Mask-to-wafer alignment utilizing zone plates

Abstract

Mask-to-wafer alignment in X-ray lithography is advantageously carried out utilizing zone plate marks formed on the mask and wafer. In practice, it has been observed that the intensity and in some cases even the location of the centroid of the light spot formed by a zone plate mask can vary during alignment as the mask-to-wafer spacing is changed. The present invention is based on the discovery and analysis of the causes of such variations. Based thereon, applicants have devised a modified mask structure which, when used with a wafer in a zone plate alignment system, enables mask-to-wafer alignment to be made more easily and more reliably than was possible heretofore. The modified mask structure includes a localized blocking layer over each zone plate on the mask. This layer allows only a negligible portion of the light employed to illuminate the zone plates on the mask to propagate into the mask-to-wafer space. Also, the modified mask structure includes an antireflection layer that reduces interference effects between light spots imaged by the wafer zone plates and light reflected from the mask. Further, one layer of the modified mask structure is designed to have a quarter-wavelength thickness thereby minimizing interfering light reflected from that layer.