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Sources of Internal Scattering of Electrons in a Cylindrical Mirror Analyser (CMA)

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SURFACE AND INTERFACE ANALYSIS, VOL. 24, 152-162 (1996)
Sources of Internal Scattering of Electrons in a
Cylindrical Mirror Analyser (CMA)*
M. M. El Gomati and T. A. El Bakush
Department of Electronics,University of York, Heslington, York YO1 5DD. UK
A number of possible sources of internal scattering of electrons in a single-pass Varian cylindrical mirror analyser
(CMA) have been identified and their relative contributions to the electron spectral background evaluated. These
sources include: the inner surfaces of the outer cylinder; the input and output apertures of the inner cylinder; the
field trimmers between the two cylinders; the walls of the exit aperture in front of the electron detector; the outer
surfaces of the inner cylinder. A tbeory/experiment comparison of the above sources has verified that the two most
significant of these are the walls of the exit aperture and the field trimmers. The other sources contribute only
negligibly to the total signal of internally scattered electrons. Simple modifications of the spectrometer and its
detector show a large reduction in the internal scattering signal. A simple experimental measurement for determining the relative contribution of the various internal scattering sources is suggested. This measurement is recommended instead of the commonly used peak-to-background ratios as a diagnostic tool for detecting and evaluating
all the above-mentioned sources of internal scattering of electrons in a CMA.
INTRODUCTION
The characterization of electron energy analysers to
produce the exact number of electrons versus their
energy distribution ( N ( E ) or E . N ( E ) us. E ) in an Auger
electron spectroscopy (AES) experiment has been a
challenging task since the development of AES as a
surface analytical technique. The identification and correction of the factors that alter the faithful reproduction
of this distribution has witnessed two stages of development that accompanied signal acquisition in AES;
namely the differential and direct energy distribution
modes. In the early days of the AES technique and up
until the early 1980s, the majority of spectra were collected in the differential mode. The spectrometer
properties that were considered to affect the Auger
signal in this case were the magnetic field around the
sample and inside the analyser,’.’ the transmission
properties (function) of the a n a l y ~ e r , ~the
- ~ analyser
field of view and its depth of ~ o c u s . ~As
- ~the direct
energy mode became more widely used, some of these
factors needed revisiting, particularly the transmission
properties of the analyser, the detector efficiency and its
operating conditions and the electronic
Internal scattering of electrons inside the analyser has
become the latest factor to be taken into consideration
in the characterization of spectrometers.’ 3-17
The importance of identifying these factors and removing their effects to some known precision has gathered
momentum as more use of the spectral background k
made in quantitative AES and in Auger mapping. It is
now the practice, for example, that Auger imaging is
carried out in the direct energy mode. The surface concentration of an element in this mode is estimated to be
proportional to the height of its Auger peak. This height
* This paper is dedicated to Professor Martin Pruttan (University
of York, Department of Physics) on the occasion of his 60th birthday.
CCC 0142-2421/96/030152- 11
0 1996 by John Wiley & Sons, Ltd
is determined by measuring the electron count at the
peak minus that due to the background at the same
energy. The latter count is estimated by extrapolating
the background from at least two counts on the highenergy side of the Auger peak used to map a solid
surface.I8 For quantifying the resulting maps, the
underlying spectral features that are used in forming it
must also be quantifiable. There are also a number of
studies that have shown the advantages in using the
direct energy mode for quantitative analysis in preference to the differential spectra where the shape of the
Auger peak in question is changed as a result of the
chemical environment of the emitting atom. In addition
to these, the use of the whole or part of the spectral
background as an aid to the quantification of spectra
and images has been suggested or demonstrated in a
number of recent studies.’8-’2
These studies have led to a marked improvement in
the manufacture and operation of spectrometers, as has
been manifested in the various round-robin experiments
that were carried out over the last 10-15 years. For
example, SeahZ3compiled experimental data collected
from various spectrometers in the differential mode and
found that the ratio between the amplitudes of the
Auger peaks of copper at 60 and 920 eV differed from
the one instrument to another by nearly a factor of ten.
Another study conducted by the ASTMZ4 showed that
intensity ratio of the Auger signals in the differentiated
mode varied from one instrument to another with a
scattering factor ranging from 0.79 to 2.30. Recently,
work carried out by the Japanese VAMAS groupz5also
showed similar results but with a smaller scattering
factor ranging from 1.03 to 1.65.
In spite of these improvements, the effects of some of
the above-mentioned factors were not included in these
reported studies, and hence an absolute spectrum is not
a product for most spectrometers available for the AES
analyst or the instrument operator. One of these factors
is the scattering of electrons inside the analyser, generating an unwanted contribution to the spectral backReceived 19 July 1995
Accepted 3 November 1995
INTERNAL SCATTERING OF ELECTRONS IN A CMA
ground of an AES experiment. This signal, referred to as
internal scattering (IS) and schematically illustrated in
Fig. 1, can be a substantial percentage of the overall
collected spectral background signal.' Its reduction or
elimination should assist the task of quantifying AES
and SAM data. Moreover, the exact electron energy distributions could be compared with Monte Carlo simulations of electrons in solids to discover the presence of
systematic trends in this distribution as a function of the
sample's atomic number, the incident beam energy or
the experimental g e ~ m e t r y .Understanding
~~~~~
the
causes of IS is therefore most useful in quantitative
Auger electron spectroscopy and imaging.
It is interesting to note that the effects of internal
scattering of electrons inside the analyser are more pronounced in the direct mode in comparison to the differential mode. This is because the latter basically
suppresses the spectral background (where this unwanted signal shows most). Consequently, addressing
the internal scattering issue has accompanied the use of
the direct energy mode in collecting spectra and images
and has therefore come much later in the characterization of electron energy analysers than the more
obvious effects like the magnetic field, the analyser
transmission properties and its field of view, etc.
