W deposition and titanium fluoride formation during WF6 reduction by Ti: Reaction path and mechanisms

Reaction of ${\mathrm{WF}}_{\mathrm{6}}$ with air-exposed 27- and 250-nm-thick Ti films has been studied using Rutherford backscattering spectroscopy, scanning and high-resolution transmission electron microscopy, electron and x-ray diffraction, and x-ray photoelectron spectroscopy. We show that W nucleates and grows rapidly at localized sites on Ti during short ${\mathrm{WF}}_{\mathrm{6}}$ exposures (≈6 s) at $\text{445\hspace{0.05em}°}\mathrm{C}$ at low partial pressures $p_{{\mathrm{WF}}_{\mathrm{6}}}\text{<0.2\hspace{0.05em}}\mathrm{Torr}.$ Large amounts of F, up to ${\text{≈2.0×10}}^{17} {\mathrm{atoms/cm}}^{\mathrm{2}}$ corresponding to an average F/Ti ratio of 1.5 in a 27-nm-thick Ti layer, penetrate the Ti film, forming a solid solution and nonvolatile ${\mathrm{TiF}}_{\mathrm{3}}.$ The large stresses developed due to volume expansion during fluorination of the Ti layer result in local delamination at the W/Ti and the ${\mathrm{Ti/SiO}}_{\mathrm{2}}$ interfaces at low and high ${\mathrm{WF}}_{\mathrm{6}}$ exposures, respectively. ${\mathrm{WF}}_{\mathrm{6}}$ exposure at $p_{{\mathrm{WF}}_{\mathrm{6}}}\text{>0.35}$ results in the formation of a network of elongated microcracks in the W film which allow ${\mathrm{WF}}_{\mathrm{6}}$ to diffuse through and attack the underlying Ti, consuming the 27-nm-thick Ti film through the evolution of gaseous ${\mathrm{TiF}}_{\mathrm{4}}.$