The present invention relates to an electron-optical system for making a substantially parallel micro electron beam.
The electron beam used in a scanning electron microscope (SEM) is made converged to an extremely small point causing the microscope to have a very high resolving power. Converging a beam to so small a point is accompanied, as a matter of course, by a very large beam-converging angle. Therefore, when an object of observation is an uneven-surfaced sample having on its surface many microscopic concaves and convexes, there are some cases where the coned flank of the beam touches the inner side wall or upper edge of some of the concaves, causing it to be impossible to observe stereoscopic micro surface structures of the sample. To overcome such a disadvantage it is desired to obtain a pseudoparallel micro electron-beam having a conversion angle of the order of 10.sup.-3 radian and a beam diameter not larger than 1 .mu.m. However, it is very difficult to obtain such a pseudoparallel micro electron-beam with a conventional electron-optical system: though such a beam is, in principle, obtained by using electron lenses with a very long focal length, the astigmatism and spherical aberration of a symmetrical electron lens become larger as the lens has its focal length made longer. Under the circumstances it is an urgent matter to create any electron-optical system capable of producing a sharp-focused pseudeoparallel micro electron-beam.
Further, such a system is desired also in some field of electron diffractometry. In analyzing the surface structure of a sample, the conventional electron diffractometers, which use an electron beam with a diameter of the order of a millimeter, can give only the data averaged over an area of the order of millimeter, and does not give information about fine structures contained in a region of the order of micrometer. Such fine structures become capable of being analyzed for the first time with a substantially parallel electron beam having a diameter not larger than the order of a micrometer.
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