Nanometer gap measurement and verification via the chirped-Talbot effect

We describe a noncontact, optical method of measuring, with nanometer-level sensitivity, the gap between two planar objects in close proximity, such as a substrate and either a proximity-lithography mask or an imprint template. Interference fringes from a chirped-checkerboard mark on one object are observed using a nonexposing wavelength with long-working-distance, oblique-incidence microscopes. The gap is determined from the spatial frequency and phase of the fringes. We verify the gap measurement using a variation of the Talbot effect with the chirped-checkerboard mark. The two forms of gap measurement are complementary since one is suited to measuring and setting gap prior to exposure, and the other is ideal for confirmation of the gap that existed during exposure. (C) 2004 American Vacuum society.