Impact of Mask Roughness on Wafer Line-Edge Roughness

The influence of line-edge roughness (LER) of an optical photomask on the resulting printed wafer LER is investigated. The LER Transfer function (LTF) proposed by Naulleau and Gallatin, and later corrected by Tanabe, is shown to be a very useful tool for evaluating the low-pass filtering behavior of the imaging tool and its impact on the transfer of mask LER to the wafer. Highfrequency mask LER can also impact wafer LER by lowering the normalized image log-slope (NILS) of the image, though it would take a large amount of mask LER before this affect would be noticeable. Low-frequency mask LER, most likely due to mask writer errors such as shot placement or rotation errors, will produce wafer LER that may be significant in magnitude. Further work characterizing the magnitude and frequency content of mask LER over many different masks and processes is needed.