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6.4 CD Measurement Function (Option)
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6.4.3 Principle of CD Measurement
6.4.3.1 Edge Detection Algorithm in Auto Measurement
Figure 6.4-2 shows the cross section and secondary electron signal of the typical specimen.
As demonstrated in the figure, the intensity of secondary electron signal is higher at the edges of
the pattern. In auto CD measurement, edge positions are detected using this characteristic of
secondary electron signal.
Fig. 6.4-2 Secondary Electron Signal Waveform
There are two kinds of edge detection algorithms available; linear approximation method and
threshold level method. Determine which method to use with reference to the description below.
(1) Linear Approximation Method
In this method calculation is made upon determining the intersecting point of baseline and
edges, and it is suitable when a standing wave is relatively sharp at the pattern edges.
In this case, it is required to specify a constant threshold level parameter for the same kind
of specimens.
L
Fig. 6.4-3 Linear Approximation Method
Measured pattern
Secondary electron
signal line profile