What changes in routing at double-patterning nodes?

Short Answer

At double-patterning nodes one metal layer is printed with two masks, so every shape and via gets a mask colour and same-mask shapes need more spacing than different-mask ones. Routing has to assign colours legally, usually automatically during detail routing. Cut metal shapes are used to separate line ends, and writing DEF or GDS needs specific options so masks and cuts carry through.

Technical Explanation

• When features are too close to print with one exposure, the layer is split across two masks. Neighbours on different masks can sit closer than neighbours on the same mask.
• Every shape and via on those layers needs a mask colour. route_detail and other routing commands normally add them automatically.
• derive_mask_constraint derives colours for uncoloured shapes and vias from the underlying tracks: -derive_cut_mask for vias, -follow_pin_mask to take colour from the pins a wire touches, and -follow_wider_object_mask to follow a wider object rather than the track.
• Via definitions can carry mask constraints on their shapes or enclosure layers, and NDRs can include mask constraints for routed nets.
• Cut metal shapes reduce line-end spacing. A cut metal layer matches the metal layer with the same mask number, and its size comes from the technology file. create_cut_metals inserts cuts where the preferred-direction spacing equals the cut width.
• If routing changes after cuts are inserted, remove them with remove_cut_metals and insert them again.
• Output needs care: DEF with cut metal shapes requires write_def -version 5.8, and GDSII or OASIS requires -connect_below_cut_metal on write_gds or write_oasis.

What To Check

• Same-mask spacing violations in check_routes.
• That every shape on double-patterning layers has a mask colour.
• Cut metal shapes present where line ends are closest, and refreshed after routing changes.
• The DEF version and GDS options used for output.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): All shapes coloured, no same-mask violations, cuts refreshed after the last routing change, and outputs written with the right options.
• Suspicious (illustrative): A cluster of same-mask violations in a dense area, usually cleared by rerouting a few nets.
• Hard stop: Cut metals left from before an ECO, or a DEF written in an older version that drops them.

What The Interviewer Is Testing

• Whether you understand why mask colour creates new spacing rules.
• Do you know the tools that assign colours and cuts?
• A strong answer shows you remember the output format requirements.

Follow-up Question & Model Response

"Why does a wire need to follow its pin's mask?"

Candidate Model Response: The pin shape already has a colour, fixed by the cell layout. If the wire touching it is on the other mask, the overlap between them is split across two exposures, which can print badly or break the connection. Following the pin mask keeps the connected shapes on one mask. The same logic applies to wider objects, which is what -follow_wider_object_mask covers.