Scattered light: the increasing problem for 193-nm exposure tools and beyond

Scattered light, especially in the mid spatial frequency range, is shown to be more prominent and critical for exposure tool at 193nm and beyond in addition to the figure error represented by 37 Zernike polynomials. Image quality is degraded by the scattered light. An imaging model based on the concept of Power Spectral Density (PSD) is developed on a telecentric exposure system with Kohler illumination. The PSD takes into account of the different regime of spatial frequency of the roughness in the system. The imaging model uses two set of PSDs (either Gaussian, K-correlation or Fractal) one from the condenser/reticle roughness and the other from projection optics roughness. Each PSD is described by only 2 parameters. The simulation shows that condenser roughness modifies the source shape and reduces spatial coherence of the source. The projection optics roughness degrades image quality and cause long range light scattering into opaque region. This model provided a efficient framework for the study of the impact of scattered light on various lithographic techniques, including double exposure.