Stress modulation of titanium nitride thin films deposited using atomic layer deposition

Abstract

Stress engineering of thin titanium nitride (TiN) films is of considerable importance to the memory industry, where these thin films are free to deform and need to be structurally robust to the deposition of overlying films and processing steps that are a part of the fabrication flow. TiN films in the thickness range of 50 to 100 Å are deposited at 425 °C using atomic layer deposition and are tensile in nature. The as-deposited films are partially surface oxidized due to exposure to atmosphere. The films are subsequently oxidized in an ozone/oxygen ambient at temperatures lower than 275 °C to form a surface oxide layer comprising of titanium dioxide (TiO2) and titanium oxynitride (TiOxNy). Volumetric expansion associated with oxide formation is found to induce compressive stress in the film, while oxidation had the undesirable effect of increasing film resistivity. A dilute hydrofluoric acid solution is used to etch the surface TiO2 layer, while a thin TiOxNy layer remains intact on the film surface. The removal of surface TiO2 results in restoring the resistivity of the films to values comparable to that of as-deposited TiN, while maintaining the compressive stress induced by film oxidation. X-ray photoelectron spectroscopy shows that the processing scheme results in increasing the amount of TiOxNy in the near-surface region of the films. The authors postulate that the higher molar volume TiOxNy layer exerts a compressive force on the underlying TiN film, and prevents a full relaxation of the films to their original tensile stress state. Further, the authors show that by controlling the oxidation conditions, it is possible to modulate the film stress in the range of −750 to +750 MPa. The processing scheme thus allows for stress engineering of thin TiN films at processing temperatures lower than 275 °C.