| 摘要 |
PURPOSE:To detect a mark surely by changing the scanning frequency of an electron beam again and performing scanning when the position of the mark fails to be detected owing to the poor S/N ratio of the reflected electron signal. CONSTITUTION:When a mark for positioning is moved up to the position of an electron beam, the electron beam receives deflection and scans on a sample. The reflected electron signal from the sample is inputted to a detector 13 for reflected electron, and is then processed with a processing unit 15. When the electron beam crosses the mark, the reflected electron signal changes; threfore, the output value of a counter 8 in this stage is taken into a CPU4 through an interface 5. Therefore, the coordinates at the edge of the mark can be determined by calculation. If a wrong timing pulse is outputted from a timing circuit 16 owing to the poor S/N of the reflected electron signal, the pulse is detected with the CPU4. The pulse is then changed over and the detection of the mark is executed again. |
