Impact of EUV photomask line-edge roughness on wafer prints

The line-edge roughness (LER) of a photomask image has a measurable impact on the corresponding printed wafer LER. This impact increases as wafer exposures move from 193nm DUV to 13.5nm EUV wavelengths since the imaging tool is a low-pass filter with EUV passing more spatial frequencies. Even the high frequency mask LER may impact the wafer image by lowering its image log-slope (ILS). Studying the magnitude and frequency content of mask LER is a first step to reducing the wafer LER. The next step is to determine which components of mask line roughness actually contribute to the wafer line roughness. Order is imposed on this study by fabricating programmed LER patterns on an EUV mask to introduce controlled variations in LER spatial frequency and magnitude. More specifically, line-width roughness (LWR), LER and power spectral density (PSD) are extracted from 64nm and 90nm (1X) pitch lines on a programmed LER EUV photomask. The same mask is then exposed on the ASML EUV Alpha Demo Tool (ADT) at best focus and dose. Three chemically amplified EUV photoresists are evaluated using the programmed LER photomask through PSD and LWR comparisons and the highest performance resist is used for a comprehensive LER transfer analysis. Wafer LWR is extracted from 64nm and 90nm pitch lines and correlated back to the base mask patterns revealing an empirical LWR transfer function (LTF). Finally, the study is extended to 45nm (1X) pitch lines by deploying a pupil filter on the ADT to explore the effect on LWR as the feature sizes shrink.