Index Introduction The Research at Rijnhuizen Results in 2008 Education, Training, Outreach and Public Information Output Appendix website Rijnhuizen 2.9 | Advanced applications of XUV optics (AXO) Division: nanolayer Surface and Interface physics Group leader: E. Louis Scientific advisors: J. Verhoeven, A.W. Kleyn Senior scientist: E. Zoethout Postdocs: E.D. van Hattum, R. Sobierajski  Graduate students: J. Chen, A.J.R. van den Boogaard Undergraduate student: S. Khorsand Technicians: S. Alonso van der Westen, A.J. van Calcar, K. Grootkarzijn, R.H. Harmsen, P. Sallé, M. Zee Funding: FP-I10, Carl Zeiss SMT, EC, TFF, M2i, ASML, STW, FOM Research programme The general aim of the AXO group is to carry out research and development of XUV- and soft X-ray single- and multilayer systems. This notably includes multilayer optics for new applications in science and technology. The AXO programme also addresses the particular physics research on these applications. The focus is currently on EUV photolithography, including the execution of the more applied and technological parts of the FOM Industrial Partnership Programme XMO (eXtreme UV Multilayer Optics). The group deals with the transfer of new techniques and processes, tested on small laboratory samples, to the miscellaneous applications. An example of this is the successful fabrication of demonstration optics which include new processes developed elsewhere in the nSI department.  The group also investigates the basic mechanisms responsible for smoothening optics substrates. It turned out that residual roughness on mirror substrates can be removed when the surfaces are polished with low energy ions. Experimental work including the modeling of these processes is the subject of PhD researches. Furthermore, the group carries out experiments to enhance the spectral purity of EUV optical systems, i.e. the suppression of parasitic wavelength bands in the light from EUV sources. Another example of EUVL imposing new research requests is found in the ISitCLEAR project (In situ monitoring of contamination layers on EUV optics at Ångstrom resolution). The aim of this project is the development of a surface sensitive method to probe ultrathin contamination layers on multilayer systems, and to build a functional model that has the potential to be used in an EUV exposure tool. Several candidate techniques have been investigated and experimentally verified, resulting in demonstrations of sub-nanometer accuracies when probing thin film contamination. The group is also involved in a joint task force on EUV optics contamination issues, an important aspect of utilising optics in photolithography equipment. This concerns a combined effort by Carl Zeiss, ASML, TNO, Philips and FOM, aimed to investigate and control the contamination of EUV optics.  A second focus of the group is the study of multilayer properties under exposure of extremely intense femtosecond pulses, carried out at the Free Electron Laser facility FLASH in Hamburg. The aim is to understand the damage mechanisms and to develop new radiation resistant multilayers. Such optics can be applied on beamlines or dedicated experimental set-ups at FLASH or XFEL, the new X-ray Free Electron Laser to be built in Hamburg. Finally, in the framework of preparations for an STW project on even shorter wavelength lithography, pilot studies on multilayers for 6.7 nm radiation have been carried out in close cooperation with the TFM group. Figure 2.12: One of the three advanced ultra high vacuum multilayer deposition facilities (background) in the nSI department. In the foreground, a Variable Angle XPS set-up is shown, connected in vacuo with the deposition set-up. Highlight The 2008 research highlight of the AXO group results from research on multilayer mirrors exposed to the very intense femtosecond pulses of the FLASH XUV free electron laser in Hamburg. Though the appliction of such mirrors in free electron lasers is optically very attractive, the mirror stability is extremely challenged at the high peak powers occuring in such lasers. The eventual damage mechanism is still largely unexplored, and the AXO staff aims at understanding the basic physics here.  Exposure at and just above the mirror damage threshold did result in craters in the multilayer that have been examined by AFM, Nomarski microscopy and Scanning Transmission Electron Microscopy (STEM). In combination with radiation absorption and heat diffusion calculations, the measurements showed that the crater is the result of compaction of the layers due to mixing of the Mo and Si layers and re-crystallisation into molybdenum-silicide crystallites. This process takes place after the actual femtosecond pulse, on a nanosecond timescale at a temperature of ~1000 °C. Thus the EUV reflection of the light during the fs-pulse is not affected by the damaging mechanism. The multilayer damage mechanism described above – essentially being ultrafast atomic diffusion – was observed for the first time by AXO. The result stands out in contrast with the relatively slow diffusion processes known up to now, indicating orders of magnitude slower diffusion processes. Such diffusion occurs during e.g. steady state thermal annealing, characterised by a low threshold temperature for silicide formation (~325 °C) and a time scale of many hours. We have now found that identical silicide formation can also be observed on a nanosecond scale.