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6.4 CD Measurement Function (Option)
6 - 78
(9) Edge Detection Area
Set parameters specific for each edge detection method.
(a) Linear Approximation button
Specifies the Linear Approximation method for auto edge detection. The following
four parameters are effective for this method.
(b) Threshold Level combo box
Specify a threshold value to be used for linear approximation. This parameter is
selectable in a range of 5 to 95. For example, if there are standing waves in the
pattern under measurement, multiple steps may be produced at the edge part,
resulting in improper edge detection. To prevent this, adjust the threshold value
properly for normal edge detection.
In the linear approximation method, a slope line is determined by identifying a local
peak (maximum value) on the signal gradient in the edge detecting direction. In this
step of operation, if there are multiple gradient peaks (local gradient peaks), a slope
line is determined using the first local gradient peak that is larger than an input
threshold value, with respect to the maximum value "100" of the signal gradient in the
edge part. Therefore, in the case of a signal waveform (line profile) having just one
peak in the signal gradient, edge detection is performed independent of the specified
threshold value.
(c) Differential Size combo box
This parameter is used for signal gradient calculation (coefficient of signal
differentiation) in linear approximation. Specify "5" in common practice. If variation
in measured value data is appreciable due to noise contained in the image, it is
advisable to specify "7". One of numeric values 3, 5 and 7 is selectable.
(d) Base Line Start Point combo box
This parameter is used to determine the start point of signal baseline readout in linear
approximation. Using a position specified for Base Line Start Point, a baseline is
determined according to an average value in a signal level range specified for Base
Line Area (described below). For Base Line Start Point, one of the following four
values is selectable; 1, 2, 4 and 8. The smaller this value, the nearer the start point of
baseline reading is brought to the maximum gradient position of the signal, thereby
shifting the baseline upward.
(e) Base Line Area combo box
This parameter indicates a range of baseline reading in linear approximation. One of
the following four values is selectable; 4, 8, 16 and 32 (represented in unit of number
of pixels).
(f) Threshold button
Specifies the threshold level method for auto edge detection.