SIRIS
Steps
Desorption and atomization of a small portion of the sample by an
energetic ion beam (sputtering)
Suppression
of ions formed during the sputtering process
Selective
laser ionization of sputtered neutrals (Resonance Ionization Spectroscopy, RIS)
Time-of-Flight mass spectrometry of ions
Features
Excellent Sensitivity
Most sputtered atoms are neutral (usually >99%). This fact
combined with the high ionization efficiency of Resonance
Ionization gives high count rates even when small volumes are
probed.
Reduced
Matrix Effects
A major source of matrix effects in secondary ion mass
spectrometry (SIMS) is the large change in ion yield (varying
typically from 10-7 to 10-3) which occurs with variations in
surface composition. This phenomenon has little effect on the SIRIS signal because the neutral yield changes very little (from
>99.9999% to 99.9%).
Reduced
Background and Isobaric Interference
The analyte is selectively ionized by Resonance Ionization
using a pulsed laser. This almost eliminates interferences.
Near-Uniform Response
For over 80% of the elements (see our RIS
periodic table ), the resonance ionization spectroscopy process can be saturated
over the full intersection of an unfocused laser beam and the
sputtered neutrals, resulting in 2% to 5% useful yield (number of
analyte ions detected per analyte atom sputtered) using an Ar+
or O2+ sputtering beam.
Quantitative
Good linearity is observed over several orders of magnitude
dynamic range.
Excellent Lateral and Depth Resolution
The high useful yield allows sampling over extremely small
areas and sub-monolayer depths. Sub-micrometer lateral resolution
and monolayer depth resolution can be achieved using a liquid
metal ion gun for sputtering.
Imaging
Capability
By rastering (deflecting the ion beam to different positions on the sample), concentration images of small areas can be constructed.
For larger area images, the sample is translated beneath a fixed
ion beam. We can use a liquid metal ion gun for high resolution (sub-mm
features) or a higher current ion gun for large features such as when we probed
a Cd precipitate in CdZnTe for anomalous Cu
concentration.
Applicable to Conductors, Semiconductors, and Insulators
Charge build-up in insulating samples is easily avoided by
flooding low-energy electrons between ion beam pulses. We have published a
comparison between SIRIS and a similar technique that uses laser desorption
instead of ion sputtering for measuring boron in
animal tissue.
(865) 483-1113 (voice), (865) 483-3316 (FAX)