In-depth and surface characterization of Al/Mo/SiC multilayers P. Jonnard 1* , M.-H. Hu 1 , K. Le Guen 1 , J.-M. André 1 , F. Delmotte 2 , E. Meltchakov 3 , A. Galtayries 4 1 – Laboratoire Chimie Physique – Matière Rayonnement, UPMC Univ Paris 06, CNRS, 75005 Paris, France 2 – Laboratoire Charles Fabry, Institut d'Optique Graduate School, 91127 Palaiseau, France 3 – Institut d’Astrophysique Spatiale, 91403 Orsay, France 4 – Laboratoire de Physico-Chimie des Surfaces, Ecole Nationale Supérieure de Chimie de Paris, 75005 Paris, France * e-mail: philippe.jonnard@upmc.fr INTRODUCTION We present the characterization of Al/Mo/SiC periodic multilayers designed for optics applications in the extreme ultra-violet range (EUV). With respect to the Al/SiC system [1], the addition of a thin Mo layer only at the SiC-on-Al interfaces decreases the roughness at both SiC-on-Al and Al-on-SiC interfaces. We combine different experimental techniques to study the bulk as well as the superficial zone of Al/Mo/SiC multilayers. EXPERIMENTS Various experimental techniques are performed: • X-ray reflectivity (XRR) at 0.154 nm to characterize the samples following their preparation and evaluate the thickness and roughness of the various layers in the stacks; • EUV reflectivity measurement at the application wavelength to experimentally estimate efficiency of the introduction of the Mo layers; • Time of flight secondary ion mass spectrometry (ToF-SIMS) to determine the depth distribution of the various elements present within the stacks and try to determine the behavior of Mo atoms within the stack; ToF-SIMS X-ray photoelectron spectroscopy (XPS) to determine the chemical state of the most superficial atoms and particularly to have an idea of the oxidation of the top layer of SiC as this can be responsible of a loss reflectivity in the EUV range. RESULTS AND DISCUSSIONS 1. XRR at 0.154 nm The multilayers are analyzed by x-ray reflectivity measurements at 0.154 nm (Cu Kα emission at 8048 eV). The fit of the reflectivity curves gives the thickness of the different layers as well as the interfacial roughness. The fitting results indicate that the deposited multilayers coincide in a satisfactory manner with those expected in the preparation step. 2. EUV reflectivity 3. ToF-SIMS CONCLUSIONS Periodic Al/Mo/SiC multilayers were studied by combining various experimental techniques providing valuable information on their optical characteristics (XRR); the ToF-SIMS results show that the Mo atoms are present at both Al-on-SiC and SiC-on-Al interfaces. This explains why roughness decreases at all the interfaces while the Mo atoms are added at only one interface; the combination of the ToF-SIMS and XPS results show that the first SiC layer is partially oxidized, but the oxygen atoms do not reach to first Mo and Al layers. An equivalent SiO 2 layer is revealed at the top of the multilayer. It explains the difference obtained between the optics simulation and the EUV reflectivity measurements. REFERENCES [1]. P. Jonnard, K. Le Guen, M.-H. Hu, J.-M. André, E. Meltchakov, C. Hecquet, F. Delmotte, A.Galtayries, Proc. SPIE 7360, 7360O (2009). [2]. E. Meltchakov, C. Hecquet, M. Rouillay, S. de Rossi, Y. Menesguen, A. Jérome, F. Bridou, F. vernière, M.