Tungsten
The metal used to fill contacts and vias ('plugs') that connect wiring levels — valued for reliable fill of tiny holes.
In short
Tungsten is used mainly to fill the tiny vertical holes — contacts and vias — that connect one wiring level to another and to the devices. It has higher resistance than copper or aluminum, but it fills small, high-aspect-ratio holes reliably and stands up well, which matters more for short plugs.
Why it matters
Connecting stacked wiring levels needs a metal that can fill narrow vertical holes without voids. Tungsten's excellent fill and reliability in these 'plugs' made it the standard for contacts and vias, even though it is more resistive than the wiring metals.
Beginner intuition
Wiring runs horizontally on many levels; to connect the levels (and reach the devices) you need vertical 'plugs' through the insulator.
For those small vertical holes, reliable, void-free filling matters more than the lowest resistance — and tungsten fills them very well, so it became the plug metal.
Material properties
- Fill (process)
- Deposited by CVD to fill narrow, deep contacts and vias with few voids.
- Electrical
- Higher resistivity than copper or aluminum, acceptable because plugs are short.
- Reliability / mechanical
- Refractory and robust; stable through subsequent processing.
A note on properties
Where it is used
- Contact plugs connecting devices to the first wiring level
- Via plugs connecting wiring levels to each other
- Some local interconnect and specialty roles
Manufacturing process connection
Tungsten is deposited by CVD (usually over a barrier/adhesion layer) to fill the holes, then CMP removes the overburden, leaving plugs. This ties it to deposition and CMP equipment.
Equipment connection
Common issues
What can go wrong:
- Voids in the plug fill → high resistance or opens
- Barrier or adhesion failure
- Dishing or erosion during CMP
Performance implications
Tungsten shows that 'best' is role-specific: a metal chosen for fill and reliability (property), deposited by CVD and planarized by CMP (process/equipment), forms plugs (device structure) that reliably connect levels (performance) — even though its resistance is higher than the wiring metals.
Alternatives & material selection
Tungsten is chosen for plugs where fill and reliability dominate, while copper or aluminum carry the longer wires — the conductors are matched to their role, not ranked overall.
- Copper
- Lower resistance for the wiring itself, and increasingly for some vias.
- Cobalt / other metals
- Explored for contacts and plugs at advanced nodes.
Advanced & research
Emerging and research directions, beyond today’s established practice.
- EMERGINGBarrier/liner scaling for tungsten plugs
- EMERGINGLow-resistance contact schemes
- EMERGINGAlternative plug metals (e.g. cobalt) at advanced nodes