Simulation of electroless deposition of Cu thin films for very large scale integration metallization

Electroless copper deposition has been shown to be a viable technique for the fabrication of very large scale integration (VLSI) interconnections. In particular, electroless deposition was found to be able to fill the high aspect ratio features in submicron VLSI technologies. In this work one- and two-dimensional models of electroless Cu deposition are presented. These models are based on the diffusion of the dominant ionic species [Cu(II)] and the surface reaction kinetics. These assumptions result in a nonlinear diffusion equation to be solved. A one-dimensional numerical model is used to determine the diffusivity of the copper ion and the reaction coefficients from experimental results. The two-dimensional model is based on SIMBAD, a thin film growth simulator, which was enhanced by the inclusion of a finite element module to solve the diffusion of the dissolved Cu(II) ion to the film surface. The module automatically generates a triangular grid and has a finite element computational engine capable of solving the diffusion equation with the nonlinear boundary conditions arising in the problem. In order to confirm the predictions of the two-dimensional model copper films were deposited over high aspect ratio trenches cut into silicon oxide. Simulations using simulation by ballistic deposition (SIMBAD) were found to compare very well with cross-sectional scanning electron micrographs of these films. The numerical models were also used to study the sensitivity of the conformal filling of trenches and vias to the diffusivity and reaction coefficients. It was found that a 200 Å seam/void would form in the film if the diffusivity was dropped by an order of magnitude.