NF3 Plasma-Enhanced Isotropic Atomic Layer Etching of Amorphous and Crystalline Hafnium Oxide Films

This study investigates a highly effective isotropic atomic layer etching (ALE) process for both amorphous and crystalline hafnium oxide (HfO2) films using fluorine (F) radicals generated from a nitrogen trifluoride (NF3) remote plasma with dimethylaluminum chloride (Al(CH3)2Cl, DMAC) as the removal reactant. While previous ALE processes using reactants such as hydrogen fluoride (HF) have demonstrated limited efficacy on crystalline HfO2, the NF3-plasma-based process in this study shows significantly improved etching performance. At a process temperature of 200{degree sign}C, the saturated etch rates were 3.1 and 2.9 Å/cycle for amorphous and crystalline HfO2, respectively, dramatically reducing the etch-rate difference between the two film structures. The process exhibits excellent geometric isotropy, confirmed by uniform thickness reduction on a high-aspect-ratio (~20) trench structure. The ALE process also improved film quality by reducing the surface roughness and leakage current of the amorphous films. For crystalline films, a significant reduction in leakage current was observed after 10 ALE cycles, despite a slight increase in roughness. These results demonstrate the effectiveness of NF3 remote plasma for precise isotropic etching of both amorphous and crystalline HfO2 films, making it suitable for advanced semiconductor devices.