Cryogenic atomic layer etching of SiO2 based on cyclic CF4/Ar plasma

This study demonstrates the feasibility of a cryogenic atomic layer etching (cryo-ALE) process for Silicon Dioxide (SiO2) using a CF4 plasma adsorption step and a surface removal step based on Argon plasma. In situ ellipsometry was used to monitor the amount of SiO2 etched during each cycle. To improve the process, several parameters were optimized. First, the sample temperature was varied from 20 degrees C, temperature of which CF4 is in etching regime, to -130 degrees C, switching the plasma process to a deposition regime. Then, CF4 plasma parameters, including the pressure, inductively coupled plasma (ICP) power and its duration were studied. For a lower pressure, a higher ICP power and a longer plasma time, the process switches from etching to over-deposition. Finally, after optimization of the CF4 plasma parameters, an average etch rate per cycle (EPC) of 0.4 nm/cycle was found with a process synergy determined to be 99%.