Selectivity between SiO₂ and SiN_x during Thermal Atomic Layer Etching Using Al(CH₃)₃/HF and Spontaneous Etching Using HF and Effect of HF + NH₃ Codosing

Selectivity was examined between SiO 2 and SiN x during thermal atomic layer etching (ALE) and spontaneous etching. Thermal ALE of SiO 2 and SiN x was explored using sequential trimethylaluminum (TMA) and hydrogen fluoride (HF) with reactant exposures of 3 Torr for 45 s at 275 °C. SiO 2 thermal ALE achieved an etch per cycle (EPC) of 0.20 Å/cycle and near-ideal synergy up to 95%. SiN x thermal ALE exhibited a higher EPC of 1.06 Å/cycle. The selectivity factor was ∼5:1 for SiN x etching compared to SiO 2 etching (preferential SiN x removal) during thermal ALE using TMA and HF. Spontaneous etching was then quantified using repeated exposures of HF vapor alone at 3 Torr and 275 °C. SiO 2 spontaneous etching was minor at an etch rate of 0.03 Å/min, enabling near-ideal synergy for SiO 2 thermal ALE. In contrast, major SiN x spontaneous etching displayed an etch rate of 1.72 Å/min and predominated over SiN x thermal ALE. The selectivity factor was ∼50:1 for SiN x spontaneous etching compared to SiO 2 spontaneous etching using an HF pressure of 3 Torr. This selective SiN x spontaneous etching was attributed to F – surface species during HF exposures. NH 3 codosing with HF was then examined during thermal ALE and spontaneous etching. Thermal ALE of SiO 2 and SiN x was examined using sequential TMA and HF + NH 3 codosing with reactant exposures of 3 Torr for 45 s at 275 °C. SiO 2 thermal ALE with HF + NH 3 codosing had a high EPC of 8.83 Å/cycle. In contrast, SiN x thermal ALE with HF + NH 3 codosing was negligible. The selectivity factor was reversed and much higher at >1000:1 for SiO 2 etching compared to SiN x etching (preferential SiO 2 removal) during thermal ALE with HF + NH 3 codosing. Rapid SiO 2 spontaneous etching with HF + NH 3 codosing at 3 Torr had an etch rate of 27.50 Å/min. In contrast, SiN x spontaneous etching with HF + NH 3 codosing produced a very low etch rate of 0.02 Å/min. The selectivity factor was >1000:1 for SiO 2 spontaneous etching compared to SiN x spontaneous etching with HF + NH 3 codosing. This selective SiO 2 spontaneous etching was attributed to HF 2 – surface species during HF + NH 3 exposures. These studies revealed that the NH 3 coadsorbate during HF exposures modified the active etch species and dramatically influenced the etch selectivity between SiO 2 and SiN x . Reciprocal etch selectivity should be important for the selective removal of SiO 2 or SiN x in composite structures.