BAW Resonator Guard Ring Layout for Spurious Mode Suppression
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Solution Overview
Problem
Bulk acoustic wave (BAW) resonators face challenges in maintaining a sufficient frequency separation between series and parallel resonant modes, leading to energy loss and increased phase noise due to spurious modes, which limits their operating range and performance.
Innovation Solution
Incorporating a metal guard ring with a width between 2.5 μm to 3.5 μm between the piezoelectric layer and the upper acoustic reflector, which suppresses spurious modes and increases the coupling coefficient, thereby enhancing the quality factor and frequency separation between resonant modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional BAW resonator structure is used, then the device can operate at the desired frequency, but the frequency separation between series and parallel resonant modes is insufficient, leading to energy loss and increased phase noise
Solution Approach 1:
The patent segments the resonator structure by introducing a guard ring that divides the electrode-piezoelectric layer interface into distinct regions: a central active region and a peripheral guard region. This segmentation creates acoustic impedance boundaries that suppress spurious modes and improve frequency separation between resonant modes, directly addressing the reliability issue while reducing energy loss.
Solution Approach 2:
The guard ring acts as an intermediary element between the electrode and the piezoelectric layer. This intermediate structure modifies the acoustic field distribution and provides a transition zone that suppresses spurious modes, thereby improving frequency separation and reducing energy loss without requiring fundamental changes to the resonator architecture.
2Speed
If the piezoelectric layer thickness is reduced to achieve higher frequencies, then the operating frequency range increases, but the coupling coefficient decreases, limiting performance
Solution Approach 1:
The patent applies local quality by creating a non-uniform structure at the electrode-piezoelectric layer interface through the guard ring. This local structural modification concentrates acoustic energy in specific regions and enhances the coupling coefficient locally, compensating for the overall thickness reduction and maintaining high-performance coupling at elevated frequencies.
Solution Approach 2:
The patent changes the geometric parameters of the resonator structure by introducing the guard ring with specific width and position parameters. These parameter changes modify the acoustic field distribution and stress patterns, thereby enhancing the coupling coefficient and allowing the device to operate at higher frequencies with improved performance.
3Reliability
If a guard ring is added to suppress spurious modes, then the coupling coefficient and quality factor improve, but the device complexity and manufacturing steps increase
Solution Approach 1:
The patent merges the guard ring formation with existing fabrication processes by integrating it into the electrode patterning step. The guard ring is formed using the same photolithography and deposition processes as the main electrode, combining multiple functions into a single structural element and minimizing additional manufacturing complexity.
Solution Approach 2:
The guard ring serves multiple functions simultaneously: it suppresses spurious modes, enhances the coupling coefficient, improves frequency separation, and defines the active region boundary. This multi-functionality reduces the need for additional separate components and simplifies the overall device architecture despite the added structural element.
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
The guard ring design increases the coupling coefficient by approximately 15% and improves the resonator's performance by reducing energy loss and phase noise, allowing for a wider operating range and increased manufacturability.
Implementation Method 1
A piezoelectric layer is formed over the lower acoustic reflector, and an upper acoustic reflector also including alternating dielectric layers of lower and higher acoustic impedance materials is formed over the piezoelectric layer. A metal guard ring is formed between the piezoelectric layer and the upper acoustic reflector
Data Source
AI summary
A BAW resonator includes first and second electrodes located over a substrate. A piezoelectric layer is located between the first and second electrodes. A guard ring is located between the piezoelectric layer and the second electrode, and is spaced apart from a perimeter of the electrode. The guard ring has a width in a range from 2.5 μm to 3.5 μm.