Thermal Atomic Layer Etching of Silicon Using O2, HF, and Al(CH3)3 as the Reactants

Thermal atomic layer etching (ALE) of silicon was performed using O2, HF, and Al­(CH3)3 as the reactants at temperatures from 225 to 290 °C. This thermal etching process is based on Si oxidation using O2 and conversion of SiO2 to Al2O3 using trimethylaluminum (TMA). Al2O3 is then fluorinated by HF to produce AlF3 prior to removal of AlF3 by a ligand-exchange reaction with TMA. Thermal Si ALE was studied using silicon-on-insulator wafers. In situ spectroscopic ellipsometry was employed to monitor simultaneously both the thickness of the top SiO2 layer and the underlying silicon film during Si ALE. These studies revealed that the silicon film thickness decreased linearly with the number of reaction cycles while the thickness of the SiO2 layer remained constant. Using an O2–HF–TMA exposure sequence, the Si ALE etch rate was 0.4 Å/cycle at 290 °C. This etch rate was obtained using static reactant pressures of 250, 1.0, and 1.0 Torr and exposure times of 10, 5, and 5 s for O2, HF, and TMA, respectively. The SiO2 thickness was 10–11 Å under these reaction conditions at 290 °C. The Si ALE etch rate increased with O2 and TMA pressure before reaching a limiting etch rate at higher O2 and TMA pressures. The order of the reactants affected the Si etch rate. Changing the exposure sequence from O2–HF–TMA to O2–TMA–HF decreased the etch rate from 0.4 to 0.2 Å/cycle at 290 °C. Decreasing the etch temperature below 290 °C also resulted in a decrease in the Si etch rate. Atomic force microscopy measurements determined that the root-mean-square (RMS) roughness of the surface was 2.0 ± 0.2 Å before and after Si ALE using the optimum reaction conditions. Decreasing the static O2 pressures below 250 Torr decreased the etch rate and also increased the RMS surface roughness. There was no evidence of any change in the Si ALE process for ultrathin Si films with thicknesses of