Blister formation on multilayer mirrors due to the exposure of hydrogen

Extreme Ultraviolet Lithography (EUVL) has been developed as a technique to reduce feature sizes on electronic chips. To reflect and focus the EUV light, multilayered mirrors form the crucial part of the lithographic machines’ optical system. Although a single multilayered mirror can have a reflectivity of up to 70% [1], the lifetime of these mirrors could be decreased by swelling and blistering due to hydrogen ion implantation in the case of excessive irradiation. In the context of EUV multilayered mirrors blistering might occur under certain hydrogen conditions. Although already successfully tested approaches to cope with this issue exist the fundamental physics lying behind blister formation is still not fully understood. This is mainly caused by the many different parameters such as mirror temperature, hydrogen fluence, hydrogen ion flux, and directionality of the feed gas that have an influence on the blister formation. To have a better control of blister formation on multilayered mirrors under various hydrogen exposure conditions more insight into the fundamental processes that result in blister formation is desired. Therefore an experimental setup has been designed in which multilayer mirrors can be exposed to three different hydrogen sources, and, thereby, intentionally produce surface blistering. The hydrogen sources produce a mixture of molecular, atomic, and ionic hydrogen. Besides studying the fundamentals of blister formation, the setup can also serve as a quick practical tool for testing the endurance of new capping layers, and the resistance of other components to blistering. As a first approach to quantify the physical and chemical changes that occur in the multilayer mirror before visible blistering, changes in the electrical and scattering properties of the multilayer mirror are being investigated. It is known from literature that, before surface blistering, stress builds up between the layers, and voids are formed inside the multilayer, due to the diffusion of hydrogen. The absorption of hydrogen in the multilayer will change the capacitance as well as the conductivity of the multilayer. At the point that surface blistering starts, visible-light scattering from the mirror will increase. Monitoring conductivity and scattering changes, combined with nuclear recoil data and modeling, will allow us to tell where hydrogen accumulates, how the hydrogen stabilizes itself in the material, and when void formation begins. With this experimental setup we will get a better understanding of the early stages of blister formation. This will eventually give us the tools to improve the protection against blistering or even prevent