# Sc/SiC/Al Multilayer Optimization for Li K Spectroscopy
## 2 Experimental Details  
In the present work, we analyzed Sc/SiC/Al multilayers of 20 periods of 11.85 nm. The first layer on the substrate is Al, followed by SiC and Sc. Scandium is proposed due to its low absorption in EUV above its M\({}_{2,3}\) absorption edge at \(\sim\)28 eV (44 nm), while aluminum presents its L\({}_{2,3}\) absorption edge near 73 eV (17 nm), making them a good combination for the requested photon energy range. SiC was introduced into the multilayer structure in order to obtain a better reflectivity as compared with the two-component Al/Sc system. It turns out that such a design helps to smooth the interfaces and prevents the development of the Al\({}_{3}\)Sc phase that was reported to form at the Al-Sc interface [13]. All the multilayers were deposited via magnetron sputtering at Laboratory Charles Fabry (LCF) of Institut d'Optique Graduate School. The magnetron sputtering system at LCF can accommodate up to four material targets and the deposition process can run either in rf- (radio frequency) or dc- (direct current) mode, according to the nature of the material being deposited. The substrates were Si wafers in the (100) crystal orientation with a surface roughness in the 0.3 nm range. The targets used were pure (99.95%) scandium and silicon carbide, as well as a Si-doped (1.5 wt.%) aluminum. The base pressure in the deposition chamber was lower than 10\({}^{-5}\) Pa. The working gas (argon) pressure was set to 0.27 Pa. We used the dc-mode operation with a 100 mA current for the Sc target and rf-mode for the SiC and Al targets with the powers of 150 and 200 W, respectively. All deposited samples were capped with a few nm thick protection layer of SiC in order to protect the multilayer from oxidation and preserve its reflectivity. In the spectral range of interest, the thickness of the capping layer can even be optimized for increasing the initial value of the peak reflectance by a few percent. A good temporal stability of this type of multilayer has been already proven previously [13]. All samples were initially characterized with a Bruker D8 reflectometer via grazing incidence X-ray reflectivity (GIXR) measurements at grazing incidence at the Cu K\(\alpha\) emission line (\(\lambda\) = 0.154 nm; E = 8.048 keV). EUV reflectivity measurements in the Li K range were performed at the bending magnet BEAR (Bending magnet for emission, Absorption, and Reflectivity) beamline [22; 23] at the Eletrta synchrotron facility, operating in the 2.8-1600 eV spectral region. An energy scan was performed in the range from 45 to 65 eV with a step of 0.1 eV. The measurements were conducted using the grazing incidence monochromator [22] of the beamline and using an exit slit aperture of 900 \(\times\) 100 \(\upmu\)m\({}^{2}\). This setup allowed for delivering a spot dimension of about 200 \(\times\) 100 \(\upmu\)m\({}^{2}\) FWHM and an energy bandwidth \(\Delta\)E of about 0.03 eV, well below the energy bandwidth of the multilayer structure. Higher-order purity was obtained using a 0.1 \(\upmu\)m thick Al filter. EUV light was delivered with 0.9\({}^{\circ}\) linear polarization, as given by the setting of the aperture of the polarization selector of the beamline. The reflectivity was measured inside the BEAR UHV end station [22], where the angular resolution and accuracy given by the manipulator and detector goniometers was 0.01\({}^{\circ}\). The reflectivity was evaluated by measuring the current from a silicon diode (IRD SXUV-100 model), positioned in correspondence with the light beam and, in parallel, the current from the last mirror of the beamline, used as an intensity monitor. Both diode and monitor currents were read with two multimeters (Keithley 6517A), used with the appropriate range. The reflectivity was calculated after a standard procedure based on 4 measurements: a reflectivity scan and relative dark measurement of the current of the diode, positioned in correspondence with the reflected beam and in parallel with the monitor current, both functions of energy.