Structure and manufacturing method of surface acoustic wave filter with interdigital transducer

1 . A surface acoustic wave (SAW) filter device, comprising:

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;

a first interdigital transducer (IDT) disposed on the bottom surface of the piezoelectric layer, an interdigital portion of the first IDT being exposed in the cavity, the first IDT having a top surface facing the piezoelectric layer and a bottom surface opposite to the top surface and facing the bottom substrate;

a second IDT disposed on the top surface of the piezoelectric layer, an interdigital portion of the second IDT being vertically aligned with the interdigital portion of the first IDT, the second IDT having a bottom surface facing the piezoelectric layer and a top surface opposite to the bottom surface;

a first pad metal layer formed on the bottom surface of the first IDT, a first section of the first pad metal layer being electrically connected to a first input and output end of the first IDT, and a second section of the first pad metal layer being electrically connected to a second input and output end of the first IDT;

a first opening formed in the piezoelectric layer and exposing the first input and output end of the first IDT;

a second opening formed in the piezoelectric layer and exposing the second input and output end of the first IDT;

a second pad metal layer formed on the piezoelectric layer, a first section of the second pad metal layer being electrically connected to the first input and output end of the first IDT via the first opening, a second section of the second pad metal layer being electrically connected to the second input and output end of the first IDT via the second opening, a third section of the second pad metal layer being formed on the top surface of the second IDT and electrically connected to a first input and output end of the second IDT, and a fourth section of the second pad metal layer being formed on the top surface of the second IDT and electrically connected to a second input and output end of the second IDT;

a first dielectric layer disposed between the piezoelectric layer and the bottom substrate;

a second dielectric layer disposed below the first dielectric layer; and

a third dielectric layer disposed below the second dielectric layer, and including a protruding structure protruding toward the first dielectric layer,

wherein the cavity is surrounded by a double-wall boundary structure constituted by the second dielectric layer and the protruding structure of the third dielectric layer.

2 . The SAW filter device of claim 1 , wherein

the bottom substrate is formed of Si, SiO 2 , polysilicon, silicon carbide, sapphire (Al 2 O 3 ), or a stacked combination of two or more of those materials, and

the bottom substrate is bonded to the third dielectric layer.

3 . The SAW filter device of claim 1 , wherein

the first dielectric layer is formed of silicon oxide, silicon nitride, or a stacked combination of those materials.

4 . The SAW filter device of claim 1 , wherein

the second dielectric layer is formed of polysilicon, amorphous silicon, AlN, SiN, TaN, GaN, or a stacked combination of two or more of those materials.

5 . The SAW filter device of claim 1 , wherein

the third dielectric layer is formed of silicon oxide, silicon nitride, or a stacked combination of those materials.

6 . The SAW filter device of claim 1 , further comprising:

a non-conductive layer disposed between the third dielectric layer and the bottom substrate, the non-conductive layer being formed of polysilicon, amorphous silicon, silicon nitride, aluminum nitride, gallium nitride, or a stacked combination of two or more of those materials.

7 . The SAW filter device of claim 1 , further comprising:

a buffer layer disposed between the third dielectric layer and the bottom substrate, the buffer layer being formed of silicon nitride, silicon oxide, or a stacked combination of those materials.

8 . The SAW filter device of claim 1 , further comprising:

a non-conductive layer disposed between the third dielectric layer and the bottom substrate, the non-conductive layer being formed of polysilicon, amorphous silicon, silicon nitride, aluminum nitride, gallium nitride, or a stacked combination of two or more of those materials; and

a buffer layer disposed between the first dielectric layer and the non-conductive layer, the buffer layer being formed of silicon nitride, silicon oxide, or a stacked combination of those materials.