Novel metrology for measuring spectral purity of KrF lasers for deep-UV lithography

Abstract

The use of higher NA lenses of next generation 248 nm microlithography systems 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 (&FWHM), at nearly every pulse. Both, experience and analysis have shown that the AX 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% ofthe laser energy, AX iflt Therefore, the lithographer is at risk of losing the image quality if the line shape, characterized by is outside its limit, even if the laser signals that the AXFW is within limits. In this paper, we describe the results of our development of new generation spectrometers that are capable of making simultaneous, high resolution AX FWHM and L\X95% 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% integral values. The implementation of these new metrology tools provides the lithographer with a correct measure of the laser spectral purity during exposure and during process optimization.

Laser spectrum line shape metrology at 193 nm

“…The signal from the PDA is read by a laser microprocessor. The intensity of transmitted light along the angle a to the etalon is determined by the following equations5:It -I/(l + (2F/ii)2 sin2 (8/2)),(2) F=icR"2/(1-R),(3) 6=27rLt/A+z1çb,(4) Lls2dflaCOSa(5) In these equations, 4 is the incident light intensity, 6 is the phase shift, Aq$ is extra phase shift on reflection from the etalon surface, ? is the laser wavelength, d is the etalon gap, F is the etalon finesse, n, is the…”