Plasma-assisted thermal atomic layer etching for palladium and surface structure analysis with density-functional theory

The plasma-assisted thermal atomic layer etching (ALE) process was performed on the palladium (Pd) using sequential, self-limiting thermal reactions with surface chlorination using Cl₂ plasma and removal with NH₃ ligand addition. The formation of PdCl₂ layer was confirmed with x-ray photoelectron spectroscopy (XPS) in the chlorination step. The chlorination depth was saturated to 16 Å after 30 s of Cl₂ plasma exposure at 150 °C. The etching was performed via NH₃ ligand addition above 100 °C. The etch per cycle (EPC) was saturated at 16 Å/cycle after 30 s of Cl₂ plasma exposure and 30 s of NH₃ ligand addition at 150 ∼ 200 °C. Surface chlorine residues after ALE were less than 1 % under process conditions with self-limiting characteristics. The surface roughness increased from 0.24 to 0.52 nm after 20 ALE cycles. No crystallinity changes after ALE, but the individual peak areas decreased differently by 61 % for Pd(111), 56 % for Pd(100), 66 % for Pd(110), and 94 % for Pd(311). Density functional theory (DFT) calculations showed that Pd(311) had the lowest reaction energy of −0.576 eV. The reaction product was proposed as trans-Pd(NH₃)₂Cl₂, showing the lowest reaction energy yield of 0.26 eV when NH₃ is adsorbed on the chlorinated Pd surface.