Deuterium retention in tungsten films after different heat treatments

Tungsten films deposited by magnetron sputtering on polycrystalline tungsten substrates were used as a model system to study the influence of the film microstructure on deuterium retention behavior. Different microstructures were produced by annealing the films up to recrystallization temperature and the corresponding structural changes were investigated by scanning electron microscopy combined with focused ion beam (FIB) cross sectioning. The influence of the induced structural changes on D retention was investigated by both nuclear reaction analysis and temperature-programmed desorption. D concentration in the investigated W films is higher than in polycrystalline bulk tungsten by a factor of 3. D retention in the films decreases as a function of annealing temperature. After annealing at 2000 K, FIB cross-section images reveal that cavities appeared at the grain boundaries within the film and at the initial interface between the W film and W substrate. This new microstructure strongly affects the D depth profile and leads to the increase of D retention. Although a further increase of the holding time at 2000 K or an increase of the annealing temperature to 2150 K lead to the reduction of the retained D amount, the D concentration in the recrystallized W films cannot be reduced to a level as low as that of bulk W recrystallized at 2000 K for 30 min.