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

Abstract

Reaction of WF6 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 WF6 exposures (≈6 s) at 445 °C at low partial pressures pWF6<0.2 Torr. Large amounts of F, up to ≈2.0×1017 atoms/cm2 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 TiF3. The large stresses developed due to volume expansion during fluorination of the Ti layer result in local delamination at the W/Ti and the Ti/SiO2 interfaces at low and high WF6 exposures, respectively. WF6 exposure at pWF6>0.35 results in the formation of a network of elongated microcracks in the W film which allow WF6 to diffuse through and attack the underlying Ti, consuming the 27-nm-thick Ti film through the evolution of gaseous TiF4.