X-RAY SPECTROMETRY
X-Ray Spectrom. 2005; 34: 493–497
Published online 5 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/xrs.872
A new microfocus x-ray source, iMOXS, for highly
sensitive XRF analysis in scanning electron microscopes
A. Bjeoumikhov,1 V. Arkadiev,2 F. Eggert,2 V.-D. Hodoroaba,3 N. Langhoff,1 M. Procop,3
J. Rabe2 and R. Wedell2∗
1
2
3
Institute for Scientific Instruments GmbH (IfG), D-12489 Berlin, Germany
Institut für angewandte Photonik eV (IAP), D-12489 Berlin, Germany
Bundesanstalt für Materialforschung und -prüfung (BAM), D-12200 Berlin, Germany
Received 14 December 2004; Accepted 14 July 2005
Scanning electron microscopes are usually equipped with energy-dispersive X-ray detectors for
electron probe microanalysis. This widespread analytical method allows investigators to determine
the elemental composition of specimens with a spatial resolution of about 1 µm. However, owing to
the electron–specimen interaction, the emitted spectra reveal, in addition to characteristic lines, also a
high level of continuous bremsstrahlung background. As a result, elements with low concentrations
cannot be identified. The minimum detection limit can be diminished by two orders of magnitude if the
characteristic lines are excited as fluorescence by an additional x-ray source. In this case, the emergence of
bremsstrahlung is considerably reduced. Combining a high-brilliance microfocus x-ray tube with efficient
polycapillary optics enables one to realize an experimental arrangement for performing local fluorescence
analysis at the same point where the electron beam hits the sample. The polycapillary optics under
consideration focuses the emitted x-radiation onto focal spots between 30 and 100 µm in diameter. Count
rates of several thousands cps have been achieved. Elemental maps have been obtained by means of the
motorized specimen stage of the microscope. Copyright  2005 John Wiley & Sons, Ltd.
INTRODUCTION
The principle of electron probe microanalysis (EPMA) was
introduced by Hillier,1 and Castaing and Guinier2 in the late
1940s. In 1951, Castaing outlined the fundamental physical
concepts of quantitative analysis.3 At present, scanning
electron microscopes (SEMs) are equipped with energydispersive or wavelength-dispersive x-ray spectrometers
so that EPMA has developed into a mature analytical
method. However, the electron-excited x-ray spectra show,
in addition to characteristic lines, also a bremsstrahlung
background resulting in detection limits in the range
0.1–1 wt%. The additional use of x-ray excitation for xray fluorescence (XRF) analysis in a SEM could considerably
improve the detection limits. A second advantage of the
combined use of EPMA and XRF in a SEM comes from
the large difference in the energy dependence of ionization
cross-sections for electrons and photons. In the case of
electrons, an effective ionization needs an overvoltage ratio
>2. Because the maximum SEM beam voltage is usually
limited to 30–40 kV, lines above 20 keV will not appear
above the bremsstrahlung spectrum. The photoionization
cross-section has its maximum for photon energies directly
above the binding energy. Therefore, a 50 kV x-ray tube
would allow the use of the full range of the spectrometer
Ł Correspondence
to: R. Wedell, Institut für Angewandte Photonik
eV (IAP), D-12489 Berlin, Germany.
E-mail: [email protected]
Contract/grant sponsor: Senat of Berlin.
(usually up to 40 keV) for the detection of lines originating
from inner shell photoionizations.
A first attempt to realize a photon source in a SEM
was made by Gould and Healey4 with a secondary massive
target. Further constructions use a thin metal foil which is
hit by the electron beam. The foil anode is positioned above
the sample, which is then irradiated by the secondary x-rays
generated in the foil.5,6 However, the excitation intensities
on the sample achieved in such a scheme are relative low
and the irradiated surface area is rather large (several mm2
and larger). A considerable improvement in the excitation
conditions became possible on the basis of a combination of
efficient polycapillary optics with high-brilliance microfocus
tubes.7 The experimental arrangement described in detail
below is a realization of a confocal scheme of electron probe
microanalysis and micro x-ray fluorescence (XRF). The
advantages of x-ray excitation in SEM are illustrated here by
selected application examples.
DESIGN OF THE m-XRF MODULE FOR SEM
A special module was developed in IfG/IAP for performing
X-ray-induced fluorescence analysis at SEMs (see Fig. 1).
