New Co-based multilayer materials for optics applications in the EUV range – COBMUL

It is necessary to design and fabricate new optics elements due to the large development of photon sources and applications working in the extreme UV (EUV) and soft x-ray ranges, such as He-II emission line at the wavelength of 30.4 nm for imaging of solar corona, Ni-like Ta x-ray laser working at the wavelength of 4.48 nm near “water window” region, for example. Periodic multilayers are suited for this purpose, because their layers of nanometric thickness lead to period of the order of the nanometer suitable to diffract EUV and x-ray radiations. To be efficient these optics should have large reflectance and suitable thermal and time stabilities. For numerous applications the multilayers should also need to be stable up to a few hundreds °C.

We expect to establish a methodology of interface analysis and a reasonable theory demonstrating what is happening at the interfaces of periodic multilayers, based on the study of Co-based multilayers. We also expect to accomplish the preparation of new Co-based multilayers with not only outstanding optical properties but also competent stability in some harsh working environment such as synchrotrons or free electron laser facilities. To achieve this goal we propose in this project to pursue our investigations along three paths:
(i) optimizing the parameters of the Co/Mg system (thickness of the layers and deposition pressure, power and temperature, …) on which preliminary promising results have already been obtained (43% reflectivity), and will be used as a prototype of the Co-based multilayers,
(ii) changing magnesium by another material chosen for its adequate optical constants and also its ability to not react with cobalt,
(iii) considering tri-layer systems, the two layers other than Co being two metals or one metal and a high formation enthalpy compound; indeed, it has been shown that tri-layer systems can present higher reflectance than bi-layer systems.

In the project the Co/Mg system will be first used as a prototype of the Co-based multilayers to be developed. A methodology combining non-destructive characterization techniques, hard x-ray and EUV reflectance measurements, x-ray emission and nuclear magnetic resonance spectroscopies, magneto-optic Kerr effect measurements, will be applied to understand the interface interactions occurring in this kind of multilayers. Then, from this knowledge, interface engineering will be applied to improve the structural quality of the stacks as well as to anticipate their behaviour as a function of annealing up to some hundreds °C, in order to design novel Co-based multilayers that can used in practical applications. The designed multilayers will be fabricated and characterized and their interfacial behaviour controlled in order to give high and reliable reflectivities. The project will be considered successful if a set of Co-based multilayers is produced, designed and optimized for the EUV range and having thermal stability well above 200°C, suitable for applications in synchrotron radiation, soft X-ray laser, EUV solar observation, …