Electromigration in VLSI Physical Design: A Brief Guide -

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Electromigration in VLSI Physical Design: A Brief Guide

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Electromigration in VLSI physical design is a major concern, particularly at lower technology nodes where the cross-sectional area of metal interconnects is relatively small. If an integrated circuit is not constructed properly, it might suffer from electromigration. The electromigration rate is influenced by various elements, and one field of reliability engineering focuses on assuring minimal electromigration. VLSI interconnect design begins with examining the conductor shape to achieve reduced electromigration.

What is Electromigration in VLSI?

Electromigration is the movement of atoms caused by a current flowing through a substance. If the current density is high enough, the heat released within the material will continually shatter and shift atoms. This will result in both vacancies and deposits.

When a high current density flows through a metal connection, the momentum of current-carrying electrons may be transferred to metal ions after impact. Metal ions may be dragged toward the direction of electron transport due to momentum transfer.

As the technological node shrinks, so does the cross-sectional area of the metal interconnects, and current density increases significantly at the lower node. Electromigration has been an issue since the 90 nm technology node or perhaps before, but it worsens at the 28nm or lower node.

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The phenomenon of Electromigration in VLSI Physical Design:

A potential difference is applied across a metal connection, creating an electric field from anode to cathode. The electron is moved in the opposite direction of the electric field by this field. The electron’s momentum causes electricity to flow in the electron. These traveling electrons have velocity, and when they collide with metal ions, the metal ions experience two opposing forces. One force is caused by an electric field, whereas the other is caused by an electron wind hit. When the current density is large, the force from the electron wind exceeds the force from the electric field.

The treated metal ion began to drift in the opposite direction of the electric field, depending on the current density. If the current density is high, the connection may be harmed by EM immediately or after months/years of operation, depending on the current density. As a result, the dependability of ASIC will be determined by this EM impact.

Techniques for Electromigration Prevention:

The connection used changes with the scalability of the technology node. Initially, pure aluminum was used as an interconnect, but the industry soon switched to an Al-Cu alloy, and ultimately to copper interconnects. Copper interconnects can sustain nearly five times the current of aluminum interconnects while meeting the same reliability standards.

Strategies that can be used during physical design to avoid the electromigration problem:

  1. Increasing the metal width to lower current density;
  2. Lowering the frequency;
  3. Lowering the supply voltage;
  4. Maintaining wire length sorting, and
  5. Reducing the buffer size in clock lines

Electromigration checks are done during the VLSI physical signoff stage with regard to the electromigration rules specified by the foundry to prevent the electromigration problem.

Conclusion

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