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.