The present study is a theory/experiment comparison
of the sources of IS in one of the most widely used electron energy analysers, the cylindrical mirror analyser
(CMA). Although a great deal is known about the performance and focusing properties of this analyser, an
analyst still faces difficulty in obtaining the true electron
intensity-energy distribution, N ( E ) [or E . N ( E ) for this
type]. This has been mainly attributed to the contribution of internal scattering of electrons inside the
analyser and is considered by some to be a fundamental
limitation of the sensitivity of the CMA. It was with this
picture in mind that we started the present investigation.
The case for identifying and correcting the internal
scattering of electrons in CHAs has been addressed in
great detail by Seahi4 and Greenwood et
and will
not therefore be discussed in detail here. Seah found out
that internal scattering comes mainly from the outer
hemisphere, as electrons of kinetic energy a few
153
electron-volts higher than the pass energy tend to strike
that part of the hemisphere near to the output position.
These in turn produce secondary electrons that find
their way to the electron detector. Seahi4 suggested that
the effect of these electrons could be reduced to negligibly low values if the analyser is operated with pass ener~
gies of -50 eV or higher. Greenwood et ~ 1 . ' have
proposed a method that corrects for the transmission
properties and the effects of internal scattering of electrons in their CHA. However, such methods cannot be
applied to CMAs and hence a study of the origin of IS
in this analyser, its effects on the collected spectra and
its elimination or minimization is required.
In the present study, we have identified a number of
possible sources that give rise to an IS signal in a singlepass CMA (and some will also apply to a double-pass
CMA). These include; the inner surfaces of the outer
cylinder; the outer surfaces of the inner cylinder; the
exit slit aperture walls in front of the electron detector;
the input aperture of the inner cylinder; the output
aperture of the inner cylinder; the field trimmers
between the two cylinders for the present type of
analyser. Although the present study considers a specific
analyser design (Model number 981-2707 made by
Varian Associates, USA), the use of field trimmers of
different shapes and position with respect to the exit
aperture at the inner cylinder is an accepted practice in
other CMA designs to correct for field penetration at
the inner cylinder apertures. The present study and its
findings may apply to such cases, particularly if these
field trimmers and the output slit aperture are of the
shape and/or the position as shown in Fig. 2.
An important finding of the present study is the elimination of some of the above listed sources as significant
contributors to internal scattering, while, on the other
hand, we have identified two of these as major sources.
Furthermore, a simple experimental measurement complimented with electron trajectory simulation is suggested as a diagnostic tool for the detection of, and in
estimating, the relative contribution of the different IS
sources. This is in preference to the more widely used
peak-to-background ratio (P/B) from elemental standard spectra as an indication of the presence and
amount of IS in spectrometers.
(b)
IS
I
E
Figure 1. The effect of internal scattering of electrons in an AES analyser on the collected spectrum. (a) Spectra with no internal scattering
showing a given P/B ratio. (b) A simple constant internal scattering (IS) contribution from the analyser internal walls. This is not necessarily
the case, and a non-uniform contribution of IS is most likely. (c) The effect of IS on an N ( E ) spectrum. Note that the P/B ratio is now worse
than in the case where B > B.
M. M. EL GOMATI AND T. A. EL BAKUSH
154
/
I
VmIMFKj
S e a m h y ~ T r a j ~MagneticShield
Figure 2. The structure of the single-pass cylindrical mirror
analyser (CMA) used in the present study. The various trajectories
depicted show that electrons emitted from the sample surface with
the correct energy (corresponding to the pass energy of the
analyser) and angle of emission pass through and are focused at
the detector (as shown in the bottom half of the diagram). Electrons emitted with higher energies than the pass energy of the
analyser (shown in the top half of the diagram) strike the inner
surfaces of the outer cylinder and generate secondary and backscattered electrons. Some of these may have the correct energies
and angles of emission to reach the electron detector, as depicted.
The theoretical investigation carried out in this study
involved the use of electron trajectory simulations. Two
computer programs were used: the SIMION electron
and ion simulation package” and a purpose-written
p r ~ g r a m . ’ The
~ latter program considers the fate of
electrons originating at the outer cylinder with random
energies and angles of emission, in addition to those
originating at the sample. Full details of this program
are given el~ewhere.’~
A new equation of motion for the
former type of electrons has been derived3’ which is different from that used in the literature31 for calculating
the trajectories of electrons originating at the focal
point of the analyser.
THE ORIGIN OF INTERNAL SCATTERING OF
ELECTRONS IN A CMA
Internal scattering (IS) of electrons occurs when some of
the electrons emitted from the surface under analysis
strike the inner surfaces of the spectrometer and generate secondary electrons that bear no direct relationship
to those emitted from the sample under analysis, except
in being generated by them. It has long been argued
that some of these secondary electrons may be generated with the appropriate energies and emission angles
to reach the detecting system and contribute to the
spectral background signal, as depicted in Fig. 1. The
overall contribution of these electrons depends upon the
geometry of the analyser, the potential applied to its
various electrodes and the degree of surface coverage of
its internal surfaces. The latter arises as a result of either
particles sputtered during specimen cleaning, particularly during depth profiling, or purposely coating with a
low secondary electron emission material (normally
used to reduce the secondary electron emission from
inner analyser walls). The current perception of the AES
community of the causes of IS in CMAs is based on the
assumption that the secondary electrons that are generated at the outer cylinder of the CMA, as a result of
energetically backscattered electrons from the surface of
the solid under investigation, tend to have a direct line
of sight with the detecting system or can find their way
to it after a number of multiple scattering events inside
the analyser. This is considered to be a limitation in the
CMA performance.
