A real-time inspection is useful and effective to optimize lens aberrations of excimer-exposure sytem, which can expose patterns less than 100 nm. We have developed a portable i.e., compact and lightweight phase measuring interferometer (P-PMI), which can be attached to a stage of the exposure system during real-time monitoring the aberration of the projection lens mounted on the exposure system. Measured repeatability of the wavefront measurement is ab out 0.1 mλ and tool-to-tool difference is 0.6mλ. Measured wavefront during adjusting a projection lens agree dwell with a simulated result. LWA was successfully optimized using P-PMI data.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.