Anisotropic etching of Pt thin films through cyclic treatment using low-energy oxygen-ion sputtering and formic acid vapor exposure

Pt thin films were anisotropically etched through cyclic treatment comprising O 2 plasma exposure, followed by formic acid (FA) vapor treatment. Pt was directionally etched by exposure to low-energy oxygen ions, resulting in the self-limited formation of a Pt oxide on the surface with a thickness of ∼ 2 nm, simultaneously. Subsequently after the O 2 plasma exposure, the Pt oxide layer was removed and deposits on the sidewall were laterally cleaned by the FA vapor treatment, resulting in a refreshed Pt surface. Surface analysis using X-ray photoelectron spectroscopy confirmed that the Pt oxide formed in the first step, mainly comprising a Pt 4+ (PtO 2 ), was removed in the second step. Energetic ions (∼70 eV) in O 2 plasma yielded an etch rate of ∼ 1.3–1.8 nm/cycle along the surface normal of the Pt film. FA vapor exposure selectively removed the PtO 2 layer and sidewall deposits, leading to anisotropic etching. The mechanism underlying the self-limited oxide formation and the reduction in metallic-Pt thickness caused by the low-energy oxygen ions in O 2 plasma was discussed using a simple model. This model assumes kinetic energy deposition from oxygen ions into the surrounding Pt atoms, which is described by a one-dimensional distribution corresponding to the ion penetration depth.