EUV interferometric testing and alignment of the 0.3 NA MET optic Kenneth A. Goldberg * , Patrick Naulleau, Paul Denham, Senajith B. Rekawa, Keith Jackson, J. Alexander Liddle, Erik H. Anderson Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT Extreme ultraviolet (EUV) interferometry has been successfully performed for the first time at 0.3 numerical aperture (NA). Extensive EUV “at-wavelength” testing including alignment, was performed on a newly created Micro Exposure Tool (MET) optic designed for sub-50-nm EUV lithographic imaging experiments. The two-mirror, 0.3 NA MET is ar- guably the highest resolution light-projection lithography tool ever made. Using both lateral shearing and phase-shifting point-diffraction interferometry, the wavefront was measured across the field of view, and the alignment was optimized in preparation for imaging. The wavefront quality reached 0.55 nm RMS (  EUV /24.5) in a 37-term annular Zernike poly- nomial series, dominated by higher-order spherical aberration. Measurements included calibrations of the interferometer accuracy, assessment of repeatability, and cross-comparisons of visible and EUV interferometric measurements. The comparisons and the final, measured wavefront quality were affected by an apparent alignment drift, several tenths of a nm in magnitude. Significant unresolved differences between testing strategies shows that continued work is needed to improve the measurement accuracy to levels required for EUV lithography. Keywords: extreme ultraviolet interferometry, extreme ultraviolet lithography, EUV, at-wavelength testing, MET. 1. INTRODUCTION A new generation of 0.3 numerical aperture (NA) prototype extreme ultraviolet (EUV) optical systems is being produced to provide an opportunity for early learning with sub-50-nm EUV imaging. Developed for static micro-field imaging, these two-mirror, 0.3 NA, Micro-Exposure Tool (MET) optics operate at 13.5-nm wavelength, and have a design Rayleigh resolution of 27-nm. 1,2,3 They hold the promise of even higher resolutions achieved with tailored illumination conditions. Visible-light and EUV wavefront measurements of the MET reveal it to be one of the highest resolution light-projection lithography tools ever made. In order to achieve optimal, diffraction-limited performance, EUV optical systems require alignment to sub-nanometer aberration tolerances. Wavefront aberrations of a few tenths of a nanometer can cause a significant reduction in the proc- ess window. While image-printing and certain aerial image monitoring configurations will provide some wavefront quality feedback, at these high resolutions, detailed quantitative system measurements are only available from interfer- ometry. Ultra-high-accuracy interferometry is a cornerstone requirement for the success of these and future optics, and represents a strategic risk-reduction step for these expensive developmental optics. Operating at the design EUV wave- length, EUV interferometry has been used in the diagnosis and remediation several types of fabrication and system- alignment errors, in the assessment of chromatic effects and flare, and most importantly, in the optimization of imaging performance. 4,5,6,7 In principle, if the mirror surfaces are clean and the multilayer profiles are well known, then predictable angle- and po- larization-dependent phase differences can be accounted for, and the EUV and visible-light measurements will agree. However, for EUV optics, the required system wavefront accuracies approach or exceed the accuracy limits of the inter- ferometers used to test them. Active collaborations between researchers at Lawrence Livermore National Laboratory (LLNL) and Lawrence Berkeley National Laboratory (LBNL), in the cooperative measurement of eight different proto- type EUV optics, have improved our confidence in ultra-high accuracy interferometry in the 100-pm domain. Careful intercomparisons have led to the discovery and remediation of several systematic error sources that would have been *KA.Goldberg.@.lbl.gov; phone 1 510 495-2261; fax 1 510 486-4550; www-cxro.lbl.gov Emerging Lithographic Technologies VIII, edited by R. Scott Mackay, Proceedings of SPIE Vol. 5374 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 · doi: 10.1117/12.546199 64 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/27/2015 Terms of Use: http://spiedl.org/terms