Optical detection of hydrogen diffusion through thin film barrier materials

The ability of hydrogen to dissolve and diffuse in solid materials causes accelerated material wear and decreased robustness. In many applications hydrogen induced embrittlement determines the lifetime of hardware components that operate in direct contact with hydrogen gas. In addition, hydrogen induced embrittlement can also play a role in components exposed to hydrogen species generated by corrosion processes. Hydrogen permeation into such components can be mitigated in several ways. For instance, metal alloying can be applied in order to reduce the hydrogen solubility. Alternatively, a protective film can be added that acts as a diffusion barrier between the component and the hydrogen environment. Such a barrier is beneficial when a component has a complex structure that is crucial for its functionality and, therefore, cannot be altered.

The choice of material for an optimal protective film will depend on the particular application. An easily accessible and preferably low cost technique for comparison of the hydrogen permeation in candidate barrier layers is needed to efficiently search for an effective material.

In this work, an optical sensor for hydrogen diffusion in thin metal and non-metal films was proposed and the relevant physical processes regarding the fabrication and use of such a sensor were investigated. This knowledge was applied for measuring and comparing hydrogen diffusion through a range of potential barrier materials and reference materials. A Y thin film was used as the sensor layer, onto which a test layer, of which the diffusion properties are to be measured, was deposited. The dielectric function of the sensor film changes upon hydrogen absorption, which was monitored with spectroscopic ellipsometry or optical transmission. The design was optimized to enable comparison of hydrogen permeability in different materials. The work in this thesis resulted in, to our knowledge, the first quantitative measurements of the hydrogen diffusion constant in Ru. Furthermore, the knowledge was applied to design a layer stack that allows a reliable comparison of hydrogen diffusion kinetics in various barrier layers with low hydrogen diffusion constants.