170
COMPARISON
OF XPS AND
AES
WITH
OTHER
TECHNIQUES
(with
a
thin oxide
film
present), obtained
in a
TEM,
is
shown
in
Figure 6.2;
the BKa
line
is
very clear
in the
spectrum.
In
the
scanning Auger microscope
the
analysis depth
is
determined
by
the
energy
of the
outgoing electrons
as
described
in
Chapter
1 and the
spatial resolution depends
on the
size
of the
electron
probe.
Thus,
the
spatial resolution attainable with
SAM can be an
order
of
magnitude
better than that recorded with SEM/EDX. With
the
development
of
sub-micron features in microelectronics SAM is finding a new use: as a
high-resolution chemical imaging facility,
the
emphasis
no
longer being
on the
need
for a
surface
analysis.
Although X-rays
do
show
a
small chemical
shift
with oxidation
state,
this feature
is not
employed
in
analytical X-ray analysis
of the
type used
in
electron microscopes; thus
EDX
only provides elemental information
unlike
the
additional chemical information provided
by XPS and
AES.
The
addition
of an EDX
facility
to a
surface analysis spectrometer
is
worthy
of
consideration.
In the SAM
mode
it is
possible
to
acquire both
Auger
(surface)
and
X-ray
(bulk)
chemical maps
of the
specimen.
A
typical set-up
is
shown
in
Figure 6.3.
In
conjunction with XPS,
an
X-ray detector
is
able
to
provide good
quality fluorescence
spectra
(XRF)
to
within about
2 keV of the
X-ray
source operating potential.
As
this will usually
be
around
15
keV, X-ray
spectra
up to 13 keV can be
obtained. This method
is
particularly
useful
for
insulating specimens
not
amenable
to
analysis
by
AES/EDX such
as
paint
films or
some catalysts.
XRF is
also
useful
in XPS
depth
profiling
where
a
global analysis
of
specimen chemistry
can be
achieved before
segregation
or
interfacial
effects
are
studied
in
detail.
6.2
Electron
Analysis
in the
Electron
Microscope
As
well
as the
possibility
of
X-ray analysis
in the TEM and
STEM,
it is
possible
to
analyse
the
energy
of the
transmitted beam which
is the
basis
of
electron energy-loss spectroscopy
(EELS).
As an
electron beam passes
through
an
electron transparent specimen,
it is
able
to
eject electrons
whose binding energies
are
less than that
of the
primary
beam
energy.
By
recording
the
depletion
of the
primary beam energy with
an
electron
spectrometer positioned below
the
specimen,
an
energy-loss spectrum