A Measurement Technique For Motion Characteristics Of The Slit Based On Machine Vision

The blade or slit system is a field stop in a step-and-scan projection lithography machine.Its main function is to maintain synchronous motion with the mask during scanning exposure stage to achieve the purpose of precisely defining exposure area of the wafer.Accurate exposure has a significant effect on improving the resolution of the lithography machine.Therefore,it is of great significance to study the motion characteristics of the slit in the development of the slit.The structure of the slit is relatively thin,easy to deformation,and the speed and acceleration are large during the whole movement,so contact measurement technique is unfavorable.The contact measurement technique needs additional load attaching to the slit,so movement of the slit will change and the measured motion characteristics are not the true motion characteristics of the slit.Consequently,a method based on machine vision is adopted to build a set of measurement system,the edge detection technology in image processing is described,and the location information of the slit is obtained using this technology.In the aspect of system calibration,considering that height errors will be introduced if a standard calibration reference is selected,it takes advantage of slit's features to select the slit as a calibration object.The relationship between spatial coordinates of the measurement system and coordinates of the camera target is calibrated by using a grating displacement sensor.This calibration method can ensure higher calibration accuracy relatively.In experimental parts,a series of related experiments are designed to show the accuracy and repeatability precision of displacement measurement.By means of laser interferometer,the relationship between slit's positions and coordinates of the camera target is calibrated,and it is verified that the accuracy of speed measurement of the measuring system is 0.31%.The motion characteristic curve of the slit is measured by the measuring system.It is concluded that the whole movement process of slit is 298.4ms,the time from the beginning to the uniform motion stage is 83.3ms,and the duration of uniform motion stage is 113.5ms.The average speed for uniform motion stage is 98.25mm/s,and the maximum velocity of which is103.458mm/s,the minimum rate of this period is 91.408mm/s,the velocity stability of the phase is 6.18%.And the speed repeatability accuracy is 4.10mm/s.At the same time,a detailed error analysis is performed for the measurement system.For some controllable error factors,quantitative analysis is carried out,and correctness of the error analysis is verified combined with the experimental data.