Reprinted with permission from (J. Phys. Chem. C, 2010, 114 (14), pp 6484-6490). Copyright (2010) American Chemical Society.International audienceWe propose a new system, namely the periodic Co/Mg multilayer system, for optics applications in the EUV range. Close to the Mg L edge, i.e., around a wavelength of 25 nm or a photon energy of 50 eV, a reflectivity of about 43% is measured at 45° for s-polarized radiation. Moreover, it appears that this system is stable over a period of time of three months. The introduction of thin boron carbide interfacial layers proves disastrous contrary to simulations that show this could be beneficial. We combine X-ray reflectivity in the hard X-ray range, X-ray emission spectroscopy, and nuclear magnetic resonance to determine the thickness and roughness of the Co and Mg layers as well as the chemical state of the Co and Mg atoms at the interfaces. This reveals that in the Co/Mg system the interfaces are abrupt and there is no interdiffusion between the Co and Mg layers. Then the difference between the experimental and simulated reflectivities is ascribed to the interfacial roughness of the order of 0.4 nm. In the Co/Mg/B4C system, evidence of a large mixing of the Co and B4C layers is presented and explains the poor reflectance of this system

Mg-based multilayers, including SiC/Mg, Co/Mg, B(4)C/Mg, and Si/Mg, are investigated for solar imaging and a He II calibration lamp at a 30.4 nm wavelength. These multilayers were fabricated by a magnetron sputtering method and characterized by x-ray reflection. The reflectivities of these multilayers were measured by synchrotron radiation. Near-normal-incidence reflectivities of Co/Mg and SiC/Mg multilayer mirrors are as high as 40.3% and 44.6%, respectively, while those of B(4)C/Mg and Si/Mg mirrors are too low for application. The measured results suggest that SiC/Mg, Co/Mg multilayers are promising for a 30.4 nm wavelength.

We study the introduction of a third material, namely Zr, within a nanometric periodic Mg/Co structure designed to work as optical component in the extreme UV (EUV) spectral range. Mg/Co, Mg/Zr/Co, Mg/Co/Zr and Mg/Zr/Co/Zr multilayers are designed, then characterized in terms of structural quality and optical performances through X-ray and EUV reflectometry measurements respectively. For the Mg/Co/Zr structure, the reflectance value is equal to 50% at 25.1 nm and 45° of grazing incidence and reaches 51.3% upon annealing at 200°C. Measured EUV reflectivity values of tri-layered systems are discussed in terms of material order within a period and compared to the predictions of the theoretical model of Larruquert. Possible applications are pointed out.

We have developed Mg/Co, Mg/Zr/Co, Mg/Co/Zr, and Mg/Zr/Co/Zr periodic multilayers and measured at 25.1 nm a reflectivity (R) highly sensitive to the material order within the period. To understand why Mg/Co/Zr is a more efficient mirror (R=50%) than Mg/Zr/Co and Mg/Zr/Co/Zr (∼40%), we have probed the interface quality through time-of-flight secondary ion mass spectrometry and nuclear magnetic resonance measurements. The Zr-on-Co interface is found quite sharp while a strong intermixing process is evidenced between the upper Co and lower Zr layers, responsible for the decrease in optical contrast and subsequent R loss.

The efficiency of B(4)C, Mo and Zr barrier layers to improve thermal stability of Mg/Co multilayer up to 400 °C is investigated. Multilayers were deposited by direct current magnetron sputtering and characterized using X-ray and extreme ultraviolet reflection. The results suggest that B(4)C barrier layer is not effective due to drastic diffusion at Mg-B(4)C interface. Although introducing Mo barriers improves the thermal stability from 200 to 300 °C, it increases the interface roughness and thus degrades the optical performances. On the contrary, Zr barriers can significantly increase the thermal stability of Mg/Co up to 400 °C without optical performance degradation. Thus, Mg/Zr/Co/Zr is suitable for EUV applications requiring both optimal optical performances and heat resistance.