Dynamic three-dimensional mask-wafer positioning with nanometer exposure overlay accuracy

In previous articles we described an x-ray mask alignment scheme called Interferometric Broad-Band Imaging (IBBI), and an interferometric gapping scheme called Transverse Chirp Gapping (TCG). Both schemes encode position in the spatial phase relation of two sets of interference fringes, observed with an oblique-incidence optical microscope. IBBI is capable of detecting sub-nanometer misalignment, and TCG has shown a phase detectivity in the nanometer regime. Here we demonstrate the efficacy of IBBI and TCG for performing dynamically aligned and gapped exposures using a relatively inexpensive (-$150k), custom-built stepper. Additionally, we describe a scheme to minimize overlay runout by direct alignment of an x-ray point source to an arbitrary fiducial mark on the mask.