Acoustic wave device and method of fabricating the same, filter, and multiplexer

An acoustic wave device comprising:

• a support substrate;
• a single piezoelectric substrate that is located on the support substrate and is single-crystal;
• first electrodes located on a first surface of the piezoelectric substrate;
• second electrodes located on a second surface of the piezoelectric substrate;
• an acoustic mirror that is located on the support substrate, is located between the support substrate and the first electrodes in resonance regions where the first electrodes and the second electrodes face each other across at least a part of the piezoelectric substrate, is not located between the support substrate and the first electrodes in at least a part of a region between the resonance regions, reflects an acoustic wave propagating through the piezoelectric substrate, and includes a first film and a second film alternately stacked, an acoustic impedance of the second film being higher than an acoustic impedance of the first film,
• an insulator film that is located between the support substrate and the piezoelectric substrate in the at least a part of the region between the resonance regions, is made of a different material from the acoustic mirror, and has a Young's modulus less than Young's modulus of the first film and the second film.

An acoustic wave device comprising:

• a support substrate;
• a single piezoelectric substrate that is located on the support substrate and is single-crystal;
• first electrodes located on a first surface of the piezoelectric substrate;
• second electrodes located on a second surface of the piezoelectric substrate;
• and
• an acoustic mirror that is located on the support substrate, is located between the support substrate and the first electrodes in resonance regions where the first electrodes and the second electrodes face each other across at least a part of the piezoelectric substrate, is not located between the support substrate and the first electrodes in at least a part of a region between the resonance regions, and reflects an acoustic wave propagating through the piezoelectric substrate,
• wherein the first surface in a first region where the first electrodes are located and the first surface in a second region where the first electrodes are not located are flat.