Transversely-excited film bulk acoustic resonator with half-lambda dielectric layer
Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode in the diaphragm. A half-lambda dielectric layer is formed on one of the front surface and back surface of the piezoelectric plate.
1. An acoustic resonator device comprising:
a substrate having a surface;
a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate;
an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode in the diaphragm; and
a half-lambda dielectric layer formed on one of the front surface and the back surface of the piezoelectric plate.
2. The acoustic resonator device of claim 1 , wherein
a thickness is of the piezoelectric plate and a thickness td of the dielectric layer are defined as follows:
2 ts=λ 0,s , and
0.85λ 0,d ≤2 td≤ 1.15λ 0,d ,
where λ 0,s is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and
λ 0,d is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.
3. The acoustic resonator device of claim 1 , wherein
the dielectric layer is one or more of SiO 2 , Si 3 N 4 , Al 2 O 3 , and AlN.
4. The acoustic resonator device of claim 1 , wherein
the piezoelectric plate is lithium niobate,
the dielectric layer is SiO 2 , and
a thickness ts of the piezoelectric plate and a thickness td of the dielectric layer are defined by the relationship: 0.875 ts≤td≤1.25 ts.
5. The acoustic resonator device of claim 4 , wherein
a temperature coefficient of frequency of the acoustic resonator device is between −32 ppm/C° and −42 ppm/C° at a resonance frequency and between −20 ppm/C° and −36 ppm/C° at an anti-resonance frequency.
6. A filter device, comprising:
a substrate;
a piezoelectric plate having parallel front and back surfaces and a thickness ts, the back surface attached to the substrate;
a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on respective portions of the piezoelectric plate suspended over one or more cavities formed in the substrate;
a first dielectric layer having a thickness tds deposited between the fingers of the series resonator; and
a second dielectric layer having a thickness tdp deposited between the fingers of the shunt resonator, wherein
ts, tds, and tdp are related by the equations:
2 ts=λ 0,s , and
0.85λ 0,d ≤2 tds< 2 tdp≤ 1.15λ 0,d ,
where λ 0,s is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and
λ 0,d is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.
7. A filter device, comprising:
a substrate;
a lithium niobate piezoelectric plate having parallel front and back surfaces and a thickness ts, the back surface attached to the substrate;
a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on respective portions of the piezoelectric plate suspended over one or more cavities formed in the substrate;
a first SiO 2 layer having a thickness tds deposited between the fingers of the series resonator; and
a second SiO 2 layer having a thickness tdp deposited between the fingers of the shunt resonator, wherein
ts, tds, and tdp are related by the equation:
0.875 ts≤ tds<tdp ≤1.25 ts.
8. The filter device of claim 7 , wherein
a temperature coefficient of frequency of each of the plurality of resonators is between −20 ppm/C° and −42 ppm/C° at the resonance frequencies and the anti-resonance frequencies of all of the plurality of resonators.
9. A method of fabricating an acoustic resonator device on a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to a substrate, the method comprising:
forming a cavity in the substrate such that a portion of the single-crystal piezoelectric plate forms a diaphragm spanning the cavity;
forming an interdigital transducer (IDT) on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm; and
forming a half-lambda dielectric layer on one of the front surface and the back surface of the piezoelectric plate.
10. The method of claim 9 , wherein
a thickness is of the piezoelectric plate and a thickness td of the dielectric layer are defined as follows:
2 ts=λ 0,s , and
0.85λ 0,d ≤2 td≤ 1.15λ 0,d ,
where λ 0,s is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and
λ 0,d is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.
11. The method of claim 9 , wherein forming a half-lambda dielectric layer further comprises:
depositing one or more of SiO 2 , Si 3 N 4 , Al 2 O 3 , and AlN.
12. The method of claim 9 , wherein
the piezoelectric plate is lithium niobate, and
forming a half-lambda dielectric layer comprises depositing SiO 2 to a thickness td, where td is greater or equal to 0.875 ts and less than or equal to 1.25 ts, where ts is a thickness of the piezoelectric plate.
13. The acoustic resonator device of claim 6 , wherein
the first dielectric layer and the second dielectric layer are one or more of SiO 2 , Si 3 N 4 , Al 2 O 3 , and AlN.