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Preliminary synchrotron radiation characterization of first multilayer mirrors for the soft X-ray water window
Spectrochimica Acta Part B 62 (2007) 586 – 592 www.elsevier.com/locate/sab
Preliminary synchrotron radiation characterization of first multilayer mirrors for the soft X-ray water window ☆ Gianfelice Cinque a,⁎, Antonio Grilli b , Giannantonio Cibin c , Agostino Raco b , Alessandro Patelli d , Valentina Mattarello d , Augusto Marcelli b , Valentino Rigato d a Diamond Light Source, RAL, Chilton-Didcot, OX11 0DE Oxon, U.K. INFN, Laboratori Nazionali di Frascati, via E. Fermi 40, I-00044 Frascati, Italy INFN, Laboratori Nazionali di Legnaro, viale dell'Università 2, I-35020 Legnaro, Italy d IMONT, Piazza dei Caprettari 70, I-00186 Roma, Italy b
c
Received 16 February 2006; accepted 17 March 2007 Available online 31 March 2007
Abstract The development of high-reflectivity devices for soft X-rays at quasi-normal incidence is a challenging research for the development of synchrotron radiation optics, particularly for soft X-ray microscopy and X-ray microprobe spectroscopy. Here we present data concerning the deposition of the first Ni/Ti and Ni/TiO2 multilayers grown at the INFN Legnaro Laboratories (LNL). These multilayers have a lattice spacing in the order of 14 Å and more than 100 of bilayers. Experimental tests on these multilayers have been performed by a vacuum compatible θ–2θ reflectometer, set up at the INFN Frascati Laboratories (LNF), where their characterization has been accomplished by means of synchrotron radiation. The first multilayer mirrors tailored in order to work at quasi-normal geometry have been measured in the lower X-ray energy domain using both white-beam and monochromatic radiation at about 1 keV. © 2007 Elsevier B.V. All rights reserved. Keywords: Ni/Ti Ni/TiO2 multilayers; SR soft X-rays; SR reflectometer; Water-window optics; Soft X-ray optics
1. Introduction The design of efficient reflective coatings utilizing multilayer has boosted up the construction of optics in the Extreme UltraViolet (EUV), especially for Solar Physics (for a review see [1]). Actually, high-resolution imagers comprising normalincidence telescope mirrors coated with narrow-band multilayers tuned to specific coronal or transition–region emission lines are in use in a variety of missions. E.g., the planned UVCI coronagraph project aims to investigate the solar wind acceleration from a range of solar source structures by the ☆ This paper was presented at the “18th International Congress on X-ray Optics and Microanalysis” (ICXOM-18) held in Frascati, Rome (Italy), 25–30 September 2005, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Diamond-RAL, Chilton-Didcot OX11 0DE, OXON, UK. Tel.: +44 1235 778410; fax: +44 1235 778448. E-mail address: [email protected] (G. Cinque).
0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.03.028
simultaneous observations of particles and radiation: it consists of an externally occulted off-axis Gregorian telescope with multilayer-coated optic. Nowadays, the project HElium Resonance Scattering in Corona and HELiosphere (HERSCHEL) of ESA will prove the validity of the new concept before the UVCI will obtain monochromatic images in the Lyman α-lines of the two most abundant elements, i.e. hydrogen (1216 Å) and singlyionized helium (304 Å). Space probes utilizing normalincidence multilayer optics for high-resolution imaging and/or spectroscopy benefit greatly from improvements in multilayer performance, specifically in higher reflectance and better spectral selectivity, as well as greater heat stability and radiation–particle resistance. The Sun Watcher using APS detectors and image Processing (SWAP) is another telescope that will provide images of the solar corona at a temperature of 1.5 MK (n.b. blackbody emission peak ∼ 2200 Å). Upon the heritage of the joint ESA-NASA mission SOHO launched in 1996, SWAP will continue the systematic coronal mass ejection
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watch program on recording global waves propagating across the solar disc, EUV dimming regions, etc. etc. SWAP will be an off-axis Ritchey–Chretien telescope with a CMOS-APS detector covered by a scintillator layer. Spectral filtering around the Fe XII emission line (195 Å) is achieved with EUV multilayers deposited on the mirrors, and completed with a set of aluminum filters blocking the visible [2]. However, the development of multilayer reflectors for Synchrotron Radiation (SR) is strongly motivated by the possibility to tailor and construct beamline optical systems in the wavelength region above 30 Å. The idea is to take advantage of radiation tolerance and high throughput of such artificial devices with respect to available organic crystals when used as X-ray monochromator according to classical Bragg's diffraction scheme [3]. In practice, at such lower energies it is difficult to fabricate standard multilayers with thickness of layer pairs around 30 Å, without mentioning that their overall number is still technically limited to few hundreds of bilayers. Both of these facts affect, respectively, the energy range accessibility and the energy band resolution of the optical systems concerned. EUV multilayers designed for relatively broad spectral response are deposited onto normal-incidence gratings in commerce for high-reflectivity spectrographs. For softer Xrays, double-crystal monochromators constructed by multilayers have been considered and experimentally used: in particular W/Si was exploited, since its second order of Bragg reflection matches to the first order of a KAP(100) crystal in a double-crystal monochromator [4]. As a matter of fact, the development of multilayer mirrors is particularly appealing for the microscopy of organic samples in the so called “water window”, i.e. in between the absorption K edges of carbon and oxygen. Within, respectively, 284 and 543 eV (44 to 23 Å), a good penetration in micrometer-thick specimens is available for X-rays, which provide very good contrast between protein and water and therefore for organic materials in their natural environment, while their wavelength value gives the potential for high-spatial resolution imaging. In such energy range the available optics for X-ray focussing are either policapillary systems or Fresnel zone plates, both in transmission. Since no natural crystal can be routinely used as diffractors, the rationale and advantage of multilayer mirrors are easily explained when considering the wider solid angle available on focussing the X-rays at near-normal incidence with respect to the grazing angles, and the need of high quality surfaces for total reflection on standard X-ray mirrors. A straightforward example of a soft X-ray microscope by SR applied to the imaging of systems of biological and medical
