In-Situ Observation of Reaction Layer in Surface for Damage-Free Atomic Layer Etching

The atomic layer etching (ALE) process, which is advantageous because of its atomic-scale precision and damage-free, uniform processing, is expected to be one of the processes used to achieve the aforementioned requirements. Here, “damage-free” etching means that the defect density in surface is maintained before and after the etching process. Although ALD is an effective method for forming thin films layer by layer, it is not capable of depositing films with high crystallinity. Therefore, CDE and ALE will be important processes for etching while maintaining the defect density. It is required to investigate the surface reaction mechanism by atomic-scale surface observation. Changes in crystal structure after ion bombardment have been reported by simulations, and it has been reported that ion-induced damage can extend over several layers. [1,2] However, experimental investigation has not yet been reported. This is because surface analysis equipment with atomic-spatial resolution is required to observe the damage formation mechanism. Furthermore, surface contamination due to air exposure of the surface interfere with the elucidation of the reaction model. We have reported that reaction layer in surfaces irradiated by ions and radicals in ALE have been analyzed by various <i>in-situ</i> analysis system, and highlights the importance of <i>in-situ</i> analysis. [3-5] It is necessary to perform all surface analysis by <i>in-situ</i> system. Horiike group had performed atomic layer etching (ALE) for Si by alternating CF₄/O₂ and Ar plasma in 1990. [6] Fluorinated Si layer is desorbed by Ar ion bombardment as volatile species like SiF₄. However, the ion bombardment induces several damages in a reaction layer within a few nanometers. Processed samples should be transported from a reactor to a surface analysis chamber without exposure to air in order to investigate the reaction layer. We report changes in the crystal structure of Si (111) surfaces during dry etching and Atomic Layer Etching (ALE) processes, which have rarely been reported. [7] The observation requires the elimination of a small amount of surface contamination, which we achieved by developing a sample transfer system to move the sample with keeping high vacuum level between the STM/AFM atomic resolved surface analyzer and the plasma reactor. The etching Si by F radicals occurred while forming a few atomic layers of fluorination layer. The layer was completely removed by heating and restored to its original crystal structure without any damage. This is a very effective process for manufacturing advanced semiconductor devices. Furthermore, it was found that irradiation with Ar ions, which have energies lower than energy threshold of sputtering, did not destroy the crystal structure. However, the combination of F radicals and Ar ions simulating ALE did not recover the crystal structure, and holes with a depth of sub-nanometer were formed on the surface. We experimentally elucidated that atomic-scale damage occurs in the reaction layer and that this damage inhibits the reconstruction of the crystal structure, such as the formation of reaction layers consisting of several atomic compositions and the destruction of the crystal structure. A damage-free ALE requires a process that does not generate such damage.<br/>[1] H. Ohta et. al., J. Vac. Sci. Technol. A <b>19</b>, 2373 (2001). [2] D. B. Graves et. al., Appl. Surf. Sci. <b>192</b>, 72 (2002). [3] K. Nakane et. al., ACS Appl. Mater. Interfaces <b>11</b>, 37263 (2019). [4] M. Hasegawa et. al., J. Vac. Sci. Technol. A 38, 042602 (2020). [5] A. P. Osonio et. al., J. Vac. Sci. Technol. A <b>40</b>, 062601 (2022). [6] Y. Horiike et. al., J. Vac. Sci. Technol., A <b>8</b>, 1844 (1990). [7] T. Tsutsumi et. al., J. Vac. Sci. Technol., A <b>42</b>, 032603 (2024).