Effect of gas impurity and ion bombardment on stresses in sputter-deposited thin films: A molecular-dynamics approach

Abstract

Intrinsic stresses in sputter-deposited thin films are studied via a two-dimensional molecular-dynamics model. Two-body potential functions, periodic boundary conditions, and a generalized Langevin equation are applied to determine the microstructure of the film. The intrinsic stresses are then calculated using a stress method. 12 layers of substrate atoms are arranged in the (111) plane at the beginning of the film growth simulation. The molecular-dynamics simulations using the constant pressure and constant temperature ensemble are first carried out to obtain the zero stress state of the substrate. A thin film of Ni atoms is deposited in the presence of a background of argon and energetic ions in order to obtain a reasonable representation of the film structure. After the deposition process is completed, the film and the substrate are allowed to contract or expand in accordance with the elastic energies. It is found that the microstructure and intrinsic stresses of the film depend upon the incident energy of incoming particles, the ion bombardment, and the amount of trapped gas impurity. The model strongly suggests that the argon impurities trapped into the deposited film are the primary cause of the state of compressive stress. It also shows that in sputter-deposited films the magnitude of the compressive stress depends more strongly on the film structure than on the quantity of the argon gas trapped in the film. A tight packing of film atoms around argon atoms is likely to lead to higher compressive stresses in the film.