Harnessing Sulfoxides as Mild Oxidants for Aprotic Oxidation of Thiosilanes

Abstract The deposition of silicon oxide (SiOx) thin films has traditionally depended on highly reactive and corrosive oxidants, limiting the development of mild thermal atomic layer deposition (ALD) processes compatible with back-end-of-line integration. To enable low-temperatures (≤400 °C) ALD, we explored the use of sulfoxides as mild, aprotic oxidants to convert tetraorthothiosilicates (Si(SR)4), thiosilanes containing four Si–S bonds, into silicon dioxide in the solution phase on the NMR scale, under aprotic and anaerobic conditions. Reactivity studies show that oxidation efficiency is strongly governed by the electronic properties of both the thiolate ligands and the sulfoxide substituents: electron-withdrawing groups on thiosilanes accelerate oxidation, while alkyl-substituted sulfoxides exhibit the highest reactivity. Polar solvents, notably acetonitrile, markedly enhance reaction rates. Fourier-transform Infrared, powder X-ray Diffraction, X-ray Photoelectron Spectroscopy, elemental analysis, and solid-state 29Si NMR analyses confirm the formation of amorphous silicon dioxide. Kinetic studies indicate first-order dependence on Si(SR)4 and second-order dependence on sulfoxide. Density functional theory calculations support a two-step oxidation pathway involving a hypervalent silicon pre-equilibrium complex and predict lower activation barriers for electron-deficient thiosilanes. The presence of the pre-equilibrium complex intermediate is supported by NMR experiments. Collectively, these results establish a new class of sulfoxide-mediated, mild oxidation chemistry and identify thiosilanes and sulfoxides as potentially interesting precursor candidates. While the present study focuses on solution-phase reactivity, these findings provide preliminary chemical insights that may inform the future design of precursors for next-generation low-temperature thermal ALD of SiOx thin films, pending validation under gas-phase ALD conditions.