METHOD AND APPARATUS FOR DETERMINING A FEATURE-FORMING VARIANT OF A LITHOGRAPHIC SYSTEM

A method of determining a feature-forming variant, such as focus or exposure, for a lithographic system or the like includes Fourier processing that extracts a figure of merit. In a preferred embodiment, the lithographic system fabricates a series of formations of a single pattern. An optical image of each exposure is formed. A Fourier power spectrum is determined for each optical image. An angular power function is extracted from each power spectrum by quantifying optical power along lines originating at various angles from an origin of the power spectrum. With regard to axes of maximum power, the distribution of on-axes and off-axes power is quantified for each angular power function. An optimum for the lithographic system is determined based upon the data extracted from the angular power functions. Preferably, the extraction of an angular power function from the associated Fourier power spectrum is an averaging of the optical power along selected portions of the lines originating from the origin, thereby improving the signal-to-noise ratio.