AIMING AND HIT VERIFICATION IN SINGLE-ION TECHNIQUES

Although ion microprobes can be used as high power microscopes to select a special target site for further investigations, single ion experiments often require additional high magnification light microscopy of the target, because imaging by ions might spoil the effect to be investigated. Ideally, the light microscope should be coaxial with the microbeam with its objective lens as near as possible to the target. In the past there have been various techniques to circumvent this obvious impossibility. Some place the microscope objective slightly out of axis, some use mirrors, with or without holes far the beam, under 45 degrees to the target (or nearly so) and long working distance microscopes. And there was even one (D. Heck, Karlsruhe) using a mirror-microscope-objective, coaxial with the beam, with a central hole to let the beam pass. All these setups have suboptimal resolution, either because only part of the target is in focus or because of too long working distance or because part of the possible resolution is compromised by the central borehole (or because of all these reasons together). The best possible resolution is only possible with a high power objective near the target. We have therefore designed a microscope which flips onto and out of the beam axis with sub-mu m precision. To find the spot, where the beam will hit, we first image a test grid with the microprobe. Then we flip the light microscope onto the beam axis and identify the center of the SIM image with the corresponding point in the light microscopic image. This is the point, where the undeflected beam will hit. Similarly important is the reliable identification of a hit independently of target thickness, composition or topography. As any separate detector in front of the target would destroy the microfocus, the target itself is the only place where the beam can be allowed to interact. Moreover this interaction must deliver an effect for every hit and this effect must be detectable with 100% efficiency. The only effect that fits to these requirements, is the emission of secondary electrons which are produced in large numbers and can be detected with high efficiency. This work reports about our light microscopic aiming system, about our (still not perfect) hit detector, and how various operating conditions influence the reliability of hit detection.