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Time-of-Flight Secondary Ion
Mass Spectrometry (TOF-SIMS) uses a pulsed primary ion beam to desorb and ionize
species from a sample surface. The resulting secondary ions are accelerated into
a mass spectrometer, where they are mass analyzed by measuring their
time-of-flight from the sample surface to the detector. An image is generated
by rastering a finely focused beam across the sample surface. Due to the
parallel detection nature of TOF-SIMS, the entire mass spectrum is acquired from
every pixel in the image. The mass spectrum and the secondary ion images are
then used to determine the composition and distribution of sample surface
constituents.
TOF-SIMS provides
spectroscopy for characterization of chemical composition, imaging for
determining the distribution of chemical species, and depth profiling for thin
film characterization. |
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TOF-SIMS
Spectroscopy |
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In the
spectroscopy and imaging modes, only the outermost (1-2) atomic layers of the
sample are analyzed. To ensure the analyzed secondary ions originate from the
outer surface of the sample, a primary ion dose of less than 1012 ions/cm2 is
employed. Below this "static limit," roughly less than one in one thousand
surface atoms or molecules are struck by a primary ion. The actual desorption of
material from the surface is caused by a "collision cascade" which is initiated
by the primary ion impacting the surface. The emitted secondary ions
are extracted into the TOF analyzer by applying a potential between the sample
surface and the mass analyzer. TOF-SIMS spectra are generated using a pulsed
primary ion source (very
short pulses of <1 ns). Secondary ions travel through the TOF analyzer with
different velocities, depending on their mass to charge ratio (ke=½mv2).
For each primary ion pulse, a full mass spectrum is obtained by measuring the
arrival times of the secondary ions at the detector and performing a simple time
to mass conversion. |
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TOF-SIMS
Imaging |
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Chemical images
are generated by collecting a mass spectrum at every pixel (256 x 256) as the
primary ion beam is rastered across the sample surface. The figure to the right
shows an example of elemental and molecular imaging. The sample is the
cross-section of a time release drug pellet. The map on the left is of the peak
intensity at 268 Da, the molecular ion of the drug Metoprolol. The map on the
right is of the peak intensity at 23 Da for sodium. |
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TOF-SIMS Depth
Profiling |
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TOF-SIMS is
capable of shallow sputter depth profiling. An ion gun is operated in the DC
mode for sputtering, and the same ion gun or a second ion gun is operated in the
pulsed mode for data acquisition. Depth profiling by TOF-SIMS allows monitoring
of all species of interest simultaneously, and with high mass resolution. The
figure to the right shows a TOF-SIMS depth profile of a thin gate oxide acquired
in the dual beam mode using a 15 keV Ga+ beam for spectral
acquisition and a 1 keV Cs+ beam for sputtering. |
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