Focussing of a transient low energy Cs+probe for improved NanoSIMS characterizations

Abstract

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 E 0 P = 0 V and E 0 S = +5930 V focuses a 100 eV caesium beam into an area 9.2 μ m in diameter. Even for a final energy as low as 25 eV, 90% and 50% of the beam is confined to areas of 30 μ m and 4 μ m in diameter respectively (with electrodes E 0 P and E 0 S 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.