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Growth and Nano-structural Studies of Metallic Multilayer for X-ray Mirrors
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2005 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

A part of the Ph.D. project focused on growth and characterization of metal multilayers is presented in this licentiate thesis. The main interest in carrying out this research is to develop highly reflective normal-incidence condenser mirrors for soft X-ray microscopy studies in the water window (λ=2.4-4.2 nm) wavelength regime.

Transition metals like Sc, Ti V, etc. have been considered because of the presence of their 2p-absorption edges within the water window. An anomalous dispersion at absorption edges has been utilized to get enhanced reflectance of soft X-rays. Since a single surface exhibits a very poor X-ray reflectivity, Cr/Sc, Cr/Ti, and Ni/V multilayers were grown in order to coherently add many reflections from several interfaces. The selection of Cr and Ni, as spacer layer, was made on the basis of their X-ray optical contrasts with the above-mentioned transition metals. The multilayer design, i.e., the individual layer thicknesses and the total number of bilayers, directly influences the resultant reflectance and careful determination was therefore made with the aid of computer simulations.

All multilayers were grown on chemically cleaned Si substrates by ion-assisted dual target magnetron sputtering under high vacuum (~10-7 Torr) conditions. The effect of low and high ion-flux bombardment of low energy (<50 eV) Ar ions, on growing surfaces was studied for all material systems. Furthermore, a two-stage deposition of each individual layer with modulated ion-energies was applied in order to obtain smooth and abrupt interfaces with as small intermixing as possible. Ion-surface interactions were also theoretically considered for estimating an appropriate ion-flux and ion-energy range desired for sufficient ad-atom mobilities.

X-ray reflectivity and transmission electron microscopy have been the main probes for multilayer characterization in this work. For the Cr/Ti multilayer designed for normal incidence and grown with optimized two-stage ion-energy modulation, a peak reflectance of 2.1% was achieved at the Ti-2p absorption edge (λ=2.74 nm). For a multilayer mirror designed for the Brewster angle a maximum reflectance of 4.3% was accomplished. These measurements were made at the synchrotron radiation source BESSY in Berlin. Specular reflectivity and diffuse scattering scans were utilized for quantitative and qualitative analysis of the vertical and lateral structure of the multilayers. At-wavelength measurements of a series of Cr/Ti multilayers revealed the accumulation of roughness with increasing number of bilayers (N>100) for this material system. Hard X-ray reflectivity and diffractometry were used for quality checks of the multilayers for rapid feedback to the deposition. In-situ annealing using hard X-ray reflectivity was also performed to assess the thermal stability of Cr/Ti multilayers. It was found that probably due to a strong thermal diffusion the degradation of multilayers (with bilayer period of 1.37 nm) in this material system occurs just above the growth temperature (~40°C). The accumulation of a low spatial frequency "waviness" with increasing number of layers in Cr/Ti multilayers was investigated by transmission electron microscopy. The influence of process conditions on multilayer structure with different periodicities was investigated by TEM analyses of a series of three samples for each of the above-mentioned material system. The Cr/Sc multilayers have shown the most flat and abrupt interface structure without any significant roughness evolution when grown with optimum process parameters.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2005. , 69 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1193
Keyword [en]
Soft X-rays, Multilayer Mirrors, X-ray microscope
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-11381ISBN: 91-85457-23-X (print)OAI: oai:DiVA.org:liu-11381DiVA: diva2:17787
Presentation
2005-10-28, Plank, Physics House, Linköping University, Linköping, 10:15 (English)
Opponent
Supervisors
Note
Report code: LiU-TEK-LIC-2005:48. On the day of the defence data the status of article I was: Accepted.Available from: 2008-03-26 Created: 2008-03-26 Last updated: 2009-05-15
List of papers
1. Interface engineered ultra-short period Cr/Ti multilayers as high reflectance mirrors and polarizers for soft X-rays of lambda=2.74 nm wavelength
Open this publication in new window or tab >>Interface engineered ultra-short period Cr/Ti multilayers as high reflectance mirrors and polarizers for soft X-rays of lambda=2.74 nm wavelength
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2006 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 45, no 1, 137-143 p.Article in journal (Refereed) Published
Abstract [en]