Another source of internal scattering of electrons in
this analyser is the surfaces of the exit aperture in front
of the electron d e te ~ to r .~In
’ this case it is not the
highly energetic electrons emitted from the sample that
are the cause of scattering but rather electrons emitted
with slightly different energies and angles of emission
from those being analysed, i.e. electrons of energies
E AE and angles 0 f A@. Some of these electrons
strike the surfaces of the exit slit aperture placed in
front of the detector and generate secondary electrons
which are subsequently collected by the detector and
appear as an increase in the electron background signal.
In addition to the above, there are four more likely
sources of IS in this analyser which have not been systematically investigated before. These include the input
and output apertures of the inner cylinder, which are
normally covered with a high-transparency mesh to
stop field penetration; the outer surfaces of the inner
cylinder; and the field trimmers between the two cylinders as depicted in Fig. 2.
MEASUREMENT OF INTERNAL SCATTERING
IN THE CMA
~~~~~
~~
~~~~~~~-
Two experiments have been carried out to estimate and
reduce internal scattering in the present CMA. The first,
used as a diagnostic tool, involves the use of a monoenergetic electron beam to represent a proportion of the
electrons that leave the sample in an AES experiment.
Figure 3 outlines a schematic illustration of this experiment. The spectral energy distribution E . N ( E ) is then
recorded from a few electron-volts up to E , + 50 eV,
where E , is the incident electron energy. Although only
one electron energy is used in this experiment, it can be
easily seen that it represents in an integral form (i.e.
when using other incident beam energies) what happens
in a real AES experiment.
The second experiment involves the measurement of
the P/B ratio from three elemental Cu spectra collected
under different electron detector configurations. This
ratio is measured in its simplest form where P = P and
B = B’ in Fig. 1. The sample used was kindly provided
by Dr M. Seah of NPL, UK and consists of a thin foil
of pure copper measuring 10 x 10 mm. Prior to data
collection, the surface of the sample was normally in situ
bombarded with a beam of 3 keV Ar’ ions until no
traces of surface impurities were present in the AES
spectra. These were collected under the following conditions: a primary beam energy, E , , of 5 key, an electron
beam current 1 PA, a dwell time of 100 ms per energy
point, an energy interval of 1 eV and spectra collected
in the energy range 50-2500 eV. The detection system
used consists of a Gallelio analogue electron multiplier
(Model 4700) followed by a purpose built signal
INTERNAL SCATTERING OF ELECTRONS IN A CMA
155
I
3
I =Outer cylinder
s
U= Field trimmas
I
Energy
0
650
EP
outer cylinder
inner cylinder
r
-
/
Sample
-
_I_
Figure 3. Schematic illustrating the concept of a mono-energetic beam to detect and estimate the contribution of internal scattering of
electrons in an energy analyser. (a) The spectral background in an AES experiment, with the shaded area being the total area under the
spectrum in (c). (b) The experimental arrangement. (c) A schematic of the measurement in (b).
digitizerz9 to produce a signal in the form of an electron
count which is fed into a counter for subsequent processing. The detection configurations used were :
(1) The detector operated in the normal mode, as
shown in Fig. 2;
(2) A pair of high-transparency metal grids placed
between the exit slit aperture and the electron multiplier
to act as a filter to low-energy secondary electrons;
(3) as above but with the grid in front of the multiplier being biased to - 10 V.
The outer cylinder
The contribution of this source to the total IS signal has
been addressed in an earlier theory/experiment comparative study3’ and the findings of this study suggest that
this is not a significant source of IS in this analyser.
This conclusion was based on trajectory simulation of
electrons originating at the outer cylinder with random
energies and angles of emission. The trajectory of electrons of energies up to the incident beam energy were
considered, in order to take account of both secondary
and backscattered electrons. A total of lo4 electrons for
each incident energy in the range 1000-5000 eV in steps
of 1000 eV were considered. The trajectories of these
electrons are described by a new equation of motion for
calculating the distance 2, that a scattered electron (off
the walls of the outer cylinder) travels along the z-axis
as depicted in Fig. 2. This has the form
2, = 2rz cos I ~ ( K ~ exp
) ~ .(KO
’ sin’ 0)
(1)
where
u1 = (KOsinZ O)’.’
M. M. EL GOMATI A N D T. A. EL BAKUSH
156
q is the electron charge, E , and V stand for the energy
of the scattered electron and the potential difference
between the two cylinders, respectively, rI and rz are the
radii of the inner and outer cylinders, respectively, and 8
is the angle that a scattered electron makes with the
cylinder surface.
In this simulation all the dimensions of the present
CMA were fixed with the exception of the output exit
slit aperture in front of the electron detector. This has a
width of 290 pm. It is considered to be of paramount
importance because of the rather small solid angle that
it makes with the output aperture in the inner cylinder.
Several other larger slit aperture widths were also considered, as well as the case of no aperture at all. Of
course the change of the slit aperture size will change
the energy resolution of the analyser. However, this is
not important at this point, where the fate of the secondary electrons from the outer cylinder is the issue
under consideration. For brevity, only the results
obtained in the case of 5 keV incident electrons are
shown in Table 1. In the case of the 290 pm aperture,
the total contribution at all incident energies is much
less than 1%, suggesting that the inner surfaces of the
outer cylinder are not a major source of IS in. this
CMA.
This result has been experimentally confirmed by
measuring the P/B of Cu and Ag spectra using two
outer cylinders of different inner surface finishes under
similar experimental conditions as stated above. These
are the original one supplied by the manufacturer
(Varian Associates, USA) with a smooth surface finish,
and another purposely fabricated at York with 0.5 mm
deep grooves. The grooves make an angle of 45" in one
direction with respect to the axis of the cylinder, while
being at right angles to the other direction and facing
the incident electrons. In comparison with a smooth
surface, this geometry should decrease the chances of
forward scattering of both secondary and backscattered
electrons off the surface of the outer cylinder towards
the electron detector. It is anticipated therefore that if
the surface of this electrode is a major source of IS then
this will be reflected in a different P/B of the collected
spectra from the two outer cylinders. The results
obtained show negligible change in the P/B ratios when
using either cylinder.
