Signal-Integrity Interview Questions

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:

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

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

·For power analysis, each cell’s power dissipation has been characterized in the library (.lib) file. For leakage power, the EDA tool simply adds up the leakage power of each cell. (Note: Leakage power is usually state dependent) For dynamic power, the EDA tool either estimates net capacitance before P&R or calculates net capacitance after P&R. The designer has to provide the toggle rate. This can be based on educated guess, experience, simulation, or emulation.

oStatic IR drop analysis aims to determine the **average, steady-state voltage drop** across the power grid.

• the baseline voltage level experienced by cells over a longer period, primarily influenced by leakage and the average rate of switching.

• Average current calculated from average leakage and average switching power provides a measure of the continuous load on the power grid.

• In some low-activity modes or technologies with high leakage, the average leakage current can be a significant contributor to the overall static power consumption and thus the static IR drop.