Line edge roughness detection using deep UV light scatterometry

Abstract: Line edge roughness (LER) is one of the problems that reported to affect the final fab products and most of the time it occurs in the photo lithography process. The industry requirements call for an in-line nondestructive and very fast measuring tool. We report on a possible in-line integrated metrology solution for alarming when LER has grown out-of a specified range. The solution is based on optical scatterometry combined with new algorithms. Once the integrated metrology tool starts alarming, the operators can initiate a more accurate and time consuming procedure at offline stand alone tools to identify and quantify the problem. Wafers with structures of lines that intentionally contain roughness have been prepared by e-beam process. The structures were characterized both by direct imaging tools (SEM and AFM with high aspect ratio tip) and by a scatterometric method using NovaScan 3090. It is shown that for specific structures, scatterometry can alarm when the nominal roughness is between 7.5 and 10 nm. Further development of the method may lead to even smaller detectable range. Hence it is recommended to use this tool for process alarming by preparing few simple test structures that will be combined in scribe lines (in unused areas within the mask).

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