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Sputtering vs Evaporation: Mechanism, Step Coverage, Adhesion and Alloy Composition

Both are physical vapor deposition (PVD) methods that turn a solid material into atoms and deposit them on the wafer. Sputtering bombards a target with argon ions from a plasma and deposits the atoms knocked out of it. Evaporation heats the material in high vacuum, with a resistive heater or an electron beam, until it evaporates. Sputtering keeps alloy composition and gives good adhesion, so it dominates chip interconnect and barrier metals. Evaporation sends atoms in straight lines, which is useful for forming electrodes by lift-off.

AspectSputteringVacuum evaporation (thermal / e-beam)
How atoms are releasedAr ions from a plasma strike the target and knock atoms out by momentum transferThe material is heated by a resistive heater or an electron beam until it evaporates
Pressure and atom energyGas at about 0.1 to 1 Pa; arriving atoms typically have several to tens of eVHigh vacuum (about 10⁻⁴ Pa or lower as a guide); arriving atoms have only about 0.1 eV
Step coverageLimited, though better than evaporation; ionized sputtering or collimators improve deposition at the bottom of holesVery poor: atoms travel in straight lines and do not reach shadowed areas
Adhesion and film qualityGood; films tend to be denseUsually weaker than sputtering; improved by heating the substrate and similar methods
Alloy and compound compositionFilms keep nearly the target composition; reactive sputtering also makes nitrides and other compoundsComposition tends to shift because the components have different vapor pressures (co-evaporation from several sources can adjust it)
Deposition ratePractical rates with magnetrons; varies by materialGenerally easy to make high
Effects on the substrateExposure to plasma and energetic particles can cause heating or damageRelatively little substrate heating; X-rays from electron-beam evaporation can cause damage
Typical usesChip interconnect (Al, Cu), barrier metals (Ta, TaN, TiN), Cu seed layers, metals for silicidesElectrodes formed by lift-off (for example on compound semiconductors), research and development, OLED emitting layers

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

When to use which

Sputtering
When you need uniform, well-adhering films over large wafers, or alloy and nitride films. Most metal films in chip interconnect processing are sputtered.
Evaporation
For lift-off, where the film should land only from directly above, for high-purity films, and for materials that should not be exposed to plasma. With an electron beam, even high-melting-point metals can be evaporated.

FAQ

Why is sputtering mainly used for chip interconnect?

It deposits alloys and compounds with nearly the same composition as the target, gives dense, well-adhering films and coats large wafers uniformly. Reactive sputtering with nitrogen also makes TiN and TaN barrier metals.

Why is evaporation suited to lift-off?

Evaporated atoms travel almost in straight lines, so hardly any film lands on the resist sidewalls. When the resist is dissolved, the film on top of it comes away cleanly and the film in the pattern stays. Sputtering also coats the sidewalls, so the film does not break cleanly.

What is the difference between thermal and electron-beam evaporation?

Thermal evaporation heats a metal boat or similar holder by passing current through it. It is simple, but high-melting-point materials are hard to evaporate and the boat can contaminate the film. Electron-beam evaporation focuses an electron beam on the material, so it can evaporate refractory metals and oxides with less contamination.

How can a film be deposited deep inside a hole?

In sputtering, ionizing the sputtered atoms and pulling them in vertically with a bias, or using a collimator, improves coverage at the bottom. If that is not enough, CVD or ALD, which deposit through surface reactions, are used.

Check your understanding

Into which category does deposition by sputtering fall?

  1. APVD (physical vapor deposition)
  2. BCVD (chemical vapor deposition)
  3. CThermal oxidation
Show answer and explanation

Answer: A. PVD (physical vapor deposition)

Sputtering accelerates ions from an argon or similar plasma at several hundred volts into a target; atoms knocked out by momentum transfer deposit on the wafer. Since no chemical reaction is involved, it is classified as PVD (physical vapor deposition). It deposits metal wiring, barrier metals (Ta, TiN), Cu seed layers and electrodes. CVD relies on chemical reactions of precursor gases, and thermal oxidation oxidizes the silicon itself.

Which correctly describes vacuum evaporation?

  1. AFilms are deposited by chemical reactions of source gases
  2. BMetal is deposited from an electrolyte
  3. CMaterial is heated in vacuum to evaporate it and deposit it on the substrate
Show answer and explanation

Answer: C. Material is heated in vacuum to evaporate it and deposit it on the substrate

Vacuum evaporation heats material by resistance or electron beam until it evaporates, and the atoms travel through vacuum (about 10⁻⁴ Pa or better) to coat the substrate. The equipment is relatively simple and films are pure, but line-of-sight travel gives poor step coverage and alloy compositions drift, so sputtering dominates IC wiring. Evaporation is used for lift-off electrode patterning on compound semiconductors and for depositing OLED emissive layers.

What is an advantage of vacuum evaporation that heats material with an electron beam?

  1. AIt can evaporate even high-melting-point materials
  2. BIts step coverage is very high
  3. CIt forms films through chemical reactions of source gases
Show answer and explanation

Answer: A. It can evaporate even high-melting-point materials

Electron-beam evaporation focuses an electron beam accelerated through several kV onto the source material, vaporizing refractory metals such as tungsten and molybdenum and oxides that resistive heating cannot, with little crucible contamination. Atoms travel in straight lines, giving poor step coverage—which is exploited in lift-off, where film deposited over patterned resist is removed along with the resist to define electrodes.

  1. In PVD such as sputtering and evaporation, why is it hard to coat the sidewalls and bottoms of deep trenches?
  2. Which is an advantage of sputtering over vacuum evaporation?
  3. Which correctly describes step coverage in deposition?

Practice Deposition (CVD and PVD) questions

Key terms explained

Sputtering
A type of PVD in which ions in a plasma bombard a target and the ejected atoms are deposited on the wafer.
Vacuum evaporation
Forming a film by heating a material in vacuum until it evaporates and condenses on the substrate. Electron-beam heating handles high-melting-point materials, but step coverage is poor.
Magnetron sputtering
Sputtering with magnets behind the target that trap electrons near it and raise plasma density. It increases deposition rate and is the mainstream sputtering method today.
Reactive sputtering
Sputtering a metal target in an atmosphere containing a reactive gas such as nitrogen or oxygen to form nitride or oxide films. It is used for films such as titanium nitride (TiN).
Step coverage
A measure of how uniformly a film covers the sidewalls and bottoms of steps and holes compared with flat areas. ALD gives excellent step coverage.
Barrier metal
A thin metal film around copper wiring that prevents copper from diffusing into insulating films or silicon, such as Ta or TaN.
Seed layer
A thin copper film deposited by sputtering or similar before copper electroplating. It provides the conductive path for the plating current.

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