How do you debug LVS opens and floating shapes?

Short Answer

An open means the pins of one net are not connected by its shapes, so one schematic net extracts as two or more layout nets. A floating shape is a piece of a net that touches none of its pins. Find both in ICC2 with check_lvs (ICC2) using full error counts and detailed open locations, fix power nets before signal nets, then confirm in IC Validator.

Technical Explanation

• An open splits a net into fragments. In extraction each fragment gets its own net, so the devices on the smaller piece no longer match the schematic.
• check_lvs -checks {open floating_routes} (ICC2) runs just those two checks. By default it reports at most 20 errors per type, so an ECO that left 380 nets open looks like 20 problems. Use -max_errors 0 (ICC2) to see them all.
• By default opens are reported as the bounding box of the whole open net, and floating pins are not reported. -open_reporting detailed (ICC2) gives the actual break locations at some runtime cost, and -report_floating_pins true (ICC2) lists pins left hanging.
• Power and ground terminals are treated as unconnected voltage sources by default. With -treat_terminal_as_voltage_source true (ICC2) they count as connected, so a net joined only through its terminals stops reporting opens. Pick the setting that matches how the net is really connected, and write it down.
• Floating shapes are often leftovers: a stub after an ECO reroute, a strap segment cut by a blockage, a route that ends short of its pin. They add metal that no net needs, can create DRC or density problems, and sometimes point to the open next to them.
• In IC Validator an open shows up as unmatched nets at a failed equivalence point in cell.LVS_ERRORS (ICV). If both fragments carry the same label, text_net() (ICV) also reports a text open.
• Common roots are a missing via stack where a strap crosses a macro ring, a rail cut when a filler was removed, and a PG pin on a macro that the grid never reached.

What To Check

• Open and floating counts with -max_errors 0 (ICC2), not the default 20.
• Which nets the opens sit on: PG first, then clocks, then data.
• Whether the opens cluster around one macro, one strap crossing or one ECO region.
• The terminal setting used, so a clean result means what you think it means.
• cell.RESULTS (ICV) for the LVS verdict after the fix.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): 0 opens and 0 floating shapes with -max_errors 0 (ICC2), and the LVS header reads PASS.
• Suspicious (illustrative): Exactly 20 opens reported, which is the default cap and hides the real count, or a few floating stubs all inside one ECO region.
• Hard stop: Any open on a power or ground net inside a macro ring or a switched domain. That block cannot sign off IR drop or LVS until it is joined.

What The Interviewer Is Testing

• Knowledge that check_lvs (ICC2) caps output at 20 per type and reports opens as bounding boxes by default.
• Understanding of what the terminal option changes and why the choice must be deliberate.
• Can the candidate tie an ICC2 routing open to the IC Validator compare result?

Follow-up Question & Model Response

"What is the difference between an open net and a floating shape?"

Candidate Model Response: An open net is a net whose pins are not all joined by its shapes, so it extracts as more than one net. A floating shape belongs to a net but touches none of that net's pins. An open always breaks LVS for the devices on the cut-off piece. A floating shape may not break the compare at all, but it is usually debris from an edit and often sits right next to a real open.