A mask-to-wafer alignment and gap setting method for x-ray lithography using gratings

Abstract

A new optical-heterodyne interferometry alignment and gap setting method for x-ray lithography is developed, where the phases of beat signals are used for detection. The fact that the lateral displacement and the gap between mask and wafer can be detected independently is shown, based on the results of analysis and experiments. Two pairs of gratings are arranged as detection marks. One pair forms a window on the mask and a checkerboard grating on the wafer, while another pair forms two linear gratings at right angles to each other. Two He-Ne laser beams with slightly different frequencies illuminate the gratings from the ±1st-order diffraction light directions. The lateral displacement is detected by measuring the beat signal phase difference between the two (0,1)th-order diffraction lights from two pairs of gratings. To detect the gap, the signal phase difference between the (0,1)th- and (1,1)th-order diffraction lights from the pair of linear gratings is used. Better than 0.02 μm (3σ) alignment repeatability and 0.1 μm gap setting accuracy have been achieved.