Challenges of laser spectrum metrology in 248 and 193-nm lithography

Several approaches for high-resolution laser metrology have been discussed. One approach is to use a multiple-etalon spectrometer, which has two or more etalons with different FSRs. This approache can increase both the resolution at FWHM and the tails, as well as increase the spectrum range of the instrument. With the proper alignment, this multiple etalon configuration can produce an instrument whose resolution is equal to or better than the highest resolution etalon while still maintaining the FSR of the lower resolution etalon. In the configuration tested, a spectrometer designed for 248nm was constructed with a 2pm etalon and a 20pm etalon. The resolution of this multi-pass, multi-etalon (MPME) spectrometer produced an instrument function of 0.086pm FWHM and 0.339pm for the integrated 95% level over an integration range of 20pm. Another approach is to use a combination of diffraction grating and etalon - based spectrometers. In this approach, the etalon provides high resolution for FWHM measurements, while diffraction grating provides accurate measurement of the spectrum tails over the wide scanning range. This idea has been tested with a 193 nm instrument.