Mask topography effects in projection printing of phase-shifting masks

Topography effects of glass edges in phase-shifting masks (PSM's) on image quality are assessed using the rigorous electromagnetic simulation program TEMPEST on three different optical systems for four PSM technologies including alternating, rim, attenuated, and chromeless. The scalar and thin mask approximations used in simulation programs such as SPLAT can be in error by as much as 20% for certain classes of shifter edges. A feature size independent bias of 0.021 /spl lambda//NA per edge is recommended for alternating masks with vertical edges because light is lost near the etched glass edges. No direct electromagnetic interaction between chromium edges and shifter edges was found for rim phase-shifting masks. The rim dimension can thus be designed solely on the basis of the sidelobe level and peak intensity. For attenuated PSM, edge effects are less severe but sidelobe problems occur. For a center to sidelobe contrast of 0.6 over a DOF of 3 RU, a lower transmission of 4% is recommended. For chromeless PSM, the imbalance in image peaks is shown to be affected by the optical stepper parameters. In any PSM technology, it appears that a 360/spl deg/ glass protrusion may produce a drastic drop in intensity due to resonant effects.<
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