The multiplexer includes a plurality of IDT electrodes on a substrate, an insulating cover located on the substrate so as to configure one or more spaces above the plurality of IDT electrodes, an antenna terminal, transmission terminal, and reception terminal which are all located on the substrate and pass through the cover, and a reinforcing layer which is located on the cover and is made of metal. By the plurality of IDT electrodes, a transmission filter located in a signal path connecting the antenna terminal and the transmission terminal and a receiving filter located in a signal path connecting the antenna terminal and the reception terminal. The reinforcing layer includes a first area part facing the transmission filter and a second area part which faces the receiving filter and is separated from the first area part.

The multiplexer includes a plurality of IDT electrodes on a substrate, an insulating cover located on the substrate so as to configure one or more spaces above the plurality of IDT electrodes, an antenna terminal, transmission terminal, and reception terminal which are all located on the substrate and pass through the cover, and a reinforcing layer which is located on the cover and is made of metal. By the plurality of IDT electrodes, a transmission filter located in a signal path connecting the antenna terminal and the transmission terminal and a receiving filter located in a signal path connecting the antenna terminal and the reception terminal. The reinforcing layer includes a first area part facing the transmission filter and a second area part which faces the receiving filter and is separated from the first area part.

A surface acoustic wave device includes a piezoelectric single crystal substrate and an electrode. The piezoelectric single crystal substrate is made of LiTaO.sub.3 or LiNbO.sub.3. The electrode includes a titanium film formed on the piezoelectric single crystal substrate and an aluminum film or a film containing aluminum as a main component. The aluminum film or the film is formed on the titanium film. The aluminum film or the film containing aluminum as the main component is a twin crystal film or a single crystal film, the aluminum film or the film has a (111) plane that is non-parallel to a surface of the piezoelectric single crystal substrate with an angle θ, and the aluminum film or the film has a [−1, 1, 0] direction parallel to an X-direction of a crystallographic axis of the piezoelectric single crystal substrate.

An acoustic wave device includes a substrate, a multilayer film on the substrate, an LT layer configured by a single crystal of LiTaO.sub.3 on the multilayer film, and an IDT electrode on the LT layer. The thickness of the LT layer is 0.3λ or less where λ is two times a pitch p of electrode fingers in the IDT electrode. Euler angles of the LT layer are (0°±20°, −5° to 65°, 0°±10°), (−120°±20°, −5° to 65°, 0°±10°), or (120°±20°, −5° to 65°, 0°±10°). The multilayer film configured by alternately stacking at least one first layer and at least one second layer. The first layer is comprised of SiO.sub.2. The second layer is comprised of any one of Ta.sub.2O.sub.5, HfO.sub.2, ZrO.sub.2, TiO.sub.2, and MgO.

An acoustic wave device includes a substrate, a multilayer film on the substrate, an LT layer configured by a single crystal of LiTaO.sub.3 on the multilayer film, and an IDT electrode on the LT layer. The thickness of the LT layer is 0.3λ or less where λ is two times a pitch p of electrode fingers in the IDT electrode. Euler angles of the LT layer are (0°±20°, −5° to 65°, 0°±10°), (−120°±20°, −5° to 65°, 0°±10°), or (120°±20°, −5° to 65°, 0°±10°). The multilayer film configured by alternately stacking at least one first layer and at least one second layer. The first layer is comprised of SiO.sub.2. The second layer is comprised of any one of Ta.sub.2O.sub.5, HfO.sub.2, ZrO.sub.2, TiO.sub.2, and MgO.

An acoustic wave device includes a silicon support substrate that includes first and second main surfaces opposing each other, a piezoelectric structure provided on the first main surface and including the piezoelectric layer, an IDT electrode provided on the piezoelectric layer, a support layer provided on the first main surface of the silicon support substrate and surrounding the piezoelectric layer, a cover layer provided on the support layer, a through-via electrode that extending through the silicon support substrate and the piezoelectric structure, and a first wiring electrode connected to the through-via electrode and electrically connected to the IDT electrode. The piezoelectric structure includes at least one layer having an insulating property, the at least one layer including the piezoelectric layer. The first wiring electrode is provided on the layer having an insulating property in the piezoelectric structure.

An acoustic wave device includes a silicon support substrate that includes first and second main surfaces opposing each other, a piezoelectric structure provided on the first main surface and including the piezoelectric layer, an IDT electrode provided on the piezoelectric layer, a support layer provided on the first main surface of the silicon support substrate and surrounding the piezoelectric layer, a cover layer provided on the support layer, a through-via electrode that extending through the silicon support substrate and the piezoelectric structure, and a first wiring electrode connected to the through-via electrode and electrically connected to the IDT electrode. The piezoelectric structure includes at least one layer having an insulating property, the at least one layer including the piezoelectric layer. The first wiring electrode is provided on the layer having an insulating property in the piezoelectric structure.

A acoustic wave resonator comprises a piezoelectric substrate and a plurality of interdigital transducer (IDT) electrodes disposed on the piezoelectric substrate, the plurality of IDT electrodes formed of a mixture of tungsten and chromium to provide for reduction in size and increase in quality factor of the acoustic wave resonator.

An elastic wave device includes a substrate, an IDT electrode, a spacer layer, a cover, and a protective layer. The spacer layer is provided on the substrate and surrounds the IDT electrode. The cover is provided on the spacer layer, is spaced apart from the IDT electrode, and includes a first main surface adjacent to the spacer layer and a second main surface facing the first main surface. The protective layer includes a third main surface contacting the second main surface, a fourth main surface facing the third main surface, and a side surface connected to the fourth main surface. In at least portion of the side surface of the protective layer, a portion including an intersection line between the side surface and the fourth main surface is located farther inward than an outer edge of the substrate in plan view in the thickness direction of the substrate.

An elastic wave device includes a substrate, an IDT electrode, a spacer layer, a cover, and a protective layer. The spacer layer is provided on the substrate and surrounds the IDT electrode. The cover is provided on the spacer layer, is spaced apart from the IDT electrode, and includes a first main surface adjacent to the spacer layer and a second main surface facing the first main surface. The protective layer includes a third main surface contacting the second main surface, a fourth main surface facing the third main surface, and a side surface connected to the fourth main surface. In at least portion of the side surface of the protective layer, a portion including an intersection line between the side surface and the fourth main surface is located farther inward than an outer edge of the substrate in plan view in the thickness direction of the substrate.