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interest is the back illuminated scheme proposed by Del Rio and co-workers [5]. Here, the SR white-beam is selected in wavelength by Bragg's diffraction onto a spherically bent crystal: thus, the incidence angle controls the spectral band midpoint in energy, while the object distance beyond the focal point sets the magnification of the projected image over the spatially sensitive detector (CCD or suchlike). Conceived for natural mica crystals (2d ∼ 11 Å), and therefore allowing at quasi-normal incidence the focussing of X-ray with energy above 1 keV, this full-field transmission microscope will be able to carry out contrast-enhanced microscopy at the absorption edge of transition metals or high Z contaminant elements present inside thin-layered samples. On exploiting the effective low Z typical of light matrixes, tiny structures and the trace content of contaminants could be imaged by difference below and above the absorption edge of the atomic species under investigation [6,7]. 2. Multilayer mirror material choice The demand in SR research of highly-reflective multilayers is strongly motivated by X-ray microscopy and microprobe spectroscopy applications in the water-window region. For such uses, reflectivity and bandwidth are among the most critical parameters determining the performances of these mirrors. In spite of the various studies carried on in this domain, the low reflectances achieved are due to the small Fresnel coefficients of materials at wavelengths between 23 and 44 Å. Conversely, a large number of bilayers is required, which limits the spectral bandwidth and introduces the problem of interface imperfections. For the selection of the atomic species to be used, a combination of binary materials whose refractive indices are as different as possible, but with the smallest absorption coefficients, should be chosen to obtain high reflectance [8]. In practice, a low absorption (low Z element) layer is intercalated between two absorber (high Z element) layers: the maximum contrast (difference in the refractive index real part) is given by the absorber material with respect to the spacer, chosen to so that the working energy range is far from absorption edges. Ni/Tibased systems are well-suited candidates for multilayer mirrors due to the optical constant matching of nickel and titanium: theoretically, the combination of these materials gives a high reflectivity just above the Ti absorption L2,3 edge nearby (around 28 Å). It was already suggested in literature the use of metal oxides in soft X-ray reflectors at water-window wavelengths. In principle, an oxide layer could prevent the formation of alloys at the metal interface, while X absorption in oxides remains
Fig. 1. Ni/Ti NT012 and Ni/TiO2 NT015 projects on Si(100) substrate grown at LNL-Legnaro.
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Table 1 Results of the characterizations of NT012 and NT015 mutilayer projects by RBS and XRR No. of Target Thickness Λ (Å) periods (Å) NT012 300
Ti Ni
6.5 7.6
No. of Target Λ (Å) periods
13.9 ± 0.1 NT015 150
Ti Ni
12.8 ± 0.2
Experiments at LNL-Legnaro Laboratories and Padua University.
negligible below the oxygen absorption K edge. In the end, multilayers fabricated with the above combination should be compatible in terms of atomic interface structures. The reflectivity of multilayer mirrors is thus strongly constrained by the layer structures and interface roughness. Intermixed widths, or thickness ratio between each layer are important parameters affecting the performances of such mirrors. 3. Nickel–titanium and nickel–titanium dioxide deposition at LNL Among the most recent works on multilayers, nickel– titanium depositions have been considered in detail and shown in Fig. 1. Ni/Ti systems are also particularly appealing for their magnetic properties. Scientifically, the research on Ni/TiO2 is even more interesting but at the price of further technological difficulties due to the metal on different metal–oxide interface. Briefly, the high vacuum chamber for the multilayer deposition presents three sputtering cathodes for RF-driven magnetron deposition of either Ti, Ni or Mo targets. The process gases typically used are Ar (operating pressure of 10− 3 mbar) and O2 (mbar). A DC biasable sample-holder rotates to face the spattering sources, while couples of Helmoltz coils are used for plasma densification on the surfaces of single-crystal Si wafer typically used as substrates. Quartz micro-balances and a cylindrical Langmuir probe accomplish for the plasma diagnostic inside the chamber. All phases of the deposition process are controlled in remote by software, e.g. the quadrupole mass spectrometer driven feedback of the oxygen flow to achieve the desired stoichiometry of TiO2. Some time has been spent to optimize the growth conditions of the single materials by the previous set-up, namely bilayers with structure Ti–Ni-substrate. On the basis of such trials, several Ni/Ti multilayers have been fabricated, and the last sample NT012 has been deposited on
Fig. 3. Schematic layout of the soft X-ray reflectometer installed at the wiggler beamline at LNF-Frascati.
applying the highest flux and energy of the ion bombardment with respect to all the others of the series. Further work has been done for depositing Ni and Ti oxide by optimizing the flux of oxygen with respect to the other parameters of the process. This study deals with the first depositions of nickel onto titanium and titanium oxide; indeed, they must be considered preliminary samples for which the various parameters of growth and protocol of control are not yet optimized at best. At the beginning, such multilayers have been characterized by Rutherford Back-scattering Spectroscopy (RBS), Nuclear Reaction Analysis (NRA), and by X-ray Reflectivity (XRR). These pre-measures are summarized in Table 1. The first spectroscopic technique uses a beam of alpha particles (by HVEC 2.5 MV and CN 7.0 MV Van deer Graaff accelerators at LNL) impinging at 2.2 MeV onto the target to detect the back-scattered ones (at angle of about 160°). Straightforward kinematics of atomic elastic collisions (RBS) or nuclear reactions (NRA) explain how the energy loss of the recoil particles is related to the penetration depth and the atomic mass of the target atoms, while the total yield is proportional to the atomic species density. The energy spectrum so achieved (see Fig. 2) reveals no contamination by Ar in the NT012 sample, and two non-broadened bands (non-interdiffused layers) of Ti and Ni are present. From the energy loss calculations, the values of both Ti and Ni layer thickness are extrapolated and given in Table 1. The second is the usual diffraction technique where a conventional X-ray source is used (Philips X'Pert Pro Diffractometer at Padua University); namely a Cu anode and its Kα emission line, whose beam is reflected on the target at grazing incidence. The spectra attained in angular scans establish in the case of atomic ordered systems the structural parameters, like lattice period and absorber-to-period thickness ratio, according to Bragg rule. The periodicity of the multilayers in Table 1 are extrapolated from the angle position of the first diffraction peak since the more apt superior orders of diffraction could not be usefully recognized over the background. 4. The soft X-ray reflectometer at LNF
Fig. 2. RBS spectrum of the NT012 project 300 periods Ni/Ti on Si(100) substrate. Experiment at the 2.2 MV accelerator of LNL-Legnaro.
