The Estimation and Improvement of Overlay Error Budgets for an Excimer Laser Aligner: Compensation for Illumination Optical System Errors

This paper describes studies on the overlay error budgets caused by an illumination optical system for an excimer laser aligner using TTL (through the lens) alignment. Illumination errors, such as chip rotation, magnification, trapezoid, and distortion have been analyzed and estimated. It was demonstrated that the total error of the illumination optical system was required to be better than 0.035 μm to satisfy the positioning strategy for a 0.05 μm(3σ) overlay accuracy. Several compensation methods using global alignment, a reticle stage, and a Z-tilt sensor have been developed and adopted to the aligner to reduce these overlay errors. As a result, it was obtained experimentally that the total overlay error of the illumination optical system satisfied better than 0.035 μm.Moreover, the excimer laser aligner showed excellent performances with a better than 0.03 μm(3σ) overlay accuracy.