Elastic Wave Filter Layout to Prevent Piezoelectric Film Cracks
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Solution Overview
Problem
Conventional elastic wave devices with a cavity portion below a piezoelectric film are prone to cracks in the piezoelectric film due to stress concentration at the corner portions of the bus bars.
Innovation Solution
The elastic wave device incorporates a support substrate with a recessed portion to create a cavity portion, using a piezoelectric film made of lithium niobate or lithium tantalate, and an electrode structure with bus bars and electrode fingers. The corner portions of the bus bars are positioned outside the cavity portion to reduce stress concentration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a cavity portion is provided below the piezoelectric film to improve resonance characteristics, then Q-value is improved, but cracks occur in the piezoelectric film due to stress concentration at bus bar corner portions
Solution Approach 1:
The bus bar corner portions are extracted from the cavity portion area and positioned outside the cavity. This separation removes the stress concentration points from the vulnerable region, allowing the cavity to provide resonance enhancement without causing cracks in the piezoelectric film.
Solution Approach 2:
The electrode structure is designed with different spatial configurations: the bus bar corner portions are positioned outside the cavity portion where stress concentration would be harmful, while the main electrode body remains inside the cavity to benefit from the resonance effect. This local differentiation optimizes both structural integrity and resonant performance.
2Reliability
If bus bars are positioned inside the cavity portion to maximize resonance effect, then Q-value is improved, but stress concentration at corner portions causes cracks in the piezoelectric film
Solution Approach 1:
The harmful corner portions of the bus bars are extracted from the cavity portion and repositioned outside it. This extraction eliminates the stress concentration effect while preserving the beneficial resonance characteristics generated by the cavity structure.
Solution Approach 2:
The design accepts that bus bars are necessary for electrode function but strategically positions their corner portions outside the cavity where they cannot cause stress concentration. This converts a potentially harmful element into a benign one while maintaining the beneficial resonance effect of the cavity.
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 effectively reduces or prevents cracks in the piezoelectric film, allowing for improved resilience and reliability of the elastic wave device while maintaining high Q-value resonance characteristics.
Implementation Method 1
a piezoelectric film made of lithium niobate or lithium tantalate, laminated on the first main surface of the support substrate... The elastic wave device uses a bulk wave in a thickness slip mode
Data Source
AI summary
An elastic wave device includes a piezoelectric film made of lithium niobate or lithium tantalate, and a first electrode finger and a second electrode finger opposing each other in a direction intersecting a thickness direction of the piezoelectric film. When an average thickness of the piezoelectric film is d and a distance between centers of the first electrode finger and the second electrode finger is p, d/p is about 0.5 or less. The first electrode finger and the second electrode finger are connected to the first and second bus bars, respectively. The first and second bus bars include corner portions. At least one of corner portions of the first and second bus bars is outside a cavity portion when viewed in plan view.