Bragg Stack Tuning for Spurious Mode Suppression in SM XBAR
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Solution Overview
Problem
The performance of Solidly Mounted XBAR (SM XBAR) filters is limited by the suboptimal frequency response of the Bragg stack, which affects the reflection and transmission of energy, leading to inefficient resonance mode reflection and spurious mode transmission.
Innovation Solution
The Bragg stack structure is optimized by configuring alternating layers of high and low acoustic impedance materials with specific thicknesses to generate a reflectance frequency band centered around a displaced frequency, maximizing energy reflection and spurious mode transmission away from the transducer structure, using thickness equations that account for harmonic spurs and error components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the Bragg stack uses conventional thickness design centered at the resonance frequency, then the resonance mode reflection is adequate, but spurious modes are transmitted inefficiently
Solution Approach 1:
The patent applies parameter changes by modifying the thickness of Bragg stack layers from the conventional design (centered at resonance frequency) to an optimized design where layer thicknesses are tuned to center the reflectance band at a frequency offset from the resonance frequency. This parameter adjustment enables simultaneous optimization of resonance mode reflection and spurious mode transmission suppression.
Solution Approach 2:
The patent implements local quality by creating different functional zones within the Bragg stack: layers are designed with specific thicknesses to provide enhanced reflection at the resonance frequency while simultaneously providing suppression of spurious modes at offset frequencies. This localized functional differentiation resolves the contradiction between adequate resonance reflection and efficient spurious mode rejection.
2Reliability
If the Bragg stack is optimized for maximum resonance reflection, then energy reflection is improved, but spurious mode transmission is not sufficiently suppressed
Solution Approach 1:
The patent converts the harmful effect of spurious mode transmission into a beneficial outcome by designing the Bragg stack to actively suppress these spurious modes. The optimized layer thicknesses create a reflectance band that coincides with spurious mode frequencies, transforming the Bragg stack from a passive structure into an active suppression mechanism that eliminates harmonic spurs while maintaining resonance reflection.
3Use of energy by moving object
If the Bragg stack layer thicknesses are increased to improve reflection, then energy reflection is enhanced, but the frequency response becomes suboptimal
Solution Approach 1:
The patent applies dynamics by creating a frequency-selective Bragg stack response through optimized layer thicknesses. Instead of a static, uniform reflection characteristic, the design produces a dynamic frequency response where the reflectance band is positioned to simultaneously address resonance reflection and spurious mode suppression, enabling the filter to adapt its energy reflection characteristics across different frequency regions.
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 optimization enhances the SM XBAR performance by improving the reflection of resonance modes and reducing spurious mode transmission, leading to better filter performance and suppression of harmonic spurs, while maintaining in-band filter response.
Implementation Method 1
an acoustic Bragg reflector between the substrate and the piezoelectric layer, the acoustic Bragg reflector comprising alternating layers of a first material and a second material having a higher acoustic impedance than the first material
Implementation Method 2
thicknesses of the first material and the second material of the acoustic Bragg reflector are configured to generate a reflectance frequency band centered around a displaced frequency
Implementation Method 3
A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm
Implementation Method 4
XBAR resonators provide very high electromechanical coupling and high frequency capability
Data Source
AI summary
An acoustic resonator device is provided that includes a substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT) at the piezoelectric layer; an acoustic Bragg reflector between the substrate and the piezoelectric layer. The acoustic Bragg reflector includes alternating layers of a first material and a second material having a higher acoustic impedance than the first material. Thicknesses of the first material and the second material of the acoustic Bragg reflector are configured to generate a reflectance frequency band centered around a displaced frequency f0′, the displaced frequency f0′ being displaced from a resonance frequency fr of the acoustic resonator device based on a harmonic spur of the resonance frequency fr. In this aspect, the thicknesses of the first material and the second material are measured in a direction normal to the substrate.