Semiconductor Microbump Structure for Low-Resistance Stacked Connections
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Solution Overview
Problem
Existing methods for connecting electrodes of stacked semiconductor elements using Sn-based solder result in high resistance due to alloy growth and oxidation at the boundary between Sn-based solder and Ti, leading to ineffective electrical connection.
Innovation Solution
A semiconductor device with a microbump formed of Sn-based solder on one electrode and a concave bump pad on the other, where a third metal layer is diffused into the microbump and a second metal layer of Co is formed on the bump pad, with specific thicknesses and materials for the metal layers to ensure electrical connection, including a first metal layer of TiN or Ta, and a third metal layer of Cu, Ni, Pd, or Pt to reduce oxidation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a microbump of Sn-based solder is formed on an electrode and connected to a barrier metal layer in a diffusing manner by formic acid reduction, then electrical connection between stacked semiconductor elements is achieved, but high resistance occurs due to alloy growth and oxidation at the boundary between Sn-based solder and Ti barrier metal
Solution Approach 1:
The patent introduces a Cu layer as an intermediary between the Sn-based solder microbump and the Ti barrier metal layer. This Cu intermediary layer prevents direct contact between Sn and Ti, thereby eliminating the harmful alloy growth and oxidation reactions at the Sn-Ti interface. The Cu layer serves as a diffusion barrier and chemical buffer, allowing electrical connection to be maintained without the harmful interfacial reactions.
Solution Approach 2:
The patent creates a composite metal layer structure consisting of multiple layers (TiN, Cu, Co) with different functional properties. The TiN layer provides barrier functionality, the Cu layer prevents Sn-Ti reaction, and the Co layer provides additional protection. This composite structure combines the advantages of different materials to achieve both electrical connectivity and protection against oxidation and alloy growth.
2Ease of manufacture
If Ti is used as barrier metal with thickness of approximately 200 nm formed by sputtering, then the barrier metal can be formed in wafer process, but high resistance due to oxidation occurs at the boundary between Sn-based solder and Ti
Solution Approach 1:
The Cu layer is introduced as an intermediary between the Sn-based solder and the Ti barrier metal that was previously formed by sputtering in the wafer process. This additional Cu layer prevents the oxidation and alloy growth problems that occurred with direct Sn-Ti contact, while maintaining compatibility with the existing sputtering-based manufacturing process.
Solution Approach 2:
The patent segments the barrier metal structure into multiple functional layers (TiN, Cu, Co) rather than using a single Ti layer. This segmentation allows each layer to perform its specific function: TiN for initial barrier, Cu for preventing Sn-Ti reaction, and Co for additional protection. This segmented approach solves the oxidation problem while maintaining manufacturability.
3Strength
If formic acid reduction is used to connect barrier metal and Sn-based solder in a diffusing manner, then physical connection is achieved, but electrical connection is ineffective due to high resistance from alloy growth
Solution Approach 1:
The Cu layer serves as a mediator that allows the formic acid reduction process to create a diffused connection without the harmful Sn-Ti alloy growth. The Cu layer enables controlled diffusion while preventing the formation of high-resistance alloy compounds, thus maintaining both physical connection strength and electrical connection reliability.
Solution Approach 2:
The composite metal layer structure (TiN-Cu-Co) enables the formic acid reduction process to achieve both strong physical connection and low electrical resistance. The different materials in the composite structure provide complementary functions: structural integrity from the barrier layers and low-resistance electrical pathways that prevent harmful alloy growth.
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 enables reliable and efficient electrical connection between the electrodes of stacked semiconductor elements, maintaining low resistance over time even at high temperatures, as demonstrated by Kelvin resistance measurements.
Implementation Method 1
a third metal layer diffused into the microbump
Implementation Method 2
reduces an oxide film on surfaces of the third metal layer and the microbump by heating treatment under a reduction atmosphere
Data Source
AI summary
An imaging device includes a first semiconductor element including at least one bump pad that has a concave shape. The at least one bump pad includes a first metal layer and a second metal layer on the first metal layer. The imaging device includes a second semiconductor element including at least one electrode. The imaging device includes a microbump electrically connecting the at least one bump pad to the at least one electrode. The microbump includes a diffused portion of the second metal layer, and first semiconductor element or the second semiconductor element includes a pixel unit.