The apparatus includes an electron source for generating an electron beam, a deflector for scanning generated electrons, {TAIL ~ an objective lens} for focusing the electron beam upon the object, a grid disposed between the object and the objective lens, a wafer holder for holding {HEAD ~ the object}, a sensor for detecting generated secondary electrons, a potential controller for controlling the potential E0, E1 and E2, and a storage for storing optimum potential conditions 
The apparatus includes an electron source for generating an electron beam , a deflector for scanning generated electrons , { TAIL ~ an objective lens } for focusing the electron beam upon the object , a grid disposed between the object and the objective lens , a wafer holder for holding { HEAD ~ the object } , a sensor for detecting generated secondary electrons , a potential controller for controlling the potential E0 , E1 and E2 , and a storage for storing optimum potential conditions
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