UBM Structure Mitigating Electromigration via IMC Diffusion Barriers
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Solution Overview
Problem
Electromigration (EM) in flip-chip solder joints leads to the formation of intermetallic compounds (IMC) at the interface between under-bump metallization (UBM) and solder alloys, causing rapid dissolution and migration of UBM materials, which results in open circuit failures and reduced joint strength in semiconductor devices.
Innovation Solution
A method for forming UBM structures in semiconductor devices using a sequence of copper, tin, and nickel metallization layers, followed by a thermal reflow process to form intermetallic compound (IMC) layers that act as diffusion barriers, mitigating EM-induced failures by slowing down UBM dissolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If lead-free solder bumps are used to form connections with integrated circuits, then environmental safety and worker safety are improved, but electromigration-induced reliability issues worsen due to faster UBM dissolution
Solution Approach 1:
The UBM structure is segmented into multiple functional layers: a copper layer for electrical connection, a tin layer that forms protective IMC compounds, and a nickel layer as a diffusion barrier. This segmentation allows each layer to perform its specific function, with the tin layer forming IMC compounds that protect the copper layer from rapid dissolution during electromigration, thus maintaining reliability while using lead-free solder
Solution Approach 2:
The patent employs a composite UBM structure combining copper, tin, and nickel layers. The copper provides electrical conductivity, the tin forms protective intermetallic compounds (Cu6Sn5, Cu3Sn) that slow down dissolution, and the nickel acts as a diffusion barrier. This composite material approach solves the contradiction by creating a multi-functional UBM that is both environmentally safe with lead-free solder and reliable against electromigration
2Reliability
If intermetallic compounds form at the interface between UBM and solder alloys during electromigration, then electrical connection is established, but rapid UBM dissolution accelerates causing open circuit failures
Solution Approach 1:
The tin layer is pre-formed on the copper layer before solder bump attachment. During the reflow process, this pre-formed tin layer reacts with copper to create a controlled IMC structure (Cu6Sn5 and Cu3Sn layers) before electromigration begins. This preliminary formation of IMC compounds creates a protective barrier that slows down subsequent UBM dissolution during electromigration, preventing rapid failure while maintaining electrical connection
Solution Approach 2:
The multi-layer UBM structure with controlled IMC formation acts as a cushioning mechanism against electromigration damage. The tin-copper-nickel layering and pre-formed IMC compounds provide a buffer that absorbs and mitigates the harmful effects of electromigration, slowing down material dissolution and extending joint lifetime while maintaining electrical functionality
3Ease of manufacture
If a simple UBM structure is used to reduce manufacturing complexity, then ease of manufacture is improved, but protection against electromigration-induced UBM dissolution is insufficient
Solution Approach 1:
The UBM is segmented into distinct functional layers (copper, tin, nickel) that can be deposited using standard sequential sputtering or electroplating processes. While this multi-layer approach increases structural complexity, each layer serves a specific function and can be manufactured using conventional semiconductor fabrication techniques, achieving a balance between manufacturability and electromigration resistance
Solution Approach 2:
The patent uses composite material layers with well-established deposition methods. The copper-tin-nickel composite structure can be formed using sequential sputtering or electroplating, which are standard industrial processes. This approach provides enhanced protection against UBM dissolution through the synergistic effects of different materials while maintaining compatibility with existing manufacturing capabilities
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
The formed IMC layers effectively prevent copper atom consumption during EM, enhancing the reliability and longevity of solder joints by restricting UBM dissolution and maintaining joint strength.
Implementation Method 1
forming a first intermetallic compound (IMC) layer between the first metallization layer and the third metallization layer
Implementation Method 2
the first IMC layer comprising the first metal, the second metal and the third metal
Data Source
AI summary
A semiconductor device comprises a semiconductor substrate, an under-bump metallization (UBM) structure overlying the semiconductor substrate, and a solder bump overlying and electrically connected to the UBM structure. The UBM structure comprises a first metallization layer comprising a first metal, a second metallization layer comprising a second metal different from the first metal, and a first intermetallic compound (IMC) layer between the first metallization layer and the second metallization layer, the first IMC layer comprising the first metal and the second metal.