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.