From the very beginning it was designed to fit to SEMs of
different manufactures. iMOXS (modular x-ray source with
optics) is an attachment to a SEM intended to excite XRF
radiation with a primary x-ray beam. It contains a lowpower microfocus x-ray tube, which is enclosed in a tube
housing with a shutter and a filter revolver. The filter revolver
can be equipped with different filter materials according to
Copyright  2005 John Wiley & Sons, Ltd.
494
A. Bjeoumikhov et al.
3
1
2
Figure 1. iMOXS (modular x-ray source) with the adjustment
support: 1, screws for adjustment of the source to the
specimen stage of the SEM; 2, turning knob for source moving
into working position; 3, screws for optics fine adjustment.
customer requirements and choice of the anode material. The
x-ray beam emitted by the tube is collimated and focused by
the integrated x-ray optics.
The capillary optics module8 – 10 is a decisive element
of the XRF module because it allows local excitation of
fluorescence lines with high intensity. The combination of
glass capillary optics with a micro-focus low-power x-ray
tube (30 W) results in an x-ray source whose emission
intensity and beam parameters are comparable to those of
conventional high-power x-ray tubes (1–3 kW). The reason
for that is that, first, the capillary optics module used has a
large solid angle of capture, second, x-radiation is transmitted
through the optics without significant intensity losses and,
third, transmitted x-rays are focused onto a small spot on the
sample.
The optics module contains the capillary optics in a metal
casing, the optics socket with adjustment screws to align the
optics relative the anode spot and the adjustment support
to align the position of the x-ray beam relative the electron
beam spot on the sample in the SEM. Depending on the
problem under consideration, different optical elements can
be installed in the module:
ž cylindrical monocapillaries with an inner diameter from
10 µm to 1 mm and with different lengths;
ž elliptical monocapillaries with a focal spot size <5 µm
diameter and a focal distance from 1 to 5 mm;
ž polycapillary semi-lenses providing a quasi-parallel exit
beam with a diameter of 5–7 mm;
ž polycapillary zoom optics with a variable distance between
the anode spot and a sample surface providing a
20–100 µm microspot.
Copyright  2005 John Wiley & Sons, Ltd.
For XRF and XRF analytical applications, the iMOXS
is normally equipped with polycapillary zoom optics. The
optics can be exchanged by unscrewing it from the optics
socket and replacing it with a new optical element in a
ready-made screw mount.
The high-voltage generator provides up to 50 kV of
positive polarity for the connection with the anode of an xray tube. Additionally it contains a regulated power supply
for the cathode filament heating current. The regulated
tube current may be varied continuously from 0 up to the
maximum of 800 µA.
The modular x-ray source can be easily attached to a
SEM of an arbitrary type by means of a special flange
adaptor. For a given SEM model, the adapting flange
guarantees a vacuum-tight connection and the optics is
preadjusted in such a way that the x-ray beam hits
the same specimen area which is also scanned by the
electron beam if the specimen is positioned at the correct
working distance from the objective lens for electron probe
microanalysis.
During assembly, the optics unit attached to the tube
housing is prealigned in such a way that its first focus
matches the anode spot of the x-ray tube. The tube housing
can be rapidly exchanged by loosening two screws if an
x-ray tube with another target material is required. When (as
an exception) it is necessary to adjust the optics relative
to the anode spot, one should use the special screws
3 (see Fig. 1). The whole of the x-ray source with the
integrated optics should be directed to the sample. The
adjustment of the source in the X- and Y-direction is carried
out with the screws 1 on the support (see Fig. 1). The
optics–sample distance is varied by the linear stage 2. A
special specimen is provided for making the fine adjustment
of the iMOXS easier (Fig. 2, left). This specimen is a disk
with concentric rings made from different elements, which
is fixed on an aluminum substrate. During the adjustment,
the specimen should be positioned on the sample holder
of the SEM and the sample holder should be placed
in its normal working position. It is favorable for the
adjustment that the holder is tilted to be perpendicular
to the optics axis. The adjustment procedure consists in
measuring the fluorescence signal from the target (see
Fig. 2, right). The adjustment is perfect when both the
electron and x-ray beams hit the iron central part of the
specimen.
The position of the x-ray spot can be additionally
controlled in the following way. The focused x-ray beam
causes charging of insulating materials due to photoelectron
emission. For this reason, the charged region presents itself
as an additional contrast in a scanning electron micrograph.
