Gas porosity formation in epitaxial TiN films deposited by reactive magnetron sputtering in mixed Ar/N₂ discharges

Transmission electron microscopy (TEM) has been used to study gas porosity in epitaxial TiN films grown on MgO (001) by reactive magnetron sputtering in mixed Ar/N₂ discharges at a total Ar+N₂ pressure of 1 mTorr while varying the Ar partial pressure P_Ar from 0 to 0.9 mTorr. During film growth the substrate temperature T_s was kept at 750 °C, and an applied negative substrate bias V_s of 350 V was used to obtain a flux of Ar⁺ and N⁺₂ ions impinging on the growing film surface. The size distributions of gas bubbles formed during deposition were obtained from TEM analysis of TiN films grown at different P_Ar. It was found that an increase in P_Ar, while changing the relative amount of trapped Ar and N which precipitate into bubbles, also leads to a decrease in mean bubble diameter from ∼8 nm at P_Ar=0 to ∼2 nm at P_Ar=0.5 mTorr and to ∼1 nm at P_Ar=0.9 mTorr. Such a strong dependence of bubble diameters on the composition of the gas trapped within the bubbles suggests that gas solubility is an important factor responsible for differences in bubble nucleation and growth kinetics. It is shown that gas dissociation controlled resolution and reabsorbtion of nitrogen is the dominating mechanism responsible for the growth of large N₂ bubbles. Surface diffusion controlled migration and coalescence of bubbles only provides a limited coarsening of predominantly the Ar‐containing bubbles.