PURPOSE: To automatically correct the shifted focus of electron beam caused through variation of converging conditions of tri-electrode electrostatic lens by providing deflecting means, means for deciding the diameter of electron beam and means for controlling the voltage of second anode.
CONSTITUTION: A field emission electron gun and a tri-electrode electrostatic lens are employed for forming a scanning electron microscope. At first, the acceleration voltage V0 and the take-out voltage V1 are applied while the control voltage V2 is applied to (1) side of switch thus to focus at the point A. Then the switch is turned to (2) side and at same level of V2, it is scanned through a deflection coil 8 to detect the secondary electrons through a detector 9 and to decide the diameter of the electron beam through a decision circuit 16 to be stored in a microcomputer 18. It is repeated for faintly incremented V2 to obtain a proper control voltage V2. Consequently, the variation of the optoelectronic system due to the variation of converging conditions of tri- electride electrostatic lens is corrected automatically resulting in improvement of the operationability.
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