The above result suggests that either both surface finishes contribute equally to the total IS signal and hence
a distinction between them is not possible, or that this
source contributes negligibly to internal scattering. If
the outer cylinder is a significant contributor, then the
former suggestion cannot be true due to the difference
in electron scattering expected from the two surface finishes. However, it is possible that the IS contribution is
only initiated at the outer cylinder by secondary and
backscattered electrons off its surface to other parts of
the analyser and that it is an electron cascade-like
process that follows afterwards and hence the surface
finish of the outer cylinder will play a less significant
role in this case. This of course could be argued based
on the electron trajectory simulations (Fig. 4) of secondary electrons generated at the outer cylinder being
accelerated towards the inner cylinder.
The second experiment, schematically depicted in Fig.
3, is considered to be more useful than the use of P/B as
an aid to identifying and quantitatively estimating the
contribution of the various IS sources considered here.
Figure 5 shows spectra in the energy range 0-1000 eV
collected with an incident electron beam energy of
Table 1. The number of electrons that strike the walls of the exit slit aperture for a total of lo4 electrons incident with an energy of
5 keV
Aperture size
Source size
0.29 mm
0.58 mm
1.45 mm
B
A+B
0
0
2
2
8
0
2.90 m m
4
11
4.35 m m
7 25 mm
20.0 mm
11
17
21
34
1112
974
outor cyllndor
0 p t l c .xi*
Figure 4. Electron trajectory simulation of secondary electrons generated at the outer cylinder with random energies up to those used to
generate them. The case depicted is for an incident beam of 1000 eV and a pass energy of 500 eV. It is obvious that all secondary electrons
are accelerated towards the inner cylinder, while for the backscattered electrons the majority strike the field trimmers but some manage to
exit from the inner cylinder aperture.
INTERNAL SCATTERING OF ELECTRONS I N A CMA
(4
700
-
Smooth surfaced cylinder
II
600 -
500
-
.i400-
300200100-
0
0
100
200
300
400
500
600
Energy (eV)
700
800
900
1000
157
rotated until a maximum current is obtained at the
outer cylinder, which is disconnected from the voltage
supply for this measurement. This ensures that the incident electrons do in fact resemble those with the same
energy emitted from the sample in the usual AES
experiments. In estimating the total contribution of the
IS signal, the area under the curve between 50 eV and
E, - AE, where E , is the incident beam energy and AE
is of the order of a few electron-volts, is ratioed to the
area under the peak at E,. In the present case this ratio
is -0.20 for both surface finishes. Similar figures to this
are obtained with different incident beam energies in the
range 500-2500 eV. This suggests that the total IS
signal resulting from the whole spectral energy distribution N ( E ) is therefore also of a similar magnitude.
On close inspection of the spectra of Fig. 5 and by
complementing them with electron trajectory simulations, one is able to draw a clearer picture of the contribution of the various parts of the analyser considered to
be likely sources of internal scattering. Figure 6 is a set
of trajectory simulations of fixed energy electrons incident on an outer cylinder which is being ramped in the
usual manner to collect an E . N ( E ) us. E spectrum as
shown in Fig. 5. Seven different analysis energies are
considered. It is clear from the trajectory simulations
that the 920 eV incident electrons strike the outer cylin-
100
-10 Vapplied 10 a grid
c
Zl00W
50Smooth surfaced cylinder
500
- - - -o
100
200
300
400
800
Energy (eV)
500
,
1
700
BM)
--a
900
1m
Figure 5. Spectra collected from the t w o cylinders with a 920 eV
beam directed towards the inner surfaces of the outer electrode by
biasing the sample with - 1 kV: (a) from the cylinder with a
smooth surface; (b) from the cylinder with a sawtooth surface as
shown in Fig. 2; (c) same as (b) but with - 10 V applied to a grid
placed on the front of the electron multiplier.
mag. x140
200
100
-920
inner
beam
tively
eV using the two outer cylinders with different
surface finishes. In this experiment the electron
is directed towards the sample, which is negabiased to about -1.0 kV. The sample is first
200
300
400
500
600
700
BOO
900
1000
Figure 6. Electron trajectories of 920 eV emitted in the angular
range 36.3-48.3" (a-g) The condition of the cylindrical mirror
analyser (CMA) at various pass energies. It is clear from the figure
that the 920 eV electrons strike the field trimmers in the energy
range from -650 t o -890 eV. This is in close agreement with the
experimental results shown in Fig. 5.
158
M. M. EL GOMATI AND T. A. EL BAKUSH
der only when the spectrometer is set in the energy
range 0-600 eV. The area under the spectrum of Fig. 5
in the corresponding energy interval is therefore the
contribution of the outer cylinder to the total IS signal.
For the present set-up and using either of the two outer
cylinder finishes, this amounts to only about 1-1.5% of
the signal under E,. In other words, the outer cylinder
of this CMA is a less significant source of IS.
The field trimmers
The expanded portions of the spectra in Fig. 6 clearly
show seven broad energy peaks in the energy region
600-950 eV. The total signal under the area of the spectrum in this energy range amounts to only 556% of that
under E,. At first sight these peaks appear as plasmon
loss peaks. However, they differ from normal loss peaks
on three accounts. First, the incident electrons are not
interacting with a sample as in the usual scattering
experiments that give rise to plasmon losses and the
appearance of loss peaks. Second, the energy separation
of these peaks ranges between 70 and 100 eV for incident beam energies in the range 1000-2000 eV. The
usual energy-loss peaks occur typically in the range
15-30 eV for most elements. Thirdly, the present peak
energies and energy separation between the individual
peaks are dependent on the incident beam energy. The
energy separation between the individual peaks
increases as the incident beam energy increases. Electron energy-loss peaks have fixed energy separation
between each other, representing multiple plasmon
losses, and are fixed with respect to the incident beam
energy.
