Design and Experiment of The Wafer Pre-alignment System

The wafer pre-alignment system is a fundamental part of the lithography. A system for automated pre-alignment of wafer with a diameter of 8 inches is developed. The system consists of three functional units that can achieve the pre-alignment task with high efficiency and high precision, including positioning unit, transporting manipulator and transferring unit. The requirement of positioning and transportation accuracy (less than 7 mum) is essential to design and development of integrated system due to the high precision of IC manufacturing. In this paper, a liner CCD camera with a resolution of 0.1 mum is used to detect the wafer edge and the notch. Major methodology issues, including least square circle fitting approach for calculating the center and radius of the wafer, an edge flexion method for detecting the range of wafer notch, of alignment strategies are introduced. The experimental results show that the accuracy of the wafer notch detection is 3.49 mum and the overall accuracy of wafer positioning is 6.11 mum.