| 摘要 |
PROBLEM TO BE SOLVED: To optimize the SN ratio of a secondary charged particle image by deciding to select either one of a first signal processing circuit based on an analog detecting method and a second signal processing circuit based on an electron coefficient method. SOLUTION: A measured value of the irradiated current amount of the primary electron beam 5 measured by a determining and control system 30 and a measured value of the maximum irradiated current amount to be forecasted on the basis of the dead time of the secondary electron detecting system while applying the electron coefficient method are compared with each other, and on the basis of a result of this comparison, any one of a first and a second signal processing circuits is selected. With other method, the relation between the irradiated current amount of the primary electron beam and the SN ratio is obtained by both the methods of the analog detecting method and the electron coefficient method, and the irradiated current amount Ip having a reversed SN ratio is output as a reference value. The irradiated current amount of the primary electron beam 5 is measured by a determining and control system 30, and in the case where the obtained measured value is larger the irradiated current amount Ip, a switch 31 is automatically changed over to an AD side, and in the case where the obtained measured value is smaller than the irradiated current amount Ip, the switch 31 is automatically switched to an EC side.
|
