Symmetric XBAR Diaphragm Structure for Spurious Mode Suppression
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Solution Overview
Problem
Existing RF filters using acoustic wave resonators are not well-suited for higher frequency communications bands above 3 GHz, such as those proposed for future wireless networks, due to limitations in spurious mode control and performance at these frequencies.
Innovation Solution
The use of a transversely-excited film bulk acoustic resonator (XBAR) with a symmetric diaphragm structure, featuring interleaved IDT fingers on both sides of the piezoelectric plate, balanced by back-side fingers to minimize spurious modes, and dielectric layers to enhance electromechanical coupling and heat dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional acoustic wave resonators are used, then they are suitable for lower frequency bands, but they cannot achieve high performance in higher frequency bands above 3 GHz due to spurious mode control limitations
Solution Approach 1:
The patent employs asymmetric IDT finger configurations where the number of fingers, their lengths, and spacing are deliberately made asymmetric between opposite sides of the resonator. This asymmetry disrupts the symmetry of spurious mode patterns, causing destructive interference and suppressing spurious mode excitation. The asymmetric design specifically targets and eliminates harmful spurious modes while maintaining the desired fundamental resonance modes, thereby improving filter performance in higher frequency bands.
Solution Approach 2:
The patent extends the conventional planar IDT design into the vertical dimension by incorporating IDT fingers that extend through the thickness of the piezoelectric substrate. This three-dimensional configuration allows for better control of acoustic wave propagation and provides additional degrees of freedom for suppressing spurious modes. The vertical extension of IDT fingers enables more effective electromechanical coupling and better control over the acoustic field distribution, reducing spurious mode generation.
2Speed
If higher frequency operation is achieved, then communication bandwidth is increased, but spurious modes become more problematic and harder to control
Solution Approach 1:
The asymmetric IDT configuration with different finger counts, lengths, and spacing on opposite sides creates frequency-selective suppression of spurious modes. The asymmetry is designed to be particularly effective at higher frequencies where spurious modes become more problematic. By making the IDT structure asymmetric, the patent creates frequency-dependent cancellation of spurious modes through destructive interference, allowing high-frequency operation with reduced spurious mode content.
3Use of energy by moving object
If IDT fingers are added to enhance coupling, then electromechanical coupling improves, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent applies local quality by making different regions of the IDT structure have different properties. Specifically, opposite sides of the resonator have IDT fingers with different characteristics (different numbers of fingers, different lengths, different spacing) tailored to local requirements. This localized differentiation allows for optimized electromechanical coupling in each region while maintaining overall symmetry for spurious mode suppression, avoiding the need for uniformly complex structures throughout.
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
The XBAR resonator achieves high electromechanical coupling, reduced spurious modes, improved power handling, and smaller resonator size, enabling effective operation in higher frequency bands with enhanced filter performance.
Implementation Method 1
A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm
Implementation Method 2
dielectric layers to enhance electromechanical coupling and heat dissipation
Implementation Method 3
back-side fingers to minimize spurious modes
Data Source
AI summary
Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a surface. A back surface of a single-crystal piezoelectric plate is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) is formed on a front surface of the piezoelectric plate with interleaved IDT fingers of the IDT disposed on the diaphragm. Back-side fingers are formed the back surface of the diaphragm.