The storage ring DAΦNE at Frascati has unique characteristics for Synchrotron Radiation research thanks to the low electron energy, 0.51 GeV, and the high circulating current, routinely over 1 A. The intense flux of soft photons is exploited in three beamlines [9]. Referring to the sketch of Fig. 3, soft Xrays are emitted from a wiggler source with a continuous SR spectrum and critical energy of circa 300 eV. Since 2002, a
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Fig. 4. UHV reflectometer experimental set-up at LNF-Frascati: SR beam coming from leftside. At left: external side view; top right: inside view from top; bottom right: schematic drawing top view.
double-crystal monochromator is operational for experiments resolved in energy (mostly X-ray absorption near edge spectroscopy) in the distinguishing energy window below 4 keV [10]. Based on the boomerang geometry, it ensures a fixed beam exit while allowing crystals scan for Bragg diffraction in a range of grazing angles from 15° to 75° by sets of different crystals to select the photon energy with a typical bandwidth ΔE / E of circa 10− 4. Changes in the softer X-ray energy side to extend the working region downward to the oxygen K-edge included leaktight and ultra-thin windows to gain in X-ray transparency but still guaranteeing the beamline vacuum. The new equipment of monochromator crystal with large lattice spacing, namely KAP (100) (and KTP(011)) crystals, are available for the lower
energy domain. Endowed with 2d = 26.6 Å (10.9 Å), a couple of such crystals fits the X-ray energy selection within the so-called water window at 480 eV (or down to1200 eV). Our KAP(100) crystals have been tested by X-ray Diffraction under a conventional Cu source (by courtesy of Claudio Veroli and Giorgio Cappuccio of CNR-ICSM). Since 2005 the commissioning and test of the vacuum compatible soft X-ray reflectometer (see Fig. 4) have been completed including the remote software-controlled unit. In brief, the whole set-up consists in a UHV chamber allocating two high precision goniometers coupled to two UHV all-magnetic rotary feedthroughs to perform angular scans of the multilayer under X-ray beam and to follow the reflected beam by the detector arm. Extreme precision, accuracy and repeatability of the
Table 2 Technical data for the UHV reflectometer, controller and acquisition system at LNF-Frascati Double UHV goniometer system Motors:
Rotation range Repeatability Reversal error Resolution UHV Rotary feedthrough Reflectometer arm and slits Remote control
Detection and acquisition
2-phase-microstep endless Contactless limit-reference switches Optical rulers for absolute positioning 360° endless ±0.0002° ±0.0004° ±0.002° open-loop ±0.0002° closed-loop Rare-earth magnets, non-lubricated b100 mm, 0.5 to 2 mm 2-axes microstep controller Manual Joystick/Display unit LabVIEW code for ω– and θ–2θ scans ADC input 12 bit 0–10 V signal Absolute photodiode 100 mm2 windowless UHV preAmp 10 V/nA, linearity ±1%
Fig. 5. Si/Mo multilayer ω-scan at 45° under SR white-beam at LNF-Frascati: narrower Bragg peak due to the Si(111) substrate at circa 2.8 keV, on right the multilayer contribution. Inset: equivalent test by conventional X source.
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Fig. 6. Si/Mo multilayer measurements by monochromatic SR at 1200 eV with LabView interface control of the X-ray reflectometer. Left panel: 2θ-scan of the detector; right panel: ω-scan.
angular movements (better than 3.5 μrad)) are guaranteed without backlash by the readout of absolute encoders for angle feedback. (Table 2). In both open- and closed-loop mode, a dedicated software code controls the angular movements of sample and detector arm during a measure scan in the vacuum chamber. The direct detection of diffracted X-ray beams is accomplished by an absolute X-ray photodiode plugged on a UHV compatible preamplifier. The experimental techniques here considered for the optical characterization of the soft X-ray devices by SR monochromatic or white-beam X-ray reflectometry include [11,12]: i) angle scan of the multilayer by fixed detector (ω-scan) for determining the device reflectivity band-pass (rocking curve); ii) correlated angle scan of both the multilayer and detector (θ–2θ) for assessing the device diffraction figure of merit; iii) energy resolved assessment of the reflectivity at fixed detector and sample angles.
Fig. 7. Ni/Ti multilayer test under white-beam at 45°: the lower peak is the average of the four scans plotted above, background subtracted and normalized to a e- circulating current of 1 A at DAFNE, LNF-Frascati.
We proceeded with the direct measure of the reflectivity of a standard Si/Mo device, a workhorse in the multilayer mirror family. The system under investigation consists in a Si/Mo series of 40 layers, deposited by a Λ = 64 Å onto a Si(111) substrate, and tailored to work at 8° with a band-pass energy spanning from 700 to 850 eV. Even if not optimized for the water window, the test of such multilayer allowed us to assess the experimental setup and acquisition system by a well-known and robust device of known reflectivity in the soft X-ray domain. At first, the multilayer was tested in vacuum by exploiting the SR white-beam and collecting an ω-scan of the mirror around 45° (detector fixed at 90°). In Fig. 5, the signal due to the SR reflection shows both the Si(111) diffraction from the substrate, which accomplishes for the narrower peak corresponding to Bragg diffraction peak around 2800 eV, and the sum of the multilayer harmonic multiple reflections, broader peak at right. Namely, this is an average due to ∼ 50% of the reflection at 550 eV ∼ 33% of the 825 eV contribution, plus ∼16% of the 965 eV beam (calculations based on Fresnel equations and Henke's optical data [13]). An XRD characterization by a Cu 8048 keV source confirms these attributions to the resolved Bragg peaks, and the difference in shape is owing to the different set-up resolution.
