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.