Abstract: An automatic focusing system is described for use with scanning systems having a poor signal to noise ratio video signal obtained by repetitive scanning. Typically the system is applicable to a scanning electron microscope.A preliminary filtering of the video signal is performed using a filter which eliminates video signal content corresponding to the fundamental and low harmonics of the repetitive scanning.Basic noise reduction is achieved using an N-path filter (typically N=8) commutated at a frequency equal to N.times.line repetition frequency of the electron beam scanning. The output signal is rectified and averaged to give a d.c. level proportional to the high frequency content of the video signal clustered around harmonics of the line scan repetition rate. This d.c. level is maximized by making stepwise adjustments in the current flowing in the focusing coil of the electron beam scanner and comparing d.c. levels before and after adjustment.

Abstract: Electron microscope apparatus for electron beam microfabrication. A mounting is described for supporting at least the final lens assembly of an electron microscope column above the vacuum chamber which also provides a rigid mounting for a stage on which a substrate is located together with reflecting mirrors which co-operate with a laser interferometer system for measuring the distances moved by the stage and therefore the substrate in two orthogonal directions.To avoid unwanted movement between the mirrors and the interferometer heads, the latter are also carried by rigid support means on the underside of the mounting.The mounting may comprise a solid ring located in an aperture in the lid of the vacuum chamber.Alternatively, the mounting may comprise a rigid and reinforced plate forming the lid of the chamber, from the underside of which extend the rigid support means for the stage and interferometer heads.