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

Sputter-deposited tungsten (W) thin films exhibit high intrinsic tensile and compressive stresses. When used as the absorber for x-ray lithographic masks, the stress induced in-plane and out-of-plane distortions produce significant very large scale integrated circuit pattern distortions. In this article, the origin of intrinsic stress and its abrupt transition from the tensile to the compressive state have been investigated theoretically as well as experimentally. A physical understanding of this transition may lead to better designs of sputter deposition processes used to create thin film x-ray mask absorbers. Using a two-dimensional molecular dynamics (MD) model for W, the microstructure and 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 characterization techniques. The MD simulations demonstrate that the transition from highly tensile to highly compressive stress depends on the ability of the process to create tightly packed gas impurity atoms in very dense tungsten films, a state achieved only at sufficiently high levels of Ar ion bombardment and adatom energies. The variations in film stress as a function of sputtering pressure using Ar and Xe as background gases are also simulated. Both experiments and simulations show that the pressure at the stress transition decreases when the background gas is changed from Ar to Xe. The simulation results for a wide range of substrate bias further demonstrate that the stresses in sputter-deposited thin films can be controlled successfully by the use of ion energy.