Of course, the micrograph must be taken at a low beam
voltage, e.g. 1 kV, and low current to avoid any charging by
the incident electrons. In this way, the position of the x-ray
spot can be made directly visible. Especially suitable are
insulating materials with a well-formed surface morphology
such as paper, because a contrast-rich image helps to adjust
the correct working distance. Figure 3 shows the position
of the x-ray spot as a dark region in the SEM image of a
common laser printer paper.
X-Ray Spectrom. 2005; 34: 493–497
iMOXS microfocus x-ray source for XRF in SEM
140000
NiKa-7.5keV
120000
I, cts
100000
80000
CuKa-8.0keV
ZnKa-8.6keV
60000
40000
FeKa-6.4keV
20000
Fe 0.065
Ni - 1.20
0
-1.5
-1
-0.5
0
X, mm
Cu - 2.30
0.5
1
1.5
Zn - 2.35
Figure 2. Adjustment specimen for the x-ray source and the corresponding -XRF line scan.
Al
Cu
normalised intensity
105
Cu
Fe
sum
sum
104
Cu
Fe
diffraction
Rh-tube
Zn
Mn?
Zn
103
Pb Bi
Pb Bi
Pb Bi
102
0
2
4
6
8
10
12
14
16
energy / keV
Figure 3. SEM image of a paper surface showing the position
of the x-ray focus as a dark spot.
EXAMPLES
Figure 4 illustrates how the excitation of characteristic xrays can be advantageously combined to determine the
concentrations of main components by EPMA and the trace
components by XRF. The figure gives the spectra of an
aluminum alloy excited by 30 keV electrons and by the xrays from a Rh tube operating at 25 kV. The concentrations
of copper and iron can be determined from the electronexcited spectrum. Deconvolution of the weak zinc lines from
Cu Kˇ would result in a large uncertainty for the zinc
concentration. XRF allows the evaluation of the Zn Kˇ line
not seen in the electron-excited spectrum. Also, the lead and
bismuth concentrations of about 0.2 wt% can be found only
from the XRF spectrum.
The microanalytical capabilities of the iMOXS are demonstrated by two more examples. The first example is the
identification of the steel type of small particles, which could
be, in principle, the result of mechanical abrasion. Owing to
the irregular shape of the particles (balls, chips, filings), the
angles of incidence and take-off, necessary for a quantitative
Copyright  2005 John Wiley & Sons, Ltd.
Figure 4. Electron-excited (30 keV) and fluorescence (25 kV Rh
tube) spectra from an aluminum alloy. Spectra are normalized
to equal the Cu K˛ intensity.
analysis, cannot be defined. Any identification can be done
only by means of the lines which appear in the spectrum.
In the case of electron bombardment, there is an efficient
excitation of characteristic lines only for overvoltage ratios
of ½2. Most SEMs have a maximum beam voltage of 30 keV.
Therefore, the spectra will show only lines originating from
the ionization of core levels with binding energies smaller
than ¾15 keV. Charging of the particles is an additional
drawback. However, in the case of excitation by x-rays also
core levels with higher binding energies can be ionized
and the related x-ray lines can be used for identification.
Figure 5 shows an SEM micrograph of a steel particle (BAM
certificated reference material ZRM 290-1) with the related
x-ray spectra. In the case of electron excitation, vanadium,
chromium and cobalt are clearly identified. However, owing
to the known overlapping of W M and Mo L with Si K
and S K, respectively, these two metals cannot be unambiguously identified as separate components. In the case of
XRF, tungsten and molybdenum can be identified by their
L and K series, respectively. In this way, particles of five
different steel reference materials (EURO 428-2, EURO 479-1,
X-Ray Spectrom. 2005; 34: 493–497
495
A. Bjeoumikhov et al.
290-1_P4P3e.emsa
105
normalised intensity
ZRM290-1
V
(a)
Fe
Co
Cr
Cr
Rh
Rh
XRF, Rh-tube 40 kV
EPMA 20 kV
Fe
W Mo
S
104
Mo
W
103
W
Mo
S Rh
W
Mo
Co
W
Cu
102
101
0
2
4
6
8
10 12
energy / keV
(b)
14
16
18
20
Figure 5. Steel particle (a) with the x-ray spectra and (b) excited with a 40 kV Rh tube and 20 keV electrons.
spec04/05.emsa
14000
Ba
intensity / counts per channel
496
10000
8000
(b)
Ba
6000
Cr
Pb
4000
Ba
2000
0
(a)
3/1
3/2
12000
Ca
Ba
Ca
4
Pb
Cr
Ba
6
Fe
Fe
Cu
Zn
Pb
Pb
8
10
energy / keV
12
14
16
Figure 6. Red paint particle (a) and x-ray fluorescence spectra from two particles of the same paint excited by a 30 kV Rh tube (b).
