The invention concerns an "in situ" ion-etching device for local thinning of a sample in a transmission electron microscope (1) with simultaneous electron microscopic observation. Towards this end, an ion beam device (2) is arranged in such a way that the finest possible ion probe is produced at the sample location and can be scanned over the sample surface. The ion beam (16) and sample (10) thereby enclose the flattest possible angle. To compensate for the magnetic field of the objective lens (5), the ion beam (16) is defected along a curved path onto the sample (10). In a preferred embodiment, an electrostatic cylinder capacitor sector field effects double focusing. The ion probe can be positioned, via the scanned ion image, onto a selected region of the sample by the secondary electrons (22) released from the sample (10). The sample location can be observed during the ion thinning process in electron transmission or electron diffraction. It is thereby possible to carry out target preparations under high-resolution observing conditions and to eliminate contaminant or reactive layers.
