EUV Interferometry of the 0.3 NA MET Optic Kenneth A. Goldberg *a , Patrick Naulleau a , Paul Denham a , Senajith B. Rekawa a , Keith Jackson a , Erik H. Anderson a , J. Alexander Liddle a , Jeffrey Bokor a,b a Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 b EECS Department, University of California, Berkeley, CA 94720 ABSTRACT A new generation of 0.3 numerical aperture prototype EUV optical systems is now being produced to provide an oppor- tunity for early learning at 20-nm feature size. Achieving diffraction limited performance from these two-mirror, annular projection optics poses a challenge for every aspect of the fabrication process, including final alignment and interfero- metric qualification. A new phase-shifting point diffraction interferometer will be used at Lawrence Berkeley National Laboratory for the measurement and alignment of the MET optic at EUV wavelengths. Using the previous generation of prototype EUV optical systems developed for lithography research, with numerical apertures up to 0.1, EUV interferom- eters have demonstrated RMS accuracy levels in the 40–70 pm range. Relative to the previous generation of prototype EUV optics, the threefold increase to 0.3 NA in the image-side numerical aperture presents several challenges for the extension of ultra-high-accuracy. Keywords: interferometry, extreme ultraviolet lithography, EUV, at-wavelength testing, MET. 1. INTRODUCTION High-accuracy interferometry is a cornerstone requirement for the success of EUV optical systems. State-of-the-art visi- ble-light testing is used in the fabrication of the individual mirror elements, 1,2 and has been used in the alignment of numerous assembled EUV optical systems. 3,4,5 Interferometer absolute accuracies in the 50 pm range are a requirement for the measurement of production-quality EUV elements and assembled systems. 6 Visible-light interferometry continues to benefit by close, ongoing comparisons with EUV interferometric measurements performed on the same optical systems. A system-level comparison performed at more than 40 points across the field of view of the Engineering Test Stand (ETS) Set-2 optic, revealed a level of agreement of 0.35±0.11 nm between EUV and visible light interferometries. 7 The discrepancy was concentrated in the lowest spatial frequency aberrations (astigmatism in particular) which are most important for the alignment of the system. Recently, several systematic measurement error sources have been identified via comparison, and subsequently addressed. This continuing learning raises the accuracy of the visible-light interferometric techniques. EUV interferometry performed with the phase-shifting point diffraction interferometer (PS/PDI) 8,9 has demonstrated accuracy levels of 40-70 pm during the testing of previous generations of prototype EUV optical systems, typically of 0.1 NA, developed for EUV lithography research. 10 Often, prior to measurements with the PS/PDI, the optics are measured with lateral shearing interferometry (LSI) which is performed with a cross-grating transmission beamsplitter placed near the image-plane. 11 As reported previously, for the initial measurement of a nominally pre-aligned test optic, the LSI has several advantages over the PS/PDI. These include ease of alignment, high efficiency, and the potential to measure aberrations of larger magnitudes. Switching between the PS/PDI and the LSI requires only a change of the image-plane mask. (Since the experimental chamber is also designed for small-field imaging experiments 12 , there is a built-in load-lock and manual wafer (or pinhole-mask) transfer system.) Measurements performed at the operational EUV wavelengths, at-wavelength, remove potential uncertainties about the response of the resonant-reflective multilayer coatings, and have provided accurate predictions of imaging * Correspondence: Email: KAGoldberg@lbl.gov; Telephone: 510-495-2261; Fax: 510-486-4550 Emerging Lithographic Technologies VII, Roxann L. Engelstad, Editor, Proceedings of SPIE Vol. 5037 (2003) © 2003 SPIE · 0277-786X/03/$15.00 69