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

Abstract

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 SOxFyClz products in oxidation states greater than 4+. These SOxFyClz 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.