These observations suggest that these peaks originate
via a different mechanism to electron losses. A comparison between the 920 eV incident electrons and the electron trajectory simulations of Fig. 6 shows that the
incident electrons at these energies strike the field trimmers placed between the two cylinders as well as small
portion of the outer cylinder near the field trimmers
(labelled C in Fig. 2) and the far edge of the inner cylinder. It is no coincidence that the present CMA also has
five such field trimmers as shown in Fig. 2, hence
making a total of seven distinct features within the
analyser that the incident electrons strike in this energy
range. The consequence of such an interaction is the
appearance of seven peaks shown in Fig. 5(a, b).
The field trimmers are made of L-shaped rings connected via a resistor chain to the outer cylinder to
correct for changes to the field distribution in this
region of the analyser. It is usually the practice to place
these rings closer to the outermost envelope of the trajectory of the analysed electrons. This allows a shorter
pair of cylinders to be used, with a ground termination
between them where the field trimmers can be mounted
on. The benefit of this arrangement is an easier access to
the various parts of the spectrometer, particularly for
the electrical connection when using a co-axial electron
gun to the CMA, as is the case in the present spectrometer design. However, placing the field trimmers at
this position makes a direct line of sight between any
electrons that are generated at this area and the exit slit
aperture and its housing. It is the aperture of the latter,
shown shaded in Fig. 7, that acts as a source of ter-
tiaries and other remotely related electrons that contribute to the IS signal.
In order to establishlconfirm experimentally that
these peaks are associated with the field trimmers, we
have replaced the latter with a single electrode placed at
ground potential. This was made out of an insulated
disk (made out of PEEK, Polypenco Ltd) coated with a
layer of gold about 200-300 nm in thickness [Plate
l(b)]. Replacing a family of field trimmer electrodes
with a single grounded electrode will have the effect of
altering the field near the exit aperture of the inner
cylinder, with a subsequent change to the spectrometer’s
resolution and an energy shift of the analysed peaks.
This, however, is not as important an issue for the
present measurement, which is primarily concerned
with establishing whether these field trimmers are the
cause of the observed peaks or not and in estimating
their contribution to the total IS signal. The presence of
a single flat disk should, therefore, based on the above
argument, replace the five peaks due to the field trimmers with two peaks. This has indeed been the result
obtained, as shown in Fig. 7(c) where a total of four
instead of seven peaks is obtained. This confirmed that
these peaks are due to energetic electrons striking this
part of the analyser and subsequently initiating an IS
contribution.
The energetic electrons that strike the field trimmers
and the ground plate behind them give rise to both secondary and backscattered electrons. The majority of
these electrons have energies in excess of a few tens of
electron-volts for 1000 eV incident electrons. By placing
a high-transparency metal mesh in front of the electron
multiplier, biased to -10 V, it was possible to reduce
the total amount of the IS signal to 1% from 8%
obtained with no biasing. In addition, the peaks due to
the field trimmers have also disappeared, as shown in
Fig. 5(c). This implies that the IS signal from this part
of the analyser is made primarily of low-energy secondary or tertiary electrons of energies < 10 eV. It is likely
that most of these electrons are indeed tertiaries at the
output aperture of the inner cylinder or at the slit aperture holder itself, as both the backscattered and secondary electrons that are produced at the field trimmers are
accelerated under the influence of the field towards the
inner cylinder, as depicted in the electron trajectory
simulation of Fig. 4.
There are two mechanisms/processes that could give
rise to these peaks. The first is from the energetic incident electrons striking the edges of the five field trimmer
metal rings. These will have a high secondary electron
yield because of their position, making a near-grazing
angle of incidence with the incident electrons compared
with the ground electrode that carries them, which
makes a near-normal angle of incidence. Figure 6 shows
these peaks to appear at an energy where electrons are
centred on the edge of the rings. Further, the edge
sharpness of the metal rings, their thickness and
material could also play a role in contributing to the
appearance of these peaks. Alternatively, these peaks
may arise from the physical presence of the metal rings,
acting to shadow part of the produced backscattered
and secondary electron signal generated at this part of
the analyser. These in turn will cause a single peak from
the earthed electrode to be divided into several peaks,
depending on the number of rings used. In the present
-
-
0'1O0
2.10~
4'10~
100
200
300
400
500
600
Energy (eV)
Field trimmers replaced with a single disk
700
800
900
1000
Plate 1. (a) A photograph of the Varian cylindrical mirror analyser (CMA) showing the position and shape of the field trimmers. (b) A single flat annular disk made out of a PEEK
insulator coated with 200-300 n m gold film that is used in place of the field trimmers shown in (a). (c) Spectrum collected with the single disk shown in (b).
2
6'103
-w
8*102
1*104
INTERNAL SCATTERING OF ELECTRONS IN A CMA
analyser, there are five such rings, and five peaks are
also observed. It is more likely, however, that both of
these processes will contribute to the appearance of
these peaks.
It is interesting to note that in a normal AES spectrum, and indeed for the present CMA, one does not see
the peaks due to the field trimmers for two reasons. The
first is fundamental and is associated with the signal-tonoise ratio obtainable in an AES experiment. This can
be understood by an inspection of Fig. 5. The peak
heights due to these field trimmers, or more appropriately the peak-to-trough heights, are only a very small
fraction of the peak height at E , , amounting to <1/
1000. This count (or peak-to-trough height) is much
smaller than the statistical fluctuations of electron emission from the sample and the stability of the incident
beam current, and would not therefore be visible in a
normal spectra. The second reason is that the peak
energies due to the field trimmers are a function of the
incident beam energy as discussed earlier. In an AES
experiment the whole spectral energy distribution, N(E),
will act as a source for the production of these peaks.
