Atomic layer etching of TiN by oxidation and SF6-H2 plasma exposure: ALE window defined by HF, H, and F

Expanding the understanding of dry-etching processes calls for the identification of species present in the plasma and how they react with the material surface. Here, we have investigated an atomic layer etching (ALE) process of TiN consisting of the oxidation of the top layer and the subsequent selective removal of this surface oxide by an SF6-H2 plasma. The self-limiting nature of the ALE steps has been demonstrated, and the ALE window has been identified in terms of the SF6:H2 gas ratio. Optical emission spectroscopy and quadrupole mass spectrometry measurements of the plasma mixtures show the formation of HF in the plasma and the presence of F and H radicals. The concentrations of these species mainly define the ALE window: HF provides the selective etching of the surface oxide over TiN, while F radical levels must be negligible to prevent the etching of the bulk TiN underneath. Moreover, a high density of H radicals in the plasma reduces the surface oxide and, therefore, inhibits the ALE process, defining the lower limit of the ALE window. The insight into the species involved in the etching process provided by this work can trigger the exploration of similar ALE processes for other materials.