The effects of broadband 250 nm illumination on process latitude

Abstract

The effects on lithography of the spectral bandwidth of the illumination used in an optical step-and-scan projection aligner have been studied J. Buckley and C. Karatzas, SPIE 1088, 424 (1989). An illumination bandwidth of 240–255 nm is compared with a single wavelength of 248 nm representing mercury–xenon lamp and excimer-laser spectra, respectively. Linewidth control for resist-thickness variations, resist-profile effects, and linewidth stability on topography are reported. The experimental results on critical-dimension variations were obtained from tests run on a deep-UV 0.35 numerical aperture (NA) 4× step-and-scan system using the Shipley XP89-142 negative resist system, and on a 0.35 NA 5× excimer stepper using both SNR248 and XP89-142 (SNR248-3×) resist. An HP 8450 photospectrometer was used to study resist reflectivity variations with wavelength and thickness. Both SEM and electrical-probe measurements are reported. It is demonstrated that resist thickness variations cause larger critical-dimension variations in monochromatic exposures than in broadband exposures. It is shown that the critical-dimension variation due to resist-thickness variation over topography reduces the process window, particularly in the case of monochromatic illumination. It is also demonstrated that the full benefit of the spectral width of the broadband illumination is only achieved when the illumination source spectrum, the deep-UV filter-transmission characteristics, and the resist transmission are all matched.