Temperature effect on kinetic friction characteristics of Cu substrate composed by single crystal and polycrystalline structures

Abstract

Cu–Cu ultrasonic bonding is a highly promising technology for advanced interconnect technology for a 3D integrated circuit micro bump. The interface kinetic friction characteristic is also a key factor in determining the level of bonding for this technology. This study uses molecular dynamics simulation to determine the effects of different temperatures on the material kinetic friction characteristics of Cu substrates in single-crystal and polycrystalline structures. The Taguchi method is used to optimize the simulation, in order to confirm that the convergent kinetic friction coefficients are not affected by size effect. A comparison between the results obtained in the present study and those of previous studies shows good agreement between these two sets of results for kinetic friction coefficients. In order to thoroughly understand the effect of temperature on material characteristics, this study uses the central symmetry parameter to monitor defected atoms and to determine the kinetic friction characteristics at various temperatures. The results show that in single-crystal structures, because the generation of dislocations is not restricted to grain boundaries, the normal force does not change significantly but the tangential force decreases as the temperature increases. Therefore, the kinetic friction coefficient for single-crystal Cu decreases as temperature increases. Because the generation of dislocations in polycrystalline structures is restricted to grain boundaries between 293K and 650K, the tangential force and the normal force do not decrease significantly as the temperature increases. Consequently, the kinetic friction coefficient for polycrystalline Cu remains stable and is unaffected by any increase in temperature. Between 825K and 1000K, although the generation of dislocations in polycrystalline structures is still restricted to grain boundaries, high temperatures soften the material, which leads to a significant decrease in the tangential force and the normal force. Therefore, the kinetic friction coefficient of polycrystalline Cu does not decrease significantly. Finally, at temperatures of less than 850K, the kinetic friction coefficient for single-crystal Cu is greater than that for polycrystalline Cu. At 850K, the kinetic friction coefficient for single-crystal Cu is equal to that for polycrystalline Cu, but at 1000K, the kinetic friction coefficient for single-crystal Cu is less than that for polycrystalline Cu. The results of the simulation show that lattice structures have a significant effect on the kinetic friction coefficient at different temperatures.