Interference lithography (IL) [Yen et al., Appl. Opt. 31, 2972 (1992)] based on reflective optics, and utilizing extreme ultraviolet (EUV) light from an electron storage ring, can be used to pattern structures with periods below 70 nm [Solak et al., Appl. Phys. Lett. 75, 2328 (1999); Ali et al., Microelectron. Eng. 65, 454 (2003)]. Previously achromatic interferometers with EUV transmission diffraction gratings were also used for this purpose [Wei et al., J. Vac. Sci. Technol. B 12, 3648 (1994)], and recently we have printed sub-50 nm high quality periodic structures in poly(methylmethacrylate) exposed with transmission-grating EUV interference lithography (EUV-fL) [Solak et al., Microelectron. Eng. 61, 77 (2002); Solak et al., ibid. 67-68, 56 (2003)]. With this technique, two-dimensional patterns are easily achievable using multiple diffraction gratings. Both types of EUV EUV-IL have been implemented on one of the undulator beam lines at the University of Wisconsin-Madison Synchrotron Radiation Center electron storage ring because this undulator source provides a tunable and coherent beam of radiation well suited for IL studies. In this article, both exposure methods were applied to explore the ultimate resolution of two positive-tone chemically amplified resists supplied by the Shipley Corporation. We have demonstrated the printing of features as small as 26 nm line and space in Shipley XP9947W-100 resist. The present results demonstrate the fine quality of the above-mentioned EUV-IL techniques to test resist formulations for high-resolution patterning, and we provide the experimental setup and discuss the observed results. (C) 2004 American Vacuum Society.