Etch selectivity during plasma-assisted etching of SiO₂ and SiN_x: Transitioning from reactive ion etching to atomic layer etching

Continued downscaling of semiconductor devices has placed stringent constraints on all aspects of the fabrication process including plasma-assisted anisotropic etching. To address manufacturing challenges associated with atomic-scale control, material selectivity, etch fidelity, and increasingly complex device architectures, reactive ion etching (RIE) is transitioning to plasma-assisted atomic layer etching (ALE). Even though the number of elements used in the semiconductor devices has increased several-fold over the last four decades, SiO₂ and SiN_x remain the most commonly used dielectric materials. In fact, fluorocarbon based, plasma-assisted ALE processes for SiO₂ and SiN_x have already been integrated into semiconductor manufacturing, including etching of self-aligned contacts for advanced transistors. However, several challenges remain in achieving ultrahigh etch selectivity of SiO₂ over SiN_x and vice versa. In this article, first, the authors provide a focused review on selective RIE of SiO₂ over SiN_x and contrast this with ALE. A particular focus is given to the etching mechanism, including the role of the mixing layer composition and thickness at the fluorocarbon-SiO₂ interface, the F-to-C ratio in the fluorocarbon parent gas, H₂ dilution, surface composition on the nonetched SiN_x, ion flux and energy, Ar plasma activation duration in ALE, and chamber memory effects. Second, we discuss the reverse case of selectively etching SiN_x over SiO₂ with careful attention given to the role of novel hydrofluorocarbon gases and dilution of the primary feed gas with other gases such as CH₄ and NO. In the second part of this review, we also discuss how novel surface chemistries are enabled by the introduction of ALE, which include selective (NH₄)₂SiF₆ formation on the SiN_x surface and selective surface prefunctionalization of SiO₂ to enable ultrahigh selectivity. Through this review, the authors hope to provide the readers with an exhaustive knowledge of the selectivity mechanisms for RIE of SiO₂ over SiN_x and vice versa, which provides a basis for developing future highly material-selective ALE processes.