9 Copper Micro and Nano Particles Mixture for 3D Interconnection …
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strength from below 5 Mpa to above 20 Mpa on average, under the sintering condition of 300 °C for 60 min. Using Cu microscale particles sintering with this oxidization removal methods demonstrated higher bond strength due to the Cu surface
modification in formic acid bonding condition [27].
Some surface modification technology are also developed to lower the oxidation
ratio e.g. Ag nano-particle precipitation assistance, which utilizes green amine treatment [28]. Other surfactant-free alternatives like syntheses methods enhancement
also help to improve the conductivity [29].
Another problem seen by many researches is that copper paste made by nanoparticles has large amount of porosity and cracks [30]. One of the possible reasons
is that the uncontrolled agglomerations of nano-particles results in inhomogeneity.
Different evaporation rate of the dispersion solvents lead to particles shrinkage. Due
to this residual stress, the paste is prone to cracking [31]. Hence, it is essential to
mitigate the cracks generation and improve the packing density in the copper paste.
Besides, one also needs to pay attention to the development of suitable industrial
processes that are low cost and environmental-friendly.
Based on these consideration, this chapter presents a method to use Cu micro—
and nano—particles mixture to mitigate cracking and voiding concerns and to determine the optimal weight ratio for the mixture. It is also important to find a scalable
preparation method for the mixed paste since low temperature sintering is desirable. Throughout this chapter, the development of Cu micro—and nano—particles
mixture interconnects is discussed in the following sequence:
(1) Establish a model to calculate the best weight ratio between micro—and nano—
particle to achieve the maximum packing density;
(2) Develop copper paste preparation method for low temperature sintering;
(3) Compare the mixed paste with the micro—and nano—paste in terms of thermal,
electrical, and mechanical properties.
9.2 Packing Density Modeling
9.2.1 Algorithm Design and Assumption
The objective of this packing density modeling is to determine the best volume/weight
mixture ratio between the Cu micro—and nano—particles. In the following model,
since the micro-particle size is fixed, the aim of this model is to find the best nanoparticle size to mix with the micro-particle to achieve the maximum packing density.
This model is mainly based on the Monte Carlo Method, which is used to randomly
generate the nano-particles in the interstitials. Some assumptions are made to simplify
the computing:
• All the particles are perfect spherical, with 1 μm sized micro-particle and uniform
sized nano-particle;
• Micro-particles are arranged in a face-centered cubic (FCC) configuration;
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strength from below 5 Mpa to above 20 Mpa on average, under the sintering condition of 300 °C for 60 min. Using Cu microscale particles sintering with this oxidization removal methods demonstrated higher bond strength due to the Cu surface
modification in formic acid bonding condition [27].
Some surface modification technology are also developed to lower the oxidation
ratio e.g. Ag nano-particle precipitation assistance, which utilizes green amine treatment [28]. Other surfactant-free alternatives like syntheses methods enhancement
also help to improve the conductivity [29].
Another problem seen by many researches is that copper paste made by nanoparticles has large amount of porosity and cracks [30]. One of the possible reasons
is that the uncontrolled agglomerations of nano-particles results in inhomogeneity.
Different evaporation rate of the dispersion solvents lead to particles shrinkage. Due
to this residual stress, the paste is prone to cracking [31]. Hence, it is essential to
mitigate the cracks generation and improve the packing density in the copper paste.
Besides, one also needs to pay attention to the development of suitable industrial
processes that are low cost and environmental-friendly.
Based on these consideration, this chapter presents a method to use Cu micro—
and nano—particles mixture to mitigate cracking and voiding concerns and to determine the optimal weight ratio for the mixture. It is also important to find a scalable
preparation method for the mixed paste since low temperature sintering is desirable. Throughout this chapter, the development of Cu micro—and nano—particles
mixture interconnects is discussed in the following sequence:
(1) Establish a model to calculate the best weight ratio between micro—and nano—
particle to achieve the maximum packing density;
(2) Develop copper paste preparation method for low temperature sintering;
(3) Compare the mixed paste with the micro—and nano—paste in terms of thermal,
electrical, and mechanical properties.
9.2 Packing Density Modeling
9.2.1 Algorithm Design and Assumption
The objective of this packing density modeling is to determine the best volume/weight
mixture ratio between the Cu micro—and nano—particles. In the following model,
since the micro-particle size is fixed, the aim of this model is to find the best nanoparticle size to mix with the micro-particle to achieve the maximum packing density.
This model is mainly based on the Monte Carlo Method, which is used to randomly
generate the nano-particles in the interstitials. Some assumptions are made to simplify
the computing:
• All the particles are perfect spherical, with 1 μm sized micro-particle and uniform
sized nano-particle;
• Micro-particles are arranged in a face-centered cubic (FCC) configuration;