20
ELECTRON
SPECTROMETER
DESIGN
The
mounting
of
conducting samples
is
best achieved with clips
or
bolt-down
assemblies.
For XPS the use of
double-sided adhesive
tape
can be
used
but
only sparingly because
it can
have mobile materials,
such
as
release
agents,
on its
surface which
can
contaminate
the
surface
under investigation.
For
conducting specimens,
a fine
stripe
of
conducting paint,
in
addi-
tion
to the
adhesive
tape,
is all
that
is
necessary
to
prevent sample
charging. Solvents
in the
conductive paint
can
cause
the
pump down
time
to be
extended
as
they
evaporate
into
the
vacuum. Alternatively,
metal tape with
a
metal loaded (conducting) adhesive
may be
used.
Most
laboratories
have
a
selection
of
sample
holders,
usually fabricated
in-house,
to
accommodate large
and
awkwardly shaped specimens. Dis-
continuous specimens present rather special problems.
In the
case
of
powders,
the
best method
is
embedding them
in
indium
foil,
but if
this
is
not
feasible, dusting them
onto
double-sided adhesive tape
can be a
very
satisfactory alternative. Fibres
and
ribbons
can be
mounted across
a gap in a
specimen holder ensuring that
no
signal
from
the
mount
is
detected
in the
analysis.
The
type
of
sample mount varies with
the
instrument design
and
most
modern spectrometers
use a
sample stub similar
to the
type employed
in
scanning electron microscopy,
or a
sample platter that
will
accommo-
date many samples.
For
analysis,
the
sample
is
held
in a
high-resolution
manipulator with
x, y, and z
translations,
and
tilt
and
rotation about
the
z-axis (azimuthal rotation).
For
scanning Auger microscopy, where
the
time taken
to
acquire high-resolution maps
can be
about
1 h, the
stabil-
ity of the
stage
is
critical, since
any
drift
during
analysis
will
degrade
the
resolution
of the
images. Image registration software, used during
ac-
quisition,
can
mitigate
the
effects
of a
small
amount
of
drift.
For
angle
resolved
XPS
(ARXPS),
the
amount
of
backlash
in the
rotary drive must
be
small
and the
scale should
be
graduated
in
increments
of 1 for
manual
operation.
Once
mounted
for
analysis, heating
or
cooling
of the
specimen
can be
carried
out in
vacuo.
Cooling
is
generally restricted
to
liquid
nitrogen
temperatures although liquid helium
stages
are
available. Heating
may
be
achieved
by
direct (contact) heating using
a
small
resistance heater
or
by
electron bombardment
for
higher temperatures. Such heating
and
cooling will either
be a
preliminary
to
analysis
or
carried
out
during
the
analysis
itself
(with
the
obvious exception
of
electron bombardment
heating). Heating
in
particular
will
often
be
carried
out in a
preparation