BAW Resonator Bi-Layer Top Electrode for Zero Etch Undercut
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Solution Overview
Problem
Conventional piezoelectric resonators face issues with etch undercutting during the fabrication of bi-layer top electrodes, particularly due to galvanic and catalytic reactions during wet etching, which negatively impacts the performance and integrity of the resonator.
Innovation Solution
A multi-layer top electrode configuration is employed, where the topmost layer protects the underlying layers from subsequent etching steps, using distinct materials for the bottom and top metal layers to prevent etch undercut, and an interconnect metal layer is used to isolate the bottom metal layer from etching, ensuring the top electrode is not undercut during fabrication.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional single-layer top electrode is used, then the fabrication process is simpler, but etch undercut occurs during wet etching due to galvanic and catalytic reactions
Solution Approach 1:
The top electrode is divided into two distinct layers: a bottom metal layer (e.g., molybdenum) and a top metal layer (e.g., aluminum or aluminum alloy). This segmentation allows each layer to serve specific functions - the bottom layer provides electrical connectivity while the top layer acts as an etch barrier, preventing galvanic and catalytic reactions during wet etching that would otherwise cause undercutting.
Solution Approach 2:
The top metal layer serves as an intermediary protective layer between the etchant and the bottom metal layer. During wet etching of the piezoelectric layer, this top layer prevents direct contact between the etchant and the bottom metal layer, thereby eliminating galvanic and catalytic reactions that cause etch undercut.
2Ease of manufacture
If the bottom metal layer is exposed during etching, then the etching process is more accessible, but undercutting of the top electrode occurs negatively impacting performance
Solution Approach 1:
The top metal layer is deposited over the bottom metal layer before the wet etching process begins. This preliminary protective layer ensures that during subsequent etching steps, the bottom metal layer remains protected and exposed only where intentionally designed, preventing unwanted undercutting and maintaining electrode integrity throughout the fabrication process.
3Manufacturing precision
If a bi-layer top electrode configuration is used, then etch undercut is prevented and active area dimensions are better controlled, but the device complexity increases
Solution Approach 1:
The bi-layer configuration assigns different local qualities to different parts of the electrode structure. The bottom metal layer (e.g., molybdenum) provides excellent electrical conductivity and adhesion, while the top metal layer (e.g., aluminum) provides etch barrier properties. This local differentiation of material properties enables precise control of active area dimensions while maintaining overall device functionality.
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 enhances the control over the active area dimensions of the resonator, reduces spurious modes, and improves the acoustic and electrical performance by preventing etch undercut, thereby providing a superior piezoelectric resonator architecture.
Implementation Method 1
galvanic and catalytic reactions during wet etching
Implementation Method 2
galvanic and catalytic reactions during wet etching
Implementation Method 3
When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer
Implementation Method 4
BAW resonators are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition
Implementation Method 5
BAW resonators are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition
Data Source
AI summary
A method of fabricating a BAW piezoelectric resonator, the method comprising the steps of providing a bottom electrode and a piezoelectric layer coupled to the bottom electrode. A bottom metal layer is deposited on a top electrode on the piezoelectric layer. The bottom metal layer is patterned and etched. A top metal layer of the top electrode is deposited on the etched bottom metal layer. The top metal layer is patterned and layered such that the top metal layer completely covers the top and sides of the bottom metal layer.