Temperature and current density distributions in via structures with inhomogenous step coverages

Abstract

Metallization systems, especially vias are a critical part of any VLSI chip. For the reliability prediction of vias it is important to get information on the temperature- and current density distributions in the metallization structure. Numerical methods like the finite element method FEM, give a better understanding of local heating and current crowding. With a user routine the calculation of the mass flux and mass flux divergences is possible and out of the simulations the weakest link in the structure can be determined. A qualitative comparison of the electromigration resistance between different structures can be done. A via filling with conventional techniques can lead to poor step coverages. An occurrence of shadowing effects during the sputter process of the second metallization can decrease the step coverage in the via as well and can lead to an inhomogeneous step coverage of the metallization material in the via. For a conventional aluminum via structure without barrier layers the current density and temperature distributions as well as the maximum mass flux divergences depending on the step coverage were investigated by finite element simulations. A phenomenological investigation of the influence of different inhomogeneous step coverages in the via on the mass flux divergence was done, and compared with the homogenous normal step coverage. The dependence of the overlap of the second metallization and the via aspect ratio as well as the applied current density from the maximum mass flux divergence will be investigated and the main influences determined.