Selectivity between SiO2 and SiNx 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 SiO₂ and SiN_x during thermal atomic layer etching (ALE) and spontaneous etching. Thermal ALE of SiO₂ 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. SiO₂ 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 SiO₂ 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. SiO₂ spontaneous etching was minor at an etch rate of 0.03 Å/min, enabling near-ideal synergy for SiO₂ 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 SiO₂ spontaneous etching using an HF pressure of 3 Torr. This selective SiN_x spontaneous etching was attributed to F^– surface species during HF exposures. NH₃ codosing with HF was then examined during thermal ALE and spontaneous etching. Thermal ALE of SiO₂ and SiN_x was examined using sequential TMA and HF + NH₃ codosing with reactant exposures of 3 Torr for 45 s at 275 °C. SiO₂ thermal ALE with HF + NH₃ codosing had a high EPC of 8.83 Å/cycle. In contrast, SiN_x thermal ALE with HF + NH₃ codosing was negligible. The selectivity factor was reversed and much higher at >1000:1 for SiO₂ etching compared to SiN_x etching (preferential SiO₂ removal) during thermal ALE with HF + NH₃ codosing. Rapid SiO₂ spontaneous etching with HF + NH₃ codosing at 3 Torr had an etch rate of 27.50 Å/min. In contrast, SiN_x spontaneous etching with HF + NH₃ codosing produced a very low etch rate of 0.02 Å/min. The selectivity factor was >1000:1 for SiO₂ spontaneous etching compared to SiN_x spontaneous etching with HF + NH₃ codosing. This selective SiO₂ spontaneous etching was attributed to HF₂^– surface species during HF + NH₃ exposures. These studies revealed that the NH₃ coadsorbate during HF exposures modified the active etch species and dramatically influenced the etch selectivity between SiO₂ and SiN_x. Reciprocal etch selectivity should be important for the selective removal of SiO₂ or SiN_x in composite structures.