Sandwich Electrode BAW Resonators With Thicker Acoustic Cavities
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Solution Overview
Problem
Conventional bulk acoustic wave (BAW) devices face challenges in achieving higher resonant frequencies without compromising manufacturing complexity, electrical resistance, capacitance, and power handling capability.
Innovation Solution
The use of sandwich electrodes in BAW devices, comprising an outer layer with higher acoustic impedance and an inner layer with lower acoustic impedance, allows for a thicker acoustic cavity length without thinning the piezoelectric layer and electrodes, thereby increasing resonant frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the thicknesses of the piezoelectric layer and electrodes are reduced to achieve higher resonant frequencies, then the resonant frequency increases, but the manufacturing difficulty increases and electrical resistance increases
Solution Approach 1:
The electrode is segmented into multiple layers (first electrode layer, second electrode layer, third electrode layer) with different acoustic impedances. This segmentation allows the electrode structure to be optimized for both high frequency operation and manufacturing feasibility, as each layer can be designed with appropriate thickness and material properties independent of the others.
Solution Approach 2:
The electrode uses composite material structure with layers of different acoustic impedances (e.g., tungsten layer combined with molybdenum layer). This composite approach enables the electrode to achieve the desired acoustic properties for high frequency resonance while maintaining manufacturability and controlling electrical resistance through material selection.
2Length of stationary object
If the piezoelectric layer is thinned to reduce total device thickness without reducing electrode thickness, then the device thickness is reduced, but the capacitance between electrodes increases
Solution Approach 1:
The invention changes the acoustic impedance parameter distribution within the electrode structure by using layers with different acoustic impedances. This allows the electrode to maintain appropriate acoustic thickness for frequency control while the piezoelectric layer can be optimized independently, managing capacitance effects through the overall device design rather than being constrained by uniform thickness reductions.
3Reliability
If the device area is reduced to avoid increasing capacitance, then the capacitance is controlled, but the power density increases and power handling capability is limited
Solution Approach 1:
The segmented electrode structure allows different regions of the electrode to serve different functions - some layers optimized for acoustic performance and others for electrical performance. This segmentation enables the device to maintain larger effective area for power handling while controlling capacitance through the layered structure's acoustic properties.
4Length of stationary object
If thinner piezoelectric layers are used to reduce device thickness, then the device becomes smaller, but the breakdown voltage decreases
Solution Approach 1:
The use of composite electrode materials with different acoustic impedances allows the piezoelectric layer to be optimized for thickness without being constrained by the need for uniformly thick electrodes. The composite electrode structure compensates for the thinner piezoelectric layer, maintaining overall device robustness and breakdown voltage through the acoustic impedance matching provided by the multi-layer electrode design.
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 enables BAW devices to resonate at higher frequencies without the manufacturing difficulties and performance limitations associated with conventional devices, while maintaining robustness and power handling capabilities.
Implementation Method 1
BAW devices receive an electrical signal that produces a varying (RF) electric field between two electrodes in a BAW device, causing the piezo layer positioned between the electrodes to expand and contract to produce acoustic waves having a resonant frequency
Implementation Method 2
A sandwich electrode includes an outer layer with a higher acoustic impedance and an inner layer with a lower acoustic impedance, with the inner layer disposed between the outer layer and the piezoelectric layer
Data Source
AI summary
A bulk acoustic wave (BAW) device comprises a piezoelectric layer disposed between a first electrode layer and a sandwich electrode. The sandwich electrode includes a first layer of a first material having a first acoustic impedance and a second layer of a second material having a second acoustic impedance that is less than the first acoustic impedance of the first layer. The second layer of the sandwich electrode having the lower acoustic impedance is disposed between the first layer and the piezoelectric layer. The sandwich electrode combined with the piezoelectric layer and first electrode can cause the BAW device to resonate at a frequency whose wavelength corresponds to an acoustic cavity length of the BAW device, depending on an acoustic mirror included on one side of the BAW device. In one example, the acoustic cavity length is about 1.5 times of the resonant frequency wavelength.