Page 195 - Inorganic Mass Spectrometry - Fundamentals and Applications

Secondary Ion X ass Spectrometry l81

of
only when the beam is located within a preselected portion the raster. Mechan-
ical aperturing is used with microscope imaging instruments.
A neutral component of the primary beam is formed when the primary ions
collide with residual gas in the primary beam column. This component is mini-
mized by designing the primary column for rapid pumping to maintain ultrahigh-
vacuum conditions. The neutral component is not focused and thus sputters sur-
faces around the crater formed by the ion beam. If elements at the surface are being
profiled, the continued presence of these elements, due to the neutral beam sput-
tering, leads to errors in the profile and limits the dynamic range of the profile.
Coating the surface with a thin layer a nonprofiled element reduces the effect.
of
Gross discrepancies from actual in-depth distribution can result from elec-
trical or chemical potential gradients produced as a result the ion bombardment,
of
by
Electrical gradients that are created charge buildup of imperfectly compensated
is
insulators can have dramatic effects, as shown in Fig. 4.23, where sodium pro-
filed in silicon dioxide on silicon
[SO]. With positive ion bombardment the insula-
tor is charged positively, driving the electropositive sodium into the matrix. When
the conducting silicon substrate was reached, the charging field collapsed and a
large spike of sodium was encountered as the sodium lost its mobility. Gibbsian,
or chemical potential, segregation effects have been demonstrated by profiling a
of
number of oxides with widely varying heats formation in a silicon matrix (with
and without an oxygen leak, Fig. 4.24) [8l]. For elements with heats of formation
less than that of silicon, the decay length with the oxygen leak increased as silicon
of
segregated to the surface to form the oxide. For elements with heats formation
greater than that of silicon, the decay length shortened as they preferentially seg-
regated to the surface.

the
The formation of secondary ions in the sputtering process depends strongly on
electronic structure of the target matrix and the ionization potential of the atoms
(positive ions) or the electron affinity (negative ions). In the sputtering process,
collisions between the bombarding ions and the target atoms cause excitation of
the atoms, which may de-excite through photon or electron emission. The proba-
bility of electron emission depends upon the ionization potential; thus, alkali and
alkaline earth metals with low ionization potentials form positive ions with much
higher probability than elements with higher ionization potentials. In the sputter-
ing process, electrons are also liberated and negative ions are formed by electron
capture. Thus, the halogens and group VI A elements preferentially form negative
ions. These periodic trends may be noted in the positive and negative secondary
ion yields of a large number of elements [82] (Figs. 4.25 and 4.26). The observed
intensity is dependent not only on the number of ions generated but also on the sur-
vivability of the ion as it leaves the surface. For positive ions, survivability de-