Surface caesium content is known to greatly influence the negative ion yield
during SIMS analyses. In NanoSIMS, a 16 keV ion probe of Cs+simultaneously performs the surface enrichment and the sample sputtering.
To increase the surface caesium content and thus generate higher ion yields,
it is suggested herein to significantly reduce the energy of the primary
ions by biasing the sample with a positive voltage slightly smaller than the
voltage of 8000 V that is used for primary ion acceleration. Then, once the
typical bias voltages are restored, SIMS analysis may carried out with
increased sensitivity right from the beginning of the sample erosion. The
use of such a procedure may improve many types of SIMS investigations such
as thin sections of biological samples, semiconductor wafers shallowly doped
over very small areas and small meteorite samples.
However, during deceleration close to the sample surface, the beam size is
markedly enlarged unless the excitation of the objective lens is corrected.
In this study, optics simulations are performed using Simion 8 in order to
facilitate adjustment of the experimental setups.
An objective lens excitation set at E0P = 0 V and E0S = +5930 V focuses a
100 eV caesium beam into an area 9.2 μm in diameter. Even for a final
energy as low as 25 eV, 90% and 50% of the beam is confined to areas
of 30 μm and 4 μm in diameter respectively (with electrodes E0P
and E0S set at 3150 and 5450 V and using a beam slightly limited in angle).
The procedures being suggested will be confirmed by experimental studies
soon to be submitted as complementary contribution. As predicted, caesium
rich surfaces greatly improve ion yield and consequently localised SIMS
analysis as well.