ALD vs CVD: Atomic Layer Deposition Compared

ALD (atomic layer deposition) is a special form of CVD in which the precursor gases are pulsed in one at a time, and each pulse reacts only until the surface is covered. The film therefore grows one atomic layer per cycle, with perfect control of thickness and excellent coverage of very deep structures. The price is speed: ALD is much slower than ordinary CVD, so it is used for the thinnest and most critical films.

AspectALDCVD
How it worksPrecursors are pulsed in turn, separated by purges. Each reaction is self-limiting, so one cycle adds about one atomic layerPrecursors flow in together and react continuously on the surface
Thickness controlAtomic-level, set by the number of cyclesSet by deposition time and rate, so less precise
ConformalityExcellent, even in very deep, narrow holes and trenchesGood, but deep structures can pinch off before they are filled
Deposition rateSlow: a fraction of a nanometer per cycleFast: many nanometers per minute or more
Uniformity and qualityVery uniform, dense and with few pinholesGood uniformity
TemperatureOften low (about 100 to 350 °C)Wide range, often higher (plasma CVD is lower)
ThroughputLow per wafer (batch tools help)Higher
Typical thicknessAbout 1 to 50 nmTens of nanometers to micrometers
Typical usesHigh-k gate dielectrics (such as HfO2), thin barriers, spacers, liners in 3D structuresThick insulating films, polysilicon, silicon nitride, tungsten plugs

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

When to use which

ALD
When the film must be extremely thin, perfectly uniform and must coat complex 3D structures: gate stacks of FinFETs and GAA transistors, DRAM capacitors.
CVD
When you need thicker films quickly and at lower cost, and the structure is not too demanding.
Trade-off
Engineers often combine them: ALD for the first thin conformal layer, then faster CVD to fill or thicken.

FAQ

Why is ALD slow?

Each cycle adds only about one atomic layer, because the precursors are pulsed one at a time with purges in between. That gives perfect thickness control but a rate of a fraction of a nanometer per cycle.

Why is ALD used for gate dielectrics?

High-k gate dielectrics such as HfO2 must be only a few nanometers thick, perfectly uniform and must coat 3D structures such as fins evenly. ALD does all three.

Is ALD a type of CVD?

Yes. ALD can be seen as a special form of CVD in which the reactions are separated in time and are self-limiting.

When is CVD preferred over ALD?

When a thicker film is needed quickly and cheaply and the structure is not too demanding, such as thick insulating films.

Check your understanding

Which is a characteristic of ALD (atomic layer deposition)?

  1. AIt can deposit films several µm thick quickly in a single step
  2. BIt repeats self-limiting surface reactions of precursors, controlling thickness at the atomic-layer level
  3. CIt is a printing technique that needs no vacuum
Show answer and explanation

Answer: B. It repeats self-limiting surface reactions of precursors, controlling thickness at the atomic-layer level

ALD repeats a cycle: dose a precursor gas that adsorbs only one layer on the surface, purge the excess, then supply a reactant gas to complete the reaction. Because reaction stops once surface sites are filled (self-limiting), thickness is controlled precisely at about 0.1 nm per cycle, and films coat trenches with aspect ratios of several tens uniformly. The trade-off is a slow deposition rate.

Which is an example of a typical material deposited by ALD?

  1. AThick copper wiring
  2. BPhotoresist
  3. CHafnium oxide for high-k films, titanium nitride and the like
Show answer and explanation

Answer: C. Hafnium oxide for high-k films, titanium nitride and the like

ALD offers atomic-level thickness control and excellent conformality, so it deposits high-k hafnium oxide gate dielectrics, titanium nitride for metal gates and barriers, zirconium oxide in DRAM capacitors, films around 3D NAND charge-trap layers, and spacers for multiple patterning—wherever thin, uniform films are needed. Its share of deposition steps keeps growing in advanced logic and memory.

Which correctly describes step coverage in deposition?

  1. AHow thick the film deposited on the sidewalls and bottom of a step is compared with flat areas
  2. BThe electrical resistance of the film
  3. CThe deposition rate per minute
Show answer and explanation

Answer: A. How thick the film deposited on the sidewalls and bottom of a step is compared with flat areas

Step coverage expresses how much film lands on the sidewalls and bottoms of trenches and holes relative to flat areas, e.g., bottom thickness ÷ top thickness as a percentage. PVD such as sputtering scores low (below 10% at the bottom of deep holes) because atoms travel in straight lines; CVD scores higher as surface reactions wrap around features; ALD reaches nearly 100%. For barriers and gate films that must be thin and uniform in tiny holes, this metric drives the choice of method.

  1. Under what condition does CVD give good step coverage (conformality)?
  2. Which is a characteristic of low-pressure CVD (LPCVD)?
  3. What is the main advantage of plasma CVD (PECVD)?

Practice Deposition (CVD and PVD) questions

Key terms explained

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.
CVD
Chemical vapor deposition. A method of depositing thin films on a wafer through chemical reactions of source gases.
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.

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