Matching of different CD-metrology tools for global CD signature on photomasks

Critical Dimension uniformity (CDU) is one of the most critical parameters for the characterization of photomasks. For years the understanding was that CDU describes a rather random fluctuation of the CD across the mask. With more advanced CD tools and mask processes the local short-range CD variation (on a length scale of micrometre) can be distinguished from the global CD signature (typically on a length scale of centimetre). Recent developments in the pattern generator sector allow correcting for such global CD signatures. This triggers the current challenge to find stable methods to characterize the global signature of photomasks. In our work we present matching results of a technique that calculates the CD signature using exponentially weighted surrounding points. We investigated different CD SEM tools of different technology generations. We show that our method allows determination of the CD signature independently of the measurement tool with low uncertainty and moderate measurement effort. This holds even true when the CDU value is mainly dominated by the measurement error. Thus our method provides a tool to extend the utilization of older generation metrology tools as well as the possibility to improve the measurement capability for CD signature of current tools.