| 摘要 |
PURPOSE:To conduct focusing quickly by a method wherein coil currents for a lens are set, the mark of a sample is scanned with charged beams, difference between waveform information, in which a secondary charged particle signal detected is A/D converted, and information, in which the waveform information is delayed, is obtained and the lens is excited by currents maximizing the cumulative adding value of a differential signal of a previously set threshold or more. CONSTITUTION:Information is set to a DAC 8 for an amplifier 7 driving a focusing coil 3 from a computer 6. The stepped mark of a sample 5 is scanned with electron beams, and a reflected electron signal is A/D converted 13, mean adding-processed 10 and stored 11. A waveform is read, difference between the waveform and information in which the waveform is delayed is acquired, and differential information of a proper threshold or more is cumulative-added 15. A cumulative adding value is increased on focusing. Specified characteristics are obtained from a current value (x) and the cumulative adding value (y). When coefficients (a), (b), (c) are acquired by application through a method of least squares to y=ax<2>+bx+c from the characteristics and x=-f/2a maximizing (y) is set to exciting currents, focusing can be performed rapidly, and focusing is hardly affected by noises.
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