Surface acoustic wave device and composite module including same
A surface acoustic wave device includes a piezoelectric substrate including an interdigital transducer located on one principal surface thereof, an insulating layer arranged around the interdigital transducer, a cover layer arranged over the insulating layer and the interdigital transducer, and a columnar electrode penetrating through the insulating layer and the cover layer, and connected to the interdigital transducer through a connection wiring, wherein a buffer layer is arranged on the other principal surface of the piezoelectric substrate.
1. A surface acoustic wave device comprising:
a piezoelectric substrate including an interdigital transducer located on one principal surface thereof;
an insulating layer arranged around the interdigital transducer;
a cover layer arranged over the insulating layer and the interdigital transducer; and
a columnar electrode penetrating through the insulating layer and the cover layer, and connected to the interdigital transducer through a connection wiring; wherein
a buffer layer is disposed on the other principal surface of the piezoelectric substrate;
the buffer layer includes a plurality of layers having different acoustic impedances from each other; and
each of the plurality of layers contains irregularities.
2. The surface acoustic wave device according to claim 1 , wherein at least one of the plurality of layers of the buffer layer is made of resin, and a filler having a smaller coefficient of linear expansion than the resin is contained in the resin.
3. The surface acoustic wave device according to claim 1 , wherein the plurality of layers having different acoustic impedances from each other are configured such that the layers having coefficients of linear expansion successively closer to a coefficient of linear expansion of the piezoelectric substrate are positioned successively closer to the piezoelectric substrate and the layer having the coefficient of linear expansion closest to the coefficient of linear expansion of the piezoelectric substrate is positioned closest to the piezoelectric substrate.
4. The surface acoustic wave device according to claim 1 , wherein the buffer layer is arranged to cover only the interdigital transducer when looking at the piezoelectric substrate in a plan view.
5. The surface acoustic wave device according claim 1 , wherein irregularities are provided in the other principal surface of the piezoelectric substrate, and the buffer layer is arranged on the other principal surface including the irregularities.
6. The surface acoustic wave device according to claim 5 , wherein the irregularities provided in the other principal surface of the piezoelectric substrate are defined by roughened surface portions of the piezoelectric substrate.
7. The surface acoustic wave device according to claim 1 , wherein the irregularities in at least one of the plurality of layers of the buffer layer are provided in a surface thereof on a side opposite to the piezoelectric substrate.
8. The surface acoustic wave device according to claim 7 , wherein the irregularities are defined by roughened surface portions.
9. The surface acoustic wave device according to claim 1 , wherein the plurality of layers having different acoustic impedances from each other have different sizes.
10. A composite module comprising:
a substrate; and
the surface acoustic wave device according to claim 1 mounted on the substrate.
11. The composite module according to claim 10 , further comprising an additional one of the surface acoustic wave device mounted on the substrate.
12. The composite module according to claim 10 , wherein the substrate is one of a ceramic multilayer substrate and a resin multilayer substrate.
13. A mobile communication device comprising the composite module according to claim 10 .
14. A mobile communication device comprising the surface acoustic wave device according to claim 1 .