HERSCHEL/HECOR mission
successful launch 14/09/2009
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HDR defense of Franck Delmotte
Auditorium 14h -
4/02/2010
XUV Optics Research
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EXTREME ULTRAVIOLET MULTILAYER MIRRORS FOR ASTROPHYSICS
After a four-years development phase, our STEREO EUV imagers were launched as part of the STEREO solar imaging mission. They now provide 3D images of the sun (first images in December 2006, see figure with solar images at 17.5, 19, 28 and 30 nm). We have also been involved in the optimization and deposition by ion beam sputtering of EUV coatings for other solar missions: SWAP and HECOR. SWAP (Sun Watcher using Active Pixel System detector and imaging Processing) is an EUV imager on the PROBA-II ESA mission that will provide solar corona images in the FeIX/X line (λ=17.5 nm). The Mo/Si multilayer coatings were similar to those of STEREO and have been calibrated on the CEMOX reflectometer called (see below). HECOR (a HElium CORonagraph aboard the Herschel sounding rocket) is a coronagraph designed to observe the solar corona at 30.4 nm. On this instrument, we were able to test a new kind of coating, developed during the PhD work of Julien Gautier (2002-2005): B4C/Mo/Si, a tri-material multilayer structure. We have shown that these multilayers provide much higher reflectivity around 30 nm than two-material multilayers and are well suited for solar imaging applications. In anticipation of the Solar Orbiter ESA mission, we have developed some bi-band mirrors designed to reflect two solar emission lines. Aging and thermal cycling have been studied in collaboration with CNES. These experimental results have been used to make a proposition within EUI European consortium for Solar Orbiter (launch date not earlier than 2015).
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X-RAY FOCUSING MIRRORS WITH CR/SC MULTILAYER COATINGS
Aspherical optical components coated with reflective multilayers for x-ray analysis have been developed in collaboration with the XENOCS company during the PhD work of Aurélie Hardouin (2005-2007). X-ray analysis of low-Z elements by electron microprobe (EPMA: Electron Probe MicroAnalysis) with emission lines in the spectral range between 180 eV and 550 eV, requires analyzer crystals based on the use of interferential multilayer coatings. Aspherical optical components, such as ellipsoids of revolution, permit to focus X-rays on two dimensions in a single reflection, thereby collecting more flux and achieving lower detection limits than the optical components currently used. We have studied and developed Cr/Sc multilayers designed to reflect the nitrogen Kα emission line, having both good optical properties (maximum reflectivity obtained: 37%), and good temporal and thermal stability. In order to improve the analyzer performances, we have developed antireflective coatings in the XUV range, in order to optimize the signal to noise ratio. The very good results achieved have led to the development of aspherical optical components. To do so, the multilayer coating requires a period gradient profile, in order to compensate the incidence angle variation on the optic. The process development has permitted to produce several prototypes of EPMA optics (see photo) for the detection of the nitrogen Kα emission line.
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MO/SI MULTILAYER COATINGS FOR EXTREME ULTRAVIOLET LITHOGRAPHY
Due to their high normal incidence reflectance achieved around 13.5 nm, Mo/Si-based multilayer mirrors have been widely studied during the last few years, mainly for applications in EUV Lithography. In collaboration with industrial partners (Xenocs and SAGEM) we have studied several issues concerning the optics required to collect the flux emitted by an EUV source. We first optimised laterally graded thicknesses of Mo/Si multilayers to take into account angle of incidence variation of the different rays on a 190 mm diameter EUV collector, using a deposition technique that consists in varying the substrate velocity when it goes above the sputter sources. We obtained highly reflective mirrors with uniformity better than ±0.5% along the diameter of the optic. A peak reflectivity of 67 % at 13.5 nm has been measured using synchrotron radiation reflectometry at ALS-Berkeley.
We have also studied in collaboration with P. Jonnard (LCPMR, Paris) the composition of Mo/Si interfaces at nanometer scale and the effect of thin barrier layers (0.3 nm to 1 nm) by different analysis techniques like X-ray emission spectroscopy and TEM microscopy (TEM picture of an Mo/Si multilayer is shown on the figure above).
