Advanced yield enhancement technique: Lithography friendly design

With 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.

The LFD 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:

1. 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.
2. 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 layer.
3. 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.
4. 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 critical topologies.

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 rules.

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.