This will therefore have the effect of smearing off their
appearance in the spectra as a result of being superim-
159
posed on top of each other. All in all, the overall contribution is to cause a smoothing effect to these peaks but
to raise the value of the spectral background of the
obtained spectra as shown in the diagram of Fig. 1.
Finally, the overall contribution of this part of the
analyser to the total IS signal is likely to be roughly
constant with respect to energy, based on the above discussion, but its magnitude will greatly depend on the
transmission and energy resolution properties of the
spectrometer.
The exit slit aperture
The contribution of the final slit aperture to the IS
signal has been suggested by El Gomati and El
Bakush3' to be an important source of IS in this class
of analysers. Figure 7 shows the trajectory of electrons
that leave the sample with an energy of 500 _+ 1 eV in a
solid angle of 42.3 k 6". With the outer cylinder biased
to the voltage required for the analysis of 500 eV electrons, it is clear that some of these electrons strike the
walls of the exit slit aperture. We have used the York
trajectory program to estimate the number of electrons
(c) E= 502 eV
Figure 7. Trajectory simulations of electrons of energies 500 * l eV in a solid angle of 42.3 *6' (a), (b and c) for electrons with energies
498 (b) and 502 (c) and emitted at three different angles of 36.3". 42.3" and 48.3". All simulations are for a CMA pass energy of 500 eV.
M. M. EL GOMATI AND T. A. EL BAKUSH
160
~
that strike the walls of this aperture in the energy and
angular range of E, 5 1 eV and 12", respectively. One
hundred incident electrons of several energies (498, 499,
500, 501 and 502 eV) and an angular interval of 1" were
used. Table 2 shows the number of electrons of various
energies that strike the walls of the exit aperture when
the pass energy of analyser is set to 500 eV. Electrons
emitted with energies (499 eV or > 501 eV do not pass
through this aperture, which has a width of 0.29 mm, as
is evident in Fig. 8(b, c). This is an expected result in
terms of the energy resolution of this analyser. The
results shown in Table 2(a) were based on the assumption of ideal sharp edges. In reality, however, edges are
blunt. T o assess the effects of non-sharp aperture edges,
an exit slit aperture with a truncated vertex was also
considered. The length of the truncated face presented
to the incident electrons was chosen to be 100 and 150
pm. The number of electrons striking the edges of the
aperture was found to increase as the vertex varied from
a point to an increasingly broad face (Table 2).
Using a monoenergetic beam of incident electrons
and negatively biasing a high-transparency metal mesh
in front of the electron detector to - 10 V should inhibit
any of the low-energy secondary electrons from reaching the electron detector. In performing this experiment,
it was possible to reduce the total electron signal at E ,
by about 18-20%, which is in very good agreement with
the simulation results. The above experiment was
repeated with the incident electron beam scanned to
fill-in the input aperture of the inner cylinder. This to
some extent simulates electrons that leave the sample
with angles in the range 36.3-48.3". Negatively biasing
the metal mesh in front of the detector to -10 V has
shown a reduction of up to 25% of the signal at E , in
this case.
The above results have been confirmed in AES by
measuring the P/B of a Cu elemental standard as
described earlier. The results obtained are given in
Table 2. The percentage of electrons with different energies
that strike the walls of the exit aperture and produce
secondary electrons: (a) for an ideal case where edges
are very sharp (i.e. assumed as a point); (b) truncated
edges of 100 pm, (c) truncated edges of 150 pm
-
Electron energy
(a) Sharp edges
498 eV
499 eV
500 eV
501 eV
502 eV
(b) Edges
498 eV
499 eV
500 eV
501 eV
502 eV
-
-
0 29 mm apeflure
0.58 mm apenure
-
13
14
9
13
11
-
-
14
12
100 pm
( c ) Edges - 1 50 pm
498 eV
499 eV
500 eV
501 eV
502 eV
13
18
13
-
-
23
28
23
-
18
19
-
19
19
29
30
-
30
27
Table 3. Peak-to-background ratios measured from the spectra
collected with different detector operating conditions
Improvement
with respectintoPJB
Detection condition
( P P ) C"
With no grid
With double grid
With double grid and a
biased grid
0 0834 0 006
0 0927 0 006
0 0967 *O 006
*
*
case with no grid
11%
15%
Table 3. These clearly confirm that an improvement of
-15% is obtained in the P/B ratio by modifying the
configuration of the electron detector. This figure of
15% should be compared with an expected total IS
signal from the deflection experiment amounting to
about 25-30%. Although there is a difference in the
overall ratio in the two classes of experiments, amounting to nearly a factor of 2, the trends of the obtained
results are consistent with the given interpretation. The
reason for this discrepancy could be due to the difference in the energy and angular distribution of the
emitted electrons in the two experiments.
The input and output apertures of the inner cylinder
The contribution of these two sources is rather difficult
to measure or quantify. Their effects are likely to come
from the high-transparency mesh that covers them, the
purpose of which is to stop field penetration. It is
unlikely that the presence of the metal mesh will act as
a source of the detected IS electron signal. This is based
on similar P/B ratios obtained from CMAs which either
do not have a metal mesh at the inner cylinder, as
reported by Frank et a1.,34or from a recent design by
Flora et al.j5 which replaced the metal mesh with a
family of longitudinal wires giving an 87.5% transparency. In addition, it has been shown in the trajectory
simulation of Fig. 7 that electrons with energies which
are greater or less by a few electron-volts than those
being analysed that manage to exit from the inner cylinder tend to strike other parts of the aperture holder and
hence are not detected. Any scattering off the mesh is
likely to cause a larger energy loss than just 2-3 eV and
hence such scattered electrons tend to have a different
trajectory from those under analysis, i.e. with respect of
the voltage of the outer cylinder. We have also halved
the size of the input aperture but have kept the aperture
at the output the same. Repeating the experiment
depicted in Plate 1 resulted in similar values in terms of
the internal scattering at E , . It is concluded therefore
from this experiment and the preceding discussion that
the input and output apertures of the inner cylinder are
not a significant source of IS in this analyser.
