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 O 3 (ozone) and SOCl 2 (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 O 3 exposure. A dramatic mass decrease occurred for every SOCl 2 exposure. The mass change per cycle (MCPC) for Mo ALE was self-limiting after long SOCl 2 exposures. The MCPC increased slightly with longer O 3 exposure times. In situ QCM studies suggested that this soft saturation with longer exposure to the O 3 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 MoO 3 on the Mo surface after exposure to O 3 at 150 °C. From the QMS studies, volatile SO 2 and MoO 2 Cl 2 were monitored when Mo was exposed to SOCl 2 during the ALE cycles at 200 °C. These results indicate that Mo ALE occurs via oxidation and deoxychlorination reactions. Mo is oxidized to MoO 3 by O 3 . Subsequently, MoO 3 undergoes a deoxychlorination reaction where SOCl 2 accepts oxygen to yield SO 2 and donates chlorine to produce MoO 2 Cl 2 . Additional QCM experiments revealed that sequential exposures of O 3 and SO 2 Cl 2 (sulfuryl chloride) did not etch Mo at 250 °C. Time-resolved QMS studies at 200 °C also compared sequential O 3 and SOCl 2 or SO 2 Cl 2 exposures on Mo at 200 °C. The volatile release of MoO 2 Cl 2 was observed only using the SOCl 2 deoxychlorination reactant. Atomic force microscopy (AFM) measurements revealed that the roughness of the Mo surface increased slowly versus Mo ALE cycles.