A secondary X-ray analyser using a flat ring-shaped radially graded multilayer
Introduction
The rapid progress in artificial magnetic nanostructures, which revealed novel magnetic phenomena such as Perpendicular Magnetic Anisotropy (PMA) and Giant Magneto-Resistance (GMR), has stimulated studies of structure and magnetic features in thin metastable films and superlattices. Among a number of various experimental techniques for thin film studies, the X-ray absorption fine structure (XAFS) spectroscopy proved to be a very powerful tool for obtaining information on local structure (by EXAFS), electronic (by XAS) and magnetic (by XMCD) properties of atoms under study.
It is well known that the energy-selective detection of the X-ray fluorescence signal associated with excitation of the inner shell of diluted atoms is the most sensitive technique to obtain their local structure by means of XAFS [1]. Application of grazing-incidence (GI) arrangement of the sample with respect to the incident beam, as can be seen in Fig. 1a, is a common approach to achieving near-surface sensitivity of probing X-rays [2], [3]. In theory, the GI geometry exploits the total external reflection (TER) effect and limits the beam penetration down to a depth of few nanometers. That greatly enhances surface sensitivity, eliminates the background from bulk scattering and increases the signal-to-background ratio to a satisfactory level.
However, an experimenter wishing to obtain fluorescence XAFS data in the GI geometry faces several problems. To acquire complete TER conditions the flatness and surface roughness parameters of the sample as well as the grazing angle deviations must satisfy very stringent conditions. In a routine GI experiment with real samples, the beam penetration depth is about 10–100 times more than the theoretical value. In the case of non- or poly-crystalline samples, that leads to a rather tolerable reduction of the signal-to-background ratio, the situation is much more complicated for single-crystal films or thin films grown on single-crystal substrates; since the penetration depth of the exciting beam becomes comparable with the extinction length of diffraction, the fluorescence XAFS spectra of these samples are generally contaminated by casual sharp reflections. Though it is often possible to remove the contribution of undesired reflections by spinning the sample around its normal axis, this trick cannot be used universally because of the inherent angular rigidity of the GI geometry. Moreover, some of the reflections can be removed (or spread out) from the XAFS spectrum only by additional rotation about an axis transverse to the normal one.
Considering the goal of flexibility in magnetic and structural studies of thin films, we adopted a reversed experimental geometry. In this arrangement, see Fig. 1b, the primary beam strikes the sample at a nearly normal-incidence angle while the fluorescence signal is detected at a small grazing-exit angle. In this case, the fluorescence emitted from the topmost layer of the sample is concentrated in a rather narrow cone near the critical angle ϕc. The GE fluorescence geometry restricts the analysed depth of the sample down to a few tens of nanometers [3]. An obvious advantage of this arrangement is that the sample can be easily set at the proper angle of incidence to avoid undesired Bragg reflections. Another attractive feature is the freedom in choosing the part of the sample to be probed. Furthermore, the GE fluorescence geometry is ideally suited for the PMA measurements of magnetic films by means of XMCD spectroscopy.
Till now there has been little attention to this geometry as an attractive technique for thin-film studies. The GE conditions were used for obtaining the fluorescence XAFS spectra of concentrated samples with negligible self-absorption distortions [4], [5]. In the case of X-ray fluorescence analysis, the GE fluorescence geometry has been applied to study thin films with a high spatial resolution [6].
To ensure the success of the GE fluorescence XAFS studies, the detector has to cover most of the grazing emergent fluorescence. A good energy resolution is necessary to extract a weak fluorescence signal from intense scattering and background fluorescence signals. A high counting-rate capability is required to measure the XAFS spectrum within a reasonable period, which is particularly necessary for XMCD spectroscopy. No such detector has been built yet, as its construction poses a significant challenge.
In this paper, we describe the conceptual design and characteristics of the novel X-ray fluorescence analyser with the flat radially graded multilayer serving as a tuneable monochromator. Its main purpose is to detect grazing-emergent fluorescence in thin-film experiments. We will show that good energy resolution, large acceptance angle, and easy tunability over a wide energy range can be achieved using a single-piece multilayer mirror. To verify the principle and potentialities of the analyser we have produced a laterally graded W/Si multilayer with linearly increasing d-spacing. A well-resolved K fluorescence series of thin copper film excited by a monochromatic beam was measured using the multilayer.
Section snippets
Concept of analyser
There were several key motivations that we used to design our analyser. Due to the axial symmetry of the GE experiment a monochromator with ring-like geometry should be designed. The symmetric design increases simplicity and reduces cost. In order to isolate the fluorescence signal from scattered radiation in fluorescence XAFS experiments, it is desirable to obtain a relative energy resolution about 2–4%. The energy scanning capability is another important aspect of the secondary monochromator.
Experimental
To demonstrate the potential of the modern multilayer technology we produced a laterally graded W/Si multilayer with linearly increasing d-spacing. The multilayer was deposited by magnetron sputtering on a flat silica substrate with a width of 2 cm and a length of 8 cm by magnetron sputtering. To achieve the needed gradient the substrate was moved with the computer-controlled velocity over the W- and Si-targets. The deposition regime was chosen so as to keep the thickness of the W layers (∼1.0 nm)
Discussion
Although the design of the GRAD analyser is dedicated to the grazing-exit fluorescence experiments, it is of interest to make comparison with other XAFS detectors of secondary fluorescence.
Stern/Heald detector is the simplest fluorescence detector commonly used in the XAFS technique [10]. It consists of an ionisation chamber combined with Soller collimator and an appropriate rejection filter. The filter reduces the signal of scattered photons much more than the fluorescence signal while
Conclusion
The potential of the graded multilayers is very promising for energy-resolved fluorescence experiments under grazing-exit conditions. It permits one to construct a new tuneable secondary monochromator for surface and thin-film structural measurements using the XAFS technique. In this work we obtained the first encouraging results on the energy resolution of the graded W/Si multilayer. The final goal of the GRAD project is to create a highly efficient energy-resolving analyser of secondary X-ray
Acknowledgements
We thank S.K. Kim for discussion of features of measurements of thin magnetic films. This work was partially sponsored by the Russian Foundation for Basic Research under grants 99-02-16671 and 00-02-17624.
References (12)
- et al.
Solid State Commun.
(1977) - et al.
Nucl. Instr. and Meth. A
(1986) - et al.
Nucl. Instr. and Meth. A
(1988) Phys. Rev.
(1954)- et al.
Phys. Rev. Lett.
(1983) - et al.
Phys. Rev. B
(1992)
Cited by (1)
X-ray spectrometry
2002, Analytical Chemistry