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TESTS OF MULTILAYERS BASED SOFT X-RAY MONOCHROMATORS FOR SYNCHROTRON
RADIATION
M. Grioni, F . Schaefers, J. Goedkoop, J. Fuggle, J. Wood, H. van Brug
To cite this version:
M. Grioni, F . Schaefers, J. Goedkoop, J. Fuggle, J. Wood, et al.. TESTS OF MULTILAYERS BASED
SOFT X-RAY MONOCHROMATORS FOR SYNCHROTRON RADIATION. Journal de Physique
Colloques, 1987, 48 (C9), pp.C9-91-C9-94. �10.1051/jphyscol:1987913�. �jpa-00227239�
Colloque C9, supplement au n012, Tome 48, dbcembre 1987
TESTS OF MULTILAYERS BASED SOFT X-RAY MONOCHROMATORS FOR SYNCHROTRON RADIATION
M
.
GRIONI' , * *,
F.
SCHAEFERS" * *,
J.
B.
GOEDKOOP*,
J. c .
FUGGLE*,
J.L. WOOD**** and H. van BRUG'
" R I M , University of Nijmegen, NL-6525 ED Nijmegen.
The Netherlands
"'LURE, Universite de Paris-Sud, Bat. 2090, F-91405 Orsay Cedex, France
8 ' .
Berliner Elektronenenspeicherring-Gesellschaft fiir Synchrotronstrahlung GmbH (BESSY), 0-1000 Berlin 33, F.R.G.
' * * * Ovonic Synthetic Materials, Troy, MI 48084, U.S.A.
'FOM, Institute for Plasma Physics, NL-3430 Nieuwegein, The Netherlands
ABSTRACT
We used multilayer/K~~ and multilayer/multilayer combinations in a double-crystal monochromator at the BESSY storage ring. Throughput and resolution of the ML/KAP combination compare favourably with known sources for spectroscopic applications in the 600-800 eV energy range. The ML/ML high throughput appears suited for trace-element analysis and fluorescence or Auger experiments that do not require a highly monocromatic light. These results suggest the use of ML/ML prefilters in high-resolution, multi-element soft X-ray monochromators.
INTRODUCTION
The philosophy of our project on use of multilayers in monochromators has been discussed elsewhere /1,2/. Briefly, one of the outstanding problems posed by present synchrotron radiation sources is the development of monochromators that cover with sufficient resolution the energy range between the C K edge and 800 ev.
High-energy grating instruments working at very grazing incidence, /3,4,5/ are necessarily quite expensive due to the use of large aspherical optical elements, and the resolution achieved in practice depends critically on the quality and on a very careful tuning of these elements. Crystal monochromators benefit from a conceptually simple design and from the fact that crystals of very good quality can be obtained routinely. The use of crystal monochromators is limited however in the low energy region by the distance between crystalline planes (dl according to Bragg's relation A = 26 sine, so that the lowest energy pratically achievable is -.
800 ev with beryl crystal (2d = 15,45 A ) . In principle organic crystals (acid phthalates) with large 2d-spacing could be used, but these organic crystals are readily damaged and eventually destroyed by synchrotron radiation. An alternative or complementary solution is offered by synthetic multilayers /1,2,6,7,8/, man-made one dimensional "crystals" obtained by electron beam or sputter deposition of alternate layers of a high-Z metal, e.g. tungsten, and spacer layers of a low-z-element (e.g. C or Si) with low absorption in the energy range of interest. One can consider two kinds of applications of multilayers in a double crystal monochromator. In the first case a multilayer is used as a wide band pass
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987913
C9-92 JOURNAL
DE
PHYSIQUEfilter and reduces the radiation load on an organic crystal (KAP) that in turn determines the overall resolution. In the second case two identical ML's are used as dispersive elements, monochromators of the latter type hve a poor resolution compared with crystal monochromators, but can reach very low photon energies, for instance around the C K edge. They are suited for experiments where low photon energies and sufficiently high flux are more important than good resolution, as in trace element analysis or some Auger experiments. In our earlier work we investigated the properties of Ni-C and W-Re-C multilayers /1,2/. The aim of the present paper is to present data on W-Si layers and to state our assessment of the developments needed for the improvement of resolution and troughput of multilayer/KAP monochromators.
