CVD vs PVD: Thin-Film Deposition Compared

Both methods deposit thin films on wafers. In chemical vapor deposition (CVD), gases react on the wafer surface to form the film. In physical vapor deposition (PVD), such as sputtering, atoms are knocked out of a solid target and land on the wafer without a chemical reaction. CVD is the usual choice for insulators and for coating deep structures evenly; PVD is the usual choice for metals.

AspectCVDPVD (sputtering)
PrincipleGaseous precursors react chemically on the heated or plasma-activated wafer surfaceAtoms are ejected physically from a solid target (for example by argon ions) and condense on the wafer
Typical materialsSilicon dioxide, silicon nitride, polysilicon, tungsten, low-k dielectricsMetals such as aluminum, titanium, titanium nitride, tantalum and copper seed layers
Step coverageGood, especially for LPCVD; ALD is even betterLine-of-sight deposition, so deep trenches and holes are covered poorly (improved by ionized PVD)
TemperatureHigher: LPCVD at about 600 to 800 °C, plasma CVD at about 200 to 400 °CLower: from room temperature to a few hundred degrees
Film purityCan include hydrogen, carbon or chlorine left over from the precursorsHigh, set by the purity of the target
EnvironmentAtmospheric to low pressure, with reactive gasesHigh vacuum with an inert gas plasma
HazardsToxic, flammable or corrosive gases (silane, WF6) need careful handling and abatementFewer hazardous chemicals, but high vacuum and plasma systems
Typical usesInterlayer dielectrics, spacers, polysilicon gates, tungsten plugsBarrier and seed layers for copper wiring, aluminum pads, contact metals

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

When to use which

CVD
When you need an insulating film, or a film that must coat steep features evenly.
PVD
When you need a pure metal film at low temperature, for example the seed layer for copper plating.
Together
A chip uses both: CVD builds the insulating layers, and PVD deposits the thin metal liners inside the openings before they are filled.

FAQ

What is the main difference between CVD and PVD?

CVD forms the film by chemical reactions of gases on the wafer, while PVD, such as sputtering, physically ejects atoms from a solid target onto the wafer.

Why is CVD better at coating steep structures?

Gas molecules can reach into trenches and holes and react everywhere on the surface. PVD deposits in straight lines, so deep, narrow features are covered poorly.

Which is used for metal wiring?

Both. PVD is used for barrier and seed layers and aluminum pads, while CVD deposits tungsten plugs. Copper is filled by electroplating over a PVD seed layer.

What is PECVD?

Plasma-enhanced CVD uses a plasma to activate the gases, so films can be deposited at lower temperatures, around 200 to 400 °C.

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.

What is the main advantage of plasma CVD (PECVD)?

  1. AIt requires higher temperatures than thermal CVD
  2. BThe film contains no hydrogen at all
  3. CIt can deposit films at lower temperatures than thermal CVD
Show answer and explanation

Answer: C. It can deposit films at lower temperatures than thermal CVD

Plasma-enhanced CVD uses energetic plasma electrons to decompose and activate precursors, so it deposits at about 300–400 °C rather than the 600–800 °C typical of thermal CVD. It is used for interlayer dielectrics and passivation after aluminum or copper wiring, when high temperatures are not allowed. Films tend to contain hydrogen from the precursors, and their quality is generally below that of thermal CVD films.

Which is a characteristic of low-pressure CVD (LPCVD)?

  1. AOperating at reduced pressure gives good thickness uniformity and allows many wafers to be processed at once
  2. BOperating at atmospheric pressure makes the equipment the simplest
  3. CUsing plasma allows deposition at room temperature
Show answer and explanation

Answer: A. Operating at reduced pressure gives good thickness uniformity and allows many wafers to be processed at once

LPCVD is thermal CVD performed at reduced pressure, tens to hundreds of Pa. Faster gas diffusion makes deposition surface-reaction-limited, so more than 100 wafers stood a few mm apart can be coated uniformly, with good step coverage. It deposits polysilicon, silicon nitride and TEOS oxide, but its 600–800 °C temperatures rule it out after metallization.

  1. What is the main purpose of placing magnets behind the target in magnetron sputtering?
  2. Which correctly describes step coverage in deposition?
  3. Under what condition does CVD give good step coverage (conformality)?

Practice Deposition (CVD and PVD) questions

Key terms explained

CVD
Chemical vapor deposition. A method of depositing thin films on a wafer through chemical reactions of source gases.
Sputtering
A type of PVD in which ions in a plasma bombard a target and the ejected atoms are deposited on the wafer.
PECVD (plasma-enhanced CVD)
CVD that decomposes source gases with plasma to deposit films. It can deposit at lower temperatures than thermal CVD and is used for insulating films in interconnect steps and the like.
LPCVD (low-pressure CVD)
Thermal CVD performed under reduced pressure. Faster gas diffusion gives good thickness uniformity and step coverage, and many wafers can be processed at once in a vertical furnace. It is used for polysilicon and nitride films.
ALD
Atomic layer deposition. A deposition method that repeats self-limiting surface reactions of precursor gases to control film thickness one atomic layer at a time.

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