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The SIMS (Secondary Ion Mass Spectrometry) is a spatially high
resolution analytical technique that is being used for analysis
of isotopic composition of elements in small volumes of solid
samples. In contrast to an electron microprobe, which utilizes
an electron beam to bombard a solid sample and analyses intensities
of characteristic X-rays of chemical elements, a SIMS analytical
system (ion microprobe) uses a finely focused beam of primary
ions (usually these are Cs+, O+, O2+, O- and O2-) directed onto
a sample to obtain mass spectra of the ionized particles ejected
from the sample due to collision cascade reactions.
In more detail, such interaction of primary ions with sample
surface causes three major effects:
- Mixing of the upper layers of the sample that results
in an amorphization of the material structure;
- Implantation of the primary ion beam atoms in the
sample, and
- Ejection of particles (atoms or small molecules) from
the sample surface which can be electrically neutral, as well
as positively and negatively charged.
The charged particles of one polarity called "secondary
ions" extracted and accelerated by electrostatic field
form a secondary ion beam directed into a double focusing
mass spectrometer where they are separated according to their
energy and mass/charge ratio before being detected.
Nearly all elements of the periodic table (from H to U) can
be quantitatively analyzed by SIMS technique. There are three
basic modes of ion microprobe operation:
- Local analysis in a point. In this mode,
a focused stationary primary beam is used to determine chemical
(element concentrations) or isotopic compositions of the
sample. The spot size ranges normally from a few to tens
of microns and is selected for specific applications.
- Depth profiling. It is based on a phenomenon
of etching a crater into the sample. Continuously collected
secondary ions as the primary ion beam erodes the sample
surface reveal compositional and isotopic changes with depth
(i.e., by measuring signal intensity vs. time and then converting
time into depth).
- Secondary ion imaging. The distribution
of elements or isotopes can be obtained as an image of a
given spatial resolution that could be reached of about
~1um over areas up to 500 μm2.
For further details, a reader could be addressed to the following
sources:
A beginner’s guide to the Cameca SIMS instruments (University
of Edinburgh, Ion Microprobe Facility).
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Last updated: November 16, 2009 |
