We describe performance modeling and design optimization of a prototype extreme ultraviolet (EUV) focus sensor (FS) designed for use with existing 0.3 numerical aperature (NA) EUV proj ecti on- lithography tools. At 0.3 NA and 13.5 nm wavelength, the depth of focus at the resolution limit shrinks to 150 nm increasing the importance of high-sensitivity focal-plane-detection tools. The FS is a free-standing Ni grating structure that works in concert with a simple mask pattern of regular lines and spaces at constant pitch. The FS pitch matches that of the image-plane aerial-image intensity: it transmits the light with high efficiency when the grating is aligned with the aerial image laterally and longitudinally. Using a single-element photodetector, to detect the transmitted flux, the FS is scanned laterally and longitudinally so the plane of peak aerial-image contrast can be found. The design under consideration has a fixed image-plane pitch of 80 nm, with aperture widths of 12-40 nm (1-3 wavelengths), and aspect ratios of 2-12. TEMPEST-313 is used to model the light transmission. Careful attention is paid to the annular, partially coherent, unpolarized illumination and to the annular pupil of the Micro-Exposure Toot optics for which the FS is designed. The system design balances the opposing needs of high sensitivity and high throughput optimizing the signal-to-noise ratio in the measured intensity contrast. (c) 2005 American Vacuum Society.