Fig. 8. XRD spectrum at 8048 eVof the 300 periods Ni/Ti onto Si substrate. Cu Kα1 emission at 40 kV/30 mA and measure by a Huber θ–2θ diffractometer at LNF.
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Secondly, the Si/Mo mirror was exposed to by monochromatic SR beam at 1200 eV: both ω-scan and 2θ-scan (both under vacuum) are reported in Fig. 6. The former measure is a direct test of the device rocking curve, i.e. its bandwidth in terms of accepted angle at fixed reflected energy. In this case the energy resolution of the monochromator crystals dominates the mutilayer expected resolving power and gives a bandwidth of circa 2.3°/87°. The latter measure is mostly a test of the resolving power of the set-up in terms of angle definition by slits and detector arm length (in practice Δθ ≤ 3.5 mrad). We carried on a test of reflectivity for the first Ni/Ti multilayer deposited at LNL: again, the ω-scan of the device around 45° and detector fixed at 90° was used. This multilayer consisted in 300 bilayers with Λ ≈ 14 Å deposited on a Si(100) wafer as substrate. Designed for the water window, precisely for soft X-rays around ∼ 440 eV in quasi back-scattering geometry, we tested its figure of merit under white-beam, i.e. like a SR first optic element. The bottom reflection signal in Fig. 7 is an average of 4 scans which corresponds to an effective e- current in DAΦNE of 1 A (n.b. Si(100) crystal diffraction prohibited). A simple simulation addresses this signal to mostly a single contribution of the 670 eV X-rays from the SR (calculations performed as before according to [13]). A further characterization of the Ni/Ti multiplayer periodicity was given by X-ray Diffraction technique. Like for XRR method, the period Λ in Angstrom is calculated according to the formula n ⁎ Λ = 1.545 / (2 ⁎ sinθ), where 1.545Å is the Cu Kα emission wavelength, and θ is the grazing angle of the nth Bragg peak. From leftside of Fig. 8 the total reflection shoulder is clearly visible, followed by the series of Bragg diffractions harmonics starting from the principal peak around 3°.
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To test new optics, at INFN Frascati Laboratories a vacuum compatible reflectometer has been set up: direct characterization of the multilayer reflectivity have been performed by SR photons emitted from the wiggler source of DAΦNE. The first experimental measurements were accomplished in 2005 with the direct measurements by means of X-rays of the first opticalgrade multilayers in terms of energy bandwidth (rocking curve) and reflectivity. Two experimental approaches have been followed, i.e. white-beam ω-scans for determining the multilayer rocking curve, and both ω- and 2θ-scans measurement by monochromatic beam at circa 1.2 keV. Experimental data by SR about the characterization of the vacuum reflectometer and the new multilayers are given. The measurements performed on a benchmark Si/Mo device and the preliminary results obtained on both Ni/Ti and Ni/TiO2 devices can be summarized as follows: by comparison of the experiments by SR and conventional X source, the overall reflectivity of the Ni/Ti device is estimated to be between 5 to 9%. RBS, NRA and XRR techniques result in Ni/Ti and Ni/TiO2 multilayer periods of, respectively, 13.86 ± 0.07 Å and 12.78 ± 0.18 Å. New experiments are in due course at LNF for better definition of the nickel titanium device performances as a function of the photon energy and in terms of θ–2θ scans. The natural development of these studies consists in reaching the project reflectivity of ∼30% nearby the anomalous diffraction at the Ni L-edges. In perspective, multiple multilayer assembly onto large substrates could gain in optical acceptance and, eventually shaped according to spherical geometries, will finally refocus the SR beam. Acknowledgments
5. Conclusions The experimental development on high-reflectivity normalincidence optics for soft X-rays is strategically important in multidisciplinary researches from astrophysics to free electron lasers. The scientific importance in Synchrotron Radiation research is of great topical interest, especially for soft X-ray (spectro)microscopy of biological structures in between oxygen and carbon absorption K-edges. This study addresses the demand on novel Ni–Ti based X-ray reflectors for the water window. Even if such material choice has already been tried (but with interleaved carbon layers [14]), these are among the first nickel–titanium devices with over 100 bilayers designed to work at quasi-normal incidence that have been grown and experimentally measured in reflectivity. Despite other studies on oxide multilayer [15], these are, to our knowledge, the first results concerning soft X-ray reflectors made of pure metal layers (Ni) deposited onto different metal oxide (TiO2) and tested under SR. The design and deposition of such devices has progressed at INFN Legnaro Laboratories, after an optimization of the deposition conditions in the new sputtering chamber. Among the best candidates for soft X-ray optics, the first Ni/Ti reflectors have been successfully grown, as well the completely new Ni/ TiO2 multilayers.
