MASS SPECTROMETRY
FOR
SURFACE ANALYSIS
177
the
influence
of the
sample matrix upon
the
ionization probability
and
therefore makes quantification more reliable.
The
ionization
step
can be
accomplished using electron beam, plasma
or
laser post
ionization.
A
method closely associated with SIMS,
in
that
the
mass anal-
ysis
of
emitted ionized atoms, molecules,
and
clusters
is
undertaken,
is
laser ablation microprobe mass spectrometry (LAMMS).
The
lateral
and
depth resolutions attainable with LAMMS
are
depen-
dent
on
both
the
power
and the
focus
conditions
of the
pulsed laser
beam.
A
fully
focused beam will provide very
good
spatial resolution
but
as
the
power density
at the
specimen surface
is
very high
the
crater
formed
will
be
relatively deep; this
is the
so-called
'hard'
laser ioniza-
tion
mode
of
LAMMS.
For the
same power output
a
fully
defocused
beam
will create
a
crater which
has a
larger diameter
but is
much
shallower;
this
is the
'soft' laser desorption mode (0.1
um is
probably
the
best 'sampling
depth'
achievable with LAMMS
at
present).
It is
interesting
to
note that this technique
was
originally developed
as a
bulk
microprobe instrument complementary
to EDX and
giving ele-
mental
and
isotopic information down
to
hydrogen.
It is
available
in
both
the
reflection mode, analogous
to
SEM/EDX,
and
transmission
configuration,
the
TEM/EDX
equivalent. Although
the
mass spectrum
obtained
by
LAMMS does
not
give
an
exclusive surface analysis
in
the
same
way
that SSIMS, XPS,
or AES
does,
it
will often provide
valuable
information concerning
the
surface
phases. This
is
because,
in
the
ablation process,
the
volatilized material
of the
crater will neces-
sarily
include
that
at the
very surface.
The
high sensitivity
of
mass
analysis
techniques such
as
SIMS
and
LAMMS ensure that
any
unsus-
pected elements present
at the
surface
are
clearly
defined
in the
resul-
tant mass spectrum.
The
major
advantages
of
LAMMS
at the
present
time
are its
ability
to act as a
light
element/isotopic
specific micro-
probe,
and the
rapidity with which
it can
profile
thin
films; the
depth
of
each laser shot
can be
matched
to film
thickness
by
varying
the
operating conditions
from
'soft'
to
'hard'.
Unlike
SEM and
SIMS,
insulating
samples
can be
analysed without
any
special sample pre-
paration
or
charge compensation being required.
The
main problem
is
quantification
of the
resultant spectrum, although semi-quantitative
data
can be
achieved quite
successfully
using standards
of
similar
composition.