BAW Resonator Top Electrode Recess Structure for Etch Undercut Control
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Solution Overview
Problem
Conventional piezoelectric resonators face issues during etching, particularly with undercutting of the top electrode, which affects the performance and accuracy of the resonator due to galvanic and catalytic reactions during wet etching processes.
Innovation Solution
A multi-layer top electrode configuration is introduced, where the top metal layer is recessed relative to the bottom metal layer, allowing for a passivation layer to cover and protect the underlying layers from subsequent etching, preventing undercutting and ensuring precise control over the active area dimensions.
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 multiple layers (first top metal layer and second top metal layer) with different etch selectivities. This segmentation allows the first layer to be etched without significant undercut while the second layer provides a protective cap, thereby preventing etch undercut and improving manufacturing precision without significantly complicating the fabrication process.
Solution Approach 2:
The first top metal layer acts as an intermediary between the piezoelectric layer and the second top metal layer. It has intermediate etch selectivity compared to the piezoelectric layer and the second metal layer, allowing it to be selectively etched while protecting the underlying piezoelectric layer from undercut, thus resolving the contradiction between process simplicity and edge control precision.
2Productivity
If wet etching is used for simplicity, then the etching process is faster and easier, but galvanic and catalytic reactions cause undercutting of the top electrode
Solution Approach 1:
The top electrode is segmented into two layers with different etch resistances. The first top metal layer has intermediate etch selectivity, allowing wet etching to proceed quickly through it without attacking the underlying piezoelectric layer, while the second top metal layer with high etch selectivity protects the structure. This maintains high productivity while ensuring reliability.
Solution Approach 2:
The first top metal layer is deposited and patterned before the second top metal layer. This preliminary action creates a protective structure that prevents galvanic and catalytic reactions from causing undercut during subsequent wet etching of interconnect layers, thereby maintaining both etching speed and electrode integrity.
3Shape
If the top metal layer is etched to the same level as the bottom metal layer, then the electrode structure is more uniform, but undercutting occurs during subsequent interconnect etching
Solution Approach 1:
The top electrode is designed with asymmetric layering: the first top metal layer extends to the same level as the bottom metal layer for uniformity, while the second top metal layer is recessed to provide protection. This asymmetric configuration maintains the benefits of uniform electrode structure while preventing undercut during subsequent etching processes.
Solution Approach 2:
Instead of varying the lateral dimensions of a single-layer electrode, the solution adds a vertical dimension with a second top metal layer that is recessed relative to the first layer. This dimensional change allows the first layer to maintain uniformity while the second layer provides the necessary protection against undercut, preserving manufacturing precision.
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 performance of piezoelectric resonators by preventing etch undercut, maintaining the integrity of the top electrode and improving the acoustic, electrical, and process integration requirements, leading to superior resonator architecture.
Implementation Method 1
The passivation layer covers and protects the underlying layers from subsequent etching, thereby preventing etch undercut of the top electrode
Implementation Method 2
When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer
Implementation Method 3
The acoustical isolation is produced with a substrate via-hole, a micromechanical bridge structure, or with an acoustic mirror structure
Data Source
AI summary
A method of fabricating a piezoelectric resonator includes providing a bottom electrode and a piezoelectric layer coupled to the bottom electrode. A bottom metal layer of a top electrode is deposited on the piezoelectric layer. A top metal layer of the top electrode is deposited on the bottom metal layer. A photoresist layer is deposited on the top metal layer. The photoresist layer is patterned and etched. The top metal layer is patterned and etched while the etched photoresist layer remains. The bottom metal layer is patterned and etched such that an entire perimeter side surface of the top metal layer is recessed relative to a perimeter edge of the bottom metal layer. The etched photoresist layer is removed. A passivation layer is deposited on the top and bottom metal layers such that the top and bottom metal layers are isolated from a subsequent metal etch step.