Multilayers for next generation EUVL at 6.X nm

Abstract

The most promising wavelength for the next generation EUV lithography in terms of maximizing throughput of an optical system was found to be 6.63nm, where highest peak reflectivity is expected at this wavelength using La(La₂O₃)/B₄C structures. The optical throughput at 6.63nm is expected to be ~6 times lower than at 13.5nm due to the higher resolution of multilayers at the smaller wavelengths. La/B₄C and La₂O₃/B₄C multilayer structures were coated at RIT by using magnetron sputtering deposition technology. EUV reflectivity of the multilayers was tested at CXRO and NewSUBARU. The round robin measurements demonstrated a maximum deviation of 1.9% in the peak reflectivity and 0.0063nm in the peak position. The big difference in the peak value can be explained by presence of the higher harmonics in the probe beam at NewSUBARU which affected the accuracy of the measurements. The maximum peak reflectivity of 48.9% was measured from La/B4C multilayer at 6.68nm. Maximum reflectivity of La₂O₃/B₄C structure at this wavelength was 39.2% while reflectivity at 6.63nm was measured to be 42.68%. The measured band width of the reflectivity curves was about 20% lower than for ideal structures. La₂O₃/B₄C structure demonstrated a larger level of the imperfections resulting in much lower performance. EUV reflectivity of one of the La/B4C multilayers deposited in December 2000 was measured at NewSUBARU in January 2011 and the results were compared with the measurements performed in January 2001 at CXRO. The maximum reflectivity dropped from 42.6% to 37.6%. Reduction of the reflectivity band width from 0.044nm to 0.04nm was also observed.