Paper PS+EM+TF-ThA3
Rapid thermal-cyclic Atomic Layer Etching of SiO₂ Using Infrared Annealing

Thursday, October 25, 2018, 3:00 pm, Room 104C

Device structures are changing from planar-types to three-dimensional types, such as fin-type field-effect transistors and gate-all-around transistors, in order to reduce leakage current and power consumption. Furthermore, fabricating these devices requires both high selectivity and precise control of device dimensions below the 10 nm scale. Atomic layer etching (ALE), which involves the layer-by-layer removal of thin films, is expected to meet these requirements. In a conventional ALE process for SiO₂[1], HF and NH₃ molecules are exposed to the surface simultaneously. In this case, the etching amount per cycle is proportional to square root of the exposure time of HF and NH₃, indicating that the process is quasi-self-limiting (not self-limiting).

We have been investigating a thermal ALE of SiO₂ in which ammonium fluorosilicate-based surface modified layer is formed after exposure to HF and NH₃ gas, and removed by using infrared annealing. Etching depth of blanket SiO₂ samples were measured by using in-situ ellipsometry. We found that self-limiting formation of the modified layer was achieved with separate exposure of HF and NH₃ gas. In addition, we successfully demonstrated that this etching process was highly selective with respective to poly-Si and SiN. Comparison between experimental results and Langmuir adsorption model for dependence of etching rate on partial pressure of HF and NH₃ clarified that the SiO₂ surface during the gas exposure is in adsorption-desorption equilibrium.

In addition to HF/NH₃ gas chemistry, we developed an ALE process utilizing plasma to generate HF molecule in stead of using HF gas. This process consists of plasma discharge for HF exposure, evacuation of the residual gas, NH₃ exposure, and infrared annealing. In the plasma discharge, fluorine based gases, such as CF₄, NF₃, and SF₆ were diluted by H₂ gas. The etching rate of SiO₂ saturated at 2.8 nm/cycle in regard to the discharge time, which indicates self-limiting behavior of the modification step. In contrast, the etching rates of poly-Si and SiN were below the detection limit of in-situ ellipsometry, demonstrating that this etching process for SiO₂ is highly selective to poly-Si and SiN.