| 摘要 |
PURPOSE:To realize highly accurate mark detection by always maintaining the optimum condition of the relation between the mark detection signal intensity and the reference signal level independent of ion seed and change of acceleration voltage. CONSTITUTION:In case the ion seed or acceleration voltage which result in emission of large amount of secondary electron is used, the amplification rate of mark detection system is kept low and the mark detection signal which is amplified and accumulated by a number of times of scanning is supplied to a comparison circuit 10 after it is set to the intensity which is just suitable to the signal level sent from the reference signal source 11. On the contrary, in case the ion seed or acceleration voltage which result in emission of less amount of secondary electron is used, the amplification rate of mark detection system is kept high and a number of times of scanning the ion beam is increased at the mark region. With increase of a number of times of scanning, the accumulator 9 is controlled and a number of times of accumulation of signals from the amplifier is matched with a number of times of scanning of ion beam. As a result, the SN ratio can be improved for the detected signal having weak intensity resulting in low SN ratio by increasing a number of times of accumulation.
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