SM-XBAR Filter Structure With Bragg Reflector for 27 GHz Bands
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Solution Overview
Problem
Existing RF filters using acoustic wave resonators are not well-suited for higher frequencies and bandwidths required by future communications networks, particularly in the 27 GHz communications bands.
Innovation Solution
The development of solidly-mounted transversely-excited film bulk acoustic resonators (SM-XBARs) with an acoustic Bragg reflector and specific dielectric layers, which enhance the Q-factor and frequency selectivity, enabling effective operation in high-frequency bands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional acoustic wave resonators are used, then the filter structure is simple, but the frequency capability and Q-factor are insufficient for higher frequency bands
Solution Approach 1:
The patent employs a composite structure combining piezoelectric material layers with acoustic Bragg reflector layers. The piezoelectric layer (e.g., lithium niobate or lithium tantalate) provides electromechanical coupling, while the acoustic Bragg reflector (alternating high and low acoustic impedance layers) provides acoustic confinement. This composite material approach enables high Q-factor and frequency capability in the 27 GHz band while maintaining a manageable device structure.
Solution Approach 2:
The patent transitions from two-dimensional surface acoustic wave resonators to three-dimensional bulk acoustic wave resonators with vertical layering. The acoustic Bragg reflector is structured as multiple alternating layers with different acoustic impedances, creating a vertical dimension for acoustic confinement. This dimensional transition enables higher frequency operation and improved Q-factor by confining acoustic energy within the piezoelectric layer.
2Manufacturing precision
If the acoustic Bragg reflector with multiple alternating layers is implemented, then the Q-factor and frequency selectivity are enhanced, but the manufacturing complexity increases
Solution Approach 1:
The acoustic Bragg reflector is segmented into multiple alternating layers of high acoustic impedance material (e.g., aluminum nitride, diamond) and low acoustic impedance material (e.g., silicon dioxide, silicon oxycarbide). Each layer has a thickness of approximately one-quarter of the acoustic wavelength at the resonant frequency. This segmentation creates constructive interference for acoustic wave reflection, enhancing frequency selectivity and Q-factor while allowing standard thin-film deposition techniques to be used.
Solution Approach 2:
The patent optimizes the thickness of each Bragg reflector layer to be approximately one-quarter of the acoustic wavelength at the desired resonant frequency. By carefully controlling the thickness parameters of alternating high and low impedance layers, the structure achieves maximum acoustic reflection and minimal transmission, thereby enhancing frequency selectivity. The piezoelectric layer thickness is also optimized to be approximately half the acoustic wavelength for maximum electromechanical coupling.
3Speed
If thin piezoelectric layers are used for high frequency operation, then the frequency capability improves, but the acoustic energy confinement becomes more challenging
Solution Approach 1:
The acoustic Bragg reflector acts as an intermediary structure between the thin piezoelectric layer and the substrate. It provides acoustic confinement by creating a high-impedance barrier that reflects acoustic waves back into the piezoelectric layer, preventing energy leakage into the substrate. This intermediary structure enables thin piezoelectric layers to maintain effective acoustic energy confinement even at high operating frequencies where the layer thickness is comparable to the acoustic wavelength.
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
SM-XBARs provide high electromechanical coupling and frequency capability, enabling the design of RF filters with improved insertion loss, rejection, and bandwidth, suitable for next-generation wireless communication systems.
Implementation Method 1
a piezoelectric layer, which may be bonded to the acoustic Bragg reflector
Implementation Method 2
an acoustic Bragg reflector, which may include alternating layers of material having different acoustic impedances
Data Source
AI summary
Resonator devices and filter devices are disclosed. A radio frequency filter includes substrate; a piezoelectric layer; a conductor pattern comprising a plurality of interdigital transducers (IDTs) on the piezoelectric layer; and an acoustic Bragg reflector between the substrate and the piezoelectric layer. The plurality of IDTs comprises a first IDT of first acoustic resonator and a second IDT of a second acoustic resonator. Moreover, a first thickness of the piezoelectric layer under the first IDT is greater than a second thickness of piezoelectric layer under the second IDT.