Thermal Migration (TM) and Stress Migration (SM) Reduction Technique
Thermal migration (TM) and stress migration (SM) are closely related phenomena in microelectronics, and they often act in the same direction. Both TM and SM are associated with atomic migration within materials due to temperature and mechanical stress gradients, respectively. To mitigate these issues in physical design, several strategies can be employed:
Reduce Electromigration (EM)
One of the first steps in reducing SM and TM is to address electromigration (EM) concerns. By minimizing the risk of EM, the potential for SM is also reduced. Additionally, TM can be lowered if local current densities and Joule heating are reduced as a result of EM-mitigation measures.
Pay special attention to the placement of high-power transistors. The goal is to distribute transistors with high loads across a larger die area to avoid local hotspots. This can help in reducing temperature gradients and minimizing TM and SM risks.
Organize the routing of interconnects to avoid high current densities and hotspots caused by Joule heating. Implement wider wires for carrying high currents, potentially using higher layers in the metal stack. Additionally, avoid routing adjacent wires carrying high currents simultaneously to reduce localized heating.
Heat Conductors (Thermal Wires and Vias)
Skilled layout designers often include additional wires and vias specifically designed for heat conduction, regardless of their current-carrying capabilities. These thermal wires and vias enhance heat conductivity, reducing thermal hotspots and gradients. Good thermal conductivity across the entire chip can be beneficial.
Reducing Power Consumption
Lowering the power consumption of transistors can help reduce temperature gradients and mitigate TM caused by internal heating of circuit elements.
Routing Along Isothermal Lines
Whenever possible, route interconnects along isothermal lines, especially in regions with significant temperature gradients. This approach can help minimize TM by reducing exposure to thermal hotspots.
For SM mitigation, consider incorporating additional features in the metal stack to normalize material distribution and create a uniform coefficient of thermal expansion (CTE). However, be cautious about how these features interact with thermal wires and vias used for TM mitigation.
The use of “soft” dielectrics with low stiffness allows for unhindered thermal expansion of interconnects, reducing the introduction of mechanical stress in the wires. Fully “relaxed” wires experience minimal SM. However, this approach may conflict with EM damage countermeasures since SM is sometimes used to counteract EM and extend wire lifetime.
Three-Dimensional Circuits (3D-ICs)
In 3D-ICs with multiple active layers, through-silicon vias (TSVs) can introduce mechanical stress due to mismatches in coefficients of thermal expansion (CTEs). Keep-out zones are often created around TSVs to avoid SM in wires and minimize stress-related mobility changes in active devices. Introducing thermal TSVs can also help reduce thermal gradients and TM in 3D-ICs.
In summary, mitigating TM and SM in physical design involves a combination of strategies, including optimizing transistor placement, routing, heat conduction elements, and material choices. These measures aim to reduce temperature gradients, mechanical stress, and hotspots, ultimately enhancing the reliability and longevity of microelectronic devices.
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