| 摘要 |
The invention relates to wafer inspection by means of a scanning electron microscope (SEM) column in which the secondary electron detector (22, 24) is positioned centrally above the objective lens of the column. Secondary electrons that leave the central part of the specimen in a direction substantially perpendicular to its surface are inevitably collected in the central part of the detector surface where the bore (36) for the primary beam (6) is situated. Consequently, such electrons do not contribute to the detector signal. In order to avoid such a detrimental loss of signal contribution, it is proposed to provide a central electrode (35) in the central bore (36) such that secondary electrons that approach the bore are driven aside towards the electron-sensitive detector region (48).
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