . 
     External connection terminal  9  is provided on protective layer  10 . External connection terminal  9  is preferably, for example, a flat pad. The pad is a thin film preferably including an electrically conductive material, for example, Ti, Cu, Ni, or Au, etc. The pad may include multiple thin films. External connection terminal  9  overlaps with at least a portion of connection electrode  6  and a portion of line electrode  7  in plan view. 
     When mounting surface acoustic wave filter  100  onto a circuit board, external connection terminal  9  is connected to a line provided on the circuit board. External connection terminal  9  may be a bump. The bump is a metal bump, for example, a solder bump or an Au bump. Note that even when the external connection terminal is a bump, the bump overlaps with at least a portion of connection electrode  6  and a portion of line electrode  7  in plan view. 
     As shown in  FIG. 1 , pillar  8  extends through protective layer  10  in the thickness direction. One end portion of pillar  8  in the thickness direction of protective layer  10  is connected to external connection terminal  9 , and the other end portion of pillar is connected to connection electrode  6  or line electrode  7 . Pillar  8  is preferably made of a conductive material, for example, Ti or Cu. Pillar  8  may be formed of the same or similar conductive material as connection electrode  6 , line electrode  7 , and external connection terminal  9 , or formed of any other conductive material. 
     Pillar  8  electrically connects external connection terminal  9 , connection electrode  6 , and line electrode  7 . Pillar overlaps with external connection terminal  9 , connection electrode  6 , and line electrode  7  in plan as viewed in the thickness direction of protective layer  10 . 
     Surface acoustic wave filter  100  shown in  FIG. 1  is mounted on a circuit board while external connection terminal  9  is abutting a line provided on the circuit board. In other words, surface acoustic wave filter  100  is flip-chip mounted on the circuit board. Surface acoustic wave filter  100  and the circuit board define a “module” according to the present invention. In the first preferred embodiment, the module is preferably a WLP surface acoustic wave filter. 
     In surface acoustic wave filter  100 , protective layer  10  includes a first portion  10   a  located above the hollow portion  11  (corresponding to “region I” in the figure) and a second portion  10   b  located above the support layer  4  (corresponding to “region II” in the figure). In other words, second portion  10   b  of protective layer  10  is located on the outer periphery of piezoelectric substrate  1 . 
     Where d2 is a distance, in second portion  10   b  of protective layer  10 , between the surface (primary surface  1   a ) of piezoelectric substrate land the surface of protective layer  10  in the thickness direction of piezoelectric substrate  1 , second portion  10   b  is disposed so that distance d2 is greatest at a location where second portion  10   b  is adjacent to or in a vicinity of first portion  10   a , and distance d2 is shortest at a location where second portion  10   b  is adjacent to or in a vicinity of an end portion of piezoelectric substrate  1  in the width direction of piezoelectric substrate  1 . 
     In surface acoustic wave filter  100  shown in  FIG. 1 , distance d2 decreases toward the end portions of piezoelectric substrate  1  in the width direction of piezoelectric substrate  1 . In other words, the surface of second portion  10   b  is inclined from primary surface  1   a  of piezoelectric substrate  1  so that distance d2 is maximum at a location where second portion  10   b  is adjacent to or in a vicinity of region I and distance d2 is minimum at a location where second portion  10   b  is furthest away from region I. 
     Protective layer  10  further includes a curved surface in an end portion  10   c  in the width direction of piezoelectric substrate  1 , the curved surface being convex in the opposite direction from the piezoelectric substrate  1 . Accordingly, the end portions of the surface of protective layer  10  are not sharply squared. 
     Moreover, where d1 is a distance in first portion  10   a  of protective layer  10 , between the surface (primary surface  1   a ) of piezo electric substrate  1  and the surface of protective layer  10  in the thickness direction of piezoelectric substrate  1 , the maximum value of distance d1 is greater than the maximum value of distance d2. According to this, the surface of first portion  10   a  is inclined from primary surface  1   a  of piezoelectric substrate  1 . 
     More preferably, the surface of first portion  10   a  of protective layer  10  is convexly curved in the opposite direction from the piezoelectric substrate  1 . According to this, the entirety or substantially the entirety of protective layer  10 , including first portion  10   a  and second portion  10   b , has a curved surface that is convex outwardly. 
