the LFD approach, process kit information, which includes RET recipes,
process models and parameterizable rules, is provided by the foundry to
run simulations on the layout under various process windows and correct
the hotspots which are prone to failures. This decreases the probability
of failure due to process variations and hence improves yield. By
utilizing a LFD flow, a layout designer can better predict the
printability of the design by manufacturing houses and thus can predict
and prevent any failures during the design phase itself.
flow is performed on the layers of the DRC-clean Graphical Data Stream
(GDS) of the design. Generally, designers perform LFD on the OD, poly,
and the metal layers of the design to identify the hotspots. The basic
steps of LFD are following:
- Resolution enhancement
techniques which includes optical proximity correction and phase shift
mask are performed precisely on the layers of the GDS to predict the
actual mask geometries. These RET recipes are specified in the LFD kit
provided by the foundry.
- After applying the RET recipes,
contours are simulated for different values of various process
parameters. These contours are used to generate the PV bands for each
- Lithography checks are performed on the PV bands of each
layer to identify the printability failures like pinching, bridging and
area overlap which are discussed in detail in the next section of the
article. These checks are coded in the rule decks.
- The design is
divided into portions and each individual portion can be assigned a
manufacturability score which defines the minimum amount of variations
which is expected to occur in that portion during manufacturing at a
given process window. Design Variability Index (DVI) is a metric which
returns a score for the design, indicating how likely it is that the
variations in printing will negatively impact yield. This metric is
useful for design teams because it helps to identify sensitive and
Identification of hotspots using PV-Bands
Some common terms used are:
Target layout is the intended pattern in the design.
PV band as discussed above is obtained by simulating the contours.
Inner tolerance band
is the edge of the printability region (region independent of process
variations) which is defined by the minimum LFD width. Minimum LFD width
is the minimum value of the width which a pattern should have to get
printed without resulting in any manufacturability failure. It is
similar to minimum allowed width of a metal route as defined in the DRC
Outer tolerance band is the outermost edge of the
PV band which is allowed so that the spacing between the PV bands of the
adjacent patterns does not go below the minimum LFD spacing between the
two adjacent PV bands.