Fluorocarbon based atomic layer etching of Si3N4 and etching selectivity of SiO2 over Si3N4

Abstract

Angstrom-level plasma etching precision is required for semiconductor manufacturing of sub-10 nm critical dimension features. Atomic layer etching (ALE), achieved by a series of self-limited cycles, can precisely control etching depths by limiting the amount of chemical reactant available at the surface. Recently, SiO2 ALE has been achieved by deposition of a thin (several Angstroms) reactive fluorocarbon (FC) layer on the material surface using controlled FC precursor flow and subsequent low energy Ar+ ion bombardment in a cyclic fashion. Low energy ion bombardment is used to remove the FC layer along with a limited amount of SiO2 from the surface. In the present article, the authors describe controlled etching of Si3N4 and SiO2 layers of one to several Angstroms using this cyclic ALE approach. Si3N4 etching and etching selectivity of SiO2 over Si3N4 were studied and evaluated with regard to the dependence on maximum ion energy, etching step length (ESL), FC surface coverage, and precursor selection. Surface chemistries of Si3N4 were investigated by x-ray photoelectron spectroscopy (XPS) after vacuum transfer at each stage of the ALE process. Since Si3N4 has a lower physical sputtering energy threshold than SiO2, Si3N4 physical sputtering can take place after removal of chemical etchant at the end of each cycle for relatively high ion energies. Si3N4 to SiO2 ALE etching selectivity was observed for these FC depleted conditions. By optimization of the ALE process parameters, e.g., low ion energies, short ESLs, and/or high FC film deposition per cycle, highly selective SiO2 to Si3N4 etching can be achieved for FC accumulation conditions, where FC can be selectively accumulated on Si3N4 surfaces. This highly selective etching is explained by a lower carbon consumption of Si3N4 as compared to SiO2. The comparison of C4F8 and CHF3 only showed a difference in etching selectivity for FC depleted conditions. For FC accumulation conditions, precursor chemistry has a weak impact on etching selectivity. Surface chemistry analysis shows that surface fluorination and FC reduction take place during a single ALE cycle for FC depleted conditions. A fluorine rich carbon layer was observed on the Si3N4 surface after ALE processes for which FC accumulation takes place. The angle resolved-XPS thickness calculations confirmed the results of the ellipsometry measurements in all cases.