Fabrication of interconnects using pressureless low temperature sintered Ag nanoparticles
Highlights
► Pressureless sintered joints are achieved at low bonding temperatures of 423–473 K. ► Sintering can take place so long as the chemical bonds on nanoparticles are broken. ► We find a novel pinecone-like recrystallization morphology of Ag nanoparticles. ► Thermal conductivity of sintered Ag layers is reduced due to the scattering effect.
Introduction
Nanojoining provides a promising Pb-free interconnects solution for electronic packaging. The melting point and the sintering temperature of nanoparticles are much lower than those of bulk state because of the size effect of nanoparticles [1], [2]. For example, the melting point of a Ag nanoparticle with size 20 nm is 898 K, which is less than the bulk value 1234 K [3]. The surface-melted nanoparticles sinter easily to form a sintered metal with bulk melting temperature [4], [5]. Ag can satisfy the requirements of heat dissipation in a high power density system because of high thermal conductivity. Therefore, Ag nanoparticles can be used as interconnects to be a promising alternative for Pb-free solders.
Numerous studies on sintering of nano-silver paste to substitute solders for bonding have been performed. In order to achieve the joints, pressure is applied generally during the sintering process [4], [5], [6], [7], [8], [9]. However, the use of pressure is difficult for automation and may damage dies, which needs to be avoided. On the other hand, a common view is that sintering takes place only when the organic shells which disperse the nanoparticles and prevent them from aggregation are decomposed [4], [5]. Thermal processing requires temperatures in excess of 473 K to remove the organic substances, which leads to a high bonding temperature in the range 523–623 K [4], [5], [6], [7], [8]. It is unsuitable for other packaging materials.
In this paper, we report a Cu-to-Cu interconnects fabrication process based on the pressureless sintering of Ag nanoparticles for electronics packaging at a temperature close to the melting point of Sn–Pb solders (456 K). It was not necessary for organic shells to be completely decomposed for sintering to take place. Instead, it was sufficient that the chemical bonds that connected the organic shells with Ag nanoparticles were broken. This new point of view led to a way to lower bonding temperature. A novel recrystallization morphology of sintered Ag nanoparticles was observed. In addition, the effect of morphology on the thermal conductivity is discussed.
Section snippets
Materials and methods
Ag nanoparticles used in this study were prepared by flocculation using sodium citrate from Carey Lea's colloidal [10]. The average diameter of Ag nanoparticles was about 20 nm detected by transmission electron microscopy (TEM, Tecnai G2 F20, FEI). Citrate was the organic shell by which Ag nanoparticles were isolated from each other. The thermogravimetric analysis revealed that the amount of Ag nanoparticles in water-based silver paste was 78%, and the final decomposition temperature of organic
Results and discussion
Fig. 1 shows the Raman spectroscopy analysis of the Ag nanoparticles sintered at different temperatures. The intermediate decomposition product of citrate, acetonedicarboxylic acid, was unstable at an elevated temperature and spontaneously decarboxylated to form acetoacetic acid [11]. These organics were reserved in the sintered layers because of an insufficient sintering temperature compared with the final decomposition temperature of citrate. With rising temperature, the intensity of bands
Conclusion
In summary, the Cu-to-Cu interconnects fabrication process based on the pressureless sintering of Ag nanoparticles at temperatures close to the melting point of Sn–Pb solders for electronic packaging was achieved. A novel pinecone-like recrystallization morphology of Ag nanoparticles was found. The bonding layers had a better capacity of heat dissipation compared with solders. Results provided in this study demonstrated that pressureless sintering of Ag nanoparticles is an effective
Acknowledgments
This work is supported by Shenzhen Science and Technology Plan Project under Grant No. XCL201110007, and Guangdong Province Scientific Project under Grant No. 2011B090400257.
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