     Next, a problem with a Comparative Example and advantageous effects of surface acoustic wave filter  100  according to the first preferred embodiment will be described, with reference to Comparative Example of the first preferred embodiment shown in  FIGS. 2 and 3 . 
       FIG. 2  is a cross-sectional view of surface acoustic wave filter  200  according to Comparative Example, which is compared to  FIG. 1 . 
     Referring to  FIG. 2 , a surface acoustic wave filter  200  according to Comparative Example has similar features as surface acoustic wave filter  100  according to the first preferred embodiment shown in  FIG. 1 , except that surface acoustic wave filter  200  according to Comparative Example includes a protective layer  12 , instead of protective layer  10  included in surface acoustic wave filter  100 . 
     Specifically, in surface acoustic wave filter  200 , protective layer  12  includes a first portion  12   a  located above a hollow portion  11  (a region I), and a second portion  12   b  located above a support layer  4  (a region II). In other words, second portion  12   b  of protective layer  12  is located on the outer periphery of piezoelectric substrate  1 . 
     In second portion  12   b , distance d2 between the surface (primary surface  1   a ) of piezoelectric substrate  1  and the surface of protective layer  12  in the thickness direction of piezoelectric substrate  1  is constant or substantially constant in the width direction of piezoelectric substrate  1 . Stated differently, the surface of second portion  12   b  is in parallel with or substantially in parallel with primary surface  1   a  of piezoelectric substrate  1 . For this reason, the surfaces of the end portions of protective layer  12  in the width direction of piezoelectric substrate  1  have square or substantially square shapes. 
     Moreover, in first portion  12   a  of protective layer  12 , distance d1 between the surface (primary surface  1   a ) of piezoelectric substrate  1  and a surface of protective layer  12  in the thickness direction of piezoelectric substrate  1  is constant or substantially constant in the width direction of piezoelectric substrate  1 . Stated differently, the surface of first portion  12   a  is in parallel with or substantially in parallel with primary surface  1   a  of piezoelectric substrate  1 . In other words, in the entirely of protective layer  12 , the distance between the surface (primary surface  1   a ) of piezoelectric substrate  1  and the surface of protective layer  12  in the thickness direction of piezo electric substrate  1  is constant or substantially constant in the width direction of piezoelectric substrate  1 . 
     Surface acoustic wave filter  200  according to Comparative Example is flip-chip mounted on a circuit board  300 , as shown in  FIG. 3 . In flip-chip mounting, surface acoustic wave filter  200  is picked up by suction of the rear surface of surface acoustic wave filter  200  (corresponding to the primary surface of piezoelectric substrate  1  opposite the primary surface  1   a ) by a mounter nozzle  350 , and surface acoustic wave filter  200  is transferred over the circuit board  300 . Surface acoustic wave filter  200  is then placed down onto circuit board  300 , bringing external connection terminal  9  into contact with circuit board  300 , thus mounting surface acoustic wave filter  200  in a predetermined location on circuit board  300 . 
     Here, when bringing surface acoustic wave filter  200  into contact with circuit board  300 , a stress F 1  is applied to circuit board  300  downwardly in the thickness direction of surface acoustic wave filter  200 . Surface acoustic wave filter  200  is subjected to a stress F 2  as are action force against stress F 1 . 
     At this time, circuit board  300  may be inclined from primary surface  1   a  of piezoelectric substrate  1 . If surface acoustic wave filter  200  is mounted on circuit board  300  while circuit board  300  is inclined, the end portion of a surface of protective layer  12  in the width direction thereof may come into contact with circuit board  300 , as shown in  FIG. 3 . For this reason, stress F 2  is applied by circuit board  300  to the end portion of the surface of protective layer  12 . 
     In Comparative Example, since the end portions of the surface of protective layer  12  are square or substantially square, and thus stress F 2  is concentrated onto the end portions. Thus, Comparative Example has the potential of causing damage to protective layer  12  at the end portions, for example, cracks and chips. As a portion of protective layer  12  is damaged, water or gas may enter protective layer  12  through the damaged portion, and connection electrode  6  and line electrode  7 , etc., may be subjected to oxidation and corrosion. As a result, there is a concern that this may lead to degradation in filter characteristics of surface acoustic wave filter  200 . 
     Moreover, in Comparative Example, circuit board  300  is required to have high coplanarity in order to prevent the damage to protective layer  12 , which complicates the mount process. 
