Immunity to signal degradation by overlayers using a novel spatial-phase-matching alignment system

We describe improvements to the interferometric broad-band imaging alignment scheme introduced in 1993. Alignment is signified by matching across the midline of a charge-coupled device, the spatial phase of interference fringes formed by diffraction from complementary marks on mask and substrate. Image contrast is enhanced by back diffraction from hatched alignment marks on the substrate. Overlayers of resist polysilicon, and aluminum have negligible effect on interferometric broad-band imaging alignment; they alter image contrast but not spatial phase. Novel alignment marks that incorporate four gratings increase the capture range to several tens of micrometers. By spatial filtering in the back focal plane of the alignment microscope, mask-sample gap may be determined from the resulting spatial phase shift. An alignment system for x-ray nanolithography (XLS-4) that incorporates the interferometric broad-band imaging scheme has been constructed. (C) 1995 American Vacuum Society.