The outer surfaces of the inner cylinder
Internal scattering from this surface can occur as a
result of energetic electrons backscattered from the
outer cylinder towards the inner cylinder. These and
other generated secondary electrons could impinge on
the outer surfaces of the inner cylinder and in so doing
INTERNAL SCATTERING OF ELECTRONS IN A CMA
initiate further secondaries or tertiaries that may later
on find their way to the electron detector. It could also
be due to low-energy secondary electrons from the
sample, with respect to those being analysed, which are
projected (i.e. turned hard on) onto the surface of the
inner cylinder as they enter the analyser. This is
depicted in the trajectory simulation of Fig. 8. These too
could initiate another cascade of secondaries and tertiaries that may contribute to the total IS signal. We
have used electron trajectory simulations to study the
fate of electrons that leave the surface of this electrode
at random energies and angles of emission. The results
obtained suggest that none of the emitted electrons as a
result of this mechanism could be detected in the
spectra.
The above result is confirmed by repeating the experimental arrangement shown in Fig. 3. By detecting the
electron energy distribution to beyond the incident
beam energy E , , it is possible to compare the obtained
results with the above simulation experiment. In this
case, the incident electrons have energies less than the
analysis energy and hence, as schematically shown in
Fig. 8, these could resemble both backscattered electrons reflected from the outer cylinder and low-energy
electrons leaving the sample. Figure 5 shows the experimental results obtained. It is clear that there is no signal
beyond the E , energy, which confirms that although
scattering off this surface must occur, the emitted electrons are collected inside the analyser (probably flowing
to ground) and hence are not detected. In an earlier
experiment, the surface of this electrode was coated with
a low secondary electron yield carbon dag to reduce
electron emission from its surface. No visible change in
the P/B ratio was obtained as a result of this treatment
either. The conclusion of these experiments suggests
that this electrode is not a significant source of internal
scattering in the output signal from a CMA.
DISCUSSION
The present study has identified a number of the most
likely sources of internal scattering of electrons in a
single-pass CMA. Of the six different sources considered, the exit slit aperture and the field trimmers are
161
found to be the most significant contributors, while the
inner surfaces of the outer cylinder were found to contribute much less than 1.5% of the total IS signal.
Further, based on the findings of the present work and
using simple geometrical arguments, this source is
unlikely to be a major IS source for other CMAs of
similar design principles. However, the only part of the
outer cylinder which could act as an IS source is that
part facing the exit aperture of the inner cylinder, i.e.
part C on Fig. 2. The effect of this source will be similar
to the field trimmers (as can be seen in Fig. 5 ) with the
peak at -622 eV. It may be argued in this case that the
field trimmers should be designed such that they are
placed as close as possible to the outermost electron
trajectory envelope as shown on the bottom part of Fig.
2. In this case the more energetic electrons will be
trapped in-between the various trimmers electrodes. In
addition, most generated secondary electrons from this
source will not have a direct line of sight to the exit
aperture, but rather they will be most likely to strike the
inner cylinder and be transmitted to ground. Such an
arrangement is expected, from simple geometrical arguments, to reduce the size of this signal by up to an order
of magnitude in comparison with the present geometry.
An alternative scheme to filter this IS signal in the case
of systems employing electron counting could be
achieved by raising the threshold of the electron detector to above the energy of the IS electrons.
Eliminating IS from spectra is fundamental for quantitative AES and SAM. It is interesting to note that the
study reported by SeahI5 for a number of different
CMA designs has shown varying degrees of IS. It is
likely that these variations are due to changes in the
different design or constructions of the analysers
studied. For example, it is very unlikely that a doublepass CMA will have an IS contribution from field trimmers. This is because the second pair of mirrors does
not see any electrons of higher energies than those being
analysed and hence any generated electrons from field
trimmers in the first pair of mirrors, if present, will be
filtered by the second pair. Also, the PHI (Perkin
Elmer) single-pass CMA design (PHI Model 590 scanning Auger microscope) is unlikely to show the effect of
the field trimmers. In this design a small ‘biased’ ball is
placed in-between the exit aperture and the electron
detector, which could be acting in this respect to deflect
outer cylinder
-Electrons with mergy less than the analysed ones falling on the inner cylinder
~
Secondary electrons generatedat the inner cylinder.
Figure 8. Trajectories of electrons entering the cylindrical mirror analyser (CMA) with energies less than those being analysed. This resembles what happens in an AES experiment where spectra are collected sequentially in energy.
162
M. M. EL GOMATI AND T. A. EL BAKUSH
the low-energy electrons produced as a result of energetic electrons striking the field trimmers. Goto et aL9 in
their design used a pair of longer cylinders to avoid field
changes around the exit aperture of the inner cylinder.
Although this eliminates the need for field trimmers, the
long cylinders are usually undesirable as they occupy
space that can be best used for other purposes.
The exit slit aperture is found to be the major source
of IS in the present design. The effects of this source,
however, are likely to be present in CMAs but with
varying degrees from one instrument to another,
depending on the detailed construction of the final exit
slit aperture. Internal scattering from this source has
been reduced in the work reported here by negatively
biasing a metal mesh in front of the electron detector to
- 10 V. This has the undesirable effect of losing the lowenergy part of the spectrum. A different solution under
investigation in the authors’ laboratory is to modify the
field trimmers so that they are placed at the outermost
trajectory as shown in the lower part of Fig. 2.
