Vacuum ultraviolet enhanced atomic layer etching of ruthenium films

Abstract

Vacuum ultraviolet (VUV) enhanced atomic layer etching (ALE) of thin (∼8 nm) Ru films is demonstrated. Oxidation half-cycles of 2–5 min VUV/O2 co-exposure are used to oxidize near-surface Ru to RuO2 at 1 Torr O2 and 100–150 °C. In situ x-ray photoelectron spectroscopy measurements indicate that RuO2 formation saturates after ∼5 min of VUV/O2 exposure at 100 and 150 °C. The depth of Ru oxidation is limited by the rate of oxidation and can be controlled with substrate temperature and exposure time. Etching half-cycles are performed by exposing the oxidized Ru film to HCOOH vapor at 0.50 Torr for 30 s isothermally, which results in the removal of the oxidized Ru layer. The amount of Ru removed per ALE cycle is determined by comparing ex situ x-ray reflectivity (XRR) measurements of the film before and after etching. When using 2 min VUV/O2 co-exposure, approximately 0.8 and 0.9 Å of Ru is etched per cycle at 100 and 150 °C, respectively. XRR and atomic force microscopy measurements indicate that the as-deposited and sputtered Ru film surface becomes smoother as ALE is performed. The etch rate decreases with ALE cycles and corresponds to a slowing oxidation rate, which is likely associated with the decrease in surface roughness. Density functional theory is used to study the adsorption of oxidants in a model Ru system, and nudged elastic band (NEB) calculations describe O diffusion into the Ru substrate by following an O “probe” atom as it moves between Ru(002) atomic planes with 0.50 monolayer (ML) O on the surface. NEB results reveal an approximate energetic barrier to diffusion, Ea, of 5.10 eV for O to move through the second and third atomic Ru layers when O, which can form an RuOx species, is subsurface. This Ea is in excess of the energetic gain of 4.23 eV in adsorbing an O atom to Ru(002) with 0.50 ML O. The difference in Ea and the adsorption energy likely contributes to the self-limiting nature of the oxidation and explains the observation that VUV/O2 co-exposure time must be increased to allow additional time for O diffusing into the subsurface as it overcomes the barrier to subsurface O diffusion. The self-limiting oxidation of Ru arising from VUV/O2 at low temperatures, in turn, enables an ALE process for Ru.