In a piezoelectric device, a layered portion includes, at a position at least above a recess, a single crystal piezoelectric layer and a pair of electrode layers to apply voltage to the single crystal piezoelectric layer. At least a portion of the pair of electrode layers includes a lower electrode layer extending along a surface of the single crystal piezoelectric layer, the surface being closer to a base. The lower electrode layer is present only inside the recess.

A filter circuit has a cascaded resonator circuit with a first acoustic resonator and a second acoustic resonator connected in series on a printed circuit board (PCB). The admittances as functions of frequency of the first and second acoustic resonators are substantially identical. The filter circuit also has a composite resonator circuit formed by a capacitor connected in parallel with the second acoustic resonator on the PCB. The capacitor improves a steepness of an upper bandpass edge of the filter circuit.

An acoustic wave resonator includes a resonating part disposed on and spaced apart from a substrate by a cavity, the resonating part including a membrane layer, a first electrode, a piezoelectric layer, and a second electrode that are sequentially stacked. 0 Å≤ΔMg≤170 Å may be satisfied, ΔMg being a difference between a maximum thickness and a minimum thickness of the membrane layer disposed in the cavity.

An acoustic wave device includes a support substrate including silicon, a piezoelectric layer provided directly or indirectly on the support substrate, and an interdigital transducer (IDT) electrode provided on the piezoelectric layer. When a wavelength defined by an electrode finger pitch of the IDT electrode is λ, a thickness of the piezoelectric layer is about 1λ or less. V.sub.L, which is an acoustic velocity of a longitudinal wave component of a bulk wave propagating through the piezoelectric layer, satisfies Unequal Equation (2) below in relation to an acoustic velocity V.sub.Si-1 determined by Equation (1) below:
V.sub.Si-1=(V.sub.2).sup.1/2 (m/sec)  Equation (1),
V.sub.Si-1≤V.sub.L  Unequal Equation (2), V.sub.2 in Equation (1) is a solution of Equation (3), and
Ax.sup.3+Bx.sup.2+Cx+D=0  Equation (3).

A bulk acoustic wave (BAW) resonator includes a piezoelectric layer oriented so that an N-polar surface forms a frontside surface that faces away from the substrate while a metal-polar surface forms the backside surface and faces toward the substrate. A process for the manufacture of a bulk acoustic wave (BAW) resonator includes orienting a piezoelectric layer on a substrate so that an N-polar surface forms a frontside surface that faces away from the substrate while a metal-polar surface forms the backside surface and faces toward the substrate; etching a via though the backside of the substrate to the metal-polar surface of the piezoelectric layer; and removing etch residue from a sidewall of the resonator cavity.

Provided is a laminate including: a substrate 210, an electrode layer 230 disposed on or above the substrate 210 and having a single-crystalline structure containing a metal element; a buffer layer 220 formed between the substrate 210 and the electrode layer 230 and configured to improve crystal orientation of the electrode layer 230; and a piezoelectric layer 240 formed on the electrode layer 230 and made of a piezoelectric body. Each of the buffer layer 220 and the piezoelectric layer 240 has a single-crystalline structure based on a composition of either ScAlN or AlN.

Apparatus and associated methods relate to forming an epitaxial layer of aluminum on an aluminum-nitride compound. The aluminum is epitaxially grown on the crystalline aluminum-nitride compound by maintaining temperature of a crystalline aluminum-nitride compound below a cluster-favoring temperature threshold within a vacuum chamber. Then, the crystalline aluminum-nitride compound is exposed to atoms of elemental aluminum for a predetermined time duration. The aluminum is epitaxially grown in this fashion for a predetermined time duration so as to produce a layer of epitaxial aluminum of a predetermined thickness. Such epitaxially-grown mono-crystalline aluminum has a lower resistivity than poly-crystalline aluminum.

A bulk acoustic wave filter includes: a first bulk acoustic wave resonator including, in an order from bottom to top, a first cavity, a first bottom electrode, a first segment of a piezoelectric layer, and a first top electrode; a second bulk acoustic wave resonator disposed adjacent to the first bulk acoustic wave resonator, and including, in the order from bottom to top, a second cavity, a second bottom electrode, a second segment of the piezoelectric layer, and a second top electrode; a boundary structure surrounding the first cavity and the second cavity, the boundary structure including a boundary portion extending between and separating the first cavity and the second cavity, and the boundary portion being disconnected at a disconnection region; and a first release hole formed in the piezoelectric layer, and overlapping the disconnection region.

An acoustic resonator includes: a resonating unit including a piezoelectric layer, a first electrode disposed on a lower side of the piezoelectric layer, and a second electrode disposed on an upper side of the piezoelectric layer; a substrate disposed below the resonating unit; a support unit forming a cavity between the substrate and the resonating unit; and a pillar extending through the cavity and connecting the resonating unit to the substrate. The resonating unit further includes a first insertion layer disposed above the pillar.

An acoustic resonator device is formed that reduces a thermal coefficient of expansion mismatch between a piezoelectric plate and a silicon substrate by bonding the front surface of the silicon substrate having a filled and planarized sacrificial tub to a piezoelectric substrate and thinning the silicon substrate by removing material from a back surface. That back surface is then bonded to a handle wafer having a thermal coefficient of expansion (TCE) closer to a TCE of the piezoelectric substrate than a TCE of the silicon substrate and thinning the piezoelectric substrate to a target piezoelectric membrane thickness to form a piezoelectric plate. A conductor pattern is formed on the thinned piezoelectric plate and the sacrificial tub is removed to form a cavity and release a membrane of the piezoelectric plate using an etchant introduced through holes in the piezoelectric plate.