XBAR Filter Dielectric Layer Split for A2 Spur Suppression
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Solution Overview
Problem
Existing RF filters using acoustic wave resonators are not well-suited for higher frequency communications bands, such as those defined in the 5 G NR standard, due to inefficiencies in handling higher transmit powers and the excitation of spurious modes like A2 spur.
Innovation Solution
The use of transversely-excited film bulk acoustic resonators (XBARs) with a divided frequency-setting dielectric layer, where a portion of the dielectric layer is placed on the back side of the piezoelectric plate, to effectively separate the resonance frequencies of shunt and series resonators and reduce the excitation of spurious modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single frequency-setting dielectric layer is used on the front side of the piezoelectric plate, then the resonance frequency can be set, but spurious modes like A2 spur are excited and filter performance deteriorates
Solution Approach 1:
The single frequency-setting dielectric layer is divided into two separate layers: a front-side frequency-setting dielectric layer and a back-side frequency-setting dielectric layer. This segmentation allows independent optimization of each layer's thickness to control resonance frequency while suppressing spurious modes through destructive interference of acoustic waves reflected from opposite sides of the piezoelectric plate.
Solution Approach 2:
The frequency-setting function is extended from one dimension (front side only) to two dimensions (both front and back sides). By placing dielectric layers on both surfaces of the piezoelectric plate, the patent creates a symmetric structure that simultaneously achieves frequency control and spurious mode suppression through balanced acoustic impedance matching.
2Adaptability or versatility
If existing acoustic wave resonators are used for higher frequency communications bands, then device compatibility is maintained, but efficiency in handling higher transmit powers decreases
Solution Approach 1:
The patent modifies the physical parameters of the resonator structure by introducing divided frequency-setting dielectric layers with specific thickness relationships (front-side layer thickness and back-side layer thickness configured to suppress spurious modes). This structural parameter change enables the resonator to maintain compatibility with existing devices while improving power handling efficiency through reduced spurious mode excitation and enhanced electromechanical coupling.
3Reliability
If the electromechanical coupling to spurious modes is reduced, then filter performance improves, but the complexity of the resonator structure increases
Solution Approach 1:
The frequency-setting dielectric layer is segmented into front-side and back-side layers, allowing independent thickness optimization. This segmentation enables suppression of spurious modes through controlled interference patterns while maintaining a relatively simple overall structure that can be integrated into existing filter designs with minimal complexity increase.
Solution Approach 2:
The patent employs asymmetric thickness configuration of the divided dielectric layers relative to the piezoelectric plate thickness, creating an optimized acoustic impedance profile that suppresses spurious modes. This controlled asymmetry achieves performance improvement without requiring complete structural redesign, thus limiting the increase in device complexity.
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 configuration enhances the electromechanical coupling to the primary shear acoustic mode, reduces the peak admittance of spurious modes by up to 28 dB, and improves the overall performance of RF filters for higher frequency communications bands.
Implementation Method 1
a piezoelectric plate; portions of the piezoelectric plate form diaphragms spanning cavities in the substrate
Implementation Method 2
frequency-setting dielectric layers configured such that a combined thickness of the front-side frequency-setting dielectric layer and the back-side frequency-setting dielectric layer is less than a thickness of the piezoelectric plate
Data Source
AI summary
An acoustic filter is provide that includes a substrate; a piezoelectric plate attached to the substrate; and a plurality of bulk acoustic resonators including one or more shunt resonators and one or more series resonators. One or more of the shunt resonators includes an interdigital transducer having interdigital fingers on the piezoelectric plate; a front-side frequency setting layer at least partially on a front side of the at least one shunt resonator; and a back-side frequency setting layer on a back side of the at least one shunt resonator. Moreover, a thickness of the back-side frequency setting layer is at least 30% of a total thickness of the front-side frequency setting layer and the back-side frequency setting layer.