STRESS AND MICROSTRUCTURE OF SPUTTER-DEPOSITED THIN-FILMS - MOLECULAR-DYNAMICS SIMULATIONS AND EXPERIMENT

Sputter-deposited tungsten (W) thin films exhibit high intrinsic tensi le and compressive stresses. When used as the absorber for x-ray litho graphic masks, the stress induced in-plane and out-of-plane distortion s produce significant very large scale integrated circuit pattern dist ortions. In this article, the origin of intrinsic stress and its abrup t transition from the tensile to the compressive state have been inves tigated theoretically as well as experimentally. A physical understand ing of this transition may lead to better designs of sputter depositio n processes used to create thin film x-ray mask absorbers. Using a two -dimensional molecular dynamics (MD) model for W, the microstructure a nd stresses of sputter-deposited films are calculated as a function of various deposition parameters and compared with the experimental data obtained by employing a variety of thin film deposition and character ization techniques. The MD simulations demonstrate that the transition from highly tensile to highly compressive stress depends on the abili ty of the process to create tightly packed gas impurity atoms in very dense tungsten films, a state achieved only at sufficiently high level s of Ar ion bombardment and adatom energies. The variations in film st ress as a function of sputtering pressure using Ar and Xe as backgroun d gases are also simulated. Both experiments and simulations show that the pressure at the stress transition decreases when the background g as is changed from Ar to Xe. The simulation results for a wide range o f substrate bias further demonstrate that the stresses in sputter-depo sited thin films can be controlled successfully by the use of ion ener gy.