Mitigating Electromigration In Chip Design

From smartphones to laptops, we use a variety of devices every day that rely on integrated circuits (ICs), or chips, to function. These chips are made up of thousands of transistors and interconnects, which transmit electrical signals from one part of the chip to another. But as demand for speed and complexity forces more energy through ever-smaller devices, this concentrated current flow can threaten chip performance in a phenomenon called electromigration.

What is electromigration?

Electromigration is the movement of atoms caused by the flow of electric current.

This movement can change the physical structure of a conductor by forming voids or hillocks that can cause shorts, open circuits, performance degradation or device failure. As the current density Increases through the Interconnects, the rate of electromigration also Increases, which makes electromigration a runaway process. In fact, electromigration is the primary cause of failure in circuits.

Images of a void and a hillock formed in semiconductor interconnect through electromigration of metal atoms away from some areas and toward others

The speed at which electromigration progresses can be accelerated by higher currents, higher temperatures, increased mechanical stress, defects in the conductor, and the presence of impurities. These are captured in Black’s equation — an empirical Arrhenius equation that describes the phenomenon in terms of mean time to failure (MTTF):

• J: current density
• N: a scaling factor
• k: Boltzmann’s constant
• T: temperature (in K)
• E_a: activation energy (in J)

Developed by Robert Black in 1969, Black’s equation helps engineers understand the potential longevity of a circuit by calculating how electromigration can affect their design. The equation relates the MTTF to the wire’s current density, temperature, and material properties.

Current Density: Because larger wires have a greater cross-sectional area, they have a lower current density and are less susceptible to electromigration.

Temperature: As the temperature rises, atoms move faster, increasing the likelihood of electromigration.

Material Properties: Some materials have greater resistance to electromigration. For example, copper’s high electrical conductivity makes it a more popular choice than aluminum.

With increasing miniaturization of our electronic devices, the probability of electromigration — and the issues it causes — grows significantly. Because chips are responsible for the reliable performance of these devices, electromigration must be mitigated to maintain a product’s functionality.

How to mitigate the effects of electromigration in chip design

To avoid the frustrations and dangers of metal migration in electronic components, chip designers can use several techniques, including:

1. Increasing interconnect width to reduce current density.
2. Using materials with high electromigration resistance, such as gold and copper.
3. Diverting current intensity using redundant vias.
4. Decreasing spacing between interconnects.
5. Designing the circuit for lower voltage levels.

Of course, solving electromigration in chip designs can require a mix of multiple techniques. When designing interconnects for very large-scale integration (VLSI), simulation software can help you determine the best options for your specific current requirements. Chip designers must analyze the current density through interconnects and various reasons for increased resistance to predict electromigration in the chips.

The increased interconnect length along with decreased widths, high current densities, and poor heat dissipation require significantly complex electromagnetic rules to address these challenges at advanced technology nodes.

Testing and monitoring for electromigration in chip design

To achieve reliability in chip design, engineers must weigh the trade-offs between many different design choices. For example, increasing the interconnect width will decrease resistance and current density and increase the capacitance. Or, choosing to decrease the spacing between interconnects will aid integration, but it will also increase crosstalk coupling.

Simulation software such as Ansys RedHawk-SC lets chip designers predict and analyze the behavior of interconnects so they can decide how to distribute the power within a system before the chip goes into fabrication. Using simulation to test, validate, and monitor chip designs not only improves electronics performance, but also helps companies avoid costly errors and product recalls.

The result: Reliable electronics

Electromigration is a critical issue in the design and manufacturing of chips. While the effects of electromigration can be devastating, there are several measures that can be taken to prevent or minimize its impact. The risk of electromigration can be reduced by optimizing the design of the interconnects, choosing the right materials, controlling the temperature, and carefully managing the manufacturing process.

With the ongoing advancement in electronics, it is essential to understand and address the challenges posed by electromigration to ensure the reliability and longevity of chips. Using advanced simulation signoff tools like Ansys Redhawk -SC, and Ansys Totem, engineers can build faster, more powerful, and more reliable systems, even within the smallest of spaces.