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Today, precision of a few nanometers is requiid to measure critical dimensions (CD's). Measurement tools must and are being designed to be more stable over both short and long periods. In addition, the high spatial resolution of a scanning electron microscope (SEM) is making it possible to determine moi about the semiconductor feature itself. This paper reports on new data collection and analysis techniques that yield more meaningful and reliable values for SEM-measured CD's including information about (1) CD variation along the feature, (2) individual edge roughness and (3) variations due to the measuring instniment. At TV rates an electron beam is raster-scanned over a small area of interest containing a pitch or line. A CD value is computed from the video signal associated with each of the raster's horizontal scans across the sample. The average of these separately computed CD values is taken to be the CD of record. This method preserves information about "apparent" edge roughness and orientation. However, the contribution of "real" edge roughness is determined only through further analysis. A correlation program was created to compare edge and CD data sets. Plots of various correlations showed that contributions to the standard deviation of edge and CD data sets were quantifiable. For instance, a correlation of a CD data set with itself, but with data acquired at a different time, generated a number that could be associated with the contribution of random video noise. A correlation of a CD data set with itself, but with its position shifted in the data collection window, differentiated contributions of the real feature roughness and other data set variations not assodated with the sample. The correlation piots also revealed information about the frequency of these various contributions.
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