Accuracy of focus wedge mark measurement using fitting function for partially coherent illumination

The theory of best focus determination in focus wedge mark (FWM) metrology is extended from coherent illumination to partially coherent illumination. Through a theoretical analysis, we propose a focus curve fitting function suitable for the FWM, which is expressed by the sinc function. The experimental results for best focus estimations using two types of fitting functions, which are a conventional quadratic polynomial and the proposed sinc function, are reported for various defocus regions to be fitted. When the first inflection points of the sinc function are included in the fitting defocus region, we can achieve stable best focus estimation within 3 nm in our i-line exposure experiments. (author)