MULTILAYER OPTICS FOR SOFT X-RAYS
H. Grimmer1, M. Horisberger1, U. Staub1, H.-Ch. Mertins2, and F. Schäfers2
1 Paul Scherrer Institut, CH-5232
Villigen PSI,
2 BESSY, Lentzeallee 100, D-14195 Berlin
E-mail of presenting author: GRIMMER@PSI.CH
Keywords: Multilayer, mirror,
polarizer, reflectance, soft X-rays, absorption edge
Multilayer structures for the soft X-ray range were designed as normal incidence mirrors (q ~ 85o) or as linear polarizers at the Brewster angle (q ~ 45°) for wavelengths close to the absorption edges of C (284 eV), Ti (454 eV), V (512 eV), Sb (528 eV) or Cr (574 eV). They were produced by DC-magnetron sputter deposition on silicon wafers and consist of alternating layers of Cr/C, W/C, W/Ti, Ni/Ti, Ni/V, W/Sb or W/Cr.
The multilayers were investigated first by small angle X-ray diffraction using Cu K radiation. The roughness of the various interfaces was determined by simulating the measured results using the program SUPREF. The simulation of measurements carried out a few months after production of the multilayers showed that the surface layer was oxidized completely in those cases where it consisted of W, which was confirmed by XPS. A cover layer of 3 nm Al reduces oxidation in air and the consequent reflectivity reduction considerably. The soft X-ray reflectivities of two identic W/Ti samples with and without cover layer differed by 10% sixteen weeks after production.
The reflectance of the mirrors for soft x-rays was measured with the UHV three circle reflectometer at the BESSY beamline PM-4. Special emphasis was put on the behaviour of the reflectance at the absorption edges of the multilayer materials [1]. Fig. 1a) gives the reflectance of a W/Ti multilayer mirror with a 3 nm layer of Al on top. The layer thickness was designed to give high reflectance close to normal incidence for photon energies E below the Ti L edge. Spin-orbit splitting leads to slightly different binding energies LII for 2p1/2 and LIII for 2p3/2 electrons. Below the L edges the reflectance R increases with increasing E due to increasing contrast between the real parts of the refraction indices of W and Ti, above the L edges R drops rapidly due to the increase in the imaginary part of the refraction index of Ti.
Fig. 1: Reflectance at the first Bragg peak as a function of the photon energy E. Notice that the angle at which the Bragg peak appears varies with E such that E sin ( q ) remains constant.
Fig. 1b) shows the reflectance of a W/C multilayer mirror with high reflectance at the Brewster angle for photon energies at the C K-edge. In this case the reason for the double edge structure cannot be spin-orbit splitting: the first reflectance minimum (at 285 eV) is due to a p* resonance, the second (around 300 eV) to s*.
The reflectance of a Ni/V multilayer mirror
designed for high reflectance at the Brewster angle for photon
energies E at the V L edge is shown in Fig. 2. The measured
values are compared with computed ones obtained as follows: The V
layers were taken twice as thick as the Ni layers. The
rms-roughness values were assumed to increase from s0=0.15
nm at the substrate to s100=0.6 nm at the surface with intermediate
values sn
at the n-th interface given by sn2=s02+nsd2.
The computation made use of the atomic scattering factors of Ni
and V given in [2]. The energy resolution of these data is not
sufficient to reproduce the spin-orbit splitting of the binding
energies of 2p electrons.
Fig. 2: The angle at which the Bragg
peak appears varies with the photon energy E such that
Esin()=constant.
The wavelike dependence of the measured reflectance on the photon energy above the V L edge (520 eV < E < 670 eV) can be interpreted as DAFS (Diffraction Anomalous Fine Structure) oscillations. They give information about the local surrounding of the absorbing V atoms and are compatible with the literature values of bulk vanadium for the sample shown in Fig. 2.
A Ni/V mirror consisting of 150 bilayers with
period 1.72 nm and produced in our new sputter chamber has been
investigated at BESSY in April 1998. It showed about four times
higher reflectivity at the V L edge than the mirror of Fig. 2.
Detailed measurements of its DAFS oscillations have been made and
will be analyzed with a view of detecting possible deviations
from bulk structure in the vanadium layers.