Cryo-ALE of Si and SiO2 using SF6 Physisorption
Résumé
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.
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. As a result, another gas chemistry has to be tested in order to
achieve higher etching selectivity.
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 cryo-ALE study based
on 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 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. 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, 2019
2. Antoun et al., Sci. Rep. 10, 2021
3. Royer et al., J. Vac. Sci. Technol. A 14, 234–239, 1996