Aberration measurement using in-situ two-beam interferometry

A reticle with phase-only blazed gratings of varying azimuthal orientations diffracts light into only two orders, 0 & +1, discretely illuminating a lens pupil. The image of each grating is a sinusoidal interference pattern and is recorded as a surface relief in a highly absorbing photoresist. The maximum image contrast occurs when focus is set such that the RMS wavefront error over the two beams is minimized. This maximum contrast vs focus is recorded by a CCD array mounted on a dark-field optical microscope and the aberrations are obtained from an analysis of this record. Repeatability of equivalent primary aberrations of less than 0.001(lambda) RMS are achieved and used to monitor lens stability.