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

Claim 1

In a method for focusing the objective lens of a scanning transmission-type corpuscular-beam microscope in which the beam of the microscope is deflected by a deflection system excited in sawtooth fashion and generates a raster consisting of parallel lines on a specimen to be examined, said microscope including 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 said raster, the improvement comprising the steps of, measuring, during the exposure of a specimen point, partial beam radiation intensities in a cone of the beam at two points disposed symmetrical with respect to the longitudinal axis of said beam cone by means of said detector, the effective input area of said detector being smaller than the cross-sectional area of said cone at the same height in said microscope, and adjusting the lens current of the objective lens of said microscope so that output signals generated by said detector are equal for both of said beam radiation measurements at said points thereby focusing said beam on said specimen.

Claim 8

An apparatus for automatically focusing the objective lens of a scanning transmission-type corpuscular-beam microscope including deflection means for deflecting the beam of said microscope and generating a raster consisting of parallel lines on a specimen to be examined, said apparatus comprising: beam radiation detector means, disposed behind a specimen to be examined in the microscope along the beam path, on the optical axis of said microscope for measuring partial beam radiation intensities of said beam; beam deflection means, disposed between said specimen and said radiation detector means; a wobble generator, coupled to said beam deflection means, for generating a square-wave output signal for deflecting a ray cone of said beam in two deflection directions disposed symmetrical with respect to the optical axis of said microscope, the frequency of said output signal being high compared to the line frequency of said specimen raster; means, coupled to said objective lens, for regulating the current of said objective lens so that the beam radiation intensities measured by said detector means are equal thereby focusing said beam on said specimen; a lens current wobble generator, coupled to said lens current regulating means, for superimposing an alternating current on said lens current; and a phase discriminator, coupled to said lens current wobble generator, said lens and said detector means, for comparing the phase between output signals generated by said lens current wobble generator and said detector means, said discriminator generating an output signal for controlling said lens current regulating means.

Claim 9

An apparatus for automatically focusing the objective lens of a scanning transmission-type corpuscular-beam microscope including deflection means for deflecting the beam of said microscope and generating a raster consisting of parallel lines on a specimen to be examined, said apparatus comprising: beam radiation detector means, disposed behind a specimen to be examined in the microscope along the beam path, on the optical axis of said microscope for measuring partial beam radiation intensities of said beam; beam deflection means, disposed between said specimen and said radiation detector means; a wobble generator, coupled to said beam deflection means, for generating a square-wave output signal for deflecting a ray cone of said beam in two deflection directions disposed symmetrical with respect to the optical axis of said microscope, the frequency of said output signal being high compared to the line frequency of said specimen raster; means, coupled to said objective lens, for regulating the current of said objective lens so that the beam radiation intensities measured by said detector means are equal thereby focusing said beam on said specimen; a lens current wobble generator, coupled to said lens current regulating means, for superimposing an alternating current on said lens current; and a phase discriminator, coupled to said lens current wobble generator, said lens and said detector means, for comparing the phase between output signals generated by said lens current wobble generator and said detector means, said discriminator generating an output signal for controlling said lens current regulating means.