EXPERIMENTAL
For this experiment we have used the double crystal monochromator beamline at the BESSY storage ring /9/. The angular acceptance of the monochromator is 10 mrad and an ellipsoidal mirror focuses the diffracted beam. The angular excursion of the monochromator is 10-80 degrees. In the first experiment we used a KAP crystal (surface area 70x20 nun2) in the second crystal position and in the first position a 200-layer W/Si multilayers with 2d = 53.2 A /lo/ matched in second order to KAP, as was done in previous experiments /I/. A disadvantage of this choice is the loss in reflectivity compared to the first order peak. In the second experiment we fitted the monochromator with 2 identical W/Si multilayers with a 2d spacing of 61.2 A, matched to better than 0.1 A. This large interplanar spacing corresponds to an energy range 200-800 eV. The surface area of the multilayers was again 70 x 20 nun2 and the number of double layers 200. Fine tuning of the parallelism between the two elements was possible in situ thanks to flexible joints and piezoeletric actuators. The experiment reported here were conducted with BESSY operating in single bunch mode : an increase of 10 to 20 times in the measured flux and a somewhat better resolution is expected for a normal mode of operation. Photon flux measurements were performed with a calibrated InGaP photodiode that could be positioned to accept the whole reflected and focused beam. Absorption spectra of clean samples were collected in the total electron yields mode in a standard UHV system operating at a base pressure better than 1 x lo-' torr.
Eiaure : ML/KAP photon flux A 0.8 pm thick carbon foil was used to filter the incoming beam.
a) ML/KAP
-
Fig. 1 shows the flux of the ML/KAP combination, measured after a 0.8 ym carbon foil. The peak in the photon flux and the subsequent decrease reflect the spectral output of the 800 MeV storage ring, while the broad dip centered around 1100 eV results from the photodiode response around the GaA f t e r c o r r e c t i n g f o r a b s o r p t i o n i n t h e C f o i l and f o r t h e r e f l e c t i v i t y o f t h e g o l d - c o a t e d m i r r o r w e o b t a i n 6.75 x
l o 7
photons/sx100mA. T h i s v a l u e h a s t o b e compared w i t h t h e incoming f l u x , 8 x 1012 photons/sxlOOmA x 0 . 1 % bandwith. The t h e o r e t i c a l peak r e f l e c t i v i t y of KAP a t t h i s e n e r g y i s 4.3 % /11/. When bandpass i s t a k e n i n t o account ( s e e below) t h i s y i e l d s a second o r d e r peak r e f l e c t i v i t y of 2 x f o r t h e w/Si m u l t i l a y e r . T h i s i s h i g h e r t h a n p r e v i o u s l y o b t a i n e d with W-Re C m u l t i l a y e r s /1,2/ b u t i s s t i l l low. I t c o u l d be s i g n i f i c a n t l y improved i f t h e m u l t i l a y e r s were o p t i m i z e d f o r second o r d e r r e f l e c t i o n . The photon f l u x a t t h e sample was a d e q u a t e f o r t o t a l y i e l d X-ray a b s o r p t i o n e x p e r i m e n t s , even w i t h t h e reduced single-bunch c u r r e n t s .F o r u s t h e most a t t r a c t i v e measure of r e s o l u t i o n of t h e multilayer/KAP monochromator i s t h e r e s o l u t i o n observed i n XAS s p e c t r a . The Fe L3 edge ( F i g . 2) i s n o t a s i m p l e s t e p f u n c t i o n / l 3 / and t h e L3 b r o a d e n i n g i n XPS i s 0.8 e v /12/.
