Growth of epitaxial TiN films deposited on MgO(100) by reactive magnetron sputtering: The role of low-energy ion irradiation during deposition

Plan-view and cross-sectional transmission electron microscopy have been used to investigate the role of low-energy ion irradiation in controlling the defect structure of epitaxial TiN(100). The films were deposited by reactive magnetron sputter deposition onto MgO(100) substrates at film growth temperatures T_s between 550 and 850°C (0.26 to 0.35 of the melting point of TiN in K) and negative substrate biases V_s between 0 and 500 V. Sputtering was carried out in pure N₂ atmospheres, the energy per N ion incident at the film surface was ∼ eV_s 2 (N₂⁺ was the predominant ionic species), the incident ion to thermal-atom flux ratio for films grown with V_s≥100 V was ∼1.3, and the deposition rate was ∼1 monolayer s^-1 (1.3 μm h^-1). The primary defects observed in the films were dislocation loops on {111} planes. The number density n_d of these loops decreased with increasing T_s (e.g., for V_s=0, n_d ranged from 5×10¹²cm^-2 at 550°C to 1.5×10¹⁰cm^-2 at 850°C). However, n_d also decreased rapidly with increasing V_s at constant T_s until a minimum defect density was attained at V_s^*(T_s) after which n_d incre ased again. Films grown at T_s≥750°C and V_s=V_s^* ≅ 300 V were essentially free of dislocation loops. On the other hand, films grown with T _s<650°C and V_s≥400V (i.e., V_s > V_s^*) exhibited very high dislocation loop densities, ≥5×10¹²cm^-2, together with the preci itation of N₂ gas bubbles. The net effect of ion irradiation on film microstructure depended upon a competition between the defect annihilation rate due to enhanced adatom mobilities during deposition and the collisionally-induced defect formation rate. The residual defect density was thus a function of both T_s and V_s. Under the proper growth conditions, ion irradiation led to a reduction in dislocation loop densities by more than 5 orders of magnitude. Cross-sectional micrographs of multilayer films grown as a function of V_s at constant T_s showed that n_d increased or decreased (depending upon the direction of the change in V_s) abruptly and reversibly.