Page 212 - Inorganic Mass Spectrometry - Fundamentals and Applications

198 Cristy

the back of a tungsten frit. The cesium diffuses through the frit, where it is ther-
mally ionized. In the presence of an electrostatic field, CS+ is accelerated into the
A
instrument (Fig. 4.32). drawback of cesium is the necessity of handling the metal
in inert gas glove boxes. Some have overcome this problem by using cesium com-
al. [ 1091 heated
et
pounds that decompose or react to release cesium in situ. Okutani
cesium chromate with silicon, which reacted to release CS and form residues of
and Cr20,. As with oxygen, cesium is sometimes evaporated onto the surface
directly. This ensures saturation of the work function effect for CS+ sputtering, or
alternatively it enhances the negative ion yield for noble gas sputtering.
The liquid metal ion source (LMIS) provides the highest current density and
the smallest probe size. In the LMIS (Fig. 4.33), a tungsten emitter tip is coated
with a liquid metal such as CS, Hg, Ga, In, Sn, Bi, or Au; a high potential field is
then applied. Under these conditions, the liquid metal forms a “Taylor” cone with
a small tip from which ions are emitted. This small, very-high-brightness, very-
high-cu~ent-density (~10~ rVcm2) source allows focusing to spots as small as 20
nm with current densities up to 5 A/cm2. High-resolution secondary ion imaging,
most commonly with Ga+ and In+ ions, is the forte of LMIS. The very fine intense
LMIS beam has also been used for micromilling and cutting in semiconductor
analysis and repair.

The primary ion column may be nothing more than a vacuum between the ion
it
source and the sample target, but generally contains some beam shaping and con-
trolling elements. The MMA diagram in Fig. 4.5 illustrates common features. The
ion extractor may be considered to be part of the ion source or the first element of
the
A
the primary column. potential difference between the extractor and ion source
accelerates the primary ions to the desired energy, generally between 0.2 and 40
keV. It is desirable to filter the primary ion beam so that only the selected species
strike the sample. The IMMA does that with a primary magnetic sector; other in-
struments use a Wien filter. Beam shaping and directing are done with electrostatic
alignment plates, apertures, and one or more electrostatic lenses. With scanning
probe-i~aging instruments, electrostatic deflection plates are required to control
the position of the beam on the target. Time-of-flight instruments require pulsed
acceleration of the primary ions and often have electric or magnetic field “bunch-
ing” elements to sharpen the pulse primary ions.
of

The sample mount requires an electrical connection to maintain a constant poten-
tial on the sample surface. In quadrupoles this generally ground, and extraction
is
lenses are biased to collect the secondary ions. Magnetic sector instruments usu-