Thermal Atomic Layer Etching of Molybdenum Using Sequential Oxidation and Deoxychlorination Reactions
Taewook Nam, Troy A. Colleran, Jonathan L. Partridge, Andrew S. Cavanagh, Steven M. George
Abstract
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.