New Method to Reduce Alignment Error Caused by Optical System

ABSTRACT Advanced stepper or scanner needs an extremely accurate alignment system. The alignment accuracy is mainly affected by the errors caused by mark deformations and by optical system. To improve the alignment accuracy of our wafer alignment system called 'FIA' (Field Image Alignment), we have developed a method called the 'FFO' (FIA Focus Optimization). Our studies have already shown that FFO has the effect of reducing the errors caused by mark deformations. To examine the errors caused by optical system, new approaches are adopted. In the new approaches a simulation method and a suitable experimental are used. The simulation results by the new method, Spatial Frequency Analysis of Image, show the relation between defocus and the errors caused by optical system and the superiority of FFO. Suitable experimental system brings us the same results as the simulation method. As a result, FFO is also has a positive effect on the errors caused by optical system. FIA with FFO is much more accurate alignment sensor for ULSI production. Keywords: Alignment, Stepper, Image Processing, Optical System, Imaging System