Atomic Layer Etching of AlF3 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and Hydrogen Fluoride

Abstract

The atomic layer etching (ALE) of AlF3 was demonstrated using sequential thermal reactions with Sn(acac)2 and hydrogen fluoride (HF) as the reactants. AlF3 ALE is the first example of the thermal ALE of a metal fluoride. AlF3 ALE was investigated using in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) measurements at temperatures from 150 to 250 °C. The QCM studies observed that AlF3 was etched linearly with atomic level precision versus number of sequential reactant cycles. QCM investigations also revealed that the sequential Sn(acac)2 and HF reactions were self-limiting versus reactant exposure. The FTIR spectroscopic analysis observed AlF3 etching by monitoring the loss of absorbance of Al–F stretching vibrations in the AlF3 film. The FTIR studies also suggested that the Sn(acac)2 reaction is self-limiting because of the buildup of acac-containing species on the AlF3 surface. The QCM measurements determined that the mass change per cycle (MCPC) increased with temperature from −2.0 ng/(cm2 cycle) at 150 °C to −18.2 ng/(cm2 cycle) at 250 °C. These MCPC values are equivalent to etch rates from 0.069 Å/cycle at 150 °C to 0.63 Å/cycle at 250 °C. In the proposed reaction mechanism for AlF3 ALE, the Sn(acac)2 reactant accepts fluorine from AlF3 and donates acac to the surface. This reaction is believed to yield SnF(acac) and AlF(acac)2 as volatile reaction products. The QCM and FTIR results suggest that the HF reaction converts AlF2(acac)* surface intermediates to AlF3* and volatile acacH reaction products. The ALE of other metal fluorides using Sn(acac)2 and HF should be possible by a similar mechanism.