34
ELECTRON
SPECTROMETER
DESIGN
The
stringent vacuum requirements
and the
relatively
poor
stability
mean that
the
cold
field
emitter
is
rarely used
for
Auger electron spec-
troscopy.
For
medium-
and
high-performance Auger instruments, either
LaB
6
or
Schottky
field
emitters
are
generally used
in
commercially avail-
able instruments.
The
spot
size
attainable
with
a
particular
electron
gun is a
function
of
the
primary beam current.
For
example,
the
smallest spot
size
obtain-
able
on a
scanning Auger
microscope
with electromagnetic lenses
and
LaB
6
filament is
about
20 nm at 0.1 nA but
increases
to l00 nm at
lOnA.
The
intensity
of
Auger electrons emitted depends
on the
speci-
men
current
and at 0.1 nA
spectrum acquisition will
be a
lengthy pro-
cess,
but at 10 nA the
current
of
Auger electrons will
be
increased
to the
point where analysis becomes practical.
A
satisfactory compromise must
be
reached between spatial resolution
and
spectral intensity.
A field
emission source
in a
similar column will give somewhat better resolution
but at a
much improved current owing
to its
superior brightness. Such
electron
guns
are the
preserve
of
high-resolution scanning Auger micro-
scopes;
such
a gun is
illustrated
in
Figure 2.6.
When
SAM is
required
on a
multi-technique
XPS
instrument,
the
best
configuration
probably includes
a
high-brightness,
hot field
emission
Figure
2.6
Schematic diagram
of a UHV
electron
gun for
Auger electron
spectroscopy