Improvement of ion beam brightness by reducing voltage ripple of an electrostatic accelerator for microbeam focusing by a triplet quadrupole lens system having large spherical aberration coefficient

The microbeam system at Tohoku University was upgraded to a triplet lens system aiming at applying to the analysis of sub-micron features. The triplet lens system has a higher demagnification than the existing doublet system. However, the introduction of the triplet system also resulted in larger chromatic and spherical aberration coefficients. To overcome these problems, the energy resolution of the accelerator was improved by developing a terminal voltage stabilization system. The energy resolution of the accelerator was improved to 1 × 10−5 ΔE/E, which resulted in an increase in the brightness of the beam. The beam brightness was 2.3 pA Δ μm−2 Δ mrad−2 Δ MeV− and was higher in the central region. The effects of the increased chromatic and spherical aberration were mitigated by restricting the divergence angle without reducing the beam current. A beam spot size of 0.6 × 0.8 μm2 was obtained with a beam current of 150 pA.