Accurate Measurement of Spherical and Astigmatic Aberrations by a Phase Shift Grating Reticle

A new grating structure is proposed for measuring aberrations in lithography lenses. The grating structure on a reticle comprising opaque lines, naked lines and π/2-phase-shifted grooves with their width ratio equal to 2/1/1 has a property such that either of the two first-order diffracted rays disappears entirely. The other first-order ray interferes with the zeroth-order ray to form tilted standing waves. Since a resist pattern of the grating is formed on top lines or valley lines of the standing waves, it moves with perfect linearity upon defocus. In this paper the quantitative metrology of even aberrations in a lithography lens is described along with a demonstration of an application of the grating reticle for a krypton fluoride excimer laser scanner with a numerical aperture (NA) of 0.73. Variation of the grating period along with a small-opening stopper inserted into the illuminator enlarge the measurable range of the pupil radius. The measurement technique described in this paper achieves high accuracy which is sufficient for small aberration lenses in state-of-the-art scanners.