Two-Layer XBAR Electrodes for Heat and Spurious Mode Control
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Solution Overview
Problem
Current RF filters using acoustic wave resonators are not well-suited for higher frequency communications bands, such as those defined in the 5G NR standard, particularly for bands n77 and n79, and millimeter wave frequencies, due to limitations in handling transmit power and dissipating heat efficiently.
Innovation Solution
The use of Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with two-layer IDT fingers and specific geometries to reduce spurious modes and enhance thermal and electrical conductivity, allowing for improved performance in high-frequency applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional acoustic wave resonators are used, then the filter can operate at lower frequencies, but the filter cannot efficiently handle high-frequency transmit power and dissipate heat in 5G NR and millimeter wave bands
Solution Approach 1:
The resonator structure is segmented into multiple functional layers including a piezoelectric layer, a sacrificial layer, and an acoustic mirror structure with multiple reflective layers. This segmentation allows each layer to perform specific functions: the piezoelectric layer generates acoustic waves, the sacrificial layer creates air gaps for thermal management, and the acoustic mirror reflects acoustic waves to enhance resonance while managing heat dissipation pathways.
Solution Approach 2:
The invention introduces a vertical dimension to heat dissipation by creating air gaps between the piezoelectric layer and the substrate using a sacrificial layer. This three-dimensional structure allows heat to dissipate through both the substrate and the air gap pathway, significantly improving thermal management capability for high-frequency operations in 5G NR and millimeter wave bands.
2Power
If the IDT finger geometry is optimized for high electromechanical coupling, then frequency capability improves, but spurious modes are generated
Solution Approach 1:
The IDT finger geometry is optimized with specific local characteristics including tapered finger widths and strategically positioned apertures. The fingers have varying widths along their length, with wider regions at the ends and narrower regions in the middle, creating localized stress distributions that enhance electromechanical coupling while suppressing spurious mode generation through controlled acoustic wave propagation.
Solution Approach 2:
The resonator structure incorporates dynamic elements including the tapered IDT fingers and the acoustic mirror structure that actively manage acoustic wave reflections. The aperture regions in the IDT fingers create dynamic acoustic pathways that enhance the desired resonant mode while dynamically suppressing spurious modes through interference patterns and controlled reflections from the acoustic mirror.
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
XBARs provide high electromechanical coupling and frequency capability, effectively handling high-frequency power and reducing spurious modes, thus enabling efficient heat dissipation and improved filter performance in 5G NR and millimeter wave communication bands.
Implementation Method 1
An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating layer, or diaphragm, of a single-crystal piezoelectric material. The IDT includes a first set of parallel fingers, extending from a first busbar and a second set of parallel fingers extending from a second busbar. The first and second sets of parallel fingers are interleaved. A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm.
Implementation Method 2
an acoustic mirror structure including a first reflective layer, a second reflective layer, and a third reflective layer. The first reflective layer is spaced from the piezoelectric material by a first air gap. The second reflective layer is spaced from the piezoelectric material by a second air gap. The third reflective layer is spaced from the piezoelectric material by a third air gap.
Data Source
AI summary
There is disclosed acoustic resonators and filter devices. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm. The interleaved fingers comprise a first layer adjacent the diaphragm and a second layer over the first layer opposite the diaphragm.