Bonding Pad Composite Metal Structure for Wire Bonding
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Solution Overview
Problem
The challenge lies in achieving a reliable wire bonding process on thick aluminum layers without causing deformation or passivation cracks, which results in a 'Non Stick on Pad' (NSOP) failure, and gold layers are not suitable for stable bonding on 8-inch wafers due to their high current-carrying capability requiring thinner thicknesses.
Innovation Solution
A bonding pad design featuring a first soft metal structure covered by a harder second metal structure, with a passivation layer and an electrically conductive protective layer, where the harder metal structure extends into recesses or overhangs to distribute bonding forces and prevent deformation, allowing for stable wire bonding without deforming the soft top metal layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick top metal layer (aluminum) is used to achieve high current-carrying capability and low resistance, then electrical performance is improved, but the metal layer becomes too soft and deforms during wire bonding
Solution Approach 1:
The bonding pad uses a composite structure with a soft aluminum layer (first metal structure) for electrical performance and a harder second metal structure (e.g., copper, nickel, or tungsten) for mechanical stability. This composite approach allows the soft aluminum to provide low resistance while the harder overlay prevents deformation during wire bonding, resolving the contradiction between electrical performance and mechanical strength.
2Strength
If the bonding force is reduced to avoid deformation of the soft aluminum layer, then mechanical stability is improved, but a sound load-bearing connection cannot be achieved
Solution Approach 1:
The harder second metal structure provides sufficient mechanical strength to withstand wire bonding forces without deformation, while still allowing the bonding wire to achieve a sound load-bearing connection. This resolves the contradiction by providing both mechanical stability and bonding reliability through the composite structure.
3Strength
If gold is used instead of aluminum to avoid softness and deformation, then mechanical stability is improved, but the cost increases and thin layer thickness is required
Solution Approach 1:
The invention uses a composite structure where inexpensive aluminum provides the base layer for electrical performance, while a thinner overlay of harder metal (which can be gold, but is not required to be) provides mechanical stability. This allows achieving the mechanical benefits of gold without requiring thick gold layers, reducing cost while maintaining performance.
Solution Approach 2:
The harder second metal structure is applied locally only at the bonding pad area where mechanical strength is needed, rather than using it for the entire top metal layer. This localized application reduces material cost while providing mechanical stability exactly where required for wire bonding.
4Reliability
If a thick aluminum layer is used, then current-carrying capability is improved, but passivation cracks develop during bonding
Solution Approach 1:
The harder second metal structure acts as a protective overlay that prevents passivation cracks from developing in the thick aluminum layer during wire bonding. This composite structure maintains the current-carrying capability of thick aluminum while eliminating the harmful crack formation.
Data Source
AI summary
A bonding pad on a substrate has a first metal structure establishing an electrical connection between a device and a bonding area, and a second metal structure arranged at the bonding area. The first metal structure extends, within the bonding area, at least over part of the bonding area between the substrate and the second metal structure, so as to contact the second metal structure, the second metal structure being harder than the first metal structure.