Electromigration (EM) and IR Drop in VLSI Physical Design
Electromigration (EM) and IR drop are two critical power integrity challenges in VLSI designs. Both impact the reliability and performance of an integrated circuit (IC).
1. Electromigration (EM)
What is Electromigration?
Electromigration (EM) is a phenomenon where high current density causes the gradual displacement of metal atoms in interconnects, leading to circuit failures.
Causes of EM
- High current density: When excessive current flows through a narrow wire, metal atoms get displaced.
- Temperature effects: Higher temperatures accelerate metal diffusion.
- Thin metal interconnects: As process nodes shrink (e.g., 7nm, 5nm), wires become thinner, increasing EM risk.
Effects of EM
- Open circuit failure: If too many metal atoms move, the wire breaks, leading to functional failure.
- Short circuit failure: Metal migration may cause a short between two adjacent wires.
- Timing degradation: Increased resistance over time slows down signals, affecting timing.
Electromigration Prevention Techniques
✅ Use wider power/ground (VDD/VSS) wires to handle high currents.
✅ Add redundant vias to improve current distribution.
✅ Use strong metal layers (M5, M6, etc.) for power/clock routing.
✅ Limit high-current paths by distributing power evenly.
✅ Perform EM analysis using tools like RedHawk, PrimeRail, or Voltus.
2. IR Drop (Voltage Drop Issue)
What is IR Drop?
IR drop occurs when voltage loss happens along the power delivery network (PDN) due to the resistance (R) and current (I) flow in metal wires. It results in reduced voltage at standard cells, affecting their operation.
Types of IR Drop
- Static IR Drop
- Happens due to constant current flow in power rails.
- Affects low-power and always-on paths.
- Dynamic IR Drop
- Happens due to sudden high current demand (during clock switching, high toggling).
- Leads to timing violations and performance degradation.
Causes of IR Drop
- High resistance in power rails (thin metal layers).
- High switching activity (clock tree, high-frequency signals).
- Insufficient power grid (poor power planning).
Effects of IR Drop
🚨 Timing failures: Lower voltage slows down circuits, causing setup timing violations.
🚨 Functional failures: If voltage drops below a critical level, logic gates may not switch properly.
🚨 Reliability issues: Long-term IR drop can increase EM effects, causing permanent damage.
IR Drop Prevention Techniques
✅ Use thick power (VDD/VSS) rails to reduce resistance.
✅ Implement decoupling capacitors (decaps) to stabilize voltage fluctuations.
✅ Optimize power grid design (add more power straps in higher metal layers).
✅ Run IR drop analysis using tools like RedHawk, Voltus, or PrimeRail.
✅ Reduce simultaneous switching activity (e.g., clock gating, staggered clocking).