Page 205 - Inorganic Mass Spectrometry - Fundamentals and Applications

Secondary Ian Mass S~ectramet~ 191

The most satisfactory results for SIMS quanti~cation have been achieved
through the use of relative sensitivity factors (RSFs). In its simplest form, RSFs
are determined by measuring ion collection efficiencies from known matrices and
recording them as a ratio to a reference element that is usually the most abundant
metallic atom in the matrix. A matrix that matches the unknown is necessary. If a
good matrix match is available, excellent results can be achieved”especial1y in the
dilute concentration range. A relative se~sitivi~~actor (RSF) may be defined as

where S,, is the RSF for element i with respect to the matrix element m; Cj and
Cm are the atomic concentrations of impurity element i and matrix element m, re-
spectively; and the isotopes measured are j for element i and y1 for element m. If
of
relative sensitivity factors for all the elements in an unknown similar matrix have
been determined, then the concentration of each element x in the unknown may be
calculated from

If only some of the elements are of concern and the concentration of the ma-
trix element is known or can be reasonably estimated, then Eq. (4.5) can be used
to solve for each unknown.
The method of defining RSFs described is traditional in analytical chemistry,
generates RSFs without units, results in larger numbers elements for which the
for
SIMS inst~ment is more sensitive, and is essentially the same as Wittmaack’s pro-
posed use of “scaled sensitivity ratios” [ 1001. However, an alternative de~nition of
sensitivity factors that has gained much popularity with semiconductor specialists
is that of Wilson [69,101]:
pi = (I~I,) RSF (4.7)
where pi is the impurity atom density in atoms per cubic centimeter (atomslcm3)
and the other symbols are as described. For practical use with nonmonoisotopic
elements, the ion intensities need to be corrected for isotopic abundance, and the
equation becomes

With this definition, RSF has units of atoms per cubic centimeter, and an element
RSF.
that is detected with more sensitivity in the SIMS instrument has a smaller