Theoretical efficiency analysis of a condenser-embedded grating-based spectral purity filter for EUV lithography

Being based on reflective optics, extreme ultraviolet (EUV) lithography systems are, in principle, relatively immune to chromatic errors. However, illumination bandwidth control is still required for EUV lithography. For example, appreciable amounts of UV power, combined with resist sensitivity to this wavelength band, would decrease image contrast. Also, appreciable amounts of IR power would place unacceptable thermal loads on the projection optics. A practical method for spectral filtering, widely used in short-wavelength synchrotron applications, is the grating monochromator. Here we present the theoretical performance analysis, of a,grating-based spectral purity filter integrated into an EUV lithography condenser system. Although the specific examples presented here are geared towards a specific condenser design, it should be noted that the methods described are generally applicable to a variety of condenser designs as might be found in future EUV lithography systems. (C) 2002 Elsevier Science B.V. All rights reserved.