Aspects of this disclosure relate to a bulk acoustic wave device with a plurality of piezoelectric layers having at least one polarization inversion. The bulk acoustic wave device can include a plurality of stacked piezoelectric layers. The plurality of stacked piezoelectric layers can include a piezoelectric layer formed by atomic layer deposition. The bulk acoustic wave device can excite an overtone mode as a main mode. Related filters, multiplexers, packaged radio frequency modules, radio frequency front ends, wireless communication devices, and methods are disclosed.

A bulk acoustic wave resonator having a central region, an outer region, and a raised frame region between the central region and the outer region is disclosed. The bulk acoustic wave resonator can include a piezoelectric layer and a top electrode over the piezoelectric layer. The top electrode is disposed at least in the central region, the outer region, and the raised frame region, the top electrode including a first layer and a second layer. A material of the first layer is different from the material of the second layer.

A tuning fork-type vibration piece is provided, in which a cushioning portion is formed on the base of a package and allowed to contact parts for contact of arm portions which are any parts but their edges, and the parts for contact of the arm portions that contact the cushioning portion are electrodeless regions, which prevents the risk of frequency fluctuations caused by any electrode being chipped off by contact with the cushioning portion.

A tuning fork-type vibration piece is provided, in which a cushioning portion is formed on the base of a package and allowed to contact parts for contact of arm portions which are any parts but their edges, and the parts for contact of the arm portions that contact the cushioning portion are electrodeless regions, which prevents the risk of frequency fluctuations caused by any electrode being chipped off by contact with the cushioning portion.

A piezoelectric stack, including: a substrate; an electrode film; and a piezoelectric film comprising an alkali niobium oxide of a perovskite structure represented by a composition formula of (K.sub.1-xNa.sub.x)NbO.sub.3 (0<x<1), wherein an average light transmittance through the piezoelectric film in a wavelength region of visible light and near-infrared ray is 65% or more.

A bulk acoustic resonator includes a substrate, a frequency control layer changing a resonant frequency or antiresonant frequency of the bulk acoustic resonator according to a thickness of the frequency control layer, a piezoelectric layer disposed between the frequency control layer and the substrate, a first electrode disposed between the piezoelectric layer and the substrate, a second electrode disposed between the piezoelectric layer and the frequency control layer, a metal layer connected to the first electrode or the second electrode, and a protective layer disposed between the second electrode and the frequency control layer, wherein the frequency control layer covers a larger area than that of the protective layer.

An electronic device that includes a base substrate having a mounting surface; an electronic component having a mechanical vibration portion mounted on the mounting surface of the base substrate; an intermediate layer mounted on the base substrate and forming an internal space with the base substrate so as to accommodate the electronic component therein, the intermediate layer having at least one through-hole that opens the internal space to an outside; and a sealing layer on the intermediate layer and sealing the internal space by closing the at least one through-hole.

An electronic device that includes a base substrate having a mounting surface; an electronic component having a mechanical vibration portion mounted on the mounting surface of the base substrate; an intermediate layer mounted on the base substrate and forming an internal space with the base substrate so as to accommodate the electronic component therein, the intermediate layer having at least one through-hole that opens the internal space to an outside; and a sealing layer on the intermediate layer and sealing the internal space by closing the at least one through-hole.

An elastic wave device includes a piezoelectric layer including a first main surface and a second main surface facing the first main surface, an acoustically reflective layer stacked on the first main surface of the piezoelectric layer, an excitation electrode disposed on the piezoelectric layer, and a support layer. The acoustically reflective layer overlaps at least the excitation electrode in a plan view of the piezoelectric layer from the side of the second main surface. The support layer surrounds the acoustically reflective layer in a plan view of the piezoelectric layer from the side of the second main surface.

An elastic wave device includes a piezoelectric layer including a first main surface and a second main surface facing the first main surface, an acoustically reflective layer stacked on the first main surface of the piezoelectric layer, an excitation electrode disposed on the piezoelectric layer, and a support layer. The acoustically reflective layer overlaps at least the excitation electrode in a plan view of the piezoelectric layer from the side of the second main surface. The support layer surrounds the acoustically reflective layer in a plan view of the piezoelectric layer from the side of the second main surface.