A special thanks is due to Claudio Veroli and Giorgio Cappuccio for their courtesy in characterizing the multilayer by XRD, and Antonietta Frani for her software support. We thank Marco Pietropaoli and Giacomo Viviani of the LNF for their technical support. Moreover, a long-term collaboration with A. Soldatov and M. Matzurisky of the Rostov University is acknowledged. This work has been financially supported by the Istituto Nazionale di Fisica Nucleare-Gruppo V in the framework of the ARCHIMEDE experiment. References [1] D. Windt, S. Donguy, J. Seely, B. Kjornrattanawanich, Experimental comparison of extreme-ultraviolet multilayers for solar physics, Appl. Opt. 43 (2004) 1835–1848. [2] D. Berghmans, J.F. Hochedez, J.M. Defise, J.H. Lecat, B. Nicula, V.A. Slemzin, G. Lawrence, A.C. Katsiyannis, R.A.M. Van der Linden, A.N. Zhukov, F. Clette, P. Rochus, E. Mazy, T. Thibert, P. Nicolosi, M.G. Pelizzo, U. Schuehle, SWAP onboard PROBA 2, A new EUV imager for solar monitoring, Adv. Space Res. 38 (2006) 1807–1811. [3] E. Ziegler, Multilayer optics for synchrotron X-ray applications, in: A. Florin (Ed.), Optical Interference Coatings, Proc. SPIE, Int. Soc. Opt. Eng., 2253 (1994) 248–259. [4] K. Yamashita, M. Watanabe, O. Matsudo, J. Yamazaki, I. Hatsukade, T. Ishigami, S. Takahama, K. Tamaura, M. Ohtani, Characterization of
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Preliminary synchrotron radiation characterization of first multilayer mirrors for the soft X-ray water window ☆ Gianfelice Cinque a,⁎, Antonio Grilli b , Giannantonio Cibin c , Agostino Raco b , Alessandro Patelli d , Valentina Mattarello d , Augusto Marcelli b , Valentino Rigato d a Diamond Light Source, RAL, Chilton-Didcot, OX11 0DE Oxon, U.K. INFN, Laboratori Nazionali di Frascati, via E. Fermi 40, I-00044 Frascati, Italy INFN, Laboratori Nazionali di Legnaro, viale dell'Università 2, I-35020 Legnaro, Italy d IMONT, Piazza dei Caprettari 70, I-00186 Roma, Italy b
c
Received 16 February 2006; accepted 17 March 2007 Available online 31 March 2007
Abstract The development of high-reflectivity devices for soft X-rays at quasi-normal incidence is a challenging research for the development of synchrotron radiation optics, particularly for soft X-ray microscopy and X-ray microprobe spectroscopy. Here we present data concerning the deposition of the first Ni/Ti and Ni/TiO2 multilayers grown at the INFN Legnaro Laboratories (LNL). These multilayers have a lattice spacing in the order of 14 Å and more than 100 of bilayers. Experimental tests on these multilayers have been performed by a vacuum compatible θ–2θ reflectometer, set up at the INFN Frascati Laboratories (LNF), where their characterization has been accomplished by means of synchrotron radiation. The first multilayer mirrors tailored in order to work at quasi-normal geometry have been measured in the lower X-ray energy domain using both white-beam and monochromatic radiation at about 1 keV. © 2007 Elsevier B.V. All rights reserved. Keywords: Ni/Ti Ni/TiO2 multilayers; SR soft X-rays; SR reflectometer; Water-window optics; Soft X-ray optics
1. Introduction The design of efficient reflective coatings utilizing multilayer has boosted up the construction of optics in the Extreme UltraViolet (EUV), especially for Solar Physics (for a review see [1]). Actually, high-resolution imagers comprising normalincidence telescope mirrors coated with narrow-band multilayers tuned to specific coronal or transition–region emission lines are in use in a variety of missions. E.g., the planned UVCI coronagraph project aims to investigate the solar wind acceleration from a range of solar source structures by the ☆ This paper was presented at the “18th International Congress on X-ray Optics and Microanalysis” (ICXOM-18) held in Frascati, Rome (Italy), 25–30 September 2005, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. Diamond-RAL, Chilton-Didcot OX11 0DE, OXON, UK. Tel.: +44 1235 778410; fax: +44 1235 778448. E-mail address: [email protected] (G. Cinque).
0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.03.028
simultaneous observations of particles and radiation: it consists of an externally occulted off-axis Gregorian telescope with multilayer-coated optic. Nowadays, the project HElium Resonance Scattering in Corona and HELiosphere (HERSCHEL) of ESA will prove the validity of the new concept before the UVCI will obtain monochromatic images in the Lyman α-lines of the two most abundant elements, i.e. hydrogen (1216 Å) and singlyionized helium (304 Å). Space probes utilizing normalincidence multilayer optics for high-resolution imaging and/or spectroscopy benefit greatly from improvements in multilayer performance, specifically in higher reflectance and better spectral selectivity, as well as greater heat stability and radiation–particle resistance. The Sun Watcher using APS detectors and image Processing (SWAP) is another telescope that will provide images of the solar corona at a temperature of 1.5 MK (n.b. blackbody emission peak ∼ 2200 Å). Upon the heritage of the joint ESA-NASA mission SOHO launched in 1996, SWAP will continue the systematic coronal mass ejection
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watch program on recording global waves propagating across the solar disc, EUV dimming regions, etc. etc. SWAP will be an off-axis Ritchey–Chretien telescope with a CMOS-APS detector covered by a scintillator layer. Spectral filtering around the Fe XII emission line (195 Å) is achieved with EUV multilayers deposited on the mirrors, and completed with a set of aluminum filters blocking the visible [2]. However, the development of multilayer reflectors for Synchrotron Radiation (SR) is strongly motivated by the possibility to tailor and construct beamline optical systems in the wavelength region above 30 Å. The idea is to take advantage of radiation tolerance and high throughput of such artificial devices with respect to available organic crystals when used as X-ray monochromator according to classical Bragg's diffraction scheme [3]. In practice, at such lower energies it is difficult to fabricate standard multilayers with thickness of layer pairs around 30 Å, without mentioning that their overall number is still technically limited to few hundreds of bilayers. Both of these facts affect, respectively, the energy range accessibility and the energy band resolution of the optical systems concerned. EUV multilayers designed for relatively broad spectral response are deposited onto normal-incidence gratings in commerce for high-reflectivity spectrographs. For softer Xrays, double-crystal monochromators constructed by multilayers have been considered and experimentally used: in particular W/Si was exploited, since its second order of Bragg reflection matches to the first order of a KAP(100) crystal in a double-crystal monochromator [4]. As a matter of fact, the development of multilayer mirrors is particularly appealing for the microscopy of organic samples in the so called “water window”, i.e. in between the absorption K edges of carbon and oxygen. Within, respectively, 284 and 543 eV (44 to 23 Å), a good penetration in micrometer-thick specimens is available for X-rays, which provide very good contrast between protein and water and therefore for organic materials in their natural environment, while their wavelength value gives the potential for high-spatial resolution imaging. In such energy range the available optics for X-ray focussing are either policapillary systems or Fresnel zone plates, both in transmission. Since no natural crystal can be routinely used as diffractors, the rationale and advantage of multilayer mirrors are easily explained when considering the wider solid angle available on focussing the X-rays at near-normal incidence with respect to the grazing angles, and the need of high quality surfaces for total reflection on standard X-ray mirrors. A straightforward example of a soft X-ray microscope by SR applied to the imaging of systems of biological and medical
