Hydrogen Radical Permeation Dynamics through Ultrathin ALD Al ₂ O ₃ Layers Revealed by In Situ Ellipsometry

This work focuses on the behavior of hydrogen permeation through ultrathin aluminum oxide (Al 2 O 3 ) layers upon exposure to hydrogen radicals (H*), based on the permeation dynamics revealed by in situ spectroscopic ellipsometry (SE). This work first demonstrated the feasibility of using real-time in situ SE measurements to differentiate between hydrogenation and oxidation processes of Hf in Pd-capped Hf (Pd/Hf) stacks with and without an atomic layer-deposited (ALD) Al 2 O 3 cap. Without applying an Al 2 O 3 cap on top of the stack, all 450 °C H* exposures led to both hydrogenation and oxidation of the Hf layer. The oxidation was presumably triggered by the residual oxidants present in the system. With increasing Al 2 O 3 thickness, a stronger retardation of both hydrogenation and oxidation of Hf was observed, consistent with ex situ X-ray diffractometry and elastic recoil detection analysis (ERDA) results. The application of an ALD Al 2 O 3 layer of a thickness of 1.8, 2.7, and 4.2 nm on the Pd/Hf stacks slowed down hydrogen permeation from H* by a factor of 48, 514, and 4751, respectively. The method demonstrated here can be used for screening other material candidates as potential H* permeation barriers. The hydrogen diffusion length ( L ) in ALD Al 2 O 3 is further obtained at 450 °C in an assumed steady-state permeation regime. Specifically, an exponential decay of the hydrogen permeation flux derived from ERDA measurements is observed with increasing Al 2 O 3 thickness, which is well described by L = 0.48 ± 0.05 nm. Further analysis using the reported values from higher temperatures reveals that hydrogen diffusion length exhibits Arrhenius-type behavior with an activation energy of 0.57 ± 0.02 eV, indicating a thermally driven diffusion of H* through the ALD Al 2 O 3 layers. These results are relevant for applications in which surfaces are exposed to a flux of active hydrogen species, like in extreme ultraviolet (EUV) lithography and fusion reactors.