Surface acoustic wave device and filter

1. A surface acoustic wave device comprising:

a pair of comb electrodes that are provided on a piezoelectric substrate, and that respectively has a plurality of electrode fingers, a plurality of dummy electrode fingers and a bus bar to which the plurality of the electrode fingers and the plurality of the dummy electrode fingers are connected, edges of the plurality of the electrode fingers of one of the comb electrodes respectively facing the plurality of the dummy electrode fingers of the other; and

an added film that is provided at least under the bus bar of the pair of the comb electrodes and under the plurality of the electrode fingers and the plurality of the dummy electrode fingers in a first region and is not provided in a crossing region where the plurality of the electrode fingers of the one of the comb electrodes and the plurality of the electrode fingers of the other cross each other, the first region being a region between front edges of the plurality of the dummy electrode fingers and edges of the plurality of the dummy electrode fingers connected to the bus bar and extending in an alignment direction of the plurality of the electrode fingers,

wherein a region between the electrode fingers and the dummy electrode fingers in the first region have a portion without the added film.

2. The surface acoustic wave device as claimed in claim 1 , wherein the added film is buried in a recess of an upper face of the piezoelectric substrate.

3. The surface acoustic wave device as claimed in claim 1 , wherein the added film is provided in clearances between the front edges of the plurality of the electrode fingers and corresponding front edges of the plurality of the dummy electrode fingers and extends under the plurality of the electrode fingers in a second region in which the clearances extend in the alignment direction of the plurality of the electrode fingers.

4. The surface acoustic wave device as claimed in claim 1 further comprising reflectors that are provided on both sides of the pair of the comb electrodes in the alignment direction of the plurality of the electrode fingers,

wherein the added film is also provided under bus bars of the reflectors and under electrode fingers of the reflectors in the first region.

5. The surface acoustic wave device as claimed in claim 4 , wherein the added film under the pair of the comb electrodes is separated from the added film under the reflectors.

6. The surface acoustic wave device as claimed in claim 1 , wherein the added film is made of a dielectric film including one of an SiO 2 film, a Ta 2 O 5 film, an Al 2 O 3 film, a SiN film, a TaN film and a SiC film.

7. The surface acoustic wave device as claimed in claim 1 , wherein:

the added film is made of a metal film including one of a Cu film, an Al film, a Ti film and an Au film; and

in the pair of the comb electrodes, a thickness of a metal film consisting of the added film and the bus bar in a region of the bus bar is larger than a thickness of the plurality of the electrode fingers of the crossing region, and a thickness of a metal film consisting of the added film and the electrode fingers or the dummy electrode fingers in the first region is larger than a thickness of the plurality of the electrode fingers of the crossing region.

8. A filter comprising a surface acoustic wave device having the surface acoustic wave device as claimed in claim 1 .

9. A surface acoustic wave device comprising:

a pair of comb electrodes that are provided on a piezoelectric substrate, and that respectively has a plurality of electrode fingers, a plurality of dummy electrode fingers and a bus bar to which the plurality of the electrode fingers and the plurality of the dummy electrode fingers are connected, edges of the plurality of the electrode fingers of one of the comb electrodes respectively facing the plurality of the dummy electrode fingers of the other; and

an added film that is provided at least under the bus bar of the pair of the comb electrodes and under the plurality of the electrode fingers and the plurality of the dummy electrode fingers in a first region and is not provided in a crossing region where the plurality of the electrode fingers of the one of the comb electrodes and the plurality of the electrode fingers of the other cross each other, the first region being a region between front edges of the plurality of the dummy electrode fingers and edges of the plurality of the dummy electrode fingers connected to the bus bar and extending in an alignment direction of the plurality of the electrode fingers,

wherein the added film is buried in a recess of an upper face of the piezoelectric substrate.

10. A surface acoustic wave device comprising:

a pair of comb electrodes that are provided on a piezoelectric substrate, and that respectively has a plurality of electrode fingers, a plurality of dummy electrode fingers and a bus bar to which the plurality of the electrode fingers and the plurality of the dummy electrode fingers are connected, edges of the plurality of the electrode fingers of one of the comb electrodes respectively facing the plurality of the dummy electrode fingers of the other;

an added film that is provided at least under the bus bar of the pair of the comb electrodes and under the plurality of the electrode fingers and the plurality of the dummy electrode fingers in a first region and is not provided in a crossing region where the plurality of the electrode fingers of the one of the comb electrodes and the plurality of the electrode fingers of the other cross each other, the first region being a region between front edges of the plurality of the dummy electrode fingers and edges of the plurality of the dummy electrode fingers connected to the bus bar and extending in an alignment direction of the plurality of the electrode fingers; and

reflectors that are provided on both sides of the pair of the comb electrodes in the alignment direction of the plurality of the electrode fingers,

wherein the added film is also provided under bus bars of the reflectors and under electrode fingers of the reflectors in the first region.