Abstract
Wideband multilayers designed for various applications in hard X-ray to Extreme UV spectral regions are based on a layered system with layer thicknesses varying largely in depth. However, because the internal structure of a thin film depends on its thickness, this will result in multilayers in which material properties such as density, crystallinity, dielectric constant and effective thickness vary from layer to layer. This variation causes the fabricated multilayers to deviate from the model and negatively influences the reflectivity of the multilayers. In this work we solve this problem by developing designs of wideband multilayers with strongly reduced layer thickness variations in depth, without essential degradation of their optical characteristics.
Highlights
Multilayer mirrors (MMs) are widely used in X-ray and Extreme UV technologies because of their high reflectivity at any angle including normal incidence
Being analogous to 1D artificial crystals and based on interference of waves reflected from different interfaces, standard periodic MMs are characterized by a narrow spectral and angular bandwidth that limits the range of their applications
For many applications it is desirable that either the spectral or angular bandwidth is essentially increased as compared to conventional MMs
Summary
Multilayer mirrors (MMs) are widely used in X-ray and Extreme UV technologies because of their high reflectivity at any angle including normal incidence. A way generally used to increase the reflectivity bandwidth of MMs is a variation of the period with the depth of a multilayer structure This way radiation of different wavelengths λ or of different angles φ incident onto the MM, according to the Bragg law, are reflected from stacks of layers placed at different depth. Notice that all approaches result in non-monotonic oscillating variation of the layer thickness with depth, even though the desired profile of the reflectivity curve is very simple (for example, constant). The question arises whether it is possible to strongly reduce the layer thickness variation in depth without essential degradation of its optical characteristics This paper answers this question positively and demonstrates such designs using a Mo/Si based multilayer stack
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