Comparison of d a t a such a s t h a t i n F i g . 2 with d a t a from Fink e t a l . /13/ suggest t h a t t h e r e s o l u t i o n of ML/KAP monochromator i s about 900 meV FWHM a t 710 e V , i . e . E / A E = 780. The r e s o l u t i o n a t t h e M5 peak o f La (E = 834 eV, n o t shown) was e s t i m a t e d t o be 850 meV. The l i m i t i n g r e s o l u t i o n of a ML/KAE monochromator i s s e t by t h e r o c k i n g c u r v e of KAP and s h o u l d b e 640 meV a t 700 eV. W h i l s t w e emphasize t h a t more work i s needed t o s y s t e m a t i z e e s t i m a t i o n of monochromator r e s o l u t i o n i n t h i s e n e r g y r e g i o n , t h e r e s o l u t i o n we a c h i e v e d was p o o r e r t h a n t h e t h e o r e t i c a l l i m i t of KAP. This is t e n t a t i v e l y a t t r i b u t e d t o d i v e r g e n c e of t h e s y n c h r o t r o n beam and t h e mosaic s p r e a d i n t h e KnP c r y s t a l .
l5&uue2 2 ML/KAP, Fe L3 a b s o r p t i o n edge from a c l e a n i r o n sample.
r
I I I I1
Fiaure 3 FeL2,3 a b s o r p t i o n edges from a Fe mesh. ML/KAP and ML/ML r e s o l u t i o n a r e compared.
C9-94 JOURNAL DE PHYSIQUE
b) U M L comblnatlon
. . -
we can estimate the ML/ML resolution at 700 eV by comparison with the ML/KAP result. The measured AWHM of the Fe L3 edge is 4.0 eV.This yields a total'instrumental broadening that lies between the limits 6.3 ev (E/AE = 112) and 7.8 eV (E/AE = 90) obtained by summing linearly and respectively quadratically all the different. contributions and which includes the contribution from beam divergence (1.9 eV). Because of the shallower angles and of the use of first order reflections the ML/ML throughput is much larger than for the ML/*
combination, down to 200 eV. The measured ML peak reflectivity at the Fe L3 edge is 5 %. The theoretical value is
-
20 % /11/, and the discrepancy is mainly due to roughness and layer thickness fluctuations throughout the multilayer, he normalized flux of 2.5 x 10 photon/sxlOOmAxeV (corrected for transmission of the A1 filter) is large compared, for example, with the value of 3 x lo8 ph/sxlOOrnA x 0.35 ev of beryl crystals at 800 eV at the same monochromator /9/.-
We have reported here on the use of W-Si multilayers in ML/ML and ML/KAP monochromators for synchrotron radiation. The performance was improved with respect to that previsoulsy found for metal-carbon multilayers /1,2/, but this may not be an indication of the ultimate merit of metal C and metal Si multilayers fully optimized for this work. The ML/KAP monochromator resolution is already nearly comparable with current grating monochromators in the range 600-800 eV. We consider optimization of the multilayers for second order reflection to be the first taks in further improvements. After this the influence of beam divergence on resolution can be cut down by narrowing the angular acceptance of the monochromator. Alternative approaches to improve resolution are the use of multilayers on curved substrate3 to collimate the beam in the dispersive plane and two-stage monochromators, the first stage based on a multilayer pair, and the second stage equipped with 2 organic crystals. Our ML/ML results indicate for the latter case a flux of 4.5 x lo7 ph/sxlOOmA x 1 eV at 700 eV with elements at present commercially available ; the limiting resolution would be-
450 meV.Further work is required to quantify estimates of resolution from measured spectra in this energy region, and has been started.
-
We wish to thank G. Van der Laan for useful discussions. The expert assistance of the BESSY staff is also kindly acknowledged. This work was supported by a grant of the European Cormnunity Stimulation Program (CODEST).REFERENCES
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