Page 148 - Inorganic Mass Spectrometry - Fundamentals and Applications

138 Olesik

more important problem as the size of the structures in the semiconductor devices
is reduced, Metal contamination levels in processing solutions as low as 50 parts
per quadrillion have been identified as potentially deleterious. The operating
are
properties, dark current, and oxide breakdown voltage dependent on contd-
nant and dopant element levels in semiconductors [356]. Particulates collected in
be
traps used to maintain clean conditions must identified in order to reduce their
occurrence. The use of ICP-MS for the analysis of samples related to semiconduc-
tor device production has recently been reviewed 1357-3591.
The needs of the semiconductor industry have been one the main driving
of
forces to attain detection limits in the low parts-per-quadrillion level. Sector-based
ICP-M~ instruments are often used in a low-resolution mode to provide the very
high sensitivities that are needed. Many the elements of interest are below mass
of
80. Therefore, the use of reaction cells in ICP-MS inst~ments in order to reduce
molecular ion signals and other background likely to have a major impact on the
is
analysis of materials used in the semiconductor industry. The importance of
is so key
procedures for analysis of these materials with very low detection limits
to the comercial success of the semiconductor manufacturers that details of the
ICP-MS-based methods may not be published. Those that are published may be
modified somewhat from what is done in practice.
igh-purity acids are used for cleaning and etching of semiconductor mate-
rials. The analysis challenges include attaining sufficiently low detection limits,
avoiding conta~nation (including from the extraction of elements on tubing and
sample introduction systems), and dealing with molecular ion spectral overlaps
from plasma species, water de~omposition products, and the acids themselves.
Among examples of ICP-MS analysis of acids used in semiconductor processing
are hydrofluo~c acid [360,361], sulfuric acid 13621, hydrochloric acid [363], and
phosphoric acid 13641.
Several approaches have been used to reduce spectral overlaps due to
molecular ions. The use of “cold plasma,’ conditions, described earlier, is well
suited to the analysis of high-purity acids. Fe, Ca, and K, which suffer from severe
spectral overlaps with Ar0+, Ar+, and 3*ArH+, under normal plasma conditions,
can be measured in the low-ppt level. The argon ion number density in the ICP is
greatly reduced by operating at lower temperatures. Analyte ion signals for
with low ionization energies are not significantly degraded, Sector-based
inst~ments have been used for acid analysis in low-resolution mode to
provide high sensitivity and in high-resolution mode to overcome spectral over-
laps [365]. ETV-ICP-MS has also been used for high-purity acid analysis [366-
3681 to avoid ArO+ and other 0- or H-containing molecular ions.
The sample introduction system, sampler, skimmer, and ion optics can be
sources of contamination that produces high ICP-MS blank signals so proper
cleaning and maintenance are essential [369]. Some have argued that no dilution
or evaporation (for preconcentration) of high-purity acid samples should be used