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-degrees-C, and an applied negative substrate bias V(s) of 350 V was used to obtain a flux of Ar+ and N2+ 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 approximately 8 nm at P(Ar) = 0 to approximately 2 nm at P(Ar) = 0. 5 mTorr and to approximately 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.