Selectivity between SiO₂ and SiN_<i>x</i> during Thermal Atomic Layer Etching Using Al(CH₃)₃/HF and Spontaneous Etching Using HF and Effect of HF + NH₃ Codosing

Abstract

Selectivity was examined between SiO2 and SiNx during thermal atomic layer etching (ALE) and spontaneous etching. Thermal ALE of SiO2 and SiNx 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%. SiNx thermal ALE exhibited a higher EPC of 1.06 Å/cycle. The selectivity factor was ∼5:1 for SiNx etching compared to SiO2 etching (preferential SiNx 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 SiNx spontaneous etching displayed an etch rate of 1.72 Å/min and predominated over SiNx thermal ALE. The selectivity factor was ∼50:1 for SiNx spontaneous etching compared to SiO2 spontaneous etching using an HF pressure of 3 Torr. This selective SiNx 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 SiNx 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, SiNx thermal ALE with HF + NH3 codosing was negligible. The selectivity factor was reversed and much higher at >1000:1 for SiO2 etching compared to SiNx 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, SiNx 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 SiNx 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 SiNx. Reciprocal etch selectivity should be important for the selective removal of SiO2 or SiNx in composite structures.