SF6 Physisorption based cryo-ALE of silicon

International audience; Cryogenic Atomic Layer Etching (Cryo-ALE) using C4F8 as a precursor gas has been previously presented as an alternative method to achieve ALE of SiO2. In this process, C4F8 is injected in gas phase during the “chemical modification” step, in order to physisorb on a cooled SiO2 surface. The etching step is then achieved using an Ar plasma with a low energy ion bombardment. The temperature window of this process was between -120°C and -90°C 1,2. A self-limiting etching regime was obtained with a regular etch per cycle (EPC) through several tens of nanometers which demonstrates the process stability without any contamination of the reactor walls. In this paper, we show the results using SF6 molecules as precursors for cryo-ALE of silicon.In 1996, Royer et al. studied the chemisorption of sulfur and fluorine on Si during a simultaneous exposure to SF6 gas and Ne+ ion beam. In this work, they showed by XPS measurements that the fluorine quantity on the Si surface tends to increase as the temperature decreases, for a process window between 20°C and 130°C 3. Therefore, a cryo-ALE study based on SF6 physisorption was carried out to extend the use of this alternative approach to other materials.The work presented in this paper 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 presented notably to determine the optimal temperature and purging time for the process. Subsequently, cryo-ALE test results on Si, SiO2 and Si3N4 will be shown. These results will finally be compared to the ones obtained previously using C4F8 physisorption.This research project is supported by the CERTeM 2020 platform, which provides most of the equipment and funded by the European Union (FEDER fund) as well as the French National Research Agency (ANR PSICRYO fund).1. Antoun et al., Appl. Phys. Lett. 115, 153109, 20192. Antoun et al., Sci. Rep. 10, 20213. Royer et al., J. Vac. Sci. Technol. A 14, 234–239, 1996