Clean SiO₂ atomic layer etching based on physisorption of high boiling point perfluorocarbon

This study aimed to evaluate the SiO₂ atomic layer etching (ALE) process that is selective to Si₃N₄ based on the physisorption of high boiling point perfluorocarbons (HBP PFCs; C₅F₈, C₇F₁₄, C₆F₆, and C₇F₈ have boiling points above room temperature). The lowering of the substrate temperature from 20 °C to -20 °C not only increased SiO₂ etch depth per cycle (EPC) but also increased etch selectivity of SiO₂/Si₃N₄ to near infinity. Due to the differences in fluorocarbon adsorption at a temperature during the physisorption depending on boiling points of PFCs, the desorption time and ion bombardment energy during the desorption step needed to be optimized, and higher ion bombardment energy and longer desorption time were required for higher HBP PFCs. Even though near infinity etch selectivity of SiO₂/Si₃N₄ was obtained, for the SiO₂ etching masked with Si₃N₄ patterns, due to the adsorption of PFC on the sidewall of the Si₃N₄ layer, the difficulty in anisotropic etching could be observed. By adding an O₂ descumming step in ALE processes, an anisotropic SiO₂ etch profile could be obtained with no adsorption of fluorocarbon on the chamber wall. Therefore, it is believed that the HBP ALE processes can be applicable for achieving high selective SiO₂/Si₃N₄ with more stability and reliability.