Bench evaluation of lithographic lenses from measurements of the point spread function

Abstract

Image quality criteria derived from the point spread function (PSF) are useful for testing lithographic lenses. The Strehl ratio (SR) is a particularly simple and powerful measure of lens quality which is readily accessible from measurements of the PSF. The SR correlates well with traditional figures of merit such as the mean square wavefront aberration and the modulation transfer function. For object dimensions on the order of X/NA, the SR is a direct measure of the field dependent variation of exposure dose caused by lens aberrations. In this sense, the SR connects the lens quality to the final pattern definition in the image recording medium. Experimental aspects of the PSF measurement are emphasized. The lens test bench used to measure the value of the SR through focus and with field position is described. Data showing the PSF, and the field and focus dependence of the SR, are presented for high resolution Hg I-line and G-line lenses. © 1988 SPIE.