| 摘要 |
In a field emission scanning type electron microscope which varies widely a primary electron beam acceleration voltage, a width of the primary electron beam is thinly focused by a large acceleration voltage when the beam passes through the focusing lens. The AC variation caused due to the gases near an electron source is included in the primary electron beam current. The variation of the primary electron beam which spreads near an objective lens after it has passed through the focusing lens is detected by a detector near the objective lens. The secondary electron beam emitted from a specimen is detected by a scintillator to produce a scanning measurement signal. When the acceleration voltage is large, the scanning measurement signal is divided by an output signal of a detector near the objective lens. When the acceleration voltage is small, the measurement signal is divided by an output signal of a detector near the focusing lens. This selection may be performed by means for selecting on the basis of a ratio R (=(acceleration voltage V0)/(lead-out voltage V1) to be obtained from the high voltage source apparatus. Alternatively there may be provided comparator means for comparing the AC variations of the two signals which are obtained by dividing a scanning measurement signal by the signals of both detectors and selector means for applying the smaller signal to a display.
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