A surface acoustic wave (SAW) filter includes a bottom substrate, a piezoelectric layer disposed above the bottom substrate, the piezoelectric layer having a bottom surface facing the bottom substrate and a top surface opposite to the bottom surface, a cavity disposed below the piezoelectric layer, an interdigital transducer (IDT) disposed on the top surface of the piezoelectric layer, and a back electrode disposed on the bottom surface of the piezoelectric layer. At least a portion of the back electrode is exposed in the cavity.

A fabrication method of a surface acoustic wave (SAW) filter includes obtaining a piezoelectric substrate, forming a first interdigital transducer (IDT) on a first portion of the piezoelectric substrate, forming a first pad metal layer on the first IDT, forming a first dielectric layer on the first portion of the piezoelectric substrate, covering the first IDT and the first pad metal layer, forming a trench in the first dielectric layer, forming a second dielectric layer on the first dielectric layer, forming a third dielectric layer on the second dielectric layer, removing a second portion of the piezoelectric substrate to obtain a piezoelectric layer, forming a second IDT on the piezoelectric layer, and etching and releasing a portion of the first dielectric layer surrounded by the trench to form a cavity.

A fabrication method of a surface acoustic wave (SAW) filter includes obtaining a piezoelectric substrate, forming a first interdigital transducer (IDT) on a first portion of the piezoelectric substrate, forming a first pad metal layer on the first IDT, forming a first dielectric layer on the first portion of the piezoelectric substrate, covering the first IDT and the first pad metal layer, forming a trench in the first dielectric layer, forming a second dielectric layer on the first dielectric layer, forming a third dielectric layer on the second dielectric layer, removing a second portion of the piezoelectric substrate to obtain a piezoelectric layer, forming a second IDT on the piezoelectric layer, and etching and releasing a portion of the first dielectric layer surrounded by the trench to form a cavity.

A composite substrate includes a final substrate, and a piezoelectric material directly molecularly bonded to the final substrate at a first interface. The piezoelectric material comprises an epitaxial layer, but does not comprise a seed layer. Additional composite substrates include a final substrate, and a piezoelectric material directly molecularly bonded to the final substrate at a first interface. The piezoelectric material comprises an epitaxial layer. The composite substrate further includes a seed layer on which the piezoelectric material has been epitaxially grown. The seed layer is disposed on a side of the epitaxial layer opposite the final substrate. An acoustic wave device comprises such a composite substrate with at least one electrode on a surface of the piezoelectric layer opposite the substrate.

A composite substrate includes a final substrate, and a piezoelectric material directly molecularly bonded to the final substrate at a first interface. The piezoelectric material comprises an epitaxial layer, but does not comprise a seed layer. Additional composite substrates include a final substrate, and a piezoelectric material directly molecularly bonded to the final substrate at a first interface. The piezoelectric material comprises an epitaxial layer. The composite substrate further includes a seed layer on which the piezoelectric material has been epitaxially grown. The seed layer is disposed on a side of the epitaxial layer opposite the final substrate. An acoustic wave device comprises such a composite substrate with at least one electrode on a surface of the piezoelectric layer opposite the substrate.

A fabrication method of a surface acoustic wave (SAW) filter, includes: obtaining a piezoelectric substrate; forming a back electrode on a first portion of the piezoelectric substrate; forming a sacrificial layer on the first portion of the piezoelectric substrate, covering the back electrode; forming a first dielectric layer on the first portion of the piezoelectric substrate, covering the sacrificial layer; bonding a bottom substrate to the first dielectric layer; removing a second portion of the piezoelectric substrate to expose the first portion of the piezoelectric substrate, the first portion of the piezoelectric substrate constituting a piezoelectric layer; forming one or more release holes through the piezoelectric layer; forming an interdigital transducer (IDT) on the piezoelectric layer; and etching and releasing the sacrificial layer via the one or more release holes to form a lower cavity exposing the back electrode.

A fabrication method of a surface acoustic wave (SAW) filter, includes: obtaining a piezoelectric substrate; forming a back electrode on a first portion of the piezoelectric substrate; forming a sacrificial layer on the first portion of the piezoelectric substrate, covering the back electrode; forming a first dielectric layer on the first portion of the piezoelectric substrate, covering the sacrificial layer; bonding a bottom substrate to the first dielectric layer; removing a second portion of the piezoelectric substrate to expose the first portion of the piezoelectric substrate, the first portion of the piezoelectric substrate constituting a piezoelectric layer; forming one or more release holes through the piezoelectric layer; forming an interdigital transducer (IDT) on the piezoelectric layer; and etching and releasing the sacrificial layer via the one or more release holes to form a lower cavity exposing the back electrode.

An acoustic wave device includes a piezoelectric body made of lithium niobate and disposed directly or indirectly on a supporting substrate, and IDT electrode disposed directly or indirectly on the piezoelectric body. When the wavelength of an acoustic wave that is determined by a pitch of electrode fingers of the IDT electrode is denoted by λ, the thickness of the piezoelectric body is equal to or less than about 1λ. The acoustic wave device uses the plate wave S0 mode propagating in the piezoelectric body. The Euler angles of the lithium niobate are (0°±10°, θ, 90°±10°), provided that θ is from about 0° to about 180° inclusive.

An acoustic wave device includes a piezoelectric body made of lithium niobate and disposed directly or indirectly on a supporting substrate, and IDT electrode disposed directly or indirectly on the piezoelectric body. When the wavelength of an acoustic wave that is determined by a pitch of electrode fingers of the IDT electrode is denoted by λ, the thickness of the piezoelectric body is equal to or less than about 1λ. The acoustic wave device uses the plate wave S0 mode propagating in the piezoelectric body. The Euler angles of the lithium niobate are (0°±10°, θ, 90°±10°), provided that θ is from about 0° to about 180° inclusive.

A frequency division duplex (FDD) first band includes a first downlink operating band and a first uplink operating band. An FDD second band includes a second downlink operating band and a second uplink operating band. In the FDD first band and the FDD second band, (1) the first downlink operating band, second downlink operating band, first uplink operating band, and second uplink operating band are positioned in order from lowest to highest frequency. The frequency range of the first uplink operating band and that of the second uplink operating band do not overlap each other. A filter is formed in or on a first substrate having piezoelectric properties and has a pass band including the first and second uplink operating bands.