Elastic Wave Pad Layout to Protect Piezoelectric Thin Films
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Solution Overview
Problem
Elastic wave devices with piezoelectric single-crystal thin films are prone to breaking and peeling due to high pressure during bonding and mounting, and thermal stress causes structural issues, leading to reliability concerns.
Innovation Solution
The design includes a multilayer film structure with a piezoelectric thin film, a high acoustic velocity film, and a low acoustic velocity film laminated on a support substrate, where the piezoelectric thin film is not present below the pad electrode portion, and the pad electrode is laminated onto a recessed surface of the substrate, ensuring the piezoelectric thin film is protected from direct pressure and thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the piezoelectric single-crystal thin film is made thin to reduce device size, then the device can be miniaturized, but the piezoelectric thin film becomes prone to breaking and peeling under pressure and thermal stress
Solution Approach 1:
The device structure is segmented into distinct functional regions: the piezoelectric thin film is confined to the active area above the interdigital transducer electrodes, while the pad electrode region is separated and positioned over the support substrate without the piezoelectric film. This segmentation allows the piezoelectric film to be thin for miniaturization while preventing it from extending into high-stress bonding regions.
Solution Approach 2:
The piezoelectric thin film is extracted or removed from the region below the pad electrode. This extraction eliminates the vulnerability of the thin piezoelectric film to bonding pressure and thermal stress in the connection area, while preserving the thin-film advantage in the active piezoelectric region for device miniaturization.
2Ease of manufacture
If the metal bump is bonded directly onto the pad electrode from above, then electrical connection is achieved, but high pressure is applied to the pad electrode causing the piezoelectric thin film to break or peel
Solution Approach 1:
The piezoelectric thin film is taken out or removed from the region where the pad electrode is formed and where bump bonding occurs. This eliminates the conflict between the bonding process requirements (direct pressure from above) and the piezoelectric film integrity, as the film no longer exists in the high-stress bonding zone.
Solution Approach 2:
The support substrate acts as an intermediary structure that provides a stable base for the pad electrode and bump connection, independent of the piezoelectric thin film. This intermediary structure absorbs the bonding stress and thermal stress, preventing transmission to the piezoelectric film while maintaining electrical connection functionality.
3Ease of manufacture
If the elastic wave device is pressed onto the circuit board from the bump side, then mounting is achieved, but the piezoelectric single-crystal thin film breaks or the laminated structure peels away
Solution Approach 1:
The piezoelectric thin film is extracted from the mounting region below the pad electrode, eliminating its exposure to mounting pressure. The support substrate and pad electrode structure remain to provide mechanical connection, while the thin piezoelectric film is preserved only in the active area where it is needed for wave generation and detection.
Solution Approach 2:
The device is segmented into a fragile active region (with thin piezoelectric film for wave generation) and a robust connection region (with support substrate and pad electrode for mounting). This segmentation allows the device to be mounted from the bump side with the connection region absorbing stress while the active region remains protected and intact.
4Ease of manufacture
If support layers or cover layers contract due to thermal processing during mounting, then thermal processing is completed, but large stress acts on the piezoelectric thin film causing breaking or peeling
Solution Approach 1:
The piezoelectric thin film is taken out from the region where thermal stress concentrates during mounting (below the pad electrode). This extraction prevents the film from being subjected to large stresses caused by differential thermal contraction of support and cover layers, while allowing thermal processing to proceed for manufacturing purposes.
Solution Approach 2:
The piezoelectric thin film is patterned and confined to specific regions before subsequent mounting and thermal processing steps. This preliminary structuring ensures that when thermal processing occurs and stress develops, the piezoelectric film is already positioned in low-stress zones, preventing breaking or peeling during later manufacturing steps.
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 reliability and yield of the elastic wave device by preventing the piezoelectric thin film from breaking or peeling during bonding and mounting, and maintains structural integrity under temperature changes.
Implementation Method 1
a piezoelectric thin film which is not present below the external connection terminal in a state where the piezoelectric thin film overlaps with the support substrate when viewed in a planar manner
Implementation Method 2
a high acoustic velocity film, a low acoustic velocity film, and the piezoelectric single-crystal thin film are laminated onto a support substrate in that order from the bottom
Data Source
AI summary
In an elastic wave device, a multilayer film including a piezoelectric thin film is provided on a support substrate, an interdigital transducer electrode is provided on one surface of the piezoelectric thin film, a wiring electrode is connected to the interdigital transducer electrode, the wiring electrode includes a lead electrode portion and a pad electrode portion, an external connection terminal is located above the pad electrode portion, the external connection terminal is electrically connected to the pad electrode portion, and the external connection terminal is bonded onto the pad electrode portion on the support substrate so that at least the piezoelectric thin film of the multilayer film is not present below the pad electrode portion.