Lithographic alignment and overlay by projection moiré interferometry

Abstract

As device structures advance into the extreme submicrometer regime, decreased alignment and overlay budgets, conservatively to 50 nm, will be required. The alignment scheme must demonstrate significantly better performance. Present alignment schemes are based on relatively large alignment marks’ or on coarse gratings.2 We report an alignment scheme based on projection moire interferometry with a resolution to better than 2 nm. The alignment scheme involves illuminating a grating on the wafer with two coherent laser sources from opposite directions arranged so that the diffraction orders are near normal to the wafer surface. Various alternative geometries have also been investigated. The interference between these two diffraction orders produces a projected moire interference pattern with a phase that contains the alignment information. This phase is periodic in the position of the wafer with a period of 1 /2 the Si grating. By monitoring the intensity of the central fringe, an alignment sensitivity of 2.5 nm for a 1 pm pitch grating and of 1 nm for a 340 nm pitch grating has been demon­strated with He-Ne laser illumination (633 nm). Similar sensitivities are observed in overlay measurement for superimposed gratings. A novel approach to spatial period doubling is demonstrated for double-exposed gratings offset by 1/2 the grating period.