Using steady-state Ar plasma in conjunction with periodic injection of a defined number of C₄F₈ molecules and synchronized plasma-based Ar⁺ ion bombardment enables controlled removal of SiO₂ layers with a thickness in the Angstrom-range.¹ The physical sputter rate of SiO₂ vanishes for Ar⁺ ion bombardment with a maximum ion energy of 20 eV and lower, whereas for a fluoroccarbon-coated SiO₂ surface, chemical modifications of the SiO₂ surface are induced by low energy ion bombardment and SiO₂ etching is initiated. In this work we examined SiO₂:Si and SiO₂:Si₃N₄ etching selectivity and the roles of maximum ion energy, etching step length (ESL), FC surface coverage, and fluorocarbon precursor selection in controlling these. By optimization of ALE process parameters, e.g. low ion energies, ESL, and FC film deposition per cycle, highly selective SiO₂ to Si₃N₄ etching can be achieved for conditions where FC selectively accumulates on Si₃N₄ surfaces.² Surface oxidation of Si takes place during the Ar⁺ ion bombardment step, and makes it more difficult to achieve abrupt and highly selective removal of thin SiO₂ layers from Si. Mechanism of the observed ALE behaviors will be discussed using real-time and post-plasma surface analysis data.

¹ D. Metzler, R. Bruce, S. Engelmann, E. A. Joseph, and G. S. Oehrlein, J. Vac. Sci. Technol. A 32, 020603 (2014).

² C. Li, D. Metzler, C. S. Lai, E. A. Hudson, and G. S. Oehrlein, J. Vac. Sci. Technol. A 34, 041307 (2016).

* I gratefully acknowledge the essential contributions and collaboration of Kang-Yi Lin, C. Li, D. Metzler, S. Engelmann, R. Bruce, E. Joseph , C. S. Lai, and E. A. Hudson on which this talk is based. Additionally, funding from Semiconductor Research Corporation (No. 2017-NM-2726), National Science Foundation (CBET-1134273), and US Department of Energy (DE-SC0001939) is thankfully acknowledged.