Stacked Redistribution Layers for Electromigration Resistance
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Solution Overview
Problem
Dies are prone to fast electromigration (EM) degradation due to high operating currents, leading to copper-tin intermetallic formation and void creation at the redistribution layer/ball region, which reduces the die's lifespan and causes defects.
Innovation Solution
Incorporating stacked redistribution layers with materials like cobalt tungsten phosphorous and nickel, along with underbump metallization layers, to slow down copper diffusion and enhance resistance to EM degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single metal redistribution layer is used, then the device structure is simple, but the resistance to electromigration degradation is insufficient
Solution Approach 1:
The single metal redistribution layer is segmented into multiple stacked redistribution layers (first metal redistribution layer, second metal redistribution layer, and third metal redistribution layer). Each layer has a thickness of 1-3 micrometers, creating a multi-layer structure that provides redundant current paths and reduces electromigration degradation by distributing the current load across multiple layers.
Solution Approach 2:
The patent employs composite material structure by stacking different metal layers (copper, cobalt tungsten phosphorous, nickel) with distinct electrical and mechanical properties. This composite approach optimizes both electrical performance and resistance to electromigration, as each material contributes its unique advantages to the overall redistribution layer system.
2Use of energy by moving object
If copper material is used in the redistribution layer, then the electrical conductivity is high, but the copper diffusion to solder ball is rapid causing void formation
Solution Approach 1:
The patent introduces intermediate barrier layers (cobalt tungsten phosphorous layer and nickel layer) between the copper redistribution layers and the solder ball. These intermediary layers act as diffusion barriers that prevent copper atoms from migrating to the solder ball, thereby eliminating the harmful copper-tin intermetallic formation and void creation while maintaining the high electrical conductivity of the copper layers.
Solution Approach 2:
The harmful copper diffusion pathway is extracted and separated from the main current path by removing copper's direct contact with the solder ball. The copper is confined to specific layers where it performs its electrical function, while the barrier layers extract and block the diffusion pathway, preventing the formation of copper-tin intermetallics at the critical interface.
3Power
If high operating current is applied, then the power delivery is sufficient, but the electromigration degradation occurs rapidly
Solution Approach 1:
The current path is segmented into multiple parallel pathways through the stacked redistribution layers. Each layer carries a portion of the total current, reducing the current density in any single layer. This segmentation allows the device to deliver high total power while maintaining lower local current densities that minimize electromigration degradation, thereby extending die lifespan.
Solution Approach 2:
The patent changes the structural parameters of the redistribution system by introducing multiple layers with optimized thicknesses (1-3 micrometers each) and material compositions. These parameter changes increase the overall current-carrying capacity and distribute the electromigration stress across multiple interfaces and materials, enabling high power delivery with improved reliability and extended operational lifespan.
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 solution significantly increases the die's resistance to electromigration, reducing the likelihood of void formation and extending the die's lifespan by limiting copper diffusion and increasing resistivity.
Implementation Method 1
The copper material of the metal distribution layer 110 may diffuse towards the solder ball 116, creating a void at the interface (e.g., region 118) of the metal distribution layer 110 and the solder ball 116. The second metal redistribution layer includes a cobalt tungsten phosphorous material... slow down copper diffusion
Implementation Method 2
limiting copper diffusion and increasing resistivity... significantly increases the die's resistance to electromigration
Data Source
AI summary
Some implementations provide a semiconductor device (e.g., die) that includes a substrate, several metal layers and dielectric layers coupled to the substrate, a pad coupled to one of the plurality of metal layers, a first metal redistribution layer coupled to the pad, and a second metal redistribution layer coupled to the first metal redistribution layer. The second metal redistribution layer includes a cobalt tungsten phosphorous material. In some implementations, the first metal redistribution layer is a copper layer. In some implementations, the semiconductor device further includes a first underbump metallization (UBM) layer and a second underbump metallization (UBM) layer.