SF4 as the Fluorination Reactant for Al2O3 and VO2 Thermal Atomic Layer Etching

Abstract

Thermal atomic layer etching (ALE) is an important technique for the precise isotropic etching of nanostructures. Thermal ALE of many materials can be achieved using a two-step fluorination and ligand-exchange reaction mechanism. Most previous thermal ALE processes have used HF as the fluorination reactant. Alternative fluorination reactants may be needed because HF is a weak nucleophilic fluorination reactant. Stronger fluorination agents may be required for the fluorination of some materials. To demonstrate the usefulness of SF4 as an alternative to HF, thermal Al2O3 ALE was compared using SF4 or HF together with Sn(acac)2 as the metal precursor for ligand exchange. SF4 and HF were observed to behave similarly as fluorination reactants during Al2O3 ALE. The mass gains during the initial SF4 and HF exposures on Al2O3 atomic layer deposition (ALD) films at 200 °C were comparable at 35 and 38 ng/cm2, respectively, using quartz crystal microbalance measurements. In addition, the etch rates were similar at 0.20 and 0.28 Å/cycle for Al2O3 ALE using SF4 and HF, respectively, at 200 °C. Thermal VO2 ALE was also performed for the first time using SF4 or HF and Sn(acac)2 as the reactants. There was evidence that SF4 is a stronger fluorination reactant than HF for VO2 fluorination. The mass gains during the initial SF4 and HF exposures on VO2 ALD films were 38 and 20 ng/cm2, respectively, at 200 °C. Thermal VO2 ALE also had a higher etch rate when fluorinating with SF4 compared with HF. Etch rates of 0.30 and 0.11 Å/cycle were measured for VO2 ALE using SF4 and HF, respectively, together with Sn(acac)2 at 200 °C. Fourier transform infrared experiments were also used to monitor fluorination of the Al2O3 and VO2 ALD films by SF4 or HF. FTIR difference spectroscopy was used to observe the increase of Al–F and V–F stretching vibrations and the loss of the Al–O and V—O/V═O stretching vibrations for Al2O3 and VO2, respectively, versus SF4 or HF exposure at 200 °C. Additional absorbance features after fluorination of the Al2O3 ALD films by SF4 were consistent with SFx surface species. SF4 is a useful fluorination agent for thermal ALE processes and can be used as an alternative to HF. In addition, SF4 may be necessary when fluorination requires a stronger fluorination reactant than HF.