WHY NOT JUST ONE LAYER

Billions of connections, one silicon surface

too many wires for one plane

A modern chip can hold billions of transistors, and connecting them all would need billions of wires. A single layer of metal can route only so many before wires start crossing each other, which a flat layer can't do. The fix is to stack more layers, each one able to run its own set of wires.

Wires that can't fit on one layer simply move to the next one up.

THE METAL STACK

Layers grow wider with height

The lowest layers sit close together and carry short, local connections between nearby transistors. Higher layers grow wider and farther apart, and carry longer connections and power across the whole chip. A leading-edge chip can use up to about fifteen metal layers.

Lower layers are dense and local; upper layers are wide and long-range.

WHY WIDTH GROWS WITH HEIGHT

A trade worth making

Thin, dense wires Pack tightly, but resist current more per unit length

Thick, wide wires Resist less, so they suit long runs and shared power

The result Local wiring stays thin; long-range wiring gets thick

A wire only needs to be as thick as the distance it has to cover justifies.

LOW-K DIELECTRIC

The insulator matters as much as the metal

less capacitance, faster signals

Every pair of neighboring wires forms an unwanted capacitor through the insulator between them, and that capacitance slows signals down. Fabs use a low-k dielectric, an insulator with a lower dielectric constant than ordinary oxide, specifically to keep that capacitance down as wires pack closer together.

A better insulator between the wires is still an upgrade to the wires.

VIAS

The connectors between layers

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What a via is

A small metal-filled hole linking one metal layer to the next

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Built with damascene

Etched into the dielectric, then filled with copper, same as the wires

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The job

Every signal that changes layers passes through one

A via is a vertical wire, doing between layers what a wire does within one.

UNIT 27 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered why a chip needs up to fifteen layers of wiring, and why they grow with height.