Proximity effects and influences of nonuniform illumination in projection lithography

Abstract

The change in the size of a feature due to the proximity of large features is explored experimentally and theoretically, and the effects of nonuniform light source intensity are examined theoretically. For proximity effects, a test mask was designed to provide test patterns for two different systems: the Perkin-Elmer “Micralign” model 220 and the Canon FPA-141 4x reduction projection printer. The mask consists of isolated features of various widths which pass parallel to large features at various spacings. Exposures made on the Perkin-Elmer showed proximity effects dependent on the amount of partial coherence present in the light. For relatively incoherent light, proximity effects are intuitive and cause linewidth and spacewidth changes for small features. For relatively coherent light, the proximity effects can be counterintuitive in that the width of a small feature can be reduced due to the presence of adjacent features. Proximity effects are particularly significant for small geometries in the range of 0.5w/(NA) and below, where w is the wavelength. Some mask design guidelines are offered, suggesting that better overall image quality may be obtained not with coherent illumination, but with a partial coherence factor of about 0.5. Several types of condenser aperture illumination sources are explored. Central obscuration is shown to be disadvantageous for traditional optical lithography where the fundamental image spatial frequency is below that of coherent cutoff. A four spot Gaussian source is shown to give image quality similar to that of a uniform source of the same diameter. It is shown that source asymmetry cannot contribute asymmetrical resist profiles in a diffraction limited system, and thus any resist asymmetry must be attributed to proximity effects. © 1982 SPIE.