EURO 480-1, BAM ZRM 290-1 and BAM ZRM 291-1) could
be clearly identified.
The second example demonstrates how, in a similar
manner as before, particles can be identified much better
from the fluorescence spectrum than their electron-excited xray spectrum. Different red paints could be discriminated by
their XRF spectra from small particles taken from varnished
parts. The lines of the metals (Pb, Cr, Ba, Fe) used as pigments
and their line ratios allowed it to be established in most cases
whether the same paint or not was used for the varnishing.
The particles could be analyzed as received without any
additional preparation, e.g. carbon coating as for EPMA
analysis. The position of the x-ray spot can be seen as a dark
region in Fig. 6(a). The permanent monitoring of the x-ray
spot position during analysis by means of a low kV SEM
image makes particle analysis very convenient.
Several commercial fundamental parameter programs
are available to accomplish quantitative analysis. Important
Copyright  2005 John Wiley & Sons, Ltd.
for use in combination with EPMA at the SEM is an import
filter for the EMSA/MAS spectral data file format, which
has been adopted in the meantime by ISO as an international
standard, ISO 22 029 : 2003.
In this work, WinAxil was used. It has the options of either
calculating the x-ray tube spectrum or editing it. The second
option can be advantageously used for taking into account
modification of the emitted tube spectrum due to the beam
transmission through x-ray optics. Related investigations
are still in progress. For the optics used in the iMOXS, the
transmission coefficient remains nearly constant between
5 and 10 keV. Therefore, we did not modify the calculated
spectrum, but used an additional 0.1 mm aluminum window
between the tube and x-ray optics to suppress the low-energy
bremsstrahlung. Using a tube with a tungsten target at 20 kV,
the excitation spectrum is dominated by the L-lines with
negligible intensity modifications by the x-ray optics. Table 1
gives as an example the result of a standardless analysis
X-Ray Spectrom. 2005; 34: 493–497
iMOXS microfocus x-ray source for XRF in SEM
Table 1. Composition of CRM JK37 (wt%)
Component
Fe
Cr
Ni
Cu
Mn
Mo
Evaluated line
Concentration
by WinAxil
Certified value
K˛
K˛
K˛
K˛
K˛
K˛
36.2 š 0.09
27.7 š 0.07
29.4 š 0.08
1.1 š 0.02
2.0 š 0.04
3.7 š 0.3
—
26.72
30.83
0.94
1.73
3.55
of the alloyed steel reference material Jernkontoret JK37.
Calculated concentrations are in reasonable agreement with
the certified values. With the exception of molybdenum, the
differences are slightly larger than the calculated standard
deviation, which takes into account the statistical error only.
Inhomogeneous specimens were analyzed by XRF line
scans and maps using the motorized specimen stage of the
SEM. Lateral resolution is approximately given by the focus
diameter of the x-ray optics. With count rates of several
thousand per second, the dwell time per pixel could be set
to 5 s or even shorter. However, a significant reduction of
the total acquisition time by decreasing the dwell time is not
possible owing to the time the stage needs for adjustment
of the next position. With modern SEMs, where low-voltage
SE images can be taken, e.g. at 1 kV, specimen positions can
be permanently inspected and SE images can be saved with
the x-ray line scan or map as for EPMA measurements. No
additional coating of insulating specimens by a conductive
layer is required for XRF analysis.
CONCLUSIONS
A new x-ray source as an attachment to SEMs has been
developed. The x-ray source consists of a micro-focus low-
Copyright  2005 John Wiley & Sons, Ltd.
power x-ray tube and a focusing x-ray optics. The optics
images the anode micro-focus onto a spot on the specimen in
the microscope analysis chamber. The spot diameter ranges
from 10 to 100 µm depending on the type of the optics.
The x-ray source can be easily aligned in such a way that
EPMA and XRF analysis can be performed for the same
specimen region as demonstrated with the examples in the
previous section. The two methods can be combined using
their special advantages, i.e. high spatial resolution and light
element detection in the case of EPMA and lower detection
limits and detection of high-energy x-ray lines, which cannot
be excited by the electron beam, in the case of XRF analysis.
Acknowledgements
Financial support by the Senat of Berlin within the framework of
a research and development project is gratefully acknowledged.
The authors thank Mrs S. Bjeoumikhova for performing test
measurements with the adjustment specimen.
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