Atomic layer etching of titanium nitride with surface modification by Cl radicals and rapid thermal annealing

Thermal atomic layer etching (ALE) is a promising method for isotropic etching with atomic level precision and high conformality over three-dimensional structures. In this study, a thermal ALE process for titanium nitride (TiN) films was developed using surface modification with a Cl₂/Ar downstream plasma followed by infrared (IR) annealing of the films. The oxygen-free Cl₂-based plasma was adopted to enable highly selective etching of TiN with regard to various materials. It was confirmed that spontaneous etching of TiN during radical exposure can be suppressed at a surface temperature of −10 °C. Measurements of etch per cycle (EPC) of TiN demonstrated that the EPC is self-limiting with respect to both the radical exposure and IR annealing times. With repeated steps of self-limiting radical exposure and IR annealing, TiN was etched at 2.0 nm/cycle, while no thickness change was observed for poly-Si, SiO₂, Si₃N₄, W, and HfO₂. The selectivity to amorphous carbon was higher than 4. X-ray photoelectron spectroscopy analysis revealed that during surface modification, NCl_x species sublimate spontaneously, while TiCl_x species remain in the surface-modified layer on TiN. This TiCl_x-based modified layer desorbs in the IR annealing step, and the TiN surface then returns to its original condition (pristine TiN) before surface modification.