Thermal Atomic Layer Etching of SiO by a "Conversion-Etch" Mechanism Using Sequential Reactions of Trimethylaluminum and Hydrogen Fluoride.

The thermal atomic layer etching (ALE) of SiO 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 SiO ALE was dependent on reactant pressure. SiO 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 SiO 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 AlO/aluminosilicate intermediate was present after the TMA exposures. The AlO/aluminosilicate intermediate is consistent with a "conversion-etch" mechanism where SiO is converted by TMA to AlO, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO + 4Al(CH) → 2AlO + 3Si(CH). 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 AlO and aluminosilicates to species such as AlF and SiOF. Subsequently, TMA can remove the AlF and SiOF species by ligand-exchange transmetalation reactions and then convert additional SiO to AlO. The pressure-dependent conversion reaction of SiO to AlO and aluminosilicates by TMA is critical for thermal SiO ALE. The "conversion-etch" mechanism may also provide pathways for additional materials to be etched using thermal ALE.