Effect of HF Pressure on Thermal Al2O3Atomic Layer Etch Rates and Al2O3Fluorination

Thermal Al2O3 atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF can be employed as the fluorination reactant, and Al(CH3)3 can be utilized as the metal precursor for ligand exchange. This study explored the effect of HF pressure on the Al2O3 etch rates and Al2O3 fluorination. Different HF pressures ranging from 0.07 to 9.0 Torr were employed for Al2O3 fluorination. Using ex situ spectroscopic ellipsometry (SE) measurements, the Al2O3 etch rates increased with HF pressures and then leveled out at the highest HF pressures. Al2O3 etch rates of 0.6, 1.6, 2.0, 2.4, and 2.5 Å/cycle were obtained at 300 °C for HF pressures of 0.17, 0.5, 1.0, 5.0, and 8.0 Torr, respectively. The thicknesses of the corresponding fluoride layers were also measured using X-ray photoelectron spectroscopy (XPS). Assuming an Al2OF4 layer on the Al2O3 surface, the fluoride thicknesses increased with HF pressures and reached saturation values at the highest HF pressures. Fluoride thicknesses of 2.0, 3.5, 5.2, and 5.5 Å were obtained for HF pressures of 0.15, 1.0, 4.0, and 8.0 Torr, respectively. There was an excellent correlation between the Al2O3 etch rates and fluoride layer thicknesses versus HF pressure. In addition, in situ Fourier transform infrared spectroscopy (FTIR) vibrational studies were used to characterize the time dependence and magnitude of the Al2O3 fluorination. These FTIR studies observed the fluorination of Al2O3 to AlF3 or AlOxFy by monitoring the infrared absorbance from the Al–O and Al–F stretching vibrations. The time dependence of the Al2O3 fluorination was explained in terms of rapid fluorination of the Al2O3 surface for initial HF exposures and slower fluorination into the Al2O3 near surface region that levels off at longer HF exposure times. Fluorination into the Al2O3 near surface region was described by parabolic law behavior. The self-limiting fluorination of Al2O3 suggests that the fluoride layer on the Al2O3 surface acts as a diffusion barrier to slow the fluorination of the underlying Al2O3 bulk. For equal fluorination times, higher HF pressures achieve larger fluoride thicknesses.