       FIG. 4  shows how surface acoustic wave filter  100  according to the first preferred embodiment is flip-chip mounted onto circuit board  300 . In  FIG. 4 , circuit board  300  is inclined, as with  FIG. 3 . For this reason, an end portion of the surface of protective layer  10  comes into contact with circuit board  300 . 
     In surface acoustic wave filter  100  according to the first preferred embodiment, the surface of second portion  10   b  is inclined from primary surface  1   a  of piezoelectric substrate  1  so that distance d2 is greatest at a location where second portion  10   b  is adjacent to or in a vicinity of region I and distance d2 is shortest at a location where second portion  10   b  is farthest away from region I. Furthermore, the surface of end portion  10   c  of protective layer  10  is convexly curved in the opposite direction from piezoelectric substrate  1 , that is, end portion  10   c  is not sharply squared. For this reason, stress F 3  by circuit board  300  is distributed and act on protective layer  10 . Accordingly, the potential of damaging the end portion of protective layer  10  can be reduced. For this reason, oxidation and corrosion of connection electrode  6  and line electrode  7  resulting from the damage to protective layer  10  can be reduced, and, consequently, degradation in the filter characteristics can be reduced. 
     Moreover, since circuit board  300  is not required to have high coplanarity for mounting surface acoustic wave filter  100  onto circuit board  300 , the mounting process is not complicated. 
     Note that while the first preferred embodiment has been described with reference to distance d2 in second portion  10   b  of protective layer  10  reducing toward the end portion of piezoelectric substrate  1  in the width direction of piezoelectric substrate  1 , the present application is not limited to the above configuration, insofar as distance d2 is greatest at the location where second portion  10   b  is adjacent to or in a vicinity of first portion  10   a  and distance d2 is shortest at the location where second portion  10   b  is adjacent to or in a vicinity of the end portion of piezoelectric substrate  1  in the width direction of piezoelectric substrate  1 , as described above. Accordingly, surface acoustic wave filter  100  according to the present invention also includes an implementation in which, for example, protective layer  10  has a surface that recedes into piezoelectric substrate  1  between the end portion of piezoelectric substrate  1  in the width direction of piezoelectric substrate  1  and the location where second portion  10   b  is adjacent or in a vicinity of to first portion  10   a.    
     Furthermore, in surface acoustic wave filter  100  according to the first preferred embodiment, the following conditions are met for the shape of protective layer  10 . 
       FIGS. 5A and 5B  are cross-sectional views of surface acoustic wave filter  100  according to the first preferred embodiment. For ease of description,  FIGS. 5A and 5B  show only piezoelectric substrate  1 , support layer  4 , cover layer  5 , and protective layer  10  of surface acoustic wave filter  100 . 
       FIG. 5A  is a cross-sectional view of surface acoustic wave filter  100  with no external stress applied to protective layer  10 .  FIG. 5B  is across-sectional view of surface acoustic wave filter  100  with external stress applied to protective layer  10 . 
     As described with reference to  FIG. 1 , the entirety of protective layer  10 , including first portion  10   a  and second portion  10   b , has a curved surface that is convex outwardly. In  FIG. 5A , the distance between the surface of cover layer  5  and the surface of protective layer  10  in the thickness direction of piezoelectric substrate  1  is defined as a “thickness of protective layer  10 .” The thickness of protective layer  10  reduces from the center region of first portion  10   a  toward second portion  10   b.    
     In  FIG. 5A , t1 denotes the maximum thickness in first portion  10   a  (the maximum distance in first portion  10   a , between the surface of cover layer  5  and the surface of protective layer  10  in the thickness direction of piezoelectric substrate  1 ), and t2 denotes the maximum thickness in second portion  10   b  (the maximum distance in second portion  10   b , between the surface of cover layer  5  and the surface of protective layer  10  in the thickness direction of piezoelectric substrate  1 ). Moreover, X denotes the minimum distance in hollow portion  11 , between the surface of cover layer  5  facing hollow portion  11  (a surface opposite the surface facing protective layer  10 ) and the surfaces of surface acoustic wave resonator  2  and line  3 , in the thickness direction of piezoelectric substrate  1 . 
     Here, when mounting surface acoustic wave filter  100  onto circuit board  300 , surface acoustic wave filter  100  is pushed against circuit board  300 . Thus, protective layer  10  receives a pressure from circuit board  300  in the thickness direction, as a reaction force. 
