LBAW Reflector Stack for Suppressing Shear-Mode Spurious Passbands
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Solution Overview
Problem
Lateral acoustic coupling in bulk acoustic wave (BAW) thin-film devices, specifically laterally coupled BAW (LBAW) filters, face challenges with spurious passbands arising from the excitation of thickness-shear (TS2) wave modes, which are close to the desired passband, making it difficult to suppress unwanted responses effectively.
Innovation Solution
A reflector stack is designed to selectively pass shear waves at the frequency of the unwanted passband while reflecting both shear and longitudinal waves at the filter passband, using an asymmetric multilayer structure with varying layer thicknesses and acoustic impedances to achieve this frequency-dependent behavior, allowing for efficient suppression of the spurious TS2 mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a conventional acoustic reflector is used to suppress unwanted passbands, then the spurious TS2 mode can be suppressed, but the filter passband characteristics deteriorate due to loss of acoustic energy
Solution Approach 1:
The acoustic reflector is designed with non-uniform layer thicknesses where the first reflector layer has a thickness different from the quarter-wavelength thickness, while subsequent layers maintain or adjust their thicknesses to compensate. This local variation in thickness creates frequency-selective reflection characteristics that suppress the TS2 mode while preserving passband energy
Solution Approach 2:
The reflector structure utilizes controlled deviations from the ideal quarter-wavelength thickness parameter. By intentionally setting the first layer thickness to a value different from the quarter-wavelength thickness and adjusting other layer thicknesses accordingly, the reflector achieves differentiated acoustic impedance profiles that selectively affect different frequency bands
2Loss of energy
If the acoustic reflector is optimized to reflect both shear and longitudinal waves at the filter passband, then energy loss is reduced, but the ability to suppress the TS2 mode deteriorates
Solution Approach 1:
The acoustic reflector is segmented into multiple layers with different thickness characteristics. The first layer serves a different function (creating impedance mismatch for TS2 suppression) compared to the subsequent layers (maintaining reflection for passband), allowing the reflector to perform both functions simultaneously through functional segmentation
Solution Approach 2:
The reflector structure introduces asymmetry in layer thickness distribution, with the first layer having a distinctly different thickness from the quarter-wavelength value. This asymmetric configuration creates the necessary conditions for frequency-selective behavior, enabling differential treatment of the TS2 mode and the filter passband
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 effectively suppresses the unwanted passband by at least 8 dB, improving the quality of the passband response and reducing manufacturing complexity by minimizing the number of layers required, making LBAW filters more suitable for commercial RF applications.
Implementation Method 1
the acoustic reflector structure is adapted to acoustically isolate the vibration layer from its surroundings at the first frequency band more efficiently than at the second frequency band
Implementation Method 2
designed so that at the frequency of the unwanted passband, shear waves pass through the reflector
Implementation Method 3
The piezoelectric thin-film layer and electrode layers that constitute the resonator transform the electric signal into acoustic vibration and vice versa
Data Source
Figure 1
Figure 2~3
Figure 4a~4b
AI summary
The invention relates to a laterally coupled bulk acoustic wave (LBAW) filter (70) comprising a vibration layer (73) for carrying bulk acoustic waves, electrode means (71, 72, 74) comprising a first electrode (71) coupled to the vibration layer (73) for exciting to the vibration layer (73) at least one longitudinal wave mode having a first frequency band and one shear wave mode having a second frequency band, and a second electrode (72) coupled to the vibration layer (73) for sensing the filter pass signal, the first and second electrodes (71, 72) being laterally arranged with respect to each other, and an acoustic reflector structure (75) in acoustic connection with the vibration layer(73). According to the invention, the reflector structure (75) is adapted to acoustically isolate the vibration layer (73) from its surroundings at the first frequency band more efficiently than at the second frequency band for suppressing the effect of the shear wave mode at the second frequency band from the filter pass signal. The invention helps to improve the quality of LBAW filter passbands.