Surface Phenomena During Plasma-Assisted Atomic Layer Etching of SiO.

Surface phenomena during atomic layer etching (ALE) of SiO were studied during sequential half-cycles of plasma-assisted fluorocarbon (CF) film deposition and Ar plasma activation of the CF film using in situ surface infrared spectroscopy and ellipsometry. Infrared spectra of the surface after the CF deposition half-cycle from a CF/Ar plasma show that an atomically thin mixing layer is formed between the deposited CF layer and the underlying SiO film. Etching during the Ar plasma cycle is activated by Ar bombardment of the CF layer, which results in the simultaneous removal of surface CF and the underlying SiO film. The interfacial mixing layer in ALE is atomically thin due to the low ion energy during CF deposition, which combined with an ultrathin CF layer ensures an etch rate of a few monolayers per cycle. In situ ellipsometry shows that for a ∼4 Å thick CF film, ∼3-4 Å of SiO was etched per cycle. However, during the Ar plasma half-cycle, etching proceeds beyond complete removal of the surface CF layer as F-containing radicals are slowly released into the plasma from the reactor walls. Buildup of CF on reactor walls leads to a gradual increase in the etch per cycle.