Cryo-ALE of Si based on SF6 physisorption

Jack Nos (1) , Thomas Tillocher (1) , Philippe Lefaucheux (1) , Jacques Faguet (2) , Kaoru Maekawa (3) , Remi Dussart (1)

1 GREMI - Groupe de recherches sur l'énergétique des milieux ionisés
2 Tokyo Electron America
3 TEL Technology Center

Cryogenic Atomic Layer Etching (Cryo-ALE) has been presented in the previous PLATHINIUM conference (2019) as a different approach to achieve ALE of SiO2. In this process, C4F8 is used as a precursor in the “modification” step in order to physisorb on a cooled SiO2 surface between -120°C and - 90°C 1,2. The “etching” step is then achieved by using an Ar plasma with a low energy ion bombardment. However, C4F8 injection at cryogenic temperatures does not allow high etching selectivity of SiO2 over Si and Si3N4 as the deposited CFx passivation layer is not thick enough to efficiently passivate Si and Si3N4 surfaces. Nevertheless, self-limiting etching was achieved and a very stable process of SiO2 etching was obtained. In 1996, Royer et al. studied the chemisorption of fluorine and sulfur on Si during a simultaneous exposure to SF6 gas and Ne+ ion beam. In this study, they showed by XPS measurements that the fluorine amount at the Si surface increases as the temperature decreases in a process window between 20°C and -130°C 3. Moreover, SF6 is a well-known gas used in Si plasma etching. Therefore, cryo-ALE based on SF6 physisorption was studied to extend the use of this new type of process to other materials and to characterize its etching properties. This work was carried out using a cryogenic ICP reactor equipped with in-situ diagnostics. Mass spectrometry measurements enabled to characterize the SF6 physisorption and its surface residence time at different temperatures. Spectroscopic ellipsometry was used to monitor the etching rate and to characterize the sample surface at the nanoscale during the three process steps: SF6 physisorption, pumping and Ar plasma etching. Tests were performed on SiO2, Si3N4 and p-Si coupons glued on SiO2 6” carrier wafers. SF6 physisorption experiments will first be studied and presented notably to find the optimal temperature and purging time for the process. Then, cryo-ALE test results on Si, SiO2 and Si3N4 will be shown. These results will finally be compared to the ones obtained using C4F8 physisorption. Thanks/Acknowledgement The authors thank S. Tahara for all the helpful discussions. This work was supported by the CERTeM 2020 platform which provided most of the equipment. References 1. Antoun et al., Appl. Phys. Lett. 115, 153109, 2019 2. Antoun et al., Sci. Rep. 10, 2021 3. Royer et al., J. Vac. Sci. Technol. A 14, 234–239, 1996

Domaines: Physique des plasmas [physics.plasm-ph]

Jack Nos, Thomas Tillocher, Philippe Lefaucheux, Jacques Faguet, Kaoru Maekawa, et al.. Cryo-ALE of Si based on SF6 physisorption. PLATHINIUM (Plasma Thin film International Union Meeting) 2021, Sep 2021, Antibes, France. ⟨hal-03539330⟩