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interest is the back illuminated scheme proposed by Del Rio and co-workers [5]. Here, the SR white-beam is selected in wavelength by Bragg's diffraction onto a spherically bent crystal: thus, the incidence angle controls the spectral band midpoint in energy, while the object distance beyond the focal point sets the magnification of the projected image over the spatially sensitive detector (CCD or suchlike). Conceived for natural mica crystals (2d ∼ 11 Å), and therefore allowing at quasi-normal incidence the focussing of X-ray with energy above 1 keV, this full-field transmission microscope will be able to carry out contrast-enhanced microscopy at the absorption edge of transition metals or high Z contaminant elements present inside thin-layered samples. On exploiting the effective low Z typical of light matrixes, tiny structures and the trace content of contaminants could be imaged by difference below and above the absorption edge of the atomic species under investigation [6,7]. 2. Multilayer mirror material choice The demand in SR research of highly-reflective multilayers is strongly motivated by X-ray microscopy and microprobe spectroscopy applications in the water-window region. For such uses, reflectivity and bandwidth are among the most critical parameters determining the performances of these mirrors. In spite of the various studies carried on in this domain, the low reflectances achieved are due to the small Fresnel coefficients of materials at wavelengths between 23 and 44 Å. Conversely, a large number of bilayers is required, which limits the spectral bandwidth and introduces the problem of interface imperfections. For the selection of the atomic species to be used, a combination of binary materials whose refractive indices are as different as possible, but with the smallest absorption coefficients, should be chosen to obtain high reflectance [8]. In practice, a low absorption (low Z element) layer is intercalated between two absorber (high Z element) layers: the maximum contrast (difference in the refractive index real part) is given by the absorber material with respect to the spacer, chosen to so that the working energy range is far from absorption edges. Ni/Tibased systems are well-suited candidates for multilayer mirrors due to the optical constant matching of nickel and titanium: theoretically, the combination of these materials gives a high reflectivity just above the Ti absorption L2,3 edge nearby (around 28 Å). It was already suggested in literature the use of metal oxides in soft X-ray reflectors at water-window wavelengths. In principle, an oxide layer could prevent the formation of alloys at the metal interface, while X absorption in oxides remains
Fig. 1. Ni/Ti NT012 and Ni/TiO2 NT015 projects on Si(100) substrate grown at LNL-Legnaro.
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Table 1 Results of the characterizations of NT012 and NT015 mutilayer projects by RBS and XRR No. of Target Thickness Λ (Å) periods (Å) NT012 300
Ti Ni
6.5 7.6
No. of Target Λ (Å) periods
13.9 ± 0.1 NT015 150
Ti Ni
12.8 ± 0.2
Experiments at LNL-Legnaro Laboratories and Padua University.
negligible below the oxygen absorption K edge. In the end, multilayers fabricated with the above combination should be compatible in terms of atomic interface structures. The reflectivity of multilayer mirrors is thus strongly constrained by the layer structures and interface roughness. Intermixed widths, or thickness ratio between each layer are important parameters affecting the performances of such mirrors. 3. Nickel–titanium and nickel–titanium dioxide deposition at LNL Among the most recent works on multilayers, nickel– titanium depositions have been considered in detail and shown in Fig. 1. Ni/Ti systems are also particularly appealing for their magnetic properties. Scientifically, the research on Ni/TiO2 is even more interesting but at the price of further technological difficulties due to the metal on different metal–oxide interface. Briefly, the high vacuum chamber for the multilayer deposition presents three sputtering cathodes for RF-driven magnetron deposition of either Ti, Ni or Mo targets. The process gases typically used are Ar (operating pressure of 10− 3 mbar) and O2 (mbar). A DC biasable sample-holder rotates to face the spattering sources, while couples of Helmoltz coils are used for plasma densification on the surfaces of single-crystal Si wafer typically used as substrates. Quartz micro-balances and a cylindrical Langmuir probe accomplish for the plasma diagnostic inside the chamber. All phases of the deposition process are controlled in remote by software, e.g. the quadrupole mass spectrometer driven feedback of the oxygen flow to achieve the desired stoichiometry of TiO2. Some time has been spent to optimize the growth conditions of the single materials by the previous set-up, namely bilayers with structure Ti–Ni-substrate. On the basis of such trials, several Ni/Ti multilayers have been fabricated, and the last sample NT012 has been deposited on
Fig. 3. Schematic layout of the soft X-ray reflectometer installed at the wiggler beamline at LNF-Frascati.
applying the highest flux and energy of the ion bombardment with respect to all the others of the series. Further work has been done for depositing Ni and Ti oxide by optimizing the flux of oxygen with respect to the other parameters of the process. This study deals with the first depositions of nickel onto titanium and titanium oxide; indeed, they must be considered preliminary samples for which the various parameters of growth and protocol of control are not yet optimized at best. At the beginning, such multilayers have been characterized by Rutherford Back-scattering Spectroscopy (RBS), Nuclear Reaction Analysis (NRA), and by X-ray Reflectivity (XRR). These pre-measures are summarized in Table 1. The first spectroscopic technique uses a beam of alpha particles (by HVEC 2.5 MV and CN 7.0 MV Van deer Graaff accelerators at LNL) impinging at 2.2 MeV onto the target to detect the back-scattered ones (at angle of about 160°). Straightforward kinematics of atomic elastic collisions (RBS) or nuclear reactions (NRA) explain how the energy loss of the recoil particles is related to the penetration depth and the atomic mass of the target atoms, while the total yield is proportional to the atomic species density. The energy spectrum so achieved (see Fig. 2) reveals no contamination by Ar in the NT012 sample, and two non-broadened bands (non-interdiffused layers) of Ti and Ni are present. From the energy loss calculations, the values of both Ti and Ni layer thickness are extrapolated and given in Table 1. The second is the usual diffraction technique where a conventional X-ray source is used (Philips X'Pert Pro Diffractometer at Padua University); namely a Cu anode and its Kα emission line, whose beam is reflected on the target at grazing incidence. The spectra attained in angular scans establish in the case of atomic ordered systems the structural parameters, like lattice period and absorber-to-period thickness ratio, according to Bragg rule. The periodicity of the multilayers in Table 1 are extrapolated from the angle position of the first diffraction peak since the more apt superior orders of diffraction could not be usefully recognized over the background. 4. The soft X-ray reflectometer at LNF
Fig. 2. RBS spectrum of the NT012 project 300 periods Ni/Ti on Si(100) substrate. Experiment at the 2.2 MV accelerator of LNL-Legnaro.
