What is EM (Electro migration)? How to solve EM violations?

o    Electromigration (EM): The gradual displacement and migration of metal atoms in an interconnect (wire or via) caused by the momentum transfer from flowing electrons. Over time, at high current densities and temperatures, this movement can lead to:

o    Voids: Depletion of metal atoms in a region, increasing resistance and potentially causing an open circuit (wire break).

o    Hillocks: Accumulation of metal atoms in another region, potentially causing shorts to adjacent wires.

o    Increase Wire Width: Widening the metal wire increases its cross-sectional area, directly reducing the current density (J=I/Area). This is the most common fix for wire EM. Often implemented using Non-Default Rules (NDRs).

o    Add Vias: For via EM violations (common due to small cross-section), increase the number of via cuts in parallel between the connecting metal layers. This provides multiple paths for current, reducing the density through any single via cut.

o    Layer Promotion: Route the high-current net on higher metal layers. Top metal layers are thicker, have lower resistance, and often have higher EM current limits.

o    Reduce Wire Length: Shorter wires have less overall resistance and potentially less susceptibility, though EM is primarily about density.

o    Buffer Insertion/Driver Downsizing: Inserting buffers can break a long net driven by a single high-current driver into segments driven by smaller buffers, potentially reducing peak currents in segments. Downsizing the driver cell reduces the current it pushes

o    Tool to Check EM: EM analysis is performed by specialized Power Integrity and Reliability analysis tools, often the same ones used for IR drop:

o    Ansys RedHawk-SC

o    Cadence Voltus

o    Synopsys PrimeSim Reliability Analysis (formerly PrimeSim EMIR)

o    These tools take inputs: the layout (DEF), parasitics (SPEF), current information (from simulation VCD/FSDB or vectorless analysis), and technology EM rules (often in the tech file or a separate rule deck)

o    https://www.synopsys.com/glossary/what-is-electromigration.html