Document 6539263

X-RAY SPECTROMETRY
X-Ray Spectrom. 2004; 33: 281–284
Published online 29 January 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/xrs.722
Characterization of x-rays emerging from between
reflector and sample carrier in reflector-assisted TXRF
analysis†
Kouichi Tsuji∗ and Filip Delalieux
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
Received 14 October 2002; Accepted 10 July 2003
The possible application of an Si reflector, which is placed just above the sample carrier in total reflection
x-ray fluorescence (TXRF) analysis, was investigated. The x-rays that were emitted from an Mo tube and
passed between the Si reflector and the Si sample carrier were analyzed with an Si drift detector. In our
experimental setup, the angle between the reflector and the sample carrier can be changed by adjusting
the inclination of the reflector. The intensity of the x-rays that emerged from between the two Si surfaces
drastically changed depending on the reflector angle. At a proper reflector angle, this intensity showed
a maximum and, in addition, the Compton peak in the x-ray spectrum was suppressed. When this x-ray
beam was used for excitation of TXRF signals, the highest intensity of x-ray fluorescence emitted from
the sample was detected, indicating that these experimental conditions are useful for the enhancement of
TXRF intensities. Copyright  2004 John Wiley & Sons, Ltd.
INTRODUCTION
Total reflection x-ray fluorescence (TXRF) is a powerful
technique for trace analysis on flat samples such as
Si wafers.1,2 TXRF instrumentation has been improved
considerably since its first introduction in the 1970s. To
improve detection limits even further, it is important
to increase the TXRF intensity while maintaining a low
background.
In the field of micro x-ray fluorescence (µ-XRF), capillary
optics such as monocapillaries and polycapillaries have been
developed to obtain micro x-ray beams.3 X-rays are totally
reflected on the inner surface of each capillary, and are thus
focused to a small area. We considered that a similar idea
could be applicable to TXRF analysis in order to obtain a
primary x-ray beam with a higher intensity. In the case of
TXRF analysis, the cross-section of the primary beam should
be a line. Therefore, we have proposed the use of a flat
reflector, which is placed above the sample carrier in TXRF
analysis.4,5 It is to be expected that the primary x-rays will
be multiply reflected between the reflector and the sample
carrier, resulting in an intense ribbon-shaped x-ray beam.
Cheburkin and Shotyk proposed a simple TXRF setup
that consisted of two flat plates (short reflector and longer
sample carrier).6 The reflector was placed on a special spacer
50 µm, fixing two plates parallel to each other. When the
distance between the two plates is very small, i.e. less than a
few hundred nanometers, coherent propagation of x-rays is
Ł Correspondence to: Kouichi Tsuji, Department of Applied
Chemistry, Graduate School of Engineering, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585, Japan.
E-mail: [email protected]
† Presented at the European Conference on EDXRS, Berlin,
Germany, 16–21 June 2002.
Contract/grant sponsor: Japan Society for the Promotion of Science.
produced in this waveguide.7 Egorov and Egorov studied the
fundamentals of waveguide resonators and their analytical
applications.8,9 However, the experimental condition of the
distance between two plates of several tens of micrometers is
still interesting from the point of view of actual application
to x-ray analysis. Sanchez called this device an ‘x-ray beam
guide’ and applied it to TXRF and grazing emission XRF.10
In our experimental setup, which has been reported on
before,5 the angle between the reflector and the sample
carrier can be adjusted. Therefore, this device could be called
a ‘tapered waveguide’ or ‘tapered beamguide’. It is important
to know the energy distribution of the x-rays that emerge
from the beamguide in order to evaluate their suitability as
an XRF excitation source. Therefore, in this paper, we report
analytical results for x-rays that emerge from between the
two Si surfaces by using an Si drift detector (SDD), which is
placed parallel to the sample carrier.
EXPERIMENTAL
The experimental setup is shown in Fig. 1. An Si wafer,
which was cut to a size of 30 ð 80 mm and pasted on a Cu
holder, was used as a reflector.5 One end of the reflector
rests on a sample holder, while the other end is attached
to a vertical linear positioning stage using two bars. The
angle between the Si reflector and the Si sample carrier is
adjusted by changing the inclination of the Si reflector. An
Al foil spacer was previously used so that primary x-rays
irradiated the sample with strong intensity. In this work,
this spacer was not used to focus the primary x-rays on
the exact sample position. The TXRF signal of the sample,
placed on the Si sample carrier, is measured with a pure
Si energy-dispersive x-ray (EDX) detector. In addition, the
primary x-rays, which pass between the reflector and the
sample carrier, are analyzed with an SDD. SDDs are a new
Copyright  2004 John Wiley & Sons, Ltd.
282
K. Tsuji and F. Delalieux
Figure 2. Mo K˛ intensities as a function of the tilting angle of
the sample (corresponding to the x-ray incident angle). The
measurements were performed at different reflector angles
(0.076 and 0.051° ).
Figure 1. Experimental setup for reflector-assisted TXRF
analysis. The x-rays that emerge from between the reflector
and the sample carrier are measured with an SDD.
and promising type of x-ray detector.11,12 One of the unique
advantages of SDDs is that they can be operated at very high
count rates of more than 106 counts cm2 s1 . Therefore, this
detector is suitable for the analysis of the strong primary
X-rays in our study. A circular sensitive area of 5 mm2 is also
sufficient for the measurement of the primary x-ray beam,
because the width of the ribbon-shaped x-ray beam is several
tens of micrometers. The energy resolution of the SDDs has
been improved to be <150 eV at 5.9 keV, which is sufficient
for the analysis of the primary x-ray spectra. In addition,
the SDD is a compact detector because it offers excellent
spectroscopic performance at 15 ° C, avoiding the use of
liquid nitrogen; therefore, it is easy to include in an already
existing experimental setup.
