XBAR Shunt Resonator Dielectric Thickness for Spurious Mode Suppression
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Solution Overview
Problem
Current RF filters using acoustic wave resonators are not well-suited for higher frequency communication bands proposed for future wireless networks, as they face challenges in achieving sufficient frequency separation and are prone to spurious acoustic modes that affect filter performance.
Innovation Solution
The use of Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with a thick frequency setting dielectric layer on shunt resonators to create an asymmetric structure, and varying the thickness of these dielectric layers across shunt resonators to prevent constructive addition of spurious modes, thereby improving frequency separation and reducing admittance spikes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a thick frequency setting dielectric layer is used on shunt resonators to achieve frequency separation, then frequency separation is improved, but spurious acoustic modes are excited causing admittance spikes
Solution Approach 1:
The patent applies different dielectric layer thicknesses to different shunt resonators within the same filter. Specifically, at least one shunt resonator has a first dielectric layer thickness while another shunt resonator has a second dielectric layer thickness that differs from the first. This local variation in dielectric thickness prevents uniform excitation of spurious acoustic modes across all resonators, thereby reducing admittance spikes while maintaining the frequency separation benefit of thick dielectric layers.
2Reliability
If conventional acoustic wave resonators are used for higher frequency bands, then existing technology is maintained, but sufficient frequency separation and performance are not achieved
Solution Approach 1:
The patent introduces asymmetry into the filter structure by varying the dielectric layer thicknesses across different shunt resonators. This asymmetric configuration breaks the uniformity that causes constructive addition of spurious modes, thereby improving filter performance in higher frequency bands. The asymmetric design allows the filter to maintain reliability while becoming adaptable to higher frequency applications up to 28 GHz.
Solution Approach 2:
The patent changes the physical parameter of dielectric layer thickness to optimize filter performance for higher frequency bands. By adjusting and varying the thickness parameter across different resonators, the filter achieves both sufficient frequency separation and suppression of spurious modes, enabling reliable operation in higher frequency communication bands.
3Ease of manufacture
If uniform dielectric layer thickness is used across all shunt resonators, then manufacturing is simplified, but spurious modes constructively add causing performance degradation
Solution Approach 1:
The patent implements local quality variation by applying different dielectric layer thicknesses to different shunt resonators. This approach maintains relative manufacturing simplicity while preventing the constructive addition of spurious modes that occurs with uniform thickness. The variation in thickness is sufficient to disrupt spurious mode coherence but can still be achieved through standard manufacturing processes with appropriate control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the performance of RF filters by reducing admittance spikes by over 17 dB, allowing them to effectively operate in wider communication channel bandwidths and higher frequency ranges, such as those up to 28 GHz, while maintaining passband performance.
Implementation Method 1
A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm
Implementation Method 2
RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost
Data Source
AI summary
Acoustic filters and methods of fabricating acoustic filters are disclosed. A filter includes a single-crystal piezoelectric plate having a front surface and a back surface attached to a substrate, and a plurality of acoustic resonators including a first shunt resonator, a second shunt resonator, and one or more series resonators. Each of the plurality of acoustic resonators includes an interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, interleaved fingers of the IDT disposed on a respective diaphragm formed by a portion of the piezoelectric plate that spans a cavity in the substrate. A frequency setting dielectric layer is formed over the first and second shunt resonators but not over the one or more series resonators. The frequency setting dielectric layer has a thickness t1 on the first shunt resonator and a thickness t2 on the second shunt resonator, where t1 is not equal to t2.