Wafer Pad UBM Layering with Step Structure for Thermal Stress Relief
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Solution Overview
Problem
The existing semiconductor packaging technologies face challenges with heat dissipation and operational stability due to thermal stress, which can cause passivation layer cracking, leading to improper stacking of the UBM layer and potential material mixing between the solder bump and electrical pad, resulting in quality defects.
Innovation Solution
The method involves forming at least one material layer of the UBM layer through electroplating to ensure complete coverage of the wafer, using a step structure created by protection layers to facilitate proper stacking and adhesion, and forming a barrier and wetting layer to improve bonding quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a polymer layer is added on the passivation layer to prevent cracking, then the reliability of the passivation layer is improved, but the difficulty of UBM layer stacking increases due to indent formation at layer edges
Solution Approach 1:
The method performs preliminary actions by forming the UBM layer materials (adhesion layer, barrier layer, wetting layer) on the wafer surface before bonding, ensuring proper coverage and stacking on complex geometries including the indent regions between passivation and polymer layers. This preliminary preparation prevents improper stacking during subsequent bonding processes.
Solution Approach 2:
The invention applies different material properties locally: the adhesion layer provides strong bonding to the wafer surface, the barrier layer prevents material diffusion at critical interfaces, and the wetting layer ensures proper solder bump adhesion. Each layer is specifically designed to address local requirements at different regions of the wafer surface.
2Ease of manufacture
If sputtering is used to form the UBM layer, then the manufacturing process is simple, but improper stacking occurs at the edges of protection layers leading to material mixing defects
Solution Approach 1:
The adhesion layer serves as an intermediary between the wafer substrate and the barrier layer, ensuring proper bonding and preventing direct contact that could lead to material mixing. The barrier layer acts as an intermediary preventing diffusion between the adhesion layer and wetting layer, maintaining distinct material boundaries at interfaces.
Solution Approach 2:
The UBM layer is constructed as a composite structure with multiple materials: adhesion layer (e.g., Ti, Cr, W), barrier layer (e.g., Ni, Pd, Pt), and wetting layer (e.g., Cu, Au). This composite structure combines the advantages of different materials to achieve both ease of manufacture and high precision stacking.
3Temperature
If the passivation layer is made thin to reduce stress, then the heat dissipation is improved, but the layer becomes more susceptible to cracking under thermal stress
Solution Approach 1:
The protection system uses a composite structure with the passivation layer (e.g., SiO2, Si3N4) combined with a polymer layer (e.g., BCB, polyimide). This composite provides both thermal management and mechanical strength, allowing thin passivation for heat dissipation while the polymer layer compensates for cracking susceptibility.
Solution Approach 2:
The polymer layer is applied beforehand to cushion and absorb thermal stress that would otherwise cause cracking in the thin passivation layer. This preliminary protective measure allows the passivation layer to remain thin for heat dissipation while maintaining reliability under thermal cycling.
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 approach enhances the yield rate and quality of bonding between the bump and pad by ensuring proper UBM layer stacking and preventing material mixing, thereby improving the reliability and stability of semiconductor packages.
Implementation Method 1
a barrier layer is electroplated onto the adhesion layer by using an electroplating solution
Data Source
AI summary
A wafer structure and a method for fabricating the same are provided. First, a wafer having a pad and a first protection layer with a first opening is provided. Next, a second protection layer with a second opening is formed on the first protection layer. Part of the pad and the first protection layer are exposed from the openings. The edges of the openings construct a step structure. Following that, an adhesion layer is formed on the pad, the step structure and the second protection layer. Afterwards, a photo-resist layer with a third opening is formed on the adhesion layer. Then, a barrier layer is electroplated onto part of the adhesion layer. Further, a wetting layer is formed on the barrier layer, and then the photo-resist layer and part of the adhesion layer exposed outside the barrier layer are removed. Finally, a solder layer is printed onto the wetting layer.