Application of interferometric broadband imaging alignment on an experimental x-ray stepper

A series of experiments were performed with an interferometric-broadband imaging (IBBI) alignment system on an experimental x-ray lithography stepper. These experiments demonstrated sub-1 nm consistency of independent IBBI measurements and the ability to feedback lock the mask relative to a wafer to within a mean of 0.0 nm and a standard deviation of 1.4 nm. Comparisons of displacement measurements made with IBBI and closed-loop piezo drives confirmed scale consistency to within about 1.5%. In the absence of feedback control, spurious relative displacements were observed, due to temperature gradients and the nonrigid mechanics. This argues in favor of feedback control during exposure. The robustness of IBBI in allowing nanometer-level alignment measurements during x-ray exposure, with remotely located long-working-distance, low-cost microscopes should enable it to be used as a stand-alone alignment system or as an adjunct to global alignment. (C) 1998 American Vacuum Society. [S0734-211X(98)10906-X].