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Pore Engineering of Sintered Silver via Electromigration
Keywords: Pore Engineering, Sintered Silver, ELectromigration
Pore Engineering of Sintered Silver via Electromigration HITEN abstract: Porosities are naturally introduced in sintered silver systems, and adversely affect the thermal conductivity of the material, among other factors including interfaces and reinforcement distribution. Therefore there are many post processing techniques such as annealing (or coating techniques) that are used to improve stability of electrical, thermal, mechanical and optical properties of these structures. In addition, the sintered silver widely used as an interconnect between different electrical components. High current densities lead to appreciable electromigration effects where momentum exchange between conduction electrons and atoms lead to atomic diffusion in the direction of electron flow. Eventually these lead to an accumulation of voids at the cathode and hillocks at the anode, and eventual failure of the conducting element. However, electromigration can be used constructively as a tool for fabrication of zero and one dimensional nanocrystals [1, 2], nanostructures for local electric field enhancement in plasmonics [3, 4], molecular-scale biochemistry measurements [5-8] and to control the kinetic faceting of surface orientations that belong to the equilibrium shape of crystals [9]. There are considerable numbers of studies on internal morphology changes of sintered silver at high temperature. For example in our previous studies a solution for thermally stable high temperature die attach proposed to change the random porosity of standard pressure-free sintered silver die attach into single voids and It was controlled by the mesh geometry [10]. The study [11] on the electrical and thermal characterisation of sintered silver shows that the oxidization of exterior surface of sintered silver could prevent the changes in surface morphology whereas the interior pore surfaces of the porous silver remain largely oxide-free therefore the pore surfaces path the way for fast atomic movement resulting in grain growth and changes in the internal microstructure [12]. Finally the recent results show that it is possible to refinement the Internal structure of porous sintered silver via electromigration. Understanding of internal structures of sintered silver and atomic flux and transport in nanoscale devices is vital for new functional devices on this scale. Here we present new results to show the changes on the internal pore structure both as statistical averages of grain size and, uniquely in this type of study, by tracking the pore size and density of internal structure at the surface, centre and bottom of sintered silver stripe before and after electromigration experiments. These results provide addition evidence for previous explanation of microstructural changes [1, 12] and oxidization of exterior surface of sintered silver. The comparison between internal morphology changes using electromigration techniques and the microstructural evolution cause at high temperature shows that the electromigration can be used as a novel promising technique to engineering the internal pore structures. However the mechanical and reliability test for different pore structures using this technique required further studies.
Ali Mansourian, PhD student
King's College London
London, London

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