Method and apparatus for focusing the objective lens of a scanning transmission-type corpuscular-beam microscope

摘要

A method and apparatus for focusing the objective lens of a scanning transmission-type corpuscular-beam microscope. In the method, the beam of the microscope is deflected by a deflection system excited in sawtooth fashion and generates a raster consisting of parallel lines on the specimen to be examined. The microscope includes a beam radiation detector disposed behind the specimen along the beam path which generates an output signal which controls the brightness of a picture tube monitor operated synchronously with the raster. The improvement of the invention comprises the steps of measuring, during the exposure of a specimen point, partial beam radiation intensities in the cone of the beam at two points disposed symmetrical with respect to the longitudinal axis of the beam cone by means of the detector, the effective input area of the detector being smaller than the cross-sectional area of the cone of the beam at the same height in the microscope. The lens current of the objective lens is then adjusted so that the output signals generated by the detector are equal for the beam radiation measurements at the two points.