     At this time, upon receipt of the pressure from circuit board  300 , protective layer  10  is pushed, preferentially, from the central region of first portion  10   a . Initially, the contact area between circuit board  300  and the surface of protective layer  10  is small, and deformation of protective layer  10  is also small. However, as the pressure increases, protective layer  10  is further deformed, increasing the contact area between circuit board  300  and the surface of protective layer  10 . 
     At this time, the rear surface of protective layer  10  deforms convexly in the direction to piezoelectric substrate  1 . As the rear surface of protective layer  10  deforms, cover layer  5  deforms convexly in the direction to piezoelectric substrate  1 . 
     Thereafter, as the protective layer  10  deforms to the border between first portion  10   a  and second portion  10   b , protective layer  10  becomes less likely to deform, as shown in  FIG. 5B . This is because the end portions of protective layer  10  in the thickness direction are supported by support layer  4 , and the rigidity of protective layer  10  is increased by a compressive stress caused in protective layer  10 . 
     In  FIG. 5B , the upper limit of the distance between the highest point and the lowest point of the convexly curved rear surface of cover layer  5  in the thickness direction thereof can be represented as a difference (t1−t2) between the maximum thickness t1 of first portion  10   a  and maximum thickness t2 of second portion  10   b.    
     Note that the highest point refers to a region of the convex rear surface of cover layer  5  that is closest to piezoelectric substrate  1  (e.g., the central region of the convexly curved rear surface). The lowest point refers to a region of the convex rear surface of cover layer  5  that is farthest away from piezoelectric substrate  1  (e.g., end regions of the convexly curved rear surface). 
     Here, in surface acoustic wave filter  100 , maximum thickness t1 of first portion  10   a , maximum thickness t2 of second portion  10   b , and minimum distance X of hollow portion  11  satisfy the relational expression expressed as (t1−t2)&lt;X. 
     In this way, a gap is provided between the rear surface of cover layer  5  and the surface of surface acoustic wave resonator  2  or line  3  even if protective layer  10  is deformed farthest. This can prevent the rear surface of cover layer  5  from contacting with surface acoustic wave resonator  2  or line  3 , and thus prevent damage to surface acoustic wave resonator  2  and line  3 . Accordingly, the deterioration can be reduced in the filter characteristics of surface acoustic wave filter  100 . 
     Note that line electrode  7  is provided between first portion  10   a  of protective layer  10  and cover layer  5 , as shown in  FIG. 1 . This allows protective layer  10  to have increased rigidly against the pressure from circuit board  300 . 
     Variation of First Preferred Embodiment 
     Next, referring to  FIG. 6 , surface acoustic wave filter  100  according to a variation of the first preferred embodiment will be described. 
     As shown in  FIG. 6 , surface acoustic wave filter  100  according to the variation has similar features as surface acoustic wave filter  100  shown in  FIG. 1 , except that surface acoustic wave filter  100  according to the variation includes a penetrating electrode  6 A, instead of connection electrode  6 . 
     In surface acoustic wave filter  100  according to the present variation, penetrating electrode  6 A is provided through cover layer  5  and support layer  4 . Penetrating electrode  6 A and pillar  8  electrically connect external connection terminal  9  and line  3  which are respectively provided on protective layer  10  and piezoelectric substrate  1 . 
     Even though line  3  and external connection terminal  9  are electrically connected using penetrating electrode  6 A as in the present variation, the surface of second portion  10   b  of protective layer  10  is inclined from primary surface  1   a  of piezoelectric substrate  1  so that distance d2 is greatest at a location where second portion  10   b  is adjacent to or in a vicinity of region I and distance d2 is shortest at a location where second portion  10   b  is farthest away from region I, similarly to the first preferred embodiment described above. Furthermore, end portion  10   c  of protective layer  10  has a curved surface that is convex in the opposite direction from piezoelectric substrate  1 . Due to this, the potential of damaging protective layer  10  can be reduced. Accordingly, oxidation and corrosion of an external connection line resulting from the damage to protective layer  10  can be reduced, and, consequently, degradation in the filter characteristics can be reduced. 
     Second Preferred Embodiment 
     Referring to  FIG. 7 , an electronic device  100  according to a second preferred embodiment of the present invention will be described. 
     Referring to  FIG. 7 , a surface acoustic wave filter  100  according to the second preferred embodiment has similar features as surface acoustic wave filter  100  shown in  FIG. 1 , except for the features of a protective layer  10 . 
