Mechanism of SiN etching rate fluctuation in atomic layer etching^a)

Atomic layer etching (ALE) enables atomic-precision control of the surface reaction for device fabrication. In this study, we investigate SiN ALE with process optimization of the surface adsorption and desorption steps, and we clarify the rate fluctuation mechanism. When we attempted CH₃F/Ar plasma adsorption followed by the subsequent Ar desorption step, an etch stop was observed owing to the excess deposition (>6 nm) of a protective film on the surface. X-ray photoelectron spectroscopy analysis revealed that a high number of C–C bonds was detected. As the bonding energy of C–C (6.4 eV) is high, these bonds remain after the desorption step. The excess C-rich polymer deposition after ALE originates from the residual C–C bonds. To suppress the C-rich polymer deposition, we studied stable SiN ALE using a desorption step with Ar/O₂ plasma (0.36 nm/cycle) and a two-step sequential desorption with Ar and O₂ plasma (0.6 nm/cycle), which resulted in stable SiN ALE processes. Because the surface condition is able to fluctuate with the number of cycles, precise surface control is strongly required to achieve stable ALE. Control of the plasma–wall interaction is also important for stable SiN ALE. We intentionally changed the chamber wall conditions and found that the polymer-deposited wall caused a fluctuation of the etched amount, which resulted from CF_x desorption from the deposited polymer. Thus, it is also important to control the influence of desorbed species from the chamber components for suppression of the ALE fluctuation.