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Recent development of high-precision aberration measurement techniques has enabled in situ characterization of the aberration response to wavelength offset. These measurements show that majority of the reconstructed Zernike terms exhibit some degree of sensitivity to wavelength. Although this dependence diminishes with the increasing order of Zernike polynomial, we consider the cumulative contribution of five Zernike terms, which have the strongest wavelength dependence (Z2, Z4, Z6, Z8, and Z11). The imaging impacts of KrF laser wavelength and spectral bandwidth are investigated using aerial image simulation; the behavior of the process window, mask error enhancement factor (MEEF), image placement, proximity effect, and sidelobe intensity is quantified. In this model, the chromatic aberrations are experimentally measured in a 0.68-NA KrF step-and-scan exposure system using the LITEL aberration test (InspecStep interferometer manufactured by LITEL Instruments, Inc., San Diego, California). The illumination spectrum input is characterized by spectroscopic measurement of a 2-KHz KrF laser source. In the lithography model, it is important to incorporate all of the wavelength-sensitive terms due to the additive contribution to the overall lens aberration balance. As shown previously, the longitudinal and lateral chromatic aberrations (image height and magnification) are the most sensitive to shift in center wavelength and have the strongest contribution to the aerial image modulation. Simulation results show several imaging changes for isolated lines and contact holes with changes in illumination spectrum. However, the rates of change are shown to decrease as bandwidth is reduced well into the subpicometer level. In the case of isolated contacts, the depth of focus (DOF) increases with the increase in bandwidth, however, at the expense of reduced exposure latitude. This suggests that engineering the spectral output of the laser can provide some process enhancement, although careful compromise is needed to utilize any DOF enhancement, since other image metrics including MEEF, side-lobe intensity, and image placement are also affected.
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