A Research On Measurement Technique Of The Iris Diaphragm Based On Machine Vision

The main function of the iris diaphragm in lithography projection lens is to control the luminous flux by adjusting the NA of objective lens tofor the resolution in different exposure conditions. It is significant to measure and research on the iris diaphragm because of the resolution is animportant performance index of the lithography systems. Owing to the thin depth and irregular edge of the iris diaphragm, the contact measurement is not fit for measuring this part. This paper adopts a way based on machine vision to establish a measuring system, and get the dimension by image processing. In practical application, the diaphragm which is a regular and curved polygonwas supposed to a round. So, it is important to evaluate the reliable dimension of the iris diaphragm.The main content of this paper is shown below:1、The structure of the iris diaphragm was analyzed first, then the measuring system was built. This measuring system is mainly composed by CCD camera, lighting source, and computer. This paper has summarized the current calibration techniques, and adopted Zhang Zhengyou calibration method to calibrate the distortion of the camera, and adjust distortions of the images collected by the system. The accuracy when transforming the image from pixel to millimeter can be improved to a certain extent, with calibration of the objective resolution of camera according to concentric template and figuring out the corresponding coefficient of objective resolution for every circularity based on the actual form of diaphragm.2 、 When processing the image, it has located the sub-pixeledge of iris diaphragm first, and then figure out its corresponding deviation from circular form according to the circle parameters which are matching various approximate circle(such as circumcircle, incircle, equivalent area of circle) of the iris diaphragm, with considering the irregularity of the edge of the iris diaphragm..3、This paper has proposed a diffraction simulation for the regular and curved hexadecagon and several kinds of coincidence circularity, and evaluated the effective sizes of the diaphragm by the approximation of its diffraction definition. This paper has implemented the Fraunhofer diffraction simulation and compared the sizes of Airy disk in the diffraction pattern. With further analysis, a circularity which shares a diameter mostly appropriate to the actual diameter value of the diaphragm is presented.4、Before the measurement, this paper has analyzed the error influencing factors of the measuring system and also implemented the error analysis test for several controllable factors. By the measurement for standard component, we find that the measurement precision of this measuring system is less than 0.02 mm, the measuring precision of repeatability is less than 0.003 mm, meeting the requirement in this paper.