| 摘要 |
PURPOSE:To allow the irradiation amount of the electron beam applied to a sample to be constant, by a method wherein an irradiation electron-beam current and a reference electron-beam current are compared with each other to obtain a division value, and multiplying the division value by a clock frequency for regulating the deflection speed of an electron beam. CONSTITUTION:A measured current value In obtained by a Faraday cup 31 is converted into a digital value in an A/D converter 32 and stored in a measured current register 33 and then compared witha reference current value Ir in a divider 34 to obtain a division value. The division value is passed through a division value register 35 and multiplied in a multiplier 36 by a clock frequency determined in a frequency register 10 and then stored in a calibrated-frequency register 37. A clock signal having a frequency determied n the register 37 is generated from a clock circuit 23. Measurement of a sample current by the cup 31 is intermittently performed, e.g., every 30 hours, and the clock speed is automatically set, thereby allowing a constant exposure amount to be obtained at all times. |
