We will introduce history of atomic layer etching (ALE) and conventional plasma etching. ALE achieves this continuous improvement, since the process has advantages such as more precise, higher controllability and repeatability compared to conventional plasma etching. The ALE concept starts with adsorption of an etchant gas or deposition of an etchant film on a target material, followed by a removal step to react between the etchant species and the target material. The reaction is promoted by using accelerated ions in plasma. An ALE process for Si by alternating subnanometer-scale fluorocarbon film deposition and Ar⁺ ion etching in two different chambers, and developed self-limiting, layer-by-layer etching by controlling the thickness of the fluorocarbon film deposited on the surface Si.

Fluorine atoms in the fluorocarbon film are known to react with Si atoms in the SiO₂ during Ar⁺ ion etching, while carbon atoms will react with oxygen atoms in the SiO₂ to generate gas phase CO_x molecules. The unreacted carbon atoms will form a carbon-rich film on the SiO₂ surface, and the thickness of this film increases with the number of cycles, eventually disturbing the etching reaction between the fluorocarbon and the SiO₂.

The carbon-rich film formed on the SiO₂ surface and the residual fluorocarbon film on the chamber walls result in difficulty in controlling the process and affect the repeatability of the etch rate per cycle (EPC) during the ALE. In industrial applications, moreover, the ALE process is required to perform both deposition and etching process in the same chamber. Therefore, we must consider the effects of fluorocarbons deposited on the chamber walls.

We will show our suggested ALE process for SiO₂ to achieve high controllability and repeatability. The ALE process is a cyclic process composed of two steps: a deposition step which forms a fluorocarbon film using an Ar/C₄F₈ plasma, followed by an O₂ plasma etching step that performs the reaction between the fluorocarbon and the SiO₂. Simultaneously, the unreacted carbon atom is evaporated as CO_x molecules. Therefore, O₂ plasma suppresses forming a carbon-rich film on the target material surface and maintains the chamber wall conditions by removing the fluorocarbon on the chamber walls.

We demonstrated that novel ALE process for SiO₂ exhibits high reproducibility and has the potential to allow uniform EPC values over large wafer surfaces. The surface chemistry and etched thickness of SiO₂ during this ALE process were investigated.