A Dual Grating Alignment Method Insensitive to Mask-Wafer Gap Variation

This paper presents an X-ray lithography alignment method insensitive to mask-wafer gap variation, using dual gratings. Diffraction light beam intensities for various dual gratings were calculated on the assumption that each grating is a sinusoidal amplitude grating. As a result, if dual gratings, consisting of a linear grating on the mask and a checkerboard grating on the wafer, are used as a alignment key, it was found that the diffraction beam intensity I (0, 1), zero order in x (displacement) direction and first order in y direction, is independent from mask-wafer gap variation. The fact was confirmed by more detail calculations and experiments. Alignment experiments were carried out using intensity difference ΔI = I1 (0, 1) -I2 (0, 1) between two pairs of the dual gratings, which were arranged to have 180° phase shifted signal with reference to each other. A resolution better than 0.01 μm and 0.03 μm (3σ) alignment repeatability have been achieved among 25 μm and 55 μm gap variations.