|
In this study, a discussion of the challenges and the general requirements associated with the possible candidates of resolution enhancement techniques for 90 nm lithography technologies are provided. Theoretical analysis of the benefits of dipole illumination will be covered. As dipole illumination performance is pitch dependent, simulation results have demonstrated superb printing performance with scattering bars added. Nevertheless, there are some forbidden pitches that are degraded by the use of dipole illumination. The motivation behind this study is to investigate the limit of dipole illumination to image for 90 nm equal lines and spaces and identify the forbidden pitches. Rayleigh Criterion applies only to a feature at the resolution limit of imaging system. Therefore, Rayleigh equations are not sufficient to address the effect of NA and coherence (σc) on DOF. Hence, we offer to extract from Fraunhofer diffraction equation to incorporate coherency factor σc and pitch changes to extend use of Rayleigh equation. Results show that the extension of Rayleigh equation is capable to map out the forbidden pitch locations for any feature size and illumination conditions. More importantly, it can complement the concept of objective lens pupil filling to provide the theoretical ground for illumination design in order to suppress the forbidden pitch phenomenon. The influences of NA, outer σo and inner σi on the depth of focus and exposure latitude on 90 nm equal lines and spaces are shown quantitatively with experiment results. Experimental results show that decreasing inner sigma results in contrast loss of the structure in the optimal orientation. Furthermore, the influence of NA is relatively stronger compared to the effect of coherence (sigma) in terms of linewidth variation through pitch.
|
