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Copper vs Cobalt vs Ruthenium Interconnects Compared

For wide interconnects, copper, with the lowest bulk resistivity, is the best material, and most interconnects are still copper today. But when line widths shrink to tens of nm or less, electrons scatter at the wire surfaces and grain boundaries, raising resistivity (the size effect), and the barrier layer copper requires takes up a large share of the cross-section. Cobalt and ruthenium have bulk resistivities several times that of copper, but their electron mean free paths are short, so their resistivity rises less as lines narrow, and their barrier layers can be thinner. That is why they are being introduced in the narrowest lines, contacts and vias.

AspectCopper (Cu)Cobalt (Co)Ruthenium (Ru)
Bulk resistivity (room temperature, typical)About 1.7 µΩ·cmAbout 6 µΩ·cmAbout 7 µΩ·cm
Electron mean free path (typical)Long, about 40 nmShort, around 10 nm or lessShort, around 10 nm or less
Resistivity rise in narrow linesLarge. Below tens of nm, surface and grain-boundary scattering increase, sometimes reaching several times the bulk valueSmaller than copperSmaller than copper
Barrier layerRequired. A few nm of TaN/Ta or similar surrounds the sides and bottomUses a thin liner (TiN, etc.), which can be thinner than for copperDiffuses little into dielectrics, so the barrier may be made very thin or omitted
Melting point (approx.)About 1085 °CAbout 1495 °CAbout 2334 °C
Electromigration resistanceNeeds countermeasures. Strengthened with top cap layers and linersHigher than copperHigh
Processing and fillDamascene process. Filled by electroplating over a seed layer. Dry etching is difficultNarrow holes and trenches are easy to fill by CVD or platingCan be deposited by CVD or ALD. Depositing a film and then patterning it by etching is also being explored
Layers where usedMost interconnects, from intermediate to upper layersContact and via fill, some narrow lines in the lowest layers, caps and liners for copper interconnectsBeing researched and developed for the narrowest lines and vias. Also used in liners
Material availabilityAbundant and inexpensiveMore expensive than copperA platinum-group precious metal; scarce and expensive

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Copper (Cu)
Intermediate and upper interconnect layers with relaxed line widths, where the advantage of the lowest bulk resistivity applies directly.
Cobalt (Co)
Places near the transistors, such as contacts and vias, where holes are very narrow and copper's effective resistance rises because of its barrier and seed. Also caps and liners that improve the reliability of copper interconnects.
Ruthenium (Ru)
A candidate for the narrowest lines and vias, with line widths in the teens of nm or less. Because the space freed by omitting the barrier can be used for conductor, its effective resistance may be lower than copper in narrow lines.

FAQ

Copper has the lowest bulk resistivity, so why are cobalt and ruthenium being considered?

In narrow lines, the effective resistance is determined less by bulk resistivity than by surface and grain-boundary scattering and by the thickness of the barrier layer. Copper's mean free path is long, about 40 nm, so its resistivity rises sharply as lines narrow, and it also needs a barrier a few nm thick. Cobalt and ruthenium show less resistivity rise as lines narrow and allow thinner barriers, so in very narrow lines their resistance may be close to or even lower than copper's.

What is the size effect?

When the width or thickness of a wire becomes comparable to or smaller than the electron mean free path, electrons scatter more readily at the wire surfaces and grain boundaries, and the resistivity rises above the bulk value. Narrow wires also have smaller grains, which further increases grain-boundary scattering.

Why do copper interconnects need a barrier metal?

Copper diffuses easily into dielectrics and silicon. In silicon it causes transistor leakage and degradation, and in dielectrics it degrades the insulation between wires. So the sides and bottom are surrounded by a barrier metal such as TaN/Ta, and the top is covered with a dielectric cap film.

Will all copper interconnects be replaced by other metals?

No. In intermediate and upper layers with relaxed line widths, copper, with the lowest bulk resistivity, remains advantageous. New materials are introduced first where copper's weaknesses stand out, such as contacts and vias near the transistors and the narrowest lines.

Check your understanding

When copper wires become very narrow, what is the main reason their effective resistivity rises?

  1. ABecause the wire temperature drops
  2. BBecause surface and grain-boundary scattering of electrons increase, and the high-resistance barrier layer takes up a larger share
  3. CBecause copper turns into a semiconductor
Show answer and explanation

Answer: B. Because surface and grain-boundary scattering of electrons increase, and the high-resistance barrier layer takes up a larger share

When linewidths approach or fall below the electron mean free path (about 40 nm in copper), scattering at surfaces and grain boundaries rises and resistivity can reach several times the bulk value; narrow lines also have smaller grains, adding grain-boundary scattering. Meanwhile, barrier layers of fixed thickness occupy a growing share of the cross-section. Metals with shorter mean free paths—cobalt, ruthenium, molybdenum—are therefore being evaluated and introduced.

What is an advantage of metals such as ruthenium (Ru), being considered as alternatives to copper for fine wiring?

  1. ATheir resistivity rises relatively little in narrow wires, and the barrier layer may be made thinner (or unnecessary)
  2. BThey are much cheaper than copper
  3. CThey transmit light, making inspection easier
Show answer and explanation

Answer: A. Their resistivity rises relatively little in narrow wires, and the barrier layer may be made thinner (or unnecessary)

Copper's resistivity is low in wide wires, but in lines only 10–20-odd nm wide, surface and grain-boundary scattering rise and a barrier a few nm thick is still required, raising effective resistance. Ruthenium and molybdenum have short electron mean free paths (a few to over ten nm), so their resistivity rises less as lines narrow, and they diffuse little into dielectrics, potentially allowing thin or no barriers. They can also be patterned by etching.

What is the main purpose of barrier metals such as Ta and TaN in copper wiring?

  1. ATo reduce wire resistance
  2. BTo prevent copper from diffusing into insulators and silicon
  3. CTo oxidize the copper surface
Show answer and explanation

Answer: B. To prevent copper from diffusing into insulators and silicon

Copper diffuses readily into dielectrics and silicon; in silicon it forms deep levels that cause transistor leakage and degradation, and in dielectrics it compromises insulation between wires. Copper lines are therefore lined on sides and bottom with a few nm of barrier metal such as Ta/TaN and capped on top with a dielectric film. The barrier also improves adhesion to the dielectric and helps texture the copper grains.

  1. Which is closest to the resistivity of copper (Cu) at room temperature? (Bulk value)
  2. What is the main purpose of placing a cap layer such as cobalt on top of copper wiring?
  3. Which correctly describes electromigration?

Practice Interconnect processes questions

Key terms explained

Barrier metal
A thin metal film around copper wiring that prevents copper from diffusing into insulating films or silicon, such as Ta or TaN.
Electromigration
The movement of metal atoms in interconnects caused by electric current. It creates voids and hillocks that lead to open or short circuits.
Damascene process
A method of forming interconnects by etching trenches in an insulating film, filling them with metal, and removing the excess metal by CMP. Used for copper wiring.
RC delay
Signal delay determined by the product of wire resistance (R) and capacitance (C). It grows as scaling makes wires thinner, limiting performance.
MOL (middle of line)
The contacts and local interconnects that link transistors (FEOL) to the multilevel wiring (BEOL). With scaling, their resistance has grown and now limits performance.

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