Thermal Atomic Layer Etching of CoO, ZnO, Fe2O3, and NiO by Chlorination and Ligand Addition Using SO2Cl2 and Tetramethylethylenediamine

Abstract

Thermal atomic layer etching (ALE) of CoO, ZnO, Fe2O3, and NiO was achieved using chlorination and ligand-addition reactions at 250 °C. This two-step process was accomplished by first chlorinating the metal oxide with SO2Cl2. Subsequently, ligand addition to the metal chloride was performed using tetramethylethylenediamine (TMEDA). In situ quadrupole mass spectrometry (QMS) studies on metal oxide powders revealed that CoO, ZnO, Fe2O3, and NiO all formed stable and volatile MCl2(TMEDA) compounds (M = Co, Zn, Fe, Ni) as etch products at 250 °C. These QMS studies of the sequential SO2Cl2 and TMEDA exposures were facilitated by a new reactor design with two nested inlet lines that transport the reactants separately to the powder substrate. The time-dependence of the reactants and products could also be monitored by the QMS investigations. The large SO2+ ion intensity observed at the beginning of the SO2Cl2 exposure was consistent with the chlorination reaction MO + SO2Cl2 → MCl2 + SO2 + (1/2)O2. The time-dependent QMS studies also observed the MClx(TMEDA)+ ion intensity peaking at the beginning of the TMEDA exposures. The subsequent decay of the MClx(TMEDA)+ ion intensity, while the (TMEDA)+ ion intensity remained constant, was evidence for a self-limiting ligand-addition reaction. The mass loss of the metal oxide powders was confirmed after sequential SO2Cl2 and TMEDA exposures. The etching of two of these metal oxides was also verified using separate experiments on flat substrates using SO2Cl2 and TMEDA exposures at 250 °C. For CoO thermal ALE, an etch rate of 4.1 Å/cycle at 250 °C was measured using X-ray reflectivity (XRR) studies. For ZnO thermal ALE, an etch rate of 0.12 Å/cycle at 250 °C was measured using quartz crystal microbalance (QCM) investigations. Other first row transition metal oxides were surveyed in addition to CoO, ZnO, Fe2O3, and NiO. QMS studies of TiO2, Cr2O3, and MnO2 showed no volatile species formation during sequential SO2Cl2 and TMEDA exposures at 250 °C. In contrast, V2O5 and CuO were spontaneously etched using SO2Cl2 at 250 °C, as determined by the observation of volatile VOCl3 and CuCl3 etch products, respectively. Calculated Gibbs free energy changes for the various etching reactions also supported the experimental observations for the first row transition metal oxides. These studies illustrate that the chlorination and ligand-addition reaction mechanism can provide a new avenue for the thermal ALE of a variety of transition metal oxides that have nonvolatile metal chlorides.