Spontaneous Etching of Metal Fluorides Using Ligand-Exchange Reactions: Landscape Revealed by Mass Spectrometry

Thermal atomic layer etching (ALE) can be performed using sequential reactions based on surface modification followed by volatile release of the modified surface layer. Surface modification can be accomplished using fluorination. Volatile release can then be achieved using precursors that undergo ligand-exchange reactions with the fluorinated surface layer. Metal fluorides can be employed to model the fluorinated surface layer. The ligand-exchange reaction between the precursor and the metal fluoride can lead to spontaneous etching of the metal fluoride. A new reactor with in situ quadrupole mass spectrometry (QMS) was constructed to observe the volatile etch products from the reaction of ligand-exchange precursors with metal fluoride powders. The metal fluoride powders were AlF3, HfF4, GaF3, InF3, and SnF4. The ligand-exchange precursors were Al(CH3)3, SiCl4, and TiCl4. A variety of studies were conducted including Al(CH3)3 + AlF3, SiCl4 + HfF4, SiCl4 + InF3, TiCl4 + SnF4, Al(CH3)3 + GaF3, and SiCl4 + AlF3. The temperature-dependent in situ QMS studies revealed the many possibilities that occur during the ligand-exchange reaction of precursors with metal fluoride powders. Various categories of behavior were observed from these studies: (i) Ligand exchange occurs at low temperature, but metal etch products from the substrate are not observed until high temperature. (ii) Ligand-exchange and metal etch products from the substrate are observed at similar temperatures. (iii) Ligand exchange occurs, but no metal etch products from the substrate are observed up to a limiting temperature. Knowledge of these possibilities for the ligand-exchange reaction between precursors and metal fluoride powders during spontaneous etching helps to further the understanding of thermal ALE.