| 摘要 |
PURPOSE:To detect the signal of charged corpuscles accurately when the signal produced from a sample is varied, and to correct the astigmatism of an optical system, by converting the peak height of a signal differentiated from an image signal into a pulse number, and utilizing the result. CONSTITUTION:Electron beams 2 from an electron gun 1 as a charged corpuscular beam generating source are focused in a sharp beams and radiated over a sample 4 by an electromagnetic lens 3. The signal of the secondary electrons or the like at each scanning position from the sample 4 is converted into an electric signal by a detector 8 and an amplifier 9, and the resultant signal is used for an image signal. Moreover, the differentiated signal from a differential circuit 12 of the system is made into two specific signals by an all-wave rectifier circuit 15, and a pulse with a frequency responding to the voltage value of the input signal is output by a voltage/frequency converter circuit 16. This pulse is computed at a computing circuit 17. The computed value is added directly to a central control circuit 10 as a signal to show the focus condition of the electron beams, and the signal of the charged corpuscles when the signal from the sample 4 is varied is detected accurately.
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