BAW Resonator Electrode Air Gap for Conductivity and Acoustic Loss
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Solution Overview
Problem
Conventional Bulk Acoustic Wave (BAW) resonators face challenges in maintaining high conductivity of metal electrodes while avoiding mass loading and acoustic loss, which degrades performance, especially at high frequencies.
Innovation Solution
Incorporating an air gap between the high conductivity and high impedance metal layers within the electrode structure of the BAW resonator, allowing for increased thickness of the high conductivity layer without mass loading and maintaining performance by terminating acoustic waves at the high impedance layer/air interface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the thickness of the high conductivity metal layer is increased to improve electrical conductivity, then electrical conductivity is improved, but mass loading increases and acoustic loss increases, degrading resonator performance
Solution Approach 1:
The top electrode is segmented into multiple metal layers with an air gap between them. The first metal layer provides high electrical conductivity while the second metal layer provides high acoustic impedance, and the air gap prevents acoustic energy from reaching the first layer. This segmentation allows each layer to perform its specialized function without the drawbacks of a single thick layer.
Solution Approach 2:
The air gap acts as an intermediary between the first and second metal layers. It serves as an acoustic barrier that reflects acoustic waves before they can reach the first metal layer, while still allowing the electrode structure to function as a unified electrical conductor. The air gap mediates between the conflicting requirements of electrical conductivity and acoustic impedance.
2Reliability
If the thickness of the top electrode is increased to gain conductivity, then electrical conductivity is improved, but the resonant frequency changes and Q factor decreases
Solution Approach 1:
The electrode is divided into multiple functional layers that can be independently optimized. The first metal layer thickness is optimized for electrical conductivity while the second metal layer and air gap combination is optimized to maintain acoustic properties and resonant frequency stability, resolving the trade-off between conductivity and frequency stability.
Solution Approach 2:
Different regions of the electrode structure are assigned different properties: the first metal layer has high electrical conductivity, the second metal layer has high acoustic impedance, and the air gap has low acoustic density. This local differentiation allows each region to contribute to the overall performance without compromising the other requirements.
3Reliability
If a single thick metal layer is used to provide both high conductivity and high acoustic impedance, then both properties are improved, but acoustic energy is driven into the metal layer causing Q factor to drop
Solution Approach 1:
The electrode is segmented into two separate metal layers with an air gap between them. The second metal layer provides high acoustic impedance to reflect acoustic waves, while the first metal layer provides electrical conductivity. The air gap prevents acoustic energy from reaching the first layer, eliminating the Q factor degradation that would occur with a single thick layer.
Solution Approach 2:
The air gap serves as an intermediary acoustic barrier between the acoustic field and the first metal layer. It reflects acoustic waves before they can enter the conductive metal layer, preventing the conversion of acoustic energy into heat that would reduce the Q factor, while still allowing the electrode to provide the necessary electrical conductivity.
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 enhances electrical conductivity while reducing resistive and acoustic losses, maintaining the quality factor and resonant frequency of the BAW resonator.
Implementation Method 1
terminating acoustic waves at the high impedance layer/air interface
Implementation Method 2
acoustic waves in the piezoelectric layer 12 within the active region 32 of the BAW resonator 10 are excited by an electrical signal applied to the bottom and top electrodes 14 and 16
Data Source
AI summary
Embodiments of a Bulk Acoustic Wave (BAW) device including a high conductivity electrode are disclosed. In some embodiments, a BAW device includes a piezoelectric layer, a first electrode on a first surface of the piezoelectric layer, and a second electrode on a second surface of the piezoelectric layer opposite the first electrode. The second electrode includes a first metal layer and a second metal layer. The second metal layer is on the second surface of the piezoelectric layer, and the first metal layer is over a surface of the second metal layer opposite the piezoelectric layer, where the first metal layer is separated from the second metal layer by an air gap. By including the air gap, the thickness of the first metal layer (e.g., a high conductivity layer) can be increased to thereby increase the electrical conductivity of the second electrode while maintaining the performance of the BAW device.