CONCLUSION
A theory/experiment comparative study of the likely
sources of internal scattering of electrons in a CMA is
reported. A total of six such sources have been considered. The results obtained have eliminated one
source, the outer cylinder, as being the most significant
contributor of 1s in this CMA. ~
~it has identified
~
~
a new source, the field t r h m e r electrodes placed inbetween the two cylinders, as a significant source of IS
in this analyser. Backscattered and secondary electrons
off this area of the analyser are thought to impinge on
the output aperture of the inner cylinder. As a result of
this process, tertiary and more remotely related electrons with energies c 10 eV are produced. These have a
direct line of sight to the electron detector and hence
contribute to the collected spectra as background. The
output exit aperture is found to be by far the largest
source of IS in this class of analysers. It was found that
the effect of internal scattering can be reduced by the
addition of a negatively biased high-transparency metal
mesh in front of the electron detector. In the present
set-up this arrangement does not allow the potential of
the front end of the electron detector to be altered. The
drawback of this solution is the loss of that part of the
spectrum up to -20 eV. However, this region does not
usually have usable Auger peaks and hence this is considered to be a relatively small price to pay for the
improvement of the quality of the spectra. Perhaps it
would be better to place a double mesh in front of the
electron detector. This will allow the potential at the
front end of the electron detector to be altered to
improve its gain at low voltage operation. Finally, a
new experimental arrangement combined with electron
trajectory simulations is suggested as a diagnostic tool
to study the relative contribution of the various sources
of IS in this class of analysers.
Acknowledgement
The authors would like to thank DI S. Bean Of York Electron Optics
h
~
~
,
(YEO) of the University of York, and Dr M. Seah of NPL, UK for
useful discussion, and D r R. Roberts for the critical reading of the
manuscript.
REFERENCES
1 . P. H. Holloway and D. M. Holloway, Surf. Sci. 66, 635
(1 977).
2. S. lngrey and W. D. Westwood, Surf. Sci. 100, 281 (1 980).
3. F. C. Eagen and E. N. Sickafus, Rev. Sci. lnstrum. 48, 1269
(1977).
4. N. E. Erickson and C. J. Powell, Surf. Interface Anal. 9, 1 1 1
(1 986).
5. D. C. Peacock, M.Prutton and R . H. Roberts, Vacuum 34,497
(1984).
6. E. N. Sickafus and P. H. Holloway, Surf. Sci. 51, 131 (1975).
7. M. P. Seah, J . Electron Specrrosc. Relat. Phenom. 48. 209
(1989).
8. M. P. Seah and G. C. Smith, Surf. lnterface Anal. 15, 751
(1 990).
9. Goto et a / . ,QSA-8. Surrey (1 994).
10. M . P. Seah and G. C. Smith, Vacuum 41,7 (1 990).
1 1 . M. P. Seah, J . Electron Spectrosc. Relat. Phenom. 50, 137
(1 990).
12. M. P. Seah and G. C. Smith, Surf. Interface Anal. 17, 855
(1991 ).
13. M. P. Seah, M. E. Jones and M. T. Anthony, Surf. Interface
Anal. 6, 242 (1 984).
14. M. P. Seah, Surf. Interface Anal. 20, 865 (1 993).
15. M . P. Seah, Surf. Interface Anal. 20, 876 (1993).
16. J. C. Greenwood, M. Prutton, R. H. Roberts and Z. Liu, Surf.
Interface Anal. 20, 891 (1 993).
17. T. A. El. Bakush, M. M.El Gomati and 2.Liu, Insr. Phys. Conf.
Ser. 138, 213 (1992).
18. M .M. El Gomati and C. G. H. Walker,Appl. Surf. Sci. 35, 177
(1 988).
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
M. Prutton and M. M. El Gomati, Surf. Interface Anal.
9, 99
(1986).
G. Ichimura, R. Shimizu, T. Sekine, K. Goto and H. Shimizu,
Surf. Interface Anal. 11, 94 (1 988).
R. Shimizu and 2.-J. Ding, Rep. Prog. Phys. 487 (1992).
I. R. Barkshire, M. Prutton, J. C. Greenwood and M . M . El
Gonati, Surf. Interface Anal. 20, 984 (1993).
M . P. Seah, Surf. Interface Anal. 9, 85 (1 986).
C. Powell and N. E. Erickson, J . Electron Specrrosc. 25, 87
(1 982).
T. Omhura and R . Shimizu, in Analytical Electron Microscopy,
edited by D. C. Joy, p. 325. San Francisco Press, CA (1987).
J. A. D. Matthew, W. C. C. Ross and El Gomati, Inst. Phys.
Conf. Ser. 130, 383 (1992).
J. C . Greenwood, M. Prutton and R . H. Roberts, Phys. Rev. B
49,12485 (1994).
D. A. Dhal, J. E. Delmore and A. D. Appelhans, Rev. Sci.
lnstrum. 61, 607 (1 990).
T. A. El Bakush, D. Phil thesis, University of York (1994).
T. A. El Bakush and M. M. El Gomati, J . Electron Spectrosc.
Relat. Phenom.. 74, 109 (1995).
V. V. Zashkvara, M . I. Korrsinsku and 0. S. Komsmacher, Sov.
Phys. Tech. Phys. (English Transl.) 11, 96 (1966).
M. M . El Gomati and T. A. El Bakush, QSA-7, Surrey (1 992).
M. P. Seah and G. C. Smith, Surf. Interface Anal. 15, 751
(1 990).
P. Vasina and L. Frank, J . Phys. E 12, 744 (1 979).
P. S. Flora, Huang Min, C. J. Harland and J. A. Venables, Inst.
Phys. Conf. Ser. 98, 299 (1 989).
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