IMMUNITY TO SIGNAL DEGRADATION BY OVERLAYERS USING A NOVEL SPATIAL-PHASE-MATCHING ALIGNMENT SYSTEM

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