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An electron beam column incorporating an asymmetrical detector optics assembly provides improved secondary electron collection. The electron beam column comprises an electron gun, an accelerating region, scanning deflectors, focusing lenses, secondary electron detectors and an asymmetrical detector

An electron beam column incorporating an asymmetrical detector optics assembly provides improved secondary electron collection. The electron beam column comprises an electron gun, an accelerating region, scanning deflectors, focusing lenses, secondary electron detectors and an asymmetrical detector optics assembly. The detector optics assembly comprises a field-free tube, asymmetrical with respect to the electron optical axis; the asymmetry can be introduced by offsetting the field-free tube from the electron optical axis or by chamfering the end of the tube. In other embodiments the detector optics assembly comprises a field-free tube and a voltage contrast plate, either or both of which are asymmetrical with respect to the electron optical axis.

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1. An electron beam column comprising:a probe optics assembly, for forming an electron probe;an electron optical axis, defined by said probe optics assembly;a secondary electron detector situated below said probe optics assembly; anda detector optics assembly, generally cylindrically symmetric about

1. An electron beam column comprising:a probe optics assembly, for forming an electron probe;an electron optical axis, defined by said probe optics assembly;a secondary electron detector situated below said probe optics assembly; anda detector optics assembly, generally cylindrically symmetric about said electron optical axis, situated below said probe optics assembly;wherein at least one component of said detector optics assembly is configured to break said cylindrical symmetry. 2. An electron beam column as in claim 1, wherein said detector optics assembly comprises a field free tube, configured to break said cylindrical symmetry, situated between said electron probe and said secondary electron detector. 3. An electron beam column as in claim 2, wherein said detector optics assembly further comprises a voltage contrast plate situated below said field free tube and said secondary electron detector. 4. An electron beam column as in claim 1, wherein said detector optics assembly comprises a voltage contrast plate, configured to break said cylindrical symmetry, situated below said secondary electron detector. 5. An electron beam column as in claim 4, wherein said detector optics assembly further comprises a field free tube situated between said electron probe and said secondary electron detector and above said voltage contrast plate. 6. An electron beam column as in claim 2, wherein said detector optics assembly further comprises a voltage contrast plate, also configured to break said cylindrical symmetry, situated below said field free tube and said secondary electron detector. 7. An electron beam column as in claim 2, wherein said field free tube has a chamfered lower end. 8. An electron optical column as in claim 7, wherein said field free tube has a circular bore and a square perimeter. 9. An electron beam column as in claim 2, wherein said field free tube is offset from said electron optical axis. 10. An electron optical column as in claim 4, wherein said voltage contrast plate is offset from said electron optical axis. 11. An electron optical column as in claim 4, wherein said voltage contrast plate has a notch in the aperture. 12. An electron optical column as in claim 4, wherein said voltage contrast plate has a bump on the aperture. 13. An electron optical column as in claim 6, wherein said field free tube has a chamfer on the lower end and said voltage contrast plate is offset from said electron optical axis. 14. An electron beam column as in claim 1, further comprising a stage situated below said detector optics assembly and said secondary electron detector. 15. An electron beam column as in claim 1, wherein said probe optics assembly comprises:an electron gun;an accelerating region situated below said electron gun;scanning deflectors situated below said accelerating region; andfocusing lenses situated below said deflectors. 16. An electron beam column comprising:a probe optics assembly, for forming an electron probe;an electron optical axis, defined by said probe optics assembly;a secondary electron detector situated below said probe optics assembly; anda field free tube with a chamfered lower end, situated between said probe optics assembly and said secondary electron detector. 17. An electron beam column as in claim 16, wherein said probe optics assembly comprises:an electron gun;an accelerating region situated below said electron gun;scanning deflectors situated below said accelerating region; andfocusing lenses situated below said deflectors. 18. An electron beam column comprising:a probe optics assembly, for forming an electron probe;an electron optical axis, defined by said probe optics assembly;a secondary electron detector situated below said probe optics assembly;a field free tube with a chamfered lower end, situated between said probe optics and said secondary electron detector; anda voltage contrast plate offset from said electron optical axis, situated below said field free tube and said secondary electr on detector. 19. An electron beam column as in claim 18, wherein said probe optics assembly comprises:an electron gun;an accelerating region situated below said electron gun;scanning deflectors situated below said accelerating region; andfocusing lenses situated below said deflectors.