Contrast studies in high-performance projection optics

Image contrast is a simple physical parameter for estimating performance Of optical systems using positive photoresist. Measurements of image contrast are reported on a variety of single and multi-wavelength projection aligners, and compared with theoretical predictions. On the best systems the dark field contrast approaches the theoretical (diffraction limited) contrast within an equivalent defocus error of about one-half Rayleigh unit. In general the contrast is lower, owing to a number of nonidealities. The effects of focus error, astigmatism, chromatic aberration, off-axis aberrations, and flare from clear-field areas are observed in the contrast measurements. On some systems, the contrast can be significantly increased by a modification of the source filtering.