| 摘要 |
PURPOSE:To perform a processing for deciding an absolute focus position by a digital control with accuracy in accordance with the necessity in an objective pattern in sampling, inspection and measurement. CONSTITUTION:A surface to be measured is automatically focused by driving an objective lens 3, or a stage with a motor M, and the controller is provided with a front pin detecting part 13 for outputting a front pin signal, a rear pin detecting part 14 for outputting a rear pin signal and a focusing pin detecting part 15 for outputting a focusing pin signal respectively, and the automatic gain control of a differential signal corresponding to the difference between the front pin signal and the rear pin signal is performed by the focusing pin signal, and the motor M is servo-driven by a servo-driving circuit 31 with a control signal for which the automatic gain control is given, and then, a focusing signal of showing a focus offset indicating the difference between the focal position and a present position is outputted from a D/A converter 51 so as to be superposed on the output of the servo-driving circuit 31, and then, accurate focusing is performed. |
