Proceedings PaperContact Hole Imaging In Stepper Lithography
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Contact hole array imaging is compared to line-space grating imaging to determine the resolution of contact hole features printed by stepper lenses. Both calculated and experimental results are reported. Image intensity profiles, IIPs, have been calculated for selected cross-sections of square contact holes and equal line-space gratings. The calculations are done according to the method of Hopkins for circular aperture optical systems, partial coherence levels, fractional pupil fill (S), of 0.5 and 0.7, and various levels of optical defocus. The calculated results show that for S = 0.7, at resolution levels (i.e., half-pitch of equal line-space grating or the width of the square contact) greater than 0.9 λ/NA, the IIP of the lateral cross-sections of contact hole arrays to be equal to or better than that of the correspondingly dimensioned gratings. At resolution levels less than 0.9 λ/NA, the contact hole IIP degrades rapidly with respect to the grating IIP. A comparison of the lateral and diagonal cross-sections of contact holes with no defocus aberration shows that the shape of the contact changes from a 'square' to a 'circle' in the 1.25 to 1.00 λ/NA range. In the 1.0 λ.NA resolution region the lateral contact hole IIP has a greater intensity than that of the space of the grating. The effects of optical defocus on contact hole imaging are examined for g, h, and i-line stepper lens at the same λ/NA resolution levels. Contact hole IIPs degrade more rapidly with optical defocus than the correspondingly dimensioned grating IIPs. Experimental comparisons between contact hole and grating resist images are presented for several stepper lens systems. The comparison between calculated and experimental results shows significantly less than 'perfect' imaging for some stepper lenses. Contact hole imaging characteristics are much more sensitive to residual aberrations in the optical system and can be used as a relative measure of the magnitude of these aberrations.