What is an RC corner?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
An RC corner is a physical interconnect extraction condition modeling the manufacturing variations in metal wire thickness, width, and inter-wire spacing (CMP polishing, etching tolerances), independent of transistor process variations.
Mentor explanation
RC corners model the geometric and material tolerances of metal interconnect.
Key terms
- Cworst (Cmax) — thick/wide wires with narrow spacing → maximum capacitance, low resistance. Setup-critical for short, capacitance-dominated paths.
- RCworst (RCmax) — thin/narrow wires with minimum spacing → high resistance and high capacitance. Setup-critical for long global routes.
- Cbest (Cmin) — thin wires with wide spacing → minimum capacitance. Hold-critical for short paths.
- RCbest (RCmin) — thick wires with wide spacing → minimum total wire delay. Hold-critical for fast global races.
Never assume RCworst is always the worst setup condition — for short paths where capacitance dominates, Cworst is often the worse corner. Both must be checked.
Practical example
RC Corner Matrix Summary:
+----------+--------------------+-------------------+-------------------------+
| RC Corner| Wire Geometry | Key Characteristic| Primary Impact |
+----------+--------------------+-------------------+-------------------------+
| Cworst | Max width/height | Maximum Cap | Setup (Cap-heavy paths) |
| RCworst | Min width, max space| Maximum R x C | Setup (Long wire routes)|
| Cbest | Min width, max space| Minimum Cap | Hold (Fast paths) |
| RCbest | Max width, max space| Minimum R x C | Hold (Fast global nets) |
+----------+--------------------+-------------------+-------------------------+Interview trap
Assuming RCmax alone covers all setup-worst wire conditions. Short capacitance-dominated paths can be worse under Cworst, while long routes are worse under RCworst.
Key takeaways
- RC corners model physical metal fabrication variations (CMP dishing, dielectric thickness).
- Cworst and RCworst constrain Setup (Max delay); Cbest and RCbest constrain Hold (Min delay).
- Both Cworst and RCworst must be verified to catch all setup violations across short and long paths.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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