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 (SO2Cl2) to form gold chloride. Subsequently, ligand addition to the gold chloride was performed using triethylphosphine (PEt3) 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 SO2Cl2 exposure when forming the gold chloride. A mass loss was then monitored during each PEt3 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 AuClPEt3 as the dominant Au-containing etch product during PEt3 exposures. Time-dependent QMS studies also observed the AuClPEt3+ ion intensity peaking at the beginning of each PEt3 exposure. The AuClPEt3+ ion intensity then decreased as the PEt3+ ion intensity remained constant. This behavior indicates a self-limiting ligand-addition reaction. X-ray photoelectron spectroscopy on these Au nanopowders showed evidence for AuClPEt3 on the surface of the Au nanopowder when the final exposure was PEt3. 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 SO2Cl2 and PEt3, Cu and Ni nanopowders were also etched at 150 °C. Cu formed a volatile Cu2Cl2(PEt3)2 dimer during PEt3 exposures at 150 °C. Likewise, Ni formed volatile NiCl2(PEt3)2 during PEt3 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.