Page 219 - Inorganic Mass Spectrometry - Fundamentals and Applications

204 Cristy

begins to increase on the surface and CrO2" decreases. When the oxide was pro-
filed, the CrO+ species dropped rapidly; the Cr0,- species remained at a constant
level for a period before decreasing. This simple experiment showed that one oxide
species was forming initially and progressed into the bulk. A later forming oxide
characterized by CrO+ formed on the surface but apparently remained in a mono-
layer on the surface even though additional oxygen exposure occurred.
In a more recent application of SSIMS, the ratios MO,-/MO- and
MO~-/~O- for transition metal oxides of the type M 0 were measured to demon-
y.
strate a fingerprint spectrum characteristic of the oxidation state of the metal. The
data for iron oxide were then compared to oxide found on a steel sample with an
oxide film 4 nm thick. The best match was to Fe,O, [ 1 151.
SSMS has been applied to the study of molecular adsorption on metal sur-
faces [ 1 161, catalytic surfaces [ 1171, surfaces of HF- and NH4F-treated silicon
is
[ 1 181, and many others in which surface chemistry important.

Profiling or other applications of SIMS that are not static are sometimes referred
of
to as dynamic, although that connotation seems superfluous. The limits SIMS
profiling resolution are being driven by requirements of the semiconductor in-
the
dustry, in which device size is getting smaller and doping implants are getting shal-
lower. As a result, primary beam energies are being reduced. ~ua~pole instru-
ments with low extraction potentials can easily accommodate lower primary beam
energies (e.g., 200 eV) [l 191. However, modifications to magnetic sector instru-
ments so they can work with lower secondary ion extraction energies have resulted
in impact energies as low as 1 key
In profiling, there is a transition region at the surface due to initial sputter-
ing where the implantation of the bombarding beam has not reached full equi-
its
librium value. By sputtering with 250 to 500 elv O,+ at normal incidence for most
rapid incorporation of the oxygen, transition depths of less than 1 m have been
observed in silicon [ 120,12 l]. Subkiloelectron-volt sputtering can also be achieved
by higher primary ion energy at glancing incidence, but tr~sition depths become
is
of
greater as incorporation the primary beam reduced. Oxygen flooding was used
to overcome this effect in profiling a boron implant in silicon, as shown in Fig.
4.36. A comparison is made by using 1 keV O,+ at 60" incidence to profiles made
by using 250 and 500 eV O,+ at normal incidence [ 1221. Another method of over-
coming the transition region
is capping the sample with a thin layer of the substrate
element so that implant equilibrium has been reached when the capped material is
sputtered through [ 1231.
In addition to the transient sputtering region, a thin layer of native oxide on
the surface complicates profile interpretation. Iltgen et al. [ 1241 described a method