Metal Layers

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Introduction

In a semiconductor chip, we have multiple metal layers for signal, clock, and power routing. Using only one metal layer for routing can cause the following problems:

• Increased congestion

• Crosstalk

• Higher Delay

• Inefficient Power Distribution

Multiple metal layers are used in complex chip designs to increase the circuit density and enable the integration of more components into a smaller footprint.

The set of metal layers used in the design of a chip is known as a metal stack.

In this article, we will be explaining the metal stack. We will consider a 13-layer metal stack to understand the concept better.

Metals Used for Routing Power Grid

The topmost layers of the metal stack are used for routing the power grid. This is because these layers are wider and provide low resistance. This ensures that IR drop is within the specified limits and power is evenly distributed to the chip. In the case of a 13-layer stack, 12 and 13 can be used for creating the top-level power grid. Block-level power grids can be routed using metals 10 and 11. Standard cell rails are routed using layers 1 or 2.

Metals Used for Routing Clock Signals

To route clock signals, two adjacent metal layers from the middle of the stack are selected. In the 13-layer stack we have taken, we can use metals 8 and 9 for routing clock signals. Choosing adjacent layers helps increase routing flexibility as there will be vertical and horizontal wires available for routing. Also, adjacent metal layers have similar properties like resistance, width, spacing, etc. Choosing at the bottom of the stack can increase congestion. Also, more vias will have to be traversed for the clock signal to reach their assigned routing resources. This can increase the delay for the clock signal.

Conclusion

The number of metal layers has increased as semiconductor technology developed, enabling more complex and powerful integrated circuits. The additional layers provide greater routing flexibility, allowing designers to optimize the chip's layout and improve its performance, power efficiency, and signal integrity.

We hope this blog post was insightful.