# Sc/SiC/Al Multilayer Optimization for Li K Spectroscopy
###### Abstract
); [EMAIL_ADDRESS] (F.D.) Footnote 19: institutext: Laboratoire de Chimie Physique—Matière et Rayonnement, Faculte des Sciences et Ingenierie, Sorbonne Universite, UMR CNRS, 4 Place Jussieu, CEDEX 05, 75252 Paris, France; [EMAIL_ADDRESS]  
Footnote 20: institutetext: Laboratoire Charles Fabry, Institut d’Optique Graduate School, Universite Paris-Saclay, CNRS, 91120 Palaiseau, France; [EMAIL_ADDRESS] (E.M.); [EMAIL_ADDRESS] (F.D. ); [EMAIL_ADDRESS] (F.D. ); [EMAIL_ADDRESS] (F.D.) Footnote 21: institutetext: Laboratoire de Chimie Physique—Matière et Rayonnement, Faculte des Sciences et Ingenier, Sorbonne Universite, UMR CNRS, 4 Place Jussieu, CEDEX 05, 75252 Paris, France; [EMAIL_ADDRESS]  
Footnote 22: institutetext: Laboratoire Charles Fabry, Institut d’Optique Graduate School, Universite Paris-Saclay, CNRS, 91120 Palaiseau, France; [EMAIL_ADDRESS] (E.M.); [EMAIL_ADDRESS] (F.D. ); [EMAIL_ADDRESS] (F.D.) Footnote 23: institutetext: Laboratoire de Chimie Physique—Matière et Rayonnement, Faculte des Sciences et Ingenier, Sorbonne Universite, UMR CNRS, 4 Place Jussieu, CEDEX 05, 75252 Paris, France; [EMAIL_ADDRESS]  
Footnote 24: institutetext: Laboratoire de Chimie Physique—Matière et Rayonnement, Facultespacings being smaller than the long wavelength of the characteristic radiations emitted by light elements. This amounts to the relatively weak intensities emitted by such elements, arising from their low fluorescence yield [4], and the strong absorption of emissions within the specimen, raising the need for optical elements with a relatively high reflectivity at a working angle suitable for direct integration into scanning electron microscopes (SEM) and electron microprobes apparatus (EPMA) [5]. Let us remark that, at the present time, none of the periodic multilayers used in these commercial SEMs and EPMAs are optimized to work in the photon energy range of Li K emissions. The use of non-optimized structures results in weak reflected intensities, leading to long acquisition times and broad emission peaks, making it difficult to disentangle spectral interferences. To tackle this challenge, optical elements can be realized by using periodic multilayers made of alternating light and heavy layers with optimized periodicity \(d\)[6; 7; 8]. The constructive interference of multiple reflections from the interfaces is analogous to a Bragg reflection from the crystals. EUV multilayers are already used in astrophysics [9] and lithography [7; 10], and they seem to be especially interesting for detecting lithium K emissions around 50 eV [11]. Indeed, the peak positions and shape of the Li K emission band depend on the chemical state of the Li atoms. The detection and quantification of such low-energy emission in WDS reveal the need for specific multilayers presenting a narrow bandwidth in order to obtain a good spectral resolution. If the quantification of lithium is the main objective, and not the determination of the chemical state of the Li atom, then the bandwidth of the diffraction pattern of the dispersive multilayer, which governs the spectral resolution, should be not too large, so that spectral interferences can be avoided, and not too small, so that the collected intensity is large. A considerable amount of research has been published on the development of multilayers in the EUV range generally designed to work at normal incidence [7; 12; 13; 14]. Al-based structures such as Al/Mo/B\({}_{4}\)C and Al/Mo/SiC achieve a peak reflectance of more than 50% at normal incidence for a photon energy lower than 70 eV [15]. Another example is Be-based multilayers, which prove to be good candidates due to the low absorption of beryllium. For instance, Mo/Be multilayers were reported to measure a peak reflectivity of 58% at 94 eV (13.2 nm) and a spectral bandwidth of 2.5 eV (0.35 nm) [7]. Another interesting system is Be/Si/Al, with a reflectivity of 61% at normal incidence at an energy of 72.5 eV (17.1 nm) and a spectral bandwidth of 1.7 eV (0.4 nm) [7; 16].