ZYGO KMS-310, Supervisor Manual

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KMS-310/400 Supervisor's Manual 3-17
Pitch Calibration
Pitch calibration is used to determine the total magnification of the optical
system. Optical systems have distortions in the image that cause blooming, a
tendency for bright regions to push into dark regions. For this reason, dark lines
may appear too small and white lines may appear too large. The pitch
measurement avoids this problem by measuring the distance between two similar
edges (e.g., from the left edge of one white line to the left edge of another white
line). The most significant aspect of pitch calibration is that this establishes the
one-to-one correspondence of actual versus measured geometry sizes. All
programs that use the same magnification objective MUST contain the identical
pitch factor in microns per pixel (as shown in the Calibration screen of each
program).
L
Note: Pitch factors can also be viewed in the Calibration menu, which
is found under the Master menu for any selected program.
Line Calibration
The errors in white or dark line measurements are compensated for by the line
factor. After the total optical magnification is calibrated, a line is measured and
compared with its known dimension. The difference is an error caused by the
optics. This value is used to adjust the measurement to the correct result. The
nominal calibration applies only to the type of substrate used during calibration
(such as a binary chrome-on-glass standard used for binary chrome-on-glass
photomasks).
Image Intensity Profile
The brightness of the video image is digitized to 256 levels of gray for the
computer to analyze. This provides a representation of the surface for
measurement. The optics blur the image slightly, so even a perfect black-to-white
transition in the subject results in a slope from almost black to nearly white in the
video. Since this slope is present, a white line is wider at the base than at the top.
For this reason, a decision must be made as to where on this slope to make the
measurement.
This decision point is the threshold, which is a percentage of the difference
between the darkest and brightest points about the line. A 50 percent threshold is
usually used, because this is halfway along the slope.
Image Measurement
The KMS-310/400 software analyzes the profile (waveform) for each line of
pixels (video scan line), detecting the relative intensity of each pixel as a function
of gray level. Each scan line is individually looked at for the threshold, based on
the peak white and minimum black levels and the resulting measurement. The
KMS-310/400 system contains both detection and measurement thresholds.