     Protective layer  10  of surface acoustic wave filter  100  shown in  FIG. 7  has a first portion  10   a  having a flat surface. Note that first portion  10   a  including a flat surface includes distance d1, in first portion  10   a , between the surface (primary surface  1   a ) of a piezoelectric substrate  1  and a surface of protective layer  10  in the thickness direction of piezoelectric substrate  1 , as being not only constant, but also substantially constant in the width direction of piezoelectric substrate  1 . Distance d1 as being substantially constant in the width direction of piezoelectric substrate  1  means that distance d1 varies within an extent of error (e.g., within plus or minus 10%). 
     The surface of second portion  10   b  of protective layer  10  in surface acoustic wave filter  100  according to the second preferred embodiment is also inclined from primary surface  1   a  of piezoelectric substrate  1  so that distance d2 is maximum at a location where second portion  10   b  is adjacent to or in a vicinity of region I and distance d2 is minimum at a location where second portion  10   b  is farthest away from region I. Furthermore, end portion  10   c  of protective layer  10  has a curved surface that is convex in the opposite direction from piezoelectric substrate  1 . Accordingly, similarly to surface acoustic wave filter  100  according to the first preferred embodiment, the potential of damaging protective layer  10  can be reduced. Accordingly, oxidation and corrosion of a connection electrode  6  and a line electrode  7  resulting from the damage to protective layer  10  can be reduced, and, consequently, degradation in the filter characteristics can be reduced. 
     Third Preferred Embodiment 
     Referring to  FIG. 8 , an electronic device  100  according to a third preferred embodiment of the present invention will be described. 
     Referring to  FIG. 8 , electronic device  100  according to the third preferred embodiment basically has similar features as surface acoustic wave filter  100  shown in  FIG. 1 , except for the features of a cover layer  5 . 
     In surface acoustic wave filter  100  shown in  FIG. 8 , cover layer  5  includes a curved portion that is convex in the opposite direction from a piezoelectric substrate  1 . 
     While  FIG. 8  shows cover layer  5  as having a single curved portion, it should be noted that cover layer  5  may have multiple curved portions. For example, in an implementation in which a support layer  4  is provided in a hollow portion  11 , cover layer  5  is divided into two portions at the connection between support layer  4  and cover layer  5 , one portion being on one side in the width direction, the other portion being on the other side in the width direction, each of which portions is curved convexly in the opposite direction from piezoelectric substrate  1 . 
     Surface acoustic wave filter  100  according to the third preferred embodiment includes a second portion  10   b  of a protective layer  10  which has the same or similar features as that of surface acoustic wave filter  100  according to the first preferred embodiment. Accordingly, surface acoustic wave filter  100  according to the third preferred embodiment can provide the same or similar advantageous effects as those of surface acoustic wave filter  100  according to the first preferred embodiment. 
     Fourth Preferred Embodiment 
     Referring to  FIGS. 9 and 10 , an electronic device  100  according to a fourth preferred embodiment of the present invention will be described. 
     Referring to  FIG. 9 , a surface acoustic wave filter  100  according to the fourth preferred embodiment has similar features as surface acoustic wave filter  100  shown in  FIG. 1 , except for the features of a protective layer  10 . 
     In surface acoustic wave filter  100  shown in  FIG. 9 , protective layer  10  includes a first curved portion  10   d  at an end portion of piezoelectric substrate  1  in the width direction, and a second curved portion  10   e  between first curved portion  10   d  and an end surface of piezoelectric substrate  1  in the width direction. First curved portion  10   d  is convex in the opposite direction from piezoelectric substrate  1 . In contrast, second curved portion  10   e  is concave in the direction to piezoelectric substrate  1 . 
     Compared to surface acoustic wave filter  100  shown in  FIG. 1 , surface acoustic wave filter  100  according to the fourth preferred embodiment has an increased bond area between protective layer  10  and primary surface  1   a  of piezoelectric substrate  1 , due to second curved portion  10   e . However, since the thickness of second curved portion  10   e  decreases toward the end portion of piezoelectric substrate  1 , protective layer  10 , which covers the end portion of piezoelectric substrate  1  in the width direction, is moderated from significantly increasing in volume that is due to the provision of second curved portion  10   e.    
