WHY RESISTANCE MATTERS HERE

A tiny problem, multiplied by billions

bare silicon resists too much

A metal wire touching bare, doped silicon or polysilicon meets more electrical resistance than a chip can afford. Multiply that small loss by billions of contacts, and it becomes real wasted power and a real limit on speed. The fix happens right at the surface, before any wire is even built.

A contact that resists too much wastes both power and speed.

SILICIDE

A metal-silicon alloy, grown on purpose

metal + silicon, reacted by heat

A thin metal film, often titanium, cobalt, or nickel, is deposited over the whole wafer and heated. It reacts with any exposed silicon or polysilicon to form silicide, a low-resistance compound. Unreacted metal sitting on the oxide is then etched away, leaving silicide only where silicon was exposed.

The reaction is self-aligned: silicide forms only where bare silicon meets the metal.

A COMPLETE CONTACT

Silicide, then a tungsten plug

Silicide caps the source, drain, and gate. A narrow hole etched through the oxide above it is filled with tungsten, forming a plug that reaches up to the first metal layer.

Every current path out of a transistor crosses silicide, then a tungsten plug.

WHY TUNGSTEN HERE

Not the copper used everywhere else

The hole is narrow and deep Tungsten deposits evenly even in a tall, thin opening

The process runs hot Tungsten tolerates heat that would damage a copper fill

Copper starts one layer up M1 and above use copper instead, once the transistor is safely capped

Tungsten handles the hardest, hottest connection; copper takes over once things cool down.

UNIT 26 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered silicide and tungsten plugs: getting current out of a transistor without much resistance.