Page 286 - Inorganic Mass Spectrometry - Fundamentals and Applications

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In any glow discharge device, there are complex inte~elationships among the dis-
charge voltage, current, and pressure, In the case of using a secondq cathode,
these dependencies take on different meanings as redeposition process is inte-
the
gral to the success of the methodology, whereas it is generally thought of as a neg-
ative attribute in other CD implementations. Simply put, these parameters must
be
balanced to achieve the fo~ation of a metallic overlayer of proper thickness. Sec-
ondary issues come about from the desire to achieve rapid plasma stabilization,
low amounts of signal drift, and unifo~ sputter-erosion in the case of depth pro-
filing of insulators. Figure ’7.4. illustrates the importance of choosing the proper
discharge pressure (presented here as the pressure in the inte~ediate chamber be-
of
tween the ion source and mass analyzer) through temporal profiles the ion sig-
nals for the case of using a tantalum mask for a glass sample [21]. At the highest
pressure (curve a), extensive redeposition is evidenced by the steady decrease in
Si+ intensity over time, The lowest pressure (curve b) effects far less redeposition
and so the opposite trend is seen. Finally, at a moderate pressure (curve c) the
plasma is seen to reach a steady state in approximately 12 min. Source stability
evaluated at this set of conditions using a NIST reference material was on order
the
of 6% RSD using ion beam ratios over a 150-min analysis period. Van Crielcen et
al. [22] also showed that variations in discharge power (voltage and current) at a
single source pressure influence the extent of redeposition (via samplelmaslc ion
beam ratios),
The group at the University of Antwerp has done extensive evaluations of
the possible scope of the secondary cathode methodology [23]. The evaluations
identified a number of other “electrical” dependencies. For example, the conduc-
tivity of the analytical sample influences the required voltage and current to
sus-
tain a stable discharge, and thus the interplay with source pressure to achieve a
proper level of redeposition. This disparity was first evidenced in a comparison be-
tween the analysis of bulk nonconductors (glass) and compacted oxides, Although
both sample forms can be grossly classified as “nonconductors”, differences in ab-
solute resistivity do affect perfo~ance, As is well known throughout the GI) lit-
erature [521, the applied potential also plays a key role in the evolution of the sput-
tered crater. As such, there may exist trade-offs between the discharge conditions
that produce the desired analyte signal qualities and those for obt~nin~ adequate
depth resolution for a given application. Finally, the surface roughness of the in-
sulating sample has an effect on the optimal discharge conditions and equilibra-
tion times E221. In the case of very rough samples, greater amou~ts of redeposition
are required to fill crevices in the surface with the conductive overcoating,

The major advantage of using the secondary cathode method for nonconductor
analysis is that the sample is analyzed directly in its native form. Because grind-