To align a grating (3a) on a mask (3) with respect to an equivalent grating (8a) of a wafer in an photolithographic system where the mask is imaged by an imaging system (7) onto the wafer, symmetrical diffraction orders (u+1 , u.1 ) are focussed on the wafer grating and diffracted a second time to return colinear with the optical axis and to be deflected by a beam splitter (5) to a photo detector (6). The intensity of the super imposed outbeams depends on the relative phase differences of the diffracted beams, and hence on the displacements of the mask and wafer gratings. The phase of the electrical output signal is determined by introducing periodic phase differences in the diffracted beams of the mask grating by a wobbling parallel glass plate (4). For simultaneous X-, Y-alignment crossed gratings are used that operate on two pairs of diffracted beams; the polarisation direction of one of these pairs is rotated by 90° before impinging on the second grating so that each pair of diffracted beams can be fed to a separate photodetector. In a step-and repeat photolithographic system optical fine alignment can be performed by adjusting a fixed tilt angle of the glass plate after having determined the amount of misalignment.
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