Elastic Wave Device Joining Layer Thickness Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing elastic wave devices face a decrease in joining strength between the pad electrode and the bump electrode due to the occurrence of Kirkendall voids when subjected to thermal shock, which affects the overall bonding strength between the elastic wave element and the package substrate.
Innovation Solution
The elastic wave device incorporates a multilayer structure for the pad electrode with a joining layer of Al or Al alloy, where the thickness of the joining layer is controlled to be 2,000 nm or less, and the alloy layer formed at the joining portion has a thickness of 2,100 nm or less, with a maximum void diameter of 250 nm or less, to prevent Kirkendall voids and enhance bonding strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a thick Al joining layer is used to ensure adequate material for bonding, then the joining strength may be sufficient, but Kirkendall voids occur during thermal shock causing decreased joining strength
Solution Approach 1:
The patent changes the thickness parameter of the Al joining layer from conventional thick designs to specifically 2000 nm or less. This parameter change prevents excessive Al diffusion into the Au bump electrode during thermal shock, thereby preventing Kirkendall void formation while maintaining adequate joining strength for reliable bonding.
Solution Approach 2:
The patent employs a composite multilayer electrode structure combining Au (bump electrode), Al (joining layer), and Ti (barrier layer). This composite structure leverages the advantages of each material: Au provides excellent conductivity and ductility, Al provides strong bonding capability, and Ti acts as a diffusion barrier to prevent intermetallic compound formation, collectively achieving both strength and reliability.
2Reliability
If the joining layer thickness is reduced to prevent Kirkendall voids, then reliability under thermal shock improves, but joining strength may decrease
Solution Approach 1:
The patent introduces a Ti barrier layer in the composite electrode structure to compensate for the reduced Al joining layer thickness. The Ti layer prevents Au-Al intermetallic compound formation and acts as a diffusion barrier, enabling the use of thin Al layers (2000 nm or less) without sacrificing joining strength, thus achieving both reliability and strength simultaneously.
Solution Approach 2:
The Ti barrier layer serves as an intermediary between the Au bump electrode and Al joining layer. It mediates the interaction by preventing direct diffusion and intermetallic compound formation between Au and Al, allowing the thin Al layer to maintain adequate bonding strength while preventing Kirkendall void formation during thermal shock.
3Ease of manufacture
If a conventional Al joining layer is used without thickness control, then manufacturing is simpler, but Kirkendall voids occur reducing device reliability
Solution Approach 1:
The patent establishes a specific thickness parameter (2000 nm or less) for the Al joining layer that balances manufacturing feasibility with reliability requirements. This quantified parameter provides clear manufacturing guidance while preventing Kirkendall void formation, making the process both simple to implement and reliable.
Solution Approach 2:
The patent adopts a composite multilayer structure (Au-Al-Ti) that maintains manufacturing simplicity through standard sputtering processes while achieving superior reliability. The composite design prevents Kirkendall voids through the Ti barrier layer and controlled Al thickness, without requiring complex 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 significantly reduces the occurrence of Kirkendall voids and maintains high joining strength between the elastic wave element and the bump electrode, even under thermal shock conditions, enabling a compact chip-size package structure with improved reliability.
Implementation Method 1
The Al joining layer and the bump electrode are joined together by fusion bonding, and an Au—Al alloy layer is formed at the joining portion between the bump electrode and the Al joining layer.
Implementation Method 2
an elastic wave element which includes a piezoelectric substrate, an IDT electrode disposed on the piezoelectric substrate
Data Source
AI summary
An elastic wave device includes an elastic wave element which includes a piezoelectric substrate, an IDT electrode, and a pad electrode including a joining layer, a package substrate provided with an electrode land, and a bump electrode which joins the pad electrode and the electrode land. The joining layer includes a first principal surface and a second principal surface, the first principal surface side of the joining layer and the bump electrode are joined together to define a joining portion, and an alloy layer is formed at the joining portion. The thickness of the joining layer is about 2,000 nm or less, the thickness of the alloy layer is about 2,100 nm or less, and the distance from a surface of the alloy layer on the piezoelectric substrate side to the second principal surface of the joining layer is about 1,950 nm or less.