Cryo atomic layer etching of SiO2 by C4F8 physisorption followed by Ar plasma

Atomic Layer Etching is performed on SiO2 samples cooled down to a very low temperature (below -100 degrees C). C4F8 gas flow is injected and molecules physisorb on the cooled surfaces. Etching is then carried out using argon plasma with a low ion energy. Atomic layer etching of SiO2 has been proved for a temperature of -120 degrees C, whereas no etching was obtained at -110 degrees C. The etched amount per cycle is 0.4 nm. Self-limiting etching was achieved and evidenced by in situ ellipsometry. It is also shown that working at low sample temperature with this type of process prevents reactor wall contamination. This enables us to process many etching cycles without drift in etched amount per cycle. In order to characterize the surface roughness after etching, Atomic Force Microscopy has been performed, showing a slight increase of about 0.8 nm for a 27 nm SiO2 etched depth.