A rotating Mo anode tube was operated at an acceleration
voltage of 30 kV and at a tube current of 90 mA. X-rays
emitted from the Mo anode were monochromatized by a
W/C multilayered monochromator. The incident angle of
the primary x-rays was changed by tilting the sample, which
was fixed on a large goniometer. A detailed description of the
goniometer performance has been given elsewhere.13 After
truncating the x-ray beam with a slit, a beam with a width of
¾0.3 mm was obtained to irradiate the sample. An Si wafer
20 ð 70 mm2 was used as a sample carrier. The samples
analyzed consisted of thin layers, which were deposited on
the Si sample carrier by vacuum evaporation.
RESULTS AND DISCUSSION
First, Mo K˛ intensities were measured by the SDD as a
function of the incident angle of the primary x-rays.
During the measurements, the reflector angle was fixed
at 0.076° and 0.051° . As can be seen from Fig. 2, the curve of
the Mo K˛ intensities shows a maximum. The tilting angle
Copyright  2004 John Wiley & Sons, Ltd.
is just the reading from a goniometer in this figure.
This result indicates that the transmission efficiency between
the reflector and the sample carrier is highest at this angle.
Therefore, the following experiments were performed at this
incident angle.
Figure 3(a) and (b) show the spectra obtained for the xrays that emerged from between the reflector and the sample
carrier. It is found that the Mo K˛ x-ray intensity depends
on the reflector angle . The Mo K˛ intensity was plotted
as a function of (Fig. 4). The Mo K˛ intensity curve has
two maxima at approximately 0.75 and 0.02° . As discussed
in a previous paper,5 at the reflector angle that corresponds
to the first peak, part of the x-rays are reflected on the
reflector surface and are focused towards the slit between
the two plates, which leads to the high Mo K˛ intensities.
The second peak in Fig. 4 is expected to be caused by more
complicated processes. One of the most likely processes is
multiple reflections between the reflector and the sample
carrier. Usually, once the x-ray beam is totally reflected on
the sample carrier, the x-rays proceed away from the sample
carrier. However, the application of a reflector allows these xrays to propagate by totally reflecting between the two plates
and to concentrate on the actual sample position. Since the
angle is very small (about 0.02° ), this geometry would be
close to the waveguide geometry.
In addition to the Mo K˛ intensity, attention should be
paid to the shape of the x-ray spectra in Fig. 3(a) and (b).
It is clear that for smaller reflector angles , the Compton
peak shifts to higher energies and its intensity decreases.
Eventually, it becomes difficult to recognize the Compton
peak near 0.02° . The Compton peak occurs as a result of
inelastic scattering. The energy shift relative to the original
(Mo K˛) spectrum depends on the scattering angle; large
energy differences occur at larger scattering angles. The
decrease in the Compton peak intensity indicates that the xray beam propagates by forward scattering, including total
reflection.
A monochromatic x-ray beam without Compton radiation is very suitable as an excitation source for application to
TXRF analysis. Therefore, TXRF analyses were performed at
X-Ray Spectrom. 2004; 33: 281–284
Characterization of x-rays in reflector-assisted TXRF
Figure 5. Intensities of Au L˛ x-rays, which were emitted from
an Au thin film on an Si sample carrier and which were
detected with an EDX detector in the TXRF configuration (i.e.
mounted perpendicular to the sample surface), as a function of
the reflector angle .
TXRF intensities show a clear angle dependence, which compare well with the results obtained with the SDD. At angles
larger than 0.05° , the Au L˛ x-ray intensities do not depend
on the reflector angle. As can be seen from Fig. 5, x-ray intensities decrease by around 0.03° , which corresponds to the
angle dependence of Mo K˛ in Fig. 4. The Au L˛ intensity
drastically increases at about 0.02° , where the Compton peak
has almost disappeared and the Mo K˛ intensity increases
again, as shown in Fig. 3(b).
CONCLUSIONS
Figure 3. Spectra (measured with an SDD) of the x-rays that
emerge from between the Si reflector and the sample carrier at
different reflector angles: (a) wide range (0.01–0.18° ) and
(b) narrow range (0.01–0.09° ).
A novel reflector-assisted TXRF setup has been proposed. To
characterize the actual x-ray beam used as excitation source
for x-ray fluorescence, the x-rays that emerged from between
the reflector and the sample carrier were analyzed using an
SDD. It was found that a monochromatic x-ray beam without
a Compton peak was obtained for small angles between the
reflector and the sample carrier. When this x-ray beam was
used for excitation of TXRF signals, the highest intensity of
X-ray fluorescence emitted from the sample was detected,
indicating that these experimental conditions are useful for
enhancement of the TXRF intensities.
Acknowledgements
Filip Delalieux was supported by the Japan Society for the Promotion
of Science (JSPS). Part of this work was financially supported by a
grant-in-aid from the JSPS.
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Figure 4. Mo K˛ intensities as a function of the reflector
angle .
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at a reading value of 1.32° (maximum in Fig. 2). The Au L˛
intensities are plotted as a function of the reflector angle. The
Copyright  2004 John Wiley & Sons, Ltd.
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