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 tungsten thin films deposited by ion beam sputtering on silicon substrates. Residual stresses, stress-free lattice parameter, crystal microdistortions, and average length of the coherently diffracting domains have been deduced from x-ray diffraction measurements. The as-deposited him is strongly compressed (-5.2 GPa) and its microstructure is very far from the bulk tungsten one: the coherently diffracting domain size is nanometric (about 5 nm), the stress-free lattice parameter is larger than the bulk one (about 0.6%), and microdistortions are considerable (0.6%). The ''atomic peening'' model is proposed to explain the mechanical state of these films. Diffraction analysis, correlated with impurity concentration measurement, evidences the main role played by backscattered Ar ions in stress genesis. Nevertheless, the contribution of the most energetic W particles to the stress generation process cannot be neglected. We have equally studied Ar+ ion (340 KeV) irradiation effects. We have found that irradiation induces a total stress relaxation, a return of the stress-free lattice parameter to the bulk one, a strong decrease of the microdistortions, and an increase of the coherently diffracting domain sizes. A thermal irradiation effect seems appropriate to explain residual stresses and microstructure modifications induced by ion irradiation. These features are in agreement with the interpretation proposed in the case of as-deposited films. (C) 1996 American Institute of Physics.