The storage ring DAΦNE at Frascati has unique characteristics for Synchrotron Radiation research thanks to the low electron energy, 0.51 GeV, and the high circulating current, routinely over 1 A. The intense flux of soft photons is exploited in three beamlines [9]. Referring to the sketch of Fig. 3, soft Xrays are emitted from a wiggler source with a continuous SR spectrum and critical energy of circa 300 eV. Since 2002, a
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Fig. 4. UHV reflectometer experimental set-up at LNF-Frascati: SR beam coming from leftside. At left: external side view; top right: inside view from top; bottom right: schematic drawing top view.
double-crystal monochromator is operational for experiments resolved in energy (mostly X-ray absorption near edge spectroscopy) in the distinguishing energy window below 4 keV [10]. Based on the boomerang geometry, it ensures a fixed beam exit while allowing crystals scan for Bragg diffraction in a range of grazing angles from 15° to 75° by sets of different crystals to select the photon energy with a typical bandwidth ΔE / E of circa 10− 4. Changes in the softer X-ray energy side to extend the working region downward to the oxygen K-edge included leaktight and ultra-thin windows to gain in X-ray transparency but still guaranteeing the beamline vacuum. The new equipment of monochromator crystal with large lattice spacing, namely KAP (100) (and KTP(011)) crystals, are available for the lower
energy domain. Endowed with 2d = 26.6 Å (10.9 Å), a couple of such crystals fits the X-ray energy selection within the so-called water window at 480 eV (or down to1200 eV). Our KAP(100) crystals have been tested by X-ray Diffraction under a conventional Cu source (by courtesy of Claudio Veroli and Giorgio Cappuccio of CNR-ICSM). Since 2005 the commissioning and test of the vacuum compatible soft X-ray reflectometer (see Fig. 4) have been completed including the remote software-controlled unit. In brief, the whole set-up consists in a UHV chamber allocating two high precision goniometers coupled to two UHV all-magnetic rotary feedthroughs to perform angular scans of the multilayer under X-ray beam and to follow the reflected beam by the detector arm. Extreme precision, accuracy and repeatability of the
Table 2 Technical data for the UHV reflectometer, controller and acquisition system at LNF-Frascati Double UHV goniometer system Motors:
Rotation range Repeatability Reversal error Resolution UHV Rotary feedthrough Reflectometer arm and slits Remote control
Detection and acquisition
2-phase-microstep endless Contactless limit-reference switches Optical rulers for absolute positioning 360° endless ±0.0002° ±0.0004° ±0.002° open-loop ±0.0002° closed-loop Rare-earth magnets, non-lubricated b100 mm, 0.5 to 2 mm 2-axes microstep controller Manual Joystick/Display unit LabVIEW code for ω– and θ–2θ scans ADC input 12 bit 0–10 V signal Absolute photodiode 100 mm2 windowless UHV preAmp 10 V/nA, linearity ±1%
Fig. 5. Si/Mo multilayer ω-scan at 45° under SR white-beam at LNF-Frascati: narrower Bragg peak due to the Si(111) substrate at circa 2.8 keV, on right the multilayer contribution. Inset: equivalent test by conventional X source.
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Fig. 6. Si/Mo multilayer measurements by monochromatic SR at 1200 eV with LabView interface control of the X-ray reflectometer. Left panel: 2θ-scan of the detector; right panel: ω-scan.
angular movements (better than 3.5 μrad)) are guaranteed without backlash by the readout of absolute encoders for angle feedback. (Table 2). In both open- and closed-loop mode, a dedicated software code controls the angular movements of sample and detector arm during a measure scan in the vacuum chamber. The direct detection of diffracted X-ray beams is accomplished by an absolute X-ray photodiode plugged on a UHV compatible preamplifier. The experimental techniques here considered for the optical characterization of the soft X-ray devices by SR monochromatic or white-beam X-ray reflectometry include [11,12]: i) angle scan of the multilayer by fixed detector (ω-scan) for determining the device reflectivity band-pass (rocking curve); ii) correlated angle scan of both the multilayer and detector (θ–2θ) for assessing the device diffraction figure of merit; iii) energy resolved assessment of the reflectivity at fixed detector and sample angles.
Fig. 7. Ni/Ti multilayer test under white-beam at 45°: the lower peak is the average of the four scans plotted above, background subtracted and normalized to a e- circulating current of 1 A at DAFNE, LNF-Frascati.
We proceeded with the direct measure of the reflectivity of a standard Si/Mo device, a workhorse in the multilayer mirror family. The system under investigation consists in a Si/Mo series of 40 layers, deposited by a Λ = 64 Å onto a Si(111) substrate, and tailored to work at 8° with a band-pass energy spanning from 700 to 850 eV. Even if not optimized for the water window, the test of such multilayer allowed us to assess the experimental setup and acquisition system by a well-known and robust device of known reflectivity in the soft X-ray domain. At first, the multilayer was tested in vacuum by exploiting the SR white-beam and collecting an ω-scan of the mirror around 45° (detector fixed at 90°). In Fig. 5, the signal due to the SR reflection shows both the Si(111) diffraction from the substrate, which accomplishes for the narrower peak corresponding to Bragg diffraction peak around 2800 eV, and the sum of the multilayer harmonic multiple reflections, broader peak at right. Namely, this is an average due to ∼ 50% of the reflection at 550 eV ∼ 33% of the 825 eV contribution, plus ∼16% of the 965 eV beam (calculations based on Fresnel equations and Henke's optical data [13]). An XRD characterization by a Cu 8048 keV source confirms these attributions to the resolved Bragg peaks, and the difference in shape is owing to the different set-up resolution.
Fig. 8. XRD spectrum at 8048 eVof the 300 periods Ni/Ti onto Si substrate. Cu Kα1 emission at 40 kV/30 mA and measure by a Huber θ–2θ diffractometer at LNF.
