Deformation mechanism in the forcefill process

Forcefill is a method used in metallization of ICs to fill via holes with metal. At elevated temperature and under applied pressure the metal flows into the via hole. The method is applicable to aluminum as well as to copper. In this article the mechanism of the process is discussed based on measurements of the kinetics and on finite element calculations of the shear and hydrostatic stress in the film covering the via hole. It is demonstrated that naı̈ve use of deformation maps constructed by Frost and Ashby for the description of the forcefill process easily leads to highly inconsistent results. The calculated stress distribution shows that the relevant shear stress is more than one order of magnitude lower that the applied pressure. Since no deformation map was available for the grain size appropriate for the forcefill experiment a map for 1 μm grains was constructed. This map indicates that for the forcefill process diffusional flow is the dominant deformation mechanism. It is shown that under these conditions the diffusional flow process described by Frost and Ashby is in essence identical to our previously reported stress-induced diffusion model.