Plasma atomic layer etching for titanium nitride at low temperatures

Abstract

Isotropic plasma atomic layer etching (ALE) was developed for titanium nitride (TiN) through a three-step process: plasma oxidation, plasma fluorination, and thermal removal at low temperatures. In the plasma oxidation step, TiN was oxidized to form a titanium oxide (TiO2) layer with O radicals generated from O2 plasma at 100 °C. The TiO2 thickness was found to be saturated with plasma after an exposure time of 300 s, and the saturated thickness increased from 0.29 to 1.23 nm with increasing temperature and RF power. In the plasma fluorination step, the TiO2 layer was converted to titanium oxyfluoride (TiO2−xFx) with F radicals generated in the CF4 plasma at 100 °C. The F atomic fraction on the surface was found to be saturated at 12%, with RF powers below 15 W in the fluorination step. The process temperature was increased during the removal step, and the TiO2−xFx formed by plasma fluorination was completely removed above 150 °C. The removal rates of TiN ranged from 0.24 to 1.71 nm/cycle by controlling the thickness of the TiO2 layer determined earlier. The average surface roughness of TiN decreased from 1.27 to 0.26 nm after 50 cycles of the ALE process. This work demonstrated that plasma oxidation and fluorination with thermal removal can remove TiN at the atomic scale at low temperatures for atomic-scale three-dimensional devices.