Thermal atomic layer etching of VO₂ using sequential BCl₃ and SF₄ exposures: Observation of conversion, ligand-exchange, and oxidation state changes

The thermal atomic layer etching (ALE) of VO₂ was demonstrated using sequential exposures of BCl₃ and SF₄. The VO₂ etch rate measured by quartz crystal microbalance investigations at 250 °C was 2.3 Å/cycle. The mass losses during individual BCl₃ and SF₄ reactions were nearly self-limiting versus BCl₃ and SF₄ exposures. The VO₂ etch rates were also dependent on temperature and varied from 0.05 Å/cycle at 150 °C to 2.3 Å/cycle at 250 °C. Fourier transform infrared (FTIR) spectroscopy studies observed VO₂ etching by monitoring the decrease in absorbance from V—O stretching vibrations in the VO₂ film. The FTIR spectra during the initial BCl₃ exposures on the VO₂ film observed the growth of absorbance from B—O stretching vibrations from B₂O₃ and the concurrent loss of V=O vibrational features. These changes were consistent with BCl₃ converting VO₂ to B₂O₃. The FTIR difference spectra during subsequent SF₄ and BCl₃ reactions also observed the growth and loss of absorbance features that were attributed to F₃V=O and V—F stretching vibrations, respectively. These changes indicate that SF₄ fluorinates VO₂ to form a VOF₃ surface layer and then BCl₃ undergoes ligand-exchange with VOF₃ to volatilize the VOF₃ surface layer as VOCl₃. There was also evidence for conversion of VO₂ to B₂O₃ during BCl₃ exposures and then removal of B₂O₃ by SF₄ exposures. In addition, quadrupole mass spectrometry (QMS) measurements observed that the SF₄ exposures produced ion intensities for SO_xF_yCl_z products in oxidation states greater than 4+. These SO_xF_yCl_z products indicate that SF₄ is being oxidized and acting as a deoxyfluorination reactant. Concurrently, the QMS analysis also monitored ion intensity for S₈⁺, S₇⁺, S₆⁺, S₅⁺, and S₄⁺. These S₈ electron impact ionization products argue that SF₄ oxidation occurs concurrently with SF₄ reduction. The QMS also observed ion intensities corresponding to VCl₄⁺ and VOCl₃⁺. The presence of VOCl₃⁺ indicates that the oxidation state of vanadium has increased to 5+ in some of the volatile etch products. The QMS also detected trichloroboroxin (B₃O₃Cl₃) during BCl₃ exposures. B₃O₃Cl₃ is a known etch product of B₂O₃ during BCl₃ exposures. BCl₃ can convert VO₂ to B₂O₃ and then proceed to etch the converted B₂O₃. Thermal VO₂ ALE using BCl₃ and SF₄ reveals the rich complexity of surface etching reactions that can proceed by multiple pathways including conversion, ligand-exchange, and oxidation state changes.