XPS
IMAGING
AND
MAPPING
5 ]
of
the
spot
of
electrons
on the
X-ray anode. Therefore,
if the
spot
of
electrons
is
scanned
on the
anode,
the
X-ray beam will
be
scanned
on
the
sample surface. Again,
the
transfer lens
can be
operated
in its
maximum
transmission mode because
it
does
not
contribute
to
the
spatial resolution.
The
advantages
of
this method
are the
same
as
those
for
source-defined small area
XPS.
The
disadvantage
is
that
the field of
view
is
very limited. This
is
especially true
in the
direction
of the
Bragg angle.
As the
electron beam
is
scanned over
the
anode,
the
diffraction angle
is
changing
and so the
wavelength
of
the
X-rays reaching
the
sample
is
changing.
In
turn, this means
that
the
kinetic energy
of the
photoelectrons will change
and so the
energy
to
which
the
analyser
is
tuned must
be
adjusted
as a
function
of
distance.
If the
deflection angle
is too
large there
will
be no
intensity
in the
AlKa radiation
at the
required wavelength
and so
photoemission
is not
possible.
If it is
necessary
to map
large areas
of
the
sample, this method must
be
used
in
combination with stage
scanning.
2.8.2
Parallel acquisition
In
parallel acquisition
of
photoelectron images,
the
whole
of the field of
view
is
imaged simultaneously without scanning voltages being applied
to any
component
of the
spectrometer.
To
obtain images
via
this route, additional lenses
are
required
in the
spectrometer. These must also
be
equipped with
a
two-dimensional
detector. Figure
2.16
shows schematically
how the
method works.
The
photoelectrons pass through lenses
1 and 2 in the
transfer lens
assembly,
producing
a
photoelectron image
of the
specimen surface
at
some plane within
the
lens column
after
each lens
(the
image planes).
Lens
3 is
operated such that
its
focal
length
is
equal
to the
distance
between
the
lens
and the
second image plane. This means
that
electrons
emanating
from
any one
point
on the
image will leave lens
3 on
parallel
paths.
The
angle between
the
beam
of
electrons
and the
lens axis will
depend only upon their distance from
the
lens axis
in the
image plane.
The
electrons then enter
the
analyser, which functions
as
both
an
energy
filter and a
lens.
If the
deflection angle
of the
analyser
is
180°
then
the
angular distribution
of the
electrons
as
they leave
the
analyser
is