The understanding of degradation phenomena in dualdamascene (DD) copper metallization structures due to high current densities and temperatures in reliability stress tests has been an important challenge during the past decades. Compared to standard Al(Cu)-based and W-plug interconnects for copper as metallization material the failure mechanism might change caused by the different architecture of a DD interconnect system. The determination of the mass transport pathway is one major point of concern. Electromigration for instance can occur through a number of different pathways like the surface, the interface, and the grain boundaries. In a DD copper metallization, interface diffision was determined as main degradation mechanism [ 13.Beside the electromigration due to the joule heating, temperature gradient driven thermomigration will occur. Out of the mismatch between the properties of the different materials in the DD interconnect like thermal expansion coefficient (TCE) mechanical stress in the metallization system arises. Out of this the reliability prediction due to the different migration mechanisms like electro-themo-and stressmigration as well as the different migration path become more and more important and have to be considered.In this study the void formation, as well as the mass flux divergence distribution out of the static simulations, in a copper metallization was investigated by finite element simulation (using ANSYSB). A calculation routine is used for the determination of the mass flux divergences and void formation in the model. First the suitability of the simulation will be shown by the calculation of the void formation for a copper interconnect structure taken from the literature [4]. For comparison of the different influences default models were created. Based on the default models the overlap of the metallization, the via height, the applied current density and stress free temperature was varied. The mechanical stress, the temperature and current density distribution as well as the influence of the different migration mechanisms were determined. Out of this the void formation and its localisation were calculated.
Mass flux divergence equationThe simulations were carried out with the finite element program ANSYSB. The program allows the simulation of current density, temperature gradient and mechanical stress distributions in the metallization structures. A user routine for the calculation of the stress gradients and the local mass flux divergences was developed and is described in previous studies [5,6]. Out of the calculation and simulation results the determination of the void formation in the metallization system can be done.The effects of high current density, the induced joule heating and the mismatch in the material properties lead to the total divergence the of three migration mechanisms. The total mass flux divergence is defined by the following equation:In this equation Do is the self-diffusion coefficient, N is the atomic concentration, the effective charge of ion is Z*e, the Bol...
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