An Monitoring Experimental Facility Based On One-Way Defocus Differential Astigmatic Principle

Laser direct writing system is the key equipment for fabrication of optical diffractive elements, and the defocus detection technique is the important sub-tech in it. The aim of defocus detection is to achieve high etching quality by providing the defocus error signal for focus-servo system which can make the etching spot focus accurately.In this paper, the differential astigmatic detection method for defocus measurement with one-way defocused detector is developed from the conventional differential astigmatic detection. Two detectors are used both in conventional differential astigmatic device and the device given in this paper. The difference is that the detectors in former device are placed in front of the focus and behind the focus separately, but the detectors in later device are placed in front of the focus and on the focus separately.Firstly, in order to meet the requirement in application, the influencing factors for defocus measurement with one-way defocused detector are analyzed and the influencing regularity of detector position are discussed, after that the relationship between defocus errors and output signals is given too. According to the optimization results, the experimental device is established for this paper.Secondly, the circuits are designed for signal conversion with low noises.Thirdly, data processing software is programmed with median method filter to decrease the noise, and the stability of indicating value is improved.At last, the principle of defocus measurement with one-way defocused detector is proved correctly. The result compared with conventional differential defocus measurement method shows that the higher measurement sensitivity can be achieved by the method discussed in this paper. According to the discussing results, it can be seen that defocus measurement method with one-way defocused detector is really an effective method for defocus monitoring.