Microstructure and residual stresses have been studied in 100 nm thin tungsten films deposited by ion beam sputtering on silicon substrates. Residual stresses, the stress-free lattice parameter, crystal microdistortions and the average length of the coherently diffracting domains (CDD) have been measured by X-ray diffraction, The as-deposited film is strongly compressed (-5.2 GPa) and its microstructure is very far from that of bulk tungsten : the size of the CDD is nanometric (about 5 nm), the stress-free lattice parameter is larger than in the bulk (about 0.6%) and microdistortions are considerable (0.6%). Our diffraction data are in agreement with the "atomic peening" model usually admitted to explain the compressive stresses in sputtered films. These Features are also confirmed by irradiations with Ari ions (340 keV). They induce simultaneously a total stress relaxation, a return of the stress-free lattice parameter to the bulk value and a strong decrease of the microdistortions.