Surface Phenomena During Plasma-Assisted Atomic Layer Etching of SiO₂

Surface phenomena during atomic layer etching (ALE) of SiO₂ were studied during sequential half-cycles of plasma-assisted fluorocarbon (CF_x) film deposition and Ar plasma activation of the CF_x film using in situ surface infrared spectroscopy and ellipsometry. Infrared spectra of the surface after the CF_x deposition half-cycle from a C₄F₈/Ar plasma show that an atomically thin mixing layer is formed between the deposited CF_x layer and the underlying SiO₂ film. Etching during the Ar plasma cycle is activated by Ar⁺ bombardment of the CF_x layer, which results in the simultaneous removal of surface CF_x and the underlying SiO₂ film. The interfacial mixing layer in ALE is atomically thin due to the low ion energy during CF_x deposition, which combined with an ultrathin CF_x layer ensures an etch rate of a few monolayers per cycle. In situ ellipsometry shows that for a ∼4 Å thick CF_x film, ∼3-4 Å of SiO₂ was etched per cycle. However, during the Ar plasma half-cycle, etching proceeds beyond complete removal of the surface CF_x layer as F-containing radicals are slowly released into the plasma from the reactor walls. Buildup of CF_x on reactor walls leads to a gradual increase in the etch per cycle.