Acoustic reflector for a BAW resonator providing specified reflection of both shear waves and longitudinal waves
A BAW resonator includes a piezoelectric layer, a first electrode, a second electrode, a substrate, and an acoustic reflector disposed between the substrate and the second electrode. The acoustic reflector has a plurality of layers. A performance of the acoustic reflector is determined by its reflectivity for a longitudinal wave existing in the BAW resonator at the resonance frequency of the BAW resonator and by its reflectivity for a shear wave existing in the BAW resonator at the resonance frequency of the BAW resonator. The layers of the acoustic reflector and layers disposed between the acoustic reflector and the piezoelectric layer are selected, with reference to their number, material, and thickness, such that the transmissivity for the longitudinal wave and the transmissivity for the shear wave in the area of the resonance frequency is smaller than −10 dB.
1. A BAW resonator with
a piezoelectric layer with a first surface and a second surface opposing the first surface;
a first electrode disposed on the first surface of the piezoelectric layer;
a second electrode disposed on the second surface of the piezoelectric layer;
a substrate; and
an acoustic reflector disposed between the substrate and the second electrode, the acoustic reflector comprising a plurality of layers from a material with high acoustic impedance and from a material with low acoustic impedance, wherein areas with layers with high acoustic impedance and areas with layers with low acoustic impedance are alternately adjacently disposed,
wherein the performance of the acoustic reflector is determined by its reflectivity for a longitudinal wave existing in the BAW resonator at the resonance frequency of the BAW resonator and by its reflectivity for a shear wave existing in the BAW resonator at the resonance frequency of the BAW resonator, and
wherein the layers of the acoustic reflector and layers disposed between the acoustic reflector and the piezoelectric layer are selected, with reference to their number, material, and thickness, such that the transmissivity for the longitudinal wave and the transmissivity for the shear wave in the area of the resonance frequency is smaller than −10 dB.
2. The BAW resonator of claim 1 , wherein the transmissivity for the longitudinal wave and for the shear wave in the area of the resonance frequency is smaller than −15 dB.
3. The BAW resonator of claim 1 , wherein the transmissivity for the longitudinal wave in the area of the resonance frequency is smaller than −20 dB, and wherein the transmissivity for the shear wave in the area of the resonance frequencies are smaller than −15 dB.
4. The BAW resonator of claim 1 , wherein the layers of the acoustic resonator are selected such that the BAW resonator has an unambiguous and desired dispersion performance.
5. The BAW resonator of claim 4 , wherein the layers of the acoustic resonator are selected such that the distance between the longitudinal main resonance and the first shear harmonic wave is greater than the bandwidth of the resonator.
6. The BAW resonator of claim 5 , wherein the layers of the acoustic resonator are selected such that the distance between the longitudinal main resonance and the first shear harmonic wave is greater than the bandwidth of the longitudinal main resonance of the resonator.
7. The BAW resonator of claim 1 , wherein the acoustic reflector includes a first layer with low acoustic impedance, a second layer with high acoustic impedance, a third layer with low acoustic impedance, a fourth layer with high acoustic impedance, and a fifth layer with low acoustic impedance.
8. The BAW resonator of claim 7 , wherein the layers with low acoustic impedance are SiO 2 layers, wherein the layers with high acoustic impedance are W layers.
9. The BAW resonator of claim 7 , comprising the layer sequence given in the following table
Layer no.
Material
Layer thickness rel. to λ long
112
SiO2
0.770
114
Tungsten
0.175
116
SiO2
0.250
118
Tungsten
0.245
120
SiO2
0.180
106a
Aluminum
0.130
106b
Tungsten
0.070
102
AlN
0.220
104a
Tungsten
0.028
104b
Aluminum
0.080 to 0.12
104c
SiN
0.008
wherein the layer thicknesses are indicated relative to the longitudinal wavelengths ·λ long in the respective layer, and
wherein deviations from these thicknesses have to be smaller than 20%.
10. Filter circuit with a plurality of BAW resonators each of which comprises:
a piezoelectric layer with a first surface and a second surface opposing the first surface;
a first electrode disposed on the first surface of the piezoelectric layer;
a second electrode disposed on the second surface of the piezoelectric layer;
a substrate; and
an acoustic reflector disposed between the substrate and the second electrode, the acoustic reflector comprising a plurality of layers from a material with high acoustic impedance and from a material with low acoustic impedance, wherein areas with layers with high acoustic impedance and areas with layers with low acoustic impedance are alternately adjacently disposed,
wherein the performance of the acoustic reflector is determined by its reflectivity for a longitudinal wave existing in the BAW resonator at the resonance frequency of the BAW resonator and by its reflectivity for a shear wave existing in the BAW resonator at the resonance frequency of the BAW resonator, and
wherein the layers of the acoustic reflector and layers disposed between the acoustic reflector and the piezoelectric layer are selected, with reference to their number, material, and thickness, such that the transmissivity for the longitudinal wave and the transmissivity for the shear wave in the area of the resonance frequency is smaller than −10 dB.
11. The filter circuit of claim 10 , wherein the resonator elements and the acoustic reflectors of the plurality of BAW resonators are formed on a common substrate.