Recent improvements in soft x-ray multilayer diffraction gratings

U.  Kleineberg, K. Osterried, H.-J. Stock, P. Muller#, D. Fuchs#, 
K.F. Heidemann* and U. Heinzmann

Universitaet   Bielefeld,   Fakultaet   fuer   Physik,   D-33615 
Bielefeld; *Carl Zeiss, D-73446 Oberkochen; #PTB, D-10587 Berlin

In recent years the development of multilayer gratings is of increasing
interest for soft x-ray optical monochromator and spectrometer applications.
The multilayer coating of phase gratings provides diffraction efficiencies
which make the operation of gratings at (near) normal incidence angles
reasonable.  Any deviation, however, of the groove profile from ideal shape
will redistribute the efficiency within the diffraction orders, while
irregularities like ruling errors and microroughness will increase straylight
between the regular diffraction peaks and reduce multilayer reflectivity,
respectively.  The goal of our work is to improve multilayer diffraction
gratings towards higher diffraction efficiencies, lower stray- light and high
ruling density.  Mo/Si multilayer coatings were deposited by electron beam
evapo- ration at UHV conditions onto a ruled blaze grating (1221 L/mm) and a
holographically recorded sinusoidal grating (6600 L/mm).  The surface
topography of the gratings have been studied by Atomic Force Microscopy (AFM)
before and after the multilayer coating procedure.  Besides for groove
geometry characterization these measurements were evaluated with respect to
surface mi- croroughness and groove efficiency.  The monochromatic diffraction
efficiency has been measured by the use of Synchrotron Radiation from the
Berlin electron stora- ge ring BESSY.  For the multilayer blaze grating coated
with 20 Mo/Si bilayers of 10.5 nm period thickness a very high +1st order
diffraction efficiency of 32% (with respect to the incoming intensity) at 13.8
nm wavelength has been measured, which can be explained by the perfect
matching of the multilayer period thickness (10.5 nm) and the blaze angle
(0.80) as well as a low facet surface roughness due to a refined groove
forming technique, which has been measured by AFM to be 0.5 nm (r.m.s.).  In
the case of the multilayer sinusoidal grating coated with 18 Mo/Si bilayers of
8.5 nm thickness we have measured 11% absolute diffraction efficiency at 13.1
nm wavelength in (both) first diffraction orders.  Due to the high ruling
density, which is near the limit of UV lithography, this grating has the
capabili- ty for high resolving power even at (near) normal incidence.

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B. Nelles, U. Heinzmann; Applied Optics 34 (28), 6506 (1995)
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Menke,  B.  Schmiedeskamp,  U. Heinzmann, D. Fuchs, P. Mller, F. 
Scholze,  G.  Ulm, K. F. Heidemann, B. Nelles; J. Electr. Spectr. 
Rel. Phenom. (in press)