Scaling of Atomic Layer Etching of SiO2 in Fluorocarbon Plasmas: Transient Etching and Surface Roughness

Atomic layer etching (ALE) is a cyclic plasma process used in semiconductor fabrication that can remove single layers of atoms during each cycle provided each step is self-terminating. In plasma ALE of dielectrics (e.g., SiO
2
, Si
3
N
4
) in fluorocarbon gas mixtures, etching proceeds through deposition of a thin polymer layer, and the process is not strictly self-terminating. As a result, ALE performance is a delicate balance between deposition of polymer during the first half of the cycle and etching (with polymer removal) during the second half. In the absence of complete removal of the overlying polymer in each cycle, ALE may be transient as the polymer thickness grows with each cycle. With small amounts of polymer left on the surface, roughening may be reinforced by there being a variation in polymer thickness.