Effectiveness of Different Ligands on Silane Precursors for Ligand Exchange to Etch Metal Fluorides

Metal fluorides can be spontaneously etched by ligand-exchange reactions. During these reactions, the exchange of ligands between the incoming precursor and the metal fluoride leads to the volatilization of the metal fluoride. These ligandexchange reactions are important in many thermal atomic layer etching (ALE) processes. To test the effectiveness of different ligands for ligand exchange, the spontaneous etching of metal fluorides by silane precursors containing different ligands was investigated using in situ quadrupole mass spectrometry (QMS). The homoleptic and heteroleptic silane precursors were SiCl4, SiCl2(CH3)(2), SiCl(CH3)(2)H, and Si(CH3)(4). These silane precursors provide Cl, CH3, or H ligands for the ligand-exchange reaction. The metal fluorides were GaF3, InF3, ZnF2, ZrF4, HfF4, and SnF4. The QMS results showed that F/Cl ligand exchange was observed for all the metal fluorides with chlorine-containing silane precursors. In addition, all the volatile metal etch products were metal chlorides, namely, GaCl3, InCl3, ZnCl2, ZrCl4, HfCl4, and SnCl4. No metal methyl complexes were detected as volatile metal etch products indicating no F/CH3 exchange. For SiCl(CH3)(2)H as the silane precursor, the observation of SiF2(CH3)(2) indicated that F/ H exchange is also possible in addition to F/Cl exchange. For the various metal fluorides, the dominance of the F/Cl exchange led to nearly equivalent onset temperatures for ligand exchange and for etching to yield metal chlorides. Thermochemical calculations of the ligand-exchange reactions also predicted the formation of metal chlorides. All F/Cl exchanges were the most thermodynamically favorable as demonstrated by their negative changes in Gibbs free energy (delta G). These experimental and theoretical results provide guidelines for designing precursors for thermal ALE processes.

Automated summary

The spontaneous etching of metal fluorides by different ligands was investigated, and it was found that F/Cl ligand exchange was the most effective, yielding metal chlorides. F/H exchange was also observed under certain conditions. These results provide guidelines for designing precursors for thermal ALE processes.