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range. Increasing the contrast further does not usually give significant gains in
accuracy.
The gain and backoff should be set so that all measured points are within the
maximum and minimum limits of the video system. The video signal levels can
be viewed by selecting “graph” in the Set Image panel (Figure 7.4) and for bright
marks the background should be at about 10% of the range above zero or higher
and the mark level should be about 10% of the range below the maximum or
lower. (For dark marks the mark level will low and the background will be high).
Marks with low inherent contrast may not produce the 10% and 90% video levels
for any gain and backoff settings. The video signal may saturate at lower backoff
levels. Saturation is characterised by the lack of noise in the video signal even
with the minimum amount of signal averaging. Set the gain and backoff to give
the maximum contrast without saturation.
When executing a jobcal or a fullcal both the gain and the backoff are set
automatically to give a contrast level of about 70% on the calibration mark. In
other cases and especially when aligning to marks on substrates, as required for
direct write or measuring stitch acceptance tests, the gain level can be adjusted
automatically but the backoff level needs to adjusted manually.
17.2.1.
Manual gain and backoff adjustment
Move the gain and backoff sliders in the Set Video Level panel (Figure 7.9) or
define values directly in the command:
VB_OPER>qset video/gain=0.3/backoff=0.8
17.2.2.
17.2.2.1.
Automatic gain and backoff adjustment
HR machines
There are two functions that automatically adjust the gain called “peak” and
“mean”. There is no function to automatically adjust the backoff.
The video gain can be switched into automatic adjustment mode by typing:
VB_OPER> VID_P
This command switches the machine into SEM mode and selects the automatic
peak level adjustment using the command:
VB_OPER>qset video/vmode=peak/mark=0.8
This command is the same as selecting the peak switch in the Set Video Level
panel (Figure 7.9). Auto peak level will adjust the gain until the peak video level
reaches the value set either in the Set Video Level panel or by the “mark”
command option (usually about 70%-80%). The final level will be affected by the
amount of noise in the signal.
Alternatively, the video gain can be automatically adjusted by switching into SEM
mode and selecting the video mean switch in the Set Video Level panel (Figure
7.9) or by using the command:
VB_OPER>qset video/vmode=mean/bckgrd=0.2
Auto mean level will adjust the gain until the median video level reaches the
value set either in the Set Video Level panel or by the “bckgrd” command option.
This is usually set to about 20-30% for bright marks when the SEM magnification
is such that the background dominates the area on the monitor. The final level is
not affected so much by noise in the signal but does depend strongly on the
Part Number:878275
Vectorbeam Operator Manual