(Invited) Advanced Plasma Etching Processing: Atomic Layer Etching for Nanoscale Devices
Authors: Tsutsumi Takayoshi , Zaitsu Masaru , Kobayashi Akiko , Kobayashi Nobuyoshi , Hori Masaru 
Published: February 2020

Plasma processes for atomic scale deposition and etching are required for nanoscale devices such as Fin-field effect transistors (FETs), nanowires, 3D NAND-type flash memory units, and others 3D devices. To achieve high performance of such nanoscale devices, the continuous development of fabrication processes is necessary. The solution is usually based on a trial and error approach where some external parameters such as RF power, feed gas, gas mixture ratio, pressure, and electrode temperature have been optimized to obtain desired etching characteristics. This spends long development time with high cost. Moreover, it is not a robust process because internal parameters such as electron and radical densities, ion energy, and wafer temperature, which define the etching performance directly, can change over time even if external parameters are kept constant.
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 COx 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 COx 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.