Page 149 - Inorganic Mass Spectrometry - Fundamentals and Applications

Induct~vely ~o~pled Plasma Mass Spectrometry 139

because any sample preparation or handling enhances chances of contamination
[368]. However, many of the acids can cause sample introduction problems if run
of
directly. Often, large sample volumes high-purity acids are evaporated in order
to improve detection limits. Great care must be taken to prevent contamination.
The purity of silicon and gallium arsenide starting materials has been
for
measured by using ICP-MS [370-3721. Impurities in materials used as dopants
semiconductor^ have also been measured by using ICP-MS [373].
Vapor phase dissolution (VPD) is comonly used for surface and contam-
ination analysis of serniconductor wafers [374-3791. HF vapor is used to remove
a silicon oxide or native silicon layer. A drop of hydrofluoric acid or deionized
water (with a volume of SO to 200 pL) is placed on the surface and rolled around
the surface to dissolve the metals. The small drop is then analyzed by ICP-
using either a direct injection nebulizer, a micronebulizer, or ETV. The ability of
ICP-MS to measure several elements rapidly in a small volume of solution is
essential.
Trace impurities in some gases used for semiconductor processing have
also been monitored using ICP-MS [380-3821. It may be necessary to use
specialized materials for the sampler and skimmer to prevent degradation. In one
study, the level of iron from a hydrogen chloride cylinder was found to increase as
the cylinder was used and to depend on the cylinder-valve package [382].
Impurities in photoresists have also been measured by ICP-MS [383,384].
Ultrasonic nebulization and electrothermal vaporization sample introduction ap-
proaches have been used.
The current analytical capabilities of ICP-MS provide a means assess new
to
low levels of contamination in the semiconductor industry [38S]. Contamination
in clean room air can be detected at very low levels. Dopant and trace metal
contamination on semiconductor wafer surfaces can be monitored. Ultratrace
metals in deionized water, high-purity acids, and other process chemicals can
often be measured at concentrations less than 1 part per trillion.

In many chemical systems, the form of an element controls its biological or
chemical effect and its transport or remediation. Elemental ions in different oxida-
tion states, as different metal-ligand complexes and various organometallic mole-
cules, often have very different toxicity. Certain species are essential nutrients
[Cr(III), for example], whereas others are toxic or carcinogenic [such as Cr(VI)].
Organo~etallic mercury compounds, such as methylmercury, are far more toxic
than inorganic mercury. Trialkyltin compounds are highly toxic and tin is present
in some food cans. The transport of metals through soils and water systems is
highly species~dependent, Industrial processes may also be highly dependent on
elemental speciation. Therefore, elemental analysis alone often provides insuffi-