     According to surface acoustic wave filter  100  of the fourth preferred embodiment, the bond area between primary surface  1   a  and protective layer  10  at the end portion of piezoelectric substrate  1  in the width direction can be increased while preventing an increase in volume of protective layer  10 . An increase in the bond area between primary surface  1   a  and protective layer  10  increases the bonding strength, thus preventing protective layer  10  from being de bonded from primary surface  1   a  when the end portion of the surface of protective layer  10  in the width direction comes into contact with the circuit board  300 . 
       FIG. 10  shows a variation of surface acoustic wave filter  100  according to the fourth preferred embodiment. Also in the present variation, protective layer  10  includes a first curved portion  10   d  at an end portion of piezoelectric substrate  1  in the width direction, and a second curved portion  10   e  between first curved portion  10   d  and an end surface of piezoelectric substrate  1  in the width direction. Thus, the present variation of surface acoustic wave filter  100  according to the fourth preferred embodiment can provide the same or similar advantageous effects as those of surface acoustic wave filter  100  shown in  FIG. 9 . 
     Fifth Preferred Embodiment 
     Referring to  FIGS. 11 and 12 , an electronic device  100  according to a fifth preferred embodiment will be described. 
     Referring to  FIG. 11 , a surface acoustic wave filter  100  according to the fifth preferred embodiment has similar features as surface acoustic wave filter  100  according to the variation of the first preferred embodiment shown in  FIG. 6 , except for the features of a support layer  4  and a protective layer  10 . 
     In surface acoustic wave filter  100  shown in  FIG. 11 , a penetrating electrode  6 A is provided through a cover layer  5  and a support layer  4 . An end surface of support layer  4  in the width direction is located outward of the end surface of protective layer  10  in the width direction (on the end surface side of piezoelectric substrate  1  in the width direction). Note that the end surface of protective layer  10  in the width direction is located outward of the end surface of cover layer  5  in the width direction. 
     Compared to surface acoustic wave filter  100  shown in  FIG. 6 , surface acoustic wave filter  100  according to the fifth preferred embodiment includes end portion  10   c  of protective layer  10  in the width direction located farther away from the end surface of piezoelectric substrate  1  in the width direction. Thus, when mounting surface acoustic wave filter  100  onto circuit board  300  (see  FIG. 4 ), the potential of the surface of end portion  10   c  of protective layer  10  coming into contact with circuit board  300 , can be reduced. Accordingly, the potential of damaging protective layer  10  can further be reduced. 
       FIG. 12  is a diagram showing a variation of surface acoustic wave filter  100  according to the fifth preferred embodiment. Also in the present variation, the end surface of support layer  4  in the width direction is located outward of the end surface of protective layer  10  in the width direction. Note that  FIG. 12  shows the end surface of cover layer  5  in the width direction that is located outward of the end surface of protective layer  10  in the width direction. The variation can also yield the same or similar advantageous effects as those of surface acoustic wave filter  100  shown in  FIG. 11 . 
     Sixth Preferred Embodiment 
     Referring to  FIG. 13 , an electronic device  100  according to a sixth preferred embodiment of the present invention will be described. 
     Referring to  FIG. 13 , a surface acoustic wave filter  100  according to the sixth preferred embodiment has similar features as surface acoustic wave filter  100  shown in  FIG. 1 , except for the features of a protective layer  10 . 
     In surface acoustic wave filter  100  shown in  FIG. 13 , end portion  10   c  of protective layer  10  does not have a curved surface that is convex in the opposite direction from piezoelectric substrate  1 . However, the surface of second portion  10   b  of protective layer  10  in surface acoustic wave filter  100  according to the sixth preferred embodiment is also inclined from primary surface  1   a  of piezoelectric substrate  1  so that distance d2 is maximum at a location where second portion  10   b  is adjacent to or in a vicinity of region I and distance d2 is minimum at a location where second portion  10   b  is farthest away from region I. Accordingly, similarly to surface acoustic wave filter  100  according to the first preferred embodiment, the potential of damaging protective layer  10  can be reduced. Accordingly, oxidation and corrosion of a connection electrode  6  and a line electrode  7  resulting from the damage to protective layer  10  can be reduced, and, consequently, degradation in the filter characteristics can be reduced. 
     The presently disclosed embodiments should be considered in all aspects as illustrative and not restrictive. The scope of the present invention is defined by the appended claims, rather than by the description above. All changes which come within the meaning and range of equivalency of the appended claims are to be embraced within their scope. 
     While preferred embodiments of the present 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 from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.