Photon flux requirements for EUV reticle imaging microscopy in the 22- and 16nm nodes

EUV-wavelength actinic microscopy yields detailed information about EUV mask patterns, architectures, defects, and the performance of defect repair strategies, without the complications of photoresist imaging. The measured aerial image intensity profiles provide valuable feedback to improve mask and lithography system modeling methods. In order to understand the photon-flux-dependent pattern measurement limits of EUV mask-imaging microscopy, we have investigated the effects of shot noise on aerial image linewidth measurements for lines in the 22 and 16-nm generations. Using a simple model of image formation near the resolution limit, we probe the influence of photon shot noise on the measured, apparent line roughness. With this methodology, we arrive at general flux density requirements independent of the specific EUV microscope configurations. Analytical and statistical analysis of aerial image simulations in the 22 and 16-nm generations reveal the trade-offs between photon energy density (controllable with exposure time), effective pixel dimension on the CCD (controlled by the microscope's magnification ratio), and image log slope (ILS). We find that shot-noise-induced linewidth roughness (LWR) varies inversely with the square root of the photon energy density, and is proportional to the imaging magnification ratio. While high magnification is necessary for adequate spatial resolution, for a given flux density, higher magnification ratios have diminishing benefits. With practical imaging parameters, we find that in order to achieve an LWR (3σ) value of 5% of linewidth for dense, 88-nm mask features with 80% aerial image contrast and 13.5-nm effective pixel width (1000× magnification ratio), a peak photon flux of approximately 1400 photons per pixel per exposure is required.