Electromigration:
Electromigration is an unwanted slow movement of materials in a semiconductor. It is similar to an Iron bridge which gets rusted over the time and is declared unsafe for use after some determined number of years.
Electromigration predicts the lifetime of a device. Life cycle is one of the inputs given to the tool before checking Electromigration effect. Electromigration is inevitable.
What is the reason for Electromigration?
One word answer – High Current Density. Wires are used inside a Chip to make connections between different devices (cells) and to power and ground supply pads.
EM check ensures that these wires and via connections do not fail due to years of continuous use. When the device is being used, the current flows through wires inside the device.
Current is the flow of charge. So current density is charge flowing through a conductor per unit time per unit area. When charge flows through a metal it causes heating due to the collision of flowing electrons with the lattice atoms which increases the resistance of the metal.
When the current density is high, due to the high momentum of flowing electrons the lattice atoms get displaced from their lattice points and gets deposited at some other location.
The location from where it gets displaced creates an open and the place of accumulation can become short with the adjacent metal wire causing functionality failure of the design.
How is it done?
- Semiconductor fabrication companies provide guidelines for the maximum value of current allowed through a metal wire/via at a particular temperature. This maximum allowed current is called Electromigration limit. It is defined in the Technology file or Design Rule Check (DRC) book provided by the fabrication company.
- Interconnects in the design are analyzed for their AC current carrying capacity and is compared with the maximum value of current limit specified by the foundry. If it exceeds, we see an electromigration violation.
- The interconnects are tested under stress at extreme PVT conditions to predict the life expectancy of the design under real conditions.
- Inputs: LEF/DEF, Netlist (.v), SDC, .lib, SPEF (optional)
- Settings for running EM checks:
Running EM check: Electromigration check is initiated using verify_AC_limit command.
Analyzing Signal EM reports:
How to fix Electromigration in VLSI?
Fixing Methods: As we know the reason for Electromigration is High current density, so our focus should be on reducing the current density.
- Increase wire width: By increasing the width of the violated net. Increase in metal width results in lower current density which fixes EM violations. As Electromigration is analyzed on routed database, care should be taken while increasing width as it can lead to DRC violations.
- By downsizing the driver cell: Bigger the size of a cell, higher is its charge storing capacity. By downsizing a driver cell of violating net, the current density can be controlled and fix the EM violation. This method does not cause DRC violations but can lead to timing issues.
- Control temperature: Implement thermal management to regulate chip temperature. Elevated temperatures accelerate electromigration. Incorporate on-chip cooling mechanisms to reduce risk.