Instrumental Aspects of Spatially 3-Dimensional SIMS Analysis

Abstract

It has become clear from detailed ion optical analyses [1,2,3] that matching analyzer acceptance with secondary ion beam emittance is essential to obtain high transmission at given mass resolution, independent of the particular imaging principle used (scanning probe, microscope, image dissecting ion probe). Close-to-optimum matching can be obtained using a “transfer optic” system (consisting of an immersion lens immediately at the target and one or more image transfer lenses between immersion lens and mass spectrometer) as described by SLODZIAN [1,4]. Future scanning probes and microscopes therefore will have to incorporate such a system, taylored to the particular mode of analysis and primary beam illustration. When comparing in particular the ion microscope and the scanning ion microprobe mode of operation for their relative merits, it is only fair to assume the same type of double focusing spectrometer for both systems. We consider an instrument which is partially corrected for second-order aberrations [5], the most important remaining aberrations being second-order chromatic aberration and third-order aperture aberration. Without taking into account detailed ion optical properties of particular instrument designs, LIEBL [6] gives an estimate for the maximum tolerated divergence angle and the slit width s′ for such an instrument in dependence on the required mass resolution R (= M/ΔM)

$$\begin{array}{*{20}{c}} {{}^{a}x = {{{(40R)}}^{{ - {{1} \left/ {3} \right.}}}}} \\ {s' = L/20R} \\ {L = 8{{r}_{m}}} \\ \end{array}$$

where L is the total ion path length and r_m the magnetic deflection radius. Owing to the second-order chromatic aberration coefficient A_δδ, the maximum allowed initial energy ^ΔΦch of the secondary ions is determined by

$$\Delta {{\Phi }_{ch}}=0.45V{{(R.{{A}_{\delta \delta }})}^{-1/2}}$$

where V is the final energy of the secondary ions and it has been assumed that the chromatic aberration contribution to the width of the slit image, r_mA_δδ(Φ/V)², is about half the slit width s′.