Achieving ultrahigh etching selectivity of SiO₂ over Si₃N₄ and Si in atomic layer etching by exploiting chemistry of complex hydrofluorocarbon precursors

The authors demonstrate that complex hydrofluorocarbon (HFC) precursors offer significant advantages relative to gas mixtures of comparable elemental ratios for plasma-based selective atomic layer etching (ALE). This work compares mixtures of a fluorocarbon precursor and H₂ with an HFC precursor, i.e., mixtures of octafluorocyclobutane (C₄F₈) with H₂ and 3,3,3-trifluoropropene (C₃H₃F₃), for SiO₂ ALE and etching of SiO₂ selective to Si₃N₄ or Si. For continuous plasma etching, process gas mixtures, e.g., C₄F₈/H₂, have been employed and enable highly selective material removal based on reduction of the fluorine content of deposited steady-state HFC films; however this approach is not successful for ALE since hydrogen-induced etching reduces the thickness of the ultrathin HFC passivation layer which is required for both etching of SiO₂ and passivation of the Si₃N₄ and Si underlayers, leading to lower materials etching selectivity. Conversely, the experimental results show that C₃H₃F₃-based ALE enables ultrahigh ALE selectivity of SiO₂ over Si₃N₄ and Si. The hydrogen in the precursor structure allows to reduce the fluorine content of the deposited HFC film without suppressing the formation of the passivation layer on the surface. Gas pulsing of complex reactive precursors in ALE provides the prospect of utilizing the precursor chemical structure for achieving high materials selectivity in ALE.