Cr-Ti multilayers with ultrashort periods of 1.39-2.04 nm have been grown for the first time as highly reflective, soft-x-ray multilayer, near-normal incidence mirrors for transition radiation and Cherenkov radiation x-ray sources based on the Ti-2p absorption edge at E = 452eV (lambda = 2.74 nm). Hard, as well as soft, x-ay reflectivity and transmission electron microscopy were used to characterize the nanostructure of the mirrors. To achieve minimal accumulated roughness, improved interface flatness, and to avoid intermixing at the interfaces, each individual layer was engineered by use of a two-stage ion assistance process during magnetron sputter deposition: The first 0.3 nm of each Ti and Cr layer was grown without ion assistance, and the remaining 0.39-0.72 nm of the layers were grown with high ion-neutral flux ratios Phi˙(PhiTi = 3.3, PhiCr = 2.2) and a low energy Eion (ETi = 23.7 and ECr = 21.2), ion assistance. A maximum soft-x-ray reflectivity of R = 2.1% at near-normal incidence (~78.8°) was achieved for a multilayer mirror containing 100 bilayers with a modulation period of 1.379 nm and a layer thickness ratio of Gamma = 0.5. For a polarizing multilayer mirror with 150 bilayers designed for operation at the Brewster angle, 45°, an extinction ratio, Rs/Rp, of 266 was achieved with an absolute reflectivity of R = 4.3%.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13066 (URN)10.1364/AO.45.000137 (DOI)
Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2017-12-13
2. Effects of ion-assisted growth on the layer definition in Cr/Sc multilayers
Open this publication in new window or tab >>Effects of ion-assisted growth on the layer definition in Cr/Sc multilayers
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2008 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 516, no 6, 982-990 p.Article in journal (Refereed) Published
Abstract [en]

Nano-structural evolution of layer morphology and interfacial roughness in Cr/Sc metal multilayers grown with ion assistance during magnetron sputter deposition has been investigated by high resolution transmission electron microscopy and hard X-ray reflectivity. Calculations based on a binary collision model predict an ion-assisted growth window for optimized Cr/Sc multilayer interface sharpness, within the ion energy range of 21 eV to 37 eV and an ion flux of 10 ions per deposited atom. Multilayers with nominal modulation periods in the range of 1.6 nm to 10.2 nm, grown with these conditions, exhibit a well-defined layer structure with an improved flattening and abruptness of the interfaces. It is shown that multilayers with a modulation period smaller than 3.4 nm have clear benefit from the reduced intermixing obtained by utilizing a two-stage ion energy modulation for each individual layer. The amorphization of Sc and Cr layers, below certain thicknesses, is found to be independent of the low energy ion-assistance. It is also shown that the Cr/Sc multilayers, containing periods less than 2 nm are ‘self healing’ i.e. they re-gain abrupt interfaces and flat layers after morphological disturbances during ion assisted growth. In comparison, multilayers grown without ion-assistance exhibited severe roughness and layer distortions.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2008
Keyword
Multilayers, X-ray mirrors, Ion assistance, Magnetron sputtering, Cr/Sc multilayers, Interface engineering, Optical coatings, Surface roughness
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-11487 (URN)10.1016/j.tsf.2007.06.108 (DOI)
Note
Original publication: N. Ghafoor, F. Eriksson, P.O.Å. Persson, L. Hultman and J. Birch, Effects of ion-assisted growth on the layer definition in Cr/Sc multilayers, 2008, Thin Solid Films, (516), 6, 982-990. http://dx.doi.org/10.1016/j.tsf.2007.06.108. Copyright: Elsevier B.V., http://www.elsevier.com/Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2017-12-13
3. Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics
Open this publication in new window or tab >>Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics
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2008 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 47, no 23, 4196-4204 p.Article in journal (Refereed) Published
Abstract [en]

Cr/Sc and Ni/V multilayers, intended as normal incidence soft x-ray mirrors and Brewster angle polarizers, have been synthesized by employing a novel modulated low-energy and high-flux ion assistance as a means of engineering the interfaces between the subnanometer layers on an atomic scale during magnetron sputter deposition. To reduce both roughness and intermixing, the ion energy was modulated within each layer. The flat and abrupt interfaces yielded soft x-ray mirrors with near-normal incidence reflectances of R = 20.7% at the Sc 2p absorption edge and R = 2.7% at the V 2p absorption edge. Multilayers optimized for the Brewster angle showed a reflectance of R = 26.7% and an extinction ratio of Rs/Rp=5450 for Cr/Sc and R = 10% and Rs/Rp=4190 for Ni/V. Transmission electron microscopy investigations showed an amorphous Cr/Sc structure with an accumulating high spatial frequency roughness. For Ni/V the initial growth mode is amorphous and then turns crystalline after ~1/3 of the total thickness, with an accumulating low spatial frequency roughness as a consequence. Elastic recoil detection analyses showed that N was the major impurity in both Cr/Sc and Ni/V with concentrations of 15 at. % and 9 at. %, respectively, but also O (3 at. % and 1.3 at. %) and C (0.5 at. % and 1.9 at. %) were present. Simulations of the possible normal incidence reflective properties in the soft x-ray range of 100-600 eV are given, predicting that reflectivities of more than 31% for Cr/Sc and 5.8% for Ni/V can be achieved if better control of the impurities and the deposition process is employed. The simulations also show that Cr/Sc is a good candidate for mirrors for the photon energies between the absorption edges of B (E = 188 eV) and Sc (E = 398.8 eV).

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13060 (URN)10.1364/AO.47.004196 (DOI)
Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2017-12-13

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