Angstrom-level precision in etching of SiO₂ is possible using steady-state Ar plasma in conjunction with periodic injection of a defined number of C₄F₈ molecules and synchronized plasma-based Ar⁺ ion bombardment.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. The question of SiO₂:Si₃N₄ etching selectivity was studied and evaluated with regard to the dependence on maximum ion energy, etching step length (ESL), FC surface coverage, and precursor selection. Since Si₃N₄ has a lower physical sputtering energy threshold than SiO₂, Si₃N₄ physical sputtering can take place after exhausting the chemical etchant at the end of each cycle for certain ion energies and leads to Si₃N₄/SiO₂ etching selectivity. By optimization of the 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.²Mechanism of the observed ALE behaviors will be discussed using real-time and post-plasma surface analysis data. We will also discuss current challenges with the application of these approaches to other materials.

Acknowledgements: I am pleased to acknowledge the essential contributions and collaboration of D. Metzler, C. Li, S. Engelmann, R. Bruce, E. Joseph, C. S. Lai, and E. A. Hudson to this work, and thank National Science Foundation (CBET-1134273), US Department of Energy (DE-SC0001939) and Semiconductor Research Corporation for funding.

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).