Download Optimizing the operation of X-Ray sources

Surface Analysis
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Optimizing the operation of X-Ray
sources
Technical Note
The sensitivity of the XPS analysis depends
on the luminosity of the used energy
analyzer and the density of the
characteristic X-ray at the sample position.
The X-ray density hitting the sample
surface depends on the X-ray power and
the distance between the anode surface
and the sample. The X-ray intensity
depends strongly on the distance (I ∝ r2),
see figure 1.
Relative XPS signal intensity dependency on sample-anode distance
0.005
0.004
Signal (arbitrary units)
Depending on the anode type there is a
specific working voltage of the X-ray
source, at which there is a maximum yield
of the characteristic X-rays. On the basis of
this dependency it is also possible to check
the surface condition of the anode.
Standard distance for specification
0.003
-2.18
y=x
0.002
0.001
0
0
20
40
Distance Sample-Anode in mm
Figure 1
Ag 3d5/2 FWHM
XR 50 @ 10 mm
RQ 20/63 @ 22 mm
0.85 eV
2500 kcps
625 kcps
0.90 eV
3700 kcps
925 kcps
1.00 eV
6700 kcps
1675 kcps
1.40 eV
17600 kcps
4400 kcps
Typical signal (intensity above background) versus FWHM of the Ag 3d5/2 peak measured using the PHOIBOS
100 MCD-5 analyzer with the Large Area Mode (achieved data). Excitation with XR 50 X-ray source at 10 mm
anode working distance (Mg Kα at 300 W, 15 kV) or RQ 20/63 at 22 mm anode working distance.
60
The anode voltage of the X-ray source
influences the X-ray density, too. Below the
ionisation energy (i.e. for Al about 1500 eV),
also for high anode power, no characteristic
X-rays can be excited. On the other hand the
ionization cross section decreases with
increasing energy. The maximum, where the
highest yield of the characteristic X-rays is
achieved, has to be found between these two
extremes.
This maximum X-ray yield was measured for
an Al and a Mg anode (Fig. 2). The maximum
yield was found for Mg at nearly 10 kV and
for Al at nearly 14 kV.
Figure 2
The graph shows a flat maximum. Within this flat region the amount of changes in X-ray intensity
induced by fluctuations of the anode voltage are smallest (1% yield variation for 10% variation of
the anode high voltage).
Coating of the anode with carbon or other materials caused by the degassing of organic samples or
operation of the anode under poor vacuum conditions will decrease the X-ray yield. This effect can
be distinguished from other effects (i.e. aging of the multiplier), by measuring the curve shown in
figure 2. The maximum will shifted to higher voltages, if the anode is coated by residual material. If
the contamination layer exceeds a given thickness no maximum will be found within the typical
working range of the X-ray source.
SPECS GmbH
Surface Analysis
and Computer Technology
Voltastrasse 5
13355 Berlin
Germany
Phone: +49 30 467824-0
Fax: +49 30 4642083
E-mail: [email protected]
http://www.specs.de
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