-F. Ravet-Krill, F. Delmotte, Appl. Phys. A98, 111 (2010). ACKOWNLEDGEMENTS Part of this work was funded by the ANR project 07-BLAN-0150. Figure 3 and 4 present the depth profiles of Si - , SiC - , Al - and Mo - from the second, third and fourth periods of the Al/Mo/SiC_1 and Al/Mo/SiC_2 multilayers. Figure 5 presents the depth profiles of the Al + , Si + and Mo + for the Al/Mo/SiC_2 multilayer. All of them show that the Mo profile has two peaks: one at the interface SiC-on-Al and the other at the interface Al-on-SiC. * Analysis of the superficial layers Figure 6 presents the depth profiles of the first and second periods of the Al/Mo/SiC_2 multilayer analyzed by negative ions. It shows that the end of the O diffusion can be attributed to a sufficiently thick SiC layer or to the presence of the Mo layer which should also act as a diffusion barrier. MULTILAYERS The multilayers are prepared using magnetron sputtering: Al/Mo/SiC_1 (mp09045): Si / [Al(11.5 nm)/Mo(1.3 nm)/SiC(3.8 nm)] 15 ; Al/Mo/SiC_2 (mp09047): Si / [Al(6.4 nm)/Mo(1.4 nm)/SiC(1.1 nm)] 25 / SiC(1.1 nm); Al/Mo/SiC_3 (mp08123): Si / [Al(6.2 nm)/Mo(1.8 nm)/SiC(0.8 nm)] 20 . The reflectivity measurements in the EUV range performed near normal incidence (80° glancing incidence around 17 nm with synchrotron radiation at Elettra. The results prove that the introduction of the Mo layers leads to reflectance values higher than 50% [2]. Figure 1 presents the reflectivity curves of the three multilayers. High reflectances are observed: 32.4% at 30.4 nm for Al/Mo/SiC_1 (mp09045); 53.4% at 17.5 nm for Al/Mo/SiC_2 (mp09047); 49.4% at 17.3 nm for Al/Mo/SiC_3 (mp08123). Both negative and positive ions are recorded in order to study the buried layers and the superficial layers. * Analysis of the buried layers Figure 2 presents the depth profiles of the Al/Mo/SiC_1 multilayer with the detection of the negative ions. The presence of oxygen at the surface and on the silicon substrate is observed. It is due to the atmospheric contamination on the surface and to the native oxide on silicon at the interface with the substrate respectively. Figure2. From top to bottom, Si-, O- and Al- ion depth profiles of the Al/Mo/SiC_1 sample. 4. XPS The XPS spectra are measured on the Al/Mo/SiC_1 multilayer which has the thickest SiC superficial layer. The maximum of Mo 3d 5/2 and Al 2p correspond to the Mo and Al metal. The C 1s peak presents two chemical states: at 283.3 eV due to SiC and at 285.0 eV due to surface contamination. For O 1s a symmetrical peak is observed which corresponds to SiO 2 . The Si 2p states exhibit two chemical states. The corresponding XPS spectrum and its decomposition are shown in the Figure 7. The peak at 100.5 eV corresponds to SiC and the one at 102.8 eV corresponds to the oxidized silicon SiO x . It is expected that SiO 2 layer is formed on the surface. Calculation of the thickness of SiO 2 gives a value of 1.6 nm. Figure 7. Si 2p XPS spectrum of the Al/Mo/SiC_1 multilayer. Solid line: experiment; dashed lines: components obtained from the decomposition of the peak. Figure 6 Figure 5 Figure 3 Figure 4 Figure 1 10 100 1000 10000 100000 0 200 400 600 800 Sputtering time (s) Surface Substrate 1 5 10 15 1 5 10 15 1 10 100 1000 10000 20 40 60 80 Sputtering time (s) 4 3 2 A B C Al SiC Mo Si Al/Mo/SiC_2 0 1 80 100 120 140 160 180 Sputtering time (s) A B C D Al/Mo/SiC_2 0 500 1000 1500 2000 2500 3000 3500 4000 96 98 100 102 104 106 Binding energy (eV) SiC 100.5 SiO x 102.8 Al/Mo/SiC_1 1 10 100 1000 10000 100000 0 20 40 Sputtering time (s) 2 1 Al SiC Mo O Al/Mo/SiC_2 10 100 1000 10000 100000 50 100 150 Sputtering time (s) 2 3 4 AC B Al SiC Mo Si Al/Mo/SiC_1