Plasma-assisted thermal-cyclic atomic-layer etching for selective removal of thin films

Plasma-assisted thermal-cyclic atomic layer etching (ALE) is an attractive approach to achieve atomically precise isotropic etching. It is also suitable for selective ALE because the ALE reaction can be controlled by temperature. The ALE of various films including Si₃N₄, TiN, SiGe, and W were evaluated using a 300-mm apparatus composed of a plasma source for surface modification and infrared heating lamps for removal of the modified surface. The results of selective removal of these films are presented, focusing on the detailed analysis of the surface atomic reactions by in situ x-ray photoelectron spectroscopy (XPS). For the Si₃N₄ and TiN films, formation and removal of the ammonium-salt-based modified layer were detected after hydrofluorocarbon-based plasma exposure and heating. High selectivity over SiO₂ and poly-Si was obtained because these films do not contain N atoms, which are essential to form modified layers. Self-limiting cyclic etching of SiGe, which is selective to Ge, was also demonstrated using the formation and removal of ammonium salt-based modified layers. The selectivity was due to enhanced formation of modified layers on Si-rich films. One important feature of plasma-assisted thermal cyclic ALE is its ability to control material selectivity by infrared heating time. The selectivity of the W and TiN films was controlled from non-selective to infinitely selective by tuning the infrared heating time. This phenomenon was due to the difference in desorption temperatures of the modified surfaces of the two films. Only the modified surface of W, which was ascribed to WF_x, was removed when infrared heating time was in the range from 4 to 8 sec, while the modified surfaces of both W and TiN films were removed when infrared heating time was in the range from 20 to 24 sec.