Thermal Atomic Layer Etching of Gold Using Sulfuryl Chloride for Chlorination and Triethylphosphine for Ligand Addition

Thermal atomic layer etching (ALE) of gold was achieved using sequential chlorination and ligand-addition reactions. This two-step process first chlorinated gold using sulfuryl chloride (SO 2 Cl 2 ) to form gold chloride. Subsequently, ligand addition to the gold chloride was performed using triethylphosphine (PEt 3 ) to yield a volatile gold etch product. Quartz crystal microbalance measurements on cubic crystalline gold films showed etching at temperatures from 75 to 175 °C. The most consistent etch rate was 0.44 ± 0.16 Å/cycle at 150 °C. A mass increase was observed during each SO 2 Cl 2 exposure when forming the gold chloride. A mass loss was then monitored during each PEt 3 dose when ligand addition yielded a volatile etch product. In situ quadrupole mass spectrometry (QMS) studies on Au nanopowder at 150 °C showed the production of AuClPEt 3 as the dominant Au-containing etch product during PEt 3 exposures. Time-dependent QMS studies also observed the AuClPEt 3 + ion intensity peaking at the beginning of each PEt 3 exposure. The AuClPEt 3 + ion intensity then decreased as the PEt 3 + ion intensity remained constant. This behavior indicates a self-limiting ligand-addition reaction. X-ray photoelectron spectroscopy on these Au nanopowders showed evidence for AuClPEt 3 on the surface of the Au nanopowder when the final exposure was PEt 3 . Transmission electron microscopy analysis revealed that Au ALE did not roughen the crystalline Au nanoparticles. Powder X-ray diffraction measurements also showed narrower diffraction peaks after Au ALE on Au nanoparticles that were consistent with larger Au nanoparticles. This sintering effect may be caused by Au redistribution resulting from the disproportionation of the AuCl surface species. Using the same alternating exposures of SO 2 Cl 2 and PEt 3, Cu and Ni nanopowders were also etched at 150 °C. Cu formed a volatile Cu 2 Cl 2 (PEt 3 ) 2 dimer during PEt 3 exposures at 150 °C. Likewise, Ni formed volatile NiCl 2 (PEt 3 ) 2 during PEt 3 exposures at 150 °C. Gibbs free energy changes from ab inito calculations support these etch product observations and offer a thermodynamic explanation for the formation of a copper dimer. These studies illustrate that sequential chlorination and ligand-addition reactions can provide a useful ALE pathway for gold and other metals.