Thermal Atomic Layer Etching of Titanium Nitride Using Sequential, Self-Limiting Reactions: Oxidation to TiO2 and Fluorination to Volatile TiF4

The thermal atomic layer etching (ALE) of TiN was demonstrated using a new etching mechanism based on sequential, self-limiting oxidation and fluorination reactions. The oxidation reactant was either O-3 or H2O2, and the fluorination reactant was hydrogen fluoride (HF) derived from HF-pyridine. In the proposed reaction mechanism, the O-3 reaction oxidizes the surface of the TiN substrate to a TiO2 layer and gaseous NO. HF exposure to the TiO2 layer then produces TiF4 and H2O as volatile reaction products. The overall reaction can be written as TiN + 3O(3) + 4HF -> TiF4 + 3O(2) + NO + 2H(2)O. Quartz crystal microbalance studies showed that HF can spontaneously etch TiO2 films. Spectroscopic ellipsometry and X-ray reflectivity analysis showed that TiN films were etched linearly versus the number of ALE cycles using O-3 and HF as the reactants. The TiN etching also occurred selectively in the presence of Al2O3, HfO2, ZrO2, SiO2, and Si3N4. The etch rate for TiN ALE was determined at temperatures from 150 to 350 degrees C. The etch rates increased with temperature from 0.06 angstrom/cyde at 150 degrees C to 0.20 angstrom/cycle at 250 degrees C and stayed nearly constant for temperatures >= 250 degrees C. TiN ALE was also accomplished using H2O2 and HF as the reactants. The etch rate was 0.15 angstrom/cyde at 250 degrees C. The TiN films were smoothed by TiN ALE using either the O-3 or H2O2 oxidation reactants. The thermal ALE of many other metal nitrides should be possible using this new etching mechanism based on oxidation and fluorination reactions. This thermal ALE mechanism should also be applicable to metal carbides, metal sulfides, metal selenides, and elemental metals that have volatile metal fluorides.