Sensing of Hydrogen Diffusion through Protective Caps: An Ellipsometric Approach to Estimate Hydrogen Content in Pd/Hf Stacks with an Al ₂ O ₃ Cap

In this work, we develop and validate a spectroscopic ellipsometry (SE) approach to first monitor the performance of a thin-film palladium/hafnium (Pd/Hf) hydrogen detection sensor suitable for applications up to a temperature of 450 °C and second to quantify the hydrogen permeation through thin capping layers. A 2.5 nm aluminum oxide (Al2O3) thin film is tested as a potential protective hydrogen barrier under hydrogen radical (H*) flux conditions found in extreme ultraviolet lithography scanners. To this end, the interaction between molecular and atomic hydrogen and a Pd/Hf stack was studied at temperatures from 120 to 670 °C. Optical and structural changes during hydrogen exposures were investigated using in situ SE and ex situ X-ray diffraction. The stack is confirmed to remain stable in the respective metallic phases during vacuum annealing up to 450 °C without forming any crystalline HfO2 by reaction with trace oxidative species. Upon molecular H2 exposure, the formation of hafnium hydride (HfHx) can be observed for temperatures up to 350 °C, while Hf oxidation occurs at higher temperatures. Upon exposure to H*, HfHx formation is observed for temperatures up to 450 °C, again followed by oxidation at higher temperatures. Capping the stack with a 2.5 nm Al2O3 layer fabricated by atomic layer deposition led to a retardation of 70 times for hydrogenation upon H* exposure at 450 °C accompanied by little oxidation. An analytical SE model was developed for analyzing the H-content incorporated into this stack through the Al2O3 capping layer, showing a decent match with that from absolute quantification by elastic recoil detection analysis.