INTERFEROMETRIC BROADBAND IMAGING

摘要

Alignment marks on first and second plates (13, 21) include a plurality of periodic gratings. A grating (gm1) on a first plate (13) has a period or pitch P1 paired up with a grating (gS1) on the second plate (21) that has a slightly different period P2. A grating (gm2) on the first plate (13) having a period p3 is paired up with a grating (gS2) on the second plate (21) having a slightly different period p4. Illuminating the gratings produces a first interference pattern (fig. 7) characterized by a first interference phase where beams diffracted from the first and second gratings overlap and a second interference pattern characterized by a second interference phase where beams diffracted from the third and fourth gratings overlap. The plates are moved until the first and second interference phases correspond to a first predetermined interference phase and a second predetermined interference phase respectively (fig. 6).