RESIDUAL-STRESSES AND MICROSTRUCTURE IN TUNGSTEN THIN-FILMS ANALYZED BY X-RAY DIFFRACTION-EVOLUTION UNDER ION IRRADIATION

Microstructure and residual stresses have been studied in 100 nm tungs ten thin films deposited by ion beam sputtering on silicon substrates. Residual stresses, stress-free lattice parameter, crystal microdistor tions, and average length of the coherently diffracting domains have b een deduced from x-ray diffraction measurements. The as-deposited him is strongly compressed (-5.2 GPa) and its microstructure is very far f rom the bulk tungsten one: the coherently diffracting domain size is n anometric (about 5 nm), the stress-free lattice parameter is larger th an the bulk one (about 0.6%), and microdistortions are considerable (0 .6%). The ''atomic peening'' model is proposed to explain the mechanic al state of these films. Diffraction analysis, correlated with impurit y concentration measurement, evidences the main role played by backsca ttered Ar ions in stress genesis. Nevertheless, the contribution of th e most energetic W particles to the stress generation process cannot b e neglected. We have equally studied Ar+ ion (340 KeV) irradiation eff ects. We have found that irradiation induces a total stress relaxation , a return of the stress-free lattice parameter to the bulk one, a str ong decrease of the microdistortions, and an increase of the coherentl y diffracting domain sizes. A thermal irradiation effect seems appropr iate to explain residual stresses and microstructure modifications ind uced by ion irradiation. These features are in agreement with the inte rpretation proposed in the case of as-deposited films. (C) 1996 Americ an Institute of Physics.