The critical areas required to calculate the average number of
faults are extracted from the layout. Critical areas such as
transistor gate can be easily identified using Boolean operations,
contact/via numbers can found by counting the number of polygons in
these mask layers. Using EYES non redundant
contacts/vias are easily found too. Other critical areas such as
extra and missing material and pinhole critical area are extracted
using specialized algorithms.
Extra material critical area
The critical area for extra material defects is defined as the
region in which the center of a circular defect of a given size
must fall in order to produce a fault. The figure below shows the
extra material critical area for the defect size shown. The area of
the polygons that define such a region is a measure of the
probability of the fault occurring in the circuit.
Extra material critical area.
Pinhole critical area
Pinhole faults are caused by defects in the dielectric separating
two conducting layers. These defects are very small and are usually
modeled as having zero radius. The regions of a layout susceptible
to these defects are those where two conductors that belong to
separate electrical nodes overlap. The figure below shows the
pinhole critical area for a small layout.
Pin hole critical area.
Other non zero sized dielectric faults can also be modeled in a
similar way to extra material critical area.
Missing material critical area
The critical area for missing material defects is defined as the
region in which the center of a circular defect of a given size
must fall in order to produce a fault. The figure below shows the
critical area for a simple layout.
Missing material critical area.
The extraction mechanism originally proposed to determine
missing critical area involved the extraction of the area of self
intersection of a polygon shrunk by an amount equal to the defect
radius. However, a simple polygon shrink will under estimate this
area as shown in below (a-b), since it not only shrinks across the
width of the conduct as required, but also along its length as
well. The required critical area for the conductor track is shown
(c). The algorithm used within the eyes system overcomes this
limitation by noting that the same missing material defects also
have the potential to cause contact/vias to fail (as indicated in
(d)). The critical area associated with the contacts (e) is
combined with the region generated from the conductor itself to
give the total critical area (f).
Generation of missing material critical
area.
The EYES tool is capable of extracting
all of these and other critical area type metrics efficiently from
even the largest designs. The PEYE-CAA
tool can extract and display these critical areas and also critical areas by electrical node
interaction.
Critical area analysis must be combined with defect date in a yield model to accurately predict yield.
