14 June 1999 Novel metrology for measuring spectral purity of KrF lasers for deep-UV lithography
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Abstract
The use of higher NA lenses of next generation 248 nm microlithography system sets tight requirements on the spectral purity of the laser, especially because these lenses are not chromatically corrected. Present day KrF excimer lasers are equipped with etalon-based spectrometers that can measure the laser linewidth at full-width-at-half maximum, at nearly every pulse. Both, experience and analysis have shown that the (Delta) (lambda) FWHM may not be the optimum measure of laser spectral purity, and that a better characterization would be the width of the line that contains 95 percent of the laser energy, (Delta) (lambda) 95 percent int. Therefore, the lithography is at risk of losing the image quality if the line shape, characterized by (Delta) (lambda) 95 percent int is outside its limit, even if the laser signals that the (Delta) (lambda) 95 percent measurements of laser line shape. The measurements can be done on a pulse-to-pulse basis or with averaging over an exposure window. Several different configurations and their comparable analysis are presented. These new spectrometers are compact, and can be integrated with a deep UV laser or used as a portable field service tool Despite the small size, the spectrometers have a resolution of about 0.1 pm when measuring FWHM values and about 0.3 pm when measuring 95 percent integral values. The implementation of these new metrology tools provides the lithography with a correct measure of the laser spectral purity during exposure and during process optimization.
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Alexander I. Ershov, Gunasiri G. Padmabandu, Jeremy D. Tyler, Palash P. Das, "Novel metrology for measuring spectral purity of KrF lasers for deep-UV lithography", Proc. SPIE 3677, Metrology, Inspection, and Process Control for Microlithography XIII, (14 June 1999); doi: 10.1117/12.350848; https://doi.org/10.1117/12.350848
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