A major issue for EUV collector coatings is their stability to the plasma source environment: heating, ion bombardment, surface pollution by plasma particles, … Part of the PhD work of Christophe Hecquet (2005-2008) concerns the optimization of multilayer stability in such an environment. Long term thermal stability (up to 200h at 500°C), the effect of ion beam erosion on multilayer structure and also exposition to a real EUV source have been studied for different kind of multilayers.__________________________________________________________________________________
MULTILAYER MIRRORS FOR ATTOSECOND PULSES
The ability of High Harmonic Generation (HHG) sources to produce attosecond pulses has been demonstrated experimentally about ten years ago. High harmonics of the fundamental IR laser beam (at 800 nm) with orders in the range 21 to 81 are thus in the X-UV spectral domain. These ultra-short light pulses of sub femtosecond duration open several new scientific and technologic challenges. One of these challenges is the manipulation of attosecond pulses: how to reflect and/or focus such pulses without altering their temporal characteristics? XUV multilayer mirrors can provide a solution if they fulfill the two following conditions. Firstly, they must have a broad spectral bandwidth in order to reflect as many high harmonics as possible. Secondly, their spectral phase has to be accurately controlled on the full mirror bandwidth. Broadband mirrors have been designed, consisting of a combination of two periodic multilayers, each including the three materials Mo, Si and B4C within each period. The reflectance of the stacks were measured on the ELETTRA synchrotron facility in the spectral range 20-40 nm and compared to calculation. A band pass of 12.6 nm and a reflectivity of 20% around λ=34 nm have been obtained. A HHG attosecond source developed at the Lund Laser Center has been used to measure the phase variation induced by reflection on such a mirror. These first experiments have shown that the broadband multilayer mirror allows reflecting attosecond pulses without altering pulse duration. Further work is planned with support of the Triangle de la Physique.
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MIRRORS AND POLARIZERS IN THE VACUUM ULTRAVIOLET RANGE
In the VUV(Vacuum Ultra Violet) domain, between 60 and 140 nm, most of materials display both high absorption and low reflectance, and so their optical complex indices are not well known. Nevertheless this wavelength domain is of interest for astrophysics. In collaboration with the MANOLIA group (M.Cuniot, J-M. Desvignes) and with the support of an R&D contract from CNES, we investigate thin layer deposition in order to achieve the highest possible reflectivity and to fabricate polarizers. Firstly, a new graphic method able to give complex optical constants of the deposited layers from spectral reflectivity measurements of test samples has been developped. First sample polarizers have been fabricated. Their application is to measure the Hanle effect (rotation of polarization plane under a magnetic fied) in the framework of the solar coronal Lyot/SMESE mission (collaboration with the Orsay Space Astrophysics laboratory IAS). On that occasion, a collaboration has been established with the Physikalisch-Technisches Bundesanstalt (Synchrotron, Bessy II, Berlin), for measurements of the polarization reflectivity in this particular wavelength domain.
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EXTREME ULTRAVIOLET INTERFEROMETRY FOR METROLOGY
Our group has been involved for more than 15 years in the development of dedicated XUV interferometric systems, based on wavefront division interferometers, typically Fresnel bimirrors, because beam splitter quality in the XUV domain is a challenge. Today, our experience with the technique enables us to perform EUV (extreme UV) metrology. We have developed phase metrology at design wavelength for multilayer structures such as those of the European Union supported EUV lithography Integrated Project "More Moore". The use of phase-shift enhanced masks is considered as a possible way to reach the 22 nm node. We have made calibrations of these structures on the Swiss Synchrotron Source (collaboration: CEA-LETI). We also developed the optical metrology of refractive indices of materials used in our multilayer coatings. We use the Fresnel’s bimirror to measure the phase shift introduced by a thin transmissive layer deposited on an ultra-thin layer membrane. The dispersion curves have shown interdiffusion between the membrane and the material teste, thus providing an inaccuracy on the indices. We intend to work with new membranes, made of silicon carbide, to reduce the phenomenon.
We have also used low coherence interferometry to investigate the depth structure of a complex multilayer stack reflector. The probing instrument is our interferometer based on a Fresnel's bimirror illuminated by relatively wide-band synchrotron undulator light near 13.5 nm (Swiss Synchrotron Source). The contrast of the interferogram has revealed effective reflective planes in the depth multilayer structure. First results in this spectral range may open the way to a new physical approach to extreme ultraviolet sample characterization in the form of line-scan optical coherence tomography with nanometer resolution.__________________________________________________________________________________
VUV FOURIER TRANSFORM SPECTROMETER
After the first validation in July 2003 of a Fourier transform spectrometer covering the vacuum ultraviolet range (100000 cm-1 to 200000 cm-1), its integration into the beamline DESIRS of synchrotron SOLEIL was possible. To that end, the both the mechanics and the optics of the instrument have been fully redesigned and a new version of the optics has been fabricated by our high precision optic shop. Our goal was to achieve a theoretical resolution at about 740000 (0.1 to 0.2 cm-1), as compared to the best diffraction grating operating in this spectral range, which has a resolution of 200000. In addition, the final system should operate in an ultra-high vacuum environment. The project was funded by (ANR) in collaboration with researchers from the SOLEIL synchrotron. In 2007, the spectrometer was assembled and installed on the beamline DESIRS. In September, the first argon spectrum was obtained and it did show the expected nominal resolution. The theoretical predictions of the spectrum corroborate perfectly the measurements and fully validate the spectrometer. In October 2007, the first user coming from Japan has benefited to one week beam time for his research on the spectrum of neon (the figure shows a tiny part of the result). In early 2008, a suitable resampling scheme has provided a resolution of 1.4 million. The international community of spectroscopy has now a tool unmatched in the world to achieve absorption spectrometry.
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