Thermal Atomic Layer Etching of Molybdenum Using Sequential Oxidation and Deoxychlorination Reactions

Thermal atomic layer etching (ALE) of molybdenum (Mo) was demonstrated using sequential exposures of O₃ (ozone) and SOCl₂ (thionyl chloride). In situ quartz crystal microbalance (QCM) studies were performed on sputtered Mo-coated QCM crystals. The QCM results revealed that Mo ALE displayed a linear mass decrease versus ALE cycles after a short etching delay. A pronounced mass increase was observed for every O₃ exposure. A dramatic mass decrease occurred for every SOCl₂ exposure. The mass change per cycle (MCPC) for Mo ALE was self-limiting after long SOCl₂ exposures. The MCPC increased slightly with longer O₃ exposure times. In situ QCM studies suggested that this soft saturation with longer exposure to the O₃ resulted from the diffusion-limited oxidation of Mo. The Mo etch rate increased progressively with etching temperature. Under saturation conditions, the Mo etch rates were 0.94, 5.77, 8.83, and 10.98 Å/cycle, at 75, 125, 175, and 225 °C, respectively. X-ray photoelectron spectroscopy (XPS) and in situ quadruple mass spectroscopy (QMS) studies were conducted to understand the reaction mechanism. XPS revealed primarily MoO₃ on the Mo surface after exposure to O₃ at 150 °C. From the QMS studies, volatile SO₂ and MoO₂Cl₂ were monitored when Mo was exposed to SOCl₂ during the ALE cycles at 200 °C. These results indicate that Mo ALE occurs via oxidation and deoxychlorination reactions. Mo is oxidized to MoO₃ by O₃. Subsequently, MoO₃ undergoes a deoxychlorination reaction where SOCl₂ accepts oxygen to yield SO₂ and donates chlorine to produce MoO₂Cl₂. Additional QCM experiments revealed that sequential exposures of O₃ and SO₂Cl₂ (sulfuryl chloride) did not etch Mo at 250 °C. Time-resolved QMS studies at 200 °C also compared sequential O₃ and SOCl₂ or SO₂Cl₂ exposures on Mo at 200 °C. The volatile release of MoO₂Cl₂ was observed only using the SOCl₂ deoxychlorination reactant. Atomic force microscopy (AFM) measurements revealed that the roughness of the Mo surface increased slowly versus Mo ALE cycles.