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Secondly, the Si/Mo mirror was exposed to by monochromatic SR beam at 1200 eV: both ω-scan and 2θ-scan (both under vacuum) are reported in Fig. 6. The former measure is a direct test of the device rocking curve, i.e. its bandwidth in terms of accepted angle at fixed reflected energy. In this case the energy resolution of the monochromator crystals dominates the mutilayer expected resolving power and gives a bandwidth of circa 2.3°/87°. The latter measure is mostly a test of the resolving power of the set-up in terms of angle definition by slits and detector arm length (in practice Δθ ≤ 3.5 mrad). We carried on a test of reflectivity for the first Ni/Ti multilayer deposited at LNL: again, the ω-scan of the device around 45° and detector fixed at 90° was used. This multilayer consisted in 300 bilayers with Λ ≈ 14 Å deposited on a Si(100) wafer as substrate. Designed for the water window, precisely for soft X-rays around ∼ 440 eV in quasi back-scattering geometry, we tested its figure of merit under white-beam, i.e. like a SR first optic element. The bottom reflection signal in Fig. 7 is an average of 4 scans which corresponds to an effective e- current in DAΦNE of 1 A (n.b. Si(100) crystal diffraction prohibited). A simple simulation addresses this signal to mostly a single contribution of the 670 eV X-rays from the SR (calculations performed as before according to [13]). A further characterization of the Ni/Ti multiplayer periodicity was given by X-ray Diffraction technique. Like for XRR method, the period Λ in Angstrom is calculated according to the formula n ⁎ Λ = 1.545 / (2 ⁎ sinθ), where 1.545Å is the Cu Kα emission wavelength, and θ is the grazing angle of the nth Bragg peak. From leftside of Fig. 8 the total reflection shoulder is clearly visible, followed by the series of Bragg diffractions harmonics starting from the principal peak around 3°.
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To test new optics, at INFN Frascati Laboratories a vacuum compatible reflectometer has been set up: direct characterization of the multilayer reflectivity have been performed by SR photons emitted from the wiggler source of DAΦNE. The first experimental measurements were accomplished in 2005 with the direct measurements by means of X-rays of the first opticalgrade multilayers in terms of energy bandwidth (rocking curve) and reflectivity. Two experimental approaches have been followed, i.e. white-beam ω-scans for determining the multilayer rocking curve, and both ω- and 2θ-scans measurement by monochromatic beam at circa 1.2 keV. Experimental data by SR about the characterization of the vacuum reflectometer and the new multilayers are given. The measurements performed on a benchmark Si/Mo device and the preliminary results obtained on both Ni/Ti and Ni/TiO2 devices can be summarized as follows: by comparison of the experiments by SR and conventional X source, the overall reflectivity of the Ni/Ti device is estimated to be between 5 to 9%. RBS, NRA and XRR techniques result in Ni/Ti and Ni/TiO2 multilayer periods of, respectively, 13.86 ± 0.07 Å and 12.78 ± 0.18 Å. New experiments are in due course at LNF for better definition of the nickel titanium device performances as a function of the photon energy and in terms of θ–2θ scans. The natural development of these studies consists in reaching the project reflectivity of ∼30% nearby the anomalous diffraction at the Ni L-edges. In perspective, multiple multilayer assembly onto large substrates could gain in optical acceptance and, eventually shaped according to spherical geometries, will finally refocus the SR beam. Acknowledgments
5. Conclusions The experimental development on high-reflectivity normalincidence optics for soft X-rays is strategically important in multidisciplinary researches from astrophysics to free electron lasers. The scientific importance in Synchrotron Radiation research is of great topical interest, especially for soft X-ray (spectro)microscopy of biological structures in between oxygen and carbon absorption K-edges. This study addresses the demand on novel Ni–Ti based X-ray reflectors for the water window. Even if such material choice has already been tried (but with interleaved carbon layers [14]), these are among the first nickel–titanium devices with over 100 bilayers designed to work at quasi-normal incidence that have been grown and experimentally measured in reflectivity. Despite other studies on oxide multilayer [15], these are, to our knowledge, the first results concerning soft X-ray reflectors made of pure metal layers (Ni) deposited onto different metal oxide (TiO2) and tested under SR. The design and deposition of such devices has progressed at INFN Legnaro Laboratories, after an optimization of the deposition conditions in the new sputtering chamber. Among the best candidates for soft X-ray optics, the first Ni/Ti reflectors have been successfully grown, as well the completely new Ni/ TiO2 multilayers.
A special thanks is due to Claudio Veroli and Giorgio Cappuccio for their courtesy in characterizing the multilayer by XRD, and Antonietta Frani for her software support. We thank Marco Pietropaoli and Giacomo Viviani of the LNF for their technical support. Moreover, a long-term collaboration with A. Soldatov and M. Matzurisky of the Rostov University is acknowledged. This work has been financially supported by the Istituto Nazionale di Fisica Nucleare-Gruppo V in the framework of the ARCHIMEDE experiment. References [1] D. Windt, S. Donguy, J. Seely, B. Kjornrattanawanich, Experimental comparison of extreme-ultraviolet multilayers for solar physics, Appl. Opt. 43 (2004) 1835–1848. [2] D. Berghmans, J.F. Hochedez, J.M. Defise, J.H. Lecat, B. Nicula, V.A. Slemzin, G. Lawrence, A.C. Katsiyannis, R.A.M. Van der Linden, A.N. Zhukov, F. Clette, P. Rochus, E. Mazy, T. Thibert, P. Nicolosi, M.G. Pelizzo, U. Schuehle, SWAP onboard PROBA 2, A new EUV imager for solar monitoring, Adv. Space Res. 38 (2006) 1807–1811. [3] E. Ziegler, Multilayer optics for synchrotron X-ray applications, in: A. Florin (Ed.), Optical Interference Coatings, Proc. SPIE, Int. Soc. Opt. Eng., 2253 (1994) 248–259. [4] K. Yamashita, M. Watanabe, O. Matsudo, J. Yamazaki, I. Hatsukade, T. Ishigami, S. Takahama, K. Tamaura, M. Ohtani, Characterization of
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