Molecular Dynamics Modeling of Microstructure and Stresses in Sputter-Deposited Thin Films

A molecular dynamics model to study thin film formation processes is presented. In this model, the Lennard-Jones and Moliere potential functions are used to describe the interaction between the atoms, and ions and atoms, respectively, and determine the atomic forces among the atoms. A third-order, nonuniform time step integration scheme is used to determine the new position and velocity of each particle within the film and the substrate. The use of nonuniform time interval results in significant savings in CPU time. The force evaluation is performed in a short cutoff range which requires that the list of neighbor particles be known. A “position index method'5 which is much more efficient than the previous schemes, is proposed and implemented in order to sort the table of atoms locally. The frictional force originating from the heat bath is represented by using the generalized Larigevin equation such that the temperature of the substrate can be kept constant during the film growth. A moveable periodic boundary condition which can allow the film to expand or contract depending on the internal stresses within the film and the substrate, is used in the present molecular dynamics (MD) model. A local stress parameter is defined in order to examine the effects of defects, voids, and gas impurity on local stress distribution. The intrinsic stresses are then calculated from the film structure. This improved MD model can predict the effect of adatom energy, ion bombardment, and gas impurity on microstructure and intrinsic stresses in thin films very successfully. © 1993, American Vacuum Society. All rights reserved.