Page 210 - Inorganic Mass Spectrometry - Fundamentals and Applications

196 Cristy

Gas Inlet

~ollow Cathode
lnte~ediate Electrode

-Magnet
Anode

ion Extractor
P

Duoplasmatron ion source schematic. (From Ref. 108.)

ionization cesium ion source (Fig. 4.329, and the liquid metal ion gun (Fig. 4.33).
Desirable features of any ion source are high brightness, high current density, low
energy spread, long-term stability, and long life.
The most comon species used with SIMS sources are h+, 02+, 0-, and
N2+. These ions and other permanent gas ions are formed easily with high bright-
ness and stability with the hollow cathode duoplasmatron. &+ does not enhance
the formation of secondary ions but is popular in static SIMS, in which analysis of
the undisturbed surface is the goal and no enhancement is necessary. 02+ and 0"
both enhance positive secondary ion count rates by formation of surface oxides
that serve to increase and control the work function of the surface. 02+ forms a
more intense beam than 0" and thus is used preferentially, except in the case of
analyzing insulators (see Chapter l l). In some cases the sample surface is flooded
with 0, gas for surface control and secondary ion enh~cement. An N,+ beam en-
hances secondary ion formation, but not as well as O,+. It is very useful for pro-
filing and analysis of oxide films on metals, however. It also is less damaging to
duoplas~atron hollow cathodes and extends their by a factor of 5 or more com-
life
pared to oxygen.
The detection sensitivity for carbon; hydrogen; the electronegative ele-
ments-F, Cl, 0, S, etc.-and certain metals with low work functions (such as Au
and Pt) is greatly enhanced by analyzing their negative secondary ions, sputtered
by a CS+ ion beam. Most cesium ion sources use heated metal to provide vapor to