Page 204 - Inorganic Mass Spectrometry - Fundamentals and Applications

190 Cristy

at
where I are the corrected intensity values infinite velocity for the element of in-
terest, i, and for all the matrix elements x, y, and z. The infinite velocity intensity
for each element is obtained by altering the extraction voltage and measuring the
secondary ion intensities over four or more increments, plotting the intensity of
the
each element versus the reciprocal velocity (obtained from kinetic energy ex-
pression, E = % mv2), and extrapolating to llv = 0. This method was tested inde-
pendently by Losing et al. [SS] on three homogeneous metal alloys with trace,
major, and minor elements. Negative secondary ions were generated by using a
02+ beam. Ex-
CS+ beam, and positive secondary ions were generated by using an
traction offsets were 80, 120, 160, and 200 V. The results on these samples were
generally within a factor 3. A major drawback found was that application of the
of
high offsets reduced sensitivity by a factor of about 100, thereby requiring higher
primary beam currents and energies to get adequate counting rates for the minor
elements. The method also requires a SIMS instrument that allows changing of the
extraction field in the necessary increments. This technique appears promising for
quantification of samples for which no standards are available, but more research
of
and application to a wider variety samples are needed to validate the theory.
Although a CS+ primary beam was used originally to enhance the formation
of negative secondary ions, its use has been further popularized by the discovery
that MCs+ ions (where M is any element in the specimen) are formed with greatly
reduced matrix effects [96]. It is believed that the MCs+ ions are formed by com-
bining an independently sputtered neutral M atom and a CS+ ion in the near-sur-
face region. With greatly reduced, and in some cases negligible, matrix effects,
the
difficulty of quantification in SIMS is reduced. Without a matrix effect, the indi-
vidual MCs+ ion yields could be determined by using a reference standard, and the
corrections applied from one type of sample to another or throughout the depth
profile. Schroeer et al. [97] measured relative yields of secondary ions from five
to
different metals and found the relative yields from matrix matrix varied no more
than a factor of 2, with the exception of Cr from Si, which varied by a factor of 4.
They also reported that the yields of the MCs+ ions were independent of the am-
up to about l O+ torr. Their primary beam was at an angle
bient oxygen pressure set
of 42" to the target normal. Wittmaack [98] found that MCs+ yields were strongly
dependent on the primary impact angle for mate~als with low sputtering yields
such as Si and Al. Homma et al. [99] studied MC,+ ion yields in Si and SiO, and
found group 11,111, and IV MCs+ yields in SiO, lower or comparable to that in Si
whereas group V, VI, and VI1 elements had higher yields in SiO,. The use of MCs+
ions lowers the ion yields for most elements but greatly improves the sensitivity
for the group I1 B elements (Zn, Cd, and Hg). These elements tend to have lower
signals when an~y~~d with conventional primary ions as a result of their high first
ionization potentials (i.e., low positive ion yields) and negative electron affinities
(i.e., no negative ion yield). Overall, the use the MCs+ cesium attachment ions
of
leads to easier quanti~cation of SIMS data.