Metallic Bragg Mirror Electrodes for High-Frequency BAW Resonators
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Solution Overview
Problem
Film bulk acoustic wave resonators face issues with spurious acoustic waves due to transverse waves generated perpendicular to the main acoustic wave, degrading frequency response, and high-frequency operation is hindered by quality factor deterioration and thin piezoelectric layers leading to insertion loss and power handling limitations.
Innovation Solution
Employing metallic Bragg mirror electrodes with alternating layers of metals like ruthenium and titanium to confine acoustic energy, maintaining piezoelectric material thickness, and dissipating transverse waves, while using a substrate to reduce overmoding effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional electrode structures are used in bulk acoustic wave resonators operating at higher frequencies, then the resonator can be fabricated, but the quality factor deteriorates and power handling capability is reduced
Solution Approach 1:
The electrode structure is segmented into multiple thin metal layers (e.g., alternating layers of ruthenium and titanium) rather than using a single thick electrode layer. This segmentation creates a Bragg mirror structure that reflects acoustic waves back into the piezoelectric film, improving quality factor while maintaining manufacturability at higher frequencies
Solution Approach 2:
The electrode is constructed as a composite structure with alternating layers of metals having different acoustic impedances (e.g., ruthenium and titanium). This composite Bragg mirror structure provides both acoustic reflection functionality and electrical conductivity, resolving the contradiction between quality factor and fabrication precision
2Speed
If the piezoelectric film thickness is reduced to maintain resonance frequency at higher operating frequencies, then the resonator operates at the desired frequency, but the quality factor and power handling capability deteriorate
Solution Approach 1:
The electrode is divided into multiple thin layers forming a Bragg mirror that reflects acoustic energy back into the piezoelectric film. This allows the use of thinner piezoelectric films for higher frequency operation while maintaining quality factor through improved acoustic energy confinement
Solution Approach 2:
The acoustic impedance parameters of the electrode layers are optimized by selecting metals with appropriate impedance mismatches (e.g., ruthenium and titanium). This parameter optimization enhances acoustic reflection at the desired resonance frequency, allowing thinner piezoelectric films to maintain high quality factor
3Device complexity
If single-layer electrodes are used to simplify the electrode structure, then the device complexity is reduced, but the electrical conductivity and acoustic energy confinement are insufficient
Solution Approach 1:
The electrode uses composite metal layers with alternating high and low acoustic impedance materials (e.g., ruthenium and titanium). This composite structure provides both electrical conductivity and acoustic reflection properties, achieving energy confinement without excessive complexity
Solution Approach 2:
The Bragg mirror electrode structure serves multiple functions simultaneously: it provides electrical conductivity for signal application, reflects acoustic waves to confine energy in the piezoelectric film, and maintains a planar structure compatible with standard fabrication processes
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
Enhances resonator quality factor, improves power handling, and maintains piezoelectric film thickness for better frequency response and reduced spurious signals, facilitating higher resonance frequencies and improved manufacturing reliability.
Implementation Method 1
metallic Bragg mirror electrodes formed by alternating layers of metals with large acoustic impedance mismatch, such as ruthenium and titanium, to confine acoustic energy within the piezoelectric material layer
Implementation Method 2
layer of piezoelectric material
Data Source
AI summary
Aspects and embodiments include a bulk acoustic wave resonator comprising a layer of piezoelectric material, an upper electrode disposed on top of the layer of piezoelectric material, the upper electrode including a metallic Bragg mirror having alternating layers of a first metal and a second metal, and a lower electrode disposed on a bottom of the layer of piezoelectric material, the lower electrode including a metallic Bragg mirror having alternating layers of a third metal and a fourth metal.