BAW Filter Structure With Isolated Piezoelectric Films for Low-Stress Bonding
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Solution Overview
Problem
Traditional bulk acoustic wave (BAW) filter manufacturing methods face issues with film stress and breakage due to thermal expansion mismatches and complex processing, leading to high device failure rates and challenges in achieving rapid mass production.
Innovation Solution
A BAW filter structure and preparation method involving independent piezoelectric film elements on an epitaxial substrate, with corresponding bottom electrode units, and a bonding structure on a supporting substrate, allowing for axisymmetric resonant regions and reduced stress through etching isolation and simplified substrate processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If entire wafer bonding method is used to transfer piezoelectric films, then mass production capability is improved, but film stress and breakage increase due to thermal expansion mismatch
Solution Approach 1:
The patent divides the piezoelectric film into independent island structures separated by etched grooves, rather than treating it as a continuous entire wafer. This segmentation reduces the overall stress accumulation and prevents catastrophic breakage across the entire film, while still allowing batch processing for mass production.
Solution Approach 2:
The patent extracts and removes the epitaxial substrate after forming the piezoelectric film elements, transferring only the necessary film structures to the supporting substrate. This extraction eliminates the source of thermal expansion mismatch stress while preserving the functional piezoelectric elements for production.
2Manufacturing precision
If chemical mechanical polishing is performed on supporting substrate, then surface evenness for bonding is improved, but process complexity and mechanical damage risk increase
Solution Approach 1:
The patent performs preliminary preparation of the supporting substrate by forming cavity structures and sacrificial layers before bonding. This preliminary action ensures that the substrate surface is properly prepared for selective bonding without requiring extensive post-bonding polishing or complex surface evenness correction processes.
3Adaptability or versatility
If interconnecting through holes are etched in piezoelectric films for cascading filter construction, then filter interconnection is achieved, but film collapse and breakage are caused
Solution Approach 1:
The patent uses metal bonding layers as intermediary structures that extend into the cavity regions to provide electrical interconnection between cascaded filter stages. This intermediary approach allows filter interconnection without etching through holes in the piezoelectric film itself, preserving film integrity while achieving the required adaptability for cascading configurations.
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 method reduces film stress, simplifies the production process, and increases yield by avoiding complex substrate polishing and interconnecting through holes, making it suitable for industrial-scale production of high-quality BAW filters with minimal stress and cost.
Implementation Method 1
a bulk acoustic wave propagating longitudinally is excited by the piezoelectric film under the action of an alternating current electric field
Implementation Method 2
the bulk acoustic wave is reflected by upper and lower interfaces of a composite film and thus confined within the composite film to oscillate back and forth to form a standing wave
Data Source
AI summary
The present disclosure provides a bulk acoustic wave (BAW) filter structure and a preparation method thereof. According to the present disclosure, piezoelectric film elements are formed on a surface of an epitaxial substrate to form a transfer structure; resonant regions are defined on a supporting substrate and covered with bonding units to obtain a bonding structure; upper and lower surfaces of the transfer structure are reversed, and bottom electrode units are bonded to the resonant regions correspondingly one to one to obtain a BAW structure; and the epitaxial substrate is removed, and top electrode units are formed on surfaces of the piezoelectric film elements that are in contact with the epitaxial substrate previously. The BAW filter structure of the present disclosure can achieve batch production with low cost, high efficiency and high yield.