Surface World May 2020 Surface World May 2020 | Page 54

TESTING & MEASUREMENT
Some instruments possess a filter turret which houses multiple filters to
give a wider range of applications the instrument can address. As an
example, a 10 μm nickel filter is used to measure thicknesses of
electrodeposited layers on various substrates, while an aluminium
(500 μm or 1000 μm) filter may be used for precious metal analysis.
Collimators
Next the primary X-ray beam is collimated to a specific spot size by
passing through a collimator. The application of the ED-XRF system is
used for usually dictates the type of collimator that is used. To
measure coatings such as electroless nickel immersion gold (ENIG) in
electronics applications, a small collimator must be used to ensure
only the feature of interest is excited to produce a signal. A
compromise must be made between the size of the beam and the
signal (count rate), as decreasing the size of the collimator (and
effective primary beam size), the signal becomes difficult to
differentiate from the background signal. One way to overcome this
issue is to use capillary optics to reduce the primary beam size.
Capillaries are normally glass cylinders which utilize internal
reflections to direct the beam down the length of the capillary into the
sample. Thus, the energy imparted onto the sample is greater even at
sizes down to 10 μm.
chambers (housing a cathode at the periphery) with a wire filament at
their core (anode), see below (Figure 6). The detector is typically filled
with a noble gas (neon, argon, krypton or xenon) and can be mixed
with a secondary gas to provide a quench once the signal is detected.
The proportional counter functions by allowing the X-ray fluorescence
signal (ionised particles) to enter the detector and collide with atoms
of the inert gas inside. The inert gas atoms are themselves ionised to
produce electrons, which migrate towards the anode, and a positively
charged ion which migrate to the cathode.
Collectively the electrons and positive ions are referred to as “ion
pairs.” As the electrons get closer to the anode they tend to get
accelerated by the electric potential of the anode and collide with
further atoms of gas and multiply the signal. These multiplications
are referred to as Townsend Avalanches, which are detected
as changes in voltage across the anode. These signals are then
interpreted by the software that is supplied with the instruments
from various manufacturers.
Figure 5: Different types of capillaries which use uses Total External
Reflection to direct the primary X-ray beam on to the focal plane of the
sample. A monocapillary (left) is a glass tube which focuses the X-ray
beam onto the sample surface. A Polycapillary (right) is a bundle of
glass capillaries, tapered at each end which deliver the X-ray beam to
the sample.
Detectors
In order to detect the signal from the samples, ED-XRF systems use
one of two types of detectors: Proportional Counter or Silicon based
detectors.
Proportional counter detectors are characteristically gas-filled
Figure 6: Schematic Diagram of a proportional counter detector (left)
and the method of signal detection within a proportional counter
(expanded).
Proportional counter detectors have relatively large apertures for
signals to enter, therefore an advantage of such a detector is that
the signal received is relatively abundant. For relatively simple
applications (for example measuring thicknesses of general
electrodeposited coatings), short measurement times can be expected
while still maintaining a high degree of accuracy. Typical resolution
of approximately 900eV can be expected for this type of detector.
For further information on these detectors please see the review
by G.F. Knoll (Knoll, 2000).
The Silicon Drift Detector (SDD) is currently the pinnacle of detector
technology. They are silicon semiconductors doped with p-type
acceptors and n-type donors to allow the controlled propagation of
the X-ray signal to the anode. It acts much like the proportional
counter detector in that is measures the incoming ionised particle
“cloud” by attracting them to the anode by applying a differential
charge between the anode and cathode. The charges at the anode
are converted to a signal by a Field Effect Transistor (FET) which
are then amplified by the preamplifier and measured by the pulse
processor. The pulse processor not only has to differentiate between
different X-ray energies but also between different events arriving at
very short intervals. For more information on SDD please see Strüder
et al. (Strüder, et al., 1998).
Silicon Drift Detectors due to their small size tend to acquire lower
count rates (rate of incoming signals). However, due to their design
they are faster, have lower noise, no requirement for external cooling
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