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LPCVD vs PECVD: Key Differences Explained

Both are CVD processes that form films by reacting gaseous precursors, but they supply the reaction energy differently. LPCVD drives the reaction with heat alone, at about 600–800 °C in a low-pressure furnace, so it deposits dense, low-hydrogen films that cover steps uniformly. PECVD decomposes the precursor gases with a plasma, so it can deposit at low temperatures of about 300–400 °C and can be used even after interconnects have been formed. However, its films contain much hydrogen and have lower density.

AspectLPCVDPECVD
Reaction energyHeat onlyA plasma (RF discharge) decomposes and activates the precursor gases
Temperature (typical range)About 600–800 °CAbout 300–400 °C
Pressure (typical range)Tens to hundreds of PaAbout hundreds of Pa
Processing styleBatch processing in a vertical furnace. 100 or more wafers are stacked and coated at onceMainly single-wafer. Wafers are processed one at a time under a showerhead electrode
Film qualityDense, with near-stoichiometric composition. Slow etch rateLower density, and composition tends to deviate. Fast etch rate
Hydrogen content (nitride example)A few % or lessAbout 10–30%, as Si-H and N-H
Step coverageSurface-reaction-limited, so films are nearly uniform in thickness even over stepsOften worse than LPCVD. Can be improved depending on the precursors and conditions
Film stressNitride has a large tensile stress of about 1 GPa. Thick films crack easilyCan be tuned from compressive to tensile, for example by the ratio of high-frequency to low-frequency power
ThroughputEach run is long, including ramp-up and ramp-down, but many wafers are processed at onceHigh deposition rate and short processing time per wafer
Usable process stageBefore interconnects are formed (while the wafer can withstand high temperature)Can be used even after interconnects are formed
Main films and usesPolysilicon, nitride for LOCOS/STI masks and spacers, TEOS oxideInterlayer dielectrics, passivation films, copper diffusion barriers, SiON anti-reflective coatings, hard masks

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

When to use which

LPCVD
When the process can withstand high temperature and density, purity and step coverage come first: gate polysilicon, nitride for isolation and spacers, and other front-end steps that build the transistors.
PECVD
When the underlying layers cannot withstand high temperature: interlayer dielectrics and protective films after aluminum or copper interconnects are formed, and nitride films whose stress needs to be tuned.
Rule of thumb
The thermal budget (how high the temperature can go) first determines which method is usable. Then compare the required film quality, coverage, stress and processing speed to choose.

FAQ

Why can PECVD deposit films at low temperature?

High-energy electrons in the plasma collide with the precursor gases and break them into reactive radicals and ions. The reaction proceeds without heating the gas or the wafer itself to high temperature, so deposition is possible at about 300–400 °C, compared with 600–800 °C for thermal CVD.

Why can't LPCVD be used after interconnects are formed?

It requires high temperatures of about 600–800 °C, which would melt or deform aluminum interconnects and degrade copper interconnects and low-k films. Interconnect processing generally has to stay below about 400 °C, so PECVD is used.

What problems does a high hydrogen content in PECVD films cause?

It lowers film density and increases the wet etch rate. In addition, if hydrogen escapes during later heat treatment, the film can shrink and its stress can change. That is why LPCVD is chosen for films that must be dense or pure.

Why is the film thickness uniform even when LPCVD processes many wafers at once?

At low pressure, gas molecules diffuse quickly, so the deposition rate is set by the surface reaction rather than by gas supply (surface-reaction-limited). As long as the surface temperature is uniform, films of nearly the same thickness form everywhere, even with wafers spaced a few mm apart.

Check your understanding

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.

For films of the same material, in what way are high-temperature thermal CVD films generally better than plasma CVD films?

  1. ALower deposition temperature
  2. BAlways faster deposition
  3. CDenser films with fewer impurities such as hydrogen
Show answer and explanation

Answer: C. Denser films with fewer impurities such as hydrogen

High-temperature thermal CVD reacts precursors fully, producing dense, near-stoichiometric films with few impurities such as hydrogen—LPCVD nitride contains only a few percent hydrogen, whereas PECVD nitride may contain around 20%. PECVD deposits at low temperature but yields less dense films that etch faster, so the choice depends on thermal constraints and required film quality.

  1. Which correctly describes silicon nitride deposited by plasma CVD?
  2. Which method is used to tune the stress of plasma CVD silicon nitride from compressive to tensile?
  3. Under what condition does CVD give good step coverage (conformality)?

Practice Deposition (CVD and PVD) questions

Key terms explained

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.
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.
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.
CVD
Chemical vapor deposition. A method of depositing thin films on a wafer through chemical reactions of source gases.
Passivation layer
A film formed as the topmost layer of a chip to protect the circuitry from moisture, contamination and mechanical damage. Silicon nitride and polyimide are typical materials.

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