Thermal Atomic Layer Etching of SiO₂ by a “Conversion-Etch” Mechanism Using Sequential Reactions of Trimethylaluminum and Hydrogen Fluoride

The thermal atomic layer etching (ALE) of SiO₂ was performed using sequential reactions of trimethylaluminum (TMA) and hydrogen fluoride (HF) at 300 °C. Ex situ X-ray reflectivity (XRR) measurements revealed that the etch rate during SiO₂ ALE was dependent on reactant pressure. SiO₂ etch rates of 0.027, 0.15, 0.20, and 0.31 Å/cycle were observed at static reactant pressures of 0.1, 0.5, 1.0, and 4.0 Torr, respectively. Ex situ spectroscopic ellipsometry (SE) measurements were in agreement with these etch rates versus reactant pressure. In situ Fourier transform infrared (FTIR) spectroscopy investigations also observed SiO₂ etching that was dependent on the static reactant pressures. The FTIR studies showed that the TMA and HF reactions displayed self-limiting behavior at the various reactant pressures. In addition, the FTIR spectra revealed that an Al₂O₃/aluminosilicate intermediate was present after the TMA exposures. The Al₂O₃/aluminosilicate intermediate is consistent with a "conversion-etch" mechanism where SiO₂ is converted by TMA to Al₂O₃, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO₂ + 4Al(CH₃)₃ → 2Al₂O₃ + 3Si(CH₃)₄. Ex situ X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction of silicon species after TMA exposures. Following the conversion reactions, HF can fluorinate the Al₂O₃ and aluminosilicates to species such as AlF₃ and SiOxFy. Subsequently, TMA can remove the AlF₃ and SiOxFy species by ligand-exchange transmetalation reactions and then convert additional SiO₂ to Al₂O₃. The pressure-dependent conversion reaction of SiO₂ to Al₂O₃ and aluminosilicates by TMA is critical for thermal SiO₂ ALE. The "conversion-etch" mechanism may also provide pathways for additional materials to be etched using thermal ALE.