Cryo-ALE of Si and SiO2 using SF6 Physisorption

Cryogenic Atomic Layer Etching (Cryo-ALE) using C4F8 as a precursor gas has beenpreviously presented as an alternative method to achieve ALE of SiO2. In this process, C4F8 isinjected in gas phase during the “chemical modification” step, in order to physisorb on a cooledSiO2 surface. The etching step is then achieved using an Ar plasma with a low energy ionbombardment. The temperature window of this process was between -120°C and -90°C 1,2.However, C4F8 injection at cryogenic temperatures does not allow high etching selectivity ofSiO2 over Si and Si3N4 as the deposited CFx passivation layer is not thick enough to efficientlypassivate Si and Si3N4 surfaces. As a result, another gas chemistry has to be tested in order toachieve higher etching selectivity.In 1996, Royer et al. studied the chemisorption of sulfur and fluorine on Si during asimultaneous exposure to SF6 gas and Ne+ ion beam. In this work, they showed by XPSmeasurements that the fluorine quantity on the Si surface tends to increase as the temperaturedecreases, for a process window between 20°C and -130°C 3. Therefore cryo-ALE study basedon SF6 physisorption was carried out to extend the use of this alternative approach to other materials.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 surfaceresidence time at different temperatures. Spectroscopic ellipsometry was used to monitor theetching rate and to characterize the sample surface at the nanoscale during the three processsteps: SF6 physisorption, pumping and Ar plasma etching. Tests were performed on SiO2, Si3N4and p-Si coupons glued on SiO2 6” carrier wafers.SF6 physisorption experiments will first be studied and presented notably to find the optimaltemperature and purging time for the process. Subsequently, cryo-ALE test results on Si, SiO2and Si3N4 will be shown. These results will finally be compared to the ones obtained previouslyusing C4F8 physisorption.This research project is supported by the CERTeM 2020 platform, which provides most of theequipment and funded by the European Union (FEDER fund) as well as the French NationalResearch 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