What is Electromigration Effect in VLSI?

Material migration encompasses several forced transport processes in solid materials, such as chemical diffusion due to concentration gradients, migration caused by temperature gradients, mechanical stress, and electrical fields. The latter, known as electromigration, is particularly relevant in modern electronics. Electromigration refers to the movement of metal atoms in a conductor due to the momentum transfer from moving electrons, often described as an “electron wind.”

Electromigration Effect in VLSI(Source)

The Mechanism of Electromigration

In electronic circuits, copper or aluminium interconnects are polycrystalline, consisting of grains with identical crystal lattices but different orientations. When current flows through these wires, interactions between the electrons and metal ions occur, particularly at grain boundaries. These interactions cause atoms to move in the direction of the electron flow, leading to the formation of “hillocks” and “voids.” Hillocks can short-circuit adjacent interconnects, while voids can interrupt current flow, eventually causing circuit failure.

Read also: Antenna Prevention Techniques(Antenna Effect Fixes)

Historical Context and Empirical Models

The importance of understanding and mitigating electromigration was highlighted by physicist J. R. Black in the late 1960s, who developed an empirical model to estimate the mean time to failure (MTTF) of a wire considering electromigration effects. The MTTF is influenced by several factors, including current density, temperature, and material properties.

Key Factors Influencing Electromigration

• Wire Material: Copper is generally more robust against electromigration than aluminum, with a higher electromigration activation energy due to its superior electrical and thermal conductivity and higher melting point. Alloying aluminum with small amounts of copper and silicon (AlSiCu) can also enhance its resistance to electromigration.
• Temperature: Higher temperatures exacerbate electromigration, reducing the MTTF of interconnects. Effective thermal management is crucial to maintain reliability, as demonstrated by the significant reduction in permissible current density at elevated temperatures.
• Wire Size and Geometry: The current density, a critical parameter for electromigration, depends on the wire’s cross-sectional area. Interestingly, reducing the wire width below the average grain size can enhance reliability due to the formation of a bamboo-like structure, where grain boundaries lie perpendicular to the current flow, reducing material transport.
• Wire Length: Short wires below a certain length, known as the Blech length, are less susceptible to electromigration due to a stress-induced reversed migration process that counteracts the material flow.
• Via Arrangements and Corner Bends: Vias and contact holes are particularly vulnerable to electromigration due to their typically higher current density. Multiple vias arranged to distribute current evenly and avoiding 90-degree bends in wires can mitigate electromigration effects.
• Terminal Connections: Proper design of terminal connections is crucial, as different connection positions can lead to varying current loads within the terminal structure. Ensuring current density verification for all terminal regions helps maintain reliability.

How to Mitigate Electromigration

Effective mitigation of electromigration involves optimizing the physical design to manage current density and thermal conditions. This includes:

• Material Selection: Using materials with higher electromigration resistance, such as pure copper or AlSiCu alloys.
• Design Adjustments: Implementing design changes like slotted wires and fine-grain power meshes to distribute current more evenly and reduce the impact of electromigration.
• Thermal Management: Enhancing the thermal conductivity of surrounding materials to dissipate heat more effectively.
• Current Modeling: Employing various current models to predict and manage the impact of different current waveforms on electromigration.

By integrating these strategies into the design process, developers can significantly improve the reliability of electronic systems, ensuring their performance over extended periods despite the challenges posed by miniaturization and increased current densities.