Interferometric-spatial-phase imaging for six-axis mask control

We describe a unified approach to measuring alignment and gap with nanometer detectivity between two planar objects (e.g., a mask and a substrate) in close proximity. The method encodes lateral position in the phase of interference fringes, formed by diffraction from grating and checkerboard alignment marks, designed to enable a wide acquisition range. For gapping, the method incorporates, in the same mark, coarse-gap detection (30–300 μm) and absolute-gap detection at sub-30 μm using a chromatic Fabry–Pérot scheme. Fine detection of sub-30 μm gaps is inferred from the frequency and phase of fringes, calibrated using the chromatic Fabry–Pérot. Illumination with a variable-bandwidth source enables either “achromatic” aligning or “chromatic” gapping. Sub-nanometer detection and feedback control of mask position is demonstrated in X, Y, and θ. Overlay of exposed patterns is demonstrated to be <3 nm.