Acoustic Wave Substrate Layout for Thermal Stress Reliability
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Solution Overview
Problem
Acoustic wave devices face reliability issues due to stress generated by differences in linear expansion coefficients between piezoelectric and substrate materials, with same-material substrates lacking sufficient strength.
Innovation Solution
An acoustic wave device design where the piezoelectric substrate is bonded to a support substrate with a frame, and the substrate is made of a material with a similar linear expansion coefficient, ensuring the piezoelectric substrate remains only in acoustic wave element regions and is removed in frame regions, reducing stress and enhancing reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the material of the piezoelectric substrate differs from that of the substrate, then the linear expansion coefficient difference increases, but stress is generated decreasing reliability
Solution Approach 1:
The piezoelectric substrate is selectively removed from specific regions (frame area and electrode pad areas) while being retained in the acoustic wave element area. This local differentiation allows the frame to be formed on the support substrate directly, enabling the use of strong materials like ceramic or metal for the substrate without generating stress, while maintaining the necessary piezoelectric functionality in the active region.
2Reliability
If the material of the piezoelectric substrate is the same as that of the substrate, then stress is reduced, but the substrate material must be lithium tantalate or lithium niobate which lacks sufficient strength
Solution Approach 1:
The device is segmented into three distinct functional regions: the acoustic wave element area where the piezoelectric substrate is retained for functional operation, the frame area where the piezoelectric substrate is removed to allow bonding of strong substrate materials, and the electrode pad areas where the piezoelectric substrate is also removed for electrical connection purposes. This segmentation resolves the contradiction by spatially separating the requirements for stress reduction and strength.
Solution Approach 2:
The support substrate acts as an intermediary between the acoustic wave elements and the final substrate. It provides a bonding interface that can accommodate strong materials (ceramic, metal, or glass) while the piezoelectric substrate is selectively removed in non-active regions, allowing the support substrate to bear mechanical loads without generating stress from linear expansion coefficient mismatches.
3Reliability
If the piezoelectric substrate is removed in frame regions, then stress is reduced and reliability improves, but the frame formation process becomes more complex
Solution Approach 1:
The piezoelectric substrate is removed from the frame area and electrode pad areas before the frame formation process. This preliminary action simplifies the subsequent frame formation by providing direct access to the support substrate surface, eliminating the need for complex through-holes or vias that would be required if the piezoelectric substrate remained in those regions. The removal is performed as a preliminary step that facilitates rather than complicates the overall manufacturing process.
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 reduces stress and improves the reliability of the acoustic wave device by minimizing the difference in linear expansion coefficients between the support and piezoelectric substrates, allowing for sufficient strength and improved temperature characteristics.
Implementation Method 1
a piezoelectric substrate bonded to a support substrate; a first acoustic wave element formed on the piezoelectric substrate
Data Source
AI summary
An acoustic wave device includes: a support substrate; a piezoelectric substrate bonded to the support substrate; a first acoustic wave element formed on the piezoelectric substrate; a frame formed on the support substrate to surround the first acoustic wave element; and a substrate formed on the frame so that a cavity to which the first acoustic wave element is exposed is formed above the piezoelectric substrate, wherein a difference in linear expansion coefficient between the support substrate and the substrate in a first direction in a surface direction of the piezoelectric substrate is less than a difference in linear expansion coefficient between the support substrate and the piezoelectric substrate in the first direction, and the piezoelectric substrate remains in a region where the first acoustic wave element is formed and is removed in a region where the frame is formed.