Stacked BAW Resonator Structure for High-Frequency Overtone Stability
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Solution Overview
Problem
High-frequency bulk acoustic wave (BAW) resonators face challenges due to thin piezoelectric material films, leading to decreased power-handling capacities, electromechanical coupling factors, and quality (Q) factors, as well as increased electrode resistance, which affects performance and resonator stability.
Innovation Solution
The BAW device employs a stack of at least two piezoelectric material layers of the same polarity with an interposer and a raised frame structure, configured to excite an even overtone mode, using thicker piezoelectric and electrode layers with doping to maintain mechanical stability and low resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If thin piezoelectric material films are used to achieve high resonance frequencies, then resonance frequency is improved, but power-handling capacity, electromechanical coupling factor, and quality factor deteriorate
Solution Approach 1:
The piezoelectric material layer is divided into multiple thinner layers stacked together, where each layer has a thickness of 50-200 nm. This segmentation allows the total piezoelectric thickness to be sufficient for high frequency operation while individual layers remain thin enough to maintain quality factors. The intermediate electrode connects these segmented layers electrically while acoustically isolating them to prevent short-circuiting.
Solution Approach 2:
The patent transitions from a single-plane electrode structure to a stacked multi-layer structure with intermediate electrodes positioned between piezoelectric layers. This dimensional change in the layer stack architecture enables simultaneous achievement of high resonance frequency (through total thickness) and high quality factor (through individual layer thinness), resolving the contradiction between frequency and reliability.
2Speed
If thin piezoelectric material films are used, then resonance frequency is improved, but electromechanical coupling factor deteriorates
Solution Approach 1:
The piezoelectric material is segmented into multiple layers with intermediate electrodes, allowing the cumulative piezoelectric effect to be sufficient for high coupling factor while individual layers remain thin for high frequency operation. Each piezoelectric layer contributes to the overall electromechanical coupling without suffering from the drawbacks of excessive thinness.
Solution Approach 2:
The patent creates a composite structure alternating piezoelectric material layers with intermediate electrode layers. This composite architecture combines the high piezoelectric coefficient materials with conductive intermediate layers to achieve both high electromechanical coupling factor and high resonance frequency, as the composite structure multiplies the piezoelectric effect across multiple interfaces.
3Speed
If thin piezoelectric material films are used, then resonance frequency is improved, but quality factor deteriorates
Solution Approach 1:
The piezoelectric layer is segmented into multiple thin layers (50-200 nm each) separated by intermediate electrodes. This segmentation prevents acoustic energy leakage and reduces damping losses that would occur in a single thin layer, thereby maintaining high quality factor while achieving high resonance frequency through the cumulative thickness of multiple layers.
Solution Approach 2:
The intermediate electrodes act as acoustic barriers and electrical connectors between piezoelectric layers. These intermediary structures prevent acoustic energy from short-circuiting through a single thin piezoelectric layer, thereby reducing energy loss and maintaining high quality factor while enabling high frequency operation through multiple stacked layers.
4Ease of operation
If lighter electrodes with smaller electrode areas are used for impedance matching, then impedance matching is improved, but electrical conductivity deteriorates
Solution Approach 1:
The electrode structure is segmented into multiple intermediate electrode layers positioned between piezoelectric layers. Each intermediate electrode can be optimized for local impedance matching requirements, while the cumulative effect of multiple electrodes provides sufficient total conductivity. This segmented electrode architecture resolves the contradiction between localized impedance matching and overall electrical conductivity.
5Speed
If simple overtone scaling with uniform piezoelectric material layer is used, then high resonance frequency is achieved, but coupling deteriorates by the square of the overtone mode
Solution Approach 1:
Instead of using a single uniform piezoelectric layer for overtone scaling, the patent segments the piezoelectric material into multiple layers with intermediate electrodes. This segmentation maintains strong fundamental mode coupling while enabling overtone operation, preventing the coupling degradation that occurs with simple overtone scaling of uniform layers.
Solution Approach 2:
The patent changes from a single-plane uniform piezoelectric layer to a stacked multi-dimensional structure with intermediate electrodes. This dimensional change in the piezoelectric architecture enables overtone mode operation without the coupling degradation proportional to the square of the overtone mode, as the stacked structure provides multiple coupling pathways.
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 configuration enables higher resonant frequencies up to 10 GHz while maintaining acoustic energy confinement and reducing electrode resistance, enhancing the BAW resonator's performance and stability.
Implementation Method 1
a stack of at least two first piezoelectric material layers of the same polarity type sandwiched between the first electrode and the second electrode
Implementation Method 2
acoustic waves propagate in a bulk of a piezoelectric material layer
Data Source
AI summary
Aspects and embodiments disclosed herein include a bulk acoustic wave (BAW) device comprising a first electrode, a second electrode, a stack of at least two first piezoelectric material layers of the same polarity type sandwiched between the first electrode and the second electrode, and a first interposer sandwiched between the at least two first piezoelectric material layers and including at least one intermediate electrode, the BAW device being configured to excite an even overtone mode as the main mode of operation.