Copper Paste Firing Method for Low Resistivity Wiring
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Solution Overview
Problem
Conventional methods for firing copper pastes fail to achieve sufficient sinterability and low electrical resistivity in copper wiring due to oxidation issues and residual binder resin, leading to increased wiring resistance.
Innovation Solution
A method involving a first heating step in a nitrogen gas atmosphere with 500 ppm to 2000 ppm oxidizing gas and a second heating step in a nitrogen gas atmosphere with 1% or more reducing gas, followed by continuous gas flow, to form a dense copper sintered body with reduced electrical resistivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If copper paste is fired in an inert gas or vacuum to prevent oxidation, then copper particle oxidation is suppressed, but binder resin remains in the wiring due to oxygen deficiency, deteriorating sinterability and increasing wiring resistance
Solution Approach 1:
The firing process is divided into three distinct stages: first heating in inert gas to prevent oxidation, second heating in oxidizing atmosphere to remove binder resin, and third heating in inert gas to reduce copper oxide. This segmentation allows each stage to address specific requirements without compromising other aspects of the process
Solution Approach 2:
The gas atmosphere is periodically changed during the firing process, switching between inert gas (for oxidation prevention), oxidizing atmosphere (for resin removal), and inert gas again (for reduction). This periodic action enables the system to achieve multiple objectives sequentially
2Reliability
If copper paste is fired in air to remove binder resin, then wiring resistance due to resin is suppressed, but copper particles are oxidized, increasing wiring resistance
Solution Approach 1:
The process separates resin removal from oxidation prevention by using distinct heating stages with different atmospheres, allowing resin elimination without copper oxidation
Solution Approach 2:
The inert gas atmosphere is applied first to prevent copper oxidation before any oxidizing conditions are introduced, ensuring copper particles remain protected throughout the subsequent resin removal stage
3Device complexity
If conventional single-stage firing is used, then the process is simple, but sinterability of copper particles is insufficient and electrical resistivity is high
Solution Approach 1:
The firing process is segmented into three heating steps with different atmospheric conditions, enabling controlled oxidation and reduction cycles that enhance copper particle sintering and reduce electrical resistivity
Solution Approach 2:
The gas atmosphere composition is changed as a controlled parameter throughout the firing process, transitioning from inert to oxidizing and back to inert conditions, which drives the chemical transformations needed for low-resistivity copper wiring formation
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 method effectively decreases the electrical resistivity of copper wiring by promoting dense sintering and reducing oxidation, achieving resistivity levels of 6 µΩcm or less.
Implementation Method 1
a first heating step of heating the substrate in a nitrogen gas atmosphere containing 500 ppm or more and 2000 ppm or less of oxidizing gas in a volume ratio, and oxidizing and sintering copper particles in the copper paste
Implementation Method 2
a second heating step of heating the substrate in a nitrogen gas atmosphere containing 1% or more reducing gas in a volume ratio, and reducing the oxidized and sintered copper oxide
Implementation Method 3
oxidizing and sintering copper particles in the copper paste at 350°C or more and 500°C or less
Data Source
Figure 1~2
AI summary
To provide a method for firing a copper paste, which improves sinterability of copper particles for the purpose of forming a copper wiring line that is decreased in the electrical conductivity. A method for firing a copper paste, which comprises: an application step wherein a copper paste is applied over a substrate; a first heating step wherein the substrate is heated in a nitrogen gas atmosphere containing from 500 ppm to 2,000 ppm (inclusive) of an oxidizing gas in terms of volume ratio after the application step, thereby oxidizing and sintering copper particles in the copper paste; and a second heating step wherein the substrate is heated in a nitrogen gas atmosphere containing 1% or more of a reducing gas in terms of volume ratio after the first heating step, thereby reducing the oxidized and sintered copper oxide.